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Plastics processing Manual (CIPET JAIPUR)


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it is PDF are typed of myself. study triks & short & sweet (Technical manual) Of Diploma in Plastics technology(DPT-DPMT). All machine knowlage in plastics processing.
Er. Naresh Dhaker

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Plastics processing Manual (CIPET JAIPUR)

  1. 1. 2014-2017 Naresh Dhaker CIPET JAIPUR 2014-2017 Plastic processing tech.
  2. 2. 1 | P a g e INJECTION MOULDING  Historical Background  A single-action hydraulic injection machine was designed in the U.S.A. in 1870 by Hyatt  Heating-cylinder design was first recognised in a patent issued to Adam Gastron in 1932.  Large-scale development of injection moulding machinery design towards the machines we know today did not occur until the 1950's in Germany  InjectionMouldingProcess – Over View Solid Wide neck, Flat Product is made like bucket, cabinets, Automobile & Industrial parts etc…. by injecting molten thermoplastic material in to a closed mould which is relatively cool.  Type of Injection Moulding Machine  Hand Injection Moulding M/C  Plunger type Injection Moulding M/C  Reciprocating Screw Type Injection Moulding M/C  Hand Injection Moulding Machine vertical machine consists of Barrel, Plunger, Band Heaters along with energy regulator, Rack & Pinion system for Injecting the material by the plunger, a torpedo and nozzle.
  3. 3. 2 | P a g e  Plunger Type Injection Moulding Machine Vertical & Horizontal Plunger Type Injection Moulding Machine  The Reciprocating Screw  The feeding zone  The compressing (or transition) zone  The metering zone Machine components
  4. 4. 3 | P a g e  The Injection Process  Plasticises the material by reciprocating Screw.  Injects the molten material to a closed mould o via a channel system of gates and runners.  Cools the Mould.  Refills the material for the next cycle.  Ejects the Product.  Closes the Mould for further cycle.  Injection Moulded Items
  5. 5. 4 | P a g e Advantages of Injection Moulding Process  Parts can be produced at high production rates.  Large volume production is possible.  Relatively low labour cost per unit is obtainable.  Process is highly susceptible to automation.  Parts require little or no finishing.  Many different surfaces, colours, and finishes are available.  Good decoration is possible.  For many shapes this process is the most economical way to fabricate.  Process permits the manufacture of very small parts which are almost impossible to fabricate in quantities by other methods.
  6. 6. 5 | P a g e  Minimal scrap loss result as runners, gates, and rejects can be reground and reused.  Same items can be moulded in different materials, without changing the machine or mould in some cases.  Close dimensional tolerances can be maintained.  Parts can be moulded with metallic and non-metallic inserts.  Parts can be moulded in a combination of plastic and such fillers as glass, asbestos, talc and carbon.  The inherent properties of the material give many advantages such as high strength-weight rates, corrosion resistance, strength and clarity. Limitations of Injection Moulding  Intense industry competition often results in low profit margins.  Mould costs are high.  Moulding machinery and auxiliary equipment costs are high.  Lack of knowledge about the fundamentals of the process causes problems.  Lack of knowledge about the long term properties of the materials may result in long-term failures. Machine operation sequence The mould closes and the screw begins moving forward for injection. The cavity fills as the reciprocating screw moves forward, as a plunger. The cavity is packed as the screw continuously moves forward.
  7. 7. 6 | P a g e The cavity cools as the gate freezes off and the screw begins to retract to plasticize material for the next shot. The mould opens for part ejection The mould closes and the next cycle begins
  8. 8. 7 | P a g e Mould system A typical (three-plate) moulding system A two-plate mould. A three-plate mould. The moulded system includes a delivery system and moulded parts. Screw Used in Injection Moulding Machines
  9. 9. 8 | P a g e The screw has three zones with a ring-plunger assembly. The Feed Zone, where the plastic first enters the screw and is conveyed along a constant root diameter; the Transition Zone, where the plastic is conveyed, compressed and melted along a root diameter that increases with a constant taper; and the Metering Zone, where the melting of the plastic is completed and the melt is conveyed forward along a constant root diameter reaching a temperature and viscosity to form parts. L/D RATIO  The L/D ratio is the ratio of the flighted length (Effective Length) of the screw to its outside diameter.  Most injection screws use a 20:1 L/D ratio. But it may range from 18:1 to 24:1  In the case of Thermoset it may range from 12:1 to 16:1. High L/D Ratio results the following ….  More shear heat can be uniformly generated in the plastic without degradation;  Greater the opportunity for mixing, resulting in a better homogeneity of the melt.  Greater the residence time of the plastic in the barrel possibly permitting faster cycles of larger shots. COMPRESSION RATIO (CR)  The ratio of the first flight depth of feed zone to the last flight depth of meter zone ,  Or,  First Channel Volume of feed zone to last channel volume of metering zone,  Typically ranges from 1.5:1 to 4.5:1 for most thermoplastic materials.  Most injection screws classified as general purpose have a compression ratio of 2.5:1 to 3.0:1.  Thermo set screws have a 1:1 ratio.
  10. 10. 9 | P a g e Higher the CR results the following ….  Greater shear heat imparted to the resin  Greater heat uniformity of the melt  High Potential for creating stresses in some resins  High energy consumption Back Pressure (Kg/Cm2 or bar) Back pressure is the amount of pressure exerted by the material ahead of the screw, as the screw is pushed back in preparation for the next shot. Effect of Back Pressure  More Homogeneous Mix  Proper Melting  More compact  Sometime leads degradation Injection Speed (cm/Sec) The injection speed is the forward speed of the screw during its injection operation per unit time. Effect of Injection Speed  Easy Injection of Material  Avoid Short-Shot  Some times leads more orientation & burn marks Screw Rotation Speed The screw rotation speed (RPM) is the rate at which the plasticizing screw rotates. The faster the screw rotation result the following ..  Faster the material is compressed by the screw flights  Increasing the amount of shear heating  Low residence time, some less melting
  11. 11. 10 | P a g e Cushion The cushion is the difference in the final forward position of the screw and its maximum allowable forward position.  More Cushion results more residence time, some time degrades.  If the screw were allowed to travel its full stroke and stop mechanically against the nozzle, the cushion would be zero.  With zero Cushion no hold on works.  Typically a cushion of 3 to 6 mm is used. Materials for Injection Moulding  Acrylonitrile butadiene styrene (ABS)  Acetal  Acrylic  Polycarbonate (PC)  Polyester  Polyethylene  Fluoroplastic  Polyimide  Nylon  Polyphenylene oxide  Polypropylene (PP) **  Polystyrene (PS)  Polysulphone  Polyvinyl chloride (PVC
  12. 12. 11 | P a g e 
  13. 13. 12 | P a g e
  14. 14. 13 | P a g e TOGGLE TYPE CLAMPING  A toggle is mechanically device to amplify force.  In a moulding machine, which consists of two bars joined, together end to end with a pivot .  The end of one bar is attached to a stationary platen, and the other end of a second bar is attached to the movable platen.  When the mould is open, the toggle is in the shape of a V.  When pressure is applied to the pivot, the two bars form a straight line. TOGGLE TYPE CLAMPING TOGGLE TYPE CLAMPING ADVANTAGE  Low cost and lower horsepower needed to run.  Positive clamp of the mould DISADVANTAGE  Do not read the clamp force.  Clamping is more difficult.  Higher maintenance as lubricant is provided. HYDRAULIC CLAMPING  A clamping unit actuated by hydraulic cylinder, which is directly connected to the moving, closed the mould. In this case ram of hydraulic system is attached to moving platen. There are two halves in hydraulic cylinder, which is actually inlet and outlet of oil.
  15. 15. 14 | P a g e  When oil goes to the cylinder with pressure oil pushes the ram to forward direction by which moving platen moves and mould closed and when oil comes from the cylinder the ram come back and mould is open. HYDRAULIC CLAMPING ADVANTAGE  Clamp speed easily controlled and stopped at any point.  Direct a read out of clamp force.  Easy adjustment of clamped force and easy mould set up.  Low maintenance as part is self lubricated. DISADVANTAGE  It is higher cost and more expensive than toggle system.  None positive clamp. TIE-BAR LESS CLAMPING  Tie-Bar less clamping system is basically Hydraulic clamping system without any tie bar.  The platen is moved on a rail system.  The main advantage of this system there is no limitation of mould platen size.  As there is no tie bar so the mould dimension is not so important.  Also mounting of the mould is easy and it is very useful when products eject from the mould is manual.  Much larger mould mounting area.  Larger stroke compared to the toggle type machines.
  16. 16. 15 | P a g e  Full machine capacity can be utilised.  Smaller machines can mould larger components.  Saves floor space.  Saves electrical energy because of reduction in the size of machine.  Has the capacity to reduce weight of the moulded component because tie- bar stretching is not there.  Machine becomes very flexible for future modification.  Easy access to mould cavity's because of the absence of the tie bars.  Robotic arm movement becomes easy.  Fewer moving parts so lesser wear and tear so longer life for machines.  Lower lubrication required.  Removal of mould plates much simple.  Greater stability.
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  18. 18. 17 | P a g e BLOW MOULDING PROCESS  INTRODUCTION  Blow Moulding process is widely used for producing bottles or other hollow objects, due to its least expensive & simplest process to manufacture.  It offers a number of processing advantages,such as moulding of irregular curves,low stresses,variable wall thickness,the use of polymers with high molecular weight & favourable moulding cost.  Blow moulding is operated using low moulding pressure & hence result in Low Internal Stresses.  Commonly used materials are:PE,PVC,PET,PC, PA, POM.  It is principally a mass production method.  Blow Moulding is an alternatives process to other process like Injection Moulding. Rotational moulding Thermoforming  Since the mould used for the process consists of female cavity, it is easy to vary wall thickness & weight of the part.  This is done either by changing machine parts or melt conditions.  PRINCIPLE  Material is fed into a heated barrel of Extruder.  With the help of screw rotation & heaters the plastic is melted and homogenised.  Melted material is forced through a set of die to form a tube or parison (Hot Plastic tube)  parison is introduced into a mould,The mould closes & pinches off.  Blow pin is inserted through the open end of the parison to form a neck.
  19. 19. 18 | P a g e  Finally air is introduced through the blow pin to inflate the parison inside the mould.  By this, the molten Polymer copies the details of the Mould.  Lastly the moulded product is cooled & ejected.  In the finishing stage, the part undergoes, trimming, finishing, Printing-Labeling & decorating.  BASIC REQUIREMENTS  Homogeneous melt of plastic material.  Formation of the molten resin into a hollow tube or parison.  Sealing the ends of the parison in the closing mould,except the area through which blowing air can be supplied.  Inserting the blow pin or mandrel through the open end of the parison.  Blowing or inflating the parison inside the mould.  Cooling the blow moulded part.  Ejecting the part & trimming flash if needed.  Finishing & decorations on the product.  BLOW MOULDING METHODS  Based on the method used to create the parison or preform, two types of blow moulding are recognised.  Extrusion Blow Moulding that uses an extruded tube.  Injection Blow Moulding that uses an Injection Moulded preform.
  20. 20. 19 | P a g e  Blow moulding methods commonly used in Industries are :-  Extrusion Blow moulding  Injection Blow moulding  Stretch Blow moulding.  Press Blow moulding & Dip Blow moulding for squeeze.  Multilayer Co extrusion Blow moulding. BLOW MOULDING MACHINES & THEIR CONSTRUCTION. Blow moulding machine consists of two parts: 1. The parison Forming Unit(Extruder). 2. The Blow moulding unit.
  21. 21. 20 | P a g e BLOW MOULDING UNIT  Made off two female cavities that close around a parison.  Pinch-off at one or both ends.  One entrance for the blowing air.  A cutout section. PLASTIC MATERIALS  Those Materials which show high melt strength & good stretch properties at the extrusion temperature are suitable for formation of parison & blowing. Following Polymers are more often blow moulded:  PE(LDPE,HDPE,HMHDPE)  PP,PVC,PA,PS,PC,PET,EVA,SAN,TPE  LDPE us used for more flexible items.  HDPE is used for Rigid Bottles, Chemical Drums, gasoline tanks.  PP is used because of high stiffness, good chemical resistance, clarity and good glass and good resistance to high temperature.  PVC & PS for general purpose articles requiring transparency at modest cost.  RPVC – Can be bi axially oriented in extrusion stretch blow moulding.  PC is used for containers which show good transparency, excellent impact strength, good heat resistance, good printability.  PET is used in basically oriented stretch blow moulding & commonly used for carbonated beverages, packaging of drinks pharmacy products & water bottles.
  22. 22. 21 | P a g e  EVAL (Ethylene Vinyl Alcohol) is used as barrier layer in Multilayer containers. PROCESSING TEMP.OF PLASTICS. Polymer Proc. Temp. Drying Temp. Cavity Temp. LDPE 160-170 50-70 5-30 LLDPE 160-180 50-70 5-30 HMHDPE 160-210 50-75 10-30 PP 170-220 50-80 20-50 PVC 170-200 50-70 15-35 PS 250-280 50-70 20-40 PC 280-300 70-100 50-70 PA 190-240 70-100 20-40 PET 240-255 120-140 10-20 ……………………………END……………………………………
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  24. 24. 23 | P a g e EXTRUSION  EXTRUSION:  Continuous Process  In principle, the plastic raw material is plasticated by means of a screw plastication unit and the molten material is continuously pumped out through a standard orifice (die) in order to take the shape and then the shape is set by cooling/sizing system. Example: Film,Pipe,Tube, Profile, Monofilament, Box Strapping etc.  CLASSIFICATION OF EXTRUDERS 1 Batch – Type 1.1 Ram Extruders 1.2 Reciprocating screw extruders g 2 Continuous –Type 2.1 Screwless Extruders 2.1.1 Disk Extruders 2.1.2 Drum Extruders 2.1.3 Other Extruders 2.2 Screw Extruders 2.2.1 Single-Screw Extruders (SSE) 2.2.2 Twin-Srew Extruders (TSE)
  25. 25. 24 | P a g e 2.2.3 Multi-Screw Extruders  SINGLE SCREW EXTRUDER  Single Screw Extruder Parts & its functions Screw Pump : Combination of Screw & Barrel Hopper : Funnel like device, mounted on Hopper throat. Holds a constant reserve of material. Barrel : Cylindrical housing in which the screw rotates. Hopper Throat : Circular opening at the feed end through which the material enters the screw pump. Drive System : AC/DC drives Speed reduction gear box Transmission system The Single-Screw Extruder consist of a screw rotating in heated barrel or cylinder to which the material is fed.  Feed hopper  Extruder Screw and Barrel  Drive system (motor, gear box, transmission)  Thrust Bearing  Heating and Cooling Elements  Screen Pack and Breaker plate
  26. 26. 25 | P a g e  Die  Temperature and pressure controls.  Definition of terms Compression Ratio - is the ratio between the channel depth is the feed zone to that of the metering zone. - Usually from 1.5 to 4:1 L/D ratio - Length to nominal dia of screw - usually 20 to 22:1 Important Specification  Nominal dia of screw  Output (kg/hr.)  Zones of Extruder & its Functions:  Feed Zone - Transport the material from hopper to compression zone. - Compacts, eliminates air gap  Compression Zone - Transport the material from compression to metering zone. - Softens the material  Metering zone - Melts, Mixes, the material pressurizes and pumps the melt.
  27. 27. 26 | P a g e  Screw Nomenclature P is the screw pitch, distance between the centre of a two adjacent flights. W is the channel width L is the land width q is the helix angle, defined as an angle between the flight to the transverse plane of the screw axis. D is the screw diameter, developed by rotating the flight about the screw axis. R.D is the root diameter Flight is the helical metal thread of the screw. C is the channel depth o radial distance form the bore of the barrel to the root  SCREW TYPES Extruder Screws  General purpose screw  PVC screw  Nylon screw  Two stage screw/vented screw  Segmented screws is also available for special purpose
  28. 28. 27 | P a g e General purpose screw PVC screw  HEATING AND COOLING ELEMENTS There are three methods of heating extruders: • Electric • Fluid • Steam Heating Electric Heating  Induction Heaters  Cast-in Heaters  Band Heaters • Mica Insulated • Ceramic Insulated The electric heating is most commonly used due to : 1 Accuracy 2 Reliability 3 Easy to hook up.  INDUCTION HEATERS  AC Current passes through coil thus setting up a magnetic flux. Heat is generated from the resistance offered to the eddy current set up by the flux.
  29. 29. 28 | P a g e  The barrel is heated directly by its resistance to the induced current Schematic Arrangement Showing an Induction Heater in Section Advantages :  Accurate Control of Temperature.  Good provision for cooling the barrel  No possibility for hot or cool spots. Disadvantages :  Relatively high cost.  BAND HEATERS They consist of Ni-chrome or other resistance wires mica or ceramic insulated, then encased in steel cover. MICA INSULATED CERAMIC INSULATED Flexible, supplied as a single piece. Rigid, supplied in 2 halves Can withstand a load of 23-31 KW/m2 Can withstand higher heating load Shorter service life Better services life Less expensive More costly  FLUID HEATING SYSTEM
  30. 30. 29 | P a g e The heating fluid, that is most commonly used for extruders is oil. It may be heated by any suitable means (mainly electrical). The heating system consists of a heater a circulating pump, a surge tank, and a heat transfer channel in the extruder barrel.  STEAM HEATING The high specific heat and latent heat of vapourisation of water makes steam an excellent heat transfer medium. However, this system is not frequently used because of low maximum temperature that can be achieved, a need of working with high pressure piping, frequent leaks of steam that require shutting down of heating for repairs, and corrosion effects.  BARREL COOLING  Barrel Cooling is needed to prevent overheating that may cause degradation.  For small extruders fans that blow air over or around the barrel are used  Other cooling system used include:  Cooling channels inside the barrel wall  Fins on the barrel or on the heaters to speedup heat transfer  A water-fog spray over barrel.  Continuous, controlled vaporization of liquid (Water)  Copper tubing carrying cold water is sometimes used.  HOPPER COOLING  Water-cooling is used to cool the hopper throat to prevent bridging and to protect the rubber parts present in the screw support assembly.
  31. 31. 30 | P a g e  SCREW COOLING  The cooling may freeze a layer of plastic on the screw root, reducing the channel depth thus producing more shear at a cost of throughout.  This may also reverse the required relationship between the friction coefficient (low friction coefficient on the screw, high on the barrel), further reducing the drag flow.  Furthermore, there is a danger that the material staying a long time near the screw root will degrade, contaminating the product.  It is important to remember that the conveying ability of the screw is controlled by the friction coefficient ratio : f(barrel) / f(screw) ó it is important to maximize this ratio.  Under normal circumstances the minimum value of the ratio that guarantees conveying is 1.4.  BREAKER PLATE AND SCREEN PACK  BREAKER PLATE Perforated circular metallic disc of about 4-5 mm thick. Functions - Support for Screen pack - Converts the Spiral flow of melt in to stream lined laminar flow - Holds back contamination and unmelted particles.  SCREEN PACK  Wire mesh 40,60,80  Arrests the unmelted particles and contamination  Helps in developing back pressure  DIE DESIGN
  32. 32. 31 | P a g e The role of a die is to form the melt into a desired linear product: fibres, films, sheets, profiles, rods,etc. The die is a channel, whose profile changes from that of the extruder bore to an orifice, which produces the required form. The dies can be classified using different criteria. For example, considering cross section of the extrudate one may recognize dies to produce:  Solid Cross-Sections  Hollow Cross –Sections Another classifications scheme is based on the die attachment to the extruder barrel:  Straight –through dies  Cross –heat dies  Offset dies  SOLID CROSS – SECTIONS  A Typical Die Design for extruding a solid rod is shown in fig.  In the figure, DD is the diameter of die orifice, DB is the diameter of bore of extruder barrel, a is the lead-in angle, and P is the die land.  Because of the screen pack and breaker plate assembly, the pressure in the extruder (PE) is reduced by the pressure loss across the assembly (PL).  Since the die outlet is at atmospheric pressure, the working pressure is the die pressure (PD) given by the difference: PD = PE – PL.  HOLLOW SECTIONS Hollow products like pipe or tubes are produced using the die design shown in Fig.  The outer diameter of tube is determined by the diameter of the outer die ring orifice.
  33. 33. 32 | P a g e  The inner diameter is determined by the mandrel diameter  To make the mandrel and outer die ring orifice concentric, centring screws are provided.  The mandrel is held in position by a spider. In the centre of the spider a hole is drilled to supply air down the mandrel.  To provide a smooth glossy extrudate, the die head is heated. A cold die may cause blockage of the die.  STRAIGHT – THROUGH DIES  Those dies whose axes are arranged to be in line with the direction of supply of melt.  Spider, Mandrel is needed for tubes  Used for the extrusion of pipe, rod, profiles and sheet  CROSSHEAD DIES  Arranged with their axes at an angle of 908 (458 and 308 are also used) to the melt feed.  No need for spider assembly.  Used for the production of insulated wires, cables  OFFSET DIES  Combination of both straight – through die and off-set die.  Used for the production of pipe.  EXTRUDATE SWELL  Extrudate is contraction in the direction of extrusion and expansion in the cross-section while emerging from the die is called Extrudate Swell.
  34. 34. 33 | P a g e  The phenomenon (previously called die swell) is illustrated in fig.  Numerically, the extrudate swell is defined as the ratio of the outer extrudate diameter (DE) to the other diameter of the die exit (DD), i.e., B = DE / DD  When the melt emerges out of the die lips, there will be expansion in the direction perpendicular to flow and contraction in the direction parallel to flow.  Constrained molecules tends to relax at the die outlet. This leads to die swell.  This is nullified by higher take off speed. Extrudate Swell may be reduced by :  Decreasing the extrusion rate  Increasing the melt temperature  Increasing the die land  Increasing the draw-down ratio.
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  36. 36. 35 | P a g e COMPRESSION MOULDING  INTRODUCTION Compression Moulding is a technique principally for thermoset plastic moulding in which the moulding compound (generally preheated) is placed in the heated open mould cavity, mould is closed under pressure, causing the material to flow and completely fill the cavity, pressure being held until the thermoset material has cured. In this process a measured quantity of a plastic material is shaped or formed by heat and pressure. A known weight or volume of the moulding powder is kept in an open mould cavity. The mould consists of two halves, a male and a female part, and is normally preheated. Mould halves are attached to the upper or lower platens of a press. These mould halves are called the core or plunger and the cavity. Heat is applied either directly to the mould or indirectly through the platens. A systematic diagram of Compression Moulding is given in Fig.1. The moulding compound, generally in powder form, is placed in the mould cavity after which the mould is closed. Heat and pressure is applied causing the plastic material to soften, flow and fill the cavity completely. Depending on the characteristics of the plastic material and the design of the mould, the temperature ranges from 140°C to 250°C & pressure range from 2000 to 10,000 psi. The mould remains heated and closed until the plastic material cures and sets. A moulding pressure of 2000 psi on the total projected area of the part is generally recommended. To this must be added approximately 800 psi for every inch of the vertical height of the moulded part, to arrive at total moulding pressure required. After the material has cured sufficiently, the mould is opened and the piece is ejected by knock out pins. The cure time depends on the size and thickness of the part. This may vary from 20 seconds to 10 minutes. The cure time cannot be predicted accurately, but must be determined by experience or by trial.
  37. 37. 36 | P a g e  MATERIALS Thermosetting resins: Phenol formaldehyde, Urea formaldehyde, Melamine formaldehyde, Polyester, Epoxy. Special Thermoplastics: Ultra high molecular weight HDPE.  VARIABLES AND FLOW PROPERTIES VARIABLES :- The following variables to pay attention during moulding. 1. Flow Characteristics 2. Curing Characteristics 3. Particle Characteristics („grinds‟) FLOW PROPERTIES:- There are three independent flow properties of concern thermoset moulding. 1. The ease of flow (fluidity), 2. The total flow that occurs before the material sets, 3. The time available for flow PROCESS OPERATION 1. Pelleting (an optional stage) 2. Preheating (also optional); 3. The moulding stage; 4. Finishing operations.  PROCESS OPERATION 1. PELLETING: Pelleting permits accurate measurement of the charge to the mould reduces contamination and facilitates preheating. It does not,however, always fit in very well with more automated compression systems. 2. PREHEATING: Preheating is useful in that it reduces expensive moulding time, allows rapid heating of large pellets or masses of powder. Helps to remove moisture and other volatiles prior to moulding and because it advances the cure has been claimed to reduce moulding shrinkage. 3. THE MOULDING STAGE: The powder material (specified) is placed inside the heated mould cavity and mould is closed, predefined pressure is continuously applied on to the moulding.
  38. 38. 37 | P a g e Since the material will start to cure as soon as it comes into contact with the heated mould. It is important that flow and shaping be completed before the gel point is reached ( i.e. the material shows first signs of being cross linked) The optimum cure (i.e. cross-linking) for one property is not necessarily the best cure for another. It is therefore necessary to establish what properties are important in the finished moulding and use cure times and temperatures that give a good compromise to the various requirements, including that of cost. It is particularly important that flow and cross-linking process are controlled and are in the correct sequence. While improvements in the operation can often be made by such refinement as breathing (venting) they will be no value if the basic process is not correct. 4. FINISHING OPERATION: The part finishing is done depending upon the product comes out after curing. Sometimes it is observed that additional material is coming out from the moulding as a flash which can be removed after curing. To make more attractive some other finishing operations can be performed on to the finished product.  MOULDING SEQUENCE The moulding sequence in compression moulding is as follows (Fig. 2) :(i) Mould open, Cavity cleaned, (ii) Pellet loaded, Mould closing, (iii) Mould closed - Curing stage, (iv) Mould open, Moulding ejected.
  39. 39. 38 | P a g e  MOULDING CYCLE The compression moulding cycle, in many instances the dwell time will comprise a larger fraction of the moulding cycle than indicated in Fig. 3.  MACHINERY & EQUIPMENTS  COMPRESSION PRESS Simplicity is the keynote of the compression moulding press. It basically consists of two platens that close together, applying heat and pressure to mould some material into the desired shape. Most compression presses are hydraulically operated though some are also operated pneumatically. The platens move up and down on four corner posts under pressures that typically can range from 20 to 1000 tons, depending upon press size. Platen themselves can range in size from 8 in. square to 5 ft.square.  COMPRESSION PRESS Various degrees of automation are available in most modern hydraulic presses to feed material and eject the part after cure. Older, simpler systems had temperature, pressure, dwell and time controls. Today‟s modern equipment has more sophisticated microprocessor controllers. Presses used for moulding thermoset are available in many different shapes and design and can be classified as either hand, mechanical or hydraulic types. Most compression moulding presses are of the upstroke type as shown in the figure 4.
  40. 40. 39 | P a g e  HYDRAULIC SYSTEM UPSTROKE TYPE: In this type, a hydraulic ram moves the bed or bolster of the press upward to close the mould. The strain rods or tie rods hold the upper and lower parts of the press in accurate alignment. So that the two parts of the mould do not mismatch. Adjusting collars on the strain rods allow the daylight opening between platens to be adjusted to the requirements of the mould. The daylight opening is the maximum distance between upper and lower p1aten. This opening must be large enough so that the compression moulding can be removed from the die. The two parts of the mould are bolted to the upper and lower platens of the press. These platens have heating channels, which are backed with asbestos board insulation. During setting-up, the two platens of the press must be checked for parallelism. UPSTROKE TYPE: The disadvantage of the simple upstroke type compression moulding press is that the return is slow, relying entirely on gravity, although pressure can be applied fairly quickly. This limitation can be sorted out by incorporating a pushback ram in the machine. DOWNSTROKE TYPE: A downstroke press has a fixed lower bolster and a moving upper bolster (fig.5). The down stroke machines are better suited to the moulding of unusually large components, which require a longer stroke. The downstroke type press, overcomes the disadvantages of the up stroke press.
  41. 41. 40 | P a g e  HEATING AND COOLING OF MOULDS Heating is generally done by cartridge type electrical heaters even though hot oil system can also be used. The mould cooling is done by circulation of cold water through the channels.  TYPES OF MACHINES Presses used for moulding thermosets are available in many different shapes and designs and can be classified as either hand, mechanical or hydraulic type.  TYPES OF MOULDS  Compression moulds are usually constructed of case-hardened tool steel, in order to withstand the high pressures of compression moulding. There are three basic types of Compression moulds :  1. FLASH TYPE  2. POSITIVE TYPE  3. SEMI POSITIVE – FLASH TYPE  FLASH TYPE The flash type of mould is used to produce a shallow shape (Fig. 6). With this type of mould. a slight excess of the moulding powder is loaded into the mould cavity and, on closing the top and bottom force. The excess powder is forced out and 'flash' is formed. This can be removed quite readily, though it tends to make the process somewhat wasteful. However the moulds are relatively cheep and are suitable for building up into tools containing multi-impressions. Another advantage is that, while to some extent it is wasteful as far as raw materials are concerned, very slight labour costs are necessary in weighing out the powder.
  42. 42. 41 | P a g e  POSITIVE TYPE The positive type of mould produces a vertical flash in the direction of moulding pressure (Fig. 7). In this mould, the upper part of the mould (the force) fits closely into the lower part (the cavity). This type of mould is also easy to manufacture, but no allowance is made for placing excess powder in the cavity. Should this occur in error, the mould will not close. If however, the correct charge added, only a small amount of the flash results and it can be easily removed. A disadvantage with such a mould is that the gas liberated during the chemical curing action will be trapped, thus resulting in blisters. This type of mould is suitable for moulding high bulk factor material and is used only on a small scale for molding thermosets. It is however, used for moulding laminated plastics and certain rubber components. Some of the reasons for discarding this type of mould are the necessity for weighing or measuring an exact charge, excessive wear on sliding fit surface of the top and bottom forces and difficulty of ejecting the moulding.
  43. 43. 42 | P a g e SEMI POSITIVE – FLASH TYPE The semi-positive flash type of compression mould combines the features of both the flash and positive type(Fig. 8). The moulding powder is forced into a complicated mould, at the same time, making allowance for excess powder and flash. This type of mould is expensive to manufacture and maintain and is therefore used where long runs are required. With such a mould, it is also possible both horizontal and vertical flash. The area of the faces of the mould which come in contact with each other when the mould is closed, is generally referred to as the 'land'. ADVANTAGES  Lowest cost molds  Little "throw away" material provides advantage on expensive compounds  Often better for large parts  Lower labor costs  Minimum amount of wasted material & Improved material efficiency  Internal stress and warping are minimized.  Dimensional accuracy & stability is excellent.  Shrinkage is minimized and closely reproducible.  Thick sections and large parts are practically possible.
  44. 44. 43 | P a g e  Lower molding pressures allow molding of large parts on presses of lower tonnage LIMITATIONS  Offers least product consistency  Not suitable for fragile mold features, or small holds  Uneven parting lines present a mold design problem  High impact composites make flash control & removal difficult.  The depth of the molded holds is limited to 2 or 3 times their diameter  Shot weight must be tightly controlled  Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control.
  45. 45. 44 | P a g e
  46. 46. 45 | P a g e TRANSFER MOULDING  INTRODUCTION For some applications, it is desirable to close the mould first and then introduce the moulding compound in its fluid state through a small opening or gate leading to the mould cavity or cavities. This technique is called transfer or plunger moulding. This is variation of compression moulding and is particularly valuable when intricate shapes have to be moulded. It is also useful when metal inserts have to be incorporated (as in electrical component). Even with the use of pellets there is danger of their moving during the liquefaction process and causing movement of the inserts. It is used frequently when the mould sections are very delicate, when the moulded parts has thick sections 3.2 mm or more, or when an insert is retained in the cavity for moulding in place. In such applications, closing the moulding containing a moulding compound that is not yet fully liquid (as in the case in transfer moulding), flow speed and pressure can be controlled to minimize the possibility of any such damage.  PROCESS OPERATIONS: In transfer moulding, a known amount of moulding powder is placed in a heated chamber (transfer pot), outside of the mould. When liquefaction is completed a piston forces the resin through a connecting channel into the mould. As in compression moulding the mould is kept heated and this completes the chemical reaction which leads to hardening of the moulding. A systematic diagram of Transfer Moulding is shown in fig.1.  TYPES OF TECHNIQUE 1. TRUE TRANSFER OR POT TYPE TRANSFER MOULDING In true transfer or pot type transfer moulding, the mould is closed and then placed in an open press. The charge of moulding compound is introduced into an open pot at the top of the mould. The plunger is then placed into the pot and
  47. 47. 46 | P a g e the press is closed. As the press closes, it pushes against the plunger, which in turn exerts pressure on the moulding compound, forcing it down through a vertical passage called a sprue and through runners and gates into the cavities. After the curing, the mould is removed from the press, the plunger is withdrawn, the mould is opened, and the parts are ejected. (Fig. 2) Pot type transfer moulding may also be done with the bottom half of the mould bolted to the lower press platen and the plunger bolted to the upper press platen. The upper mould half, containing the pot, may then be manually placed over the lower mould half. Or, it may be suitably supported and guided so that the opening of the press separates the lower mould half from the upper mould half and in the same motion, pulls the plunger out of the pot. The procedure is reversed when the press closes. 2. PLUNGER TRANSFER MOULDING As much material is wasted in the large pot, it is generally more economical to use plunger transfer moulding instead. In plunger moulding, the plunger is essentially a part of the press rather than part of the mould. It is usually driven by a hydraulic circuit and a cylinder attached to the head of the press and can, therefore, be considerably smaller in diameter than the pot type plunger. The mould is held closed by the clamping action of the press, independent of the plunger movement or force. The behavior of the moulding compound is identical, however, channels, called sprues and runners, direct the flow of
  48. 48. 47 | P a g e material from the pot to the cavities passing through a restriction, or gate, just before entering the cavity. A single pot can feed many cavities. Air in the cavities displaced by the incoming materials must be expelled through strategically placed vents. When the material is introduced into the pot, it is usually a measured charge in compacted form, preheated to a temperature approaching that of polymerization. Only sufficient material for a single shot is loaded at one time. The force that moves the charge of preheated material out of the pot is transmitted through a plunger, which is closely fitted to the pot to prevent leakage of material through clearances between the plunger and the sides of the pot. Sealing grooves usually are cut into the plunger to further reduce leakage. When a transfer mould reaches the end of its cure cycle, the entire shot is ejected, including the gates, runners, sprues and the cushion of cured material (called the cull) formed in the pot. Preheating of material is very important in transfer moulding. Cold material flows slowly, with the result that the first material to enter the cavity may polymerize before reaching its final destination. If that occurs, poor quality mouldings can then be expected, both from the standpoint of appearance and that of their mechanical properties. 3. SEMI AUTOMATIC TRANSFER MOULDING In semi automatic transfer moulding, the operator actuates the press each cycle to close the mould and then manually introduced the moulding compound into the transfer pot, which is in top half of the mould. Next, the operator actuates appropriate controls to cause the plunger to descend into the pot. The press controls take over to time the cure cycle, open the mould, and eject the parts from the cavities using ejector pins in the mould. The operator lifts the parts from the ejector pins and initiates another cycle. 4. FULLY AUTOMATIC TRANSFER MOULDING In fully automatic transfer moulding, the press recycles automatically thereby feeding moulding compound into the transfer pot and removing parts at the end of each cycle, Automatic transfer presses are often configured horizontally so
  49. 49. 48 | P a g e that when the moulded parts are ejected from the cavities, they will readily fall into a container or conveyor belt below the open mould.  MOULDING SEQUENCE The mould is closed and material is placed into the hot transfer chamber of fully automatic transfer moulding and plastication. The plunger descends into the pot, causing material to melt and flow through runners into hot mould cavities. Curing under the pressure of transfer ram. After cure, the press opens, the plunger retracts, and parts are ejected with cull and runners.
  50. 50. 49 | P a g e Deflashing and mould cleaning  TYPES OF MOULD 1. INTEGRAL POT TRANSFER MOULD The integral pot transfer mould was the first to be used and is so called because the pot and plunger are built as an integral part of the mould. Round pots are the most common, but other shapes also can be used to accommodate particular sprue locations that might be needed with minimum material waste. The integral pot mould frame is a three plate type with the pot contained in the middle section. The transfer plunger is mounted in the top section of the mould frame and the cavity in the bottom section. The area of the pot must be at least 10% larger than the total clamping area (horizontal surfaces that are in contact with plastic material) in the cavity section. This is to prevent the unclamping force from overcoming the clamping force and causing the mould to flash. After the material has cured, the moulded parts are ejected by the action of the press moving the mould ejector bar, but the cull and sprue are held to the bottom of the plunger by one or more moulded dovetails. A lateral blow with a wooden stick or a soft hammer releases the cull and thereby clears the mould. 2. PLUNGER TRANSFER MOULD Plunger transfer moulds or plunger moulds, describe mould that use an auxiliary press ram to force the plunger into the pot (or cylinder), moving the material out of the pot and into the cavities. Transfer pressure and speed of transfer are readily controllable, independent of clamping pressure. Pot size (hence cull size) in a plunger mould need only be large enough and deep enough to accommodate the full charge of material.
  51. 51. 50 | P a g e Maximum pot area, on the other hand is established by determining the force in tons that can be delivered by the auxiliary ram of the press and dividing that figure by 3.5. This will assure that about 540 kg/cm2 force will be available as moulding pressure, which is sufficient for most transfer grade material formulations. The auxiliary rams usually are atop the upper, fixed platen of the press and are downacting. The clamping ram moves the lower platen upward to close the mould. After the mould is clamped, the material is loaded into the pot, and the auxiliary ram is energized. Ratio of the clamp ram to the auxiliary ram is generally 3 to 1 or 4 to 1. When the material is cured, the auxiliary ram retracts and the press is opened. The moulded parts, cull and runners are ejected simultaneously by the action of the mould ejected bar.  ADVANTAGES  Loading a preform into the pot takes less time than loading preforms into each mold cavity.  Tool maintenance is generally low, although gates and runners are susceptible to normal wear.  Longer core pins can be used and can be supported on both ends, allowing smaller diameters.  Because the mold is closed before the process begins, delicate inserts and sections can be molded.  Higher tensile and flexural strengths are easier to obtain with transfer molding.  Automatic de-gating of the mold's tunnel gates provides cosmetic advantages.  LIMITATIONS  Molded parts may contain knit lines in back of pins and inserts.  The cull and runner system of transfer molding leaves waste material, but this scrap can be greatly reduced by injection molding with live sprues and Runner less Injection Compression (RIC).
  52. 52. 51 | P a g e  Fiber degradation of orientation occurring in the gate and runner system reduces the molded part's impact strength.  Compared to compression molding, high molding pressures are required for the transfer process, so fewer cavities can be put into a press of the same tonnage.
  53. 53. 52 | P a g e
  54. 54. 53 | P a g e ROTATIONAL MOULDING INTRODUCTION  Rotational moulding is a process of making hollow articles.  The part is formed inside a closed female mould.  In this process the mould rotates biaxially during heating and cooling cycle.  Rotational moulded pieces are stress free because the pieces are produced without any external pressure.  The ability to manufacture large containers of capacity 30,000 gallons as well as small items like golf ball is responsible for the growth of this process.  The Process requires relatively in expensive equipment and exerts on only small pressure on the material being formed. PRINCIPLE  The principle of the process is that finely divided plastic material becomes molten when comes in contact with hot metal surface of the mould and takes up the shape of that surface.  As only female mould is used, the only pressure exerted are those induced by gravity and centrifugal force.  The polymer is then cooled while still in contact with the metal mould to get the solid copy of the surface.  Rotational moulding permits to make a wide variety of fully and partially closed items. Articals of Roto moulding
  55. 55. 54 | P a g e ADVANTAGES AND DISADVANTAGES Advantages  The major advantage of rotational moulding as compared to other plastic moulding processes is that it can make very large parts.  It requires comparatively low cost input.  The products are stress free with strong out side corners. There are no weldlines, sprue or gate marks.  Here impact toughness is improved and failure due to brittleness is reduced  The external dimensional details can be easily moulded with better surface glossiness .  The colour changes in the product can be made easily. Similarly mould changes can also be done rapidly.  Multilayer moulding is also possible for providing chemical resistance and strength to the part.  Good control over wall thickness variation is also achievable as compared to blow moulding or thermoforming.  Moulding can be done with metal inserts and minor undercuts.  No scrap or very little scrap is produced.  Low tooling cost. DISADVANTAGES  The moulding cycles are longer compared to blow moulding and thermoforming.  In case of big parts loading and unloading is very labour intensive.  The process is not suitable for parts with wall thickness less than 0.03”.  The conversion of plastic granules to powder form increases the equipment and process cost.
  56. 56. 55 | P a g e LIMITATIONS 1. It is an open moulding process and so there are no cores inside the hollow parts 2. Surface details and dimensions can only be provided and controlled on the side of the part. 3. The process requires heating and cooling of not only plastic material but also the mould as well. 4. The long heating cycle increase the possibility of thermal degradation. 5. It is not suitable for materials with less heat resistant to withstand the long heating cycle. 6. The material must be capable of being pulverised into fine powder that flows like liquid. 7. Removal of plastic sticking onto the surface of cavity requires careful application of mould release agent. ROTATIONAL MOULDING PROCESS  Loading  Heating & Moulding  Cooling  Unloading
  57. 57. 56 | P a g e LOADING  This step includes weighing of the charge for a particular product then transferring it to the open cold mould.  The mould surface usually coated with a mould releasing agent.  The raw material can be in the form of powder or liquid state.  The wall thickness can be controlled by varying the amount of raw material charged.  After the material is charged the mould is closed and clamped to the arm of the machine.  Then the mould is moved to an oven for heating HEATING & MOULDING  The mould fixed to the arm now moved to a closed chamber where it undergoes intense heating.  During heating the mould rotates in two planes perpendicular to each other.  The rotational speed varies in the range of 0-40 rpm on minor & 0-12 rpm on the major axis.  4:1 ratio is the most commonly used for symmetric article.  For moulding unsymmetrical products a wide variability of ratios is necessary.  The revolving motion distributes the plastic material uniformly over the inside surface of the mould.  The plastic material fuses into layers to form a hollow article.  In case of hot air oven the temperature should be between 200ºc to 500ºc.  The moulding cycle time varies from 2 to 20 minutes depending upon the wall thickness of the article.  The wall thickness can vary from 2 to 12mm or more.  The heating chamber should be large enough to house the mould and rotate it freely.
  58. 58. 57 | P a g e COOLING  For cooling the mould is transferred to the cooling station while still rotating.  The cooling should be made as quickly as possible to avoid the plastic part to shrink away form the mould.  Otherwise the part will get distorted.  Cooling can be done by air or water. To provide faster cooling cold water is sprayed over the mould. UNLOADING  After cooling the mould is transferred to the unloading station.  In this step the mould is opened and the cooled part is taken out.  It can be done manually or with mechanical assistance.  The ejection can also be done by forced air.  The mould is cleaned and the charge is loaded for the next cycle. HEATING SYSTEM IN ROTATIONAL MOULDING  The rotational molding process heats and cools both the mold and the plastic material.  Cavities are build up with materials having high thermal conductivity, in order to minimize the time required for heat to pass through the wall of the cavity.  Rotational molds may be heated by either an open-flame method, a hot air re-circulating oven method, or by a hot-oil jacketed mold system.  Molten Salt-it leads to corrosion.  Infrared Heater-Very efficient but costly method.  The most used system is a re-circulating hot-air oven. RE-CIRCULATING HOT AIR OVEN METHOD  In this system a positive displacement circulating fan distributes air through a system of ducts into the swept volume of the oven.
  59. 59. 58 | P a g e  The capacity of the fan (cubic meters of air per minute), will determine the number of air changes per minute.  On contemporary machines, air should be changed in the oven approximately 25-30 times per minute in order to provide an effective heating for the mold.  Direction of the air in the oven is generally caused by the directional louvers so that no “dead spots” are created.  The static pressure capability of the fan system provides force to push the air over the mold and provide the scrubbing action of the hot air on the mold.  On contemporary machines, air should be changed in the oven approximately 25-30 times per minute in order to provide an effective heating for the mold.  Direction of the air in the oven is generally caused by the directional louvers so that no “dead spots” are created.  The static pressure capability of the fan system provides force to push the air over the mold and provide the scrubbing action of the hot air on the mold.  The absorption of the heat by the mold transmits through to the powder to create the molded parts.  The medium for heating hot-air ovens may either be natural gas or oil with a modulating burner.  In some cases, electric heaters are used to generate the hot air environments.  The regulation of air temperature in the swept volume of the oven is controlled by sophisticated electronic temperature control devices.
  60. 60. 59 | P a g e  The time that the mold remains in the oven is known as “Oven residence time”.  The medium for heating hot-air ovens may either be natural gas or oil with a modulating burner.  In some cases, electric heaters are used to generate the hot air environments.  The regulation of air temperature in the swept volume of the oven is controlled by sophisticated electronic temperature control devices.  The time that the mold remains in the oven is known as “Oven residence time”.  The oven residence time necessary to cure a part will depend upon the wall thickness of the part, the type of plastic material being used to mold the part, and the conductivity of the metal of the mold.  Aluminum with a higher conductivity, allows heat to transfer from the air stream to the mold and the powder at a much faster rate than does steel.  Thinner gauge aluminum helps to increase the conductivity. OPEN FLAME HEATING  In the case of the rock and roll machines there was no heated oven; an open –flame method used whereby a manifold of gas jets was placed to evenly heat the mold.  As the mold rotated about the major (rolling) axis, the heat was imparted directly onto the mold surface, and transferred through to the plastic.  The machine was inexpensive to manufacture, but the operating cost were significantly more than the closed oven type of heating system.  All of the thermal energy not imparted to the mold went into the atmosphere creating increased temperature in the work environment and the loss of energy.  Open flame machines are still used for very large tanks that are too large to fit in conventional re-circulating ovens or where the quantity of tanks is so small as to not justify the expense of building a large oven.
  61. 61. 60 | P a g e HOT OIL HEATING METHOD  The hot oil jacketed mold system was one of the earliest systems used for rotational molding.  In closed oven with re-circulating hot air, their will be heat losses due to the extra volume in the oven not filled by the mold.  The jacketed mold maintains the mold temperature very close the the temperature of the hot oil being used.  Therefore, the hot oil system generally uses a lower temperature for molding since the oil is in direct contact with the mold and imparts the heat energy very quickly.  The difficulty of using jacketed hot-oil molds is that the expenses of the molds is considerably more than used in the other types of heating. THE HEATING TIME DEPENDS UPON THE FOLLOWING FACTORS.  Part Size  Wall thickness  Resin
  62. 62. 61 | P a g e THE SPEED DEPENDS UPON THE FOLLOWING FACTORS  Part size  Geometry  Resin  Heating rate  Thermal conductivity of mould metal  Sometimes due to over heating the air inside the mould gets expanded and some internal pressure builds up, which may distort the mould as well as the part.  To avoid this, mould can be vented.  To provide vent a small pipe is placed which runs from inside to outside of the mould. To prevent entry of the vent pipe a small amount of glass fiber can be added. ROTATIONAL MOULDING MACHINES Three basic types of machines are : i. Batch type ii. Semiautomatic type iii. Continuous or rotary type  Batch type is used in prototype or low volume production. This method requires less capital but most involvement of manual labour.  Continuous or rotary type method include three basic stations arranged 120º apart from arms attached to a central hub containing the drive mechanism.  Advantage of this system is minimal labour and high production rate.
  63. 63. 62 | P a g e ROTATIONAL MOULDING BY CAROUSEL-TYPE MACHINES  The carousel type machine is a three-station rotary indexing type with a central turret and three cantilevered mould arms.  Individual arms are involved in different operations simultaneously so that no arms are idle at any time.  All operations are automated and at the end of each cycle the turret is indexed 120º, thereby moving each mould arms to its next station.  Newer carousel machines being offered today have four arms.  The additional arm can be used in a second oven, cooler or load station, depending on, which is the most time-consuming part of the over all cycle.  The four-arm carousel machines increase the production by allowing the indexing from station to station to occur more frequently than could be managed on a three-arm machines. . MULTILAYER ROTATIONAL MOULDING
  64. 64. 63 | P a g e  It is used to combine two different colours of the same material or two dissimilar material into one part.  It offers potential advantages of increased stiffness.  When solid and foam are combined, improved barrier properties and permeation resistance by using thin inner or outer layer of low permeable material.  It is a two staged process in which the skin of one material is combined with an inner layer of another material.  The first shot of material moulds in the normal fashion and the material adheres to the mould.  When adhering and curing of the layer is completed the mould is removed from oven and second shot material is added.  While producing very thick parts care should be taken not to thermally degrade the outer layers at the expense of optimizing properties of inner layers.  The double process is at its best when two walls adhere to each other.  Two different colors of virgin and reprocessed combinations of the same material would be ideal.  Dissimilar materials such as nylon and PE that don‟t bond to each other are being used, but there are some limitations.  Two materials must be chemically compatible.  They should have similar processing temperature and similar co-efficient of thermal expansion. MATERIAL CONSIDERATIONS FOR ROTATIONAL MOULDING  All thermoplastic materials can be rotationally moulded.  HDPE, LLDPE, LDPE,PVC,PC,ABS,PS,Acrylics, Nylon , TPU, SAN Polyesters are the materials which are commonly used.  The various properties considered in selecting the proper material are grindability, particle distribution,particle mesh size, pourability, bulk density and fusability.
  65. 65. 64 | P a g e  The material should be able to ground to a fine powder and the common size is about 300µ and maximum size is upto 400/500 µ.  To provide fine grinding the high speed impact mills are used.  The particle size distribution should also be uniform to provide uniform conductance of heat.  The most common mesh size for rotational moulding ranges from 16 to 50.  The material should produce less volatiles during heating.  The most common mesh size for rotational moulding ranges from 16 to 50.  The material should produce less volatiles during heating. MATERIAL PREPARATION  A process used to reduce the pallets or granules to a smaller size is called grinding or milling.  In this process the granules fed into the centre of two plates, each with a series of radially arranged cutting edges.  One plate is held stationary while other is rotated at high speed.  The gap between the cutting edges of the two plates is narrower at their peripheries than the centres.  Any individual granules subjected to cutting action, generates frictional heat and increases the temperature of metal cutting face.  Hence the temp must be controlled so that it doesn't raise beyond the melting point of granules.  This ground particles will be passed through a series of vibrating sieves through which the finer particles will fall and be collected for use.  The oversized particles held on the sieve are conveyed back to the mill for further disintegration.
  66. 66. 65 | P a g e MOULD MATERIALS  Moulds are not so expensive, but entirely depends upon the quality level of the moulded parts and the method of heating to be used in the process.  Three types of mould materials in common use are  Cast aluminum  Steel sheet metal  Electroformed copper-nickel  Cast aluminium moulds are widely used for small to medium sized parts requiring number of cavities.  Steel sheet is preferred where surface finish is not critical and for the larger moulds of simple design.  Electroformed copper-nickel moulds are most expensive but offer a very smooth finish.  This type of moulds are best when very intricate surface and precise detail is required on the finished part.
  67. 67. 66 | P a g e PROCESS VARIABLES  There are many potential variable in the rotational moulding process, that can affect the size of the part being produced.  If there is any variation in the amount of plastic material charged into the cavity, the wall thickness will change accordingly.  The shrinkage and part dimension also vary with a change in wall thickness.  The speed and ratio of rotation determine the number of times a specific location on the cavity passes through the puddle of plastic material and the direction in which it enters and exits the puddle.  A change in these molding machine settings can affect the uniformity of the wall thickness of part.  The molding machine speed, ratio of rotation, oven temperature and other processing parameters must accommodate all the parts being moulded.  Variation in over time, temperature and air velocity can affect final part size.  The hotter the plastic material becomes, the more it expands, the material will then contract or shrink more as it returns to room temperature.  The speed with which plastic material is cooled will affect shrinkage. Cooling the material quickly will result in a low shrinkage factor.  Cooling the material slowly increase shrinkage, but the shrinkage will be more uniform.  These Variations in shrinkage encourage warpage and make it difficult to maintain uniform dimensions.
  68. 68. 67 | P a g e  Variations in the amount of mold release used can increase or decrease the tendency of a hollow part to pull away from the cavity as the part cools and shrinks.  The best approach is moulding parts to close tolerance to establish the optimum moulding cycle and then maintain those conditions.  The speed of rotation of the mold must be slow enough to ensure the gravity holds the plastic material in a puddle in the bottom of the cavity. FAULTS AND REMEDIES IN ROTATIONAL MOULDING PROBLEMS POSSIBLE SOLUTIONS Production rate is too low (or the heating cycle too long) Raise the oven temperature. Use a higher-melt index powder. Use a lower density powder. Powder fuses across deep narrow draws in the mould resulting in incomplete mould fill Increase the speed of rotation. Use a lower-density powder. Use a higher-melt index powder. Modify the mould, if possible. Moulded piece contains bubbles Or Piece has a rough inside surface Raise the oven temperature. Increase the heating cycle. Use a higher-melt index powder. Use a lower-density powder. Improve mould wall uniformity. Decrease the wall thickness, if possible by decreasing the amount of powder in every charge. Make sure the interior surface of the mould is dry.
  69. 69. 68 | P a g e Piece is yellow, brown or other discolouration action. Decrease the heating cycle. Lower the oven temperature. Check the mould wall for contamination such as rust. Piece sticks in the mould Clean the mould surface and apply a suitable mould release agent. Decrease the heating cycle or oven temperature. Increase the cooling cycle. Vent the mould. Modify the mould so that the part tapers to a slightly larger dimension toward the end of the mould through which it is removed
  70. 70. 69 | P a g e Piece is brittle Use a low density powder. Use a lower-melting index powder. Redesign the part to eliminate sharp corners. Piece is warped Decrease the cooling rate. Lower the oven temperature. Rotate the mould during cooling. Vent the mould. Improve the wall uniformity. Allow short air cooling before water cooling. ROTATIONAL MOULDING PROCESS Vs BLOW MOULDING PROCESS  Rotational moulding process have clear advantages over other process like blow moulding and injection moulding.  The ideal shape for a part for Blow Moulding is a cylinder that is closed on one end with a small opening at the other end. The best shape for a rotationally moulded part is ball.  Extrusion blow moulding machine cost more than the rotational moulding machine for a given size and capacity.  Blow moulding machines are powered by electricity which is 40 % more costly than the natural gas that is typically used for heating in the rotational moulding process.  The moulds for blow moulding are normally higher in cost than rotational moulding.  Blow moulding has advantage over rotational moulding of being able to process many thermoplastic materials including ABS and PPO.  Multilayered walled parts like fuel tanks are blow moulded but much more costly moulding machines are required.  The blow moulding process has advantage of being able to process materials as-received in pallet form. It eliminates the cost of pulverizing the pallets into fine powder.
  71. 71. 70 | P a g e  Blow moulding process ideally suitable for capacity upto 1000 its . But container with capacity 10,000 or 50,000 Hrs are most common in rotational moulding process.  Blow moulding is preferred for larger volume, lighter duty barrels. Rotational moulding dominates the market for smaller volume specially barrels with improved toughness. ROTATIONAL MOULDING & ITS APPLICATION Polyethylene  Industrial products : Tanks drums, containers, nesting pallets, floor maintenance machine components and tanks, medical carts, viedo game housings, news paper and magazine vending machines, tool chests, shipping cases.  Transport Products.  Consumer Products.  Recreational Products.  Agricultural products. POLYPROPYLENE  PP is specified for many of the same applications as PE. PP‟s higher stiffness and increased temperature resistance allow it to perform in applications where PE isn‟t quite enough.  All these application takes advantage of PP‟s chemical resistance, stiffness, and heat deflection temperature.  Other applications include large chemical shipping drums, radio active material containers and high-temperature air ducts.  PP is a relatively new as rotational molding material. PVC  PVC material has many applications like.  Industrial Products : Flexible and rigid airducts, machine feet, air and water filters, gaskets, tires and floor scrubber squeegees and bladders.
  72. 72. 71 | P a g e  Medical Products : Examination chair arms, Flexible anesthesia face masks, blood pumps, respiration squeeze balloons and anatomical teaching models.  Consumer products : Figurines and life-size statuary, soft cushioning furniture, picture frames, artificial fruit and toys.  Recreational Products : Soft, cuddly, and noise-making squeeze toys; life like doll and animal heads and body parts; toy wheels, energy-absorbing sports helmet liners. NYLONS  Nylon becomes the choice where PE isn‟t quite good enough.  Nylon is a choice when the application requires more temperature resistance, tensile strength, or chemical resistance in contact with oil and gasoline.  Typical uses for nylon include fuel storage tanks, high temperature ducts, truck radiator surge tanks, large chemical shipping bottles, air horns, pressurized water treatment tanks, air intake manifolds. POLYCARBONATE  PC‟s impact strength allowed it to take this application away from glass. This material‟s excellent out door weatherability is another important consideration.  Other uses include pressurized beer containers and dispensers, air-cleaner housings, heating and intake ducting, illuminated traffic signage, transparent food and medical containers. …………………………………………………………………………
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  74. 74. 73 | P a g e Thermoforming process  Introduction Thermoforming secondary processing technique. The process involves heating a thermoforming sheet to its softening temp. or forcing the hot and flexible sheet the contours of mould by vacuum where it is held until cool.  Thermoforming Process  Clamping  Clamping of Sheet .  Heating  Heat the sheet by Radiant heaters  Pre-stretch  Air is introduced to blow a small "bubble"  The mould is then raised into the pre-stretched sheet.  Vacuum  A vacuum is applied on Sheet.  Plug Assist  Where a deep draw is required a top "plug" may be used.  Cooling and Release  The material is allowed to cool by air or water spray.  The moulding is released from the mould by introducing a small air pressure.  Finishing  After moulding, any mould finishing may be performed, trimming, cutting, drilling, polishing, decorating etc.
  75. 75. 74 | P a g e  Material  Acrylonitrile Butadiene Styrene ABS  Polystyrene PS  Polycarbonate PC  Polypropylene PP  Polyethylene (sheet and foamed sheet) PE  Polyvinyl Chloride PVC  Acrylic PMMA The softening range and hot strength are important properties. Types of mould  Plaster  Wood  Plastic mould  Aluminium OR  Good quality wood  Aluminum  Mils Steel  Plaster of Paris  Epoxy  Plaster(Plaster of Paris) Most commercial moulding plasters are not strong enough to be in prototyping. Plaster are inorganic calcious materials that hydrolytically react and harden when mixed with water, soap such as murphy‟s oil soap. Advantage :- Cheap, quick, no venting is necessary and so intimate production of details is possible. Disadvantage :- A max. of only about 50 forming is possible. The surface being very soft.
  76. 76. 75 | P a g e  Wood The wood is kiln-dried before shaping to minimize stress relief during fabrication. After through drying the surface can be sealed with temp. Advantage :- Cheap, longer life span then plaster moulds, higher impact strength.  Disadvantage :- Limited life say approximately 500 forming. During repeated forming, wooden mould should not be allowed to become too hot and its dimensions should be checked regularly.  Plastic moulds Plastic mould are used where mould surface temp. is not exceed 600 C where drape vacuum forming used epoxy and unsaturated polyester resin together with glass fiber are the mould materials of choice. Advantage :- easily manufactured, low thermal conductivity ,little or no finishing is required. Disadvantage :- Some materials are sensitive to high forming temp.  Applications (Thermoforming)  Baths & Shower Trays Yoghurt Pots  Ski-Boxes Boat Hulls  Machinery Guards Vehicle Door Liners  Refrigerator LinersSandwich Boxes  Parts of vehicle cabs Exterior Shop Signs  Heating systems  Convection ovens  Infrared radiant heaters  Electrically power infrared heaters Types of forming processes 1. Straight vacuum forming 2. Pressure forming 3. Plug-Assist forming
  77. 77. 76 | P a g e 4. Free forming 5. Drape forming 6. Snap-back forming 7. Matched –die forming 8. Mechanical forming  Straight vacuum forming In this process a plastic sheet is clamped in a frame and heater. When the hot sheet becomes rubbery or elastic, it is placed on female mould cavity. The vacuum is applied and atmospheric air is cooled the sheet.  Pressure forming In this process a plastic sheet is formed in a mould. The air pressure on the plastic is used of force the material against the and heated till softened. Or the mould is airtight seal the mould and air pressure is than applied or made the product.  Plug-assist-forming A male plug is used in this method of vacuum forming. Plastic sheet is clamped in the female mould and after the heat- softened sheet is sealed across the mould cavity. A push the sheet causing to stretch. When the plug has completed its penetration stroke and Sheet is transfer from the plug surface to the cavity mould. This method permits deeper with more wall thickness. The formed article in shaped. The plug are made up of metal , wood , or thermoset plastic. These should be heated to a few degree less then the temp. of the plastic in order prevent premature cooling.  Free forming Some of the products need very high optical clarity.if the forming is very done using a mould would result in undesirable changes in the surface quality of the part. To avoid this ,the techniques known as free forming or free blowing is used. In this technique, the part is expanded with air pressure. The size of the bubble is often monitored by an electric eye. When the bubble reached desired size. The air pressure is reduced to a level maintains the size of the bubble while part cooling.
  78. 78. 77 | P a g e  Drape forming In this process the plastic sheet is clamped and heated then drawn over the mould either by pulling it over it over the mould or by forcing the mould into the sheet. When the mould has been forced into the sheet and a seal created. Vacuum applied beneath the mould forced the sheet over the male mould. by draping the sheet over the mould , that part of the sheet touching the mould remains close to the original thickness of the sheet.  Snap-Back Forming Snap back forming is the modified form the drape forming. In snap back forming the material is heated to the sag point and then drawn slightly into a vacuum box below the part. This pre- stretching this centre of the part and usually about one- half to two-third of the total draw that the part with receive. A second step is then activated to give more draw. the male mould is pressed against the material to draw it farther. During this drawing step , the thickness of the centre of the material by contact the mould and thinning near the edges. Finally the part is formed by applying a vacuum through the male mould and causing the part to „ snap back‟ against the outside the male mould. the advantage of this process is the uniform wall thickness. Only drawback being that it requires longer cycle time.  Matched Die Forming This method resembles to that of compression moulding. The material is heated to the sag point and is trapped and formed between male and female dies that are made up of wood, plaster, epoxy, or other material. the Clearance between male and female mould decides the wall thickness. No vacuum or no air is applied in this process. mould are placed until the plastic cools & cures.  Comparison of thermoforming & injection moulding Thermoforming Injection moulding Sheet & raw stock Granule & raw material Die are made wood ,plaster of paris The mould cost is very high ( mild steal) If the no of articles to be moulded is less then High production
  79. 79. 78 | P a g e Operation set up quickly Operation takes more tune Sheet can be printed or decorated before forming Processing Before decoration not possible Holes and undercuts can‟t be produce by thermoforming Holes and undercuts be produce by Injection moulding Pressure required is lower then injection moulding Pressure required is very high Lower cost is original cost Higher original machine cost.  THERMOFORMED DISPOSABLE CUPS Introduction: Wide applications such as Ice-cream cups, Tea cups, Thmblers for serving cool drinks, coffee etc. Plant Capacity: 10,000 cups per day on single shift. Sizes 100 ml. cups and 250 ml. cups. Raw materials: - HIPS/PVC sheets of up to 0.2mm thickness. Plant & Machinery: Thermoforming machine Thermoformed article cutting press Compressor Process: Fix the sheet on the frame above the mould and clamp properly. Heat the sheet to softening point. Remove heaters, Lift the mould up.Apply vacuum to get the desired shape.Remove the moulded item from the mould.Separate the moulded cups from the sheet by a cutting press. Land & Building: Total land is around 3000 sq. ft. Total covered area is around 1500 sq. ft.
  80. 80. 79 | P a g e Utilities: Total connected load is around 15 KW. Manpower: Around 3 persons. Estimated cost oc project: Cost of Plant & Machinery is around 3 lakhs. Total capital investment is around 5 lakhs.
  81. 81. 80 | P a g e CALENDERING  Basic principle It is process of squeeze a plastic melt between two or more rotating or rolls to from continuous film and sheet. It is technique of making paper , metal and rubber industries.  Materials The PVC – resin which is generally processed in calendaring plant. 1. PVC 2. PS 3. PP 4. PE 5. ABS Are also used for calendaring process.  Types of calendaring 1. L-CALENDER 2. INVERTED L- CALENDER 3. F- CALENDER 4. Z-CALENDER
  82. 82. 81 | P a g e  Heating system a) High Pressure steam is used on calendaring plants. Wark pressure range 680-1380 KN/m and temperature range 120-1850 C. b) High pressure hot water (H.P.H.W.) Advantage of calendar process 1. Delivery/output of a calendar id high. 2. Increase in maximum width of film/sheet. 3. Better control of film/sheet thickness 4. Better optical properties.
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  84. 84. 83 | P a g e TECHNICAL MANUAL ON “MODEL QUESTION AND ANSWERS IN PLASTICS PROCESSING” INJECTION MOLDING 1 MARK QUESTIONS AND ANSWERS 1. Maximum daylight = Minimum daylight +_______________. Ans. maximum mold opening 2. Polyacetal material requires no predrying before molding. State true or false Ans. False 3. A higher screw rpm will cause the screw to return to its starting position sooner. Say True / False Ans. True 4. Increase of screw rpm will result in larger cushion say True / False Ans. False 5. Depression on molded component is called ______________ defect. Ans. Sink marks 6. Cushion size is the amount of plastic ahead of the screw before the injection Stroke begins say True or False Ans. False 7. An increase in the cushion size a)Reduces the effective operating length of the screw. b)Increase the effective length of the screw c)Does not change the effective operating length of the screw d)None Ans. a
  85. 85. 84 | P a g e 2 MARK QUESTIONS AND ANSWERS 1. Why hopper throat cooling is necessary for barrel? Ans. Hopper throat cooling is necessary for barrel to avoid bridge formation. If too cold granules also may slip each other and there by the output may be reduced. 2. What is cavity pressure? Ans. The pressure generated in the cavity when the molten material is forced inside the cavity is called as the cavity pressure 3. Distinguish between speed & injection pressure Ans. Higher injection speed (cc/sec) can push the melt to farthest part of mould at a shorter time before the melt freezes. Injection pressure (kg/cm2) is required to overcome the resistance to the flow of the melt in the mould. It depends on the temperature of mould, flow ratio and melt viscosity. 3. Match the following. 1. Thin wall molding requires a) Dehumidifier 2. Molding will have burn mark b)High pressure injection machine 3. Moisture remover device c) Non return valve 4. To prevent melt leakage into barrel d) Inadequate venting Ans.1-b,2-d,3-a,4-c 4 MARK QUESTIONS AND ANSWERS 1. Draw the neat sketch of an injection-moulding machine and name its parts.
  86. 86. 85 | P a g e 2. State the merits of preheating Ans. The merits of preheating are, 1) Reduce the cycle time 2) To improve the quality of product. 3) To obtain the desired property like clarity etc in the moulding. 4) Energy saving 3. Discuss in brief the process of Gas assisted injection moulding. Ans. Gas assisted injection Moulding is used to produce hollow out injection moldings by controlled injection of an inert gas (nitrogen) into the polymer melt. The gas does not mix with the plastic but instead continuos channel through the hotter, less viscous section of the stream. The gas maintains the pressure throughout the Moulding cycle. During the cooling phase the gas ensures the positive contact between the polymer and the surface of the mould. Swirl marks free surface is possible in this process. 8 MARK QUESTIONS AND ANSWERS 1. Compare and contras hydraulic clapping system verses Toggle clapping systems. Ans. HYDRAULIC SYSTEM: • .Much higher original cost • .Higher horse power needed. Therefore more expensive to run. • Unlimited stroke potential • .Direct readout of clamp force • .Easy adjustment of clamp force • .Easy mold setup • .Varies stroke to mold height • .Clamp speed easily controlled or stopped at any point • .Low maintenance as parts are self lubricated TOGGLE SYSTEM:
  87. 87. 86 | P a g e • .Lower original cost • .Lower horse power needed, more economical to run • .Limited stroke potential • .No Direct readout of clamp force • .More difficult • .More involvement in mold setup • Constant mold stroke • Clamp speed more difficult to control and stop 2. .With a neat sketch explain the In line- reciprocating screw type injection moulding process. Ans. In this type of machine the plunger/spreader that is the key to the plunger type machine is replaced by the reciprocating screw, which moves back and forth inside the heating cylinder. as the screw rotates the flights pick up the feed of the granular material from the hopper and force it along the hot cylinder barrel. As the material comes off the end of the screw the screw moves back to permit the plastic material to accumulate. At proper time the screw is moved forward, acting as a plunger and propelling the plasticized material through the nozzle and sprue into the mould cavities. Measuring the back travel of the screw regulates the size of the change.
  88. 88. 87 | P a g e EXTRUSION 1 MARK QUESTIONS AND ANSWERS 1. Increase in back pressure will result in ___________(increase / decrease) in output. Ans. Decreases 2. What is the relation between die diameter, bubble diameter and layflat width. Ans. Blow ratio (BR) = Dbmax / Dd or Blow up ratio = 0.637 x lay flat width Die diameter Lay flat width = (πDd/2) BR Or Lay flat width = 1.57 x bubble diameter. Where, Dbmax = maximum diameter of bubble Dd = die diameter. 3. Hopper cooling is done. a) To prevent slipping of granules b) To avoid bridge formation b) To increase output. d) All of the above Ans. b 4. The die used for coating of wire and cable ___________ a) Cross head/offset die b) straight die c) coat hanger die d) T- Die Ans. A 2 MARK QUESTIONS AND ANSWERS 1.State the importance of screw cooling Ans. Screw cooling is recommended to prevent the decomposition of the heat sensitive material 2. Name the different extruder screws.
  89. 89. 88 | P a g e Ans. The types of screws used are, a) General purpose or the Polyolefin screw b) PVC screw c) Polyamide screw d) Barrier screw e) Twin screw f) Vented screw 3. .What is guesseting device? Ans The guesseting device is used for folding of the film so that the appearance of the bag is small but it can take large amount of material. 4. Why Pretreatment of the substrate / web is essential before coating? Ans. The pretreatment of the substrate / web is essential before coating so as to increase the adhesion of the surface with the polymer. 5. State the different pipe-sizing methods Ans. The different pipe sizing methods are, 1) Vacuum trough 2) Sizing sleeve 3) Extended mandrel 4) Sizing plates. 4 MARK QUESTIONS AND ANSWERS 1. .State the functions of screen pack and breaker plate. Ans. The functions are, Breaker plate: Its principle function is to provide support to screen pack. The breaker plate helps to convert the spiral flow of the polymer melts leaving the screw into a linear, axial one. It also supports the screen pack.
  90. 90. 89 | P a g e Screen pack: Its main function is to eliminate the particulate contaminants. 2. Explain in detail the causes and remedies for “Melt fracture”. Ans Melt fracture or rough surface finish is caused when the melt temperature is too low. It can also happen due to narrow die gaps.  It is a die-entry effect  In any converging flow there are tensile and shear forces  If tensile stresses become large and if they exceed the tensile strength of melt, the desirable smooth laminar flow is lost completely.  The extrudate emerging from die exit will be of irregular shape. This phenomenon is called “Melt fracture”. 3. Enlist the factors, which affect the output of an Extruder. Ans. The factors that affect the output of the extruder are, Factor Direction of change Output Screw speed Increase Increase Channel depth Increase Increase Helix angle Increase Increase ( max. upto 300) Back Pressure Decrease Increase Length of Metering Increase Increase Viscosity Increase Decrease .
  91. 91. 90 | P a g e 4. What is bambooing ? Explain the root cause in detail. Ans. Bambooing is the defect that is observed during the extrusion of pipe etc. this defect arises due to improper die design, die not having proper polish , defective or intermittent working of melt pump which causes melt surge.  The melt as it proceeds along the die channel, has a velocity profile with maximum at the centre and zero at the wall.  As it leaves the die lips, the material at the wall has to accelerate to the velocity at which the extrudate is leaving the die.  This generates tensile stress and if the stress exceeds Tensile strength, the surface ruptures causing the visual defect - “shark skin”.  If the conditions causing shark skin becomes more intensive, Eg. Pressure at the extruder becomes excessive or die temperature drops, the extrudate “snaps back” -- “Bambooing effect”. BLOW MOLDING 1 MARK QUESTIONS AND ANSWERS 1. Jerry can with handle is produced by _____________ a. Injection blow moulding b. Extrusion blow moulding c. Injection stretch blow moulding d. Extrusion stretch blow moulding
  92. 92. 91 | P a g e Ans.b 2. Small pharmaceutical bottles with accurate neck finish is produced by ________ Process. Ans. Injection blow molding 3. 200litres capacity oil barrel are economically produced by ______ Ans. Intermittent blow molding 4. The main root cause for „pitted surface‟ defect in blow molding is____________ a) Excessive melt temperature b) Inadequate air venting c) High blow ratio d) None of the above Ans. d 5. All bottles are Bi-axially oriented. Say true or false. Ans. False 6. Odd shape containers are made by die shaping say true or false. Ans. True 2 MARK QUESTIONS AND ANSWERS 1. Name any two merits of Injection blow moulding. Ans. The two merits of injection blow molding are, • Perfect neck finish. • Scrap free product. 2. State the different types of blow moulding processes? Ans. Injection blow moulding, Extrusion blow moulding, Injection stretch blow molding, Dip blow molding, multilayer blow molding, special blow molding. 3. State the various moulds materials used in blow moulding.
  93. 93. 92 | P a g e Ans. The various materials used in blow Molding are Al, steel, beryllium- copper alloy, kirksite. 4. State the merits of biaxially oriented blow molding products. Ans. It enhances the mechanical properties, optical properties, chemical resistivity & improves barrier properties. 4 MARK QUESTIONS AND ANSWERS 1. Compare & contrast extrusion blow molding and injection blow molding Ans. 2. What do you mean by stretch blow molding? Explain the process Ans. Stretch blow molding is the technique in which the use of mechanical assistance is done to stretch the part in the longitudinal direction at the same time the blowing of the part stretches it in the hoop direction or the radial direction. This biaxial stretching occurs when the parison or the preform is blown into the desired shape in the blow mold. SR. NO EXTRUSION BLOW MOLDING INJECTION BLOW MOLDING 1 Best suited for shorter runs and quick tool change Best suited for longer runs 2 Machine costs are less Machine costs are more 3 Tooling costs are 50 to 75% less than injection molding. Tooling costs are more 4 Accuracy depends on die design Very high accuracy can be obtained 5 Seam lines or pinch marks are possible in the product No seam lines or pinch marks possible in injection blow molding 6. Scrap generation is more Scrapless or negligible scrap generated.
  94. 94. 93 | P a g e The longitudinal stretching of the parison is accomplished using a telescopic mandrel or core pin that extends to push on the bottom of the preform at the same time the air is being injected to push against the walls to stretch the material radially. The temperature of the process is properly selected so as to obtain proper orientation. 8 MARK QUESTIONS AND ANSWERS 1. .Mention types of blow molding techniques and explain with neat sketch of ram type of blow molding process. Ans. The different types of blow molding techniques are, • Injection blow molding • Extrusion blow molding • Multicolor blow molding • Dip blow molding Ram type or the accumulator type of blow molding is the process in which the extrudate flows from the extruder into an external chamber or accumulator. At an appropriate moment in the cycle, a ram in the chamber advances and injects the resin in the die that is mounted on the outlet of the accumulator to form the parison. Very large parts can be made using the accumulator system because the volume of the accumulator is several times larger than the injection volume possible with the reciprocating screw machine and the injection could be very fast using the ram type of machine. The accumulator can be heated to maintained the proper temperature of the resin so only resin with good heat stability should be used since the time that the resin may be at high temperature could be quite long.
  95. 95. 94 | P a g e THERMOFORMING 1 MARK QUESTIONS AND ANSWERS 1.In thermoforming plastic sheet heated to the – a) Melting point b) Sag point c) Tg –point d) none of the above Ans. b 2. In Drape forming technique vacuum is used for forming say true or false. Ans. True 3. Canopies for racing vehicles is produced by- a)free forming or free blowing b) drape forming c) Pressure forming d)Mechanical forming Ans b 4. What is the other name for free forming Ans free blowing. 5. Bath tubs are made out by _____________ thermoforming technique Ans. Drape forming 6. Which of the following materials not usually thermoformed a) HIPS & ABS b) PMMA c) PVC d) POM & Nylon Ans. d 7. Where the production runs are very short in thermoforming _______ molds is preferred. Ans. wood 2 MARK QUESTIONS AND ANSWERS 1. Enlist thermoforming techniques. Ans. The different thermoforming techniques are, 1. Straight vacuum thermoforming
  96. 96. 95 | P a g e 2. Pressure thermoforming 3. Plug assist thermoforming 4. Reverse draw thermoforming 5. Match die thermoforming 6. Drape forming 7. Ring thermoforming 8. Free forming 9. Snap back forming 2. .State any two merits of Plug –assisted forming. Ans. The advantage of plug assisted forming are, • Better wall thickness uniformity • Exact shape of required can be obtained. • 3. State the merits of die-matched thermoforming. • Ans. The advantages of die matched thermoforming are, • 1.Very large parts can be manufactured • 2. Complicated shapes can be easily manufactured • 4. .State the various mould materials used in Thermoforming • Ans. The materials used are, • 1. Wood • 2. Aluminum • 3. Epoxy • 4. Steel • 5. .Match the following: • a) Refrigerator liners 1) Extrusion • b) Air bubbles film 2) Co-injection • c) Two colour moulding 3) pad printing
  97. 97. 96 | P a g e • d) Curved surface printing 4) Thermoforming. • Ans. a-4, b-1, c-2,d-3 8 MARK QUESTIONS AND ANSWERS 1. Classify Thermoforming processes. Explain the process of plug-assisted forming with merits over drape forming. Ans. The thermoforming process are classified as, • Straight vacuum forming • Pressure forming • Plug assisted forming • Reverse draw forming • Free forming • Drape forming • Mechanical forming • Snap back forming • Matched die forming Plug is used in the plug assisted thermoforming to push the material into the mold. The plug can be used to carry the material towards the area where it would be too thin if straight vacuum or pressure forming were used. Plugs can be made of wood, metal. the plug is generally heated to few degrees less than the temperature of the plastic. The advantages of the drape forming over the plug assisted forming are, – Better wall thickness can be accurately obtained as compared to the drape forming. – Less space is required around the mold, so the trim scrap is less in plug forming.
  98. 98. 97 | P a g e ROTATIONAL MOLDING 1 MARK QUESTIONS AND ANSWERS 1. In rotational molding process plastics material perfectly- a) Melts b) fuses c) Both (a) & (b) d) None of the above. Ans. B 2. Rotational molding process has four principle steps. ( 1) loading (2) ___ (3) Cooling & (4) _____. Ans. Heating and unloading 3. Rotational speed of major axis should be slow & it should not be more than – a) 100 rpm b) 85 rpm c) 60 rpm d) 75 rpm Ans. C 4. State true or false: The amount of material controls the wall thickness of rotational molded products. Ans.True 2 MARK QUESTIONS AND ANSWERS 1. What do mean by grindability for rotational molding? Ans. Grindability of the material means that the resin can be ground to fine powder. Resin grades that have very low melting may not be easy to grind in the high speed impact mills. 2. What is Pourability in rotational molding? Ans. To tumble properly in the mold the plastic powder must flow easily without any external pressure. This property is called pourability. A minimum flow rate of 185 gm/min characterizes acceptable rotational molding powders. 3. What is mixed resin in rotational molding process?
  99. 99. 98 | P a g e Ans. The mixed Resin are characterized by two different melting points and thus will have different softening points. Due to different softening points it would fuse and stick to the walls of the mold and have two layer molding. 4. How do you achieve the openings in rotational molded tanks? Ans. Openings can be obtained in the rotational molded tanks using metal inserts at the position where the inserts are required with the insulated plugs. 4 MARK QUESTIONS AND ANSWERS 1. What are the advantages & disadvantages of rotational molding. Ans. Advantages of rotational molding: • Very large parts can be made with relatively low cost equipment. • Low pressures are used and the temperature used are also less • Stress free parts can be made using the rotational molding • If parts of equivalent sizes are made then less investment as compared to injection or blow molding. • Less cost of equipment and ease of making the mold. • Multilayer and Multicolor product possible. Disadvantages: • simple shapes can only be manufactured • poor dimensional tolerance control • generally thicker overall walls • low part mechanical properties. 8 MARK QUESTIONS AND ANSWERS 1. What is rotational molding process? Explain the process with a neat sketch. Ans. Rotational molding uses the rotation of the mold in the heated chamber to form the part. It is mainly suited for the formation of very large, seamless and hollow stress free parts. The process uses no pressure and hence the
  100. 100. 99 | P a g e mold can be simple and inexpensive. The temperatures used in the rotational moldings are less as compared to other processes. Process: The rotational molding process has four principle steps, these are 1. Loading The loading steps begins with careful weighing of the charge of the starting material. The starting material is usually a finely ground thermoplastic powder. The charge of the starting material is loaded manually into an open , cold mold that is prepared from inside by coating the mold release agent. 2. Heating and cooling The heating cycle is done in an oven. The heating oven must be large enough that the entire mold assembly can be placed inside it and rotated freely. The mold assembly is rotated throughout the heating and cooling cycle. The heating cycle for the mold is often quite long and so it is common to have multiple molds that are cycled together. The mold is rotated biaxially so that is even mixture of the molten plastic inside the mold. The rotation speed determines the thickness of the molded product.
  101. 101. 100 | P a g e 3.Unloading Unloading steps begins with the removal of the lid of mold. The cold part is removed manually or with a mechanical assist. Mold release is sufficient to allow removal. . CALENDERING 1 MARK QUESTIONS AND ANSWERS 1. In L-Type calender – a) offset roll on the top & the take off is from the bottom roll b) offset roll is at the bottom & the take off is from the top roll c) Both (a) & (b) d) Neither (a) nor (b) Ans. b 2. which of the following is not a type of Calender a) „L‟ type b) „F‟ type c) „E‟ type d) „Z‟ type Ans. E -Type
  102. 102. 101 | P a g e 3. Most common type of intensive mixer using calendering is______ a) Braw blender b) Planetory mixer c) Banbury mixer d) None of the above Ans. c 4. State true or false: The golden rule as such in calendering is to use temperature as high as possible provided no sticking to rolls & any degradation is caused. Ans. True 2 MARK QUESTIONS AND ANSWERS 1. What is plate out in case of calendering? Ans. Transfer of tacky deposit of compound to the roll is plate out defect. 2..Match the following for the calendering take off: a)Bright film finish 1) Engraved roll b)Mat finish 2) Shot peened roll c) Orange peel 3) Chrome plated roll d) Embossed 4) Sand blasted roll Ans. a-3, b-4, c-2, d-1 COMPRESSION AND TRANSFER MOLDING 1 MARK QUESTIONS AND ANSWERS 1. State true or false: Storing under cryogenic can enhance shelf life of thermoset compounds. Ans. True 2. .State true or false