1. • Plastics began to be developed in the 1920s
• Rapid progress in the 1940s
• In the 1970s, reinforced plastics began
• Plastics in general can be molded, cast, formed, and
machined into complex shapes in few operations
• Plastics are shaped into discrete products or as sheets,
plates, rods, and tubing that may then be formed by
secondary processes into a variety of discrete products.
• Plastics can be as pellets, granules, or powders and are
melted (for thermoplastics) just before the shaping
process.
• Liquid plastics that cure into solid form are used
especially in the making of thermosets and reinforced-
plastic parts.
2.
3.
4. Injection Molding
• Injection molding is similar to hot-chamber die casting
• The pellets or granules are fed into the heated cylinder, and the melt is
forced into the mold either by a hydraulic plunger or by the rotating screw
system of an extruder.
• As in plastic extrusion, the barrel (cylinder) is heated externally to promote
melting of the polymer
• In injection-molding machines, greater portion of the heat transferred to
the polymer is due to frictional heating
• Modern machines are of the reciprocating or plastic acting screw type
• pressure builds up at the mold entrance, the rotating screw begins to move
backwards under pressure to a predetermined distance
• This movement controls the volume of material to be injected
• The screw then stops rotating and is pushed forward hydraulically, forcing
the molten plastic into the mold cavity
• The pressures developed usually range from 70 to 200 MPa.
• For thermoplastics, the molds are kept relatively cool at about 90°C.
• Thermoset parts are molded in heated molds at about 200°C, where
polymerization and cross-linking take place.
• After the part has cooled sufficiently (for thermoplastics) or cured (for
thermosets), the molds are opened and ejectors are used to remove the
part from the mold.
6. Sequence of operations in the injection molding of a part with a reciprocating screw. This
process is used widely for numerous consumer and commericial products, such as toys,
containers, knobs, and electrical equipment
7. • Elastomers also are injection molded into discrete products by these
processes.
• the material is molten when injected into the mold, complex shapes with
good dimensional accuracy can be obtained.
• because of uneven cooling of the part in the mold, residual stresses develop
• Molds with moving and unscrewing mandrels also are used in injection
molding,
• they allow the molding of parts having multiple cavities or internal and
external threaded features
• To accommodate part design, molds may have several Components
including runners (such as those used in metal-casting dies), cores, cavities,
cooling channels, inserts, knockout pins, and ejectors
Process Capabilities
• Injection molding is a high-rate production process and permits good
dimensional control
• most parts generally weigh from 100 to 600 g, they can be much heavier,
such as automotive-body panels and exterior components.
• Typical cycle times range from 5 to 60 seconds
• they can be several minutes for thermosetting materials
8. Machines.
• Injection-molding machines are usually horizontal, Vertical machines are
used for making small, close-tolerance parts and for insert molding.
• clamping force on the dies generally is supplied by hydraulic means,
although electrical means (which weigh less and are quieter than hydraulic
machines) also are used
• Modern machines are equipped with microprocessors in a control panel
and monitor all aspects of the operation.
• Injection-molding machines are rated according to the capacity of the mold
and the clamping force
• In most machines, this force ranges from 0.9 to 2.2 MN.
• The largest machine in operation has a capacity of 45 MN, and it can
produce parts weighing 25 kg.
• The molds generally are made of tool steels, beryllium-copper, or
aluminum.
• They may have multiple cavities, so that more than one part can be made in
one cycle
Reaction-injection Molding
• In the reaction-injection molding (RIM) process, a monomer and two or
more reactive fluids are forced at high speed into a mixing chamber at a
pressure of 10 to 20 MPa and then into the mold cavity
9. • Chemical reactions take place rapidly in the mold, and the
polymer solidifies
• Typical polymers are polyurethane nylon, and epoxy
• Cycle times may range up to about 10 minutes, depending on
the materials, part size, and shape.
• Major applications of this process include automotive parts
(such as bumpers and fenders, steering wheels, and
instrument panels), thermal insulation for refrigerators and
freezers, water skis, and stiffeners for structural components
• Parts made may range up to about 50 kg.
• Reinforcing fibers (such as glass or graphite) also may be used
to improve the product’s strength and stiffness
• Depending on the number of parts to be made and the part
quality required, molds can be made of common materials,
such as steel or aluminum.
• |
10. Schematic illustration of the reaction-injection molding process. Typical parts made are
automotive-body panels, water skis, and thermal insulation for refrigerators and freezers
11. Blow Molding
• Blow molding is a modified extrusion- and injection-molding process
• In extrusion blow molding, a tube or preform (usually oriented so that it is
vertical) is first extruded.
• lt is then clamped into a mold with a cavity much larger than the tube
diameter and blown outward to fill the mold cavity
• Depending on the material, the blow ratio may be as high as 7:1
• Blowing usually is done with a hot air blast at a pressure ranging from 350
to 700 kPa
• Drums with a volume as large as 2000 liters can be made by this process.
• Typical die materials are steel, aluminum, and beryllium copper.
• In some operations, the extrusion is continuous and the molds move with
the tubing.
• The molds close around the tubing, sealing off one end, breaking the long
tube into individual sections, and moving away as air is injected into the
tubular piece
• The part is then cooled and ejected from the mold
• Corrugated-plastic pipe and tubing are made by continuous blow molding in
which the pipe or tubing is extruded horizontally and blown into moving
molds.
12. • In injection blow molding, a short tubular piece (parison) is injection
molded into cool dies.
• The dies then open, and the parison is transferred to a blow-molding die by
an indexing mechanism
• Hot air is injected into the parison, expanding it to the walls of the mold
cavity.
• Typical products made are plastic beverage bottles (typically made of
polyethylene or polyetheretherketone, PEEK) and small, hollow containers
• A related process is stretch blow molding, in which the parison is expanded
and elongated simultaneously, subjecting the polymer to biaxial stretching
and thus enhancing its properties.
• Multilayer blow molding involves the use of coextruded tubes or parisons
and thus permits the production of a multilayer structure
• A typical example of such a product is plastic packaging for food and
beverages, having such characteristics as odor and permeation barrier, taste
and aroma protection, scuff resistance, the capability of being printed, and
the ability to be filled with hot fluids.
• Other applications of this process are for containers in the cosmetics and
the pharmaceutical industries.
13. Schematic illustrations of (a) the extrusion blow-molding process for making plastic
beverage bottles; (b) the injection blow-molding process; and (c) a three-station injection
molding machine for making plastic bottles.
14. Thermoforming
• Thermoforming is a process for forming thermoplastic sheets or
films over a mold through the application of heat and pressure.
• In this process,
• a sheet is (a) clamped and heated to the sag point (above the
glass-transition temperature, Tg, of the polymer), usually by
radiant heating, and (b) forced against the mold surfaces through
the application of a vacuum or air pressure.
• Sheets used in thermoforming are available as a coiled strip or as
lengths and widths of various sizes.
• The mold is generally at room temperature
• Shape produced becomes set upon contact with the mold.
• Because of the low strength of the materials formed, the pressure
difference caused by a vacuum usually is sufficient for forming.
• However, thicker and more complex parts require air pressure,
ranges from about 100 to 2000 kPa, depending on the type of
material and thickness of the sheet.
• Mechanical means, such as the use of plugs, also may be
employed to help form the parts.
15. Various thermoforming processes for a thermoplastic sheet. These processes commonly are
used in making advertising signs, cookie and candy trays, panels for shower stall, and packaging
16. • Process Capabilities.
• Typical parts made by thermoforming are packaging, trays for
cookies and candy, advertising signs, refrigerator liners, appliance
housings, and panels for shower stalls.
• Parts with openings or holes cannot be formed by this process
because the pressure difference cannot be maintained during
forming.
• Because thermoforming is a combination of drawing and
stretching operations (much like in some sheet-metal forming), the
material must exhibit high, uniform elongation; otherwise, it will
neck and tear.
• Thermoplastics have high capacities for uniform elongation by
virtue of their high strain-rate sensitivity exponent, m,
• Molds for thermoforming usually are made of aluminum because
high strength is not required hence, tooling is relatively
inexpensive.
• Thermoforming molds have small through-holes in order to aid
vacuum forming.
• These holes typically are less than 0.5 mm in diameter; otherwise,
they would leave marks on the parts formed.
17. • Defects encountered in thermoforming
(a) tearing of the sheet during forming
(b) nonuniform wall thickness
(c) improperly filled molds
(d) poor part definition, and
(e) lack of surface details.
18. Compression Molding
• In compression molding, a pre-shaped charge of material,
pre-measured volume of powder, or viscous mixture of liquid-
resin and filler material is placed directly into a heated mold
cavity.
• Typically is around 200°C but can be much higher.
• Forming is done under pressure from a plug or from the
upper half of the die
• process is somewhat similar to closed-die forging of metals.
• Pressures range from about 10 to 150 MPa.
• There is a flash formed, which subsequently is removed by
trimming or by some other means.
• Typical parts made are
dishes handles, container caps, fittings
electrical and electronic components,
washing-machine agitators housings.
• Fiber-reinforced parts with chopped fibers also are formed
exclusively by this process.
19. • Compression molding is used mainly with thermosetting plastics, with the
original material being in a partially polymerized state.
• However, thermoplastics and elastomers are also processed by compression
molding.
• Curing times range from about 0.5 to 5 minutes, depending on the material
and on part thickness and shape.
Process Capabilities.
Three types of compression molds are available:
• Flash type: for shallow or flat parts
• Positive type: for high-density parts
• Semi-positive type: for quality production.
• Undercuts in parts are not recommended
• dies can be designed to open sideways to allow removal of the molded part.
• In general, the complexity of parts produced is less than that from injection
molding, but the dimensional control is better.
• Surface areas of compression-molded parts may range up to about 25 m.
• Because of their relative simplicity, dies for compression molding generally
are less costly than those used in injection molding.
• They typically are made of tool steels and may be chrome plated or
polished for an improved surface finish of the molded product.
20. Types of compression molding – a process similar to forging: (a) positive, (b) semipositive, and
(c) flash, which is later trimmed off. (d) Die design for making a compression-molded part with
external undercuts.
21. Transfer Molding
• Transfer molding represents a further development of compression molding.
• The uncured thermosetting resin is placed in a heated transfer pot or chamber, and
after the material is heated
• it is injected into heated closed molds.
• Depending on the type of machine used, a ram, plunger, or rotating-screw feeder
forces the material to flow through the narrow channels into the mold cavity
• pressures up to 300 MPa.
• This viscous flow generates considerable heat, which raises the temperature of the
material and homogenizes it.
• Curing takes place by cross-linking.
• Because the resin is in a molten state as it enters the molds, the complexity of the
parts and the dimensional control approach those of injection molding.
Process Capabilities.
• Typical parts made by transfer molding are electrical connectors and electronic
components, rubber and silicone parts, and the encapsulation of microelectronic
devices.
• The process is suitable particularly for intricate shapes with varying wall
thicknesses.
• The molds tend to be more expensive than those for compression molding, and
some excess material is left in the channels of the mold during filling, which is later
removed.
22. Sequence of operations in transfer molding for thermosetting plastics. This process is suitable
particularly for intricate parts with varying wall thickness.
24. Plastics Processing:
Some parts made by Compression and Transfer Molding
dishes, handles for cooking pots
skis, housing for high-voltage switches
some rubber parts like shoe soles
and even composites such as fiber-reinforced parts
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25. Compression Molding
• A widely used molding process for thermosetting plastics
• Also used for rubber tires and polymer matrix composite parts
• Molding compound ( i.e. the raw materials) are available in several
forms: powders or pellets, liquid, or pre-form
• Amount of charge must be precisely controlled to obtain repeatable
consistency in the molded product
Fig: Compression molding for thermosetting plastics: (1) charge is loaded,
(2) and (3) charge is compressed and cured (increase temp & hold for
some time), and (4) part is, (after cooling ) ejected and removed.
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26. Molds for Compression Molding
• Simpler than injection molds
• No sprue and runner system in a compression mold
• Process itself generally limited to simpler part geometries due to
lower flow capabilities of TS materials
• Mold must be heated, usually by electric resistance, steam, or hot oil
circulation (for curing purposes)
• Molding materials:
– Phenolics, melamine, urea-formaldehyde, epoxies, urethanes,
and elastomers
• Typical compression-molded products:
– Electric plugs, sockets, and housings; pot handles, and dinnerware plates
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27. Transfer Molding
In transfer molding, the TS charge is loaded into a chamber
immediately ahead of mold cavity, where it is heated; pressure is
then applied to force soft polymer to flow into another heated mold
where it cures, finally acting like a injection molding. (note that, in
compression molding, the TS-charge is directly loaded, right into the mold cavity)
• Two variants:
– Pot transfer molding - charge is injected from a "pot" through a
vertical sprue channel into cavity
– Plunger transfer molding – plunger injects charge from a heated
well through channels into cavity
– TS …..thermoset
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28. Figure (a) Pot transfer molding: (1) charge is loaded into pot, (2)
softened polymer is pressed into mold cavity and cured, and (3) part is
ejected.
Pot Transfer Molding
Note that (1) Both upper & Lower dies are present; See the parting line (2)Raw mtl is loaded into the
back of the upper die. (3) the solidified sprue portion goes along with the plunger as a ‘cull’ and shall
be knocked off, automatically as an operation; finall comment is that, it is as if a shorter version of
Injection molding; Only thing is that the upper die itself is doing the function of the shot chamber.; Raw
matl is loaded as discrete heated slugs, either manually or in an automated way; Simplified version of
an IM process..
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29. Figure (b) Plunger transfer molding: (1) charge is loaded
into pot, (2) softened polymer is pressed into mold cavity and
cured, and (3) part is ejected.
Plunger Transfer Molding
Note that the cull & two runners go along with the molded part and shall
be subsequently cut off, by manual labor. Note the absence of the sprue
in plunger transfer molding11:34 AM
30. Plastics Processing: Compression and Transfer Molding
• used mostly for thermosetting polymers
• mold is heated and closed using pressure
• plastic flows to fills the cavity
• flash must be trimmed by finishing
dishes, handles for cooking pots
skis, housing for high-voltage switches
some rubber parts like shoe soles
and even composites such as fiber-reinforced parts
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31. Plastics Processing: Compression and Transfer Molding (from another book material)
compression molding
transfer molding
(more complex shapes)
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32. Compression vs. Transfer Molding
1. In both processes, scrap is produced each cycle as leftover material, called the cull
2. In compression molding there is no question of sprue, since the input mass of melt is
placed directly on to the cavity. But this is not the case in (pot) Transfer
molding……(for u to think..)
3. In case of Plunger transfer molding, there is no sprue; But the runner or runners are
present.
4. Transfer molding is a special example of injection molding without an inbuilt injector.
The input mass is charged discretly from outside.
5. Transfer molding lends itself to molding with inserts, in which a metal or ceramic
insert is placed into cavity prior to injection, and the plastic bonds to insert during
molding (Screw driver with a plastic handle is a good eg for insert-molded part.
1. Mold making cost for transfer molding is more expensive.
2. But Transfer molding can give short cycle times in automated production.
3. Transfer molding is capable of molding more intricate part shapes than compression molding but not as
intricate as injection molding
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33. Thermoforming
Flat thermoplastic sheet or film is heated and deformed into
desired shape using a mold, by pressing against the
mold or vacuum forces against the mold.
• Heating usually accomplished by radiant electric heaters located on
one or both sides of starting plastic sheet or film
• Widely used in packaging of products and to fabricate large items
such as bathtubs, contoured skylights, and internal door liners for
refrigerators
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34. • Step(1)
• The plastic sheet is first clamped on both
sides with clamps.
• The plastic sheet is then softened by heating
• Heating is mostly done by a radiant electric
heater, as shown in the figure.
Steps in the Vacuum
Thermoforming process
• Step(2) :
• The softened sheet is placed over a
concave mold cavity….in clamped condition
• It may be possible that the heated sheet
remains stationary and the bottom mold
could move up.
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35. • Step(3) :
• A vacuum draws the sheet into the cavity.
• The heated sheet spreads over the mold
shape and occupies all features that are
given in the mold.
• The vacuum force is so great that the sheet
material shall get into even the intricate
shapes of the mold
Steps in Vacuum Thermoforming
Step(4) :
Plastic hardens on contact with the cold
mold surface, and
the part is removed and subsequently
trimmed from the web.
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36. Use of a positive mold in vacuum
thermoforming: (1) the heated plastic
sheet is positioned above the convex
mold
Vacuum Thermoforming
Use of a positive mold in vacuum
thermoforming: (2) the clamp is lowered
into position, draping the sheet over the
mold as a vacuum forces the sheet
against the mold surface
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37. Applications of Thermoforming
• Thin films: blister packs and skin packs for packaging commodity products
such as cosmetics, toiletries, small tools, and fasteners (nails, screws, etc.)
– For best efficiency, filling process to containerize item(s) is immediately
downstream from thermoforming
• Thicker sheet stock: boat hulls, shower stalls, advertising displays and
signs, bathtubs, certain toys, contoured skylights, internal door liners for
refrigerators
Complete Plastic
Manufacturing
Processes_(360p).flv
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•Raw mtl for Thermoforming process is
thermoplastics in …………… form.
38. Plastics Processing: Thermoforming (another figure …you can
choose any one you feel good with..)
Sheet of plastic Heated (soft) Molded using a shaped die
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40. Rotational molding…Trash cans,
buckets, barrel drums…
Note that the pressurizing fulid is needed to keep the mold cover in a clamped
condition during the time of 2-axis rotation.
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FI
41. Operational principle of a rotomolding
• This processes is meant for hallow parts, whose size is very large, but having a thin wall
thickness.
• The mold is made of metal. Consists of a body and a cover.
• A pre-measured quantity of powdered polymer material (monomer) is placed inside the mold as
raw material.
• the mold must be warm at this time.
• This powder is a monomer, obtained from some preparatory polymer process.
• The mold is in fact housed inside a oven itself. Close the door of the mold. Heating starts.
• The mold is rotated about two axes as shown in the figure, simultaneously.
• This action tumbles the powder against the mold walls. Note that the powder tumbling means that
the powder is not forced by the centrifugal force, but the g-effect. This is how it differs from the CF
casting process. In RM, the rpm is very low.
• The heating fuses the powder without melting it.
• Some chemical cross linking agents may be added too, to make it from a mer to polymer.
• Cross linking of the molecules occur following the shape of the part all around
• Typical parts made by this method are water tanks, (sintex), toys, boat hulls, buckets,footballs
etc.
• Note that we can attach metallic or plastic inserts to these parts (like cast in place)
• Wall thicknesses as small as 0.4mm are possible. At the same time parts as large as 1.8m x 1.8m
x3.6m have been rotomolded so far.
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FI
•Complete
plastic parts
mfg
