Basic of Blow molding .

2.  TOPIC

3.  Definition
 Blow molding is a plastic processing technique where air is
blown in a heated plastic until it fills a mold and forms the
desired shape to produce hollow parts.
Raw materials:
Thermoplastics in the form of pellets or beads(Resin
beads)
 Basic procedure of blow molding :
 Melting the thermoplastic resin by extruder.
 Forming molten resin into the parison (or preform).
 The parison is placed inside a mold. The air is inflated
inside the plastic that push outward against the cavity wall.
 The formed product is then cooled & ejected from the mold
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4. Figure : Blow molding

5. Two important terms:
Parison and preform :
Parison is the part that is used in blow molding where
air is inflated to from the desired shape. It is closed on
one end. Here the preform is same as parison but there is
thread in the open end in preform. Preform is mostly used
in bottles .
Figure: Parison Figure :Preform

6.  Types of Blow molding :
Blow molding is of three types
1) Extrusion Blow molding
2) Injection Blow molding
3) Stretch Blow molding
 Extrusion blow molding :
 Plastic is heated and extruded into a hollow parison .
 Then parison is captured by cold metal mold
 Air is blown into the parison ,inflated into the mold to
make desired shape .

7.  Types of Extrusion blow molding:
 Continuous Extrusion molding:
In extrusion molding, plastics is continuously extruded and the
individual parts are cut off by knife.
 Intermittent extrusion molding:
In straight intermittent extrusion, extruder screw run ,stops and
pushes melt out. Here, accumulator is used to gather the mold.
Extrusion
blow molding
Continuous extrusion
molding
Intermittent extrusion
molding

8.  Extrusion blow molding:
1
2 3
4
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9.  Extrusion blow molding:
 Advantage:
 Low molding cost resulting from lower pressure used
 Low tool cost due to simple structure
 Fast production due to small processing cycle
 Ability to mold complex part
 Disadvantage:
 Limited to hollow parts
 Low strength of the product

10.  Injection blow molding:
 Injection blow molding is used for producing hollow objects in
large quantity .the main product of this process are jars ,PET
bottles etc.
The main steps of the injection blow molding are:
Injection:
 Extruder barrel and screw assembly melts the polymer .
 Then it is injected through heated cavity and core pin, where
preform forms .
Blowing :
 Preform passes to the hollow chilled mold. Then compressed air
is blown through core rod into preform.
Ejection:
 The final product is cooled and ejected from blow mold and
stripped off the core rod.

11.  Injection blow molding:

12. Injection blow molding:
 Advantages:
 Ability to produce threaded mold neck.
 Minimizes weld line
 Disadvantages:
 Small capacity of the product

13.  Injection stretch blow molding:
 The plastic is first molded into a preform by injection molding.
 Then the preform are heated and blown in high temperature
pressure ratio.
 Here, the heated preform is always stretched with a core.
 The basic advantage of stretch blow molding is fast production and
less pressure (as core pin is used to stretch the heated preform),
also the product’s excellent visual and dimensional quality.
 Disadvantage of this process is that it is not suitable for every
product. It is usually used to manufacture champagne bottles.

14. Materials used in blow molding:
Most thermoplastics can be blow molded
 Some typical applications are
HDPE : High Density Polyethylene (stiff bottle, toys, cases, drum)
LDPE : Low Density Polyethylene (flexible bottle)
PP : Polypropylene (higher temperature bottle)
PVC : Polyvinyl Chloride (clear bottle, oil resistant containers)
PET : Polyethylene terephthalate (soda pop bottle)

15.  Factor effecting blow
molding:
 Parison characteristics:
Size, shape, straightness, wall thickness, mechanical
strength.
 Inflation pressure:
Pressure will be set according to the process.
 Mold design :
Size of the mold, the material the mold is made with.
 Blow ratio 
Here, the blow ratio must be 1.5 to 3
Mold diameter
Parison diameter

16.  Advantage of Blow molding:
 Low tooling cost.
 Fast production rate.
 Ability to mold complex.
 Little scrap generated.
 Recycling.
 Producing large hollow parts.
 Disadvantage of blow molding:
 Only for hollow parts.
 Defect may be seen.
 Thick parts can not be manufactured.

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