Blow molding is a plastic processing technique where a heated plastic material is extruded into a hollow shape called a parison, which is then placed inside a mold and inflated with air until it takes the shape of the mold cavity. There are different types of blow molding depending on how the parison is formed, such as extrusion blow molding where plastic is continuously extruded and cut into parisons. Common materials used for blow molding include HDPE, LDPE, PP, PVC, and PET. Key factors that affect blow molding include the parison characteristics, inflation pressure, mold design, and blow ratio.

1. Blow molding
• Blow molding is the plastic processing technique where air is
blown in a heated plastic untill it fills a mold and forms the
desired shape to produce hollow parts.
Procedure of blow molding :
The basic procedure of blow molding are:
• Melting the thermoplastic resin by extruder .
• Forming molten resin into the parison .
• The parison is placed inside a mold .the air is inflated plastic
that push outward against the cavity wall.
• The formed is then cooled & ejected from the mold
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2. Raw material for blowing into material :
Parison and preform :
Parison is the part that is used in blow molding where air is inflated
to from the desired shape. It is one end closed test tube shaped
thing .Here the preform is same as parison but the there is thread
in the open end in prefom .Preform is mostly used in bottles .
Fig: Parison Fig :Prefom

3. Figure : Blow molding

4. Types of Blow molding :
• According to the parison making blow molding is three types,
• Extrution Blow molding
• Injection Blow molding
• 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 .

5. 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 now a days.
Extrusion blow
molding
Continous extrusion molding
Intermittent extrusion
molding

6. Extrusion blow molding:
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7. Extrusion blow molding:
Advantage:
• Low tool & die cost
• Fast production
• Ability to mold complex part.
Disadvantage:
• Limited to hollow parts
• Low strength

8. 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 passed to the hollow chilled mold .The compressed air is
blown through core rod into preform .
Ejection :
• The final product is cooled ejected from blow mold and stripped off
the core rod.

9. Injection blow molding:

10. Injection blow molding:
Advantages:
• Produce an injection mold neck for accuracy
• Eliminates weld line
Disadvantages:
• Small capacity

11. 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 .
 Disadvantage of this process is not suitable for every product (except
champne bottle)

12. 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)

13. Factors in blow molding:
• Parison characteristics:
Size, shape ,straightness, wall thickness, mechanical strength.
• Inflation pressure:
pressure will be set according to the process. Here, stretch blow molding
pressure is less needed .
• Mold design
• Blow ratio=
Here ,the blow ratio must be 1.5 to3
Mold diameter
Parison diameter

14. Blow molding :
Advantage:
• Low tooling cost
• Fast production rate.
• Ability to mold complex.
• Little scrap generated.
• Recycling.
• Large hollow parts.
Disadvantage:
• Only for hollow parts .
• Defect may be seen .
• Thick parts can not be manufectured.