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The different types of blow molding

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Blow molding ( also known as blow forming) is a manufacturing process through which hollow plastic parts are formed such as plastic bottles and pipes.


The blow molded parts can be made from various thermoplastic materials, including the following:

  • Polyethylene ( Both at low and high density)
  • Polyethylene terephtalate (PET)
  • Poly propylene (PP)
  • Polyvinyl Chloride (PVC)

Blow Molding Process

Plastic is heated until it melts down and becomes a viscous fluid. Then this plastic is extruded through a blow molding die to form a tube shape called a Parison. A mold containing a cavity is closed around a parison before the pressurized air is blown inside so that the parison expand against a cavity wall. A plastic is cooled and hardened and the air is released. Then the mold opens and the part is carried out.

Types of Blow Molding

There are three types of blow molding:

  • Extrusion blow molding
  • Injection blow molding
  • Stretch blow molding

Extrusion Blow Molding

Plastic is melted and extruded in a hollow parison before it is closed in to a cold metal mold. Next air is introduced in to a Parison, making it into a hollow bottle , can,glass or part by pushing it against a cavity wall.

Types of Extrusion Blow Molding

  • Continuous extrusion molding
  • Intermittent extrusion molding

Continuous extrusion molding

In this type plastics are continuous extruded by a extruder and the parts are cutoff by a knife.

Intermittent extrusion molding

In this type extruder screw runs, then halt for a while and melted plastic is driven out. Now accumulator is utilized for gathering melted plastic.

Injection Blow Molding

It is used for the manufacturing of hollow objects in large quantity.

The steps of injection molding are

  • Injection
  • Blowing
  • Ejection


Polymer is melted and it is then transferred into a heated preform mold where preform forms. It is a temporary shaped product which is used to make a final product.


The preform is then passed to the hollow chilled mold. Then through the core rod the air is blown in to a perform material around which the mold is clamped.


The final product is cooled and then removed from the blow mold and the core rod.

Stretch Blow Molding

In this process plastic is converted into a preform by using injection molding process. These performs are then transferred to a machine where they are heated beyond glass transition temperature by using infrared heaters or radiators. The air is then blown to high pressure by utilizing metal blow molds. Now core rod is used to pull the preform. This method is used for making bottles and other things.

Advantages of Blow Molding

  • It is core less
  • Rapid cooling is done in blow molding
  • Complex hollow shaped products can be manufactured
  • High production rate
  • Low tooling and die cost
  • Recycling of flash can be done

Disadvantages of Blow Molding

  • It is only used for hollow parts
  • Thick hollow parts cannot be form
  • Trimming is required
  • Low Strength

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