Blow Molding

Blow Molding

Blow molding is used to produce sturdy plastic containers and parts of all sizes. There are three types of blow molding machines: Extrusion, injection, and injection stretch blow molding machines.

Blow Molding
(Copyright Wilmington Machinery, Inc., Wilmington, NC)

Polymer Product Manufacturing Steps

Blow molding is one of the ways you can process the polymer in the manufacture of polymer products:


Extrusion blow molding is used to make a variety of complex parts.

( Copyright Milacron, Batavia, OH)

General Information

In extrusion blow molding, the melted polymer is first pushed into a mold. Air is then injected into the mold and used to blow the polymer to the walls. After the newly formed part is cooled, the mold opens and the part is ejected.

extrusion blow molding
( Copyright Milacron, Batavia, OH)

Equipment Design

In extrusion blow molding the annular die first fills with melted polymer, as shown in the animation below. A plunger then pushes the melt into the open mold cavity, forming a tube of a melted polymer called a parison. Next, the mold closes, sealing the bottom of the parison and forming the neck of the bottle. Compressed air enters through the gray tube, pushing the parison to the mold wall. The bottle is then cooled and ejected from the mold.

Parison formation is an important step in the extrusion blow molding process: If the parison sags before the mold close, the part will have an irregular wall thickness. To avoid sag, manufacturers use high molecular weight polymers.

Many systems have automatic controllers that vary the die orifice diameter as the parison is extruded to match the mold shape. Precise parison control conserves raw material and yields a more consistent part thickness after blowing.

Blow molding machines may also be modified to extrude more than one layer of plastic at a time, as shown below. Coextruded parts might include an internal layer of recycled plastic or a barrier film to prevent leakage of toxic materials. The multiple layers can be seen in the container on the left.

Blow molding machine
( Copyright Milacron, Batavia, OH)
Blow molding machine
( Copyright Davis-Standard, LLC., Fulton, NY)

In order to increase productivity and optimize floor space, many manufacturers use a multi-parison system such as the one shown in the diagram on the top right. By increasing the number of die heads used, production volume is easily increased.

blow molding
( Copyright Milacron, Batavia, OH)

Usage Examples

Extrusion blow molding is most commonly used to make tanks, bottles, and containers. These products are used in many industries, including pharmaceuticals, toys, personal care products, automotive, lawn and garden supplies, and medical products.

Extrusion blow molding
( Copyright Milacron, Batavia, OH)


  • Better than other blow-molding machines for making complex parts.
  • Short runs or prototype parts are more economical than using injection blow molding.
  • Higher productivity than rotational molding.


  • Large initial capital investment.
  • Inefficient material usage compared to injection blow molding and rotational molding.
  • Irregular wall thickness.
  • Added labor and floor space required for product finishing, such as grinding and trimming.


Injection blow molding is used to produce containers to exact specifications, usually in long production runs.

( Copyright Milacron, Batavia, OH)

General Information

Injection blow molding consists of three steps: formation of the preform, expansion of the preform, and part ejection.

Injection blow molding
( Copyright Milacron, Batavia, OH)

Equipment Design

This animation shows the key steps in injection blow molding. Melted polymer is injected into the yellow preform mold, forming the bottle’s neck. The preform is then rotated to the blue bottle mold. The mold closes and compressed air enters through the gray rod, expanding the preform to the cool mold walls. The mold opens and the finished bottle is ejected.

A typical injection blow molding machine has the same three stations. However, each mold may contain several cavities so that many bottles may be produced at one time.

At station 1 injection molding occurs after the mold closes. The core rods and preforms rotate to station 2, where the preforms are blow-molded. The mold opens when the parts have cooled allowing the core rods and bottles to rotate to station 3, where the bottles are ejected.

 blow molding machine
( Copyright Milacron, Batavia, OH)

Usage Examples

Injection blow molding is used for many of the same applications as extrusion blow molding. Small containers requiring high tolerance necks are the most common products. These bottles are used in the pharmaceutical, food, cosmetic, and personal products industries.

Injection blow molding applications
( Copyright Milacron LLC, Batavia, OH)


  • Little or no variation in weight and wall thickness.
  • No trimming is needed vs. extrusion blow molding.
  • No scraps produced.
  • No variation in bottle quality from recycling of scrap.
  • Molds have a long lifespan, suited for long runs.
  • Uses less floor space than extrusion blow molding.
  • No finishing equipment needed.
  • Low labor costs.


  • High capital costs for short runs.
  • Unsuitable for complex shapes with handles or off-center necks.
  • Long cycle times.

Injection Stretch

Injection stretch blow molding is a variation of injection blow molding used to produce clear plastic bottles.

General Information

Injection stretch blow molding is a four-step process. First, the preform is injection molded to the core rod. Next, the preform is elongated by a stretching rod. The stretched preform is then blown to the mold walls. After cooling, the part is released.

Equipment Design

The movie below depicts the steps in injection stretch blow molding: The preform is the first injection molded in the yellow mold, then rotates to the blue bottle mold. The gray rod elongates, stretching the polymer preform to the end of the mold. Compressed air then enters through the rod, expanding the preform to the mold walls. The mold then opens and the transparent bottle is ejected.

Injection stretch machines typically consist of two or three stations. At the first station, the polymer is injection molded to form a preform. Next, at the second station, the preform is conditioned or heated to an optimal temperature for blow molding. However, not all machines require the second step. During the steps of the last station, the bottle is formed through blow molding and then released.

Usage Examples

Injection stretch blow molding is used to make clear, thin-walled containers. The most recognizable shape produced using this method is the plastic soda-pop bottle.

Containers produced by injection stretch blow molding are widely used in the food, pharmaceutical, and personal product industries.

Containers produced by injection stretch
Containers produced by injection stretch blow molding

(Copyright Milacron, Batavia, OH)


  • Lower container weight than injection or extrusion blow molding.
  • Produces containers with higher longitudinal and hoop strength than other blow molding techniques.
  • No pinch-off scar.
  • High tolerance neck design.
  • Clear, biaxially oriented products.
  • No scrap or finishing equipment.


  • Not economical for short runs.


Davis-Standard, LLC. , Fulton, NY

Milacron, Batavia, OH

Wilmington Machinery, Inc., Wilmington, NC


Gabriele, Michael C. “Multi-parison blow molding meets new volume needs.” Modern Plastics Feb. 1997: 48-53. Print.

Gabriele, Michael C. “PET Finds Growing Use in Non-food Containers.” Modern Plastics April      1997: 60-65. Print.

Halasz, Laszlo. Control Methods in Polymer Processing. Amsterdam, The Netherlands: Elsevier      Science Publishers, 1993. Print.

Pearson, J.R.A.. Mechanics of Polymer Processing. London: Elsevier Applied Science      Publishers, 1985. Print.

Rogers, Jack K. “Blow molding industry takes on recycling, PET extrusion.” Modern Plastics Aug. 1990: 46-9. Print.

Rosen, Stephen L. Fundamental Principles of Polymeric Materials. New York: John Wiley &      Sons, 1982. Print.


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