What is the Difference Between Injection and Blow Moulding?

If you’re not in the industry, we understand that it can sometimes be confusing when trying to understand the different moulding procedures to make your favourite plastic bottles. You may think that all bottles are made the same way, with the same machine.

This is simply not the case. In this blog post, we’ll clear the confusion by talking about the different kinds of blow moulding.


Blog Moulding

Blow Moulding technology has been developed to make hollow parts such as plastic bottles with uniform wall thicknesses that retain the integrity of the outside shape. About 10% of all worldwide plastic is used in this process.

There are three types of Blow Moulding Processes:

  1. Extrusion Blow Moulding
  2. Injection Blow Moulding
  3. Stretch Blow Moulding


Let’s discuss each in more detail:


Extrusion Blow Moulding

Extrusion Blow Moulding is the most common blow moulding process. Hot plastic material is dropped from an extruder and into a water-cooled mould. The mould is then closed, air is injected through the neck of the bottle, like a balloon, until the hot plastic material hits the sides of the cold mould and then the bottle freezes and becomes rigid.

This process can be used to create diverse bottle and container shapes, sizes with varying neck openings. Depending on the machine, up to 350 bottles can be made per hour. Extrusion blow moulds are usually less expensive than injection blow moulds and faster to produce. It can be used with PET, HDPE, PVC, PC, and PP plastics. 


Advantages of Extrusion Blow Moulding

  • High rate of production
  • Low capital investment to set up
  • Good for small production runs


Disadvantages of Extrusion Blow Moulding

  • High scrap rate
  • Less control over wall thickness
  • Not easy to trim excess plastic


Injection Blow Moulding

Injection blow molding is the process whereby the plastic preform is injection molded and the preform travels on the core rod to the blow mold station, where blow air enters through the core rod and lifts the hot preform material off the core rod and forms it by air pressure to the design of the female blow mold.

Bottles made using this technology have an exact weight that cannot be changed and there is an even and more precise distribution of the plastic material. They do not require trimming.


Advantages of Injection Blow Moulding

  • No unwanted scrap material is generated during the process.
  • Better, tighter part tolerances that are safe and secure are created.
  • The procedure produces good surface finishes.


Disadvantages of Injection Blow Moulding

  • Compared to extrusion-based blow moulding, the injection blow-moulding process has a lower production rate (why it is less widely used)
  • Only suits small capacity bottles, as it is difficult to control the base centre during blowing.
  • Requires a higher capital investment


Stretch Blow Moulding

Stretch blow moulding is best known for producing PET bottles commonly used for water, juice and a variety of other products Compressed air is then blown into the stretched parison to expand to the bottle’s mould. Once the bottle is cooled the mold is opened and the finished bottle is emptied from the mold cavity.

The production rate can be much higher than the other two forms of blow moulding, but it is much more expensive to set up.


Advantages of Stretch Blow Moulding

  • High rate of production
  • Low capital investment to set up
  • Good for small production runs


Disadvantages of Stretch Blow Moulding

  • Highly dependent on petroleum prices.
  • Requires a lot of production methods. 
  • Has limited uses. 


See it in action

As you can see, producing different bottle-types requires vastly different methods. B&I Polycontainers offers extrusion, injection and stretch blow-moulding facilities to create the perfect bottle for our customers. If you would like to see the difference for yourself, please contact us. We’ll be happy to make an appointment with a member of the sales team for a full factory tour. 

B&I Polycontainers is a Proudly South African plastic bottle manufacturer and has decades of experience in supplying the best plastic bottles locally and internationally. Contact us today for your packaging needs.


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