Skip to content

Stretch Blow Molding In the P.E.T Industry: What to Know

Stretch blow molding is the plastic process you never knew you appreciated. Here’s why this process is so important in bottle manufacturing.

Stretch Blow Molding in the PET Industry | KB Delta

The P.E.T industry is often ignored even though it is all around people every day.

Plastics are used for packaging food, drinks, medicines, and a variety of other products. There are projections that the market for P.E.T packaging will be more than $55 billion by the year 2021.

When looking at the weight of materials, beverages make up just over 80% of all packaging in the P.E.T industry and that does not include other bottled liquids like oils, syrups, dressings, and more. Many people drink from or use a product out of a plastic bottle every single day.

However, most people do not know the process behind the making of each bottle.


How Are Bottles Made?

While plastic may seems simple or mundane, the processes by which plastic containers are produced can be quite complex.

Many people think of blowing a hollow area into a container as an old fashioned practice used on glass and only by a select number of dedicated artisans. However, your water bottles, sports drinks, and soda bottles all come from the very similar practice of blow molding.

With stretch blow molding, polyethylene terephthalate or another synthesized polymer similar to it, is expanded using specific amounts of air pressure and shaped into the desired form.

It is clear upon a simple investigation that a variety of plastic materials are used to hold different products. They come in different colors, shapes, and sizes depending on the brand, and the type of contents inside. All of these things must be taken into account prior to molding any plastic into the correct shape.

Different properties within a type of plastic have the ability to impact different steps of the formation process.

This complicates stretch blow molding because manufacturers must have the correct tools and variance of things like air pressure to ensure that their process best matches the material they use.


Intricacies of Bottle Design

When you think of designing a bottle, you likely consider that someone must think of what shape and size they would like the bottle to be. While you must make that decision, that is only the end goal of the entire design and production process.

Even within stretch blow molding, there are many small steps within each stage of production.

Because of this, there are many factors that have to be considered prior to creating bottles to be filled and put on shelves in the supermarket.

For example, if you want a bottle to be transparent, you must take great care to find the perfect heat for your equipment and materials.

If you try to use too much or too little heat, your container will likely be hazy, pearlescent, or uneven in its appearance.

Another consideration you must make is the stretch ratio of the material with which you are working. This is an integral part of forming the container. It will determine the possible length and diameter you can reach as well as what diameter and length will perform best together.

It also impacts the thickness of the bottle as well as how much material you need for each bottle to have even thickness throughout the entire container.


Contact Us to Learn About Our Compressor Parts for Bottle Manufacturing | KB Delta


The Steps and Stages of the Stretch Blow Molding Process

This process takes place in three steps, but the final step includes three stages.

In the first step, injection molding is used for a preform. A preform is a small vial, tube, or bottle shape from which the full sized bottle will be made. Then the preform is set aside after this initial molding is complete.

Then the second step is to heat the preform again in preparation for the final step of forming by stretch blowing.

This final forming itself is broken down as well.

  • Low blow delay: in this stage, the preform that has been reheated is first stretched by a rod.
  • Pre blow: While still being stretched with the rod, the preform begins to receive low pressure inflation with air inside a mold.
  • Blowing period: This is the final step of the formation. The rod ceases to stretch the material and the air pressure is increased to fully expand the preform to fit the mold. This is where the bottle forms to its final and desired shape.



While this is the most common practice with blow molding, there are some other methods. For example, when the low blow delay and pre blow stages are merged so that the material can be stretched without touching the rod used for stretching.

Other details vary between different products, but the overarching process is fairly universal.

The P.E.T industry surrounds people everywhere, every day. In a multibillion dollar industry, this process of stretch blow molding is included in more than three fourths of all products.

As the plastic packaging industry grows, the importance and prevalence of the method only becomes more valuable.

Even those who produce the small percentage of P.E.T products should utilize such process.

Posted in
Skip to content