Blow Molding 101

We are pleased to introduce our reference guide which defines the terms and processes used in the world of blow molding.
This guide will be contunously updated, because we can only help and understand each other, if we speak the same language.

For more detailed questions or any suggestions on a specific topic, we are looking forward to receiving your feedback in the comments.

Your W. MÜLLER-Team


Often times, a bottle is not a product until it is filled. It can be food, drink, cosmetics, toiletries, chemicals, or even medicine. What type of product the bottle will hold has a huge impact on its design and materials!

For example, chemicals must often be bottled in special materials that do not react with the chemical. This allows the chemical to stay pure in the bottle, and prevents hazardous chemicals from being released.

Foods, especially dairy products, also need a special bottle design because they react with light and quickly spoil. Other foods can also last longer thanks to light or oxygen impermeable packaging.

Bottles filled with hot liquids must also be specially designed, as not to deform. This design can often be very different than the designs for bottles that are filled cold, and the materials must be able to withstand the increased temperature.

A thin-walled PET beverage bottle shrinks when you fill it with hot liquids.

Bottle designs can also allow for pleasing aesthetic and eye-catching marketing.



The flash is the "waste product" of the blow molding process, created by excess plastic being pressed to the edges of the mold. When the two halves of the mold press onto the suspended plastic, the excess plastic is squeezed to the top and bottom of the mold, where it stays during blowing. After the bottle is blown, the flash stays attached to the bottle during the cooling process, and must be removed.

This isn’t really “waste” though! After the flash is removed, it can be reground and introduced back into the extrusion process. This can help provide a cost-effective and environmentally sustainable alternative to the “virgin,” or new material, which comprises parts of the bottle.

Plastic Extruded with an Open Mold
The plastic, pictured in green, is first extruded from the head with the mold open.

Mold Closed and Plastic Blown
Then, when the mold closes around the suspended plastic, the plastic is blown to the edges of the mold.

Bottle with Neck and Bottom Flash
Here you can see the excess plastic at the bottom and neck of the bottle.


The screw is the main component of an extruder. It transports the plastic, which is poured into the extruder as granules through a hopper. Due to the heat that results from the friction of mixing of the granules, and from the heating bands, which are located around the extruder, the granules are plasticized.

The screw ensures the best possible mixing (homogenization) and transports the plastic compound to the extruder outlet. In some cases, new plastic granules and recycled material are mixed together, so the screw helps to ensure a uniform distribution of materials. If color is added in the form of a masterbatch, the screw ensures that the color is homogeneously mixed with the neutral plastic.

There are special screws for various applications and plastics.

Extruder screw with mixing tip 

Extruder screw for PA material


Extruder screws have different sections; the feed section, the compression section, and the metering section. The size and layout of the sections are often customized for the specifc material or application, and special applications can add special sections to the screw, such as a mixing tip.