Process Overview

Blow molding is probably the second most common method to manufacture plastic parts. There are two types of blow molding processes: extrusion blow molding and injection blow molding. In extrusion blow molding, a tube of molten plastic, called a parison, is vertically extruded. A mold, usually constructed of two aluminum halves, clamps around the parison after it reaches a certain length. A blow pin enters the top of the mold, punctures the parison, and blows compressed air into the parison. The compressed air forces the soft plastic against the walls of the mold, where it cools and is then ejected. This is a common method used to make milk jugs, shampoo bottles, and many other types of bottles.

Injection blow molding combines elements of injection molding and blow molding processes. First, a preform is injection molded. A preform is an injection moldable part that will later be blown into a bottle (it is analogous to a parison). Sometimes preforms are made in an injection molding machine and then transferred to the blow molding machine, and sometimes they are made directly in the injection blow molding machine. Regardless, a heated preform is placed in a blow mold, where compressed air is forced inside of it, stretching the plastic against the mold walls. This is a common method of making soda bottles.

Materials

Many thermoplastics can be blow molded, but a few dominate the industry because of they’re processing characteristics. As in extrusion, the polymer must have high melt strength so the melted plastic won’t rip when it is stretched. Some of the most common plastics and their applications are listed below:

HDPE (high density polyethylene): stiff bottles, toys, cases

LDPE (low density polyethylene): flexible bottles

PP (polypropylene): higher temperature bottles and cases

PVC (polyvinyl chloride): clear bottles, oil resistant containers

PC (polycarbonate): housings

Nylon: automotive coolant bottles, power steering reservoirs

FEP (fluorinated ethylene propylene): chemical resistant bottles

Summary

Some characteristics of blow molding are:

· Bottles are the most common product

· Parts with large, mostly enclosed, volumes of air are usually produced with blow molding

· Not all materials are blow-moldable. Most commonly used are polyethylene, polypropylene, PET

· Blow molding cannot make complex geometry or sharp corners