While thermosets can be extruded by heating them, the majority of extruded plastics are thermoplastics. Within that classification there is a wide range of high performance, engineering grade, and commodity plastics to choose from, all of which can include amorphous or crystalline molecular structures.
High performance plastics are the rarest and most expensive types to be extruded; these plastics are made for harsh environments and feature especially good thermal and mechanical properties. One such plastic is polyetherketone (PEK), which is used in the aerospace and automotive industries because it’s resistant to chemicals, flame, and shrinkage, as well as being remaining strong over a range of temperatures.
One step below are engineering plastics, which are specialty plastics designed to include unique properties without being as expensive as high performance materials. These include TPE, ABS, nylon, and polycarbonate. Finally, commodity plastics are more general use materials that make up the majority of extrusions. These plastics, which include polypropylene, polyethylene, and PVC, are the least expensive plastic types.
Finally, any plastic from these three groups can either have amorphous or crystalline polymer structure, which lends certain properties. Amorphous structures, such as those found in PVC, ABS, and polycarbonate, have messy, mobile polymer chains. This lends them flexibility, elasticity, and impact and shrinkage resistance. However, they’re not as strong or chemically resistant as crystalline structure plastics, like HDPE or polypropylene. These plastics have rigid structures that lend heat resistance, strength, and stiffness. However, they also tend toward shrinkage and don’t have much impact resistance.
Plastic Extrusion Process Basics
The plastic extrusion process begins with the placement of raw resin into the extruder’s hopper. If the resin lacks additives necessary for the particular application (such as UV inhibitors, anti-oxidants, or colorants), then they are then added. Once in place, the resin is typically gravity-fed through the feed throat of the hopper down into the extruder’s barrel. Within the barrel is a long, rotating screw that feeds the resin forward in the barrel towards the die.
As the resin moves along within the barrel, it is subjected to extremely high temperatures until it starts to melt. Depending on the type of thermoplastic, barrel temperatures can range between 400 and 530 degrees Fahrenheit. Most extruders have a barrel that gradually increases in heat from the loading end to the feed pipe to enable gradual melting and minimize the possibility of plastic degradation.
Once the molten plastic reaches the end of the barrel, it is forced through a screen pack and fed into the feed pipe that leads to the die. The screen, reinforced by a breaker plate due to high pressures in the barrel, serves to remove contaminants that may be present in the molten plastic. The porosity of the screen, number of screens, and other factors can be manipulated until uniform melting occurs as a result of the right amount of back pressure.
Once in the feed pipe, the molten metal is fed into the die cavity, where it cools and hardens. To expedite the cooling process, the newly formed plastic commonly receives a sealed water bath, in which it is pulled through a water bath in a vacuum to prevent any hollow form from collapsing. In the case of plastic sheeting extrusions, cooling rolls replace the water bath. We go into more detail on the types of cooling equipment farther down.
Read more: Types of Plastic Extrusion Materials