The plastic extrusion process starts with what are called thermoplastic resins. Thermoplastic resins are a type of plastic which can be melted, processed, and then re-melted to be used again. These resins are generally delivered in a pellet or bead form to be used in the plastic extrusion machinery.
The pellets or beads can come in several different forms. There are plastic resin beads, which come in what is referred to as a virgin form. These are beads which have never been processed before and generally come with certifications of purity. The beads are also available in grades of quality that can be purchased for specific uses. Waste plastic from the extrusion process can be reprocessed into beads that may be used again, which reduces the overall waste generated in the process.
Machinery and Melting
Extrusion machinery can be complicated to operate, but the overall process is relatively straightforward. The heart of the machine is the screw, which sometimes is referred to as an auger. The screw is turned by a gearbox, which is powered by a motor. It is enclosed in a tight, heated barrel, which helps to provide friction.
The thermoplastic pellets are delivered into the machine through a hopper. The hopper is located at the rear of the barrel/screw assembly and the pellets drop into the barrel from there. As the screw turns, it slowly drags the thermoplastic pellets forward. The heat from the friction of the screw turning inside the barrel--along with external heating--melts the plastic as it moves forward in the barrel. The melted plastic is pushed into a section designed to meter the plastic for the next stage in the process. It also may be subjected to pressurized pumping at this phase of the process.
Once the plastic has entered into metering section of the barrel, it is ready to be extruded into a die. The die is attached to the barrel and it represents the final shape or profile that the plastic is intended to take. The plastic is forced into the die. As the plastic moves forward into the die, it will be separated by a mandrel, which is centered in the extrusion channel.
Pressurized air is forced though the mandrel structure as a means of keeping the plastic from collapsing as it moves through the die. As the plastic leaves the die, it will enter into a vacuum environment. Inside the vacuum, there are sizing rings meant to keep the plastic in the desired shape. The vacuum environment also will be filled with water as a means of cooling the extruded plastic. After the extruded plastic has passed through the water-filled vacuum environment it can be cut or spooled as appropriate.