Plastic Extrusion 101
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections, and to work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms parts with an excellent surface finish.
In the extrusion of plastics, thermoplastic material in the form of granules is fed from a top-mounted hopper into the barrel of the extruder and heated and mixed so it is fully plasticised.
The fully plasticised compound is then pushed through the extrusion tool, passed through a series of forming plates with vacuum calibration and at the same time is cooled with chilled water.
Additives such as colourants and UV inhibitors (in either liquid or pellet form) are often used and can be mixed into the compound prior to arriving at the hopper. While pultrusion can offer many similar profiles in continuous lengths, usually with added reinforcing, this is achieved by pulling the finished product out of a die. This is then pulled through an automatic cutting system and cut to a pre-determined length to suit the customer’s requirements.
Step 1 - Loading the pellets or resin
The raw thermoplastic is first loaded into a hopper located above the rest of the machine. This allows for gravity to do most of the work in ensuring plenty of material is feeding through for a good extrusion. If required, additives are also loaded into the hopper to be mixed with the plastic and to provide various properties to the end extrusion, depending on the additive selection.
Once inside the machine, the pellets have to pass a rotating screw. This ensures regulation of the amount of material coming through, as well as mixing the pellets and additives a little.
Step 2 - Melting and filtering
Bypassing through the screw, the pellets are forced into the barrel. This is where the heat is applied and the plastic is melted. To ensure the best quality extrusion, via avoiding burning or overheating the plastic, the barrel becomes progressively hotter towards the end, melting the plastic gradually.
The temperature of the barrel, and the speed the plastic moves through it, are different for every material type and have significant implications as to the properties of the resulting extrusions.
At the end of the barrel is the breaker plate, used to filter contaminants in the plastic as it passes through the screen.
Step 3 - The Extrusion Itself
After exiting the barrel, the plastic will meet the die. This is where the plastic is forced into the shape of the final extrusion and is where the most attention to detail must be paid. The die design has to make heavy considerations as to how the plastic will flow through since even the smallest faults can result in significant warping of the final extrusion.
Today it is possible to make an almost infinite number of extrusion designs work.
Step 4 - Cooling it down
Now that the extrusion has passed through the die, it needs to be set in shape and since plastic cools much slower than the majority of extrusion materials, and therefore has to undergo a more effective cooling process.
The plastic is fed straight into water baths from the die.
Step 5 - Adding the finishing touches
Adhesive strips, punching, cutting, bending, drilling, printing etc. are just a few of the in-line fabrication and finishing possibilities available in most extrusion facilities.