The Plastic Extrusion Process
In the plastics
extrusion process, raw thermoplastic material, or resin, is gravity fed from a
top mounted hopper into the barrel of an extruder. Additives, such as colorants
and UV inhibitors, in either liquid or pellet form are often used and can be introduced
into the resin below arriving at the hopper. The process has much in common
with plastics injection molding though differs in that the process is usually
continual. While injection molding can offer many similar profiles in
continuous lengths, usually with added reinforcing, the finished product is
pulled out of a die instead of extruding the fluid resin through a die.
As the material enters the feed throat near the rear of the barrel it comes in contact with the screw. The rotating screw forces the plastic resin forward into the barrel that is heated to the desired melt temperature depending on the resin. In most processes, a heating profile is set for the barrel utilizing three or more independent PID (proportional-integral-derivative controller) controlled heat zones that gradually increase the temperature of the barrel from the rear where the resin has entered to the front. This allows the plastic resin to melt gradually as it is pushed through the barrel and lowers the risk of overheating which may cause degradation in the polymer.
At the front of the barrel, the resin leaves the screw and travels through a reinforced screen to remove any contaminants. A breaker plate generally reinforces screens because the pressure at this point can exceed 5000 psi (34 MPa).
After passing through the breaker plate resin enters the die. The die is what gives the final product its profile or shape and must be designed so that the molten plastic evenly flows from a cylindrical profile, to the product's profile shape. Uneven flow at this stage would produce a product with unwanted stresses at certain points in the profile. These stresses can cause warping upon cooling. Almost any shape imaginable can be created so long as it is a continuous profile.
The product must now be cooled which is usually achieved by pulling the extrudate through a water bath. Plastics are excellent thermal insulators and are therefore very difficult to cool quickly. Compared with steel, plastic conducts its heat away 2000 times more slowly. In a tube or pipe extrusion line, a sealed water bath utilizes a carefully controlled vacuum to keep the newly formed and still molten tube or pipe from collapsing. A set of cooling rollers is generally used in the sheet extrusion process to cool sheet as it exits the extruder.
Sometimes, on the same line, a secondary process may occur before the product has finished its run. In the manufacture of adhesive tape, a second extruder melts adhesive and applies this to the plastic sheet while it’s still hot. Once the product has cooled, it can be spooled, or cut into lengths for later use.
Plastic extruders are also extensively used to prepare recycled plastic waste and/or raw materials after cleaning, sorting and/or blending into filaments suitable for blending into the resin pellet stock used by the plastics industry at large.