The Extrusion Process
The 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.
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.
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