Thursday, 14 July 2011

Plastic Extrusion Basics

We use the by-products of plastic extrusions every day. The toothbrush that you used this morning was created by large plastic extrusion machines. Not to mention the computer, keyboard and mouse you use on a regular basis were all manufactured by the plastic extrusion process. Plastic extrusion is a high volume manufacturing process where raw plastic material is melted and shaped into a continuous profile.
The extrusion process produces end products like pipe/tubing, weather stripping, fence, deck railing, window frames, adhesive tape and wire insulation.

During the extrusion process, raw thermoplastic material in the form of small beads, typically called resin, fed from a top mounted hopper into the barrel of the plastic extruders. Additives like colorants and UV inhibitors, in either liquid or pellet form, are often used and can be mixed into the resin prior to arriving at the hopper.

The material enters through an opening near the rear of the barrel and comes into contact with the screw. The rotating screw forces the plastic beads forward into the barrel which is heated to the desired melt temperature of the molten plastic. Usually this temperature can range from 392 °F to 527 °F depending on the type of polymer. In most cases, a heating profile is set for the barrel in which three or more independent PID controlled heater zones gradually increase the temperature of the barrel from the rear to the front. This allows the plastic beads to melt gradually as they are pushed through the barrel and lowers the risk of overheating which may destroy the polymer.

Extra heat is contributed by the intense pressure and friction taking place inside the barrel. If an extrusion line is running a certain material fast enough, the heaters can be shut off and the melt temperature maintained by pressure and friction alone inside the barrel. In most extruders, cooling fans are present to keep the temperature below a set value if too much heat is generated.

More and more plastic products are finding their way into our homes everyday. Recycling of the extruded materials can create a large cost benefit not only for the manufacturer and consumer, but as new science facts have come to surface perhaps our own environment as well.

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