Plastic Injection Molding
Plastic injection molding
Plastic injection molding is the process of injecting melted plastic resin, then forcing it under pressure into a plastic mold, where it cools and hardens. The plastic mold is then opened, and the part is removed or “ejected”, usually automatically by “ejector pins”, long thin metal rods inside the mould that push through the cavity forcing the plastic part out of the mold cavity automatically as the molding machine opens the mold.
The plastic mold must have a very heavy wall thickness to keep the mold cavity from bulging since the plastic resin, is injected with as much as 20,000 pounds per square inch of pressure. And for the same reason molding machines must be able to clamp the mold shut with tremendous force.
On the top of the molding machine there is usually a hopper which is filled with the plastic resin to be molded, usually in the form of small pellets about 1/3 the size of a grain of rice. The hopper feeds the pellets into the “barrel” of the machine.
The barrel has a long screw or auger inside, that rotates, forcing the pellets toward the front of the barrel. The process of auguring the pellets produces a lot of heat, that melts the pellets. Additional heat is supplied to the barrel through the use of heating bands, electric resistance heating elements that wrap around the barrel.
At the end of the barrel there is a nozzle from which the melted plastic resin is ejected into the mold. The nozzle also has electric resistance heating bands wrapped around it. The temperature of the nozzle is very important, and the temperature is monitored and controlled at the operator control panel.
If the nozzle temperature is not hot enough the plastic can “freeze off”, forming a solid plug that has to be removed before plastic can be injected into the mold. If the nozzle is too hot, it can burn the resin destroying the plastic before it makes it into the mold.
Most injection molding machines can be run in either manual mode, or automatic mode. In the manual mode, an operator slides open a door providing access to the mold itself. The operator reaches in and pulls the part out of the mold, then closes the door, and the mold closes, is filled with plastic, cools of for a few moments, and then opens up again. At that point the operator opens the door again, and the whole process repeats.
In automatic operation there is no operator. The machine just hums along by itself, automatically ejecting the part or parts when the mold opens, and then re closing the mold, injecting the plastic etc.
Plastic injection molds can become quite complex. They usually require water lines to be built into the mold to cool of the melted plastic resin and harden the part, and they have ejector pins that push into the mold cavity as the mold opens, to eject the part out of the cavity. Often the molded part requires moving metal pieces called “slides” that must pull metal cores out of areas of the part where there is no plastic such as the inside of pipe fittings etc.
There are many different brands and types of plastic injection molding machines that vary in the way they clamp the mold shut and other differences as well. What I’ve talked about is very very basic.
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