Types of Injection Molds and the Injection Mold Process
Injection molds are the containment shells that give plastic parts their shapes. When used for casting, EDM and other types of machining, they are constructed out of aluminum, beryllium copper, brass, kirksite and stainless steel. Each interior surface is specially coated and polished in order to reduce wear and make ejection of plastic parts easier and with fewer errors. There are four types of Injection Mold Designs: Cold-Runner two-plate molds, Cold-Runner three-plate molds, Hot Runner molds, and Insulated Runner molds.
There are five key parts to each type of mold:
Runners are the channels that the resin (liquid plastic) flows through.
A Gate is the entry point where the resin actually enters the mold to form the part.
Vents are positioned on each parting line or where the mold will separate to eject the part. They allow gas and heat to be released as the resin is injected.
The Internal Water Cooling Network controls the speed of the mold process which in turn controls the strength of the part as well as how much it will shrink due to heat and pressure.
Bushings and Pins work as one piece to help eject parts once the process has been completed. They are powered by either a hydraulic or pneumatic system.
The process of Injection Molding is simple, but each step must be accomplished without error:
Clamping - The two halves of a mold must be secured together before the plastic can be inserted or injected. Most are pushed and held together by a hydraulic clamp. They are kept in place by the fixed and moving platens of the injection machine as plastic is injected.
Injection - Plastic pellets are heated by the injection machine. Once they are melted to the proper temperature, this 'shot' is injected into the mold. The pressure of injection holds the shot together in order to form the part. Time needed to accomplish the process can be estimated by the amount of shot used factored with the amount of force or power used to inject the shot along with the resulting pressure.
Cooling - Upon contact with the interior of the mold, the shot begins to cool and harden in the desired shape. To control shrinkage, more shot is added to maintain temperature. It is this thermodynamic property of the plastic used and the required thickness of the part that determines how long cooling will take.
Ejection - Once that time is finished, the mold is separated and the completed part ejected from the two halves by a mechanism using the bushings and pins described above. If required, a special release agent may be sprayed over the part and ejection mechanism.