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A Basic Plastic Injection Molding Machine ProcessMaterial Drying and Material Hoppers
Material Drying Depending on the resin you are using and whether or not it is a “hydroscopic” resin (absorbs moisture form the air), you may have to first dry the resin. This is a process of removing any moisture from the individual plastic pellets, so as to make them usable in the injection molding process. This is done with a material dryer, which is provided my many different manufactures, and it’s sole purpose is to remove moisture from plastic. This is accomplished by circulating heated “dry” air through the resin inside a drying hopper at a manufacturer specified temperature. This causes the plastic resin to release it’s moisture and most commonly this moist air is run through the material dryer through a bed of molecular sieve (desiccant) which retains the moisture inside the dryer bed and sends dry heated air back out to the hopper. This air needs to be maintained at a dew point level somewhere between -10 and -40 degrees F. Any higher than this and you run the risk of having wet material. The material can be checked for moisture using a couple of glass slides and a hot plate, looking for small pockets of gas in the pellet. This is done by heating the slides on the hot plate to a high enough temperature to soften or melt the plastic you are checking, laying a few pellets on one of the heated slides and sandwiching the pellets between the plates. You can then inspect the flattened pellets between the plates for small gas bubbles which would indicate moisture. The other method involves a lab device called a moisture analyzer, which is considerably more expensive but much more accurate. Once the plastic resin is dry, which usually is anywhere from 2 -4 hours on average, it is ready for processing and we are going to feed the resin into the machine hopper to begin the molding process.
The Machine Hopper These can be very small (shot hoppers) to very large in size but they all serve the same purpose and that is to hold plastic pellets or resin, which will be used in the injection molding process. The smaller shot hoppers, are typically used when you are drying resin either in a mezzanine or machine side drying hopper. These small hoppers only bring a small amount of resin each cycle to the screw feed area, thus controlling the likely hood of the pellets regaining moisture while waiting to be fed to the screw as can be the case with small shot sizes and larger machine hoppers that are not being dried on very humid days. The other very common method, but will require some reinforcement of the hopper mount, is to mount a sufficiently sized drying hopper directly over the feed throat of the machine. This method is commonly used but presents it’s own challenges as they are much more difficult to move out of the way when you have to inspect the feed throat. Some machine makers have added an air assisted hopper slide option to aide in the process of moving the hopper out of the way as needed. All of these options, should be mounted on a magnetic drawer base. The purpose of this unit is to try and eliminate any metal objects that may have gotten into the material, virgin or regrind, and yes this does happen from time to time. Sometimes it’s a malicious act but most of the time it’s carelessness or disrepair that causes this to occur. Metal and screws don’t mix and it can make a mess out of your molding process depending on where the metal ends up. It can get lodged in the screw, nozzle or nozzle tip, in your gates, or simply scratch your beautiful mold surfaces should it make it’s way through the entire system. Each case has it’s related costs to repair and these magnetic bases can prevent much of it from ever happening.
Written by: WM8C, July 28th, 2006. Not for use without written permission
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