Material Drying and Material Hopper

Material Drying

Depending on the resin you are using and whether or not it is a hydroscopic resin (absorbs moisture from 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 its 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 manufacture specified temperature. This causes the plastic resin to release its 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 going to feed the resin 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, s