One key aspect of plastic injection molding revolves around pre-treating resin. If we needed a theme, we could look to the iconic lyrics from Irving Berlin’s “‘The Near Future’ written in 1919. Written with prohibition in mind, it went viral (1920’s style). A less drinking referenced version “How dry I am, how dry I am, nobody knows how dry I am…” made it into many Warner Brothers Cartoons. We can personally attest to remembering Daffy Duck in particular.
It should have become the injection molders’ theme song as well, but even today, drying is a process that both escapes and confounds many injection molders.
The vast majority of polymers (resins) are hygroscopic or due to their polarity are able to absorb moisture. The two major exceptions to this are polyethylene and polypropylene and are referred to as hydrophobic. They may suffer from surface moisture, but in very small amount relative to the resin’s mass. Those moisture absorbing hygroscopic category resins, can be divided into two groups. The first group will exhibit cosmetic concerns if the molder does not manage the absorbed moisture. The second group will actually experience performance degradation i.e. structural damage due to hydrolysis and ultimate depolymerization. If your resin is a blend, such as PC/ABS, you must consider the more hygroscopic of the two.
The actual process of resin drying is a posting (actually more like a book) for another time. Suffice it to say, talk to your molder. Whether they have a controlled environment or not, ask about their drying protocols, (their equipment, their processes, and their methodologies). They should have a definitive plan for how they manage their drying. Another day we will go into more about what to ask and look for to achieve effective resin drying.
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