Water Droplets in the Feed Throat

While this may be an extreme case, if you are processing hygroscopic resins and not drying them properly, not only can you get water in the feed throat, but you also will have parts failing in service. What happens to your day when a customer calls in with part failures? Your next few days become chaotic. The extra time and effort required to find the cause—and 100% part inspections—will cost you money. By the way, you may not see splay on the parts, so visual inspection will not find the bad ones.

If you are still in any doubt about the ramifications of not paying appropriate attention to the details of drying, let’s consider a more basic question: Why do you need to dry moisture-sensitive resins—such as PC, PET, nylon, PBT, and certain elastomers—in the first place? The common answer is, “You dry these resins to get the moisture out.” While true, this is not an acceptable answer if you are practicing Scientific Molding. You need to state why these resins need to be dried.

Drying moisture-sensitive resins is important to maintain their physical properties. Processing these resins with small amounts of water causes the polymer chains to undergo a chemical reaction known as hydrolysis. Trace amounts of water react with the polymer chains in the barrel at melt temperature and literally chop the long chains into shorter chains. Water actually is consumed by the hydrolysis reaction so you will not see splay. Parts may look fine and even pass tests at room temperature, but the parts fail in use. Short chains do not provide the properties of strength, chemical resistance, etc. for long-term use or use at high or low temperatures. Often, part failure comes with liability issues that can really jack up the costs. Bottom line, the better answer to the question of “why dry” is, “to maintain part performance.” The resin needs to be dry, often below 200 parts/million (ppm) before processing.