Indispensable Tools for Вђњmoisture Controlвђќ on Plastic -Mark 3

Moisture Analyzing is an important process in plastic molding industry but mostly neglected. Engineering plastic, such as PA (Nylon), PC and PET, are hygroscopic. They adsorb moisture from the humid ambient air and give it back during the dry days. And every type of resin can hold a certain amount of moisture between its molecular chains.

According to the research, the presence of excessive moisture in resin has significant effect on its physical properties, i.e. weight, density, viscosity, melt flow index and conductivity. These properties are directly related to the resin’s working conditions and will finally affect the molded parts. i.e. mechanical strength and appearance.

During the injection process, excessive moisture in resins vaporizes and condenses on the surface of the mold. Splay, silver streaking, internal bubbles or craters are commonly found defects on the molded parts. In the worst case, the moisture may even cause attrition to the mold over a period of time.

To avoid these problems; pre-drying of resins becomes a very important step in the molding process. Various dryers i.e. hot air type or dehumidifying type are commonly used to dry the resins. However, this drying process cannot tell if the resins are adequately dried, we must use a high precision Moisture Analyzer to determine the moisture content after drying.

The efficiency of dryers determines the drying time needed. The drying time has to be controlled properly. If the drying time is too short, the excessive moisture remains; but if it is too long, there will also be negative effect. Like PBT and PA, over-drying will cause the migration of additives from the resins or even degradation finally. PBT molded part will become more brittle, and the surface color appears to be darken and looks dully. For PA, its stability against heat will be affected and its viscosity will increase that finally affects the shaping during molding.

Therefore, high precision moisture analyzer is critical to ensure the moisture content is within the specified range of a particular plastic and resin.

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