Abstract
The first part of the paper presents the specific issues from the injection molding associated with the water content of the hygroscopic plastics (water adsorption, equilibrium moisture level, chemical affinity, hydrolysis) and recommendations regarding the drying of plastics for injection, the drying methods and defects specific to products injected due to the moisture content. The experimental results on the injection of a polyamide (AKULON) and of a thermoplastic polyurethane (DESMOPAN) presented in the second part of the paper are focused on the surface appearance fault splay associated with the different values of moisture content for these two materials, verify the value for the admitted moisture content for two hygroscopic materials, polyamide and thermoplastic polyurethane, and ends with conclusions on the residual humidity allowed and opinions on choosing the drying technology and parameters.
Highlights
Moisture absorption is the capacity of a material to absorb moisture from its environment
The goal of this paper is to highlight and to clarify essential details regarding the drying of raw polymeric materials for injection molding, in general, and to verify the value for the admitted moisture content for two hygroscopic materials, polyamide and thermoplastic polyurethane by monitoring the silvers streaks on the injected parts molded from raw material with various moisture content
Exposure to environmental moisture can result in distinctly different responses of the polymeric material according to the affinity of the macromolecule for water molecules
Summary
Moisture absorption ( known as water absorption) is the capacity of a material to absorb moisture from its environment. In a typical environment to which polymers are exposed, water molecules may be present in the forms of vapor, liquid or both. A polymeric material contains a certain amount of water. Dehumidifying or drying plastics before processing is a vital part of injection molding process. In the case of polymeric raw material, the purpose is to minimize or eliminate complications that may be caused by too much moisture in a plastic material. Even a reduced amount of moisture in the polymeric bulk will turn into steam that mixes with the melt and give, on the injected parts, at least a distinctive fault: splay or "silver streaks"
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