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Surface Quality - Processing Optimization and In-mold Solutions

SpecialChem / Jan 3, 2011

Surface is the first contact between a part and the customer. Its nonconformity is perceived as negative and a single faulty part brings discredit on the entire batch or the complex device including it. Surface quality mainly depends on processing parameters easing or disrupting flows, or damaging compounds; Design parameters favoring or blocking flows; plastics grades and additives chosen to optimize optical properties, rheology, warpage etc. In-line treatments can be used to create a high quality surface. For a fair compound processed on an adapted machine, surface quality depends on numerous parameters including resin moisture; pressure, temperature, time and speed profiles of the machine and tools including venting; gates, sprues, runners and others. For each issue it is necessary to identify the cause(s) and react on the main involved parameter(s) to avoid compound degradation, to ease flowing, to favour homogeneity and consistency of flows, temperatures and pressures. Colorants, pigments, fillers and other additives must preserve optical features, dimensional stability without shrinkage and warpage and must withstand high temperatures of processing. The right gloss/mat balance depends on the basic polymer, additives, and mold wall finish. Adding special mineral fillers and/or a secondary polymer or using proprietary additives can enhance matting. Warpage comes from non-uniform cooling, inconsistent shrinkage, and non-uniform stress due to excessive anisotropy, abnormal or inconsistent pressures and/or temperatures, material variations, inconsistent melt and pigmentation. Weld lines are induced by holes, inserts and other obstacles or result from multiple gate mold feeding. Incomplete or inadequate filling of the mold leads to short shots. Surface quality can be enhanced using processing aids, plasticizers, isotropic reinforcements, nucleators (for polyolefins) and selecting specific resin grades and additives. Structural foams; mat, polished or grained mold walls can enhance part surface quality or modify the aspect to be less sensitive to optical defects. In-mold decoration (IMD) and various similar techniques –FIM, IML, IMC etc- hide the surface of the core with another material creating a high quality surface.

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