Surface finishes mold components

Polymerization directly into the finished product, in the form of films or blends, on the surface of molded components or in their morphology. Here there are limitations on practical flexibility and purity, constraints on the shape or design of the finished products and their reproducibility and the chemical process. This approach is not possible for converters (for lack of chemical infra.struc-ture) and is unsuitable even for basic research because of poor reproducibility. 

Injection-compression molding first injects the material into a partially opened mold, and then squeezes the material by closing the mold. Injection-compression molding is used for polymer products that require a high quality surface finish, such as compact discs and other optically demanding components because it practically eliminates tangential molecular orientation. 

As with the chemical components, it is necessary to maintain constant surface temperatures in the mold for a reproducible surface finish and constant chemical reactivity. This temperature varies according to the chemical system being used and has been determined empirically. 

Seals are those elements that create or maintain process boundaries between system components and/or subassemblies in order to ensure system integrity in validated process and utility systems. Seals must be biocompatible (able to be in contact with bacteria or mammalian cells without interfering with their metabolism or ability to live and multiply), must be corrosion and permeation resistant, their surface finishes must be free of molding imperfections and foreign matter on surfaces within the sealing area, and shall not generate particulate that may entrain the product. 

The surface finish of the mold should be of a very high standard so as to provide the basis for the gloss requirements on the molded component some molds may be chrome plated. 

As fast injection speeds are used for LLDPE, burn marks or matt patches may appear on the molding, as a result of inadequate venting, so the position and size of the vents play an important part on the surface finish of the molded component. For example, the location of the vent could be at the end of the melt flow path, at the junction of melt flows or, at the base of deep (or blind) pockets. Vent sizes of 0.025 to 0.08mm (0.001 to O.OOSin) deep by 4 to 6mm (0.157 to 0.236in) wide) are used. 

The mold filling speeds used with PPVC need to be relatively slow as excessive shear can cause the material to degrade or its use will effect the surface finish of the molding (i.e. splash/mica marks or surface delamination). Profiled mold filling speeds are often used to produce components which require a good surface gloss. 

Component lead finish is another important aspect that must be considered to achieve total lead-free implementation. The matter of component and PWB finish is discussed in detail in Chap. 12. Both Ni/Pd and Ni/Pd/Au are among the major surface finish candidates for lead-free component leads. Important aspects to consider are the adhesion of a lead finish to the molding compound and process compatibility. The typical thicknesses for these finishes are listed in Table 13. Inadequate adhesion can lead to delamination, followed by moisture ingress and possible popcorn effect. 

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