Summary Matching Process and Plastic

As reviewed throughout this book it can be said that there is a basic thought of designing. 

It is that the plastic and the process selected profoundly affect the quality and appearance of the product. For this reason, it is usually unwise to create a design first, and then decide on material and process at least consider all three factors from the start. This seems obvious and logical, but it is frequently ignored in practice, especially when converting a metal design to a plastic material. 

As reviewed there is much to consider. Examples include cooling as the product sets up results in different shrinkage rates for thicker versus thinner sections in the different processes. This results in either external waviness or sink marks, or warpage and internal voids, as the product contracts. Flat surfaces are difficult to maintain but not impossible to attain using certain processes. High speed of flow to fill the cavity of the mold is impeded going around square corners, so provision for radii and fillets are important. 

Attempting to flow past thin sections to fill wider sections is difficult or may be impossible, because the flow thickens enroute acts like plaque forming along walls of a human artery. Even if both of these ills are avoided, the final result may still contain areas of high shear stresses invisible to the eye but waiting in ambush to cause failure later under extreme conditions previously thought to be well within the material s specifications. 

Preliminary consideration of candidate materials, processes and tooling factors, configuration, thicknesses in section, ribs, bosses, holes, surface characteristics, color, graphics, decoration, and assembly methods will begin to impose some discipline on the product design as it evolves. In the middle and latter phases of the design cycle, two or three concepts should make their validity apparent to all involved. With luck (logic), one will 

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