Plastic parts design tolerances

In many cases, a product fails when the material begins to yield plastically . In a few cases, one may tolerate a small dimensional change and permit a static load that exceeds the yield strength. Actual fracture at the ultimate strength of the material would then constitute feilure. The criterion for failure may be based on normal or shear stress in either case. Modes of failure include excessive elastic deflection or buckling. The actual failure mechanism may be quite complicated each failure theory is only an attempt to explain the failure mechanism for a given class of materials. In each case a safety factor is employed. However, with proper part design, these failures are eliminated or can be permitted since part performance is met. 

Physical dimensions of many processed parts must be held to fairly close tolerances to ensure proper assembly of parts into a complete structure, as, for example, molded fender panels bolted to steel chassis cars, plastic screw caps for glass jars, etc. In general, the final dimensions of the processed part will differ from the dimensions of the mold cavity or the pultrusion die. Such differences are somewhat predictable, but are usually unique to the specific material and to the specific process. The dimensions of a mold cavity for a phenolic part requiring close tolerances will often be different from dimensions of a cavity for an identical polyester part. Both the part designer and the mold or die designer must have a full understanding of the factors affecting final dimensions of the finished product, and often need to make compromises in tolerances of both part and cavity dimensions (or even in plastic material selection) in order to achieve satisfactory results with the finished product. 

Corner binds. Corner bind is a condition where the comers of the parts, often the stiffest portions of a part, bind and prevent assembly of the parts. This can occur even if the sides of the parts warp inward, as illustrated in Fig. 8.28a, which is a common problem in plastic part assembly. The solution is to radius the corners of the part, a practice which should be followed to reduce the stress concentration factor in any case. However, the radii tolerances can still cause an interference. This problem can be avoided by designing the part so the inside radius on the outer part (i l) is smaller than the outside radius on the inner part (i 2). 

Semidovetail joint. The sernidovetail joint is probably the most common joint used for position and contour control in plastic parts. The semidovetail joint is used in place of a full dovetail joint because the latter is not required since location in the inside direction is controlled by the joint on the other side of the part. More importantly, the semidovetail joint requires two-thirds the wall thickness of a fiill dovetail joint. A semidovetail joint around the entire perimeter of the part provides location, tends to mask minor warpage and debris from joining devices (surplus adhesive, solvent, welding flash, etc.), and is reasonably tolerant of dimensional variations. In addition, if designed steel-safe, it can be readily adjusted if the molded parts turn out to have too... 

TMConceptfCSE (Computerized Shrink Evaluation, Plastics Computer, Inc., Montclair, NJ). Develops the actual mold dimensions needed to meet specific product tolerances, taking into account part design, gate location and geometry, mold filling, process conditions, and postmold stabilization. 

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