Tolerances

Establishing the correct tolerances with respect to the product function is of economic importance. The designer should be aware that dimensions with tight tolerances have a big influence on the costs of both product and mold. Even slightly over specifying tolerances may adversely influence tool costs, injection molding conditions, and cycle time. It is recommended to indicate only critical dimensions with tolerances on a drawing. Depending on the application, a division into three tolerance classes can be made: normal; price index 100 accurate; technical injection molding; price index 170 precise; precision injection molding; price index 300 The most important characteristics of the tolerance classes are shown in the figure below. Characteristics of tolerance classes.
Mold design, mold cavity dimensions, product shape, injection-molding conditions and material properties determine the tolerances that can be obtained.  The figure below provides a summary of the factors that play a major role in establishing dimensional accuracy. Factors affecting parts tolerance.