Part Details Review

Even though your primary responsibility is part design, you have to be concerned with the tooling issues also, as they can influence the part quality and the molded-part tolerance. The mold that produces your part obviously has to be designed and constructed to significantly tighter tolerance limits than the specified part tolerances. Tooling tolerances are, therefore, generally kept at 25% to 50% of the part tolerance limits.

Inadequate design of the mold cooling system can also negatively influence molded-part tolerances. If one side of the mold is at a higher temperature than the other, the temperatures of the part surfaces will have a similar relationship at ejection. In practice, the core side of the mold is difficult to cool. Therefore, it generally remains hotter than the cavity side. This differential cooling of the part induces uneven cooling stresses on the part surfaces that result in warpage and ultimately in loss of dimensional control. The following figure shows examples of inadequate and proper cooling.

During injection molding, areas close to the gate are packed more than those at the end of flow. This is mainly attributed to greater packing and holding pressures around the gate as compared to those at the end of flow due to melt compressibility. Another factor responsible for this differential packing is the melt temperature differential between the two sites. Densely packed areas show less shrinkage than those that are not as dense. In long parts, which are most prone to cavity pressure variation, multiple gates can be used to help minimize this packing differential.

It is difficult to maintain control of dimensions that span across mold inserts or slides. If the part features dictate the necessity of these, avoid tight tolerances in the affected areas. The same applies to the dimensions affected by the alignment of the mold halves. Sometimes, relocating the mold parting line can bring features on the same side of mold for better dimensional control.

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