Holes & Slots
Holes and slots can be readily produced by the injection molding process used by MIM, and generally, can be accomplished at no additional cost to the piece price. However, adding these features does increase the cost and complexity of the mold. Also keep in mind that beyond representing obvious functional features, holes and slots can also be used to reduce part mass and provide uniform wall thicknesses.
It is important to be aware of the type and direction of a hole and how it could affect the cost and the robustness of the mold. Fig. 14 shows several types of holes, their direction relative to the parting-line, and their impact on the mold. Holes that are perpendicular to the parting-line represent the easiest mold design approach and the lowest cost to incorporate in the mold. Holes that are located parallel to the parting line are readily applied, but the tooling costs more than holes located perpendicular to the parting-line as they require mechanical slides or hydraulic cylinders to actuate them during part ejection. Holes that are set at an angle to the parting-line are also possible, but the mold construction and the mechanism to actuate them becomes very expensive and in many cases the mold features mandate more frequent maintenance downtime and related upkeep costs.
Cores and slots that intersect one another can also create complex part features. However, when employing intersecting features, the mold construction and robustness must be considered. Fig. 15 shows the advantages of using a D-shaped hole as an ideal seal-off surface for an intersecting hole. In this case, two flat surfaces are sealing against one another providing a tool that will be easy to maintain and less likely to generate unacceptable flash during the molding process. The alternative displayed in the figure shows the least attractive approach, which requires one of the cores to have a contoured or profiled face to match the core or hole that it will be sealing against during the injection portion of the molding process. In circumstances like these, the core orientation is critical and the feathered edges are likely to wear more rapidly affecting the shape and size of the molded feature. Mold flash is also a concern in these situations.
