Uniform Wall Thickness, Coring & Mass Reduction
Since injection molding is employed as the shape forming process step in MIM, part designs can avoid the limitations of traditional metalworking processes. For example, machining involves the removal of material from a solid shape to get to the desired final component design. As a result, design engineers are limited to design decisions that can be readily produced on an economical basis and those which do not violate the design limitations of machining. The benefits of removing excess material for reduced part mass is generally not considered as this design approach would add incremental machining costs. With MIM, as is the case with plastic injection molding, design engineers have the freedom of starting with a "clean slate", and building up their component geometry by placing material only where it is needed for function and strength. This serves several benefits for the MIM process and the customer. The very fine metal powders used in the MIM process are expensive, and any opportunity to limit the amount of material required in a component helps minimize the final MIM part cost. Additionally, maintaining a uniform wall thickness throughout a component reduces the likelihood of molding process flaws, thus improving the overall part quality, cosmetics, and generally improves the resulting dimensional tolerances that the MIM process can provide.
Fig. 4 illustrates several preferred geometries accomplished through coring to create uniform walls. You will also note instances where unnecessary material has been removed or cored out in areas with thick cross sections. Coring can be done either parallel or perpendicular to the parting line. Fig. 5 illustrates both types of coring. Coring perpendicular to the parting line (Section A-A) can be produced with cores, which are fixed features on either half of the mold. Coring parallel to the parting line (Section B-B) can be produced with slides, which are moving components in a mold. The slides are usually placed at the parting line and move parallel to it. Slides add complexity and costs to a mold, so if the design permits, coring perpendicular to the parting line is preferred approach.
Remember, when designing a MIM part, or when coring out an existing design, maintaining a consistent uniform wall thickness throughout the part is the primary objective. Again, in a MIM component, uniform walls are desired for higher precision, more repeatable dimensional capability, lower processing costs and improved aesthetics. If, however, varying wall thickness cannot be avoided, a gradual transition between differing wall thicknesses should be provided and every attempt should be made to avoid abrupt changes. Fig. 6 provides a recommended wall thickness transition ratio for those situations when uniform walls cannot be achieved.
