Sintering Supports
During the debinding and high temperature sintering processes, molded parts (or green parts) shrink about 20%. While the parts are shrinking and before the parts can fully sinter, the forces of gravity and friction (from shrinking) may distort the parts if they are not adequately supported. Ideally, MIM components should be designed with a large flat surface or with several component features that have a common plane. This design approach allows the use of standard or flat debinding and sintering plates or trays, and eliminates the need for custom or part specific debinding and sintering supports. These custom or part-specific supports can be expensive to produce and represent added tooling costs for the customer. Fig. 7 illustrates a MIM component that is fully supported and placed onto a standard plate without the need for special supports.
However, if a single flat surface or plane cannot be provided, part specific debinding and sintering supports will be needed. There are various types of specialized supports that can be used. The simplest type of debinding and sintering support is a ceramic strip. Fig. 8 illustrates a typical use for a ceramic strip, which is often used to support cantilevered features that could "sag" in the high temperature sintering process. The strips come in different heights and widths to meet the finished part's dimensional requirements. If the design permits, ceramic strips can be avoided by designing "molded-in" supports. This would eliminate the need for the additional tooling costs, but would add a non-functional feature to the component. Fig. 9 shows how a "molded-in" feature could eliminate the need for supports.
An increase in complexity and cost from ceramic strips are ceramic plates with machined features. Attempts are made to minimize the cost of these machined plates by limiting the plate features to holes or grooves. These types of supports are more expensive than simple ceramic strips, but can fully support features that are more complex. Fig. 10 illustrates an example geometry that would require this type of support.
It is also possible to machine custom ceramic plates for supporting highly complex part geometries. Fig. 11 shows a MIM part that is placed on machined posts. If the part were simply placed on the thin walled legs, the legs would likely "drag" open when the part shrinks 20% during the sintering process. Placing the part upside down is not an option due to the small feature on the top. The intent of the posts on the custom ceramic plate is to suspend the part so the bottoms of the legs are not making contact with the base of the plate. In this example, the effects of gravity can actually help keep the legs straight. This type of support plate represents among the most expensive type of supports used by the MIM process.
