Flash & Witness Lines
While designing a MIM component, witness lines and areas of potential flash should be taken into consideration. Critical areas from both an aesthetic and functional standpoint should be assessed for possible effects of witness lines or for minimizing the potential for flash. It should be noted that MIM feedstock tends to flash more readily than most plastic materials, and as a result, MIM molds require very precise fits between each of the mold components such as slides, cores, and parting line. Remember, flash generated on a MIM part becomes a metal burr after sintering and is difficult to remove.
Witness lines are an unavoidable result of two mating mold components. Whether along a parting line, or where a core pin seals off against a slide or other mold feature, injection molded material under pressure will be imprinted with the witness mark of two pieces of steel meeting one another. Fig. 27 illustrates the typical witness line to be expected along a parting line. In this example, the parting line is just above the fillet and the part will have a witness mark all around the part at that point. The witness line can often be minimized or removed with a secondary tumbling operation. As discussed in the Corner Breaks & Fillets section of this design guide, if the bottom fillet is not needed and a sharp corner can be tolerated, the full part geometry can be kept in the upper half the mold. This would move the parting line to the bottom of the part and no witness line would be present. A tumbling operation could be performed that would give the part a slight corner break as an alternative to containing the part geometry in both mold halves in order to accommodate a radius along the edge of the part.
The potential for flash will always exist and in many cases the construction of the mold plays a big role in minimizing this potential. However, there are design actions that can be taken that will improve the robustness of the mold, thus decreasing the chances of flash on the part. One major way for avoiding flash is to have "flat-on-flat" contact for the mold seal-off features. Fig. 28 shows how an intersection of 2 holes can be redesigned to reduce the potential of flash using a D-shaped hole as an ideal seal-off surface for the 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.
Whenever possible, areas of potential flash, and/or witness lines are moved away from critical areas. In the circumstances where this is not possible, there may be alternatives to ensure any witness lines and/or flash does not interfere with the function of the part. Fig. 29 illustrates one of these alternatives. On a cylindrical component with an external undercut, the parting line would run lengthwise, down the center of the part. To avoid a situation where any witness on the O.D. could interfere with the function of the component, small flats are added along the parting line to ensure that any witness line and/or flash would occur below the functional diameter of the part.
