Witness lines are an unavoidable result of two mating mold components. However, while designing a MIM component, witness lines and areas of potential flash should be taken into consideration. When you work with an OptiMIM Engineer, we will work with you from your initial design so that you can rest assured you are getting a fully optimized component.
Designing Your Part for MIM
When designing your component for the metal injection molding process, 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 plastics, and as a result, MIM molds require very precise fits between each of the mold components such as slides, cores, and parting lines.
Witness Lines During Molding
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. 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 of the mold. This will move the parting line to the bottom of the part and no witness line would be present. A tumbling operation can be performed that will 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.
Removing & Avoiding Flash
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, decreasing the chances of flash on the part. Flash generated on a MIM part becomes a metal burr after sintering and is difficult to remove, so it is best to work with a design team that knows how to design to remove or reduce the possibility of flash.
One major way for avoiding flash is to have “flat-on-flat” contact for the mold seal-off features. Whenever possible, areas of potential flash and/or witness lines are moved away from critical areas. Where this is not possible, there may be alternatives to ensure any witness lines and/or flash do not interfere with the function of the part
To avoid a situation where any witness could interfere with the function of the component, small flats can be added along the parting line to ensure that any witness line and/or flash will occur below the functional diameter of the part
Leading Metal Injection Molding Manufacturer
On every project, we aim to deliver more consistent parts, more efficiently, at lower costs. Our goal is to do away with the expenses associated with secondary processes such as machining, achieving net-shape the first time. So, we build molds that are more efficient for high-volume production and put in as much complexity as needed up front to avoid costly machining and secondary operations. Contact our team today to get the conversation started about your next project.