Sink & Knitlines
Similar to a plastic injection molded part, a MIM part may contain sinks and knit-lines caused by improper part and mold design. Sink (a physical depression on the surface of a part) frequently occurs around thicker sections. The example shown in Fig. 24 illustrates how sink can occur when a supporting rib intersects a wall. If the rib is the same thickness as the wall, the intersection of the two, creates a localized thick wall and is susceptible to sink. Decreasing the thickness of the supporting rib eliminates or reduces the potential for sinks. Generally, the thickness of the rib should be about 75% of the thickness of the wall.
Knit-lines can occur when two flow paths of material meet in the cavity and when the flow path is relatively long. Fig. 25 shows a knit-line occurring in a cylindrical part with a core in the center and a single gate. The two flow paths have to go around the core before meeting on the opposite side. Due to the long flow path, the two flow fronts of material have cooled down, which creates a visually evident knit-line. Fig. 25 also shows how dual gating the part can substantially reduce and at times, eliminate visible knit-lines. You should keep in mind that visually negligible knit-lines on properly designed MIM parts are superficial and do not represent a structural defect or part performance issue. Generally, knit-lines of this type have a shallow witness that is a little as .0005" deep to .005" deep.
