Sink Marks defects

How to prevent Sink Marks defects in advance

Sink marks defects are localized depressions in the surface of injection molded parts caused by a non uniform shrinking of the plastic during the cooling process. In cosmetically critical parts, they can be a serious problem. Sink marks are dependent on part geometry and material shrink rates. Sink marks often come as a surprise when the mold is complete.

If the sink marks defects are objectionable to the customer, the plastic mold manufacturer is often tasked with “eliminating them”. Since the mold is complete, the mold manufacturer does not have the luxury of changing the part geometry. At that point, changing the material to one with a lower shrink factor is usually not an option due to sizing issues with other parts.  At this stage of the game, the only option left to the mold company is to adjust the processing conditions to try to eliminate the sink. This usually means running the molding machine at the extremes of the process window, which can lengthen cycle times and increase the amount of molded in stresses.

Mostly, the best solution to solve the sink marks issue is reducing the wall thickness, since mold is made by the mold company, so will be high cost to solve this issue, to avoid this sink marks issue in advance will be the idea to prevent this issue, so mold flow analysis and DFM report (design for manufacturability) is the most important role to avoid this sink marks defects.

The part below was suffering sink marks defects beyond what was allowed by the part designer’s requirements. If the tool was built to this geometry, they would have had a serious problem. Looking at the graduated scale we can see a maximum of .004″ of sink.  It doesn’t sound like much, but .005″ is the limit on the print. To fix the issue, the two thick ribs on the underside of the part were reduced in thickness.