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How to minimize sink marks of injection molded parts?

Minimizing of sink marks can be achieved by following proper part design. The sink marks will not be eliminated if you have any ribs or structural features behind the 'A' surface. But with proper part design they can be minimized to a point that they are not visible.

Go back to basics. If the part is designed correctly and the rib to top wall section ratios are adhered to. I would not go above 50% you will be good. Also make sure your gating is correct and that your flow lengths are not too long and fill pattern are good. (No idea what your part looks like). Cooling is also critical.

It will depend on the product, thickness, ribs location, ejection pins, etc. A good methods learnt from German is to try different processing condition varying the holding pressure time and checking the weight of the finished parts. It might be possible that the gate is cooling to fast and does not allow the holding pressure to do the job of compacting the parts. Also, you can modify the injection pressure, speed, mold temperature but really important is to do only one change at a time.

You will never cover sink marks up with the paint, they will appear worse than they do on the plastic part. There are a few good way to check for sink on a part before painting. these are:
  • Project a grid pattern of bright lines on the part, using an overhead projector, distortion in the grid pattern shows sinks easily.
  • Wipe the part with Alcohol and observe the drying pattern. Sink areas take longer to dry.

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