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How to avoid sink mark on A surface?

The root cause of sink marks is almost always either poor part design or poor gate design. Make your gate diameter (subgate) or thickness (edge gate) should be 80% of the nominal wall thickness to avoid the gate freezing off too soon during packing. Wall thickness should be uniform (no thick sections). The ribs and bosses should be short, not tall, and their base thickness less than 2/3 of the nominal wall thickness for low-shrinkage plastics like ABS and less than 1/2 of the nominal wall thickness for high-shrinkage plastics like nylon or PP.

Begin by recognizing that paint usually accentuates flaws in the injection molded part, any sinks, flow lines, scratches, dust, oil, etc. will show up so take special care-concentrate on your part design. Minimize ribs, bosses, focus on basic part design rules and good process will take care of the rest. Minimize the use of mold release, keep it clean and you should have good success.

Try to simulate first and compare with the adjustment into the machine, also take note of the part weight (take pictures to verify, and compare), adjust with speed first and second balance with pressure, is possible that your part require 3 profiles at the second stage in the injection molding process.

In the other hand maybe in the injection mold, at the parting line you can create a tab area, an small rib or a flow leader just to take out or minimize any weld or melt line on the surface part and force the flow front to fill the problem area more uniform, also if the gate is far from the problem is better.

Also run some trails with the next tonnage machine maybe you are close to reach the max share rate of the material, so try some samples with the next flow index material, take note and compare the final weight.

If you haven't built the injection mold yet, be sure to have plenty of cooling lines in both halves of the injection mold but especially the A half. Make sure to core out the underside of the part on the b half so that the wall thicknesses are less than the main wall thickness on the A half. Material selection can be important too. If you've already built the injection mold, the processing may be your best bet. Packing the injection mold out with higher back pressure and hold time can make a difference. Sometimes running the melt temp lower and a slower injection speed can help. Sometimes having the A side mold temp cooler that b side mold temp can help, or vice versa. Try both. Additives may help but sometimes they affect the surface finish or may affect dimensional stability.

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