Design issues are very difficult to process around. Your defects indicate a potential venting problem which should be addressed first by reducing clamp tonnage and/or adding more vents.
Question: I am processing both clear and red transparent PMMA of the same grade. For the red part, the thickest area is 4mm compared the average wall thickness 2.5 mm. It’s easy to get shrinkage in the 4 mm position. Each component gets one hot runner gate. Thick area is far from gate compare the conjunction area with clear component. Here is the problem. When you increase the holding pressure or holding pressure time, it easily leads to shining line even break in the conjunction area. I also try to set the holding pressure very high at the first 0.3 second till reduced pressure to 13 seconds. But it doesn’t work really. The mold gets some problem, but I want to compensate it with the processing.
My Response: You are correct, this is a design problem which is difficult to process around. The shiny line and breakage defects may indicate a potential venting problem which should be addressed by reducing tonnage and/or adding more vents. The best way to attempt processing this part is to fill the mold with a short shot during 1st stage fill, and then use one pressure for the entire second stage packing until the gate is sealed. To determine the best pressure, perform a study to determine the highest and lowest pressure which will provide an acceptable part.
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-Andy
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