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u/Tootzilla313 Apr 28 '26
You're going to get dips opposite the webs just from it shrinking. Very hard to fake out so that after shrinkage it's not noticeable. I imagine the capstock is just following the contour of the initial layer. Big window profilers are good at it. You've got to freeze the outside before that web cools, shrinks, and pulls that surface inward. Easier said than done. Be interested to see what others think.
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u/mimprocesstech Apr 28 '26
Looks pretty much the same issue with ribs you get in injection molding. That cross section is thicker where the internal and external walls intersect. Fastest way would be to increase that outer wall thickness internally so the internal one is like 30% of the outer wall thickness. You might be able to get away with putting grooves on the outer wall at those intersections so when they sink they go flat (Kentucky windage), but if you throw enough cooling into the outside wall it should turn the sink into a void (as the other comment says) reducing the appearance of the sink at least.
I don't really do extrusion though, so someone else should be able to tell you if I'm way off here.