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[drcrash]

I strongly suspect that a somewhat taller riser will help a bit, even if you can't afford to have one that's actually tall.

At the problematic points, you want the riser to be fairly steep-sided.

Think about how the plastic thins as its being pulled toward the mold---where it touches the mold, it cools a bit and mostly stops stretching. What's left of the plastic keeps stretching. That's why you get progressive thinning toward the bottom of the part, and also down the riser, if the riser is steep.

Carried too far, that's bad. You can get really thin plastic, or even a blowout where the last bit of plastic stretches too much.

Up to a point, though, it can be good, because when you stretch the plastic in one direction, it may contract a little bit in the other direction, at right angles to the stretching. (E.g., when you stretch a rubber band, which is really a narrow sheet, it gets even narrower.)

To the extent that stretching the rubbery plastic one way makes it contract the other, that can help you use up the extra plastic area, pulling plastic down and away from the part, rather than having it be too big horizontally and fold in on itself to make a web.

Watching the video of a pull may help you tell if this is on the right track. Does the web form while the plastic is being sucked into the part, but before it's sucked onto the riser, or after it meets the riser below, when it's being sucked into the angle formed between the part and the riser? If it's the latter, delaying when it meets the riser will likely help.

You probably want the riser pretty steep all the way around, and at least anywhere near the problematic corner(s). You want the plastic to be free to contract in any direction it can, anywhere near a problematic corner(s), so that it pulls some of the extra plastic area away from that corner and along the sides, instead of having to fold up there.
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Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com

[crashmann]

That's a pretty neat looking part, and definitely a good challenge to get a clean pull. Have you tried pushing down the plastic in the troublesome corners just before applying vacuum?

Sometimes when I have parts laid out too close on my platen, I get webbing unless I jam down the plastic between the parts before the vacuum kicks in.

Charlie