(Continued from this thread on making canopies:
http://www.rcuniverse.com/forum/m_20...tm.htm#5086196 )
About sag (a.k.a. droop) and webbing (a.k.a. wrinkles):
Webbing happens when the plastic is stretched too much before being sucked inward onto the buck (mold).
When it's sucked inward, there's too much plastic area, and it doesn't contract enough, so it has to fold in on itself. (And often sticks to itself.)
Sometimes this happens because of too much sag. Some pro machines have hot air blowers that suspend the plastic while it's being heated, so that it can get hot without actually sagging and stretching. Unfortunately, in a simple vacuum former, sag serves two purposes: it tells you whether the plastic has gotten stretchy enough, and it actually stretches the plastic. This can be good, up to a point, if it's stretching toward the shape you want, but not overstretching. It is bad if it goes too far and results in webbing.
To avoid webbing, you need to somehow control how the plastic stretches, or how it's sucked inward toward the mold, so that it never stretches too much anywhere, and can contract enough to meet the mold without folding up.
One way to do this is often with a riser. A short riser is usually enough to allow the plastic to form to the whole part, with the not-sucked-in-far-enough radius below the part, where the riser sits on the platen. To avoid webbing, though, you may need a substantially taller riser.
This often works because it changes how the plastic is pre-stretched when you lower it over the buck. The buck basically acts like a tent pole, and the plastic stretches into a tent shape, more or less pyramidal or conical, with a saddle-type curvature. (Convex wrapping around the buck, concave upward and outward.)
This is easiest to understand if you think about forming something tall and skinny, and radially symmetric like a rocket nosecone, poking up under a rubbery sheet. It's basically tent pole, which makes a steep round tent that gradually transitions to a flatter pyramid as you go toward the edges and corners.
Using a taller riser takes advantage of this shape---the slope is steeper near the top, and flattens out as you go downward and outward toward the plastic-holding frame. The top part of the tent is narrower than the bottom part, so if you just use that, you've got a piece of plastic that's closer to the shape you want, fitting more tightly around the buck before you suck it inward. (Further down, the circumference at a given level is bigger and more likely to result in webbing when sucked inward.)
So tip #1 is: try a taller riser. Since you're only using the inner part of the plastic, right around the buck, you'll get more stretching and thinning of that smaller area of plastic, so you may need to use somewhat thicker plastic to compensate.
Another way to get a similar effect is to use a smaller or better-fitting piece of plastic. When you stretch it over the buck, you'll get steeper sides all the way down, because the angle to the plastic-holding frame is just steeper. That way you don't have to suck as much plastic in, or suck it in as far, to meet the buck.
One neat way to get this effect is by "vignetting." Rather than using a smaller or different-shaped piece of plastic, you can selectively heat the part of the plastic you want to stretch, and not the parts that you don't. You do this by shadowing the plastic partially with something like aluminum window screening, or entirely with something like aluminum foil. (Similar techniques can be used to even out the heat distribution of an oven that heats unevenly. That's one of two basic techniques I used to get my portable electric barbecue grill to heat plastic very evenly.)
For example, for a rocket nosecone, you might heat a circle around the part you'll lower onto the buck, using an aluminum foil mask with a circular hole in it. Outside that circle, the plastic won't stretch, or not nearly as much, so when you drape the plastic over the buck, you'll get a steep-sided symmetrical tent around your steep-sided symmetrical buck.
(Now think of something like a bubble canopy. If you stretch a big piece of plastic over it, it will conform to the canopy on top, but you'll get a fairly flat surface outward from the lowest points of contact to the edges of the plastic. When you think about which parts of the plastic will be sucked down and inward, you can visualize some pretty big circumferences. But if you stretch a closer-fitting piece of plastic over it, the sides will be steeper and only need to be sucked in a little bit. The plastic may be rubbery enough to contract while being sucked in, rather than folding up.)
So tip #2 is: shape the plastic, or the stretchy area of the plastic, to avoid overstretching the plastic in places that are likely cause webbing. This may be all around the part, as in the examples above, or only in a few problematic places. Visualize the tent you make by draping the plastic, and how it will be sucked inward, to figure out where it's likely to overstretch and be unable to snap back enough.
If you're getting webbing on one side only, that's likely because the sides of your buck are too steep there, and/or the plastic in the area around there is stretching too much. You may be able to fix that by simply tilting the buck away from the problematic area. That reduces the gap between the buck and the plastic on that side.
So tip number #3 is: if your buck causes asymmetrical webbing, put it on a wedge-shaped riser, so that the angle isn't as steep on the problematic side.
If that doesn't work because you're getting webbing in more than one area, more or less opposite each other, you may need to find some other way of reducing the slack overstretched plastic, or carefully taking up the slack.
Sometimes it works to guide the plastic inward toward the buck, so that you don't get too much plastic area being sucked in onto a particular smaller area of the buck. Where you're getting webbing, try putting things on the platen around the buck to take up the slack, and guide the plastic away from those points. If you put two pieces of stuff on either side of the problematic spot, the plastic will touch them before being pulled into the buck in that area. After that, no plastic will be sucked inward to that spot from further around the buck, and the plastic will stretch parallel to the edge of the buck as it comes down into the valley between them. This may make it contract in the direction toward the buck, or at least not stretch much more, as it's sucked inward.
If you go too far with this, you may just create new webs. As the plastic is sucked down over the buck and the other objects, you may get ridges in the "tent" from the buck to those other objects, and the plastic may suck in on either side of the ridges to create a web with the ridge as its edge. Ooops. So don't use things that are too tall or too pointy, or you may create more problems than you solve.
So tip #4 is: guide the plastic onto the buck by placing objects on the platen around the problematic area.
(I confess I don't really have a good grasp of this one myself; the above discussion is just my current understanding. If other people experiment with this, I'd be very interested in what happens. Relatively soon I'll probably get more experience than I want with this, because I have a project that involves some very tricky shapes...)
All of this involves some tricky spatial reasoning, and everything happens very fast when you're actually vacuum forming. If you have a video camera or a webcam that can capture video, it may be worth videotaping your mistakes, and reviewing them in slow-mo to see where and how the plastic stretches and too much gets sucked in. If you're doing large stuff with expensive plastic, it may be worth scaling things down, using a smaller similar-shaped buck---it doesn't have to be smooth and pretty---and/or cheaper, thinner plastic. (You could even put a grid of marks on the plastic, so that you can see and interpret the distortions... but I've never had the need and the patience for that, yet.)