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  1. #1

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    avoiding webbing (wrinkles) in vacuum forming

    (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.)

  2. #2

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    RE: avoiding webbing (wrinkles) in vacuum forming

    Another one...

    Tip #5: use a tapered riser, which flares out toward the base. (E.g., if your part is circular, a pedestal in the shape of a truncated cone.)

  3. #3

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    RE: avoiding webbing (wrinkles) in vacuum forming

    Interesting and well written drcrash. I have tried most of the tips you have listed, most of the time with success. (though maybe not on the first try!) Probably the best thing I have done to help prevent webbing is to build different size vacuum boxes so that I can choose the one that best fits the part being molded. Lots of excess plastic flopping around is the recipe for webbing. The added benefit is less wasted plastic as well. I also use the tapered riser on a regular basis. Often a riser won't get rid of the webbing completely but it will force the webs to form on those areas of the part that will be trimmed away. I have not tried the selective heating idea but it does sound like it has possibilities. One other idea not discussed here which I have tried is pulling the plastic into a female mold rather than over a male plug. Some time ago a friend of mine asked my to pull some parts for his project and instead of sending me the plug he sent me some plaster molds. I asked him if he wanted me to use the molds to make a plug but he said I should be able to vac-form right down into the female mold. I had never tried it, in fact it had never even ocurred to me to try it. However, this particular gentleman's Father had once owned a company that made vac-formed plastic models so I figured he must know something about it! Long story short, it worked perfectly and there is not really any chance of webs forming when you are sucking the heated plasitc down into a cavity. Two other benefits can be had with this method, one is the fact that the part does not "grow" by the thickness of the plastic, and the second is that you can mold much finer detail into the surface of the part such as rivets and panel lines. Obviously not every part can be made in a mold when vac-forming but many can. I plan on expiramenting more with this in the future. Good subject, I look forward to more input from other folks.

    Chad Veich

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    RE: avoiding webbing (wrinkles) in vacuum forming

    One thing I have tried is to use a sheet of thin latex rubber placed in the frame and drawn to the shape (not heated). The rubber will generally web where the heated plastic would have, so you can try different risers, etc. to predict what they will do.

  5. #5

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    RE: avoiding webbing (wrinkles) in vacuum forming

    ORIGINAL: Chad Veich

    Probably the best thing I have done to help prevent webbing is to build different size vacuum boxes so that I can choose the one that best fits the part being molded. Lots of excess plastic flopping around is the recipe for webbing. The added benefit is less wasted plastic as well.
    Another way to get the same effect is to have different-sized frames and gaskets for one platen. The platen I usually use is just a board with a hole in it. (For making one thing at a time, that works as well as anything else.) To use different-sized frames, I put down a sheet of flexible plastic with the right gasket stuck to it, and tape it down around the edges.

    By the way, you don't really need pairs of frames; just one rigid frame will do. Then you can clip strips of something to the top to hold the plastic down along each edge---that's how lots of industrial ther****rmers do it. The strips don't even have to fit---only one does. If you want to uses a bunch of different sizes of plastic, that can save time and money on frames. At most you need one good rigid frame per size.

    For thin plastic, aluminum windowscreen edging is good enough, if you use aluminum corners. You can mix and match the side parts, so you only need one pair in each length to make the frame you need. (Unless you want to make squares... then you need four of each size. So I don't do squares. :-) For the upper strips, you can use anything that's at least long enough---say, the two next-longer sizes you've got lying around. You just leave the extra hanging off the corners, like this:

    Code:
                                                    +----+
                                                    |    |
                                                    |    |
     +----------------------------------------------+    |
     |                                              |    |
     +------+----+----------------------------------+    |
            |    |                                  |    |
            |    |                                  |    |
            |    |                                  |    |
            |    |                                  |    |
            |    |                                  |    |
            |    +----------------------------------+----+------+
            |    |                                              |
            |    +----------------------------------------------+
            |    |
            |    |
            +----+
    ...and clip them on with binder clips. If you're using a gasket sealing system and use binder clips, they create a potential leak, because they go under the bottom frame as well as over the top strips. I've found that they work fine anyway for a vacuum-cleaner based system, if you're careful to make sure that they're flat against the bottom frame, and the rolled edge is inward of where you'll hit the gasket. The leakage is small enough that a high-volume low-vacuum system just doesn't care. (Likewise for the very small seams at the corners when you plug the bottom frame together using the aluminum corners.)

    Right now I'm making a heavier-duty set of adjustable frames for a no-gasket high-vacuum system, so that I can do all sizes and shapes in thick plastic. That's easy enough for a system where the plastic seals directly to the edge of the platen, because the bottom frame doesn't have to be airtight. Unfortunately it means that the hard problem is making all the different raised platen edges you want---they do have to be airtight. I'm thinking of trying to make a set of telescoping platens with silicone edging. (I'm thinking silicone edges should work as well as metal, and sometimes better, and should be able to take the heat.) Or I might just make a telescoping mold and cast a bunch of platen-tops out of solid silicone caulk... not sure yet.

    One other idea not discussed here which I have tried is pulling the plastic into a female mold rather than over a male plug. [...] [T]here is not really any chance of webs forming when you are sucking the heated plasitc down into a cavity.
    Yes. Female molds can be great. They can be harder to make, though, especially if they're non-small.

    For bubble canopies, if the exact shape isn't critical, an easier way to do it is by gravity forming and/or "free blowing." Instead of making an actual female mold, you just make a teardrop-shaped cutout, put it on top of a box, and suck air out of the box, pulling the plastic down into it to make an actual bubble. (When they started calling them "bubble" canopies, they weren't kidding.) People making full-size vehicles do this, so I'm wondering why more modelers don't.

    For full-size stuff, at least, you can actually get a fair bit of control of the shape by varying how much gravity-sag you use and how much vacuum, and by tilting the box. If you just use gravity, you'll get a catenary-like curve that never reaches 180 degrees. If you use vacuum, you'll get something more bubbly, that's rounder and may flare outward and come back in a bit. You can also use gravity to change the front-to-back profile; if you put the nose-end down, the plastic will sag toward that end and enlarge the front, making a more exaggerated teardrop shape.

    Industrial ther****rming people do this sort of thing for actual production, without ever making a mold. They work out how hot to get the plastic, how far to let it sag under gravity, how long to apply how much vacuum, etc.... and then do it many times with surprisingly consistent results, all without ever making an actual mold.

    I'm not sure how well that works at model scales, because the force of gravity isn't much on a little thin piece of plastic. I'm thinking it may work okay with thick PETG for fairly large model canopies, and it's on my list of things to try. You may also need fairly high-quality plastic. (Most full-size bubbles are done in cast plate plastic, not the usual extruded stuff. It's got a more consistent thickness, and doesn't have the residual stresses from the extrusion process.)

    Once you get one canopy you like, you can use it as a mold, and vacuum form more the normal way. All with no sanding.

    (I just hate sanding things to shape. I'm bad at it, and if I work that hard on something, I don't want to risk breaking it by flying it or something crazy like that. :-) I'm also not the best pilot. Bad combination. That's one reason I took up vacuum forming---so I could make things easily enough that I wouldn't be too afraid of breaking them. Lately I've been hung up on designing vacuum formers, but some day I'll get back to smashing planes into things. :-) )

    BTW, the "cutout" doesn't have to be a two-dimensional hole in a flat plate. It can be the three-dimensional shape of where the canopy is supposed to meet the fuselage. (Or if you want to get fancy, you can intentionally make it somewhat different and oversized, to control the shape of the bubble, and then cut the resulting bubble it down to fit the fuselage. If you're willing to work that hard, you could probably make some pretty-close-to-scale canopies.)

    And if what you're making is going to be used as a mold for the actual canopies, you can use any material that's easy to form the way you want. (Such as really thick plastic, so that you can use gravity better, and then form thin plastic over it.)

    One thing I really want to try is making custom fuselages that way. I'm pretty sure some full-sized ultralight stuff has been made that way, either free-blowing the body halves and reinforcing them with foam and carbon spars, or free-blowing a shape and using it as a mold for composite layup. One problem there is that if you don't have very consistent technique and materials, it's not going to be symmetrical enough. One thing I want to try is making one half, casting over it in something thin and rubbery, and then using that as a mold, turning it inside out to make the other half. That's tricky because it has to be very flexible but not distort too much when you cast in it. I think that's doable if you're clever, and you could take advantage of the flexibility to "fix up" the shape a little.

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    RE: avoiding webbing (wrinkles) in vacuum forming


    ORIGINAL: allanflowers

    One thing I have tried is to use a sheet of thin latex rubber placed in the frame and drawn to the shape (not heated). The rubber will generally web where the heated plastic would have, so you can try different risers, etc. to predict what they will do.
    I've thought of doing something like that, but haven't come across a nice rubber sheet... where did you get yours?

    It's good to know that it works for you; I should make an effort to find some thin rubber and try it.

    It seems like it would be very especially for free-blowing, to get a rough idea of what kind of shapes you can by sucking something rubbery into a cutout; the trial and error part of free blowing can waste a lot of plastic.

    (By the way, if anybody knows where to find thin silicone rubber sheet or strips reasonably cheaply, that would be useful, too. I'm thinking of putting a rubber edge on a telescoping platen by simply stretching a sheet of rubber over it, or just wrapping a hefty strip of silicone around the edge. Silicone should be able to take the heat of plastic forming to it and staying hot for 20 seconds at a time.)

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    RE: avoiding webbing (wrinkles) in vacuum forming

    It was years ago and at my work, before I retired. I think it was obtained from McMaster-Carr.
    http://www.mcmaster.com/

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    RE: avoiding webbing (wrinkles) in vacuum forming

    I tried it, sort of, with a piece of rubber glove latex cut out of (you guessed it) a rubber glove.

    It didn't work very well, but I can see why, and I find it moderately encouraging.

    I was trying to form a canopy mold about two by four inches, the biggest I could do with the biggest piece I could cut out of a glove.

    It's not stretchy enough to gravity form a decent canopy at that size. There's not enough weight of plaster to stretch the rubber enough. It should work better for larger canopies or even thinner rubber.

    I built a dam around the (teardrop-shaped) cutout, so I could pile more plaster on. That helped, making a less-shallow canopy shape, but my dam leaked and I couldn't keep the shape. (This was all done on the fly before the plaster set.)

    I also got wrinkles around the edges. Now I realize that the rubber must be taut to start with, especially if it's not tacked down near the cutout. As the rubber stretches, more will slide into the hole from further out. That will be too big, and wrinkle up, like webbing when actually vacuum forming. This problem was exacerbated by the fact that the piece of glove rubber I used wasn't really flat---it was saddle-shaped, so it was tight in the middle with excess rubber around the edges---just the opposite of what you'd want. (Maybe rubber from a big balloon would be the right thing, starting out more bowl-shaped.)

    Using a dam and piling more plaster on amounts to pressure-forming the rubber. That means that you'll get a more bulbous shape (as you would with vacuum under the rubber), but you have less control of the shape by tilting the cutout.

    My current guesses are that:

    (1) this will work with very thin, stretchy rubber
    (2) the smaller the canopy, the thinner the rubber must be, and
    (3) For a given size canopy, the thickness of the rubber will determine whether the gravitational effects dominate or the pressure effects dominate.
    (4) If the rubber is very thin, you won't need much depth of plaster, so it'll be mostly gravity forming, resulting in catenary-type curves that don't reach 180 degrees, but you can tilt the cutout to change the fore-and-aft size distribution somewhat.
    (5) If the rubber is thick, you'll need a deeper column of plaster to get the stretch you need, and the pressure will be more uniform. On one hand, that will give more bulbous shapes, but on the other it will give you less fore-and-aft proportion control using gravity.

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    RE: avoiding webbing (wrinkles) in vacuum forming

    ok. sag is not always a bad thing you can actually use the sheet sag to your advantage, for example if you're making a nose cone or male molds invert your mold upside down
    and you could actually eliminate alot of problems. also try to control the flow of your vacuum, too fast of a suction will also produce webs. preheating your tool will also help.
    and finally if you can get a hold of silicone spray for your mold will do it alot of good.

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    RE: avoiding webbing (wrinkles) in vacuum forming


    ORIGINAL: marty-vf

    ok. sag is not always a bad thing you can actually use the sheet sag to your advantage, for example if you're making a nose cone or male molds invert your mold upside down
    and you could actually eliminate alot of problems.
    Do you mean suck the plastic upward onto an inverted mold, or use a female (cavity) mold?

    also try to control the flow of your vacuum, too fast of a suction will also produce webs.
    Any hints on how fast is too fast? How long should it take to suck the plastic down?

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    RE: avoiding webbing (wrinkles) in vacuum forming

    I came across a simple trick for forming rocket nosecones that should also apply to many difficult cowlings. (This is a simple version of what's called "plug assist" in industrial vacuum forming.)

    After stretching the plastic over the mold, with the vacuum OFF, use something with a round opening about the right size---such as a roll of masking tape, for a small cowling---to push the plastic straight down to the platen in a circle with the mold dead center. Then hit it with vacuum.

    That will create a steep-sided cone of plastic around the mold, so that it fits better, and is less likely to web when sucked in. It avoids the hassle of making a close-fitting frame, or figuring out how to hold a vignette in place while heating the plastic. A roll of masking tape may be all the extra equipment you need. (But you might need three or four hands---two to hold the frame down and one or two to lower the plug-assist thing as straight-down as you can.)

    Thanks to Doug Walsh for this one. (He's the author of Do It Yourself Vacuum Forming for the Hobbyist, but this one's not in the book.)

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    RE: avoiding webbing (wrinkles) in vacuum forming

    Good tip DR, thanks for sharing!

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    RE: avoiding webbing (wrinkles) in vacuum forming

    How do you calculate the amount of plastic (size of sheet) for the buck being used? There must be a basic rule based on the height and width of the mold?


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