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