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Vacuum forming clear plastic.


Clunkmeister
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My Lanc is supposed to have bulged out rear observation windows, almost a bay window look.

I have never even played with or contemplating vacuum forming in my life. Anyone have any ideas?  I’m illiterate and clueless with this stuff.

A2E2C093-E5EF-4134-B34E-BF97AD5084DE.jpeg

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Yes, there is a specific plastic, I have some in different thicknesses for my project. It’s called PETG, they use it for RC car bodies. I ordered some to experiment with, .020, .030, and .060 to use with my old Mattel Vac-u-former. If you make a buck of the shape you need, I can try to pull a couple parts for you. 

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3 hours ago, Clunkmeister said:

Huh, well it’s a skill I need to develop. The only issue I see is the right angle on the top and bottom edges.. will it make that sharp a turn?

Yes, PETG will not tear, especially not as small a part as that. If it’s a slice off a cylinder, then it might be a good start to use some large styrene rod.

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If you use male molds (we keep in the same vein as krow, don’t we :) ? ) you will not have sharp corners, and your mold will in theory have to be smaller by the thickness of the plastic you are forming on top of it.

If you want better definition, even panel lines, and mold a piece the same size as your original, then you need to use a female mold, i.e. 1) make your master, then 2) make a mold in a rigid matter (resin, plaster) around this master. You will need to have a suction hole in this female mold. Best is to insert a piece of monofilament - that will not adhere to the mold matter - or wire, « planted » in you master part. If its diameter is half the thickness of the plastic you are forming, or less, there will be no dimple where the suction hole is. Make sure your mold is large enough to cover the grid of your vacforming machine, so that air can only go through the small hole you have done. 3) heat and form your plastic sheet on ths female mold.

If all vacs used the female mold method, then it would have a greater success than it sees nowadays : better formed parts, surface detail where it matters, i.e. on the surface of the molded part, easier to achieve conformity to the desired dimensions, and finally clear definition of where to cut the part from the backing sheet. Unfortunately, only a handful of British kit manufacturers ever used it... :(

A final note. PETG (one common commercial brand in Europe is Vivak, by Bayer) is the only transparent material that does not yellow with age. It has one drawback however, in that it will trap ambient moisture, a bit - to a much lesser degree - like cotton. When you form it, controlling the temperature is critical. A bit too cold, and the sheet will not form. A bit too hot, and steam bubbles from trapped moisture will form inside the sheet. On typical chinese dental vac-forming machines that you find relatively cheap on ebay, the window between too cold and bubbles is something like 2 or 3 ... seconds. To get better results, it is recommended to do what plastic injecters do, i.e. dehydrate the PETG sheet. Two hours in your kitchen oven at low temerature (50 to 60 ° C) should do the trick.

HTH

 

Hubert

 

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Interested stuff, Hubert. This is another skill I’d really like to learn. The female mold method to me, seems like something that’s be easy enough to master, given time. So do you just use one mono sized hole in the missile of the female mold, or do you spread them out around the part?

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I have tried both, with a single hole or many holes. It works about the same, and that the air could be sucked through a tiny (less than 0.5 mm) hole amazed me the first time. (And made me wonder what it can be when this hole happens in the hull of a spacecraft, or a submarine).

I’d say the number of holes is just commen sense judgement. If you have a big cavity, more than one hole will help suck air more quickly. And you want the process to be as fast as possible before the plastic cools down and cannot be formed.

Hubert

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