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Vacuum forming questions

Old 07-12-2007, 09:32 PM
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Campy
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Default Vacuum forming questions

I am going to try my hand at vacuum forming. I have built a 10" x 12" x 3" box and a 750 watt hot plate in a reflective foil lined box for a heater and I have a 10 gallon shop vac for the suction (bag and filter removed for this).

Everything I have read has indicated that PETG is what to use for the canopy.

The canopy questions are:

The canopy will be about 2 1/2" wide x 3" high x 3 1/2" long.

What Thickness PETG would you suggest ? I have .030, .040 and .060 thick PETG, or do I need something thicker.

I have the plug already carved from balsa. Will regular balsa be OK to use or do I need to harden it with something ? Trying to carve the plug from pine would be extremely difficult considering the raised framework and rivet heads.

Can I also use the PETG for the wheel pants ? If not, what would you suggest using, and again, what thickness ?

Each half of the wheel pant is 7" long x 3 1/2" wide and (with the 1/2" backing ) 1 1/4" thick. The wheel pants are made from pine.

Thanks
Old 07-13-2007, 08:45 AM
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Steve Collins
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Default RE: Vacuum forming questions

Balsa will not work well. I have vacuformed several canopies using balsa molds that were finished with about 20 - 25 coats of clear dope. The point of the clear dope was to build up a surface that could be sanded lightly to remove the wood grain.

I still had some grain showing in the clear canopy and the dope melted slightly under the heat of the plastic. You definitely need a hard surfaced mold that will stand up to the heat of the hot plastic.
Old 07-13-2007, 09:14 AM
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LesUyeda
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Default RE: Vacuum forming questions

.030 should be plenty heavy enough. For wood grain, if you pull a canopy, leave it in place. The canopy next to the wood will take the wood grain, but the exterior will be smooth. Pull your final canopy over the first.

No reason not to use the petg for wheel pants.

Les
Old 07-13-2007, 09:29 AM
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spiral_72
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Default RE: Vacuum forming questions

We always used MDF (at the local home improvement store) for building molds. It comes in 1/2" - 1" sheets. You glue and stack them to the height you want and cut the mold. The mold box can be made of two MDF sheets also with a plenum between the two (built using 1/2" thick strips and double sided tape). Put a pipe flange on the bottom plate for your vac source.

The MDF sands smooth if you work it down with 220grit, 320grit, 400grit then spray silicon spray on the mold liberally. Wait 2 minutes and sand the silicon coated wood HARD with 400 or 600 grit. It'll leave a surface that should be good for up to about 50 pulls. You can silicon and sand again if you develop a problem area on the mold.

0.30 - 0.40 PETG should work VERY well.

Drill 1/16" vacuum holes around the mold into the plenum and in cavities in the mold for a excellent pull.

I can draw something up if your interested.
Old 07-13-2007, 11:28 AM
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Default RE: Vacuum forming questions

OK. Thanks for the replies.

I will try the balsa canopy plug 1st and leave the 1st pull on the plug as suggested to eliminate the grain problem.

A question on pulling a canopy over the 1st one - will they stick together ? One of the articles I read mentioned something about dusting the plug with talcum powder to prevent the plug sticking. Is this a good idea ?

Thanks
Old 07-13-2007, 01:05 PM
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Default RE: Vacuum forming questions

talc sound like it might leave spots on the final thing, ive heard somewhere that balsa dust can cause spots in parts.
Old 07-13-2007, 03:32 PM
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Default RE: Vacuum forming questions

Campy,

just sand the 1st pull with 320 to smooth out any bumps etc and you will be fine.
Clean it off real well 1st
I pulled PETG itself, no problem and mine was 5" high and 20 " long

Mike
Old 07-13-2007, 06:29 PM
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Default RE: Vacuum forming questions

OK I will try the sanding/cleaning on the 1st pull before doing one over it.
Old 07-23-2007, 09:01 AM
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drcrash
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Default RE: Vacuum forming questions

ORIGINAL: Campy

I am going to try my hand at vacuum forming. I have built a 10" x 12" x 3" box and a 750 watt hot plate in a reflective foil lined box for a heater and I have a 10 gallon shop vac for the suction (bag and filter removed for this).
I'm guessing that your "hot plate" is a single round burner thing. You may need to position a reflector over it to block the direct infrared to the plastic directly above it, and bounce the IR around to the sides and corners. Otherwise you're likely to have a serious hot spot in the middle. (And the ends of your rectangular plastic may not get hot enough before the middle gets too hot.)

That's pretty much what I did for my two-burner-hot-plate-and-disposable-aluminum-pans oven:

http://www.rcgroups.com/forums/showthread.php?t=621858

Also, a shop vac isn't especially appropriate for vacuum forming. (But it will probably work fine for canopies.) It's a myth that shop vacs pull harder than household vacs. (The strongest-pulling vacuum cleaners are canister vacuums with two-stage blowers.) Any high-amp (or high-wattage) vacuum cleaner is probably fine for thin plastic, including a $5 canister vacuum from Goodwill.

(At any rate, the "gallons" aren't relevant. That's just the size of the bucket that the air gets sucked through, not the strength of the motor. Some wimpy shop vacs have big buckets, and some powerful ones have small buckets.)




Old 07-23-2007, 09:18 AM
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MinnFlyer
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Default RE: Vacuum forming questions

Hi Campy, I just read your post about vacuum forming. Please let me know how it worked out for you. I am especially interested in knowing if the shop vac worked.

The reason for my curiosity is that I have a really good vacuum pump that I salvaged from a piece of office equipment a year or so ago. I will never get around to using it, so I'm willing to give it to someone, but I want to make sure that it will go to someone who will get some good use out of it. (If it's just going to sit and collect dust, it can do that here )
Old 07-23-2007, 09:28 AM
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RCKen
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Default RE: Vacuum forming questions

Mike,
I thought you said that I could have that pump!!!

Ken

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