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Old Jun 26, 2012, 08:47 PM
Balsa&Tissue
payne9999's Avatar
United States, OR, Beaverton
Joined Jan 2011
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Question
Making casts of vacuum formed parts?

I often damage the vac formed parts from kits and there is always some hanger rash or scrape from an ugly landing. So, I want to make castings of my vac formed parts so I can make new pieces if I screw up or if I just want to make parts from better raw material.

Can anyone give me some good advice about what materials will work best?

Plaster? Resins? which release agents are safe for the plastics? Any other advice....

Thanks,

Dave
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Old Jun 26, 2012, 09:27 PM
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United States, WA, Marysville
Joined May 2009
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Dave,
There is a really good and inexpensive book called Do it Yourself Vacuum Forming for the Hobbyist by Walsh. He has a chapter on that exact subject. He uses plaster of paris. http://www.build-stuff.com/001book_vacuum_forming.htm. It would be a well spent 13 dollars.

Did you get my PM on nacelle measurments you requested?

Best,
Robert
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Old Jun 26, 2012, 09:34 PM
Balsa&Tissue
payne9999's Avatar
United States, OR, Beaverton
Joined Jan 2011
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Quote:
Originally Posted by Robert R View Post
Dave,
There is a really good and inexpensive book called Do it Yourself Vacuum Forming for the Hobbyist by Walsh. He has a chapter on that exact subject. He uses plaster of paris. http://www.build-stuff.com/001book_vacuum_forming.htm. It would be a well spent 13 dollars.

Did you get my PM on nacelle measurments you requested?

Best,
Robert
Robert,

Yes I did and the Blue Wonder motors fit just great. Plenty of room for tolerances of the build.

Thanks,

Dave
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Old Jun 26, 2012, 09:56 PM
This is a fine fiddly business
Robert R's Avatar
United States, WA, Marysville
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Dave,
You are welcome, glad to help.
Robert
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Old Jun 27, 2012, 05:45 AM
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USA, FL, Tampa
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I used to use plaster of paris but it's too soft. It works ok but is just not durable.
Switched to Durham's Rock Hard water putty. It's about the same as working with plaster of paris but cures to a much harder material with a very smooth surface. Durham's is available at the home improvement store.
Others swear by hydrocal but I have yet to try it.
Glenn
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Old Jun 27, 2012, 07:19 AM
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I second the vote for the Rockhard putty. The best thing I ever found was Diamondcrete but it's no longer available.
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Old Jun 27, 2012, 10:11 AM
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Fix-All has worked great for me. It is so hard you don't want to sand large amounts of it. Mix with water like any plaster. Best of all I don't seem to get air bubbles. We used to add a bit to drywall plasters to get them to set up faster so we could get repairs done real fast. I know it cures fast, and seems to dry out way faster than most plasters I have used.

No release agent used on plaster to plastic BTW. If you want to fiberglass over a plaster form then you need to seal and wax it, but that is a whole different subject.

Fred
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Old Jun 27, 2012, 02:04 PM
Balsa&Tissue
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United States, OR, Beaverton
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Quote:
Originally Posted by glewis View Post
I used to use plaster of paris but it's too soft. It works ok but is just not durable.
Switched to Durham's Rock Hard water putty. It's about the same as working with plaster of paris but cures to a much harder material with a very smooth surface. Durham's is available at the home improvement store.
Others swear by hydrocal but I have yet to try it.
Glenn
This material sounds like the best choice for me. I found some at Home Depot.

What release agent do you use if any?

Thanks,

Dave
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Old Jun 27, 2012, 03:09 PM
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No release agent used on plaster to plastic. Plastic will release the plaster with little massaging. Most molds to make plaster parts are thermal formed plastic.

Fred
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Old Jun 27, 2012, 03:16 PM
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Originally Posted by Freddie B View Post
No release agent used on plaster to plastic. Plastic will release the plaster with little massaging. Most molds to make plaster parts are thermal formed plastic.

Fred
Are you saying with Fixall or with Durhams Rock Hard?

Thanks,

Dave
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Old Jun 27, 2012, 03:38 PM
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Originally Posted by payne9999 View Post
Are you saying with Fixall or with Durhams Rock Hard?

Thanks,

Dave
Yes, most plaster is similar. It is a mix with water. Plaster of paris, Fix-All, Durhams Rock Hard, Drywll Gypsum Plaster, all seem to be similar is mix, dry, and such. The plaster is smooth and plastic is slippery. Even epoxy will peel easy from clear shinny plastic.

Try a small drop of plaster mix, on your 'mold' in a not critical area. You will see. It may pop off as soon as dry when yu pick it up!

Fred
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Old Jun 27, 2012, 03:48 PM
Balsa&Tissue
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Originally Posted by Freddie B View Post
Yes, most plaster is similar. It is a mix with water. Plaster of paris, Fix-All, Durhams Rock Hard, Drywll Gypsum Plaster, all seem to be similar is mix, dry, and such. The plaster is smooth and plastic is slippery. Even epoxy will peel easy from clear shinny plastic.

Try a small drop of plaster mix, on your 'mold' in a not critical area. You will see. It may pop off as soon as dry when yu pick it up!

Fred
Thanks Fred.

Well, I went ahead a did one set. Mixed it a little thicker than heavy cream.

So, How long does it take to set? Dang, I am curious now to see how good they might come out...


Dave
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Old Jun 27, 2012, 04:41 PM
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A bit late,,, but..
I spray the inside of the part with silicone spray. I feel better having some kind of mold release. Note: don't use any kind of oil, the plaster will never set up. Guess how I know.... Support the part somehow. I use globs of clay stuck on.

Heavy cream is good. You want it thin enough to pour. I like to overfill as much as possible, gives some room to sand the bottom nice and flat.

Next, tap the part to make the bubbles rise. You want to really vibrate it good to get as many bubbles out as possible.

You will notice the color changes some and it will start to pull away around the edges. Usually takes a few hours to be dry enough to pop out of the mold.

It takes a good while to set up completely. The outside will be hard but it will still have a chewy center.... You will notice the part will feel cold indicating the damp insides. On thick parts it will take a week to fully dry out. Don't try to use it until it dries fully.

Glenn
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Old Jun 27, 2012, 04:48 PM
Balsa&Tissue
payne9999's Avatar
United States, OR, Beaverton
Joined Jan 2011
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Quote:
Originally Posted by glewis View Post
A bit late,,, but..
I spray the inside of the part with silicone spray. I feel better having some kind of mold release. Note: don't use any kind of oil, the plaster will never set up. Guess how I know.... Support the part somehow. I use globs of clay stuck on.

Heavy cream is good. You want it thin enough to pour. I like to overfill as much as possible, gives some room to sand the bottom nice and flat.

Next, tap the part to make the bubbles rise. You want to really vibrate it good to get as many bubbles out as possible.

You will notice the color changes some and it will start to pull away around the edges. Usually takes a few hours to be dry enough to pop out of the mold.

It takes a good while to set up completely. The outside will be hard but it will still have a chewy center.... You will notice the part will feel cold indicating the damp insides. On thick parts it will take a week to fully dry out. Don't try to use it until it dries fully.

Glenn
Glen,

I sunk the plastic parts into some sand. Then I just poured the material in and left them. They are firm now but maybe a little soft. I did tap the box for a while and a lot of bubbles did come to the surface.

Thanks,

Dave
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Old Jun 27, 2012, 04:49 PM
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Great advice Glenn, and it was worth mentioning it. A slow bake well onder 170 degree F, will help dry out the parts, but I never do that untill a day or 2 went by first.

I don't use silicone spray. If you ever need to paint the original part used for a mold, or something you vac-form, if any silicone is left at all, the paint won't stick. I don't think you can ever gett all the silicone off anything.

Fred
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