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Tiny bubbles: unfortunately not a song title but a problem

 
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addled



Joined: 05 Oct 2015
Posts: 4
Location: Virginia USA

PostPosted: Sat Oct 10, 2015 8:10 pm    Post subject: Tiny bubbles: unfortunately not a song title but a problem Reply with quote

I'm totally new to this. I built a small vacuum forming box (internal dimensions about 6x6x1.5") for a one-off project: a double windscreen for a 1/12 scale formula 1 car from the late '60s.

I'm using PETG sheet, the only thermoform plastic available from a local hobby shop. The slug is plaster of paris (which I had around and needed take a mold from a kit windscreen that I modified). The box is small enough to heat the sheet using a heat gun.

The first and only pull so far was successful except for lots of tiny bubbles: not acceptable for a windscreen! I looked online for help and found that someone had posted to an RC group with the same problem 10 years ago (also using a plaster slug, for a cockpit canopy). Every reply suggested something different, but no clear solution. PETG needed drying. Moisture in the plaster. Temperature too high.......

One suggestion was to form a vinyl part, finish and sand it if necessary, leave it on the slug, then form the PETG part over the vinyl. The poster said the two materials would not stick as PETG forms at a lower temperature. Is there anything in this suggestion?

I'm hoping that 10 years on, someone knows the cause of the bubbles and how to resolve the problem and I would appreciate any help!

Thank you

Mark
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kayaker43
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Joined: 13 Jun 2007
Posts: 171

PostPosted: Tue Oct 13, 2015 7:45 pm    Post subject: Reply with quote

Pet-g is generally not regarded as hygroscopic and I've never seen it bubble but other swear that it can. It may be possible but its unlikely. Most times its really polycarbonate mistakenly sold as pet-g, or an unfinished mold with surface dust or texture that makes little dents that seem like bubbles.

Its possible you re severely overheating it with the hear gun, PET-G is normally a really user friendly plastic
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Hobby-Vac and Proto-Form machine plans

Also other plans books and videos for people who like to build things
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addled



Joined: 05 Oct 2015
Posts: 4
Location: Virginia USA

PostPosted: Tue Oct 13, 2015 10:22 pm    Post subject: Those bubbles Reply with quote

Thanks for your ideas/advice. I don't think I'm severely overheating the material, which is sold in small sheets labelled "polyester", though a little investigating online reveals that the distributor's (Midwest Products) product is pet-g.

If I interpret you correctly, you are saying that polycarbonate is susceptible to 'bubbling'.

The problem may well be 'surface dust'. These are nothing larger than pinprick sized dents on the inside surface. For the initial 'pull' (so far the only one, as I don't want to waste material unnecessarily) I dusted the plaster very lightly with talcum powder, blowing over it - again online 'advice' - as a mold release agent.

The following seems to be an important issue then. Can/should one coat plaster of paris with something (resin?) to seal it and try to eliminate dust? Or is a fully dried and cleaned plaster surface okay? What about painting the plaster with a high temperature paint, like the type used for spray painting gas grills? I happen to have some of this.
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kayaker43
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PostPosted: Tue Oct 13, 2015 10:43 pm    Post subject: Reply with quote

Plaster can be smooth if pulled from a smooth mold or slightly grainy if you sand off the skin, You don't need mold release and talc will cause surface defects so that's probably it.

Avoid paint if possible. Try without talc to see if it improves. Resin may not stick well either. Try forming a thin styrene sheet over the mold, trim it out and leave in place. This gives the plaster a temporary shell that leaves a good finish when you form PET-g over it.
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Doug Walsh
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Hobby-Vac and Proto-Form machine plans

Also other plans books and videos for people who like to build things
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addled



Joined: 05 Oct 2015
Posts: 4
Location: Virginia USA

PostPosted: Wed Oct 14, 2015 8:45 am    Post subject: Reply with quote

Doug

Thank you again. I'd read of the PET-G over styrene 'trick,' but was skeptical. With your advice it will be worth trying. I made some adjustments to the mold, so smooth plaster became somewhat grainy (though I've smoothed it again with 6000 grit micro mesh) and a styrene skin would be helpful.

Mark
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addled



Joined: 05 Oct 2015
Posts: 4
Location: Virginia USA

PostPosted: Tue Oct 20, 2015 3:04 pm    Post subject: The styrene did the trick Reply with quote

Thanks Doug

A thin layer of styrene over the plaster mold has produced a much more acceptable windscreen

Kind regards

Mark
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