Advice for Heat Forming Acrylic Sheet

[Usagi Pilgrim]

Hey all. Even though this is technically a Marvel related prop, it's not a build thread, so I thought I'd get more responses here.

I'm going to be attempting Agent Coulson's energy shield by etching a 24 inch round piece of light blue acrylic, then heating it with a heat gun inside an old satellite dish to make the dish shape. The plexiglass is 1/8 thick.

So... Does anyone have any experience with anything similar, that can give me any tips or warnings? I'm a trial & error guy most of the time, but acrylic isn't exactly cheap. & I don't want to waste anything.

 

[division 6]

I've done blow forming (Captain Picard's fish tank)

Plastic is heated on a sheet of metal in oven type arrangement like used for a vacformer.
Once soft it's placed on a board with an air fitting attached (metal plate tacked over opening to disperse air).
Top framing with opening clamped on top, then air valve is slowly opened until the desired height is achieved.
monitor till cool.

 

[lmgill]

Usagi,
Your going to have a couple of issues.
First: A heat gun will not get the plastic hot enough everywhere, at the same time to allow it to form nicely.
Second: "Mark off". As hot plastic bends, any area that cools first, will move less than the surrounding area, this causes a minor change in thickness resulting in a distortion of light passing through it. It's know as "Mark off". If the plastic is opaque* then the mark off appears as a surface blemish.
Third: You are wanting to make a compound curve (a curve in two directions) To do this, the plastic needs to either stretch, in the middle or compress at the edges, or both.

To help solve these issues:
One; use an oven, not a heat gun. Acrylic will start to soften around 215 degrees Fahrenheit, so you can do this in a houshold oven. My guess is you will want to be at 250 - 275 F. You do not want the plastic too hot, as this will just cause more mark off. You want it hot enough to take the shape required. Acrylic melts at 300-315F

Second: To help reduce mark off, cover the surface your plastic is to touch with wool felt. (fabric store should have this) You want wool, since synthetic felt could melt.
This goes for the tray you are going to heat the plastic on in the oven. I use 3M 77 spray adhesive to hold the felt in place. Remember, this is a "Contact" adhesive, and against logic, you need to spray both surfaces and let them dry before you stick them together.

Third: This one is tricky. Without seeing how deep you satalite dish is, it's difficult to say whether you can make the plastic compound form this much by laying it into just one surface. You will need to experiment with a test piece the same size to see how hot the plastic needs to be, and if it will form the way you want it. The nice thing with this test piece is, you can reheat it and try several times till you find the right combination.

Hope this helps some.





* Opaque= does not pass light
Translucent= passes light, but does not pass image
Transparent= Passes light and Image
Clear= has no color

 

[cavx]

Not to hijack your thread, but Perspex is one of the more tricky plastics to vacuum from.

Here is my tests on "female tooled" vacuum forming.

I did these smaller scale test pulls made from 3mm (1/8th") Perspex. I didn't have clamping plates, so the sheet bowed and eventually broke the seal. The taller dome is thin, but optically clear at the thinnest parts. I want to make a small jig and see if I can't make bubbled lenses for Stormtroopers this way after seeing how bad the "male tooled" vacuum from parts look (to look through) andhow clear this dome is.

My new rig, scaling it up from 250mm to 700mm.

I did my seal tests (3 times) yesterday and all worked a treat, so off to get the fan forced heater built. That is 10mm Perspex on the form.

 

[TJackFx]

Usagi,
you can do this in a houshold oven.

Please do not heat plastic in any oven you use to cook food in. Bad idea.

 

[lmgill]

Please do not heat plastic in any oven you use to cook food in. Bad idea.

No, not a problem. As long as you do not burn the plastic. Acrylic is pretty stable,chemically.

 

[lmgill]

Cavx,

When Vacuum forming, the mark-off from a "female" tool and a "male" tool on clear material would be the same, just on the other face of the plastic. In real industry this is often referred too as "first surface" and "second surface", as the surface you use (the one seen) is the surface touching the tool (1st /female), or not touching the tool (2nd/ male).
If i understand your tooling, you seem to be describing "Blow forming". This would result in clearer lenses, as the plastic never touches a colder surface.

If you have to vacuum form you lens, the one advantage in a first surface tool, is this puts the mark-off is on the convex side of the lens, allowing easier sand and polish to remove the distortion.

 

[TJackFx]

No, not a problem. As long as you do not burn the plastic. Acrylic is pretty stable,chemically.

The heat of thermoforming acrylic sheet may result in the release of vapors or gases, including methyl methacrylate (MMA) monomer. MMA vapors can cause eye and respiratory irritation, headache and nausea. MMA is toxic to lungs and mucous membranes. All I'm trying to say is that heating the acrylic in your house oven is likely not going to allow you to ventilate the area well enough to remove the vapors.

 

[lmgill]

Most household ovens have a fan system and the vapors given off are not extreme in volume, unless you burn it by getting it too hot too quickly. However, if you are sensitive to these things, then you shouldn't be heating it with a heat gun either.

 

[Mr. Nagata]

Wow, I never knew about the wool felt thing. Can you vac form clear visors this way?

Usagi,
Your going to have a couple of issues.
First: A heat gun will not get the plastic hot enough everywhere, at the same time to allow it to form nicely.
Second: "Mark off". As hot plastic bends, any area that cools first, will move less than the surrounding area, this causes a minor change in thickness resulting in a distortion of light passing through it. It's know as "Mark off". If the plastic is opaque* then the mark off appears as a surface blemish.
Third: You are wanting to make a compound curve (a curve in two directions) To do this, the plastic needs to either stretch, in the middle or compress at the edges, or both.

To help solve these issues:
One; use an oven, not a heat gun. Acrylic will start to soften around 215 degrees Fahrenheit, so you can do this in a houshold oven. My guess is you will want to be at 250 - 275 F. You do not want the plastic too hot, as this will just cause more mark off. You want it hot enough to take the shape required. Acrylic melts at 300-315F

Second: To help reduce mark off, cover the surface your plastic is to touch with wool felt. (fabric store should have this) You want wool, since synthetic felt could melt.
This goes for the tray you are going to heat the plastic on in the oven. I use 3M 77 spray adhesive to hold the felt in place. Remember, this is a "Contact" adhesive, and against logic, you need to spray both surfaces and let them dry before you stick them together.

Third: This one is tricky. Without seeing how deep you satalite dish is, it's difficult to say whether you can make the plastic compound form this much by laying it into just one surface. You will need to experiment with a test piece the same size to see how hot the plastic needs to be, and if it will form the way you want it. The nice thing with this test piece is, you can reheat it and try several times till you find the right combination.

Hope this helps some.





* Opaque= does not pass light
Translucent= passes light, but does not pass image
Transparent= Passes light and Image
Clear= has no color

 

[lmgill]

Wow, I never knew about the wool felt thing. Can you vac form clear visors this way?

No
Just drape form

 

[cavx]

If i understand your tooling, you seem to be describing "Blow forming". This would result in clearer lenses, as the plastic never touches a colder surface.

The domes shown technically were "untooled" apart from the chamber they were pulled into. So I guess you could say blow formed, except these were done under vacuum. I took the 2nd done to 20"HG from memory. Both are made from the same 3mm white Perspex, just the 2nd dome (the taller one) I decided to go all or nothing and wanted to see how far I could go before it tore. It didn't tear, but is paper thin at the top.

The big rig is just a bigger version with steel rings to hold the plastic edges flat.

Hopefully I get do some work on the heater today. I am using halogen lights. Instant heat and they get really hot, like they will start a fire type hot. Before I knew the risks of working PVC, I used the heater on some sheet of the 3mm PVC. It was vaporing in under 3min!

The tool I have to make half cylinders for a project has some small imperfections, and as I understand this, these will be show on the surface of the parts unless I can skin the inside of the tool with a thin rubber of similar heat resistant material.

- - - Updated - - -

No
Just drape form

Is that the same as "slump" forming?
You frame the plastic, heat it up to the point where in slumps, then gravity do it job?

 

[lmgill]

The tool I have to make half cylinders for a project has some small imperfections, and as I understand this, these will be show on the surface of the parts unless I can skin the inside of the tool with a thin rubber of similar heat resistant material.


Is that the same as "slump" forming?
You frame the plastic, heat it up to the point where in slumps, then gravity do it job?

The heat resistant material will not have an effect. as long as the tool surface is cooler than your plastic, it will cool the plastic and cause mark off. A mirror polished tool, preheated to 80% of you plastic temp would be the way to go. Production tools for this type of work, I believe are almost always made of metal, steel or aluminum and often have internal cooling system with liquid.
Slump forming refers to, as you describe, plastic in a frame, allowed to slump downward by gravity. Drape forming is laying the plastic on a form. Since the need for the felt on this form.

 

[cavx]

The heat resistant material will not have an effect. as long as the tool surface is cooler than your plastic, it will cool the plastic and cause mark off. A mirror polished tool, preheated to 80% of you plastic temp would be the way to go. Production tools for this type of work, I believe are almost always made of metal, steel or aluminum and often have internal cooling system with liquid.

The parts I am making will be from solid black materials. No optical clarity required. I just don't want any surface lumps. As I understand it, this process can pick up fine details like text.

 

[lmgill]

In that case, just make sure your mold surface is very smooth, at least 600 grit.

 

[cavx]

In that case, just make sure your mold surface is very smooth, at least 600 grit.

I might skin it with 0.6mm metal then.