Heatgunning a Dome with PETG? Is it possible?

I'm preparing the process I'll take for making an Alien head. I'm going to carve the bulk of it out of extruded styrofoam but I want the dome to be clear, allowing people to see the skull underneath it and also allowing me to see through it. I've bought a couple of sheets of clear A1 1mm PETG for this task. My plan was to carve the dome first out of the foam, coating it with plaster of paris for strength and heat protection and then heatgunning the PETG over the shape. After forming the dome, I'd cut into the foam buck and continue carving the skull and details underneath and glue the clear plastic shell over the top of it.

My question is how much flexibility will I get from the PETG? I've curved simple visors in the past but the curve on this dome will be on more than one axis so is this doable?



The shape isn't terribly detailed.

The alternative is vac forming, but that's more time and money that I'd want to spend building one.

PETG is a difficult plastic to work with (not my fav) but saying that, I think the heat gun route will be very difficult if not impossible. You'll have a better result with vac-forming than with the heat gun (un-even heat mostly, or the danger of melting your plastic). Also, the transparency could be affected with the heat gun.

I second joberg's suggestion. Vacuum forming PETG is pretty simple. I've done it with an old toaster oven and a shop vac. Punished Props has a good video on how to do it.

http://punishedprops.com/2015/11/23/vacuum-forming-machine/

If your piece is too big for a toaster oven and you don't want plastic in your kitchen oven, X Robots has a simple plastic heater built around a space heater.

https://www.youtube.com/watch?v=maH5Ech0wK8


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P.S. if you have access to basic woodworking tools, a vacuum forming box is simple and cheap to build. I made mine out of half a sheet of 1/4" MDF. It took half an afternoon.

Zinger said:

I second joberg's suggestion. Vacuum forming PETG is pretty simple. I've done it with an old toaster oven and a shop vac. Punished Props has a good video on how to do it.

http://punishedprops.com/2015/11/23/vacuum-forming-machine/

If your piece is too big for a toaster oven and you don't want plastic in your kitchen oven, X Robots has a simple plastic heater built around a space heater.

https://www.youtube.com/watch?v=maH5Ech0wK8


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P.S. if you have access to basic woodworking tools, a vacuum forming box is simple and cheap to build. I made mine out of half a sheet of 1/4" MDF. It took half an afternoon.

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Given the size of the project (80cm long), the price of 6mm MDF alone comes to £107 to make a hot box, vacuum box and a frame.

Jesuit24 said:

Given the size of the project (80cm long), the price of 6mm MDF alone comes to £107 to make a hot box, vacuum box and a frame.

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How expensive is MDF in the UK? A 4ft x 8ft x 1/2in sheet is $25 here.

80 cm is quite a bit bigger than my project was.

Is there a MakerSpace near you? My MakerSpace has real vacuum forming machine that could handle 80 cm. Maybe you'll get lucky and find one relatively close.

Jesuit24 said:

Given the size of the project (80cm long), the price of 6mm MDF alone comes to £107 to make a hot box, vacuum box and a frame.

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That sounds expensive for MDF, it's only $20 per sheet for 2400mm x 1220mm (8ft x 4ft) here.

Regardless, building your own decent-size vacuum-forming machine is something you won't regret. I've used the one I built over and over again, even for things I never thought to vacuum form. In short, it is invaluable.

Xolin said:

That sounds expensive for MDF, it's only $20 per sheet for 2400mm x 1220mm (8ft x 4ft) here.

Regardless, building your own decent-size vacuum-forming machine is something you won't regret. I've used the one I built over and over again, even for things I never thought to vacuum form. In short, it is invaluable.

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The table is also useful for dust collection when sanding and other work.

There are shops that do vacuum forming. Might be worth calling around and seeing if there's someone with a large enough machine to do a couple of pulls for you.

I've halved my initial estimate now to £55. Before I was thinking of buying a few smaller boards rather than cutting up a few bigger ones. But the question is how high does my hot box have to be in relation to the size of the project in order to heat the plastic evenly? All the smaller MDF boards I've seen are cut to roughly 600mm by 1200mm. Would 600mm be a tall enough for a project 800mm wide?

Jesuit24 said:

I've halved my initial estimate now to £55. Before I was thinking of buying a few smaller boards rather than cutting up a few bigger ones. But the question is how high does my hot box have to be in relation to the size of the project in order to heat the plastic evenly? All the smaller MDF boards I've seen are cut to roughly 600mm by 1200mm. Would 600mm be a tall enough for a project 800mm wide?

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You might try asking James at X Robots. His heater is 75 cm high and 60 cm square. It as a 45 cm square work area.
http://www.xrobots.co.uk/vacuumtable/

Going by those dimensions, 60 cm high would be too short. But he's using an IR heater, so he just needs to get the light evenly spread out plus space to make sure the plastic doesn't sag down into the heater. That would be bad.

Ask James. He's got the most experience.

One more thing. How wide is the piece going to be? If you need an 90cm x 90cm table, that's going to be 4 times the area of James' table. That might be too much for a 2Kw patio heater. Plus it would take a lot of suction to get a good pull. Possibly too much for one shop vac.

If you need to go that big, finding someone with a commercial machine would probably be the better option.

At that size, 2 shop vacs will be needed.

Zinger said:

One more thing. How wide is the piece going to be? If you need an 90cm x 90cm table, that's going to be 4 times the area of James' table. That might be too much for a 2Kw patio heater. Plus it would take a lot of suction to get a good pull. Possibly too much for one shop vac.

If you need to go that big, finding someone with a commercial machine would probably be the better option.

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The piece is only 20cm wide, and maybe 30cm deep. One problem is that 80cm length is that the A1 size comes to 84cm, leaving not much room for the depth of the piece. I'd have to place it on the buck diagonally which would give me 102cm, which still may not be enough? How do you gauge how much space you leave for the depth of the piece?

I was planning on making a vac former A1 sized since that would come in useful for other projects down the line, but I could cut the cost further if I made the vac former specifically to the size I need for this project. It's just then I'd have a chunk of wood in my garage that lets me only make xenomorph head domes.

I spent half an hour yesterday emailing vacuum forming companies in the UK. Only two have gotten back to me so far, one didn't specialise in clear plastics and the other had a minimum order quote of £250.

Before he even considers vacuum forming this dome, does he have a buck?

The price of the MDF does sound expensive as a 2440mm x 1220mm x 16mm sheet is $31 at Bunnings.

However, for that size, I will suggest he slump forms it. He can use a heater like X-Robots and heating the plastic from underneath will be more efficient as heat rises. He just needs to be on the ball to shut off the heat at the right time.

Slump forming has several advantages -

1. (and this is a big one with a dome for an Alien) is optical clarity of the plastic. It won't suffer the usual optical distortions like vacuum forming tends to do due to the temperature differences once hot plastic is stretched over a colder buck. . Just ask anyone with bubble lenses in their Stormtrooper bucket about this.

2. He only need to cut out the hole the same shape as the final dome. He can use screws to attach the plastic to the MDF.

3. He can take his time here and being in the UK, might have to do this in stages anyway.

Jesuit24 said:

The piece is only 20cm wide, and maybe 30cm deep. One problem is that 80cm length is that the A1 size comes to 84cm, leaving not much room for the depth of the piece. I'd have to place it on the buck diagonally which would give me 102cm, which still may not be enough? How do you gauge how much space you leave for the depth of the piece?

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I'd worry about the 30 cm draw depth. With my shop vac I can get decent draws 6 - 8 cm deep, but struggle after that. Real vacuum forming machines use large evacuated tanks to create their suction. They get much better draw than a shop vac.

You might want to consider cavx's suggestion below. I hadn't heard for slump forming before, but it looks pretty simple.


Plus, if you were going to build a vacuum form, you'd have to build the heater and frame anyway. You could try slump forming one dome. If it doesn't work, you're only out a sheet of plastic.

Jesuit24 said:

The piece is only 20cm wide, and maybe 30cm deep.

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At my MakerSpace this evening, I was talking with someone who knows way more about vacuum forming than I do. He said one danger of vacuum forming deep draws is they tend to form "ribs" especially on near vertical sides. Here's a photo of a piece showing the problem.



That piece is only about 12 cm tall.

He said there are ways to avoid this. Like using what he called a clam shell mold to pre-shape the plastic before you hit the vacuum or making a negative buck instead of a positive. Both of those sounded complicated.

Trying slump forming first is looking better and better.

Zinger said:

At my MakerSpace this evening, I was talking with someone who knows way more about vacuum forming than I do. He said one danger of vacuum forming deep draws is they tend to form "ribs" especially on near vertical sides. Here's a photo of a piece showing the problem.

That piece is only about 12 cm tall.

He said there are ways to avoid this. Like using what he called a clam shell mold to pre-shape the plastic before you hit the vacuum or making a negative buck instead of a positive. Both of those sounded complicated.

Trying slump forming first is looking better and better.

Click to expand...

The only issue I foresee with slump forming is the depth without suction. Basically the bigger the surface area, the deeper the slump will be. At the right temp, you should be able to get at least 50% of the width in height (depth of slump). But your plastic may need to be thicker than 1mm to achieve this without a tear.

Ribs are not always going to happen but that part is way taller than it is wide. Some of that might be uneven heating or just not enough suction at the crucial time, so the plastic folded instead of stretching down. One way to prevent that is to raise the buck into the plastic as it pulls down.

Female tooling is pretty much the ideal method for deep pulls, but as you have thought it out, tooling is tricky to say the least. You need to know what the "indent" of your part will look like in order to make a successful female tool. The way I've designed these is to make a male part in a 3D CAD (I only used SketchUp), built a box around that, then delete the original part. What you are left with is a female tool for that part.

The clam method is basically a combo of male and female tooling systems where the female tool is larger and placed over the male tool. The heated plastic is mechanically shaped as well as pulled by suction. You can simply make a slightly over size frame around the male buck as well to help push the plastic down to get a seal so that you get a proper pull. I've done this on 3mm plastic pulls.

In short, vacuum forming pulls that are deeper than they are wide are going to be challenge. Watch all the YouTube videos showing home made vacuum forming systems and none of them (except that C-3PO butt made by X-Robots) are very high. Normally the ratio of width to height is like 4:1 or the height is about 25% of the width.