Quick and Easy Pressure Forming Makes Plexiglas Domes

Thermoplastics are amazingly versatile materials. Apply some heat, add a little force, and within seconds you’ve got a part. It’s not always quite that simple, but as [maxelrad] discovered, sometimes thermoforming can be as easy as blowing up a balloon.

In need of a cowling for an exterior light fixture on an experimental aircraft, [maxelrad] turned to pressure forming of Plexiglas for the hemispherical shape he needed. His DIY forming rig was a plumbing-aisle special: PVC pipe and caps, some air hose and fittings, and a toilet flange for the pressure chamber. The Plexiglas was softened in a toaster oven, clamped over the business end of the chamber, and a few puffs of air inflated the plastic to form a dome. [maxelrad] points out that a template could be applied over the plastic sheet to create the streamlined teardrop shape he needs, and he notes that the rig would likely work just as well for vacuum forming. Of course, a mold could be substituted for the template to make this a true blow-molding outfit, but that would take away from the simplicity of this solution.

There have been a fair number of thermoforming projects featured on Hackaday before, from this DIY vacuum former to a scratch-built blow molder. And while we really like the simplicity of [maxelrad]’s technique, what we’d really love to see is some details on that airplane build.

29 thoughts on “Quick and Easy Pressure Forming Makes Plexiglas Domes

  1. I’m surprised he went with positive pressure instead of a vacuum. Sure, the first way is cheaper, but the second is much safer. Should it fail, it fails inward instead of blasting hot plastic outward :)

    1. The Plastic doesn’t get that all that hot and the pressure shouldn’t be all that high. Its a fairly safe process. A face shield and gloves are likely not a bad idea just in case.

      I think vacuum forming might cool the acrylic down too much before the dome could properly form. But I guess that depends on how leaky the rig is. If you did it with a vacuum pump and chamber it might work just the same as under pressure. But the typical vac rig I’ve seen uses a vacuum cleaner and is pretty leaky so there is a constant flow of air across the plastic cooling it. Usually its done with softer plastics like PETG rather than acrylic plexiglas.

      1. I’ve never done pressure forming, so I’ll have to take your word on pressures and temps. I would argue, though, that the same rig is used for both positive and negative pressure forming, so the air gaps (aka leaks) will be roughly the same except a negative pressure rig will somewhat self seal while a positive pressure rig would expand the holes in the seal. I’d be curious to see what the effects of gravity are on the shape between vacuum molding and pressure molding in this application are though.

        1. Pressures are fairly low (low 10’s of PSI tops), the plastic is pretty weak when its hot, think a sheet of rubber band. sealing isn’t that hard and it doesn’t need to be that good.

        2. The exact same rig cannot do both, the vacuum forming rig requires the chamber to be larger than the part. Making a larger air tight area is hard. This blow mold rig has a tiny volume of pressurized air making it easier to build. That is the tldr for this hack a day post.

      2. I also imagine it is much easier to eyeball the results and judge when to stop with positive pressure (so it grows OUT like in the photo) vs negative (where the dome is sucked downwards into a tube and not easily seen.)

    2. Shouldn’t be an issue in this case. An embolism will how well before failure if pressure is applied slowly. If applied quickly, it is still likely that failure will be at a ductile point (warm an soft) relieving pressure without throwing shrapnel. If overpressure after it is cool enough to throw shrapnel, the fixture looks like it will leak more than sufficiently to relieve pressure well before failure with nothing but pony clamps holding it. Rough calc: 4″PVC flange is about 12 in^2. The pony clamps are probably providing about 20lb each (I measured mine that look similar in size) for 80lbs before relief. That is about 7PSI.

      The plexi should be able to take 30PSI or more when flat in the state where it can throw chunks (safety factor 2, when used as glazing, 6″ by 6″ square. From a table, not calculated by me). With the spherical curvature and thinning considered (presuming uniform thinning), it should be good for more (using the calc in ASME Sec8 and common propertied for acrylic).

      That said, I would wear a face shield. Spare eyes are not yet on the market.

      1. “using the calc in ASME Sec8 and common propertied for acrylic”

        Back away from the engineering… hoe lee sheet… dude, no. This is not a pressure vessel, and the code doesn’t even remotely cover this. Everything you said is wrong and people are dumber for having read what you wrote.

  2. I like this because while plastic domes are available from many Chinese suppliers, they are kind of bulky (they take up room and don’t ship easily or small) and therefore kind of expensive.

  3. I could need a similar process for underwater cameras, to combat the narrowing of the FOV by the higher refractive index medium water. His process seems to create very wobbly surfaces, insufficient for a camera dome. Anybody with a idea on how to get a smooth and even surface? I’d need only an inch or so of dome diameter.

    1. Used to make windscreens for motorcycles this way, getting an even finish is mostly about getting even temperature in the plastic. To the point where a 12mm steel bar running 5cm above a bubble a meter long and 30cm high would cause a noticeable dip in the the plastic there where it cooled it

    2. any fluid, be it gas or liquid, would reshuffle inside the chamber towards the weaker spot (thinner material, or higher spot temperature), which will stretch it and weaken it even more. Assuming fairly even heating, and homogenity, this is only an issue for something as sensitive as optics.

      what you need is a positive and a negative mold with a gap in between to apply the pressure, such that when you press it down, it doesnt give, and applies more pressure on the thicker spots.

      think sheet metal press, for plastics.

      i am not an engineer, and i hope i have explained my thoughts clear enough.

  4. I’m not really talking about the surface per se but the variation in material thickness. Sanding isn’t going to change this. And for polishing a 10 mm dome is too small on the inside.

  5. A source of clear acrylic domes is the old rear projection TVs. They have three CRT projection tubes with a mineral oil-filled focusing units and clear acrylic domes in red, green and clear. They’re about 6-7″ in diameter with a nice flat flange for mounting and sealing.

  6. IIRC the original formed “perspex” canopies on british WWII fighers were pressure formed using hot oil in a mold cavity. I think that definitely wouldn’t be for amateurs but the heat transfer would be good. I’ve seen some bubble canopies for homebuilts made by simple draping plastic over an elliptical hole cut in a sheet of plywood – heating it properly and letting gravity pull it into shape. Again, I think the failure/disappointment rate might be significant without a little practice.

  7. I’d use a high capacity bicycle pump to have good control over the inflation. Not one of those cheap ones, the fancy ones like serious water rocket builders use. Orrrrr, since this is HaD, build your own pump.

    1. I would just use the air compressor and the tire fill set with the pressure gauge – which I also use for the bicycle. So technically you can call it a high capacity (1,5kW) bicycle pump. :-)

  8. Wow, this is a nice method for making acrylic domes.

    And seriously, for the folks running around saying the sky is falling for the lack of safety… natural selection. Anyone that gets hurt trying something they saw on the internet without doing their own safety work gets what they deserve, and the rest of mankind is better off for it. And the muggle gets to wear a swank eye-patch for the rest of their life, problem solved.

  9. How low have we sunken that people are concerned for safety during dome making with super low pressures? Guys, be careful you don’t drown in your bath tub. Wear life vests!

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