Precise bending of plexiglass

Machine Shop Plastics & Composites
#1

I’d like to make a plexiglass cover for my Godbout S100 system, so the innards are visible. Was considering a simple inverted-U design.

I can design and cut the flat plexiglass, but am looking for the equipment and advice/help to achieve bends that are precise enough to not leave noticeable gaps (< 1/16"?) on the sides. Before the design can be done, an allotment of linear distance for the corner bends needs to be made. Target thickness is TBD, but given the weight of the system probably at 3/16 - 1/4".

Another consideration is whether the bend location and radius can be controlled well enough so that mounting holes created in the laser process will line up with the holes on the system. Is this doable or best to drill the holes post-bend? The concern is cracking the plexiglass while drilling, especially since the holes would be on the sides/wings of the cover.

I’m certain that the knowledge of how to do this exists at the space, but not sure if the equipment does.

An alternative would be to use a local plastics fab service, if the price wasn’t excessive. Any recommendations for plastic fabricators?

Recommendations, advice, puns, and pithy comments all welcome.

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#2

There’s no simple answer, but this article is a good treatment of the subject.

Based on the initial question posed in the article, it looks like you should start with a K factor estimate of about 0.4. But it also looks like you’ll be doing a bunch of tests to make measurements.

However, I suspect that with the material thickness you propose you will have a fairly large bend radius that may not be suitable for your computer cover.

I believe that commercially a heated die/press is used for the bend and AFAIK we don’t have anything like that. We have (had?) a heated-strip bender, but it’s highly dependent on your own personal experience to get reliable results - regarding distance, angle, and perpendicularity. Someone else can chime in here.

WRT the drilling, if you can support the area under your drill, you shouldn’t have any trouble drilling post-bending. Plexiglas (i.e., cell cast acrylic) is soft and not particularly brittle. Personally, I would recommend drilling as a secondary step anyway because it will be easier to get the holes located/aligned properly.

For that matter, if you get the distance across the top of the case correct, you can also saw/route the U-legs to length post-bending. I would leave the protective film on the plastic (or make your own) to prevent scarring from the saw and router.

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#3

Cut a piece of wood to the dimensions you need and use it as a template with a heat gun to put the bends in place. I’ve done that with 1/8" poly sheets that I cut down with a dremel to bend and make custom reinforcements for fragile playfield plastics on my Blackwater 100 pinball.

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#4

Go slow when heating to avoid scorching, bubbles, or craze lines in the bends.

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#5

Would cutting three pieces, then welding the top piece to the top edges of the two sides get you results you want?

There are specialty welding “glues” for plexiglass which give a clear finish and are thin enough to wick into the gap between the top and the side pieces. Allied Plastics sell the solvents you need. https://www.tapplastics.com/product_info/diy_articles/plexiglass_glue

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#6

That’s a great suggestion, Raymond. As long as had the plexiglass was centered and square with the top of the wooden template, that should give a fairly snug fit.

#7

My original plan was to use the approach you suggested, making the enclosure from 3 separate pieces. And I still may go that way. The good thing is that I have some experience with this technique. The ‘glue’ is really just a solvent that liquefies the plexiglass at the interface and ‘welds’ the mating surfaces together.

I saw a pic of a single piece cover on an IMSAI system that was on ebay, and it looked really clean. Hence my desire for that approach.

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#8

If you want any help with this project I used to work at TAP Plastics in Sacramento. I spent a lot of time bending and welding acrylic.

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