How to design and assemble layered acrylic case

[kingberg]

Hi!

I'm in the process of designing my own keyboard and the next step is to design the case. My plan is to use layered acrylic sheets, but I'm not sure how to best assemble them. I think I've seen some kind of hollow, threaded cylinder (kind of like a hollowed out screw) that you can screw the screw into and lock it into place (sorry for the terrible explanation). Anyone know what I'm talking about, and is this a good way to go about it?

[vvp]

It is called insert: https://www.youtube.com/watch?v=MoAji1iq_nk

I do not use them. You will get slightly weaker joints by cutting threads into acrylic or by using wood crews into softer plastic (e.g. ABS) without threads (wood screws will cut it's own thread).

[kingberg]

Do you just screw into the plastic, or is there another solution that you use?

[DMA]

How about just gluing layers together? You'll still need screws for the bottommost layer, but otherwise glue should provide better joints.

[vvp]

Do you just screw into the plastic, or is there another solution that you use?

When I use a soft plastics (like ABS) then I just screw them in. The (wood) screw will cut its own thread.If the plastic is a bit harder (e.g. PLA) then you can heat the screw with e.g. a cigarette lighter before screwing it in.

When I use a hard plastic (like acrylic) then I just cut a metric thread in it (e.g. M4) and then use a bolt.

You can use inserts if you want a bit more strength. Glue may be a good option too if the plastic is not transparent ... or it is transparent and you do not care about the looks.

[DMA]

Glue may be a good option too if the plastic is not transparent ... or it is transparent and you do not care about the looks.

Why not use solvent as a glue and press things together? Should result in decently transparent interface

[Muirium]

and you do not care about the looks.

^ That bit. Transparent acrylic is all about the looks. You’ll definitely see imperfections in the glue. It’s like why cars are spray painted instead of painted with a brush. “Mere cosmetics” are paramount, even on the most workaday stuff.

[vvp]

Glue may be a good option too if the plastic is not transparent ... or it is transparent and you do not care about the looks.

Why not use solvent as a glue and press things together? Should result in decently transparent interface

I never tried it. It may work well. Some practice may be needed so that the joint is bubble free. Also some solvent will be pushed over the edges. That will likely create visual defects.

I'm joining ABS 3D-printed parts with "ABS glue" a lot. It works very well but it is opaque.

[DMA]

and you do not care about the looks.

^ That bit. Transparent acrylic is all about the looks. You’ll definitely see imperfections in the glue. It’s like why cars are spray painted instead of painted with a brush. “Mere cosmetics” are paramount, even on the most workaday stuff.

LOL. Cars are spray-painted because it's way cheaper. When they are spray-painted, that is - they're mostly dipped in paint nowadays, because, again, it's way cheaper than even spray-painting

[Muirium]

They do a good job of it then.

My strange analogy is because, once upon a time, my old man ruined a panel in his little Renault 5 (presumably reversing into another car or a wall at speed, knowing him) but said car was 10-20 years old already and the only replacement he could find was some other colour entirely. He was too cheap to paint it right. He literally got out a brush and slathered some navy blue on it. I was about 8 at the time and I still remember just how AWFUL that looked, and the looks we’d get from other people driving that thing around until the engine mercifully blew a gasket at an arterial roundabout and the car got ditched for good.

[Findecanor]

I think I've seen some kind of hollow, threaded cylinder (kind of like a hollowed out screw) that you can screw the screw into and lock it into place (sorry for the terrible explanation).

Like these?
More common in hardware stores are spacers without any threading though.

There are also "rivet nuts"/"rivnuts"/"T-nuts" that are threaded, with a hat.
I have also seen acrylic-sandwich keyboards with just hexnuts or capnuts.
Beware that bolts only come in certain lengths, so you'd have to design the thickness of the case for bolts to fit, or otherwise you'd have to cut the bolts to length.

I'm joining ABS 3D-printed parts with "ABS glue" a lot. It works very well but it is opaque.

You can find thinner "ABS cement" in model stores that is transparent and evaporates quickly. However, these solvent-based adhesives for ABS don't work with acrylic. I think adhesives for PVC containing MEK (Swe: Butanon) should work, but those glues might not be particularly thin or transparent (and some moderately large things I've glued with "PVC glue" have never really dried completely...)

I have been recommended Acrifix for acrylic but not tried it.
Superglue bonds well, but beware that superglue emits fumes that will react with and fog up clear acrylic (even without contact!), so if you use it, it is best used if you had already decided to paint it.

[kingberg]

There are also "rivet nuts"/"rivnuts"/"T-nuts" that are threaded, with a hat.
I have also seen acrylic-sandwich keyboards with just hexnuts or capnuts.
Beware that bolts only come in certain lengths, so you'd have to design the thickness of the case for bolts to fit, or otherwise you'd have to cut the bolts to length.

Rivnuts seems like the closest to what I had in mind so far since I would prefer to not damage the plastic or risk wearing it out from opening the keyboard (for whatever reason). I could maybe do it with cap nuts. Thanks!

[vvp]

Can acrylic handle the pressure from a rivnut?
I know they can be used with carbon fiber composite but I have never seen them in acrylic.