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Very small and thin transparent enclosure

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Nusa:
I've used hot glue with personal one-off projects. Should be fine for a prototype...you'll  have to evaluate it for production. Can be smoothed when hot and trimmed with a knife when cold.

For cold glues, I suspect super glue would work. Never tried myself.

My other idea would be using the right size end plug for the tube to shrink around. In your case, perhaps a 4-5 mm thick sheet of plastic or wood cut to the width of the pcb. Butt pieces up to the pcb ends, shrink the tube, then cut both ends off to get nice edges. If you make the plugged ends long enough, you can drill them for mounting holes.

reboots:
This is a little more than 6mm thick, but it might fit your other requirements:

http://www.witzsportcases.com/id-badge-holders/see-it-safe-new.html

Waterproof PVC badge holders are a common product. This listing ships from USA, but something similar may be available within your locale:

https://www.ebay.com/itm/183568594557

You will definitely want to test the waterproof claim. As other commenters have mentioned, clear plastics (especially PVC) won't last forever outside.

jmaja:
Clear polyolefin heatsink from 3M worked OK and it was possible to seal the ends by melting. I could not find one with glue.

However, I'm trying to go forward with a custom enclosure made from polycarbonate. I have designed it to be openable and sealed with an O-ring. The O-ring is set up as piston cylinder style.

The PCB became even thinner (3.1 mm) allowing the enclosure to be only 4.7 mm. Now my problem is to find suitable screws for such a small and thin enclosure. The screws need to be recessed into the enclosure. The total lenght of the screw (including the head) must be less than 4.5 mm, preferably 4 mm to allow some tolerance. The thickness of the enclosure halfs at the screws is 2.2 mm, since there will be 0.3 mm thick fabric in between. Due to O-ring configuration no compression from screws is needed, it's enough to just keep the halfs together.

The first installation and possible battery change after a few years is done by the user. The screws must work at least 3 times.

What kind of screws would fit this application? Thread forming screws or machine screws? What kind of nut arrangement for the latter?

The suitable size would be M2 (or #2). Thread forming at only 4 mm length are very hard to find and surpricingly expencive. Will they hold with such a short thread? Only 1.5-2 mm.

Also M2 thread incerts for only 2.2 mm thickness are hard to find. I found one, but it seems not designed for recessed installation. Square or hexagonal nuts can be found at 1.2 mm thickness. Would making a recession for it work?

The screws need to tolarate weather. Maybe A2 or brass is OK?

At least the first enclosures will be CNC machined, since the mold cost approximations I have got have been 5 000-10 000 €. Is that expected?

jmaja:
Would even tapping M2 threads to this 2.2 mm thick half work? Obviously not anymore, if the enclosure will be molded.

Skashkash:
Since we are throwing ideas out there.

Impulse seal it in pouch?

Like these marker lights (as an example)
https://countycomm.com/collections/all/products/maratac-mas-micro-adhesive-strobe-3-pack-u-s-a-nsn?variant=20534147317830

May want a top stiffener piece of pcb or plastic on top of circuitry. Extend ends and punch mounting holes like a luggage tag.

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