Electronics > Manufacturing & Assembly

tough sheet dielectric material?

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coppercone2:
What is a good brand of tough dielectric material to cover a chassis segment?

I use double sided sticky high temp silicone glue kapton sheet. but its still kinda thin, also expensive

Is kapton the best material for this? I mean something that can armor it a bit from getting banged by a loose transformer or something like that. like in the inside of a welding machine

Kapton thick is only 10 thousanths or so usually. I mean something thick like 30 thousanths and pre adhesive lined (sheet adhesion always comes out like shit), and durable for high temps. Maybe kapton can go on top of this material that acts more like armor then insulation..

Kean:
Look for "Presspaper", "Presspahn", or "Elephantide".  This is very often used with transformers.  It is made from cotton, and can be quite thick and is similar to heavy cardboard.  Not sure of its temperature rating, but I think I've seen it rated for 100C up to 150C.

Another common insulating material is polypropylene, e.g Formex insulation.  https://www.itwformex.com/

T3sl4co1l:
Vulcanized, "hard", or "fish" paper is a very old-fashioned solution, but it is indeed still available; there's even a UL94 classification for it:


--- Quote ---HB: slow burning on a horizontal specimen; burning rate < 76 mm/min for thickness < 3 mm or burning stops before 100 mm
--- End quote ---

which based on my specimen from McMaster, they've formulated it pretty accurately.

Needless to say it's just cellulose (or, the "vulcanized" type is supposed to be rubberized -- there are different products under similar trade names, beware what you're actually getting!), so is class A insulation, and generally frowned upon these days.

Polyester film/sheet (Mylar(R)) is probably the most common material, often with a rubber or acrylic adhesive, for example lining the inside of desktop PSUs.  Polypropylene may also be used, depending on insulation system and etc.  PET is good up to over 100C, give or take grade, required strength, possible shrink rate, etc.

Polyimide (Kapton(R) is the brand name) is indeed quite strong stuff, extremely high temperature rated as well, as polymers go; but on the brittle side, which may be problematic.  (There was a time when aircraft were being wired with the stuff; after a number of high-profile failures due to fatigue, abrasion and cracking, this was phased out.  It looks like a lot of space hardware still uses it though, or at least as a wrap if not as direct insulation, and that's probably fine for something floating in space without any bumps after it's orbited.)

If high temperatures and working voltages are required, consider something like resin-bonded mica paper.  Microwave oven transformers regularly use(d?) it, soldering irons use it (give or take the resin, or which kind), heaters use it, it's great.  Brittle obviously, but at such temperatures, you are forced to compromise on toughness.  Other ceramics are an option as well (porcelain, steatite, Al2O3, etc.; mostly moldable/machinable when green/unfired, but machinable e.g. Macor(R) products also exist, or, good old grinding if you must).

Other resin-bonded products; phenolic paper (cellulose based); fiberglasses with polyester, phenolic, epoxy or silicone resins; bakelite itself is still available and used for many things (phenolic resin with fillers, molded; just not with long-distance paper/textile fibers); etc. Sheets are available of course (e.g. Garolite and other brands).  Downside, being rigid materials, they aren't as usable as a lining or wrapping material.

Tim

coppercone2:
hmm that is some stuff to look at.

The idea is lets say a transformer dismounts, and its connected to wiring. The transformer edge hits the chassis as it falls off the table and then wiring can touch that area. Lets say like a 3x3 inch transformer


I have a feeling 10thousanth kapton (even though the sheets I find are only 0.002-0.003) won't be enough. I only see tape in the 0.01, I wanted sheets.


What kind of thickness of material is required so that a corner hit from a height will protect it electically?


My my little power supplies I have been making (94% efficency AC/DC converter + fuses, fan) I am putting kapton in the area under the PCB and on the lid bottom. Its all grounded but it can't hurt. Say a part desolders. The kapton is rated for 300C. But I see in the welding machines there is like more of a thick sheet glued to the chassis. I doubt its as high temperature rated, but it looks relatively puncture proof.


I forgot about polyester. That might be the best bet, I think it glues OK. I thought the bigger temp is better because it might prevent damage from arcing and stuff (harder to destroy it).



Getting thin strip of garolite or bakelite I guess is also an option. But I have a feeling their too stiff and it will pop off the chassis after a impact, the plastic might conform to chassis dents. This is a limiting factor of maximum thickness IMO.

ajb:

--- Quote from: coppercone2 on April 24, 2024, 04:47:12 pm ---The idea is lets say a transformer dismounts, and its connected to wiring. The transformer edge hits the chassis as it falls off the table and then wiring can touch that area. Lets say like a 3x3 inch transformer
--- End quote ---


--- Quote --- Say a part desolders. The kapton is rated for 300C. But I see in the welding machines there is like more of a thick sheet glued to the chassis. I doubt its as high temperature rated, but it looks relatively puncture proof.
--- End quote ---

You're concerned with the broken-off transformer or some self-desoldered component damaging the internal insulation of the enclosure, and *then* some wiring coming into contact with that specific now-uninsulated spot on the enclosure?  That seems like a rather convoluted failure method to be concerned with, and one that implies the device is probably in a bad way already before the potential short to the housing. 

Usually you see the relatively beefy polyester or similar sheets where there is high voltage/energy and a relatively hostile environment (ie welders), HV clearance is marginal to begin with, and/or to provide shielding where someone might have the device open for servicing while powered on (and it's not practical to make everything in the interior touch-safe).  Plastic sheeting is dirt cheap in volume, even cut to custom shapes, so it's pretty cheap insurance for commercial goods.  Unless your power supplies are intended to take a real beating on the regular, it doesn't sound like you really need to worry about it. If you have high voltages inside, then a non-metallic or grounded enclosure is important for user safety in any case.  Additional internal insulation wouldn't hurt, but if you've got wires at mains voltage or whatever flopping around inside of a box with a bunch of other electronics, then there are more potential failure/hazard modes than just those wires coming into contact with the housing.

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