Plastics not subject to viscoelasticity (cold flow)

Question

Plastics not subject to viscoelasticity (cold flow)

bcavender (Electrical)

(OP)

31 May 18 17:28

I have been asked to find an economic, machinable plastic (or dielectric) for a continuously loaded application (20 year life) at a stress of appx 0.1 MPa at a max temp of 50C. While not a high speed or high energy situation, it is a rotating, close clearance application and the material cannot expand from its original dimensions without creating a potential frictional failure that could generate unacceptable heat.

Initial stress here isn't great, but I have read that 'cold flow' over a long time period can become a significant percentage of the initial geometry. The information I have been able to pull up so far is very broad and gives me little confidence about selecting a material. Not being in materials science, my ignorance of creep is probably has me looking in all the wrong places.

Can anyone shed some light about the cold flow being largely a thermoplastic problem (if true at all) and/or recommend a different source of viscoelastic data that I can use to find a plastic NOT subject to this long term creep phenomenon and point me into a productive direction?

All comments welcome!

Best regards,

Bruce

btrueblood (Mechanical) · Answer 1 · 2018-05-31 17:35:13

Hmm. In general, if the absolute temperature is more than about 80% of a materials melting point, it's subject to creep - viscoelastic behavior. Your description (2nd sentence) though seems to say you want a low expansion coefficient? That sounds more like a ceramic to me.

Compositepro (Chemical) · Answer 2 · 2018-05-31 19:19:49

Fiberglass/epoxy laminates are generally used for your type of application. The same as used for circuit boards.FR-4 or G-10.

https://www.mcmaster.com/#grade-g-10-garolite-shee...

bcavender (Electrical) · Answer 3 · 2018-05-31 20:01:10

btrueblood,

Machinable ceramics are very interesting. Temp/chemistry/strength/zero creep seem to easily be right there.

Macor from Corning looked perfect for this app, until I saw the price of $2500 for a 12" long round. Just started studying this area, but most sites I have run across are serving the high end academic or military complex and not general commercial.

Do you have any experience with sources for machinable ceramics in a workable price range?

Thanks for the comment!

Bruce

bcavender (Electrical) · Answer 4 · 2018-05-31 20:34:11

CompositePro,

Garolite looks to fill the bill technically/economically, but requires harder tooling and makes a good bit of tiny grit that is hard to get out of flood coolant when machining. Hard, maybe even damaging, on the CNC if you are going to do much of it. Need a material I can get into the 0.001" per tooth chip range so I can be well sure to filter all the cuttings out when milling.

Very close! Thank you!!!

Bruce

Pud (Mechanical) · Answer 5 · 2018-06-01 10:55:43

Torlon (PAI) has a low cte for a plastic. It's also cross-linked so good creep characteristics. Can be bought as bar stock - expensive but not as ceramic.

Torlon

www.tynevalleyplastics.co.uk

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btrueblood (Mechanical) · Answer 6 · 2018-06-01 14:54:40

Mycroy/Mycalex, Aremco are a couple of names.

http://www.cotronics.com/vo/cotr/cm_machinable.htm

Look into sintered boron nitride also.

None of them are cheap, per se. All of them are brittle, and may shed particulates if they are in sliding contact with other stuff (but so will filled plastics). And all of the machinable ceramics (save BN) can give wear/abrasion trouble; machining dry, with a vacuum cleaner nozzle close to the cutting tool, is a good option.

Boron nitride, fwiw, forms a graphite-like film when mixed with water. Neat stuff.

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Plastics not subject to viscoelasticity (cold flow)