Machining Plastic - Nylon Acetal UHMW LDPE HDPE machining qualities?

I know nothing of Acetal or UHMW (until reading this message, not even the fact that they existed) but I can tell you with certainty that the UV resistance of PE (Either HD or LD "flavors") is pretty much nonexistent. They'll both "brittle up" and start to crumble in short order. Depending on the thickness, coloring (if any), exact amount of exposure, intensity of the UV source, and so on, they may start showing signs of problems in just a few hours, or they may stand up for a few days, but they *WILL* disintegrate into a worthless scattering of plastic dust in far less time that what you're likely to call "acceptable" for your application.

Nylon also gets a bit weird when left in the sun, but not in every case, so I would presume there are variations (probably from additives aimed specifically at altering UV characteristics) within the overall type-designation "nylon" that have at least some UV resistance.

offrey W. Schultz" wrote: I know nothing of Acetal or UHMW (until reading this message, not even

It depends on the additives. I've got an around 10 year old poly-tunnel, which is ~0.5mm polythene sheet covering aluminium hoops. Works well, the plastic has not become brittle enough to tear even in relatively high winds.

My choice from these would be UHMW. Best machining of the group and at least in my very limited experince doesn't seem to break down when used outside.

Use a VERY SHARP HSS cutter and a cold air gun if you have it or an air hose if you don't. The problem here is melting on the cutter, so slow the rpm's down if you get that. All plastics don't hold their shape well, UHMW is no exception, light cutting and clamping forces help. Also water can absorb and change dimensions on UHMW, I think.

My knowledge here ain't perfect. Maybe others would care to comment.

Karl

Best plastic for a bushing would be Nylon or Delrin.

"Polythene"? (Tries to figure out why that rings a bell... And fails... I've heard of the stuff before, but I can't remember where or in what context)

Idunno what you've actually got, but if it has survived 10 years in sunlight, I'm certain beyond any possibility of doubt that it's something other than Polyethylene.

Whether high or low density, polyethylene is notorious for disintegrating in sunlight - 6 months, tops, is all the longer a piece, especially a thin sheet, of it can be reasonably expected to last when directly exposed to sunlight. If it lasts beyond that, it's a safe bet that, whatever it is, it ain't PE.

I'm not convinced either of them would be the best plastic, but each of them are available as bushing material and would be a good choice, assuming either variety that was chosen was of that nature. Nylon so found is black in color as I recall, and has graphite included in its makeup for lubricity. Delrin is available with Teflon included (Delrin AF), which makes it a superior bearing over typical acetyl, but I have no clue about either's ability to resist UV, which, for sure, is death on white acetyl plastic, albeit a slow one in some cases. Delrin AF is a light milk chocolate color and is a pleasure to machine. Likewise, the black nylon machines better than the ivory colored stuff. Apparently the graphite helps lessen the stringiness for which nylon is so well known. If the piece in question can be covered by anything that would shield it from UV, I'm of the opinion that material choice would not be so critical.

Harold

Delryn, aka acetal, machines very nicely. Don't know how UV stable it is. It (and nylon) will absorb moisture a little with consequent slight dimensional change.

Another possibility might be Noryl.

Whichever you choose, the material will need to have a UV stabilizing additive. Generally this is simply carbon and turns the material black. I would probably choose black UHMW (some people call it UHMWARCB for Ultra high molecular weight abrasion resistant carbon black, but that's a mouthfull). This assumes that the piece is a bushing and takes no actual loads itself, except to fill a gap. UHMW will tend to cold flow a little when it gets hot and is used as s structural member of some kind. Machines beautifully though.

Delrin (acetal) is probably a good choice too. Delrin is often used in bearings (as is UHMW) but tends to be slightly higher in yeild strength when warm so may be more structurally stable. The negative about delrin is it can absorb some water and swell and can also crack. UHMW is almost impossible to crack (until it breaks down from UV)

koz

Geoffrey W. Schultz wrote:

Balance it more carefully.

Are you _sure_ that's the problem? This is a support pipe, not a running fit, right? Oscillations are supported by springy supports, not by loose fitting ones.

If that's what you want to do we can help.

Use the web engine to find your stock, then show the catalog page for machining data there.

Acetal is Delrin, right? That's the one you want. All the others melt during machining. Delrin is pretty stiff and strong. A good all around engineering plastic. UV grade is black.

Yours,

Doug Goncz (at aol dot com) Replikon Research, Seven Corners, VA

I think it's a more commonly used in the UK synonym for polyethylene.

It is, it's all in the additives. Polythene tape of a similar thickness is translucent, "white", and degrades in a few months. This has a fairly pronounced yellow cast, due to the UV blockers.

For UV resistance select from........

Abs Acetal (delrin) Noryl Nylon PBT Polycarbonate Glass Filled all above in color black

PEEK - if you have LOTS of money, ie 1993 per foot, 2" dia, $231.88, unless you would like a "sheet", which is only 12" x 48" x 2" thick, sit down........$7163.20 Acrylic (UF-3, UF-4, Solar Shield) Polycarb (Tuffak XL, Hyzod SR) Poly-Teak Board PVDF (Kynar) Starboard Starlite 6000 UHMW Ultem 1000

source was a Laird Plastics selection guide

Someone else can please narrow down the other factors.

michael

Hey Geoff,

Have you considered Micarta or Bakelite? Works pretty well, and machines OK too.

Take care.

Brian Lawson, Bothwell, Ontario.

If you select to use Nylon you should slit that bushing lengthwise. Moisture and/or temperature changes will otherwise change the fit and could grab the shaft. The slit allows for expansion. HTH

Based upon input from this forum, rec.rec.boats.cruising and all of the pointers that were provided, I decided to go with black UHMW. As described on some web sites it's the "poor man's teflon" with a very low coefficient of friction, good UV stability and machines well. On top of that, a 3"x1' piece of rod UHMW is only $11. I ordered it yesterday from MSC and it'll be here today.

I've opted to simply machine the UHMW to form the sleave bearing surface rather than purchase a sleave bearing and mount it in the housing. I'll let you know how it turns out,

Thanks again for all of the help!

-- Geoff

John wrote in news: snipped-for-privacy@rogers.com:

Getting back to METALWORKING ...

Is there a *reason* you're not making this out of bronze or steel? Just because there was a plastic piece there *before* doesn't necessarily mean you gotta have one now.

It seems that about half of you wear concerns and UV deterioration problems would go away with a good bronze or steel sleeve bearing. (Might help solve the vibration issue too.)

My two cents.

- Carl

The most critical factor is a scary sharp cutter. Certain carbide inserts work quite well if they are uncoated and of the positive/negative type e.g. TNMP-321. You can hone these on a fine diamond hone. Otherwise, a carefully honed HSS cutter.

I would choose one of those.

Could you avoid the UV resistance problem by placing a thin walled stainless shield over the bearing area? Attach it to the mast but have it not in contact with the bushing.

Ted

This is going on a boat in salt water. Steel would rust and there would be a big glavanic corrision problem at the stainless/bronze interface.

-- Geoff

"Carl Hoffmeyer" wrote in news:CJRsb.57482$ snipped-for-privacy@bgtnsc05-news.ops.worldnet.att.net:

Actually, Geoff, it used to be that good turnbuckles were made with stainless screws and bronze bodies (or vice versa). The reason is that stainless galls badly against itself (as many of us know, although we may not know the word for it other than, "!#$%&*?!" .) Stainless against bronze runs freely and happily and is a much better quality device. Unfortunately, they cost more and looked strange, and are now, I think, gone from the market.

Galvanic doesn't become a real issue unless the two pieces are immersed. Above deck, there may be a little salt around, but it mostly gets washed off by the rain, I suspect.