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Polishing Acetal/Delrin and other plastics

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Mike Donnerstag01/11/2019 18:38:42
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101 forum posts
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Can anyone tell me the best way to polish plastics such as acetal once hand-turned in the lathe? I'm just turning some knobs for the Myford lathe.

Many thanks,

Mike

Ian Welford01/11/2019 18:51:58
283 forum posts

sand it from 180 grit through to 600 preferably with steam ate lubricated paper or ABRANET at slow speed in the lathe .Then wet sand with wet n dry upto about 1200 grit, follow with metal polish eg t cut if you want a really gloss finish.

doesnt take long with each grit but wipe the work between grits with. damp paper towel to remove dust and possible grit parictles before using the next grit.

Mike Donnerstag01/11/2019 19:05:37
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101 forum posts
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Many thanks Ian. Just the advice I needed. I’ll give it a try.

Mike

JasonB01/11/2019 19:45:28
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16530 forum posts
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I use Micro-Mesh to finish Clear acrylic to a glass clear finish.

As turned

Polished inside and out

Edited By JasonB on 01/11/2019 19:50:24

Ian P01/11/2019 20:03:55
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2248 forum posts
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A lot of plastics like, acrylics (Perspex) and hard PVC are easy to polish, some like Acetal (Delrin) are more of a challenge and probably near impossible. I machine a lot of parts out of black Acetal and have not found any practical way of making even a very fine machining marks disappear.

Jason, the ER collet in your pictures look a tad unusual in that the gaps between the tines look unequal, is it just an optical illusion?

Ian P

JasonB01/11/2019 20:14:38
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Ian, I think it is just teh angle of the photo, will check tomorrow and report back if it is something else.

I wonder if flame or chemical polishing would work one some of these plastics like it does with perspex

Alan Wood 401/11/2019 20:46:29
130 forum posts
4 photos

Slightly relevant but Polywatch is a compound for polishing scratches out of watch faces and is particularly good. It appears to fuse the material over the blemishes. Think it is on Amazon.

Ian P01/11/2019 20:50:15
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2248 forum posts
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Acetal is quite a low friction slippery material a bit like PTFE, filing or abrading with even the finest wet and dry just leave a matt, white-ish surface. I'm not sure what temperature it melts at but I have never heard of it being flame polished.

Most, maybe its all, commercially manufactured Acetal components have visible signs of the machining so I have just assumed its not economic or possible to polish.

Ian P

Derek Lane01/11/2019 20:56:45
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236 forum posts
56 photos

Like Jason I use micro mesh to 12,000 grit sanded wet, also another method I have used is sand with wet and dry to 1200 grit and then apply burnishing cream. I get a good result with either method.

I have heard of people using T Cut to polish as well as brasso

This is one done with micro mesh

 

dscf5276-001.jpg

Edited By Derek Lane on 01/11/2019 20:57:38

Edited By Derek Lane on 01/11/2019 20:59:07

Cornish Jack01/11/2019 20:59:20
954 forum posts
134 photos

Final gloss finish using Jeweller's rouge, perhaps? or, for cheapskates like me , toothpaste. Won't do for anything other than the very fine stage. Use to use it for watch glasses.

rgds

Bill

Paul Lousick01/11/2019 21:59:33
1212 forum posts
502 photos

Toothpaste is a fine abrasive and often suggested for cleaning and polishing the outer glass on car head lights.

Paul.

Mick B101/11/2019 22:15:47
1241 forum posts
70 photos

I've been machining black and white acetal for years and it's never occurred to me to polish it - I've always thought the machined surfaces looked fine. If the tool's sharp and nicely stoned on the edge, you can get a very attractive finish.

Edited By Mick B1 on 01/11/2019 22:16:16

Sam Stones01/11/2019 22:21:30
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663 forum posts
255 photos

The two basic families of materials being discussed here are either crystalline or amorphous.

I'd like to suggest (although I haven't tried), that polishing items of crystalline materials, e.g. polyacetal, Nylon, polyethylene, polypropylene, etc. need to be deep frozen, or at least taken below their glass transition temperature.

I have machined (trued up) with some success, polyester (rubber) skate wheels directly from the freezer.

I also question, during the abrasion and polishing, if the surface of crystalline materials is being continually 'pulled apart', thus introducing (let me call it), a broken surface. Someone with good microscopy (and photographic) skills might like to intervene here.

Acrylic, and other amorphous materials on the other hand, are (generally) a breeze to polish.

Good luck,

Sam

Ian Johnson 101/11/2019 23:12:30
174 forum posts
50 photos

Try flame polishing with a Butane/Propane torch, just waft it carefully over the surface and it will leave a very pleasant looking satin/gloss finish. It also has the effect of making the surface more 'tactile', so knobs are ideal for this technique as all the edges are smoothed over and machining marks are erased. I've had good results with black Acetal but don't linger with the flame or it will melt, burn or bubble.

Ian

Sam Stones02/11/2019 00:05:21
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663 forum posts
255 photos

And ... don't sniff the smoke if you happen to overdo the flame treatment of polyacetal. You'll know why (and never forget) if you do.

Other than this (after a bit of practice on something which doesn't matter), I'd go along with Ian's 'flame polishing'.

It's also a useful (actually a commercial) technique for oxydising (oxidizing US) the surface of a crystalline polymer (plastic) prior to painting.

Incidentally, there are many grades of amorphous plastics which are blended or otherwise modified with other materials, rendering them less polish-able (Ugh!)

Sam

Play safe

Ian Johnson 102/11/2019 01:17:15
174 forum posts
50 photos

Here's a mobile phone holder with a fluted screw I made a few months ago. It's made from black Acetal, and flame polished. Although some machining marks are visible because of the hand ground ball cutter, I think it looks pretty good, and it feels nice to hold, it's weird how Acetal feels different after flame polishing.

20190820_143440.jpg

Oh and like Sam says don't breath in the fumes if it starts burning, it's probably Phosgene or something equally horrid!

Ian

Sam Stones02/11/2019 01:47:32
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663 forum posts
255 photos

Nice work Ian, and I trust you didn't mind my interjection re the fumes. I'm no chemist, but I think burning POM liberates formaldehyde.

Compared with nylon and polypropylene, I much preferred machining p.acetal.

Sam

Sam Stones02/11/2019 02:02:12
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663 forum posts
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I didn't have this picture loaded in my albums, hence this delayed follow up.

This device for testing rotary switches features various uses of polyacetal. Black for bearings, and I think there's a slab of white with all those 'spikes' poking through. It may however have been high density polyethylene but I can't remember.

Sam

crw_6651---eaton---02.jpg

Circlip02/11/2019 09:24:37
983 forum posts

For Acrylics, solvent vapour polishing works.

Regards Ian.

Ian Welford02/11/2019 10:09:43
283 forum posts

I have the micromesh system and use it but for making a few knobs it seemed a bit OTT so hence the advice given. It shou”d have said STEARATE lubricated paper, fingers faster than brain as usual !

Circlip, I have no ever heard of sovent vapour polishing, please elaborate on that one !

regards Ian

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