Machining clear plastic and keeping it that way

I was going to say "acrylic", but then I realized that maybe I would be narrowing it down too much.
How hard is it to machine a part out of hard clear plastic and then make
it clear again? Any gotchas? What plastic should one start with? What question am I failing to ask?
The part will be cylindrically symmetric (i.e., turned on a lathe), about 1.5" diameter, 0.5" tall, and accuracy can be as sloppy as 0.01 in all directions. But it would need to be "pretty".
Some plastic that is a bit more rugged in impact than acrylic would be nice (do they make crystal clear Delrin?) but I can't think of anything like that which would actually work in this case.
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Tim Wescott
Wescott Design Services
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flame polishing is one way to make hazed plastic clear again, but it depends on what you're working with.
Another way is to use a very dissolvable plastic and dip it in solvent and let it dry. This works with acrylic and the stuff they make clear screwdriver handles out of. I seal my name into handles like this with acetone.
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On Tue, 05 Nov 2013 12:26:16 -0600, Tim Wescott

cracking is the biggest issue i had doing this. I "made a sandwich" with AL above and below, clamped together. Don't know all the names for sure, I think it was called lexan.
Only did it once, but it came out nice on my second try.
karl

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On 6/11/2013 2:52 AM, Karl Townsend wrote:

I think Lexan is polycarbonate.
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On Friday, November 8, 2013 11:22:18 PM UTC-5, Glenn B wrote:

Yes, it is.
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On 11/5/13, 10:26 AM, Tim Wescott wrote:

I really have no plastics experience, but once saw some guy cleaning up (what I think was) plexiglass edges with a propane torch.
It was obvious he'd had a lot of experience doing so, as they came out smooth, transparent and magically perfect like... he said they called it 'flame polishing'
Just did a YouTube search and got a bunch of hits, but on time to watch any now.
BTW, there is a polymer newsgroup... though the time or two I looked there was nothing but Crickets chirping there.
Good Luck!
Erik
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Erik pretended :

Flame polishing with a small propane or similar torch is fairly easy. Practice on a scrap first. It is easy to set the edge on fire.
Polycarbonate (Lexan) is NOT suitable for flame polising it bubbles and cracks. Acrylic is best.
--
John G



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From what I have read the biggest thing is to get the feed and speed right. I don't think any method of cutting is going to leave you a perfect finish, but cutting with feeds to slow for the RPM is going to leave the hard white stringy melted surface that you hate. I've not done any "clear" plastic, but I occasionally cut stencils out of PETG photo mounted to a sacrificial backer. On my little CNC router I turn the spindle speed down to about 18,000 RPM, and scream along with a fresh sharp down cut carbide router bit at a very high feed. Usually around 100 IPM with my smallest bits and even faster with larger ones. Since these are usually one offs and not frequent I have not played with getting it exactly right, but I've found in general the faster I feed (and the fresher the cutter) the better my finish.
I think my approach would be to define the types of cuts first. Cutting a stencil is certainly going to be different than cutting a mold, and both would be different from cutting a part with all outside profile machining. Then I would wander over to CNCzone and ask some of the sign makers for some help with the speed/feed for your chosen plastic material.
After you have figured out how to get the best possible cut to begin with then I would look at ways to improve the surface if desired.
<Tangent Mode On>
... And after I had it mostly figured out but before I set it up I would also consider other approaches for atleast some types of cuts. For instance if I was making a 3D clear bust I would be more likely to consider making a mold and selecting a nice casting resin rather than machining the bust itself out of clear acrylic. I know some folks will freak at that last one because they are mostly familiar with the thermal issues of thick pours of epoxy resins (usually not good because most yellow when cured) and most more common poly resins, but there are resins designed to be poured thicker. There are also techniques like filling a mold and then emptying it leaving a thin layer of resin to form a thin shell in the mold.
There are so many other things that can be done when it comes to plastics and resins that it really depends on what you are making, how it will be used, and how many of them you need to make. I bet there are even clear plastics that can be blow molded economically.
<Tangent Mode Off>
Bob La Londe
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Tim Wescott wrote:

Plexiglas can be polished easily after machining. Jeweler's rouge or similar stages of roughing and finishing compound, then a final pass with toothpaste to remove any red tint left behind.
If you want pretty and clear, you can't do better than cast acrylic, it is as clear as glass, maybe even more so.
Jon
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On Tue, 05 Nov 2013 14:21:53 -0600, Jon Elson wrote:

I'm considering casting, although at this point I just want onesies, not any great quantities.
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Tim Wescott
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On Tue, 05 Nov 2013 14:29:05 -0600, Tim Wescott

Acrylic stock shapes are available either cast or extruded. The cast stuff has better optical properties.
As for tough clear plastics, polycarbonate (Lexan) is the most obvious alternative to acrylic, but more difficult to polish.
--
Ned Simmons

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Tim Wescott wrote:

No, the stuff you buy in blocks is "cast", not meaning that YOU do the casting, but the thing is cast to net shape. It is allowed to set up over some time, and when done right is very clear and uniform. You can also get extruded acrylic, it is not as clear, and has a slightly wavy surface. It could be polished after machining, though. But, the extruded stuff is usually tubes and rods with fairly small cross sections.
Jon
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Tim - I drilled the center of a piece of epoxy bar stock about 5/8 in. thick. Then a brass weight was glued with epoxy to the bottom and a piece of paper with the scale for a hydrometer was placed inside. Then a thin top was glued on with the tiniest gooping onto the the first number of the paper scale. Suprizingly, it looked noticebly better than the others.
Hul

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A good solvent to clean up the interior void would be glacial acetic acid. It melts the acrylic and then it hardens with a smoother surface. A couple of droppersfull would do a small volume like this. Must be "glacial"(99%), not "normal" or vinegar(~5%)
http://www.grainger.com/Grainger/LABCHEM-CHEMICAL-GLACIAL-ACETIC-ACID-AMERICAN-8EL40?gclid=CKLOyLr_0roCFQyg4AodTXoAHA&cm_mmc=PPC:GooglePLA-_-Lab%20Supplies-_-Lab%20Chemicals-_-8EL40&ci_src588969&ci_skuL40&ef_id=S-1LhkNIYWUAAEudcyoAAAMA:20131107153158:s
The thinnest Weld-on products would work too.
(Amazon.com product link shortened)
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Just a heads up to anybody using glacial acetic acid- it's really nasty stuff, mostly due to the vapors. It's more irritating than even strong ammonium hydroxide solutions. Good ventilation or a respirator is a must, not a suggestion.
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On Tue, 05 Nov 2013 12:26:16 -0600, Tim Wescott

just sand it with progressively finer finishing paper and polish. Made a couple yoyos with the young french exchange student we had with us 15 or so years ago out of clear acrylic and they are just like glass
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If you want the face clear, screw a piece of half inch thick plexi to a wood covered faceplate, cut a circle, and buff the edge
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On Tue, 05 Nov 2013 12:26:16 -0600, Tim Wescott

Two things that might be of use if you go for acrylic. Little experience Im afraid practically. 1. I read long ago that you must not use any oil based contact in your machining or polishing so that includes most polishing (eg. buffing) compounds, machine oils etc. - it leads to those hairline fractures not immediately but eventually, especially in stressed areas. 2. Chloroform is a proper solvent for things like perspex. You can make perfect joints with it, I think. Though not easy to obtain cheaply, used to be available in chemists here in uk but no longer! ...C+
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Tap suggests a mapp torch in this video, but well, they don't make mapp anymore. video has other tips for acrylic work
http://www.tapplastics.com/product/repair_products/plastic_adhesives/tap_acrylic_cement/130
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