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Machining plastics

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bfk

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#1
I'm looking for tips on machining plastics. I know acetal is said to be easy to machine, but what little things will make it easier, or less expensively destructive?
And how about UHMW, nylon, or even HDPE. I suspect that there could be a problem with melting, but I'd rather benefit from your experience than end up with a bunch of tools with baked on plastic.
Part of the reason is that I mentor the high school robotics team in town and they now have a mill (LMS 3990) and an Emco Maximat 7 (gifted from the university, and needing checked out), and it might be safer to have them experiment on plastic rather than metal as a first step. But I might be totally wrong about that.

Brian
 

KMoffett

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#2
One issue that you might run into with slick plastics, is material holding.

Ken
 

mmcmdl

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#3
Flood coolant is your friend . Or just blast air or even a spray mist to clear your chips . If you're trying to feed too quick for your rpm's , you may run into trouble .
 

Metal

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#4
Another thing I've found is uncoated, really sharp tools with one flute (on the mill) really does the job
You need to manage the long, long, long, stringy chips so they don't tangle up on everything though
 

GoceKU

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#5
I've machined plenty of nylon and is not difficult at all, lots of chips, i've even used worn out brazed carbide tools still cuts well, sharp radius HSS tool leaves best finish.
 

ch2co

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#6
I guess I never thought about it. I just blindly started with some nylon and it cut pretty well and then tried delrin and was very impressed
with how easy it was to mill or turn. No special tooling, just watch out for taking too deep a cut because it will melt on you. Neither tend
to produce chips, just long tangling threads that get wound around everything.
 

Ulma Doctor

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#7
delrin is forgiving with a wide range of tooling and is just beautiful to machine on a lathe or mill.
if you work delrin too hard or generate excessive heat in it's working, formaldehyde will be released from the acetal and a strong smell will be noticed.
i have used sharp and well radiused tooling with great success.

Nylon is similar but sharp generously radiused tools seem to finish nylon nicely.

i don't have enough practical experience to share an intelligent opinion on tooling for HDPE or UHMW.

sharp tooling is a good thing for most plastics in my experience
 

Asm109

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#8
Once you try machining HDPE you will never want to do it again. It just doesn't cut clean.
 

mksj

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#10
Similar experience to Ulma Doctor. HDPE or High Density Polyethylene, there are lots of cross linked (high density) variations, often seen it in medical practice as a mating surface in joint replacement UHMWPE and HXLPE. Very durable and tough stuff. I mostly have machined delrin on the lathe and mill, using very sharp uncoated cutters at fairly high speed. Comes out with a very nice finish. Polyethylene is softer and deforms more, and may be stringier, you also may get more deflection in the work piece and if you advance the cutter too quickly at low speed it could catch. I was planning on machining some HDPE for mill covers on either side of my vice, so will see how that machines.

I also would be very careful of turning some plastics on a lathe, I recall one case were an individual was practicing on some heavy wall PVC pipe, it deformed while turning and got caught up on the cutter, made a real mess. Also some plastics like acrylics will shatter and also the heat from the cutter will melt the plastic and can be catastrophic. Lexan or polycarbonate will not shatter, but it requires the right cutters and speeds. My vote would be for delrin using very sharp cutters, I recently did some work with delrin and the finished milling was absolutely smooth.
 

richl

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#11
Only 1 experience with UHMW, and that was today. In the mill I was making a couple way wipers for my lathe, I cut them out in the bandsaw and just cleaned up the edges in my mill. 1" cutter, very sharp American end mill cobalt, no coating, cutting around 230 rpms, slow feed. It cleaned up the edges well, not a mirror finish, but for what I am doing it will be fine.
As mentioned by others, they do not hold well in a vise.

Ymmv
Rich
 

Wreck™Wreck

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#12
Your biggest problem will be chip control and deburring, I just finished 1700 ABS parts in a lathe today which was a miserable mess of chips that will not break. Push the feed rate as hard as possible in an effort to make the chip as thick as you can.

With UHMW Polyethelene all bets are off, this is a messy material at best.
 

rzbill

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#13
I don't care for machining any version of Polypropylene or Polyethylene. I do it when I have to. Their modulii are very low ( think wax ) which means they move under cutting loads. Drilled holes always shrink and one has to have a good grip in the part or the cutting tool can yank it out of the fixture. The molecules for both of these are very long and "smooth" so they slide easily against each other (think worms). This is unlike delrin and nylon that have a much bulkier and branched structure (including some molecule to molecule crosslinking) that inhibits molecule to molecule motion. (think trees packed tightly together). As a result, nylon and delrin have a higher modulus and are able to be used at higher temperatures.
 

bfk

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#14
Many thanks. Unless we can score some cheap Delrin or nylon, I think maybe we'll stick to 6061 which we can get from a local manufacturer who has offered us access to their drops. Sounds like the potential for mess, and/or flying plastic isn't worth the effort.
 

JimDawson

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#16
I have machined a lot of HDPE/UHMW with no problems, cuts like butter. No coolant needed, but use air or a shop vac to remove chips. Use razor sharp tools, and turn slower than you might think is correct and keep the feed rates up. Ignore what the machining charts say about feeds & speeds. I treat it a lot like like aluminum. Woodworking solid carbide router bits work great. The best is Onsrud 0 flute bits, designed for cutting the stuff. You want to cut, not rub the tool bit on the work. The biggest problem is generating heat caused by rubbing, this is where turning slower comes in. An end mill will take a pretty big bite in most plastics and that's what you want. Turning too fast just welds plastic to the tool bit, this is true for all plastics

Acetal machines like a dream, but it's not cheap. Again, razor sharp tools. Acrylic and polycarbonate are also easy to machine. You can use kerosene on acrylic, but the best I have found for polycarbonate is dish soap and water. Petroleum products can cause crazing on the cut edges of polycarbonate.
 

crazypj

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#17
Surprised no one mentioned Teflon?
It's OK as long as you don't overheat it, then you get really toxic gas produced (cyanide or something?)
 

rzbill

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#18
Should be Fluorine since that is the only atom other than Carbon and Hydrogen in Teflon (PTFE or PolyTetraFluoroEthylene).

As you already know, it is soft, like Polyethylene. The reason is the molecule. PTFE is essentially Polyethylene with some Hydrogens removed and replaced with Flourine. The long snake-like molecule structure is the same for both.
 

crazypj

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#19
OK, sound good to me. Must have been some other plastic as I was doing prototypes last 3 months I worked for Tuscan Engineering.
I do remember general manager, foreman and charge-hand warning me about cyanide gas being produced from some stuff I was machining. (hey, I was 19~20 and already had the 'crazy' nickname ;))
I did wonder if I was posting about the right material as I know Polyflurotetraehelyne shouldn't make cyanide (my brother was working for a plastic supply company and could get hold of any spec sheet)
 

Metal

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#20
I have machined a lot of HDPE/UHMW with no problems, cuts like butter. No coolant needed, but use air or a shop vac to remove chips. Use razor sharp tools, and turn slower than you might think is correct and keep the feed rates up. Ignore what the machining charts say about feeds & speeds. I treat it a lot like like aluminum. Woodworking solid carbide router bits work great. The best is Onsrud 0 flute bits, designed for cutting the stuff. You want to cut, not rub the tool bit on the work. The biggest problem is generating heat caused by rubbing, this is where turning slower comes in. An end mill will take a pretty big bite in most plastics and that's what you want. Turning too fast just welds plastic to the tool bit, this is true for all plastics

Acetal machines like a dream, but it's not cheap. Again, razor sharp tools. Acrylic and polycarbonate are also easy to machine. You can use kerosene on acrylic, but the best I have found for polycarbonate is dish soap and water. Petroleum products can cause crazing on the cut edges of polycarbonate.
thats the key
If the cutter has touched metal, if it doesn't slice you at the slightest touch, it is not sharp enough.
I used a 1 flute 1/4" hss bright endmill on uhmw and it cuts /really/ well, universes different than just throwing one of my regular cutters on the mill.
Note that if you aren't climb milling your finishing pass you'll get these long flappy pain in the ass burrs
 

Steve Peterson

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#21
I cut a few small sheets of HDPE in my CNC router and it cut really easily. I was using a 2 flute 1/8" endmill at 16000 rpm and 40-50 ipm. Cut depth was 0.08" per pass. It could have easily handled a more aggressive cut. It produced small chips, no strings.

The only slight issue was a few drill holes where the drill pecked, but didn't completely raise out of the hole between steps. A small amount of melting occurred here.

The end mill had probably seen 10 hours of use cutting cherry, so it was not very sharp. Most people recommend sharp tools.

Steve
 
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