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View Full Version : Anyone with experience machining HDPE and/or LDPE?



Buckshot
12-12-2004, 03:47 AM
.......For a project in mind I've figured to use one of these as a bearing material. The plastic (refering to either one) will be pressed inside a possibly 1" ID steel pipe. Or it may need to be turned smaller to fit inside a 3/4" ID pipe. Right now the plastic is in a 1x12" bar.

Once inserted into this sleeve, the plastic will need to be bored to accept a steel rod. The steel rod will rotate back and forth maybe 90* repeatedly, but not fast.

This assembly will be used as a valve to start and stop the flow of a very heavy grease/waxy semi-solid. There will be possible artificial temps of maybe 130 degrees, and I have no idea of pressure but would guess maybe 40 psi?

The whole reason for the plastic liner type design is to stay away from high precision boring and honing of steel on steel surfaces. Ideally I'm hoping that an interferance fit can be made that will seal and still allow the inner (say, 1/2" dia) steel rod to rotate this short distance back and forth.

I need to know if the idea seems feasible for the application and any input on boring, turning or working with either of these plastics? I'm sure one requirement will be sharp tools http://bbs.homeshopmachinist.net//biggrin.gif.

The reason I bought these two materials was based primerily upon their price. There were a couple other much more expensive materials which also might be better?

Best,
Rick

Ian B
12-12-2004, 05:54 AM
Rick,

Polyethylene is a thermoplastic, ie. it gets soft when it gets hot - I have no idea how much load you'll be putting on the bearings, or what the bearing area will be, but if there's an elevated temperature, you may find that the plastic squeezes out of the pipe.

There are certainly more suitable plastics - Nylatron (MOS2 filled Nylon) is one. It's a good bearing material, it'll withstand 220F and it's easily machineable. Have a look at http://www.quadrantepp.matweb.com/SpecificMaterialNew.asp?bassnum=P1SM21&group=General for its properties.

hth,

Ian

Lee
12-12-2004, 06:39 AM
This type plastic will melt just cutting it with a saw blade. It has only fair properties with regard to friction.
You can do a search for plastic at Mcmaster Carr. It will produce all the types of plastic that they carry with its uses and properties there as well.

Other plastics places are US Plastics and Professional Plastics.

------------------
Lee

SGW
12-12-2004, 07:30 AM
I've done a little with the stuff. My main memory of working with it is the astounding quantity of chips that stuck to everything because of static electricity. I think I finally rigged up my shop vac to catch the chips coming off the end mill, to cut down on the blizzard.

But it machined easily, with a sharp end mill.

rockrat
12-12-2004, 09:19 AM
FYI - HDPE and LDPE will both change size on you after cutting. I normally cut the first and let it sit overnight to see where it ends up the next day. Then program the offset in the lathe to compensate.

If you need to make a choice on what type of plastic to use, you might try calling your local dealer. Our local dealer will give me suggestions on what others have used in similar situations. They always get me right were I need to be. And normally they are priced the same as McMaster, plus I can pick it up the same day.

As far as cutting the stuff, I pour lots of coolant on it. Keeps things from melting. Sharp tools. Too light of a cut and the chip wont break, turns into string and wraps, quite nicely, around the part just after the tool. Not a big deal but just not efficient.


[This message has been edited by rockrat (edited 12-12-2004).]

Spin Doctor
12-12-2004, 09:26 AM
I agree with Ian, go with the Nylatron

Evan
12-12-2004, 09:28 AM
I have machined HDPE and Nylatron, Delrin, Teflon, etc. HDPE is easy to machine BUT, it has between 7 to 10 times the linear cooeficient of expansion of steel. That means that when it warms it will expand .010 for every .001 of steel. A friction fit will be just that, a friction fit, lots of friction. Drilling plastics can be interesting as they are poor conductors of heat. The result is that the hole closes in on the bit, binding it. Boring it is no problem after the initial hole has been made but as said, wait for it to cool before measuring. I use isopropanol as a cutting lube. It evaporates quickly and carries away a lot of heat. It is readily available as gas line deicer.

I also agree that Nylatron is the best choice for this, unless you can obtain carbon filled nylon.

In this pic there is on the left Nylatron, in the middle is carbon filled nylon and on the right is HDPE. The carbon filled nylon is superior for applications that involve repetitive cyclical motion and lubricants such as grease. I have used it to make replacement swing arm bearings for a motorcycle, it lasted years.

http://vts.bc.ca/pics/plastic1.jpg



[This message has been edited by Evan (edited 12-12-2004).]

wierdscience
12-12-2004, 09:53 AM
I have machined tons of HDPE,it will work fine for your application,so long as you have a handle on the valve other than a knurled knob.

Even a .001" undersize will turn fairly easy with hand pressure if the handle is say 4" long.The properties of HDPE are similar to Teflon,but doesn't have the temp range or chemical resistance of Tef.

Razor blade sharp tools are needed to machine plastics period.I don't think shrinkage will be much of a problem for you since you making a seal and not a bushing.
You might consider one alernative design.

Chevron valve stem packings are availible in a variety of sizes and are cheap.All you would need is to cut a snap ring groove in the shaft on each end,load the chevrons stack a couple of wave washers on each end to provide sealing tension and snap the rings on.What you would have then is a frictionless seal that is pressure compensating,the more pressure it sees the tighter it seals.Just a thought.

motorworks
12-12-2004, 02:37 PM
Buckshot
Have a look at the plastic section on:
http://www.eng-tips.com/