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Best plastics for fixtures?

sneasle

Plastic
Joined
Jan 28, 2014
Location
United States, Alabama
Hi All,

I am working on a fixture to hold a rather large object and it needs to be cut from a non-conductive material. It needs to hold a grab onto the lip of a cylindrical object. The object is 23" diam, the lip is 24" diam and 1.250" thick and it weights about 80lbs total.

I'm considering using two pieces of 36"x36"x3/4" thick material and cutting out a center opening and then cutting a recess that would clamp onto either side of the ring. That said, I'm not really sure what plastic would be the best choice.

McMaster is near me, so I tend to order most of my raw materials from them, and in this size I have the following options:
Chemical Resistant PVC
Easy-to-Machine Impact Resistant ABS
Impact-Resistant UHMW Polyethylene
Abrasion-Resistant Glass-Filled UHMW Polyethylene


I'm leaning toward the Polyethylene right now, but I'm not sure if there is a better choice. I don't have much experience with plastics (being an electronics guy), so if there are any veterans here that could make some suggestions I would appreciate it. I need to be able to support the weight in both the plane of the material, and perpendicular to the plane of the material (the fixture rotates to allow for loading/unloading of the cylinder).
 
Be aware that plastic tends to be thermally unstable. Uhmw is particularly bad for moving around. If you go with pvc wear a respirator when you machine it. It lets off something awful that burns your nose, I want to say chlorine but I might be wrong. Whatever it is it doesn't tell good.
Jordy
 
My only concern with PVC is it's cracking from impact. I'd hate for this fixture to get let down 'hard' when being rotated and it cause it to crack. If it's not a problem with this kind of PVC, then great!

Oh, I won't actually be doing the cutting. I don't have anything large enough for this. Will farm it out to a local shop, so it will be up to them if they have the controls in place to handle the fumes.

There won't be any chemicals involved with this fixture thankfully. So if chemicals are the only concern with ABS, it might be a good option. I think the only chemical that would ever come near this would be windex.
 
Be aware that plastic tends to be thermally unstable. Uhmw is particularly bad for moving around. If you go with pvc wear a respirator when you machine it. It lets off something awful that burns your nose, I want to say chlorine but I might be wrong. Whatever it is it doesn't tell good.
Jordy



Ya, thermal stability concerns me, but I can't use anything metalic. I suppose I could use a fiberglass, but in some cases fiberglass isn't any better. What about the glass filled Polyethylene, might that be an OK compromise?
 
Acetal every time and I've made dozens of fixtures and assembly jigs from it, - reasonably impact resistant, dimensional stability amongst the best.
 
I'd be Ok with Acetal/Delrin, I just haven't been able to find someone that can sell me a piece that large yet. If you've got a supplier you could share, I'll price it and see what it looks like. The PVC and Polyethylene are all ~$300 for the sheet, so I'd rather not go too far much more than that.
 
I would lean toward linen/cotton cloth laminated phenolic. Its easy to machine, stable, heat resistant and available in large sheets and of varying thickness's.

Tom
 
I would lean toward linen/cotton cloth laminated phenolic. Its easy to machine, stable, heat resistant and available in large sheets and of varying thickness's.

Tom

Got any sources to share? This isn't a material I am familiar with.

edit: Found some here, it's more expensive than the Delrin...

Another vote for Acetal/Delrin Same stuff different name.

Although pvc is much cheaper but doesn't machine quite as well as delrin.


Agreed, I'd use delrin if I could. Looks like it's almost $800 for a 36"x36"x.750" sheet, which is a bit pricey.
 
what about wood? I don't know how it compares to plastics on thermal stability but it might work if you are dry machining.
 
lke Kingbob said, wood. look at high grade plywood like Baltic Birch or Maple Die board. Die board is very flat, stiff and strong. a coat of varnish will protect it from moisture. easy to work with and is stiffer and stronger than any of the plastics you are thinking about other than the fabric filled phenolics. Check out Paragon Steel Rule Die in Rochester NY. They will laser cut the parts to size for you in die board n a few days, for less than you can buy the material for. if you need a recess, just design a multi layered stack and laminate together
 
Good suggestion on the die board material, although if the intent is to use the clamping ability over and over, screw inserts or through holes with nuts/washers will be required. If plastic is the choice, I would use PVC. It will drill and tap fine, as long as you use reasonable size fasteners (1/4-20 or larger, I'd say). The Delrin is easier and more fun to machine, but will cost a good deal more.
 








 
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