Monday, March 16, 2009

UHMW Polyethylene - the Slickest Plastic of the Bunch

I remember when UHMW was introduced to the market - Ultra High Molecular Weight Polyethylene was supposed to replace Teflon in every "wear" application. It was supposed to last longer. The scientists had no idea what they had created and what an industry would be spawned. The first UHMW was called Pactene and was green. It's naturally white, but some marketing guru decided that if they made it green, and trademarked it, everyone else would have to sell white to compete. That was a pretty smart guy. Turns out UHMW has allowed many industries to be much more efficient in their production. Funny thing is... UHMW doesn't wear out very fast and after it was first introduced and all that Teflon was replaced, sales of UHMW fell. Until we all thought of some more places it could be used.

Keep in mind, UHMW does not glue to itself. Or to anything. It's one of the group of plastics that are so chemical resistant that they don't bond with solvent adhesives. LDPE, HDPE, UHMW, Polypropylene, UHMW polyethylene.... so you have to mechanically fasten, or use contact cement or 2 sided tape to stick it down. There is a product out there called "Slick Strips" with adhesive already applied to the sheet for wear applications. Peel and Stick.

UHMW also has some amazing physical properties - with a density of 0.96 it could float, and can take extremely cold temperatures. Cryogenic.

UHMW is used around the world in 1000's of applications. It has the lowest coefficient of friction for a plastic. And it's extremely chemical resistant. That means that metal chains can rumble over a wear guide of UHMW without wearing down. In the mining industries, gravel and ore can travel over UHMW plates easily (insteadof metal which will either wear or decompose - or both). In the marine industry, dock bumpers are made from thick UHMW so boats from dinghies to aircraft carriers can bump against the docks. My first use was a wear plate under the mast of my Hobie Cat to replace the Teflon disc that came with it. I made a toboggan for my kids by heating and wrapping the front end. That thing was out of control. Literally. Why do we love sliding on snow so much? To get on AFV? I digress. Now there are special formulations for specific industries - even formulas to make it even slicker! More on that below...

Know your UHMW! Here's a legend to some of the formulas:
Virgin UHMW Polyethylene - this means you are getting the original resin in your sheet. It can be made 2 ways: Extruded (most common) and Compression Molded (less common). Extruded will always be cheaper and has a lower molecular weight. For most applications, it's fine. Compression Molding offers a sheet that has far less internal stress from production and a modestly higher molecular weight. If you need to do a LOT of machining to the sheet, ask for compression molded UHMW. It is worth the investment.
Reprocessed UHMW Polyethylene - this is the most common form and the least expensive. The low bidder is usually quoting this. If it is white, then only white scraps are ground and added to the extrusion. This is OK for many applications - like bumpers or other plates that get hit, bumped and scuffed. If the repro UHMW is black, then there is typically a mix of scraps in the formula. Not recommended for a wear application.
Exotic UHMW Formulas - OK, maybe not exotic, but there are a lot of formulas available that will perform in a superior way for your application. Specific grades are made for very specific applications. Not only is the density a variable, but the molecular weight is a variable. There are fills with silicone, oil, fiberglass and mysterious proprietary additives the manufacturers guard like the Coke recipe that change the physical properties - and that may make the particular formula work better or worse for your application. ASK questions and make sure you get a spec sheet from the company - so you get what you asked for. The best plastic companies will ask you lots of questions about your application and exactly what you are trying to achieve by using UHMW.

Parting thought: I have seen customers try to use HDPE instead of UHMW. It may come from the same family, but everything is different about the 2 plastics. They cut and machine differently. The form different. They have completely different properties and pricing. As they say - you get what you pay for. While the short term performance of HDPE might seem like a cost savings, long term detriments will more than cancel out the savings - and if you have a brand to protect, try to remember how hard you worked to build the brand....

3 comments:

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