Ultra High Molecular Weight Polyethylene (UHMW) is one the most widely used thermoplastic materials today. It is defined as having a minimum molecular mass (molecule length) of 3.2 million; some resins go as high as 7.5 million (for reference, HDPE = ~1MM, LDPE = ~500,000)
UHMWPE advantages in its virgin form:
- odorless, tasteless, and nontoxic
- highly resistant to corrosive chemicals except oxidizing acids
- almost no moisture absorption
- extremely low coefficient of friction (comparable to PTFE (“Teflon®”))
- self-lubricating
- excellent abrasion resistance, especially wet (~15X better than carbon steel)
- highest impact resistance of any thermoplastic (IZOD = no break)
Easily modified, this base material is used in applications as diverse as cargo dock impact bumpers to dump truck liners to food equipment to orthopedic joint replacements. Essentially a “poor man’s Teflon®”, it shares similar properties, with some significant limitations.
Like PTFE, UHMW cannot be melt processed, as this breaks down the molecular chain. It is made using compression and heat to “fuse” the material below its melting point. It is available in compression molded sheets up to 8’ x 20’, and ram-extruded rod, tubes & profile shapes.
The largest industrial markets for UHMW are food processing and heavy equipment, both primarily for bearing & wear components. For such applications, the limitations versus nylon or acetal are:
- Heat Resistance – the heat deflection temperature is 116⁰F, compared to ~200⁰F
- Thermal Expansion – UHMW changes at 2X the rate of nylon with temperature changes
- Compressive Strength – its compressive strength = 3000 psi, versus 15,000 psi for nylon – UHMW’s safe working load is 1000 psi maximum
- Inserts – since the material is only 25%as strong as nylon, larger / specialty inserts with a foot extension on the bottom (prevents pull-through), and limited torqueing is needed
Where food contact is not an issue, reprocessed UHMW is a cost effective alternative. Using clean regrind mixed with virgin black resin yields a material with the same low friction, low moisture absorption and impact resistance of virgin at lower material cost. Due to the inclusion of the larger regrind particles, it is actually slightly harder and some OEMs prefer it over virgin.
There many modifications of UHMW available, including colors; UV stabilized; lubricant filled (solid, oil or MoS2), ceramic filled; anti-static/conductive; metal/X-ray detectable; high temperature.
This last version needs come clarification. Some mills offer an anti-oxidant filled UHMW touted to withstand 275⁰F – HOWEVER -THE HEAT DEFLECTION TEMPERATURE IS THE SAME AS VIRGIN, 116⁰F! What the 275F means is that, where pure virgin UHMW starts to oxidize (turn brown) over time @ ~175⁰F, this version adds ~100⁰F heat resistance before oxidation.
Too much information? Give us a call, the experts at WS HAMPSHIRE can walk you through all the data to find the right material for your needs – you’re in the right place!
Tom Connelly is a self proclaimed “street engineer” with over 40 years in the plastics industry.