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2 min read

UHMW 101: From Molecular Weight to Machining

March 20, 2018

UHMW 101: From Molecular Weight to Machining

UHMW remains a hot topic here on Tech Talk, and on our Ask the Experts portal, too. Why is this true? For one, the material offers great value over other plastics, and has good abrasion-resistance properties, plus it is processed via a unique control (no molding, only machining). This trifecta of properties values make for a distinctive material, read on for more top FAQs:

UHMW is short for ultra-high molecular weight polyethylene; or a semi-crystalline polymer. As a point of reference, high-temperature polymers are classified based on their molecular structure. Semi-crystalline polymers are solid until heated to certain temperatures, where they will quickly turn to liquid. Amorphous polymers do not exhibit any crystalline properties and resist liquid melting. UHMW is a considered a high-density polyethylene with a median molecular weight (falling within a range of 3.1 to 5.0 million).

1) UHMW has a low melt point, but a high COF

Like all polyethylenes, UHMW has a low melting point (270°) and a high COF (120 x10” In/In/°F). It gives one of the lowest wear rates (even better than steel, nylon or fluoroplastics) to resist most forms of abrasive media. These qualities give the material good impact resistance.

2) Molecular weight = better abrasion resistance

Molecular weight has a direct impact on a material’s ability to resist abrasion. For instance, a molecular weight grade of 4 million has an abrasion resistance of 100 when measured using a sand slurry test. Yet when you increase to the molecular weight to 6 million, the abrasion rate goes to 75, or an improvement of 25%. Compare this with steel, which has a resistance of 160 and it becomes clear why the material is a good choice for abrasive wear environments.

3) No molding, machining only

Though known as a tough material, UHMW cannot be molded; machining is the best processing method. In fact, waving and warping are common to large sheets of virgin UHMW, so molding is nearly impossible without compromising the integrity of the material. And unless you use the right tools and techniques (revealed in this technical guide), your machine cutting tools are known to actually become melting tools instead! Coolants are critical to maintaining the right heat levels.

4) Beware of expansion to avoid deformation

When machining UHMW, special attention must be paid to how quickly the material expands (up to 20x the rate of steel expansion). Anytime you machine UHMW, or any material with instability, it’s critical to consider the final operating conditions of the part and machine accordingly. Materials that are fabricated in a warm climate will expand and contract and otherwise behave differently when installed and operated in cold-weather environments, and vice versa.

5) Inert to most chemicals

Most liquid solutions are compatible with UHMW, including various forms of alcohol, ketones, and acids. But one should beware of chemicals with high-oxidation, like bleach, and hydro carbons like gasoline.

UHMW is available in many variations, including glass or moly-enhanced or cross-linked. Submit your specs to our team, and we can walk you through the pros and cons of each.

Custom Plastic Fabrication: Get the Guide!

Topics: UHMW
Dave Biering

Written by Dave Biering

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