Garland Manufacturing Company
Gar-Dur® brand UHMW Extrusion and Machined Parts
Frequently Asked Questions About UHMW
Identification of UHMW
- Burn Test – light with a match – smell smoke – smells like candle wax – indicates polyethylene – Gar-Dur does not drip as readily as HDPE but it will drip
- Oven Test – place on aluminum dish in a 300° oven – regular HDPE will slump or melt – Gar-Dur will not change size or shape (it could warp or distort due to built in stresses)
- Saw Test – regular HDPE gives sawdust or filings – Gar-Dur gives strings or nothing
- Only to be sure of UHMW 20-30 I. V. is a sand slurry test. If sample is partially crosslinked the only way to tell if material is high I. V. is, again, sand slurry
- Does a sample of UHMW contain regrind? Hard to tell, maybe some black specks or other colors. The only way to determine is a sand slurry test. It tends to feel stiffer/harder. There may be some increase in modules. If the resin has been reprocessed many times abrasion and impact resistance can be greatly affected.
- Is the 20-30 I.V. the highest impact? No, 10-20 I. V. could have the higher impact but most application for UHMW require high abrasion resistance and Gar-Dur with an I/V/ of 24 has the highest abrasion resistance. Generally abrasion resistance is the most sought after feature with impact second, low coefficient of friction third and chemical resistance fourth.
- wind – no effect
- rain or sea water – no effect
- sun without protection – depends on thickness and location. The thicker – the better.
- Sustained high temperature use – 180°F, intermittent use – 200°F. Does it degrade at higher temps? No for short time sup but for longer time above 200°F it loses mechanical properties, abrasion and impact
- Water solutions generally safe except highly oxidizing chemicals such as bleach. Hydrocarbons: gasoline, kerosene, oil and grease cause swelling. Chlorinated solvents cause swelling. Organic alcohols, ketones and acids have little effect.
- Resistant to two types of irradiation – beta – electron beam, Gamma-cobalt 90.
- Cryogenic – Liquified Gases
- Hydrogen and nitrogen don’t effect. Oxygen could effect.
- Fire and Flammability
- Gar-Dur has no UL recognition. Would be HB on UL-1 – that is slow burning (less than 3″ per minute)
- No info but Gar-Dur produces less smoke than most plastics and no toxic fumes.
- General Rule – if abrasion or impact are not a requirement, Gar-Dur may be over engineered.
- If Gar-Dur is designed as a strength member, unless the load is low, the part is doomed to failure.
- Impact Superior in all tests. Standard notched izod, double notched izod, charpy impact, drop weight and tensile impact.
- Strength Important Notice – Nobody designs UHMW parts on strength – Exception, many do but it is bad practice. Creep modules is the only way. Tensile strength of Gar-Dur = 5000 – 6000 psi
- Fatigue – Cyclical Loading – Vibration resistance best of any thermoplastic.
Fabrication - How to form or machine
- Molding Typical question: I’m an injection molder, how do I mold UHMW Answer: “You can’t.” “Why not?” – The melt flow index is 0. To “Sure I can etc.” the answer is – If it molded readily, you probably destroyed the high molecular weight.
General Information – Regular tool steel will dull easily on UHMW. Dull tools overheat the surface, causing internal stresses and possibly warping or distortion. Use Stellite alloy or tungsten carbon tipped cutting edges. Keep surface cool. Keep cuttings or shaving away from surface. See Gar-Dur machining information sheet.
- Warping & Distortion of machined parts. Cause by built in stresses or poor machining techniques which overheat the surface.
Typical questions – Can I hold -.005 on a machined UHMW part?
Answer – It depends on
- Machining technique
- Size of part and symmetry
- How the part is fastened and supported in the machine tool.
Typical question: How thick should they be? No universal answer.
- Thicker at impact area
- Thinner in slide area
- Minimum thickness 1/4″
- Adhesive Can they be used? Not for most applications – reason: The large difference between coefficient of thermal expansion of metal and UHMW puts high stresses on bond if the bond sees temperature extremes. For inside factory use – adhesives may be okay.
- Fasteners – Best way to secure UHMW to metal UHMW must be allowed to expand or float Large flat head fasteners must be used. Fastener shaft holes in UHMW must be oversized to allow for sheet expansion and contraction.
- Forming Bending or folding sheet, rod and bar stock Easy process above the melt point – at 300°F it shapes easily. Must be fastened in the bent or folded position until cooled.
- Stamping – Form or contour sheet on metal stamping equipment Note: Stamping equipment must be modified to staying down position. This is called dwell time. The cost is low. All stamping machine manufacturers can provide dwell time modification at low cost.
- Forging – Reforming a molded block in a hydraulic press A two step Process: Step One: Make a bloke Step Two: Reform it to a part
- Thermal Welding Can UHMW be thermal welded? Yes. It behaves much like regular HDPE. What temperatures and pressures are required? Minimum temperature 400°F. Minimum pressure of 300 psi recommended. What percentage of parent strength can be attained? 80%
- Ultrasonic Welding Can UHMW be ultrasonically welded? Yes. Again the process is much like regular HDPE. Thick sections of UHMW may be more difficult than HDPE because of lower modules. If plastic is softer, like UHMW, more attenuation of the ultrasonic energy occurs.
- Stress Relief – Annealing Most UHMW sheet, rod, bar and board stock have some stresses. Reason: When it is made it is cooled non-uniformly – the outside cools faster than the core. Typical question: Can I buy annealed stock? Yes, but it costs more. Can I anneal it myself? Yes. Ask Garland for instructions. Warning – if you try to anneal after machining, parts will shrink.
- Grinding – For regrind or to reduce flake particle size. Can it be done? Yes.
- Coating – Putting a thin layer on a surface
Many people ask can I coat UHMW on wear surfaces. No.
- It does not melt to form a continuous surface.
- It requires pressure to weld all the unmelted particles at high temperatures.
Testing - Is it or is it not UHMW?
IV and sand slurry test procedures are the two ways to test material.
Product testing is available but is expensive.
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