Compression Molding and Skiving of GUR® Ultra High Molecular Weight Polyethylene (UHMWPE)

The extremely high melt viscosity of GUR® UHMWPE makes conventional processing techniques, including screw extrusion and injection molding, difficult to accomplish.  The most commonly used processing method is compression molding of the powder into sheets in multi-opening hydraulic presses equipped with heating and cooling channels.

GUR UHMWPE has an average bulk density of 450 grams/liter versus a specific gravity of 0.93 for the finished sheet.  Thus, powder fill height is typically 2.0-2.2 times the intended sheet thickness.  Recommended press conditions are a sheet core (centerline) temperature to 190-200°C (approx. 375-390°F) at 70 to100 bar (1000 to 1500 psi), in order to achieve an optimum combination of tensile, abrasion, and impact properties.  

The press should be able to provide a minimum pressure of 70 bar (1000 psi) over the projected surface area and should be oversized to accommodate up to 6-10% shrinkage between initial pressing and finished sheet.  Molds can be either matched metal or flat with disposable edging, and should be hardened.  The gap between mold sections should be about 0.05 mm (0.002”), in order to prevent dusting as the mold is closed.  Mold surfaces should be chromium-plated for corrosion resistance.  To obtain the best surface finish, molds should be as smooth as possible, although in some cases, light texturing may be desirable to reduce sticking.  Silicone (polysiloxane) or calcium stearate can be used as mold release agents.  It is advisable to use multiple hydraulic cylinders for uniform pressure distribution.  Thermal oil is typically used for heating and water for cool down.  Platen operation is generally 10°C (18°F) above desired core temperature.

Processing steps are usually as follows:

1. Condition the mold platens
2. Fill the mold filling [weigh & level] with material
3. Expel the air from the mold using 1500 psig pressure and no heat
4. Reduce the pressure to 500-750 psig, then begin the heat cycle (the sintering time is approximately 1 hour per 10mm of sheet thickness)
5. Increase the pressure to 1500 psig, then begin the cooling cycle (the cooling time is approximately 50% of the sintering time)
6. Reduce the pressure to 0 psig, then open the mold

In USA produced sheet sizes are most commonly 4’ x 8’, 4’ x 10’, and 5’x 12’, with 20’ lengths available.  The thinnest sheet directly pressed by this method is 0.25”.  Many processors will press thicker sheets or billets and skive or slice sheets as thin as 0.067”.  Tape from cylindrical billets can be skived to 0.005-0.010”.  Thickness tolerance of pressed sheet is generally ±5-10% with length and width tolerance typically +1/2”/-1/4”.  Skived or planed sheet tolerance is typically ±0.020”.

Since UHMWPE does not liquefy above its melting point of 135°C (275°F), no mixing or flowing action occurs and special care must be taken to uniformly distribute pigment and/or stabilizers prior to mold filling.  This can be accomplished by a variety of mixer types, including both intensive and non-intensive devices.  Intensive mixers, such as the Henschel mixer, are used for quick dispersal of ingredients.  Other methods, including ribbon blenders, cement mixers, or drum tumblers can also successfully utilized.  Equipment selection should be based upon an evaluation of test results.

Besides pressing sheet, parts can be direct compression molded from powder or preforms.