Several years ago, an ASTM task force agreed on the value of 3,100,000 molecular weight as the dividing line, above which the UHMW description should apply. Resin properties increase with increasing MW and start to level off at the 3,100,000 value. Also, processibility is more difficult above that dividing line, and material cost rises more rapidly. Methods used currently are compression molding, ram extrusion, and warm forging of extruded slugs. UHMW polyethylene has outstanding abrasion resistance and a low coefficient of friction.
Impact strength is high, and chemical resistance is excellent.
Most PP parts are produced by injection molding, blow molding, or extrusion of either unmodified or reinforced compounds. Polypropylene is a low-density resin that offers a good balance of thermal, chemical, and electrical properties, along with moderate strength and moderate cost. Strength properties are increased significantly with glass-fiber reinforcement. Electrical properties of PP moldings are affected to varying degrees by service temperature. Dielectric constant is essentially unchanged, but dielectric strength increases and volume resistivity decreases with increased temperature.