PE can be processed and manufactured in many ways. With ethylene as the main raw material and propylene, 1-butene and hexene as copolymers, the polymer is polymerized by slurry polymerization or gas-phase polymerization process under the action of catalyst, and the resulting polymer is flash-steamed, separated, dried and granulated to obtain a finished product with uniform particles. This includes processes such as sheet extrusion, film extrusion, pipe or profile extrusion, blow molding, injection molding and rotational molding.
1 Extrusion: Grades used for extrusion production generally have a melt index of less than 1 and a medium to wide MWD. low MI during processing results in a suitable melt strength. Wider MWD grades are more suitable for extrusion because they have higher production speeds, lower die opening pressures and a reduced tendency for melt fracture.
PE has many extrusion applications, such as wire, cable, hose, tubing and profiles. Pipe applications range from small cross-section yellow pipe for natural gas to thick-walled black pipe of 48in diameter for industrial and municipal pipelines. Large diameter hollow wall pipe is growing rapidly as an alternative to storm drains and other sewer lines made of concrete.
Sheet and Thermoforming: The thermoformed lining of many large picnic-type coolers is made of PE for toughness, light weight and durability. Other sheet and thermoformed products include fenders, tank liners, pan and basin guards, transport boxes and tanks. A large number of fast-growing sheet applications are mulch or pool-bottom Muri, based on the fact that MDPE is tough, chemically resistant and impermeable.
2 Blow Molding: More than 1/3 of the HDPE sold in the U.S. is used for blow molding applications. These range from bottles containing bleach, motor oil, detergent, milk and distilled water to large refrigerators, automotive fuel tanks and cartridge cans. Blow molding grades have similar property specifications, such as melt strength, ES-CR and toughness, to those used for sheet and thermoforming application grades, so similar grades can be used.
Injection-blow molding is typically used to make smaller containers (less than 16oz) for packaging pharmaceuticals, shampoos and cosmetics. An advantage of this process is that the bottles are produced with automatic debottlenecking, eliminating the need for a post-trimming step that is typically associated with blow molding processes. Although some narrow MWD grades are used to improve surface finish, medium-wide to wide MWD grades are generally used.
3 Injection molding: HDPE has countless applications ranging from reusable thin-walled beverage cups to 5-gsl cans, consuming 1/5 of the HDPE produced domestically. injection molding grades generally have a melt index of 5 to 10 and are available in lower flow grades with toughness and higher flow grades with processability. Uses include daily necessities and food thin-walled packaging; tough, durable food and paint cans; high resistance to environmental stress cracking applications, such as small engine fuel tanks and 90-gal garbage cans.
4 Rotational molding: Materials using this processing method are typically crushed into a powdered material that is allowed to melt and flow in a thermal cycle. Rotational molding uses two types of PE: general purpose and cross-linkable grades. General purpose MDPE/HDPE typically have densities ranging from 0.935 to 0.945 g/CC with narrow MWD, resulting in high impact products with minimal warpage, and a melt index range of 3-8. Higher MI grades are generally not suitable because they do not have the desired impact and environmental stress cracking resistance of rotomolded products.
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