PE can be manufactured in a wide variety of different processing methods. Using ethylene as the main raw material, propylene, 1-butene and hexene are copolymers. Under the action of the catalyst, the obtained polymer is flashed, separated, dried, granulated, etc. by slurry polymerization or gas phase polymerization. In the process, a finished product having uniform particles is obtained. These include, for example, sheet extrusion, film extrusion, tube or profile extrusion, blow molding, injection molding, and rotational molding.
▲Extrusion: Grades used for extrusion production generally have a melt index of less than 1 and a medium to wide MWD. A low MI gives a suitable melt strength during processing. The wider MWD grade is more suitable for extrusion because of their higher production speed, lower die pressure and reduced melt fracture tendency.
PE has many extrusion applications such as wires, cables, hoses, tubing and profiles. Pipe applications range from small-section yellow tubes for natural gas to thick-walled black tubes for industrial and urban pipelines up to 48 in. Large diameter hollow wall tubes are used as an alternative to rainwater drains and other sewer lines made of concrete to grow rapidly.
Sheets and thermoforming: The thermoformed linings of many large picnic-type reefers are made of PE for toughness, light weight and durability. Other sheet and thermoformed products include fenders, tank liners, tray guards, shipping boxes and cans. A large number of rapidly growing sheet applications are in the mulch or pool bottoms, which are based on MDPE's toughness, chemical resistance and impermeability.
▲Blow molding: HDPE 1/3 or more sold in the US is used for blow molding. These range from bottles containing bleach, motor oil, detergents, milk and distilled water to large refrigerators, car fuel tanks and canisters. Blow molding grade characteristics such as melt strength, ES-CR and toughness are similar to those used for sheet and thermoforming applications, so similar grades can be used.
Injection-blow molding is commonly used to make smaller containers (less than 16 oz) for packaging pharmaceuticals, shampoos and cosmetics. One advantage of this process is the automatic cornering of the bottles, without the need for post-finishing steps like normal blow molding. Although some narrow MWD grades are used to improve surface finish, medium to wide MWD grades are typically used.
▲ Injection molding: HDPE has numerous applications ranging from reusable thin-walled beverage cups to 5-gsl cans, which consume 1/5 of domestically produced HDPE. The injection molding grade generally has a melt index of 5 to 10, and has a higher flowability grade with a lower toughness grade and a processability. Uses include daily necessities and food thin wall packaging; tough, durable food and paint cans; high resistance to environmental stress cracking applications such as small engine fuel tanks and 90-gal trash cans.
▲Rotational molding: Materials using this processing method are generally pulverized into powder materials to melt and flow in the thermal cycle. Rotational molding uses two types of PE: generic and crosslinkable. General-purpose MDPE/HDPEs typically have densities ranging from 0.935 to 0.945 g/cc and have a narrow MWD for high impact and minimal warpage, with melt indices typically ranging from 3-8. Higher MI grades are generally not applicable because they do not have the desired impact and environmental stress crack resistance of the rotomoulded article.
High performance rotational molding applications utilize the unique properties of their chemically crosslinkable grades. These grades are fluid in the first stage of the molding cycle and then cross-linked to form their excellent resistance to environmental stress cracking and toughness. Abrasion resistance and weather resistance. Crosslinkable PE is uniquely suited for use in large containers ranging from 500-gal to various chemical storage tanks to 20,000-gal agricultural storage tanks.
▲ Film: PE film processing is generally performed by ordinary blown film processing or flat extrusion processing. Most PEs are used for film, general low density PE (LDPE) or linear low density PE (LLDPE). HDPE film grades are generally used where superior stretchability and excellent barrier properties are required. For example, HDPE films are commonly used in commodity bags, grocery bags, and food packaging.