The sheet extrusion process is a continuous operation of melting and conveying a polymer in a heated screw-and-barrel assembly (an extruder). The homogenous melt is forced to flow through a flat sheet die from which it exits at the desired width and thickness. The molten sheet is wound through the nips of a three-chill-roll stack for thickness sizing and cooling. The solid sheet is then cooled further on a cooling conveyor while the edges are trimmed to final desired sheet width. The sheet is either rolled or sheared into sections for later use.
Equistar offers both polyethylene (PE) and polypropylene (PP) resins for sheet extrusion applications.
Benefits and Applications
Sheet extrusion products are an ideal replacement for wood, paper and metal products, where durability, cost and weight are an issue. It is common for polyolefin sheet products to be used as feedstock for the thermoforming process, wherein flat sheet products are converted into three dimensional products
Some of the many applications in which sheet extrusion products are used include: restroom stalls and partitions, cutting boards, boat decking, bacon boards, folders, toys, tanks, plastic envelopes and printer cartridges.
Properties
The preferred resins for sheet extrusion and thermoforming are high molecular weight (MW) or low melt flow resins (as indicated by the melt index (MI) of a polyethylene resin or the melt flow rate (MFR) of a polypropylene resin). These properties ensure the resin has adequate melt strength (or melt stiffness) to produce a uniform gauge thickness sheet, under optimum conditions, and hold its own weight during subsequent thermoforming PE resins are available in a wide range of densities. High density polyethylene (HDPE) homopolymer has the highest density (0.960 g/cc or higher) and, as a result, has the highest stiffness, moisture resistance and heat deflection temperature (HDT). HDPE copolymers, which have densities between 0.940 and 0.959 g/cc, have lower stiffness, but enhanced impact and low temperature properties. Linear low density polyethylene (LLDPE), with densities between 0.918 g/cc and 0.940 g/cc, and low density polyethylene (LDPE), with densities between 0.915 g/cc and 0.935 g/cc, can be successfully converted into sheet for applications requiring the flexibility afforded by the lower stiffness polymers.
PP resins are not categorized by resin density, but are instead divided into three categories. Homopolymer PP resins are the most rigid. Random copolymer PP resins are more flexible and can be formulated to provide good optical properties (clarity). Impact copolymer PP resins provide enhanced impact strength to the finished goods, particularly at lower temperatures.