Extruded plastic sheets are an essential part of the plastic fabrication industry, since most thermoforming processes use plastic sheeting of various gauges as a raw material. Consumer products, packaging, plastic containers and many other industries use plastic sheets in thermoforming processes such as vacuum forming, pressure forming and inline thermoforming. The automotive, aerospace, petrochemical, food and marine industries use raw, thick gauged plastic sheets and blocks to machine industry-specific parts.
Plastic sheets are used as signage, and clear acrylic and Plexiglas sheets make excellent windows, large picture frames, barriers and point of purchase displays. Plastic sheeting is extruded the same way plastic channels and profiles are, with an additional end process. Plastic pellets or flakes are fed into a hopper that then feeds the raw plastic into a screw conveyor. The screw conveyor shears and pushes the material along, heating and "plasticizing" the pellets into molten plastic. As the conveyor continues to turn, molten plastic is pushed out through a flat die. Instead of being instantly cooled, the flat shape is pulled and stretched by grips into wider sheets, which are then fed into a series of round metal cooling "calenders" and are ultimately wound onto spools. Thicker gauge sheets are cut and stacked flat, ready to be thermoformed into shapes. Dies can also be round, so that as the plastic is extruded through the conveyor its tube shape is sheared in half, and both top and bottom are stretched into flat sheets separately. Sometimes additives and coating resins are added to the surface of the plastic sheet during the calendering process.