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.
Plastic sheets may be fabricated from a number of different plastic materials, including HDPE, LDPE, PETG, PVC
, polypropylene, polystyrene
, vinyl and acrylic
; different applications call for different properties of strength, flexibility, hardness, corrosion resistance and color, which is why so many styles of plastic have been created. Some industries use plastic sheets with minor alterations for applications such as cutting boards, business signs, Plexiglas windows and silk-screening. Vacuum and pressure forming plastics manufacturers, however, choose plastic sheets with fine to medium gauges to be further processed into three dimensional products. Vacuum formed
products are made by heating a plastic sheet until it is flexible, then introducing a male mold underneath the plastic sheet and vacuuming the heated plastic sheet to the mold so it takes the mold's exact shape. The pressure forming process is identical, except that a female mold is introduced and the heated plastic sheet is vacuumed into it. Blister packs and many packaging items are fabricated this way, as well as disposable utensils, containers, bathtubs and showers, marine seats and parts and all other hollow, shaped plastic parts made for industrial, commercial and residential use.