Plastic Sheets
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. Other industries
in automotive, aerospace, petrochemical, food and marine use raw thick gauge
plastic sheets and blocks to machine industry-specific parts. Plastic sheets
are used as signage, and clear acrylic and Plexiglass 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
which 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 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. Some industries use plastic sheets
with minor alterations for applications such as cutting boards, business signs,
Plexiglass 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.