Sheet extrusion is a technique for making flat plastic sheets from a variety of resins. The thinner gauges are thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers and margarine tubs. Another market segment uses thick sheet for industrial and recreational applications like truck bed liners, pallets, automotive dunnage, playground equipment and boats. The third primary use for extruded sheet is in geomembranes, where flat sheet is welded into large containment systems for mining applications and municipal waste disposal.
Thermoplastic sheet production is a significant sector of plastics processing. Thermoplastic sheets are flat, plastic materials with a gauge of at least 250 microns and which include both flexible and rigid materials, as well as solid, foamed, and hollow materials.
Solid sheet extrusion units consist of at least one extruder and one sheet extrusion die. They are followed by the polishing stack, in general comprising 3 calenders, calibrating and cooling the sheet with their surfaces or calender nips. Behind this the roller conveyor and the draw-off rolls for air cooling are located. The sheet is finally cut and stored. Sheet extrusion characteristics:
- width in excess of 2 m
- thicknesses ranging from approx. 0.5 to 15 mm
- no limitations as to length
- setup as multilayer sheets with functional surfaces (colour, haptics, UV-protection …)
- grain/structured surfaces
- easier forming possible (corrugated panels, folding, thermoforming …)
Polystyrene continues to be the most common polymer for use in sheet extrusion. It is the dominant material for thermoformed packaging and competes with ABS and PP in technical markets. End use applications include tubs and pots for yogurt, margarine, and desserts. Thermoformed packaging is also used in many other applications in the food industry.
There are three primary techniques used to manufacture thermoplastic sheet. These are:
- Extrusion through a flat die onto casting rolls.
- Extrusion through an annular die onto a sizing mandrel. The pipe-like cross section that is extruded will be slit in one or more places and then flattened and handled as sheet.
- Resins and additives will be plasticated between large rolls and then sized through a series of additional rolls into a flat sheet. This process is known as calendering.
Each of these methods has advantages and disadvantages depending on factors such as type of polymer being processed, thickness and width of sheet, and surface quality desired.
Single Layer Flat Sheet extrusion is the most common technique used in extruding plastic sheet for the thermoforming industry. The classic machinery components for this process can be described as follows:
Resin is fed into an extruder where it is plasticated into a melt.
The extruder, consisting of a heated barrel with an internal rotating screw, pumps the melted resin into a flat sheet die which sizes the sheet (thickness and width).
The sheet exits the die in a semi-viscous state and travels through a series of rolls to cool. These rolls also determine final sheet size, thickness, and width.
The flat sheet may then be wound onto continuous rolls, or “pre-sheared” into discrete lengths.
Coextrusion is a process that allows the combination of different materials and colors in a single sheet. This is done to achieve special properties which are specific to a certain polymer, or for aesthetic effects with color, or for economic reasons where an inexpensive material “sub-strata” is combined with a more expensive material “cap”.
Within the building and construction industries, sheet extrusion is used for a variety of applications. One of the main uses of extruded PS sheet is for thermal insulation materials for walls, roofs, and under floors.
In the automotive industry, sheet is currently used to produce interior trim, panels, and dashboards. Foamed polyolefin sheet, both cross-linked and non-cross-linked, is also used in automotive applications.
There are a number of other applications where thermoformed sheet plays a significant role. These include the manufacturing of luggage, refrigerator liners, and shower units.