SHEET EXTRUSION
Sheet extrusion is a technique for making flat plastic sheets from a variety of resins.
1. Solid sheet extrusion units consist of at least one extruder and one sheet extrusion die.
2. They are followed by the polishing stack, in general comprising 3 calenders, calibrating and cooling the sheet with their surfaces or calender nips.
3. Behind this the roller conveyor and the draw-off rolls for air cooling are located.
4. The sheet is finally cut and stored.
Sheet extrusion characteristics:
1. width in excess of 2 m
2. thicknesses ranging from approx. 0.5 to 15 mm
3. no limitations as to length
4. setup as multilayer sheets with functional surfaces (colour, haptics, UV-protection ...)
5. grain/structured surfaces
6. easier forming possible (corrugated panels, folding, thermoforming)
Materials
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:
1. Extrusion through a flat die onto casting rolls.
2. 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.
3. 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.
Applications
1. 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.
2. 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.
3. 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.
PROFILE EXTRUSION
This process is used to manufacture plastic products with a continuous cross-section such as; drinking straws, plastic evestroughing, decorative moulding, window trimming and a wide variety of other products polymer melt into the hollow mould cavity under high pressure.
Process
1. The plastic is fed in pellet form into the machines hopper (this machine is known as an Extruder), the material is softened by both friction & heat and conveyed continuously forward by a rotating screw inside a heated barrel.
2. The softened plastic is then forced out through a die and directs into cool water where the product solidifies. Subsequently it is conveyed onwards into the take-off rollers which pulls of the softened plastic from the die.
Raw Materials
Most common thermoplastic polymers can be used for extrusion and the material choice is dependent on both the performance requirements and on the economic constraints.
Typical Materials for Plastic Profiles:
1. HDPE (High Density Polyethylene)
2. LDPE (Low Density Polyethylene)
3. LLDPE (Linear Low Density Polyethylene)
4. PETG
5. Flexible PVC
6. Butyrate
7. Polypropylene
8. Polystyrene
9. ABS
The most commonly used material for general purpose extrusions is PVC.
Advantages
1. Equipment widely available in all geographical areas. Short lead times.
2. Relatively low tooling costs
3. Inexpensive process
4. Product combinations possible
5. Design freedom
Disadvantages:
1. Design possibilities severely limited because of linear nature of process.
Applications
1. Window profiles
2. Sealing sections
3. Modular drawer profiles
4. Decorative trim
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