Extrusion - Sheet
Extrusion is one of the most common plastic processing method that is used to create rod, sheet, film, profile or pipes & tubes.
It is a continuous, high temperature and high pressure manufacturing method that results in lengths or rolls/coils of finished or semi-finished product.
Most plastics and elastomers can be extruded including particle and fibre filled grades and best results are achieved if a uniform thickness is produced.
Extruders range in size and capability to suit the material or end product being manufactured. Typical hourly outputs range from a few kgs per hour to up to 20 tonnes per hour.
The key components in an extruder are a barrel and an Archimedian screw. The screw is coupled to an electric motor that drives it within the barrel.
The plastic, usually in the form of powder or granules, is fed from a hopper onto the screw. It is then conveyed along the barrel where it is heated by conduction from the barrel heaters and shear due to its movement across the screw flights. The depth of the screw flights channel is reduced along the length of the screw so as to compact the material.
A die is fitted to the end of the barrel and the molten plastic is formed into the desired shape. Before the plastic is formed by the die it passes through a breaker and/or screen which serves to create back pressure that ensures a consistent linear flow of plastic to the die. The screen and breaker plate assembly also removes impurities from the plastic melt.
Extruders can be single or twin screw. The twin screw and barrel design can be parallel or conical and the screws can co-rotate or counter-rotate.
As extruder screw has three different zones along its length. Namely feed, compression and metering.
The feed zone pre-heats and conveys the plastic to the compression zone. In the compression zone the flight channel depth decreases to compact the plastic and squeeze out any trapped air pockets. In the metering zone the flight channel depth is constant but more shallow than the feed zone which ensures a constant homogenous supply of molten plastic to the die.
Different types of plastic require different screw designs and for certain materials additional zones to decompress and re-meter the plastic are present to allow for a vent port in the extruder barrel. Venting is necessary to remove volatiles such as moisture that has turned into steam from the melt. If not removed, these gases can accumulate in the finished product and cause a defect such as a surface blemish or a weaknesss.
Sheet extrusion can be through a flat die onto casting rolls or through an annular die onto a sizing mandrel. Where the pipe-like cross section that is extruded is slit in one or more places and then flattened and handled as sheet.
Common sheet extrusion flat die designs include T-type, Coathanger and Curved manifold and the die design is specific to the material type, sheet geometry and quite often the extruder. The melted plastic exits the flat die which is followed by the polishing stack, in general comprising 3 sets of rolls which calibrating and cooling the sheet with their surfaces or nip gap. Further cooling of the sheet by air then takes place before the sheet is cut and stacked. Sheet extrusion characteristics:
- width in excess of 2 m
- thicknesses ranging from approx. 0.5 to 15 mm
- as with all extrusion methods there are no limitations on length
- in line slitting, corrugation and printing of sheet is possible
Further processing of extruded sheet by vacuum-forming is common and 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 Polypropylene for technical applications. 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.
Extrusion - Sheet Companies