The main part of the extrusion line is the extruder itself. Its role is vital as its design determines the final quality of the product. An extruder consists of a metal barrel which is and cooled evenly from the outside. In the barrel is the screw, the design of which is largely responsible for which type of raw material can be extruded. There is a vast amount of technology built into the screw, which not only aids the processing itself, but also helps determine the quantity of output for the respective plastics. The drive capacity of the extrusion line must be compatible with the size of the extruder as well as the type of product which can be processed.
Precision weigh scale blenders dose and mix materials to a specific recipe, before loading the mix to the hopper opening into the feed section of the screw. As it is fed through the screw flights, it is melted and mixed into a homogenous blend by a combination of heat and carefully controlled shearing force. From the screw tip, the material is filtered and the pressure is stabilised, before extrusion through the sheet die, the design of which ensures that an even flow of material is extruded across the whole web width.
A restrictor bar and adjustable lips, together with the heating of the die, all ensure an even distribution of the extruded material. Whilst continually flowing out from the die, the plastic material is then smoothed out on a three-roller stack system kept at a constant temperature. Apart from enabling the even cooling of the thermoplastic material, the polishing rolls determine the surface finish and the required thickness of the product.
In addition, other rolls can be fitted at this stage to allow embossing or lamination of the sheet with decorative films. The thermoplastic material is lifted up over the polishing rolls by the pull rolls and down the track, where atmospheric cooling of the sheet takes place. In the cutting unit, it is cut by either a guillotine or a saw at the required length and the finished sheet is then stacked and packed.
If the sheet is required on a roll, then a winder is fitted instead of a stacking system. A prerequisite for winding is that the material is of a suitable thickness and flexibility.