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Types of Extrusion
Stretch film can be manufactured using two manufacturing techniques – the blown and the cast extrusion methods.
The Blown Method
A blown extruder basically takes the polythene granules from the mixing hoppers, screws them along a heated tube which melts the granules changing them from solid to molten during their passage so that the blend ends up at approximately 200 degrees centigrade. The molten mass is forced through a circular die to emerge as a very thin tube of polythene. This tube is hauled upwards and away from the extruder through driven rollers at the same time as air is introduced into the tube causing it to expand. The amount of air in the tube determines its diameter and hence the final width of the film whilst the speed with which it is hauled away from the extruder determines the film’s final thickness.
The collapsed tube of polythene leaving the nip rollers is led away over various tensioning rollers to be wound up into reels. When sheet film is required, the edges of the collapsed tube are removed to leave two sheets running together. These are then separated and further slit into the final required width.
The Cast Method
In the cast process the extrusion process is exactly the same as for the blown process. The difference is in the machine die and the method of cooling the film.
Here the molten polythene is forced through a die that terminates in a horizontal slot – so forming a single thickness of sheet rather than a tube.
The sheet leaves the die and is “cast” immediately on to a roller having a highly polished surface and which is maintained at a low temperature by being cooled internally with chilled water. This means the film can be cooled very rapidly and evenly to produce sheet that is both optically clear and thickness controlled within fine limits.
The sheet of film is drawn away from the die at a rate that determines its final thickness. The width of the sheet is determined by the length of the slot in the die. This cannot be adjusted.
After passing over various tensioning rollers the sheet is trimmed at the edges and then slit into a number of narrower widths before being wound onto cores.
Both blown and cast films can be manufactured as either mono-extruded or co-extruded types.
In mono-extrusion, the die of the machine is fed with molten polythene from a single extruder only. This material may be one grade of polythene or a mixture of grades. The whole of the thickness of the polythene film produced therefore comprises this grade or mixture.
In co-extrusion, a complex die is fed with materials from more than one extruder, each of which can contain a different grade or mixture of grades of polythene.
The flow of materials from each extruder are only united just before they leave the die and cool. Film produced from this process is therefore composed of a number of layers joined at their faces. This process is used to manufacture films having layers with different characteristics.
The multi-layer production process allows manufacturer more flexibility to place different ingredients where they are needed most. It also enables a more controlled spread of all the ingredients, thus making a stronger film.
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