One of the most impressive type of extrusion is blow film extrusion, a combination of extrusion and blowing which allows to produce a tube of thin plastic film. The most common material which is used in blow extrusion is Polyethylene (HDPE, LDPE, LLDPE).
Here is a movie of this impressive process:
The process begins in an extruder where the plastic material is melted and the tube is formed. The tube is drawn upward when is still hot and soft while the air is provided constantly in order to expand it through the die. Then the material of the tube is going upwards by the air and also cooled.
During this process there are present several types of rolls which have different functions, such as guiding, collapsing or pinching. The roll gives the possibility to provide the foil in one piece. The thickness of the foil depends on the production speed and the diameter of the blowned plastic.
Below you can see on a schematic drawn how the process looks.
Quite a large number of the foil production is made by this process.
The advantages of manufacturing plastic film by this process includes the regulation of film thickness and eliminate end effects such as edge bead trim. This process can be used for the manufacture of co-extruded, multi-layer films for high barrier applications such as food packaging.