The manufacture of plastic thin film (polymeric) used for packing or coating is done in the following way :
the polymer is molten in an extruder (i.e. a screw ensuring fusion by self-heating), then it is pushed in a die (plane or circular),
finally it is stretched in the air and is cooled (it is "the cast-film" process or film blowing).
Many usual products of packing (food packing for example) consist of several layers of polymer (5 or 9 in certain cases) having each one a specific function (materials in contact with food, materials barriers tight with the air, with water...). For the manufacture of this kind of films, one uses several extrusion machines, then the various polymers are put in contact in the "feed-block" (see Figure 1 ). Then, drawing is done as explained above.
In some industrial circumstances, this process becomes unstable. So the final product has a loss of transparency incompatible with the severe industrial criteria (see Figures 2, 3 and 4 ). The regularity thickness is also an important factor which determines the quality of the product.