Some Basic Facts about Shrink Bundling Film
Flexible Packaging Films – Some Basic Facts about Shrink Bundling Film
The most usual methods of manufacturing plastic film are inflation and extrusion. The following information will only deal with inflation.
In the inflation method, the film is manufactured in the form of a tube, which is either rolled up in the form of a tube, or else the edges of the flattened tube are trimmed and the two sheets of film remaining are rolled up on individual bobbins. A machine for the inflation of film consists of four sections: extruder, ring shaped nozzle, cooling system and winder.
The extruder melts and plasticizes the resin granulate and extrudes a thin walled tube through the ring shaped nozzle. An air cushion inside the tube expands it to the requisite diameter, whereupon the tube is captured and runs through a pair of rollers to the rolling unit. The tube is usually cooled by air being blown on it externally through a ring shaped slit. There is also another cooling device in which the film runs over a water-cooled inner mandrel, external cooling being cared for by air or running water.
The shrinkage properties of the various plastic films are obtained by stretching the film during the manufacturing process and cooling it in such a way that the shrinkage tendencies become frozen. In principle, this is possible with all types of plastic film. The ratio between longitudinal and transverse shrinkage can be controlled to a certain extent by the inflation ratio.
Certain types of shrink films shrink at such a low temperature that boiling water can be used for shrinking. For instance, this is the case with Cryovac bags which are used for poultry, meat, cheese, etc. However, the most usual method involves the use of hot air circulation or heat radiation. The following listing will give you an overview of the degrees C temperatures which cause the more usual films to shrink: Polyester = 70 – 120; LDPE = 90 – 150; Radiated PE = 70 – 140; Polypropylene = 105 – 180; Polystyrene = 100 – 130; PVC = 65 – 150 and PVDC Copolymer = 65 – 100. Please note that these temperatures given are approximate and variations could arise due to the thickness of the film etc. Finally, and most important here, is that the requisite tunnel temperatures are much higher than the actual shrinking temperature of the film.