Canning and Fish Packaging Technology

Unit 10 - Manufacture of plastic packages

10.1.2.1.Film blowing
Blowing is one of the two methods used for making thin films. In this method, molten plastic is extruded by the screw extruder (described earlier) into a circular die (slit in the shape of a circle). As soon as the film comes out of the die as a tubing, it is blown into a bubble. As the bubble is hauled up, it is cooled and finally wound on reels as a lay flat tube.

The ratio of final diameter of the tube to diameter of the die is important. For most common application, the ratio of 2:1 is used. Higher blow up ratios are used in making shrink wrap films as higher the ratio the more are the stresses incorporated. They get released when heated and hence results in more film shrinkage.

Extrusion of certain polymers into blown film will also result blocking unless antiblock agents are used. The extruded tube of the film is usually slit open to form 2 plies and would up as reels and usually used for the purpose of stretch wrapping or shrink wrapping.