Canning and Fish Packaging Technology

Unit 10 - Manufacture of plastic packages

10.1.1.1.Extrusion
Extrusion is used in most forms of plastic conversion and is important process in both in rigid and flexible packaging. An extruder, is a machine which has a single screw that is made to rotate inside a barrel. The geometry of the screw is important and ideally each plastic would use one type of screw. At the resin feed end (feed zone) the screw diameter is constant and smallest. In the next region (compression zone) the core diameter increases. The relative length of this compression zone is one of the main differences between different types of screw of the extruder. The final region (metering zone) is again constant in screw core size and shape throughout its length.

Solid resin usually in granule form is fed to the extruder through a hopper. The plastic is fed forward by the rotating screw and melts by the combination of heat from the heated barrel and by the friction created by the turning of the screw. The molten polymer moves forward and fed into a die that starts to create a shape typical of the final product. A die with a long thin slit can give a cast film or a sheet. A similar die can be used for extrusion coating. Molten polymer may also be made to come out as a small tube and that can be the basis for blown film manufacture. Molten polymers may also be prepared in an extruder to be forced into a mould as in injection moulding.