Plastic hollow articles can be found everywhere. Plastic hollow articles are used for packing, storing, transport or guiding of liquids or other bulky goods such as pwerders or the like. These plastics hollow articles are bottles, cans, drums, and tanks, but also pipes, tubes, or ducts. Plastic hollow articles are used besides bormal packaging applications to provide protection; for example protective cases for sensitive electronic equipment. Due to the double wall structure, rigid lightweight structureal components can be made of plastic hollow articles as well, e.go, transportation pallets, seat structures or all kinds of panels.
The equipment is presented that help understand extrusion blow moulding fundamentals as below:
A. BASIC STRUCTURE OF A BLOW MOULDING MACHINE
A blow moulding machine can be devided into several subsystems, which will be described below. The extruder, including material hopper, screw, DC (mostly) or AC drive, and heating for plasticizing and preparation of the thermoplastic melt.
In extrusion blow moulding, extruder barrels with grooved feed zones are most used. These extruders feature an output behavior commonly is independent from the counter pressure. This is important considering that the wall thickness cantrol works with variable die gaps, which obviously leads to varying counter pressures. In order to prevent melting in the grooved feed zone ( especially with polyolefins), this feed zone has to be cooled. In most cases, this is performed by a separate water cooling circuit controlled via a proportional valve.
The screws used in extrusion blow moulding are basically the same as those used in conventional extrusion processes. There are three-section screws, barrier screws, screws with different mixing and dispersion devoices, and certain specially developed screws, such as screws for processing 6-layers coextrusion regind.