Blow moulding is the process by which articles are formed by in ation of a molten resin to Žfill a mould cavity having the desired shape and dimensions.Compression moulding is the oldest technique for the production of polymer products,and is mainly used for thermosets.
BLOW MOULDING
Blow moulding is the process by which articles are formed by in ation of a molten resin to Žfill a mould cavity having the desired shape and dimensions. Bottles for soft drinks and other liquids are blow moulded. The two most important process variants are extrusion blow moulding and injection blow moulding.In extrusion blow moulding, an extruder pumps the melt through an annular die to form a molten tube or parison with well deŽ ned and controlled dimensions. The parison is clamped between the two mould halves and is in ated by internal air pressure to take the shape of the mould cavity, which is usually cooled. Finally, the formed article solidiŽ es as a result of cooling and the mould is opened to eject the article without damage. Depending on the thermal stability of the material, the extrusion process can be continuous or intermittent. Intermittent systems can use reciprocating screws which operate as rams, in the same manner as in injection moulding.
Injection blow moulding is a two stage process. In the Žfirst stage, the plastic is injected into a cavity where the preform is moulded. The preform is then transferred to the blow mould for infiation. Injection blow moulding offers the advantages of accurate dimensional control, elimination of scrap and the moulding in of threads before blowing. Extrusion blow moulding is preferred for contain- ers with high length/diameter ratios and for products with handles.
Polyethylene terephthalate or PET is the polymer most widely used in injection blow moulding, for carbonated drinks and water bottles. Stretch blow moulding is used to produce PET bottles of enhanced physical properties. In this process, stretching induces the formation of small lamellar crystals. These crystals result in more transparent and tougher products than those produced without stretch blowing, which have spherulitic crystals.
COMPRESSION MOULDING
Compression moulding is the oldest technique for the production of polymer products,and is mainly used for thermosets. In this process the compound is pressed in the mould by the heated platens of a hydraulic press. This pro- cess is to some extent analogous to sheet metal stamping. Injection moulding of polymers has replaced compression moulding for some polymers, because of the advantages in materials handling and automation. However, compression moulding has an advantage in the processing of reinforced polymers.Owing to modest levels of deformation and stress involved in compression moulding, the reinforcing fibres are not damaged. Very high fibre concentrations and longer Žfibres can be included in compression moulded products.
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