Interaction between plastics in fluid state and HR components

Chemical interaction. Decomposition products of molten plastics may have a corrosive effect on the mould, and particularly on HR components. PVC has a strong corrosive action at the processing temperature because of the release of hydrogen chloride and the formation of hydrochloric acid. Overheating of POM and PET releases formaldehyde and acetic aldehyde, respectively. Fluid plastics of the fluorine group, like PFA, FEP, polyvinylidene fluoride (PVDF) or ethylene tetrafluoroethylene copolymer (ETFE) are highly corrosive, as in these conditions fluorine readily enters into a chemical bond with iron. Too high a temperature may be the cause of decomposition of flame retardants (antipyrenes) and of halogen release, as compounds of bromine, fluorine, etc. Stainless steels or protective coatings then need to be used. Copper alloys are especially vulnerable to corrosion.

The reverse action also occurs, e.g., entry of PP into a chemical reaction with copper alloys, and hence with copper/beryllium. This reaction may cause the surface of a copper/beryllium component to become corroded, with the moulded parts being contaminated by alloy molecules. At the same time macromolecules of plastic are damaged through the catalytic action of copper on molten PP. Copper/beryllium nozzles for injection of PP should have a nickel or silicon carbide coating.

Mechanical interaction. Glass fibre (GF), mineral fillers for plastics and some inorganic pigments like titanium dioxide (TiO2; titanium white) and chromium oxide (Cr2O3; chrome green) cause abrasive damage to all injection machine components coming into contact with the plastic, especially the screw and its non-return valve, and also to the HR, chiefly the ends of the nozzle tips, the gates, the valve pins and so on. A particularly strong abrasive effect is brought about with PA 66 and GF. Research has shown that it is 7 times greater than that found with PES and GF. With these plastics, special abrasion-resistant nozzle types and replaceable inserts, tips and gate apertures, should be used.

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