Relationship between plastic in flowing state and hot runner parts
1. Chemical relationship
The decomposition products of molten plastics have a corrosive effect on the mold, so special hot runner parts are required. PVC has strong corrosion at processing temperature, because it will release chlorine and hydrogen and form hydrochloric acid. During the whole process of heating POM and PET, formaldehyde and acetaldehyde are released respectively. Melts of fluoroplastics such as PFA, FEP, PVDF (polyvinylidene fluoride) or ETFE (ethylene/tetrafluoro ethylene copolymer) are highly corrosive. Under this condition, chlorine and iron quickly form chemical bonds. Too high temperature will cause decomposition of smoldering agent; Make bromine or fluorine and other compounds release halogen atoms. Stainless steel or protective coating is required. Copper alloys are highly susceptible to corrosion.
The chemical reaction of PP with copper alloy and beryllium copper alloy also has adverse effects. This reaction will cause the surface of beryllium copper alloy to be corroded, and the injection parts will be polluted by alloy molecules. Copper has catalytic effect on PP melt, and the long molecular chains of plastics are damaged at the same time. When PP is injected, beryllium copper alloy parts should be coated with nickel or silicon carbide.
2. Mechanical action
Glass fiber (GF), mineral filler and some inorganic pigments, such as titanium oxide and chromium oxide, are added into the plastic. As long as all parts of the injection machine that contact the plastic are subject to abrasion damage, especially the screw and its check valve. There are also hot runner, mainly nozzle top, gate and valve core. PA66 reinforced by GF has a particularly strong wear effect, which is 7 times that of PES reinforced by GF. Special wear resistant nozzles and replaceable inserts such as thimbles and sprue rings should be used.
Many hot runner mold customers have a misunderstanding: the plastic solution in the hot runner is always hot, so the injection pressure loss of the hot runner is far less than the cold runner.
This is not the case. In order to meet the needs of hot runner structure design, the flow interval of solution in hot runner system will be greatly increased, so the loss of injection pressure in hot runner system cannot be ignored. In practical application, due to the excessive injection pressure loss in the hot runner system, the injection molding is difficult, the processing of plastics with poor fluidity (such as PC, POM, etc.), the flow interval of solution in the hot runner system is large, and the product weight is large, CAE runner analysis accounting software should be selected.
