Waste recycled plastic raw material contains residual chlorine, sulfur and miscellaneous heavy metal additives left from previous product processing; these impurities decompose into complex acidic corrosive mist under molten high-temperature condition on domestic and modified Husky recycled-material dedicated hot runner, triggering typical electrochemical pitting perforation damage of conventional 304 stainless steel sheath thermocouple. When acidic corrosive vapor floats in workshop air and adheres on thermocouple sheath surface under alternating hot-cold temperature between hot runner and ambient environment, condensed tiny electrolyte liquid film covers stainless steel outer wall, different metal crystalline phases and surface impurity inclusion spots form countless micro primary battery units. Metal at anode position continuously dissolves inward to develop pinpoint corrosion pit gradually expanding depth month by month; initial invisible tiny corrosion spot has already destroyed stainless steel inherent passivation protective film and accelerates subsequent corrosion developing speed sharply.
After continuous three to six months of uninterrupted production, corrosion pit penetrates entire sheath wall completely, enabling corrosive gas and damp air to enter internal magnesia insulation layer and erode core thermo-alloy wire from inside out, leading to sharp insulation resistance drop and irregular random temperature hopping fault on controller display screen. 316L molybdenum-containing stainless steel sheath with higher acid-resistant component can obviously slow down pitting corrosion developing rate, while Inconel nickel-based alloy sheath achieves long-term stable anti-corrosion effect despite higher unit procurement cost. Many small recycled plastic processing factories select cheap ordinary 304 stainless steel thermocouple blindly to cut initial auxiliary material purchasing cost, ignoring later frequent whole-set sensor replacement expenditure and batch defective products loss caused by unstable temperature control from corroded drifting thermocouple.
Standard preventive maintenance regulation requires cleaning accumulated plastic coke and corrosive residue on thermocouple sheath surface with neutral degreasing solvent every two weeks for recycled plastic production line; regularly spray thin layer of neutral anti-corrosion protective coating on exposed sheath outer surface to reduce corrosive medium direct adhesion probability. Formal purchasing technical clause specifies minimum 316L stainless steel sheath as compulsory configuration for all thermocouple matched with recycled plastic hot runner, fundamentally restricting inferior 304 specification products from entering high-corrosion production environment and lowering long-term equipment operation hidden quality risk.
