High-purity calcined magnesia powder is the core filling raw material for Inconel-sheathed thermocouple matched with Männer ultrahigh-temperature PEEK, PI and fluoroplastic hot runner as well as domestic new-energy high-temperature molding equipment, and the calcination temperature, heat preservation duration and impurity removal degree directly decide finished thermocouple's long-term insulation performance under over 500℃ continuous working condition. Ordinary industrial raw magnesia contains carbonate crystal water, organic impurity and soluble salt substance; if without high-temperature calcination above 1600℃ in professional kiln, these impurities will gradually decompose and release water vapor and acidic corrosive gas under ultrahigh hot runner temperature, generated internal gas pressure expands continuously and breaks compact mineral insulation structure, then corrosive gas penetrates inward to corrode noble metal core wire and trigger rapid measuring drift and insulation breakdown within short production cycle.
Standard standardized calcination process strictly controls kiln temperature curve and staged heat preservation parameter: keep raw magnesia at 1620℃ constant for more than 4 hours to fully decompose carbonate and evaporate crystal water completely, then conduct closed cooling under inert gas protection to avoid secondary moisture absorption from ambient air after high-temperature calcination. Premium calcined magnesia maintains stable high insulation resistance from room temperature up to 650℃ without gas precipitation or volume shrinkage, effectively ensuring long-term stable signal transmission of S-type and R-type platinum-rhodium noble metal thermocouple under ultrahigh-temperature corrosive environment. In contrast, inferior raw material adopts low-temperature quick calcination below 1200℃ to reduce production energy consumption cost, residual undecomposed impurities become hidden hazard after thermocouple installed on high-temperature hot runner.
Procurement technical specification clearly distinguishes calcined magnesia filled thermocouple and common industrial magnesia filled products: only configure high-temperature calcined powder finished goods for hot runner whose continuous working temperature exceeds 480℃, select economical ordinary magnesia specification for conventional PP, ABS medium-low temperature commodity hot runner to avoid over-spec cost waste. Incoming inspection sets high-temperature heating spot-check item for high-end customized thermocouple, randomly take out internal filler powder and heat up to 550℃ inside laboratory equipment, observe whether volatile gas generates to screen unqualified low-calcination inferior products before warehousing and installation.
