From the outside to the inside shell (stainless steel / carbon steel / copper / titanium heating wire (nickel, chromium, lead alloy), fillers (magnesium oxide, the role of thermal conductivity, insulation), housing (stainless steel, carbon steel, titanium, copper, etc.), ceramic head (role: Insulation), explosion-proof box. 
 Shape classification: 
 Single-ended straight, double-ended straight, U-shaped, W-type, shaped