Structure and Materials of the Heating Wire
The heating wire should be resistant to tension, bending, corrosion, moisture, aging, and have good insulation. It is recommended to adopt the following two structures.
Heating Wire Structure
(1) Multi-layer composite structure (Figure 17). The glass fiber layer 2 bears the tensile force (its tensile strength generally exceeds 15 kg), and its strength should be suitable for spring mattresses. The plastic layer 1 provides insulation, corrosion resistance, and moisture resistance. 3 is the resistance wire.
(2) Spiral twisted structure (Figure 18). The polyester-coated resistance wire 3 is twisted on the glass fiber core wire 2, and the outer layer is the plastic insulation layer 1. In order to improve the flexibility of the resistance wire, multi-strand resistance wire should be used, especially when the diameter of the resistance wire is greater than 0.14 mm.
Insulating Materials
Some factories use polyvinyl chloride as the insulating layer. Compared with rubber, it has advantages such as aging resistance, oil resistance, chemical resistance, moisture resistance, and good coloring properties. The biggest disadvantage of using polyvinyl chloride as the heating wire insulation material is that its maximum continuous operating temperature cannot exceed 105°C (it becomes soft at 75–80°C). Chlorinated rubber ethylene is a plastic material with relatively good comprehensive performance, and its maximum continuous operating temperature can reach 150°C. Chlorinated rubber ethylene has excellent heat resistance, aging resistance, and chemical corrosion resistance, and the product does not deform in moist water.
Polytetrafluoroethylene (F4) is an excellent plastic. Its tensile strength is 150–300 kg/cm². Under high temperatures, its mechanical properties are better than those of general plastics, and its fatigue life is similar to that of metals. Without external force, it can still maintain dimensional stability at 250°C. Its maximum continuous operating temperature is 260°C, and it also has excellent electrical insulation properties. It can be used in a wide range of chemically corrosive media.
Polyethylene-propylene (FEP) is a copolymer of tetrafluoroethylene and hexafluoropropylene, very light, with similar electrical insulation properties, and similar applications.
Polyethylene (PE) has excellent dielectric properties, high specific gravity, low water absorption, and good chemical stability and mold resistance. Due to different polymerization methods, polyethylene is divided into high-pressure, medium-pressure, and low-pressure types, and its maximum continuous operating temperature is 120°C.