1.1 Cable conductor
The conductor of the fire resistant cable is a copper conductor, the conductor geometry is circular, and the plurality of stranded conductors should be pressed. Compared with the fan-shaped conductor, the circular pressed copper conductor has many advantages: the mica tape is tightly combined with the conductor when the mica tape is wrapped, which is favorable for uniform distribution of the electric field, enhances the electrical insulation performance of the cable, and can save the amount of the mica tape and reduce the amount of the mica tape. cost.
1.2 refractory layer of cable
Two or more layers of mica tape are wrapped around the conductor, and the general overlap rate is not less than 30%. Sometimes the overlap rate is 50% to meet the fire test requirements, so as to achieve the fire test. The wrap angle is controlled at 40-50 degrees.
1.3 Cable insulation and sheath
Cable insulation is allowed to use cross-linked polyethylene or halogen-free low-smoke flame-retardant polyolefin insulation material according to different flame retardant grades. Because of the overlapping of two or more layers of fire-resistant mica tape on the outer conductor, the cable with conductor cross-section of 25mm2 and above The thickness of the insulation layer is allowed to be reduced by 20%, but is based on the ability to pass the fire resistant cable test. The cable sheath is made of halogen-free low-smoke flame-retardant polyolefin sheathing material, and its structural size is implemented according to relevant standards.
1.3.1 Extrusion die
In general, the extrusion of halogen-free low-smoke flame-retardant polyolefin insulation is extruded, and the jacket is produced by a squeeze tube or a semi-extruded tube. When the extrusion die is used, the melt viscosity of the halogen-free low-smoke flame-retardant polyolefin is increased, the pressure of the head is increased, and the pressure of the extruded product is relatively compact, resulting in expansion during the die-off, so the inner diameter of the die is selected to be larger than the finished product. The nominal diameter is about 5% smaller. When using extruded or semi-expanded tube type, the draw ratio must be considered. The halogen-free low-smoke flame-retardant polyolefin has a draw ratio of 2.5-3.2. Theoretically, the smaller the stretch ratio, the smoother the surface, and the sheath is practical. Mode: core diameter = outer diameter of the cladding + (0.6 ~ 1.5) mm, inner diameter of the die = nominal outer diameter of the cable + (2 ~ 7) mm.
1.3.2 Extrusion process
The initial temperature setting is generally about 5 to 10 ° C lower than the normal extrusion temperature so that when the extrusion is stable, the temperature is just within the extrusion temperature range of the material. In general, the halogen-free low-smoke flame-retardant polyolefin cable material has a faster frictional rise than the low-smoke low-halogen and ordinary flame-retardant polyolefin cable material, and the process temperature range is narrow.
The screw speed is fast, the shearing effect is increased, and the mechanical thermal decomposition of the flame retardant is easily caused; the rotation speed is increased, the friction heat generation is also large, and the flame retardant is decomposed to affect the quality of the extruded surface. At the same time, when the screw speed is large, the motor will be overloaded.
1.3.3 Extrusion equipment
A screw having a length to diameter ratio of L/D of 20 or 25 can be used to obtain a preferred extrusion surface, and a screw compression ratio of 1:1 to 2.5:1 is suitable. The extrusion equipment should have a good cooling device.