The conductors of fire-resistant cables are copper conductors, the conductor geometry is circular, and multiple stranded conductors should be pressed tightly. The conductors are wrapped with two or more layers of mica tape overlapping each other, and the wrapping angle is controlled at 40-50 degrees.
Low smoke halogen-free flame retardant cable production process
The conductor of the fire-resistant cable is a copper conductor, the conductor geometry is circular, and multiple stranded conductors should be pressed tightly. Compared with sector-shaped conductors, circular pressed copper conductors have many advantages: when wrapping mica tape, the mica tape is closely combined with the conductor, which is conducive to uniform distribution of the electric field and enhances the electrical insulation performance of the cable. At the same time, it can save the amount of mica tape and reduce the cost of the cable. cost.
The outside of the conductor is wrapped with two or more layers of mica tape. Generally, the overlap rate is not less than 30%. Sometimes the overlap rate reaches 50% to meet the fire resistance test requirements, and the fire resistance test is used as the benchmark. The wrapping angle is controlled at 40-50 degrees.
Cable insulation is allowed to use cross-linked polyethylene or halogen-free low-smoke flame-retardant polyolefin insulation materials according to different flame retardant levels. Since the conductor is overlapping and wrapped with two or more layers of fire-resistant mica tape, for cables with a conductor cross-section of 25mm2 and above, The thickness of the insulation layer is allowed to be reduced by 20%, but it 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 sheath material, and its structural dimensions are in accordance with relevant standards.
Generally speaking, the extrusion of halogen-free low-smoke flame-retardant polyolefin insulation is extruded, and the sheath is produced by extruded tube or semi-extruded tube. When using an extrusion mold, the melt viscosity of the halogen-free low-smoke flame-retardant polyolefin is high, which increases the pressure on the machine head and presses the extruded product more densely, causing expansion when leaving the mold. Therefore, a mold with an inner diameter larger than the finished product is selected. The nominal diameter is about 5% smaller. The draw ratio must be considered when using extruded tube or semi-extruded tube production. The draw ratio of halogen-free low-smoke flame-retardant polyolefin is 2.5-3.2. Theoretically, the smaller the draw ratio, the smoother the surface. In practice, sheath matching Mold: inner diameter of mold core = outer diameter of wrapping layer + (0.6~1.5) mm, inner diameter of mold sleeve = nominal outer diameter of cable + (2~7) mm.
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. Generally speaking, the friction of halogen-free low-smoke flame-retardant polyolefin cable materials heats up faster than low-smoke, low-halogen and ordinary flame-retardant polyolefin cable materials, and the process temperature range is narrower.
The screw rotates quickly and the shearing effect increases, which can easily cause mechanical thermal decomposition of the flame retardant; as the rotation speed increases, the frictional heat generation also increases, which can also cause the flame retardant to decompose and affect the extrusion surface quality. At the same time, when the screw speed is high, the motor will be overloaded.
Use a screw with a length-to-diameter ratio L/D of 20 or 25 to obtain a more ideal extrusion surface, and a screw compression ratio of 1:1 to 2.5:1 is more suitable. Extrusion equipment must have a good cooling device.
All in all, the production of low-smoke halogen-free flame-retardant cables requires ultra-high production technology and professional production equipment to ensure high-quality completion of the product. If you do not understand the production process of low-smoke halogen-free flame-retardant cables, I hope this article will describe It can bring you some help.