Low-smoke halogen-free flame-retardant cable production process

1.1 Fire Resistant Cable Conductor

The conductor of fire resistant cables typically uses a copper wire, with a circular geometry. The multiple strands are often compacted together. This design offers several advantages over fan-shaped conductors. When mica tape is applied, it wraps tightly around the conductor, promoting an even distribution of the electric field. This improves the cable's electrical insulation properties, reduces the amount of mica tape required, and ultimately cuts costs.

1.2 Fire Retardant Layer

Two or more layers of mica tape are wrapped around the conductor. Generally, the overlap rate is no less than 30%, but sometimes reaches up to 50% to satisfy fire testing requirements. The wrapping angle is usually controlled between 40-50 degrees.

1.3 Cable Insulation and Sheath

The insulation of fire resistant cables can be made from cross-linked polyethylene or halogen-free low-smoke flame-retardant polyolefin materials, depending on the flame retardancy grade. Since there are two or more layers of fire-resistant mica tape wrapped around the conductor, the thickness of the insulation layer for cables with a conductor cross-section of 25mm² and above can be reduced by 20%, provided they pass the fire resistance tests. The cable sheath is made from halogen-free low-smoke flame-retardant polyolefin sheathing material, following relevant standards for structural dimensions.

1.3.1 Extrusion Die

In general, halogen-free low-smoke flame-retardant polyolefin insulation is extruded, while the jacket is manufactured using either a full or semi-extruded tube. When using an extrusion die, the melt viscosity of the halogen-free low-smoke flame-retardant polyolefin increases, raising the pressure at the die head. This results in a more compact extruded product with some expansion after exiting the die. Therefore, the inner diameter of the die is typically chosen to be about 5% smaller than the nominal diameter of the finished product. When using full or semi-expanded tube extrusion, the draw ratio must be considered. For halogen-free low-smoke flame-retardant polyolefin, the draw ratio ranges from 2.5 to 3.2. In theory, the smaller the draw ratio, the smoother the surface. The practical mode is: core diameter = outer diameter of the sheath + (0.6 to 1.5) mm, inner diameter of the die = nominal outer diameter of the cable + (2 to 7) mm.

1.3.2 Extrusion Process

The initial temperature setting is usually about 5 to 10°C lower than the standard extrusion temperature to ensure that once extrusion stabilizes, the temperature remains within the appropriate range for the material. Halogen-free low-smoke flame-retardant polyolefin cable materials tend to experience faster frictional heating compared to low-smoke low-halogen and ordinary flame-retardant polyolefin materials, and their process temperature range is narrower. Higher screw speeds increase shear effects, leading to mechanical thermal decomposition of the flame retardants, which can compromise the quality of the extruded surface. Additionally, high screw speeds result in excessive motor loads.

1.3.3 Extrusion Equipment

An extruder with a length-to-diameter ratio (L/D) of 20 or 25 can produce a preferred extrusion surface, and a screw compression ratio of 1:1 to 2.5:1 is suitable. The extrusion equipment should feature a reliable cooling system to maintain optimal operating conditions.