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

1.1 Fire Resistant Cable Conductor

The conductor of a fire-resistant cable typically consists of a copper core, which is shaped into a circular geometry. The use of multiple strands ensures better conductivity and durability. These stranded conductors are often pressed together, which provides several advantages over fan-shaped conductors. For instance, when mica tape is applied around the conductor, it adheres more tightly, ensuring an even distribution of the electric field. This enhances the cable's electrical insulation properties while reducing the amount of mica tape required, thus lowering costs.

1.2 Fire-Resistant Layer

Around the conductor, two or more layers of mica tape are wrapped, with an overlap rate of at least 30%. In some cases, the overlap may reach up to 50% to meet specific fire testing requirements. The wrap angle is usually controlled between 40 and 50 degrees to optimize performance during fire tests.

1.3 Cable Insulation and Sheath

Cable insulation can vary depending on the flame-retardant grade, allowing the use of either cross-linked polyethylene or halogen-free, low-smoke, flame-retardant polyolefin materials. Given the presence of two or more layers of fire-resistant mica tape 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 the cable still passes the necessary fire-resistant tests. The cable sheath is made from halogen-free, low-smoke, flame-retardant polyolefin material, following relevant industry standards for structural dimensions.

1.3.1 Extrusion Die

Halogen-free, low-smoke, flame-retardant polyolefin insulation is typically extruded, while the jacket is produced using either a squeeze tube or semi-extruded tube method. When using an extrusion die, the melt viscosity of the material increases, raising the head pressure, which results in a more compact extruded product. Consequently, the inner diameter of the die is chosen to be approximately 5% smaller than the nominal outer diameter of the finished product. When employing extruded or semi-expanded tube methods, the draw ratio must be taken into account. For halogen-free, low-smoke, flame-retardant polyolefin materials, the draw ratio ranges from 2.5 to 3.2. Theoretically, the smaller the draw ratio, the smoother the surface, and the more practical the sheath. A recommended formula for the core diameter is: core diameter = outer diameter of the cladding + (0.6 to 1.5) mm, and the 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. This allows the extrusion process to stabilize, ensuring the temperature remains within the optimal range for the material. Halogen-free, low-smoke, flame-retardant polyolefin materials tend to experience a faster rise in frictional heat compared to low-smoke, low-halogen, and ordinary flame-retardant polyolefin materials. As a result, their process temperature range is narrower. High screw speeds increase the shearing effect, potentially causing mechanical and thermal decomposition of the flame retardant, which can impact the quality of the extruded surface. Additionally, high screw speeds can lead to motor overload.

1.3.3 Extrusion Equipment

An extruder with a length-to-diameter ratio (L/D) of 20 or 25 is preferred for obtaining a superior extrusion surface. A screw compression ratio of 1:1 to 2.5:1 is also suitable. The extrusion equipment should feature an effective cooling system to maintain consistent temperatures and improve overall performance.