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Causes Of Cracking In Halogen-free Low Smoke Flame-retardant Cables
- Jun 25, 2018 -

The common and troublesome quality problem of halogen-free low-smoke flame-retardant cables is the environmental stress cracking of the insulation material. The main reasons for the environmental stress cracking of halogen-free and low-smoke flame retardant materials are as follows.


Causes of Cracking in Halogen-free Low Smoke Flame-retardant Cables

The common and troublesome quality problem of halogen-free low-smoke flame-retardant cables is the environmental stress cracking of the insulation material. The main reasons for the environmental stress cracking of halogen-free and low-smoke flame retardant materials are:


(1) Polymer properties: Due to the high loading of flame-retardant fillers such as aluminum hydroxide and magnesium hydroxide, the strength of the polymer body is reduced, the elongation and flexibility are also reduced, and the tear resistance and crack resistance are inherently insufficient. At present, the commonly used halogen-free material carriers generally use low-density polyethylene and EVA. After the flame-retardant compounding, the hardness of the mixture is high, especially to meet the IEC332-1 flame retardancy (single-burning level, material The corresponding oxygen index is generally greater than 28), and the material hardness at this time is often greater than the Shore A90.


(2) Extrusion process of materials: Based on the high filling characteristics of halogen-free low-smoke materials using metal hydrate as a flame retardant, such as the extrusion process temperature, improper compression ratio of the screw will cause decomposition and high hydrated oxides. Cut the polymer broken bond. Due to the relatively large compression, the speed of the extruder cannot reach the normal level, resulting in too long time for the material to stay between the barrel and the screw. Under the high temperature caused by the friction, the hydrous oxides decompose in advance and produce bubbles inside the material (micro hole). The mechanical strength and anti-winding ability of the finished product after extrusion are greatly reduced.


Sudden cooling also increases the disadvantages.


(3) Conditions of use and storage conditions: Polyethylene-based halogen-free low-smoke materials are generally sensitive to temperature. In environmental conditions with large temperature changes, internal stresses caused by thermal expansion and contraction tend to cause cracking in the interface region. For example, if a user in the north places the finished cable tray of the halogen-free flame-retardant cable on the user's premises without external packaging, one week later, almost all of the cables on the disk were found to have longitudinal cracks on the sunny side, and the cable length was up to the cable. 1/2 circumference of the disk. The analysis is due to the fact that the cable tray has a sun-facing side and a backside, and the two sides are cracked by the large temperature difference and bending stress.


(4) Extrusion Mould: The structural type of the extrusion die, the draw ratio and the tension balance are also crucial to eliminate the internal stress after extrusion. Semi-extruded molds with lower pressure are generally used. The stretching ratio and the tension equilibrium ratio of the inner and outer molds should be controlled at 1:2 and close to 1, respectively.


The equation for the tensile equilibrium ratio is as follows:


In the formula: Dd——diameter of the mold sleeve (mm)


DT——The outer diameter of the core (mm)


dcw—Outside diameter (mm) after cable insulation or jacket extrusion


Dbw——core diameter (mm)