In the production process of wire and cable, we often encounter conductors that need to be preheated, but the reasons for preheating conductors and the setting of process parameters are not well understood.
Ways of preheating conductors
1. Drying the conductor or stranded conductor in an oven.
2. Using a gun barrel pre-heater to dry the conductors simultaneously when the insulation is extruded.
3. Use of a hot air blower, directed at the conductor.
4. Use of a heater for uninterrupted baking of the payoff spool.
5. Use of induction preheaters to dry the conductors simultaneously when the insulation is extruded.
The role of conductor preheating
1. Remove moisture and avoid bubbles in the insulation
As the conductors we use are mostly filled with fibers, the fibre material has a water absorption rate of 3% to 5%. When the fibers are not dried, when they are extruded, at an extrusion temperature of 150°C or more, the moisture evaporates and the steam expands rapidly, resulting in bubbles when the insulation thickness is thin and the strength of the solution is not great, while causing the phenomenon of broken glue. At the same time, the surface of the copper conductor can also produce moisture, causing the same consequence.
In addition to drying, this can be improved by lowering the temperature of the eye mould to increase the strength of the solution, vacuuming to eliminate the effects of steam, changing the distance between the inner and outer eye mould to increase the elimination of gas, cooling the insulation quickly to increase the strength of the insulation and increasing the thickness of the insulation.
The bubbles mentioned here refer to the bubbles generated along the surface of the conductor, excluding the bubbles inside the insulation caused by the moisture of the insulation material itself.
2. Improve the adhesion of the insulation material
When the insulation is extruded the temperature of the conductor is usually at room temperature, around 15°C to 30°C, while the temperature of the insulating rubber (at the eye mould) is around 120°C to 220°C, which means that the temperature difference between the two is around 100°C to 200°C. With such a large temperature difference, when the insulation material touches the surface of the conductor, the material on the contact surface cools rapidly, and after the wire is out of the mould, the outer layer of material cools slowly. The rapid cooling of the adhesive material increases the surface hardness and reduces the adhesion of the adhesive material to the conductor. At the same time, the cooling of the adhesive material is a shrinkage process. When the insulation thickness is relatively thick, the inner and outer layers of the insulation do not shrink synchronously and the shrinkage force generated by the cooling of the outer layer of adhesive material causes the inner layer to be stretched outwards, resulting in a reduction in adhesion. To increase the adhesion of the insulation, it is therefore necessary to reduce the temperature difference between the two in terms of the conductor.
Other methods to increase the adhesion of the insulation are: extrusion, vacuum, increase the distance between the inner and outer die, change the size of the outer die, hot water cooling, increase the extrusion pressure of the rubber and other measures.
3. Improve the crystalline state of the insulation material and eliminate residual stress
The processing of plastic is a polymer chain open melt --- rearrange the process of recrystallization, melt process, the molecular chain in the temperature and the role of shear force is disrupted, cooling molecular chain rearrangement, the arrangement of the molecular chain needs time and certain temperature conditions. If the conductor is cold, the rearrangement process of the molecular chains is frozen before it is completed and this force with a tendency to rearrange itself remains in the insulating material. On the one hand, the irregular arrangement of the molecular chains causes the performance of the material not to be fully developed and the strength, elongation and other characteristics to be reduced; on the other hand, the existence of this residual stress, in the subsequent processing and storage, use process, residual stress release causes the insulation to crack.
Temperature differences are particularly severe for crystalline materials such as PE.
Other ways to improve the crystalline state of the insulation are: using hot water cooling to reduce the temperature difference between the inside and outside of the insulation; reducing the compression ratio of the screw to reduce the residual shear stress; improving the design of the head and mould runners to avoid stress concentration points; baking the insulation core in an oven to eliminate stress, which requires setting the temperature and time according to the insulation thickness; designing the mould to reduce the stretching ratio, etc. The mould is designed to reduce the stretch ratio, etc.
The conductor preheating process settings
The conductor preheating temperature or preheating current should be set according to the equipment specifications, insulation thickness, insulation material process temperature, production speed, ambient temperature, distance between the preheater and the eye mould, etc. At the same time, the preheating process is not a fixed value because the fibre material filled in the centre of the conductor has a certain heat-resistant temperature. Therefore, the preheating process is set to minimize the temperature difference between the conductor and insulation without changing the conductor's performance, and to match the hot water cooling process.
Conductor preheating considerations
1.Under the premise of ensuring the stability of the discharge line, let the preheater and the head as close as possible to avoid heat dissipation.
2.In the production of debugging, broken line processing and other abnormal processing, should pay attention to the changes in preheating temperature; if necessary, need to temporarily shut down the preheater.
3.When the conductor surface moisture is more serious, it should be wiped before entering the preheater to avoid oxidation and spots on the conductor.
4. Preheating during induction is only for bare conductors and is not valid for insulated core wires.
5. Too high a preheating temperature may cause the filling fibers to shrink or even carbonise, resulting in a reduction in tensile strength, so please check.
6. When the cable is extruded outside the need for a certain adhesion requirements, the adhesion of the insulation should be slightly greater than the adhesion of the outside, the preheating temperature is set to assess the adhesion as a guideline.
7. For UL wire, the tensile strength of the insulation is required, and the physical and mechanical properties test is arranged during the first inspection.
8. For pre-heating for the purpose of eliminating moisture, please use dry filled fibre material as far as possible.