The plastic extrusion of wires and cables is using a continuous extrusion process. The plastic is extruded onto the conductor or wire core through a screw of extruder, forming the insulation layer, shielding layer, inner sheathe layer, and outer sheath layer of the wires and cables.
A typical plastic extruding machine consists of a pay-off device with tension control, straightening device, preheating device, extruder (main machine), cooling device, spark tester, meter counter, traction device, take-up device, and control system.
The most widely used extrusion machine for wires and cables is the single-screw extruder, which determines the main performance of the production line. After decades of innovation, the basic performance of single-screw extruders has been continuously improved. In general, the factors affecting the output of extruders can be considered from the following aspects.
1.Increase the extruder screw speed
This is the most critical factor influencing the capacity of an extruding machine. The screw speed not only increases the extrusion speed and output of materials, more importantly, ensures good plasticization while achieving high production rates.
In the past, the main way to increase the output of extruders was to increase the screw diameter. Although the material extruded per unit time will increase as the screw diameter increases, the extrusion machine is not a screw conveyor. In addition to extruding materials, the screw also extrudes, stirs, shears, and plasticize. Under the premise that the screw speed remains unchanged, the mixing and shearing effect of a screw with a large diameter and a large groove on the material is not as good as that of a screw with a small diameter.
When the screw diameter remains unchanged and the screw speed is increased, the torque endured by the screw will increase. When the torque reaches a certain level, the screw is in danger of being twisted. However, by improving the material and production process of the screw, rationally designing the screw structure, shortening the length of the feed section, increasing the flow rate of the material, and reducing the extrusion resistance, the torque can be reduced and the screw's bearing capacity can be improved. How to design the most reasonable screw and maximize the screw speed while the screw can withstand it requires professionals to obtain it through a large number of experiments.
2.Improve the extruder screw structure
The screw structure is a primary factor affecting the extruder's capacity. Simply increasing the screw speed to enhance output without a rational screw structure is contrary to objective laws and is unlikely to succeed.
The design of high-speed, high-efficiency screws is based on high rotation speeds. This screws may exhibit suboptimal plasticization effects at low speeds, but as the screw speed increases, plasticization gradually improves, reaching optimal levels at the design speed. This results in both high productivity and satisfactory plasticization. For example, the BM-type screw is an efficient low-resistance screw, but a simple BM-type screw may not meet requirements. Improvements, such as adding suitable kneading and pin shear elements to the basic BM-type screw, prove effective in increasing material throughput and improving plasticization effects during high-speed operation. High-speed, high-efficiency screws can be optimized in various aspects through extensive experiments, resulting in outstanding performance.
3.Design advanced barrel structures
Improving the barrel structure involves enhancing temperature control in the feed section and incorporating feed slots. The independent feed section is essentially a water jacket running its entire length, with advanced electronic control for temperature regulation.
Proper water jacket temperature is crucial for the stable and efficient operation of the extruder. Excessive water jacket temperature softens the raw material prematurely, potentially causing surface melting of the material particles and weakening the friction between the material and the barrel wall, thereby reducing extrusion thrust and output. However, excessively low barrel temperatures increase the resistance to screw rotation, and when it exceeds the motor's bearing capacity, it can lead to difficult motor starting or unstable speeds. Advanced sensors and PLC control technology should be applied to monitor and control the water jacket temperature automatically within the optimal process parameters.
The implementation of trapezoidal high-force feed groove technology in the barrel's feed section significantly improves the extrusion capacity of the extruding machine. Some companies now adopt a modular design for the barrel, with a detachable feed section barrel, allowing easy replacement of the more easily worn feed section, contributing to an extended equipment lifespan. Under essentially the same structure, the manufacturing cost of the reducer is roughly proportional to its external dimensions and weight. Larger external dimensions and weight of the reducer imply higher material consumption during manufacturing, and the use of larger bearings increases manufacturing costs.
4.Implement multiple vibration reduction measures
High-speed extruders are prone to vibration, and excessive vibration is highly detrimental to equipment operation and component lifespan. Therefore, multiple vibration reduction measures must be taken to minimize extrusion machine vibration and improve equipment lifespan.
The motor shaft and the high-speed shaft of the reducer are the most susceptible points to vibration in wire and cable extruders. Firstly, high-speed extruders should be equipped with high-quality motors and reducers to avoid vibration caused by the rotor of the motor and the high-speed shaft of the reducer. Secondly, a well-designed transmission system is essential. The use of a belt pulley system to transmit motor power, with a vibration-reducing belt, is advantageous in preventing motor vibration from transmitting to the reducer. However, if the dynamic and static balance of the belt pulley itself is poor, it can also introduce new vibrations. Therefore, high-quality pulleys and belts subjected to dynamic and static balance tests should be employed.
Through optimization and careful manufacturing at each stage, the overall performance of high-speed and high-efficiency extruders has significantly improved. This has greatly reduced energy consumption, enhanced product quality, and aligned with the current trend of energy conservation and emission reduction in society.