I. Introduction
Silicone rubber, due to its high heat resistance and excellent cold resistance (with a long-term operating temperature range of -90 to 250℃), along with its excellent electrical insulation and aging resistance, has seen rapid development in the cable industry.
Over the past few decades, the silicone rubber industry has continuously faced the challenge of meeting the growing needs of the modern extruded product market. Silicone rubber has seen improvements in abrasion resistance, cut resistance, chemical resistance, oil resistance, and mechanical strength. As a material with a high heat aging temperature, its value and reliability have promoted its widespread use by manufacturers and users. Today, the application of silicone rubber in the wire and cable industry continues to expand, primarily used as insulation and sheathing for ship cables, aviation cables, motor lead wires, heating wires, and many special-purpose cables (such as those used in the nuclear power, aerospace, and metallurgical industries).
In the past two years, our company has received numerous orders from customers for silicone rubber insulated and sheathed cables. However, since this type of product is not our main product, our processing technology and production equipment are not yet fully developed. Although we encountered many difficulties during production, through everyone's joint efforts, the products were ultimately delivered to customers on schedule, and we learned a great deal from the experience.
II. Problems Encountered During the Production of Silicone Rubber Cables
1. After vulcanization and compounding, the rubber compound is prone to scorching and contains a large number of impurities during extrusion, leading to insulation voltage breakdown.
2. Loose shells and blistering occur during sheath extrusion.
III. Solutions
1. Mixing and Re-mixing Technical Requirements
Although the processing equipment for silicone rubber is similar to that for organic rubber, it is best not to use the same open mill to process both organic and silicone rubber. Ideally, a dedicated silicone rubber processing room should be available, and the environment should be kept clean, as contaminated silicone rubber will have reduced mechanical and electrical properties. If dedicated processing equipment and facilities cannot be provided for silicone rubber, it is crucial to ensure complete isolation of contaminating materials from silicone rubber and its compounding agents, as most impurities originate from the compounding process.
Due to the inherent characteristics of silicone rubber, compounds requiring reprocessing will undergo plasticity changes after reprocessing, making them prone to scorching on high-speed rollers. Cooling water should be circulated through the rollers of the open mill to prevent scorching, especially for compounds using bis(2,4-dichlorobenzoyl) peroxide as the vulcanization system. This is because the decomposition temperature of bis(2,4-dichlorobenzoyl) is approximately 45°C, and the decomposition products, 2,4-dichlorobenzoic acid and 2,4-dichlorobenzene, are not easily volatile, leading to easy burning of the compound. To obtain high-quality products, the following basic steps must be followed during silicone rubber compounding:
(1) Carefully weigh each compounding component to be used (e.g., flame retardant, vulcanizing agent, color masterbatch, etc.). (2) After placing the pure silicone rubber or reinforcing compound on the open mill, adjust the roller gap so that the silicone rubber wraps around the faster-moving roller and is thoroughly re-mixed. Pure silicone rubber usually only requires slight re-mixing or no re-mixing before adding fillers. However, because reinforcing silicone rubber contains silica, it must be thoroughly re-mixed. The re-mixing is complete when the compound wraps around the faster-moving roller.
(3) If necessary, flame retardants, color masterbatches, etc., should be added to the compound. Some fillers may fall into the receiving tray during mixing; these should be collected and added to the compound before the next filler addition. A rubber scraper is commonly used to scrape the fillers from the receiving tray; a brush should be avoided as some bristles may fall off and mix into the compound. It is especially important not to add all fillers to the compound at once, but rather in 2-3 batches. The compound should be thoroughly mixed after each batch of filler is added. This ensures uniform dispersion of the filler and avoids the formation of hard filler lumps. A reasonable roller gap ensures optimal mixing speed and quality of the rubber compound.
(4) The last ingredient added to the rubber compound is the vulcanizing agent. Since we currently use 2,4-benzoyl dichloride peroxide as the vulcanizing agent, do not add it when the rubber compound is too hot (not exceeding 40°C). Otherwise, partial premature vulcanization will occur, leading to loss of the rubber compound or vulcanizing agent. Sufficient cooling water should be introduced into the rollers to prevent overheating of the rubber compound. Finally, to ensure uniform dispersion of the vulcanizing agent, the entire roll of rubber compound should be circulated through it several times.
2. Correct Use of Filter Mesh and Filter Pads
Filter mesh typically consists of a 20-40 mesh filter plate and a 60-100 mesh stainless steel filter mesh with finer openings. Some manufacturers prefer not to use filter mesh and extrude directly because this increases the extrusion speed and sometimes eliminates the possibility of heat generation and scorching near the filter plate. However, the use of filter screens is crucial, as they play a significant role in removing impurities and undispersed filler particles from the rubber compound. Simultaneously, the filter screens also remove air trapped in the rubber compound during mixing and reprocessing, especially for softer compounds.
Since silicone rubber is only slightly thermoplastic and is not easily subjected to flow stress or shear strain in its uncured state, the design of the filter plate is not critical. Most filter plate designs are suitable for silicone rubber processing.
3. Selection of Extrusion Equipment
Regarding equipment selection, since we do not have dedicated silicone rubber cable production equipment, we have no choice. Of the three continuous vulcanization production lines we currently use, the Φ65/Φ90 steam continuous vulcanizing extruder and the PLCV salt bath continuous vulcanizing extruder are suitable for silicone rubber production.
4. Process Exploration and Improvement
Since silicone rubber cables are not our regular product, we are always learning by trial and error in their production. For insulation extrusion, we used both a Φ65/Φ90 steam continuous vulcanizing extruder and a PLCV continuous vulcanizing extruder. However, the Φ65/Φ90 steam continuous extruder cannot connect a telescopic tube to the die head because doing so would cause the die head temperature to rise rapidly under steam heating, resulting in premature vulcanization of the silicone rubber during extrusion. Therefore, in actual production, we did not connect the telescopic tube to the die head, but instead used steam in an open manner. This resulted in the inability to apply excessive steam pressure, affecting production speed and wasting a significant amount of steam. When using the PLCV unit for insulation extrusion, under controlled extrusion temperature conditions, extrusion itself did not present major problems; only 0.2 MPa pressure or no pressure was needed to meet product requirements.
We are currently producing a batch of silicone rubber insulated and silicone rubber sheathed single-core cables. Because the customer requires easy peeling between the insulation and sheath, this means that the double-layer co-extrusion process we normally use in producing single-core cables cannot be used. When preparing to extrude the sheath after the insulation was extruded, a problem arose: the silicone rubber insulation layer of the insulated core was easily pushed up in the inner mold, becoming blocked and causing blistering and bursting of the sheath after extrusion. After repeated experiments, we adopted a method of longitudinally wrapping a layer of non-woven fabric around the insulated core to provide a storage space for the gas released during the secondary vulcanization of the silicone rubber insulation. This effectively solved the problems of insulation being pushed up in the inner mold and sheath blistering at a pipeline pressure of 0.2–0.3 MPa.
In September 2006, a customer ordered a silicone rubber insulated and silicone rubber sheathed frequency converter cable. Because copper tape was wrapped around the insulated core as a shielding layer after cabling, we initially used a method of applying pressure only after the finished head was sealed during sheath extrusion to prevent water ingress. However, the same problems of sheath loosening, blistering, and bursting occurred. Through verification, our method of wrapping a layer of non-woven fabric around the copper tape shielding layer and applying pressure when the cable head enters the vulcanizing tube 5m has effectively solved the problems of sheath loosening, blistering, and bursting.
Based on previous successes, we blindly believed that non-woven fabric could solve the problems encountered in the production of silicone rubber cables. However, the reality is that not all silicone rubber cables will produce qualified products simply by wrapping non-woven fabric around the insulation. When extruding the sheath after multi-core cable cabling, although non-woven fabric is also wrapped, insufficient filling during cabling and insufficient melt pressure during sheath extrusion often prevents the silicone rubber from being squeezed into the insufficiently filled gaps, resulting in leakage after pressure vulcanization in the vulcanizing tube. Therefore, we do not recommend adding filling and wrapping non-woven fabric during sheath extrusion of multi-core cables after cabling, because the cable cores have sufficient gaps to store the gas released by the silicone rubber insulation during secondary vulcanization. As long as the pipeline pressure is applied appropriately, sheath loosening, blistering, and leakage can be completely avoided.
IV. Conclusion
In summary, by analyzing the problems encountered in the production process, focusing on their unique aspects, and looking beyond the surface to the essence, we were able to fundamentally identify the causes of the problems and adopt practical and feasible process and technical measures to solve them. We believe that through our efforts, the production process of silicone rubber cables will continue to mature, product quality will continue to improve, silicone rubber cables will gradually become our leading product, and the company's market competitiveness in rubber-sheathed cables will continue to strengthen.