Application of insulation and sheathing made of organosilicon rubber (silicone)

Where cable products with insulation and sheathing made of organosilicon rubber are used?

For the manufacture of thermoresistant and frost–resistant cable products, it’s used special compounded rubber - organosilicon (silicone) rubbers. This rubber has high resistant to moisture, heat and cold, ultraviolet and gamma radiation, oxygen and ozone, and also have high biological inertness. Products with insulation and sheathing made of organosilicon rubber are used as leading-out wires of electric motor windings, wires for use inside electric and gas stoves, wires for the underground railway. The operational temperature of these wires can reach 180˚С with continuous service of operation.

Composition, technology features and extrusion lines equipment for applying insulation and sheathing made of organosilicon rubbers.

The line usually includes the following equipment:

1. Pay-off machine.
2. Pre-heating device (for insulating operation)
3. Extruder with a feeding roller and crosshead.
4. Vulcanizing chamber
5. Haul-off unit
6. Printer for shell marking.
7. Device for testing ”one direction” insulation.
8. Countdown device;
9. Diameter gauge
10. Coolant bath
11. Sootblower.
12. Talc spreader.
13. Take-up machine.

The composition of lines equipment for applying insulation and sheathing made of organosilicon rubber, is similar in many aspects with the lines for applying insulation made of polymers. At the same time, there are significant differences related to the properties of the processed material.

Namely:

• Require pre-production of rubber for extrusion line reprocessing. The rubber product from the manufacturer, that coming in leafed blocks or briquettes, requires preheating and cutting into strips. This operation is performed on the heating rolls. This is a machine in which two horizontal, hollow massive rolls rotating towards each other with the same or different peripheral speeds. To maintain the required mode, the rolls have devices that regulate the gap size between the rolls, control and regulate the temperature of the rolls and the pressure in the gap, as well as a knife for producing stripes from heated rubber. The rolls body is heated by using steam or heated oil. Tapes made of rubber, that prepared for using, are placed on pallets or hung on special frames (racks).

• Proper planning of organosilicon rubber stocks is required due to its tendency to auto-vulcanization during long-term storage.
• A device is required to preheat the core to a temperature of 100 – 150˚С. This is necessary to avoid sudden cooling, when the core contact with the rubber in the extruder head and, accordingly, increases the adhesion of the rubber for insulation. The core heating, as a rule, is performed by induction method.
• The extruder has a short screw with a special cutting profile, the L/D ratio = 12:1 (for comparison, extruders for polymer reprocessing have the L/D ratio = 24 ÷ 30:1). There are no heaters on the cylinder body, the rubber is heated and plasticized due to the pressure and friction, that resulting from the screw rotation. To maintain an optimal extrusion temperature condition, the cylinder has a cooling-water jacket. The screw is also cooled by water. Depending on the design, the extrusion die is cooled by water or oil. Using of such measures is caused by the need to exclude premature vulcanization of rubber, that is caused by high temperature.
• The feed of the rubber tape into the extruder is performed by the operator, whish places the end of the tape into the extruder loader and pressed it down by pay-off roll. During the operation, due to the screw rotation, there is a continuous feed of the tape into the extruder.
• Vulcanization of organosilicon rubber is happened in an air environment (at high temperature, up to 250 – 450˚С) of vulcanization chambers, that integrated into the extrusion line.
• Because of the cable products shells have the ability to clump together during the storage of the cable product, the finished product is talcated before being placed on the drum.
• The linear speed of the extrusion line essentially depends on the shell thickness, the diameter of the insulated product, the length of the vulcanization chamber, the vulcanization time and, as a rule, does not exceed 40-50 m/min for the wire section of 1.5 mm².
• During products manufacturing with using organosilicon rubbers, it’s used one– and two-stage vulcanization.

— During single-stage vulcanization, the finished product is made directly on the extrusion line. The long-term operating temperature of the cable product, in this case, reaches 150˚С.

— During two-stage vulcanization (thermostat control), coils or drum with the product after leaving the extrusion line are placed in a thermostat, in which hot air circulates at a temperature of 200C for 1-3 hours. As a result, the product increase its physical, mechanical and dielectric characteristics, and also increases a long-term operating temperature up to 180C.

— During one– and two–stage vulcanization, in order to exclude the process of rubber destruction during the product using, it is necessary to use devices for exhaustion of cross-linking agents’ decomposition products - peroxides, moisture, and etc. substances.