Silicone rubber can be processed by ordinary rubber processing equipment, but attention should be paid to: (1) the processing process should be kept clean, and other rubber, oil or impurities should not be mixed, otherwise the vulcanization and properties of silicone rubber will be affected; (2) silicone rubber products need to be vulcanized by hot air for a long time to improve the properties of vulcanizates.
1. Blending
Silicone rubber raw rubber is relatively soft and has certain plasticity. It can be directly mixed by mixer or internal mixer without plasticizing. There are several mixing methods:
(1) Refiner mixing
The optimum speed ratio of the rollers of the twin-roll mill is (1.2-1.4):1. After the fast rollers, the higher speed ratio leads to faster mixing, and the lower speed ratio can make the film smooth. The roller must be provided with cooling water and the mixing temperature should be below 40 (?) C to prevent the loss of coke burning or volatilization of vulcanizate. When mixing, the roll spacing is small (1-5 mm), and then increases gradually.
Feeding and operation sequence: raw rubber (wrapping roll) - reinforcing filler - structure control agent - heat-resistant additive - colorant - thin pass 5 times - blanking, oven heat treatment - refining - vulcanizing agent - thin pass - parking overnight - refining - sheeting. Rubber can also be treated without oven heat treatment. After adding heat-resistant additives, vulcanizing agent can be thinned through again. It can be put in overnight refining, and then put in refining tablets for several days. The mixing time is 20-40 minutes.
(2) mixing in internal mixer
When mixing with 2L mixer in laboratory, the mixing time is 6-16 minutes. There is no special difficulty in mixing. When the charging coefficient is 0.74, the mixing can also be carried out normally in a 160mm smelter. The discharging temperature is related to the type of reinforcing filler. When the weak reinforcing filler and precipitation silica are used, the discharging temperature is below 50 and when the fumed silica is used, the discharging temperature is about 70.
2. Extrusion
Silicone rubber is generally soft, good extrusion effect and easy to operate. It can extrude products of different shapes and sizes. Its processing equipment and tools are basically similar to those of ordinary rubber. Extruders usually use single threaded screw of 30 or 65 mm, and the ratio of length to diameter is (10-12):1. The extrusion temperature should be kept as low as possible, and the optimum temperature should not exceed 40 C. Therefore, the barrel and screw must be cooled by cooling water. The products with high quality requirements can be equipped with 80-140 mesh filter near the head of the machine to remove impurities in the rubber and improve the extrusion quality.
Silicone rubber extruded semi-finished products are soft and easily deformed, so it must be vulcanized immediately. The most commonly used method is hot air continuous vulcanization; high pressure steam continuous vulcanization is usually used in wire and cable industry. If continuous vulcanization is not possible after extrusion, in order to prevent deformation, it should be immediately connected with a disc, drum or conveyor belt after extrusion, and separated by talcum powder to avoid bonding with each other. If it is found that the compound is too soft to be extruded, 3-5 phr of fumed silica can be added into the compound.
3. Calendering
Vertical three-roll calender is generally used in calender. When used in film production, the middle roll is fixed, the speed of the middle roll is faster than that of the upper roll, and the speed ratio is (1.1-1.4):1. The speed of the lower roll is the same as that of the middle roll. When the calender starts, the upper roll temperature is 50 C, the middle roll should be kept at room temperature, and the lower roll should be cooled with cooling water. Calendering speed is generally 60-300 cm? Min-1, not too fast, first adjust the roll spacing (middle and lower rolls) at low speed to ensure a certain calendering thickness, and then increase to normal speed for continuous operation.
When the three-roll calender is used for silicone rubber pasting and rubbing, the fabric replaces the pad (polyester film) to pass between the middle roll and the lower roll. Three-roll calender is only suitable for one-sided glue coating. Four-roll calender should be used in long-term production.
4. Gluing
Glue coating means that the silicone rubber mortar is evenly distributed on the fabric by dipping or scraping to improve the strength and flexibility of the film products, to make the fabric moisture-resistant, to produce high temperature electrical insulation materials, etc.
