Dear, I have a complete set, let’s learn some science about Minasan~
Open mill mixing
The roller speed ratio of the double-roller open mill is 1.2 ~1.4: It is appropriate, with the fast roller at the back. A higher speed ratio results in faster mixing, while a low speed ratio can make the film smooth. The roller must be flowed with cooling water, and the mixing temperature should be below 40°C to prevent scorch or volatilization loss of the vulcanizing agent. When mixing, the roller distance is initially small (1~5mm), and then gradually enlarged. Feeding and operation sequence: raw rubber (roller wrapping)—→reinforcing filler—→structural control agent—→heat-resistant additive—→colorant, etc.—→thin pass 5 times—→unloading, oven heat treatment—→return to refining — → Vulcanizing agent — → Botong — → Park overnight — → Return to refining — → Take out the tablets. The rubber material can also be heat-treated in an oven without heat treatment. After adding the heat-resistant additive, add the vulcanizing agent and then thin it out, leave it overnight for re-refining, and then leave it for a few days for re-refining and tablet use. The mixing time is 20 to 40 minutes (the specifications of the open mill are φ250mm × 620mm).
If precipitated silica or weakly reinforcing fillers (titanium dioxide, zinc oxide, etc.) are used alone, there is no need to add a structure control agent to the rubber compound. Fillers should be added slowly to prevent the spherical bodies formed by fillers and raw rubber from floating on top of the accumulated rubber and causing uneven dispersion. If you want to add a large amount of filler, it is best to add it in two or three times, and score in between to ensure good dispersion. It is found that the rubber has a tendency to granulate, and the roller distance can be tightened to improve mixing. The rubber particles falling on the receiving tray should be cleaned and collected with a brush and returned to the roller of the rubber mixing machine immediately. Otherwise, the mixed rubber material will contain rubber lumps, resulting in poor appearance of the product. The incremental filler should be added after the reinforcing filler is added, and a wider roller distance can be used.
Glubber loading capacity (mixed rubber): 1~2 kg for φ160mm×320mm rubber mixing machine; 3~5kg for φ250mm×620mm rubber mixing machine.
When the silicone rubber is added to the rubber mixing machine, it is wrapped with a slow roller (front roller), and when mixing, it is quickly wrapped with a fast roller (rear roller). It must be able to operate on both sides when mixing rubber. Since the silicone rubber material is relatively soft, an ordinary knives can be used during mixing. When thinning, the flakes cannot be pulled off like ordinary rubber. Instead, a steel, nylon or wear-resistant plastic scraper can be used to scrape off the flakes. In order to facilitate cleaning and prevent lubricating oil from leaking into the glue, a movable baffle should be used. Vapor phase silica is easy to fly and is harmful to the human body. Corresponding labor protection measures should be taken. If powdered peroxide is used directly during mixing, explosion-proof measures must be taken. It is best to use paste peroxide.
If there are impurities, hard lumps, etc. mixed in the rubber material, the mixed rubber can be filtered through a rubber filter. When filtering, a 80~140 mesh screen is generally used.
2. Internal mixer mixing
Using this method can improve production efficiency and improve working conditions. Tests have shown that the performance of closed-mix compounds is similar to that of open-mix compounds. The mixing time of the 2L internal mixer in the laboratory is 6 to 16 minutes, and there are no special difficulties in mixing. When using a φ160mm open mill, mixing can proceed normally when the charging coefficient is 0.74. The discharging temperature is related to the type of reinforcing filler: when weakly reinforcing filler and precipitated silica are used, the discharging temperature is below 50°C; when gas phase silica is used, the discharging temperature is about 70°C .
3. Parking and re-mixing of the rubber compound
After the silicone rubber compound is mixed, it should be parked for a period of time (generally not less than 24 hours) to allow it to be used. Various compounding agents (especially structural control agents) can fully interact with raw rubber. After being parked, the rubber becomes hard and its plasticity is reduced, so it must be re-melted before use. An open mill is used for back-refining. At the beginning, the roller distance is large (3~5mm). At this time, the rubber material is hard and the surface is wrinkled, and it is wrapped on the front roller (slow roller). As the re-refining time prolongs, the rubber material gradually becomes softer; the roller distance is slowly reduced (0.25~0.5mm), and soon the rubber material is wrapped on the rear roller (fast roller). "After the rubber material is fully soft and the surface is smooth and flat, the film can be cut out. Insufficient re-refining will cause wrinkles on the surface of the film; excessive re-refining will cause the rubber material to become sticky and cause roller sticking.
Re-refining The temperature is generally controlled at room temperature. If the glue becomes sticky and soft and wrinkles appear on the surface after being stored for a long time (more than one month), you can add about 5 to 10 parts of fumed silica to improve the glue. Process performance and ensure the quality of vulcanized rubber.
(2) Extrusion
Generally, silicone rubber is relatively soft, so its extrusion effect is better and easy to operate.
Silicone rubber can be used to extrude products of various shapes and sizes, and its processing equipment and tools are basically similar to ordinary rubber.
Extruders generally use single-thread screws with φ30mm or φ65mm, and the length-to-diameter ratio is 10~12:1 for better results. When extruding, try to keep the temperature as low as possible, preferably no more than 40°C, so both the barrel and screw must be supplied with cooling water. For products with higher quality requirements, an 80~140 mesh filter can be installed near the machine head to remove impurities in the rubber material and improve the extrusion quality. Silicone rubber will expand when it comes out of the mouth shape. The expansion rate depends on the flow properties of the rubber material, the thickness of the blank and the pressure on the rubber material when it enters the mouth shape. However, increasing or decreasing the extraction speed changes the elongation of the unvulcanized extruded article, thereby slightly changing its dimensions. According to experience, the rubber hose expands about 3% compared to its mouth size, while the expansion rate of very soft rubber is relatively large, and the expansion rate of hard rubber is relatively small. When extruding products of other shapes, the orifice of the die is rarely the same as the cross-section of the extruded product. This is due to the different frictional forces of the flowing rubber at different points. Therefore, for a certain mouth shape, it is necessary to go through repeated trials many times in order to obtain the product with the required shape.
For extrusion of coated wires, a T-shaped machine head is required on the extruder. The mouth shape is installed on the T-shaped machine head at right angles to the extruder barrel. In this way, the core wire can be covered with a silicone rubber sheath through the hollow orifice conduit (core type). Reinforced hoses can also be continuously produced using a T-shaped machine head. First, use the normal method to press the inner rubber layer and pre-vulcanize it, then weave reinforced steel wire or nylon on the outside, and then make it pass through the T-shaped machine head on the outside. It is covered with an outer rubber layer and is finally sent to vulcanization. During the molding process, if compressed air is filled into the hose, it can prevent the inner rubber from collapsing.
Silicone rubber extruded semi-finished products are soft and easily deformed, so they usually must be vulcanized immediately. The most commonly used method is hot air continuous vulcanization; the wire and cable industry usually uses high-pressure steam continuous vulcanization. If vulcanization cannot be continued after extrusion, it is necessary to prevent deformation. After extrusion, it should be picked up immediately with a disc, drum or conveyor belt, and separated with talcum powder to avoid sticking to each other. If it is found that the rubber material is too soft and is not suitable for production, the rubber material can be mixed with 3 to 5 parts of fumed silica.
Generally used for extruded rubber formulations, the amount of vulcanizing agent should be appropriately increased compared to molded products. The extrusion speed of silicone rubber is lower than that of other rubbers. When the same extrusion speed as other rubbers is required, a higher screw speed should be used.
(3) Calendering
Silicone rubber calenders generally use vertical three-roller calenders. When used to produce film, the middle roller is fixed. The rotation speed of the middle roller is faster than that of the upper roller. The speed ratio is 1.1~1.4:1. The rotation speed of the lower roller is compared with that of the middle roller. When the calender is started, the temperature of the upper roller is 50°C. , the middle roller should be kept at room temperature, and the lower roller should be cooled with cooling water. The calendering speed should not be too fast, generally 60~300cm/min. First adjust the roller distance (middle and lower rollers) at low speed (30~60cm/min) to ensure a certain rolling thickness, and then increase to normal speed (150~300cm/min) for continuous operation. The pad cloth (often made of polyester film) passes between the middle and lower rollers, and a small amount of glue should be kept between the middle and lower rollers so that the entire cloth and the rubber material can be closely adhered. After calendering, the film roll is tied tightly and sent to an oven or vulcanization tank for vulcanization. The mandrel of the take-up roller should be a hollow metal tube, and the thickness of the film should not exceed 12cm, otherwise it will not be fully vulcanized.
Generally, the middle and upper rollers can be used for tablet discharging, and the temperature of the rollers is normal temperature. There is a way to make vulcanized film or thin rubber sheets directly on the post-extension machine. At this time, the roller temperature is: the upper roller is 60~90℃, the middle roller is 50~80℃, and the lower roller is 110~120℃. The rubber material passes through the upper and middle rollers to remove bubbles, obtain the required specifications and preheat, and then transfers from the middle roller to the lower roller for a period of vulcanization before being rolled up.
When the three-roller calender is used for silicone rubber gluing and gluing, the fabric passes between the middle roller and the lower roller instead of the pad (polyester film). The three-roller calender is only suitable for single-sided gluing. If double-sided gluing is necessary, a four-roller calender should be used for long-term production.
The degree of re-refining of the rubber used for calendering must be correctly controlled. It is best not to fully re-re-refined on the rubber mixing machine in order to obtain sufficient re-refining during the calendering process, thus preventing the rubber from being During the calendering process, the rollers stick due to excessive re-refining. The rubber compound formula also has a certain impact on calendering. The rubber compound using reinforcing fillers has better calendering process performance.
(4) Gluing
Gluing refers to evenly distributing silicone rubber slurry on the fabric by dipping or scraping to improve the strength and strength of film products. The flexural properties make the fabric resistant to moisture to make high-temperature-resistant electrical insulation materials, etc.
1. Glue preparation
The vulcanizing agent of the silicone rubber compound used for making glue is mostly the vulcanizing agent BP (dibenzoyl peroxide). This is because The vulcanizing agent BP is not easy to volatilize at room temperature and has good adhesion to fabrics. The dosage is slightly more than that of general model products. If vapor phase silica is used as reinforcing filler, the dosage should not exceed 40 parts, and the dosage of structure control agent should be appropriately increased. The solvent should be volatile, such as toluene, xylene, etc.
After sufficient re-refining, the mixed rubber is cut into thin slices, then cut into small pieces, soaked in a solvent overnight, and stirred with a mixer or mixer until the concentration is 15~25% (solid content ) glue. After the glue is made, it should be stored in an environment below 40°C.
2. Fabric pretreatment
For the bottom fabric used for silicone rubber coating, glass cloth, nylon and polyester are used in one section. Because glass cloth has good heat resistance, high strength and low hygroscopicity, it is widely used.
The surface of glass fiber is coated with paraffin lubricant (accounting for 0.2~0.5% of the weight of the fabric) during the drawing process. It is not easy to volatilize at the vulcanization temperature and affects the combination of the rubber material and the fabric. It must be carried out before gluing. Remove. In industry, heating is often used to remove lubricants. The heating method is divided into low temperature treatment and high temperature treatment. The former is to bake the glass cloth continuously at a temperature of 200~300℃ for 20~30 minutes, or pass it through a hot roller heated to 275~325℃ at a constant speed, so that the lubricant is thermally decomposed and volatilized; the latter is Bake the glass cloth at a temperature of 500°C for 2 to 4 hours, or pass it through an oven heated to 600°C at a speed of 2 to 6m/min to remove the lubricant. The strength loss of glass cloth treated at low temperature is small (a decrease of 15~25%), but the lubricant is difficult to remove (the content is reduced to about 0.2~0.5%); the strength loss of high temperature treatment is large (a decrease of 30~70%), but the lubricant is The content can be removed to less than 0.2%. In order to reduce strength loss, low temperature treatment is generally used.
The thermal deformation of nylon is large, which affects the combination of rubber and fabric. Therefore, heat setting is required before gluing, that is, the fabric is heat treated for a short period of time under a certain draft. The processing temperature is 170~175℃.
Polyester, like nylon, also needs heat setting treatment, and the treatment temperature is 215~220℃. The difference is that the polyester needs to undergo surface chemical treatment, that is, it is soaked in 25% sodium hydroxide aqueous solution at room temperature for 6 hours to make the surface easy to bond with the rubber.
3. Gluing
After the fabric is pre-treated, the surface must be treated with adhesive before gluing. The adhesive is a solution consisting of an alkoxysilane, a borate ester, a vulcanizing agent, and a solvent (ethyl acetate or ethanol).