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Introduction to rubber production process

1 overview

the main raw materials of rubber products are raw rubber, various compounding agents, as well as fiber and metal materials as skeleton materials. The basic production process of rubber products includes six basic processes: molding, mixing, calendering, extrusion, molding and vulcanization

the processing process of rubber is mainly the process of solving the contradiction between plasticity and elasticity. Through various processing means, the elastic rubber is changed into plastic rubber, and then semi-finished products are made by adding various compounding agents, and then semi-finished products with plasticity are changed into rubber products with high elasticity and good physical and Mechanical properties through vulcanization

2 rubber processing technology

2.1 plasticizing process

raw rubber plasticizing is a process in which raw rubber is transformed from a strong elastic state to a soft and easy to process plastic state by means of mechanical stress, heat, oxygen or the addition of some chemical reagents

the purpose of raw rubber plasticizing is to reduce its elasticity, increase its plasticity, and obtain appropriate fluidity, so as to meet the requirements of various processing processes such as mixing, sub derivatization, extrusion, molding, vulcanization, as well as slurry manufacturing, sponge rubber manufacturing, etc

it is very important to master the appropriate plasticity for the processing of rubber products and the quality of finished products. On the premise of meeting the requirements of processing technology, the plasticity should be reduced as much as possible. With the appearance of constant viscosity rubber and low viscosity rubber, some rubber has been directly mixed without molding

in the rubber industry, the most commonly used molding methods are mechanical molding and chemical molding. The main equipment used in mechanical plasticizing method is open type rubber mixer, closed type rubber mixer and screw plasticizer. Chemical plasticizing is a method of adding chemicals in the mechanical plasticizing process to improve the plasticizing effect

the temperature of the open mixer is generally below 80 ℃, which belongs to the low-temperature mechanical mixing method. The discharge temperature of internal mixer and screw mixer is above 120 ℃, or even as high as ℃, which belongs to high-temperature mechanical mixing

before mixing, raw rubber needs to be processed in advance, such as baking, cutting, selecting and breaking

plasticizing characteristics of several rubbers:

when natural rubber is plasticized with an open mixer, the roller temperature is ℃, and the time is about min; When the temperature reaches above 120 ℃, the time is about min

the Mooney viscosity of styrene butadiene rubber is mostly between, so styrene butadiene rubber can also not be plasticized, but after plasticization, it can improve the dispersion of the mixing machine.

cis butadiene rubber has cold fluidity and lacks plasticizing effect. The Mooney viscosity of cis-1,4-polybutadiene rubber is low, so there is no need to plasticize it

neoprene has high plasticity. It can be thinned several times before molding, and the thinning temperature is ℃

the molecular main chain of ethylene propylene rubber is saturated structure, and it is difficult to cause molecular cracking during plastic refining. Therefore, we should choose varieties with low Mooney viscosity instead of plastic refining

nitrile rubber has small plasticity, high toughness and high heat generation during molding. At the beginning of refining, low temperature below 40 ℃, small roll spacing, low capacity and segmented plastic refining should be adopted, which can achieve better results

2.2 mixing process

mixing refers to the process of uniformly mixing various compounding agents into raw rubber seeds on the rubber mixer. The quality of mixing has a decisive impact on the further processing of rubber and the quality of finished products. Even if the rubber with good formula is not mixed well, there will be uneven dispersion of compounding agent, too high or too low plasticity of rubber, easy to scorch, spray frost, etc., so that the processes of calendering, extrusion, gluing and vulcanization cannot be carried out normally, and the performance of products will also be reduced

mixing methods are usually divided into open mixer mixing and internal mixer mixing. These two methods are batch mixing, which is the most widely used method at present

the mixing process of the open mill is divided into three stages, namely, roll wrapping (the softening stage of adding raw rubber), powder eating (the mixing stage of adding powder) and refining (the stage of uniformly dispersing raw rubber and compounding agent after eating powder)

the mixing process of the open mixer is different according to the type, purpose and performance requirements of the rubber compound. During mixing, attention should be paid to various factors such as the amount of glue added, feeding sequence, roll distance, roll temperature, mixing time, roller speed and speed ratio. Neither insufficient mixing nor excessive refining is allowed

the mixing of internal mixer is divided into three stages, namely, wetting, dispersion and nirvana, and the mixing of stone by internal mixer is carried out under high temperature and pressure. The operation methods are generally divided into one-stage mixing method and two-stage mixing method

the one-stage mixing method refers to the method that the mixing is completed by the internal mixer at one time, and then the rubber mixture is obtained by pressing tablets. It is applicable to all natural rubber or rubber mixed with synthetic rubber with no more than 50%. In one-stage mixing operation, batch and step-by-step feeding method is often used. In order to prevent the sharp rise of rubber, slow internal mixer or double speed internal mixer is generally used, and the temperature when sulfur is added must be lower than 100 ℃. The feeding sequence is raw rubber - small material - reinforcing agent - filler - oil softener - discharging - cooling - adding sulfur and super accelerator

the two-stage mixing method refers to the method of making rubber mixture by mixing and pressing tablets in an internal mixer twice. This method is applicable to the compound containing more than 50% synthetic rubber, and can avoid the disadvantages of long mixing time and high temperature of the compound in the process of one-stage mixing method. The first stage of mixing is the same as the first stage of mixing, except that there is no accelerator with vulcanization and high activity. After the first stage of mixing, the next piece is cooled, parked for a certain period of time, and then the second stage of mixing is carried out. After mixing evenly, feed it to the tablet press, add vulcanizing agent, and then unload it after turning. The segmented mixing method has shorter mixing time, lower mixing temperature, more uniform dispersion of compounding agent and high rubber quality

2.3 calendering process

calendering is the process of making mixed rubber into film on the calender or semi-finished adhesive tape with framework materials. It includes tabletting, laminating, molding, textile glue hanging and other operations

the main equipment of calendering process is calender, which generally consists of working roller, frame, base, transmission device, speed regulation and distance adjustment device, roller heating and cooling device, lubrication system and emergency shutdown device. There are many kinds of calenders. There are two, three or four working rollers, and the arrangement form of the two rollers is vertical and horizontal; The three rollers are vertical Γ Type and triangle; Four rollers yes Γ Type, L type, Z type and S type. According to the process use, there are mainly tablet calenders (used for calendering film or textile gluing, most of which are three or four rollers, with different plasticity of each roller), rubberized calenders (used for textile gluing, three rollers, each roller has a certain speed ratio, and the speed of the middle roller is large. With the help of the speed ratio, it is rubbed into the textile), general calenders (also known as universal calenders, which have both tabletting and gluing functions, three or four rollers, adjustable speed ratio), profiled calenders Laminating calender and steel wire calender

the calendering process generally includes the following processes: preheating and glue supply of rubber mixture; The textile fabric is guided and dried by pressing the "check and hit/1" key (sometimes there is dipping)

the lamination of the rubber material on the four roll or three roll calender or the hanging of glue on the textile fabric depends on the cooling, coiling, cutting and placement of the semi-finished products calendered by the machine

before calendering, the rubber and textile need to be pre processed. Before entering the calender, the rubber needs to be turned over on the hot mixer. This process is hot refining or preheating, and its purpose is to improve the mixing uniformity of the rubber, further increase the plasticity, improve the temperature and increase the plasticity. In order to improve the adhesion between rubber and textile and ensure the calendering quality, the fabric needs to be dried. The moisture content is controlled at%, the moisture content is low, the fabric hardens, it is easy to be damaged during calendering, the moisture content is high, and the adhesion is poor

calendering performance of several common rubbers natural rubber has large thermoforming, small shrinkage, easy calendering and easy adhesion to hot rollers. The temperature difference of each roller should be controlled so that the film can be transferred smoothly; Styrene butadiene rubber has small thermoplastic and large shrinkage, so the rubber used for calendering should be fully molded. Due to the remarkable heat sensitivity of styrene butadiene rubber to calendering, the calendering temperature should be lower than that of natural rubber, and the temperature difference of each roll is so high that the hook should not be upward and downward; Neoprene is easy to stick to the roll at ℃ and difficult to calender. Low temperature method or high temperature method should be used. Calendering should be cooled quickly. Mixing paraffin and stearic acid can reduce the phenomenon of sticking to the roll; Ethylene propylene rubber has good ductility, and the impact knife is tightly installed with screw fastening wedge block, which can operate continuously in a wide temperature range. When the temperature is too low, the rubber shrinkage is large, and it is easy to produce bubbles; Nitrile rubber has small thermoplastic and large shrinkage. Adding fillers or softeners to the rubber can reduce the shrinkage. When the weight of fillers accounts for more than 50% of the weight of raw rubber, the film with smooth surface can be obtained. Nitrile rubber has small viscosity and is easy to stick to the cold roll

2.4 extrusion process

the extrusion process is to achieve the purpose of extrusion and preliminary modeling of rubber through the action of the barrel wall of the extruder and screw parts. The extrusion process also becomes an extrusion process

the main equipment of extrusion process is extruder

extrusion characteristics of several rubbers: natural rubber has fast extrusion speed and small shrinkage of semi-finished products. Body temperature ℃, nose ℃, mouth shape ℃; Styrene butadiene rubber has slow extrusion speed, large compression deformation, rough surface, body temperature ℃, nose temperature ℃, mouth temperature ℃; Neoprene does not need to be fully heated before being extruded. The body temperature is 50 ℃, the head temperature is 70 ℃; Ethylene propylene rubber has fast extrusion speed and small shrinkage. The body temperature is ℃, the head temperature is ℃, and the mouth shape is ℃. Nitrile rubber has poor extruding performance, so it should be fully heated during extruding. Body temperature ℃, nose temperature ℃

2.5 injection process

rubber injection molding process is a production method that injects rubber directly from the barrel into mold vulcanization. It includes feeding, plasticizing, injection, pressure maintaining, vulcanization, molding and other processes. The biggest feature of injection vulcanization is that the temperature of the inner and outer rubber is relatively uniform, and the vulcanization speed is fast, which can process most molded products

the equipment for rubber injection molding is the rubber injection molding vulcanizer

2.6 die casting process

die casting method is also called transfer mold method or shift mold method. This method is to install the rubber in the plug cylinder of the die-casting machine, and then lower the rubber under pressure and cast it into the mold cavity for vulcanization. Similar to injection molding. Such as skeleton oil seal, this method has less overflow and good product quality

2.7 vulcanization process

in the early days, the main purpose of natural rubber was to make word eraser; It was later used to make small rubber tubes. It was not until 1823 that British chemist mackintosh invented that rubber was dissolved in coal tar and then coated on cloth to make waterproof cloth, which could be used to make raincoats and boots. However, this kind of raincoat and boots will melt in summer and become hard and brittle in winter. In order to overcome this shortcoming, many people were thinking of ways at that time. American inventor Charlie Goodyear is also conducting experiments on rubber modification. He puts natural rubber and sulfur together to heat, hoping to obtain a substance that remains dry and elastic at all temperatures throughout the year. It was not until February 1839 that he succeeded. One day, he mixed rubber, sulfur and turpentine oil and poured them into the pot (sulfur is only used for dyeing). Accidentally, the mixture in the pot splashed on the hot stove. To his surprise, the mixture did not melt after falling into the fire, but remained charred as it was, and the incomplete charred mixture left in the furnace was elastic. He took the splashes from the

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