Drying of raw materials
DATE：2022/8/9 8:48:41 / READ： / SOURCE：This station
Drying of raw materials
Some plastics are easy to absorb moisture in the air during the storage period, which increases the moisture content of raw materials. Using this kind of raw material with high moisture content to form products will cause the appearance quality problems such as bubbles or lines, and the performance index of the products will be reduced in serious cases. Therefore, raw materials such as ABS, polycarbonate, polyamide, polymethyl methacrylate and cellulose coolate used for molding products must be dehumidified and dried before production. For resins such as polyethylene, polypropylene and polyoxymethylene, if the packaging bag is intact, generally drying treatment is not required. There are many methods for dehumidification and drying of raw materials, such as hot blast drying and microwave oven. The specification parameters of hopper dryer are specified in JB / t6494-92. See table 3-16. See Table 4-2 for drying temperature and drying treatment time of common raw materials. See table 4-3 for water absorption of common plastics and allowable water content of raw materials during production.Raw materials (such as polyamide) that are easy to be oxidized and discolored after drying under high temperature shall be dried by vacuum drying. The dried raw materials should not be stored for a long time. A better way is to use the continuous production method to directly transport the dried raw materials to the hopper of the molding machine and put them into production immediately. The dried raw materials are directly sent to the injection molding machine from the dehumidification dryer (4)
4.4 preparation before mixingThe preparation before raw material mixing refers to the pre production preparation of those plastic products which are composed of a variety of raw materials. For example, the material for PVC products is a mixture composed of PVC resin and various auxiliary materials. For the pre production preparation of this mixture, due to the difference of varieties in the mixture, the treatment methods before mixing are different.
4.4.1 raw material mixing equipment
PVC resin is used as the main raw material and mixed with other additives before forming products. The production equipment mainly includes vibrating screen for screening resin and filler; Gear pump for conveying liquid plasticizer; A mixer for mixing solid auxiliary materials (stabilizers, pigments, etc.) and liquid plasticizers; A three roll grinder for refining solid particles in slurry; A plunger metering pump for conveying and metering the slurry after grinding and refining; A mixer for mixing all kinds of raw materials required in the formula.
4.4.2 raw material mixing process and operation points
The mixing of raw materials, the mixing and feeding sequence of components, the reasonable control of processing parameters, and whether the selected production equipment and operation mode meet the process requirements in the process of forming PVC products. The application of many projects will have a great impact on the performance and product quality of the formed polyvinyl chloride products. Therefore, the following points should be paid attention to in the mixing and production of raw materials. ① Resin screening is to remove impurities in raw materials. Generally, vibrating screen or flat screen material is used, and the screen mesh is 40 mesh. ② For the resin transportation after screening, the spiral mode (the feeding distance is relatively close), the elevator or the pipeline air pressure transportation mode can be selected according to the height of the storage tank and the feeding distance. At present, the most widely used material transportation mode is the air pressure transportation, and 0.5MPa positive pressure wind force is selected during operation. ③ Most of the resins and fillers are measured by electronic scales. ④ Several plasticizers may be used simultaneously in the formulation of PVC products. During production, several plasticizers can be metered into the mixer after filtration; It can also be metered separately according to the proportion required by the formula and then added into a mixing tank. Under the temperature condition of about 80 ℃, the plasticizer mixing liquid in the tank is agitated with 5 ~ 6pa air pressure to make it mixed evenly, and then it is sent to the mixer with a metering pump. ⑤ In the application of additives such as powdered pigment and stabilizer, the proportion of [Formula] and plasticizer shall be 1 '(2 ~ 3), and the proportion of stabilizer and plasticizer shall be 12] and the plasticizer shall be mixed into a slurry, which shall be stirred by a mixer to make the mixture uniform; Then grind with research materials to refine the powder particles. During grinding, the speed ratio of three rollers is 1:3:9, and the gap between them should be gradually reduced. After several times of repeated grinding, the particle fineness is within 30 ~ to facilitate the uniform diffusion of pigment. ⑥ Since the amount of additives used in the formula is very small, the amount of various additives should be particularly accurate. ⑦ For the mixing of raw materials, first input 0.2MPa steam into the outer wall cavity of the mixing chamber to preheat the mixing chamber of the mixer; When mixing raw materials, PVC resin and liquid plasticizer shall be mixed first, and the mixer shall be started. The two raw materials shall be mixed for 1 ~ 2min, or when the mixture temperature reaches about 65 ℃, solid stabilizer and internal lubricant shall be added for mixing; When the material temperature reaches about 95 ℃, add external lubricant for mixing; When the material temperature reaches about 115 ℃, the mixture is transported to the cooling cold mixing mixer until the material temperature is lower than 45 ℃, and then it can be put into the extruder for extrusion granulation. When mixing raw materials, attention shall be paid to: if there are chlorinated polyethylene (CPE) or ethylene vinyl acetate copolymer (EVA) and high impact Methacrylate Butadiene vegetable ethylene copolymer (MBS) in the mixture, the temperature of the mixture shall not exceed 110 ℃, and the mixing and mixing time shall not be too long, so as to prevent the toughening agents such as CPE in the mixture from sticking and agglomerating; If there is titanium dioxide in the mixture, the titanium dioxide must be mixed within 1-2min before the end of the mixture to avoid reducing the white of the titanium dioxide; For the mixing of multi-component raw materials, it is not allowed to pour all kinds of raw materials into the mixing chamber at the same time, which will affect the use effect of plasticizer and the product quality.As follows. Raw material mixing process conditions when different types of mixers are used to mix different PVC products ① mixing process of soft PVC products: soft polyethylene product slurry → filler. When mixing raw materials, the feeding sequence is: polyvinyl chloride resin → plasticizer → stabilizer → color order, feeding amount 160kg, heating steam pressure 0.2MPa, and raw material mixing time no more than Tomin. The temperature of mixed materials shall not exceed 120 ℃. ② Mixing process of raw materials for rigid polyvinyl chloride products when mixing raw materials for rigid polyvinyl chloride products, the feeding sequence is: polyvinyl chloride resin → stabilizer → color paste → filler → lubricant.
Taking the combined operation of grh-500 as an example, the production process conditions are as follows: (1) 10min, and the temperature of the mixed material shall not exceed 100 ℃. The secondary feeding amount is 200kg, the heating steam pressure is 0.2MPa, and the mixing time of raw materials does not exceed. If the grh-200 high-speed mixer is used, the feeding amount is 90kg, the heating steam pressure of the mixing chamber can be slightly smaller, the mixing time is 10-15min, and the discharge temperature is about 90 ℃. ③ The mixing process of raw materials for semi-rigid PVC products takes the operation of grh50 high-speed mixer as an example. The feeding amount is 150kg, the mixing chamber is slightly heated, the mixing time of raw materials is 6min, and the temperature of mixed materials is between 100 ~ 110 ℃.
4.5 mixture granulation
In the production plants that use extruders to form plastic products, the mixed raw materials are usually granulated by special extrusion granulators. This kind of extrusion granulator has simple production operation, small floor area and low production cost. There are many manufacturers producing extrusion granulators in China. See table 3-22 for the models and technical parameters of extrusion granulators produced by the main manufacturers. The extruder is used to mix raw materials, and the barrel temperature (from the feeding section to the homogenization section) during plasticizing and granulation is 80 ~ 120 ℃, 130 ~ 160 ℃ and 165 ~ 170 ℃. The mold temperature is 170 ~ 180 ℃. There is no uniform regulation on the quality of granular materials. The specific requirements are: the size specification of granules can be a cube with a side length of 3mm or 4mm, or a cylinder with a diameter of 3mm and a length of 1-6mm; The particle size shall be uniform and free of debris; Uniform plasticization, bright and uniform color, no impurities; There is little particle adhesion, and long (no more than 30mm) strip particles are not allowed.
Author：admin