PVC compound granulation process
The PVC compound granulation process is a process of making high-polymer resin and various additives and auxiliaries into granulated plastics after metering, sticking, plasticizing, and pelletizing. Plastic granules are semi-finished products of the plastic molding processing industry and are also extruded , Injection molding, hollow blow molding, foaming and other forming and processing raw materials.
There are two types of resin: powder and granular.
Extrusion granulation is the most basic and simple granulation method, which is widely used.
For various plastic molding and processing methods, compared with direct processing of powder, the advantages of using granulated pellets are as follows:
(1) Convenient feeding, there is no need to install a forced feeder in the hopper.
(2) The relative density of granular materials is greater than that of powder materials, and the strength of plastic products is better.
(3) The resin is mixed with various solid powder materials or liquid additives more uniformly, and the physical properties of plastic products are more uniform.
(4) The color of plastic products is uniform.
(5) Granules contain less volatiles of air-containing agents, making it difficult for plastic products to generate air bubbles.
(6) The pellets have no pollution to the extruder and production environment.
1. Preparations before ingredients
The preparation work before batching includes resin sieving, plasticizer filtration, powdered additive refining, color masterbatch powder preparation raw material drying, blocky additive heating and melting.
High polymer resins may be mixed with mechanical impurities or other impurities during the production, packaging and transportation. To prevent damage to the granulation equipment and reduce product quality, the resin must be used after sieving. Powder polyvinyl chloride generally uses a 40 mesh screen. Granular polyethylene or polypropylene can be sieved with a fine wire mesh slightly larger than the resin particle size.
In order to prevent mechanical impurities or black garbage in the plasticizer from mixing into the product and affecting the performance of the product, plasticizers are generally filtered through a 60-120 mesh filter screen [60 mesh for large viscosity and 120 mesh for small viscosity.
For powdery additives with coarse particles or easy to agglomerate, it is best to first use a plasticizer to grind the slurry, add it to the slurry, and mix and disperse it in the resin more uniformly. The refining equipment is a three-roller grinder. The ratio of the general additives to the plasticizer in the refining formula is about 1: 1, and the slurry fineness index is 60 ~ 80 μm.
The proportion of the slurry formula is the ratio of parts by weight, and the proportion is mainly determined by the apparent density of the powdery material. If the apparent density of the powdery material is large, a large amount of plasticizer needs to be added. Plasticizers generally use low-viscosity DOP. In the amount of plasticizer input, the amount added during refining should be deducted.
For rigid polyvinyl chloride products, because no plasticizer is added, generally a high-concentration polyvinyl chloride color masterbatch powder is produced first, and then added to the kneader and mixed. For example, this method is used for dyeing rigid PVC pipes. When using a kneader to manufacture the masterbatch, a stabilizer should be appropriately added to prevent the resin from decomposing.
For moisture-absorbing resins and additives, they must be dried to remove water before kneading or extrusion. For example, nylon resin and ABS resin are easy to absorb water, and must be dried to less than 0.1% water content. Drying equipment includes boiling dryer, vacuum dryer or vacuum drying hopper.
Some functional masterbatches, such as antistatic masterbatch, anti-blocking masterbatch, pearlescent masterbatch, filler masterbatch, etc., are easy to absorb moisture, and should be dried before adding to the extruder, otherwise it will affect production or reduce products quality.
Both stearic acid and paraffin are lubricants that are used more frequently in plastic processing. If it is directly added to the kneader, it will hit the baffle due to high-speed rotation in the high-speed kneader and damage the baffle, so it must be heated and melted before adding to the kneader. The melting point of stearic acid is 69.60 ° C, and the melting point of paraffin is 60 ° C.
2. Weighing formula
Put into the kneader or mixer to mix, and the raw materials for plasticizing should first be estimated according to the equipment volume and the feeding coefficient.
Weigh accurately according to the amount of input, the weighing must be accurate, otherwise the quality of the product will be unstable. Weighing scales and automatic measuring devices should be regularly checked and corrected to prevent quality fluctuations due to measurement errors.
Kneading can be divided into heating kneading and cooling kneading, using a high-speed kneader.
Heating and kneading process conditions: soft PVC kneading time 10 ~ 15min, discharge temperature 95 ~ 100 ℃; hard PVC kneading time 5 ~ 10min, discharge temperature 100 ~ 110 ℃. In addition, the order of addition during kneading has an effect on the kneading quality.
Cooling and kneading The temperature of the material coming out of the high-speed kneader is relatively high, exceeding 100 ° C. If it is placed in a storage barrel, it is prone to discoloration, agglomeration and even material decomposition. Therefore, it is necessary to immediately cool and mix to reduce the temperature of the material below 50 ° C before it can be stored for the extruder. The cooling and mixing process conditions are kneading time of 5 ~ 10min for both soft and hard polyvinyl chloride, and the temperature of the discharge material is below 50 ° C.
The 50L internal mixer is used for the internal mixing. The process conditions for mixing the color masterbatch of soft polyvinyl chloride and polyethylene are as follows. The soft vinyl chloride internal mixing time is 3 ~ 5min, the internal mixing temperature is 90 ~ 110 ℃, and the output temperature 100 ~ 110 ℃; polyethylene mixing time is 8 ~ 10min, mixing temperature is 80 ~ 100 ℃, output temperature is 140 ~ 145 ℃. The compacted material is agglomerates, and no unplasticized powder or granules can be released.
Extrusion granulation can use single screw and twin screw extruder. It mainly controls extrusion temperature, screw speed, cutter speed and pellet cooling. So that the pellets do not occur sticky particles, the particle size is uniform, and plasticization is better. The cutter speed is adjusted to 3 ~ 4mm of pellet length. The screw speed is adjusted so that the pellets do not stick.
For extruding soft PVC cable material, the temperature of the single screw extruder with a diameter of 65mm is 140 ~ 150 ℃, 160 ~ 170 ℃, 150 ~ 160 ℃, 150 ~ 160 ℃, and the speed is 20 ~ 30r / min; diameter 51 The temperature of the ~ 150mm twin screw extruder is 130 ~ 140 ℃, 140 ~ 145 ℃, 150 ~ 155 ℃, 145 ~ 150 ℃, and the screw speed is 15 ~ 20r / min. The diameter of 6mm parallel twin-screw extruder for pelletizing polyethylene is 200 ~ 220 ℃, 230 ~ 240 ℃, 240 ~ 250 ℃, 250 ~ 260 ℃.