Modern rigid PVC profile processing uses a twin-screw e […]
Modern rigid PVC profile processing uses a twin-screw extruder to directly extrude the mixed powder, and its plasticizing mixing capacity and output are greatly improved. However, the positive displacement conveying capacity of the twin-screw extruder is much greater than the mixing capacity of the screw. The product extruded directly from the twin-screw extruder still cannot obtain a well-distributed and plasticized product, so it must be The raw materials of the formula are pre-mixed, including two stages of hot mixing and cold mixing, so that the mixture obtains a better pre-dispersion and plasticizing effect, and at the same time achieves a higher apparent density.
1. The purpose of the mixing: the mixing is to accurately measure the PVC and the auxiliary agent according to the formulation requirements, and then achieve high semi-gel degree, good fluidity and uniform density after hot mixing and cold mixing in a high-speed mixer. Dry mix.
2. Mixing equipment: conveying and storage part of raw materials, metering part, box conveyor belt, hot mixing and cold mixing pot, dry mixing storage and conveying part, power fan and central control part. 3, the process of mixing:
PVC storage tank PVC scale
Auxiliary tank hot mixing cold mixing buffer tank screening machine
Micro-tank micro-weighing scale
4. Mixing process: On the one hand, the mixing process is uniform dispersion of the auxiliary agent, on the other hand, the resin is semi-gelled and gelled: the PVC resin has grain refinement and uniform particle size change. Form a loose powder. Its process control points, feed rate, cooling water temperature, mixing temperature and time are key factors in controlling dry mix quality and yield.
A. There is a certain relationship between the amount of feed and the heating rate. That is, the heat of the material in the hot mixer is only the shearing of the stirring blade and the heat generation between the materials. Therefore, there is an optimum value for the amount of feed. If the amount of feed is too much, the material's tumbling resistance will greatly affect the heating rate, causing the rotation speed to drop, which will bring disadvantage to the mixture. The amount of feed is less than the internal shear heat, and the long mixing time directly affects the mixing efficiency. It is best to control about 70% by experiment.
B. Mixing temperature: One of the main factors affecting the performance of the dry powder. The hot mixing temperature is generally around 120 ° C. The cold mixed discharge temperature is generally lower than 40 ° C. The moisture content of the raw materials must meet the requirements of quality products and should be removed by venting to remove moisture from the materials.
C. Cooling water temperature: The cooling water temperature of the cold mixing tank is usually controlled at 13 to 15 °C.
D. Mixing time: The process of material undergoing compaction and homogenization needs to have a certain time to complete. The mixing time is affected by the mixing temperature, and the longer time is beneficial to the uniform dispersion of the material. Usually in 7-8 minutes. Also subject to the amount of feed and the order of addition in the mixer