After the dry mix leaves the mixing system, it usually […]
After the dry mix leaves the mixing system, it usually passes through a screening step before entering the storage system to screen out the coarse particles in the dry mix. The incorporation of these particles into the dry blend can affect the quality of the extruded product, such as the formation of yellow lines or even pastes. There are also some large-grain raw materials that are not completely broken, and the quality of the products will also be affected during production. Screening is a good way to avoid the entry of such abnormal materials into the extrusion system. For large mixing systems, the dry mix after sieving is usually fed into a storage tank by positive or negative pressure air for static curing. process. Storage of dry mix: The mixture should be allowed to stand in the storage tank for more than 24 hours to further mature the material, eliminate the static electricity generated during the mixing process, and improve the fluidity of the dry mix. Tests have shown that the apparent density of the mixture increases significantly after standing for 24 hours. The mixing time is short, and the profile is prone to asthma (especially in the parallel twin-screw reaction). The equipment that is fed by self-weight is prone to “bridge” phenomenon.
The rigid PVC profile extrusion is directly carried out using the powder. The main equipment used is a twin-screw extruder, which has a conical twin-screw extruder and a parallel twin-screw extruder, respectively. The conical twin-screw extruder has better compression ratio and larger plasticization, and the extrusion amount is not large. The parallel twin-screw extruder has a large amount of extrusion. At present, large profile extruders are parallel twin screws. The screw of the parallel twin-screw extruder is divided into four sections according to the function, which are respectively the plasticizing section, the exhaust section and the homogenizing section of the feeding section. The feeding section is the longest section of the four-stage screw structure, mainly for preheating the raw materials, and gradually compressing and compacting the raw materials; in order to accommodate more raw materials, the feeding section has a larger thread pitch. In the plasticizing section, the thread pitch is obviously changed, the material is greatly strengthened by the shearing action and the pressure. Under the action of shearing and external heat, the material is rapidly plasticized, and the powdery material in the feeding section is gradually melted into a semi-molten shape and packaged. Cover the surface of the screw. When the exhaust section is reached, the thread pitch is large, so that the material is fully dispersed, and the moisture and volatile matter in the material are discharged as much as possible by vacuum negative pressure. The function of the homogenization section is to maintain a certain melt temperature, further promote the plasticization of the material, not only to achieve uniform plasticization of the various points, uniform temperature, but also to achieve uniform movement speed of each particle. In actual production, observe the vacuum hole. The state of the material can also be judged by its plasticization, thus guiding the setting and adjustment of the process.
