In the PVC-U process, whether in the production process of PVC-U plates, tubes (including wire bushings), or profiles (including wire troughs), a special phenomenon has been discovered: sometimes a process with excellent processing properties is processed. The car is quickly stabilized and then replaced with a formulation with poor processing properties (such as increased filler, reduced processing modifier or increased molecular weight of PVC resin), and the processing can be carried out smoothly. On the other hand, if you start driving with a formula that is difficult to process, it is difficult to drive. If it is to be successful, it is necessary to take harsh measures such as increasing the processing temperature and/or increasing the shearing of the screw and/or reducing the extrusion speed in order to process properly. But once the processing is normal, the processing temperature can be reduced by the screw speed. The extrusion speed can be reduced, and even the normal process level can be restored. What is this? Obviously, there is an "induction plasticization" effect in the processing of PVC-U. This paper describes the experimental basis, the cause of the occurrence, and the use of the "induced plasticization" effect of "induced plasticization". According to the single screw "melt pool theory", the plastic composite forms a plug in the conveying section under the push of the screw. After entering the melting section (compression section), the plug contacts the surface of the heated cylinder during advancement, and the melting starts from the contact surface and leaves a melt film on the surface of the cylinder upon melting. If the thickness of the melt film exceeds the gap between the spiral rib and the barrel, it will be scraped off by the rotating spiral ridge and forced to accumulate in the front row of the spiral rib to form a "melt pool", while on the back side of the spiral rib Solid bed. Thus, during the forward movement along the groove, the width of the solid bed is gradually reduced until it disappears completely, that is, it is completely melted. According to the theory of melt transport, the flow of the melt in the metering section of the screw has four types: positive flow, reverse flow, cross flow and leakage flow. The positive flow is the flow of the plastic melt between the barrel and the screw along the Z axis, that is, spirally along the groove toward the die. Countercurrent is the flow caused by the pressure gradient of the stream. Since the stream pressure gradually increases as the stream advances, the direction of the countercurrent is opposite to the normal flow and flows in the direction of the Z-axis feed port. The cross flow is the flow of the plastic melt along the x-axis, that is, the flow of the melt between the two spiral edges. The leakage current is caused by the pressure gradient, or the head pressure (head pressure of No. 13-No. 15 in Table 4 (ie, melt pressure) is 15 MPa, sometimes as high as 25 MPa). It refers to the amount of material flowing out of the gap δ between the screw rib and the barrel along the screw axis toward the feed port. If the temperature of the metering section is too high, the melt viscosity is small, and the temperature of the die is low, the head pressure is increased, and the leakage flow is sometimes increased from the vent hole. The molten pool theory more accurately describes the plasticizing process when processing a polymer with a single screw. However, for the twin-screw extrusion processing of PVC-U, the situation is not so simple. The plasticizing process cannot be explained by the molten pool theory alone. There is leakage at the rear of the melting section and throughout the metering section, not just the metering section. For each groove of the melting section, there is a melt in the melt pool at the back of the groove, a melt from the front of the groove, a melt in the front and back of the groove, and a raw material in the middle, in the double Under the frequent shearing action of the screw, the melt and the raw material are mixed with each other, heat and mass transfer, and finally all melted. In the metering section, the leakage current dominates, the melt from the previous groove, the melt with a better degree of mastication and the latter, the self-generated, less masticized melt, the frequent shearing of the twin-screw Under the action of cutting, they are mixed with each other, heat and mass transfer, and improve the plasticizing effect. Each time a groove is advanced, the mastication effect is increased by one level. For a well-designed extruder, the material is completely homogenized at the end of the metering section. Each micro-volume has the same composition. The existence of leakage current will undoubtedly promote the plasticization process. At the beginning of the processing, that is, when the vehicle is just driving, there is no melt leakage in the melting section and the metering section, so processing is difficult, either increasing the processing temperature or increasing the shear rate. After a period of processing, the metering section is completely plasticized, the melt viscosity is reduced, and the leakage current is increased, which results in an "induction plasticization" effect. Tape Measure Length,Stainless Tape,Waterproof Tape Measure,Flexible Steel Measuring Tape Henan Liangjin Tools Co.,Ltd , https://www.liangjintools.com