PVC pipe process technic

March 19, 2020

PVC pipe process technic

The operation process of the host is mainly two parts: host current control and temperature control.
1: Current control: The host current is mainly adjusted by the host speed and feed speed. Maintaining a certain speed and increasing the feeding speed can increase the host current. When maintaining a certain feeding speed, appropriately increasing the speed can reduce the host current. During production, the proportion of the main engine speed and the feeding speed is adjusted reasonably, and a certain current is maintained to keep the melt plasticized to achieve the production requirements. In actual production, the usable current of the host is restricted by the screw structure, formula and temperature design, so there will be a large deviation in the control range. The current value used is generally adjusted by the mechanic according to the material performance of the observation port. If the feeding speed is too fast, the current increases significantly, it is easy to observe the mouth and the material is easy to fall, and if the current is too low, it will easily affect the melt kneading.
     Increasing the feeding speed can increase the pressure of the material and improve the kneading ability. By increasing the shear heat of the melt in advance, the plasticization can be improved. However, the dispersion properties of the formulation components in the melt are weakened. It is also possible to increase the melt plasticizing ability by increasing the speed of the host and increasing the heat of shear and friction. This can improve the full dispersion of the melt components. These two processes should be appropriately selected based on the screw structure and formula structure. The choice can also be made according to the kneading degree of the material at the observation port. This requires a technician to have certain technical experience to operate correctly. So based on the speed of the host and the current used, the basic skills of the mechanic can be seen. At the same time, observe the performance of the raw materials and adjust the basis of the basic formula (see the formula adjustment tips for details).
2: Temperature control. Temperature control is generally determined by the proportion of calcium powder and the speed of production capacity. The use temperature without adding plasticizer is relatively high. The lower the calcium powder content, the lower the relative use temperature. Without considering stability. The temperature of the screw degree lead-in and the higher temperature of the reflux section can effectively improve the distribution and plasticization of the melt components in the early stage of PVC. The exhaust section and lead temperature generally control the thinness of the melt. In rigid PVC, the temperature does not have the absolute temperature of the common pipe, 190, 195, 175, 165 merge core 170, runner 175, small mill 185, die 205-225. This temperature is relatively high at high speeds. Generally, the cone double adopts the hammer pendulum temperature curve (hard). The main sources of temperature are external heating, and shear and friction temperatures of the materials. If the shear heat is too high, the melt will become weak and the four-zone high temperature phenomenon is common. So temperature control consists of two parts: heating and cooling. The lead (zones 3 and 4) is generally best when the air cooling is stopped intermittently for 10-20 seconds. If the air cooling system is not activated frequently in the four zones, the general melt plasticization is relatively poor. The pressure in the fourth zone is relatively high at this position of the screw lead, and this position is also the end of the melt uniform distribution (the components are dispersed). When the temperature is high, the melt softness can be increased, the melt will cause partial reflow in the four zones, and the shear heat will be improved to make the PVC melt component better plasticized. If the four-zone shear is very low, the shear heat can be increased by increasing the reflux section and the stroke temperature (barrels 1 and 2), or by increasing the speed of the host to increase or decrease the high-pressure shear heat to increase the melt. Plasticizing ability. If the temperature in the third and fourth zones is too high, it will easily cause the melt to over-plasticize, and the adjustment direction is opposite to the operation direction of the above example. The barrel lead (three or four zones) temperature control can also be solved through formula adjustment, which is also the basis for process formula adjustment (follow-up).
     Many technicians do not pay attention to the temperature in the second zone, and do not even install the second zone cooling fan when installing the equipment. The temperature in the second zone of the barrel should be kept relatively small. When the temperature error is large, it has a very significant effect on the melt softness, a significant impact on the material's later fluidity, and a significant impact on the weight of the extruded product. The two-zone temperature observation is also an important basis for the components used in the pre-lubrication and external lubrication formulations (follow-up of formulation techniques).
Through the overall temperature, the mold temperature is reasonably designed, and the personal temperature of the confluent core is higher than the temperature of the four zones of the barrel. The mold temperature gradually increases according to the temperature of the confluent core. The temperature of the die is adjusted according to the expansion performance when the melt is out of the mold. The temperature of the die is too low or too high to affect the brightness of the appearance. If the temperature is too high, the melt expansion rate will be too high, which will cause the melt to break and the brightness will decrease. If the temperature is not enough, the melt can not be lubricated under high pressure. The brightness will also decrease. The influence of the die temperature adjustment on the melt to determine the overall equipment operating temperature is one of the core skills that every technician should have.
3: Switch on and off skills
      The startup procedure is a personal habit. Keep the cooler at about 130 degrees for half an hour. When the temperature in each zone reaches the temperature, keep it for 10-20 minutes to start (except when the mold silo is too large or the large mold does not have internal heating). Check whether each potentiometer returns to zero. Start synchronously, confirm that the host and feeding start, and adjust the speed synchronously for three to five laps. Slowly start the potentiometer of the host, pay attention to the current change, and adjust the feeding at the same time. When the speed of the host reaches about 500-600 rpm, keep the current for 3-5 minutes to observe the current change. If the host is hot, you can use the higher speed. After increasing the speed to about 800 rpm, observe that the current is stable and increase the speed after the work is completed.
      The difficulty of shutting down is much greater than turning it on. When shutting down, reduce the speed of the main machine to 800-500 rpm for 10-20 minutes. Reasonably reduce the shear heat and internal heat of the screw, turn off the temperature of each zone, and the lead zone is not closed. Then use the compound stabilizer to generally keep the lubrication and add the blanking uniformly. Mouth, it is better to use a feeder. There is no experience in the early stage. When the material is out of the mold, you can choose to use colored particles as a mark, turn off the feeding, and slowly stop when the material is out of the second zone. Turn off the temperature in the remaining two zones and keep some plasticized material in the lead of the screw. When the machine is turned on again, the powder in the early stage can pass the plasticized melt to increase the shear and guide the plasticization to reduce the waste rate at startup. If the material goes empty and starts up again, the PVC will be severely decomposed because the melt cannot be reasonably formed under no pressure. If the parking material is properly reserved in the screw, the new and old materials can not be seen when the machine is turned on again. This shutdown technique requires experience and requires long-term exploration to achieve. If the material goes empty, it is very difficult to start the machine, and you must wait for a long time for plasticizing. If the melt plasticization control is poor when turned off, the reject rate will increase when the machine is turned on again. In the normal high-speed state, the ratio of the host and the feed is larger than in the low-speed state. According to this characteristic, the plasticized melt can be effectively controlled during shutdown. The reserved melt must exceed the reflow section. If it is turned on, the decomposition is also serious. So the difficulty of shutting down the technology is far more difficult than the startup. Turn off the synchronous start button, and each potentiometer will return to zero. Power off the host.
       Few PVC extrusion equipment and process control techniques are formally trained. During the visits and exchanges, it was found that the mechanics used different methods. All rely on long-term experience to improve their own technology. Providing reference to everyone in this discussion process does not mean that it is completely correct. If the equipment and process technology are not selected properly, many raw material properties will not function properly, which will cause serious losses to the company's product quality and formulation costs. Reasonable choice of extrusion equipment can better understand the process and master the process.

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