Mixing process flow and operating notes of high speed and low speed pvc mixer
The hot and cold pvc mixer also known as the high and low speed pvc mixer, is the first process for producing PVC profiles, sheets, pipes, sheets and other PVC sheets, which plays an important role.
In the extrusion process, a conical twin-screw extruder is generally used, and the orthotopic conveying effect is much larger than the mixing effect on the materials, so the uniform mixing and mixing of various auxiliary agents is mainly completed by the mixing before extrusion. .
Good pvc mixer can compensate the extruder with poor plasticizing performance, but the extruder with better plasticizing performance can not compensate the performance of the profile caused by the poor mixing equipment. Therefore, if the batching work is wrong, The quality of PVC profiles is greatly affected. The operation rules of the ingredients are as follows: Work clothes must be worn during work. It is strictly forbidden to carry any sundries that are not related to work, so as not to fall into the ingredient bucket during work. Ready for the operation, all kinds of assistance The agent should be opened for inspection, and the raw material code must be neat, orderly, and check-in.
Different materials should use different precision measuring instruments. Larger amounts (such as stabilizers, impact modifiers, titanium dioxide, calcium carbonate, etc.) can be measured with an electronic scale with an accuracy of 10g; smaller amounts (such as processing aids, lubricants, masterbatches, antioxidants) Etc.) must be measured with an electronic scale with an accuracy of 1g. Measurement deviation: CaCO3 ≤ 50 g, UV-531, antioxidant ≤ 0.1 g, other materials ≤ 5 g. Strictly follow the established procedures to prevent omissions or repeated measurements.
When handling ingredients, it should be handled gently, and materials should not be leaked to the ground or dust should be raised to pollute the environment.
Measuring instruments such as electronic scales must be inspected, maintained and inspected regularly, and problems should be dealt with or replaced in a timely manner. After the work is finished, the floating material adhered to the instrument must be cleaned to avoid affecting the accuracy of the measuring instrument. The ingredients for each shift should be recorded in detail, including the raw material grade, origin, dosage, and operator. In particular, the raw materials of the new plant must be recorded in detail. After the batching work is completed, all kinds of raw materials must be sealed and treated, and the amount of the symmetrical amount of the auxiliary agent and the amount of the remaining auxiliary agent are consistent with the total amount of the auxiliary agent to prevent leakage. Match or repeat the ingredients.
Second, hot mixing and cold mixing process on pvc mixer , Hot mixing is not a simple mixing and mixing process of materials, is the material of different components and dosage in the formula, driven by the high-speed rotating stirring blade, spreads rapidly along the inner wall of the mixing machine And falling from the center of the machine to form a process of swirling motion. Under the shearing action caused by the collision and friction between the material and the stirring blade and the inner wall deflector of the mixer, the material is transformed from solid, single phase, heterogeneous to multiphase, homogeneous and partial gel state, and As the temperature of the material continues to rise, the PVC powder and other components continue to penetrate each other. The surface of the PVC particles gradually absorbs or adsorbs stabilizers, lubricants, etc., and the density can be measured by the increase in apparent density. The average density of the hot mix can be increased to 0. 63~0. 70 g/cm.
The hot pvc mixer includes three processes of diffusion, convection, and shear. The diffusion effect makes the distribution between the material components more uniform, so that the component particles migrate from more areas to less areas, so that the composition is uniform; convection and shearing are mechanical forces for irregular flow, so that the material group The fraction is evenly dispersed, and the shear force is generally about 3 in general. As the temperature continues to rise, the PVC powder continuously absorbs stabilizers, lubricants, etc., and is gradually coated with each component; when the temperature of the material rises to about 100 ° C, it is easy to remove the moisture in the PVC resin and the auxiliary agent. Other volatile substances avoid the generation of air bubbles during the extrusion process; when the material is raised to 110~120 °C, a uniform and stable dry mixed powder can be formed.
Gelation hot mixing of the pvc mixer is also the process of gelling the PVC blendstock. Unmixed PVC resin has different particle sizes and small particles at normal temperature, and it is easy to cause plasticization unevenness during extrusion molding. In the hot mixing process, the PVC resin is gradually melted, plasticized, and recrystallized to form a network morphology structure.
From normal temperature to below 50 °C, after the PVC agglomerated powder and larger particles are rubbed and impacted, although the apparent density does not change much, the large aggregated particles are sheared and broken into small particles, and After absorbing heat and mechanical energy, the activity increases, and gradually combines with other particles, and the particle size increases.
When the material temperature is below 90 °C, the temperature increase of the mixer is faster, then the temperature rises slowly, and the diffusion, shearing and convection effects are more severe.
When the material temperature reaches 115 ° C, the small particles of PVC resin become large and uniform, the small particles gradually disappear, and the edge of the particles becomes transparent due to partial gelation. Under certain conditions, the degree of gelation of the PVC resin particles is Determined by the discharge temperature.
When the temperature of the material reaches 120 ° C, the PVC particles tend to be stable and the degree of gelation is intensified. The particles in the whole hot mixing process are repeatedly combined and homogenized, the particle size is increased, and the densification effect is achieved, so that the apparent density of the PVC particles reaches a maximum value, which is favorable for increasing the extrusion yield, promoting the uniform plasticization of the material, and improving the PVC profile. Welding strength, impact properties and other properties. PVC dry mixed powder with good uniformity and good dry flow can be observed. The PVC resin has the characteristics of grain refinement and uniform particle size change, and exhibits high apparent density and partial gelation.
The temperature of the hot mixed material determines the progress of the uniform plasticization of the PVC material. The hot mixing temperature is too low, the PVC material is not plasticized unevenly, affecting the performance of the PVC profile; the hot mixing temperature is too high, the PVC material is prone to degradation in the mixing tank, and even the “paste” affects the production. Therefore, under normal circumstances Most PVC profile manufacturers use 120 °C as the rated temperature index for hot mixing. When the amount of calcium carbonate is large, the temperature of the hot mixed material can be appropriately increased to facilitate the balanced adsorption of calcium carbonate and the apparent density of the PVC material and the low melting point component. In addition to the hot mixing time, the hot mixing time needs to be controlled. It is also necessary to pay close attention to the hot pvc mixer time in order to monitor the discharge temperature control.
When continuously mixing, if the hot mixing reaches a shorter discharge time (such as less than 7min, the interval between batches should be extended;
When the pvc mixer is used for a long time, if the temperature rises too slowly (if the temperature exceeds the discharge temperature for more than 12 minutes), the material can be discharged in time, and the thermocouple and the thermometer are inspected for failure and processed.
When the mixer blade is found to be seriously worn, it should be replaced in time to prevent the mixing quality. In general, when the mixing time exceeds 2 times the normal mixing time or the wear at the end of the blade is greater than 15 mm, the blade should be replaced.
The hot pvc mixer sequence has a certain influence on the mixing quality and the mixing tank wear. PVC resin and heat stabilizer are added at low speed;
Adding impact modifier, processing aid and internal lubricant at high speed and 60 °C;
Add filler, titanium dioxide, ultramarine blue, external lubricant, UV-531, color masterbatch antioxidant, etc. at high speed and 80 °C.
First, adding a heat stabilizer can fully exert its heat stabilizing effect on the PVC resin;
The early addition of the internal lubricant facilitates the initial gelation of the PVC material;
Premature addition of titanium dioxide will affect the surface finish and color of the product, and the mixing tank will wear more;
Premature addition of calcium carbonate not only wears out the mixing tank, but also preferentially absorbs the internal lubricant;
Premature addition of processing aids will preferentially absorb the lead salt stabilizer, so that the amount of lead salt stabilizer absorbed by PVC is reduced, resulting in a decrease in material stability;
Premature addition of external lubricant will reduce the frictional heat and shear heat of the mixture, affecting the mixing effect and the dispersion performance of the stabilizer;
Premature addition of color masterbatch will affect the color of PVC profiles, and the total amount of hot mixes cannot be ignored.
Generally, each batch of material should be controlled at about 75% of the volume of the hot mixing tank to avoid excessively long (or too short) mixing time, resulting in over-plasticization of the material (or uneven plasticization).
Cold mixing technology principle The working principle of cold mixing and hot mixing is different.
Hot mixing generally does not have external heating equipment, mainly relying on the high-speed rotating blade to bring the animal material and materials into collision with each other and the heat energy generated by the friction and shear of the material and the barrel, so that the normal temperature material becomes a hot mix.
The barrel of the cold mixing machine has a sandwich with cooling water flowing in the interlayer; and the volume of the general cold mixing machine is about 3 times that of the hot pvc mixer . The hot mix entering the cold mixer is in a relatively spacious environment and the rotating blades are continuously stirred and tumbling, and the heat is absorbed by the cooling water in the interlayer of the inner wall of the cylinder, and gradually cools itself.
PVC materials should be cold mixed immediately after hot mixing to prevent degradation of the hot mix after storage at high temperatures for a long time. At the same time, if the hot mix is not subjected to cold mixing, it is easy to re-absorb the water in the natural state, which has an adverse effect on the extrusion.
Cold mixing can not only prevent the water-absorbing and moisture-reducing phenomenon of PVC materials cooling at high temperatures, but also the process of further discharging the intermolecular moisture of PVC materials during the exothermic process.
The cold mixed discharge temperature of PVC materials plays an important role in ensuring the quality of dry mix.
The PVC material is to be plasticized into the extruder, and it can be added as hot. It does not need to be cold-mixed. Therefore, the hot compound is directly added to the extruder without cold mixing, which may result in unqualified product quality. Generally, the temperature of the cold mixed discharge is 40 ° C, and the temperature of the cooling water is usually controlled at 13 to 15 ° C.
Mixing operation procedure
1. All kinds of raw materials must be listed, and they should be stacked in an orderly, clear, and neat manner; the raw materials should be separated by a corresponding distance and should not be confused with each other.
2. Operate strictly according to the mixing procedure and the discharge temperature. During the mixing process, it is necessary to observe whether the hot mixing tank and the cold mixing tank discharging valve leak.
3. Thermocouples and meters are important instruments for displaying and monitoring the temperature of the mixture. It must be regularly checked to keep the thermocouple and the instrument in a good state of sensitivity. Before mixing, be sure to check whether the thermocouple is installed, whether it is wrapped or damaged by materials, and whether the meter displays failure. Mixing operations should not be carried out when there is a quality problem with the thermocouple or meter.
4Be sure to check whether the PVC feeding tank filter is in good condition before work.
5. Regularly check the pvc mixer . If it is found that the filler cover, blade adhesion material or mixing tank has accumulated material, it should be cleaned up in time. When cleaning, it is strictly forbidden to carry hard materials such as steel, iron and copper. It is strictly forbidden to drop any debris into the mixing tank.
6. In the middle and late stages of the mixing process, it is necessary to regularly check whether the exhaust bag of the mixing tank is blocked.
7. The mixed raw materials should be transported to the storage tank in time. It should be stored for 24 hours under normal temperature conditions to eliminate the static electricity generated during the mixing process and improve the apparent density and fluidity of the dry mix. If the standing time after mixing is too short, the phenomenon of “swinging” of the profile is likely to occur during extrusion, and the “bridge” phenomenon is likely to occur on the extruder feeding by self-weight.
8. When mixing the colored materials, it must be separately received, and the mixed materials are strictly prohibited from being transported to the storage tank to avoid polluting the automatic conveying system. Color materials must be closed after manual bagging, and should not be stacked open to avoid moisture and affect the quality of the mixture. After the coloring material is mixed, the tank must be carefully cleaned and further cleaned with PVC resin until it is clean and free of mottling. The PVC resin used for cleaning the pvc mixer tank can be used as a compounding cleaning material and must not be used as an extrusion material. After the “paste” failure occurs during mixing, be sure to clean the tank with soft metal in time. Wear a gas mask when cleaning to protect personal safety. During the period of production stoppage, be sure to leave the staff on duty and start the pvc mixer regularly to prevent the computer program from being lost. High-speed mixing unit operating process specification Preparation before starting: Keep all parts of the machine clean, especially the inner cavity of the mixer and the discharge valve should be cleaned to ensure that there are no hard objects or dirt. Check the connection parts of the machine for looseness, whether there is leakage of compressed air and pipeline connection, and whether the pressure gauges indicate normal. Check each rotating part for defects such as scratches and stagnation, and lubricate all parts in strict accordance with the lubrication procedure. Check that the pneumatic valves are open and closed flexibly and without blockage. Check whether the stirring paddle is installed firmly, whether there is grinding between the scraper and the bottom of the tank, and maintain a certain clearance; when rotating by hand, the spindle should be light and flexible. The “D” belt should be evenly tensioned, and the motor bottom plate bolts should be loose. Check that electrical equipment is safe and reliable. Check whether the water, electricity and gas pressure indications are normal; the cooling water pressure should not be too high, and the water supply valve should be opened 60% ~ 70%, so as to avoid excessive pressure, causing deformation or even bursting of the cold mixing tank. If "automatic" is adopted The program must check whether the various temperature settings meet the production process. Startup, operation precautions and safety measures,
Non-professionals are strictly prohibited to operate the equipment
It is strictly forbidden to change the device setting parameters privately during the work process.
It is strictly forbidden to operate the switchboards in violation of regulations.
Automatic and manual switching should be carried out under stop or no load, and it is strictly forbidden to stand under the jack when loading and unloading.
During the operation, close attention should be paid to whether the current of the high-speed mixer is in the normal range; the high-speed mixing is generally between 80 and 100 A, and the cold mixing is generally between 18 and 20 A. If the current is too low, it may be that the feeding amount does not meet the requirements or the pulley is slack and slippery, and should be dealt with in time; if the current suddenly increases and does not fall back, and the motor is hot or noisy, there may be foreign matter or loose bearing in the pvc mixer . Should stop immediately, find out the reasons, and rule out.
For further information: