Views: 381 Author: Site Editor Publish Time: 2020-09-29 Origin: Site
The strips discharged from the pelletizing template of the water ring pelletizer are immediately cut into pellets by the rotating cutter, so the PP plastic melt must be cuttable. The main factors affecting the pelletizing effect of the water ring pelletizer are the following factors: extruder temperature, extrusion speed, connecting body temperature, melt pressure, the temperature of the discharge template, number of holes in the discharge template, and cooling temperature of the water, the pressure of the cooling water and the uniformity of the mixing ratio of the raw materials. When these eight conditions are suitable, it can ensure the normal operation of the pelletizing system when the water ring pelletizer is made of PP with a high melt index, and qualified PP plastic particles can be produced.
Polypropylene (PP) is a general-purpose thermoplastic resin with excellent mechanical properties, heat resistance, chemical resistance, and low price, so it has been widely adopted. PP granulation can adopt processes such as strand pelletizing and hot cutting with water ring die surface. The hot pelletizing of the water ring die surface has the characteristics of high pelletizing efficiency, small footprint, smooth and beautiful pellets. However, PP with a high melt index has low viscosity and easy adhesion, which has always been a problem in the granulation process.
Because PP extrusion is in the molten state, the melt index will change greatly under the small difference in temperature. Therefore, if the process is not well mastered, the use of water ring pelletizers to cut pellets will easily cause the quality of plastic particles to decline, and the particles will adhere to each other and agglomerate, which will seriously block the water ring pelletizing cover and cause the water ring pelletizer to stop.
Through a large number of process experiments and explorations, Chuangbo has mastered the relatively stable water ring pelletizing process of high melt index PP. Whether to create suitable process conditions is the key factor to ensure the quality of high melt index PP water ring pelletizing.
When the temperature of the extruder is too high, the melt index will increase and the melt viscosity will decrease. The extruded melt is easy to stick, which is not conducive to the water ring pelletizer. When the temperature of the extruder is low, the PP plasticization is not good, the viscosity increases, the mold release expansion rate increases, and there is no problem in cutting pellets, but the outer diameter of the particles becomes larger, the shape is rough, and the appearance quality of the particles becomes worse. Therefore, it is very important to first find out the suitable working temperature of the extruder. Through experiments, it is known that the extruder has a high temperature in the feed section and a gradual decrease in the temperature at the discharge end, which can be well compatible with the plasticization of raw materials and the problems of excessively high discharge port temperature. The actual temperature should be adjusted according to the different extruders and material formula.
The plastic extruder is an important part of these links. The different design of the extruder screw will affect the plasticization of the raw materials. At different speeds, the frictional heat of the screw is also different, which will affect the temperature of the raw material at the exit of the extruder, thereby affecting the melt index of the raw material. The high temperature will cause the pellets to stick, so the appropriate extrusion speed is also the influencing factor of the PP water ring pelletizer.
The temperature of the extruder connecting body will affect the melt temperature. When the temperature is high, the raw material is not shaped and cannot be cut into pellets. The temperature of the connecting body is gradually reduced until the raw material is cut into normal particles, there are no sticky particles, and the particles have no swelling phenomenon. This is the more suitable temperature. But in actual operation, the temperature of the extruder barrel, the temperature of the connecting body, and the temperature of the discharge template will all affect the temperature of the melt at the discharge port.
When other conditions do not change, the melt pressure changes with the melt temperature. When the melt pressure is high, it indicates that the material temperature is low, the plasticization state is not good, and the melt release expansion rate increases. Although the material is cuttable and has no sticky particles, the appearance of the particles deteriorates, the surface is rough, and the particle size increases. According to actual operating experience, the melt pressure is within the range of 5-5.5MPa, the particles have no adhesion, the surface is smooth, the particle size is appropriate, and the cut particles have a beautiful appearance.
Maintaining the temperature of the discharge template is the key to ensuring proper melt pressure. When the water ring pelleting system is running, there will always be a small amount of water splashing on the template, causing the template to cool down locally and causing a blockage. The design of the water ring flow channel of the water ring pelletizer should try to avoid this phenomenon. In order to reduce the influence of the temperature of the template, firstly the heating power of the mold body should be appropriately increased, and secondly, it is necessary to reduce the temperature of the template caused by other factors. In addition, the overall temperature balance of the pelletizing template will also affect the uniformity of the discharge.
The low temperature of the cooling water will cause the plastic particles to cool too quickly, which will not achieve the purpose of using the waste heat to dry the particles. If the temperature of the cooling water is too high, the particles will not be cooled enough and the particles will stick later. Should the temperature of the cooling water be higher or lower? In fact, the solution to this problem is that the water temperature is determined by the characteristics of different plastic particles.
In the water ring pelletizing system, the pressure of the water is critical, because only under a certain pressure can the water form a high-pressure water flow, so that the water circulates in the preset flow channel. Proper water pressure can cool the plastic particles without wetting the template, causing the template to cool down rapidly and causing the template to block. In order to achieve this goal, in addition to designing a scientific water flow channel, it is also necessary to equip a pump with appropriate flow and head.
The higher the uniformity of the mixing ratio of the raw materials, the more stable the characteristics of the materials, so that the fluidity and viscosity of the export materials are more consistent. The melt pressure of the discharge die is relatively stable, and the uniformity of the pellets is better!
Do you now know how to better display the real performance of the extruder? Chuangbo is a professional twin screw extruder manufacturer and supplier in China. We can provide customers with different extruders with excellent performance, such as masterbatch extruder, PVC compounding, thermoplastic elastermor, reaction extrusion line, devolatilization extruder, pelletizing system, etc.