The main control factors in the extrusion blow molding process are: extruder temperature, screw speed, blowing rate, blowing pressure inflation ratio, mold temperature, cooling time and cooling rate, etc.
1. The temperature of the extruder In the process of extruding the parison, the temperature control of the extruder directly affects the forming process of the parison and the quality of the parison. Increasing the temperature of the extruder barrel can reduce the viscosity of the melt, improve the fluidity of the melt, reduce the power consumption of the extruder, and at the same time help to improve the strength, surface gloss and transparency of the product. However, if the melt temperature is too high, it will not only prolong the cooling time and increase the shrinkage rate of the product, but also cause the extruded parison to sag under its own weight, causing the longitudinal wall thickness of the parison to be uneven, and it is also easy to degrade plastics with poor thermal stability, such as PVC, etc., will also easily cause the strength of engineering plastics (such as PC) to decrease significantly. If the temperature is too low, the plasticization of the material will not be good, the surface of the parison will be rough and not bright, and the internal stress will increase, which will easily cause the product to break during use. Therefore, considering many aspects, the barrel temperature should be as high as possible under the premise of ensuring that the extruded parison has good surface gloss, uniform plasticization, high melt strength, and under the premise of not overloading the transmission system, the lower temperature should be used as much as possible.
2. During the screw speed extrusion process, the screw speed is high and the extrusion rate is fast, which can increase the output of the extruder and reduce the sagging of the parison, but it is easy to cause the surface quality of the product to decline, and due to the increase of the shear rate, it can cause melt fracture of PE plastics and degradation of plastics with poor thermal stability such as PVC. Usually in the extrusion blow molding process, the control of the screw speed should be as fast as possible under the premise of being able to extrude a smooth and uniform parison without overloading the extrusion drive system, but generally controlled below 70r/min. In order to control the screw speed below 70r/min, a larger extrusion device should be selected for the general hollow blow molding machine.
3. Blowing rate: The volumetric rate of the air introduced during blow molding should be as high as possible to shorten the inflation time, so that the product can get a more uniform thickness and better surface quality; the speed of the air should not be too large, otherwise it may be at the air inlet. The formation of low pressure makes this part of the parison indent, or may break the parison at the die mouth, so that it cannot be inflated.
4. Blowing pressure: The blowing air should have sufficient pressure, otherwise, it is difficult to inflate the parison or the pattern on the surface of the product is not clear. Generally, the pressure of thick-walled products can be lower, and the pressure of thin-walled products and materials with high melt viscosity needs to be higher. The general blow molding pressure is 0.2~1.0MPa.
5. Inflation ratio: it refers to the ratio of the maximum diameter of the container to the maximum diameter of the parison, which is the multiple of the parison inflation. The size of the parison and the inflation ratio directly affect the size of the container. When the parison size and quality are constant, the larger the inflation ratio of the parison, the larger the container size. The expansion ratio of the parison is large, and the thickness of the container wall becomes thinner. Although raw materials can be saved, inflation becomes difficult, and the strength and rigidity of the container decrease. If the inflation ratio is too small, the consumption of raw materials will increase, the wall thickness of the product will decrease, the effective volume will decrease, the cooling time of the product will be prolonged, and the cost will increase. Molding should generally be determined according to the type and characteristics of the plastic, the shape and size of the product, and the size of the parison. Generally, the inflation of large thin-walled products is relatively small, which is 1.2~1.5 times; the inflation of small thick-walled products is relatively large, which is 2~4 times.
6. Mold temperature: The temperature of the blow mold should usually be determined according to the properties of the material and the wall thickness of the part. For general-purpose plastics, it is generally 20~50°C. For engineering plastics, due to the high glass transition temperature, the mold can be released at a higher mold temperature without affecting the quality of the product. High mold temperature also helps to improve the surface smoothness of the product. Generally, it is advisable to control the temperature of the blow molding mold at about 40°C lower than the softening temperature of the plastic.
When the mold temperature is too low during the blow molding process, the extensibility of the plastic clamped at the nip will become lower, and this part will be thicker after inflation. Too low temperature often causes spots or orange peels on the surface of the product. When the mold temperature is too high, the phenomenon at the nip is just the opposite of that when it is too low, and it will also prolong the molding cycle and increase the shrinkage of the product.
7. Cooling time and cooling rate: After the parison is inflated, it is cooled and shaped. Generally, water is used as the cooling medium. The heat is taken out through the cooling channel of the mold, and the cooling time controls the appearance quality, performance and production efficiency of the product. Increasing the cooling time can prevent the plastic from deforming due to elastic recovery. The product has a regular shape, clear surface patterns, and good quality, but the production cycle is prolonged and the production efficiency is reduced. And reduce the strength and transparency due to the crystallization of the product. If the cooling time is too short, the product will generate stress and porosity will appear.
Usually, on the premise of ensuring that the product is fully cooled and shaped, the cooling rate is accelerated to improve production efficiency; methods to increase the cooling rate include: Expand the cooling area of the mold, use refrigerated water or refrigerated gas to cool in the mold, and use liquid nitrogen or carbon dioxide to inflate and internally cool the parison.
The cooling rate of the mold depends on the cooling method, the choice of cooling medium and the cooling time, and is also related to the temperature and thickness of the parison. Generally, as the wall thickness of the product increases, the cooling time prolongs. However, different plastic varieties have different cooling times due to different thermal conductivity. Under the same thickness, HDPE has a longer cooling time than PP. For PE products of general thickness, after cooling for 1.5s, the temperature difference on both sides of the product wall is nearly equal, and there is no need to extend the cooling time too much.
For products with large size, wall thickness and special configuration, balance cooling is adopted, the cooling medium with high cooling efficiency is used for the neck and cutting parts, and the general cooling medium is used for the thinner part of the main body of the product. For special products, a second cooling is required, that is, air cooling or water cooling is used after the product is demoulded, so that the product can be fully cooled and shaped to prevent shrinkage and deformation.
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