Views: 0 Author: Site Editor Publish Time: 2022-04-27 Origin: Site
I The main control process factors
The main control factors in the extrusion blow molding process are: extruder temperature, screw speed, blowing rate, blowing pressure blowing ratio, mold temperature, cooling time and cooling rate.
II The influence of technological factors on the molding process
The temperature of the extruder In the process of extruding the parison, the temperature control of the extruder directly affects the molding 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 also help to improve the strength, surface gloss and transparency of the product. However, if the melt temperature is too high, it not only prolongs the cooling time and increases the shrinkage rate of the product, but also causes the extruded parison to sag under its own weight, causing uneven longitudinal wall thickness of the parison, and at the same time, it is easy to cause poor thermal stability. Degradation of plastics, such as PVC, can also easily cause the strength of engineering plastics (such as PC, etc.) to be significantly reduced. If the temperature is too low, the plasticization of the material is not good, the surface of the parison is rough and not bright, and the internal stress increases, which is easy to cause the product to break during use. Therefore, considering various aspects, the barrel temperature should be as high as possible on the premise of ensuring that the extruded parison has good surface gloss, uniform plasticization, high melt strength, and will not overload the transmission system. low temperature.
Screw speed During the 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. It causes melt fracture of PE plastics and degradation of plastics with poor thermal stability such as PVC. Usually in the extrusion blow molding process, the speed control of the screw should be as fast as possible on the premise that a smooth and uniform parison can be extruded without overloading the extrusion drive system. , but generally controlled below 70r/min. In order to control the screw speed below 70r/min, generally a larger extrusion device should be selected for the hollow blow molding machine.
Blowing rate When blowing, the volume rate of the air introduced should be as large as possible, so as to shorten the blowing time, so that the product can obtain a more uniform thickness and better surface quality; the speed of the air should not be too large, otherwise it may form at the air inlet. The low pressure causes this part of the parison to sink in, or it may break the parison at the die so that it cannot be inflated.
Blowing pressure The air for blowing should have enough pressure, otherwise it will be difficult to blow the mold or the pattern on the surface of the product is not clear. Generally, the pressure of thick-walled products can be lower, while thin-walled products and materials with high melt viscosity need to be used. Higher pressure, general blow molding pressure is 0.2 ~ 1.0MPa.
The inflation ratio refers to the ratio of the maximum diameter of the container to the maximum diameter of the parison, which is a multiple of the parison inflation. The size of the parison and the size of the inflation ratio directly affect the size of the container. When the size and quality of the parison are constant, the larger the inflation ratio of the parison, the larger the container size. The inflation 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 are reduced. If the inflation ratio is too small, the consumption of raw materials will increase, the wall thickness of the product will be reduced, the effective volume will be reduced, the cooling time of the product will be prolonged, and the cost will increase. When molding, it should generally be determined according to the variety 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, taking 1.2 to 1:5 times; the inflation of small thick-walled products is relatively large, taking 2 to 4 times.
Mold temperature The blow molding mold temperature should usually be determined according to the nature of the material and the wall thickness of the part. For general-purpose plastics, it is generally 20 to 50 °C. For engineering plastics, due to the high glass transition temperature, the mold can be demolded at a higher mold temperature without affecting the quality of the product. The high mold temperature also helps to improve the surface smoothness of the product. Generally, the temperature of the blow molding mold should be controlled at about 40°C lower than the softening temperature of the plastic.
When the temperature of the mold is too low during the blow molding process, the extensibility of the plastic clamped at the jaw will become lower, and this part will be thicker after inflation. Too low temperature often causes spots or orange peel on the surface of the product. When the mold temperature is too high, the phenomenon at the jaw is just the opposite, and it will also prolong the molding cycle and increase the shrinkage rate of the product.
Cooling time and cooling rate After the parison is inflated, it is cooled and shaped, and water is generally used as a cooling medium. The heat is taken out through the cooling water 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 being deformed due to elastic recovery. The shape of the product is regular, the surface pattern is clear, and the quality is good, but the production cycle is prolonged and the production efficiency is reduced. And the strength and transparency are reduced due to the crystallization of the product. If the cooling time is too short, the product will be stressed and voids will appear.
Usually, the cooling rate is accelerated on the premise of ensuring that the product is fully cooled and shaped to improve production efficiency; the methods of accelerating the cooling rate are: expanding the cooling area of the mold, using frozen water or frozen gas for cooling in the mold, and using liquid nitrogen or carbon dioxide for cooling Inflation and internal cooling of 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 is prolonged. However, different plastic varieties have different cooling time 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 it is not necessary to extend the cooling time too much.
Balanced cooling is used for products with large, thick wall and special configuration, cooling medium with high cooling efficiency is used for the neck and cutting parts, and general cooling medium is used for the thinner part of the product body. For special products, a second cooling is required, that is, air cooling or water cooling is used after the product is demolded, so that the product is fully cooled and shaped to prevent shrinkage and deformation.