1、 Main process factors controlled

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The main factors controlled in the extrusion blow molding process include the temperature of the extruder, screw speed, blowing rate, blowing pressure and inflation ratio, mold temperature, cooling time, and cooling rate.

2、 The influence of process factors on the forming process

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01

The temperature control of the extruder directly affects the forming process and quality of the billet during the extrusion process. Raising the temperature of the extruder barrel can reduce the viscosity of the melt, improve its flowability, lower the power consumption of the extruder, and also improve the strength, surface glossiness, 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 billet to sag due to its own weight, resulting in uneven longitudinal wall thickness of the billet. At the same time, it is also prone to degradation of plastics with poor thermal stability, such as PVC, and can significantly reduce the strength of engineering plastics (such as PC) billets. If the temperature is too low, the plasticization of the material is not good, the surface of the billet is rough and not shiny, the internal stress increases, and it is easy to cause the product to break during use. Therefore, considering multiple factors, the barrel temperature should be as low as possible while ensuring good surface gloss, uniform plasticization, high melt strength, and not overloading the transmission system of the extruded billet.

02

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 billet. However, it is easy to cause a decrease in the surface quality of the product, and due to the increase in shear rate, it can also 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, but generally controlled below 70r/min, on the premise of being able to extrude smooth and uniform billets without overloading the extrusion transmission system. In order to control the screw speed below 70r/min, a larger extrusion device should generally be selected for hollow blow molding machines.

03

The volume rate of air introduced during blow molding should be as high as possible to shorten the inflation time and achieve a more uniform thickness and better surface quality of the product; The speed of the air should not be too high, otherwise a low pressure may form at the air inlet, causing this part of the billet to sink in, or it may break the billet at the mold mouth, making it unable to inflate.

04

The air used for blow molding should have sufficient pressure, otherwise it will be difficult to blow and expand due to mold failure or unclear patterns on the surface of the product. Generally, the pressure for thick walled products can be lower, while for thin-walled products and materials with high melt viscosity, higher pressure is required. The blow molding pressure is generally 0.2-1.OMPa.

05

The inflation ratio refers to the ratio of the maximum diameter of the container to the maximum diameter of the billet, which is a multiple of the inflation of the billet. The size of the embryo and the inflation ratio directly affect the size of the container. When the size and quality of the billet are constant, the larger the blowing ratio of the billet, the larger the container size. The blowing ratio of the billet is high, and the thickness of the container wall becomes thinner. Although it can save raw materials, blowing becomes difficult, and the strength and stiffness of the container decrease. If the inflation ratio is too small, the consumption of raw materials increases, the thickness of the product wall decreases, the effective volume decreases, the cooling time of the product is prolonged, and the cost increases. When forming, it is generally determined based on the variety and characteristics of the plastic, the shape and size of the product, and the size of the preform. Usually, the inflation of large thin-walled products is relatively small, ranging from 1.2 to 1:5 times; Small thick walled products have a relatively large inflation rate, which is 2-4 times higher.

06

The temperature of blow molding molds should usually be determined based on the properties of the material and the wall thickness of the workpiece. For general plastics, it is generally between 20-50 ℃. For engineering plastics, due to their high glass transition temperature, they can be demolded at higher mold temperatures without affecting the quality of the product. High mold temperatures also help improve the surface smoothness of the product. It is advisable to control the temperature of blow molding molds at around 40 ℃ lower than the softening temperature of plastics.

When the mold temperature is too low during the blow molding process, the extensibility of the plastic clamped at the mouth will decrease, and after inflation, this part will become thicker. The low temperature often causes spots or orange peel on the surface of the product. When the mold temperature is too high, the phenomenon that occurs at the clamp is exactly the opposite of when it is too low, and it can also prolong the molding cycle and increase the shrinkage rate of the product.

07

Cooling time and cooling rate: After the billet is inflated, it is cooled and shaped, usually using water as the cooling medium. The heat is carried 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 plastic deformation caused by elastic recovery, resulting in a regular appearance, clear surface patterns, and excellent quality of the product. However, the production cycle is prolonged and 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 experience stress and porosity.

Usually, the cooling rate is accelerated to improve production efficiency while ensuring sufficient cooling and shaping of the product; The methods to accelerate the cooling rate include expanding the cooling area of the mold, using chilled water or gas to cool inside the mold, and using liquid nitrogen or carbon dioxide to blow and internally cool the billet.

The cooling rate of the mold depends on the cooling method, the selection of cooling medium, and the cooling time, as well as the temperature and thickness of the billet. Generally, as the wall thickness of the product increases, the cooling time prolongs. However, different types of plastics have different cooling times due to varying thermal conductivity. For the same thickness, HDPE has a longer cooling time than PP. For PE products of general thickness, after cooling for 1.5 seconds, the temperature difference on both sides of the product wall is almost equal, and there is no need to extend the cooling time too much.

For large, thick walled, and specially shaped products, balanced cooling is used. High efficiency cooling media are selected for the neck and cutting parts, while general cooling media are used for the thinner parts of the product body. For special products, a second cooling is required, which means using air or water cooling after demolding to fully cool and shape the product, in order to prevent shrinkage and deformation.

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