The main influencing factors and control methods in the blow molding process are the focus of this article.
In the blow molding process of the parison, the factors that affect the molding process of the parison mainly include the temperature of the parison, the inflation pressure, the blowing speed, the inflation ratio, and the temperature of the blow mold.
(1) Parison temperature
The temperature of the parison directly affects the stability of the shape of the parison, the surface finish of the product, the strength of the joint, and the cooling time.
When the preform temperature is higher, its strength decreases. Although the product is prone to inflation and deformation, it is difficult to ensure the uniformity of the wall thickness of the product, especially the uniformity of the longitudinal thickness of the parison. At the same time, the cooling time of the inflation material is prolonged, and the production efficiency of the blow molding machine decreases.
Conversely, lowering the temperature of the preform can ensure that it has sufficient strength, but blow molding will become difficult. If the temperature of the preform is too low, it will also cause the length of the tube embryo to shrink and the wall thickness to increase. There may be greater residual stress inside the product, resulting in a decrease in the strength of the product, a rough surface, and a significant decrease in appearance quality.
In general, under the premise of not seriously affecting the shape stability of the parison, to ensure that the product has sufficient surface finish and seam strength, the temperature of the parison can be appropriately increased.
More information on the factors that affect the quality of the extrusion blow molding parison:5 Factors Affecting the Quality of the Extrusion Blow Molding Parison.
(2) Blowing pressure and blowing speed
After the parison is clamped by the mold, compressed air is injected.
Here, compressed air plays three roles:
①Using the pressure of compressed air to make the rubber-like tubular preform swell and close to the inner wall of the inflation model cavity;
② Apply pressure to the inflated parison to obtain a product with the correct shape and clear surface text and images;
③Promote product cooling.
The pressure of compressed air is called blowing pressure. There are differences in the physical properties of different raw materials (molecular flexibility and the melt strength and melt elasticity of the preform). The rubber-like preforms have different moduli, and the air pressure required for inflation is also different. the difference. Generally, between 0.2-0.7MPA, the blowing pressure is also related to factors such as the preform temperature, mold temperature, preform thickness, inflation ratio, and product shape and size.
At that time, materials with low melt viscosity, small elastic modulus, variability, low cooling rate, high preform temperature, and high mold temperature can be used with lower blowing pressure.
Conversely, when the mold temperature is low, the mold temperature is low, or the material with high melt viscosity and high elastic modulus is used, higher blowing pressure should be used for inflation.
Besides, the blowing pressure is also affected by the wall thickness and volume of the product.
Generally speaking, for hollow products with thin walls and large volumes, and hollow products with patterns, patterns, and threads on the surface, higher blowing pressure should be used for blow molding. For products with small volume and large wall thickness, lower blowing pressure should be used.
The increase in blowing pressure helps the product to be in closer contact with the mold cavity, the heat of the product can be quickly removed, and the cooling efficiency is improved. Correspondingly, the temperature of the product during demolding can be reduced.
In terms of blowing speed, if the volume of air-filled into the cavity per unit time is increased, that is, the inflation speed is increased, the inflation time of the parison can be shortened, and it is easier to obtain products with uniform wall thickness and excellent surface quality.
However, if the inflation speed is too high, it may lead to low pressure in the air inlet area, which is likely to cause a vacuum here, resulting in indentation and deformation. In severe cases, the excessively fast airflow may even tear off the parison of the part of the fracture die, making the inflation process unable to proceed.
Therefore, in the inflation stage of the parison, a large flow of air should be injected while maintaining a low air velocity to ensure uniform and rapid expansion of the parison, shorten the cooling time before the parison contacts the mold cavity and Improve product performance.
The air blowing speed can be achieved by changing the size of the blowpipe aperture. Generally speaking, increasing the blowpipe diameter can achieve the purpose of low air velocity and large air volume flow. However, to shorten the blowing time, the blowing speed of the preform should be increased as much as possible under the premise of ensuring that the product has a more uniform thickness and better surface quality.
(3) Blow up ratio
The blow-up ratio refers to the ratio of the size of the product to the size of the parison, that is, the multiple of the blow-up of the parison. When the size and weight of the parison are the same, the larger the size of the final product, the larger the blow-up ratio.
When the same amount of raw materials are used for production, increasing the blow-up ratio of the parison can produce larger volume products. In other words, to produce products with the same volume, using a large inflation ratio can save raw materials to some extent. However, as the wall thickness of the product gradually becomes thinner, inflation molding will become more difficult, the strength and rigidity of the product will also decrease, and the stability of the product shape will become worse.
Conversely, if the blow-up ratio is too small, the consumption of plastic will increase, the effective volume of the product will decrease, and the wall thickness of the product will increase, and it will take longer to cool it down. This undoubtedly increases production costs and easily causes uneven wall thickness.
During production, the blow-up ratio should be determined according to the specific types and properties of the material, the shape and size of the product, and its value is generally between 2-4.
(4) Mold temperature and size of preform
The mold temperature usually cannot be controlled too low.
If the mold temperature is too low, the temperature of the plastic at the nip will drop quickly, and the parison will be cooled prematurely during the inflating period when positioned in the inflation mold, making it difficult to inflate and form. And it may thicken the jaws, markings appear on the surface of the plastic parts, the smoothness of the surface becomes worse, and the contour and patterns of the product become unclear.
If the mold temperature is too high, the cooling setting time will be prolonged, the production cycle will be longer, and the productivity will decrease. Also, the shrinkage rate of the part becomes larger, the size and shape are difficult to accurately control, and the surface lacks gloss.
The mold temperature should be selected according to the type and physical properties of the plastic. For those with higher glass transition temperature TG or heat distortion temperature TD, a higher mold temperature can be selected. On the contrary, the mold temperature should be reduced as much as possible.
(5) Cooling time
After the parison is inflated and close to the wall of the blow mold, do not rush to open the mold at this time. Instead, it should be cooled evenly for a while maintaining certain air pressure inside. Uneven cooling and insufficient cooling will cause different degrees of shrinkage in various parts of the product, causing product warpage, bottleneck skew, and other phenomena.
The cooling time of blow-molded products varies with different raw materials and the shape, size, wall thickness, etc. of the product, and generally can account for more than 60% of the entire molding cycle. The length of the cooling time directly affects the appearance quality, physical and mechanical properties, and production efficiency of the final product. Normally, the thicker the product, the longer the cooling time should be.
Improve the cooling efficiency of blow-molded products and shorten the cooling time, which can shorten the entire molding cycle and increase production efficiency. Sometimes to achieve this purpose, in addition to cooling the mold, internal cooling can also be performed in the molded product, that is, a strong cooling medium such as liquid nitrogen, liquid carbon dioxide, etc. is passed into the cavity of the inflation product for cooling. However, if the cooling time is too short and the cooling is too fast and insufficient, the product is prone to severe deformation due to elastic recovery after the mold is opened, the shrinkage increases, and the surface is dull.
(6) Molding cycle
Including the process of extruding the parison, cutting the parison, closing the mold, blowing, cooling, deflating, opening the mold, and taking out the product.
The selection principle of the molding cycle is to shorten it as much as possible while ensuring that the product can be shaped. An unreasonable molding cycle often leads to product quality defects.
The above is the whole content of the main influencing factors and control methods in the blow molding process.
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