Author: Huan Machinery Publish Time: 2021-07-31 Origin: Huan Machinery
Blow-molded product deformation means that its external shape deviates from the shape of the mold cavity, or convex or concave, or longitudinal or radial, or partial shape deviation. It is one of the common defects of blow molding products, and it is also one of the most comprehensive and difficult to deal with. The degree of deformation of blow molded products and the excellent degree of performance are used as one of the important indicators for evaluating product quality. Since the deformation of blow-molded products also has a great impact on the production cost, it has attracted more and more attention and attention from the industry.
Analysis and Solution of Deformation Defects of Blow Molded Products
The greater the degree of deformation of the blow-molded product, the satisfaction with its appearance and the use performance will decrease in direct proportion, and the product will be scrapped in severe cases. Deformation of blow-molded products is not only detrimental to product quality, but also detrimental to the output and production cost of blow-molded products. Therefore, the solution of the deformation defects of blow-molded products is an important subject discussed in the technical aspects of various plastic processing enterprises.
There are many reasons for the deformation of blow molded products, and it is far from enough to solve the problem by adjusting the process parameters. Through long-term practical work experience and analysis and summary of relevant data, the thermophysical properties of plastic materials and the conditions and parameters of the blow molding process have varying degrees of influence on the deformation of blow molded products, which must be comprehensively considered and reasonably resolved.
The effect of material's thermophysical properties on deformation
(1) The use of cyanide pigments in the formulation of blow molding plastics will affect the crystallinity of polyethylene and cause product deformation;
(2) Primary color blow-molded products produced from raw materials without any pigments are not easy to deform, which can improve product quality and output;
(3) The use of plastics with low melting point and good fluidity can improve product quality and output due to its low shrinkage rate, fast cooling, and resistance to deformation.
The effect of product wall thickness gap and cooling on deformation
(1) The structural design of blow molded products directly affects the wall thickness gap and cooling of the products. The large difference in wall thickness results in a large difference in cooling rate and deformation.
The wall thickness is small where the transition part of the product structure changes suddenly, and vice versa, the wall thickness is large. Therefore, the transition part of the blow molded product structure needs to be smoothly transitioned to avoid sharp corners and sharp edges. If due to the special structure of the product, the wall thickness varies greatly, the cooling should be strengthened at the thick wall, and the cooling circuit of the mold should be set according to the cooling requirements;
(2) The greater the wall thickness difference between the longitudinal wall thickness position and the radial wall thickness position of the blow molded product, the easier it is to deform. The shrinkage formed by the position of the parting line of blow-molded container products is more serious than that of the adjacent position, which is that the cooling rate of the thicker area is much slower than that of the surrounding area. The difference in cooling rate results in different shrinkage and deformation. This defect is best solved by using servo longitudinal wall thickness control and servo radial wall thickness control at the same time, but the equipment cost is relatively high. It is also possible to replace the radial wall thickness control by trimming the core mold of the machine head, but the effect is not as good. For small and medium blow molded containers of flat and square shapes, the wall thickness difference of blow molded products can be reduced by setting the size of the mouth mold and core mold slightly larger, so as to achieve shrinkage balance, thereby solving deformation defects.
Influence of molding process on deformation
(1) The temperature of the extruder barrel, head temperature, and mold temperature should not be too high. High temperature will lead to slow cooling of blow molded products and easy deformation;
(2) The molding cooling time is not enough, resulting in uneven cooling and deformation of the blow molded product after being ejected from the mold;
(3) According to the needs of the product shape, finely adjust the servo wall thickness curve to make the product wall thickness as uniform as possible;
(4) Use a chiller to provide circulating cooling water to the mold to improve the cooling effect;
(5) Improper adjustment of the mold opening stroke parameters will cause the product to be deformed when the mold is opened.
The influence of poor demoulding on deformation
(1) Blow-molded container recesses (under the handle, bottom groove, etc.) are not sufficiently demoulded, which will cause deformation when opening the mold;
(2) The blow-molded container’s mouth is not cooled well or the mouth flash is too long, Both will lead to tensile deformation when opening the mold;
(3) The sandblasting effect of the two side molds of the blow molding container mold is different, which will cause the product to be attached to the mold with better sandblasting when the mold is opened.
Influence of uneven force of ejector device on deformation
When producing blow-molded board products, it is generally necessary to design an ejector device on the mold. Uneven ejection force will eject the product out of shape. When designing the ejector device, it should be balanced with the ejection resistance. It would be better to use a combination of pneumatic or hydraulic pressure and machinery.
The effect of premature sealing of the mouth on deformation
The product just produced has not been sufficiently cooled. If the mouth is fully sealed with a cap prematurely, the product will be deformed due to the different cooling speeds inside and outside and atmospheric pressure.
The effect of stress on deformation
Residual stress, shear stress, and cooling stress in the molding process of blow molded products can also cause deformation defects of blow molded products. Due to its complexity, it will not be explained in detail here.