The process of blow molding is complex, and there are many factors affecting the quality of products, which generally include the shape of products, the performance of raw materials and the process parameters of processing molding. Although there are many factors affecting the product performance, when the product requirements and process conditions are determined, the quality of the product can be optimized by changing the influencing factors, which can achieve the purpose of reducing the consumption of raw materials, reducing the production time and optimizing the product performance.
1、Type of material
The different properties and types of resin raw materials will make the processing and molding technology and equipment change. The melting index, molecular weight and rheological properties of resin raw materials will affect the shaping of products, especially in the extrusion stage of the billet, the melting fluidity of raw materials will make the billet easy to produce sag phenomenon, will lead to the wall thickness of products thinner and uneven distribution.
2、Shape of product
As the appearance of blow molding products is more and more complex, resulting in blow molding products at each position of the blow expansion ratio is different. The convex edge, handle, corner and other positions of the product due to the shape variable is relatively large, the wall thickness of the product should be thin, so in the process of blow molding to increase this part of the billet wall thickness. The appearance of industrial products is more complex, with many corners and convex edges. The blowing ratio of these parts is larger than that of other flat parts, and the wall thickness is relatively thin, so the thickness distribution of hollow blow molded products is not uniform.
3、Mold expansion and vertical extension of the parison
One of the key links in the hollow blow molding method is the extrusion forming of the blank. The size and thickness of the blank fundamentally determine the size and wall thickness of the product. The phenomenon of melt vertical extension and mold expansion will be produced in the forming process of billet. The vertical extension of the billet is the effect of its own gravity, which makes the length of the billet increase and the thickness and diameter decrease. When the raw material is heated and melted by the extruder, the nonlinear viscoelastic deformation occurs when the material is extruded through the head, which makes the length of the billet shorten and the thickness and diameter increase. In the process of extrusion and blow molding, the two phenomena of vertical extension and mold expansion influence at the same time, increasing the difficulty of blow molding, but also make the product thickness distribution is not uniform.
4、Temperature of processing
HDPE processing temperature is generally 160~210℃. Processing temperature is too high, will make the type of billet sag phenomenon is obvious, the wall thickness distribution is not uniform, but the surface of the product will be smooth; The temperature of the die head should be as close as possible to the temperature of the heating section. The temperature of the mouth of the cup should be properly lower than that of the die head, which can reduce the influence of mold expansion of the parison.
5、Rate of extrusion
With the increase of extrusion speed, the larger the mold expansion of the billet, the thickness of the billet will increase. If the extrusion speed is too slow, the longer the billet is affected by its own weight, the more serious the sag phenomenon of billet is. Extrusion speed is too fast, will cause the type of billet shark skin phenomenon, serious will lead to the type of billet rupture. The extrusion speed will be affected by the blowing time, too fast speed will reduce the blowing time, may make the product can not be formed. The extrusion speed will affect the surface and wall thickness of the product, so the extrusion speed range needs to be adjusted constantly.
6、Ratio of blow to expansion
The melt of the inner and outer surface of the blank will be blown and extended rapidly in the mold and close to the surface of the mold until it is cooled and formed. The blank with larger diameter inside the mold will be subjected to greater stress (the ratio between the diameter of the mold with larger size and the diameter of the blank at this time is the blowing ratio). Air leakage is easy to occur during the blowing and swelling of the larger bottle shape, resulting in the failure of blowing and forming. The appearance of the product greatly affects the blowout ratio during blow molding. When blowing products with irregular shape, the blowing ratio should not be too large, otherwise it is easy to lead to melt rupture.
7、Blowing pressure and time
In the process of blow molding, the compressed gas can make the billet blow and form and cling to the inside of the mold. The forming speed of the billet is determined by the gas pressure. When the gas pressure is too large, the deformation speed of the blank is fast, which will make the plane part of the blank quickly close to the inside of the mold, so that the temperature of the blank is reduced under the influence of the mold, and the blank is gradually formed, which can not continue to deform. At this time, because of the large shape variable, the corner part of the billet has not been attached to the mold, and the deformation continues, resulting in uneven distribution of the wall thickness of the product. When the gas pressure is too small, the molding of the product is difficult, and because the pressure holding pressure is too small, the billet will shrink and can not get better products, so it is necessary to control the gas pressure reasonably when blowing. The blowing pressure of hollow products is generally controlled in 0.2~1 MPa. Blow time is mainly determined by blow molding time, pressure holding time and cooling time of the product. If the blowing time is too short, will make the product blow molding time is short, there is not enough pressure holding and cooling time, the billet will obviously shrink inward, the surface becomes rough, affect the appearance of the product, even can not be formed; If the blowing time is too long, the product can have a good appearance, but it will prolong the production time.
8、Mold temperature and cooling time
The incision of the die is generally made of steel products with greater toughness, so it needs to have excellent cooling effect. The mold temperature is too low will make the mold cut cooling faster, no ductility; The high temperature will make the billet cooling is not enough, the mold cut will be relatively thin, the product shrinkage phenomenon is obvious when cold, making the product serious deformation. The cooling time is longer, the influence of mold temperature on the product is relatively small, the shrinkage is not obvious; The cooling time is too short, the billet will have obvious shrinkage phenomenon, the product surface will become rough, so it is necessary to reasonably control the mold temperature and cooling time.
9、Speed of screw
The speed of the screw will affect the quality of the billet and the efficiency of the extruder. The size of the screw speed is limited by the raw materials, the shape of the product, the size and shape of the screw. When the rotating speed is too low, the working efficiency of the extruder is obviously reduced, and the vertical stretch time of the billet is long, which leads to the uneven distribution of the wall thickness of the product. Increasing the rotational speed reduces the operating time and increases the energy consumption. At the same time, the increase of screw speed can improve the shear rate of the screw to the raw material and optimize the appearance of the product. But the screw speed should not be too high, because the speed is too high will make the raw material in the head and the mouth of the cup stay too short, the temperature distribution is not uniform, the wall thickness of the billet is affected, and then affect the appearance of the product. Excessive rotation speed will also increase the friction force, generate a lot of heat may cause the degradation of raw materials, may also appear melt rupture phenomenon.
Post time: Nov-19-2022