The die profiled means that the die of the extruder head or the core bar is partially grooved, and the die gap at the corner of the corresponding component is increased to increase the wall thickness of the local position of the parison, and the shaped parison is obtained. To compensate for the uneven thickness of the radial wall caused by the inconsistent inflation ratio, thereby obtaining a workpiece having a relatively uniform wall thickness.
I. Shaped die
It refers shaped die of the extruder head or die mandrel local position defines a recess, corresponding to increased die gap at the corner parts, the wall thickness of the parison in order to increase the local position of the obtained shaped parison To compensate for the uneven thickness of the radial wall caused by the inconsistent inflation ratio, thereby obtaining a workpiece having a relatively uniform wall thickness. By adopting the special-shaped die, and through multiple trial-and-error, optimization, design and processing of a reasonable profiled die, the uniformity of the wall thickness distribution of the workpiece can be greatly improved.
Second, axial wall thickness control technology
The function of the axial wall thickness control technique is to make the extruded plastic parisons obtain different thicknesses in the axial direction according to the different inflation ratios of the products, thereby ensuring a relatively uniform wall thickness of the final product. It is to change the opening amount of the head by making the mandrel or the die move axially according to the preset position, thereby achieving the purpose of changing the wall thickness of the plastic parison. At present, the hopper head of the hollow forming machine generally has an axial parison control function, and its control points range from 30 to 256 points.
Third, radial wall thickness control technology
Although the axial wall thickness control technology can improve the wall thickness distribution in the height direction of the blow molded product, the horizontal section of the extruded plastic parison is still equal in thickness, and the part is blown in a certain part in the radial direction. The product required for the expansion ratio is still not optimal, so radial wall thickness control technology is produced. The radial wall thickness control technique allows the extruded parison to undergo a non-circular cross-section variation within the desired section. Radial wall thickness program control technology has been developed to date, and two typical designs have been formed, one is called a flexible ring type, and the other is called a mouth edge repair form.
(1) Flexible ring technology
The flexible ring type changes the thickness of the extruded parison by electrohydraulic servo control of deformation of the thin-walled flexible ring in one direction or two symmetrical directions. It is characterized by the fact that regardless of the shape of the blown product, radial control can function as long as the die diameter is constant. The development of the radial wall thickness control system for hollow formed plastic parisons has recently achieved a critical technological breakthrough in Jiangsu. This control technology can realize multi-point accurate control of the radial thickness of the plastic parison. The control point can easily realize 2 to 16 points of control or even more point control, and is currently being industrialized.
(2) lip edge repair technology
The edge modification is to change the wall thickness of the parison by moving up and down the modified die ring. Compared with the flexible ring structure, its biggest advantage is long service life and low processing technology. In some designs, the trimming portion of the die ring is embedded as a movable block for easy replacement and reduces the cost of replacement. This form of design also requires in-depth research to reduce costs and speed up the pace of promotion.
Radial wall thickness control technology is an effective method to improve the quality of large hollow products, and can also reduce the quality of products. Taking a 200L plastic drum container as an example, at least 5% to 10% of raw materials can be saved. At present, the additional cost of processing a large radial wall thickness control device is relatively high. With the in-depth research and development of radial wall thickness control technology, it will be applied to more large and medium extrusion blow molding hollow forming machines. The combination of axial wall thickness control and radial wall thickness control results in an optimum plastic parison for a more desirable wall thickness distribution. At present, many domestic manufacturers of hollow forming machines can select matching radial wall thickness control systems on large-scale hollow forming machines.
(3) parison temperature difference method
The deformation resistance of the parison can be expressed by the viscosity, and the magnitude of the viscosity is related to the temperature. The parison has high temperature, small viscosity, small deformation resistance, easy deformation during blow molding, and large deformation; on the contrary, the parison has low temperature, high viscosity, large deformation resistance, is not easy to be deformed during blow molding, and has small deformation. . During the extrusion process, the cooling parison is forced to cool the relatively large portion by the cooling device, so that the parison has a reasonable temperature gradient. The relatively large portion of the parison inflates the temperature, the viscosity increases, and the deformation resistance increases. In the free inflation stage, the relatively large portion of the inflation is difficult to deform, the amount of deformation is reduced, and the amount of deformation of the portion where the inflation is relatively small is increased. When the deformation is small, the deformation is completed, and the constraint inflation stage is entered, and the inflated portion is further deformed until the inflation molding member is formed, thereby improving the uniformity of the wall thickness distribution of the workpiece.
(4) Vacuum blister combined with extrusion blow molding
The different blow ratios of the various parts of the parison result in uneven wall thickness of the product. The inflation process of the product can be divided into two stages: free inflation and constrained inflation. The free inflation phase is referred to as the compressed air enters the parison until the parison begins to contact the inner wall of the mold. At this stage, the parisons have the same inflation ratio, the deformation in all directions is unconstrained, and the deformation can be expanded and deformed in any direction, and is relatively uniform. The confining inflation phase is called from the beginning of contact between the parison and the mold cavity until the parison is completely bonded to the inner wall of the mold. At this stage, the outer surface of the parison is cooled by the mold, the temperature is lowered, the viscosity is increased, the deformation is difficult, or even no deformation, resulting in a large wall thickness of the product; the blank without contact with the mold cavity has a relatively high temperature. The viscosity is small, the deformation is easy, the thickness is quickly thinned, and it is close to the inner surface of the mold, resulting in a small wall thickness of the product, and finally the wall thickness of the whole part is not uniform. If the parison is simultaneously freely inflated, a part with a completely uniform wall thickness can be obtained. Under the action of the vacuum negative pressure, the parison is deformed to a relatively large portion of the inflation, and then the compressed air is injected to improve the inflation ratio of the parison, and the workpiece having a relatively uniform wall thickness is successfully obtained.
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