YIXIN SCREW

1. Special-shaped die

Die irregularization refers to the opening of grooves at local positions on the die or mancore of the extruder head to increase the die gap at the corresponding corner of the workpiece, thereby increasing the wall thickness at local positions of the parison and obtaining irregularized parison. This compensates for the uneven radial wall thickness caused by inconsistent blow-up ratios, and thus results in a workpiece with relatively uniform wall thickness. The targeted adoption of special-shaped die molds and the processing of reasonable special-shaped die molds through multiple mold trials, optimization and design can greatly improve the uniformity of the wall thickness distribution of the workpiece.

2. Axial wall thickness control technology

The function of axial wall thickness control technology is to enable the extruded plastic preforms to obtain different thicknesses along the axial direction according to different blow-up ratios of the products, thereby ensuring that the final products have relatively uniform wall thickness. It achieves the purpose of changing the wall thickness of the plastic preform by making the mandrels or die move axially according to the preset position to alter the opening of the die head. At present, the storage die heads of hollow forming machines generally have axial parison control functions, with control points ranging from 30 to 256.

3. Radial wall thickness control technology

Although the axial wall thickness control technology can improve the wall thickness distribution in the height direction of blow-molded products, since the horizontal cross-section of the extruded plastic preform is still an equal-thickness circle, it is still not the best for some products that have a large blow-expansion ratio requirement at a certain part in the radial direction. Therefore, the radial wall thickness control technology was developed. The radial wall thickness control technology can make the extruded parison have a non-circular cross-section change within the required section. Up to now, the radial wall thickness program control technology has roughly formed two typical designs: one is called the flexible ring type, and the other is called the flange modification form.

Flexible ring technology

The flexible ring type changes the thickness of the extruded parlet by controlling the deformation of the thin-walled flexible ring in one direction or two symmetrical directions through electro-hydraulic servo control. Its feature is that no matter what shape the product is blown into, as long as the diameter of the die remains unchanged, the radial control can play a role. The research and development of the radial wall thickness control system for hollow forming plastic preforms has recently achieved a key technological breakthrough in Jiangsu. This control technology can achieve multi-point precise control of the radial wall thickness of plastic preforms. The control points can be conveniently controlled from 2 to 16 points or even more points. It is currently in the process of industrialization.

(2) Edge trimming techniques

The flange modification is achieved by the up and down movement of the modification die ring to change the wall thickness of the parison. Compared with the flexible ring structure, its greatest advantage is its long service life and low processing technical difficulty. In some designs, the trimming part of the die ring is made into an embedded movable block, which is convenient for replacement and reduces the cost during replacement. This form of design still requires in-depth research to reduce costs and accelerate the pace of promotion.

Radial wall thickness control technology is an effective method for improving the quality of large hollow products and can also reduce the weight of the products. Take a 200L plastic bucket container as an example, at least 5% to 10% of raw materials can be saved. At present, the additional cost of processing a set of large-scale radial wall thickness control devices 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-sized extrusion blow molding hollow forming machines.

The combined effect of axial wall thickness control and radial wall thickness control can obtain the best plastic parison, and thus a more ideal wall thickness distribution of the product can be achieved. At present, many domestic hollow forming machine manufacturers can choose to match the radial wall thickness control system on large hollow forming machines.

(3) Parison temperature difference method

The deformation resistance of the parison can be expressed by viscosity, and the magnitude of the viscosity is related to the temperature. The parison temperature is high, the viscosity is low, and the deformation resistance is small. It is prone to deformation during the blow molding process, and the deformation amount is relatively large. On the contrary, when the parison temperature is low, its viscosity is high, and its deformation resistance is large, it is less likely to deform during the blow molding process, and the deformation amount is relatively small. During the extrusion process, the parts of the parison that have expanded relatively large are forcibly cooled through cooling equipment, so that the parison has a reasonable temperature gradient. At the parts where the parison is expanded relatively large, the temperature is low, the viscosity increases, and the deformation resistance increases. During the free blow-up stage, the parts with greater blow-up are difficult to deform and the deformation decreases, while the parts with smaller blow-up have an increased deformation. When the smaller deformation area completes the deformation and enters the constrained blow-up stage, the blow-up area continues to deform until the blow-up formed part is formed, thereby improving the uniformity of the wall thickness distribution of the part.

(4) The combination of vacuum forming and extrusion blow molding

The different blow-up ratios of various parts of the parison result in uneven wall thickness of the products. The blow-up process of the product can be divided into two stages: free blow-up and constrained blow-up. The period from when compressed air enters the parison to when the parison begins to come into contact with the inner wall of the mold is called the free blow-up stage. At this stage, the parison has the same blow-up ratio, and its deformation in all directions is not restricted. It can expand and deform in any direction and is relatively uniform. The stage from the moment the parison comes into contact with the mold cavity until it fully adheres to the inner wall of the mold is called the constrained blow-up stage. At this stage, the outer surface of the parison is cooled by the mold, the temperature drops, the viscosity increases, and deformation becomes more difficult, or even stops deforming, resulting in a larger wall thickness of the product. The billet that does not come into contact with the mold cavity has a relatively high temperature, low viscosity, and is more prone to deformation. It quickly becomes thinner and closely adheres to the inner surface of the mold, resulting in a smaller wall thickness of the product and ultimately causing an uneven wall thickness throughout the entire workpiece. If the parison finishes free blow-up at the same time, a workpiece with completely uniform wall thickness can be obtained. Under the tensile force of vacuum negative pressure, the parison is first deformed towards the larger part of the blow-up area, and then compressed air is injected to improve the blow-up ratio at all parts of the parison, successfully obtaining a part with relatively uniform wall thickness.