Extrusion blow molding is one of the main forming methods for producing hollow plastic products with the largest output and good economic efficiency. It is suitable for hollow-forming materials such as polyethylene, polyvinyl chloride, polypropylene, polystyrene, polyester, polyamide, and polycarbonate.
It is mainly used to form container products, the minimum volume of the container can be 1mL, and the maximum volume can reach 20000L. Extrusion blow molding products are widely used in food, beverages, cosmetics, medicines, washing products, children’s toys, and other fields.
Since the mid-1980s, extrusion blow molding technology has developed greatly, and the product shape has become more complex and has unique functions. The application field has expanded to office supplies, household appliances, furniture, cultural and entertainment products, and automotive industry parts, such as bumpers, gasoline tanks, oil pipes, gas pipes, fuel tanks, etc., which have high technical content and functionality, so it is also called “engineering blow molding”.
Due to the limitation of installation space, the shape of extrusion blow molding industrial parts is becoming more and more complex, and the uneven distribution of wall thickness has become a bottleneck in the development of extrusion blow molding technology.
Next, we will introduce 5 methods to improve the uniformity of the wall thickness of blow-molded products.
1. Special-shaped die
Die profiling refers to the opening of grooves at the local position of the die or core rod of the extruder head, increasing the die gap at the corner of the corresponding part, so as to increase the wall thickness of the parison at the local position, obtain a special-shaped parison, and make up for the uneven radial wall thickness caused by inconsistent blow-up ratios, so as to obtain a part with relatively uniform wall thickness.
Targeted use of special-shaped die, and through multiple mold trials, optimization, design and processing to produce a reasonable special-shaped die, can greatly improve the uniformity of the wall thickness distribution of the part.
2. Axial wall thickness control technology
The function of axial wall thickness control technology is to make the extruded plastic parison obtain different thicknesses along the axial direction according to the different blow-up ratios of the product, to ensure that the final product has a relatively uniform wall thickness.
It achieves the purpose of changing the wall thickness of the plastic parison by making the core rod or die move axially according to the preset position to change the opening amount of the die head.
At present, the material storage head of the blow molding machine generally has an axial parison control function, and its control points range from 30 to 256 points.
3. Radial control technology
Although the axial wall thickness control technology can improve the wall thickness distribution of the blow-molded product in the height direction, the horizontal cross-section of the extruded plastic parison is still a uniformly thick circle, which is still not the best for some products with a large blow-up ratio requirement in a certain radial position. Therefore, radial wall thickness control technology was developed.
The radial wall thickness control technology can make the extruded parison present a non-circular cross-section change within the required section.
The development of radial wall thickness program control technology has roughly formed two typical designs, one is called flexible ring type, and the other is called mouth edge repair type.
(1) Flexible ring technology
The flexible ring type changes the thickness of the extruded parison by controlling the deformation of the thin-walled flexible ring in one direction or two symmetrical directions through an electro-hydraulic servo.
Its characteristic is that no matter what shape of product is blown, as long as the die diameter remains unchanged, radial control can play a role. The research and development of the radial wall thickness control system for hollow-molded plastic preforms has recently achieved a key technical breakthrough in Yankang.
This control technology can achieve multi-point accurate control of the radial wall thickness of plastic preforms. The control points can easily achieve 2 to 16 points of control or even more points of control. It is currently being industrialized.
(2) Edge shaping technology
The edge shaping is achieved by moving the shaping die ring up and down to change the wall thickness of the preform.
Compared with the flexible ring structure, its biggest advantage is its long service life and low processing technology difficulty.
In some designs, the edge shaping part of the die ring is made into an embedded movable block, which is convenient for replacement and reduces the cost of replacement.
This form of design still needs to be further studied to reduce costs and accelerate the pace of promotion.
Radial wall thickness control technology is an effective method to improve the quality of large hollow products and reduce the weight of the products. Taking a 200L plastic barrel 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-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, that is, a more ideal product wall thickness distribution.
(4) Parison temperature difference method
The deformation resistance of the parison can be expressed by viscosity, and the viscosity is related to its temperature.
When the parison temperature is high, the viscosity is low, the deformation resistance is small, and it is easy to deform during the blow molding process, and the deformation amount is large; on the contrary, when the parison temperature is low, the viscosity is high, the deformation resistance is large, and it is not easy to deform during the blow molding process, and the deformation amount is small.
During the extrusion process, the cooling equipment is used to force the cooling of the parison with a relatively large expansion, so that the parison has a reasonable temperature gradient. The temperature of the parison with a relatively large expansion is low, the viscosity increases, and the deformation resistance increases.
In the free expansion stage, the part with a relatively large expansion is difficult to deform, the deformation amount is reduced, and the deformation amount of the part with a relatively small expansion increases. When the part with a smaller deformation amount is deformed, it enters the constrained inflation stage, and the part with a larger inflation ratio continues to deform until the part is blown into shape, thereby improving the uniformity of the wall thickness distribution of the part.
(5) Combination of vacuum forming and extrusion blow molding
The different inflation ratios of various parts of the parison lead to the uneven wall thickness of the product.
The inflation process of the product can be divided into two stages: free inflation and constrained inflation.
The period from the compressed air entering the parison to the parison contacting the inner wall of the mold is called the free inflation stage. In this stage, the parison has the same inflation ratio, and the deformation in all directions is not constrained. It can expand and deform in any direction and is relatively uniform.
The period from the parison contacting the mold cavity to the parison completely fitting the inner wall of the mold is called the constrained inflation stage. At this stage, the outer surface of the parison is cooled by the mold, the temperature drops, the viscosity increases, and it is difficult to deform, or even no longer deforms, resulting in a thicker wall of the product; the blank that does not contact the mold cavity has a relatively high temperature, a low viscosity, is easier to deform, and quickly thins, close to the inner surface of the mold, resulting in a thinner wall thickness of the product, and ultimately uneven wall thickness of the entire part.
If the parison ends free inflation at the same time, a part with completely uniform wall thickness can be obtained. Under the tensile force of the vacuum negative pressure, the parison is first deformed to the part with relatively large inflation, and then compressed air is injected to improve the inflation ratio of the parison at various places, and successfully obtain a part with relatively uniform wall thickness.
The above are 5 methods summarized by Yankang to improve the uniformity of the wall thickness of blow-molded products. For more technical sharing on blow molding machines, please get in touch with Yankang Blow Molding Machine.