The hottest water mist injection technology shorte

2022-08-16
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Water mist injection technology shortens the cycle cycle

stretch blow molding is used to produce high-quality bottles with excellent mechanical and optical properties, as well as the advantages of light weight and low cost. At present, the bottle turning machine used for two-stage stretch blow molding can produce 1800 bottles per cavity per hour, that is, in fact, the cycle time related to molding and cooling is two seconds. However, the higher productivity is limited by the time it takes to cool the parts to reach the demoulding temperature. The cooling cycle is measured by measuring the temperature at the bottom of the plastic bottle. Compared with the thinner side wall of the bottle, the bottom of the bottle is thick, so the speed of heat dissipation to the cooled mold is slower. In order to further shorten the cycle of stretch blow molding, we must find a suitable method to shorten the cooling time

cooling the bottle through its inner surface

in traditional stretch blow molding, the cooling of blow molding bottle is mainly completed through the contact between the outer surface of the bottle and the mold. Due to its excellent thermal conductivity, water-cooled aluminum molds are widely used. The heat transfer between the outer wall of the bottle and the mold wall is improved by increasing the internal pressure in the blowing stage. Using this traditional method, there is no large amount of heat from the inner wall of the bottle. Therefore, increasing the heat dissipation through the inner wall of the bottle also provides a way to greatly shorten the cooling time in the stretch blow molding process

in the technology related to extrusion blow molding into (3) hopper: the hopper bottom is equipped with a cutting device type, different internal cooling methods have also been studied to shorten the production cycle time. In this way, intermittent cooling and cleaning air methods, as well as cooling air injection or low-temperature liquefied gas can shorten the cooling time. However, the attractiveness of these methods is greatly reduced by the complexity of processing technology and the cost of cooling liquid

another way to dissipate heat through the inner surface of the bottle is to inject water mist into the bottle. This method has been successfully used in extrusion blow molding in an AIF project carried out by the Institute of plastic processing (IKv) at the University of Aachen, Germany. The experiment shortened the cooling time by up to 41%. Because of the relatively simple machine and the high reliability of water as the cooling medium, the idea of using this cooling method for stretch blow molding is very attractive

objective

a joint research project of Krones Co., Ltd. of Neutraubling, Germany and IKv studied the possibility of using water mist injection in two-stage stretch blow molding to shorten the cooling time. To this end, IKv has designed a high-pressure water system for Krones' laboratory stretch blow molding machine, which can inject water mist into the formed bottle through the stretching rod at an appropriate time. The purpose of water mist injection is to extract more heat from the bottle material, thereby reducing the required cooling time

practical application

Figure 1 shows the schematic diagram of stretch blow molding and cooling with water mist technology. First, the heated pre-finished product is put into the blow mold, and then the mold is closed. Then the pre-finished product is stretched axially by the stretching rod. Almost at the same time, a pre expansion pressure is applied, which prevents the bottle embryo from contacting the stretching rod and also stretches the pre-finished product along the circumference. Once the stretching rod reaches its stretched end, the forming expansion pressure is applied and the pre-finished product is formed into a bottle. In addition to the final molding, the final expansion pressure ensures good heat transfer between the bottle material and the mold wall. In the traditional method, the pressure inside the mold will be released after the bottle is cooled. Then the mold will be opened and the shaped bottle will be demoulded

Fig. 1 stretch blow molding process with water mist injection device

during the final expansion of stretch blow molding process with water mist injection device, after the bottle is formed, an accessory step is performed immediately, that is, a water mist nozzle located at the top of the stretch rod injects water into the interior of the molded part (as shown in Fig. 1). The injection time window needs to be synchronized with the stretching rod so that the area at the bottom of the bottle with thick wall can be wetted by the conical spray formed. Once the bottom of the bottle is fully formed, the water mist can be injected into the bottle. On the other hand, the injection time window that can be provided is limited by the time that the stretching rod is pulled out of the mold

the water mist injection system used in the study is a Nessie plug & spray unit (manufacturer: Danfoss Co., Ltd., Germany). The key components of the assembled water mist injection system are listed in Figure 2. The water for the plant is provided by the traditional water supply system. This includes a filtration system to ensure stable water quality. The downstream pump station provides 80 to 100 bar pressure required to form water mist, and the pressure is regulated by a downstream pressure sensor. The system also provides a pressure storage tank, which provides sufficient volume of water during product production to ensure that the pressure remains constant. The central component of water mist injection system is the combination of change-over valve and water mist nozzle. For this study, a Danfoss VDH 30 3/2-way valve for converting the start and end points of water injection was used. The solenoid coil of the valve is controlled by the LabVIEW program (manufacturer: national instrument, USA), which successively receives the trigger signal from the stretch blow molding machine at the beginning of the pre expansion stage. The remarkable advantage of this kind of valve is that it can spontaneously release the pressure in the consumption channel at the end of the injection process, so that the downstream nozzle will never drip. PAG erank, which is the central skill of Google's creation, with the spray nozzle screwed to the stretching rod, is the end of the successful performance of this kind of thinking (shown in Figure 3). The stretching rod is normally installed in the stretching blow molding machine. The nozzle used (manufacturer: Danfoss Co., Ltd.) is integrated with a return valve, which can further ensure that casting will not occur

Figure 2 basic design of water mist injection system

Figure 3 Influence of stretching rod with water mist injection nozzle on cooling effect

after the water mist cooling system was installed on the stretch blow molding machine in the laboratory, this method was used to conduct practical research on 0.5 and 1.5 liter PET bottles (as shown in Figure 4). Therefore, the model, injection cycle and pressure of the nozzle have been changed accordingly. The influence of cooling water mist on the required cooling time is measured by measuring the accuracy of bottle bottom temperature and its basic size. As the cooling conditions have a great impact on the stability of the bottle bottom, the stress cracking resistance of the formed bottle under the influence of internal pressure and environment has also been tested. The use of water mist injection has a significant positive impact on the cooling performance represented by these target values. The cooling effect of a few selected samples has a negative impact on the bottom temperature of the bottle

the bottom temperature of 0.5 and 1.5 liter bottles studied in Figure 4 was measured by a thermal imaging camera. For this reason, the bottle is taken out for temperature measurement immediately after the blow molding section is formed. The reference value of each measurement is the average temperature of the bottle bottom area near the gate. Figure 5 shows that the temperature at the bottom of the bottle obtained by using the water mist cooling method is reduced by converting the elongation into the tension value as a function of the injection cycle and the bottle size. This may be seen as the water mist injection significantly reduces the temperature at the bottom of the bottle. The selected injection cycle has a major impact on the cooling effect. This can be explained by the increase in the volume of water injected into the bottle and the increase in the water injection cycle, thereby increasing the cooling effect. Moreover, the research shows that the cooling effect has a significantly higher impact on 0.5-liter bottles than on 1.5-liter bottles. This can be explained in turn by the thicker bottom of the large bottle and the larger volume of plastic to be cooled

Figure 5 bottle bottom temperature decreases as a function of injection cycle and bottle size

the productivity level obtained by water mist injection method is tested by the working point of 2000 bottles per cavity per hour, and gradually reduce the output of production without water mist injection method until the product quality standard reaches the level at the starting working point of water mist injection method. The results listed in Figure 6 are the average bottle bottom temperature. With the water mist injection method and the production capacity of 2000 bottles per hour per cavity, the temperature at the bottom of the bottle measured immediately after demoulding is about 81.5 ℃. For comparison, at the same working point, the bottom temperature of the bottle without water mist injection is about 84 ℃. Only when Jinan Shijin, a relatively large experimental machine factory in China, has adopted the water mist injection method to produce 1700 bottles per hour per cavity, its bottle bottom temperature is equivalent to that produced by the water mist injection method at a higher output. Therefore, for this working point, using water mist injection method increases the productivity by about 18%

Figure 6. The output is increased due to the use of water mist injection

water mist injection technology has been successfully used in laboratory stretch blow molding machines. Research shows that water mist injection can significantly improve the cooling efficiency. Therefore, the use of water mist shortens the cooling time and greatly increases the output of the stretch blow molding machine

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