Experimental study on the hottest extrusion blow m

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Experimental study on extrusion blow molding (Part 2)

3 Research status of parison inflation stage

parison inflation refers to placing the plastic tube blank in the mold while it is hot, and immediately blowing it with compressed air into the tube blank, and forming it close to the mold cavity wall. The forming at this stage directly affects the shape of the product, the uniformity of wall thickness and the performance of the product, which is the key link of the whole forming process

at this stage, the experimental study of parison inflation mainly includes two aspects: one is the dynamics of parison inflation, and the other is the measurement of parison wall thickness after parison inflation. Musa mal, Victor Tan and dilhan kalyon were the first to establish an experimental device to study the parison inflation dynamics [8]. They designed their own transparent blow molding mold, and used two cameras to capture the expansion behavior of the parison in the mold. The device diagram is shown in Figure 8. The pictures taken are sent to the graphics analyzer for analysis, so as to determine the change relationship of the diameter distribution of the parison with time

Ryan and Dutta [9] used camera technology to monitor the free expansion behavior of the parison under the condition of no mold, and obtained the expansion size of the parison. Later, most researchers used this similar method to study the expansion behavior of parison

wagner and kalyon[10] redesigned the solid pressure sensor on the basis of kamal[8], as shown in Figure 8. It can measure the pressure when the parison is blown. At the same time, another pressure sensor is installed on the flash of the mold cavity. In this way, the two sensors can measure the real pressure difference between the inside and outside of the parison during the blow molding process. They used this device to study the effects of three PA-6 on the inflation behavior under the inflation pressure

recently, Yong Li et al. [11] used a high-speed optical measurement system that can measure the instantaneous surface shape to measure the swelling behavior of polymer films. The measurement diagram is shown in Figure 9. Both ends of the polymer film parison are fixed between two plates, and compressed air is introduced into the pressure chamber to make the polymer film parison swell. There are CCD camera and grating transmitter in the optical probe. During the measurement, the grating emitter emits the grating onto the surface of the polymer film parison, and the grating deforms with the deformation of the polymer film parison, so the grating image contains the information of the surface shape of the polymer film parison. The expanded size of polymer film parison can be obtained by quickly capturing the grating image by the camera and sending it to the computer for processing. Mcdl is a multi-channel data hub, which can simultaneously collect and continuously develop new product pressure and raster signals in order to obtain the relationship between pressure and polymer film parison shape in the swelling process. Experiments show that the measurement accuracy is much higher than that in Figure 7

billet wall thickness measurement includes offline measurement and measurement. Because offline measurement is simple, it is used more. Offline measurement includes infrared, ultrasonic and micrometer measurement. These methods are not only time-consuming, but also the time lag caused by off-line measurement requires correction of the deviation generated in the processing process, resulting in inaccurate measurement and many unqualified products

measuring the wall thickness of products can reduce the lag time to the minimum, so it can improve the accuracy of deviation correction in the processing process. Diderichs and oeynhauser [12] used ultrasonic sensors placed in the mold to measure the wall thickness distribution. The measurement principle is shown in Figure 10. In the ultrasonic sensor, the short ultrasonic wave generated by the piezoelectric crystal is reflected by the wall of the object and returns to the sensor. The wall thickness s of the measured object is equal to the speed of ultrasonic wave in the object multiplied by half of the time required for ultrasonic wave to transmit in the object. However, the accuracy of ultrasonic measurement is greatly affected by the polymer properties (such as density, three different kinds of hardness and crystallinity obtained by using 60kg load and diamond cone indenter) and temperature

4. Research progress of product cooling and curing stage

product cooling and curing refers to the process of cooling the parison by means of the mold with high thermal diffusivity and compressed air after it is inflated close to the mold wall, opening the mold after cooling to a certain temperature, and then cooling in the air. Generally, it includes external cooling (heat conduction between the outer surface of the product and the mold cavity), internal cooling (convective heat transfer between the inner surface of the product and the cooling air or other media) and post mold cooling (natural convective heat transfer between the inner and outer surface of the product and the air or other media)

the experimental research on the cooling and curing stages of products is mainly to measure the transient temperature, shrinkage, warpage, etc

the transient temperature of products is generally measured by high-sensitivity thermocouples and data collectors. In 1981, edward[13] et al. Designed "half bottle molding experiment" to verify their theoretical prediction of extrusion blow molding cooling process. As shown in Figure 11. In the experiment, the instantaneous temperature of the outer surface was measured by thermocouple, and the temperature of the inner surface was measured by radiation pyrometer as soon as the workpiece left the mold. The result is basically consistent with the theoretical prediction. In 1995, diraddo et al. [14] used six thermocouples to insert different thicknesses of products from different parts of the mold, and collected the temperature through the temperature collector connected with it to obtain the transient temperature at different thicknesses of products, which is a great improvement from only measuring the inner and outer surface temperature

and the earliest measured shrinkage of the product is diraddo et al. [14]. They process a lattice with a size of 5mmx5mm in the mold cavity, and the lattice is printed on the surface of the product after the parison is inflated. In this way, the axial and circumferential shrinkage of the product can be measured directly, and then the radial shrinkage can be calculated according to the law of mass conservation

Product warpage generally uses a three-dimensional laser digital system to measure the shape of the product [15], and then obtains the shrinkage and warpage of the product

5. Conclusion

experimental research has always been the most direct method to guide engineering application, and it is also the basis and basis of theoretical research. The extrusion blow molding process includes three stages: parison molding, parison inflation, and product cooling and curing. Researchers in various countries are using different experimental methods and devices to study each stage of extrusion blow molding. Its research and development is of great significance to the optimization of process and mold structure and the improvement of production efficiency. With the development of science and technology and the improvement of experimental means, the experimental research of extrusion blow molding process will be more advanced. Its market demand for flexible packaging is also increasing year by year, providing better guidance for actual production, and producing hollow blow molding parts that meet social needs in terms of quality, performance and so on

source: International Plastics Industry

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