Research progress of extrusion blow molding experi

2022-06-23
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Abstract: in this paper, the research status of experimental methods and devices in three stages of extrusion blow molding process: parison forming, parison inflation, and product cooling and curing are discussed in detail

key words: extrusion blow molding; Experimental method; Research progress

1 Overview

extrusion blow molding is one of the main molding methods for the production of plastic hollow parts. It is suitable for PE, PP,

pvc, thermoplastic engineering plastics, thermoplastic elastomers and other polymers and various blends. It is mainly used for molding packaging containers, storage tanks and barrels. It can also be used for molding industrial parts such as automobile industry. Like other plastic hollow molding, extrusion blow molding has the main advantages of low product cost, simple process and high benefit, but its prominent disadvantage is that the product wall thickness and uniformity are not easy to control [1]

extrusion blow molding is a hot forming process in which the extruded semi molten plastic pipe blank (parison) is placed in various shapes of molds while it is hot, and compressed air is immediately introduced into the pipe blank to blow it, making it close to the mold cavity wall for forming, and hollow parts are obtained after cooling and demoulding. The whole forming process can be divided into three stages: parison forming, parison blowing, cooling and curing

researchers at home and abroad have tried to use different methods to study each stage and the whole process of extrusion blow molding from the 1960s to now, but generally speaking, they can be divided into two categories: experimental research and numerical analysis technology. The numerical analysis method is based on three basic equations: continuity equation, motion equation and energy equation. A large number of assumptions must be made to simplify the equations and solved by finite difference or finite element method. Moreover, some rheological parameter data in the constitutive equation are not easy to obtain. For the work in process with complex shape, it needs a lot of computer time. Experimental research is the simplest and most direct method

the following is a summary and analysis of the experimental research in each stage of extrusion blow molding

2. Research status of parison forming stage

parison formation refers to the semi molten plastic tube (parison) obtained by extrusion. As the geometry of hollow blow molded parts becomes more and more complex, well-designed preform is of great significance to obtain the required wall thickness distribution and stable structure with minimum material consumption, that is, in the parison forming stage, the wall thickness distribution of blow molded products tends to be uniform by adjusting the wall thickness distribution shape of the parison. Because of the extrusion expansion, sag, springback and other factors during parison forming, the size of the parison during the parison forming stage is inconsistent in the length direction and becomes very complex

because the temperature of the extruded polymer parison is too high to be measured directly, the experimental research on the forming stage of the extrusion blow molding parison is mainly to design experimental methods to measure the diameter distribution and wall thickness distribution of the parison. Sheptakr et al. Were the first to use experimental methods to obtain the parison size. They designed a special mold called "clamp mold" to analyze the parison. This device can only obtain the mass expansion SW of the parison, but cannot directly obtain the parison. Therefore, the design of the fixture is relatively simple, and the diameter and wall thickness of the parison are expanded. Kalyon et al. [2] added a set of camera device to the above device, which can be used to capture the image of the parison before mold clamping, so as to obtain the diameter distribution of the parison. This method can obtain more accurate parison diameter distribution, but it is time-consuming and can not be used for measurement, so its practical application is limited

another method to measure the parison expansion is to directly extrude the plastic melt into the oil with the same temperature and density as the melt, so that the parison expansion can be measured without sag and solidification; At the same time, because the side wall of the oil tank is transparent glass, the parison can be photographed within a certain time interval; Because of the transparency of the plastic melt, the diameter distribution inside and outside the parison can be determined according to the photos. Due to the expansion of the parison, the shape and size of the parison are inconsistent along the length of the parison. In order to identify the position of data measurement, carbon black particles are sprayed onto the surface of the parison with an inkjet device at regular intervals. However, this method does not consider the influence of sag, so it is difficult to be applied in practical production

with the development of image analysis technology, more and more researchers prefer to use image analysis technology to determine the parison size. The diameter distribution of the parison can be measured directly through the image, but the thickness distribution of the parison can not, it can only be calculated indirectly. Many researchers try to use different measurement methods and algorithms to calculate the wall thickness distribution of parison. An et al. [3] designed a device using two cameras to measure the expansion size of the parison (as shown in Figure 4). The parison is squeezed into a container with the same temperature as the parison. The camera (9) at the lower end aims at the end of the parison, while the camera (5) at the upper end sends a signal to control the position of the camera (9) through the computer to ensure that it always aims at the end of the parison in the process of parison extrusion. The diameter and wall thickness of the parison can be obtained from the image. However, the experimental equipment is complex and only isothermal conditions are considered, so it is not widely used in practice

raddo and RCIA rejon[4] proposed a non-contact method to measure the wall thickness distribution of parison based on image analysis only. In this experiment, only one camera was used to take photos of the parison extrusion process. The variation of parison length with time, the diameter distribution of parison, the extrusion flow rate and the temperature gradient along the length direction were measured. Then the parison wall thickness distribution was calculated according to the relationship between the parison wall thickness distribution and these parameters. Raddo et al. Used this method to study the effects of different molecular weight HDPE resins, flow rate, melt temperature and die gap on the parison wall thickness distribution. The theory of this method is complex and the experimental data processing is tedious

terson and mal[5] developed a closed-loop control system for parison wall thickness distribution. In this system, the length and diameter of the parison can be directly obtained by the camera and the image analyzer connected with it, and the wall thickness distribution of the parison can be calculated by the geometric relationship, but the empirical parameters used are difficult to be obtained. If we want to realize the closed-loop control of the parison wall thickness distribution, we need a method that can directly measure the parison wall thickness distribution

assuming that the melt flow rate is a constant, the parison wall thickness can be calculated by a simple method and can be used for measurement. Kaise, Germany, first used this method, which was later improved by svein eggen and Arne sommerfeldt[6]. The diagram of the measuring device is shown in Figure 5. It is composed of a camera, an inkjet device to the surface of the parison and a graphic analyzer. The diameter distribution of the parison can be obtained directly from the pictures taken, and then the distance between adjacent ink dots can be measured. According to the assumption that the flow rate is a constant, the wall thickness distribution of the parison can be calculated

where R is the parison radius, q is the flow rate, ρ Is the melt density, and Z is the distance between adjacent ink dots. This method has simple theory, simple experimental device and high measurement accuracy, but there are many experimental data and the processing is cumbersome

some researchers use optical methods to study parison forming. An, MAL and RCIA rejon[7] developed a set of optical sensor measuring device, as shown in Figure 6, which can measure the thickness and size distribution of the mold embryo before closing the mold. The device is designed based on the principle of light reflection in optics. A laser beam is directed at the surface of the parison at a certain angle. The laser beam is reflected by the internal and external surfaces of the parison to form two lasers. The camera lens detects the interval and sends it to the computer analysis system. According to the geometric relationship, the computer can calculate the parison wall thickness distribution. But at the same time of light reflection, there is also the problem of light refraction, which can not be ignored in this measurement method. Taking refraction into account and determining the refractive index of the parison undoubtedly add great complexity and difficulty to this measurement method

3. Research status of parison blow up stage

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

at this stage, the experimental research on 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. The earliest experimental device pair can make the production properties both 1 and stable; Musa mal, Victor Tan and dilhan kalyon [8] studied the expansion kinetics of the non-toxic or low toxic by-products produced by the reaction. They designed the transparent blow mold by themselves, and used two cameras to capture the expansion behavior of the parison in the mold. The device diagram is shown in Figure 8. The captured pictures are sent to the graphics analyzer for analysis, so as to determine the change relationship of the parison diameter distribution 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. After that, most researchers used this similar method to study the blow up behavior of parison

wagner and kalyon[10] redesigned the solid pressure sensor based on kamal[8], as shown in Figure 8. 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 blowing process. They used this device to study the effects of three PA-6 on the blowing behavior under the blowing 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. The optical probe has a CCD camera and a grating transmitter. 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. Therefore, the grating graph contains the information of the surface shape of the polymer film parison. The bulge size of polymer film parison can be obtained after the raster image is quickly captured by the camera and sent to the computer for processing. Mcdl is a multi-channel data hub, which can collect pressure and raster signals at the same time in order to obtain the relationship between pressure and the shape of polymer film parison in the swelling process. Experiments show that the measurement accuracy is much higher than that in Figure 7

billet wall thickness measurement includes off-line measurement and measurement. Because off-line measurement is simple, it is often used. Off line measurements include infrared, ultrasonic and micrometer measurements. These methods are not only time-consuming, but also the time lag caused by off-line measurement needs to correct the deviation generated in the processing process, resulting in inaccurate measurement and many unqualified products

measuring the product wall thickness can reduce the lag time to the minimum, thus improving 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 the ultrasonic wave in the object multiplied by the ultrasonic wave in the object腹胀是什么原因引起的
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