OPTIMIZATION OF THE DEVOLATILIZATION PROCESS IN THE INJECTION MOLDING CYLINDER

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1 OPTIMIZATION OF THE DEVOLATILIZATION PROCESS IN THE INJECTION MOLDING CYLINDER Hisakura Yuuki, Kawakubo Mitsuhiro, Kitahara Kenichi, Sugihara Makoto, Konica Minolta Inc., Aichi, Japan Hamada Hiroyuki, Kyoto Institute of Technology, Kyoto, Japan Abstract In the recent years, many of the past demerits and all vent up problems that the past vent-type injection molding machines had were solved, so such a vent-type injection molding machine is being focused. However, the plasticization process of vent-type molding is not solved theoretically and systematically. In order to use a venttype molding widely in the future, the technology needs to be built systematically by theoretical break-through of the devolatilization of gas and moisture, and by studying the influence on the molding materials. In our study, PA6 materials with dry pellets and water absorption pellets were molded by the normal barrel and the vent barrel injection molding. The systematic construction of the technology studied by grasping injection molding parameters which would influence the devolatilization of gas and moisture in the cylinder during the plasticization process for the vent-type molding by the system optimizations using Quality engineering. Introduction The injection molding processing of thermoplastics is applied to various products. It is known that the volatile constituent, which occurs at the time of resin fusion, will become the cause which induces various molding faults in the injection molding processing of the thermoplastics. Furthermore, it makes productivity worse because preparation process for the molding of the material dryness before molding, maintenance of the metallic-mold are becoming regular work to prevent molding fault occurrence. The history of a thermoplastics injection molding processing is no exaggeration to say that is the history of struggle with the molding trouble by the various volatile ingredients which are generated from the pellet of raw material at the time of plasticization fusion. It is far from the end of battle, and becoming increasingly intense in the recent years as the raw material becomes complicated and diversified. In the past 50 years, the ijection molding machine which the vent port for degassing was set in the middle position of a plasticization cylinder called vent-type injection molding machine was developed. The vent-type molding is a molding method using the cylinder which has a degassing port (vent port) in the middle position of the plasticization cylinder of the injection molding machine. Although this molding method is the conventional molding method from the past, due to the issues such as resin slippage from the vent port or difficulty of material replacement. However, in the recent years, these issues has been solved by degassing the volatile and moisture which are raised from the resin in the melting process, which the large effect of the productivity and quality improvement of resin parts are expected. In addition, the control of the mechanical strength of the material parts by the technique of supplying reinforcing materials directly from the vent port called Direct Feeding Fiber Injection Molding (DFFIM) is also expected. The degassing ability of the vent-type injection molding machine, especially the complete dehydration molding can be obtained by degassing moisture. This raw material contains as water vapor, is expected as the indispensable molding method for the injection molding. The injection molding machine of the vent-type has large merits, such as preliminary drying of raw material needed usually becomes unnecessary by degassing all the volatile ingredients from the vent port. Additionally, molding failure caused by the moisture is solved by completely dehydration from the vent-type molding. Nevertheless, it is not easy due to the vent up trouble which the melted resins spout out from the vent port or to find out the optimum conditions for the plasticization in order to guarantee the completely dehydration process. In this study, the mechanical strengths of the nylon (PA6) molding products were obtained by the vent-type injection molding machine. Water absorption was used as the evaluation index of devolatilization. Quality engineering was used for grasping the effect of injection molding parameters. Therefore, the optimization of the devolatilization process was considered by the adjustment of the injection molding parameters. Experimental Materials Nylon 6 (PA6), standard grade Amilan CM1017, was supplied by Toray Industries, Inc., Japan which can easy absorb water. Relationship between exposure time of PA6 in the thermostatic chamber and the water absorption rate are shown in Figure 1. The water absorption rate is almost saturated at 8% in about 50 h. In order to confirm the devolatilization efficiency, the materials used in the vent- SPE ANTEC Anaheim 2017 / 1566

elastic region of tensile properties by normal molding and by vent-type molding. The acquisition method of the experimental data is shown in Figure 3.

2 type injection molding were PA6 with the water absorption rate of 2% and 8%. PA6 was left in a thermostatic chamber at 60 C for a certain period of time and dried at 80 C for 4 h by dehumidifying dryer. Three PA6 materials in total were prepared. elastic region of tensile properties by normal molding and by vent-type molding. The acquisition method of the experimental data is shown in Figure 3. The signal factor was Stress (σ) corresponding to the Strain (ε) at 0.01, 0.02, 0.03 of the tensile characteristic of the test specimen with normal molding using the dry pellets. With the vent-type molding, the experimental data was Stress σ corresponding to ε= 0.01, 0.02, 0.03 of the tensile characteristic molded with the materials which absorbed water. One example of the signal factor and the obtained experimental data are shown in Figure 4. Y is data obtained from tensile properties of vent-type molding and M is the data of tensile characteristics of normal molding. This is the ideal state if both are in proportional relationship. Figure 1. Water absorption Sample Preparation The schematic of the vent-type injection molding machine is shown in Figure 2. The samples were prepared by 110 tons injection molding machine (JSW: J110AD). The barrel was changed from the normal barrel (normal molding) into the vent hold made by Nihon Yuki Co., Ltd., Japan. Then both of the test specimens were fabricated. Moreover, it was equipped with the feeder which supplies material in a certain amount with the vent-type molding. For the devolatilization process, it was assumed that the devolatilization by the update of the resin surface in the vent part. The resin pellets supplied from the feeder are plasticized by a screw, and volatile ingredients such as decomposition of the resin, and contained moisture would be distributed in the resin. This volatile ingredient was degassed from the vent part by the update on the surface of the resin in the vent part. In this case, it is known that the devolatilization thermal efficiency is changed by the variation of the screw speed, filling rate, barrel temperature, etc [1-4]. Therefore, The barrel temperature, screw speed and material feeder speed were selected as the parameters for this experiment. Figure 3. Acquisition of the tensile strength experimental data Figure 4. One example of the signal factor and the obtained experimental data Figure 2. Vent-type cylinder structure Study of the material valuation in Quality engineering Quality engineering (Taguchi method) is known as a general-purpose technique for evaluating technology. In order to optimize the devolatilization efficiency in venttype molding, evaluation was made with the data of In order to find out optimum conditions with the vent barrel, the zero-point proportional L9 dynamic characteristics of parameter design was used. The settings of each parameter are shown in Table 1, and the experimental condition is shown in Table 2. Formula (1) indicates the S/N ratio (η), and Formula (2) indicates the sensitivity (S) calculations. 1 ( S Ve ) 10log r ( db) V (1) e SPE ANTEC Anaheim 2017 / 1567

3 1 S 10log ( S Ve ) ( db) r (2) where r is a valid divisor, S is a regression component of the signal factor, and V e is an error dispersion. Table 1. The settings of each parameter Screw temperature temperature H1 [ C] H2 [ C] Table 2. L9 Experimental conditions Experi mental No temperature H1 [ C] temperature H2 [ C] Screw Mechanical Testing Tensile properties were carried out by Intron universal testing machine (Intron 5500R, Instron, USA). The size of specimen is shown in Figure 5. The testing speed of tensile test was 1 mm/min. The gauge length was set at 115 mm. At least five specimens were tested. Figure 6. Comparison of the stress-strain diagram at the time of water absorption Figure 7. Hydrolysis of the nylon A comparison of tensile strength dispersion with normal barrel and vent barrel is shown in Figure 8. The dispersion was calculated from the finite difference of the stress mean value at the strain of 0.03 in the tensile strength specificity with 15 samples each for the normal molding and for the vent molding as a dispersion. In 8% water absorption rate in the vent molding, it had larger dispersion against the strength of dry goods of the normal molding. It is estimated that devolatilization is not still enough for the normal molding. Figure 5. Size of Specimen for tensile testing Results and Discussion Comparison of tensile properties between the normal molding and the vent-type molding In the case of normal molding, with the 8% of water absorption material, strength falls 20% under the condition of 8% water absorption as a saturated state against the test specimen of the dry material in the normal molding. On the other hand, by the vent barrel, 8% water absorption material also becomes equivalent. Strength reduction is brought about, because the nylon causes moisture and the hydrolysis within the barrel and molecular weight falls as Figure 6. The hydrolysis process of the amide linkage included in nylon is shown in Figure 7. In the vent molding, strength reduction was not occurred, because the devolatilization of the moisture is made from the vent port. Figure 8. Tensile strength dispersion Optimization of plasticization condition of the vent-type molding The result of the SN ratio of parameter design in the vent molding is summarized in Table 3 and shown in Figure 9. For the SN ratio, in the case that the SN ratio of the barrel temperature after the vent is higher, there is a tendency of reducing the dispersion. The viscosity of the resin falls, which it is thought that it turned the condition to cause the devolatilization of the volatile ingredient easily. On the other hand, the higher one in the screw speed reduced the dispersion. By increasing the screw SPE ANTEC Anaheim 2017 / 1568

4 speed, updates of the top layer near the vent port increases, and it is considered that it raised the devolatilization of the thermal efficiency. The speed of rotation in the feeder and the parameter of barrel temperature had maximum peak. The result of the sensitivity is shown in Table 4 and Figure 10. From the sensitivity result, the broad sensitivity width turned to 0.3 db or less, and there were no large influence to the strength absolute value. Reproduction confirmation result is shown in table5. The optimum condition was selected as the parameter level at which the SN ratio was the highest. On the other hand, the worst condition was selected as the parameter level at which the SN ratio was the lowest. The value of estimated gain is close to that of confirmed gain. It is shown that the reproducibility of this experiment is high. Table 3. SN ratio (S) Temperature H1 [ C] Temperature H2 [ C] Screw Feed Figure 9. SN ratio Table 4. Sensitivity ( ) Screw temperature temperature H1 [ C] H2 [ C] Table 5. Reproduction confirmation result Estimated gain SN ratio Sensiti vity Confirmed gain Optimum condition Worst condition Gain Optimum condition Worst condition Gain The confirmation of the strength dispersion with 1 factor as carried out by another experiment, since the speed of rotation in the feeder and the parameter of barrel temperature had maximum peak.. The influence of the barrel temperature H1 on the tensile strength dispersion is shown in Figure 11. In accordance with the temperature rising, the strength dispersion was becoming larger from around 300 C. The photo of the sample in the case of 270 C and 310 C are shown in Figure 12. As for the sample surface, yellowing occurs at the barrel temperature over 300 C. It turns out that having given the excess heat to the resin, which was the tear down by oxidization of the resin occurred. Hence, it is considered that the strength dispersion became larger. When determining the injection parameters, it is necessary to be aware of material oxidization. The optimum condition of the barrel temperature H1 was determined at 270 C which becomes the smallest strength dispersion. The influence of the material supply amount (feeder speed) on the tensile strength dispersion is shown in Figure 13. At these parameters, when speed of rotation decreased, yellowing was also observed on the sample. Therefore, the feeder of 80 rpm was selected as the optimum value of the feeder rotation speed. Figure 14 shows tensile strength dispersion after optimization with Quality engineering. After the optimization, it was confirmed that the strength dispersion was reduced to half or less than before the optimization process. In other words, it was found that stable of tensile strength can be realized in vent molding than the normal molding with dry pellets. The moisture slightly remains in the inside material even if the pellets are dried. Therefore, it was considered that moisture would not be removable completely. On the other hand, with the vent molding, it was considered that the moisture inside the pellet was possible completely removed because the devolatilization of the moisture were made after the resin fused. Therefore, it was considered that the vent molding decreased the strength dispersion in comparison with the normal molding with dry pellets. Figure 10. Sensitivity result SPE ANTEC Anaheim 2017 / 1569

Figure 11. The relationship with temperature H1 and strength dispersion 270 310 Figure 12. H1 temperature factor experiment sample photo Figure 13.

5 Figure 11. The relationship with temperature H1 and strength dispersion Figure 12. H1 temperature factor experiment sample photo Figure 13. Influence of the material supply amount on the tensile strength dispersion devolatilization thermal efficiency. When the material which absorbed water is molded normally, the ultimate tensile strength falls as physical characteristics by hydrolysis. In the vent molding, the physical characteristics fall according to the devolatilization effect from the vent port can be controlled. However, in the vent molding, when compared with the normal molding with dry pellets, ultimate tensile strength dispersion, it turned out that the direction of vent molding is getting worse in dispersion. Then, the ultimate tensile strength was used as the evaluation index of the devolatilization thermal efficiency in the vent molding. This study was able to extract the parameter, which influences the degassing efficiency of the vent molding as well as able to optimize the plasticization conditions by using Quality engineering. When determining the plasticization conditions, for the degree of heat which was given to the resin, the amount of supply at the feeder controller should be decrease and the barrel temperature should be set to high. However, if the degree of heat is excess given, the oxidization of material would be in progress. Hence, it would be getting worse in the strength dispersion. It is also necessary to select the optimum values. Moreover, in order to increase the frequency of updates of the resin surface in the vent, it turned out that the setting at higher screw speed can raise the devolatilization efficiency. As a result of tensile strength dispersion after optimization, it was found that the stability of the tensile strength can be realized in the vent molding than the normal molding of dry pellets. It was due to moisture removal by the vent molding since the water was efficiently devolatilized as the fused resin state so that the strength dispersion seems to be reduce in the vent injection molding process. References 1. G.A. Latinen, Adv. Chem. Series, 34, 235 (1962). 2. C.T. Yang, and T.G. Smith, SPE-ANTEC Tech. Papers, 350 (1996). 3. N.H. Wang, N. Hashimoto, H. Mizuguchi, K. Nakamura, Seikei-kakou, 10, 628 (1998). 4. N.H. Wang, N. Hashimoto, H. Mizuguchi, K. Nakamura, Seikei-kakou, 9, 565 (2007). Figure 14. Tensile strength dispersion after optimization with Quality engineering Conclusions In this study, the devolatilization function of the vent molding was optimized for the ultimate tensile strength of PA6 material as the alternative specificity for SPE ANTEC Anaheim 2017 / 1570

Effect of Processing Parameters on Polypropylene Film Properties

Vol.2, Issue.5, Sep.-Oct. 2012 pp-3056-3060 ISSN: 2249-6645 Effect of Processing Parameters on Polypropylene Film Properties Ikilem Gocek 1, Sabit Adanur 2 1 (Department of Textile Engineering, Istanbul