AN INVESTIGATION OF INJECTION MOLDING PARAMETERS ON A SINGLE-STAGE INJECTION STRETCH BLOW MOLDING

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1 AN INVESTIGATION OF INJECTION MOLDING PARAMETERS ON A SINGLE-STAGE INJECTION STRETCH BLOW MOLDING Meng-Chih Chen 1*, Chih-Lin Hsu 2, Chao-Tsai Huang 3, Wen-Hsien Yang 4, Chia-Hsun Chen 5, and Kun-Chang Lin 6 1* CoreTech System Co., Ltd., Hsinchu, Taiwan jasonchen@moldex3d.com; 2 CoreTech System Co., Ltd., Hsinchu, Taiwan shirleyhsu@moldex3d.com; 3 CoreTech System Co., Ltd., Hsinchu, Taiwan cthuang@moldex3d.com ; 4 CoreTech System Co., Ltd., Hsinchu, Taiwan anthonyyang@moldex3d.com; 5 Department of Chemical Engineering, Far East University, Tainan, Taiwan. chchen@cc.fec.edu.tw; 6 Department of Chemical Engineering, Far East University, Tainan, Taiwan. kclin@cc.fec.edu.tw Abstract - The advantages of one-stage injection stretch blow molding (ISBM) are energy efficiency and high productivity as reheating is not necessary. However, due to the complexity of operation parameters from raw material through injection molding to the end products, a conventional trial-and-error method is ineffective to predict and control this process. Hence, how to correctly integrate injection and blow molding sequentially is in a great demand. In this study, firstly, we have systematically investigated one-stage injection stretch blow molding process by numerical simulation. Since various process conditions will affect the properties of preform, they will also influence the quality of final blown products. Furthermore, a comparative investigation between one-stage and two-stage processes is performed. Results showed that the various process conditions both in injection stage and in blow stage will affect the final products significantly. Introduction In the past decades, blow molding has been used widely to produce thousands of hollow plastics products, such as drink bottles, automobile tanks, containers, and so on. One of the main forming methods is injection blow molding with or without stretched processes. An injection stretch blow molding (ISBM) method can be performed in either a one-stage or a two-stage process. In a two-stage process, the preform is made from injection stage and stored. Not until required is the preform reheated to desired temperatures, stretched and blown to the final shape in the mold. On the other hand, in one-stage process, preform is moved to stretching and blowing processes immediately after injection phase. The advantages of one-stage injection stretched blow molding (ISBM) are energy efficiency and high productivity as reheating is not necessary. Also, it is more flexible to mold different circumferential wall thickness to optimize final wall thickness. However, due to the complexity of operation parameters from raw materials to injection phase, the transient behavior of preform is very difficult to monitor. In addition, how to transfer preform to further stretching and blowing processe properly is not an easy job. A conventional trial-and-error method is ineffective to predict and control this process. Hence, how to correctly integrate injection and blow molding sequentially is in great demand. [1-4] In this study, we have integrated injection molding and blow molding simulations to investigate the molding behaviors for one-stage ISBM. Firstly, we have systematically investigated one-stage injection stretch blow molding process by numerical simulation. Since various process conditions will affect the properties of preform, they will also influence the quality of final blown products. Furthermore, a comparative investigation between one-stage and two-stage processes is performed. Results showed that the thickness of the blown tube through one-stage process can be different significantly. Theory and Assumption The major analysis procedures for one-stage or two-stage ISBM can be divided into two stages. First stage is injection molding simulation for preform preparation. The injection molding stage includes filling, package, cooling, and warpage phases. In filling and packing phases, the polymer melt is assumed to behave as Generalized Newtonian Fluid (GNF). Hence the non-isothermal 3D flow motion can be mathematically described. The FVM (finite volume method), due to its robustness and efficiency, is employed in this study to solve the transient flow field in complex three-dimensional geometries. During the molding cooling phase, a three-dimensional, cyclic, transient heat conduction

2 problem with convective boundary conditions on the cooling channel and mold base surfaces is involved [5-7]. The overall heat transfer phenomenon is governed by a three-dimensional Poisson equation. We assume there is a cycle-averaged mold temperature that is invariant with time. In the warpage analysis, the mechanical properties are assumed as linear elasticity. The stress-strain equilibrium equations enable us to solve the problems. To handle the second stage, the blow molding process, the viscoelastic material model (K-BKZ) and WLF temperature dependency are considered. Also, during the blowing process, heat transfer and friction between tools and material are included. In one-stage injection stretch blow molding process simulation, the temperature distribution of the preform at the end of cooling during the injection stage is used to be the initial conditions for blow molding simulation. In two-stage process, the preform is prepared in injection molding process and stored. When blow molding process gets started, the preform is reheated to a constant temperature. It is further stretched and blown by air to form the products. Investigation Procedures To get better understanding of one-stage injection stretch blow molding, the processes can be divided into two stages: preform preparation in injection stage and blow molding stage, as listed in Table 1. Both stages are conducted numerically. In injection stage, Moldex3D software is applied to study the process from filling/packing to warpage[8]. In blow molding stage, B-SIM software is used to investigate the process from pre-stretch to blowing[9]. They are described as below: A. Preform preparation in Injection Molding Stage Fig. 1 shows the geometry of preform in injection stage. The outer diameter is 48 mm, height is 122 mm, and thickness is from 3 to 7 mm. The process conditions are: filling time is 1 sec, packing time is 1 sec, and cooling time is 10 sec. The melting temperature is 230 o C, mold temperature is 70 o C, and ejection temperature is 120 o C. The material is PET (Easeapak PET 7352). Various operation conditions are considered, including melt temperature effect, mold temperature effect, injection speed effect, and so on. B. Blow Molding Stage The procedures of blow molding are the same in one-stage and two-stage processes. Fig. 2 illustrates the basic procedures of the blow molding stage. The mold is about 230mm in height and 100mm in maximum diameter. For the plug, the diameter is 16 mm and the height is 180 mm. The preform, either in one-stage or in two-stage, is installed into mold. After pre-heated (two-stage process) or using the inner thermal distribution (one-stage process), the preform is extended by the plug. Finally, the gas comes to blow the preform and complete the process. Finally, the forming results due to the time period ratio for injection stage to blow stage are also discussed. Results and Discussions Fig. 3 shows the filling behaviors of preform during the injection stage. The melt goes through the entrance and fills the cavity from time to time. After filled, cavity is packed. Then the cooling analysis is performed. The temperature distribution of the preform is shown in Fig. 4. Clearly, at the end of cooling, the inner melt is still at high temperature. In one-stage process, this temperature distribution is taken into account as the initial temperature for further blow molding. Also, the geometrical model after cooled is forwarded to blow molding system. The preform is stretched by plug and is then blown to final product. Here, the period of injection stage including filling, packing, cooling, is 12 sec. Then the time of blow stage, including the 0.3 sec on pre-stretch, is 1 sec. The time period ratio, injection stage to blowing stage, is 12. Furthermore, to catch the crucial injection molding parameters, both of shell model and solid model have been applied. After the end of cooling, the preform has been moved to blow molding. Since the main model is based on shell model in blow molding stage, the initial temperature distribution for blow molding is based on average temperature normalized by area. Fig. 5. shows the final thickness distribution of the blown product. Clearly, in region (1), the mold constraint is strong and difficult to extend, so it is thicker. In region (3), much material is accumulated and not uniform at this situation. In addition, during the injection molding stage, both shell model and solid model have been applied to investigate the preform conditions. Considering the initial average temperature distribution, both shell and solid models for injection process prediction are in a good agreement. Furthermore, various process conditions are considered in the one-stage process in this study. The melt temperature effect is illustrated in Fig. 6 and 7. Fig. 6 shows the temperature distribution at the end of cooling phase in injection stage at three different melt temperature cases. Clearly, the inner portion is still at high temperature situation at the end of

3 cooling phase. Later, these temperature distributions are forwarded to as the initial conditions to perform blow molding analyses. Results indicate that the final thickness distributions show similar trends for different melt temperatures as in Fig. 7. The melt temperature effect on the final product is not significant. The other key issue is the time period ratio, injection stage to blowing stage. When we keep the same conditions for preform preparation (i.e., including filling, packing, cooling, is 12 sec) and changed the pre-stretch period from 0.3 sec to 1.3 sec in blow stage, the time period ratio will be modified into 6. The thickness of the final blown product is illustrated in Fig. 8. Obviously, pre-stretch will affect the final thickness distribution significantly. A two-stage stretch injection blow molding simulation is also performed to make some comparison. Fig. 9 illustrates thickness distribution for both one-stage and two-stage processes. In one-stage process, all the parameters, as mentioned earlier, are shown in Fig 5. In two-stage process, the preform is pre-heated to 100 o C before the blow molding stage. It is clear that the final thickness distribution is dramatically different. In region (1), the thickness of one-stage process is not uniform due to lower initial temperature. In region (3), the thickness of one-stage process has a significant variation. In addition, various pre-heating temperatures of the preform are conducted as well. Fig. 10 shows thickness distribution results at different pre-heating temperatures. When the pre-heating temperature is higher, the region (3) is easier to stretch but less uniform. This behavior is similar to that in one-stage process. Tables Table 1. Geometrical dimension and differential variation effect during injection blow molding process Injection Molding Process Blow Molding Process Dimension (mm) 1-stage H=122 W=48 T=3~7 Period Time (s) F=1 P=1 Melt Temperature ( ) Mold Temperature ( ) C= stage H=122 W=48 T=3~7 F=1 P=1 C= Material PET PET Mold Dimension (mm) Plug Dimension (mm) H=230 W=100 H=180 W=16 H=230 W=100 H=180 W=16 Period Time (s) 1 1 Pre-heating temperature ( ) H is height, W is width, T is thickness. F is filling; P is packing; C is cooling. : PET is Eastman PET

4 Figures Fig. 1. The geometry of preform in this study. Fig. 2. The procedures for injection stretch blow molding. Fig. 3. The filling behaviors of injected preform.

5 Fig. 4. Temperature distribution of the preform after cooling phase in injection molding process. Fig. 5. The comparison of thickness distribution between shell model and solid model in one-stage injection stretch blow molding. Here, injection stage is 12 sec; total blow stage is 1 sec, including pre-stretch of 0.3 sec. Fig. 6 (a). The temperature distribution at the end of cooling phase during the injection with different melt temperature: 230 o C

6 Fig. 6 (b). The temperature distribution at the end of cooling phase during the injection with different melt temperature: 220 o C. Fig. 6 (c). The temperature distribution at the end of cooling phase during the injection with different melt temperature: 210 o C. Fig. 7. The thickness distribution of the blown products at different melt temperatures during injection molding stage.

7 Fig. 8. When the ratio of (injection stage to blow stage) is changed from 12 to 6. The thickness distribution of final product is affected significantly. Fig. 9. The comparison between one-stage and two-stage effect on injection stretch blow molding product development. Fig. 10. Different pre-heating condition effects, from 100 o C to 120 o C, on the blow molding. Conclusion In this study, we have systematically investigated one-stage injection stretch blow molding process by numerical simulation. Results show that various process conditions both in injection stage and in blow stage will affect the quality of final product based on the evaluation of the final product s thickness. Furthermore, we also performed two-stage conventional blow molding analysisin comparison, the quality of two-stage process can be very different from that of

8 one-stage process. In addition, the study can be a good guideline to help people understand the mechanism and make proper design of products. References Modeling and simulation of stretch blow molding of polyethylene terephthalate, X. T. Pham et al., Polym. Eng. Sci. 44, 1460 (2004). Experimental study and numerical simulation of the injection stretch/blow molding process, F. M. Schmidt et al., Polym. Eng. Sci. 38, 1399 (1998). NPE 2006 News Wrap-Up: Blow Molding, J. A. Grande, Web source: news_e_28.html 5. The warpage simulation with in-mould constraint, Yi-Hui Peng et al, ANTEC The warpage simulation with in-mould constraint effect in injection moulding, Yi-Hui Peng et al, Moldex3D paper. 7. Three-Dimensional Insert molding simulation, Rong-Yeu Chang et al, ANTEC