THERMAL FEATURE OF VARIOTHERM MOLD IN INJECTION MOLDING PROCESSES

Transcription

1 THERMAL FEATURE OF VARIOTHERM MOLD IN INJECTION MOLDING PROCESSES Yan-Chen Chiou 1, Hung-Chou Wang 1, Hsien-Sen Chiu 1, Chau-Kai Yu 1, Wen-Hsien Yang 1, and Rong-Yeu Chang 2 1. CoreTeh System (Moldex3D) Co., Ltd., ChuPei City, Hsinhu, Taiwan 2.Dept. of Chemial Engineering, National Tsing-Hua Uniersity, HsinChu, Taiwan Abstrat With the deelopment of ariotherm mold temperature ontrol, people use this tehnique to fit the speial demands in injetion molding. Not only the present mold temperature settings, but also the temperature distribution of the preious state will affet mold temperature. Espeially when there are multi-layout of mold temperature ontrols with different heating soures and ooling soures, whole mold temperature properties would be ery sensitie. As a onsequene, we study mold temperature ontrol system through CAE simulation, and disuss thermal features of different ariotherm proesses. Introdution In the injetion proess, the design of the ooling system is an important fator. A good ooling system an redue yle time and improe the part quality. In the onetional ooling system, suppose the temperature distribution between ooling hannels and part stays stable. It is reasonable to use the yle-aerage temperature to analysis the molding proess. Howeer, when the oolant temperature no longer keeps onstant in the ariotherm proess, the yle-aerage approah is not applied suitably any more. As a result, it is important to simulate the temperature distribution of moldbase by the transient analysis. No matter what time to swith oolant settings, the temperature distribution an be obsered at any time in the ariotherm proess. Rising the mold temperature in the filling stage of injetion molding has a lot of adantages: to enhane the fluidity, to improe the produt quality [1] and to strengthen the intensity of welding line. Howeer, high mold temperature will inrease ooling time to ool down part temperature. In the ariotherm proess, with the adantage of high mold temperature during filling stage and proper settings, the ooling time will not inrease, and then the yle time will not rise aordingly. There are a lot of kinds of ariotherm proess, suh as rapid heat yle molding (RHCM) proess [2], pulsed ooling proess, oolant temperature swith, et. The mold temperature ariation is usually signifiant in these ariotherm proesses. It is worth studying how to ontrol temperature to reah the expet result in the ariotherm proesses. Theory Energy Conseration The oerall heat transfer phenomena is goerned by a three-dimensional Poisson equation, T T T T ρ C P = k + + (1) t x y z where T is the temperature, t is the time, x, y, and z are ρ the Cartesian oordinates, is the density, C P is the speifi heat, k is the thermal ondutiity. Equation (1) holds for both moldbase and plasti part with modifiation on thermal properties. Initial Condition The initial mold temperature is assumed to be equal to the input of initial settings. The initial part temperature distribution is obtained from the analysis results at the end of filling and paking stages. T, for r Ωm T( 0, r ) = (2) T p ( r ), for r Ω p Boundary Conditions Heat of the molten plasti part is remoed by the oolant flowing through the ooling hannel as well as the ambient air surrounding the exterior surfaes of the mold base ia a heat onetion mehanism. In this study, the effet of thermal radiation is ignored. The onditions defined oer the boundary surfaes and interfaes of the mold are speified as, T for t 0, km = h( T T o ) (3) n Where n is the normal diretion of mold boundary. On the exterior surfaes of the mold base Γ m : h = h, T = T for r Γ (4a) air 0 air m

2 On the ooling hannel surfae Γ : h = h, T = T for r Γ (4b) 0 h and hair The heat transfer oeffiients are obtained from the empirial equations ited in the standard text of transport phenomena. Numerial Disretization Method In this study, a numerial soler based on Finite Volume Method (FVM) is deeloped to sole the goerning equations. The soler has been suessfully applied in injetion molding filling simulation [2]. Numerial experiments onfirm the reliability and effiieny of the soler. Currently the proposed soler an handle tetra, hexa, prism, pyramid, and mixing elements. Prism layer element an also be used for analysis to improe thermal boundary resolution while without extensie refining of mesh. This is aluable in mold ooling analysis that may inole millions of elements. Results and Disussions In the ariotherm proess, the ooling hannels may arry high temperature steam as heating rods for filling phase and arry low temperature oolant as ooling hannels for paking and ooling phases. Figure 1 shows the geometry of the molded part and ooling hannels in the model for this study. The dimension of moldbase is 17.5 m x 21.2 m x 8.9 m. The part thikness is 1mm. Figure 2 shows sensor loations in the moldbase. Fig. 2 The loation of sensors in the moldbase In this study, a time ontrol ariotherm molding proess is adopted to ompare with a temperature ontrol ariotherm proess. In the time ontrol ariotherm molding proess, the timings of swithing oolant temperature had been deided before molding. The proess ondition settings for the time ontrol ariotherm molding proess are listed in Table 1. In the temperature ontrol ariotherm proess, the temperature riteria of proess are listed in Table 2. And oolant temperature of heating is set as 120 to inrease the heating speed. This analysis result will proide an optimized time results for time ontrol ariotherm molding proess. Simulation results will be disussed in the following desriptions. Table 1 Proess settings for the time ontrol ariotherm molding proess Melt Temperature 225 Ejetion Temperature 110 Coolant temperature (Heating stage during filling) Coolant temperature (Cooling stage during paking and ooling) Table 2 Temperature riteria of the temperature ontrol ariotherm proess Fig. 1 Geometry of molded part and ooling hannels Heating stage riteria (Coolant temperature 120 ) Part Surfae temperature greater than 100 Cooling stage riteria (Coolant temperature 25 ) Part temperature smaller ejetion temperature 110 The time ontrol ariotherm proess Table 3 shows the timings of swithing oolant

3 temperature. Figure 3 shows the aerage moldbase temperature of ariotherm proess in yles. Figure 4 (a) shows the mold temperature distribution in the heating stage, and Figure 4(b) shows the mold temperature distribution in the ooling stage. In order to heat up the mold to the initial temperature, the preheating time is an issue for ariotherm molding proess. The proper preheating time will sae energy and sure the suess of first shot. By traing the mold temperature history rising from room temperature, heating time an be obsered from the history diagram (Figure 5). Table 3 The oolant swith time for the time ontrol ariotherm molding proess Preheating time Cooling time Heating time 124 se 5 se 30 se Fig. 3 The aerage moldbase temperature ariotherm proess with yles (a) (b) Fig. 4 Moldbase temperature distribution in onentional molding proess in period of (a) Heating stage (b) Cooling stage Fig. 5 The aerage moldbase temperature ariotherm proess from room temperature to initial heating temperature The temperature ontrol ariotherm proess One of hallenges in the ariotherm proess is to deide the time of heating and ooling. It is hard to make sure that the mold temperature has reahed the required temperature. Sine the melt only ontats the aity and the other areas in the moldbase do not influene part diretly, the heating time an be dereased by only onsidering the surfae temperature of part. Through the CAE tool, the temperature of the whole part surfae an be monitored. Heating time is deided by that if the minimum mold temperature is high enough. And the ooling time is deided by that if the whole part temperature reahes the ejetion temperature. Based on the aboe two riteria, the optimized history result shows on Figure 6. From the optimized result, the settings of oolant swith timing an be found and shown in Table 4. Figure 7 shows the temperature history whih is the simulation result by applying new oolant swith timing. Figure 8 shows the temperature history on sensor whih is nearby the part, omparing with temperature history whih is nearby oolant layout (Figure 9). The temperature history of oolant layout shows greater ariation than that nearby part. Furthermore, from the temperature history whih is nearby moldbase surfae (Figure 10), the temperature ariation is more stable than preious ones. That shows that the heat transfer effet from ooling hannels is not signifiant on regions far from ooling hannel layouts. Table 4 The oolant swith time from optimizing result of the temperature ontrol ariotherm proess Preheating time Cooling time Heating time 51 se 6 se 26 se

4 Fig. 6 The optimizing history result whih follows the temperature riteria Fig. 9 The temperature history whih is nearby oolant layout Fig. 7 The history result with optimizing time settings Fig. 10 The temperature history whih is nearby moldbase surfae Conlusions Fig. 8 The temperature history on sensor whih is nearby the part In this researh, the mold temperature histories at the sensor loations an be obsered. The temperature ariations whih are at different sensor loations show different thermal haraters. By means of a further appliation of optimizing the time of heating and ooling by monitoring temperature histories in speifi areas, the effiieny of ariotherm proess an be inreased without proiding too muh heat on the unneessary area. We an see that molding by optimizing ariotherm molding proess, the yle time an be dereased. This result also demonstrates the adantage of the simulation tool to handle the properties of ariotherm proess. Referenes

5 [1]. R. Y. Chang and W. S. Yang, Numerial Simulation of Mold Filling in Injetion Molding Using a Three-dimensional Finite Volume Approah, Int. J. Numer. Meth. Fluids, Vol.37, (2001) [2]. YuFeng Chen, Venny Yang, WenHsien Yang and Rong-Yeu Chang, Temperature harateristis of moldbase in RHCM proess with 3D Fully Transient Simulation, 1295, ANTEC 2006 [3]. J. A. Chang, S. C. Chen and J. C. Cin, Rapid Mold Temperature Control on Miro Injetion Molded Parts with Aspet Ratio Miro-features, 1275, ANTEC 2006 [4]. Y. C. Chiou, Y. Y. Chou, H. S. Chiu, C. K. Yu and C. H. Hsu, Integrated true 3d simulation of rapid heat yle molding proess, ANTEC 2007 Key Words: 3D, RHCM, Mold Cooling, simulation, fully transient