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Thermal and stress analysis of rapid electric heating injection mold for a large LCD TV panel
Applied Thermal Engineering 31 (2011) 3989e3995
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermal and stress analysis of rapid electric heating injection mold for a large LCD TV panel Xi-Ping Li a, Ning-Ning Gong a, *, Yan-Jin Guan b, Guang-Ming Cheng a a b
College of Engineering, Zhejiang Normal University, Jinhua 321004, PR China Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2011 Accepted 29 July 2011 Available online 6 August 2011
Rapid electric heating cycle injection molding technology has been used widely in injection engineering application. Using this technology, excellent products with no weld marks, flow marks and other surface defects can be produced. However, as the special mold structure and its worse working process, the rapid electric heated injection (EHI) mold is easy to fail than that of the conventional injection mold along with the injection process carried out repeatedly. In this paper, an EHI mold for a large liquid crystal display (LCD) TV panel was first presented. Then the heat transfer process of the mold during the working process was studied. Lastly, through finite element simulation, the reasons that cause the large thermal stress and deformation of the mold were analyzed. The practical failure of the mold proved that the analysis presented in the paper was right. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Heat transfer process Stress analysis Electric heated injection mold
1. Introduction In conventional injection technology, the temperature of the mold is very low due to the time limit of the injection cycle, and it is usually less than 80 C. When the polymer melt is injected into the mold cavity, a frozen layer easily forms on the surface of the cavity. That’s because when the polymer melt with high temperature contacts the cool cavity surface, the temperature of the surface melt drops down rapidly to the low temperature leading to the surface melt solidified firstly. As a result, the melt filling into the cavity is influenced and its capacity to replicate the cavity surface is decreased as well. Therefore, the conventional injection method cannot be used to produce products with microstructures. The thinwall products are also very difficult to produce due to the decrease of the melt’s flow ability. In addition, the surface defects such as weld mark and flow mark are often caused due to the low temperature of the cavity surface as demonstrated in literatures [1e3]. The subsequent processes-spraying and coating which seriously pollute the environment must be used so as to improve the surface quality. These operations increase the manufacturing processes of parts, waste raw material and energy and increase the production cost as well. It is obvious that the conventional injection technology is difficult to satisfy the requirements of the modern injection industry development. For injection technology, the ideal * Corresponding author. Tel.: þ86 579 82286043; fax: þ86 579 82288020. E-mail address: [email protected] (N.-N. Gong). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.07.050
temperature of the mold during the working process is that when polymer melt injects into the mold, the cavity surface should be in a high temperature state in order to increase the flow ability of the melt. When the filling process is finished, the molded parts should be rapidly cooled down to ejection temperature so as to improve the production efficiency. The electric heating injection molding is such an injection technology that can control the mold temperature dynamically [4,5]. First, the stationary mold is rapidly heated to a high temperature by the heating rods in the mold. Then the polymer melt begins to inject to the mold cavity and in the later period of the packing stage, the mold and the melt are rapidly cooled down by coolant. When the mold is opened, the molded part is ejected; meanwhile, the heating rods begin to heat the mold again. Thus, the next injection cycle begins. As the temperature of the mold cavity is elevated before injecting, the flow ability of the melt is increased and the viscosity is decreased. The melt in the cavity can completely bended before it is cooled. So the surface defects on the polymer parts are eliminated and the high-quality products can be produced. Further more, the replication capacity of the melt is improved and the thin-wall polymer parts or products with microstructures [6e8] that usually have a bad filling process can be molded very well. Fig. 1 is the comparison of the products injected by conventional injection method and rapid electric heating injection method. It is observed that there are no weldlines on the products surface injected by electric heating injection molding method and the surface is so bright that the even the camera used to shot the photo can be mirrored.
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that, compared with the conventional injection mold, the thermal load suffered by the EHI mold during the injection cycle is the main reason to cause the mold failure. Up till now, there are many literatures [9e13] concerning with influence of thermal load on engineering applications, but there are few aiming at the injection mold. In this paper, in order to study the thermal load suffered by the EHI mold, the typical structure of an electric heated injection mold for an LCD TV panel was presented firstly. By setting the proper boundary and initial conditions, the heat transfer process of the mold was studied. Through finite element simulation (FEM), the thermal stress and deformation of the mold cavity plate were analyzed. At last, effective measures that could decrease the thermal stress and improve the lifetime of the mold were proposed. 2. Electric heated injection mold structure for an LCD TV panel Fig. 1. Comparison of panel products for different injection methods.
However, as the special structure and worse working process of the electric heated injection mold, large stress concentration can be easily induced in the mold. The large stress can cause the mold to fail very easy. Based on the descriptions above, it can be known
The conventional injection mold is mainly composed of cavity plate in the stationary mold, core plate in the movable mold. As we have known that in electric heating injection process, the mold needs to be heated and cooled rapidly repeatedly, therefore, the structure of the mold is different from that of the conventional mold. Fig. 2 shows the structure of the electric heated injection
Fig. 2. EHI mold structure of an LCD TV panel. 1. Mold cavity plate, 2. Heating rod, 3. Moveable fixed plate, 4. Stationary mold fixed plate, 5. Cooling plate, 6. Cooling channel, 7. Mold core plate.
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mold which was designed for the front panel of an LCD TV. Fig. 2(a) is the main structure of the mold; (b) is back view of the cavity plate and layout of the heating rods in the mold. In electric heated mold, except for the plates that the conventional mold possesses, there is another one cooling plate. It is assembled with the fixed plates 4 and the cavity plate 1. Besides, there are many heating rods and cooling channels in the cavity plate and the cooling plate, respectively. The back of the cavity plate is also very special, there are many reinforced ribs in order to improve the strength of the plate and be assembled with the cooling plate conveniently. In this way, the cavity plate is heated by the heating rods in heating stage, and in cooling stage, the channels in the cooling plate are filled with coolant so that the cavity plate and the melt can be cooled down rapidly, then the molded parts can be ejected from the mold in a short cycle.
after about 5000 injection cycles. When the fatigue cracks are formed on the cavity plate surface, the production process would be influenced. If the cavity plate cannot be used any more, new plate must be designed and manufactured. Thus, the cost of the mold is inevitably increased. Compared with the conventional injection mold, the thermal stress caused by the high temperature is considered as the main factor to influence the lifetime of the electric heated mold. So it is very necessary to research the heat transfer process of the mold during the electric heating injection process. According to the conservation principle of energy exchanged [18,19], the general form of heat conduction in Cartesian coordinate system can be expressed by the following Eq. (1):
3. Heat transfer process of the electric heated injection mold
where, T is the transient temperature of the object; t is the time for the heat transfer process; k is heat transfer coefficient; r is the material density; Cp is constant pressure specific heat; qv is heat flux density; x, y are orthogonal coordinates. The heat flux density qv produced by a heating rod on the interface in this paper can be approximately calculated by the following Eq. (2):
Until now, there are many ways to heat the injection mold. However, the electric heating injection technology has fewer and simpler auxiliary equipments compared with the steam-assisted heating method, the infrared heating method, and the induction heating method proposed in the literatures [14e17]. Therefore, it has been widely used in injection industry. During working process of the injection technology, the mold must be heated to a high temperature. The average temperature of the mold is usually about 125e145 C after heating stage, especially, in order to make the production process favorably, the peak temperature sometimes could reach about 150e165 C. Compared with the conventional injection mold, the peak temperature and temperature gradient are both much higher. Consequently, thermal stress of the electric heated injection mold caused by the temperature is greater than that of the conventional mold. At the same time, the mold must stand great clamp force repeatedly in injection and packing stages, therefore, the stress suffered by the mold is the combination of thermal and mechanical stress. As the stress is large and cycled, fatigue cracks can generate much more easily on the electric heated injection mold than that on the conventional mold. Fig. 3 shows the fatigue cracks appeared on an electric heated injection mold surface in practical production process. The mold was for one type of LCD TV panel and failed
vT k v2 T v2 T v2 T qv þ þ þ ¼ rCp vx2 vy2 vz2 k vt
qv ¼
P ðp dÞ L
(1)
(2)
where, P, d, L are the power, diameter and length of the heating rod, respectively. Based on Eq. (1), the temperature field could by obtained by using finite element simulation method. During the working process of electric heated mold, the heat transfer process can be divided into two main stages. In heating stage, cavity plate is heated by the heating rods. With the elevation of the temperature, the thermal stress in the mold is gradually increased. When the temperature reaches the designated value, the heating rods are controlled to stop heating the mold and this stage is finished. In cooling stage, the mold is rapidly cooled down to low temperature and the thermal stress is relaxed little by little. So the thermal stress of the mold which is caused by the temperature is much greater in heating stage than that in cooling stage. In this case, the stress of the mold only in heating stage is researched in this paper. The relaxing process of the stress in the cooling stage is not considered. 3.1. Boundary and initial conditions for heat transfer process In heating stage, the cavity plate 1 is separate from the cooling plate 5 shown in Fig. 2, and there is some adiabatic material between the cavity plate and the stationary mold plates 4. Therefore, the cavity plate is considered as heat-insulated with the other mold plates. The thermal resistance from the guide pins, fixed bolts in the mold could be neglected, too. Consequently, heat transfer can be considered to occur only in the mold cavity plate in this stage. According to the practical production, the initial temperature of the mold and the environment are both 25 C. The heat transfer coefficient between the air and the mold is set to 20 W/m2 K. The calculated heat flux of the heating rods is 3 105 W/m2 according to the parameters of the heating rods proposed by the manufacturer. 3.2. Properties of the mold material
Fig. 3. Fatigue cracks of electrical heating injection mold.
According to the requirements of the mold material mentioned above, the CEAN1 steel produced in Japan is selected as the mold material. It has a uniform microstructure. The polish ability, thermal fatigue resistance, abrasion resistance, and corrosion
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Table 1 Properties of mold material. Specific heat (J/Kg K)
Thermal conductivity (W/m K)
Modulus of elasticity (MPa)
Coefficient of thermal expansion
Hardness
7.78 103
460
30
2.05 105
1.16 105
HRC38-43
resistance of the material are all very good. The main properties of the material are listed in Table 1. Based on the practical injection parameters and the production process, the heating time of the mold is usually set to about 15 s. When all the parameters needed for the simulation are determined, the 3-D finite element model can be established and the computation of the temperature field could be carried out. 3.3. Discussion of the mold temperature field The 3-D CAD model of the cavity plate shown in Fig. 2 is meshed in tetrahedral element in order to do the finite element (FE) analysis. The total number of the element is 65870 and the nodes number is 16473. After all the boundary and initial conditions are added to the model, the temperature field of the mold is obtained by using the FE simulation based on Eq. (1) and MSC Marc software. The temperature field of the mold cavity plate after heating stage is shown in Fig. 4. In order to observe the temperature values of the cavity surface definitely, the temperature values on the typical place I, II and III of the cavity are listed by the temperature tracking points in Fig. 5. From the above two figures, it is found that when the heating stage is finished, the temperature distribution on the cavity surface is not uniform. The temperatures in the middle of the three places are much higher than that in the two sides. The highest temperatures of the three places are 154.2, 156.8 and 157.1 C respectively, and the lowest temperatures are 95.5, 94.9 and 93.1 C respectively. The temperature difference is very large. It means that the places at sides of the cavity are heated insufficiently. However, as the lowest temperature is at the side, in most of time, it cannot influence quality of the molded part. So the temperature of the cavity surface can still satisfy the technology on the whole. In the production process, sound polymer parts are produced proved that the above analysis is right. In general, larger thermal stress and deformation of the mold often generates at the place where there is a higher temperature according to the engineering heat transfer theory. But it is not suitable for this kind of mold. The stress and deformation of the mold plates are mostly concerned with their constraint states in the mold.
Fig. 4. Temperature field of the cavity plate after heating stage.
I II III
140
Temperature (oC )
Density (Kg/m3)
120 100 80 60
0
2
4
6
8
10
12
Temperature tracking points Fig. 5. Temperature values of the cavity surface. I. High temperature stage, II. Middle temperature stage, III. Low temperature stage.
4. Analysis of the thermal stress and deformation of the cavity plate Considering the convenience of the mold assembling, the cavity plate is usually fixed in the mold as the Fig. 6 shows. It is acquired that the top and right sides of the cavity plate shown in the figure are closely affixed with the stationary mold plate. The other two sides are not affixed with the plate, with some clearance left to make sure that the cavity plate can be easily put into the stationary mold plate. Then the bolts 4 and the fixed inserts 1 between the plates are used to fix the cavity plate and the mold plate firmly. In this assembling mode, the cavity plate can be installed and uninstalled much more easily. Therefore, in practical injection mold, the mold cavity plate is constrained by the stationary mold plate, the fixed inserts, and the fixed bolts. Accordingly, when the stress and deformation FE analysis of the mold is carried out, the cavity plate is defined as a deformable body. The stationary mold plate and the inserts are defined as rigid bodies. The bolts are also considered as one boundary condition to constrain the displacement of the cavity
Fig. 6. Fixing mode of the cavity plate. 1. Fixed insert, 2. Cavity plate, 3. Stationary mold plate, 4. Fixed bolts.
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Fig. 7. Deformation of the cavity plate in X direction.
plate. In addition, in every injection cycle, the temperature and temperature gradient of the mold vary greatly, so the thermal load obtained in Section 3.1 of this paper is applied to the cavity plate, too. As described above, in heating stage, with the elevation of the temperature, the thermal stress and the strain of the mold are gradually increased. The stress suffered by the mold is the combination of thermal and mechanical stress. Then the physical equations for the total stress and strain of the mold can be expressed as the follows:
9 m vux 1 > sx sx þ sy þ sz þ aT > ¼ > > vx 1þm 2G > > = vuy m 1 sy sx þ sy þ sz þ aT εy ¼ ¼ > vy 1þm 2G > > > m vuz 1 > ; sz sx þ sy þ sz þ aT > εz ¼ ¼ 2G vz 1þm εx ¼
gxy ¼
sxy 2G
; gyz ¼
syz
szx
;g ¼ 2G zx 2G
Fig. 8. Stress distribution at the back of the cavity plate.
(3)
(4)
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where, εx ; εy ; εz and sx ; sy ; sz are the total normal strains and stresses of the object in x,y,z direction respectively; T is the variable temperature values; a is the coefficient of linear expansion; gxy ; gyz ; gzx and gxy ; gyz ; gzx are the total shear strains and stresses; G is the shear elasticity modulus; m is Poisson’s ratio. Based on the equations and the determined boundary and initial conditions, the distributions of the stress and strain of the model can be calculated through the transient heat and stress FE analysis. Fig. 7 shows the cavity plate deformation after the heating stage. Fig. 7 shows that the deformation of the cavity plate is not uniform due to the non-uniform constraint. The place constrained by rigid body deforms very little, while it deforms very large where there is no constraint. The largest deformation in X direction generates in the top left corner, and the value is about 0.23 mm. As the non-uniform constraints of the cavity plate, it can be forecast that the stress concentration can be caused, and the place with large stress concentration could be easily failed. Fig. 8 is the stress distribution at the back of the cavity plate. There are the inserts constraints near the ribs numbered from 1 to 5, and the ribs numbered from 6 to 10 are near the mold plate constraints. It shows that the stresses are very large at the places with reinforced ribs constraints and they are different. Fig. 9 shows the stress values of each rib suffering. Usually, the deformation caused by the thermal expansion is concerned with the length and temperature difference of a heated object. The relationship of them can be expressed in the following Eq. (5):
e ¼ LkDT
(5)
Thermal stress (MPa)
where, e is the thermal expansion value, L is the length of the heated object, DT is the temperature difference, and k is coefficient of thermal expansion. In this paper, as the length of the cavity plate in X direction is larger than that in Y direction, the expansion deformation of the plate would be larger in X direction than that in Y direction in a condition that the values k and DT are the same. However, as the cavity plate is constrained by the inserts in the right and bottom sides as shown in Fig. 6, and the constraint is not uniform, we can see that the number 1 rib suffers the greatest stress and the value is about 995.3 MPa. That’s also because this rib faces against the insert rightly, and it cannot be extended any more. The number 5 rib suffers the least stress. It mainly because that this place is not constraint by any rigid bodies when it expanding. So the stress value is only about 449.1 MPa. Through the figure, we also found
1000 900 800 700 600 500 400 300 200 100 0
Fig. 10. Cracks formed around the ribs.
that the stress difference of the ribs numbered from 6 to 10 near the mold plate is only 214.4 MPa, while the stress difference of the ribs numbered from 1 to 5 near the inserts is about 592.1 MPa. It denotes that the top and left sides of the cavity plate suffer uniform stress as they are uniformly constrained by the mold plate during the working process. The bottom and right sides suffer nonuniform stress as they are constrained non-uniformly by the inserts. On the basis of the analysis above, it is known that the mold suffers great thermal stress during the working process. In some local place, the stress is very great. In heating stage, the cavity plate expands due to the temperature rising, but because of the constraints around it, the cavity plate is in compressive state and suffers the compressive stress. At first, the stress and strain of the plate are both small and they are in the elastic range. When the temperature drops down in the cooling stage, the stress and strain can be relaxed. With the injection cycles carried out repeatedly, the place of the plate suffering the greatest stress will be first in a fatigue state, then the compress strain cannot be relaxed and the plastic deformation forms. At last, the mold is in a failure state and cannot be used any more. According to the simulation, it is learned that the number 1 rib at the back of the cavity plate suffers the greatest stress, so it can be forecast that the fatigue cracks will generate around this place first. Fig. 10 is the photo of the failed mold in production process. The cracks generated on the practical mold are around the ribs, it is very in accordance with the above analysis. Therefore, it is proved that the analysis done by the authors is proper. As the ribs at the back of the cavity plate suffer the greatest stress during the working process, in order to improve the lifetime of the mold, there should be more reinforce ribs on the cavity plate or the thickness of the ribs should be increased or the fixed inserts should not face against the ribs. In this way, the lifetime of the mold could be improved. 5. Conclusions
0
1
2
3
4
5
6
Rib number
Fig. 9. Stress of the ribs suffering.
7
8
9
10
In this paper, the structure of the electric heated injection mold was first presented, and then the heat transfer process and stress and deformation analysis models were established. Through studying, the reasons causing the large thermal stress and deformation of the mold were analyzed. Then the following conclusions are drawn:
X.-P. Li et al. / Applied Thermal Engineering 31 (2011) 3989e3995
(1) The heat transfer process of the EHI mold in heating stage is obtained by using finite element simulation. Results show that the temperature distribution of the cavity surface is not uniform after the heating stage, however, it can still satisfy the requirement of the electric heating injection technology and good products could be produced in practical production. (2) Based on the physical equations for the total stress and strain of the mold and finite element software, the stress and deformation of the mold cavity plate is calculated. The deformation of the cavity plate is constrained non-uniformly in the thermal expansion process resulting in great stress concentration around the reinforce ribs at the back of the plate. The greatest stress that the ribs of the mold suffered is about 995.3 MPa and the fatigue cracks will first appear at this place. (3) The analysis for the failure of the electric heated mold is proved to be right by the practical failed mold. In order to improve the lifetime of the mold, there should be more reinforce ribs on the cavity plate or the thickness of the ribs is increased or the fixed inserts should not face against the ribs. Acknowledgements The research work was supported by Program for New Century Excellent Talents in University(NCET-08-0337) and Natural Science Foundation of Zhejiang Province (Y1110509). References [1] Tham Nguyen-Chung, Flow analysis of the weld line formation during injection mold filling of thermoplastics, Rheol. Acta. 43 (2004) 240e245. [2] Jong Cheol Lim, Kuk Young Cho, Jung-Ki Park, Weld line characteristics of PC/ ABS blend, J. Appl. Polymer. Sci. 108 (2008) 3632e3643. [3] Sung Yong Kang, Seung Mo Kim, Woo Lee II, Finite element analysis for wavelike flow marks in injection molding, Polym. Eng. Sci. 47 (2007) 922e933.
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[4] S.C. Chen, H.S. Peng, J.A. Chang, W.R. Jong, Rapid mold surface heating/cooling using electromagnetic technology SPE ANTEC Conference Proceedings. Society of Plastics Engineers, Dallas, USA, 2001. [5] Guoqun Zhao, Guilong Wang, Yanjin Guan, Huiping Li, Research and application of a new rapid heat cycle molding with electric heating and coolant cooling to improve the surface quality of large LCD TV panels, Polym. Advan. Technol. 22 (2011) 476e487. [6] U.A. Theilade, H.N. Hansen, Surface microstructure replication in injection molding, Int. J. Adv. Manuf. Technol. 33 (2007) 157e166. [7] D.S. Kim, J.S. Kim, Y.B. Ko, J.D. Kim, K.H. Yoon, C.J. Hwang, Experimental characterization of transcription properties of microchannel geometry fabricated by injection molding based on Taguchi method, Microsyst. Technol. 14 (2008) 1581e1588. [8] M.C. Yu, W.B. Young, P.M. Hsu, Micro-injection molding with the infrared assisted heating system, Mater. Sci. Eng. A 460e461 (2007) 288e295. [9] Shaolin Mao, Changrui Cheng, Xianchang Li, Efstathios E. Michaelides, Thermal/structural analysis of radiators for heavy-duty trucks, Appl. Therm. Eng. 30 (2010) 1438e1446. [10] Zhihui Zhang, Hong Zhou, Luquan Ren, Xin Tong, Hongyu Shan, Li Liu, Effect of units in different sizes on thermal fatigue behavior of 3Cr2W8V die steel with biomimetic non-smooth surface, Int. J. Fatig. 31 (2009) 468e475. [11] Balázs Illés, Distribution of the heat transfer coefficient in convection reflow oven, Appl. Therm. Eng. 30 (2010) 1523e1530. [12] M.R. Eslami, M.H. Babaei, R. Poultangari, Thermal and mechanical stresses in a functionally graded thick sphere, Int. J. Pres. Ves. Pip. 82 (2005) 522e527. [13] H.Y. Wang, X.L. Bi, L.L. Zhao, Q.T. Zhou, H.T. Kim, Z.G. Xu, A study on thermal stress deformation using analytical methods based on the temperature distribution of storage material in a rotary air-preheater, Appl. Therm. Eng. 29 (2009) 2350e2357. [14] Xiping Li, Guoqun Zhao, Yanjin Guan, Huiping Li, Research on thermal stress, deformation, and fatigue lifetime of the rapid heating cycle injection mold, Int. J. Adv. Manuf. Tech. 45 (2009) 261e275. [15] S.C. Chen, W.R. Jong, J.A. Chang, Dynamic mold surface temperature control using induction heating and its effects on the surface appearance of weld line, J. Appl. Polymer. Sci. 101 (2006) 1174e1180. [16] P.C. Chang, S. Hwang, Simulation of infrared rapid surface heating for injection molding, Int. Comm. Heat Mass. Tran 49 (2006) 3846e3854. [17] Xi-Ping Li, Guo-Qun Zhao, Yan-Jin Guan, Ming-Xing Ma, Optimal design of heating channels for rapid heating cycle injection mold based on response surface and genetic algorithm, Mater. Des. 30 (2009) 4317e4323. [18] M.W. Rohsenow, J.P. Hartnett, Handbook of heat transfer. McGraw-Hill, New York, 1973. [19] L.A. Luo, Application and analysis of heat transfer. Tsinghua University Press, Beijing, 1981.
Contents lists available at ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Thermal and stress analysis of rapid electric heating injection mold for a large LCD TV panel Xi-Ping Li a, Ning-Ning Gong a, *, Yan-Jin Guan b, Guang-Ming Cheng a a b
College of Engineering, Zhejiang Normal University, Jinhua 321004, PR China Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2011 Accepted 29 July 2011 Available online 6 August 2011
Rapid electric heating cycle injection molding technology has been used widely in injection engineering application. Using this technology, excellent products with no weld marks, flow marks and other surface defects can be produced. However, as the special mold structure and its worse working process, the rapid electric heated injection (EHI) mold is easy to fail than that of the conventional injection mold along with the injection process carried out repeatedly. In this paper, an EHI mold for a large liquid crystal display (LCD) TV panel was first presented. Then the heat transfer process of the mold during the working process was studied. Lastly, through finite element simulation, the reasons that cause the large thermal stress and deformation of the mold were analyzed. The practical failure of the mold proved that the analysis presented in the paper was right. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Heat transfer process Stress analysis Electric heated injection mold
1. Introduction In conventional injection technology, the temperature of the mold is very low due to the time limit of the injection cycle, and it is usually less than 80 C. When the polymer melt is injected into the mold cavity, a frozen layer easily forms on the surface of the cavity. That’s because when the polymer melt with high temperature contacts the cool cavity surface, the temperature of the surface melt drops down rapidly to the low temperature leading to the surface melt solidified firstly. As a result, the melt filling into the cavity is influenced and its capacity to replicate the cavity surface is decreased as well. Therefore, the conventional injection method cannot be used to produce products with microstructures. The thinwall products are also very difficult to produce due to the decrease of the melt’s flow ability. In addition, the surface defects such as weld mark and flow mark are often caused due to the low temperature of the cavity surface as demonstrated in literatures [1e3]. The subsequent processes-spraying and coating which seriously pollute the environment must be used so as to improve the surface quality. These operations increase the manufacturing processes of parts, waste raw material and energy and increase the production cost as well. It is obvious that the conventional injection technology is difficult to satisfy the requirements of the modern injection industry development. For injection technology, the ideal * Corresponding author. Tel.: þ86 579 82286043; fax: þ86 579 82288020. E-mail address: [email protected] (N.-N. Gong). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.07.050
temperature of the mold during the working process is that when polymer melt injects into the mold, the cavity surface should be in a high temperature state in order to increase the flow ability of the melt. When the filling process is finished, the molded parts should be rapidly cooled down to ejection temperature so as to improve the production efficiency. The electric heating injection molding is such an injection technology that can control the mold temperature dynamically [4,5]. First, the stationary mold is rapidly heated to a high temperature by the heating rods in the mold. Then the polymer melt begins to inject to the mold cavity and in the later period of the packing stage, the mold and the melt are rapidly cooled down by coolant. When the mold is opened, the molded part is ejected; meanwhile, the heating rods begin to heat the mold again. Thus, the next injection cycle begins. As the temperature of the mold cavity is elevated before injecting, the flow ability of the melt is increased and the viscosity is decreased. The melt in the cavity can completely bended before it is cooled. So the surface defects on the polymer parts are eliminated and the high-quality products can be produced. Further more, the replication capacity of the melt is improved and the thin-wall polymer parts or products with microstructures [6e8] that usually have a bad filling process can be molded very well. Fig. 1 is the comparison of the products injected by conventional injection method and rapid electric heating injection method. It is observed that there are no weldlines on the products surface injected by electric heating injection molding method and the surface is so bright that the even the camera used to shot the photo can be mirrored.
3990
X.-P. Li et al. / Applied Thermal Engineering 31 (2011) 3989e3995
that, compared with the conventional injection mold, the thermal load suffered by the EHI mold during the injection cycle is the main reason to cause the mold failure. Up till now, there are many literatures [9e13] concerning with influence of thermal load on engineering applications, but there are few aiming at the injection mold. In this paper, in order to study the thermal load suffered by the EHI mold, the typical structure of an electric heated injection mold for an LCD TV panel was presented firstly. By setting the proper boundary and initial conditions, the heat transfer process of the mold was studied. Through finite element simulation (FEM), the thermal stress and deformation of the mold cavity plate were analyzed. At last, effective measures that could decrease the thermal stress and improve the lifetime of the mold were proposed. 2. Electric heated injection mold structure for an LCD TV panel Fig. 1. Comparison of panel products for different injection methods.
However, as the special structure and worse working process of the electric heated injection mold, large stress concentration can be easily induced in the mold. The large stress can cause the mold to fail very easy. Based on the descriptions above, it can be known
The conventional injection mold is mainly composed of cavity plate in the stationary mold, core plate in the movable mold. As we have known that in electric heating injection process, the mold needs to be heated and cooled rapidly repeatedly, therefore, the structure of the mold is different from that of the conventional mold. Fig. 2 shows the structure of the electric heated injection
Fig. 2. EHI mold structure of an LCD TV panel. 1. Mold cavity plate, 2. Heating rod, 3. Moveable fixed plate, 4. Stationary mold fixed plate, 5. Cooling plate, 6. Cooling channel, 7. Mold core plate.
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mold which was designed for the front panel of an LCD TV. Fig. 2(a) is the main structure of the mold; (b) is back view of the cavity plate and layout of the heating rods in the mold. In electric heated mold, except for the plates that the conventional mold possesses, there is another one cooling plate. It is assembled with the fixed plates 4 and the cavity plate 1. Besides, there are many heating rods and cooling channels in the cavity plate and the cooling plate, respectively. The back of the cavity plate is also very special, there are many reinforced ribs in order to improve the strength of the plate and be assembled with the cooling plate conveniently. In this way, the cavity plate is heated by the heating rods in heating stage, and in cooling stage, the channels in the cooling plate are filled with coolant so that the cavity plate and the melt can be cooled down rapidly, then the molded parts can be ejected from the mold in a short cycle.
after about 5000 injection cycles. When the fatigue cracks are formed on the cavity plate surface, the production process would be influenced. If the cavity plate cannot be used any more, new plate must be designed and manufactured. Thus, the cost of the mold is inevitably increased. Compared with the conventional injection mold, the thermal stress caused by the high temperature is considered as the main factor to influence the lifetime of the electric heated mold. So it is very necessary to research the heat transfer process of the mold during the electric heating injection process. According to the conservation principle of energy exchanged [18,19], the general form of heat conduction in Cartesian coordinate system can be expressed by the following Eq. (1):
3. Heat transfer process of the electric heated injection mold
where, T is the transient temperature of the object; t is the time for the heat transfer process; k is heat transfer coefficient; r is the material density; Cp is constant pressure specific heat; qv is heat flux density; x, y are orthogonal coordinates. The heat flux density qv produced by a heating rod on the interface in this paper can be approximately calculated by the following Eq. (2):
Until now, there are many ways to heat the injection mold. However, the electric heating injection technology has fewer and simpler auxiliary equipments compared with the steam-assisted heating method, the infrared heating method, and the induction heating method proposed in the literatures [14e17]. Therefore, it has been widely used in injection industry. During working process of the injection technology, the mold must be heated to a high temperature. The average temperature of the mold is usually about 125e145 C after heating stage, especially, in order to make the production process favorably, the peak temperature sometimes could reach about 150e165 C. Compared with the conventional injection mold, the peak temperature and temperature gradient are both much higher. Consequently, thermal stress of the electric heated injection mold caused by the temperature is greater than that of the conventional mold. At the same time, the mold must stand great clamp force repeatedly in injection and packing stages, therefore, the stress suffered by the mold is the combination of thermal and mechanical stress. As the stress is large and cycled, fatigue cracks can generate much more easily on the electric heated injection mold than that on the conventional mold. Fig. 3 shows the fatigue cracks appeared on an electric heated injection mold surface in practical production process. The mold was for one type of LCD TV panel and failed
vT k v2 T v2 T v2 T qv þ þ þ ¼ rCp vx2 vy2 vz2 k vt
qv ¼
P ðp dÞ L
(1)
(2)
where, P, d, L are the power, diameter and length of the heating rod, respectively. Based on Eq. (1), the temperature field could by obtained by using finite element simulation method. During the working process of electric heated mold, the heat transfer process can be divided into two main stages. In heating stage, cavity plate is heated by the heating rods. With the elevation of the temperature, the thermal stress in the mold is gradually increased. When the temperature reaches the designated value, the heating rods are controlled to stop heating the mold and this stage is finished. In cooling stage, the mold is rapidly cooled down to low temperature and the thermal stress is relaxed little by little. So the thermal stress of the mold which is caused by the temperature is much greater in heating stage than that in cooling stage. In this case, the stress of the mold only in heating stage is researched in this paper. The relaxing process of the stress in the cooling stage is not considered. 3.1. Boundary and initial conditions for heat transfer process In heating stage, the cavity plate 1 is separate from the cooling plate 5 shown in Fig. 2, and there is some adiabatic material between the cavity plate and the stationary mold plates 4. Therefore, the cavity plate is considered as heat-insulated with the other mold plates. The thermal resistance from the guide pins, fixed bolts in the mold could be neglected, too. Consequently, heat transfer can be considered to occur only in the mold cavity plate in this stage. According to the practical production, the initial temperature of the mold and the environment are both 25 C. The heat transfer coefficient between the air and the mold is set to 20 W/m2 K. The calculated heat flux of the heating rods is 3 105 W/m2 according to the parameters of the heating rods proposed by the manufacturer. 3.2. Properties of the mold material
Fig. 3. Fatigue cracks of electrical heating injection mold.
According to the requirements of the mold material mentioned above, the CEAN1 steel produced in Japan is selected as the mold material. It has a uniform microstructure. The polish ability, thermal fatigue resistance, abrasion resistance, and corrosion
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Table 1 Properties of mold material. Specific heat (J/Kg K)
Thermal conductivity (W/m K)
Modulus of elasticity (MPa)
Coefficient of thermal expansion
Hardness
7.78 103
460
30
2.05 105
1.16 105
HRC38-43
resistance of the material are all very good. The main properties of the material are listed in Table 1. Based on the practical injection parameters and the production process, the heating time of the mold is usually set to about 15 s. When all the parameters needed for the simulation are determined, the 3-D finite element model can be established and the computation of the temperature field could be carried out. 3.3. Discussion of the mold temperature field The 3-D CAD model of the cavity plate shown in Fig. 2 is meshed in tetrahedral element in order to do the finite element (FE) analysis. The total number of the element is 65870 and the nodes number is 16473. After all the boundary and initial conditions are added to the model, the temperature field of the mold is obtained by using the FE simulation based on Eq. (1) and MSC Marc software. The temperature field of the mold cavity plate after heating stage is shown in Fig. 4. In order to observe the temperature values of the cavity surface definitely, the temperature values on the typical place I, II and III of the cavity are listed by the temperature tracking points in Fig. 5. From the above two figures, it is found that when the heating stage is finished, the temperature distribution on the cavity surface is not uniform. The temperatures in the middle of the three places are much higher than that in the two sides. The highest temperatures of the three places are 154.2, 156.8 and 157.1 C respectively, and the lowest temperatures are 95.5, 94.9 and 93.1 C respectively. The temperature difference is very large. It means that the places at sides of the cavity are heated insufficiently. However, as the lowest temperature is at the side, in most of time, it cannot influence quality of the molded part. So the temperature of the cavity surface can still satisfy the technology on the whole. In the production process, sound polymer parts are produced proved that the above analysis is right. In general, larger thermal stress and deformation of the mold often generates at the place where there is a higher temperature according to the engineering heat transfer theory. But it is not suitable for this kind of mold. The stress and deformation of the mold plates are mostly concerned with their constraint states in the mold.
Fig. 4. Temperature field of the cavity plate after heating stage.
I II III
140
Temperature (oC )
Density (Kg/m3)
120 100 80 60
0
2
4
6
8
10
12
Temperature tracking points Fig. 5. Temperature values of the cavity surface. I. High temperature stage, II. Middle temperature stage, III. Low temperature stage.
4. Analysis of the thermal stress and deformation of the cavity plate Considering the convenience of the mold assembling, the cavity plate is usually fixed in the mold as the Fig. 6 shows. It is acquired that the top and right sides of the cavity plate shown in the figure are closely affixed with the stationary mold plate. The other two sides are not affixed with the plate, with some clearance left to make sure that the cavity plate can be easily put into the stationary mold plate. Then the bolts 4 and the fixed inserts 1 between the plates are used to fix the cavity plate and the mold plate firmly. In this assembling mode, the cavity plate can be installed and uninstalled much more easily. Therefore, in practical injection mold, the mold cavity plate is constrained by the stationary mold plate, the fixed inserts, and the fixed bolts. Accordingly, when the stress and deformation FE analysis of the mold is carried out, the cavity plate is defined as a deformable body. The stationary mold plate and the inserts are defined as rigid bodies. The bolts are also considered as one boundary condition to constrain the displacement of the cavity
Fig. 6. Fixing mode of the cavity plate. 1. Fixed insert, 2. Cavity plate, 3. Stationary mold plate, 4. Fixed bolts.
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Fig. 7. Deformation of the cavity plate in X direction.
plate. In addition, in every injection cycle, the temperature and temperature gradient of the mold vary greatly, so the thermal load obtained in Section 3.1 of this paper is applied to the cavity plate, too. As described above, in heating stage, with the elevation of the temperature, the thermal stress and the strain of the mold are gradually increased. The stress suffered by the mold is the combination of thermal and mechanical stress. Then the physical equations for the total stress and strain of the mold can be expressed as the follows:
9 m vux 1 > sx sx þ sy þ sz þ aT > ¼ > > vx 1þm 2G > > = vuy m 1 sy sx þ sy þ sz þ aT εy ¼ ¼ > vy 1þm 2G > > > m vuz 1 > ; sz sx þ sy þ sz þ aT > εz ¼ ¼ 2G vz 1þm εx ¼
gxy ¼
sxy 2G
; gyz ¼
syz
szx
;g ¼ 2G zx 2G
Fig. 8. Stress distribution at the back of the cavity plate.
(3)
(4)
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where, εx ; εy ; εz and sx ; sy ; sz are the total normal strains and stresses of the object in x,y,z direction respectively; T is the variable temperature values; a is the coefficient of linear expansion; gxy ; gyz ; gzx and gxy ; gyz ; gzx are the total shear strains and stresses; G is the shear elasticity modulus; m is Poisson’s ratio. Based on the equations and the determined boundary and initial conditions, the distributions of the stress and strain of the model can be calculated through the transient heat and stress FE analysis. Fig. 7 shows the cavity plate deformation after the heating stage. Fig. 7 shows that the deformation of the cavity plate is not uniform due to the non-uniform constraint. The place constrained by rigid body deforms very little, while it deforms very large where there is no constraint. The largest deformation in X direction generates in the top left corner, and the value is about 0.23 mm. As the non-uniform constraints of the cavity plate, it can be forecast that the stress concentration can be caused, and the place with large stress concentration could be easily failed. Fig. 8 is the stress distribution at the back of the cavity plate. There are the inserts constraints near the ribs numbered from 1 to 5, and the ribs numbered from 6 to 10 are near the mold plate constraints. It shows that the stresses are very large at the places with reinforced ribs constraints and they are different. Fig. 9 shows the stress values of each rib suffering. Usually, the deformation caused by the thermal expansion is concerned with the length and temperature difference of a heated object. The relationship of them can be expressed in the following Eq. (5):
e ¼ LkDT
(5)
Thermal stress (MPa)
where, e is the thermal expansion value, L is the length of the heated object, DT is the temperature difference, and k is coefficient of thermal expansion. In this paper, as the length of the cavity plate in X direction is larger than that in Y direction, the expansion deformation of the plate would be larger in X direction than that in Y direction in a condition that the values k and DT are the same. However, as the cavity plate is constrained by the inserts in the right and bottom sides as shown in Fig. 6, and the constraint is not uniform, we can see that the number 1 rib suffers the greatest stress and the value is about 995.3 MPa. That’s also because this rib faces against the insert rightly, and it cannot be extended any more. The number 5 rib suffers the least stress. It mainly because that this place is not constraint by any rigid bodies when it expanding. So the stress value is only about 449.1 MPa. Through the figure, we also found
1000 900 800 700 600 500 400 300 200 100 0
Fig. 10. Cracks formed around the ribs.
that the stress difference of the ribs numbered from 6 to 10 near the mold plate is only 214.4 MPa, while the stress difference of the ribs numbered from 1 to 5 near the inserts is about 592.1 MPa. It denotes that the top and left sides of the cavity plate suffer uniform stress as they are uniformly constrained by the mold plate during the working process. The bottom and right sides suffer nonuniform stress as they are constrained non-uniformly by the inserts. On the basis of the analysis above, it is known that the mold suffers great thermal stress during the working process. In some local place, the stress is very great. In heating stage, the cavity plate expands due to the temperature rising, but because of the constraints around it, the cavity plate is in compressive state and suffers the compressive stress. At first, the stress and strain of the plate are both small and they are in the elastic range. When the temperature drops down in the cooling stage, the stress and strain can be relaxed. With the injection cycles carried out repeatedly, the place of the plate suffering the greatest stress will be first in a fatigue state, then the compress strain cannot be relaxed and the plastic deformation forms. At last, the mold is in a failure state and cannot be used any more. According to the simulation, it is learned that the number 1 rib at the back of the cavity plate suffers the greatest stress, so it can be forecast that the fatigue cracks will generate around this place first. Fig. 10 is the photo of the failed mold in production process. The cracks generated on the practical mold are around the ribs, it is very in accordance with the above analysis. Therefore, it is proved that the analysis done by the authors is proper. As the ribs at the back of the cavity plate suffer the greatest stress during the working process, in order to improve the lifetime of the mold, there should be more reinforce ribs on the cavity plate or the thickness of the ribs should be increased or the fixed inserts should not face against the ribs. In this way, the lifetime of the mold could be improved. 5. Conclusions
0
1
2
3
4
5
6
Rib number
Fig. 9. Stress of the ribs suffering.
7
8
9
10
In this paper, the structure of the electric heated injection mold was first presented, and then the heat transfer process and stress and deformation analysis models were established. Through studying, the reasons causing the large thermal stress and deformation of the mold were analyzed. Then the following conclusions are drawn:
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(1) The heat transfer process of the EHI mold in heating stage is obtained by using finite element simulation. Results show that the temperature distribution of the cavity surface is not uniform after the heating stage, however, it can still satisfy the requirement of the electric heating injection technology and good products could be produced in practical production. (2) Based on the physical equations for the total stress and strain of the mold and finite element software, the stress and deformation of the mold cavity plate is calculated. The deformation of the cavity plate is constrained non-uniformly in the thermal expansion process resulting in great stress concentration around the reinforce ribs at the back of the plate. The greatest stress that the ribs of the mold suffered is about 995.3 MPa and the fatigue cracks will first appear at this place. (3) The analysis for the failure of the electric heated mold is proved to be right by the practical failed mold. In order to improve the lifetime of the mold, there should be more reinforce ribs on the cavity plate or the thickness of the ribs is increased or the fixed inserts should not face against the ribs. Acknowledgements The research work was supported by Program for New Century Excellent Talents in University(NCET-08-0337) and Natural Science Foundation of Zhejiang Province (Y1110509). References [1] Tham Nguyen-Chung, Flow analysis of the weld line formation during injection mold filling of thermoplastics, Rheol. Acta. 43 (2004) 240e245. [2] Jong Cheol Lim, Kuk Young Cho, Jung-Ki Park, Weld line characteristics of PC/ ABS blend, J. Appl. Polymer. Sci. 108 (2008) 3632e3643. [3] Sung Yong Kang, Seung Mo Kim, Woo Lee II, Finite element analysis for wavelike flow marks in injection molding, Polym. Eng. Sci. 47 (2007) 922e933.
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