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Rapid mold temperature control in injection molding by using steam heating
International Communications in Heat and Mass Transfer 37 (2010) 1295–1304
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International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Rapid mold temperature control in injection molding by using steam heating☆ Ming-Chang Jeng a, Shia-Chung Chen b,c,d,⁎, Pham Son Minh b,c,d, Jen-An Chang b,c,d, Chia-shen Chung a a
Department of Mechanical Engineering, National Central University, Chung-Li, Taiwan, ROC Department of Mechanical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC c R & D Center for Membrane Technology, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC d R & D Center for Mold and Molding Technology, Chung Yuan Christian University, Chung-Li 32023, Taiwan ROC b
a r t i c l e
i n f o
Available online 4 August 2010 Keywords: Dynamic mold temperature control Steam-assisted heating Heat transfer coefficient
a b s t r a c t The rapid heating cycle has the advantage of improving product quality in injection molding. In this study, steam heating was combined with cool water on the same mold design to achieve dynamic mold surface temperature to establish control. By applying the steam system on a TV housing mold, the advantage of using steam heating for injection molding was then evaluated and compared with water heating by experiment and simulation. The effect of steam on the quality of the part was also studied. Results showed that as steam was used, the heating time of the simple mold plate can be reduced from 18 s to 8 s with the heating rate of 9 °C/s, and the cooling time is reduced over water heating. When the target temperature is changed from 70 °C to 110 °C, the heating time of the TV housing mold plate varies from 7 s to 19 s. For the product quality, steam heating showed an improvement in both the gloss and hardness of the TV housing. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction Injection molding is one of the most widely used processing technologies in the plastic industry. The mold surface temperature is critical in the plastic injection molding process. With a high mold surface temperature, the surface quality of the part will be better, although the cooling time will increase and, accordingly, the cycle time will rise as well. The decreasing of the mold surface temperature will reduce the cooling time, but there is no benefit for the surface quality of the part. A critical requirement for the current studies is to increase the mold surface temperature and still maintain a cycle time which is not too long. In recent years, the requirement for a much thinner, lighter and better mechanical performing product is more and more important for the company. There are many challenges and difficulties in producing this type of traditional injection molding. Problems such as warpage, flow mark and weld line often appear. Consequently, a new injection molding technology, rapid heat cycle molding (RHCM) is proposed. Contrasted with traditional injection molding, in RHCM, the mold temperature is increased to the target value, then, after the filling step is finished, the mold will be cooled with cold water. With the help of the heating process, the melt can easily fill into the cavity under a low injection pressure. Besides that, the surface defect such as weld line, flow mark, and floating fibers can be eliminated. For the heating process in RHCM, there are two main types of heating systems in use, surface heating and volume heating. In the previous group, several techniques have been researched. An insulation layer is ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (S.-C. Chen).
coated onto the mold base then a heating layer is applied to the insulation layer as the cavity surface. The heating layer can be quickly heated with a pair of electrodes and the insulation layer is used to enhance heating efficiency and decrease consumption [1,2]. On the other hand, for increasing the mold surface temperature in the filling process, a coating on the cavity surface with TiN and Teflon has reduced the heat transfer from the melt to the mold material, which increased the temperature on the cavity surface to 25 °C [3,4]. In another research project, on the heating surface, an electromagnetic induction coil with a different configuration was used to heat the cavity surface to reduce the weld line, shrinkage and other defects of the part surface [5,6]. Furthermore, an infrared heating system was also applied for heating the mold surface. This system can heat the surface of one or two mold halves using a suitable design [7,8]. For the newest application of surface heating, the hot air flowed into the cavity and heat convection from the hot air can directly heat the surface of the cavity [9]. The advantage of surface heating is the high rate in heating, so, the cycle time can be reduced. However, the user must have a special mold design when the mold is complex, and more equipment is needed to calculate the parameters for a high quality product. For volume heating, the most inexpensive way to achieve high mold temperature is to use hot water at a temperature as high as 90 °C or 100 °C for both heating and cooling. If the mold temperature needs to be higher than 100 °C, either a high pressure water supply system or a hot oil may be used [10]. The former may damage the channel connection and safety may be an issue after long-term use. Also, the latter may not be energy-efficient due to the low heat transfer coefficient of the oil. Local mold heating using an electric heating element is sometimes used to assist in high mold temperature control, especially for a thin-wall product. However, this still requires extra design and tool costs [11]. Further, a heater is usually used for
0735-1933/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.07.012
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auxiliary heating and is limited to increase in mold temperature of roughly several tens of degrees centigrade. Another research had used the water vapor in heating the mold with the special mold design. This research has increased the mold temperature from 30 °C to 110 °C to help the melt easily fill the cavity to reduce some defects in the product [12,13]. However, in the real mold for the TV housing of LCD (liquid crystal display), the heating and cooling channels are different. Therefore, both the heating and cooling efficiency are affected. In addition, with this design, the mold is much more complex and the cost of tooling will increase. In this study, a steam-assisted heating system combined with water cooling was applied for a simple model to establish the heating capacity of the steam in injection molding. Furthermore, for the real application, a TV housing mold was operated with the assistance of this system for confirming the efficiency of steam heating on the mold heating process and for product quality. In both designs, the heating and cooling processes were used with the same channel system. To gain efficiency in heating, at the end of the cooling period, an air flow will be blown into the channel for removing all the cooling water. The stages of steam, air and water will be considered. Besides the experiment, a 3D simulation was created for both models under different conditions and compared with the experiment results. 2. General process of steam heating in injection molding In this research, the water vapor (steam) will be used as the heating source to increase the cavity temperature of the injection mold. Then, the hot melt will be solidified by using cool water. The efficient use of steam heating will increase by the cleaning of water before it flows into the channels because of combining 2 different fluids for heating and cooling. Therefore, in general, the application of steam in injection molding will be achieved using the five steps as shown in Fig. 1. First, by opening the valve, the hot water vapor will flow into the channel. At this time, the air still appears inside the channel, so that the steam and air will be mixed together. At this point, the stage of these fluids is not
stable, and the result is the low heat transfer coefficient between the water vapor and the wall of the channel. At this time, the mold is still open after rejecting the previous part and is waiting for the next cycle. Second, when the steam flow stabilizes, all air is pushed out and only steam appears inside the channels. Therefore, the heat transfer coefficient will increase very fast. The energy transferred from steam to the mold plane will heat the cavity surface. This is the main heating process in steam heating. When the cavity surface is heated to a high temperature, the mold will completely close in preparation for the filling process. Third, when the heating process is finished, the temperature at the cavity surface will reach the target value for assistance in the filling and packing of the melt. At this time, the valve control will be switched for the water flow into the channels, and the cooling process will begin. At the beginning of this step, water will mix with steam and create an unstable stage, so, the heat transfer coefficient between channel and water is still very low. Fourth, after a short time, all the steam will be pressed out, and only water flows remain inside, so, the heat transfer coefficient will be increased, and the cooling efficient will be higher. Because of the heat relieving by cool water, the mold plane and melt will cool down to reject temperature. Fifth, after all melt reaches to the eject temperature, the mold will open for rejection of the part and prepare for the next cycle. At this time, the valve control unit will switch on for air flow into the channel to clean out all water. This step will finish when all channels are clear of water. In traditional injection molding, the cavity surface did not need heating. Hence, when the melt filled the mold cavity, the cavity surface was heated. After the filling process was finished, the melt was cooled down by cold water in the cooling system. In such a process, the quality of the product is not high, and the quality of the surface often has many problems which need to be solved. With the application of hot water vapor into injection molding, the cavity temperature in the filling period can be increased to a higher temperature, which helps the melt flows easily resulting in better packing. On the other hand, by using a low water temperature for the cooling period, the total cycle time is almost the same with traditional
Fig. 1. General process of steam heating.
M.-C. Jeng et al. / International Communications in Heat and Mass Transfer 37 (2010) 1295–1304 Table 1 The thermal property of cooling water and the steam [15]. Fluid
Temperature (°C)
Density (kg/m3)
Thermal conductivity (W/m K)
Dynamic viscosity (kg/ms)
Water Steam
20 150
998.1 0.516
0.5984 0.028
1.00E-03 1.40E-05
dimension of the coolant tank. Thermal properties of water are listed in Table 2. When fluid is steam or air, the heat transfer coefficient h can be calculated according to the following expression [12]:
h = 0:555x injection molding. Meanwhile, the melt flow ability can increase and melt viscosity can be maintained at a lower value. In addition, the surface brightness and harness also improve. 3. Theoretical model The government equation used to describe the heat transfer between fluid (steam, air and water) and mold is [15]: q = hΔt where: q is heat flux from fluid transfer to mold at the channel wall. Δt is the difference temperature between channel wall and fluid in each process. h is the heat transfer coefficient between the mold and fluid flow inside the channel system. When fluid is water, the heat transfer coefficient is calculated with the following correlation advised by Sleicher and Rouse [14]: 8 λ > > h = 5 + 0:015Rea Prb > > D > < 0:24 a = 0:88− > > 4 + Pr > > > : b = 0:333 + 0:5e−0:6Pr where λ is thermal conductivity of water, D is the diameter of the channel, the Reynolds number and the Prandl number are defined by: Re =
ρVL μ
Pr =
μCp VL λ
In which, ρ, V, μ and Cp are the density, flow velocity, viscosity and the specific heat of cooling water, respectively, and L is the characteristic
1297
" # 3 k ρw ðρw −ρs Þgr μDΔt
where k is the thermal conductivity of steam, g is the acceleration of gravity, r is the latent heat liquefaction, ρw and ρs are density of water and steam, and Δt is the difference temperature between steam and wall channel. Thermal property of steam is listed in Table 1. 4. Simulation and experiment work The mold temperature control process with steam heating consists of a steam system, a coolant system, a valve exchange unit, a control and monitor unit, and an injection molding machine. All system structures are shown in Fig. 2. For the connection between these units, the continuous lines present for the pipeline of steam, air and cool water. The steam source is made by a boiler to support enough high temperature steam for the heating process. The steam unit can reach to 10 kg/cm3 steam pressure with the chiller tank at 230 L and the max power is 20 kW. For the coolant system, a mold temperature control was used to provide a low water temperature to cool the mold. Because of the lower water temperature, the faster cooling, the 20 °C water temperature was used for all cooling process in this research. In addition, the velocity of the cooling water must be high enough to maintain the turbulent flow for cooling the mold. To improve the heating efficiency after the cooling step is finished; the air is forced into the channel system for cleaning all water and preparing a good contact of steam on the walls of the channel. On the other hand, there is a saving of energy because the fluids are recovered. A valve exchange unit can alternate the steam, air and cooling water flow in to the channel system by switching the status of corresponding control valves. The control and monitor system was used to control all units. A 3D thermal analysis of ANSYS on a simple mold plate was first run to check the analysis ability of steam heating. Fig. 3 shows the dimension and the mesh model of the simple plate. The material of plate was chosen as P20 with the density, specific heat and thermal conductivity at
Fig. 2. Mold temperature control units with steam heating.
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Fig. 3. Plate dimension and mesh model.
7800 kg/m3, 460 J/kg °C and 54 W/m °C, respectively. In simulation, the heat transfer mode around all external surfaces of plate was set at a free convection to the air; air temperature is 25 °C and heat transfer coefficient is 10 W/m2 K [15]. Multi cycles of steam heating were done based on this model. At the beginning, all plate was setup at 50 °C, and then in each cycle, 5 steps (as shown in Fig. 1) were setup for the heat convection at the channel wall. The heat transfer coefficient and fluid temperature inside channels are shown in Table 2. In general in simulation, the steam at 150 °C will heat plate until the surface temperature reaches to 140 °C, then, cool water (20 °C) will cool down the plate. Simply, by using the same channel, the plate will be heated by 150 °C steam until the surface reaches over 140 °C, then, it will be cooled down by 20 °C water. The temperature history at the point C and D of the plate was collected in multi cycles. Then, experiment result will be used to verify the simulation prediction at point C, and the comparison temperature between point C and D will show the delay time of temperature history. It is the same with simulation, a steel plate of 100 × 100 × 42.5 mm3 with 4 cooling channels (10 mm diameter with the center 15 mm beneath the surface) was used for the experiment. By using the steam system as in Fig. 3, this plate was heated by hot steam and cooled down by cool water. Then, the temperature distribution and value at the center of the surface plate were collected by an infrared thermal measurement system. This result will be used to verify the simulated prediction at point C. After that, a real application case for the TV housing mold was designed. The TV housing with 400 mm× 339.5 mm dimension is shown in Fig. 4. A P20 steel plate of 600 × 500 × 140 mm3 was designed with 8 channels (10 mm diameter and 15 mm beneath the cavity surface) for steam heating and water cooling. Fig. 5 shows the mold dimension, channel layout and the mesh model for the TV housing mold plate. For the operation of this case, the mold will first pre-heat by hot water to 50 °C, then; the cycle with steam heating, as shown in Fig. 1, will be applied. When the cavity surface at point P (Fig. 4) reaches to the
Table 2 The change of the heat transfer coefficient and the temperature of fluid in 1 cycle of steam assisted by injection molding. Process
1 2 3 4 5
Heat transfer coefficient (W/m2 K)
Temperature (°C)
3000 15,000 100 5000 100
150 150
Steam
Water
20 20 20
target temperature, the mold plate will close and the melt will start to fill cavity. For observing the effect of steam heating on heating time, the heating process for the mold plate will be run by steam 150 °C and then water 120 °C, next, the cooling process will be done with the same cool water at 20 °C. In all cycles, the temperature at point P will be measured and compared together. Then, with the same process and parameters, a simulation by ANSYS is done and compared with the experiment results. Finally, the same with the simple plate above, running coolant is 20 °C to cool down the mold, and steam at 150 °C for heating. By variance in the target temperature of the heating step from 70 to 110 °C, the temperature history was collected at point P for both heating and cooling. These results will be collected together with simulation results to show the effect of steam heating on the cavity temperature and time for heating. Besides that, for observation of the effect of steam heating and water heating on the product quality, the gloss and hardness at point P of the TV housing surface will be tested. 5. Results and discussions 5.1. Effect of steam heating on simple mold plate The variation in mold temperature (at the center of the surface) versus time under different step in 2 cycles was shown in Fig. 6a. By simulation with the same parameter in Table 1, the simulation and experiment result gets high agreement. Base on this result, the change of temperature at the center point has shown the effect of steam, air and cool water in 2 continuous cycles (from the heating stage until the final cooling step). It can be seen that with the first step, when hot steam is at 150 °C, it is ready to start for heating the mold, it will mix with the air which is still inside the channel, so that, the heat transfer coefficient will increase in time. In this time, the change of mold temperature is not clear. By simulation, the heat transfer coefficient for this period was detected at about 3000 W/m2K. However, when all the air was pressed out (step 2 in Fig. 1), the heat transfer coefficient will reach to the max value, and the temperature of the mold will increase very fast. When the heat transfer coefficient was set at 15,000 W/m2 K, the result will be the same with experiment. Next step, when the temperature at the center point of the mold surface reaches to the target value, the valve will be switched for 20 °C water flow into the channels. In this step, because hot steam is still inside the channel, there is a short time that water will mix with steam. This is the reason for the reduction of energy moving from hot mold to cool water. Moreover, the result is the lower cooling rate at the beginning of the cooling process (step 3 in Fig. 1). After that, when all steam was pressed out, the cooling rate was increased very quickly. At this time, the heat transfer coefficient between cool water and the
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Fig. 4. TV housing model and measured points.
channel wall was set at 5000 W/m2 K for simulation. When the temperature at the mold surface decreased to 40 °C, the cool water will be stopped, and the valve will be opened for the air flows into the channel for cleaning the water and preparing for the next cycle. Fig. 6b shows the temperature history between the surface (point C) and the near channel wall (point D). By 3D simulation, this result shows the
main property of volume heating; it is the delay of the temperature history between the heating location and the cavity surface. In this case, the different temperature between point C and D is about 5 °C. In the heating process and in the previous time of cooling, the temperature at point D is higher than point C; however, at the end of the cooling, the temperature at point C is higher than point D. The temperature
Fig. 5. Mold plate of the TV housing.
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Fig. 6. Temperature history at point C and D of the simple plate.
distributions (Fig. 7) also show these differences when point C reaches to 100 °C and 140 °C. In general, when combining hot steam, air and cool water into 1 cycle; the heat transfer coefficient will be changed by the change of the fluid type as well as mixing them quickly. In experiments, by using an infrared thermal measurement system, the temperature distribution at the mold surface was observed. Then, simulation and experiment results were compared together in Figs. 6a and 7. This result showed that the center temperature at the end of the period heating could reach to 140 °C. The higher temperature was concentrated at the center of the surface, and near the inlet or outlet connection. For this simple mold, the
mold surface temperature can increase from 50 °C to over 135 °C in 9 s (9 °C/s), and cool down to 50 °C in 44 s (2 °C/s). 5.2. Temperature distribution of the TV housing mold at the end of steam heating By simulation, the temperature distribution of the TV housing mold plate was observed (Fig. 8). Based on this result, in 3D and cross-section pictures, the high temperature of the mold plate is concentrated near the cavity surface. In this design, the channels were close to the cavity surface, so, both the heating and the cooling processes can get a higher
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1301
Fig. 7. Temperature distributions by steam heating and water cooling.
efficiency because the volume for heating was limited near the cavity surface (section F-F). At the end of heating process, the temperature range at the cavity surface is from 100 °C to 112 °C (section E-E). However, with the same simple plane, in the TV housing mold plane, simulation results showed that the temperature at the center of the channel system is higher than the cavity surface (section D-D). 5.3. The comparison between steam heating and water heating To compare the efficiency of steam heating and water heating, based on the 5 steps in Fig. 1, first the steam heating system was applied for the mold plate. Further, in the heating process, the steam was replaced by hot water at 120 °C. The target of the heating process is 80 °C, 1 cycle of temperature at point P (Fig. 4) was measured and compared together when steam and hot water were used for the heating stage. In these cases, cool water at 20 °C was used for cooling the mold. Fig. 9 shows the temperature history of these cases. By steam heating, the temperature at P can increase from 50 °C to 80 °C in 8 s, and then cool down until 50 °C in 12 s. However, when hot water was used for heating, it takes 18 s for raising the mold temperature from 50 °C to 80 °C, and 16 s for cooling down to 50 °C. Therefore, when steam was used for the TV housing mold, the time for the heating process can be reduced from 18 s to 8 s. This may be due to the fact that the heat transfer coefficient between the
steam and channel walls is much higher than the water and channel walls. On the other hand, the steam temperature can easily reach to the high temperature, conversely, with the high temperature, water stays at a high pressure. Therefore, the steam temperature is often higher than the water temperature. This is another reason for the higher efficiency of the steam heating method. For the cooling process, although the same 20 °C water temperature and the same flow rate are used, with the steam system, the cooling time is shorter than the water heating. This is because the steam heating rate is faster, and the volume heating, which is effected in the heating process, is smaller than the water heating, therefore, in the cooling step, the first is easier to cool down than the second. By simulation with the same parameter in Table 1, the temperature history at the center point was compared with experiment. In general, the simulation result and experiment had the same value. 5.4. Steam heating with different target temperatures Variations in target temperatures with heating times for the TV housing mold plate under 5 steps in each cycle can be found in Fig. 10. The result shows that the mold surface temperature can increase from 50 °C to 70, 80, 90, 100 and 110 °C after 7, 9, 13, 17 and 19 s, respectively. It can be seen that when the heating time increases from 7 s to 19 s, the mold surface temperature increases significantly. Both simulation and
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Fig. 8. Temperature distributions of various mold surfaces at the end of the steam heating process (Target temperature is 110 °C).
Fig. 9. Comparison of temperature history for the steam heating/water cooling and water heating/water cooling.
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Fig. 10. Steam heating and water cooling with different target temperature.
Finally, the PMMA + ABS at 245 °C were injected into the cavity, with the target temperature at 80 °C, and initial mold temperature at 50 °C. After that, by using the Multi gloss 268 m and based on the standard of ISO 2813, the gloss of the product was measured at location P (Fig. 4) with the range of 85°: 0–160 GU and a resolution of 0.1 GU. Fig. 11 shows the comparison of gloss when steam heating was used with the target temperature from 70 °C to 100 °C and melt temperature was fixed at 245 °C. It was found that with the higher target temperatures, the gloss was clearly improved. This can be explained in the effect of the heating
process. Mold temperature can be kept at high temperatures for a longer time than in water heating. Therefore, the filling and packing can be improved, making the contact between mold surface and melt improved. Hardness is one of the most important properties of a material. It can be used as an indicator of the polymer product. When the other parameters are fixed, based on the standard of ISO 868, type D and resolution of 1%, Fig. 12 shows the Shore D hardness of the TV housing (measured at point P, Fig. 4) when the steam heating process with different mold target temperatures was used. It can be seen that the higher the target temperature, the higher the hardness of the TV housing. This can be explained by the effect of crystallinity in the product after molding. As mentioned above, by using steam heating, the mold temperature at the cavity surface can rise to a higher value, hence, after a filling period, the melt can be kept at high temperatures for a longer time. In this period, the crystals will continuously increase until
Fig. 11. Gloss of the TV housing with different target temperatures.
Fig. 12. Hardness of the TV housing with different target temperatures.
experiment results show their positive agreement. In general, the rate of steam heating is about 3 °C/s. However, mold surface temperature will reach to the saturated value when the heating time is longer. This is due to the limit of the steam temperature which in this case is only 150 °C. 5.5. Effect of steam heating on the product quality
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melt temperature is lower than tg. This process shows that the heating time and target temperature are the important factors for hardness. 6. Conclusions In this study, a steam heating system combined with water cooling to achieve rapid mold temperature control for injection molding was established. The effect of steam heating and the uniformity of mold temperature for simple plate mold were first evaluated. Then the effect of steam heating was applied for a TV housing mold and compared with water heating. By experiment and simulation, the temperature at the surface was observed and compared. In addition, the product quality was also compared between two types of heating. Based on the result, the following conclusions can be made: • For the simple mold plate, steam can increase the mold surface from 50 °C to over 135 °C in 9 s (9 °C/s), and cool down to 50 °C in 44 s (2 °C/s). In this case, 5 periods of temperature history in each cycle can be observed clearly by experiment and simulation. • For the real TV housing mold, steam system had shown a higher efficiency in both heating and cooling. Compared with water heating, steam heating can reduce from 18 s to 8 s for the heating step and 16 s to 12 s for the cooling step. By changing the target temperature from 70 °C to 110 °C, the heating time varies from 7 s to 19 s, with the highest heating rate at about 3 °C/s. • Part quality is also improved when the steam was used for heating. Both hardness and groove were increased due to the high temperature maintained at the cavity surface in the filling and packing processes.
Acknowledgment The authors would like to thank the financial support from The Centerof-Excellence Program on Membrane Technology from the Ministry of Education and the project of the specific research fields in the Chung Yuan Christian University, Taiwan, under grant CYCU-98-CR-ME.
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International Communications in Heat and Mass Transfer j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i c h m t
Rapid mold temperature control in injection molding by using steam heating☆ Ming-Chang Jeng a, Shia-Chung Chen b,c,d,⁎, Pham Son Minh b,c,d, Jen-An Chang b,c,d, Chia-shen Chung a a
Department of Mechanical Engineering, National Central University, Chung-Li, Taiwan, ROC Department of Mechanical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC c R & D Center for Membrane Technology, Chung Yuan Christian University, Chung-Li 32023, Taiwan, ROC d R & D Center for Mold and Molding Technology, Chung Yuan Christian University, Chung-Li 32023, Taiwan ROC b
a r t i c l e
i n f o
Available online 4 August 2010 Keywords: Dynamic mold temperature control Steam-assisted heating Heat transfer coefficient
a b s t r a c t The rapid heating cycle has the advantage of improving product quality in injection molding. In this study, steam heating was combined with cool water on the same mold design to achieve dynamic mold surface temperature to establish control. By applying the steam system on a TV housing mold, the advantage of using steam heating for injection molding was then evaluated and compared with water heating by experiment and simulation. The effect of steam on the quality of the part was also studied. Results showed that as steam was used, the heating time of the simple mold plate can be reduced from 18 s to 8 s with the heating rate of 9 °C/s, and the cooling time is reduced over water heating. When the target temperature is changed from 70 °C to 110 °C, the heating time of the TV housing mold plate varies from 7 s to 19 s. For the product quality, steam heating showed an improvement in both the gloss and hardness of the TV housing. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction Injection molding is one of the most widely used processing technologies in the plastic industry. The mold surface temperature is critical in the plastic injection molding process. With a high mold surface temperature, the surface quality of the part will be better, although the cooling time will increase and, accordingly, the cycle time will rise as well. The decreasing of the mold surface temperature will reduce the cooling time, but there is no benefit for the surface quality of the part. A critical requirement for the current studies is to increase the mold surface temperature and still maintain a cycle time which is not too long. In recent years, the requirement for a much thinner, lighter and better mechanical performing product is more and more important for the company. There are many challenges and difficulties in producing this type of traditional injection molding. Problems such as warpage, flow mark and weld line often appear. Consequently, a new injection molding technology, rapid heat cycle molding (RHCM) is proposed. Contrasted with traditional injection molding, in RHCM, the mold temperature is increased to the target value, then, after the filling step is finished, the mold will be cooled with cold water. With the help of the heating process, the melt can easily fill into the cavity under a low injection pressure. Besides that, the surface defect such as weld line, flow mark, and floating fibers can be eliminated. For the heating process in RHCM, there are two main types of heating systems in use, surface heating and volume heating. In the previous group, several techniques have been researched. An insulation layer is ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (S.-C. Chen).
coated onto the mold base then a heating layer is applied to the insulation layer as the cavity surface. The heating layer can be quickly heated with a pair of electrodes and the insulation layer is used to enhance heating efficiency and decrease consumption [1,2]. On the other hand, for increasing the mold surface temperature in the filling process, a coating on the cavity surface with TiN and Teflon has reduced the heat transfer from the melt to the mold material, which increased the temperature on the cavity surface to 25 °C [3,4]. In another research project, on the heating surface, an electromagnetic induction coil with a different configuration was used to heat the cavity surface to reduce the weld line, shrinkage and other defects of the part surface [5,6]. Furthermore, an infrared heating system was also applied for heating the mold surface. This system can heat the surface of one or two mold halves using a suitable design [7,8]. For the newest application of surface heating, the hot air flowed into the cavity and heat convection from the hot air can directly heat the surface of the cavity [9]. The advantage of surface heating is the high rate in heating, so, the cycle time can be reduced. However, the user must have a special mold design when the mold is complex, and more equipment is needed to calculate the parameters for a high quality product. For volume heating, the most inexpensive way to achieve high mold temperature is to use hot water at a temperature as high as 90 °C or 100 °C for both heating and cooling. If the mold temperature needs to be higher than 100 °C, either a high pressure water supply system or a hot oil may be used [10]. The former may damage the channel connection and safety may be an issue after long-term use. Also, the latter may not be energy-efficient due to the low heat transfer coefficient of the oil. Local mold heating using an electric heating element is sometimes used to assist in high mold temperature control, especially for a thin-wall product. However, this still requires extra design and tool costs [11]. Further, a heater is usually used for
0735-1933/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2010.07.012
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auxiliary heating and is limited to increase in mold temperature of roughly several tens of degrees centigrade. Another research had used the water vapor in heating the mold with the special mold design. This research has increased the mold temperature from 30 °C to 110 °C to help the melt easily fill the cavity to reduce some defects in the product [12,13]. However, in the real mold for the TV housing of LCD (liquid crystal display), the heating and cooling channels are different. Therefore, both the heating and cooling efficiency are affected. In addition, with this design, the mold is much more complex and the cost of tooling will increase. In this study, a steam-assisted heating system combined with water cooling was applied for a simple model to establish the heating capacity of the steam in injection molding. Furthermore, for the real application, a TV housing mold was operated with the assistance of this system for confirming the efficiency of steam heating on the mold heating process and for product quality. In both designs, the heating and cooling processes were used with the same channel system. To gain efficiency in heating, at the end of the cooling period, an air flow will be blown into the channel for removing all the cooling water. The stages of steam, air and water will be considered. Besides the experiment, a 3D simulation was created for both models under different conditions and compared with the experiment results. 2. General process of steam heating in injection molding In this research, the water vapor (steam) will be used as the heating source to increase the cavity temperature of the injection mold. Then, the hot melt will be solidified by using cool water. The efficient use of steam heating will increase by the cleaning of water before it flows into the channels because of combining 2 different fluids for heating and cooling. Therefore, in general, the application of steam in injection molding will be achieved using the five steps as shown in Fig. 1. First, by opening the valve, the hot water vapor will flow into the channel. At this time, the air still appears inside the channel, so that the steam and air will be mixed together. At this point, the stage of these fluids is not
stable, and the result is the low heat transfer coefficient between the water vapor and the wall of the channel. At this time, the mold is still open after rejecting the previous part and is waiting for the next cycle. Second, when the steam flow stabilizes, all air is pushed out and only steam appears inside the channels. Therefore, the heat transfer coefficient will increase very fast. The energy transferred from steam to the mold plane will heat the cavity surface. This is the main heating process in steam heating. When the cavity surface is heated to a high temperature, the mold will completely close in preparation for the filling process. Third, when the heating process is finished, the temperature at the cavity surface will reach the target value for assistance in the filling and packing of the melt. At this time, the valve control will be switched for the water flow into the channels, and the cooling process will begin. At the beginning of this step, water will mix with steam and create an unstable stage, so, the heat transfer coefficient between channel and water is still very low. Fourth, after a short time, all the steam will be pressed out, and only water flows remain inside, so, the heat transfer coefficient will be increased, and the cooling efficient will be higher. Because of the heat relieving by cool water, the mold plane and melt will cool down to reject temperature. Fifth, after all melt reaches to the eject temperature, the mold will open for rejection of the part and prepare for the next cycle. At this time, the valve control unit will switch on for air flow into the channel to clean out all water. This step will finish when all channels are clear of water. In traditional injection molding, the cavity surface did not need heating. Hence, when the melt filled the mold cavity, the cavity surface was heated. After the filling process was finished, the melt was cooled down by cold water in the cooling system. In such a process, the quality of the product is not high, and the quality of the surface often has many problems which need to be solved. With the application of hot water vapor into injection molding, the cavity temperature in the filling period can be increased to a higher temperature, which helps the melt flows easily resulting in better packing. On the other hand, by using a low water temperature for the cooling period, the total cycle time is almost the same with traditional
Fig. 1. General process of steam heating.
M.-C. Jeng et al. / International Communications in Heat and Mass Transfer 37 (2010) 1295–1304 Table 1 The thermal property of cooling water and the steam [15]. Fluid
Temperature (°C)
Density (kg/m3)
Thermal conductivity (W/m K)
Dynamic viscosity (kg/ms)
Water Steam
20 150
998.1 0.516
0.5984 0.028
1.00E-03 1.40E-05
dimension of the coolant tank. Thermal properties of water are listed in Table 2. When fluid is steam or air, the heat transfer coefficient h can be calculated according to the following expression [12]:
h = 0:555x injection molding. Meanwhile, the melt flow ability can increase and melt viscosity can be maintained at a lower value. In addition, the surface brightness and harness also improve. 3. Theoretical model The government equation used to describe the heat transfer between fluid (steam, air and water) and mold is [15]: q = hΔt where: q is heat flux from fluid transfer to mold at the channel wall. Δt is the difference temperature between channel wall and fluid in each process. h is the heat transfer coefficient between the mold and fluid flow inside the channel system. When fluid is water, the heat transfer coefficient is calculated with the following correlation advised by Sleicher and Rouse [14]: 8 λ > > h = 5 + 0:015Rea Prb > > D > < 0:24 a = 0:88− > > 4 + Pr > > > : b = 0:333 + 0:5e−0:6Pr where λ is thermal conductivity of water, D is the diameter of the channel, the Reynolds number and the Prandl number are defined by: Re =
ρVL μ
Pr =
μCp VL λ
In which, ρ, V, μ and Cp are the density, flow velocity, viscosity and the specific heat of cooling water, respectively, and L is the characteristic
1297
" # 3 k ρw ðρw −ρs Þgr μDΔt
where k is the thermal conductivity of steam, g is the acceleration of gravity, r is the latent heat liquefaction, ρw and ρs are density of water and steam, and Δt is the difference temperature between steam and wall channel. Thermal property of steam is listed in Table 1. 4. Simulation and experiment work The mold temperature control process with steam heating consists of a steam system, a coolant system, a valve exchange unit, a control and monitor unit, and an injection molding machine. All system structures are shown in Fig. 2. For the connection between these units, the continuous lines present for the pipeline of steam, air and cool water. The steam source is made by a boiler to support enough high temperature steam for the heating process. The steam unit can reach to 10 kg/cm3 steam pressure with the chiller tank at 230 L and the max power is 20 kW. For the coolant system, a mold temperature control was used to provide a low water temperature to cool the mold. Because of the lower water temperature, the faster cooling, the 20 °C water temperature was used for all cooling process in this research. In addition, the velocity of the cooling water must be high enough to maintain the turbulent flow for cooling the mold. To improve the heating efficiency after the cooling step is finished; the air is forced into the channel system for cleaning all water and preparing a good contact of steam on the walls of the channel. On the other hand, there is a saving of energy because the fluids are recovered. A valve exchange unit can alternate the steam, air and cooling water flow in to the channel system by switching the status of corresponding control valves. The control and monitor system was used to control all units. A 3D thermal analysis of ANSYS on a simple mold plate was first run to check the analysis ability of steam heating. Fig. 3 shows the dimension and the mesh model of the simple plate. The material of plate was chosen as P20 with the density, specific heat and thermal conductivity at
Fig. 2. Mold temperature control units with steam heating.
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Fig. 3. Plate dimension and mesh model.
7800 kg/m3, 460 J/kg °C and 54 W/m °C, respectively. In simulation, the heat transfer mode around all external surfaces of plate was set at a free convection to the air; air temperature is 25 °C and heat transfer coefficient is 10 W/m2 K [15]. Multi cycles of steam heating were done based on this model. At the beginning, all plate was setup at 50 °C, and then in each cycle, 5 steps (as shown in Fig. 1) were setup for the heat convection at the channel wall. The heat transfer coefficient and fluid temperature inside channels are shown in Table 2. In general in simulation, the steam at 150 °C will heat plate until the surface temperature reaches to 140 °C, then, cool water (20 °C) will cool down the plate. Simply, by using the same channel, the plate will be heated by 150 °C steam until the surface reaches over 140 °C, then, it will be cooled down by 20 °C water. The temperature history at the point C and D of the plate was collected in multi cycles. Then, experiment result will be used to verify the simulation prediction at point C, and the comparison temperature between point C and D will show the delay time of temperature history. It is the same with simulation, a steel plate of 100 × 100 × 42.5 mm3 with 4 cooling channels (10 mm diameter with the center 15 mm beneath the surface) was used for the experiment. By using the steam system as in Fig. 3, this plate was heated by hot steam and cooled down by cool water. Then, the temperature distribution and value at the center of the surface plate were collected by an infrared thermal measurement system. This result will be used to verify the simulated prediction at point C. After that, a real application case for the TV housing mold was designed. The TV housing with 400 mm× 339.5 mm dimension is shown in Fig. 4. A P20 steel plate of 600 × 500 × 140 mm3 was designed with 8 channels (10 mm diameter and 15 mm beneath the cavity surface) for steam heating and water cooling. Fig. 5 shows the mold dimension, channel layout and the mesh model for the TV housing mold plate. For the operation of this case, the mold will first pre-heat by hot water to 50 °C, then; the cycle with steam heating, as shown in Fig. 1, will be applied. When the cavity surface at point P (Fig. 4) reaches to the
Table 2 The change of the heat transfer coefficient and the temperature of fluid in 1 cycle of steam assisted by injection molding. Process
1 2 3 4 5
Heat transfer coefficient (W/m2 K)
Temperature (°C)
3000 15,000 100 5000 100
150 150
Steam
Water
20 20 20
target temperature, the mold plate will close and the melt will start to fill cavity. For observing the effect of steam heating on heating time, the heating process for the mold plate will be run by steam 150 °C and then water 120 °C, next, the cooling process will be done with the same cool water at 20 °C. In all cycles, the temperature at point P will be measured and compared together. Then, with the same process and parameters, a simulation by ANSYS is done and compared with the experiment results. Finally, the same with the simple plate above, running coolant is 20 °C to cool down the mold, and steam at 150 °C for heating. By variance in the target temperature of the heating step from 70 to 110 °C, the temperature history was collected at point P for both heating and cooling. These results will be collected together with simulation results to show the effect of steam heating on the cavity temperature and time for heating. Besides that, for observation of the effect of steam heating and water heating on the product quality, the gloss and hardness at point P of the TV housing surface will be tested. 5. Results and discussions 5.1. Effect of steam heating on simple mold plate The variation in mold temperature (at the center of the surface) versus time under different step in 2 cycles was shown in Fig. 6a. By simulation with the same parameter in Table 1, the simulation and experiment result gets high agreement. Base on this result, the change of temperature at the center point has shown the effect of steam, air and cool water in 2 continuous cycles (from the heating stage until the final cooling step). It can be seen that with the first step, when hot steam is at 150 °C, it is ready to start for heating the mold, it will mix with the air which is still inside the channel, so that, the heat transfer coefficient will increase in time. In this time, the change of mold temperature is not clear. By simulation, the heat transfer coefficient for this period was detected at about 3000 W/m2K. However, when all the air was pressed out (step 2 in Fig. 1), the heat transfer coefficient will reach to the max value, and the temperature of the mold will increase very fast. When the heat transfer coefficient was set at 15,000 W/m2 K, the result will be the same with experiment. Next step, when the temperature at the center point of the mold surface reaches to the target value, the valve will be switched for 20 °C water flow into the channels. In this step, because hot steam is still inside the channel, there is a short time that water will mix with steam. This is the reason for the reduction of energy moving from hot mold to cool water. Moreover, the result is the lower cooling rate at the beginning of the cooling process (step 3 in Fig. 1). After that, when all steam was pressed out, the cooling rate was increased very quickly. At this time, the heat transfer coefficient between cool water and the
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Fig. 4. TV housing model and measured points.
channel wall was set at 5000 W/m2 K for simulation. When the temperature at the mold surface decreased to 40 °C, the cool water will be stopped, and the valve will be opened for the air flows into the channel for cleaning the water and preparing for the next cycle. Fig. 6b shows the temperature history between the surface (point C) and the near channel wall (point D). By 3D simulation, this result shows the
main property of volume heating; it is the delay of the temperature history between the heating location and the cavity surface. In this case, the different temperature between point C and D is about 5 °C. In the heating process and in the previous time of cooling, the temperature at point D is higher than point C; however, at the end of the cooling, the temperature at point C is higher than point D. The temperature
Fig. 5. Mold plate of the TV housing.
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Fig. 6. Temperature history at point C and D of the simple plate.
distributions (Fig. 7) also show these differences when point C reaches to 100 °C and 140 °C. In general, when combining hot steam, air and cool water into 1 cycle; the heat transfer coefficient will be changed by the change of the fluid type as well as mixing them quickly. In experiments, by using an infrared thermal measurement system, the temperature distribution at the mold surface was observed. Then, simulation and experiment results were compared together in Figs. 6a and 7. This result showed that the center temperature at the end of the period heating could reach to 140 °C. The higher temperature was concentrated at the center of the surface, and near the inlet or outlet connection. For this simple mold, the
mold surface temperature can increase from 50 °C to over 135 °C in 9 s (9 °C/s), and cool down to 50 °C in 44 s (2 °C/s). 5.2. Temperature distribution of the TV housing mold at the end of steam heating By simulation, the temperature distribution of the TV housing mold plate was observed (Fig. 8). Based on this result, in 3D and cross-section pictures, the high temperature of the mold plate is concentrated near the cavity surface. In this design, the channels were close to the cavity surface, so, both the heating and the cooling processes can get a higher
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Fig. 7. Temperature distributions by steam heating and water cooling.
efficiency because the volume for heating was limited near the cavity surface (section F-F). At the end of heating process, the temperature range at the cavity surface is from 100 °C to 112 °C (section E-E). However, with the same simple plane, in the TV housing mold plane, simulation results showed that the temperature at the center of the channel system is higher than the cavity surface (section D-D). 5.3. The comparison between steam heating and water heating To compare the efficiency of steam heating and water heating, based on the 5 steps in Fig. 1, first the steam heating system was applied for the mold plate. Further, in the heating process, the steam was replaced by hot water at 120 °C. The target of the heating process is 80 °C, 1 cycle of temperature at point P (Fig. 4) was measured and compared together when steam and hot water were used for the heating stage. In these cases, cool water at 20 °C was used for cooling the mold. Fig. 9 shows the temperature history of these cases. By steam heating, the temperature at P can increase from 50 °C to 80 °C in 8 s, and then cool down until 50 °C in 12 s. However, when hot water was used for heating, it takes 18 s for raising the mold temperature from 50 °C to 80 °C, and 16 s for cooling down to 50 °C. Therefore, when steam was used for the TV housing mold, the time for the heating process can be reduced from 18 s to 8 s. This may be due to the fact that the heat transfer coefficient between the
steam and channel walls is much higher than the water and channel walls. On the other hand, the steam temperature can easily reach to the high temperature, conversely, with the high temperature, water stays at a high pressure. Therefore, the steam temperature is often higher than the water temperature. This is another reason for the higher efficiency of the steam heating method. For the cooling process, although the same 20 °C water temperature and the same flow rate are used, with the steam system, the cooling time is shorter than the water heating. This is because the steam heating rate is faster, and the volume heating, which is effected in the heating process, is smaller than the water heating, therefore, in the cooling step, the first is easier to cool down than the second. By simulation with the same parameter in Table 1, the temperature history at the center point was compared with experiment. In general, the simulation result and experiment had the same value. 5.4. Steam heating with different target temperatures Variations in target temperatures with heating times for the TV housing mold plate under 5 steps in each cycle can be found in Fig. 10. The result shows that the mold surface temperature can increase from 50 °C to 70, 80, 90, 100 and 110 °C after 7, 9, 13, 17 and 19 s, respectively. It can be seen that when the heating time increases from 7 s to 19 s, the mold surface temperature increases significantly. Both simulation and
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Fig. 8. Temperature distributions of various mold surfaces at the end of the steam heating process (Target temperature is 110 °C).
Fig. 9. Comparison of temperature history for the steam heating/water cooling and water heating/water cooling.
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Fig. 10. Steam heating and water cooling with different target temperature.
Finally, the PMMA + ABS at 245 °C were injected into the cavity, with the target temperature at 80 °C, and initial mold temperature at 50 °C. After that, by using the Multi gloss 268 m and based on the standard of ISO 2813, the gloss of the product was measured at location P (Fig. 4) with the range of 85°: 0–160 GU and a resolution of 0.1 GU. Fig. 11 shows the comparison of gloss when steam heating was used with the target temperature from 70 °C to 100 °C and melt temperature was fixed at 245 °C. It was found that with the higher target temperatures, the gloss was clearly improved. This can be explained in the effect of the heating
process. Mold temperature can be kept at high temperatures for a longer time than in water heating. Therefore, the filling and packing can be improved, making the contact between mold surface and melt improved. Hardness is one of the most important properties of a material. It can be used as an indicator of the polymer product. When the other parameters are fixed, based on the standard of ISO 868, type D and resolution of 1%, Fig. 12 shows the Shore D hardness of the TV housing (measured at point P, Fig. 4) when the steam heating process with different mold target temperatures was used. It can be seen that the higher the target temperature, the higher the hardness of the TV housing. This can be explained by the effect of crystallinity in the product after molding. As mentioned above, by using steam heating, the mold temperature at the cavity surface can rise to a higher value, hence, after a filling period, the melt can be kept at high temperatures for a longer time. In this period, the crystals will continuously increase until
Fig. 11. Gloss of the TV housing with different target temperatures.
Fig. 12. Hardness of the TV housing with different target temperatures.
experiment results show their positive agreement. In general, the rate of steam heating is about 3 °C/s. However, mold surface temperature will reach to the saturated value when the heating time is longer. This is due to the limit of the steam temperature which in this case is only 150 °C. 5.5. Effect of steam heating on the product quality
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melt temperature is lower than tg. This process shows that the heating time and target temperature are the important factors for hardness. 6. Conclusions In this study, a steam heating system combined with water cooling to achieve rapid mold temperature control for injection molding was established. The effect of steam heating and the uniformity of mold temperature for simple plate mold were first evaluated. Then the effect of steam heating was applied for a TV housing mold and compared with water heating. By experiment and simulation, the temperature at the surface was observed and compared. In addition, the product quality was also compared between two types of heating. Based on the result, the following conclusions can be made: • For the simple mold plate, steam can increase the mold surface from 50 °C to over 135 °C in 9 s (9 °C/s), and cool down to 50 °C in 44 s (2 °C/s). In this case, 5 periods of temperature history in each cycle can be observed clearly by experiment and simulation. • For the real TV housing mold, steam system had shown a higher efficiency in both heating and cooling. Compared with water heating, steam heating can reduce from 18 s to 8 s for the heating step and 16 s to 12 s for the cooling step. By changing the target temperature from 70 °C to 110 °C, the heating time varies from 7 s to 19 s, with the highest heating rate at about 3 °C/s. • Part quality is also improved when the steam was used for heating. Both hardness and groove were increased due to the high temperature maintained at the cavity surface in the filling and packing processes.
Acknowledgment The authors would like to thank the financial support from The Centerof-Excellence Program on Membrane Technology from the Ministry of Education and the project of the specific research fields in the Chung Yuan Christian University, Taiwan, under grant CYCU-98-CR-ME.
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