Abstract
A hot gas is used for heating the cavity surface of a mold. Different mold gap sizes were designed. The mold surface temperature was heated to above the glass transition temperature of the plastic material, and the mold then closed for melt filling. The cavity surface can be heated to 130°C to assist the melt filling of the microfeatures. Results show that hot gas heating can improve the filling process and achieve 91% of the high aspect ratio microgrooves (about 640.38 μm of the maximum of 700 μm). The mold gap size strongly affects the heating speed and heating uniformity. Without surface preheating, the center rib is the highest. When the heating target temperature is 90°C or 100°C, the three microribs have a good uniformity of height. However, when the target temperature exceeds 100°C, the left side rib is higher than the other ribs.
1. Introduction
Nowadays, injection molding is one of the most widely used processing technologies in the manufacture of plastic products. Among typical molding parameters, the mold surface temperature is critical. At higher mold surface temperatures, the surface quality of the part will improve [1, 2]. In the injection molding field, microinjection molding is used to manufacture a variety of polymer components, because of its low cost and potential for high-volume production. Most applications are in the field of microoptics (such as CDs and DVDs) and microfluidic devices. Production of other molded microoptical components including optical gratings, optical switches, and waveguides [3–5] as well as a variety of molded microfluidic devices including pumps, capillary analysis systems, and lab-on-a-chip applications [6, 7] is ongoing.
In general, to improve an injection molding part, it requires higher mold temperatures during injection to minimize part thickness and injection pressure. However, maintaining high mold temperature during the filling process and lowering the mold temperature to below the deflection temperature during the postfilling process, while avoiding great increases in cycle time and energy consumption, is not easy. To address this problem, a variety of dynamic mold temperature controls (DMTC) have been explored in recent years. Their purpose is to eliminate the frozen layer, ideally producing a hot mold during the filling stage and a cold mold for cooling. The most inexpensive way to achieve high mold temperature is to use cooling water at temperatures as high as 90°C or 100°C [8].
Local mold heating using an electric heater [9] is sometimes used to assist high mold temperature control. However, this requires additional design and tool costs. Further, electrical heating is usually used as auxiliary heating and is limited to increases in mold temperature of roughly several tens of degrees centigrade.
Mold surface heating, such as induction heating [10–12], high-frequency proximity heating [13, 14], and gas-assisted mold temperature control (GMTC) [15, 16], can provide sufficient heating rates without significant increases in cycle time. In recent years, we have conducted systematic study of mold surface heating and mold surface localization heating of the processing characteristics.
In this study, gas-assisted mold temperature control (GMTC) combined with water cooling is used with different mold gap sizes (4 mm, 6 mm, and 8 mm) to achieve rapid mold surface temperature control for high aspect ratio microinjection molding. A set of systematic experiments were conducted to correlate the effect of heating conditions, including heating efficiency and temperature distribution uniformity. The feasibility of gas-assisted heating for mold surface temperature control during the injection process to improve the microfeatures was evaluated.
2. Experimental Method
Gas-assisted mold temperature control (GMTC) is a new technique in the field of mold temperature control, which can heat and cool the cavity surface rapidly during the injection molding process. In general, the goal of mold temperature control is to increase the mold surface to the target temperature before filling with the melt and then cooling the melt to the ejection temperature. In this research, the GMTC system consists of a GMTC controller, a hot-gas generator system, and a water mold temperature controller, as shown in Figure 1.
Schematic of the GMTC system.
The hot gas generator consists of an air compressor, an air dryer, a digital volumetric flow controller, and a high efficiency gas heater. The function of the high power hot gas generator system is to support a heat source, which provides a flow of hot air up to 500°C with a flow rate up to 500 l/min. For the coolant system, a mold temperature control was used to provide water at a defined temperature to cool the mold after the filling process and to warm the mold to the initial temperature at the beginning of the process. The valve system was used to control the water for the cooling channels and the air for the heating stage. To both control and observe the temperature at the cavity surface, two temperature sensors were used to obtain the real time mold temperature and to provide feedback to the GMTC controller. After achieving the target temperature, the gas valve closes and the mold will then completely close for the melt injection. A Sodick-TR85EH injection molding machine is used for the molding experiments.
In this paper, the hot gas will be used as a heating source to increase the cavity surface temperature of the injection mold. After the filling process is finished, the hot melt is solidified using the cool water. During operation, first, when the mold is closed, the core will move to the heating position (Figure 2—step 1). Next, the hot gas will flow into the cavity, pushing out the cool air and leaving only the hot gas (Figure 2—step 2). Therefore, the heat transfer coefficient will rapidly increase. The energy transferred from the hot gas to the mold wall will heat the cavity surface. This is the heating process of gas-assisted mold surface heating in injection molding. Finally, when the cavity surface is heated to the target temperature for assistance in the filling and packing of the melt, the mold will completely close in preparation for the filling process (Figure 2—step 3).
Mold position in the heating stage of GMTC process.
Figures 3 and 4 show the injection mold with the microstructure blocks inserted into the center of the mold. The heating area is 80 mm × 40 mm which covers the molding area of 60 mm × 25 mm. The structure of the gas flow, gas inlet, and gas outlet channel was built into the mold for ease of operation during the gas heating period to the melt injection. For observing the heating effect of the microstructure, three types of temperature sensors were used.
Experimental mold design and microstructure block insert.
Dimensions of microstructure block insert (a) and microstructure block insert (b).
Figure 5 shows the sensor positions: one near the cavity surface (Type A), one 0.3 mm beneath the groove (Type B), and one 0.3 mm (Type C) beneath the cavity surface. The temperature measurement was performed at the location of the three microgrooves (gas inlet, center, and gas outlet groove). The ABS PA758 plastic, which has a glass transition temperature of about 105°C, was used as the molding resin. The operating parameters are shown in Table 1.
Experimental parameters for microinjection molding.
Sensor location.
3. Results and Discussions
3.1. Effect of GMTC on the Heating Process
The variation in the mold temperature (at the center area—Figure 5) versus time for a heating time of 50 s is described in Figure 6. For an initial mold temperature of 70°C, the GMTC can heat the temperature at the groove bottom to above 130°C. After approximately 2 seconds as the mold closes completely (Step 3 in Figure 2), the groove temperature will cool to about 110°C. This temperature value is higher than the glass transition temperature of ABS PA 758 material.
Temperature history with gas heating and water cooling of center sensors.
In our former study, when the GMTC was used for mold surface heating, there is a temperature difference between the inlet and outlet area [2, 15]. Therefore, in this research, to evaluate the uniformity of the heating process for various mold gap sizes, the temperature at the gas inlet area and the gas outlet area was collected. In previous research, the GMTC heated the mold surface very efficiently [2, 11]. However, in this research, because the GMTC is applied to improve the filling of the melt into the microgroove, the temperature at the bottom groove is collected and compared in Figure 7. Based on these results, the mold gap size has a clear impact on the temperature uniformity of the inlet and outlet area. When the mold gap size is larger, the difference in temperature between the inlet and outlet sensor was reduced. Figure 7 shows that the temperature difference is 44.3°C, 11.3°C, and 0.3°C when the mold gap size is 4 mm, 6 mm, and 8 mm, respectively. This result is in good agreement with our previous research [2, 15].
Comparisons of heating uniformity results for the various gas inlet designs (initial temperature: 70°C, heating time: 50 seconds, sensor type B).
Using an infrared camera, the temperature distribution with the mold gap size of 4 mm, 6 mm, and 8 mm was observed and compared in Figure 8. In the heating process, this result also shows that a higher temperature is concentrated near the gas inlet gate. However, the larger the mold gap size is, the more uniform the temperature becomes. This result is in good agreement with Figure 7.
Temperature distribution of the molding area with an initial temperature of 70°C, a gas temperature of 500°C, a gas flow rate of 400 l/min, and a heating time of 50 s.
3.2. Effect of Gas-Assisted Mold Surface Heating on the Microstructure
For high aspect ratio microinjection molding, the mold surface temperature was heated from 70°C to the target temperatures of 90, 100, 110, 120, and 130°C. After heating, the mold plates required roughly two seconds to close. The injection molding cycle was then completed. After the molding cycle finished, the molded part was removed for measurement of the microribs. Figure 9 shows the molding product and the microstructure (micro rib).
Molded product (a) and microstructure (b).
For the application of GMTC on the melt filling into the microgroove, a melt of ABS PA758 was injected into the cavity, with a mold temperature of 70°C. The molding conditions are shown in Table 1, where the part size is 60 mm × 25 mm × 1 mm. After molding, a height comparison of the micro rib was performed. With regular molding operations, the mold temperature is set at 70°C, meaning that the cavity temperature at the filling state remains at 80°C. With GMTC, the mold surface temperature can rise from 70°C to the heating target temperature. The mold then needs 2 s to close completely. The real mold surface temperature during the melt injection is about 230 ± 5°C.
Figure 10 shows a 3D laser microscope image of the center rib under different heating target temperatures. Based on these results, it is clear that without the GMTC, the height of the rib is only 212.37 μm. Using GMTC with its higher heating target temperatures, a higher rib can be reached. The highest rib is 640.38 μm when the heating target temperature is 130°C. This provides clear evidence of the effect of GMTC on high aspect ratio microinjection molding. In this study, the microgroove with a maximum depth of 700 μm can be filled to over 91% of its full height.
3D laser microscope images of the plastic stamp at the center point.
To observe the effect of temperature uniformity on the height of the microribs, using the same molding process as in Table 1, the rib height was measured under different heating target temperatures. The results are shown in Table 2 and compared in Figure 11. At a mold temperature of 70°C, the center rib (at point P2) is the highest (212.37 μm), while the left and right side ribs (Figure 11) are clearly lower. In the next case, when the mold surface was preheated to 90°C, the height of the center rib increases slightly. However, with the left and right side ribs, the height has a significant improvement (87% higher than without GMTC). This strength is maintained as the heating target temperature rises to 130°C˙ At this temperature target, all ribs were filled to over 91% of the full height.
Experimental results for the microstructure.
Height of micro-structure at different heating target temperatures.
Comparison of the height of the ribs shows that without heating, the center rib is the highest, and the left and right side ribs are much lower. This is because in this part design, the center rib is closest to the injection gate. Therefore, the filling pressure and the packing pressure are much higher than for the other ribs. This difference in height negatively affects product quality. However, this difference was reduced when the GMTC was used at target temperatures of 90°C and 100°C. This result may be explained by the temperature distribution at the end of the heating step as shown in Figure 8. Using the GMTC, the molding area was heated and the filling step of the molding cycle was assisted. Therefore, melt filled into the groove more easily, resulting in higher ribs with better rib uniformity. When the heating target temperature exceeded 100°C, the height of the three ribs increased continuously. However, the left side rib (at point P1) became the highest. This result is due to the heating effect of the GMTC. In this case, the surface temperature near the gas inlet is higher than the other areas, meaning that the melt filled into the left side rib more easily than into the other two ribs.
4. Conclusions
In this study, a gas-assisted mold surface heating system combined with water-cooling to achieve rapid mold temperature control for microstructure injection molding was established. The effect of GMTC and the uniformity of the rib height were evaluated. Based on the results, the following conclusions were obtained.
The mold gap size at the heating position can affect the heating speed and heating uniformity.
By using the GMTC for preheating, the temperature of the microgroove can be higher than the glass transition temperature of ABS PA 758.
The application of hot gas heating could improve the height of the microrib to 91% of the maximum height of 700 μm (640.38 μm).
At heating target temperatures of 90°C and 100°C, the uniformity of the rib was improved. However, when the heating target temperature exceeded 100°C, the left side rib became higher than the other two ribs.
Footnotes
Acknowledgment
This research was supported by the Center-of-Excellence Program on Membrane Technology of the Ministry of Education and the R&D Center for Mold and Molding Technology.