Rapid mold temperature variation for assisting the micro injection of high aspect ratio micro-feature parts using induction heating technology

Abstract

Hot embossing and injection molding are popular methods to duplicate micro features formed during polymer micro-fabrication of MEMS devices. However, both methods face challenges in filling the polymer melt completely into a micro-featured geometry of a high aspect ratio. In this study, electromagnetic induction heating combined with water cooling is used to achieve rapid mold surface temperature control during the micro-feature injection molding process. A CAE simulation was also developed through integration of both thermal and electromagnetic analysis modules of ANSYS, and its capability and accuracy were verified experimentally. Efficiency evaluations of induction heating and the uniformity of mold temperature control were conducted on a micro-featured mold. This mold was designed with a micro channel array of 30–50 µm in width and 120 and 600 µm in depth, corresponding to aspect ratios ranging from about 2.4 to 12. The accuracies of the micro channels in molded PMMA parts can be used to evaluate the effect of mold temperature on replication accuracy. It was found that rapid mold surface heating with temperature rising from 60 °C to between 100 °C and 140 °C by induction heating requires 2–3.5 s, while the mold temperature returns to 60 °C in about 70–110 s. The simulated mold surface temperature results are consistent with measured results. Achieving the same temperature variation by switching circulation coolants of different temperatures requires at least 7 min. The simulation also reveals that the electromagnetic wave can penetrate into the bottom of the micro channel and results in only about a 2 °C difference in temperature uniformity. For mold temperatures of 100 °C, 120 °C and 140 °C, the molded channel depths were 94.9 µm, 105.4 µm and 116.0 µm, respectively, when the ideal channel depth was 120 µm. When the channel depth is 600 µm, the mold temperature must exceed 120 °C, so that reasonable accuracy in micro-feature replication can be achieved. Our results to date indicate that the aspect ratio for molded PMMA micro channels can be as high as 12. Efficient mold temperature variation by induction heating to improve the replication accuracy in molding micro features is successfully illustrated.

Three-dimensional numerical modeling of an induction heated injection molding tool with flow visualization

“…S. C. Chen et al [2][3][4][5][6][7] have investigated induction heating with experiments and simulations on weld lines and their strengths, micro-features with high aspect ratio, comparing with electrical heaters, and pulsed cooling. Overall conclusions were induction heating had faster heating, using multiturn coil gives better temperature uniformity and that induction heating does improve the filling of micro features/cavities and weld lines strength, as well as their visual appearance.…”

Gas-Assisted Heating Technology for High Aspect Ratio Microstructure Injection Molding

“…Mold surface heating, such as induction heating [10][11][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.…”

Polymer bonding by induction heating for microfluidic applications

“…Induction heating is employed for forging, extrusion, welding, and brazing, melting of metals, hardening of steel, and annealing [76; 77; 78]. It can also be used for processing small parts, for instance annealing copper crimp connectors or brazing diamond tips on cutting tools as shown in Figure 2 Induction heating is used for rapid temperature variations during micro injection moulding to achieve structures with high aspect ratio [79]. It can also be used for heat staking as shown in A metal thread is heated up inductively and inserted into a plastic part.…”