Shia-Chung Chen

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

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.

Study on rheological behavior of polymer melt flowing through micro-channels considering the wall-slip effect

Micro molding is attracting more attention nowadays and determination of the rheological behavior of the polymer melt within micro structured geometry is considered to be very important for the accurate simulation modeling of micro molding. The lack of commercial equipment is one of the main hurdles in the investigation of micro melt rheology. In this study, the melt viscosity measurement system for PS (polystyrene) melt flowing through a micro-channel was established using a micro-channel mold operated at a mold temperature as high as the melt temperature. From measured pressure drop and volumetric flow rate both the capillary flow model and the slit flow model were used for the calculation of viscosity utilizing Rabinowitsch and Walters corrections. It was found that the measured viscosity values in the test ranges are significantly lower (decreased by a factor of about 1.4–4.1) than those obtained from the traditional capillary rheometer at a melt temperature of 200 °C using both the capillary flow model and the slit flow model. As the micro-channel size decreases, the reduction in the viscosity value increases when compared with data obtained from the traditional capillary rheometer. The ratio of slip velocity relative to mean velocity was also found to increase with decreasing size of micro-channels. It seems that wall slip plays a dominant role when melt flows through micro-channels and would result in a greater percentage in apparent viscosity reduction when the size of the micro-channel decreases. In addition, the wall-slip effect becomes more significant as the melt temperature increases. In the present study we emphasize that the rheological behavior of the melt in the microscopic scale is different from that of the macroscopic scale and that current simulation packages are not suitable for micro molding simulation without considering this difference.

Study on the molding characteristics and mechanical properties of injection-molded foaming polypropylene parts

This study investigates the molding characteristics and mechanical properties of injection-molded foaming polypropylene (PP) parts and coinjection-molding PP parts for foaming core material embedded in nonfoaming skin material. Effects of processing parameters including injection-velocity, melting temperature, mold temperature and back pressure on part weight and part mechanical properties including tensile strength, flexural strength and stiffness are investigated. Influence of part thickness and foaming agent content on the degree of foaming is also studied. Based on the measured results, it was found that for thin-wall tensile foam specimens with 0.5 mm thickness, weight reduction is about 4–9% whereas in the thick-wall bending foam specimens with 15 mm thickness, the reduction is approximately 43–50%. Part thickness is a dominant factor to determine the degree of foaming. For injection-molded foaming parts, part weight, tensile strength, flexural strength and stiffness decrease with increasing melt temperature, mold temperature and injection velocity, whereas they increase with increasing back pressure. Foaming results in weight reduction but it also reduces part mechanical properties. The foaming agent content increases from 0.8 to 1.6%, the mechanical properties reduces significantly. Moreover, the coinjection-molding thin-wall parts process for foaming core material embedded in nonfoaming skin material results in a foam part with very good esthetics and higher tensile strength than injection-molded foaming parts. Meanwhile, the foam core is well bonding to the nonfoam skin in the coinjection-molding parts. The present investigation provides a molding guideline for injection-molded foaming parts and coinjection-molding parts to achieve a specific objective of part quality.

Three-dimensional simulations of the droplet formation during the inkjet printing process

The digital inkjet printing technology has been widely explored by the electronic industry in developing new manufacture processes. To shorten the design cycle, numerical simulations seem to be the required tool. In this paper, three-dimensional simulations of printhead printing process based on a numerical scheme comprised of a finite volume formulation for discretizing governing equations of the flow field and a volume-of-fluid method to predict the fluid interface are presented. The surface tension is modeled using a continuum surface force concept and thus computed as a function of the interfacial curvature. The contact angle between the fluid and the solid wall is explicitly enforced at the fluid interface. The parallel computation based on a domain decomposition strategy is employed to reduce the massive computation time required by fully three-dimensional computations. The numerical predictions of meniscus shape are similar to those published measurement results.

Simulation of injection–compression-molding process. II. Influence of process characteristics on part shrinkage

A numerical algorithm is developed to simulate the injection–compression molding (ICM) process. A Hele–Shaw fluid-flow model combined with a modified control-volume/finite-element method is implemented to predict the melt-front advancement and the distributions of pressure, temperature, and flow velocity dynamically during the injection melt filling, compression melt filling, and postfilling stages of the entire process. Part volumetric shrinkage was then investigated by tracing the thermal–mechanical history of the polymer melt via a path display in the pressure–volume–temperature (PVT) diagram during the entire process. Influence of the process parameters including compression speed, switch time from injection to compression, compression stroke, and part thickness on part shrinkage were understood through simulations of a disk part. The simulated results were also compared with those required by conventional injection molding (CIM). It was found that ICM not only shows a significant effect on reducing part shrinkage but also provides much more uniform shrinkage within the whole part as compared with CIM. Although using a higher switch time, lower compression speed, and higher compression stroke may result in a lower molding pressure, however, they do not show an apparent effect on part shrinkage once the compression pressure is the same in the compression-holding stage. However, using a lower switch time, higher compression speed, and lower compression stroke under the same compression pressure in the postfilling stage will result in an improvement in shrinkage reduction due to the melt-temperature effect introduced in the end of the filling stage.

Effects of shape, porosity, and operating parameters on carbon dioxide recovery in polytetrafluoroethylene membranes.

In this study, the recovery of carbon dioxide using an absorbent composed of 2-amino-2-methyl-l-propanol (AMP)+monoethanolamine (MEA)+piperazine (PZ) in polytetrafluoroethylene (PTFE) membrane contactors was investigated. Experiments were conducted using various gas flow rates, liquid flow rates, absorbent blends, and pore size membranes. CO(2) recovery increased with increasing liquid flow rates. The blended amine absorbent had a synergistic effect on CO(2) recovery. CO(2) recovery increased as the pore size of the PTFE membrane decreased. An asymmetric membrane had a better CO(2) recovery than that of symmetric membrane. Besides, membrane mass transfer coefficient and operational stability of asymmetric membrane were enhanced. For an asymmetric membrane, the smaller pore-size side of the membrane surface contacting the liquid phase can reduce the level of wetting of the membrane.

Chemical absorption of carbon dioxide with asymmetrically heated polytetrafluoroethylene membranes

In this study, the absorption of carbon dioxide using an absorbent composed of 2-amino-2-methyl-L-propanol (AMP) + monoethanolamine (MEA) + piperazine (PZ) in asymmetric and symmetric polytetrafluoroethylene (PTFE) membrane contactors was investigated. Experiments were conducted using various gas flow rates, liquid flow rates, and absorbent blends. CO(2) recovery increased with increasing liquid flow rates. The mean pore size of PTFE membrane reduced via heating treatment. An asymmetric membrane had a better CO(2) recovery than a symmetric membrane. For the asymmetric membrane, placing the smaller pore-size side of the membrane in contact with the liquid phase, reduced the level of wetting of the membrane. The membrane mass transfer coefficient and durability of the PTFE membrane were enhanced by asymmetrically heating.

Investigations of the tensile properties on polycarbonate thin-wall injection molded parts

Effect of processing conditions including injection speed, melt temperature, mold temperature and packing pressure on tensile properties of polycarbonate (PC) thin-wall parts were investigated. Tensile test specimen with thickness of 2.5, 1.0 and 0.8 mm were injection molded under specified conditions. Tensile properties and residual stresses were measured experimentally. It was found that the residual stress plays a more significant role in influencing part tensile properties than flow-induced molecular orientation. Part thickness and injection speed are two key factors that affect residual stress most significantly within the current molding window. As part thickness decreases, residual stress increases resulting in the greater influence in reduction of part tensile strength, yield stress as well as Young’s modulus. Higher melt temperature and higher mold temperature would reduce residual stress whereas higher injection speed and higher packing pressure would increase residual stress. Associated with the decrease of residual stress, part’s tensile strength, yield strength and Young’s modulus increase with increased melt temperature, mold temperature and injection speed. On the other hand, higher packing pressure results in a decrease of part mechanical strength.

Investigations into moulding window of gas assisted injection moulded polystyrene plates with various gas channel designs

Abstract The design of the gas channel plays an important role in the successful application of gas assisted injection moulding. Although empirical guidelines for gas channel design have been proposed by the various equipment suppliers, quantitative rules based on well designed experiments have not been reported previously. To investigate the effects of geometry on gas penetration for two plate thicknesses, transparent polystyrene (PS) plates designed with semicircular gas channels of differing radii and with rectangular gas channels of differing width to height ratios have been produced using gas assisted injection moulding. Moulding windows and criteria for gas penetration were also chosen so that design rules could be defined quantitatively. The mouldability index was also classified into five levels (excellent, good, fair, poor, and bad) based on the relative areas of the moulding windows. From a plot of mouldability index against R eq , the ratio of equivalent gas channel radius to plate thickness, it was found that to obtain an appropriate moulding window (i.e at least a fair mouldability index) R eq should be greater than 2. Gas channels with a semicircular cross-section provide better mouldability than those with rectangular cross-sections of the same cross-sectional area. For the same equivalent radius, the ratio of width to height in rectangular gas channels also affects the mouldability index. The present investigation provides part designers with preliminary quantitative design/moulding guidelines for choosing the effective gas channel design that allows the parts to be moulded within an appropriate moulding window, so that the uncertainty in both simulation and process control can be overcome. A methodology for the establishment of guidelines for quantitative design of gas channels is also proposed.

Mechanical properties of gas-assisted injection moulded PS, PP and Nylon parts

Abstract PS, PP and Nylon plate parts designed with gas channels having five different types of cross-section but with same section area were gas-assisted injection moulded (GAIM). Mechanical properties of GAIM parts were investigated via tensile and bending tests. Effects of part thickness, shape and associated dimensions of gas channels on tensile and bending properties of GAIM parts were examined. It was found that maximum tensile load and ultimate tensile stress show only slight influence from gas channel design and part thickness except Nylon parts which exhibit significant dependence on part thickness due to degree of crystallinity. Gas channel design, introducing additional moment of inertia, results in part structural reinforcement. Part stiffness and maximum bending load basically increases linearly with the moment of inertia and the section modulus of the plate, respectively. Gas channel design attached with top rib (shapes D and E) show the best effect of structural reinforcement. For brittle PS parts, plates with semicircular gas channel (shape A) exhibit maximum flexural strength. PS parts with rectangular gas channel design (shape B) can absorb more bending energy than the other designs. The present study provides part designers with a design guideline for choosing the most effective gas channel design to achieve a specific objective of part structural performance.