Haseung Chung

Optimization of micro-grinding process with compressed air using response surface methodology

This paper addresses the optimization of a micro-grinding process using compressed air to minimize specific grinding forces and surface roughness while maximizing specific material removal rate (MRR). The design-of-experiments (DOE) approach and response surface methodology (RSM) are introduced to obtain the optimal grinding conditions. In the DOE approach, a central composite design approach is used for experimental design. Micro-grinding experiments are conducted, and the experimental results are used to obtain response surface models of specific grinding forces and surface roughness in terms of depth of cut, feed rate and air temperature. Multi-objective optimization is then conducted by introducing desirability functions, and the optimal values of depth of cut, feed rate and air temperature are obtained for minimum specific grinding forces and surface roughness and maximum specific MRR. The experimental results under the optimal grinding conditions are similar to those estimated from the response surface models, and thus the validity of the models is verified.

Review: Dimensional Accuracy in Additive Manufacturing Processes

This paper reviews the state-of-the-art research related to the dimensional accuracy in additive manufacturing (AM) processes. It is considered that the improvement of dimensional accuracy is one of the major scientific challenges to enhance the qualities of the products by AM. This paper analyzed the studies for commonly used AM techniques with respect to dimensional accuracy. These studies are classified by process characteristics, and relevant accuracy issues are examined. The accuracies of commercial AM machines are also listed. This paper also discusses suggestions for accuracy improvement. With the increase of the dimensional accuracy, not only the application of AM processes will diversify but also their value will increase.Copyright

Modeling of Melting, Evaporating, and Resolidifying Procedure in Laser-Induced Metal Processing

In this paper, we present a one-dimensional (ID) melting, evaporating, and resolidifying model describing the interaction of a scanning laser beam with a metal surface wherein the continuous and stepwise heat flux is applied. One set of ID conduction equations, which is valid in all phases including solid, liquid, and vapor, has been developed along with two phase boundary conditions between the solid/liquid and liquid/air using an appropriate scaling law. The ID heat equation has been solved separately in each phase using a sharp interface temperature technique based on the front tracking method. The generalized relations of non dimensional maximum melting depth, final evaporation depth, and maximum melting time related to nondimensional interaction time and heat flux factor are established.

Study on the compressive modulus of nylon-11/silica nanocomposites

This paper investigates the unusual characteristics regarding the mechanical properties of Nylon-11 filled with different volume fractions of silica nanoparticles by selective laser sintering (SLS) from numerical simulation. The compressive modulus was predicted by two different numerical models and compared with the experimentally measured one. While the two-phase model has a limited capability in explaining the unusual behavior shown in the compressive modulus obtained by experiments with 2% volume fraction of nanoparticles, the effective interface model can simulate the unexpected characteristic of nanocomposites according to the volume fraction of nanoparticles. We can conclude that the effective interface model should be employed to predict the mechanical properties of nanocomposites for efficiency and accuracy.

Numerical Prediction of Product Deformation and Identification of Major Controlling Factors during the IML Process Including Thermal Impact

The in-mold labeling (IML) process has become a popular choice for the exterior decoration of many mobile devices due to its capability of rich color representation with a relatively easy manufacturing procedure. A molten polymer is injected into a cavity where pre-decorated film has been initially inserted. A heterogeneous characteristic of involved material coupled with various processing parameters can induce defects of the final product such as film delamination, wash-out, and flow-mark. In this study, major controlling parameters for the deformation of the final product have been identified with the design of experiment (DOE) method using several nondimensional quantities of temperature, material property, and thickness ratio. MOLDFLOW and ABAQUS software has been simultaneously used to simulate the injection process and thermal impact process, respectively. We found that the thickness ratio is a critical factor on final deformation, and the material property ratio has a relatively minor effect.

Application of 532 nm YAG-Laser Annealing to Crystallization of Amorphous Si Thin Films Deposited on Glass Substrates

A 532 nm Nd-YAG laser was applied to crystallize amorphous Si thin films in order to evaluate the applicability of a Nd-YAG laser to low-temperature polycrystalline Si technology. The irradiation of a green laser was controlled during the crystallization of amorphous Si thin films deposited onto glass substrates in a sophisticated process. Raman spectroscopy and UV-Visible spectrophotometry were employed to quantify the degree of crystallization in the Si thin films in terms of its optical transmission and vibrational characteristics. The effectiveness of the Nd-YAG laser is suggested as a feasible alternative that is capable of crystallizing the amorphous Si thin films.

Measurement of Bow in Silicon Solar Cell Using 3D Image Scanner

To reduce the cost per watt of photovoltaic power, it is important to reduce the cell thickness of crystalline silicon solar cells. As the thickness of the silicon layer is reduced, two distinctive thermal expansion rates between the silicon and the aluminum layer induce bowing in a solar cell. With a thinner silicon layer, the bowing distance grows exponentially. Excessive bowing could damage the silicon wafer. In this study, we tried to measure an irregularly curved silicon solar cell more accurately using a 3D image scanner. For the detailed analysis of the three-dimensional bowing shape, a least square fit was applied to the point data from the scanned image. It has been found that the bowing distance and shape distortion increase with a decrease in the thickness of the silicon layer. An Ag strip on top of the silicon layer can reduce the bowing distance.

Fabrication of three-dimensional functionally graded materials using controlled polycaprolactone powder characteristics and laser material processing

The functionally graded material is characterized by its gradual change in material compositions and properties over the volume of a component to optimize its functional value. To fabricate functionally graded material parts by additive laser material processing, appropriate powders are critical in term of their size, shape, and mechanical properties. This paper presents new methods to mass produce polycaprolactone powders, a type of biodegradable polymer, with deliberately modified properties as well as to fabricate functionally graded material parts using the modified powders by laser material processing. To modify the properties of the biodegradable polymer powder, this research used two different approaches: recrystallization and spraying. The two different modification methods enabled the significant change of the polycaprolactone powders’ molecular weight that strongly affects mechanical properties. This controlled molecular weight change enabled the gradual variation of the mechanical properties in functionally graded material. By the two different modification methods, the raw materials with irregular shape and size were changed into powders with a round shape and similar sizes, so that they became more suitable for the laser material processing. We verified the modified powder properties such as the size distribution, shape, crystallinity degree, thermophysical properties, and molecular weight by using a variety of methods. The modified polycaprolactone powders with varying mixture ratios were sintered by Nd:YLF laser irradiation with 349-nm wavelength, and the different mechanical properties were verified by micro-tensile tests. As an application of functionally graded material, a biomedical trigger capsule was fabricated and its performance evaluation demonstrated the desired results according to three-dimensional functionally graded material design.

Experimental study of machining process of polymer mold for fabrication of three-dimensional hydrogel scaffold

A three-dimensional hydrogel scaffold has been proposed for the effective production of biomimetic intestinal villi to reduce ethical and cost problems caused by animal testing in pharmaceutical development. This study explores an experimental approach to develop a new technique based on laser machining and microdrilling processes to produce a plastic mold for the fabrication of a three-dimensional hydrogel scaffold. For process optimization, both the laser machining and the microdrilling experiments are conducted by varying the experimental conditions, and structural characterization of the mold and intestinal villi were performed using SEM (scanning electron microscope) and OM (optical microscope) images. Polycarbonate (PC) was used as a candidate material. The experimental results show that intestinal villi can be fabricated by both laser and microdrilling machining processes.

Heat transfer enhancement in a high aspect ratio channel with system optimization

Outdoor digital information display (DID) installed near public areas has become a very popular medium of communication. Cooling performance of an outdoor DID system has become a critical issue from increasing system temperature due to heat generation at the back panel, in addition to irradiation from the sunlight. Gap distance between the heating plate and glass could control the main heat transfer rate considering its positive or negative effect on the pressure and temperature gradient at the wall. Additional reduction of maximum surface temperature can be achieved using tapered geometry with increased inlet gap distance, as well as guide vane installation at the system exit to lower pressure loss. Full system optimization has also been performed, including correlation for tapered geometry and guide vane effect. With system optimization, we can find the optimal gap clearance of parallel or tapered geometry for best performance.