Bo Sung Shin

Development of a direct metal freeform fabrication technique using CO2 laser welding and milling technology

Abstract Since the first introduction of rapid prototyping in 1986, several techniques have been developed and successfully commercialized in the market. However, most commercial systems currently use resins or waxes as the raw materials. Thus, the limited mechanical strength for functional testing is regarded as an obstacle towards broader application of rapid prototyping techniques. To overcome this problem, direct metal deposition methods are being investigated worldwide for rapid prototyping and even for rapid tooling applications. As a contribution to this development, a fundamental study on a process combination of wire welding technology using CO2 laser radiation with milling was carried out and is reported in this paper. Laser welding enables accurate deposition of metals and the subsequent milling increases the surface quality and accuracy to machining standard. Compared to powder, the use of wire is of advantage in terms of a simple feeding mechanism as well as a higher deposition rate. The main focus of the experimental investigation is to find the basic process characteristics. For this purpose, basic parts were fabricated as a function of process parameters such as laser power, welding speed and bead distance. The microstructure, hardness and tensile strength are then examined as a function of these process parameters. In conclusion, the advantages and disadvantages of this process are discussed in comparison with other direct metal fabrication techniques.

Replication of polyethylene nano-micro hierarchical structures using ultrasonic forming

We present the replication of polyethylene (PE) nano-micro hierarchical structures and their application for superhydrophobic surfaces. A commercial ultrasonic welding system was used to apply ultrasonic vibration energy to the forming of nano-micro hierarchical structures. To evaluate ultrasonic formability, Ni nanomold and nano-micro hierarchical mold were designed and fabricated. The optimal weld times were 1.5 s and 3.0 s for PE nanoprotrusions and nano-micro hierarchical structures, respectively. The forming process was conducted at atmospheric pressure. The PE structures were well replicated without a vacuum. The trapped air in the microcavity of the nano-micromold was dispersed and absorbed into the molten PE. Ultrasonic nano-microreplication technology showed an extremely short processing time and did not require a vacuum environment. To investigate the applicability of ultrasonic forming, the fabricated nanoprotrusions and nano-micro hierarchical structures were coated with plasma polymerized fluorocarbon (PPFC) of a hydrophobic nature and were applied to modify superhydrophobic surfaces. The contact angle was increased from 106° (smooth surface) to 125° (nanostructured surface) and finally to 160° (nano-microstructured surface) so that the surface became superhydrophobic.

Polymer microreplication using ultrasonic vibration energy

Polymethyl methacrylate (PMMA) microstructures were fabricated by a polymeric microreplication technology using ultrasonic vibration energy. A commercial ultrasonic welder system was used to apply ultrasonic vibration energy for micromolding. Two different types of nickel micromolds, which were equipped with pillar-type and pore-type microstructures, were fabricated. PMMA was used as the polymer microreplication material, and the optimal molding times were determined to be 2 s and 2.5 s for the pillar-type and pore-type micromolds, respectively. Compared with conventional polymer microreplication technologies, the proposed ultrasonic microreplication technology showed an extremely short processing time. Heat energy generated by ultrasonic vibration locally affected the vicinity of the contact area between the micromold and the polymer substrate. Consequently, only that very limited area was melted so that the bulk material was not seriously affected by the thermal effect and thermal shrinkage could be minimized. Furthermore, although the replication process was not performed in vacuum conditions, the ultrasonic micromolding showed high fidelity in polymer microreplication using the pore-type micromold.

A new rapid prototyping system using universal automated fixturing with feature-based CAD/CAM

Abstract Machining is the most commonly used process in the manufacturing of prototypes. This process offers several advantages, such as rigidity of the machine, precision of the operation and an especially quick delivery. The weight and immobility of the machine support and immobilize the part during the operation. However, despite these advantages, machining still presents several limitations. The immobilization, location and support of the parts are referred to as fixturing or workholding and present the biggest challenge for time-efficient machining. Thus, it is important to select and design an appropriate fixturing assembly. This assembly depends on the complexity of the part and the tool path and may require the construction of dedicated fixtures. With traditional techniques, the range of fixturable shapes is limited and the identification of suitable fixtures in a given set-up involves complex reasoning. To solve this limitation and to apply the automation, this paper presents reference- free part encapsulation and the implementation of the encapsulation system. The feature-based modeling and encapsulation systems are explored. A small part for which it is difficult to determine the appropriate fixturing assembly is made by this system.

High-Performance Optical Gating in Junction Device based on Vanadium Dioxide Thin Film Grown by Sol-Gel Method

In this paper, a high-performance optical gating in a junction device based on a vanadium dioxide dioxide (VO 2 ) thin film grown by a sol-gel method was experimentally demonstrated by directly illuminating the VO 2 film of the device with an infrared light at ~1554.6 ㎚. The threshold voltage of the fabricated device could be tuned by ~76.8 % at an illumination power of ~39.8 ㎽ resulting in a tuning efficiency of ~1.930 %/㎽, which was ~4.9 times as large as that obtained in the previous device fabricated using the VO 2 thin film deposited by a pulsed laser deposition method. The rising and falling times of the optical gating operation were measured as ~50 ㎳ and ~200 ㎳, respectively, which were ~20 times as rapid as those obtained in the previous device.

Surface Wettability in Terms of the Height and Depression of Diverse Microstructures and Their Sizes

Surface wettability in terms of the height, depression, and sizes of diverse microstructures was examined. We considered three different types of microstructures: square pillars, square pores, and hexagonal pores. An increase in pore fraction causes a decrease in the contact area between a water drop and the microstructures, resulting in an increase in contact angle. It was verified that the contact angle of the square pillar microstructure is higher than those of the square pore and hexagonal pore microstructures for the same pore fraction. Of the pore-based microstructures, the square-pore microstructure has a higher contact angle than the hexagonal pore microstructure. However, it was observed that water drops were more stable with respect to pore fraction on the hexagonal pore microstructures. In addition, compared with the pillar microstructure, the pore-based microstructures exhibited more stable contact angles for different structure heights.

Micromachining of Cr Thin Film and Glass Using an Ultrashort Pulsed Laser

Materials processing by ultrashort pulsed laser is actively being applied to micromachining technology due to its advantages with regard to non-thermal machining. In this study, materials processing with ultrashort pulses was studied by using the high repetition rate of a 800 nm Ti:sapphire regenerative amplifier. This revealed that the highly precise micromachining of metallic thin film and bulk glass with a minimal heat affected zone (HAZ) could be obtained by using near damage threshold energy. Grooves with diffraction limited sub-micrometer width were obtained with widths of 620 nm on Cr thin film and 800 nm on a soda-lime glass substrate. The machined patterns were investigated through SEM images. We also phenomenologically examined the influence of variations of parameters and proposed the optimal process conditions for microfabrication.

Measuring the Optical Rotation Angle and Circular Dichroism of Anisotropic Optical Media Using a Heterodyne Polarimeter

A method is proposed for measuring the circular birefringence (CB) and circular dichroism (CD) properties of anisotropic optical samples using a heterodyne polarimeter and an electronic signal processing scheme. Importantly, the CB and CD properties of the sample are decoupled in the analytical model, and thus the accuracy of the measurement results is improved. Furthermore, the proposed method enables the CB and CD properties of the sample to be measured over the full range. The validity of the proposed method is demonstrated by measuring the optical rotation angle and circular diattenuation of pure CB and CD samples and a composite sample with both CB and CD properties. The standard deviations of the optical rotation angle and circular dichroism are found to be 3.07× 10-3 degrees and 9.3×10-4 for a hybrid CD/CB sample, respectively.

Rapid manufacturing of SiC molds with micro-sized holes using abrasive water jet

Abstract Silicon carbide (SiC) is highly wear resistant with good mechanical properties, including high temperature strength, excellent chemical resistance, and high thermal conductivity and thermal shock resistance. SiC molds, which can be produced with diverse microstructural features, are now widely used in glass molding owing to their excellent characteristics, and also have potential applicability in IT industries. SiC molds are traditionally fabricated by silicon micromachining or dicing. The fabrication cost of silicon micromachining is very high, however, because several expensive masks are needed. Furthermore, the fabrication time is very long. Meanwhile, it is difficult to make micro-patterned molds with arbitrary shapes using dicing saws. Abrasive water jet (AWJ) is widely applied to cut and drill very brittle, soft and fibrous materials. It offers high energy density, the absence of a heat affected zone(HAZ), high performance, and an environment friendly process. In spite of these advantages, micro-hole drilling via conventional AWJ processing suffers from notable shortcomings. We proposed a new abrasive supplying method of AWJ. The proposed method reduces frosting phenomena, and provides micro-machining of AWJ. The characteristics of a hole machined was investigated by the proposed AWJ process according to the ratio of water and abrasives. With the optimal experimental conditions, 3×3 array SiC molds with the diameter of 700 μm and depth of 900 μm were successfully manufactured.

Development of a Micro Machining Technology for Fabrication of Micro Parts

Recently, ultra-precision machining and MEMS technology have taken an increasingly important position in machining of micro parts, such as PDP and IT components, as the application field of micro parts is extended. The micro machining center is effective for the fabrication of micro parts because of its benefits of low power consumption, high precision, and low machining cost. Therefore, we studied the possibility of its application to micro machining and analyzed the machining characteristics of micromachining. Furthermore, in this paper the application technology on micro parts is reported.