• Transactions of Materials Processing
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• v.8 no.3
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• pp.269-275
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• 1999

A variable blank holding force method is proposed to improve deep drawing characteristics of sheet materials. In this method, the blank holding force (BHF) is controlled throughout a drawing process so that the punch load does not exceed a critical value, which is slightly less than the conventional process with the conforming process with the variable BHF is more flexible than the conventional process with the constant BHF and it could be used for improving the product's quality and drawability. In this paper we suggest a method controlling the BHF as a function of punch travel during the forming process. The optimization BHF curves are determined theoretically and experimentally. It is concluded that for the case of optimum BHF control methods the drawn cup height and the drawing formability achieved by this method are increased than those for constant BHF method. Also, as comparing the wall thickness distribution of the cup drawn by the constant BHF and the optimum BHF control, the BHF control reduce the wall thickness variation of the drawn cup at the cup wall and make the cup thickness distribution more uniformly than the constant BHF.

• Journal of the Korean Society of Industry Convergence
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• v.10 no.1
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• pp.5-14
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• 2007

It is necessary for development of drawing product with press to have suitable material selection & all process design and the problem during press process has been cleared from judgement of experience & trial and error. Recently we can estimate press process result from computer aided design & FEM. But we can get more reliable result when we can put more precise process variants during FEM. In case of using a drawbead that is used for the material inflow, it is considered for us to put material property, other analysis condition & friction figure when material is passing through the drawbead for better FEM. From our study, we have drawn an analogy bead connection depth, friction figure & drawing and restraining load according to kinds of lubrication from experiment & FEM for the drawbead. We applied above result to the drawing experiment & FEM and confirmed the validity. We could notice the relation between friction figure & drawing load and the friction figure variation according to kinds of lubrication. It is expected to draw more precise analogy that can be used for real process due to more precise process variants application to FEM.

• Journal of Electrical Engineering and information Science
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• v.2 no.3
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• pp.71-76
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• 1997

A lot of experiments and analyses have been done to determine the aging mechanism of mica-epoxy composite material used for large generator stator windings in order to estimate remaining life of the generator for last decades. After degrading artificially the mica-epoxy composite material, the surface analysis is performed to analyze breakdown mechanism of insulation in air and hydrogen atmosphere; i) In the case of air atmosphere, it is observed that an aging propagation from conductor to core by partial discharge effect and the formation of cracks between layers is widely carbonized surface. ii) In case of hydrogen atmosphere, the partial discharge effect is reduced by the hydrogen pressure (4kg/$\textrm{cm}^2$). Potassium ions forming a sheet of mica is replaced by hydrogen ions, which can lead to microcracks. It is confirmed that the sizes of crack by SEM analysis are 10∼20[$\mu\textrm{m}$] in length under air, and 1∼5[$\mu\textrm{m}$] in diameter, 10∼50[$\mu\textrm{m}$] in length under hydrogen atmosphere respectively. The breakdown mechanism of sttor winding insulation materials which are composed of mica-epoxy is analyzed by the component of materials with EDS, SEM techniques. We concluded that the postassium ions of mica components are replaced by H\ulcorner, H$_3$O\ulcorner at boundary area of mica-epoxy and/or mica-mica. It is proposed that through these phenomena, the conductive layers of potassium enable creation of voids and cracks due to thermal, mechanical, electrical and environmental stresses.

• Macromolecular Research
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• v.12 no.4
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• pp.391-398
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• 2004

Thick-film photolithography is a new technology that combines lithography processes, such as exposure and development, with the conventional thick-film process applied to screen-printing. In this study, we developed a low-temperature cofireable silver paste applicable for thick-film processing to form fine lines using photolitho-graphic technologies. The optimum paste composition for forming fine lines was investigated. The effect of processing parameters, such as the exposing dose, had on the fine-line resolution was also investigated. As the result, we found that the type of polymer and monomer, the silver powder loading, and the amount of photoinitiator were the main factors affecting the resolution of the fine lines. The developed photoimageable silver paste was printed on a low-temperature cofireable green sheet, dried, exposed, developed in an aqueous process, laminated, and then fired. Our results demonstrate that thick-film fine lines having widths < 20 $\mu\textrm{m}$ can be obtained after cofiring.

• Journal of the Korean Society for Precision Engineering
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• v.25 no.7
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• pp.55-63
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• 2008

Fine blanking is a process of press shearing which makes it possible to produce the thick sheet metal of the finished surface and the close dimensional accuracy over the whole material thickness in the single blanking operation. In this paper, a plate holder of automotive seat recliner is manufactured by FCF(Flow Control Forming) method using the conventional mechanical press instead of the fine blanking press. Main processes for manufacturing of the plate holder by FCF method are embossing, half blanking and trimming processes. Optimal clearance, stripper force and counter force to increase the dimensional accuracy of the plate holder have been investigated by FE-analysis. As a result of FE-analysis, the clearance for both embossing and half blanking processes was -2%t and the forces of stripper and counter were 25ton and 15ton, respectively. After manufacturing the plate holder by FCF method, the measured dimensional characteristics have been compared with the required specifications as the final product. Although the dimensional accuracy of the plate holder manufactured by FCF method was a little inferior to that by fine blanking process, it was satisfactory in a general sense.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2010.11a
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• pp.5-8
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• 2010

The GCSC injectors studied in this paper are those applied to the combustion chamber of staged combustion engines. Liquid fuel is injected through tangential holes along the outer wall of the GCSC injector forming a swirling sheet and oxygen rich gas generated by a preburner enters axially through the center orifice of the injector to form a gaseous jet. The spray characteristics of GCSC injectors under ambient/high pressure conditions and the effect of recess on spray characteristics have been examined in this paper. These results are expected to be used as fundamental data to develop of a staged combustion engine.

• Steel and Composite Structures
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• v.20 no.1
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• pp.21-41
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• 2016

Cold-formed steel structures are increasingly attractive due to their benefits of good mechanical performance and constructional advantages. However, this type of construction is still not fully exploited as a result of the acknowledged difficulties involved in forming construction-efficient and cost-effective connections. Furthermore, there is a lack of information on the structural behavior of the cold-formed steel connections. In this study, the research on various cold-formed steel connections was comprehensively reviewed from both fundamental and structural points of view, based on the available experimental and analytical data. It reveals that the current design codes and guidelines for cold-formed steel connections tend to focus more on the individual bearing capacity of the fasteners rather than the overall structural behavior of the connections. Significant future work remains to be conducted on the structural performance of cold-formed steel connection. In addition, extensive previous research has been carried out to propose and evaluate an economical and efficient connection system that is obtained from the conventional connecting techniques used in the hot-rolled industry. These connecting techniques may not be suitable, however, as they have been adopted from hot-rolled steel portal frames due to the thinness of the sheet in cold-formed steels. The review demonstrates that with the increasing demand for cold-formed steel constructions throughout the world, it is crucial to develop an efficient connection system that can be prefabricated and easily assembled on site.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.12
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• pp.3728-3740
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• 1996

High-quality cups of deep drawing ratio of more than four cannot be simply drawn by conventional drawing and redrawing processes. In the present study, after the first deep drawing process, subsequent hydromechanical reverse redrawing with controlled radial pressure is employed. In order to increase the deep drawing ratio up to muchmore than four, the radial pressure should be controlled independently of the chamber pressure and thus an optimum forming condition can be found easily by varying the radial pressure. The process has been subjected to finite element analysis by using the rigid-platic material modeling considering all the frictional conditions induced by the hydrostatic pressure. In order to consider the pressure effect on the sheet, the pressure distributions on the flange part and the side wall part are calculated mumerically from simplified Navier-stokes equation. The comparison of the computation with the experiment has shown that the finite element modeling can be conveniently emplyed for the design of the process with reliability from the viewpoint of formability.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.05a
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• pp.159-162
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• 2005

The milli-components for electronic and medical device etc. have been manufactured by conventional process. Forming and machining process for those milli-components need tremendous cost and time because products require higher dimensional accuracy than the conventional ones. For instance, conventional mechanical punching process has many drawbacks for applying to high accuracy products. The final radius of hole can be varied and burr which interrupting another procedure is generated. Hydro-mechanical punching process makes possible to reduce amount of burr and obtain the fine shearing surface using the operating fluid. Hydrostatic pressure retards occurrence of initial crack and induces to locate the fracture surface in the middle of sheet to thickness direction. In this paper, Hydro-mechanical punching process is analyzed using finite element method and the effect of hydrostatic pressure is evaluated during punching process. The prediction of fracture is performed adopting the various ductile fracture criteria such as Cockcroft, Brozzo and Oyane's criterion using a user subroutine in ABAQUS explicit.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.5
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• pp.654-661
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• 2004

In deep drawing of sheet metal, there are many cases in which the uniform and thin wall thickness of the drawn products is more important than the bottom thickness. In this case, we can not easily get the deep drawn products with the uniform and precise wall thickness by only drawing process. Therefore in general the manufacturing processes which both the drawing and the ironing process are proceeded sequentially are used. But this method has the disadvantages of a cost-up, decrease of productivity and degradation of quality, because the ironing process is added after the drawing process. In this study, in order to improve those problems and to enhance the effect of deep drawing, the combined process of redrawing and ironing fur multistep drawing of cylindrical cups is used. In this experiment, we considered the characteristics of the combined process such as the relation between the drawing and ironing rates, the drawing limits and the forces needed for operations. The suggested force prediction shows that it can successfully represent experimental results.