• Journal of Chest Surgery
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• v.38 no.2 s.247
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• pp.132-138
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• 2005

Background: Mitral valve repair (MVP) is the optimal procedure for mitral regurgitation (MR), however, failure and subsequent reoperations are the limitations. The current study assessed the procedure in relation to the primary valve related causes of recurrent MR. Material and Method: MR was treated in 493 patients undergoing MVP from January of 1994 to January of 2002. The causes of MR were degenerative $(n=252,\;51.5\%),$ rheumatic $(n=156,\; 31.6\%),$ and others $(n=85,\; 16.9\%).$ Surgery comprised 446 ring annuloplasties $(90.5\%),$ 227 new chordae formations $(46\%),$ 125 quadriangular resections $(25.3\%),$ 28 chordae transfers $(5.7\%),$ and 8 Alfieri's stitches $(1.6\%).$ The mean follow up was $29.04\pm22.81$ months. Result: There were 5 early $(1.01\%)$, and 5 late deaths $(1.01\%).$ The reoperation rate was $1.42\%$. There were 45 $(9.1\%)$ recurrent MR (grade III or IV). Of these, 24 were procedure related including incomplete repair (n=14), discordant new chordae length (n=8) and others (n=2). In 21 patients, the cause was valve related including rheumatic disease progression (n=10), recurrent chordae elongation or prolapse (n=5) and others (n=6). Severe MR was higher after incomplete repair (p < 0.001), and valve related failure strongly correlated with rheumatic progression (p < 0.05). Conclusion: Since completeness of operation is the prime risk factor that determine the repair durability, intra-operative assessment of the initial repair with trans-esophageal echocardiography is essential.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1993.04b
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• pp.49-52
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• 1993

금속의 가공공정에 있어 중요한 인자중의 하나인 재료의 연성은 환경뿐만 아니라 주변압력에 따라 변화한다는 사실이 20세기에 들어 여러 연구자들에 의해 밝혀졌고 1950년대 Birdgman에 의해 최초로 실험적인 방법으로 증면되었다. 재료의 이와같은 현상을 압력유도연성이라 하는데 이는 주변압력이 재료 내부에서의 공동발생 및 그 성장에 관계하기 때문인 것으로 알려졌다. 공동의 합체 및 성장은 연성파괴의 전체조건이 되므로 이를 억제하면 성형성 및 연성이 증가됨을 알 수 있다. 금속의 압력 유도 연성을 이용한 가공방법을 가압금속형형이라 한다. 최근에 가압금속형의 범주는 크게늘고 있 는데 특히, 성형,블랭킹, 분말야금법 등에 널리 이용되고 있다. 정수압의 크기에 다른전단면의 상태 변화 즉, 소성 변형능에 관한 연구는 Fine-Blanking가공이론에 적용, 해석되고 있다.

• The Korean Journal of Rheology
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• v.5 no.1
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• pp.85-92
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• 1993

수지이동 성형공정에서 수지가 섬유직조망에 함침될 때의 투과계수와 수지의 표면 장력으로 인하여 유동진전면에서 발생하는 모세관압을 실험적으로 측정하였다. 두 종류의 섬유조직망에 대해서 가공율이 증가함에 따라투과계수는 증가한다. 수지, 섬유 그리고 공기 가 서로 다른 세 개의 상을 구성함으로써 수지의 표면장력의 영향을 받는 비정상상태의 투 과계수가 수지의 섬유직조망에 포화된 정상상태에서 측정된 투과계수보다 본 실험에서 수행 된 모든 기공을 범위에서 크다는 것을 보여주었다. 수지 유동진전면에서 발생되는 모세관압 은 기공율이 감소함에 따라 증가하였고 섬유직 조망의 기공율이 0.469인 경우에는 금형입구 에서의 수지주입압력의 25%에 해당되는 모세관압이 발생되는 것으로 나타났다. 따라서 모 세관압을 가공조건으로 고려해다 한다는 것을 제시할수 있었다. 또한 본연구에서 사용된 임 의 배향 섬유직조망에 대해 실리콘 오일과 유리섬유 계면의 동적접촉각 측정을 통하여 섬유 배향 및 기공의 분포를 나타내는 형상인자의 값을 구함으로써 실제 RTM 공정에서 발생되 는 모세관압을 예측할 수 dLT는 기초자료를 제시하였다.

• Proceedings of the Korean Society of Tribologists and Lubrication Engineers Conference
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• 1992.11a
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• pp.31-37
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• 1992

금형을 이용하는 소성가공에서는 형 사이에서 셩형되는 소재가 소성변형되지만 이때 소재는 형의 표면을 따라 미끄러지면서 유동하므로 형재료와 소재간에는 큰 마찰력이 작용하고, 형재료는 이 마찰력에 의하여 전단응력이 작용하게 된다. 따라서 이러한 전단응력이 반복하여 작용하는 프레스가공 등에서는 형의 수명이 항상 문제가 된다. 물론 형의 수명은 형재료가 프레스압력에 견디지 못하여 형이 균열되거나 펀치가 절단되는 것 같은 형재료의 강인성이 불충분하기 때문에 비교적 조기에 일어나는 파손에 의하여 수명의 대상으로 되는 경우도 있지만 무엇보다도 형의 수명을 죄우하는 가장 중요한 사항은 형재료의 내마멸성이다. 따라서 본 연구에서서는 이들 제인자를 고려하여 Die 및 Punch의 재료로 널리 사용되는 냉간성형용 Die강인 STD 11의 마멸특성을 알기 위하여 미끄럼 속도, 접촉압력 및 미끄럼 거리등을 변화시켜 내마멸성을 나타내는 최적사용조건을 찾고자 하는 것을 목적으로 하였다.

• Tribology and Lubricants
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• v.18 no.2
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• pp.127-132
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• 2002

The Taguchi method, a robust experimental design, was used to optimize manufacturing parameters of a brake lining during hot pressing and heat treatment. A friction material containing 15 ingredients was employed fur this experiment and friction and wear tests were carried out by using a pad-on-disk type tribotester. Sixteen brake linings with different manufacturing conditions were examined according to a parameter design. From the results of the signal-to-noise (S/N) ratio and the analysis of variance (ANOVA), the cause and effect of the manufacturing parameters on physical properties (hardness and porosity) and friction and wear characteristics of brake linings was obtained.

• Transactions of Materials Processing
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• v.19 no.4
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• pp.203-209
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• 2010

In the hot press forming process (HPF), a martensitic structure is obtained by controlling the cooling rate when cooling a boron sheet that is heated up to over $900^{\circ}C$. The HPF process has various advantages such as the improvement in formability and material properties and minimal spring back of the deformed materials. The factors related to the cooling rate depend on the heat transfer characteristics between heated materials and dies. Therefore, in this study, the cooling rate is controlled by adjusting the heat transfer coefficient of the material at the pressing process. And, the mechanical properties and microstructure of the deformed material is demonstrated during the HPF process where cold dies are used to form the heated steel plate. This is achieved by varying the major forming conditions that control the cooling rate regarded as the most important process parameter.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.10 no.6
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• pp.70-76
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• 2011

This paper presents an optimization procedure for reducing warpage of injection molded part by using a volumetric shrinkage deviation as an objective function. A design of experiments based on orthogonal arrays was used in the optimization procedure, and the entire optimization was performed through a two stage process - a preliminary experimentation and a principal experimentation. Proposed optimization method was applied to the design of a CPU-base part in computer. With the moderate number of experiments, an optimal molding condition for uniform distribution of volumetric shrinkage was obtained, as a result, the warpage of the molded part was significantly reduced.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.09a
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• pp.147-152
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• 2005

In recent industry, according to pursuit the miniaturization and high-precision of machine part with development of new technology as IT, BT the development of mold manufacturing technology for mass production is accompanied. In this study, the spiral type injection mold with a $200{\mu}m$ thickness shape is made for investigation of influence for injection molding process variables and the flow length is measured through an experiment. Beside, the result of each experiment Is compared with the analysis result of CAE. Taguchi method is used in this experiment and the obtained data are analyzed using ANOVA method.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.4
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• pp.144-150
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• 2005

Gas-assisted injection molding (GAIM) process is reducing the injection pressure during mold filling required as well as the shrinkage and warpage of the part and cycle time. Despite of these advantages, this process introduces new parameters and makes the application more difficult because the process interacts between gas and melt during injection molding process. Important GAIM factors that involved in this process include gas penetration design, locations of gas injection points, shot size, gas injection delay time as well as common injection molding parameters, gas pressure and gas injection time. In this study, the experiments were conducted to investigate effects of GAIM process variables on the gas penetration for PP and ABS moldings by changing gas injection point. Taguchi method was used fer the design of experiment. When the gas was injected at cavity's center, the most effective factor was shot size. When the gas was injected at cavity's end, the most effective factor was melt temperature. Injection speed was also an effective factor in GAIM process.

• Resources Recycling
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• v.17 no.5
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• pp.19-27
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• 2008

In this study, the porosity and electrical resistivity of the formed coke produced by sintering the mixture of domestic anthracite and phenolic resin were mainly investigated, when the effect of the amount of binder, the hydration temperature and time, the hardening temperature and time, sintering temperature and time, the particle size of anthracite, the grade of anthracite, and the mixing ratio of phenolic resin on the physical properties of the coke were studied. As a result, It was found that the electrical resistivity and porosity of the formed coke are varied in the range of $0.3\sim1.2\Omega{\cdot}cm$ of $10\sim30%$, respectively, in accordance with the variation of factors.