• Journal of Dental Rehabilitation and Applied Science
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• v.38 no.3
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• pp.127-137
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• 2022

Zirconia is fabricated through various processes. Each element in fabricating process can affect the physical properties of the definitive prosthesis. In particular, both the milling process and the sintering process can affect the final integrity of the zirconia prosthesis. Most of the milling machines adopt the ultra-precision 5-axis machining method, and the results vary depending on which milling method was used and how the milling equipment was managed. Milling blocks are selected according to cutting efficiency and aesthetic reproducibility. The sintering method can affect the grain growth and optical properties, and an accurate evaluation can be made only with additional research on the recent speed sintering procedure. Not only the sintering temperature but also the temperature holding time can affect the quality of definitive prosthesis.

• International Journal of Naval Architecture and Ocean Engineering
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• v.11 no.1
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• pp.294-306
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• 2019

On the basis of the Computational Fluid Dynamics technique (CFD) combined with the overlap grid method, this paper establishes a numerical simulation method to study the problem of ice-propeller interaction in viscous flow and carries out a simulation forecast of the hydrodynamic performance of an ice-class propeller and flow characteristics when in the proximity of milling-shape ice (i.e., an ice block with a groove cut by a high-speed revolving propeller). We use a trimmed mesh in the entire calculation domain and use the overlap grid method to transfer information between the domains of propeller rotation calculation and ice-surface computing. The grid is refined in the narrow gap between the ice and propeller to ensure the accuracy of the flow field. Comparison with the results of the experiment reveals that the error of the hydrodynamic performance is within 5%. This confirms the feasibility of the calculation method. In this paper, we calculate the exciting force of the propeller, analyze the time domain of the exciting force, and obtain the curve of the frequency domain using a Fourier transform of the time-domain curve of the exciting force. The existence of milling-shape ice before the propeller can greatly disturb the wake flow field. Unlike in open water, the propeller bearing capacity shows a downward trend in three stages, and fluctuating pressure is more disordered near the ice.

• Journal of Technologic Dentistry
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• v.40 no.3
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• pp.125-131
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• 2018

Purpose: Automatic dental prosthesis manufacturing process was accelerated by the spread of dental CAD / CAM system. The CAD / CAM system with milling alloys were needed supplement. So, sintered alloy blocks were introduced. In this study, we want to study sintered alloy block. And to evaluate the alloy block manufacture and alloy properties. Methods: The alloy powders were prepared by high pressure water dispersion method. The sintered alloy blocks were prepared by low temperature pressing method. Their components observation were EDX, and the alloy structure was observed by XRD. Results: Co-Cr alloy powders were observed to have a circle shape with an average diameter of about $100{\mu}m$ and a Ni-Cr alloy powder had a circle shape with an average diameter of about $50{\mu}m$. The Co-Cr alloy block is composed of Co (34.62 wt%), Cr (17.33 wt%), Mo (2.98 wt%), Si (0.36 wt%) and C (44.17 wt%). The Ni-Cr alloy powder was composed of Ni (40.29 wt%), Cr (19.37 wt%), Mo (3.53 wt%), Si (0.52 wt%) and C (33.18 wt%). The peak of the Co and CoCr peaks were observed in the CoCr alloy body by the means of XRD study. Cr2Ni3 of the peak was observed in the Ni-Cr alloy material. Conclusion : As a result, the following conclusions were obtained. 1. Prepared by high-pressure water-law Co-Cr alloy powder has an average diameter $100{\mu}m$, Ni-Cr alloy powder was found to have the form of sphere having an average diameter $50{\mu}m$. 2. Co-Cr alloy and Ni-Cr alloy block produced by low-temperature processing showed a certain ratio. 3. In the XRD study, Co phase appeared in Co-Cr alloy block after sintering. and Cr2Ni3 phase appeared in Ni-Cr alloy block after sintering.

• Design & Manufacturing
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• v.8 no.1
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• pp.27-30
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• 2014

Alloy tool steel such as SKD11 and SKD61 or high speed tool like SKH51 are used as materials for semiconductor dies. Cavities, curl blocks, pot blocks and housings are made from those materials. To make those parts from alloy tool steel or high speed tool, one utilizes discharge machining, and mechanical machining including machining center, milling, drilling, forming grinding and others. In the process of cutting machining and polishing, the die materials become unsuitable for machining owing to bubbles and foreign substances in them, which hinders production process. Therefore, this study focuses on die material selection criteria, and on analysis and comparison of material characteristics to help companies to solve their problems, make die manufacture less burdensome and extend die life.

• Journal of Technologic Dentistry
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• v.34 no.2
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• pp.75-81
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• 2012

Purpose: All ceramic crown, made from zirconia instead of metal for core material, is recognized the best esthetical prosthesis. Recently, high-priced zirconia blocks and expensive CAD/CAM machines come into use for making zirconia core. In this study, slip casting process is adapted to evaluate the possibility of the recycling the remained parts of zirconia block after machining. Methods: Remained zirconia blocks were reduced to powders with zirconia mortar, and screened with 180 mesh sieve. Passed powders were ball milled under various conditions to obtain the optimum zirconia slip for casting. Solid casting method was used for casting the specimens with plaster mold. Formed specimens were dried and biscuit fired at $1,000^{\circ}C$ for 1 hour. Biscuit fired specimens were finished with exact shape of square pillar. Finished specimens were fired from $1,200^{\circ}C$ to $1,550^{\circ}C$ at $50^{\circ}C$ intervals for 1 hour. Linear shrinkage, apparent porosity, water absorption, bulk density, and flexural strength were tested. Microstructures were observed by SEM. Results: Above examinations indicated that the optimum firing temperture was $1,500^{\circ}C$, and when fired at this temperature for 1 hour, apparent porosity was 0% and flexural strength was 680MPa. SEM photomicrographs showed uniform 200~300nm grain size, which is equal with microcture of sintered commercial zirconia block. when compare 24% linear shrinkage of cast specimen with 20% linear shrinkage of CAD/CAM machined block, it was estimated that the size controlling of cast core was not so difficult. Conclusion: According to the all of this experimental results, the cast zirconia core produced from the remained parts of zirconia block was possible to use for all ceramic denture.

• Journal of the Korean Ceramic Society
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• v.53 no.4
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• pp.435-443
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• 2016

Coal ash, a material generated from coal-fired power plants, can be classified as fly ash and bottom ash. The amount of domestic fly ash generation is almost 6.84 million tons per year, while the amount of bottom ash generation is 1.51 million tons. The fly ash is commonly used as a concrete admixture and a subsidiary raw material in cement fabrication process. And some amount of bottom ash is used as a material for embankment and block. However, the recyclable amount of the ash is limited since it could cause deterioration of physical properties. In Korea, the ashes are simply mixed and used as a replacement material for cement. In this study, an attempt was made to mechanically activate the ash by grinding process in order to increase recycling rates of the fly ash. Activated fly ash was prepared by controlling the mill types and the milling times and characteristics of the mortar containing the activated fly ash was analyzed. When the ash was ground by using a vibratory mill, physical properties of the mortar mixed with such fly ash were higher than the mortar mixed with fly ash ground by a planetary mill.

• Journal of Technologic Dentistry
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• v.41 no.1
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• pp.9-19
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• 2019

Purpose: This study was conducted to comparative evaluate the marginal and internal gap of three-unit metal frameworks(Co-Cr) fabricated by wax milling method and digital light projection method of CAD/CAM systems. Methods: All the specimens were fabricated by three different fabrication methods: conventional wax up with casting(CWC), milled wax block with casting(MWC), digital light projection with casting(DLPC) (n=10 each). The marginal and internal fits of specimens were examined using a replica technique. The light-body silicone thickness was measured at 8 reference points(each abutment: 16 measurements). All measurements were conducted by a stereomicroscope. Digital photo were taken at $150{\times}$ magnification and then analyzed using a measurement software. The Mann-Whitney test was used for analyzing the results. Results: Statistically significant differences were found between the fabrication methods(p<0.001). The mean(SD) is ${\mu}m$ for fabrication methods, the mean marginal fit were recorded respectively, CWC 63(38), MWC group 50(33), DLPC 103(54) and the mean internal fit CWC 96(47), MWC group 116(41), DLPC 138(66). Conclusion : The marginal and internal fit were statistically different according to the fabrication methods(p<0.001). In all fabrication methods, the greatest misfit was found the occlusal area of all specimens.

• Journal of Technologic Dentistry
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• v.34 no.2
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• pp.95-103
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• 2012

Purpose: Fracture strength of all-ceramic 3-unit fixed partial dentures manufactured by CAD/CAM and copy-milling systems were evaluated. Methods: Zirconia cores were made by milling the pre-sintered zirconia block by CAD/CAM or copy milling method followed by subsequent sintering. By building-up the corresponding porcelains on the core, all-ceramic bridges were fabricated, and those were evaluated in comparison with PFM fixed partial denture. Results: During the flexural test of the 3-unit PFM bridge, the porcelain started to chip or break at 507.28(${\pm}62.82$)kgf and the metal framework did not break until the maximum load level of 800kgf which was set in the testing instrument of this study. However, among all-ceramic restoration test groups, Everest(EV) group showed a peeling off or breakage of the porcelain from 365.64(${\pm}64.96$)kgf and the core was broken at 491.77(${\pm}55.62$)kgf. Those values of Zirkonzahn(ZR) were 431.03(${\pm}58.47$)kgf and 602.74(${\pm}48.44$)kgf, respectively. The break strength of the porcelain of PFM(PM) group was significantly higher than that of EV (p<0.05) group and there was no significant difference when comparing to that of ZR (p>0.05). ZR group showed higher break strength than that of EV group however there was no significant difference (p>0.05). The break strength of cores were in the increasing order of EV < ZR < PM (p<0.05). Conclusion: We could find that even though the PM group fractured at much higher value than all-ceramic cores, the breakage values of the porcelain of PM group with crack formation or delamination, which will be regarded as clinical failure, was significantly higher than that of EV group and not significantly higher than that of ZR group at p-values of 0.05. The break strength of ZR group was higher than that of EV group at an insignificant level(p>0.05).

• Applied Microscopy
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• v.41 no.3
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• pp.147-154
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• 2011

Backscattered electrons (BSE) are generated at the impact of the primary electron beam on the specimen. BSE imaging provides the compositional contrast to resolve chemical features of sectioned block-face. A focused ion beam (FIB) column can be combined with a field emission scanning electron microscope (FESEM) to ensure a dual (or cross)-beam system (FIB-FESEM). Due to the milling of the specimen material by 10 to 100 nm with the gallium ion beam, FIB-FESEM allows the serial block-face (SBF) imaging of plastic-embedded specimens with high z-axis resolution. After contrast inversion, BSE images are similar to transmitted electron images by transmission electron microscopy. As another means of SBF imaging, a specialized ultramirotome has been incorporated into the specimen chamber of FESEM ($3View^{(R)}$). Internal structures of plastic-embedded specimens can be serially revealed and analyzed by $3View^{(R)}$ with a large field of view to facilitate three-dimensional reconstruction. These two SBF approaches by FESEM can be employed to unravel spatial association of (sub)cellular entities for a comprehensive understanding of complex biological systems.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.19 no.8
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• pp.176-182
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• 2018

One of the most important factor of esthetic prosthesis is the color of the teeth. Zirconia is produced in the process of design, milling, coloring, drying, and sintering. The effect of the drying degree of zirconia colorant on the color tone is examined. For the experiment, a total of 45 zirconia specimens(15 for each) were fabricated by using cad/cam system. The zirconia specimens(L block(LAZOR), Z block(Zircos-E block posterior), A block(AlphaZ)) were then subjected to zirconia sintering in accordance with the manufacturer's instructions, using a chemical coloring agent to Non-drying(0sec), intermediate (10sec), and completely dried (10min) temperature holding times. The color tone was measured using a spectrophotometer. The results were analyzed with a One-way analysis of variance and the Tukey post-hoc test (${\alpha}=.05$) The $L^*$ values of L, A specimens, $a^*$ values of A specimens, and $b^*$ values of Z specimens were not statistically different according to the degree of drying (p>0.05), While the $L^*$ value of Z specimen, $a^*$ value of L, Z specimen, $b^*$ values of L, A specimens were statistically different according to the degree of drying(p <0.05). In conclusion, the drying of zirconia colourants improves zirconia brightness and increases the optical properties of the enamel.