Study of resin-bonded calcia investment. Part 3: Hardness of titanium castings.
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The Vickers hardness of the cross-sectioned area of titanium castings made from an experimental resin-bonded calcia investment and three commercial investments was evaluated. The microstructure and element distribution of the surface zone were analyzed using an EPMA. The results showed that the high hardness of the casting surface could be decreased using the experimental investment. The hardness of the castings made from the experimental investment at a 25-50 microm depth was lower than those from the other investments, and the thickness of the hardened casting surface was 125 microm. Layered structures with fewer layers were formed on the surfaces of the castings made from the experimental investment. The layered structures were influenced by both the investment components and the mold temperature at casting. The less contaminating nature of the experimental investment components and the technology of the room temperature mold contributed to the improved surface properties of the resulting castings. (+info)
Determination of the fabricating conditions for the preferable marginal and internal adaptation of the mica crystal castable ceramic crown.
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This study was performed to find an acceptable internal adaptation of castable ceramics containing mica and beta-spodumene crystals. The influences of factors, expansion rate of phosphate-bonded investment (A), anisotropic expansion (B), diecoating (C), shrinkage during crystallization (D), and interaction (A x B), and (A x C) were tested by twice repeated experiments under block design according to L8(2)(7) orthogonal array. Estimated mean ranges under the conditions combined with significant factors were judged by considering the criteria of the ideal internal gap (about 50 microm). The ideal marginal fit of less than 50 microm and uniform cement space about 50 microm around the axial wall could be achieved by a combination of optimum levels of A1B1C2D2. However, the estimated mean gap at the cusp tip and central fossa of occlusal inside by this combination were about 120 microm. The near intolerable gaps could not be reduced. (+info)
Development of casting investment preventing blackening of noble metal alloys part 3. Effect of reducing agent addition on the strength and expansion of the investments.
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Different reducing agents (B, Al, Si and Ti) were individually added to two gypsum-bonded investments to prepare investments preventing surface blackening of some noble cast alloys. The effect of different additive contents on green-body and burnout compressive strength, setting and thermal expansion of the investments were evaluated. The strength and expansion of the investments were changed by the additives. The compressive strength of Al-, Si- and Ti-added investments decreased with the increase of additive contents. The burnout strength of B-added investments significantly increased while green-body strength remained unchanged. The setting expansion of the B-added investments increased while those of the Al-, Si- and Ti-added investments decreased with the increase of additive contents. The thermal expansion of the Si- and Ti-added investments decreased, and that of the Al- and B-added investments remained unchanged. Further study is necessary to evaluate the effects of these additives on the accuracy of dental castings. (+info)
Dimensional changes of ring-shaped pattern.
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Ring shaped wax patterns, having the same outside diameter and different inside diameters, were invested with a gypsum-bonded cristobalite investment. The wax pattern was eliminated in an electric furnace at 120 degrees C. A fusible alloy with a melting point of 47 degrees C was cast at room temperature. The dimensional deviations between the fusible alloy casting and the wax pattern were calculated using the inside diameter, ring width and outside diameter. On the other series, a gold alloy casting of the same size was fabricated in the usual manner of the dental precise casting procedure, and the dimension was compared with that of the wax pattern. In the comparison of 2 types of patterns, dimensional change by setting expansion was different. Dimensional change of the small inside diameter specimen differed at 3 portions measured, but that of the large inside diameter specimen was comparable at 3 portions. Concerning the resultant gold alloy casting, dimensional change at the outside diameter differed from each other, but those at ring width and inside diameter were comparable to each other. The difference in the inside diameter influenced dimensional change by setting expansion as well as that of the resultant casting. (+info)
Gypsum-bonded investment and dental precision casting (III) Composition of investment for the quick casting technique.
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A simultaneous differential thermal analysis and thermogravimetry method previously established was used to estimate the composition of gypsum-bonded investment marketed for the quick casting technique. Three commercial investments of this type were heated to 700 degrees C at 10 degrees C/min and the hemihydrate content was estimated by the mass decrease reached at 300 degrees C after subtracting the mass decrease at 100 degrees C as moisture content. The hemihydrate contents were between 25% and 30%, which appears to be the range also chosen for the conventional gypsum-bonded investment of cristobalite type over 70 years by the industry. However, the new type of investment contained both cristobalite and quartz. The small sample size is a disadvantage of the present method but this can be overcome by more frequent use of the method by investigators. (+info)
Phosphate-bonded ZrSiO4 investments added with ZrC and ZrN for casting titanium.
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In this study, new investments for titanium were developed by adding ZrC or ZrN as chemical additive for thermal expansion to a phosphate-bonded zircon (ZrSiO4) investment. The following effects were then examined: setting expansion, residual thermal expansion, and compressive strength of these experimental investments; surface roughness of cast plate; and casting accuracy of titanium crown. For residual thermal expansion, it occurred even while investments were cooled to room temperature after firing in air atmosphere. This was due to the additives' oxidation to ZrO2--suggesting that residual thermal expansion increased with increased amount of these additives. As for casting accuracy of full-crown cast into molds at room temperature, it correlated with the ZrN content. Hence by adding the right amount of ZrN, cast titanium crowns with low surface roughness and good adaptability could be obtained. (+info)
Gypsum-bonded investment and dental precision casting (IV) transformation of III-CaSO4 to II-CaSO4.
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The degree of III-CaSO4 to II-CaSO4 transition was estimated on wet specimens, 25 mm in diameter and 50 mm high, prepared from a gypsum-bonded investment for quick casting. After 30 min from mixing the specimens were heated in a 700 degrees C furnace until a specimen temperature of 200 degrees C to 700 degrees C was reached. The estimation was made by measuring mass changes of the specimens before and after heating. The transition reached 39% at 350 degrees C. The same estimation method, when applied to a previous study, showed no transition to this temperature in dry specimens heated slowly (5 degrees C/min). The large difference in transition found between the wet and dry specimens was attributed to the formation of alpha- and beta-hemihydrate, respectively. At 350-450 degrees C, the transition was 3% and 48%, respectively. The pronounced latter transition, generally identified by differential thermal analysis of gypsum, appears as the major investment shrinkage demonstrating its dry dehydration process. (+info)
The thickness effects of titanium castings on the surface reaction layer.
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In this study, wedge-shaped CP titanium castings and Ti-6Al-7Nb alloy castings were produced by three types of investment. The effects of the castings' thickness on surface reaction layer were investigated by measuring Vickers hardness, observing the microstructure, and analyzing the oxygen concentration. It was found that the thickness of the surface reaction layer was affected by the thickness of the wedge-shaped castings, and that the hardness value near the surface became lower toward the tip of the wedge-shaped CP titanium and Ti-6Al-7Nb alloy castings. The surface reaction layer of the Ti-6Al-7Nb alloy castings was thinner than that of the CP titanium castings. (+info)