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At least 37 records · Page 2Linked to original sources
PubMed2005

Modeling the surface contamination of dental titanium investment castings.

OBJECTIVE: The objective of this study was to develop a computational tool for assisting the design of titanium dental castings with minimal defects and to compare computational simulations with casting experiments. METHODS: Modeling. An in-house cellular-automata solidification and finite-difference diffusion program was coupled with a commercial casting program and applied to (a) simple geometric wedge models and (b) a 3D-laser scan of a molar crown casting. Experimental. Wedges and molar crowns were hand-waxed and investment cast in commercial purity grade 1 (CP-1) titanium by a commercial dental laboratory. The castings were sectioned and analyzed using light and scanning electron microscopy, X-ray microanalysis, and microhardness testing. RESULTS: In the wedge sample, contamination with impurities (Al, Si), including intermetallic precipitates, was found to extend to a depth ranging from 30 to 120 microm depending on the section thickness and hence the local cooling rate. Microstructural and mechanical (hardness) effects were found to a depth ranging from 80 to 250 microm. The coupled micro/macro model predictions showed reasonable agreement for the pattern of contamination. SIGNIFICANCE: Dental and medical applications demand close dimensional tolerance and freedom from surface impurities and structural flaws in castings having unique shapes. The ability to predict the structural, mechanical, and chemical changes resulting from the casting process will help to design the casting and post-casting processes to minimize these problems.

Aluminum↗
PubMed1975

Permeability and porosity of dental casting investments.

Permeability of various gypsum- and phosphate-bonded investments was measured during conventional burn-out procedures. Porosity determinations were made on specimens cooled to room temperature after burn-out. As a group, the gypsum-bonded investments were found to be more permeable than the phosphate-bonded investments. Two phosphate-bonded investments were determined to be relatively impermeable to gas flow, while another exhibited permeability comparable to that of the gypsum-bonded investments. In spite of differences in permeability, the porosity of each type of investment was nearly constant. The porosity of the phosphate-bonded investment was approximately three-fourths that of the gypsum investments. These investments were modified by the addition of varying amounts of acrylic polymer for the purpose of altering permeability. The addition of acrylic polymer increased porosity and permeability of all of the materials included in this investigation. The acrylic additives, however, had no effect on the permeability of relatively impermeable investments unless used in high concentration. The results of this investigation would tend to substantiate the need for special spruing and venting procedures.

Acrylic Resins↗
PubMed1986

Thermal behavior of the gypsum binder in dental casting investments.

This study examined the thermal behavior of cast gypsum specimens, with and without additives, by means of simultaneous differential thermal analysis-thermogravimetry (DTA-TG) and dilatometry. Specimens were prepared from wet-calcined hemihydrates (Hydrocal and Densite). The additives studied were boric acid (H3BO3) and sodium chloride (NaCl), and these were added to the hemihydrate powders in concentrations of 2 wt% (in the case of H3BO3) and 0.5 wt% (in the case of NaCl). A large shrinkage was observed in the range of 300 to 500 degrees C, and this was greatly reduced when either H3BO3 or NaCl was present. The dehydration of gypsum (calcium sulfate dihydrate) was not completed until the initial stage of this large shrinkage was reached, but the phase transition of calcium sulfate anhydrite (III-CaSO4 to II-CaSO4) was the major cause for the large shrinkage. This phase transition occurred over a much wider temperature range than that suggested by the DTA-TG results. Dehydration conditions similar to those employed in wet calcination of gypsum appeared to be produced under atmospheric pressure when NaCl was present.

Boric Acids↗
PubMed2004

The longitudinal accuracy of fit of titanium implant superstructures superplastically formed on investment models.

OBJECTIVES: To measure the effect of three special liquid proportions and five forming temperatures on the length of superplastically formed titanium dental implant superstructures made using Rematitan dental casting investment. METHODS: Rematitan dental casting investment was used to make models that contained two implant abutment analogues 8 mm apart. The investment was hand-mixed with 30, 50 or 70% special liquid proportions and ten models of each were made. Dies for superplastic forming (SPF) were made that contained three models (one for each special liquid proportion). Five dies were made for forming at 810 degrees C and five dies for forming at 810, 840, 873, 900 and 927 degrees C using 2 mm thick Ti-6Al-4V discs. The distance between the abutment impressions in the formed discs was measured with a traveling microscope and compared with the distance between the abutments of the master model to determine the most accurately fitting superstructure beam. RESULTS: The proportion of special liquid used to make the SPF models had an unpredictable effect on the length of the superstructure at each forming temperature. Superstructure beams formed at lower temperatures had a better fit compared to the master model for all special liquid proportions. The most accurately sized beams were formed at 810 degrees C. SIGNIFICANCE: SPF can provide an alternative method for the production of dental implant superstructures. The fit was affected predominantly by the forming temperature, but also by the special liquid proportion used for making the SPF investment model.

Alloys↗
PubMed1998

Gypsum-bonded alumina dental investment for high-fusing casting.

In this study, we developed a new gypsum-bonded investment for high-fusing alloys. The investment was composed of gypsum as a binder and alumina as a refractory. Effects of type of alumina powder and gypsum content on characteristics of the gypsum-bonded alumina investment were investigated. Obtained characteristics of this experimental investment were as follows: fluidities ranged from 48.8 to 88.9 mm; setting times ranged from 21.2 to more than 120 minutes; setting expansions ranged from 0.4 to 1.3%; green strengths showed 0.5 to 4.5 MPa; fired strengths ranged from 0.2 to 1.7 MPa; thermal expansions after firing were -1.60 to 2.16%. Thermal expansion occurred because of the chemical reaction between Al2O3 and CaO decomposed from gypsum. These results suggest that this gypsum-bonded alumina investment with 20 or 25 mass% gypsum content possessed the fundamental properties for high-fusing alloy casting.

Aluminum Oxide↗
PubMed2004

Gypsum-bonded investment and dental precision casting (III) Composition of investment for the quick casting technique.

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.

Calcium Sulfate↗
PubMed2003

Gypsum-bonded investment and dental precision casting (I) two investments.

The present study compared the TE of two gypsum-bonded dental casting investments, the oldest K and more recent G. They had almost identical composition, 70% refractory (cristobalite) and 30% binder, but had different recommended water/powder ratios, 0.40 (K) and 0.33 (G). The average TE was significantly less (1.19%) in K than in G (1.45%); the volume decrease accompanied by the phase change of gypsum was more pronounced in the less dense K. When the dehydration conditions around gypsum are similar to those prevailing in wet calcinations methods, an expansion is likely to occur in the mold due to the formation of dental stone by recrystallization. This additional expansion has not been detected in ordinary laboratory measurement but can effectively increase the actual TE of an investment. The present study has proved the formation of dental stone by rapid heating of an investment. Specific laboratory techniques may have been supplementing low TE by this mechanism.

Calcium Sulfate↗