A comparison of the casting ability of precious and nonprecious alloys for porcelain veneering

Analysis of four dental alloys following torch/centrifugal and induction/ vacuum-pressure casting procedures

STATEMENT OF PROBLEM

Previous studies have shown casting methodology to influence the as-cast properties of dental casting alloys. It is important to consider clinically important mechanical properties so that the influence of casting can be clarified.

PURPOSE

The purpose of this study was to evaluate how torch/centrifugal and inductively cast and vacuum-pressure casting machines may affect the castability, microhardness, chemical composition, and microstructure of 2 high noble, 1 noble, and 1 base metal dental casting alloys.

MATERIAL AND METHODS

Two commonly used methods for casting were selected for comparison: torch/centrifugal casting and inductively heated/ vacuum-pressure casting. One hundred and twenty castability patterns were fabricated and divided into 8 groups. Four groups were torch/centrifugally cast in Olympia (O), Jelenko O (JO), Genesis II (G), and Liberty (L) alloys. Similarly, 4 groups were cast in O, JO, G, and L by an inductively induction/vacuum-pressure casting machine. Each specimen was evaluated for casting completeness to determine a castability value, while porosity was determined by standard x-ray techniques. Each group was metallographically prepared for further evaluation that included chemical composition, Vickers microhardness, and grain analysis of microstructure. Two-way ANOVA was used to determine significant differences among the main effects. Statistically significant effects were examined further with the Tukey HSD procedure for multiple comparisons. Data obtained from the castability experiments were non-normal and the variances were unequal. They were analyzed statistically with the Kruskal-Wallis rank sum test. Significant results were further investigated statistically with the Steel-Dwass method for multiple comparisons (α=.05).

RESULTS

The alloy type had a significant effect on surface microhardness (P<.001). In contrast, the technique used for casting did not affect the microhardness of the test specimen (P=.465). Similarly, the interaction between the alloy and casting technique was not significant (P=.119). A high level of castability (98.5% on average) was achieved overall. The frequency of casting failures as a function of alloy type and casting method was determined. Failure was defined as a castability index score of <100%. Three of 28 possible comparisons between alloy and casting combinations were statistically significant. The results suggested that casting technique affects the castability index of alloys. Radiographic analysis detected large porosities in regions near the edge of the castability pattern and infrequently adjacent to noncast segments. All castings acquired traces of elements found in the casting crucibles. The grain size for each dental casting alloy was generally finer for specimens produced by the induction/vacuum-pressure method. The difference was substantial for JO and L.

CONCLUSIONS

This study demonstrated a relation between casting techniques and some physical properties of metal ceramic casting alloys.

Castability optimization of palladium based alloys

Poor or inconsistent castability often necessitates costly laboratory remakes of cast restorations. This study investigated the effects of burnout temperature and alloy investment compatibility on the castability of selected alloys. In part A nine alloy investment combinations were compared. In part B two alloy-investment combinations at four burnout temperatures were evaluated. Differences in investment, alloy, and/or burnout temperature significantly affected castability. Castability with a poorer casting investment improved with a hotter propane/oxygen fuel source compared with natural (city) gas/oxygen.

Button versus buttonless castings for removable partial denture frameworks

Casting removable partial dentures (RPDs) without completely filling the sprue channels and generating casting buttons would result in saving metal and making more defect-free castings. This investigation tested whether a complete and sound RPD casting can be obtained when a minimal amount of metal is used. A factorial experimental design, three spruing methods, two metal feeding directions, and two different weights of metal were used to cast 60 Kennedy class II, modification 1 RPDs. The metal used to cast each framework was either enough to result in a full button or in no button. Visual and radiographic examinations and counting of defects were made by two independent operators who were unaware of the spruing method, feeding direction, or amount of metal used to make the framework. The completeness of the casting and the presence of porosities were evaluated for clasps, major connectors, and meshworks. The use of minimal metal to cast RPDs was equally as successful as using enough for a full button, provided that the appropriate spruing arrangement and metal feeding direction were chosen. Indirect metal feeding for maxillary RPDs was successful with the proper spruing arrangement.

Digital imaging techniques for dental alloy castability quantification

In this study, mesh monitors cast from experimental compositions of a Ni-Cr-Be alloy are evaluated by the application of image analysis techniques. Castability values obtained by this method are then contrasted with those from three commonly employed manual counting procedures. While castability values obtained by all methods reflect the effect of compositional variations, a comparison of results with respect to evaluation method indicates that the image analysis technique consistently yields higher castability values, especially evident in the poorly casting groups. The apparent explanation for these observed differences is that with imaging, segments that are partially cast to varying degrees are not arbitrarily eliminated from the data, as is the usual practice in manual counting methods; therefore, castability values obtained by using the imaging technique will very closely reflect an actual alloy volume of each cast monitor.

Casting system effectiveness--measurement and theory

Evaluating castability as a property of dental casting alloys is an unworkable concept, since any measure is demonstrably affected by several external factors. Casting System Effectiveness (CSE) is shown experimentally and theoretically to depend primarily upon the time taken for the advancing front to freeze. A spiral tube mold was used to measure casting length obtained under variations in casting temperature; this pattern is proposed as a standard measure of CSE. A Bernoulli 'free-fall' numerical model is shown to reproduce the principal features of such casting, with some evidence of viscosity limitation of the turbulent flow at long casting lengths. Direct measurement of melt velocities confirmed turbulent flow in the mold. A method is also suggested for assessing the influence of investment porosity on CSE.

Veterans Administration Cooperative Studies Project No. 147. A precementation comparison of metal ceramic restorations made with a gold-containing alloy or alternative alloys

This analysis compared the precementation quality of metal ceramic restorations made from different alternative alloys after they had been returned from the central dental research laboratory. Ticon, Micro-Bond N/P2, Ceramalloy II, and W-1 materials were evaluated by use of 11 well-defined criteria and for overall performance. The evaluations were compared with those of Olympia, a gold-containing alloy, which served as the control. The results showed statistically significant differences between Olympia alloy and some of the alternative alloys for six of the 11 criteria. These differences, however, were not sufficient to be considered clinically significant (clinically detectable). In the comparison of their overall performances no statistically significant differences were found.

Mechanical properties of metal-ceramic alloys at high temperature

Four alloys used for the construction of porcelain-fused-to-metal restorations were tested at high temperatures (1000 degrees C) so that loss of properties could be ascertained. In addition, thin flat plates of metal (30 x 11 x 1 mm) with a 9-mm recess and a metal thickness of 0.35 mm were processed for porcelain application and distortion measured at 15 points along the surface prior to and following each firing cycle. Yield and ultimate tensile strengths and elongation were determined on specimens at room temperature and at 1000 degrees C. None of the alloys tested showed a great amount of distortion during the various firing cycles, yet they showed increased change with each successive heating. The greatest distortion occurred with Jelenko "O" alloy. When the mechanical properties were compared, there was a marked loss of both yield and ultimate strength values for each alloy (Jelenko "O"-Y.S., 95.64%, U.T.S., 96.22%; Olympia--Y.S., 95.08%, U.T.S., 96.76%; Tempo--Y.S., 91.23%, U.T.S., 93.46%; Biobond II--Y.S., 83.72%, U.T.S., 86.21%). Percent elongation increased by 272% with Biobond II and 370% with Jelenko "O", whereas Tempo and Olympia increased by 120 and 155%. It is apparent that the palladium-based alloy and the nickel-chromium-based alloy have higher mechanical properties at the temperature where porcelain will be applied.

Veterans Administration Cooperative Studies Project No. 147. Part IX: A comparison of the mechanical properties of several alternative metal ceramic alloys cast in clinical and research laboratories

Comparison of mechanical properties of five metal ceramic alloys cast in clinical and research laboratories resulted in significant differences. Tested alloys included one Au-Pd (control), one Pd-Ag, and three Ni-Cr. Mechanical properties included strength, elongation, and microhardness. Tensile bars were torch cast simultaneously with restorations in the clinical laboratory and induction cast by using a preformed design in the research laboratory. Mechanical properties of most alloys showed larger variations when cast in the clinical laboratory. In the clinical laboratory the Au-Pd alloy showed significantly higher values for all properties. One Ni-Cr alloy was not significantly affected by environment for all properties. The remaining alloys were significantly affected for some properties by environment. The different laboratories had no greater or lesser effect on the base metals as a group than on the noble metals. Variation in handling, spruing, casting, and specimen size may account for some of the differences between laboratories.

The effect of sprue attachment design on castability and porosity

Sprue attachment design is an important and controversial variable that can affect casting characteristics of castability and porosity. Four sprue attachment designs--straight, flared, abrupt constriction, and gradual constriction--were investigated. Standardized wax copings simulating complete veneer metal ceramic crowns with knife-edge margins were sculpted on refractory investment dies, sprued, invested, and cast with Olympia alloy. The specimens were embedded, sectioned, and polished. Castability was analyzed by measuring the width of the cast meniscus of the margins. The margin widths of the flared and the straight sprue attachment groups were significantly less than the abrupt or gradual constriction attachment group (p less than 0.05). Photomicrographs revealed discernible differences in the relative quantity and location of porosity in the sprue-coping junctions. The straight and flared sprue attachment groups were less porous than the abrupt or gradual constriction groups (p less than 0.01). Flared and straight sprue attachments optimized castability and minimized porosity.

Comparison of casting ability of castable ceramic and type III gold

This study compared the property of castability of a castable ceramic to an American Dental Association-certified type II casting alloy. A wax pattern was made with a manifold-type structure from which monofilament lines projected approximately 5 to 6 mm. The diameter of the nylon lines ranged from 132 micron to 1270 micron. Each pattern was invested and cast according to the manufacturer's specifications. Each casting was divested and measured for length of casting of the monofilament on a measuring microscope. The castable ceramic and type II gold cast completely at diameters of 1270 micron, and 724 micron, but only type II gold cast completely at the 152 micron and 132 micron diameters.

Evaluation of the castability of four different alternative alloys by measuring the marginal sharpness

The castability of two low-gold and two silver-palladium alloys was evaluated, using a Type III gold alloy as reference. Six castings from each alloy were subjected to a castability test, in which the marginal sharpness of simulated crowns--that is, the edge diameter--was used to assess the castability. The mean crown edge diameters of the silver-palladium alloys were three to four times the corresponding diameters of the gold-based alloys. The differences between these two groups were statistically significant. It is concluded that the castability of the low-gold alloys studied was comparable to that of the Type III alloy, whereas the silver-palladium alloys studied had a castability that may result in technical and clinical problems.

Evaluation of an improved centrifugal casting machine

A Type III gold alloy, a silver-palladium alloy, and a base metal alloy were cast in two different centrifugal casting machines. With the number of complete cast mesh squares as an indicator of castability, the Airspin casting machine produced superior castings with all three alloys. The base metal alloy produced the greatest number of complete squares with both casting machines.

A technique for characterizing casting behavior of dental alloys

A technique for characterizing casting behavior of dental alloys has been developed and tested. The method employs easily reproducible specimen patterns and uses equipment and procedures generally available in dental prosthetic laboratories. A castability value is arrived at by counting complete segments of a cast alloy grid. Test results indicate that the castability evaluation procedure described can be a valuable aid in the laboratory for both selecting and using dental materials for producing cast restorations. The test method would also appear useful for the evaluation of products and techniques during research and development. An absolute minimum for the castability value determined relative to clinical and laboratory efficacy has yet to be established.

The metallurgy of Ni-Cr alloys for fixed prosthodontics

Ni-Cr alloys continue to be an important part of dental restorative materials. It behooves those interested to study these alloys and develop an understanding of their chemical and physical characteristics. Introduction of new alloys almost monthly presents an educational challenge to the entire profession. Many dental schools do not use Ni-Cr alloys for fixed prosthodontics because questions exist about the efficacy and performance of such alloys. These doubts may be resolved by continued research. Concern about mechanical properties can be alleviated if an understanding of the requirements for successful use, for example, casting, burnishing, polishing, and stresses encountered in service, is achieved. Information on the nature of corrosion intraorally is needed, particularly with regard to correlating in vitro testing with in vivo performance. Further studies of porcelain-alloy compatibility are indicated to understand properties affecting compatibility. Performance of high-temperature porcelain-alloy systems could influence processing techniques so that specific materials could be used more appropriately. In addition, the nature of porcelain-to-alloy bonding may be revealed by use of advanced chemical analysis techniques to identify processes occurring at the atomic level, with identification of oxides and reacted zone produces may also provide data for design of new alloys for porcelain-metal systems. Finally, factors related to the clinical performance of nickel-based alloys, including their allergenic potential, have not been considered in this report.

Effect of investment on casting high-fusing alloys

A definite interaction in castability was found between the three different phosphate-bonded investments and four different alloys tested in this study. 1. Casting completeness of all alloys, as measured by the length of test specimens, was affected by the investment. The high-gold content alloy, SMG II, yielded the most complete castings of all combinations teted when invested with Ceramigold. Of the nonprecious alloys, Biobond CB combined with Ceramigold II resulted in the longest samples. 2. The surface quality of Jelbon and SMG II castings was good, regardless of the investment used. Wiron S and Biobond CB varied in surface quality with different investments, Ceramigold II giving the smoothest castings. 3. All alloys, especially the nonprecious ones, should be tested for compatability with different investments before asserting their qualities or lack of qualities for clinical use.

Castability of low gold content alloys

A test pattern composed of 14 gauge sprue wax and various gauges of nylon lines was constructed, and used to differentiate the castability of several alloys. The castability of seven commercial low gold content alloys for porcelain fused to metal restorations was compared. Five commercial low gold content yellow alloys for full cast restorations were compared to each other and to a Type III certified control.