The effect of potential investment expansion and hot strength on the fit of full crown castings made with phosphate-bonded investment.

In an earlier investigation, it was shown that when full crowns are cast in gypsum-bonded investments, their relative inaccuracy is affected by both the investment's potential expansion and its hot strength. This study repeated the earlier one, but used a high-melting gold alloy and two phosphate-bonded investments. The investments were used under conditions which gave a range of potential expansions and hot strengths. Casting inaccuracies were determined both diametrally and axially. All castings showed distortion, which varied under the different conditions. All were oversized axially, by amounts varying from + 0.8% to +2.3%. Diametral inaccuracies ranged from -0.2% to +0.7%. Investment expansion had a strong effect on axial inaccuracy, but a negligible effect on diametral inaccuracy. Conversely, hot strength had a strong effect on diametral inaccuracy, but only a very weak effect on axial inaccuracy. With phosphate-bonded investments, both potential expansion and hot strength are important parameters of relative casting inaccuracy. In combination, these properties showed very strong correlations with both diametral and axial inaccuracies. The observed distortions were the result of anisotropic mould expansion and anisotropic alloy shrinkage. The best fit, and least distortion, occurred with an investment setting under dry conditions.

The effect of potential investment expansion and hot strength on the fit of full-crown castings made with a gypsum-bonded investment.

OBJECTIVES: It has been claimed that the strength of the investment mold at the casting temperature affects the dimensional accuracy of castings (e.g., Markley, 1953; Earnshaw, 1957; Asgar, 1972), but the relationship has not been studied quantitatively. In this investigation, the effects of both mold expansion and hot strength on the relative inaccuracy of full-crown castings have been measured and analyzed. The likelihood that a strong investment could cause distortion of the casting by non-uniform restriction of casting shrinkage (Earnshaw, 1969b) was also considered. METHODS: Castings were made with a commercial gypsum-bonded inlay investment, used both as supplied and with modifications that increased its expansion and reduced its hot strength. In both series of tests, the investments were used over a wide range of liquid/powder (L/P) ratios in casting rings fitted with dry ceramic liners, and set under dry conditions. RESULTS: Casting inaccuracy showed a significant linear correlation with total expansion and a highly significant linear correlation with the combination of total expansion and hot strength. The modified investment, with its low hot strength, gave less distortion of casting shape than did the much stronger unmodified material. However, it was found that to ensure sound castings, the hot compressive strength should not be less than 1.8MPa. SIGNIFICANCE: This investigation showed that while investment expansion is the major variable affecting casting inaccuracy, hot strength is an important modifying factor which also has to be considered when predicting casting inaccuracy from measured properties of the investment.

The fit of gold-alloy full-crown castings made with pre-wetted casting ring liners.

Measurements were made of the fit of full-crown gold-alloy castings produced with two types of pre-wetted ring liner (asbestos and cellulose) and a typical gypsum-bonded investment (Cristobalite Inlay, Sybron/Kerr Products, Romulus, MI; W/P = 0.40). Laboratory measurements were made of the effects of the liners on potential investment expansion, and properties of the lining materials considered relevant to casting accuracy were also measured. There was a wide variation in values for mean dimensional inaccuracy. One liner produced a series of castings all of whose inaccuracies lay within the range +/- 0.1%, with a mean value of + 0.01%. With the other five liners, all or most castings were undersize. With three, all or most castings showed inaccuracies worse than -0.2%. The values for casting inaccuracy with the various liners showed a probable correlation with potential investment expansion (p < 0.05); however, no correlation was found between casting inaccuracy and any apparently relevant liner properties, alone or in combination. In casting techniques which use a pre-wetted ring liner, the choice of a specific lining material is an important factor which has a significant effect on casting inaccuracy. With at least three of the six liners tested, a higher investment expansion was needed for accurate full-crown castings to be ensured.

The fit of gold-alloy full-crown castings made with ceramic casting ring liners.

Measurements were made of the fit of gold-alloy full-crown castings produced with dry ceramic ring liners. When used with vacuum investing, these liners absorb relatively large amounts of water from the investment mix (thereby reducing its original W/P ratio) and then function as wet liners, thus increasing the investment's potential expansion and giving castings which are consistently larger than when air investing is used. With four of the five liners tested, investing in air produced many castings which were unacceptably undersized (inaccuracy worse than -0.2%). The fifth liner, an industrial material 2 mm thick, gave only one casting out of 12 which was outside this limit, although all castings were undersized to a lesser extent. Vacuum investing gave improved casting accuracy; with four of the five liners, the improvement was highly significant (p < 0.001), and with the fifth, probably significant (p < 0.05). Even with vacuum investing, however, with only two of the liners did all castings show inaccuracies within +/- 0.2%. With the other three liners, some castings (ranging from 2/10 to 7/9) had inaccuracies worse than -0.2%. With both air and vacuum investing, changing from one liner to another caused changes in relative casting accuracy which were often significant (p < 0.01) or highly significant (p < 0.001). In casting techniques where a ceramic ring liner is used, the choice of specific lining material and the choice between investing in air or under vacuum are important factors which can have a major effect on the fit of castings.

Dimensional accuracy of small gold alloy castings. Part 4. The casting ring and ring liners.

The role of the casting ring and its asbestos liner is discussed. Asbestos as a liner has now largely been replaced by two alternative materials, one based on cellulose and the other on ceramic fibres. The limited literature on the effect of these newer materials on casting accuracy is also reviewed as their introduction may require significant changes in the traditional technology of dental casting.

Dimensional accuracy of small gold alloy castings. Part 3. Gypsum-bonded investment expansion.

The literature on the setting and thermal expansion of investments under both laboratory and practical conditions is reviewed.

Dimensional accuracy of small gold alloy castings. Part 1. A brief history and the behaviour of inlay waxes.

Part 1 reviews published studies which relate to materials and techniques contributing to the dimensional accuracy of small dental castings. A brief history of the cire perdue or lost-wax method of casting alloys and the development of modern dental casting materials and techniques are presented. Data on the behaviour of inlay wax, in particular its contraction and distortion, are reviewed as is the extensive literature about the casting shrinkage of gold alloys (Part 2) and the setting and thermal expansion of investments under both laboratory and practical conditions (Part 3). Part 4 discusses the role of the casting ring and its asbestos liner. Asbestos as a liner has now largely been replaced by two alternative materials, one based on cellulose and the other on ceramic fibres. The limited literature on the effect of these newer materials on casting accuracy is also reviewed, as their introduction may require significant changes in the traditional technology of dental casting.