Evaluation of an improved centrifugal casting machine

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.

Influence of the final temperature of investment healting on the tensile strength and Vickers hardness of CP Ti and Ti-6Al-4V alloy

The aim of the work was to evaluate the influence of the temperature of investment healting on the tensile strength and Vickers hardness of CP Ti and Ti-6Al-4V alloy casting. Were obtained for the tensile strength test dumbbell rods that were invested in the Rematitan Plus investment and casting in the Discovery machine cast. Thirty specimens were obtained, fifteen to the CP Titanium and fifteen to the Ti-6Al-4V alloy, five samples to each an of the three temperatures of investment: 430 degrees C (control group), 480 degrees C and 530 degrees C. The tensile test was measured by means of a universal testing machine, MTS model 810, at a strain of 1.0 mm/min. After the tensile strength test the specimens were sectioned, embedded and polished to hardness measurements, using a Vickers tester, Micromet 2100. The means values to tensile tests to the temperatures 430 degrees C, 480 and 530: CP Ti (486.1 - 501.16 - 498.14 -mean 495.30 MPa) and Ti-6Al-4V alloy (961.33 - 958.26 - 1005.80 - mean 975.13 MPa) while for the Vickers hardness the values were (198.06, 197.85, 202.58 - mean 199.50) and (352.95, 339.36, 344.76 - mean 345.69), respectively. The values were submitted to Analysis of Variance (ANOVA) and Tukey Test that indicate differences significant only between the materials, but not between the temperature, for both the materials. It was concluded that increase of the temperature of investment its not chance the tensile strength and the Vickers hardness of the CP Titanium and Ti-6Al-4V alloy.

Report of Committee on Scientific Investigation of the American Academy of Restorative Dentistry

A few personal points seem appropriate in summary. It is axiomatic that while research solves problems, it also creates them. As an example, it is an accepted fact that dentistry more than any other profession has made serious inroads into putting itself out of business through research. Each year this report takes note of the reduction in dental caries in children, which has thus triggered a change in the nature of general practice. Be this as it may, it is well to acknowledge that major dental diseases are not disappearing, but their patterns in the population are changing, accompanied by an expansion of other services. Despite the optimistic picture painted earlier in this report, it is grossly premature to acclaim the elimination of caries, and thereby a significant segment of restorative dentistry. Of course the younger age groups have been the major beneficiaries of caries reduction. As the child ages from 9 to 16 years, the percentage of mouths free of caries drops by one half. In addition, the disease pattern in the adult population is being altered because of a longer life span and loss of fewer teeth. Unquestionably what we will, and are already seeing, is not an elimination of restorative dentistry but a different target and changes in procedures because of new materials, therapy, and expanded scope of services. There is yet another subtle change in the trend of dental research and it deals with the reduction in the number of dental schools worldwide and smaller class sizes in those schools. Dental research thereby suffers because of the traditional link between education and research. Thus the potential pool of researchers becomes smaller and research monies for training programs for dental investigators, particularly in clinical research, decreases. This year a surprising number of articles are concerned with manpower as it relates to research priorities. So, it is indeed a time that both the profession and the scientific community find filled with controversy and frustration. Yet never has there been a time that offers more in challenges and rewards.