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METAL 2004 Hradec nad Moravicí

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CONSIDERATIONS REGARDING THE METHODS USED FOR THE ASSESSMENT OF CASTABILITY OF DENTAL CASTING ALLOYS

Marius Bibua

Carmen Simiona

Valentin Petrescua

Toderita Nemesa

a "Lucian Blaga" University of Sibiu, Emil Cioran str. 4, 550025 Sibiu, Romania,

[email protected]

Abstract The paper comprises a comparative analysis of the most widely used theoretical and practical criteria for the assessment of the castability in the case of casting alloys employed for the realising of dental prosthetic parts. Also, a new practical method (and the corresponding test pattern) for the castability assessment is presented and some results of its practical usage are presented and discussed. 1. INTRODUCTION

Currently, more than ever, the specialists from any technical branch are confronted with a large number of materials that they could use for various applications. In order to remain up-to-date with the newest achievements with regard to materials, they need to upgrade continuously their technologies and their equipment. This is true, not the least, for the case of dental technicians, who must keep in touch not only with developments in the medical domain, but also with the researches in the field of materials science and technology. For example, titanium has become in the past years the material of choice in dental restorations, because it offers a series of advantages compared to other metals: it has a low specific weight, is absolutely biocompatible, has an excellent corrosion resistance, a low heat conductivity etc. Among its disadvantages, however, there is a very high melting point and a high chemical reactivity. This makes its processing very difficult, requiring special laboratory equipment. Also, the casting of dental parts is itself a high-precision casting technology, very pretentious in its execution, as many parts have wall thicknesses of the order of tenths of millimeters, while in some cases the allowable dimensional errors must be limited to ?0,05...?0,1 mm. The causes for the failure of dental casting processes and the failure types are very numerous, and they demand the full attention and experience of the specialists working in dental laboratories, as well as the usage of high-end techniques and equipment. It is therefore important to analyse the casting operations and the obtained parts also from a qualitative point of view, taking into account the materials used and the accuracy of the employed technologies. 2. CASTABILITY AND THEORETICAL CASTABILITY ASSESSMENT METHODS The notion "quality of the cast parts", which is related to the presence or absence in the cast parts of defects such as pores, shrinkage holes etc. is strongly related to the notion of "castability" of the material of which the parts are made. The latter can refer to both the completeness of the casting, i.e. to the cast part proportion which could be correctly reproduced during the casting process, and to the casting precision, i.e. to the minimal


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dimensions of the details which could be reproduced. In the particular case of dental technics, the notion of castability is aiming exclusively at the first-mentioned meaning. Because of the current diversity of cast dental prosthetic parts, as well as of the variety of possibilities to realise the casting sprues and molds, the assessment of the castability gains a crucial importance, especially when handling materials with new components or new composition ratios. The casting process and the castability of materials can be influenced by a wide range of factors and therefore it is very difficult to find a single definition to cover all aspects of this notion, or to define a dimension of the castability. Various authors [4, 5] have tried to determine a castability value by means of theoretical calculations. However, in order to keep these calcula tions in a reasonable range of complexity, they can take into account only a part of the influencing factors while neglecting others. For example, taking into account only the effect of the alloy's overheating and of the mould preheating temperature, S. Hirano [6] has determined following equation for the castability calculus, applied to a plane mesh pattern:

a2mt,v TbTaC ?? , (1)

where v

v

t,v

C132

C32

lnC??

?? , (2)

Cv = the fraction of segments which were completely cast; Tm = the mould's preheating temperature; Ta = the alloy's overheating temperature; a, b = material variables.. On the other hand, if we take into account only the interaction between the solidifying time of the alloy and the advancement speed of the molten alloy through the casting sprue, the distance which can be covered by the alloy, as a measure of castability, can be calculated as [4]:

? ? ? ????

??? ????? bttlnbbtt

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hK2L 22 , (3)

where: L = advancement distance of the molten alloy;

h = height of the molten metal' s surface above the reference point: K = 4? 2a2R; a = acceleration of the centrifugal casting machine's arm; R = length of the centrifugal casting machine's arm; t = time needed for the alloy's advancement;

b = rhK2

4?

? (4)

? = superficial stress of the olten alloy; ? = the molten alloy's density; r = radius of the gate runner through which the molten alloy flows.


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3. EXPERIMENTAL CASTABILITY ASSESSMENT METHODS Currently, the most widely used experimental castability assessment methods are related to the usage of test patterns with special shapes and sizes. Such test patterns are the saucer (disc) pattern, the wedge patterns, flat or spatial mesh patterns, flat or spatial coil patterns. Figure 1 presents two examples of such patterns [5].

a b Figure 1 a - flat mesh pattern; b - saucer pattern [5] The methods and casting patterns currently in use attempt to provide the dental technicians with informations regarding the castability of metallic materials. However, with all their diversity, the information provided by these methods are relatively limited, or have a much too particular character. The saucer patterns, for example, offer only information limited by the fixed diametre of the patterns, by the fixed thicknesses of the pattern's cross-section and mainly by the relatively short distance which the molten material must go through. Thus, the assessment of castability with this method becomes subjective, not least because of the equal importance granted to the completeness of the various cross-sections found on the test patterns. Similar critiques can be brought also against the other types of test patterns. A direct consequence of these limitations is the fact that, currently, there exists no casting pattern that would be easy to manufacture, simple to use and that would offer complete information on the casting behaviour of a certain metallic material. A perfect method, which would offer complete information on the multiple aspects implied by a dental alloy's castability is, certainly, difficult to conceive. The methods used for assessing the castability should focus on offering the most important data, namely data related to both the influence of the size of the runner cross-section, and the changes of direction in the flow of the molten metal, on the obtained part, for a specific casting procedure. It is also useful to express the castability as precise as possible, based on widely-accepted criteria or measurement units. Furthermore, a good castability assessment method should allow an easy manufacturing of the pattern associated with it, an easy adaptation to the various shapes and wall thicknesses encountered in dental technics and the collecting of a maximal amount of data with the help of a minimal number of cast patterns.


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3. A NEW METHOD FOR THE ASSESSMENT OF CASTABILITY As a result of the analysis of the advantages and disadvantages displayed by the test patterns which are currently in use for the assessment of the castability in dental laboratories, a new assessment method was developed, based on a new type of test pattern. As shown in figure 2, the new proposed test pattern, labelled "D pattern", consists of three separate runners of circular cross-section with different diameters, each of them being bent along sinusoidal curves. The diameters of the branches can be varied according to the various thicknesses present in the prosthetic part which is about to be cast and for whose manufacturing the best-suited material is sought. The number of branches (runners) can also be varied according to the needs.

a

b Figure 2. D pattern: a – sketch; b – example cast from Wirolloy alloy, with branch diameters

of 2.5, 2 and 1.5 mm


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The casting cone which is formed at the casting of the pattern can be preserved as part of the pattern, as it facilitates its manipulation. Some of the advantages of the "D pattern" over other pattern types are the possibility to gain simultaneous information on the behaviour of the tested metallic material during its flow (in molten state) through long runners with various cross-section sizes and with several changes of direction, the capability of adapting the "D pattern" to virtually any cross-section size or wall thickness used in the prosthetic parts employed in dental technics and, not least, the relative simplicity of manufacturing this pattern type. For the assessment of the castability of dental metallic alloys by using the new test pattern D, a castability coefficient Ct,di can be introduced (for the diameter di of the branch i of the test pattern): Ct,di = (5 - di).5 + (20 – mi) + 2ni, (5) where di and mi represent the dimensions as indicated in figure 1 a; ni represents the number of completely cast "si" distances resulted on the cast pattern. As a practical example of using the new method, figure 1 b shows a D pattern which was cast from a Ni-Cr-based alloy, Wirolloy (63.2 % Ni, 23.0 % Cr, 9.0% Fe, 3.0 % Mo, 1.8 % Si, less than 0.1 % C), with runners of diameters Ø2.5, Ø2 and Ø 1.5 mm, respectively. The castability coefficients for the three branches of this pattern are: Ct,2,5 = (5-2.5).5 + (20-18) + 2.5 = 24,5 Ct,2 = (5-2).5 + (20-18) + 2.5 = 27 Ct,1,5 = (5-1.5).5 + (20-18) + 2.5 = 29,5. These values indicate a high castability of the Wirolloy alloy, so this material is suitable for casting dental prosthetic parts with even very fine details. 4. CONCLUSIONS The assessment of the quality of cast prosthetic parts, together with the assessment of the castability of metallic casting materials, occupies an important place in the activities of dental laboratories. The various theoretical or practical methods which are currently used for the assessment of the castability, cannot characterise this property by taking into account all factors involved. Therefore, a new method, based on the usage of a new type of test pattern, has been developed and tested in practice, with good results. The new "D pattern" offers several advantages over the classical patterns, such as the possibility of assessing the capability of a metal to enter and fill out in its molten state thin and/or long segments of a casting mold by means of one single casting operation and the capability to adapt the pattern to different cross-sections as needed. Furthermore, the new test pattern is easy to manufacture and use in dental laboratories. BIBLIOGRAPHY [1] DEAC, V., BIBU M. a.o. The casting of titanium in dental prosthetics (in Romanian), Publishing House of the University of Sibiu, Sibiu, 1995 [2] DEAC, C. Contributions to the technological development of the melting and casting of some metallic materials used in dental technics (in Romanian), Ph.D. thesis, "Lucian Blaga" University of Sibiu, 2003, 167 p. [3] KROSZEWSKY, K., HÄSSLER, C., GEHRE, G. Nachweis von Strukturinhomogenitäten zahnärztlicher Gussobjekte mittels Farbstoffdiffusion, Deutsche Zahnärtzliche Zeitschrift vol. 54, no. 10, 1999, pages 641-643.


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[4] LUK, H.W.-K., DARVELL, B.W. Casting system effectiveness – measurement and theory. Dental Materials, vol. 8, no. 3, 1992, pages 89-99. [5] WATANABE, I. a.o. Effect of casting method on castability of titanium and dental alloys. Journal of Materials Science: Materials in Medicine, vol. 11, no. 6, 2000, pages 547-553 [6] HIRANO, S. a.o. Casting of dental alloys: mold and alloy temperature effects. Dental Materials, vol. 3, no. 4, 1987, pages 307-314.

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