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Table of Contents   
ORIGINAL RESEARCH  
Year : 2011  |  Volume : 22  |  Issue : 5  |  Page : 733
Evaluation and comparison of castability between an indigenous and imported Ni-Cr alloy


1 Department of Prosthodontics, Sree Balaji Dental College and Hospital, Chennai, India
2 Department of Prosthodontics, Faculty of Dental Sciences, Sri Ramachandra University, Porur, India
3 Department of Prosthodontics, Saveetha Dental College and Hospitals, Saveetha University, Chennai, India
4 Department of Prosthodontics, Noorul Islam College of Dental Sciences, Aralumoodu, Neyyantinkara, Trivandrum, Kerala, India

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Date of Submission29-Sep-2010
Date of Decision31-Mar-2011
Date of Acceptance02-Aug-2011
Date of Web Publication7-Mar-2012
 

   Abstract 

Context: Since 1907 casting restorations have been in use in dentistry. Numerous companies have been manufacturing and marketing base metal alloys. Gold was a major component of casting alloys. But alloys with less than 65% gold tarnished easily and the increase in cost of gold post-1970s lead to the revival of base metal alloys such as nickel-chromium and cobalt-chromium alloys which were in use since 1930s.
Aim: This study was conducted to evaluate and compare the castability between an indigenous alloy and an imported alloy, as imported base metal alloys are considered to be expensive for fabrication of crowns and bridges.
Materials and Methods: This study was conducted to evaluate and compare the castability (for the accurate fabrication of crowns and bridges) between an indigenous base metal alloy-Non-ferrous Materials Technology Development Centre (NFTDC), Hyderabad (Alloy A) -and an imported base metal alloys (Alloy B). Castability measurement was obtained by counting the number of completely formed line segments surrounding the 81 squares in the pattern and later calculating the percentage values. The percentage obtained was taken as the castability value for a particular base metal alloy. The percentage of castability was determined by counting only the number of completely cast segments in a perfect casting (81 × 2 = 162), and then multiplying the resulting fraction by 100 to give the percentage completeness.
Statistical Analysis Used: The Student t-test was used.
Results: When the castability of alloys A and B was compared, the calculated value was less than the tabular value (1.171 < 2.048) leading to the conclusion that castability between alloys A and B is insignificant. Therefore we conclude that both the alloys have the same castability.
Conclusions: Using the above-mentioned materials and following the method to test castability, we were able to derive favorable results. As the results were satisfactory, we can conclude that the castability of the indigenous alloy is on par with the imported alloy.

Keywords: Castability, plastic mesh, sprue

How to cite this article:
Ramesh G, Padmanabhan T V, Ariga P, Subramanian R. Evaluation and comparison of castability between an indigenous and imported Ni-Cr alloy. Indian J Dent Res 2011;22:733

How to cite this URL:
Ramesh G, Padmanabhan T V, Ariga P, Subramanian R. Evaluation and comparison of castability between an indigenous and imported Ni-Cr alloy. Indian J Dent Res [serial online] 2011 [cited 2020 Jan 17];22:733. Available from: http://www.ijdr.in/text.asp?2011/22/5/733/93471
Casting restorations were introduced in 1907 and Dr. William H. Taggart was given a major credit for its introduction. Casting restorations have numerous applications in restorative dentistry namely inlay, onlay, removable partial denture framework, and fixed partial denture frame work. Fixed partial denture is a frequently used prosthesis in today's practice. Initially the most common alloy used to fabricate the substructure was gold alloy. Prior to 1969, over 95% of FPD in the United States were made of alloys, containing a minimum of 75% gold and metals of the platinum group.

Gold alloys were used due to their good castability, marginal fit, and modulus of elasticity. But studies have proved that gold alloys have low strength. Due to this drawback traditional expensive gold alloys have paved way to newer less expensive alloys of similar properties. Base metal alloys replaced gold alloys.

Cast nickel-chromium alloys are used in the fabrication of crown and bridges [1] since the 1930s. These alloys were developed as substitutes for type III gold alloys. One of the prerequisite for an alloy is its castability to replicate the mold space. Nickel and cobalt-chromium alloys are used in porcelain fused-to-metal restoration.

Numerous companies have been manufacturing and marketing base metal alloys, the basic composition being nickel and chromium of various propositions with other modifiers. This study was conducted to evaluate and compare the castability between an indigenous alloy and an imported alloy. The indigenous alloy was procured from NFTDC, which is an Institute run by the Government of India. It is an R&D center which is into the development of specialty alloys and materials, components, process know-how and optimization, material testing and characterization, technical consultancy service, and complete technology development package on a turnkey basis.


   Materials and Method Top


Materials

This study was conducted to evaluate and compare the castability between an indigenous base metal alloys (alloy A)* - Nonferrous Materials Technology Development Center (NFTDC), Hyderabad [Figure 1] - and an imported base metal alloy (alloy B)** - Heraenium S from Heraeuskulzer [Figure 2]. The composition is summarized in [Table 1].
Figure 1: Indigenous alloy

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Figure 2: Imported alloy

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Table 1: Composition of alloy

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Method

Plastic meshes [2] [Figure 3] square diamond shaped, each 20 × 20 mm with 0.5 mm diameter cross-filaments, were attached with inlay wax covering the diamond square in the meshes for uniformity. The plastic mesh along with the sprue was rinsed in water to remove the superficial debris and sprayed with debubblizer (silik on and wachsEntspanner) to provide better surface contact of the investment material to the plastic mesh pattern. The ringless casting system (Thermofix - DFS Germany) was used [Figure 4]. The benefits of this system have already been recognized. Investment was done with phosphate-bonded investment (Heravest Universal N, Heraeuskulzer) [Figure 5]. The plastic meshes were attached to the sprue and mounted on crucible former [Figure 6].
Figure 3: Plastic mesh

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Figure 4: Wax sprue and ringless casting system

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Figure 5: Phosphate bonded investment material

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Figure 6: Sprued mesh pattern mounted on crucible former

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They were allowed to bench set for 1 hour and placed in a cool burnout furnace [Figure 7]. In a similar manner the other specimens of plastic patterns were invested. Five meshes were invested at a time; hence a total of six molds were obtained. A two-stage burn-out procedure was followed. The furnace was heated from room temperature upto 250°C and maintained at the same temperature for 30 minutes. The temperature was raised to 950°C and maintained for 30 minutes (heat soak period). The ceramic crucibles to be used in the induction casting machine were also heated in the same furnace. The molds were divided into two groups of 15 specimens each (alloys A and B).
Figure 7: Invested mesh

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The induction casting machine was used for casting the base metal alloys. Before keeping the molds in the furnace they are balanced on the centrifugal arm by adjusting the counter weight scale (this counter weight scale has markings for mold sizes 1-9 and weights of 200-600 g).

One pellet of alloy A (8 g) and two pellets of alloy B (6 g) were used per specimen.

The formula:

(A - B) × C = D

where A = weight of the sprued pattern, B = average weight of the button portion of the preformed wax sprue, C = specific gravity of the alloy, D = amount of alloy to be used, was used and the values were as follows:

A = 1.5 g

B = 0.92 g

C = 13.5 g

D = 7.83 g.

After the mold was cooled the specimens were divested and sandblasted using aluminum oxide of 250 mm diameter. The mesh portion of casting was separated from the sprue with fine disks. The weight of each sectional mesh was recorded in grams [Figure 8].
Figure 8: Divested and sand blasted mesh

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Measurement of castability

Castability measurement was done by counting the number of completely formed line segments surrounding the 81 squares in the pattern and later calculating the percentage values. The percentage obtained was taken as the castability value for a particular base metal alloy. The percentage of castability was determined by counting only the number of completely cast segments in a perfect casting (81 × 2 = 162), and then multiplying the resulting fraction by 100 give the percentage completeness.

The sample castings obtained with selected alloy have been designated as follows:

group I = 15 and group II = 15.

Thus, a total of 30 specimens were prepared. The above samples were subjected to statistical analysis.


   Results Top


The castability measurement was done by counting the number of completely formed line segments surrounding the 81 squares in the pattern and later calculating the percentage value. The percentage obtained was taken as the castability value for a particular alloy. The percentage of castability was determined by counting only the number of completely cast segments in a perfect casting and then multiplying the resulting fraction to give the percent completeness.

Total number of fully formed squares - alloy A [Table 2]

Total number of fully form squares (X) = 1199.

Total number of expected squares (Y) = 1215.

Castability percentage = X /Y × 100.

1199/1215 × 100 = 98.68%.
Table 2: Group A, indeginous alloy

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Total number of fully formed squares - alloy B [Table 3].

Total number of fully form squares (X) = 1212.

Total number of expected squares (Y) = 1215.

Castability percentage = X/Y × 100.

1212/1215 × 100 = 99.75%.
Table 3: Group B, imported alloy (heraenium "S")

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The castability of alloys A and B is compared using the statistical technique. On the assumption that the values follow normal distribution and that the two samples are independent, the Student t-test is performed.

Using the t-test procedure the value is 1.171. The table value for t is 28 degrees of freedom at 5% level of significance is 2.048. The calculated value is less than the tabular value (1.171 < 2.048) leading to the conclusion that castability difference between alloys A and B is insignificant.

Therefore we conclude that both the alloys have the same castability.

The mean value of castability of alloys A and B was derived by using the formula:



where x i = total no. of squares formed by the i th specimen, n = number of specimens.

The mean value of alloys A and B were 79.93 and 80.80, respectively [Table 4].
Table 4: Study results

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The standard deviation of alloy was derived by using the formula:



where x i = total no. of squares formed by the i th specimen, n = number of specimens, x = arithmetic mean.

The standard deviation of alloys A and B were 4.13 and 0.56, respectively [Table 4].

Even though there seems to be an insignificant difference between the mean value of castability of the two alloys, there is a slight difference between the variance of the two alloys because of only one value appearing in alloy A (specimen A6) which is different from the corresponding value in alloy B.

As the error in specimen A6 is due to technical fault, if the castability test is repeated once again by the same method, there is a high probability that the variation be insignificant.


   Discussion Top


The high cost of gold alloys has stimulated the search for alternative dental alloys. These alternative alloys, however, may be considered viable only if their use is safe for dentists, technician, and patients. Safety must be associated with qualities that guarantee the longevity of the treatments performed. There is no doubt that gold alloys are more biocompatible. However, clinical practices show the increased use of Ni-Cr alloys for ceramo-metal restoration. In addition to the basic requirement of good resistance to corrosion that is indispensable for any restorative metal, Ni-Cr alloys also must have good castability, good interaction with dental ceramics, and ease of manipulation. These characteristics are improving with new formulation. For a successful casting to be obtained according to Bruce, certain procedures are essential namely use of adequate pressure at the time of castings, use of sprues and a reservoir of adequate size that aids in stabilizing the pattern during investing, casting procedures, use of adequate quantity of alloy to fill the mold, the master sprue reservoir.

A variety of forces act upon a liquid metal subjected to a centrifugal force during the casting process: Acceleration forces act opposite to the direction of rotation, centrifugal forces act in an outward direction, and gravitational forces in a downward direction. [4] The resultant of these forces is directed into a localized area of the object to be cast. [4] The lower weight of the base metal alloy requires that a greater force be employed to propel the mass into the investment mold. It must be kept in mind that the nearer the pattern to the frontal plane the lower the thrust; conversely, the nearer the pattern to the vertical plane the greater the thrust. [5]

Various types of sprue designs have been advocated such as cylindrical, flat, conical, with constriction or flaring at its point of attachment, curved sprues, V-shaped sprue, sprues with reservoir, and Rousseau spruing technique. [6] Preston and Berger have stated that "spruing is an art which is not well understood." [7] The efficacy of the spruing system depends on how easily the metal can flow through it and fill the mold cavity within the shortest time. [8],[9]

Most research has found a direct proportionality between flow ability and channel radius. Larger diameter sprues of shorter length than those recommended for noble alloys are used when casting base-metal alloys because of the lower density and greater temperature difference between molten alloy and investment mold. Adequate sprue way for the alloy to flow into the mold space is an important factor related to this study.

Another factor to be considered regarding the castability is the investment material used. One of the important requirements of the investment is that it should be sufficiently porous and should allow the escape of gases. [5],[10],[11],[12] Thus the sprue diameter and pressure of the casting machine have an effect not only on the rate of filling the mold cavity but also on the rate at which elimination of gases takes place from the mold cavity that also plays an important role. [10]

The flow property of an alloy depends on its density, which is related to its composition, temperature difference between mold investment, alloy, and the centrifugal force.

Usage of adequate quantity of alloy to fill the mold and the master sprue reservoir can be achieved by using the formula: (A - B) × C = D. By using this formula we obtain a buttonless casting, which in turn redresses the amount of alloy to be used, hence reducing the cost of fabrication. Studies have shown that mechanical properties may be improved with addition of 1-2% beryllium [13] to nickel alloy due to the following reasons: Decrease in the fusion temperature by about 100°C, decrease in grain size, and decrease in melting point. [14],[15],[16]

Even though there seems to be an insignificant difference between the mean value of castability of the two alloys, there is a slight difference between the variance of the two alloys because of only one value appearing in alloy A (specimen A6) which is different from the corresponding value in alloy B. As the error in specimen A6 is due to technical fault, if the castability test is repeated once again by the same method, there is a high probability that the variation be insignificant. As shown in [Table 2], mesh A6 has 16 incompletely formed squares and in [Table 3], mesh B1 and B10 have 1 and 2 incompletely formed squares respectively.

The cause of the gross castability error in specimen A6 cannot be scientifically explained, but causes can be attributed to any of the following reasons: Damaged plastic mesh, damage during investment, incomplete flow of alloy, or damage during divesting. Due to ever increasing cost of imported nonprecious Ni-Cr alloys, in particular, the urgent need for a cost-effective indigenous replacement was fulfilled by alloy A.

To conclude, using the above-mentioned materials and following the method to test castability, we were able to derive favorable results. As the results were satisfactory we can conclude that the castability of the indigenous alloy is on par with the imported alloy.


   Acknowledgments Top


0We thank Dr. K Mahendranadh Reddy for helping in procuring the indigenous alloy from NFTDC, Hyderabad.

 
   References Top

1.Baran GR. The metallurgy of Ni - Cr alloys for fixed prosthodontics . J Prosthet Dent 1983;50:639-50.  Back to cited text no. 1
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2.Agarwald D, Ingerson C. Evaluation of various castability patterns by comparison with practical castings. J Dent Res 1982;61:345.  Back to cited text no. 2
    
3.Bauer JR, Loguercio AD, Reis A, Rodrigues Filho LE. Microhardness of Ni-Cr alloys under different casting conditions. Braz Oral Res 2006;20:40-6.  Back to cited text no. 3
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4.Dewald E. The relationship of pattern to the flow of gold and casting completeness. J Prosthet Dent 1979;41:531-4.  Back to cited text no. 4
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5.Martignoni M, Schonenberger L. Precision fixed prosthodontics: Clinical and laboratory aspects, 1 st ed. Chicago: Quintessence Publishing; 1990. p. 33-56.  Back to cited text no. 5
    
6.Rousseau CH. The Rousseau casting system: A foundation for esthetic restorations. Trends Tech Contemp Dent Lab 1984;1:26-9.  Back to cited text no. 6
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7.Preston JD, Berger R. Some laboratory variables affecting ceramo-metal alloys. Dent Clin North Am 1977;21:717-28.  Back to cited text no. 7
[PUBMED]    
8.Compagni R, Faucher RR, Yuodelis RA. Effects of sprue design, casting machine, and heat source on casting porosity. J Prosthet Dent 1984;52:41-5.  Back to cited text no. 8
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9.Peregrine AM, Rieger MR. Evaluating six sprue designs used in making high-palladium alloy castings. J Prosthet Dent 1986;56:192-6.  Back to cited text no. 9
    
10.Anusavice KJ. Philips science of dental materials. In: Anusavice KJ, editor. 10 th ed. India: Prism Books Pvt Ltd; 1996.  Back to cited text no. 10
    
11.Mahler DB, Ady AB. The influence of various factors on the effective setting expansion of casting investments. J Prosthet Dent 1963;13:365-73.  Back to cited text no. 11
    
12.Shillingburg HT, Hobo S, Whitsett LD . Fundamentals of fixed prosthodontics . In: Shillingburg HT, editor. 2 nd ed . Chicago: Quintessence Publishing Co; 1981. p. 79-96.  Back to cited text no. 12
    
13.Cohen SM, Vaidynathan TK, Schulman A. The effect of limited beryllium additions on a Ni-Cr alloy. J Prosthet Dent 1988;60:688-92.  Back to cited text no. 13
    
14.Cohen SM, Kakar A, Vaidyanathan TK, Viswanadhan T. Castability optimization of palladium based alloys. J Prosthet Dent 1996;76:125-31.  Back to cited text no. 14
    
15.Craig RG. Restorative Dental Materials, 9 th ed. St. Louis, Toronto: Mosby-Year Book Inc.; 1993.  Back to cited text no. 15
    
16.Presswood RG. The castability of alloys for small casting . J Prosthet Dent 1983;50:36-9.  Back to cited text no. 16
    

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Correspondence Address:
Ganesh Ramesh
Department of Prosthodontics, Sree Balaji Dental College and Hospital, Chennai
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.93471

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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]

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