Sprue design alterations and its effect on the properties of base metal alloy castings: An in vitro study.

INTRODUCTION: To study the effect of various sprue designs on the properties of base metal alloy castings. The base metal alloys are extensively used for their excellent properties such as an increase in hardness, high melting range, high elastic modulus, its compatibility with ceramic material and low cost. However, to improve the properties of the base metal alloys is an ongoing research leading to various modifications in their fabrication procedure - which include sprue designs and their mode of attachment to the wax pattern. AIMS AND
OBJECTIVES: Study compared the effect of three sprue designs viz. conical sprue, cylindrical sprue and bottleneck sprue on the properties of mass and microhardness of base metal alloy castings.
MATERIALS AND METHODS: A prefabricated wax mesh pattern was selected for the fabrication of the alloy test samples in the study. The cylindrical sprue design was connected to the mesh pattern with a straight attachment. The bottleneck sprue design was connected to the mesh pattern with a constricted attachment. The conical sprue design was connected to the mesh pattern with a flared attachment. In this manner, ten samples for each of the three different sprue designs were prepared.
RESULTS: The obtained value for mass and microhardness were subjected to statistical analysis. ANOVA test was performed to determine the difference between the sprue designs.
CONCLUSION: The bottleneck sprue, conical sprue, and cylindrical sprue designs did not exert any apparent influence on the mass and microhardness.

Sprue design is a key and controversial variable that can affect the properties of metal alloy castings. For high-density alloys like gold, the recommended sprue design includes the use of, wider, larger diameter sprues of shorter length and have a flared point of attachment to the wax pattern for better results. The escalated cost of precious alloys led to the use of base metal alloys which are mostly nickel, and chromium-based alloys. The base metal alloys have a reputation for being technique-sensitive, and their inherent physical, and chemical characteristics make them susceptible to minor changes in the casting environment.[1] The effect of various sprue designs and their mode of attachment to the wax pattern on parameters such as porosity castability and density of base metal alloy castings have been studied by various researchers.[23] This explains that sprue design is a key and controversial variable that can affect the properties of base metal alloy castings. Mass, porosity, surface texture, microstructure and abrasion resistance of the castings can influence the mechanical properties such as microhardness, tensile strength, proportional limit, yield strength, and fatigue life of the restorations during use. Hence this study was aimed to compare the effect of three different sprue designs on the properties of mass and microhardness of base metal alloy castings.

Materials and Methods

The prefabricated, square wax mesh pattern (BEGO, Germany), each measuring 16 mm × 16 mm, with 6 × 6 square spaces with a total of 36 squares per mesh pattern were cut out [Figure 1]. Ten wax mesh patterns for each of the three test sprue designs to obtain a total of 30 wax mesh patterns for the fabrication of test samples. To standardize the test sprue patterns, a stainless steel mold with two parts [Figure 2] and with three separate mold cavities corresponded to the selected sprue designs was fabricated. The three sprue designs selected for this study were (1) cylindrical sprue design with a diameter of 1.5 mm and length of l0 mm [Figure 3a], (2) bottleneck sprue design with a diameter of 1.5 mm, length of 9 mm, and with a 1 mm long taper at the attachment end to obtain the bottleneck sprue design [Figure 3b], (3) conical sprue design with a length of 9 mm, base of 5 mm, and with a 1 mm long and 1.5 mm diameter cylindrical extension at the attachment end [Figure 3c]. 10 resin sprue patterns were made for each of the three sprue designs using Pattern Resin (GC Corporation, Tokyo, JAPAN). The attachment area between the wax mesh pattern and the sprue patterns varied for the three sprue designs. The cylindrical sprue design was joined to the mesh pattern with a straight attachment [Figure 4]. The bottleneck sprue design was joined to the mesh pattern with a constricted attachment [Figure 5]. The conical sprue design was joined to the mesh pattern with a flared attachment [Figure 6]. In this manner, 10 samples for each of the three different sprue designs were prepared for the casting procedure. All patterns were invested with a phosphate-bonded investment. Castings were done with base metal nickel-chromium alloy according to the manufacturer's instructions. The cast samples were then divested and sandblasted using 50 microns aluminum oxide. In this manner, 10 test samples for each sprue design were obtained for the evaluation of the properties [Figure 7].

Figure 1

Dimension of the mesh pattern

Figure 2

Stainless steel mold

Figure 3

Dimension of the three different sprue designs. (a) Cylindrical, (b) bottle neck, (c) conical

Figure 4

Cylindrical sprue design connected to the mesh pattern with a straight attachment

Figure 5

Bottle neck sprue design connected to the mesh pattern with a constricted attachment

Figure 6

Conical sprue design connected to the mesh pattern with a flared attachment

Figure 7

Test Samples obtained from each of the three sprue designs. (a) Conical, (b) bottle neck, (c) cylindrical

Evaluation of mass and microhardness of the test samples obtained from the three different sprue designs

Mass

The mesh portions of the castings were separated from the sprue with a thin, fine carborundum disk, by carefully following the shape and contours of the casting [Figure 8]. The mass of each of the cast mesh pattern was then measured using a physical balance (Mettler Toledo weighing machine Co., Ohio, USA) (accurate to the 0.0001 g). All the test samples; that is, 10 each obtained from the three test sprue designs were evaluated for the property of mass and the results were noted.

Figure 8

The mesh portions of the castings separated from the sprue for the evaluation of mass. (a) Conical, (b) bottle neck, (c) cylindrical

Micro hardness

After the evaluation of the mass, the test samples were evaluated for the microhardness of the cast mesh pattern. The cast mesh patterns were specifically prepared to enable the evaluation of microhardness. All the cast mesh pattern samples obtained from the three different sprue designs, were individually embedded in a poly vinyl chloride (PVC) ring with a diameter of 1 inch with self-cure clear acrylic resin (DPI-RR cold cure, Dental Products India Ltd.) [Figure 9]. After embedding, the mesh samples were polished using mechanical polishers (Vibron Polisher and Bainpol Polisher). Alumina powder of 0.3 microns was used as polishing abrasive. After the test samples were polished, they were washed and cleaned with running water and air dried. All the samples obtained from the three different sprue designs were subjected to microhardness tests. Vickers microhardness test was employed for the evaluation using a microhardness tester (Reichert Polyvar 2 Met Microhardness tester, Reichert, Austria) [Figure 10]. Microhardness was determined at nine selected sites on each of the test sample [Figure 11]. The micro hardness tester was equipped with a 1360 diamond indenter with a square pyramidal face, and was forced into the embedded test samples with a force of 150 g for 5 s. After the indenter had been withdrawn, the dimensions of the indentations all the nine sites were calculated for each of the test samples, based on which the microhardness were calculated. The microhardness values were obtained for the test samples as vickers hardness number (VHN). All the test samples; that is, 10 each obtained from the three test sprue designs were evaluated for the property of microhardness and mass. The results were tabulated and statistically analyzed and compared.

Figure 9

Cast mesh patterns embedded in self-cure clear acrylic resin

Figure 10

Micro hardness tester

Figure 11

Nine different sites on the test pattern for testing micro hardness

Results

The mass and the microhardness values for the test samples obtained from the three sprue designs was given in Tables 1 and 2a–c respectively. The statistical analysis of the mean mass and microhardness with the standard deviation (SD) for the test samples was obtained by ANOVA method. The samples obtained by the bottleneck sprue design had a mean mass of 0.90 g with a SD of 3.14, those obtained by the conical sprue design had a mean mass of 0.89 g with a SD of 3.18 and those obtained by the cylindrical sprue design had a mean mass of 0.88 g with a SD of 3.09. The test of significance indicates that the P = 0.612, which is statistically insignificant (P < 0.05) [Table 3]. The samples obtained by the bottleneck sprue design had a mean microhardness of 311.7 VHN with a SD of 22.1, those obtained by the conical sprue design had a mean microhardness of 286 VHN with a SD of 17.19 and those obtained by the cylindrical sprue design had a mean microhardness of 284 VHN with a SD of 20.8. The test of significance indicates that the P = 0.556, which is statistically insignificant (P < 0.05) [Table 4].

Table 1

Mass of test samples obtained from the three sprue design

Table 2a

Micro hardness (VHN) of the test samples obtained from the conical sprue design

Table 2c

Micro hardness (VHN) of the test samples obtained from the cylindrical sprue design

Table 3

Mean mass of the test samples obtained from the three sprue designs (ANOVA method)

Table 4

Mean micro hardness (VHN) of the test samples obtained from the three sprue designs (ANOVA method)

Discussion

Mass

The proper selection of size and configuration of the sprue is critical for the production of a complete, dense and accurate casting.[4] To achieve complete and dense castings, McLean, Anusavice and Naylor advocated sprue designs that tapered at the point of attachment of the sprue to the wax pattern.[5] However, Verrett and Duke concluded from their investigations using sprue attachments that were straight, abrupt constriction, flared and gradual constriction and found that flared and straight sprue attachments optimized castability and minimized porosity for base metal alloys.[3] Thus, there are differences of opinion among various researchers, regarding the necessary optimal sprue design and its mode of attachment to the pattern for casting base metal alloys. This study was done to evaluate the efficacy of cylindrical, conical, and bottleneck sprue designs on the properties of mass and microhardness of base metal alloy castings. In the study, the mass of the cast mesh pattern samples was measured as it is directly proportional to its density and in turn a direct indicator of dense castings. Furthermore, dense castings imply less porosity. In a previous study conducted by Rieger et al., the conical test sprue yielded castings that had superior values for mass.[1] It has been postulated that constricted sprue attachment increases the velocity of the molten alloy with minimum turbulence entering the mold space by creating a Venturi effect at the point of entry of the alloy, thus improving the density of the casting.[3] In the present study, the castings obtained from the bottle neck sprue design, cylindrical sprue design, and conical sprue design, did not exhibit any statistically significant difference in their mean mass values of the mesh pattern. Hence, there appeared to be no apparent effect of the test sprue design and their attachment designs to the wax pattern on the factor of mass (density) in this study.

Micro hardness

Microhardness of a material has a bearing on properties such as burnishability, surface wear, and shock absorbent characters. It is also representative of the microporosity and grain structure of the alloy.[6] Microhardness of base metal alloys has been evaluated with respect to the effect of parameters such as different alloy composition,[7] various heating sources used for casting and heat treated alloys[89] but data on the effect of various sprue designs on microhardness of base metal alloy is lacking. In the present study, microhardness was selected as a test parameter in view of the above considerations. In the present study, the castings obtained from the bottle neck sprue design, cylindrical sprue design and conical sprue design, did not exhibit any statistically significant difference in their mean microhardness values. The performance of dental restorations under in vivo conditions, the effect of sprue design on this parameter merits further investigations. The restricted entry point prevents suck back of metal, controls the velocity of the metal and ensures an even distribution of metal.[510] Conical sprue design had a uniform taper along its length, this reduction in the diameter of the sprue could have aided in reducing metal turbulence at the point of entry leading to reduced mean percentage porosity. There was no statistically significant difference between the mean percentage porosity obtained with the bottle neck and the conical sprue designs and both these designs exhibited significantly less values for porosity compared to the cylindrical design, suggesting their use in casting low density alloys. Studies on the effect of various sprue designs on the grain structure and properties such as tensile strength, yield strength, coefficient of thermal expansion, porcelain bond strength are also needed, to enhance the results of the study and to recommend specific sprue and attachment design for a particular alloy system.

Conclusion

The results of this study yielded the following conclusions:

The differences in the mass values between the bottleneck, conical, and cylindrical sprue designs were not statistically significant

The differences in the mean microhardness values between the bottleneck, cylindrical and conical, and sprue designs were not statistically significant.

Table 2b

Micro hardness (VHN) of the test samples obtained from the bottle neck sprue design