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Table of Contents   
ORIGINAL RESEARCH  
Year : 2014  |  Volume : 25  |  Issue : 3  |  Page : 340-345
Casting made simple using modified sprue design: An in vitro study


1 Department of Prosthodontics, Thai Moogambiga Dental College and Hospitals, Chennai, Tamil Nadu, India
2 Department of Prosthodontics, Karpaga Vinayaga Institute of Dental Sciences, Kolampakkam, Kanchipuram, Tamil Nadu, India

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Date of Submission08-Jan-2014
Date of Decision02-Feb-2014
Date of Acceptance20-May-2014
Date of Web Publication7-Aug-2014
 

   Abstract 

Background: Success in dental casting restorations for fixed partial dentures (FPDs) depends on the castability. Castability is described as the ability of an alloy to faithfully reproduce sharp detail and fine margins of a wax pattern. The goal of a prosthodontist is to provide the patient with restorations that fit precisely. Regardless of the alloy used for casting, the casting technique should yield a casted alloy, which should possess sufficient mass, surface hardness and minimal porosity after casting.
Materials and Methods: Twenty patterns for casting were made from three-dimensional printed resin pattern simulating a 3 unit FPD and casted using modified sprue technique. Later test samples were cemented sequentially on stainless steel model using pressure indicating paste and evaluated for vertical marginal gap in eight predetermined reference areas. Marginal gap were measured in microns using Video Measuring System (VMS2010F-CIP Corporation, Korea). A portion of the axial wall of the cast abutments depicting premolar and molar were sectioned and embedded in acrylic resin and tested for micro hardness using Reichert Polyvar 2 Met Microhardness tester (Reichert, Austria) and porosity using Quantimet Image Analyzer (Quantimet Corporation London, England).
Results: The results obtained for marginal gap, micro hardness, and porosity of all test samples were tabulated, descriptive statistics were calculated and the values were found to be within the clinically acceptable range.
Conclusion: The new sprue technique can be an alternative and convenient method for casting which would minimize metal wasting and less time consuming. However, further studies with same technique on various parameters are to be conducted for its broad acceptance.

Keywords: Cobalt-chromium alloy, marginal gap, microhardness, modified sprue, porosity, three-dimensional printed resin pattern

How to cite this article:
Baskaran B E, Geetha Prabhu K R, Prabhu R, Krishna G P, Eswaran M A, Gajapathi B. Casting made simple using modified sprue design: An in vitro study. Indian J Dent Res 2014;25:340-5

How to cite this URL:
Baskaran B E, Geetha Prabhu K R, Prabhu R, Krishna G P, Eswaran M A, Gajapathi B. Casting made simple using modified sprue design: An in vitro study. Indian J Dent Res [serial online] 2014 [cited 2020 Sep 26];25:340-5. Available from: http://www.ijdr.in/text.asp?2014/25/3/340/138333
Fundamental principle of casting is to adapt the restorative margin to the finish line of the prepared teeth with minimal discrepancy, which is the area of chief concern. [1],[2],[3],[4],[5],[6],[7]

Once the desired pattern is formed it is to be attached to a sprue, which has a major impact on the success of casting. [8],[9],[10] The alloy used for casting may be a base metal or noble metal alloys. [11],[12],[13],[14],[15] Many of the previous studies have focused on evaluation of marginal fit of cast restoration and effect of sprue designs on them, [16],[17] yet studies using modified sprue designs minimizing metal wastage and laboratory timing are lacking.


   Materials and methods Top


A standardized custom made stainless steel master model was made resembling a 3 unit fixed partial denture (FPD) [Figure 1]. Model depicted two abutments, a premolar and a molar with a pontic space in between abutment units in this study had a deep chamfer margin with 6° axial wall taper and 10° occlusal inclination. [18] A single stage impression of the master model with polyvinylsiloxane was made and a master die was made using type 4 die stone. This master die was used for fabricating all the 20 patterns used in this study. Master die was scanned by three shape D700 scanner for making three-dimensional printed resin patterns [Figure 2].
Figure 1: Stainless steel master model representing 3 unit fixed partial denture preparation

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Figure 2: Type 4 die stone model with three-dimensional printed resin pattern

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Once resin patterns are made, they are prepared for casting procedure. A 3.5 mm single sprue was attached to the pontic alone [Figure 3]. The 3.5 mm sprue is designed in such a way that it places the pattern 6 mm short from the free end of the rubber casting ring. The sprue has a reservoir, which is placed in the heat center of the casting ring [Figure 4]. The molten alloy from reservoir feeds the mold space with continuous flow of metal during casting. Twenty samples obtained using three-dimensional printed resin pattern were subjected to conventional casting procedures. Investment was heat treated for 3 h in a burnout furnace. During the 1 st h, the temperature was raised from room temperature to 380°C, for the 2 nd h, the temperature was raised to 900°C to accomplish complete burnout of the patterns with no residues remaining. The investment mold was initially placed in the furnace such that the crucible end was in contact with the floor of the furnace for the escape of resin material. The investment mold was reversed later near the end of burnout cycle with sprue hole facing upward to enable the escape of entrapped gases and allow oxygen contact to ensure complete burnout of patterns and allow mold expansion. [19],[20],[21],[22]
Figure 3: Modified sprue attachment with reservoir in the heat center

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Casting was accomplished with a cobalt-chromium (Co-Cr) alloy (Denchrome C, CE, Germany) melted in an induction casting machine. The casting procedure was performed quickly to prevent heat loss resulting in the thermal contraction of the mold. The Co-Cr alloy was heated sufficiently until the alloy ingot turned to molten state and the crucible was released and centrifugal force ensured the completion of the casting procedure and Co-Cr cast copings were obtained [Figure 5]. These 20 samples were then divested, sandblasted, steam cleaned and examined for presence of nodules in the internal surface of cast copings that would interfere with complete seating of the copings over the metal model. [23],[24] Nodules present in few samples were viewed through a magnified glass and removed using a carbide round bur. All the test samples were coated with a thin layer of pressure indicating paste (Fit checker II GC Corporation, Tokyo, Japan) on the internal aspect. They were then seated on stainless steel master model sequentially and loaded with a 2 kg load, which was placed on a platform mounted on the vertical arm of a surveyor for 2 min to simulate a coping cemented in the oral condition. Fit checker was used because it was easy to be removed from inner surface without damaging the inner surface of the copings. After setting, the assembly was removed from the surveyor and excess paste was removed as well. The assembly was placed under Video Measuring System (VMS2010F-CIP Corporation, Korea) for evaluation of vertical marginal gap [Figure 6]. This gap was measured in eight predetermined reference points, four areas in each abutment.

After evaluation of marginal fit, a portion of the axial wall was sectioned from both the retainers and was embedded in a polyvinyl chloride ring of 1 inch diameter with self-cure acrylic. They were then polished in two stage using a mechanical polisher (Vibron Polisher, METCO, Chennai) which consists of a rotating disc fixed with carborundum abrasive papers of grain size 120, 320,400 and 600 followed by 1/0, 2/0, 3/0, 4/0 respectively. Finally, alumina powder of 0.3 μ was used as polishing abrasive followed by washing in running water and air dried.
Figure 4: Diagrammatic representation of modified sprue with reservoir at the heat center

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Figure 5: Completed casting with the modified sprue design

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Vickers micro hardness test was employed for micro hardness, which was done in four randomly selected areas with 136° diamond indenter with a square pyramidal face. This was then embedded into the test samples with a force of 150 g for 5 s. After the indenter head had been withdrawn the dimensions of the indentations [Figure 7] at all four sites were calculated for each of the test samples using Reichert Polyvar 2 Met Microhardness tester (Reichert, Austria), based on which the micro hardness value was calculated as Vickers hardness number (VHN). The sites around these indentations were selected for the analysis of porosity of the test samples. The porosity was analyzed at ×400 magnification using Quantimet Image Analyzer (Quantimet Corp London, England). Each analyzed site covered an area of approximately 0.39 mm 2 [Figure 8]. The mean amount of porosity from all the analyzed sites for each test sample was calculated as a percentage of porosity.
Figure 6: Marginal gap of the test sample as viewed under VMS 2010F

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Figure 7: Diamond indentations on the test sample as created by Reichert Polyvar 2 Met Microhardness tester

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Figure 8: Micro porosity seen on the test sample under ×400 magnification

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   Results Top


The results obtained for marginal gap, microhardness, and porosity were tabulated and descriptive statistics were calculated using statistical package for social science version 19. Descriptive statistical analysis was calculated and expressed in terms of mean and standard deviation. There was no significant deviation from the mean value. All the values obtained were well within clinically acceptable range [Table 1], [Table 2], [Table 3], [Graph 1-3].
Table 1: Vertical marginal gap of the test samples at eight predetermined points and their mean value in microns


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Table 2: Micro-hardness values of the test samples obtained from four different indentation sites


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Table 3: Percentage porosity of the test samples obtained from four indentation sites


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   Discussion Top


The efficacy of a sprue design depends on how easily metal can flow through it and fill the mold cavity. The sprue must supply molten metal continuously so that the gases are forced out of the mold cavity. It should also compensate for the shrinkage of alloy in the casting as it solidifies. Ever since sprue techniques have been followed, various views are suggested regarding the selection and application of sprue designs and their mode of attachment to the pattern to be casted. [25]

Direct sprue former design attaches the primary sprue, directly with the pattern. In case of indirect sprue design a horizontal reservoir runner bar is positioned near the heat center of the investment ring and then pattern is connected to the horizontal runner bar using individual manifold sprue. Both direct and indirect casting techniques have yielded promising results.

In case of indirect method, individual manifold sprue which is attached to each unit of the pattern costs more metal to fill the mold. Furthermore after casting, sectioning of each manifold sprue becomes tough and time consuming job. In case of full metal restorations, the effort made to get a fine contour on the restoration surface is again compromised as the junction of sprue to the pattern is to be contoured again by trimming the hard metal. Moreover, much of the alloy used for casting is consumed by horizontal runner bar and manifold sprue areas.

Base metal alloys require casting techniques specifically designed for their physical properties because of their low density and high melting range. [26],[27] The low density of base metal alloys presents problems of castability because it allows these alloys to absorb gases more easily at high temperatures. Hence, greater centrifugal pressure is needed to cast them. Thereby the sprue designs for base metal alloy castings vary according to the physical properties of the alloys. To achieve complete and dense casting authors like McLean, Peregrina and Schorr, Anusavice have recommended constricted sprue design for low density alloys. [28],[29] However Verrett and Duke concluded from their investigations using sprue attachments that were straight, flared, abrupt constriction, and gradual constriction and found that flared and straight sprue attachements optimized castability and minimized porosity for base metal alloys. [30],[31] Authors like Rieger studied different sprue designs and recommended a conical sprue design. Thus, there are differences of opinion among the various authors, regarding the optimal sprue design and its mode of attachment to the pattern for casting base metal alloys.

In this study, 20 three-dimensional printed resin patterns each replicating a 3 unit FPD was fabricated. This pattern was attached with the special sprue design (single sprue with reservoir) only to the pontic region. They were then casted using conventional casting procedure. All the samples resulted in marginal adaptation with the mean value of 48.12 μm, (standard deviation-6.7), which falls within the clinically acceptable range of 10-160 μm. The dimension of the indentation created by microhardness tester were evaluated and results were obtained as VHN, which had a mean range of 302.14 (VHN) (standard deviation-38.77). The results obtained were well within the hardness range value of base metal alloys and in consensus with those of Bailey, Bezzon et al. [32],[33] The site around these indentations were evaluated using Quantimet Image Analyzing computer for measuring the microporosity in the test samples which again revealed presence of minimal number of porosities within the acceptable range of 1.2% (standard deviation-0.30) and in consensus with those of Chan et al., Compagni et al. [34],[35] The results obtained from these tests favor the modified sprue design used in this study, which is simple, needs less lab time and less alloy consuming. Also this technique yields values within clinically acceptable range.

One problem with direct spruing with no reservoir is the increased likelihood of suck back porosity, which happens due to improper cooling pattern, at the sprue casting junction. This problem was avoided here as the reservoir was placed in the heat center. As reservoir is in the center of the investment, the time for heat dissipation may also be comparatively longer than the periphery. This makes the alloy in the reservoir to cool lastly maintaining the correct pattern of cooling. Presence of reservoir ball at the heat center provides continuous supply of molten metal. The molten alloy entering the mold cavity gets constricted automatically after filling the reservoir. It then enters to the pontic region and fills it after which it gets constricted again in the connector regions on both the sides and fills in the retainer parts. The constricted paths at three different sites in the sprue give the necessary venturi effect. The continuous feeding of metal from reservoir and correct cooling pattern could be the reason for the complete casting with acceptable marginal gap, hardness, and porosity values within the acceptable range in this study.

The less number of test samples is one of the limitations in this study. The internal marginal gap was not evaluated. Further studies with multiunit FPD should be done evaluating the efficacy of modified single sprue technique.

Nevertheless, the limitation mentioned above, the present study using single sprue with reservoir design used as a replica of 3 unit FPD yielded results within the acceptable range. The added advantage was less wastage of metal, reduced trimming work and maintaining the form of the retainer designs.


   Conclusion Top


This study was conducted to evaluate the micro hardness, porosity and vertical marginal gap of a 3 unit cast Co-Cr copings fabricated using modified single sprue technique where the sprue was attached only to the pontic. The single sprue had a reservoir in the heat center. The casted samples were evaluated for marginal gap, porosity and micro hardness. The results obtained of all test samples were statistically analyzed, and the values fall within the clinically acceptable range. This modified design caused less metal wastage and reduced lab work and yielded results well within the clinically acceptable range. Hence this simplified technique could be applied to yield promising results by the dental students and technicians.


   Acknowledgments Top


My sincere thanks to Professor Dr. Karthik Bhanushali, Imaginarium dental lab, MIDC, Andheri East, Mumbai, for helping me with three-dimensional printing (3DP). I would like to thank the Department of Mechanical Engineering, AnnaUniversity for helping me in measuring the values with the help of Video Measuring System (VMS2010F) Reichert Polyvar 2 Met Microhardness tester and Quantimet Image Analyzer.

 
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Correspondence Address:
B Eswaran Baskaran
Department of Prosthodontics, Thai Moogambiga Dental College and Hospitals, Chennai, Tamil Nadu
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0970-9290.138333

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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]

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