- Email: [email protected]
Effect of different investments and mold temperatures on titanium mechanical properties
Available online at www.sciencedirect.com
Journal of Prosthodontic Research 56 (2012) 58–64 www.elsevier.com/locate/jpor
Technical procedure
Effect of different investments and mold temperatures on titanium mechanical properties Renata Cristina Silveira Rodrigues DDS, MSc, PhD*, Elanio Pereira de Almeida DDS, Adriana Cla´udia Lapria Faria DDS, MSc, PhD, Ana Paula Macedo MSc, Eng, Maria da Gloria Chiarello de Mattos DDS, MSc, PhD, Ricardo Faria Ribeiro DDS, MSc, PhD Department of Dental Materials and Prosthodontics, Dental School of Ribeira˜o Preto, University of Sa˜o Paulo, Av. do Cafe´, s/n, Monte Alegre, 14040-904 Ribeira˜o Preto – SP, Brazil Received 15 February 2010; received in revised form 6 January 2011; accepted 11 January 2011 Available online 15 February 2011
Abstract Purpose: The aim of the present study was to evaluate commercially pure titanium (CP Ti) casting quality when a specific to titanium and a conventional phosphate bonded investments were used under different mold temperatures. For this, the evaluated parameters were surface roughness, bending strength, Vickers microhardness, casting quality by radiographies and microstructure of CP Ti. Methods: Wax patterns (28 mm 3 mm 1 mm) were invested using two phosphate bonded investments: Rematitan Plus (REM), specific to titanium, and Castorit Super C (CAS), a conventional investment, fired and cooled until reaching two mold temperatures: 430 8C (430) and room temperature (RT). Specimens were cast from CP Ti by plasma. After casting, specimens were radiographically examined and submitted to Vickers microhardness, roughness and bending strength evaluation. Microstructure was analyzed in the center and at the surface of specimen. Results: Qualitative analysis of radiographs showed that specimens which were cast using CAS-RT presented more casting porosities while the specimens which were cast with REM-430 did not present any casting porosity. No significant difference was noted among the groups in the surface roughness and Vickers microhardness data, but the bending strength of the specimens cast using CAS was greater than REM groups. The microstructure of the specimens of the different groups was similar, presenting a feather-like aspect. Conclusion: Casting porosities found in the specimens cast using conventional investments (CAS) and lower mold temperatures would limit their use, even mechanical properties were similar than in specimens cast using specific to titanium investment (REM) at temperatures recommended by the manufacturer. # 2011 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. Keywords: Titanium; Dental casting investment; Radiography; Microscopy
1. Introduction The use of commercially pure titanium (CP Ti) has increased in dental appliances because of its good mechanical properties, excellent corrosion resistance, good biocompatibility and high strength-to-weight ratio [1–4]. However, difficulties related to casting process have been restrained titanium application, especially in prosthodontics. Casting problems are caused by high melting point, highly reactive behavior with investment materials at high temperatures, and low density, creating
* Corresponding author. Tel.: +55 16 3602 4005; fax: +55 16 3602 4780. E-mail address: [email protected] (R.C.S. Rodrigues).
difficulties in achieving complete mold filling [5,6]. To overcome these difficulties, new casting machines have been produced combining arc or induction melting, within inert atmosphere through the use of argon gas [7–9]. In addition, investment materials for titanium castings have been widely studied in recent years because, at high melting point, titanium reacts with some elements such as oxygen, hydrogen, carbon and investment surface, creating a brittle and hard surface layer, so-called alpha-case, which affects surface properties of titanium casting [10,11]. Because this layer interferes with ductility, fatigue resistance of removable partial denture frameworks and clasps, roughness and metal–ceramic bonding resistance [12], new investment materials such as Al2O3- and MgO-based materials have been
1883-1958/$ – see front matter # 2011 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. doi:10.1016/j.jpor.2011.01.002
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
developed to minimize the influence of alpha-case layer formation [9–11,13,14]. Phosphate bonded investments with silica have been used for casting dental alloys whose melting point is higher. Although these conventional investments have been used for casting titanium, because investments specific to titanium present high cost, some elements (Si, O, P, Fe and Al) of their composition react with titanium surface [15]. Consequently, some properties need evaluation before routine use of these investments. SiO2-based phosphate bonded investments were routinely used to cast dental alloys and with some modifications, as addition of Al2O3 and MgO, are used to specifically cast CP Ti and titanium alloys. Guilin et al. [10] stated that SiO2 is unstable and easily react with titanium to form more TixOy, increasing the oxide content and providing a greater microhardness to the surface reaction layer. The authors also pointed out that the Al2O3 based investments reduce these reactions and the thickness of reaction layer. However, according to some authors [16,17], despite investments based on Al2O3, MgO and ZrO2 are less reactive, they present low expansions and are more expensive, and your utilization is limited. Thermal expansion and misfit are important aspects to analyze in investments for casting titanium, once titanium requires casting at lower temperatures due to the reactivity of silica and titanium at temperatures above 500 8C, and thermal expansion at lower temperatures cannot be enough to compensate casting shrinkage, affecting misfit [18–20]. Furthermore, it is believed that mold temperature investment interferes with properties of casting titanium by reducing interfacial reactivity, and some authors have casted titanium using different mold temperatures searching one whose thermal expansion could compensate casting shrinkage [16–21]. At lower temperatures, the hardness values became constant at depths exceeding 300 mm while mold temperatures above 600 8C could increase hardness only at depth of 500 mm, suggesting that oxidation effects reach deeper in the cast body at higher mold temperatures [22]. In addition, some authors advocate the use of room temperature of the mold when titanium is casted in vaccum-pressure casting machine [23]. Nevertheless, any study has evaluated the effect of the room temperature in the microstructure and mechanical properties of titanium castings. The null hypothesis is that casting titanium using mold temperatures lower than that recommended by the manufacturer could decrease interfacial reactivity and improve compensation of the casting shrinkage, interfering with misfit. However, mechanical properties need to be maintained in these new casting conditions. In addition, the hypothesis that a conventional investment, whose cost is lower and expansion is greater, used at lower mold temperatures, could provide adequate casting, related to alpha-case formation and mechanical properties when compared to that specific to titanium. Thus, the aim of the present study was to evaluate titanium casting quality when a specific to titanium and a conventional investment were used under different mold temperatures. The evaluated parameters were surface rough-
59
Table 1 Composition of the conventional (CAS) and specific to titanium (REM) investments. Investment composition (%)a
SiO2
MgO
NH4h2PO4
Al2O3
REM CAS
55–75 60–80
10–30 6–19
5–10 10–20
10–25 –
a
Manufacturer information.
ness, bending strength, Vickers microhardness, casting defects by radiographies and microstructure of CP Ti. 2. Materials and methods 2.1. Difference from conventional methods Titanium castings are performed with phosphate bonded investments specific to titanium using mold temperature recommended by the manufacturer. The present study evaluated a conventional phosphate bonded investment at lower mold temperatures to cast titanium. 2.2. Specimen preparation The specimens were obtained from rectangular wax patterns (28 mm 3 mm 1 mm) which were invested using two phosphate bonded investments: Rematitan Plus (REM) with mixing liquid for partial denture frameworks (Dentaurum, Pforzheim, Germany), which is an investment specific to titanium, and Castorit Super C (CAS) (Dentaurum, Pforzheim, Germany). Investment compositions are presented in Table 1. Mixing of the investments was made according to manufacturer’s recommendations, under vacuum (Turbo Mix; EDG Equipamentos e Controles Ltda., Sa˜o Carlos, Brazil) before being poured into the ring. After setting, the investment blocks were put in an electric furnace (EDGCON 5P; EDG Equipamentos e Controles Ltda., Sa˜o Carlos, Brazil) and fired according to manufacturer’s instructions. The molds were cooled in the furnace to the different final mold temperatures: 430 8C (430) and room temperature of 22 8C (RT). The schedules used for dewaxing and thermal expansion are described in Table 2. Ten castings were obtained for each condition investment/mold temperature. The specimens were cast from grade I CP Ti in a vacuum-pressure casting machine (Discovery Plasma, EDG Equipamentos e Controles Ltda., Sa˜o Carlos, Brazil), where the melting was made by arc melting in a vacuum and argon inert atmosphere, with injection of the alloy/ metal into the mold by vacuum-pressure. After casting, investment blocks were quenched in cold water, as manufacturer’s instructions, until reaching room temperature, and then divested. With this procedure a sudden change in temperature and rapid steam generation occur, and the investment breaks away from the casting. So, investments adhered to castings were firstly removed by gently brushing of the investment surface using a wire brush suitable for bur cleaning. When a thin layer of investment yet remained adhered
60
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
Table 2 The schedule used for dewaxing and thermal expansion of the investments. Investment and mold temperature
Stage 1
REM-430 REM-RT CAS-430 CAS-RT
150 8C 150 8C 250 8C 250 8C
Stage 2 (90 min) (90 min) (60 min) (60 min)
250 8C 250 8C 950 8C 950 8C
(90 min) (90 min) (30 min) (30 min)
Stage 3
Stage 4
Casting
1000 8C (60 min) 1000 8C (60 min) 430 8C RT
430 8C RT – –
430 8C RT 430 8C RT
Schedules suggested by the manufacturer. REM investment: 150 8C (90 min) - 250 8C (90 min) - 1000 8C (60 min) - 430 8C and casting. CAS investment: 250 8C (60 min) -950 8C (30 min) and casting.
to castings, they were immersed in ultrasonic bath. This cleaning procedure was used to avoid alpha-case removal. 2.3. Radiographic evaluation Before the tests, all specimens were examined radiographically to detect possible casting defects that would contraindicate their use in the tests. A laboratorial unit XControl (Dentaurum, Ispringen, Germany), was set to 70 kV and 8 mA for a 5-s exposure time at 20 cm from the test specimen, and a film Polapan 57 high speed panchromatic black and white film (Polaroid Corp., Cambridge, USA), presenting an exposure area of 9 cm 12 cm, was used and auto-processed for 20 s. 2.4. Surface roughness Surface roughness of the specimens was measured with a profilometer (Mitutoyo SJ201-P, 300 mm accuracy, 0.5 mm/s speed, and five 0.8 mm cut-offs). Three readings were made in each specimen (on the center of the specimen, 1 mm to the right and 1 mm to the left) and a mean value was calculated for each specimen.
Fig. 1. Figure illustration of specimens cut transversally for Vickers microhardness evaluation. Indentations where Vickers microhardness was measured can be noted.
performed in each lateral of the cut specimen and three in the central part, as is shown in Fig. 1, permitting to analyze microhardness in the interior of the specimen.
2.5. Bending strength 2.7. Microstructure analysis Three-point bending tests were performed, at room temperature, on a universal testing instrument EMIC MEM 2000 (EMIC, Sa˜o Jose´ dos Pinhais, Brazil) using crosshead speed of 0.5 mm/min and load cell of 500 kgf. The bending strengths were determined using the equation s = 3PL/2bh2, where s is the bending strength (MPa), P is the load (N), L is the span length (mm), b is the specimen width (mm), and h is the specimen thickness (mm). 2.6. Vickers microhardness The Vickers microhardness of the specimens was measured with a load of 19.614 N applied for 30 s (Microhardness tester HMV-2 Shimadzu Corp., Kyoto, Japan). The specimens were cut in the transversal axis (3 mm 3 mm 1 mm) permitting to analyze the Vickers microhardness in the interior of specimen. After cutting, specimens were embedded using autopolymerizing acrylic resin and polished with sequential silicon carbide papers in the sequence 320, 400 and 600. So, three measures were
To analyze microstructure, one specimen of each group was embedded using autopolymerizing acrylic resin and then polished with silicon carbide papers in the sequence 320, 400, 600 and 1200. The final polishing was reached with a colloidal silica solution (OPS, Struers A/S, Denmark) + H2O2 30%. After this, the samples were etched with Kroll solution (6 mL HNO3 + 3 mL HF + 91 mL H2O) for 40 s and examined using an optical microscope Neophot 30 (Jena-Carl Zeiss, Jena, Germany). Microstructure images were obtained using a digital camera (CC-8703, GKB, Tai Chung, Taiwan). 2.8. Statistical analysis The effect of the investment and mold temperature on roughness, bending strength and Vickers microhardness was evaluated using 1-way analysis of variance (ANOVA), followed by post hoc Tukey test (a = 0.05) using the software SPSS for Windows (SPSS Inc., Chicago, USA).
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
61
Fig. 2. Radiographic images of titanium specimens cast in different conditions: REM-430 where any casting porosity was noted; REM-RT which present casting porosities pointed by arrows; CAS-430, where some casting porosities are pointed by arrows; and CAS-RT, presenting the higher quantity of casting porosities, as pointed by arrows.
3. Results
3.3. Bending strength
3.1. Radiographic evaluation
Comparison between bending strength data (Table 3) revealed that specimens cast with the conventional investment CAS presented greater bending strength than that cast using the REM ( p 0.05).
Digital images of radiographs are presented in Fig. 2. Qualitative analysis of radiographs showed that specimens which were cast using the conventional investment CAS-RT presented more casting porosities while the specimens which were cast using the investment specific to titanium REM-430 did not present any casting porosity, representing the best results.
3.4. Vickers microhardness The results of Vickers microhardness evaluation are presented in Table 3. No significant differences were noted in the Vickers microhardness values among the groups ( p 0.05).
3.2. Surface roughness 3.5. Microstructure analysis The data of surface roughness (Table 3) measured in the specimens cast using different investments and mold temperatures did not reveal any significant difference ( p 0.05).
Microscopy images (Fig. 3) of the cast specimens revealed that all the specimens presented a feather-like microstructure.
62
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
Table 3 Surface roughness (Ra), bending strength (GPa), and Vickers microhardness (VHN) of the specimens cast using a conventional and a specific to titanium investment under different mold temperatures. The results are expressed as mean (standard deviation).
Surface roughness (Ra) Bending strength (GPa) Vickers microhardness (VHN)
REM-430
REM-RT
CAS-430
CAS-RT
6.57(0.80) 0.59(0.03) 142.25(18.43)
6.23(1.00) 0.60(0.06) 128.33(9.10)
5.85(0.82) 0.70(0.03) 136.11(12.71)
5.63(0.72) 0.67(0.06) 139.67(11.84)
The specimens cast using the different investments at two different mold temperatures presented a similar microstructure at the surface (Fig. 3S) and in the central area (Fig. 3C). 4. Discussion Considering the relevancy of the investment cost in the final price of prosthodontics, the present study compared a conventional phosphate bonded investment (CAS) to a phosphate bonded investment specific for titanium (REM), at two different mold temperatures: 430 8C (430), recommended by the REM manufacturer, and room temperature (RT), suggested by some authors [22,23] to minimize reactivity of CP Ti and investment. The images of CP Ti microstructure presented in Fig. 3 revealed that alpha-case layer present at the surface of the samples (Fig. 3S) cast with CAS and REM at different mold temperatures (430 and RT) showed a similar aspect and thickness, although any quantitative evaluation of this layer thickness had not been made. Based on these images, it is observed that the investment and mold temperature did not interfere significantly with reactivity of investment and CP Ti, once alpha-case layer formation was similar in all groups. Although these results are different of some results publicized in the literature which argued that alpha-case layer is affected by mold temperature [16,18], it is necessary to consider the difference in mold temperatures evaluated in the present (430 8C and room temperature) and in the other studies (430 8C, 480 8C, 530 8C, 550 8C and 670 8C), whose mold temperature was increased [16,18]. Because specimens were not sandblasted or polished, the alpha-case layer was maintained at the surface of specimens, once only investment residues were removed using a brush and ultrasonic bath. In addition, the procedure used in all specimens was similar, affecting in a similar way all the groups. Although the same authors have argued that increased mold temperature improved titanium fluidity, casting quality, and misfit [16,18]; casting quality was evaluated in the present study only by radiographic images and the better sample quality was noted at samples cast with REM-430 (mold temperature suggested by the manufacturer), once any casting porosity was noted. However, samples cast with CAS-430 presented some casting porosities and this result can be attributed to the fact that mold temperature recommended by this investment manufacturer is 950 8C. Similarly, samples cast with CAS-RT were the group that presented more casting porosities and the great difference of mold temperature used from the suggested by the manufacturer can have contributed for this result. In addition,
these results can be attributed to the fact that the CAS investment presents inferior permeability, and the association to the rapid cooling rate due to the high difference between the molten titanium and mold temperature, mainly at room temperature, reduces the available time for gas to escape. Because alpha-case layer affects some titanium properties compromising dental prosthesis [24], the present study evaluated surface roughness, Vickers microhardness and bending strength. Vickers microhardness was measured in different regions of deep areas of the sample, once samples were transversally sectioned in order to measure microhardness out of alpha-case layer because it is known that this contamination layer interferes with specimen properties. Microhardness is affected by the microstructure of the specimens. As no difference was noted in the microstructure of the specimens cast in the different conditions (Fig. 3), the results of Vickers microhardness were similar in all the groups. Although another study had related a decrease in the Vickers microhardness from the surface to the interior for titanium casting [11], Vickers microhardness was evaluated in different deep regions as the surface was polished and alpha-case layer was partially removed. Thus, the regions evaluated in the studies were different, which could justify the different results. Similarly, no significant differences were noted in the surface roughness of samples cast in the different conditions. As tensile strength was related to surface hardness and roughness in another study that evaluated phosphate, magnesia and alumina bonded investments [11], it is possible that tensile strength would be similar if it was evaluated in the present study. However, the present study evaluated the bending strength and samples cast with CAS presented higher bending strength than that cast with REM. Thus, further studies are necessary to evaluate other properties, such as modulus of elasticity, and justify this difference. The use of non-specific to titanium investments have been searched by other studies and the results seems to be affected by the investment type. Blackman et al. [25] did not find difference in properties such as tensile strength and elongation for Rema Exakt and Ohara investments, but they argued that some investments (Dicor and Biovest) could not be used for CP Ti castings. Ferreira et al. [19] evaluated thermal shrinkage and the setting and thermal expansion of phosphate bonded investments Rema Exakt, Castorit Super C and Rematitan Plus and related that only Rema Exakt and Castorit Super C demonstrated sufficient expansion to compensate titanium casting shrinkage. Because many aspects need to be considered in a choice of an investment, the similarity in the results of Vickers microhardness and surface roughness of the samples cast with conventional
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
63
Fig. 3. Light micrographs of CP Ti: REM-430; REM-RT; CAS-430; CAS-RT in the central area (CA); and REM-430, REM-RT, CAS-430, CAS-RT, at the surface (S).
(CAS) and specific for titanium (REM) investments demonstrated the possibility of clinical application of the conventional investment; however, casting porosities found in the samples cast with CAS revealed that some care is necessary to cast frameworks with this investment, mainly for removable partial dentures, once these casting porosities could represent a problem, especially when they are present in the clasp regions [4,26,27]. 5. Conclusion As mechanical properties of the samples cast using Castorit Super C were similar that cast using the specific
to titanium investment Rematitan Plus, the conventional investment Castorit Super C could be used since rigorous care in the casting process, such as additional sprues or centrifugal injection of the alloy into the mold, was taken to decrease porosity occurrence, the main problem of these castings. Thus, radiographic evaluation is required to ensure the success of the castings. Furthermore, because the use of lower mold temperatures did not interfered with the alpha-case layer and increased the occurrence of porosities, castings in temperatures lower than that recommended by the manufacturer are not justified.
64
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
Acknowledgments The authors thank FAPESP (#2006/06426-6) for the financial support and Mr. Luiz Se´rgio Soares for the technical support. References [1] Blackman R, Barghi N, Tran C. Dimensional changes in casting titanium removable partial dentures frameworks. J Prosthet Dent 1991;65:309–15. [2] Vallittu PK, Kokkonen M. Deflection fatigue of cobalt–chromium, titanium, and gold alloy cast denture clasp. J Prosthet Dent 1995;74:412–9. [3] Okabe T, Hero H. The use of titanium in dentistry. Cells Mater 1995;5: 211–30. [4] Rodrigues RCS, Ribeiro RF, Mattos MGC, Bezzon OL. Comparative study of circumferential clasp retention force for titanium and cobalt– chromium removable partial dentures. J Prosthet Dent 2002;88:290–6. [5] Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental materials. Int Dent J 1993;43:245–53. [6] Wang RR, Fenton A. Titanium for prosthodontic applications: a review of the literature. Quintessence Int 1996;27:401–8. [7] Sunnerkrantz PA, Syverud M, Hero H. Effect of casting atmosphere on the quality of Ti-crowns. J Dent Res 1990;98:268–72. [8] Zinelis S. Effect of pressure of helium, argon, krypton, and xenon on the porosity, microstructure, and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2000;84:575–82. [9] Eliopoulus D, Zinelis S, Papadopoulus T. Porosity of cpTi casting with four different casting machines. J Prosthet Dent 2004;92:377–81. [10] Guilin Y, Nan L, Yousheng L, Yining W. The effect of different types of investments on the alpha-case layer of titanium castings. J Prosthet Dent 2007;97:157–64. [11] Hsu HC, Kikuchi H, Yen SK, Nishiyama M. Evaluation of different bonded investments for dental titanium casting. J Mater Sci Mater Med 2005;16:821–5. [12] Ida K, Togaya T, Tsutsumi S, Takeuchi M. Effect of magnesia investments in the dental casting of pure titanium or titanium alloys. Dent Mater J 1982;1:8–21. [13] Hung CC, Lai PL, Tsai CC, Huang TK, Liao YY. Pure titanium casting into titanium-modified calcia-based magnesia-based investment molds. Mater Sci Eng A 2007;454–455:178–82.
[14] Fischer J, Ebinger A, Hagi T, Stawarczyk B, Wenger A, Keller E. Mold filling and dimensional accuracy of titanium castings in a spinel-based investment. Dent Mater 2009;25:1376–82. [15] Low D, Mori T. Titanium full crown casting: thermal expansion of investments and crown accuracy. Dent Mater 1999;15:185–90. [16] Oliveira PCG, Adabo GL, Ribeiro RF, Rocha SS. The effect of mold temperature on castability of CP Ti and Ti–6Al–4V castings into phosphate bonded investment materials. Dent Mater 2006;22:1098– 102. [17] Pieralini AR, Benjamin CM, Ribeiro RF, Scaf G, Adabo GL. The effect of coating patterns with spinel-based investment on the castability and porosity of titanium cast into three phosphate-bonded investments. J Prosthodont 2010. doi: 10.1111/j.1532-849X. 2010.00629.x. [18] Fragoso WG, Henriques GEP, Contreras EFR, Mesquita MF. The influence of mold temperature on the fit of cast crowns with commercially pure titanium. Braz Oral Res 2005;19:139–43. [19] Ferreira AR, Adabo GL, Peitl Filho O, Rocha SS, Fonseca RG. Evaluation of the thermal shrinkage of titanium and the setting and thermal expansion of phosphate-bonded investments. J Prosthet Dent 2007;98:24–9. [20] Low D. Differing expansion contributions of three investment materials used for casting titanium. J Prosthodont 2009;18:444–9. [21] Sakai T, Hideshima M, Takahashi H, Ichinose S, Igarashi Y. Effect of mold temperatures on interface between primary and secondary castings of cast-on method for precision metal frameworks. J Prosthodont Res 2009;53:60–6. [22] Iiyama K, Doi H, Hanawa T. Effect of mold temperature on the mechanical durability of titanium casting clasp model. Dent Mater J 2009;28: 610–9. [23] Watanabe I, Woldu M, Watanabe K, Okabe T. Effect of casting method on castability of titanium and dental alloys. J Mater Sci Mater Med 2000;11:547–53. [24] Eliopoulus D, Zinelis S, Papadopoulus T. The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2005;94:539–48. [25] Blackman R, Tonaka T, Baez RJ. Strength comparisons of cast titanium using three phosphate type investments. J Dent Res 1991;70(Sp. Iss):485 [Abstract 1756]. [26] Hero H, Syverud M, Waarli M. Mold filling and porosity in casting of titanium. Dent Mater 1993;9:15–8. [27] Chai T-I, Stein RS. Porosity and accuracy of multiple-unit titanium castings. J Prosthet Dent 1995;73:534–41.
Journal of Prosthodontic Research 56 (2012) 58–64 www.elsevier.com/locate/jpor
Technical procedure
Effect of different investments and mold temperatures on titanium mechanical properties Renata Cristina Silveira Rodrigues DDS, MSc, PhD*, Elanio Pereira de Almeida DDS, Adriana Cla´udia Lapria Faria DDS, MSc, PhD, Ana Paula Macedo MSc, Eng, Maria da Gloria Chiarello de Mattos DDS, MSc, PhD, Ricardo Faria Ribeiro DDS, MSc, PhD Department of Dental Materials and Prosthodontics, Dental School of Ribeira˜o Preto, University of Sa˜o Paulo, Av. do Cafe´, s/n, Monte Alegre, 14040-904 Ribeira˜o Preto – SP, Brazil Received 15 February 2010; received in revised form 6 January 2011; accepted 11 January 2011 Available online 15 February 2011
Abstract Purpose: The aim of the present study was to evaluate commercially pure titanium (CP Ti) casting quality when a specific to titanium and a conventional phosphate bonded investments were used under different mold temperatures. For this, the evaluated parameters were surface roughness, bending strength, Vickers microhardness, casting quality by radiographies and microstructure of CP Ti. Methods: Wax patterns (28 mm 3 mm 1 mm) were invested using two phosphate bonded investments: Rematitan Plus (REM), specific to titanium, and Castorit Super C (CAS), a conventional investment, fired and cooled until reaching two mold temperatures: 430 8C (430) and room temperature (RT). Specimens were cast from CP Ti by plasma. After casting, specimens were radiographically examined and submitted to Vickers microhardness, roughness and bending strength evaluation. Microstructure was analyzed in the center and at the surface of specimen. Results: Qualitative analysis of radiographs showed that specimens which were cast using CAS-RT presented more casting porosities while the specimens which were cast with REM-430 did not present any casting porosity. No significant difference was noted among the groups in the surface roughness and Vickers microhardness data, but the bending strength of the specimens cast using CAS was greater than REM groups. The microstructure of the specimens of the different groups was similar, presenting a feather-like aspect. Conclusion: Casting porosities found in the specimens cast using conventional investments (CAS) and lower mold temperatures would limit their use, even mechanical properties were similar than in specimens cast using specific to titanium investment (REM) at temperatures recommended by the manufacturer. # 2011 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. Keywords: Titanium; Dental casting investment; Radiography; Microscopy
1. Introduction The use of commercially pure titanium (CP Ti) has increased in dental appliances because of its good mechanical properties, excellent corrosion resistance, good biocompatibility and high strength-to-weight ratio [1–4]. However, difficulties related to casting process have been restrained titanium application, especially in prosthodontics. Casting problems are caused by high melting point, highly reactive behavior with investment materials at high temperatures, and low density, creating
* Corresponding author. Tel.: +55 16 3602 4005; fax: +55 16 3602 4780. E-mail address: [email protected] (R.C.S. Rodrigues).
difficulties in achieving complete mold filling [5,6]. To overcome these difficulties, new casting machines have been produced combining arc or induction melting, within inert atmosphere through the use of argon gas [7–9]. In addition, investment materials for titanium castings have been widely studied in recent years because, at high melting point, titanium reacts with some elements such as oxygen, hydrogen, carbon and investment surface, creating a brittle and hard surface layer, so-called alpha-case, which affects surface properties of titanium casting [10,11]. Because this layer interferes with ductility, fatigue resistance of removable partial denture frameworks and clasps, roughness and metal–ceramic bonding resistance [12], new investment materials such as Al2O3- and MgO-based materials have been
1883-1958/$ – see front matter # 2011 Japan Prosthodontic Society. Published by Elsevier Ireland. All rights reserved. doi:10.1016/j.jpor.2011.01.002
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
developed to minimize the influence of alpha-case layer formation [9–11,13,14]. Phosphate bonded investments with silica have been used for casting dental alloys whose melting point is higher. Although these conventional investments have been used for casting titanium, because investments specific to titanium present high cost, some elements (Si, O, P, Fe and Al) of their composition react with titanium surface [15]. Consequently, some properties need evaluation before routine use of these investments. SiO2-based phosphate bonded investments were routinely used to cast dental alloys and with some modifications, as addition of Al2O3 and MgO, are used to specifically cast CP Ti and titanium alloys. Guilin et al. [10] stated that SiO2 is unstable and easily react with titanium to form more TixOy, increasing the oxide content and providing a greater microhardness to the surface reaction layer. The authors also pointed out that the Al2O3 based investments reduce these reactions and the thickness of reaction layer. However, according to some authors [16,17], despite investments based on Al2O3, MgO and ZrO2 are less reactive, they present low expansions and are more expensive, and your utilization is limited. Thermal expansion and misfit are important aspects to analyze in investments for casting titanium, once titanium requires casting at lower temperatures due to the reactivity of silica and titanium at temperatures above 500 8C, and thermal expansion at lower temperatures cannot be enough to compensate casting shrinkage, affecting misfit [18–20]. Furthermore, it is believed that mold temperature investment interferes with properties of casting titanium by reducing interfacial reactivity, and some authors have casted titanium using different mold temperatures searching one whose thermal expansion could compensate casting shrinkage [16–21]. At lower temperatures, the hardness values became constant at depths exceeding 300 mm while mold temperatures above 600 8C could increase hardness only at depth of 500 mm, suggesting that oxidation effects reach deeper in the cast body at higher mold temperatures [22]. In addition, some authors advocate the use of room temperature of the mold when titanium is casted in vaccum-pressure casting machine [23]. Nevertheless, any study has evaluated the effect of the room temperature in the microstructure and mechanical properties of titanium castings. The null hypothesis is that casting titanium using mold temperatures lower than that recommended by the manufacturer could decrease interfacial reactivity and improve compensation of the casting shrinkage, interfering with misfit. However, mechanical properties need to be maintained in these new casting conditions. In addition, the hypothesis that a conventional investment, whose cost is lower and expansion is greater, used at lower mold temperatures, could provide adequate casting, related to alpha-case formation and mechanical properties when compared to that specific to titanium. Thus, the aim of the present study was to evaluate titanium casting quality when a specific to titanium and a conventional investment were used under different mold temperatures. The evaluated parameters were surface rough-
59
Table 1 Composition of the conventional (CAS) and specific to titanium (REM) investments. Investment composition (%)a
SiO2
MgO
NH4h2PO4
Al2O3
REM CAS
55–75 60–80
10–30 6–19
5–10 10–20
10–25 –
a
Manufacturer information.
ness, bending strength, Vickers microhardness, casting defects by radiographies and microstructure of CP Ti. 2. Materials and methods 2.1. Difference from conventional methods Titanium castings are performed with phosphate bonded investments specific to titanium using mold temperature recommended by the manufacturer. The present study evaluated a conventional phosphate bonded investment at lower mold temperatures to cast titanium. 2.2. Specimen preparation The specimens were obtained from rectangular wax patterns (28 mm 3 mm 1 mm) which were invested using two phosphate bonded investments: Rematitan Plus (REM) with mixing liquid for partial denture frameworks (Dentaurum, Pforzheim, Germany), which is an investment specific to titanium, and Castorit Super C (CAS) (Dentaurum, Pforzheim, Germany). Investment compositions are presented in Table 1. Mixing of the investments was made according to manufacturer’s recommendations, under vacuum (Turbo Mix; EDG Equipamentos e Controles Ltda., Sa˜o Carlos, Brazil) before being poured into the ring. After setting, the investment blocks were put in an electric furnace (EDGCON 5P; EDG Equipamentos e Controles Ltda., Sa˜o Carlos, Brazil) and fired according to manufacturer’s instructions. The molds were cooled in the furnace to the different final mold temperatures: 430 8C (430) and room temperature of 22 8C (RT). The schedules used for dewaxing and thermal expansion are described in Table 2. Ten castings were obtained for each condition investment/mold temperature. The specimens were cast from grade I CP Ti in a vacuum-pressure casting machine (Discovery Plasma, EDG Equipamentos e Controles Ltda., Sa˜o Carlos, Brazil), where the melting was made by arc melting in a vacuum and argon inert atmosphere, with injection of the alloy/ metal into the mold by vacuum-pressure. After casting, investment blocks were quenched in cold water, as manufacturer’s instructions, until reaching room temperature, and then divested. With this procedure a sudden change in temperature and rapid steam generation occur, and the investment breaks away from the casting. So, investments adhered to castings were firstly removed by gently brushing of the investment surface using a wire brush suitable for bur cleaning. When a thin layer of investment yet remained adhered
60
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
Table 2 The schedule used for dewaxing and thermal expansion of the investments. Investment and mold temperature
Stage 1
REM-430 REM-RT CAS-430 CAS-RT
150 8C 150 8C 250 8C 250 8C
Stage 2 (90 min) (90 min) (60 min) (60 min)
250 8C 250 8C 950 8C 950 8C
(90 min) (90 min) (30 min) (30 min)
Stage 3
Stage 4
Casting
1000 8C (60 min) 1000 8C (60 min) 430 8C RT
430 8C RT – –
430 8C RT 430 8C RT
Schedules suggested by the manufacturer. REM investment: 150 8C (90 min) - 250 8C (90 min) - 1000 8C (60 min) - 430 8C and casting. CAS investment: 250 8C (60 min) -950 8C (30 min) and casting.
to castings, they were immersed in ultrasonic bath. This cleaning procedure was used to avoid alpha-case removal. 2.3. Radiographic evaluation Before the tests, all specimens were examined radiographically to detect possible casting defects that would contraindicate their use in the tests. A laboratorial unit XControl (Dentaurum, Ispringen, Germany), was set to 70 kV and 8 mA for a 5-s exposure time at 20 cm from the test specimen, and a film Polapan 57 high speed panchromatic black and white film (Polaroid Corp., Cambridge, USA), presenting an exposure area of 9 cm 12 cm, was used and auto-processed for 20 s. 2.4. Surface roughness Surface roughness of the specimens was measured with a profilometer (Mitutoyo SJ201-P, 300 mm accuracy, 0.5 mm/s speed, and five 0.8 mm cut-offs). Three readings were made in each specimen (on the center of the specimen, 1 mm to the right and 1 mm to the left) and a mean value was calculated for each specimen.
Fig. 1. Figure illustration of specimens cut transversally for Vickers microhardness evaluation. Indentations where Vickers microhardness was measured can be noted.
performed in each lateral of the cut specimen and three in the central part, as is shown in Fig. 1, permitting to analyze microhardness in the interior of the specimen.
2.5. Bending strength 2.7. Microstructure analysis Three-point bending tests were performed, at room temperature, on a universal testing instrument EMIC MEM 2000 (EMIC, Sa˜o Jose´ dos Pinhais, Brazil) using crosshead speed of 0.5 mm/min and load cell of 500 kgf. The bending strengths were determined using the equation s = 3PL/2bh2, where s is the bending strength (MPa), P is the load (N), L is the span length (mm), b is the specimen width (mm), and h is the specimen thickness (mm). 2.6. Vickers microhardness The Vickers microhardness of the specimens was measured with a load of 19.614 N applied for 30 s (Microhardness tester HMV-2 Shimadzu Corp., Kyoto, Japan). The specimens were cut in the transversal axis (3 mm 3 mm 1 mm) permitting to analyze the Vickers microhardness in the interior of specimen. After cutting, specimens were embedded using autopolymerizing acrylic resin and polished with sequential silicon carbide papers in the sequence 320, 400 and 600. So, three measures were
To analyze microstructure, one specimen of each group was embedded using autopolymerizing acrylic resin and then polished with silicon carbide papers in the sequence 320, 400, 600 and 1200. The final polishing was reached with a colloidal silica solution (OPS, Struers A/S, Denmark) + H2O2 30%. After this, the samples were etched with Kroll solution (6 mL HNO3 + 3 mL HF + 91 mL H2O) for 40 s and examined using an optical microscope Neophot 30 (Jena-Carl Zeiss, Jena, Germany). Microstructure images were obtained using a digital camera (CC-8703, GKB, Tai Chung, Taiwan). 2.8. Statistical analysis The effect of the investment and mold temperature on roughness, bending strength and Vickers microhardness was evaluated using 1-way analysis of variance (ANOVA), followed by post hoc Tukey test (a = 0.05) using the software SPSS for Windows (SPSS Inc., Chicago, USA).
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
61
Fig. 2. Radiographic images of titanium specimens cast in different conditions: REM-430 where any casting porosity was noted; REM-RT which present casting porosities pointed by arrows; CAS-430, where some casting porosities are pointed by arrows; and CAS-RT, presenting the higher quantity of casting porosities, as pointed by arrows.
3. Results
3.3. Bending strength
3.1. Radiographic evaluation
Comparison between bending strength data (Table 3) revealed that specimens cast with the conventional investment CAS presented greater bending strength than that cast using the REM ( p 0.05).
Digital images of radiographs are presented in Fig. 2. Qualitative analysis of radiographs showed that specimens which were cast using the conventional investment CAS-RT presented more casting porosities while the specimens which were cast using the investment specific to titanium REM-430 did not present any casting porosity, representing the best results.
3.4. Vickers microhardness The results of Vickers microhardness evaluation are presented in Table 3. No significant differences were noted in the Vickers microhardness values among the groups ( p 0.05).
3.2. Surface roughness 3.5. Microstructure analysis The data of surface roughness (Table 3) measured in the specimens cast using different investments and mold temperatures did not reveal any significant difference ( p 0.05).
Microscopy images (Fig. 3) of the cast specimens revealed that all the specimens presented a feather-like microstructure.
62
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
Table 3 Surface roughness (Ra), bending strength (GPa), and Vickers microhardness (VHN) of the specimens cast using a conventional and a specific to titanium investment under different mold temperatures. The results are expressed as mean (standard deviation).
Surface roughness (Ra) Bending strength (GPa) Vickers microhardness (VHN)
REM-430
REM-RT
CAS-430
CAS-RT
6.57(0.80) 0.59(0.03) 142.25(18.43)
6.23(1.00) 0.60(0.06) 128.33(9.10)
5.85(0.82) 0.70(0.03) 136.11(12.71)
5.63(0.72) 0.67(0.06) 139.67(11.84)
The specimens cast using the different investments at two different mold temperatures presented a similar microstructure at the surface (Fig. 3S) and in the central area (Fig. 3C). 4. Discussion Considering the relevancy of the investment cost in the final price of prosthodontics, the present study compared a conventional phosphate bonded investment (CAS) to a phosphate bonded investment specific for titanium (REM), at two different mold temperatures: 430 8C (430), recommended by the REM manufacturer, and room temperature (RT), suggested by some authors [22,23] to minimize reactivity of CP Ti and investment. The images of CP Ti microstructure presented in Fig. 3 revealed that alpha-case layer present at the surface of the samples (Fig. 3S) cast with CAS and REM at different mold temperatures (430 and RT) showed a similar aspect and thickness, although any quantitative evaluation of this layer thickness had not been made. Based on these images, it is observed that the investment and mold temperature did not interfere significantly with reactivity of investment and CP Ti, once alpha-case layer formation was similar in all groups. Although these results are different of some results publicized in the literature which argued that alpha-case layer is affected by mold temperature [16,18], it is necessary to consider the difference in mold temperatures evaluated in the present (430 8C and room temperature) and in the other studies (430 8C, 480 8C, 530 8C, 550 8C and 670 8C), whose mold temperature was increased [16,18]. Because specimens were not sandblasted or polished, the alpha-case layer was maintained at the surface of specimens, once only investment residues were removed using a brush and ultrasonic bath. In addition, the procedure used in all specimens was similar, affecting in a similar way all the groups. Although the same authors have argued that increased mold temperature improved titanium fluidity, casting quality, and misfit [16,18]; casting quality was evaluated in the present study only by radiographic images and the better sample quality was noted at samples cast with REM-430 (mold temperature suggested by the manufacturer), once any casting porosity was noted. However, samples cast with CAS-430 presented some casting porosities and this result can be attributed to the fact that mold temperature recommended by this investment manufacturer is 950 8C. Similarly, samples cast with CAS-RT were the group that presented more casting porosities and the great difference of mold temperature used from the suggested by the manufacturer can have contributed for this result. In addition,
these results can be attributed to the fact that the CAS investment presents inferior permeability, and the association to the rapid cooling rate due to the high difference between the molten titanium and mold temperature, mainly at room temperature, reduces the available time for gas to escape. Because alpha-case layer affects some titanium properties compromising dental prosthesis [24], the present study evaluated surface roughness, Vickers microhardness and bending strength. Vickers microhardness was measured in different regions of deep areas of the sample, once samples were transversally sectioned in order to measure microhardness out of alpha-case layer because it is known that this contamination layer interferes with specimen properties. Microhardness is affected by the microstructure of the specimens. As no difference was noted in the microstructure of the specimens cast in the different conditions (Fig. 3), the results of Vickers microhardness were similar in all the groups. Although another study had related a decrease in the Vickers microhardness from the surface to the interior for titanium casting [11], Vickers microhardness was evaluated in different deep regions as the surface was polished and alpha-case layer was partially removed. Thus, the regions evaluated in the studies were different, which could justify the different results. Similarly, no significant differences were noted in the surface roughness of samples cast in the different conditions. As tensile strength was related to surface hardness and roughness in another study that evaluated phosphate, magnesia and alumina bonded investments [11], it is possible that tensile strength would be similar if it was evaluated in the present study. However, the present study evaluated the bending strength and samples cast with CAS presented higher bending strength than that cast with REM. Thus, further studies are necessary to evaluate other properties, such as modulus of elasticity, and justify this difference. The use of non-specific to titanium investments have been searched by other studies and the results seems to be affected by the investment type. Blackman et al. [25] did not find difference in properties such as tensile strength and elongation for Rema Exakt and Ohara investments, but they argued that some investments (Dicor and Biovest) could not be used for CP Ti castings. Ferreira et al. [19] evaluated thermal shrinkage and the setting and thermal expansion of phosphate bonded investments Rema Exakt, Castorit Super C and Rematitan Plus and related that only Rema Exakt and Castorit Super C demonstrated sufficient expansion to compensate titanium casting shrinkage. Because many aspects need to be considered in a choice of an investment, the similarity in the results of Vickers microhardness and surface roughness of the samples cast with conventional
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
63
Fig. 3. Light micrographs of CP Ti: REM-430; REM-RT; CAS-430; CAS-RT in the central area (CA); and REM-430, REM-RT, CAS-430, CAS-RT, at the surface (S).
(CAS) and specific for titanium (REM) investments demonstrated the possibility of clinical application of the conventional investment; however, casting porosities found in the samples cast with CAS revealed that some care is necessary to cast frameworks with this investment, mainly for removable partial dentures, once these casting porosities could represent a problem, especially when they are present in the clasp regions [4,26,27]. 5. Conclusion As mechanical properties of the samples cast using Castorit Super C were similar that cast using the specific
to titanium investment Rematitan Plus, the conventional investment Castorit Super C could be used since rigorous care in the casting process, such as additional sprues or centrifugal injection of the alloy into the mold, was taken to decrease porosity occurrence, the main problem of these castings. Thus, radiographic evaluation is required to ensure the success of the castings. Furthermore, because the use of lower mold temperatures did not interfered with the alpha-case layer and increased the occurrence of porosities, castings in temperatures lower than that recommended by the manufacturer are not justified.
64
R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64
Acknowledgments The authors thank FAPESP (#2006/06426-6) for the financial support and Mr. Luiz Se´rgio Soares for the technical support. References [1] Blackman R, Barghi N, Tran C. Dimensional changes in casting titanium removable partial dentures frameworks. J Prosthet Dent 1991;65:309–15. [2] Vallittu PK, Kokkonen M. Deflection fatigue of cobalt–chromium, titanium, and gold alloy cast denture clasp. J Prosthet Dent 1995;74:412–9. [3] Okabe T, Hero H. The use of titanium in dentistry. Cells Mater 1995;5: 211–30. [4] Rodrigues RCS, Ribeiro RF, Mattos MGC, Bezzon OL. Comparative study of circumferential clasp retention force for titanium and cobalt– chromium removable partial dentures. J Prosthet Dent 2002;88:290–6. [5] Lautenschlager EP, Monaghan P. Titanium and titanium alloys as dental materials. Int Dent J 1993;43:245–53. [6] Wang RR, Fenton A. Titanium for prosthodontic applications: a review of the literature. Quintessence Int 1996;27:401–8. [7] Sunnerkrantz PA, Syverud M, Hero H. Effect of casting atmosphere on the quality of Ti-crowns. J Dent Res 1990;98:268–72. [8] Zinelis S. Effect of pressure of helium, argon, krypton, and xenon on the porosity, microstructure, and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2000;84:575–82. [9] Eliopoulus D, Zinelis S, Papadopoulus T. Porosity of cpTi casting with four different casting machines. J Prosthet Dent 2004;92:377–81. [10] Guilin Y, Nan L, Yousheng L, Yining W. The effect of different types of investments on the alpha-case layer of titanium castings. J Prosthet Dent 2007;97:157–64. [11] Hsu HC, Kikuchi H, Yen SK, Nishiyama M. Evaluation of different bonded investments for dental titanium casting. J Mater Sci Mater Med 2005;16:821–5. [12] Ida K, Togaya T, Tsutsumi S, Takeuchi M. Effect of magnesia investments in the dental casting of pure titanium or titanium alloys. Dent Mater J 1982;1:8–21. [13] Hung CC, Lai PL, Tsai CC, Huang TK, Liao YY. Pure titanium casting into titanium-modified calcia-based magnesia-based investment molds. Mater Sci Eng A 2007;454–455:178–82.
[14] Fischer J, Ebinger A, Hagi T, Stawarczyk B, Wenger A, Keller E. Mold filling and dimensional accuracy of titanium castings in a spinel-based investment. Dent Mater 2009;25:1376–82. [15] Low D, Mori T. Titanium full crown casting: thermal expansion of investments and crown accuracy. Dent Mater 1999;15:185–90. [16] Oliveira PCG, Adabo GL, Ribeiro RF, Rocha SS. The effect of mold temperature on castability of CP Ti and Ti–6Al–4V castings into phosphate bonded investment materials. Dent Mater 2006;22:1098– 102. [17] Pieralini AR, Benjamin CM, Ribeiro RF, Scaf G, Adabo GL. The effect of coating patterns with spinel-based investment on the castability and porosity of titanium cast into three phosphate-bonded investments. J Prosthodont 2010. doi: 10.1111/j.1532-849X. 2010.00629.x. [18] Fragoso WG, Henriques GEP, Contreras EFR, Mesquita MF. The influence of mold temperature on the fit of cast crowns with commercially pure titanium. Braz Oral Res 2005;19:139–43. [19] Ferreira AR, Adabo GL, Peitl Filho O, Rocha SS, Fonseca RG. Evaluation of the thermal shrinkage of titanium and the setting and thermal expansion of phosphate-bonded investments. J Prosthet Dent 2007;98:24–9. [20] Low D. Differing expansion contributions of three investment materials used for casting titanium. J Prosthodont 2009;18:444–9. [21] Sakai T, Hideshima M, Takahashi H, Ichinose S, Igarashi Y. Effect of mold temperatures on interface between primary and secondary castings of cast-on method for precision metal frameworks. J Prosthodont Res 2009;53:60–6. [22] Iiyama K, Doi H, Hanawa T. Effect of mold temperature on the mechanical durability of titanium casting clasp model. Dent Mater J 2009;28: 610–9. [23] Watanabe I, Woldu M, Watanabe K, Okabe T. Effect of casting method on castability of titanium and dental alloys. J Mater Sci Mater Med 2000;11:547–53. [24] Eliopoulus D, Zinelis S, Papadopoulus T. The effect of investment material type on the contamination zone and mechanical properties of commercially pure titanium castings. J Prosthet Dent 2005;94:539–48. [25] Blackman R, Tonaka T, Baez RJ. Strength comparisons of cast titanium using three phosphate type investments. J Dent Res 1991;70(Sp. Iss):485 [Abstract 1756]. [26] Hero H, Syverud M, Waarli M. Mold filling and porosity in casting of titanium. Dent Mater 1993;9:15–8. [27] Chai T-I, Stein RS. Porosity and accuracy of multiple-unit titanium castings. J Prosthet Dent 1995;73:534–41.