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 Claudia 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 Ribeirao Preto, University of Sao Paulo, Av. do Cafe,

s/n, Monte Alegre, 14040-904 Ribeirao 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

www.elsevier.com/locate/jpor

Available online at www.sciencedirect.com

Journal of Prosthodontic Research 56 (2012) 58–64

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: renata@forp.usp.br (R.C.S. Rodrigues).

1883-1958/$ – see front matter # 2011 Japan Prosthodontic Society. Published b

doi:10.1016/j.jpor.2011.01.002

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

y Elsevier Ireland. All rights reserved.

Table 1

Composition of the conventional (CAS) and specific to titanium (REM)

investments.

Investment composition (%)a SiO2 MgO NH4h2PO4 Al2O3

REM 55–75 10–30 5–10 10–25

CAS 60–80 6–19 10–20 –

a Manufacturer information.

R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64 59

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 micro-

hardness 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 manufac-

turer 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-

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 manufac-

turer’s recommendations, under vacuum (Turbo Mix; EDG

Equipamentos e Controles Ltda., Sao 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., Sao 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., Sao

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

Table 2

The schedule used for dewaxing and thermal expansion of the investments.

Investment and mold temperature Stage 1 Stage 2 Stage 3 Stage 4 Casting

REM-430 150 8C (90 min) 250 8C (90 min) 1000 8C (60 min) 430 8C 430 8CREM-RT 150 8C (90 min) 250 8C (90 min) 1000 8C (60 min) RT RT

CAS-430 250 8C (60 min) 950 8C (30 min) 430 8C – 430 8CCAS-RT 250 8C (60 min) 950 8C (30 min) RT – 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.

Fig. 1. Figure illustration of specimens cut transversally for Vickers micro-

hardness evaluation. Indentations where Vickers microhardness was measured

can be noted.

R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–6460

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 radio-

graphically to detect possible casting defects that would

contraindicate their use in the tests. A laboratorial unit X-

Control (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.

2.5. Bending strength

Three-point bending tests were performed, at room

temperature, on a universal testing instrument EMIC MEM

2000 (EMIC, Sao 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 autopolymeriz-

ing acrylic resin and polished with sequential silicon carbide

papers in the sequence 320, 400 and 600. So, three measures were

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.

2.7. Microstructure analysis

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).

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.

R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64 61

3. Results

3.1. Radiographic evaluation

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.

3.2. Surface roughness

The data of surface roughness (Table 3) measured in the

specimens cast using different investments and mold tempera-

tures did not reveal any significant difference ( p � 0.05).

3.3. Bending strength

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).

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).

3.5. Microstructure analysis

Microscopy images (Fig. 3) of the cast specimens revealed

that all the specimens presented a feather-like microstructure.

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).

REM-430 REM-RT CAS-430 CAS-RT

Surface roughness (Ra) 6.57(0.80) 6.23(1.00) 5.85(0.82) 5.63(0.72)

Bending strength (GPa) 0.59(0.03) 0.60(0.06) 0.70(0.03) 0.67(0.06)

Vickers microhardness (VHN) 142.25(18.43) 128.33(9.10) 136.11(12.71) 139.67(11.84)

R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–6462

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 manufac-

turer 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

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).

R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–64 63

(CAS) and specific for titanium (REM) investments demon-

strated 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.

R.C.S. Rodrigues et al. / Journal of Prosthodontic Research 56 (2012) 58–6464

Acknowledgments

The authors thank FAPESP (#2006/06426-6) for the

financial support and Mr. Luiz Sergio Soares for the technical

support.

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