The effects of sprue design on the roughness and porosity of titanium castings

Daniel Chan, DDS, M S , a Villa Guillory, DDS, b Ronald Blackman, DDS, MSD, c and Kwok-hung Chung, DDS, PhD a Dental School, The University of Texas, San Antonio, Texas; Goodfellow Air Force Base, Texas; Faculty of Dentistry, National Yang-ming University, Taiwan, Republic of China

Purpose . This study measured the effects of the sprue number and position on the roughness and porosity of cast titanium crowns. Material and methods. Twenty-four complete veneer crown wax patterns were fabricated on a stainless steel die with a 30-degree bevel finish line. Twelve wax patterns were sprned with one 8-gange wax sprue and the remaining 12 were double sprued. All patterns were invested with a phosphate bonded investment. Castings were made with a titanium casting, according to the manufacturer's instructions, with commer- cially pure titanium (> 99.5%) ingots. The castings were carefully cleaned and the surface roughness was measured with a profilometer. The specimens were then embedded and sectioned. Internal porosities were quantified with photographs by computerized image analysis. Data were analyzed with an ANOVA and the Student's t test with a confidence level of 95%. Results . The roughness value of the occlusal third of the crowns for the single sprue group (Ra = 3.0 _+ 0.9 pm) was significantly higher than other measurements (p < 0.05). There were statistically significant differences in values of porosity areas between the single sprue group 0 .5 _+ 0.7 mm 2) and the double sprue group (0.2 _+ 0.2 mm 2) (p < 0.01). The double sprue design resulted in a relatively smoother casting surface and less internal porosity than the single sprue design. Conclusions. Improvements in the degree of roughness and porosity of titanium crown castings were the result of the double sprue design. (J Prosthet Dent 1997;78:400-4.)

M a n y low gold and base metal alloy systems are available for crown construction. One alternative to gold or base metal alloys is titanium, which, for the past two decades, has mainly bccn used in dentistry in the manu- facturing of dental implants. >3 Recently, there has been great interest in the use of titanium for fixed and remov- able prostheses. 4-a° The high melting temperature and chemical reactivity of titanium necessitates casting ma- chines different from those used in conventional cast- ing. Current titanium casting systems are based on an electric arc design, and melting takes place in an argon or helium atmosphere to reduce titanium oxidation dur- ing casting. Greener et al. n found that a vacuum ar- gon/electric arc pressure casting machine produced only

aAssociate Professor, Department of Restorative Dentistry, Dental School, The University of Texas.

bResident, Goodfellow Air Force Base. cAssociate Professor, Department of Prosthodontics, Dental School,

The University of Texas. dVisiting Professor, Department of Restorative Dentistry, Dental

School, The University of Texas; and Professor, Faculty of Den- tistry, National Yang-ming University.

10% to 20% castability using 1000 lam and 500 pm mesh patterns, with both commercially pure titanium and Ti- 6A1-4V alloy. The use of an argon/electr ic arc vertical centrifugal casting machine (Ohara Inc., Ltd., Osaka, Japan) produced 100% castability with 1000 pm mesh and commercially pure titanium. The casting machine was equipped with a high temperature heating source (a nonconsumable tungsten arc), an inert argon atmo- sphere, and a durable copper crucible.

Sprue design is a factor that controls the velocity and adequate supply of metal to the mold. Several studies in dental casting have demonstrated the importance of not only the size o f the sprue but also its type, shape, loca- tion and direction. Sprue diameter may be the most sig- nificant variable that influences the completeness and soundness of the casting. In crown fabrication, the sprue is a variable that affects casting S u c c e s s . 12-14 Preston and Bcrgcr ~4 revealed that commonly used sprue designs were inadequate when casting titanium. Sprue geomet ry was also identified as one of the major factors producing dif- ferent castability and porosity effects with low density ti tanium alloys. ~s When casting titanium, a modified

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I 7.0_+0.1

I 0.35 ._,,. ~ _

+ 0.02 I I

I I

_ 7.70

I Fig. 1. Oimensions of stainless steel die (in ram).

I

I~ - 5 °

6.0 +0 .1

\ \ \ \

\

sprue design is imperative to ensure proper mold filling. Hero and his colleagues 16 disclosed that argon pressure, permeability o f the investment material, and venting af- fected the quality of titanium castings. Rygc et al.17 char- acterized the porosities in dental castings into two dis- tinct groups: (1) porosities caused by cooling and so- l id i f ica t ion (namely, shr inkage , subsur face , and microscopic porosities) and (2) porosities caused by gas (pinhole porosity and gas inclusion). They recommended the use ofa sprue diameter larger than the thickest cross- section of the casting to eliminate shrinkage porosity. Strickland and Sturdervant 12 studied 23 casting variables affecting porosity in gold alloy castings. They concluded that porosity was probably caused by a lack o f rapid vent- ing for gases in the mold cavity, higher melt tempera- tures, positioning in the ring, and choice of investment, all o f which, if properly controlled, could assist in re- ducing or eliminating porosity. However, runner-bar sprue design for titanium casting was tested and dem- onstrated to have a tendency to increase internal poros- ity.IS,16. I 8

The purpose of this study was to measure the effect o f the number ofsprues and their positions on the rough- ness and porosity o f simulated titanium crown castings.

M A T E R I A L A N D M E T H O D S

A stylized die, machined from a stainless steel rod with a 7.7 mm diameter, simulated the preparation for a com-

I I I I I I I I I

crown pattern

I I I I I I I l l

vent

sprue

Fig. 2. Schematic drawing of crown pattern positioned in cast- ing ring.

plete veneer metal crown. The test casting design repre- sented a complete veneer crown. The core was 6 mm high and 7 mm in diameter, with a uniform sharp cervi- cal margin o f 30 degrees (Fig. 2). 2nstead o f the shape of a tooth, a cylindrical form of wax pattern was con- structed with a 2 mm thickness o f the occlusal surface. The stainless steel dic was conditioned in a 37 ° C oven for 20 minutes to simulate oral cavity temperature and to decrease the thermal stress created during wax up. Wax patterns wcrc prepared by dipping the preheated die into molten inlay casting wax (Blue inlay casting wax, Type II, Kerr Mfg. Co., Romulus, Mich.). The metal die with wax in excess of the final wax pattern was placed in a sculpting device, as described by Philp and Bruld, 19 and secured with a set screw. A razor blade was posi- tioned vertically on the sculpturing device, and the screw attaching the movable vertical blade to the base was gradually tightened.

The wax was gently shaved by the razor blade to ob- tain a cylindrical external form as uniform in thickness as possible. The crown margin was then precisely carved to a knife-edge margin o f 30 degrees on the metal die. There were two groups of castings. The first group had one 8-gauge sprue centered occlusally on the pattern. The o the r g roup had two 8-gauge sprues placed occlusally on the patterns at the occlusal-axial line angles, flush with external pattern walls, and directed straight toward the margins. Sprue lengths for both groups were 5 mm in straight port ion and 20 mm with a 4S-degree terminal pattern angle, attached to a special crucible former (Fig. 2). In the double sprue group, an addi- tional 18-gauge wax sprue was attached to the pattern between the sprues, 2 mm from the margin at the sur- face of the pattern, to serve as an opened vent. In the

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Table I. Casting mold preparation

Phase Time

Burn 200 to 800 o C 60 minutes

Hold temperature 30 minutes Processing cycle

800 ° to 950 ° C 15 minutes Hold temperature 15 minutes 950 ° to 1200 ° C 30 minutes

Hold temperature 30 minutes Cool to 25 ° C 80 minutes

(a) (b)

Fig. 3. Schematic drawings of crown pattern; A, single sprue pattern; B, double sprue pattern.

single sprue group, a similar vent was located along the straight port ion of the main sprue (Fig. 3). The sp rue / wax pattern junction was carefully refined under 10x magnification.

The complete wax pattern with sprue former was in- vested with a phosphate-bonded investment (Ohara Ti- tanium Vest for Crowns, Ohara Co. Ltd.) according to the manufacturer's instructions ( l iquid/powder ratio, 35 ml /17S gm). The investment was spatulated under a vacuum for 15 seconds and then slowly vibrated into the ring, which was filled to approximately 5 mm above the pattern. The vent remained visible. The location of the crown pattern in the ring was marked on the invest- ment, so rings could be properly oriented for casting. The investment was allowed to bench set at room tem- perature (approximately 25 ° C) for 45 minutes before beginning burnout procedures.

Burnout procedures and temperature setting followed manufacturer's recommendations (Table I). Wax was eliminated and molds were heat soal~ed in a conventional burnout furnace for 30 minutes. The refractory molds were then transferred to a high-temperature processing furnace (Ohara Co. Ltd.) for mold expansion. After heat processing, the molds were allowed to cool in the furnace until they could be picked up with unprotected hands and placed in the casting machine. Castings were made with an argon/electric arc vertical centrifugal casting machine according manufacturer's instructions (Ohara Co. Ltd. ). A 7 gm titanium (> 99.5 %, commercially pure ) ingot was used to cast each specimen. The refractory mold was secured in the casting machine with the pattern mold oriented to the casting arm motion, according to the rec- ommendations reported by DeWald. 2° The casting pro- cedure was repeated for each of the specimens with the shortest possible delay between castings. Twelve castings were made for each group, providing a total of 24 com- plete veneer crown castings.

After bench cooling, the castings were divested manu- ally, sandblasted lightly with 50 pm aluminum oxide abrasive to remove residual investment grossly, and then the buttons of the castings were sectioned off. All cast- ings were ultrasonically cleaned in distilled water for 2 minutes. No grinding, deburring, or polishing was at- tempted, and no effort was made to reseat the castings on the original die.

The average surface roughness (Ra, #m) of the oc- clusal, middle, and gingival thirds along the mesial and distal surfaces related to the vent of the cast crown were measured by a surface profilometer (Surtronic 3, Tay- lor-Hobson, Leicester, England). A tracing length of 1.75 mm and a cutoffvalue o f 0.25 mm were adapted to maximize filtration of surface waviness. 21

After roughness assessment, the cast crowns were e m b e d d e d in p o l y m e t h y l m e t h a c r y l a t c mater ia l (Transoptic powder, Buehler Ltd., Lake Bluff, Ill.) with a specimen mounting press. The location and quantity of porosity were determined by evaluation of radiographs and photographs. Radiographic screening analysis was carried out with a dental x-ray machine that used 85 KVP, 15 mA, and a 36-second exposure time. The dis- tance between the cone and the titanium casting was 10 cm with focusing at the center of the crown. Two expo- sures with direction of 90 degrees related and parallel to the long axis of the vent were performed for each cast- ing. The dental radiographic films were developed at 27 ° C for 3.6 minutes with an autodevcloper. The films were used to confirm and locate the presence of poros- ity. The embedded castings wcrc then sectioned with a diamond saw (Isomer, Low Speed Saw, Buehler Ltd.) through the center of the crown and the long axis o f the vent. Each section surface was photographed and a 3 x S-inch color photograph, with a magnification of 10x, was made.

The surface area of the internal porosities of the sec- tioned castings was measured in an image analyzing com- puter (NIH Image 1.SS, National Institutes of Health, Bethesda, Md.). Twenty-four photographs of the sec- tion surface were measured. The total porosity area val- ues were statistically analyzed with the Student's t test

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Fig. 4. Radiographs of casting. A, Representative radiograph of single sprue group with 90 degree related to vent; B, Rep- resentative radiograph of double sprue group with paralleling to long axis of vent.

between the means and standard deviations for each group of castings with confidence level at 95%.

Statistical analysis of the roughness values was accom- plished by one-way analysis of variance (ANOVA) pro- cedure. A Schcffd F-test was used for all post hoc pairwise comparisons at the 95% confidence level.

R E S U L T S

The mean roughness values of the crown surface and standard deviation for each experimental group are listed in Table II. The roughness values (Ra) obtained range from 2.1 _+ 0.4 ~am to 3.0 + 0.9 ~m. The analysis of variance and Scheffd F-test found no significant differ- ences among the values of surface roughness for the double sprue group and between the middle and cervi- cal thirds of the single sprue group (p > 0.05), but de- termined a significantly rougher occlusal third of the single sprue group (p < 0.05). The internal porosities of the castings as disclosed by the radiographs were mainly at the opposite side to the vent from the radiographs. No significant porosity was revealed at the sprue junc- t ion with the crown por t ion (Figs. 4 and 5). The Student's t test showed that the sectioned surface of the single sprue group contained a significant greater area of porosity (1.5 + 0.7 mm 2) than the double sprue group (0.2 _+ 0.2 mm 2) analyzed (p < 0.01).

D I S C U S S I O N

The disadvantage of casting lower density metal, such as titanium, with centrifugal casting machines can be coun- teracted by higher rotational speed or by using special sprue designsY The results of previous studies influenced the specific spruc design used in this study to resolve the mold filling problem? 5,23-2s Large angulated sprues were used with the open vents in this investigation instead of conventional straight direct sprues. The spruc and vent

Fig. 5. Graphic representation of sectioned surfaces of cast- ings. A, Representative sectioned surface of single sprue group cast with paralleling to vent. B, Sectioned surface of double sprue group cast with 90-degree related to vent.

Table II. Results of surface roughness measured in this study

Roughness (Ra, I~rn) Position Single sprue group Double sprue group

Occlusal 1/3 3.0 (0.9) 2.1 (0.4) Middle 1/3 2.3 (0.6) 2.1 (0.5) Gingival 1/3 2.3 (0.4) 2.1 (0.4)

Mean of 12 measurements with standard deviation in parentheses, Scheff~ interval calculated at the 95% level of comparisons was 0.2 pm.

designs represcnt a compromise bctween rapid solidifica- tion of the molten metal and an inadcquate mold gas escape route during casting. It was reported that vents close to the margin act as an additional reservoir of mol- ten metal during the solidification process and chill set for the rapid elimination of heat from the casting. 26,27 Complete casting without any visible porosity over the side with vent in this study confirmed the positive effect of venting in this sprue design (Fig. 5, A).

Another study that investigated uncast titanium res- torations reported, 28 the surface of the spark erosion produced a mean roughness, Ra of 3.6 e 0.4 pm, which was higher than the roughness values of castings deter- mined in this investigation. The relatively dense invest- ment coupled with the low density of titanium would tend to trap gas resulting in subsurface porosities. 2s In addition, the wax is eliminated and the mold is cooled to room temperature before titanium casting in this cold mold casting technique. An expected loss of fluidity of molten titanium metal as it meets the cold mold may reduce the ability to consistently wet the surface of the cold mold for complete casting. The extreme difference between mold and melting temperatures create rapid cooling of the metal and thereby shortened the time for the gases to escape. 16 The use ofa double-sprue attach- ment to the pattern seems to provide the shortest and fastest possible route, as well as sufficient molten tita-

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THE JOURNAL OF PROSTHETIC DENTISTRY CHAN ET AL

nium metal, to thc mold cavity to eliminate porosity. The porosity distribution observed was localized to the area of the crown portion opposite to the venting (Fig. 5, A). This observation may be the result of trapping air either from the residual air or argon gas carried with the stream of molten titanium during casting. Similar re- suits were observed and reported by Hamanaka et al. 29 The surface roughness was reported to be related to the mold surface and the surface porosities of the casting. A significant decrease in surface roughness was obtained with the double sprue group, confirming that the suffi- cient molten metal was critical for complete casting of titanium especially with the cold mold casting technique. From the microstructure study in the vicinity of the oc- clusal surfaces of castings, Takahashi et al.30 observed many voids and cracks within the reacted layer, which suggest that titanium castings reacted with the phos- phate-bonded type investment. A significant smoother surface resulted from the double sprue group, along with fewer internal macrodefccts in the castings compared with those from the single sprue group (Table II and Fig. 5).

C O N C L U S I O N S

1. There was a significantly lower in roughness values of the casting surface for the double sprue group than the single sprue group (p < 0.05).

2. The double sprue design is more effective than the single sprue design in decreasing the internal porosity in titanium castings.

3. The double sprue design resulted in smoother sur- face in titanium crown casting.

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Repflnt requests to: DR. KWOK-HUNG CHUNG DEPARTMENT OF RESTORATIVE DENTISTRY UTHSCSA - DENTAL SCHOOL 7703 FLOYD CURL DR. SAN ANTONIO, TX 78284-7890

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