A castability study of nonprecious ceramometal alloys

A castability alloys R. H. Jarvis,

D.D.S.,

study of nonprecious M.S.D.,*

T. J. Jenkins,

D.D.S.,**

Eastman Dental Center, Rochester, N.Y., and State University Buffalo, N.Y.

W

ith the increased use of nonprecious metals for ceramometal restorations, many new alloys have been developed. Manufacturers’ instructions for casting the alloys can be confusing, particularly where burnout temperature is concerned.Burnout temperaturesin the range of 1,500” to 1,800” F have been recommended for investments used with nickel-base alloys. This study investigated the effect of burnout temperature on the castability of somenickel-basealloy systemsand the effect of burnout temperature on the roughness of castings. Both beryllium and nonberyllium alloy systems were analyzed. The problems associatedwith casting base metal as an alternative to noble metal in ceramometal alloys have been considered by several authors.‘-9 Early studiescomparedthe newer base metal alloys and their handling properties with those of noble metals by the use of techniques developed for noble alloys. Often the results of the studieswere disappointing; they suggestedthat the new alloys were not acceptable for clinical use. Complete castingscould not be made, and many did not fit.’ Some reports suggestthat the alloys available are acceptable, and that newer techniques, materials, and equipment matched with the alloys could show results that equal or surpassthose obtainable with noble alloys.*.3 Recent studies varied mold designs while others varied combinations of investments and casting conditions to determine optimal conditions for the useof base metal alloys5-’ The term “metal mold equilibrium,” as defined by Preston and Berger: signifiesthe goal of the latter group of investigations. They attempted to find

Presented at the ,\cademy of Denture Prosthetics, Boston, Mass. l C:l~nical Associate. Department of Prosthodontics, Eastman Dental Crntcr. *‘CXnic~al Assistant Professor, Department of Fixed Prosthcdontics, State University of New York at Buffalo, School of Dentistry. l **:Zssistant Professor, Departments of Fixed Prosthodontics and Behavioral Sciences. State Cniversity of New York at BufTalo, Srhwl of Dentistry.

490

ceramometal

and L. A. Tedesco,

Ph.D.+**

of New York at Buffalo, School of Dentistry,

the optimum mold burnout and metal melting temperatures to allow the most fluid metal flow into the mold for the most complete filling and the smoothestcasting with the best physical properties. Whitlock et a1.9proposedthe useof a square pieceof polyester sieve cloth of a specified dimension with runner bars along two adjacent edges and a sprue attached at the junction. Becausethe polyester sieve cloth complies with dimensional requirements of the American Society for Testing and Materials No. E-11-70, uniform filament size and spacing are assured.The usual technical proceduresused for wax patterns are applied to the sievecloth specimens.Since the sievecloth is available in many sizes,test specimens can be made from a size specific for any alloy system. With this procedure, Whitlock et a1.9evaluated 14 alloy systems of both noble and nonnoble metals. Castability ranged from 50% to 92%. They concluded that their method provided an objective evaluation for the castability of metals. Testing for castability can be affected by minute differences in materials, equipment, and procedures.In addition, the types of instruments used and the time schedules for their use can significantly affect the results of such castability evaluations. The purpose of this study was to evaluate the proceduresspecified by Whitlock et al.,9 asan absolutemeasurementtechnique for castability, modified for greater laboratory constraints on material, equipment, and procedures. MATERIAL

AND METHODS

Four alloys were evaluated; two contained beryllium: Lite Cast B (LCB; Williams Gold Refining Co., Inc., Buffalo, N.Y.) and Rexillium III (RIII; jeneric Gold Co., Wallingford, Conn.), and two nonberyllium: Unibond (Unitek Corp., Monrovia, Calif.) and N/P’ (Howmedica, Inc., Chicago, Ill.). Six burnout temperatures, ranging from 800” to 1,800” F, were set at 200” F intervals (SOO’, l,OOO”, 1,200”, 1,400”, 1,600”) and 1,800” F).

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CASTABILITY

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Fig. 2. Cast specimen superimposed (right).

Fig. 1. Pattern former.

assembly

with

sprue

and crucible

Each specimen was made from sieve cloth (Nalge Co., Sybron Corp., Rochester, N.Y.), size No. 20, 24 filaments/in with a filament diameter of 0.30 mm. The screen size used for each specimen was 18 X 18 filaments. A V-shape runner bar was designed that measured 20 mm for each leg and used a square IO-gauge wax bar with a groove 0.5 mm wide by 1 mm deep. The procedures were designed to minimize the influence of size and type of runner bar on castability. The mesh squares were placed into the groove on the wax pattern runner bar and luted into position with cyanoacrylate cement (Ross Chemical Company, Detroit, Mich.). The cemented patterns were allowed to set for 1 hour before placement on the sprue. Twenty-four castings were made with seven specimens fabricated on each casting to provide 168 specimens for castability evaluation. Specimens that tested roughness were cut from a piece of 0.06 clear surgical splint vacuform material. The specimens were approximately ti inch wide by % inch long. The */i-inch side of the pattern was attached to the sprue with utility wax. Twenty-four roughness specimens were prepared, one for each casting. The sprue method used was the Williams MultiSpoke Sprue system (Williams Gold Refining Company, Inc.). The system makes use of a central casting rod with eight projections or spokes radiating out from the center. Each ring was aligned in the casting machine in the same relationship. Six specimen assemblies were made for each metal to be evaluated for a total of 24 sprue assemblies (Fig. 1). Each completed pattern was sprayed with Ney Wax-Wet (J. M. Ney Co., Bloomfield, Conn.) to reduce the surface tension and then air-dried before investment.

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(left) with

counting

frame

Whip-Mix Hi-Temp (Whip-Mix Corp., Louisville, Ky.) investment was used because of its reported compatibility with the metals investigated.’ The liquid:powder ratio for investing was 80% special liquid and 20% distilled water. All the investment was from the same batch. The liquid and powder were mixed for 60 seconds under vacuum and vibrated under vacuum for 10 seconds, after which the vacuum was released and the contents were poured slowly into the Styrofoam investing ring. The investment was allowed to set overnight and was soaked for 15 minutes in water prior to burnout. A two-stage burnout procedure was used for all rings. First, they were heated at 500” F and heatsoaked for 30 minutes; and second, they were heated to the experimental temperature and held for 1 hour at 800”, l,OOO”, 1,200”, 1,400”, 1,60O”,or 1,800“ F. The 800” F burnout specimens were heated at 1,200”, held for Yr hour, brought down to 800”) and held for 1 hour. This was done to ensure complete burnout. The casting machine used was a Williams Inductocast (Williams Gold Refining Co., Inc.) without an electric eye. An optical pyrometer was used to check the melts of each casting. Manufacturer’s instructions in regard to melting or casting temperatures and hold times were followed. Approximately Iti ounce of new metal was used for each casting. The castability specimens were cleaned with 50 pm glass beads in an air abrasive unit. Photographs were made of each castability specimen after it was cleaned in the ultrasonic cleaner (Fig. 2). The photographs were used for counting the number of completed segments in each casting. A frame, divided into a 16 X 16 square grid, was constructed to facilitate the counting procedure. The total number of segments cast was divided by the total segments possible (256) and multiplied by 100, with castability expressed as a percent. Analysis of variance techniques were used to assess

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JARVIS,

Fig. 3. Readout of Cleavite surface analyzer for Rexillium tures. castability for the six burnout temperatures on the four metals. In addition, an initial inspection of the data was made for sprue position effect on castability. The roughness samples were tested on a Cleavite surface analyzer (Fig. 3).

RESULTS Percent castability values for each position on the sprue, as well as the mean percent and standard deviation for each burnout temperature, were determined. A one-way analysis of variance for sprue specimen position did not find significant mean differences for percent castability (F= .1442; df= 6,161; p = .99). Analysis of variance results for overall differences among metals for mean percent castability were significant (F = 507.59; df - 3,144; p = .OOOl). When compared with Unibond and N/P*, LCB and RI11 had significantly higher castability means (p = .OOOOOl). However, mean castability was not significantly different when LCB was compared with RI11 &I = .03) or when Unibond was compared with N/P2 (p = .88). Overall castability mean differences temperature were statistically different for (F = 190.66, df= 5,144; p = .OOOl); mean castability increased in a linear fashion as temperature increased. The composite graph (Fig. 4) shows the similar casting characteristics for the two beryllium alloys, A (LCB), and B (RIII). At 1,400” F burnout there was roughly 98% to 99% castability for the two beryllium

492

III at all burnout

JENKINS,

AND

TEDESCO

tempera-

alloys. At 1,600” and 1,800” F burnout, 100% complete castings were made. Even at 800” F burnout there was between 60% and 66% castability for each alloy. For the nonberyllium alloys the highest mean castability achieved was 76.5% for Unibond and 89.1% for N/P* at 1,800” F burnout. As the burnout temperature was reduced, the percent castability was reduced also, more steeply for N/P’ than for Unibond. However, both showed the same pattern of castability. At 800” F burnout mean castability for Unibond was 25.1%, and for the N/P* mean castability was 28%. Roughness results, except for Unibond, indicate that the lower the burnout temperature the smoother the casting (Fig. 5). Unibond displayed the same uniform roughness whether it was burned out at 800” or at 1,800” F. There seemed to be no difference in the roughness pattern. This alloy showed excessive oxidation on casting. Generally speaking, LCB, RIII, and N/P* had smooth surfaces in the 1,ooO” F burnout specimens. The decrease in roughness at 1,000” F burnout for LCB and RI11 can be attributed to the fact that the 800” F specimens were first brought up to 1,200” F for 30 minutes and then dropped to 800” F. The roughness of specimens cast at 1,000” through 1,600“ F for LCB, RIII, and N/p seemed to be fairly evenly distributed, with a slight increase in roughness as burnout temperature increased. At 1,800” F bumout there was a dramatic rise in the roughness pattern for LCB, RIII, and N/P2 (Fig. 5).

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Composite

1000

1200

WRNOUl

Fig. 4. Composite percent castability vs. burnout temperature.

DISCUSSION By modifying the technology developed by Whitlock et al.,9 this study compared nickel-base ceramometal systems for castability. A significant castability difference was found between beryllium-containing alloys and nonberyllium alloys. Nonberyllium alloys appear to have less castability than beryllium-containing alloys. In addition, the data suggest an optimal burnout temperature of approximately 1,500“ F for berylliumcontaining alloys. An optimal burnout temperature for the nonberyllium alloys was not established. A substantial increase in specimen roughness occurred as burnout temperature increased from 1,600” to 1,800” F. The uniform roughness for the Unibond specimens may be due to the heavy oxide layer present on all roughness specimens of this alloy. By means of the method described to compare the castability of the four metals, beryllium appears to be an essential element in the castability process for nonprecious metals. This study demonstrates clearly the important relationship between burnout temperature and casting temperature to achieve the optimal combination from the standpoint of castability. In addition, preliminary evidence suggests that there is no need for burnout of a casting ring at temperatures beyond those required to give the proper amount of pattern expansion. Manufacturers’ recommended burnout temperatures should take into consideration the metal-mold equilibrium point. This would provide

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Fig. 5. Composite ture.

roughness

la0

TEMFERATURE

lW0 lF

vs. burnout

moo

tempera-

the smoothest casting from minimum temperature burnout with the best fit and proper expansion. Casting techniques that recommend an 1,800” F burnout temperature should be used with caution, because a sharp increase in roughness is associated with this burnout temperature.

CONCLUSIONS In summary, the following conclusions can be made. 1. Beryllium-containing alloys were more castable than nonberyllium-containing alloys. 2. Roughness of castings increased as burnout temperatures increased. 3. There was an optimal burnout temperature of 1,500” F for the beryllium alloys studied. REFERENCES 1.

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TO APPEAR IN FUTURE for maxillary

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AND

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Jack W. Martin,

ceramo-metal

JENKINS.

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Ph.D., M. L. Shakun, D.D.S., M.S.,

relevant to treating the fractured tooth

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Intrinsic color of isophorone Part II: Color stability G. E. Turner, D.M.D., J. E. Lemons, Ph.D.

for maxillofacial

M.S.D., T. E. Fischer, D.D.S., D. J. Castleberry,

Soldering index for minimal C. A. Ullo, D.M.D.,

polyurethane

S. Lyman, D.M.D.,

Major maxillomandibular pain-dysfunction

prosthetics.

D.M.D.,

M.S.D.,

and temporomandibular

joint

coverage retainers and A. Shiu

malrelations

L. George Upton, D.D.S., M.S., Richard F. Scott, D.D.S., M.S., and James R. Hayward, M.S.

494

and

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