A comparison of the casting ability of precious nonprecious alloys for porcelain veneering

and

P. F. Vincent, M.D.Sc., F.R.A.C.D.S.,* 1. Stevens, M.D.Sc.,** and K. E. Basford, B.Sc.(HONSl*** The Dental School, University of Queensland, Brisbane, Australia

D

ental casting alloys should have the capability of being cast in thin sections of appropriate length to obtain the best marginal fit. Due to the different natures of dental casting alloys used for porcelain veneering (ceramic alloys), differences in their abilities to be cast in such sections are to be expected. The new base-metal alloys may have superior mechanical properties when compared with the precious metalceramic alloys. I-4 However, the integrity of these restorations could be compromised because of poor marginal fit due to inaccuracy in casting thin sections. Regarding the casting of nonprecious ceramic alloys, Asgars discussed the need for a meaningful test to measure an absolute value of fluidity or “castability.” He described a test using a spiral-shaped wax pattern. Phillips6 expressed concern about securing well fitting castings from nonprecious alloys. Anderson? warned that these alloys are difficult to cast with accuracy, particularly in thin sections. While other factors must be considered in the selection of an alloy for cast dental restorations, castability is of prime importance. Therefore, this study was designed to compare the property of castability for certain precious and nonprecious ceramic alloys by measuring pattern reproducibility over a selected range of diameters. MATERIALS

AND

METHODS

Five alloys were compared: Thermocraft,t Degudent Universal,* tratek,j( and Wiron S.n The first two contain precious metal. *Temporary

Lecturer

**Lecturer

SDegussa,

Department Garrett

Pty.

Pforzheim,

$Unitek

Corporation,

j/Metals

for Modern

TiBego,

Dentistry,

University

Ul-

of Queensland.

in Restorative Dentistry, University of Queensland.

***Statistician, tMatthey

in Restorative

Victory,5

Bremen,

West

of Agriculture,

Ltd., West

Brisbane,

University Queensland,

of Queensland. Australia.

Germany.

Monrovia, Dentistry,

Calif. Inc.,

Danville,

Calif.

Germany.

527

528

Fig. from

Vincent,

Stevens,

1. The sprued test piece the wax cylinder.

pattern

Fig. 2. The attachment for indicates its correct vertical

holding position.

Fig.

Degudent

3. The

cast test pieces:

.I. Prosthet. Dent. May. 1977

and Basford

and

the nylon

the inlay Universal

ring

fishing

line

to the casting at left

and

Ultratek

with

squared

cradle.

The

c:nds projecting

arrow

on the ring

at right.

The patterns were made using nylon fishing line. The diameter of each line was measured using a dial gauge comparator calibrated to 1 pLm. (Table I). Nylon line was cut square at one end. The other end was positioned so that a length of approximately 5 mm. projected from a cylinder of wax formed in a split mold. This was done for each diameter of line. The lines were arranged in ascending order of diameter. A plastic sprue was fixed to the side of the cylinder of wax, opposite the projecting nylon. The sprue was attached to a sprue former and built up in modeling wax

Volume Number

37 5

Table

Casting

I. Diameters

of nylon

ability

of

precious

line (21 measurements

Nominal

and

lurnl

S.D. (urn/

670 570 450 360 260 I10

670.00 566.57 448.76 362.05 262.00 113.00

5.67 3.65 2.21 1.35 0.94 1.18

Diameters Alloys

)

Thermocraft Degudent Universal Victory Ultratek Wiron S

670

1

570

10 IO 3 3 10

Ill. Completeness

Thermocraft Wiron S Degudent Victory

)

450

IO 8 2 0 9

of casting:

10 2 I 0 6

comparison

Ultralek * *

Victory * *

N.S.t N.S.

N.S.

at the 5 per cent

529

at each diameter)

Table II. Completeness of casting: number of complete diameter (10 castings for each alloy at each diameter)

*Significant

alloys

Mean diameter (urn I

diameter

Table

nonprecious

castings of the alloys at each

(pm/ )

360

1

9 2 I 0 3

260

1

110

9 0 0 0 5

0 0 0 0 0

of alloys over all diameters Degudent N.S. N.S.

Wiron S N.S.

level.

tN.S. = not significant.

(Fig. 1) . The sprue lengths were so arranged that the patterns were 3 to 3.5 mm. from the open end of the inlay rings. Each inlay ring was lined with one layer of asbestos. The patterns were invested under vacuum using Deguvest H. F. G. investment* with undiluted liquid at the recommended powder-liquid ratio. Ten inlay rings were placed in a furnace at 200’ C. for 30 minutes; the furnace was then allowed to heat slowly to the desired temperature (85OO C. for Thermocraft, Degudent Universal, and Ultratek; 788O C. for Victory; and 950’ C. for Wiron S). The molds were allowed to heat-soak for 60 minutes before casting. A special attachment was made to allow each inlay ring to be secured to the casting cradle of a horizontally rotating, induction melting-casting machine. Each inlay ring was marked to’allow alignment of the mold spaces in a vertical plane when seated in the casting cradle (Fig. 2) . The alloys were cast according to the manufac*Degussa,

Pforzheim,

West

Germany.

530

Vincent,

Fig. 4. Square right.

(Original

Stevens,

and

end of the complete magnification x8.)

.J. Ptosthet. Dent. May. 1977

Basford

casting

at left.

Rounded

end

of the incomplete

casting

at

turers’ instructions concerning the appearance of the alloy at casting temperature. The molds were allowed to bench cool for 10 minutes before quenching. The castings were then cleaned by blasting with a fine powder of acrylic resin polymer. The projecting portions of the castings (Fig. 3) were examined and their lengths measured using a traveling microscope calibrated to 0.1 mm. The cast projections were classified as : ( 1) complete, indicated by a square end (Fig. 4) ; (2) incomplete, taken to be a projection of greater than 1.0 mm. with a rounded end (Fig. 4) ; and (3) noncast, projections of less than 1.0 mm. The length of each projection of each casting was recorded to the nearest tenth of a millimeter. Each casting was weighed using an electrical top pan balance calibrated to 0.01 Gm. This procedure was carried out for 10 castings of each of the five alloys.

RESULTS The results of the test regarding completeness of casting are shown in Table II. These results were subjected to Wilcoxon’s test for paired comparisons to determine significant differences between alloys (Table III) .8 There was no significant difference (P < 0.05) between Thermocraft and an “ideal” alloy, that is one that can be cast completely at all diameters. Significant differences (P < 0.05) existed between the “ideal” and the remaining alloys. A Wilcoxon’s test for paired comparisons was calculated from the data (Table II) to determine the significance of the effect of diameter on completeness of casting over all alloys (Table IV). Compared with the best possible result obtainable at each diameter (i.e., 50 complete castings), it was shown that diameters of 670, 570, and

Casting

3

ability

of

4

precious

5

and

nonprecious

alloys

531

6

MASS (6)

Fig. 5. Graph of length against mass for a set of typical diameters

(calculated

from model).

450 ,um were not significantly different (P < 0.05). Significant differences (P < 0.05) existed between each of the other diameters and the best possible result. The means and standard deviations of the measurements of length cast at each diameter and over all diameters for each alloy are given in Table V. For the analysis of length cast at each diameter, an F test was used to compare variances, and this was followed by a t test. If variances were found to be significantIy different, an adjusted t test was used. For each diameter, Thermocraft was considered to be the standard. The other alloys were tested against this (Table VI). With respect to the mean total length of all diameters cast, it was shown that significant differences (P < 0.01) existed between Thermocraft and all other alloys. The mean mass and standard deviation of cast test pieces of each alloy are given in Table VII. Regression models expressing length as a linear function of mass were fitted for each diameter for all alloys and for all diameters for all alloys (Table VIII). Regression analyses were also carried out expressing completeness of casting as a linear function of mass for each diameter for all alloys and for all diameters for all alloys (Table IX). Regression analysis of length on mass for each diameter for all alloys showed a significant relationship (P < 0.01) . As the diameter decreased, an increasing percentage of the variation in length was accounted for by its linear dependence on the variation in mass, For all diameters for all alloys, a significant relationship (P < 0.01)

532

L’incent,

Stevens,

Table

IV. Completeness

.I. P~octhet. Dent. May. 19ii

and Basfwd of casting:

comparison

of diameter

Diameter

Diameter (rmi 670 570 450 360 260 *Significant tN.S.

=

Grn,

110 t

260 *

360 *

N.S.t N.S. N.S. N.S.

N.S. N.S. N.S.

N.S.

at the 5 per cent not

over all alloys

450

5 70

N.S. N.S.

N.S.

level.

significant.

Table V. Length of casting: length in millimeters of castings obtained for each diameter with each alloy (10 measurements for each alloy at each diameter; 60 measurements for each alloy over all diameters) Alloys Diameter

(gm)

670 570 450 360 260 I IO

VI. Length

Viclory

Ultratek

5.47 0.0949 5.27 0.424 3.94 1.534 4.26 1.24 3.01 1.313 0.74 0.324 3.782 1.863

4.27 1.7173 4.16 0.944 3.23 I .62 2.54 I.495 0.91 0.98 0.04 0.07 2.525 2.004

3.93 1.45 3.02 1.47 2.09 I.22 1.73 1.09 0.92 0.81 0.01 0.03 1.95 1.689

5.51 0.0738 5.53 0.0949 5.49 0.110 5.35 0.414 5.40 0.33 2.03 1.11 4.885 1.377

i( S.D. x S.D.

All

Table

Thermocraff

x S.D. ii, S.D. x S.D. zf S.D. STD.

Degudent Universal

of casting : comparison

Thermocraft Universal Thermocraft Thermocraft Thermocraft

670

1

570

1

450

/Mm/ 1

360

1

260

1

110

and Degudent and Victory and Ultratek and Wiron S

Comparison Victory, N.S., *N.S.

compared

S

5.42 0.92 5.02 I .38 3.62 2.38 2.14 2.32 3.22 2.67 0.00 0.00 3.237 2.531

of alloys at each diameter Diamerers

Alloys

Wiron

=

d and not

alloys Ultratek,

N.S.*

N.S.

t

t

$

5 N.S.

: N.S.

: t

: $

: t

at diameter N.S.

significant.

*Significant

at the 5 per

cent

level.

SSignificant

at the

cent

level.

1 per

110

tested

against

zero:

Thermocraft,$

$ i $ DegudentJ

S%Z57 Table

VII,

Casting

Mass of test pieces

ability

A 110.~

VIII.

diameters

and

precious

Mean

Regression coefficient (a,\

670 570 450 360 260

0.35 N.S.* 0.40 N.S. 0.53 N.S. 0.73t 0.74t I IO 0.37 N.S. All diameters 0.52t *N.S. = not significant. tsignificant at the 5 per cent level. SSignificant at the 1 per cent level.

(Grn.)

Regression coeJjjcienr

(rml

(61

670 570 450 360 260

1.35 N.S.* 1.58 N.S.

1.26 N.S.

All diameters

6.45 2.93 3.19

0.45 0.05 0.85 I .06

3.51

0.18

for each diameter

Regression constanf (ad

3.28 2.71 1.19

-0.23 -0.75 -1.16

0.84

I.31 N.S. I .OJ N.S. 1.lON.S.

and all

R’

F

0.34 0.35

24.21$ 25.40$

0.31

2I.60$

0.53 0.43 0.59 0.22

54.38$ 36.364 69.49$ 81.97$

on mass: for each diameter

Regression constant /a,/

0.88 -1.60 -2.12 -3.16

-2.22 - I .37

R=

F

0.52 0.52 0.39 0.58 0.29 0.25

3.27 N.S. 3.25 N.S. I .93 N.S.

4.20N.S. I .22 N.S.

9.41t

*N.S. = not significant. tsignificant at the 1 per cent level.

Table

X.

equation,

533

S.D. (Gm.1

Table IX. Regression analyses of completeness of casting and all diameters (model equation, C = a, t aI W) Each diameler

alloys

7.34

Regression analyses of length on mass: (model equation, L = a, t a, W)

Each diameter f@l

nonprecious

(10 test pieces of each alloy)

Thermocraft Degudent Universal Victory Ultratek Wiron S

Table

of

Analysis of variance (summary) : for testing L = a,, + a, W + a2 D + a3 DW + a,D* W)

DF Source 4 Regression Deviations 295 *Significant at the 1 per cent level.

Mean

regression

square

231.69

1.62

model

(model

F 571.25’

534

Vincent,

Stevens,

and Basford

.I. P,o\thet. Drnt. May, IY;i

was shown. Twenty-two per cent of the total variation in length could be accounted for by the variation in mass. Regression analysis of completeness of casting on mass showed no signiticant linear relationship for any diameter. However, for all diameters, a significant lincnt relationship (P < 0.01) existed, showing that 25 per cent of the total variation ill completeness of casting could be accounted for by the variation in mass. The significant straight-line relationships for length (L) on mass (W) for caclt diameter (D) had different slopes. The arrangement of these values suggested :I curvilinear relationship between length cast (at all diameters) and mass over all alloys, expressed by the following model: L = a,, + a, W + a, D + a:, DW + arDS W, where the ai (i = 0, 1, 2, 3, 4) represents suitable constants. The model was fitted to 300 data points, and the following estimates of the ai resulted, all of which were significant (P < 0.001) : a,, = 2.69, a, = 0.37, a2 = 0.0088, a3 = 0.0015, a, == -0.0000022. An analysis of variance of this regression model (Table X) showed that the variance attributable to the relationship yielded a significant F value (P < 0.0 1) . Sixty-six per cent of the total variation in length cast was accounted for by this relationship of length, mass, and diameter. The proportion of the variation of length accounted for by this model is sufficiently great for use as a predictor. A graph of length against mass for a set of typical diameters was calculated from this model (Fig. 5) . DISCUSSION Three alloys, including the two precious alloys, proved superior under the conditions of the experiment. Wiron S could be categorized with the precious alloys in terms of its ability to be cast completely. Completeness of casting is affected by factors other than the properties of the alloy being cast. Probably the most significant of these is casting force. The effective casting force is a combination of resolved forces due to the geometry of the mold and centrifugal force in the direction of the acceleration. In this experiment, since each mold was placed in the cradle of the casting machine so that the projections of the test piece were orientated in the vertical plane, then the casting force acting on the molten alloy in the projections would be centrifugal and would follow the direction of the acceleration. Thus, the test piece used for this experiment minimizes any effect of resolved casting force and, for this reason, is thought to be an improvement on the test pieces for assessment of castability described by Civjan, Huget. and Marsden and Asgar.” Completeness of casting could be affected by the amount of superheat. Increase in superheat to improve fluidity of molten nonprecious alloys is not indicated, since some of their constituent elements can volatilize and internal porosity can occur; both these effects will cause alterations to the properties of the alloys. The problem of back pressure is minimized by the use of vents, but these can act as “chill sets” rather than regions into which gas can escape from the mold space. Rawson, Gregory, and LundlO concluded that cooling characteristics of the mold were of greater importance than back pressure. Also, venting is considered unneces-

Volume 31 Number 5

Casting

ability

of

precious

and

nonprecious

alloys

535

sary if the object is placed 3 to 3.5 mm. from the open end of the ring.* Even if back pressure were a factor resulting in incomplete casting, it might be overcome using sufficient casting force. The use of reservoirs, among other things, allows the melt to enter the mold space rapidly at the temperature of the melt in the reservoir.11s I2 This reduces the possibility of premature solidification of the melt, and therefore, it has an effect on completeness of casting. The wax cylinder, from which the nylon pieces projected, acted as a reservoir. This reservoir was placed in such a position in the inlay ring that solidification of the projections occurred before that in the reservoir. The reservoir was large in comparison to the objects being cast. This meant that a ready supply of molten alloy was available to fill the mold spaces. The rate of filling of the mold space is related to the fluidity of the melt. The nonprecious alloys do not appear to have the same fluidity as the precious alloys as evidenced by their melting characteristics. The size and shape of the reservoir also may have reduced the amount of turbulent flow produced by molten alloy moving in a confined space. When considering the effect of diameter on completeness of casting, provided the diameter was one of the three largest, any of the five alloys could be expected to produce a complete casting. A consideration of the mean total length of cast projections obtained demonstrated the superiority of Thermocraft over all other alloys. The reason for this is its ability to be cast into long, fine mold spaces. This has potential clinical application. The linear regression analysis of length cast on mass for each diameter showed that as the diameters decreased, an increased percentage of the variation in length could be accounted for by its dependence on the variation in mass. Since all casting was carried out under constant conditions (i.e., constant centrifugal acceleration), it can be interpretated that if finer objects of any length are to be cast, more casting force is required. In general, an increase in casting force results in an increase in length of casting. This can be achieved by increasing the mass or the centrifugal acceleration. Increasing the quantity of alloy is impractical, as nonuniform heating can result. The solution is to increase the centrifugal acceleration. However, since density equals mass per unit volume, under fixed casting conditions, more dense alloys would cast better with respect to length. This was shown to be predictable under the experimental conditions. CONCLUSION The alloys tested varied with respect to their castability. This variability is related to density. Problems caused by low density may be solved by increasing the casting force. For all parameters measured in the laboratory, Thermocraft, a precious alloy, proved to be the most satisfactory. Wiron S, a nonprecious alloy, was comparable to Degudent Universal, a gold alloy. The application of Wiron S as a successful gold substitute should, therefore, be further investigated clinically. Equipment, investment, and casting techniques, all designed for the casting of *Wagner,

E.: Personal communication,

1975.

536

Vincent,

Stevens,

.I. I’roathet. Dmt. May. 1977

and Basford

gold alloys, were also used for the nonprecious readily available in clinical practice. Modifications factors may result in more successful application

alloys. These or technical to the other

are standard improvement nonprecious

facilities to these alloys.

Acknowledgment is made to Matthey Garrett Pty. Ltd., Degussa, Unitek Corporation, Metals for Modern Dentistry, Inc., Beg-o, and their Australian agents for the use of their respective alloys. We also thank Degussa and their i\ustralian agents for the investment.

References 1.

2. 3. 4. 5. 6. 7. 8. 9.

10. 11. 12.

Moffa, J. P., Lugassy, A. A., Guckes, A. D., and Gettleman, L.: An Evaluation of Nonprecious Alloys for Use With Porcelain Veneers. Part I. Physical Properties, J. PROSTHET. DENT. 30: 424-431, 1973. Moffa, J. P., and Jenkins, W. A.: Status Report on Base-Metal Crown and Bridge Alloys, J. Am. Dent. Assoc. 89: 652-655, 1974. Seed, I. R., and McLean, J. W.: The Strength of Metal/Ceramic Bonds With Base Metals Containing Chromium, Br. Dent. J. 132: 232-234, 1972. Kobes, L.: Solution of Prosthetic Problems With the Wiron Metal Fused to Ceramic Method, Quintessence Int. 4: 1-7, 1973. Asgar, K.: Metal Castings in Dentistry, in Wachtel, L. W., editor: Symposium Dental Biomaterials-Research Priorities, 1973, HEW Publication No. (NIH) 74-548, pp. 27-44. Phillips, R. W.: Skinner’s Science of Dental Materials, ed. 7, Philadelphia, 1973, W. B. Saunders Company, p. 549. Anderson, J. N.: Applied Dental Materials, ed. 4, Oxford, 1972, Blackwell Scientific Publications, p. 85. Hodges, J. L., and Lehmann, E. L.: Basic Concepts of Probability and Statistics, ed. 1, San Francisco, 1964, Holden-Day Inc., pp. 318-321. Civjan, S., Huget, E. F., and Marsden, J. E.: Characteristics of Two Gold Alloys Used in Fabrication of Porcelain-Fused-to-Metal Restorations, J. Am. Dent. Assoc. 85: 1309-l 315, 1972. Rawson, R. D., Gregory, G. G., and Lund, M. R.: Photographic Study of Gold Flow, J. Dent. Res. 51: 1331-1337, 1972. Brumfield, R. C.: Dental Gold Structures, Analysis and Practicalities, New York, 1949, J. F. Jelenko & Company, Inc., pp. 92-94. Phillips, R. W.: Skinner’s Science of Dental Materials, ed. 7, Philadelphia, 1973, W. B. Saunders Company, pp. 433-434. DENTAL

SCHOOL

TURBOT BRISBANE,

ST. QUEENSLAND

AUSTRALIA

4000