Fit uf cnwvn wax Chassiel Zeltser,
Israel Lewinstein, Faculty of Dental Medicine,
hrinkage of wax patterns occurs on the die and during rcmova1 of the pattern from the die.’ Patterns removed from the die after initial carving and remodeled at the margin after replacement on the die had a more acceptable fir. This investigation evaluated the fit of patterns removed from the die for investment following pressure application after remodeling.
Stone (Silky Rock with gypsum hardener, Whip-Mix Corp., Louisville, Ky.) working dies were prepared from polyether (Impregum, ESPE, GmbH, Seefeld-Oberbay, West Germany) impressions of a brass master die with beveled shoulder.’ The working dies were covered with a separating agent, preheated to 45” C, and dipped twice in type C inlay wax (Sybron/Kerr, Romulus, Mich.) at Y7;‘5”C:. The thickness of the wax coping was increased with a mixture of the type C wax and casting wax sheets (Sybron/Kerr) in a 3:l ratio and applied with a heated spatula. The pattern was shaped with a carving instrument to form reproducible patterns with 0.56 mm axial walls. The instrumentation with jig consisted of a carving ring with a knife, ground to the outer contour of the desired wax pattern, and a base with the waxed die. The carving ring was clamped to a heavy stand and the base was placed on a laboratory jack (Fig. 1). The pattern was carved by turning the base concentrically with the ring and raising it periodically with the jack. The patterns were removed from the dies with a wax sprue attached to the occlusal surface. They were reseated on the dies and subjected to a force of 50 gm on the sprue for 3 minutes (Fig. 2). After removal of the sprue, the margins were remodeled. A hot spatula with a small quantity of wax was inserted over the shoulder up to the axial die walls, and the margin was recarved with the instrument. A sprue was attached to the patterns, and loads of 250 to 1000 gm were applied to the sprues for 5 minutes. Some patterns were immediately invested in stone to evaluate the final adaptation of the pattern to the die. Other patterns were removed for the second time from the die, replaced on the die after 10 minutes with a ‘Instructor. **lnstructnr, ***Flmtl. 144
I,ahoratory for Dental Materials. I,abtrratorv for Dental Materials.
from the die
and Rafael Grajower,
load of 50 gm, and invested. The latter procedure determined the effect of pattern removal from the die before investment in routine castings. Ordinarily, the pattern is not reseated on the die before investment. Replacement in this study was necessary to compare the fit before and after removal from the die. The relatively small load of 50 gm was sufficient to overcome friction with the die while the pattern was seated but insufficient to cause deformations. The invested patterns were sectioned, polished under running water, and inspected with a measuring microscope (Durimet, Leitz, Wetzlar, West Germany). The elevation of the pattern above the die was measured at the center of the shoulder. The gap width at the center of the bevel represented the marginal adaptation. Additional experiments were performed to evaluate plastic deformations and changes in elastic properties that occurred during the application of loads of wax. For this purpose the movement of the piston with load tray (50 gm) of the jig was recorded with a displacement transducer (24 DCDT 500, Hewlett Packard, Waltham, Mass.) during the application of additional loads of 200 or 950 gm (Fig. 2). Experiments were performed on five cylindric wax samples of both the type C and casting wax, and on three wax patterns, before removal from the dies. Sprues of impression compound (type 2, Sybron/Kerr) were attached to the patterns, because the deformation of this material was negligible under the load cycles. The wax cylinders were 10 mm in diameter and 10 mm high.
RESULTS The experimental results (Fig. 3) are summarized in Table I. The elevation and gap width at the bevel of patterns not removed from the die before investment was immeasurable. The results for the patterns removed from and replaced on the die investment varied with the load applied after remodeling. Significantly greater @ < .Ol, t test) elevations and gap width values at the bevel were obtained for the patterns to which 250 gm was applied than for the patterns that were not loaded after remodeling. However, with a load of 1 kg instead of 250 gm the elevation and marginal gap width decreased (p < .Ol). The differences obtained with no load and 1 kg were not statistically significant. Fig. 4 shows a typical curve for dimensional changes MARCH
Fig. 1. Instrument
for carving wax patterns.
that occur vertically in cylindric type C wax specimens by application of loads and relaxation. Each load of 250 and 1000 gm was applied twice for 5 minutes. Partial dimensional recovery was obtained for 5-minute periods with a load of 50 gm. Each load cycle exhibited the following characteristic parts: AB, rapid compression; BC, slow compression; CD, rapid elastic recovery; and DE, anelastic (slow elastic) recovery. The compression in ABC is due to elastic and anelastic displacement with plastic deformation. The vertical distance between points A and E represents the plastic deformation (PD) that developed during the cycle measured after a relaxation time of 5 minutes. Similar curves were noted for cylinders prepared from the casting wax sheets and for the wax patterns on dies. However, the latter curves displayed greater displacements than cylindric samples. All curves showed that a second load application resulted in less plastic deformation (PD.2; PD4) than the first application of the same load (PD I; PD3).
DISCUSSION The plastic deformation decreased when a load cycle was repeated, which indicated that pressure affects the mechanical properties of wax. Partial realignment of THE
Fig. 2. Jig for application of pressure on samples. dt = Displacement transducer; It = load tray; sp = sprue; wp = wax pattern on die connected to base of carving instrument; W = weights placed on load tray.
large wax molecules that result from pressure also affected the tensile properties of the material. The value of 5 pm was assigned to gaps that were immeasurable. Removal of the pattern from the die then caused dimensional changes in average elevations of 29 to 56 pm depending on the load applied before removal. The variations in wax pattern adaptation to the die after being subjected to increasing loads are attributed to two factors: the increase in adhesion of the wax pattern to the die and changes in the mechanical properties of the wax. Adhesion to the die will increase the force required to remove the pattern from the die. This causes deformation when the pattern is removed from the die and impairs adaptation after replacement. Conversely, pressure induced in the wax may improve the resistance to plastic deformation during removal from the die. The latter phenomenon explains the rationale of hydrolytic pressure or “swagging” wax patterns to reduce the effects of stress relaxation and to improve the accuracy of castings.2*3 A 250 gm load before investment compro345
50 250 50 250 50 1000 50 188) 50 250 50
:: 3. Margin of wax pattern (wp) on die Cd). Load of 1000 gm was applied to pattern. Pattern was removed from die, replaced on die, and invested. Note excellent adaptation at bevel and inferior adaptation at shoulder
Table I. Elevation and marginal adaptation to dies of invested wax patterns subjected to various loads after remodeling -.-.- _--...-.. ._--____ Elevation Gap at Load after Removal bevel (pm) (wn) remodeling before No. of investment samples Ave. SD Ave. SD km) [email protected]
: 7 6
12 17 19
10 16 35 12
-t + Jr
50 250 I 000
.\;c = .\verage; Sl) = standard :r~vesrmem. + = runovrd before
- = not
7 17 7 before
mises the accuracy of the patterns. However, a load of 1 kg may slightly improve the accuracy and can be explained. The plastic deformation in the pattern caused by a load of 250 gm increases the contact surface between pattern and die by forcing the wax into crevices and surface porosities of the stone. Adhesion of the pattern to the die is therefore increased. The mechanical properties of the wax are not improved sufficiently by a 250 gm load to prevent plastic deformation during removal from the die. The increase in contact surface, and hence adhesions, caused by the use of a 1 kg load is probably most crevices are filled already under a small, because load of 250 gm. The larger load may improve resistance to plastic deformation by reducing pattern deformation during removal from the die. The results do not indicate that the application of pressure causes deformation at the bevel (Fig. 3). The gap width at the bevel was smaller than anticipated, despite the elevation and geometric considerations.’
Fig. 4. Typical curves for dimensional changes that occur in wax cylinders and patterns after repetitive loading. PD = Plastic deformations.
SUMMARY The effects of loading the wax pattern before investment was determined. Various loads were placed on the pattern after its margin was remodeled. The patterns were replaced on the die with a load of 50 gm before investment. For loads of 0, 250, and 1000 gm, the average elevation of the pattern on the die was found to be greater by 29, 56, and 19 pm, respectively, than before removal from the die. Repetitive loading showed that the plastic deformation in wax was less in the second than in the first cycle for a specific load. This decrease in plastic deformation may explain the improved adaptation of the pattern after a load of 1000 gm. REFERENCES 1.
Grajowrr, R., and Lewinstein, I.: The effect of manipulative variables on the accuracy of crown wax patterns. J PROSTHEY DENT (In press). Haberman, J. D., Davidson, G. B., and Christensen, G. Y.: A controlled method of swagging wax patterns. Int Assoc Dent Res Abstr No. M43, 1963, p 144. Christensen, G. Y.: The effect of water swagging on stress and strain in dental wax patterns. Int Assoc Dent Res No. M44, 1963, p 144.
Xeprinl reyueiit to: DR. RAFAEI. GRA,JOWEX HEBREW
1172 9 1010