Evaluation of finishing and polishing techniques on surface r o u g h n e s s of chromium-cobalt c a s t i n g s A. K e v s e r
Aydin, D.D.S., Ph.D.*
University of Ankara, Faculty of Dentistry, Ankara, Turkey The effect of finishing and polishing t e c h n i q u e s on surface r o u g h n e s s of a chromiu m - c o b a l t alloy w a s e v a l u a t e d by m e a n s of a s t y l u s profile i n s t r u m e n t a t i o n and s c a n n i n g electron microscopy. S c a n n i n g electron micrographs, surface profile tracings, surface r o u g h n e s s recordings, and s t a t i s t i c a l a n a l y s i s o f data support the finding that the best surface finish is o b t a i n e d w h e n s a n d b l a s t i n g , hard stone, m e d i u m a b r a s i v e disk, second s a n d b l a s t i n g , electropolishing, hard rubber point, hard felt d i s k w i t h p u m i c e slurry, and felt d i s k and soft brush w i t h p o l i s h i n g paste are u s e d p r o g r e s s i v e l y . The results o f this study indicate that the finishing procedure should be carried out in a logical, s y s t e m a t i c s e q u e n c e of steps. (J PROSTHET DENT 1991;65:763-.7.)
C h r o m i u m and cobalt (Cr-Co) casting alloy is widely used for making removable partial denture frameworksJ 3 These alloys were introduced for dental use in 1929 and since that time have gradually gained widespread acceptance. 1,4, 5 The Cr-Co alloys have replaced gold-base alloys mainly on the basis of their lower cost and adequate mechanical properties. 6,7 These alloys are harder than most gold alloys. Their increased hardness necessitates the use of special finishing tools to cut and smooth the prostheses, s12 Polishing is essential for the completion of removable partial denture frameworks. Dental plaque accumulates readily on unpolished surfaces and produces conditions suitable for corrosion of metals present and for tooth decay. For optimum comfort, oral hygiene, low plaque retention, and resistance to corrosion, the surface of removable partial denture frameworks should be as smooth as possibleJ 3 Studies of the surface roughness of castings have drawn the attention of investigators for many years, and most were concerned with variables other than the finishing and polishing techniques. This investigation was done to select the polishing method that provides the smoothest surface on Cr-Co castings and to compare the effectiveness of different polishing techniques. MATERIAL
AND
METHODS
The test specimens were prepared in tapered form, 0.8 cm in diameter at the bottom, 0.8 cm high, and 0.6 cm in diameter at the top. A wax sprue was attached at the bottom of the lower surface of the pattern. Ten patterns were mounted on a crucible former. The samples were produced with a Cr-Co removable partial denture casting alloy (Kentzalloy, Top Dent E Kentzler, Ellwangen/J, Mtihlgr-
*Associate Professor, Department of Prosthodontics. 10/1/27547 THE JOURNAL OF PROSTHETIC DENTISTRY
Table I. Finishing and polishing procedures tested Group
1 2 3 4 5 6 7 8 9 l0 11
Technique
Untreated control A A+B A+B+F A+B+C A+B+C+D A+B+C+D+E A+B+C+D+E+F A+B+C+D+E+F+G A+B+C+D+E+F+G+H A+B+C+D+E+F+G+I
A, First sandblasting; B, hard stone; C, medium abrasive disk; D, second sandblasting; E, electropolishing; F, hard rubber point; G, hard felt disk with pumice slurry; H, hard bristle brush with aluminium oxide; I, felt disk and soft brush with polishing paste.
aben, Germany) and an oxyphosphate investment (Dentex Aquamex, Research Development Inc., Long Island City, N.Y.) according to the instructions of the manufacturers. The casting surface for the wax pattern that had been in contact with the glass surface was used as the test surface. An induction crucible was used for the fusion of alloy, and the castings were allowed to bench cool before divesting. Following recovery, the castings were sandblasted, and 10 test specimens were prepared for each group. Different finishing and polishing techniques were applied to the test specimens (Table I). Each grinding and polishing technique was applied by the same dentist with a high-speed polishing motor (Krupp Dental, Essen, Germany) and light pressure under standard conditions. The sprues were cut off with a separating disk. Techniques for finishing included the use of special stones (Top Dent E Kentzler), medium abrasive disks (waterproof, silicon carbide paper No. 166 P600A), sandblaster equipment (Blastomatic, Schiitz-Dental GmbH, Rosbach, Germany), electrolytical polishing apparatus with 763
AYDIN
D
:!
: ! :i!i!i:!~i~=:~iil i
i
:il ili~ ~i!iiili¸ ¸¸ ~¸~¸
F i g . 1. Typical surface profile tracings of groups 1 through 4 specimens. A, U n t r e a t e d control group; B, group 2; C, group 3; D, group 4.
T a b l e II. Average surface roughness (in microns)* R~
Rz
Rmax
Groups
X
SD
X
SD
X
SD
1 2 3 4 5 6 7 8 9 10 11
5.7700 3.5000 0.9220 0.1990 0.1850 1.2430 1.8840 0.2270 0.2360 0.1470 0.1410
0.4761 0.0650 0.0408 0.0143 0.0138 0.0411 0.2602 0.0087 0.0090 0.0065 0.0092
23.9900 17.0000 5.1800 1.2840 1.1250 7.6280 7.3760 1.2970 1.2740 0.7630 0.6490
1.7691 0.3004 0.2702 0.0648 0.1142 0.4022 0.8544 0.0576 0.0581 0.0423 0.0255
36.8000 22.2300 7.8440 1.6700 1.9220 9.2250 12.4030 1.5980 1.5980 1.0390 0.7230
3.5378 0.8831 0.9175 0.0810 0.3173 0.4435 1.5684 0.0922 0.1156 0.0986 0.0864
* M e a n s of 10 specimens.
automatic control (Aurodent, K r u p p Dental), rubber points (Top Dent E Kentzler), felt disk with pumice slurry, brush wheel with aluminium oxide, and felt mops and soft brush wheels in conjunction with polishing paste (Ivoclar Universal Polier paste, Ivoclar AG Schaan, Liechtenstein, Germany). During abrasion, the direction of movement of the abrasive particles over the surface was changed constantly. When a finer abrasive was used after a coarse one, the specimen was positioned to a 90-degree angle when compared with its first location. A systematic method was followed in which each instrument or abrasive was used until its work had been completed before the next step was begun. T h e abrasive stone was applied for 90 seconds. The medium abrasive disk was applied for 90 seconds, 45 seconds for each direction. Time duration given for the sandblaster equipment was determined to be 45 seconds. The switch of the electrolytic polishing a p p a r a t u s was turned on so t h a t a reading of 2 to 3 amperes was produced. T h e specimen was k e p t in the acid solution for 5 minutes. Rubber point, felt disk, felt mop, and brush wheel were applied for 90 seconds each. Residual polishing paste was removed in an ultrasonic bath.
764
Surface roughness of the test specimens was measured in microns with a P e r t h o m e t e r {model S5P, M a h r - P e r t e n GmbH., Hannover, Germany) instrument. Arithmetic average roughness (Ra), average peak-to-valley height (Ra), and m a x i m u m peak-to-valley height (Rmax) values 13 were recorded by the P e r t h o m e t e r device from one surface of each specimen in every test group. The d a t a were statistically analyzed for differences among the 11 finishing and polishing procedures. Means and s t a n d a r d deviations were calculated for each grinding and polishing technique, and the d a t a were compared by Kruskal-Wallis one-way analysis of variance test and multiple comparisons test (Wilcoxon-Wilcox test).14 T e s t specimens were also examined by a scanning electron microscope (Cambridge stereoscan $4-10, Cambridge, England) and photographs were made at ×950 magnification.
RESULTS T h e average surface roughness values and their s t a n d a r d deviations obtained after application of the various finishing procedures to the Cr-Co casting alloy are shown in Table II. According to Rz values, finishing and polishing
J U N E 1991
V O L U M E 65
NUMBER 6
F I N I S H I N G AND P O L I S H I N G TECHNIQUES
~B
A
llm
't
C
~gm
[Ioi 5oo Fig. 2. Typical surface profile tracings of groups 5 through 8 specimens. A, Group 5; B, group 6; C, group 7; D, group 8.
i
! i¸¸~
i
ii
~i
i 50o [5 ~, : I000 :T~ i 250: T :pm ;lum: : : j,m::: l m::
Fig. 3.
Typicalsurfaceprofiletracingsofgroups9through 11 specimens. A, Group 9; B, group 10; C, group 11.
methods were ranked in the order of decreasing smoothness as follows: 1, group 11; 2, group 10; 3, group 5; 4, group 9; 5, group 4; 6, group 8; 7, group 3; 8, group 7; 9, group 6; 10, group 2; and 11, group 1 (Table II). The results of the Kruskal-Wallis test and multiple comparisons test for Ra, Rz, and Rmax values showed significant differences at the p < 0.05 level with Rz (average peak-to-valley height). These significant differences were found between group 11 and groups 7, 6, 3, 2, and 1. Selected profile tracings that are representative of the surfaces developed by the procedures tested are shown in Figs. 1 through 3. Fig. 1, A shows the highly irregular and rough surfaces of untreated samples. Surfaces of samples treated by the second method were rough and uneven (Fig. 1, B). The sixth group samples, which were treated first with sandblasting, then stone, abrasive disk, and a second sandblasting exhibited gross surface irregularities (Fig. 2, B). Samples treated by the seventh (Fig. 2, C) and third (Fig. 1, C) polishing methods showed comparatively smooth surfaces. Surfaces of samples in the eighth (Fig. 2, D), fourth (Fig. 1, D), and ninth (Fig. 3, A) groups were quite smooth with minimal irregularities. The tenth (Fig. 3, B) and eleventh (Fig. 3, C) polishing methods produced the
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Fig. 4. Scanning electron photomicrographs of groups 1 through 4 specimens (original magnification x950). A, Surface of untreated control group. B, Surface after finishing with second method. C, Surface after finishing with third method. D, Surface after finishing with fourth method.
smoothest surfaces. The surface of samples in the eleventh group was obviously the smoothest (Fig. 3, C). Selected scanning electron photomicrographs of representative surfaces are shown in Figs. 4 through 6 to further substantiate the results obtained from statistical analysis of the data. Fig. 4, A shows the highly irregular and rough surfaces of untreated samples. Surfaces of samples treated by the second (Fig. 4, B) and sixth (Fig. 5, B) methods were rough and uneven. Specimens in group No. 7 exhibited some surface irregularities (Fig. 5, C). The samples in the third group, which were treated first with sandblasting and then stone, exhibited a deeply grooved surface (Fig. 4, C).
765
AYDIN
Fig. 5. Scanning electron photomicrographs of groups 5 through 8 specimens (original magnification x950). A, Surface after finishing with fifth method. B, Surface after finishing with sixth method. C, Surface after finishing with seventh method. D, Surface after finishing with eighth method.
The specimens of groups 8, 4, and 5 resulted in smooth surfaces (Figs. 4, D and 5, A and D). Surfaces of the samples in group 10 exhibited smoother surfaces with only minimal irregularities (Fig. 6, B). The method for group 11 produced the smoothest surface (Fig. 6, C).
DISCUSSION The Cr-Co alloys have a high resistance to abrasion; thus they are more difficult to grind than the gold-base alloys. However, they retain their luster for a longer period in service10,15 and are less liable to wear when exposed to occlusal contactJ 5 The high hardness of these alloys presents a real problem at the finishing stage. 4, 6,10,16-1s The importance of the finishing and polishing technique for removable partial denture frameworks cannot be underestimatedJ 9 After casting, it is usual to sandblast the metal to remove surface roughness or adherent refractory material and to clean away the oxide film. 2, s. 1~. 16,20-22 Sandblasting removed the adherent film of investment particles and oxides, which are tenacious, but did not improve the surface. The highly irregular and rough surfaces were apparent (Figs. 1, B and 4, B). The third group to which sandblasting and the abrasive stone were applied exhibited deeply grooved surfaces (Figs. 1, C and 4, C) that had to be removed by progressively finer abrasives. Medium abrasive disks applied after a hard stone in the fifth samples group introduced only fine
766
Fig. 6. Scanning electron photomicrographs of groups 9 through 11 specimens (original magnification ×950). A, Surface after finishing with ninth method. B, Surface after finishing with tenth method. C, Surface after finishing with eleventh method.
scratches (Figs. 2, A and 5, A). After a second sandblasting, the sixth group samples exhibited irregular and rough surfaces (Figs. 2, B and 5, B). The electrolytic polishing process is invaluable for producing a bright surface on the fitting surface of Cr-Co frameworks. 2 Electrolytic polishing eliminates or reduces additional polishing on the tissue side of the c a s t . 2, 8 Material can be removed from the casting by the process, leaving a surface that is smooth and bright without damaging the detail and accuracy of the casting. 16,23 Because rough and uneven surfaces can never be made smooth, the nature of the surface after this polishing process depends on the condition of the surface before polishing commenced. 16 Surfaces that are rough must therefore be ground and smoothed with stones before they are placed in the polishing bath. 16 The electrolytic polishing procedure exhibited a significantly smoother surface texture than was found after a second sandblasting (Figs. 2, C and 5, C). Polishing with rubber wheels is the first step after the finishing process. 2' 24 Unlike an abrasive, a polishing material does not cut a groove but causes the fine scratches to be filled to produce a smooth surface. The application of rubber points applied after hard stone or electropolishing in groups 4 and 8 removed all scratches (Figs. 1, D; 2, D; 4, D; and 5, D). A felt disk with pumice removed all traces of rubber point marks (Figs. 3, A and 6, A). Polishing samples with 15 #m aluminium oxide slurry at this time reduced the grinding marks to a fine texture and gave a smooth surface with minimal irregularities (Figs. 3, B and 6, B).
J U N E 1991
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FINISHING AND POLISHING TECHNIQUES
Although groups 10 and 11 resulted in a polished surface (Figs. 3, B and C and 6, B and C), the best surface finish was produced in the group 11 samples, which were treated with a polishing paste after felt disk-pumice slurry. Polishing samples with polishing paste at this time gave a smooth and lustrous surface. The best surface finish was produced in group 11, to which all steps had been applied (Figs. 3, C and 6, C). Scanning electron micrographs and surface profile tracings in this study closely supported the results of the statistical analysis. An evaluation of the results indicates that a positive plan should be applied in the finishing operation with coarser stones and disks used at first and then a finer grade of abrasive. The finishing operation should be carried out in a logical, systematic sequence of steps. CONCLUSIONS The effect of grinding and polishing techniques on surface roughness of a Cr-Co casting alloy was evaluated by use of a surface analyzer and by observation of the casting surfaces. The best surface finish was obtained when sandblasting, hard stone, medium abrasive disk, second sandblasting, electropolishing, hard rubber point, hard felt disk with pumice slurry, and felt disk and soft brush with polishing paste were used progressively. The finishing procedure should consist of smoothing the metal with progressively finer abrasive agents until all scratches are removed, after which a polishing material is applied to give a high luster to the finished work. Results from the evaluations made in this study indicate that a definite sequence for finishing and polishing should be adopted and followed in every prosthesis.
3. Taylor DF, Leibfritz WA, Adler AG. Physical properties of chromiumcobalt dental alloys. J Am Dent Assoc 1958;56:343-51. 4. Lane JR. A survey of dental alloys. J Am Dent Assoc 1949;39:429-30. 5. Taylor DF, Sweeney WT. A proposed specification for dental chromium-cobalt casting alloys. J Am Dent Assoc 1957;54:44-8. 6. Craig RG, O'Brien WJ, Powers JM. Dental materials {properties and manipulation). St Louis: CV Mosby Co, 1975;192-5. 7. Cunningham DM. Comparison of base metal alloys and type IV gold alloys for removable partial denture frameworks. Dent Clin North Am 1973;17:719-22. 8. Collett HA. Casting chrome-cobalt alloys in small laboratories. J PROSTHET DENT 1969;21:216-26. 9. Craig RG, Peyton FA. Restorative dental materials. 5th ed. St Louis: CV Mosby Co, 1975;361-5. 10. Peyton FA. Cast chromium-cobalt alloys. Dent Clin North Am 1958;Nov:759-71. 11. Skinner EW, Phillips RW. The science of dental materials. 6th ed. Philadelphia: WB Saunders Co, 1967;581-7. 12. Smith EA. Vitallium as a substitute for dental gold casting alloys. Br Dent J 1948;85:180-1. 13. DIN Standard 4768. Ermittlung der RauheitsmessgrSssen Ra, R2, Rm~ mit elektrischen Tastschnittger~iten. Alleinverkauf der Normen dutch Beuth Verlag GmbH, Berlin und KSln 1, part 1, Aug 1974;1-4. 14. Gibbons JD. Nonparametric methods for quantitative analysis. New York: Holt, Rinehart, Winston Co, 1976;181. 15. Earnshaw R. Cobalt-chromium alloys in dentistry. Br Dent J 1956; 101:67-75. 16. Harcourt HJ. Chrome cobalt castings, Part 3. Dent Techn 1956;9:99104. 17. Osborne J, Lammie GA. Some observations concerning chrome-cobalt denture bases. Br Dent J 1953;94:55-67. 18. Phillips RW, Swartz ML, Norman RD. Materials for the practicing dentist. St Louis: CV Mosby Co, 1969;201. 19. White TD, Tarr JL. Dental laboratory technicians' manual. Washington: Air Force Manual Department of the Air Force, 1959;46-315. 20. Marxkors R. The partial denture with metal framework. Bremen: BEGO Bremer Goldschliigerei Wilh.Herbst GmbH & Co, 1984;100-4. 21. Neill DJ, Walter JD. Partial denture prosthetics. Oxford: Blackwell Scientific Publications, 1977;64-7. 22. Stewart KL, Rudd KD, Kuebker WA. Clinical removable partial prosthodontics. St Louis: CV Mosby Co, 1983;360-4. 23. (~alikkocao~lu S. Di~hekimli~indecila. I 0 Di~ Hek F Der 1973;7:230-43. 24. Henderson D, Steffel VL. McCracken's removable partial prosthodontics, 5th ed. St Louis: CV Mosby Co, 1977;349-51.
REFERENCES 1. Lanier BR, Rudd KD, Strunk RR. Making chromium-cobalt removable partial dentures: a modified r.echnique. J PROSTHETDENT 1971;25:197205. 2. Martinelli N. Dental laboratory technology. 2nd ed. St Louis: CV Mosby Co, 1975;31-358.
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DR. A. KEVSERAYDIN GENqLIK CAD KmASI SOKAKNO. 13/15 06570 MALTEPE ANKARA TURKEY
Guides for authors available The Guide to Preparing Articles for THE JOURNAL OFPROSTHETICDENTISTRY,revised by Professor Paul Barton, Editorial Consultant to the JOURNAL,and the editors, is available to prospective authors. The guide provides the format for developing different types of scientific manuscripts, a checklist for effective writing, and detailed instructions for preparing manuscripts in the style acceptable by the JOURNAL. Also available are the Guidelines for Reporting Statistical Results and an Author's Guide to Controlling the Photograph. Guides can be obtained from the office of the Editor (Dr. Judson C. Hickey, The Journal of Prosthetic Dentistry, Medical College of Georgia, School of Dentistry, Augusta, GA 30912).
THE JOURNAL OF PROSTHETIC DENTISTRY
767
Aydin, D.D.S., Ph.D.*
University of Ankara, Faculty of Dentistry, Ankara, Turkey The effect of finishing and polishing t e c h n i q u e s on surface r o u g h n e s s of a chromiu m - c o b a l t alloy w a s e v a l u a t e d by m e a n s of a s t y l u s profile i n s t r u m e n t a t i o n and s c a n n i n g electron microscopy. S c a n n i n g electron micrographs, surface profile tracings, surface r o u g h n e s s recordings, and s t a t i s t i c a l a n a l y s i s o f data support the finding that the best surface finish is o b t a i n e d w h e n s a n d b l a s t i n g , hard stone, m e d i u m a b r a s i v e disk, second s a n d b l a s t i n g , electropolishing, hard rubber point, hard felt d i s k w i t h p u m i c e slurry, and felt d i s k and soft brush w i t h p o l i s h i n g paste are u s e d p r o g r e s s i v e l y . The results o f this study indicate that the finishing procedure should be carried out in a logical, s y s t e m a t i c s e q u e n c e of steps. (J PROSTHET DENT 1991;65:763-.7.)
C h r o m i u m and cobalt (Cr-Co) casting alloy is widely used for making removable partial denture frameworksJ 3 These alloys were introduced for dental use in 1929 and since that time have gradually gained widespread acceptance. 1,4, 5 The Cr-Co alloys have replaced gold-base alloys mainly on the basis of their lower cost and adequate mechanical properties. 6,7 These alloys are harder than most gold alloys. Their increased hardness necessitates the use of special finishing tools to cut and smooth the prostheses, s12 Polishing is essential for the completion of removable partial denture frameworks. Dental plaque accumulates readily on unpolished surfaces and produces conditions suitable for corrosion of metals present and for tooth decay. For optimum comfort, oral hygiene, low plaque retention, and resistance to corrosion, the surface of removable partial denture frameworks should be as smooth as possibleJ 3 Studies of the surface roughness of castings have drawn the attention of investigators for many years, and most were concerned with variables other than the finishing and polishing techniques. This investigation was done to select the polishing method that provides the smoothest surface on Cr-Co castings and to compare the effectiveness of different polishing techniques. MATERIAL
AND
METHODS
The test specimens were prepared in tapered form, 0.8 cm in diameter at the bottom, 0.8 cm high, and 0.6 cm in diameter at the top. A wax sprue was attached at the bottom of the lower surface of the pattern. Ten patterns were mounted on a crucible former. The samples were produced with a Cr-Co removable partial denture casting alloy (Kentzalloy, Top Dent E Kentzler, Ellwangen/J, Mtihlgr-
*Associate Professor, Department of Prosthodontics. 10/1/27547 THE JOURNAL OF PROSTHETIC DENTISTRY
Table I. Finishing and polishing procedures tested Group
1 2 3 4 5 6 7 8 9 l0 11
Technique
Untreated control A A+B A+B+F A+B+C A+B+C+D A+B+C+D+E A+B+C+D+E+F A+B+C+D+E+F+G A+B+C+D+E+F+G+H A+B+C+D+E+F+G+I
A, First sandblasting; B, hard stone; C, medium abrasive disk; D, second sandblasting; E, electropolishing; F, hard rubber point; G, hard felt disk with pumice slurry; H, hard bristle brush with aluminium oxide; I, felt disk and soft brush with polishing paste.
aben, Germany) and an oxyphosphate investment (Dentex Aquamex, Research Development Inc., Long Island City, N.Y.) according to the instructions of the manufacturers. The casting surface for the wax pattern that had been in contact with the glass surface was used as the test surface. An induction crucible was used for the fusion of alloy, and the castings were allowed to bench cool before divesting. Following recovery, the castings were sandblasted, and 10 test specimens were prepared for each group. Different finishing and polishing techniques were applied to the test specimens (Table I). Each grinding and polishing technique was applied by the same dentist with a high-speed polishing motor (Krupp Dental, Essen, Germany) and light pressure under standard conditions. The sprues were cut off with a separating disk. Techniques for finishing included the use of special stones (Top Dent E Kentzler), medium abrasive disks (waterproof, silicon carbide paper No. 166 P600A), sandblaster equipment (Blastomatic, Schiitz-Dental GmbH, Rosbach, Germany), electrolytical polishing apparatus with 763
AYDIN
D
:!
: ! :i!i!i:!~i~=:~iil i
i
:il ili~ ~i!iiili¸ ¸¸ ~¸~¸
F i g . 1. Typical surface profile tracings of groups 1 through 4 specimens. A, U n t r e a t e d control group; B, group 2; C, group 3; D, group 4.
T a b l e II. Average surface roughness (in microns)* R~
Rz
Rmax
Groups
X
SD
X
SD
X
SD
1 2 3 4 5 6 7 8 9 10 11
5.7700 3.5000 0.9220 0.1990 0.1850 1.2430 1.8840 0.2270 0.2360 0.1470 0.1410
0.4761 0.0650 0.0408 0.0143 0.0138 0.0411 0.2602 0.0087 0.0090 0.0065 0.0092
23.9900 17.0000 5.1800 1.2840 1.1250 7.6280 7.3760 1.2970 1.2740 0.7630 0.6490
1.7691 0.3004 0.2702 0.0648 0.1142 0.4022 0.8544 0.0576 0.0581 0.0423 0.0255
36.8000 22.2300 7.8440 1.6700 1.9220 9.2250 12.4030 1.5980 1.5980 1.0390 0.7230
3.5378 0.8831 0.9175 0.0810 0.3173 0.4435 1.5684 0.0922 0.1156 0.0986 0.0864
* M e a n s of 10 specimens.
automatic control (Aurodent, K r u p p Dental), rubber points (Top Dent E Kentzler), felt disk with pumice slurry, brush wheel with aluminium oxide, and felt mops and soft brush wheels in conjunction with polishing paste (Ivoclar Universal Polier paste, Ivoclar AG Schaan, Liechtenstein, Germany). During abrasion, the direction of movement of the abrasive particles over the surface was changed constantly. When a finer abrasive was used after a coarse one, the specimen was positioned to a 90-degree angle when compared with its first location. A systematic method was followed in which each instrument or abrasive was used until its work had been completed before the next step was begun. T h e abrasive stone was applied for 90 seconds. The medium abrasive disk was applied for 90 seconds, 45 seconds for each direction. Time duration given for the sandblaster equipment was determined to be 45 seconds. The switch of the electrolytic polishing a p p a r a t u s was turned on so t h a t a reading of 2 to 3 amperes was produced. T h e specimen was k e p t in the acid solution for 5 minutes. Rubber point, felt disk, felt mop, and brush wheel were applied for 90 seconds each. Residual polishing paste was removed in an ultrasonic bath.
764
Surface roughness of the test specimens was measured in microns with a P e r t h o m e t e r {model S5P, M a h r - P e r t e n GmbH., Hannover, Germany) instrument. Arithmetic average roughness (Ra), average peak-to-valley height (Ra), and m a x i m u m peak-to-valley height (Rmax) values 13 were recorded by the P e r t h o m e t e r device from one surface of each specimen in every test group. The d a t a were statistically analyzed for differences among the 11 finishing and polishing procedures. Means and s t a n d a r d deviations were calculated for each grinding and polishing technique, and the d a t a were compared by Kruskal-Wallis one-way analysis of variance test and multiple comparisons test (Wilcoxon-Wilcox test).14 T e s t specimens were also examined by a scanning electron microscope (Cambridge stereoscan $4-10, Cambridge, England) and photographs were made at ×950 magnification.
RESULTS T h e average surface roughness values and their s t a n d a r d deviations obtained after application of the various finishing procedures to the Cr-Co casting alloy are shown in Table II. According to Rz values, finishing and polishing
J U N E 1991
V O L U M E 65
NUMBER 6
F I N I S H I N G AND P O L I S H I N G TECHNIQUES
~B
A
llm
't
C
~gm
[Ioi 5oo Fig. 2. Typical surface profile tracings of groups 5 through 8 specimens. A, Group 5; B, group 6; C, group 7; D, group 8.
i
! i¸¸~
i
ii
~i
i 50o [5 ~, : I000 :T~ i 250: T :pm ;lum: : : j,m::: l m::
Fig. 3.
Typicalsurfaceprofiletracingsofgroups9through 11 specimens. A, Group 9; B, group 10; C, group 11.
methods were ranked in the order of decreasing smoothness as follows: 1, group 11; 2, group 10; 3, group 5; 4, group 9; 5, group 4; 6, group 8; 7, group 3; 8, group 7; 9, group 6; 10, group 2; and 11, group 1 (Table II). The results of the Kruskal-Wallis test and multiple comparisons test for Ra, Rz, and Rmax values showed significant differences at the p < 0.05 level with Rz (average peak-to-valley height). These significant differences were found between group 11 and groups 7, 6, 3, 2, and 1. Selected profile tracings that are representative of the surfaces developed by the procedures tested are shown in Figs. 1 through 3. Fig. 1, A shows the highly irregular and rough surfaces of untreated samples. Surfaces of samples treated by the second method were rough and uneven (Fig. 1, B). The sixth group samples, which were treated first with sandblasting, then stone, abrasive disk, and a second sandblasting exhibited gross surface irregularities (Fig. 2, B). Samples treated by the seventh (Fig. 2, C) and third (Fig. 1, C) polishing methods showed comparatively smooth surfaces. Surfaces of samples in the eighth (Fig. 2, D), fourth (Fig. 1, D), and ninth (Fig. 3, A) groups were quite smooth with minimal irregularities. The tenth (Fig. 3, B) and eleventh (Fig. 3, C) polishing methods produced the
THE J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Fig. 4. Scanning electron photomicrographs of groups 1 through 4 specimens (original magnification x950). A, Surface of untreated control group. B, Surface after finishing with second method. C, Surface after finishing with third method. D, Surface after finishing with fourth method.
smoothest surfaces. The surface of samples in the eleventh group was obviously the smoothest (Fig. 3, C). Selected scanning electron photomicrographs of representative surfaces are shown in Figs. 4 through 6 to further substantiate the results obtained from statistical analysis of the data. Fig. 4, A shows the highly irregular and rough surfaces of untreated samples. Surfaces of samples treated by the second (Fig. 4, B) and sixth (Fig. 5, B) methods were rough and uneven. Specimens in group No. 7 exhibited some surface irregularities (Fig. 5, C). The samples in the third group, which were treated first with sandblasting and then stone, exhibited a deeply grooved surface (Fig. 4, C).
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Fig. 5. Scanning electron photomicrographs of groups 5 through 8 specimens (original magnification x950). A, Surface after finishing with fifth method. B, Surface after finishing with sixth method. C, Surface after finishing with seventh method. D, Surface after finishing with eighth method.
The specimens of groups 8, 4, and 5 resulted in smooth surfaces (Figs. 4, D and 5, A and D). Surfaces of the samples in group 10 exhibited smoother surfaces with only minimal irregularities (Fig. 6, B). The method for group 11 produced the smoothest surface (Fig. 6, C).
DISCUSSION The Cr-Co alloys have a high resistance to abrasion; thus they are more difficult to grind than the gold-base alloys. However, they retain their luster for a longer period in service10,15 and are less liable to wear when exposed to occlusal contactJ 5 The high hardness of these alloys presents a real problem at the finishing stage. 4, 6,10,16-1s The importance of the finishing and polishing technique for removable partial denture frameworks cannot be underestimatedJ 9 After casting, it is usual to sandblast the metal to remove surface roughness or adherent refractory material and to clean away the oxide film. 2, s. 1~. 16,20-22 Sandblasting removed the adherent film of investment particles and oxides, which are tenacious, but did not improve the surface. The highly irregular and rough surfaces were apparent (Figs. 1, B and 4, B). The third group to which sandblasting and the abrasive stone were applied exhibited deeply grooved surfaces (Figs. 1, C and 4, C) that had to be removed by progressively finer abrasives. Medium abrasive disks applied after a hard stone in the fifth samples group introduced only fine
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Fig. 6. Scanning electron photomicrographs of groups 9 through 11 specimens (original magnification ×950). A, Surface after finishing with ninth method. B, Surface after finishing with tenth method. C, Surface after finishing with eleventh method.
scratches (Figs. 2, A and 5, A). After a second sandblasting, the sixth group samples exhibited irregular and rough surfaces (Figs. 2, B and 5, B). The electrolytic polishing process is invaluable for producing a bright surface on the fitting surface of Cr-Co frameworks. 2 Electrolytic polishing eliminates or reduces additional polishing on the tissue side of the c a s t . 2, 8 Material can be removed from the casting by the process, leaving a surface that is smooth and bright without damaging the detail and accuracy of the casting. 16,23 Because rough and uneven surfaces can never be made smooth, the nature of the surface after this polishing process depends on the condition of the surface before polishing commenced. 16 Surfaces that are rough must therefore be ground and smoothed with stones before they are placed in the polishing bath. 16 The electrolytic polishing procedure exhibited a significantly smoother surface texture than was found after a second sandblasting (Figs. 2, C and 5, C). Polishing with rubber wheels is the first step after the finishing process. 2' 24 Unlike an abrasive, a polishing material does not cut a groove but causes the fine scratches to be filled to produce a smooth surface. The application of rubber points applied after hard stone or electropolishing in groups 4 and 8 removed all scratches (Figs. 1, D; 2, D; 4, D; and 5, D). A felt disk with pumice removed all traces of rubber point marks (Figs. 3, A and 6, A). Polishing samples with 15 #m aluminium oxide slurry at this time reduced the grinding marks to a fine texture and gave a smooth surface with minimal irregularities (Figs. 3, B and 6, B).
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FINISHING AND POLISHING TECHNIQUES
Although groups 10 and 11 resulted in a polished surface (Figs. 3, B and C and 6, B and C), the best surface finish was produced in the group 11 samples, which were treated with a polishing paste after felt disk-pumice slurry. Polishing samples with polishing paste at this time gave a smooth and lustrous surface. The best surface finish was produced in group 11, to which all steps had been applied (Figs. 3, C and 6, C). Scanning electron micrographs and surface profile tracings in this study closely supported the results of the statistical analysis. An evaluation of the results indicates that a positive plan should be applied in the finishing operation with coarser stones and disks used at first and then a finer grade of abrasive. The finishing operation should be carried out in a logical, systematic sequence of steps. CONCLUSIONS The effect of grinding and polishing techniques on surface roughness of a Cr-Co casting alloy was evaluated by use of a surface analyzer and by observation of the casting surfaces. The best surface finish was obtained when sandblasting, hard stone, medium abrasive disk, second sandblasting, electropolishing, hard rubber point, hard felt disk with pumice slurry, and felt disk and soft brush with polishing paste were used progressively. The finishing procedure should consist of smoothing the metal with progressively finer abrasive agents until all scratches are removed, after which a polishing material is applied to give a high luster to the finished work. Results from the evaluations made in this study indicate that a definite sequence for finishing and polishing should be adopted and followed in every prosthesis.
3. Taylor DF, Leibfritz WA, Adler AG. Physical properties of chromiumcobalt dental alloys. J Am Dent Assoc 1958;56:343-51. 4. Lane JR. A survey of dental alloys. J Am Dent Assoc 1949;39:429-30. 5. Taylor DF, Sweeney WT. A proposed specification for dental chromium-cobalt casting alloys. J Am Dent Assoc 1957;54:44-8. 6. Craig RG, O'Brien WJ, Powers JM. Dental materials {properties and manipulation). St Louis: CV Mosby Co, 1975;192-5. 7. Cunningham DM. Comparison of base metal alloys and type IV gold alloys for removable partial denture frameworks. Dent Clin North Am 1973;17:719-22. 8. Collett HA. Casting chrome-cobalt alloys in small laboratories. J PROSTHET DENT 1969;21:216-26. 9. Craig RG, Peyton FA. Restorative dental materials. 5th ed. St Louis: CV Mosby Co, 1975;361-5. 10. Peyton FA. Cast chromium-cobalt alloys. Dent Clin North Am 1958;Nov:759-71. 11. Skinner EW, Phillips RW. The science of dental materials. 6th ed. Philadelphia: WB Saunders Co, 1967;581-7. 12. Smith EA. Vitallium as a substitute for dental gold casting alloys. Br Dent J 1948;85:180-1. 13. DIN Standard 4768. Ermittlung der RauheitsmessgrSssen Ra, R2, Rm~ mit elektrischen Tastschnittger~iten. Alleinverkauf der Normen dutch Beuth Verlag GmbH, Berlin und KSln 1, part 1, Aug 1974;1-4. 14. Gibbons JD. Nonparametric methods for quantitative analysis. New York: Holt, Rinehart, Winston Co, 1976;181. 15. Earnshaw R. Cobalt-chromium alloys in dentistry. Br Dent J 1956; 101:67-75. 16. Harcourt HJ. Chrome cobalt castings, Part 3. Dent Techn 1956;9:99104. 17. Osborne J, Lammie GA. Some observations concerning chrome-cobalt denture bases. Br Dent J 1953;94:55-67. 18. Phillips RW, Swartz ML, Norman RD. Materials for the practicing dentist. St Louis: CV Mosby Co, 1969;201. 19. White TD, Tarr JL. Dental laboratory technicians' manual. Washington: Air Force Manual Department of the Air Force, 1959;46-315. 20. Marxkors R. The partial denture with metal framework. Bremen: BEGO Bremer Goldschliigerei Wilh.Herbst GmbH & Co, 1984;100-4. 21. Neill DJ, Walter JD. Partial denture prosthetics. Oxford: Blackwell Scientific Publications, 1977;64-7. 22. Stewart KL, Rudd KD, Kuebker WA. Clinical removable partial prosthodontics. St Louis: CV Mosby Co, 1983;360-4. 23. (~alikkocao~lu S. Di~hekimli~indecila. I 0 Di~ Hek F Der 1973;7:230-43. 24. Henderson D, Steffel VL. McCracken's removable partial prosthodontics, 5th ed. St Louis: CV Mosby Co, 1977;349-51.
REFERENCES 1. Lanier BR, Rudd KD, Strunk RR. Making chromium-cobalt removable partial dentures: a modified r.echnique. J PROSTHETDENT 1971;25:197205. 2. Martinelli N. Dental laboratory technology. 2nd ed. St Louis: CV Mosby Co, 1975;31-358.
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