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Method for measuring impression deformations with a modified photoelastic coating technique
Method
for
a modified
measuring
impression
photoelastic
coating
deformations
with
technique
Rafael Grajower, Ph.D.,* and Noah Stern, D.M.D., M&D.** The Hebrew University-Hadassah School of Dental Medicine,
Jerusalem,
Israel
D
eformations of impressions recorded with elastomeric materials are due mainly to the incomplete recovery from deformations which take place during the removal of the impression from the mouth and to dimensional changes which occur in the period between taking the impression and pouring the cast. The tests prescribed by the American Dental Association1 or the International Standards Organization2 measure the dimensional stability and permanent deformation of elastomeric impression materials in one dimension only. The properties of various materials, as measured according to these or similar specifications, have been published.3s 4 These tests, however, do not provide information regarding the irregular three-dimensional deformation which is to be expected in impressions of various shapes. The measurement of three-dimensional deformation has been reported for impressions of coronal cavity preparations5 and stylistic molar@ and has been carried out by comparing several corresponding dimensions of the impression and of the original object. This report describes a method for obtaining information simultaneously at many locations concerning deformation of the impression by employing a modified photoelastic coating technique. Photoelastic materials become doubly refracting when subjected to stress.’ When circularly polarized light is passed through the material and the material is then viewed through an appropriate analyzer, colored fringes are observed, indicating the amount and direction of the strain. A photoelastic material in the form of a semicured sheet can be applied as a coating which follows the contours of an object. The strain in the coating is determined with reflected light from a layer of reflecting paint already applied beneath the coating. For conventional stress analyses, the coating should adhere to the object under investigation. With the present modification, a releasing agent prevents this bonding, so that the Supported in part by a grant from the Joint Research Fund of The Hebrew Hadassah School of Dental Medicine. *Head, Laboratory of Dental Materials. **Lecturer in Fixed Prosthodontics, Department of Oral Rehabilitation.
University-
431
432
Grajower
and Stern
J. Prosthet. Dent. April, 1974
Fig. 1. View through the reflection polariscope as set for analyses of the strain magnitude at the sides: a photoelastic cap, a stone cast prepared for full coverage, and (in the center) an assembly of these.
Fig. 2. View of assembled cap through the polariscope when set for measuring the directions of principal strain. The black bands are the locus of points having a principal direction of strain parallel to the base of the cast. coating may be removed from an object and replaced on another. If the second object is larger than the first one, the coating will be subjected to stresses and will show a fringe pattern.
PROCEDURE An enlarged dental stone cast, in the shape of a full-coverage tooth preparation with a chamfer-type finishing line, was used as master model I and had a diameter of 5 cm. at the widest point. A mercaptan rubber impression was made of this
Volume Number
Measuring
31 4
impression
deformations
433
cast, and a stone cast II was prepared from this impression after 48 hours. A flat photoelastic epoxy layer 1.5 mm. thick (PL-1”) was prepared in a Tefloncoated mold. Half an hour after mixing the resin and hardener, a thin layer of r‘ewas sprayed on the layer. When the mixture layer had set to flective paint (RSO-1”) a pliable sheet, a small amount of releasing agent* was applied to it in order to prevent the adhesion of the coating to stone. For the same reason, this agent was applied to the stone casts. The still-pliable sheet was then removed from the mold, placed on top of the cast with the reflective spray facing the cast, and formed according to the contours of the cast. The photoelastic “caps” I and II were thus prepared on the corresponding casts. After 24 hours, the caps were removed from the casts and trimmed to the finishing line with a spray-cooled high-speed diamond bur. Cap I was forced on cast II, and cap II was placed on cast I. A rather irregular fringe pattern became visible on cap I when viewed through a reflection polariscope (131”) showing a maximum fringe order of two thirds. No fringes were visible on cap II. The observed fringes were due to the stresses in the cap which developed when it was forced on a cast that was larger than the one on which it was prepared. The irregular fringe pattern rendered information concerning the magnitude and location of the shrinkage in the impression (Figs. 1 and 2) . The percentage of elongation for a plane sheet of the resin material for a cycle of color fringes is given by the fringe value f = 11, 35 x lO+/tkt (Technical Bulletin TDG-1”). Assuming that this value will be similar to those values for the contoured layers we used, one may conclude that one fringe represents an elongation of 0.2 per cent in our experiments. The sensitivity of the method is approximately one fifth of a fringe or 0.04 per cent deformation.
SUMMARY AND CONCLUSIONS A new method for measuring the deformation of impression materials, employing a modified photoelastic coating technique, was described. This method enables the simultaneous observation of deformations at many locations. The quality of impression materials, with respect to deformations occurring during impression making, and dimensional stability may thus be determined. The sensitivity of the method is approximately 0.04 per cent.
“Photoelastic jt
=
thickness
Inc.,
Malvern,
of resin;
k =
Pa. constant
of resin
material
equalling
0.1
References 1. 2. 3. 4.
Guide to Dental Materials and Devices, ed. 5, 1970-71, A. D. A. Specification No. 19, pp. 185-191. I. S. 0.: Elastomeric Dental Impression Materials, Draft spec. 106/W62, Dot. 87, April, 1971. Schwindling, R. : Lineare Dimensionsveranderungen von Thiokol-Abformmassen, Dtsch. Zahnaerztl. Z. 25: 1121-1126, 1970. Schwickerath, H. : Zur Prufung der Form-und Wiedergabegenauigkeit von Abformmaterialen, Dtsch. Zahnaertzl. Z. 27: 478-484, 1972.
434 5.
6. 7.
Grajower
.J. 1’1ostlrct. Dent. April, 1974
and Stern
Podshadley, Mercaptan 28: 58-65, Schoenmakers, 106/W. G., Redner, S.: 1968.
A. C., Birtless, J. T., Neiman, R., and Dilts, W. E.: Accuracy of Pouring Rubber Impressions at Various Intervals After Separation, J. PROSTHET. DEXT. 1972. H. P. L.: Pilot Investigation on the Accuracy of Impressions, I. S. 0. ‘I‘. C. Dot. 58, Sept., 1970. Photoelasticity, Encyclopedia of Polymer Science and Technology 9: 590-610,
HAUASSAH
SCHOOL
P. 0. B. 1172 JERUSALE~~,
ISRAEL
OF DENTAL
MEDICINE
for
a modified
measuring
impression
photoelastic
coating
deformations
with
technique
Rafael Grajower, Ph.D.,* and Noah Stern, D.M.D., M&D.** The Hebrew University-Hadassah School of Dental Medicine,
Jerusalem,
Israel
D
eformations of impressions recorded with elastomeric materials are due mainly to the incomplete recovery from deformations which take place during the removal of the impression from the mouth and to dimensional changes which occur in the period between taking the impression and pouring the cast. The tests prescribed by the American Dental Association1 or the International Standards Organization2 measure the dimensional stability and permanent deformation of elastomeric impression materials in one dimension only. The properties of various materials, as measured according to these or similar specifications, have been published.3s 4 These tests, however, do not provide information regarding the irregular three-dimensional deformation which is to be expected in impressions of various shapes. The measurement of three-dimensional deformation has been reported for impressions of coronal cavity preparations5 and stylistic molar@ and has been carried out by comparing several corresponding dimensions of the impression and of the original object. This report describes a method for obtaining information simultaneously at many locations concerning deformation of the impression by employing a modified photoelastic coating technique. Photoelastic materials become doubly refracting when subjected to stress.’ When circularly polarized light is passed through the material and the material is then viewed through an appropriate analyzer, colored fringes are observed, indicating the amount and direction of the strain. A photoelastic material in the form of a semicured sheet can be applied as a coating which follows the contours of an object. The strain in the coating is determined with reflected light from a layer of reflecting paint already applied beneath the coating. For conventional stress analyses, the coating should adhere to the object under investigation. With the present modification, a releasing agent prevents this bonding, so that the Supported in part by a grant from the Joint Research Fund of The Hebrew Hadassah School of Dental Medicine. *Head, Laboratory of Dental Materials. **Lecturer in Fixed Prosthodontics, Department of Oral Rehabilitation.
University-
431
432
Grajower
and Stern
J. Prosthet. Dent. April, 1974
Fig. 1. View through the reflection polariscope as set for analyses of the strain magnitude at the sides: a photoelastic cap, a stone cast prepared for full coverage, and (in the center) an assembly of these.
Fig. 2. View of assembled cap through the polariscope when set for measuring the directions of principal strain. The black bands are the locus of points having a principal direction of strain parallel to the base of the cast. coating may be removed from an object and replaced on another. If the second object is larger than the first one, the coating will be subjected to stresses and will show a fringe pattern.
PROCEDURE An enlarged dental stone cast, in the shape of a full-coverage tooth preparation with a chamfer-type finishing line, was used as master model I and had a diameter of 5 cm. at the widest point. A mercaptan rubber impression was made of this
Volume Number
Measuring
31 4
impression
deformations
433
cast, and a stone cast II was prepared from this impression after 48 hours. A flat photoelastic epoxy layer 1.5 mm. thick (PL-1”) was prepared in a Tefloncoated mold. Half an hour after mixing the resin and hardener, a thin layer of r‘ewas sprayed on the layer. When the mixture layer had set to flective paint (RSO-1”) a pliable sheet, a small amount of releasing agent* was applied to it in order to prevent the adhesion of the coating to stone. For the same reason, this agent was applied to the stone casts. The still-pliable sheet was then removed from the mold, placed on top of the cast with the reflective spray facing the cast, and formed according to the contours of the cast. The photoelastic “caps” I and II were thus prepared on the corresponding casts. After 24 hours, the caps were removed from the casts and trimmed to the finishing line with a spray-cooled high-speed diamond bur. Cap I was forced on cast II, and cap II was placed on cast I. A rather irregular fringe pattern became visible on cap I when viewed through a reflection polariscope (131”) showing a maximum fringe order of two thirds. No fringes were visible on cap II. The observed fringes were due to the stresses in the cap which developed when it was forced on a cast that was larger than the one on which it was prepared. The irregular fringe pattern rendered information concerning the magnitude and location of the shrinkage in the impression (Figs. 1 and 2) . The percentage of elongation for a plane sheet of the resin material for a cycle of color fringes is given by the fringe value f = 11, 35 x lO+/tkt (Technical Bulletin TDG-1”). Assuming that this value will be similar to those values for the contoured layers we used, one may conclude that one fringe represents an elongation of 0.2 per cent in our experiments. The sensitivity of the method is approximately one fifth of a fringe or 0.04 per cent deformation.
SUMMARY AND CONCLUSIONS A new method for measuring the deformation of impression materials, employing a modified photoelastic coating technique, was described. This method enables the simultaneous observation of deformations at many locations. The quality of impression materials, with respect to deformations occurring during impression making, and dimensional stability may thus be determined. The sensitivity of the method is approximately 0.04 per cent.
“Photoelastic jt
=
thickness
Inc.,
Malvern,
of resin;
k =
Pa. constant
of resin
material
equalling
0.1
References 1. 2. 3. 4.
Guide to Dental Materials and Devices, ed. 5, 1970-71, A. D. A. Specification No. 19, pp. 185-191. I. S. 0.: Elastomeric Dental Impression Materials, Draft spec. 106/W62, Dot. 87, April, 1971. Schwindling, R. : Lineare Dimensionsveranderungen von Thiokol-Abformmassen, Dtsch. Zahnaerztl. Z. 25: 1121-1126, 1970. Schwickerath, H. : Zur Prufung der Form-und Wiedergabegenauigkeit von Abformmaterialen, Dtsch. Zahnaertzl. Z. 27: 478-484, 1972.
434 5.
6. 7.
Grajower
.J. 1’1ostlrct. Dent. April, 1974
and Stern
Podshadley, Mercaptan 28: 58-65, Schoenmakers, 106/W. G., Redner, S.: 1968.
A. C., Birtless, J. T., Neiman, R., and Dilts, W. E.: Accuracy of Pouring Rubber Impressions at Various Intervals After Separation, J. PROSTHET. DEXT. 1972. H. P. L.: Pilot Investigation on the Accuracy of Impressions, I. S. 0. ‘I‘. C. Dot. 58, Sept., 1970. Photoelasticity, Encyclopedia of Polymer Science and Technology 9: 590-610,
HAUASSAH
SCHOOL
P. 0. B. 1172 JERUSALE~~,
ISRAEL
OF DENTAL
MEDICINE