Copperplating copper band-compound impressions at constant voltage

Rafael Grajower, Ph.D.,* Noah Stern, D.M.D., M.S.D.,** and Judith Kaffman, D.M.D.*** Hebrew University-Hadassah School of Dental Medicine, Jerusalem, Israel

Lopperplating impressions produces dies with high dimensional accuracy which will withstand abrasion during the waxing of patterns and fitting castings on the die. The plating process should be as rapid and simple as possible to reduce working time and prevent failures.

No estimate has been given in the literature as to the required thickness of the copperplated layer. However, layers as thin as 30 pm in the incisal region have been reported.’

The plating current during copper deposition is limited by three factors. First, a dark, porous copper layer may initially be deposited on the metallized impression surface, probably through the evolution of hydrogen. The overvoltage for hydrogen evolution on copper is 1.3 V,’ but this value for graphite has not been reported. Second, irregular deposition (“mushrooming”) of copper may occur. Third, the throwing power of the system may become insuffi- cient for penetration of copper into the incisal end of thin and long impressions.

For large impressions the first and second limiting factors are most important, and for narrow ones the third factor is important. In order to obtain copper deposition at the incisal end of impressions, the copper ions should diffuse to this region without being “scavenged” to the sides of the impression by an excessively large electric field. This factor limits the potential difference which may be applied for plating small impressions. Scavenging gives rise to preferential deposition near the cervical end, causing constriction of the opening and preventing ionic penetration to a greater extent as the plating process continues and the cervical opening becomes more narrow.

Industrial electroplating is usually carried out

*Senior Lecturer, Head, Laboratory of Dental Materials.

**Senior Lecturer, Department of Oral Rehabilitation.

***Practicing dentist.

OOZZ-3913/80/040413 + 02$00.20/O 0 1980 The C. V. Mosby Co.

with current-regulated power supplies to obtain coatings with a high-quality outer layer, on objects which are generally free of deep cavities. Current regulation is also used for electroforming dental impressions, although the requirements are very different. Dental impressions are generally deep cavities, and the quality of the first-deposited, inner- most layer is of prime importance. The dentist has to adjust the current according to the size and number of impressions and according to the stage (beginning or end) of the plating process. When impressions of similar diameter and depth are plated simultaneous- ly, it may be known empirically which current is to be applied to obtain plating with sufficient penetra- tion in a minimum of time. However, when simulta- neous plating of impressions of different sizes is carried out, it is impossible to know from the total current if the maximum current which allows for penetration in the narrowest sample is not exceeded, or if the potential difference has become larger than the overvoltage for hydrogen evolution and the formation of a porous surface. Optimal plating conditions, if initially obtained with current regula- tions, may be affected by changes in the resistance of the system during the plating process. These changes may be due to the coverage of the metallizing agent with copper, the progressive change of geometry of the cathodes, and the formation of parasitic growths on areas which had not been isolated adequately.

Because of these problems, and since both ionic penetration and hydrogen evolution are directly dependent on the potential difference between the electrodes, a series of experiments were carried out on copperplating with voltage regulation. Voltage regulation should eliminate the need for adjustments to the number of plated samples. Attempts also were made to find one potential difference which would be suitable for impressions of all sizes and would thus eliminate the need for any adjustment by the dentist.

THE JOURNAL OF PROSTHETIC DENTISTRY 413

GRAJOWER, STERN, AND KAFFMAN

Table I. Thickness of electrodeposited copper layers at various potential differences

Thickness (pm)

Large Narrow impressions impressions

Potential Plating Ce rvical Occlusal Cervical In&al difference period -

(V (hours) Mean SD Mean SD Mean SD Mean SD

0.5 4 256 22 113 15 412 56 - - 0.4 6 197 23 86 11 276 31 12.5 10 0.3 6 85 9 72 6 188 45 38 4

METHODS AND MATERIALS

Impressions of two types of plastic tooth prepara- tions were made with modeling compound* in copper bands. Long abutments of incisors were represented by a tapered tooth form 11 mm long with a diameter of 5.5 mm at the base and a radius of curvature of 1.5 mm at the incisal end. Truncated cones of 6 mm length and diameters of 13 and 9 mm at the base and the top, respectively, were used to represent molar abutments. The impressions were coated with graphite,t attached to cathode holders,” and isolated with .wax.$ Six cathodes were placed symmetrically around a cylindrical copper anode with initial length and diameter of 45 and 35 mm, respectively. The open ends of the impressions were at a distance of 25 mm from the anode surface. A commercial acid copper sulphate solutions was used. Preliminary experiments showed that similar results are obtained with a ripple-free DC source and with a power supply having full-wave rectification only./ 1 Leads of 3 mm thickness and 200 mm length were used between the electrodes and the power supply. The potential difference between the electrodes remained constant to + 4% during each experiment, as measured between the anode holder and one cathode holder. Twelve specimens were coated for each potential difference and tooth form used.

Dies were prepared by filling the copperplated impressions with acrylic resin and were eventually removed from the modeling compound and the copper band. After inspection, the dies were embed- ded in acrylic resin. The samples were ground until a longitudinal section of the copper layer became

*Modeling compound, Low Heat, J. Bird Moyer Co., Philadel- phia, Pa.

tFlash Metalliser, Hanau Engineering Co., Buffalo, N. Y. $Green Casting Wax Sheets, Kerr Mfg. Co., Romulus, Mich. BCopper Electrolyte, Hanau Engineering Co., Buffalo, N. Y. l(6284A DC power supply, Hewlett Packard, Palo Alto, Calif.

apparent, and, this was subsequently, polished. A measuring microscope* was used to determine the thickness of the copper layer at the occlusal end and 1 mm from the cervical end of the impressions.

RESULTS AND DISCUSSION

The thickness of the copper layer formed at varied potential differences is given in Table I for both types of tooth forms.

Using potential differences of 0.6 V or more gave rise to dark porous areas on the dies. Hydrogen evolution on graphite apparently occurs above this potential difference. At 0.5 V satisfactory coatings were formed on the large impressions, but in the narrow impressions no copper was deposited at the incisal end, and “mushrooming” occurred at the open cervical end. At 0.4 V a complete though thin layer of copper was formed in only four of the 12 narrow specimens. At 0.3 V however, a complete layer of copper was formed on all narrow samples.

The thickness of the incisal region should be sufficient to allow cleaning of the die with cotton and solvents and to withstand the appropriate manipula- tions with wax. However, the thickness in the cervi- cal area should be greater to prevent tearing during removal of the impression from the copper band, carving of the pattern, and fitting of the casting. It was found that all copper layers obtained at 0.3 V during 6 hours satisfied these criteria.

CONCLUSIONS

According to the experimental results, the electro- deposition of copper on impressions should be carried out with a voltage-stabilized power supply at 0.3 V. This method permits the copperplating of tube impressions of various sizes without the need for further adjustments of controls on the plating appa- ratus.

*Durimet Leitz Wetzlar, West Germany.

REFERENCES

1. Smith, G. A.: Laboratory errors in,porcelain crowns. Br Dent J 139:98, 1975.

2. Lowenheim, F. A.: Modern Electroplating. New York, 1963, John Wiley & Sons, Inc., pp 21, 743.

3. Grajower, R., and Stern, N.: A simple cathode holder for electroplating copperband impressions. J PROSTHET DENT

29:285, 1978.

Reprint requests to:

DR. NOAH STERN HEBREW UNIVERSITY-HADASSAH SCHOOL OF DENTAL MEDICINE P.O.B. 1172 JERUSALEM, ISRAEL

414 APRIL 1980 VOLUME 43 NUMBER 4