Permeability and porosity of dental casting investments

Permeability

and

G.

1.

Ballard,

D.

F.

Taylor,

University

porosity

D.M.D., B.S.E.,

of

M.S.,* M.S.E.,

K.

of F.

dental

casting

Leinfelder,

D.D.S.,

M.S.,**

investments and

Ph.D.***

North Carolina, School

of

Dentistry,

Chapel Hill, N. C.

T

1 n order

to permit the rapid entry of metal into the mold during investment casting, some means must be provided for the escape of gases. If gaseous emission is blocked or sufficiently impeded, defective castings may be produced. The problems are most likely to occur when the fusion temperature of the metal is high, the permeability of the investment is low, and the metal fills the mold in a shorter than usual time. In an attempt to eliminate this problem, special venting procedures have been recommended.ll 2 In addition, special methods of spruing may be required to regulate the entry rate of molten metal .s These spruing procedures undoubtedly serve to increase the surface area through which the gases may escape, as well as slow the ingress of the molten alloy. Such special sprueing and venting methods are more commonly employed with the phosphate-bonded investments. While effective, these procedures usually increase the amount of precious metal required in the casting process. Although industrial specifications and test standards for permeability or refractoriness exist, there are no permeability standards for dental investment materials. Several studies concerning the gas permeability of dental investments have been conducted,4-’ but only limited and somewhat conflicting data have been made available as a result. It was the purpose of this investigation to measure gas permeability and porosity of various gypsum-bonded and phosphate-bonded dental casting investments at recommended burn-out temperatures. The effectiveness of an acrylic resin polymer This investigation Grant RR-5333 from Bethesda, Md.

was supported in part by United States Public Health Service Research the National Institute of Dental Research, National Institutes of Health,

The opinions or assertions contained to be construed as official or as reflecting at large. *Commander *“Associate

(DC) Professor,

herein are the private the views of the Navy

ones of this author and Department or the naval

USN. Operative

Dentistry,

University

of North

Carolina,

School

of Dentis-

try. ***Professor, 170

Operative

are not service

Dentistry,

University

of North

Carolina,

School

of Dentistry.

Volume Number

31 2

Table

I. Investment

Permeability

Investment

materials I

R & R hygroscopic Beauty Cast HFG Ceramigold Biovest

included

and

porosity

of

171

investments

in study Manufacturer

Liquid/powder

The Ransom and Randolph Company Whip-Mix Corp. The Ransom and Randolph Company Whip-Mix Corp. Dentsply International, Inc.

ratio

15150 15/50 I3/60 9.5/60 1 l/60

additive as an agent for increasing both porosity and permeability was also evaluated. Porosity is a measure of the volume percentage of an investment not occupied by a solid. Permeability is a measure of the ability of gas to flow through an investment. Unconnected or discontinuous porosity does not contribute to permeability. MATERIALS

AND

METHODS

Included in this study were two gypsum-bonded and three phosphate-bonded proprietary investment materials (Table I) . Additional specimens were made by adding acrylic resin polymer to the investment powder to provide additional porosity after burn-out. Additions of 5 and 10 per cent by weight of polymethyl methacrylate* were blended with each of the investments. The polymer had a particle size ranging from 5 to 71 p. The liquid/powder ratio of the modified investments was slightly decreased in order to maintain a constant mixed consistency (viscosity). Nevertheless, the effective liquid/investment ratio was slightly increased in each situation. It was estimated that the addition of 5 per cent acrylic resin to the gypsum-bonded investments resulted in a 2.5 per cent reduction in the amount of unmodified investment per unit of volume, while those containing 10 per cent polymer were reduced by approximately 5 per cent. For example, a standard mix of Beauty Cast investment was made with 15 C.C. of water per 50 Gm. of investment. With a 10 per cent polymer addition, 50 Gm. of powder would contain 45 Gm. of investment and 5 Gm. of polymer. This was mixed with 14.25 C.C. (15 x 0.95) of water to produce the same consistency as the standard mix. This produced a water/powder ratio of 0.285 and a water/investment ratio of 0.317, as compared to the standard 0.300 C.C. per gram. All samples were mixed under vacuum? for 25 seconds and poured into wet asbestos-lined casting rings. Beauty Cast was evaluated using both hygroscopic and high-heat techniques. The hygroscopic and Ceramigold specimens were placed in a water bath for 30 minutes and allowed to bench set for 15 minutes. All permeability test runs were initiated within one hour of sample preparation. Initial heating of the gypsum-bonded specimens was begun at a furnace temperature of 600’ F., as compared to 300’ F. for the phosphate-bonded investments. All phosphate-bonded investments were heated to 1,300’ F., with the temperature being increased at an approximate rate of 25” F. per minute. The two hygroscopic gypsum*SR-100, type 5 superfine polymer, Sartomer Resins, Inc., Essington, Pa. TWhip-Mix Vat-u-vestor, Whip-Mix Corp., Louisville, Ky.

172

Ballard,

Leinfelder,

and Taylor

J. Prosthet. Auqst,

Dent. 1975

Fig. 1. Apparatus used during this investigation for measurement of gas permeability of dental investment material: A, pressure regulator; B, bleed-off valve; C, air filter; D, desiccator chamber; E, mercury manometer; F, flowmeter; G, investment adaptor; H, furnace; and I, potentiometer.

bonded investments were heated to 900’ F., while the high-heat samples were heated to 1,200° F. The heating rate for the gypsum-bonded samples was approximately 35’ F. per minute. Investment samples for porosity measurements were made in rings of the same size used for permeability samples. The same mixing and burn-out procedures were followed. After burn-out, the samples were cooled to room temperature in a desiccator, and the rings were removed. The specimens were weighed. They were then immersed in USP mineral oil and subjected to several cycles of vacuum and pressure to facilitate the infiltration of oil. Vacuum was maintained on each cycle until no additional oil was evolved. Cycles were repeated until no more air was released on evacuation. The specimens were then allowed to stand for 16 to 20 hours in the oil. Excess surface oil was removed, and the specimens were reweighed. The weight changes were used to calculate the volume of open porosity. All samples were tested twice. A modified form of the gas-flow apparatus designed by Shell8 was used in this study and is illustrated in Fig. 1. Modifications consisted of a flowmeter” calibrated for air flow at room temperature under standard pressure, a thermocouple imbedded in the investment, and an air filter. From a compressed air source, filtered, desiccated air was passed through a flowmeter to a furnace containing the adaptor and investment specimen. Inlet pressure was monitored by reference to an open-end mercury

Porter

*Lab-Crest Company,

series 100 Centure flowmeter, Metering Tube Cat. No. 448-035, Warminster, Pa. (Meter float-l&r inch stainless steel ball.)

Fischer

and

Volume Number

34 2

Permeability

and porosity

of

investments

173

L-L ADAPTOR ROD FORMED CHANNEL 2MM BEYOND ADAPTOR TIP GROOVES-2MM

WIDTH 8 DEPTH

ASBESTOS INVESTMENT

RING

ADAPTOR

THERM Y 25 CM (IO”)

Fig.

2. Adaptor

assembly.

manometer. Investment temperature was measured by means of a potentiometer.* This design permitted continuous monitoring of the inlet air pressure and flow rate through the investment, throughout the course of each sample measurement. Pressurized gas was introduced into the casting investment through an adaptor similar to that described by Shell.” A rod within the adaptor assembly was used to form the mold cavity (Fig. 2). Since the length of the rod was adjustable, the thickness of the investment between the mold cavity and the end of the ring could be varied. For these experiments, however, investment thickness was held constant at the 9.5 mm. used by Shell8 An inlet pressure of 15 5 0.5 cm. Hg was maintained during each experimental run by means of the pressure regulator. Flowmeter readings were converted to absolute values using a conversion chart provided by the manufacturer and calibrated for air-flow measurement under standard conditions. Chart values were corrected by a factor of 1.2, since a differential pressure of 0.2 atm. (15 cm. Hg) was used to force the air through the system. Since the flowmeter was substantially distant from the furnace, correction for the effect of temperature on gas was unnecessary. The apparatus was evaluated for leakage prior to testing each specimen. RESULTS Standard investment. The results of the permeability tests are shown in Table II and graphically in Fig. 3. It was found that, after being held at the manufacturers’ recommended burn-out temperatures for 45 minutes, the proprietary unmodified investments varied considerably in gas permeability. The gypsum-bonded investments demonstrated greater permeability than the phosphate-bonded specimens. Flow *L

&

N

students’

potentiometer,

Leeds

and

Northrup

Company,

Philadelphia,

Pa.

174

Ballard,

40-i

Leinfelder,

200 ’

and

TEMPERATURE Co 300 400 500 I I

J. Prosthet. Augnt,

Taylor

600

Dent. 1975

700 I

Le z

BEAUTY CAST I HIGH HEAT) BEAUTY CAST (HYGROSC~PIC) HFG RRR

30

f z 3 y 20 5 IL IO

CERAMI GOLD

0I 800 400 600 TEMP. INVESTMENT

1000 F”

1200 TIME-

MIN. AT BURN-OUT

investment Fig. 3. Gas permeability of standard (unmodified) (kfl :) during investment heating and (right) at burn-out.

TEMP INVESTMENT C’ 200 300 400 500

400 TEMP

600 600 1000 INVESTMENT F*

600

TEMP.

materials.

Mean

flow rates

700

1200

Fig. 4. Effect of polymer additives on permeability

5 I5 TIME-HIN

25 AT 1300’

35 F

of a phosphate-bonded

45

investment.

values for this group of investments ranged from 16 to 32 C.C. per minute, while the phosphate-bonded investments exhibited flow values ranging between 0 and 23 C.C. per minute. When used as a high-heat investment, Beauty Cast (Fig. 3) was found to be the most permeable of the unmodified investments. Two of the three phosphatebonded investments proved to possess little or no permeability at the pressures employed in this study. Modified investments. As seen in Table II, the polymeric additive was found to be effective in increasing the permeability of all of the investments tested. The greatest increase was exhibited by Beauty Cast when used with the hygroscopic technique. Flow rates exceeding maximum calibration on the flowmeter (103 C.C. per minute) were noted in some gypsum-bonded samples containing 5 per cent polymer and in all such sarnples containing 10 per cent polymer. The change effected when similar amounts of acrylic resin polymer were added to the same investment was appreciably less when a high-heat technique was employed. Additions of acrylic resin polymer were not as consistently effective in increasing the permeability of phosphate-bonded investments as they were in increasing that

Permeability Table

II. Gas permeability

Gypsum bonded Beauty Cast (hygroscopic)

Beauty Cast (high-heat)

R&R hygroscopic

Phosphate bonded HFG (high-fusing gold)

Biovest

Ceramigold

insertion

+After

45 minutes

$Flow

increase

in

Flow

investments

Flow burn-out temperaturef (c.c.lmin.)

began*

Min.

F.’

I1 12 7 14 11 11 11 16 13 13 13 15

475 515 367 637 445 476 495 440 603 740 559 724

28 26 86 103+ 32 31 37 43 16 20 37 45

N.A. N.A. 59 76+ N.A. N.A. 5.5 11.5 N.A. N.A. 19 27

Standard Standard 5% 10% Standard Standard 5% 10% Standard Standard 5% 10%

15 18 20 17 -

400 430 390 525 1300 928 740 960 738

23 20 43 58 0 0 0 10 3 0 7 40

N.A. N.A. 21.5 36.5 N.A. N.A. 0 10 N.A. N.A. 5.5 38.5

62 27 20 32 21

furnace

mean

at 600”

flow

F. for recommended

rate

175

Flow increase with polymer # (c.c.jmin.)

Standard Standard 5% 10% Standard Standard 5% 10% Standard Standard 5% 10%

at manufacturers’ over

of

investments

Composition, % polymer

Investment

*After bonded.

of dental

and porosity

of standard

gypsum

bonded

burn-out

and

800”

F. for

phosphate

temperature.

samples.

of the gypsum-bonded products. The addition of 5 per cent acrylic resin to HFG (high-fusing gold) and 10 per cent to Ceramigold increased the permeability to a rate similar to or greater than that of the unmodified gypsum-bonded investments. Other polymer-investment combinations produced negligible flow increases. The flow rate of a modified phosphate-bonded investment (HFG) is shown in Fig. 4. The effect of the polymer additives on internal porosity may be seen in Table III. The addition of 10 per cent polymer by weight increased the porosity of all the samples by an average of 9 per cent. DISCUSSION A meaningful discussion of the results, as related to the permeability of the unmodified investments, should be predicated upon a determination of the minimal rate of gas flow required for successful casting. Many factors, such as investment thickness, volume and surface area of the mold cavity, mass of molten metal, and casting pres-

176 Table

Ballard,

Leinfelder,

III. Volume

J. Prosthet. Dent. August, 1975

and Taylor

porosity-standard

investment

vs. polymer Porosity

Investment

Gypsum bonded Beauty Cast (hygroscopic) Beauty Cast (high-heat) R & R hygroscopic Phosphate bonded HFG (high-fusing gold) Biovest Ceramigold

Composition.

% polymer

modified

volume (%)

Standard 10% polymer Standard 10% polymer Standard 10% polymer

49.0 56.4 47.0 53.5

Standard 10% polymer Standard 10% polymer Standard 10% polymer

31.4 41.3 35.6 45.0 32.5 43.5

46.6

51.6

investment Increase with polymer (%J

1I.0 1.4 6.5

9.9 9.4 11.0

sure, would have to be considered. If, however, it can be assumed that gypsumbonded investments possess adequate permeability, some conclusions may be made in this regard. The minimum flow exhibited by any of the gypsum-bonded specimens at burnout temperature was 16 cc. per minute (0.3 C.C. per second) through a 9.5 mm. (s/s inch) investment thickness, under a pressure of 0.2 atm. Assuming that most dental castings are completed in approximately 0.1 second, as reported by Sounder9 the investment and Myers and Pfeiffer, lo 0.03 C.C. of gas would be forced through during that time at the pressure used during this investigation. Since the volume of individual castings generally ranges from 0.2 to 0.5 sq. cm. (10 dwt. per square centimeter), this flow rate would be grossly inadequate to provide for escape of the gas. Since these investments do permit the fabrication of complete castings, either the mold gas must escape by some other route or the pressure must be raised. Under actual casting conditions, mold pressures several times that used in this study will undoubtedly develop. The low permeability demonstrated by two of the phosphate-bonded investments was not unexpected. Many manufacturers and technicians have reported that some type of supplemental venting is needed to permit adequate gas release. The flow values obtained in this investigation are considerably lower than those reported by Shell and DootzF They reported an average flow of 190 C.C. per minute through a 9.5 mm. (s/s inch) thickness of gypsum-bonded investment at a pressure of only 5 cm. Hg. Although their measurements were made under conditions somewhat different from those of this study, no apparent cause could be determined for such disparity of results. The results of this investigation also differ with those of Tsutani,7 who found the phosphate-bonded investments to be more permeable than the gypsum-bonded investments. The reason for differences would be difficult to explain since the burnout temperatures used were different. With the exception of HFG, the proprietary investments which he investigated were also different.

Volume Number

31 2

Permeability

and porosity

of investments

177

The acrylic resin polymer additive was found to increase the porosity of each investment material included in this investigation. Only the 10 per cent polymer addition was evaluated in the volume porosity determinations (Table III) . However, total investment porosity might be expected to be produced both by the polymer and by any water in excess of that necessary for the setting reaction of the investment. Changes in the liquid/powder ratio actually represented a small increase in the true liquid/investment ratio. By determining the densities of the investment material (polymer and water), it was possible to calculate the percentage volume in the set investment occupied by polymer and extra water. For example, the total porosity increase for a gypsum-bonded investment was calculated to be approximately 13 per cent. Approximately five-sixths of the porosity increase may be attributed to the presence of polymer in the set investment, the remainder being produced by the extra nonreactive water required to maintain the proper consistency of mix. The 10 per cent polymer addition increased the porosity of all of the samples by an average of 9 per cent. All measured values, however, were less than might have been expected. The range in porosity increases may be attributed to density variation among proprietary samples. In addition, the volume porosity measurements were made at room temperature and, therefore, may not provide a true index of porosity existing at burn-out temperature. The flow rate through the investment remained rather constant once the furnace was stabilized at burn-out temperature (Fig. 3). Generally, as soon as the investment was held at constant temperature, no change in permeability occurred. SUMMARY

AND

CONCLUSIONS

Permeability of various gypsum- and phosphate-bonded investments was measured during conventional burn-out procedures. Porosity determinations were made on specimens cooled to room temperature after burn-out. As a group, the gypsumbonded investments were found to be more permeable than the phosphate-bonded investments. Two phosphate-bonded investments were determined to be relatively impermeable to gas flow, while another exhibited permeability comparable to that of the gypsum-bonded investments. In spite of differences in permeability, the porosity of each type of investment was nearly constant. The porosity of the phosphate-bonded investment was approximately three-fourths that of the gypsum investments. These investments were modified by the addition of varying amounts of acrylic polymer for the purpose of altering permeability. The addition of acrylic polymer increased porosity and permeability of all of the materials included in this investigation. The acrylic additives, however, had no effect on the permeability of relatively impermeable investments unless used in high concentration. The results of this investigation would tend to substantiate the need for special sprumg and venting procedures. References 1. Brumfield, R. C.: How to Vent Full Cast Crown Molds to Avoid Gas Porosity, Thermotrol Technician (J. F. Jelenko & Co.) 4: 1-2, 1950. 2. Shell, J. S.: Spruing-Theory and Practice, Thermotrol Technician (J. F. Jelenko & Co.) 14: 1-2, 1960. 3. Wetterstrom, E. T.: An Innovation in Sprue Design for Ceramco Castings, Thermotrol Technician (J. F. Jelenko & Co.) 20: 3-4, 1966.

178

Ballard,

Leinfelder,

and

Taylor

4.

J. I’msthet. Dent. August, 1975

Coleman, R. L.: Physical Properties of Dental Materials, III. Progress Report of Research on the Dental Casting Procedure, Dent. Cosmos 68: 743-764, 1926. 5. Coleman, R. L.: Physical Properties of Dental Materials, Research Paper No. 32, J. Res. Natl. Bur. Stand. 1: 867-898, 1928. 6. Shell, J. S., and Dootz, E. R.: Permeability of Investment at the Casting Temperature, J. Dent. Res. 40: 999-1003, 1961. 7. Tsutani, I.: A Study on Permeability of Investments, J. Osaka Univ. Dent. Sot. 21: 69-84, 1968. 8. Shell, J. S.: Measurement of Permeability of Investments, Precis. Metal Mold. 18: 33, 1960. Required to Cast Dental Restorations From Molten Alloy, J. Am. 9. Sounder, W.: Time Dent. Assoc. 20: 1010-1114, 1933. Conditions on the Time Required to 10. Myers, R. E., and Pfeiffer, K. R.: Effect of Varying Cast Gold Under Air Pressure, J. Am. Dent. Assoc. 27: 530-549, 1940. DR. BALLARD DENTAL OFFICER U.S.S. JOHN F. KENNEDY F.P.O. NEW YORK 09501

(CVA

DRS. LEINFELDER AND TAYLOR DENTAL MATERIALS SECTION DENTAL RESEARCH CENTER UNIVERSITY OF NORTH CAROLINA CHAPEL HILL, N. C. 27514

67)