Internal volumetric expansion of casting investments

Internal volumetric investments

expansion

of casting

F. A. Marsaw, D.D.S.,* W. G. de Rijk, D.D.S., Ph.D.,** R. A. Hesby, D.D.S., M.S.D.,*** R. W. Hinman, D.D.S., M.S.,**** and G. B. Pelleu, Jr., Ph.D.***** Naval Dental School, Naval Medical Command, National Capital Region, Bethesda, Md., and National Bureau of

Standards,Washington, D.C.

T

he substitution of base metal alloys for noble metals in fixed prosthodontics has placed greater demands on dentists in the selection of casting materials and techniques. A common problem with base metal alloys is undersized castings, a result of the greater thermal contraction from higher solidification temperatures than occurs with noble metals.“3 The higher fusing temperatures of these alloys require the use of a phosphate-bonded investment instead of the conventional gypsum-bonded investment3 In casting dental alloys, the setting and thermal expansion of the investment compensate for shrinkage of the metal during solidification and cooling.3 The linear expansion required for these alloys varies

The opinions or assertions contained herein are the private ones of the writers and are not to be construed as offtcial or as reflecting the views of the Department of the Navy, the National Bureau of Standards, or the American Dental Association. Certain commercial materials and equipment are identified in this paper to specify the experimental procedure. In no instance does such identification imply recommendation or endorsement by the National Bureau of Standards or the ADA Health Foundation or that the material or equipment is necessarily the best available for the purpose. Recipient of Honorable Mention, John J. Sharry Prosthodontist Research Award Competition. American College of Prosthodontists, 1983. This study was supported by the Bureau of Medicine and Surgery under the Naval Medical Research and Development Command Research ‘Task No. M0095-003-3014 and by the American Dental Association Health Foundation Research Unit, National Bureau of Standards, Washington, D.C., under National Institute of Dental Research Grant No. DE 05353. *Lieutenant Commander (DC) USN; resident, Prosthodontics Department, Naval Dental Clinic; currently, Dental Department, USS Nirnitz. **ADA Research Associate, American Dental Association Health Foundation Research Unit, National Bureau of Standards, Washington, D.C. ***Captain (DC) USN; Chairman, Prosthodontics Department, Naval Dental Clinic. ****Captain (DC) USN; staff, Prosthodontics Department, Naval Dental Clinic. *****Chairman, Research Department, Naval Dental Clinic. THE JOURNAL

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between 1.4% for gold alloys and 2.5% for base metal alloys43 5 Jones and Wilson6 have demonstrated that investment setting expansion is less consistent than thermal expansion. A better understanding of setting expansion would be beneficial in the development of new techniques to produce satisfactory castings.’ Past studies on setting expansion used dial gauges, photography, and displacement transducers to record setting expansion measurements on a core of investment material. To date, only one study has been reported on measurements in the center of the mold where the wax pattern is located.* Internal measurements seem appropriate, because this is the area that determines the size and fit of the casting. The purpose of this study was to develop a technique to evaluate setting expansion in the pattern region of an investment mold. The technique should exert a minimal restraining effect on the behavior of the investment. MATERIAL

AND

METHODS

Two types of casting rings were used: a four-way split ring and a conventional rigid metal ring (WhipMix Corp., Louisville, KY.). The rings measured 42 X 32 mm. The four-way split rubber ring was made from rubber tubing cut to dimension on a lathe and then split into four longitudinal sections designed according to the technique of Menis et al.’ Each ring was lined end-to-end with a single layer of ceramic asbestos substitute (Kaoliner, Dentsply International, Inc., York, Pa.). Each ring was closed at the bottom with a plastic base. Ceramigold (Whip-Mix Corp.), Complete (J.F. Jelenko & Co., New Rochelle, N.Y.), and Hi-Temp (Whip-Mix Corp.) phosphate-bonded investments were used in this study. Sixty grams of investment was mixed with the undiluted special liquid according to the manufacturers’ recommendations for slow-speed vacuum mixing with a combination power mixer (Vat-U-Vestor, Whip-Mix Corp.). The feasibility of using a reservoir system to deter-

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ET AL

.Pipette plastic Filler Tube

-liner

-Metal CastingRing

Fig. 1. Schematic drawing of volumetric reservoir system

Table I. Equipment system

for volumetric

Equipment Finger

cot

Orthodontic hpette

elastic

Disposable pediatric mtravenous injection set Cyanoacrylate glue Shellac gasket compound Coloring

agent

Plastic disposable syringe Plastic tubing, intravenous citension kit

reservoir

Manufacturer Tyman-Templeton Co., Inc., Akron, Ohio Unitek, Monrovia, Calif Kimax 51 No. 37022 Kimbee, Sargent-Welch Scientific Co., Skokie, 111. Sherwood Medical Ind., Inc., St. Joseph, Mo. Red Devil, Inc., Union, N.J. Permatex Co., Inc., Kansas City, Kan. Wilton Enterprises, Inc., Chicago, 111. Sherwood Medical Ind., Inc. Travenol Laboratories, Inc., Deerfield, 111.

mine a volumetric change was demonstrated in a pilot study. The volumetric reservoir system (VRS) was made in the following manner (Fig. 1). The information on equipment is found in Table I. A medium linger cot (the reservoir) was attached with a l/8-inch orthodontic elastic to a 0.1 ml pipette 3 mm from the larger end. A 15 mm length of plastic filler tube was connected to a plastic syringe receptacle and inserted into the reservoir 7 mm from the inferior edge of the orthodontic elastic. The junction was sealed with ryanoacrylate glue and allowed to set for 24 hours. 362

Next, the junction between the pipette and the reservoir was sealed with a shellac gasket compound and cyanoacrylate glue, allowed to set for 24 hours, and resealed with cyanoacrylate glue. The system remained undisturbed for an additional 24 hours. The VRS was filled through the plastic filler tube (Fig. 1) with a 12 cm’ syringe that contained distilled water and a coloring agent. An additional piece of plastic tubing connected to a 12 cm3 syringe was attached to the opposite end of the system. This allowed the system to be placed under pressure so that the investigators could check its integrity and purge it of visible air bubbles, while control over the total amount of liquid in the system was maintained. The system was adjusted to contain 1.2 ml of water. A curved hemostat was clamped adjacent to the syringe receptacle to seal the system (Fig. 1). In the first part of the study, half the samples of each investment had a chromel-alumel thermocouple centered adjacent to the reservoir. This was attached to the pipette with adhesive tape. The thermocouple leads were connected to a potentiometric strip chart recorder with two pens and metric scales (Model 2125-M, MFE Corp., Salem, N.H.) with the reference junction in an ice point cell (TRC-III, Omega, Stamford, Conn.). The thermocouple and the strip chart recorder were calibrated to a mercury thermometer and found to be within 1” C of each other. The mixed investment was gently poured around the reservoir system centered in a metal or split rubber ring that rested on a vibrator (Syntron Co., Homer City, SEPTEMBER

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a-

INVESTMENTS

i

;

I

\

- - -)c -

COMPLETE CERAMICOLD HI-TEMP

oi ’ 1’ ’ ’ ’ ’ ’ ’ a ’ ’ ’ ’ 6 10 16 20 26 30 36 40 46 6066 66

66

76

106

126

TME (WUTES) Fig.

2. Time and temperature relationships

Pa.). If a split rubber ring was used, it was held together lightly for 7 minutes with a 17 X 10.5 mm paper strip, and then the strip was cut. This technique has been reported previously.’ Expansion of the investment was represented by a fall in the water level and contraction by a rise. Recordings were made every 5 minutes from the start of the mix for the first hour and every 15 minutes for the second hour. Temperature changes were recorded automatically in samples that contained a thermocouple. Temperature determinations were necessary to calculate the effect of the temperature on the water volume within the reservoir system. The coefficient of volumetric thermal expansion for water, (Y,varies with the temperature of the water. For each temperature Ti, an Lyiwas determined. The determination of the volume correction AV, caused by a temperature change from To (initial temperature, 22” C) to Tk, the temperature for which the volume correction is to be made, is given

for three phosphate-bonded investments.

Table II. Setting times (in minutes) Setting time Investment Ceramigold Complete Hi-Temp

Initial 5.95 2 .lO 4.58 + 20 4.54 + .24

AVk = Vk - V. = V,

i + C~,AT~ > I=1

where V, = volume at beginning of the experiment. The increments in temperature are chosen so that IT,+,-T,I= THE JOURNAL

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8.95 6.63 6.83

No.

f .I0 f .14 + .26

6 6 6

The percent of linear expansion (PLE) is derived from the observed change in volume AV+,, by PLE,, T, = -

100 3

AVo,, + Av, VO

>

Note that the minus sign (-) is used because a drop of the liquid in the capillary tube was read as a negative volume change. In addition, at the end of the experiment, T, = To yields

bY k

Final

PLE=-p

100 AV,,,, 3

v,,

The temperature correction is necessary only when expansion data are obtained during the setting process. Initial and final setting times for each investment 363

MARSAW

Table III. Volumetric comparison and split rings with and without thermocouple”

-Investment

Ring

Ceramigold

Split Metal Split Metal Split Metal

Complete H-Temp

With thermocouple Volume

23.0 2 2.6 20.3 +- 2.6 18.0 18.3 i 0.6 12.0 16.0

-. No. 3 4 2 3 2 2

of metal

Table V. Percent of linear setting expansion

Without thermocouple Volume 19.3 20.0 21.8 17.6 14.3 15.0

Investment No.

-+ 2.1 _+ 3.2 _+ 4.7 k 3.5 -1-3.8 + 3.5

3 4 5 3 3 3

*i oiumetric measurements in microliters for 120 minutes. Statistical comparisons between samples with and without thermocouple showd no significant diffrrrnce for each investment (p > .05. Sladerlt I test)

Table IV. Volumetric change for split rubber and metal castings rings* Split ring Investment Ceramigold C‘crmplrte t-11.TNt1p

Volume 21.2 t 2.9 20.7 .t 4.2 14.3 k 2.6

Metal No. 6 7 .5

Volume 20.1 i 2.7 18.5 rt 2.6 15.4 + 2.5

ring No. 6 6 5

and ring type were determined by a hand-held Vicat needle, according to the criteria established by Menis et al.- Six samples were recorded for each investment. All volumetric testing was conducted in an environmentally controlled room with a temperature of 22” t1” C and 50% humidity. The Student t test was used for a statistical analysis of the volumetric data. Linear setting expansion of each investment was also determined by American Dental Association (ADA) Specification No. 2, which is the trough method with a microscope micrometer comparator.” RESULTS The setting times for each investment are listed in Table II. Fig. 2 illustrates the temperature-time relationship for the combined split ring and metal ring population for each investment. The peak temperatures were 66.3” _+2.9” for Complete, 56.6” +- 4.9” for Ceramigold, and 49.3” + 0.8” for Hi-Temp. In each investment the initial and final set occurred before the peak temperature was reached. Each investment followed a similar pattern, with a peak temperature 364

ET AL

Trough’ (external)

Manufacturer’s data

Ceramigold Complete Hi-Temp

1.0 1.5 1.05

%

Volumetrict (internal) No.

0.75 t 0.22 0.64 rt 0.50 0.86 ? 0.63

%

No:

5 0.57 i 0.08 14 5 0.56 zk 0.10 13 5 0.39 ic 0.08 10

*Measurements made in accordance with ADA Specification i\‘f> 2 iLinear values calculated from volumetric measurements. Statistical comparisons between trough and volumetric measurements showed no significant differences for any of the three investments (p > .05, Student : tesr ,.

reached between 12 and 16 minutes followed by a gradual decline until room temperature was reestablished after 75 to 90 minutes. .4t 120 minutes, there was no significant difference (p > .05) in volume between samples that contained a thermocouple and samples without a thermocouple (Table III). There was no significant difference (p > .lO) in volume between the metal ring and the split rubber ring samples (Table IV). Therefore, the results were combined for each investment. Fig. 3 illustrates the PLE determined internally from the VRS data. After 2 hours the total linear setting expansions were 0.57 +- 0.08% for Ceramigold, 0.56 + 0.10% for Complete, and 0.39 + 0.08% for Hi-Temp. A comparison of the values for linear setting expansion from the manufacturers’ literature, the ADA trough method, and the volumetric method is presented in Table V. DISCUSSION The volumetric method provides reproducible data on the effect of setting expansion on the mold cavity volume, and it is the only technique to date that is capable of measuring setting expansion where it is most important; that is, the area of the invested wax pattern. The values determined in this study for internal linear setting expansion are not in agreement with reported values determined by external measurements. The value of externally determined linear setting expansion most commonly listed in the manufacturers’ literature is 1% (Fig. 3).“-” The different values may indicate that setting expansion occurs differently internally and externally. There is also the possibility that this variance in values is the result of the method used to make the measurements. Although the trough method has been used historically to determine setting expansion, it is difficult to obtain consistent results SEPTEMBER

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INTERNAL

EXPANSION

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INVESTMENTS

1.2 1.1 1.0 0.6 0.6

'! ?!F

EXTERNAL

.

OS7 0.6

(N=14)

0.6 0.4 0.3 0.2 0.1

L

CERAMIGOLDCOMf'lETE HI-TEMP LTT.VALUE

INVESTMENTS Fig. 3. Percent of linear expansion of three investments. Bar that illustrates percent determined externally represents value in manufacturers’ literature.

with phosphate-bonded investments, because they are inherently more fluid than gypsum-bonded investments. In Table V a large range (up to 70%) can be observed in the linear setting expansion values as measured by the trough method. This is in agreement with the findings of Jones and Wilson6 on the behavior of setting expansion. Measurements by the volumetric method show less variation (<20%). This finding is more consistent with clinical laboratory experience with castings, which normally do not show variation up to 70%. It is commonly thought that when less restriction is placed on a setting investment, there is an increase in setting expansion.6s’3 However, no significant difference in internal setting expansion was noted in a comparison of results from the nonyielding metal ring and the yielding split rubber ring. It is possible that the ring liner provided a sufficient cushion or that the restriction in two dimensions had little or no influence on the internal expansion. For an explanation of these findings, some other physical phenomena might be examined. If the investment is acting as a true solid after the initial setting time has elapsed, a cavity in the center will expand in an amount directly proportional to the expansion at the periphery. In other words, any constraint on the investment will affect the size of the cavity. In this study, the investment appears to be acting as a liquid or semisolid at the time setting expansion occurs and

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resembles the inner tube of a tire. The internal dimension of the inner tube (the hole) changes little as air pressure is increased; however, the periphery of the inner tube will enlarge significantly with increased air pressure. Restriction of the periphery, as by a metal ring, will result in limited expansion at the periphery; but the internal dimension of the inner tube will remain nearly the same as before the restriction was placed. This model correlates with our finding that there was no significant difference in internal setting expansion between the split rubber ring and the metal ring.

CONCLUSIONS The following can be concluded from these findings. 1. The volumetric system provides a method to acquire information on setting expansion at the location of the wax pattern. 2. Internally determined setting expansion values do not agree with values derived from external measurements (ADA trough method). 3. Measurements obtained by the volumetric method show less variability than those obtained by the trough method. 4. No significant difference in volume was seen between a restricted metal ring and an unrestricted split rubber ring, which suggests a semisolid behavior of the investment during the period of setting. The findings indicate a need to reevaluate the 365

MARSAW

1977. Departmenr of Health, Education, and Welfare. Pub1 NII. (NIH)77-1227, pp 186.201. ,Jones, D. W.. and W’l1 son, H. J.: Setting and hygroscopic expansion of investments. Br Dent J 129:22, 1970. Menis, D., Mahie. C.. Waterstrat, R., and Whitenton, E.: Factors aflecting a pattern invested in phosphate-bonded investment. Int ASSOLDent Res Ahstr No. 514, p 438, 1981. Asgar, K., Lawrence, W. N., and Peyton, F. A.: Further investigations into the nature of hygroscopic expansion of dental casting investment. J PROSTHET DENT 8:673, 1958. American Dental Association: Guide to Dental Materials and Devices, ed 6. Chicago, 1972. American Dental Association, pp 171-175. Ceramigold Investment. Louisville, 1979, Whip-Mix Corp Hi-Temp Investment. Louisville, 1977, Whip-Mix Corp. Complete Investment. New Rochelle. N.Y.. 1975. J. F. .Jelenko Rc co Eden, G. ‘I‘.. Franklin, 1). M., Powell, J. M.. Ohta, Y., and Dickson, C;.: Fit of porcelain fused to metal crown and bridge [astings. ,J Dent Ros X3:2360. 1970

methods by which setting expansion is measured, as well as the mechanism by which the expansion takes place. The need for further study by means of multiple VRSs or strain gauges embedded in the investment is indicated, and investigation is currently in progress. The influence of variously shaped casting rings and pattern position on resulting accuracy of the casting could be determined by this method of investigation. REFERENCES

I

ET AL

Baucr, K. W., and Eden, G. T.: NADL survey of casting alloys ,n commercial dental laboratories. J Dent Res 56(Special issue Bl. 214. 197: (Abstr No. 650). AI:rhir, (1 Watcrstrat, R., LLlenis, D., and Whitenton, E.: \chievcment of complerr base-metal castings which fit. ,J Dent Rrs 59(Special issue A). 474. 1980 (Abstr No. 474). Phillips, R. !t’ : Skinner’s Science of Dental ~latcrials, cd 8 I’hiladelph~a. 1982 W. B. Saunders Co.. pp 193-411, and 4~‘1-11(1.

4. ,Jrndresen, M. D.. and Stocks, C. I,.: Invesung procedures. In Eissmann H F., Kutld. K. D., and Morrow, R. RI.: Dental I~aboratory Procedures. Vol. I: Fixed Partial Dentures. St Louis, 1980. The C:. V Mosby Co., pp 150-l 58. > hlnhie, C Investments for chromium-based alloys In Valega. SI ., T bl.. .\lternatives to Gold 11lloys in Dentistry, Conlerencr Proceedings ~Januarv 24-26. 1977 Washington. I).(:.,

Delayed “hygroscupic” products

expansion of gypsum

Jo& Fortunate F. Santos, D.D.S., Ph.D.,* and Rafael Y. Ballester** University

of S%o Paula, Faculdade

de Odontologia,

SZo Paulo. Brazil

T

he expansion of gypsum bonded investments on immersion in water during the setting process was observed many years ago.’ This finding gave rise to the use of hygroscopic techniques for precision casting.2s’ It has also been theorized that the absorption of water would permit a less-restricted outward growth of gypsum crystals.4 According to this theory, the magnitude of the hygroscopic expansion depends on the stage of the gypsum setting reaction at which the mixture is

l’rrsrntcd at the General Session of the Intrrnational ‘Associatmn for Dtwral Research, Xew Orleans, La. supported in part by the Foundation for Assistance of Research 01 the State of SLo Pa&, Grant No. 80/1264-Z. -‘.\ssociate Professor, Department of Dental Materials. ‘*.+nior dental studrni

366

immersed in water.” This investigation was designed to verify whether the expansion would be influenced by immersion of gypsum products in different liquids, as well as to investigate the effect of immersion at times later than those normally used.

MATERIAL

AND METHODS

The commercial products used in this investigation are shown in Table I. They were mixed with water in the proportions recommended by the manufacturers (Table I). Mixing was performed by hand spatulation for 45 seconds followed by 15 seconds of mild vibration. Immediately after vibration the mixture was poured into cylindric split molds to set for 30 minutes. Specimens thus obtained were 1.9 cm in diameter and 3.8 cm in height. Glass spheres 4.8 mm in diameter were embedded in the open ends while the materials were SEPTEMBER

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