- Email: [email protected]
Practical application of current casting research
OPERATZVE
PRACTICAL
DENTISTRY
APPLICATION
OF CURRENT
CASTING
RESEARCH
WILLIAM J. O’BRIEN, B.S.* New York, N. Y. WITH GOOD FIT, a sound, dense casting is a necessity in crown and fixed partial denture work. In the past few years, considerable research has been done on the factors in technique which produce sound, porosity-free castings on a routine basis. It is the purpose of this article to review some of this information in the light of practical applications in the dental laboratory. In the three most recent studies on this subject, over 1,100 castings were made under different conditions and the results carefully noted. Considering the high costs of scientific research, the readers will certainly benefit by studying the results obtained. The original articles contain detailed classifications of the types of porosity and their causes. Here we are concerned with the practical applications of these findings. Since these studies were made on gold alloys, the results are a function of these materials. Porosity results from improperly made sprues, back pressure of air in the mold, and too high or too low casting and burnout temperatures.
A
LONG
SPRUES
It was found that the dimensions of sprues are more critical in preventing porosity than had previously been known. l Thin sprues (18 gauge) produced shrinkage porosity with different sprue lengths. The diameter of the sprue had to be larger than the thickest portion of the casting in order to prevent this porosity. For most crowns and inlays, this means that a 12- or lo-gauge sprue is needed to prevent localized shrinkage porosity. The reason for this is that the sprue channel, as the supplier of molten metal to the solidifying casting, must be the last to solidify. As molten metals pass from the liquid to the solid state, a contraction in volume takes place. If this decrease is not followed by a flow of molten metal from the sprue, pores will be left in the casting. It is well known that coffee will cool more quickly in a saucer than in a cup because the surface area is greater in the saucer. The same holds true for metal *Associate
Director
of Research,
J. F. Jelenko 558
& Company,
Inc.
Volume Number
10 3
APPLICATION
OF
CURRENT
CASTING
RESEARCH
559
castings, and the relationship of surface area to bulk of the sprue has to be adjusted so that the sprue is last to freeze. In addition to the thickness of the sprue, the sprue length was closely linked to por0sity.l The optimum sprue length was found to be 9 mm. Shorter sprues, even with the correct diameter, gave more porosity than the g-mm. sprue. The sprue should be placed at the bulkiest portion of the casting, and if thin sections separate bulky ones, sprues should be made in both heavy portions. CASTING
TEMPERATURE
The casting temperature was also found to have an ideal range.l This was found by experiment to be approximately 100” to 150” F. above the melting range of the gold. Lower casting temperatures produced microporosity and gas inclusions. Subsurface porosity was found with higher casting temperatures. In order to regulate the temperature factor, a pyrometer is necessary. BURNOUT
TEMPERATURE
AND
BACK-PRESSURE
POROSITY
Efect of Burnout Temperature on Soundness of Castings.-The temperature of the investment mold was found to influence “back-pressure porosity.“2 Backpressure porosity is usually found as pits on the inner surface of crowns. This
vent
Fig. rod.
l.--As This
gold enters is symbolized
the mold, by the little
trapped stick
air men,
escapes
through
the
investment
and
the
air
O’BRIEN
560
J. Pros. May-June,
Den. 1960
results when mold gases do not escape quickly enough as the molten alloy shoots down the sprue channel.4 In other words, the air in the mold pushes back on molten gold during the casting operation. Hygroscopic low burnout techniques produce more of this porosity.2*3 This has been attributed to a low water-powder ratio and incomplete elimination of the wax.2 Several techniques may be used to eliminate this porosity when a low burnout temperature is desired. 2-4 All of these methods aim at increasing the pressure of the molten alloy over that of the gases in the mold. This can be done by increasing the number of turns on a centrifugal nlachine,2 increasing the casting pressure on a pressure casting machine,4 performing the casting operation in a vacuum,2 flaring the sprue and its attachment to the casting, s keeping the distance from the top of the pattern to the outside of the investment to g inch, providing vents through which the mold gases can escape (Fig. 1 ), and using more metal.2 Factors in Elivhzating Back-Pressure Porosity.-The following factors provide the best conditions to eliminate this type of porosity with a low burnout temperature. It is important to note that each one of these factors works separately, so that all of them may not be necessary. 1. Three to four turns on a centrifugal casting machine increase the casting pressure.s 2. When using a pressure casting machine, 20 pounds per square inch is recommended.2 3. The distance of the wax pattern from the end of the investment should be G inc11.2 4. In order to provide a path for the mold gases to escape, a vent rod SUSpended in the center of the crown, but not touching the actual casting, has been found to be helpful2 5. Slight flaring of the sprue at its attachment to the pattern3 has been found to reduce this type of porosity. 6. Enough gold should be used so that the button obtained weighs the same as the casting proper.2 7. The mold temperature should be between 900” and 1,250“ F.l Time should be allowed in order for the mold to reach the temperature of the furnace. REFERENCES
1. Ryge, G., Kozak, S. F., and Fairhurst, C. W.: Porosities in Dental Gold’ Castings, J.A.D.A. 54 :746, 1957. 2. Strickla;ii9W. D., and Sturdevant, C. M. : Porosity in the Full Cast Crown, J.A.D.A. 58:69, 3. Asgar, K., akd Peyton, F. A.: Pits on Inner Surfaces of Cast Crowns, J. PROS. DEN. 9:448, 4. Phillips,
1959.
R. W.: Studies on the Density of Castings as Related to Their Ring, J.A.D.A. 35:329, 1947.
136 WEST 52~~. ST. NEW
YORK
19, N. Y.
Position
in the
PRACTICAL
DENTISTRY
APPLICATION
OF CURRENT
CASTING
RESEARCH
WILLIAM J. O’BRIEN, B.S.* New York, N. Y. WITH GOOD FIT, a sound, dense casting is a necessity in crown and fixed partial denture work. In the past few years, considerable research has been done on the factors in technique which produce sound, porosity-free castings on a routine basis. It is the purpose of this article to review some of this information in the light of practical applications in the dental laboratory. In the three most recent studies on this subject, over 1,100 castings were made under different conditions and the results carefully noted. Considering the high costs of scientific research, the readers will certainly benefit by studying the results obtained. The original articles contain detailed classifications of the types of porosity and their causes. Here we are concerned with the practical applications of these findings. Since these studies were made on gold alloys, the results are a function of these materials. Porosity results from improperly made sprues, back pressure of air in the mold, and too high or too low casting and burnout temperatures.
A
LONG
SPRUES
It was found that the dimensions of sprues are more critical in preventing porosity than had previously been known. l Thin sprues (18 gauge) produced shrinkage porosity with different sprue lengths. The diameter of the sprue had to be larger than the thickest portion of the casting in order to prevent this porosity. For most crowns and inlays, this means that a 12- or lo-gauge sprue is needed to prevent localized shrinkage porosity. The reason for this is that the sprue channel, as the supplier of molten metal to the solidifying casting, must be the last to solidify. As molten metals pass from the liquid to the solid state, a contraction in volume takes place. If this decrease is not followed by a flow of molten metal from the sprue, pores will be left in the casting. It is well known that coffee will cool more quickly in a saucer than in a cup because the surface area is greater in the saucer. The same holds true for metal *Associate
Director
of Research,
J. F. Jelenko 558
& Company,
Inc.
Volume Number
10 3
APPLICATION
OF
CURRENT
CASTING
RESEARCH
559
castings, and the relationship of surface area to bulk of the sprue has to be adjusted so that the sprue is last to freeze. In addition to the thickness of the sprue, the sprue length was closely linked to por0sity.l The optimum sprue length was found to be 9 mm. Shorter sprues, even with the correct diameter, gave more porosity than the g-mm. sprue. The sprue should be placed at the bulkiest portion of the casting, and if thin sections separate bulky ones, sprues should be made in both heavy portions. CASTING
TEMPERATURE
The casting temperature was also found to have an ideal range.l This was found by experiment to be approximately 100” to 150” F. above the melting range of the gold. Lower casting temperatures produced microporosity and gas inclusions. Subsurface porosity was found with higher casting temperatures. In order to regulate the temperature factor, a pyrometer is necessary. BURNOUT
TEMPERATURE
AND
BACK-PRESSURE
POROSITY
Efect of Burnout Temperature on Soundness of Castings.-The temperature of the investment mold was found to influence “back-pressure porosity.“2 Backpressure porosity is usually found as pits on the inner surface of crowns. This
vent
Fig. rod.
l.--As This
gold enters is symbolized
the mold, by the little
trapped stick
air men,
escapes
through
the
investment
and
the
air
O’BRIEN
560
J. Pros. May-June,
Den. 1960
results when mold gases do not escape quickly enough as the molten alloy shoots down the sprue channel.4 In other words, the air in the mold pushes back on molten gold during the casting operation. Hygroscopic low burnout techniques produce more of this porosity.2*3 This has been attributed to a low water-powder ratio and incomplete elimination of the wax.2 Several techniques may be used to eliminate this porosity when a low burnout temperature is desired. 2-4 All of these methods aim at increasing the pressure of the molten alloy over that of the gases in the mold. This can be done by increasing the number of turns on a centrifugal nlachine,2 increasing the casting pressure on a pressure casting machine,4 performing the casting operation in a vacuum,2 flaring the sprue and its attachment to the casting, s keeping the distance from the top of the pattern to the outside of the investment to g inch, providing vents through which the mold gases can escape (Fig. 1 ), and using more metal.2 Factors in Elivhzating Back-Pressure Porosity.-The following factors provide the best conditions to eliminate this type of porosity with a low burnout temperature. It is important to note that each one of these factors works separately, so that all of them may not be necessary. 1. Three to four turns on a centrifugal casting machine increase the casting pressure.s 2. When using a pressure casting machine, 20 pounds per square inch is recommended.2 3. The distance of the wax pattern from the end of the investment should be G inc11.2 4. In order to provide a path for the mold gases to escape, a vent rod SUSpended in the center of the crown, but not touching the actual casting, has been found to be helpful2 5. Slight flaring of the sprue at its attachment to the pattern3 has been found to reduce this type of porosity. 6. Enough gold should be used so that the button obtained weighs the same as the casting proper.2 7. The mold temperature should be between 900” and 1,250“ F.l Time should be allowed in order for the mold to reach the temperature of the furnace. REFERENCES
1. Ryge, G., Kozak, S. F., and Fairhurst, C. W.: Porosities in Dental Gold’ Castings, J.A.D.A. 54 :746, 1957. 2. Strickla;ii9W. D., and Sturdevant, C. M. : Porosity in the Full Cast Crown, J.A.D.A. 58:69, 3. Asgar, K., akd Peyton, F. A.: Pits on Inner Surfaces of Cast Crowns, J. PROS. DEN. 9:448, 4. Phillips,
1959.
R. W.: Studies on the Density of Castings as Related to Their Ring, J.A.D.A. 35:329, 1947.
136 WEST 52~~. ST. NEW
YORK
19, N. Y.
Position
in the