Technical considerations

Observation:    Voids in casting or incomplete casting.
Possible Solutions:    

1. Do not wax to less than 0.2 mm.

2. For bridgework use indirect spruing. See Spruing Tips.

3. Ensure all of the alloy is melted before casting.

4. Review the burnout program. A cold mold will prevent the alloy from completely filling the mold. A cold mold may result from too low a temperature or too short a time at temperature.

Observation:    Castings distorted after divesting.
Possible Solutions:    

1. Rough handling will easily distort the casting. Do not beat/hammer on the rings or buttons.

2. Using a high blasting pressure to remove investment may distort the casting. . Blast investment from the button and sprues with 50µ aluminum oxide at a pressure of 20 psi (1.5 bar). Remove the last traces of investment by using a stripping solution in the ultrasonic.

Observation:    Alloy moves or smears when grinding.
Possible Solutions:    

Use only crosscut carbide burs to prepare alloy surface. Do not use stones. Grind with a slow speed and light pressure.

Observation:    Castings distort after degassing or porcelain firing.
Possible Solutions:    

1. Metal should be no thinner than 0.2mm.

2. Do not use ceramic cone shaped pins. Use quick cool trays with thin metal pins.

3. Calibrate furnace. Degas no higher than 1800°F (790°C).

Observation:    Bubbles after porcelain firing.
Possible Solutions:    

1. Using a high blasting pressure to remove investment may force the media into the casting. Blast investment from the button and sprues with 50µ aluminum oxide at a pressure of 20 psi (1.5 bar). Remove the last traces of investment by using a stripping solution in the ultrasonic.

2. Use only crosscut carbide burs to prepare alloy surface. Do not use stones. Grind with a slow speed and light pressure.

3. Do not steam clean. Do not use alcohol, or any other cleansers or solvents. Clean alloy only with distilled water in the ultrasonic.

4. Do not sandblast surface after finishing.

Observation:    Porcelain fractures or cracks.
Possible Solutions:    

1. Review the design for unsupported porcelain. Design for 1.5 to 2.0 mm porcelain thickness and no less than 0.2 mm alloy thickness

2. Block out undercuts on dies.

3. Spreading hemostats can flex the metal causing the porcelain to fracture.

4. Under fired opaque will cause subsequent porcelain build-ups to crack. Review the porcelain firing programs. Paste opaque must be thoroughly dried before being put in the furnace. It is best to dry the work on a hot plate rather than at the mouth of the oven.

5. For bridges use the hollow pontic design. Solid pontics may induce porcelain fractures. Incorporate a 18 or 20 ga. wax wire into pontic design. Grasp with hemostats to draw heat out of pontic.

6. Avoid using ceramic conical sagger pins.

Observation:    Porcelain fractures when casting is seated.
Possible Solutions:    

The porcelain will fracture whenever the alloy flexes. Warn the dentist about a compromised path of insertion or undercut problems.

Observation:    Internal surface of copings lack a rich gold color after glazing.
Possible Solutions:    

Blast with 50µ aluminum oxide at less than 20 psi (1.5 bar) air pressure to remove any adhering porcelain and remove oxide. Subsequent glaze procedures will not darken internal surface.

Spruing Tips

1.Use reservoir sprues or indirect spruing

2. Overall spruce length should be long enough to assure investment is no thicker than _" (6mm) over the top of the pattern.

3. Spruce length from reservoir/runner bar should be no longer than 1/8" (3mm)

4. Do not place a runner bar directly on a sprue former.

5.Orient patterns to take advantage of centrifugal force: orient thin surfaces to the trailing edge.