It is important to fully understand why an error in casting and finishing dental crowns will result in problems at the clinic fit appointment. Here we discuss errors and their solutions.
Failing to use accurate burn-out temperatures and time
If the burn-out temperature is too low and the burn-out time is too short, the investment will not experience enough thermal expansion. As a result, the casting may fit the definitive cast too tightly. If the burn-out temperature is too high, the investment will decompose and cause the casting to be pitted and rough.
Use therefore the recommended burnout temperature. Accurately set the equipment up for time and temperature, and calibrate the equipment on a regularly scheduled basis.
Improperly positioning the mould in the casting machine
If the mould is positioned in the casting machine so that the thinner sections of the framework are towards the rotating direction of the spinning machine, a negative pressure may cause some parts of the pattern to be miscast.
Take advantage of the centrifugal force. Orient thin sections away from the rotating direction of the casting arm (towards the trailing edge).
Additional centrifugal pressure will be placed on the molten metal as it enters the mould. When investing the framework, mark the position of the thin framework sections so that they can be identified easily when the mould is placed in the casting machine.
Failing to heat the metal to the specified temperature before casting
If the metal is slightly below the casting temperature, a cold melt will result, and the clasp tips and thin sections will not cast completely. If the metal is slightly overheated, the casting will be porous.
Verify that the temperature gauge reads the correct temperature and that the casting machine has been correctly set and properly calibrated.
In the absence of an expensive casting machine that measures the temperature of the metal, the operator must have the knowledge to judge when the molten metal is within the casting range.
Failing to allow the metal to cool to room temperature in the investment
Quenching the mould to speed up cooling after casting may cause serious warping of the metal casting and make the investment more difficult to remove.
Allowing the mould to cool to room temperature will result in uniform cooling and is a form of tempering the metal.
When the mould is cool enough to handle with bare hands, pick it up and tap the sides of the mould lightly with the handle of a plaster spatula. Much of the investment will break away. Then grasp the button firmly with sturdy pliers and tap on the end of the sprue button with a hammer. Most of the remaining investment will fall off easily. An air-abrasion machine with a walnut shell is recommended for the removal of the remaining investment. The oxide can be removed with 50-μm-grit aluminium oxide.
Improperly using an air-abrasion machine to clean the casting
The air pressure at the nozzle of the air-abrasion machine is about 100 psi. Thin areas of a framework, especially a thin horse-shoe major connector, can be warped easily if the framework is held too close to the nozzle.
Hold the framework approximately 5 to 7 inches away from the orifice of the nozzle so that the aluminium oxide can spread out and the force of the air can dissipate.
Maintain the general shape of the plastic pattern while shaping clasps with stones and wheels. For best results, a retentive clasp should taper uniformly, from the rest or attachment to the framework to the tip. After all, shaping has been completed, use 50 μm aluminium oxide to lightly abrade the framework. This will prepare the framework for electrical polishing and establishing a clean, oxide-free surface. Abrading should be performed carefully, especially inside the clasps, to prevent the removal of an excessive, amount of metal.
Errors When Fitting The Framework To The Cast And Mouth
Laboratory fitting the casting to the definitive cast rather than to a duplicate of the definitive cast
If the technician fits the casting to the definitive cast, the cast will be scarred and scratched. If the casting does not fit the mouth, the cast cannot be used again to remake the framework. A new impression and cast must be made.
The advantage of the dentist could be the framework that has been fitted to a duplicate of the definitive cast at the laboratory and the framework has never been on the definitive cast.
The dentist has the opportunity to evaluate the casting and the unscarred cast and to compare them to the mouth. If the casting does not fit, the dentist can decide whether to remake the impression and make a new cast or send the original definitive model back for a remake. Without the unscarred model, there is no choice but to remake the impression and make a new cast.
There are some disadvantages to this procedure. Additional costs are incurred if the laboratory makes a duplicate definitive cast. Moreover, if the duplicate cast is not accurate, the casting may fit the duplicate cast and still not fit the definitive cast or the mouth. Some laboratories may tend to excessively relieve areas of the framework where it contacts the blocked-out areas if no effort to seat it quickly and completely.