Defects in the Casting.

Investing and casting requires meticulous attention to detail to obtain a successful, properly fitting casting.

Roughness.

The surface of a casting should be smooth, although finishing and polishing are still required. Lines or grooves in the casting were usually present but overlooked in the wax pattern. They may necessitate a remake, particularly if they were positioned near the margin or on the fitting surface. Generalized casting roughness may indicate a breakdown of the investment from excessive burnout temperature.

Nodules.

Bubbles of gas trapped between the wax pattern and the investment produce nodules on the casting surface. Even minute nodules can limit the seating of the casting to a considerable degree. When they are large or situated on a margin, they usually necessitate remaking of the restoration.

When small, they can often be removed with a no.1/4 or 1/2 round bur. A binocular microscope is extremely helpful to detect and remove nodules. Remove a slight excess of metal to ensure the nodule does not interfere with complete seating.

The key to avoiding nodules is a careful investing technique, a surfactant, vacuum spatulation, and careful coating of the wax pattern with investment.

Castings made with phosphate-bonded investment are especially prone to such imperfections, and experience and care are required to produce castings that are routinely free of nodules.

Fins.

Fins are caused by cracks in the investment that have been filled with molten metal. These cracks can result from a weak mix of investment (high water-powder ratio), excessive casting force, steam generated from too-rapid heating, reheating an invested pattern, an improperly situated pattern (too close to the periphery of the casting ring), or even premature or rough handling of the ring after investing.

Incompleteness.

If area of wax is too thin (less than 0.3 mm), which occurs occasionally on the veneering surface of a metal-ceramic restoration, an incomplete casting may result. Thickening of the wax in these areas is recommended. Incomplete casting of normal-thickness wax patterns may result from inadequate heating of the metal, incomplete wax elimination, excessive cooling ("freezing") of the mold, insufficient casting force, not enough metal, or metal spillage.

Voids or Porosity.

Voids in the casting (in particular in the margin area) may be caused by debris trapped in the mold (usually a particle of the invest ment undetected before wax elimination). A wellwaxed smooth sprue helps prevent this. Porosity resulting from solidification shrinkage ("suck back") occurs if the metal in the sprue solidifies before the metal in the mold, as may happen when a sprue is too narrow, too long, or incorrectly located or when a large casting is made in the absence of a chill vent. Gases may dissolve in the molten alloy during melting and leave porosity defects. Back pressure porosity may be caused by air pressure in the mold as the molten metal enters. Its occurrence is reduced by using a more porous investment, locating the pattern near the end of the ring (6 to 8 mm), and casting with a vacuum technique.

Marginal Discrepancies.

Inaccuracies of fit at the margin can be caused by distortion during removal of the wax pattern from the die. They may also result from increased setting expansion (hygroscopic technique) following uneven expansion of the mold.

Dimensional Inaccuracies.

The casting can be either too small or too large. Attention to detail is essential for an accurately expanded mold. A standardized procedure is needed in regards to liquid-powder ratio, spatulation, the ring liner, the amount of liquid added, and mold heating.

Materials for self-control:

Materials for self-control:

1. Classification of the casting alloys.

2. Composition of base alloys.

3. Composition of casting gold alloys.

4.Technique of casting.

5. Composition of investment material.

6. Defects in the Casting.

Date: 2015-12-24; view: 1149