Precision Dental Casting: Sprue and Venting Techniques

Fountains, Sprues, and Jitos in Dental Casting

Fountains (sprues) or jitos allow communication between the wax pattern and the crucible. They provide the path that moves the molten metal to fill the void left by the lost wax after it is burned inside the investment coating. There are three basic requirements:

1. The sprue should allow the molten wax to escape the mold.
2. It should allow the molten metal to flow inside the mold with minimum turbulence.
3. The metal should continue to stay molten for a time slightly longer than the alloy that has filled the mold.

Sprues (drinkers) can be metallic, wax, or plastic. The attachment point (lid) can be placed on the most voluminous part of the wax pattern, separated from the margins. Normally, these are the largest non-functional cusps (buccal and lingual). The insertion point should allow the molten metal to reach all sectors of the mold without having to flow in the opposite direction to the strength of the casting. The angle of placement of the fountain should avoid right angles, very sharp edges, or steep angles.

Factors Influencing Cylinder Placement

1. Cylinder Heat Center: Proper positioning relative to the thermal center.
2. Sprue Thickness: This allows the filling of the mold as quickly as possible. The solidification (salinization) time of the sprue can be increased for the parts to be fed (put food) by using a thick bar.
3. Shrinkage Reservoir (Chamber of Sink Marks): This allows the metal casting to cool by drawing metal to ensure a heavy cast without pores. To be effective, it should be placed at a distance equal to or less than 1 mm from the surface of the wax.
4. Sprue Length: The ideal distance is 6 mm and should not exceed 1 cm. This allows for the escape of gases that are in the mold when the molten metal enters.

Auxiliary Sprues for Precision

Auxiliary sprues are used for very thin castings to ensure quality. In labial veneers of reduced thickness, pores can be produced due to a lack of metal filling. In these cases, place a wax bar 1 mm thick extending from the cone formation to the area considered problematic.

Venting Systems for Gas Escape

Vents have the mission of facilitating the exit of gas from the mold to allow for better casting levels and very thin margins.

• Open vents: These are placed on the surface of the wax pattern, extending (brewing up) outside the crucible near the cylinder edge. They are made with a 0.8 mm wax bar.
• Closed vents: These bars are made with wax 2-3 mm thick and are glued to the edge of the cylinder in the direction of the wax pattern, without connecting, leaving a gap of about 0.6 mm.