Describe using illustrations thedifferences when spruing differentdental appliances and when usingdifferent dental alloys

Description: A sprue is a channel through which molten alloy passes to reach the mould formed by a wax pattern. They must be carefully positioned in order to be effective. A sprue used for fixed prosthodontic restorations serves three purposes:
1. 1. It serves as a structure to aid the positioning of the wax pattern correctly within the investment ring during investing.
2. 2.It serves as a channel through which molten wax can escape during the burn-out stage.
3. 3. It is a channel through which molten alloy travels to reach the void formed by the eliminated wax pattern.
Sprue position: Location and geometry of the sprue depends upon recommendations of the alloy manufacturer.
1. Ticonium- is cast using a single sprue that approachs through the refractory cast (through the base of model).
2. Type IV gold and chrome-cobalt alloys- are cast using a sprue that approaches from above and gives rise to a number of smaller, accessory sprues.
3. Denture & Partial Dentures- The sprue should be attached to the thickest part of the wax pattern. The thicker sections of a partial denture wax pattern, such as a major connector (e.g a lingual bar), are ideal. If the sprue is attached to a thin section, this area may solidify before the metal can spread throughout the mould causing porosity or a casting which is under extended (short).
4. Gold crown- sprue to thicker sections such as cusp tips, are ideal.
Sprue thickness: Size of sprue is very important, sprues must not contain constrictions that would cause the molten metal to flow from a thick to a thin area and then back to a thick area. The turbulence set up by this type of sprue often results in internal mold deformation and castings that contain unwanted inclusions.
1. Chrome cobalt- much more sluggish when molten than other dental alloys and so larger 4 mm diameter sprues are often used.
2. Gold alloys- up to a 3mm sprue diameter is sufficient when used in conjunction with a reservoir. This is because gold alloys are generally higher in density and lower solidification temperatures, and fill the mould at a higher speed and solidify before any defects can develop.
  1. Reservoirs- They should be positioned in the hottest part of the mould (i.e. the centre) and around 2mm from the wax pattern. Any contraction porosity should then occur in the reservoir and not in the casting.
    1. Reservoirs are most predominantly used for gold castings, because on Co/Cr castings the thick sprues themselves take the place of the reservoir as long as the sprue is tapered at the point of attachment.
Sprue Design: It is also advisable to taper or flatten then flare end of the sprue so the alloy enters the mould at speed and disperses.
1. Cobalt Chrome- Due to the significant contraction of Cobalt Chrome (Co/Cr) alloys, there is a need for multiple sprues on complex designs. Therefore, the sprue angle of attachment is complicated by the need for curved sprues versus the limited space in many dental casting rings. Special care must be taken to round all sprue connections, sharp edges in the refractory material can break off as the molten alloy enters the mold. These fragments can be carried deep within the mold and may ruin the casting.
2. Dentures/Partial Dentures: Auxiliary sprues- Any area that is separated from the bulk of the framework by a long span of latticework or meshwork requires a secondary sprue. The secondary sprue is a large-diameter wax cylinder that emerges from the main sprue to ensure molten alloy reaches the remote part. They are also used to support heavy metal pontics where the molten metal might be required to flow from the thinner areas into the thicker metal pontics. They should be one third to one forth the diameter of the main sprue and should be gently curved.

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Describe using illustrations thedifferences when spruing differentdental appliances and when usingdifferent dental alloys

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