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

Firstly, a sprue is a channel that is created to give passage for molten alloy to flow through, filling a mould formed by a wax pattern. For them to be effective they must be positioned carefully. They can either be direct sprued from the casting crucible to the pattern used mostly for single units (crowns, onlays, inlays etc.) Or indirect which has a connector or reservoir bar, positioned between the pattern and crucible former. Commonly used for multiple single units or partial dentures.

The alloy Ticonium is cast using a single sprue approaching from underneath the refractory cast.

Both type IV gold and chrome-cobalt alloys are cast using a sprue from above.

Co-Cr partial denture castings have multiple sprues, a main sprue generally 4mm in diameter attaches to the thickest part of the pattern e.g. major connector. Areas separated by a long span of latticework or meshwork have auxiliary sprues attached to disperse the flow of molten metal reaching these areas further away from the main sprue. They are slightly smaller in diameter than the main sprue curving down from the entrance to the pattern. Molten Co-Cr is sluggish to flow which is why a large diameter sprue is used compared to single units i.e.. Crowns. These also in turn act as a reservoir. Without the additional sprues miscasts will occur as Co-Cr contracts at a significant rate.

Spruing gold, like Co-Cr, is sprued to the thickest part of the pattern often the cusp tips. 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. Also, if the metal if forced to travel between thin-thick-thin areas it may cause a turbulence causing internal deformations. Smaller sprues are used with single units unlike denture framework. A 3mm sprue is enough when used together with a reservoir. These are a bulbous area built up in wax at double the size of the thickest part of the cast, they should be positioned at the hottest part of the mould (central) around 3mm from the wax pattern itself. They are used so that any contraction porosity should then occur in the reservoir and not in the casting. 

Unlike chrome, which is sluggish to flow, gold is high in density flowing at a higher speed and solidifies quicker meaning it is less likely for defects to occur therefore a smaller sprue can be used.

For all types of castings and materials sprue connections should be rounded. If sharp edges are left in the pattern the refractory material may break off as the molten metal enters, carrying the fragments deep into the casting.