Crown and Bridge
Posted Apr 28, 2011 in Technical
Q:Is it possible to cast a three-unit bridge using an #8 gauge runner bar on the pontic without having porosity or a miscast?
A: An #8 gauge sprue will only work if the thickest part of the bridge is not as thick as the sprue, according to Patrick Naylor, DDS, MPH, MS. In his book, Metal Ceramic Technology (Quintessence Publishing), Naylor details The Fourth Law of Casting: "A reservoir must have sufficient molten alloy to accommodate the shrinkage that occurs within the restorations. Alloy that fills the restorations will solidify first. As that molten alloy solidifies, it shrinks and creates a vacuum. For a complete casting, the vacuum must be able to draw additional metal from an adjacent source--the reservoir. A runner (connector) bar can be an effective reservoir if it is equal to or greater than the thickest cross-sectional area of the wax pattern. The penalties for not obeying this law are: shrinkage porosity and/or suck-back porosity." When in doubt of which is thicker--the...
