When Spruing Partials... Go with the Flow!
Proper spruing techniques are a major factor in eliminating miscasts, porosity, and short castings when fabricating partial frameworks. Instead of adjusting burn-out and casting temperatures when such problems occur, remember the following spruing guidelines:
- Sprue to the thickest area of the casting. Alloy should flow from the thickest to the thinnest area of the framework. Spruing to thinner areas may cause the alloy to freeze and stop flowing to the remainder of the restoration resulting in a miscast.
- Connect the sprue so the flow of alloy travels in the same direction as the design. If the sprue is placed at a right angle to the design, the result is the alloy displacing the refractory material and moving the investment with the alloy which creates porosity. An example is like a garden hose spraying directly onto the ground and digging up the soil.
- Two to three larger sprues are adequate for most castings. If auxiliary sprues are required for more remote areas of the framework, then they should always be smaller in diameter. The smaller diameter allows a proper alloy flow speed while maintaining accurate pressure.
- When a full tooth design is being cast, place a reservoir in the sprue (equal in size to the tooth) 4 to 5 mm away.
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