Casting procedures
Fabrication of metallic restorations is a challenge and this section covers the step by step procedures involved in making such restorations, the materials used, procedures involved and the possible defects and trouble shoots during fabrication.
1. What are die spacers?
Die spacers are resins that are applied in several coats within 0.5 mm of the finish line to provide relief for the cement luting agent and ensure complete seating of precisely fitting casting.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn, 415
2. What is the use of sprue formers?
A sprue former is used to provide a channel through which the molten alloy can reach the mold in an investing ring after eliminating the wax.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn, 417
3. What are the general guidelines to select a sprue former?
- The sprue former should approximate the size of the wax pattern
- The sprue former should be attached to the largest cross sectional area in the pattern
- Enough length of the sprue former is necessary
- Pattern of spruing either direct or indirect
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn, 418
4. Why the sprue former is directed away from the thin parts of the pattern?
It is because; the molten metal may abrade or fracture the investment in that area resulting in a failure.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn, 419
5. What is Hot Spot porosity?
When the sprue is attached at right angles to a broad flat surface, the metal that enters impinges the mold surface at this point and creates a hot spot. This in turn can produce suck back porosity. After the casting is solidified, a localized lingering of molten metal is produced.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn, 419
6. What is Gas inclusion porosity?
The gas inclusion porosity is related to the inclusion of gas during solidification. In a molten state many metals occlude gases. When solidified, these occluded gases are expelled out and a porosity results. Severely contaminated castings are usually black when removed from the investment and does not clean easily on pickling.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn, 440