Dental Casting Techniques Quiz

Dental Casting Investment

• The lost wax casting technique dates back to ancient times, converting wax patterns into metal castings.
• Prefabricated plastic and wax sprues are preferred over metal sprues because the material is eliminated during the heating cycle.

Sprue Functions and Requirements

• Function 1: Allows molten wax to escape from the mold.
• Function 2: Enables molten metal to flow into the mold with minimal turbulence.
• Sprue Types: Wax, Plastic, Metal
• Sprue Diameter: A 2.5mm (12 gauge) is recommended for molar metal-ceramic patterns, while a 2.0mm (10 gauge) is recommended for premolar casting and most partial coverage restorations.
• Sprue Location: Attachment point should be on the bulkiest, non-critical part of the wax pattern. It shouldn't be restricted or have a neck to avoid porosity. Optimal placement avoids occlusal and proximal contact areas.

Attachment Considerations

• Avoid restrictions or neckings at the sprue attachment point to prevent porosity.
• Widening the attachment will lead to Shrink Spot Porosity.

Venting

• Vents aid in gas escape during casting and act as heat sinks.

Casting Ring and Liner

• The casting ring holds the investment in place and prevents mold expansion.
• Wax patterns are sprayed with a wetting agent (debubblizer) to decrease surface tension and avoid air bubble formation. Excessive wetting agent leads to a rough surface.

Ringless Investment Technique

• The ringless technique uses paper or plastic instead of a traditional casting ring.
• This method allows for unrestricted expansion, particularly useful with higher-melting alloys that shrink more due to their larger cooling trajectories.

Sprue Technique

• Step 1: Attach the wax sprue to the largest non-functional cusp of the wax pattern.
• Step 2: Add wax to the attachment point and smooth it.
• Step 3: Remove the pattern from the die.
• Step 4: Using forceps, insert the sprue into the crucible former hole.
• Step 5: Line the casting ring with the open end flush to the investment and moisten the liner.
• Step 6: Position the ring over the pattern, ensuring it extends at least 6mm below the pattern.

Procedure for Multiple Casting

• When casting multiple units, each unit attaches to a runner bar.
• A single sprue can feed the runner bar, or each unit can receive its own sprue.

Investment Materials

• Investment materials form the dental cast molds.
• They consist of refractory materials like silica and binders that provide strength.
• Classification by Binder: Gypsum Bonded, Phosphate Bonded, Silica Bonded

Differences Between Gypsum and Phosphate Bonded Investments

Selection of Materials

• Factors that increase expansion in gypsum-bonded investments: full-width ring liners, prolonged spatulation, storage at 100% humidity, lower water/powder ratio, and use of two ring liners.
• Factors that increase expansion in phosphate-bonded investments: heat from setting reaction softens the wax, free setting expansion, high temp. restricts shrinkage, and powder mixed with colloidal silica reduces surface roughness, and increases expansion.

Investing and Wax Elimination

• Investing technique: the process of applying the investment material to the pattern.
• Wax elimination: The process of removing the wax pattern to create a mold.
• Aims of wax elimination: Complete wax removal, carbon vapor elimination, sufficient thermal expansion, and investment heating for casting.
• Burnout Temperature: Gypsum bonded investments use 650-687°C (gold alloy). Phosphate-bonded use up to 834°C (base metal alloys). A gradual temperature increase prevents cracking.

Casting Procedure

• Step 1: Preheat the crucible.
• Step 2: Melt the alloy. Casting ring removed from the furnace and placed in the cradle.
• Step 3: Use tongs to place the crucible onto the casting ring.
• Step 4: Align the crucible orifice with the sprue.
• Step 5: Continue heating until melting is complete.
• Step 6: Pull the casting arm forward until the pin drops.
• Step 7: The melt (seconds before casting).
• Step 8: Centrifugal force carries the molten metal into the mold cavity.

Causes of Cast Failure

• Evaluation: The casting is evaluated under magnification to ensure the inner surface is correctly fitted. Proper fitting is crucial.
• Defects: Poor investment and casting procedures can lead to casting failures.
• Potential problems: Rough casting, nodules, multiple nodules, nodules on the occlusal surface, fins, incomplete casting, shiny, rounded defect, "suck-back" porosity, inclusion porosity, and marginal discrepancy. These are visualized to identify the cause of failure. Inadequate or excessive expansion results in some of these failures.