Dental Casting Techniques Overview

Introduction to Dental Casting Techniques

• The desired accuracy of a casting is approximately 0.1%.
• Compensating for dimensional changes is crucial; shrinkage in wax and alloy equals investment expansion.

Spruing

• Before investing the wax pattern, it's attached to a sprue with a crucible former.
• Sprue types include metallic, wax, or plastic, solid, or hollow.
• Purpose of wax pattern spruing:
  • Securely fix the wax pattern for mold creation.
  • Channel wax removal during burnout.
  • Provide a channel for molten alloy entry during casting.
  • Compensate for alloy shrinkage during solidification.
• Sprue size and design:
  • Diameter must be large enough to remain open until solidification.
  • Diameter should be short enough to allow for rapid filling.
  • Large castings may require multiple sprues to reach all parts.
  • Sprue attachment must be at the pattern's bulkiest area.

Sprue Selection

• Sprues can be made from wax, metal, or plastic.
• Wax sprues are most commonly used.
• Plastic sprues are not recommended due to high flow temperatures and thermal expansion characteristics, making removal difficult.

Orientation in the Mold

• The pattern should be 6 mm from the end of the investment ring.
• Insufficient distance will cause the molten metal to break through.
• Excessive distance may lead to incomplete castings or mold fracture.
• Phosphate bonded investment uses a reduced distance of 4mm.

Lining the Ring with a Liner

• Liners are placed inside the casting ring to allow for lateral investment expansion.
• Liner functions:
  • Provide a flexible lining to the inner surface of the ring.
  • Provide thermal insulation to prevent rapid cooling and contraction of the investment.
  • Aid easy separation of the investment from the ring.
  • Allow for hygroscopic expansion in the presence of water.

Casting Ring Liner Types

• Ceramic aluminous silicate.
• Cellulose paper.
• Ceramic cellulose combinations.
• Asbestos sheet (no longer used due to being carcinogenic).

Mixing the Investment

• Accurate water-powder ratio is mixed under vacuum.
• The wax pattern is painted with the mix using a brush.
• The casting ring is applied to the crucible former and filled under vibration.
• The ring holds the investment, restricting mold expansion during setting.
• The mix is first hand-spatulated, then the crucible former and pattern are placed in the bowl.
• The mixing bowl is attached to the vacuum hose.
• The bowl is inverted, and the ring is filled under vacuum and vibration.

Advantage of Vacuum Mixing

• Reduced air bubbles and porosity in the investment.
• Increased investment strength.
• Smoother cast surface with less nodules.
• Better casting fits.

Filling the Ring

• A vibrator is often used to aid in investment material flow.
• Investment is added until the ring is full.

Wax Burnout

• The wax pattern is removed by burning off in a furnace.
• The type of investment material is dependent on the alloy to be cast.
• The proper heating temperature of the investment mold is critical to control thermal expansion.

Alloy and Mold Temperature

• Heating rate of the investment should allow gas liberation without cracking the mold.
• Proper alloy and mold temperature balance is crucial for complete, accurate castings.
• The alloy temperature should be high enough for complete melting and entering the mold, but not too hot to oxidize or damage the mold walls.
• Mold temperature must be high enough for complete expansion to avoid delayed crystallization.

Casting Machines

• Casting machines cause molten alloy to fill the investment mold cavity.
• Alloy melting is accomplished through various methods, including:
  • Gas-air torch
  • Oxyacetylene torch
  • Electrical induction heating
  • Electrical resistance melting
• The alloy can be driven into the mold by:
  • Gravity
  • Air pressure
  • Steam pressure
  • Centrifugal force.

Finishing & Polishing (Noble Alloys)

• After casting, the ring is removed and the casting quenched in water.
• Quenching provides advantages:
  • Creates a soft condition for burnishing and polishing.
  • Creates a more easily cleaned casting due to an aggressive reaction in the investment.
• The casting's surface may require pickling (heating in acid such as 50% HCl, or sulfuric acid).
• Base metal alloys do not require pickling, instead a bench cooling process.

Casting Defects

• Finning and bubbling: Occur when the investment is heated too rapidly. This causes cracks, and molten alloy flows into them, creating fins.
• Incomplete casting:
  • Problems with melting or low mold temperature.
  • Insufficient push force, causing incomplete filling.
• Porosity: Surface pitting or internal holes, caused by dissolved gases, or the inability of gases to escape from the mold.
• Oversized/Undersized casting: The casting might be too large or too small due to insufficient compensation for expansion or shrinkage.

Porosity

• Gaseous porosity: Caused by gases dissolved in the alloy. Solutions for reduction include using an inert gas atmosphere during casting or avoiding overheating.
• Back pressure effect: Results from the inability of gases to escape the mold, and causes rounding of the cast edges. Solutions include making the investment thin, and adding vents to facilitate gas escape.

Undersized Casting

• Insufficient compensation for casting shrinkage is a potential issue.
• Proper heating of the investment and mold is crucial to address this.

Additional Information

• The investment ring can be set in air or in a water bath.
• Liners are placed inside the ring for expansion, insulation, thermal properties, and easy removal from the mold cavity.
• Wax removal (burnout) involves heating the investment mold in a furnace until the wax is completely removed.
• The ring is positioned with the sprue hole facing downward to allow molten wax to escape.
• A casting machine forces the molten alloy into the mold cavity.
• The completed casting is then detached from the investment, finished and polished.