CASTING: casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy. The casting process is used to make dental restorations such as inlays, onlays, crowns, bridges and removable partial dentures.

Objectives of casting

1) To heat the alloy as quickly as possible to a completely molten condition.
2) To prevent oxidation by heating the metal with awell adjusted torch .
3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold.

STEPS IN MAKING A CAST RESTORATION
1. TOOTH PREPARATION
2. IMPRESSION
3. DIE PREPARATION
4. WAX PATTERN FABRICATION
5. SPRUING

Principles of cutting, polishing, and surface cleaning

• Surface mechanics for materials

Cutting-requires highest possible hardness materials to produce cutting

Finishing-requires highest possible hardness materials to produce finishing, except at margins of restorations where tooth structure may be inadvertently affected

Polishing- requires materials with Mohs ./ hardness that is 1 to 2 units above that of substrate

 Debriding-requires materials with Mohs hardness that is less than or equal to that of substrate to prevent scratching

•    Factors affecting cutting, polishing. and surface cleaning
  • Applied pressure
  • Particle size of abrasive
  •  Hardness of abrasive
  •  Hardness of substrate
•      Precautions
  • During cutting heat will build up and change the mechanical behavior of the substrate from brittle to ductile and encourage smearing
  • Instruments may transfer debris onto the cut surface from their own surfaces during cutting, polishing, or cleaning operations (this is important for cleaning implant surfaces)

CLEANING AND PICKLING ALLOYS

The surface oxidation or other contamination of dental alloys is a troublesome occurrence. The oxidation of base metals in most alloys can be kept to a minimum or avoided by using a properly adjusted method of heating the alloy and a suitable amount of flux when melting the alloy . Despite these precautions, as the hot metal enters the mold, certain alloys tend to become contaminated on the surface by combining with the hot mold gases, reacting with investment ingredients, or physically including mold particles in the metal surface. The surface of most cast, soldered, or otherwise heated metal dental appliances is cleaned by warming the structure in suitable solutions, mechanical polishing, or other treatment of the alloy to restore the normal surface condition.

Surface tarnish or oxidation can be removed by the process of pickling. Castings of noble or high-noble metal may be cleaned in this manner by warming them in a 50% sulfuric acid and water solution . . After casting, the alloy (with sprue attached) is placed into the warmed pickling solution for a few seconds. The pickling solution will reduce oxides that have formed during casting. However, pickling will not eliminate a dark color caused by carbon deposition 

The effect of the solution can be seen by comparing the submerged surfaces to those that have still not contacted the solution. the ordinary inorganic acid solutions and do not release poisonous gases on boiling (as sulfuric acid does). In either case, the casting to be cleaned is placed in a suitable porcelain beaker with the pickling solution and warmed gently, but short of the boiling point. After a few moments of heating, the alloy surface normally becomes bright as the oxides are reduced. When the heating is completed, the acid may be poured from the beaker into the original storage container and the casting is thoroughly rinsed with water. Periodically, the pickling solution should be replaced with fresh solution to avoid excessive contamination.

Precautions to be taken while pickling

With the diversity of compositions of casting alloys available today, it is prudent to follow the manufacturer's instructions for pickling precisely, as all pickling solutions may not be compatible with all alloys. Furthermore, the practice of dropping a red-hot casting into the pickling solution should beavoided. This practice may alter the phase structure of the alloy or warp thin castings, and splashing acid may be dangerous to the operator. Finally, steel or stainless steel tweezers should not be used to remove castings from the pickling solutions. The pickling solution may dissolve the tweezers and plate the component metals onto the casting. Rubber-coated or Teflon tweezers are recommended for this purpose.

Impression Material

Mechanical properties

1.  Resolution of forces

Uniaxial (one-dimensional) forces-compression, tension, and shear

Complex forces-torsion, flexion. And diametral

2. Normalization of forces and deformatations

Stress

 Applied force (or material’s resistance to force) per unit area

Stress-force/area (MN/m²)

Strain

Change in length per unit of length because of force

Strain-(L- L_o)/(L_o); dimensionless units

3. Stress-strain diagrams

Plot of stress (vertical) versus strain (horizontal)

• Allows convenient comparison of materials
• Different curves for compression, tension, and shear
• Curves depend on rate of testing and temperature

4. Analysis of curves

• Elastic behavior
  • Initial response to stress is elastic strain
  • Elastic modulus-slope of first part of curve and represents stiffness of material or the resistance to deformation under force
  • Elastic limit (proportional limit)- stress above which the material no longer behaves totally elastically
  • Yield strength-stress that is an estimate of the elastic limit at 0.002 permanent strain
  • Hardness-value on a relative scale that estimates the elastic limit in terms of a material’s resistance to indentation (Knoop hardness scale, Diamond pyramid, Brinnell, Rockwell hardness scale, Shore A hardness scale, Mohs hardness scale

• Resilience-area under the stress strain curve up to the elastic limit (and it estimates the total elastic energy that can be absorbed before the onset of plastic deformation)

• Elastic and plastic behavior

• Beyond the stress level of the elastic  limit, there is a combination of elastic  and plastic strain
• Ultimate strength-highest stress  reached before fracture; the ultimate compressive strength is greater than the ultimate shear strength and the ultimate tensile strength
• Elongation (percent elongation)- percent change in length up to the point of fracture = strain x 100%

• Brittle materials-<5% elongation at fracture

• Ductile materials->5% elongation  at fracture
• Toughness-area under the stress strain  curve up to the point of fracture (it estimates the total energy absorbed up to fracture)

• Time-dependent behavior

the faster a stress is applied, the more likely a material is to store the energy elastically and not plastically

• Creep-strain relaxation
• Stress relaxation

Manipulation

1. Selection-based on strength for models, casts, or dies

2. Mixing
(1)Proportion the water and powder 
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds

3. Placement

(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression