Finishing and Polishing

Remove oxygen-inhibited layer .Use stones or carbide burs for gross reduction.Use highly fluted carbide burs or special diamonds for fine reduction.Use aluminum oxide strips or disks for finishing. Use fine aluminum oxide finishing pastes. Microfills develop smoothest finish because of small size of filler particles
 

Casting ring

CASTING RING LINERS

Most common way to provide investment expansion is by using a liner in the casting ring .Traditionally asbestose was used .
Non asbestose ring liner used are :
1) Aluminosilicate ceramic liner .
2) Cellulose paper liner .

The aim of using a resilient liner is to

-. allow different types of investmentbexpansion (act as a cushion)
_. facilitate venting during casting procedure.
_. facilitate the removal of the investment block after casting.&. prevent the distortion by permitting the outward expansion of the mold.
The casting ring holds the investment in place during setting and restricts the expansion of the mold. Normally a resilient liner is placed inside the ring leaving about 2-3 mm from both ends to allow for supporting contact of the investment with the casting ring.

Purpose of Casting Ring Liner

Ringer liner is he most commonly used technique to provide investment expansion. To ensure uniform expansion , liner is cut to fit the inside diameter of the casting ring with no overlap. 

Non-asbestos Ring Liners: Ceramic (aluminum silicate) Cellulose (paper) Ceramic-cellulose combination Safety of the ceramic ring liners remains uncertain, because aluminum silicate also appears capable of producing hazardous-size respirable particles
 

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.

Acrylic Appliances

Use - space maintenance  or tooth movement for orthodontics and pediatric dentistry

1. Components

a. Powder-PMMA powder. peroxide initiator, and pigments

b. Liquid-MMA monomer, hydroquinone inhibitor, cross-linking agents, and chemical accelerators (N, N-dimethyl-p-toluidine)

2. Reaction

 PMMA powder makes mixture viscous for manipulation before curing . Chemical accelerators cause decomposition of benzoyl peroxide into free radicals that initiate polymerization of monomer .  New PMMA is formed into a matrix that surrounds PMMA powder. Linear shrinkage of 5% to 7% during setting. but dimensions of appliances are not critical

Tooth Polishing and Cleansing Agents

1. Cleansing-removal of exogenous stains, pellicle, materia alba, and other oral debris without causing undue abrasion to tooth structure
2. Polishing-smoothening surfaces of amalgam, composite, glass ionomers, porcelain, and other restorative materials

Factors influencing cleaning and polishing

-    Hardness of abrasive particles versus substrate
-    Particle size of abrasive particles
-    Pressure applied during procedure
-  Temperature of abrasive materials

Structure

 Composition

-contain abrasives, such as kaolinite, silicon dioxide, calcined magnesium silicate, diatomaceous silicon dioxide, pumice. Sodium-potassium
-aluminum silicate, or zirconium silicate; some pastes also may contain sodium fluoride or stannous fluoride, but they have never been shown to produce positive effects
 

Reactions-abrasion for cleansing and polishing

Properties -  Mechanical
 
- Products with pumice and quartz produce more efficient cleansing but also generate greater abrasion of enamel and dentin
-Coarse pumice is the most abrasive
-The abrasion rate of dentin is 5 to 6 times faster than the abrasion rate of enamel, regardless of the product
-Polymeric restorative materials, such as denture bases, denture teeth, composites, PMMA veneers, and composite veneers, can be easily scratched during polishing
-Do not polish cast porcelain restorations (e.g., Dicor) that are  externally characterized or the color will be lost
 

INVESTING
Mixing investment with distilled water is done according to the manufacturers ratio in a clean dry bowl without entrapment of the air into the mix.

Mixing methods:
a.    Hand mixing and the use of the vibrator to remove air bubbles.
b.    Vacuum mixing- This is the better method because it removes air bubbles as well as gases that are produced and thus produces a smoother mix.

Methods of investing:
a.    Hand investing
b.    Vacuum investing

Hand investing:

First the mixed investment is applied on all the surfaces of the pattern with a soft brush. Blow off any excess investment gently, thus leaving a thin film of investment over the pattern, then apply again.
Then the coated pattern can be invested by two methods;
1. Placing the pattern in the ring first and then filling the ring full with investment.
2. Filling the ring with the investment first and then force the pattern through into it.

Vacuum investing :

Vacuum investing unit: This consists of the chamber of small cubic capacity from which air can be evacuated quickly and in which casting ring can be placed.
Evacuation of air can be done by electrically or water driven vacuum pump.

Procedure:

The ring filled with investment is placed in the vacuum chamber. Air entry tube is closed. Then the vacuum is applied. The investment will rise with froth vigorously for about 10-15 sec and then settles back. This indicates that air has been extracted from the ring. The pressure is now restored to atmospheric by opening the air entry tap gradually at first and then more rapidly as the investment settles back around the pattern. Then the ring is removed from the chamber and the investment is allowed to set. Modern investing unit does both mixing and investing under vacuum and is considered better than hand mixing and pouring.
Then there are two alternatives to be followed depending upon what type of expansion is to be achieved in order to compensate for metal shrinkage. They are:

1. If hygroscopic expansion of the investment is to be achieved then immediately immerse the filled ring in water at the temperature of 37C.
Or “under controlled water adding technique”. A soft flexible rubber ring is used instead of usual lined metal ring. Pattern is invested as usual. Then specified amount of water is added on top of the investment in the rubber ring and the investment is allowed to set at room temperature. In this way only enough water is added to the investment to provide the desired expansion.

2. If thermal expansion of the investment is to be achieved, then investment is allowed to set by placing the ring on the bench for 1 hour or as recommended by the manufacturer.