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

Denture Cleansers

Use -  for removal of soft debris by light brushing and then rinsing of denture; hard deposits require professional repolishing

a. Alkaline perborates-do not remove bad stains; may harm liners .
b. Alkaline peroxides-harmful to denture liners
c. Alkaline hypochlorites-may cause bleaching, corrode base-metal alloys, and leave residual taste on appliance
d. Dilute acids-may corrode base-metal alloys
e. Abrasive powders and creams-can abrade denture surfaces

Denture cleaning Method

a. Full dentures without soft liners-immerse denture in solution of one part 5% sodium hypochlorite in three parts of water
b. Full or partial dentures without soft-liners immerse denture in solution of  1 teaspoon of hypochlorite with 2 teaspoons of  glassy phosphate  in a half of a glass of water
c. Lined dentures -- clean any soft liner with a cotton swab and cold water while cleaning the denture with a soft brush

Properties

1. Chemical-can swell plastic surfaces or corrode metal frameworks
2. Mechanical-can scratch the surfaces of denture bases or denture teeth
 

Structure of gypsum products

Components
 
a. Powder (calcium sulfate hemihydrate = CaSO4½H2O)
b. Water (for reaction with powder and dispersing powder)

Applications/Use

• Load -bearing restorations for posterior  teeth  (class I, II)
• Pinned restorations
• Buildups or cores for cast restorations
• Retrograde canal filling material

 (1) Alloy. An alloy is a solid mixture of two or more metals. It is possible to produce a material in which the desirable properties of each constituent are retained or even enhanced, while the less desirable properties are reduced or eliminated.

(2) Amalgam. When one of the metals in an alloy mixture is mercury, an amalgam is formed. A dental amalgam is a combination of mercury with a specially prepared silver alloy, which is used as a restorative material.

(3) Mercury. Mercury is a silver-white, poisonous, metallic element that is liquid at room temperature

Dental Porcelain and PFM Porcelains

Applications/Use

a. Porcelain inlays and jacket crowns
b. PFM crowns and bridges
c. Denture teeth

Terms

PFM-porcelain fused to metal
Fusing-adherence of porcelain particles into a single porcelain mass

Classification

 Dental porcelain is manufactured as a powder. When it is heated to a very high temperature in a special oven, it fuses into a homogeneous mass. The heating process is called baking. Upon cooling, the mass is hard and dense. The material is made in a variety of shades to closely match most tooth colors. Baked porcelain has a translucency similar to that of dental enamel, so that porcelain crowns, pontics, and inlays of highly pleasing appearance can be made. Ingredients of porcelain include feldspar, kaolin, silica in the form of quartz, materials which act as fluxes to lower the fusion point, metallic oxide, and binders. Porcelains are classified into high-, medium-, and low-fusing groups, depending upon the temperature at which fusion takes place. 
 
High-Fusing Porcelains. High-fusing porcelains fuse at 2,400o Fahrenheit or over. They are used for the fabrication of full porcelain crowns (jacket crowns). 

Medium-Fusing Porcelains. Medium-fusing porcelains fuse between 2,000o and 2,400o Fahrenheit. They are used in the fabrication of inlays, crowns, facings, and pontics. A pontic is the portion of a fixed partial denture, which replaces a missing tooth. 

Low-Fusing Porcelains. Low-fusing porcelains fuse between 1,600o and 2,000o Fahrenheit. They are used primarily to correct or modify the contours of previously baked high- or medium-fusing porcelain restorations. Eg  for PFM restorations

Structure

Components

a. Large number of oxides but principally silicon oxide, aluminum oxide. and potassium oxide    
b. Oxides are supplied by mixing clay, feldspar, and quartz.

Manipulation

Porcelain powders mixed with water and compacted into position for firing
Shrinkage is 30% on firing because of fusing and so must be made oversized and built up by several firing steps

Properties

1. Physical

a. Excellent electrical and thermal insulation
b. Low coefficient of thermal expansion and contraction
c. Good color and translucency; excellent aesthetics

2. Chemical

a. Not resistant to acids (and can be dissolved by  contact with APF topical fluoride treatments)
b. Can be acid-etched with phosphoric acid or  hydrofluoric acid for providing microll1echanical retention for cements

3. Mechanical

a. Harder than tooth structure and ,will cause opponent wear
b. Can be polished with aluminum oxide pastes

Zinc Phoshate Cement

Uses. Zinc phosphate cement is used both as an intermediate base and as a cementing medium. 

(1) Intermediate base. A thick mix  is used under permanent metallic restoration. This layer of cement protects the pulp from sudden temperature changes that may be transmitted by the metallic restoration. 

(2) Cementing medium. Zinc phosphate cement is used to permanently cement crowns, inlays, and fixed partial dentures upon the remaining tooth structure. A creamy mix of cement is used to seat the restoration or appliance completely into place. The cementing medium does not cement two objects together. Instead, the cement holds the objects together by mechanical interlocking, filling the space between the irregularities of the tooth preparation and the cemented restoration

c. Chemical Composition. 

(1) Powder. primary ingredients - zinc oxide and magnesium oxide. 
(2) Liquid. Phosphoric acid and water in the ratio of two parts acid to one part water. The solution may also contain aluminum phosphate and zinc phosphate Liquids exposed in open bottles will absorb moisture from the air in high humidity. The liquids will lose moisture if humidity is low. Water gain hastens setting; water loss lengthens setting time.
 
PROPERTIES OF ZINC PHOSPHATE CEMENT

a. Advantages. Some advantages of zinc phosphate cement as a cementing medium are:

o    Inconspicuous appearance. 
o    Speed and ease of usage. 
o    Sufficient flow to form a thin layer for the cementing of closely adapted crowns, fixed partial dentures, and inlays. 
o    Low thermal conductivity beneath a metallic restoration.

b. Disadvantages. Some disadvantages of zinc phosphate cement as a cementing medium are:

o    Low crushing strength that varies between 12,000 and 19,000 psi. 
o    Slight solubility in mouth fluids. 
o    Opaque material not suitable for visible surfaces. 

c. Strength. The ratio of powder to liquid increases the strength of phosphate cements to a certain point. For this reason, the dental specialist must use as thick a mix as practical for the work being performed. 

SETTING REACTIONS OF ZINC PHOSPHATE CEMENT 

a. Chemical Reaction. The chemical reaction that takes place between the powder and liquid of setting phosphate cement produces heat. The amount of heat produced depends upon the rate of reaction, the size of the mix, and the amount of heat extracted by the mixing slab. 

b. Powder to Liquid Ratio. The less powder used in ratio to the liquid, the longer the cement will take to harden. Good technique minimizes the rise in temperature and acidity of the setting cement that can injure the pulp. Generally, for increased strength, decreased shrinkage, and resistance to solubility, it is advisable to blend as much powder as possible to reach the desired consistencies. 

c. Setting Time. The setting time of zinc phosphate cement is normally between 5 and 9 minutes. 
 Lower the temperature of the glass mixing slab to between 65° and 75° F (18° to 24° C), if the glass mixing slab is not already cooled below the temperature at which moisture will condense on it. → Blend the powder slowly. →  Mix the powder over a large area of the cool slab. →  Use a longer mixing time, within optimum limits. 
 
Precautions. The following precautions should be observed. 

o    Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered. 
o    Dispense drops only when ready to mix. 
o    Use a cool, dry glass slab (65° to 75° F). 
o    Use the same brand of powder and liquid. 
o    Add increments of powder slowly. 
o    Use the maximum amount of powder to obtain the desired consistency. 

(To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.)