SELECTION OF SPRUE 

1 . DIAMETER :
It should be approximately the same size of the thickest portion of the wax pattern .
Too small sprue diameter suck back porosity results .

2 . SPRUE FORMER ATTACHMENT :
Sprue should be attached to the thickest portion of the wax pattern .
It should be Flared for high density alloys & Restricted for low density alloys .

3 . SPRUE FORMER POSITION

Based on the
1. Individual judgement .
2. Shape & form of the wax pattern .

Patterns may be sprued directly or indirectly .
Indirect method is commonly used

Denture Teeth

Use-complete or partial dentures

Type

a. Porcelain teeth
b. Acrylic resin teeth
c. Abrasion-resistant teeth (microfilled composite)

Structure and properties

1. Porcelain teeth (high-fusing porcelain)
Only bonded into denture base mechanically. Harder than natural teeth or other restorations and abrades those surfaces. Good aesthetics.Used when patients have good ridge support and sufficient room between the arches

2. Acrylic resin teeth (PMMA  [polymethyl methacrylate])

Bonded pseudochemically into the denture base. Soft and easily worn by abrasive foods . Good initial aesthetics
Used with patients with poor ridges and in cases where they oppose natural teeth

3. Abrasion-resistant teeth (microfilled resins)
Bonded pseudochemically into the denture base.Better abrasion resistance then  acrylic resin teeth
 

POLYCARBOXYLATE CEMENT 

Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material. 

Clinical Uses

Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium. 

c. Chemical Composition. 

(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included. 
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous). 

d. Properties. 

The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength. 

e. Setting Reactions: 

The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix. 

Precautions. 
The following precautions should be observed. 
o    The interior of restorations and tooth surfaces must be free of saliva. 
o    The mix should be used while it is still glossy, before the onset of cobwebbing. 
o    The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening. 
 

POLISHING MATERIALS

1 Tin Oxide. Tin oxide is used in polishing teeth and metal restorations. Tin oxide is a fine, white powder that is made into a paste by adding water or glycerin.

2. Pumice. Pumice is used as an abrasive and polishing agent for acrylic resins, amalgams, and gold. It consists mainly of complex silicates of aluminum, potassium, and sodium. Two grades--flour of pumice and coarse pumice--are listed in the Federal Supply Catalog.

3. Chalk (Whiting). Chalk is used for polishing acrylic resins and metals. It is composed primarily of calcium carbonate.

4.Tripoli. Tripoli is usually used for polishing gold and other metals. It is made from certain porous rocks.

5. Rouge (Jeweler's). Rouge is used for polishing gold and is composed of iron oxide. It is usually in cake or stick form.

6. Zirconium Silicate. Zirconium silicate is used for cleaning and polishing teeth. It may be mixed with water or with fluoride solution for caries prevention treatment. For full effectiveness, instructions must be followed exactly to obtain the proper proportions of powder to liquid.

PFM Alloys

Applications-substructures for porcelain-fused-to-metal crowns and bridges
 
Classification
o    High-gold alloys
o    Palladium-silver alloys
o    Nickel-chromium alloys

Structure

Composition
o    High-gold alloys are 98% gold. platinum. And palladium
o    Palladium-silver alloys are 50% to 60% palladium and 30 to 40% silver
o    Nickel-chromium alloys are 70% to 80% nickel and 15% chromium with other metals

Manipulation
o    Must have melting temperatures above that of porcelains to be bonded to their surface
o    More difficult to cast (see section on chromium alloys)

Properties - Physical

Except for high-gold alloys, others are less dense alloys
Alloys are designed to have low thermal expansion coefficients that must be matched to the overlying porcelain

Chemical-high-gold alloys are immune, but others passivate

Mechanical-high modulus and hardness
 

Classification of Dental amalgam

1. By powder particle shape .

• Irregular (comminuted, filing, or lathecut)
• Spherical (spherodized)
• Blends (e.g., irregular-irregular, irregularspherical, or spherical-spherical)

2. By total amount of copper

• Low-copper alloys (e.g., conventional, traditional); <5% copper
• High-copper alloys (e,g.  corrosion resistant); 12% to 28% copper

3.By presence of zinc

Examples

• Low-copper, irregular-particle alloy-silver (70%)-tin (26%)-copper (4%)
• High-copper, blended-particles alloy-irregular particles, silver (70%) –tin (26%) -Copper (4%); spherical particles, silver (72%)-copper (28%)
• High-copper, spherical-particles alloy-silver (60%) - tin (27%)-copper (13%)