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

Casting Alloys

Applications-inlay, onlay,  crowns, and bridges

Terms

a. Precious-based on valuable elements
b. Noble or immune-corrosion-resistant element or alloy
c. Base or active-corrosion-prone alloy
d. Passive -corrosion resistant because of surface oxide film
e. Karat (24 karat is 100% gold; 18 karat is 75% gold)
f. Fineness (1000 fineness is I00% gold; 500 fineness is 50% gold)

Classification

High-gold alloys are > 75% gold or other noble metals

Type 1-    83% noble metals (e.g., in simple inlays)
Type II-≥78% noble metals (e.g.,in inlays and onlays)
Type IlI-≥75% noble metals (e.g., in crowns and bridges)
Type IV-≥75% noble metals (e.g., in partial dentures)

Medium-gold alloys are 25% to 75% gold or other noble metals

Low-gold alloys are <25% gold or other noble metals

Gold-substitute alloys arc alloys not containing gold

(1) Palladium-silver alloys-passive .because of mixed oxide film
(2) Cobalt-chromium alloys-passive because of Cr203 oxide film
(3) Iron-chromium alloys-passive because of Cr203 oxide film

Titanium alloys are based on 90% to 100% titanium ; passive because of TiO2 oxide film

Components of gold alloys

-    Gold contributes to corrosion resistance
-    Copper contributes to hardness and strength
-    Silver counteracts orange color of copper
-   Palladium increases melting point and hardness
-    Platinum increases melting point
-    Zinc acts as oxygen scavenger during casting

Manipulation

-    Heated to just beyond melting temperature for casting
o    Cooling shrinkage causes substantial contraction

Properties

Physical

-    Electrical and thermal conductors
-   Relatively low coefficient of thermal expansion

Chemical

-    Silver  content affects susceptibility to tarnish
-   Corrosion resistance  is attributable to nobility or passivation

Mechanical

-   High tensile and compressive strengths but relatively weak in thin sections, such as margins, and can be deformed relatively easily
-    Good wear resistance except in contact with Porcelain
 

Introduction

The science of dental materials involves a study of the composition and properties of materials and the way in which they interact with the environment in which they are placed

Selection of Dental materials

The process of materials selection should ideally follow a logical sequence involving

(1) analysis of the problem,

(2) consideration of requirements,

(3) consideration of available materials and their properties, leading to

(4) choice of material.

Evaluation of the success or failure of a material may be used to influence future decisions on materials selection.

I . Procedure for single casting :

A 2.5 mm sprue former is recommended
for molar crowns 2.0 mm for premolars & partial coverage crowns .

II . Procedure for multiple casting :

Each unit is joined to a runner bar .

A single sprue feeds the runner bar

4 . SPRUE FORMER DIRECTION
Sprue Should be directed away from the delicate parts of the pattern
It should not be at right angles to a flat surface .(leads to turbulance  porosity .)
Ideal angulation is 45 degrees .

5 . SPRUE FORMER LENGTH

Depends on the length of casting ring .. Length of the Sprue former should be such that it keeps the wax pattern about 6 to 8 mm away from the casting ring. Sprue former should be no longer than 2 cm. The pattern should be placed as close to the centre of the ring as possible.

Significance

Short Sprue Length:

The gases cannot be adequately vented to permit the molten alloy to fill the ring completelyleading to Back Pressure Porosity.

Long Sprue Length:

Fracture of investment, as mold will not withstand the impact force of the entering molten alloy.

Top of wax should be adjusted for :

6 mm for gypsum bonded investments .

3 -4 mm for phosphate bonded investments .
TYPES OF SPRUES

I . - Wax . II . Solid

- Plastic . Hollow
- Metal .

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

Lost Wax Process

The lost wax casting process is widely used as it offers asymmetrical casting withnvery fine details to be manufactured relatively inexpensively. The process involves producing a metal casting using a refractory mould made from a wax replica pattern.
The steps involved in the process or the lost wax casting are:

1 . Create a wax pattern of the missing tooth / rim
2 . Sprue the wax pattern
3 . Invest the wax pattern
4 . Eliminate the wax pattern by burning it (inside the furnace or in hot water). This will create a mould.
5 . Force molten metal into the mould - casting.
6 . Clean the cast.
7 . Remove sprue from the cast
8 . Finish and polish the casting on the die .

The lost-wax technique is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out ("lost") to create a space into which molten metal is placed or cast. The entire lost-wax casting process . 

Wax pattern removal:

Sprue former can be used to remove the pattern. If not the pattern is removed with a sharp probe. Then the sprue former is attached to it. The pattern should be removed directly in line with the principle axis of the tooth or the prepared cavity. Any rotation of the pattern will distort it. Hollow sprue pin is advisable because of its greater retention to the pattern.