CASTING DEFECTS

Classification :

1) Distortion.
2) Surface roughness .
3) Porosity .
4)Incomplete casting .
5) Oxidation .
6) Sulfur contamination .

Distortion
It is usually due to the distortion of wax pattern.

To avoid this :
Manipulation of the wax at its softening temp
Invest the pattern at the earliest .
If storage is necessary store it in a refrigerator .
Surface roughness

May be due to :
Air bubbles on the wax pattern .
Cracks due to rapid heating of the investment .
High W/P ratio .
Prolonged heating of the mold cavity .
Overheating of the gold alloy .
Too high or too low casting pressure .
Composition of the investment .
Foreign body inclusion.

POROSITY
May be internal or external .
External porosity causes discolouration .
Internal porosity weakens the restoration .

Classification of porosity .
I .Those caused by solidification shrinkage :
a) Localised shrinkage porosity .
b) Suck back porosity .
c) Microporosity .

They are usually irregular in shape .

II ) Those caused by gas :

a) Pin hole porosity .
b) Gas inclusions .
c) Subsurface porosity .

Usually they are spherical in shape .

III ) Those caused by air trapped in the mold :

Back pressure porosity .

Localised shrinkage porosity

Large irregular voids found near sprue casting junction.
Occurs when cooling sequence is incorrect .
If the sprue solidifies before the rest of the casting , no more molten metal is supplied from the sprue which can cause voids or pits (shrink pot porosity )

This can be avoided by -
- using asprue of correct thickness .
- Attach the sprue to the thickest portion of the pattern .
-Flaring of the sprue at the point of atttachment .
-Placing a reservoir close to the pattern .

Suck back porosity

It is an external void seen in the inside of a crown opposite the sprue .
Hot spot is created which freezes last .
It is avoided by :
Reducing the temp difference between the mold & molten alloy .

Microporosity :

Fine irregular voids within the casting .
Occurs when casting freezes rapidly .
Also when mold or casting temp is too low .

Pin hole porosity :
Upon solidification the dissolved gases are expelled from the metal causing tiny voids .
Pt & Pd absorb Hydrogen .
Cu & Ag absorb oxygen .

Gas inclusion porosities

Larger than pin hole porosities .
May be due to dissolved gases or due to gases Carried in or trapped by molten metal .
Apoorly adjusted blow torech can also occlude gases .

Back pressure porosity

This is caused by inadequate venting of the mold .The sprue pattern length should be adjusted so that there is not more than ¼” thickness of the investmentbetween the bottom of the casting .
This can be prevented by :
- using adequate casting force .
-use investment of adequate porosity .
-place the pattern not more than 6-8 mm away from tne end of the casting .
Casting with gas blow holes
This is due to any wax residue in the mold .
To eliminate this the burnout should be done with the sprue hol facing downwards for the wax pattern to run down.

Incomplete casting

This is due to :
- insufficient alloy .
-Alloy not able to enter thin parts of the mold .
-When the mold is not heated to the casting temp .
-Premature solidification of the alloy .
-sprues blocked with foreign bodies .
-Back pressure of gases .
-low casting pressure .
-Alloy not sufficiently molten .

Too bright & shiny casting with short & rounded margins :
occurs when wax is eliminated completely ,it combines with oxygen or air to form carbon monoxide .

Small casting :

occurs when proper expansion is not obtained & due to the shrinkage of the impression .

Contamination of the casting
1) Due to overheating there is oxidation of metal .
2) Use of oxidising zone of the flame .
3) Failure to use a flux .
4) Due to formation sulfur compounds .

Black casting

It is due to :
1) Overheating of the investment .
2) Incomplete elimination of the wax .
 

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%)

Properties of Acrylic Resins.

• They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
• Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
• Mixing. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed
• Poor distortion resistance at higher temperatures, therefore dentures should not be cleaned in hot water
• Good resistance to color change
• Absorbs water and must be kept hydrated  (stored in water when not in mouth) to prevent dehydration cycling and changes in dimensions
• Not resistant to strong oxidizing agents
• Low strength; however, flexible, with good fatigue resistance
• Poor scratch resistance; clean tissue-bearing surfaces of denture with soft brush and do not use abrasive cleaners

Dental Solders

Applications-bridges and orthodontic appliances

Terms

Soldering -joining operation using filler metal that melts below 500° C

Brazing -joining operation using filler metal that melts above 500°C

Welding-melting and alloying of pieces to be joined

Fluxing
 -Oxidative cleaning of area to be soldered
 - Oxygen scavenging to prevent oxidation of alloy being soldered
16- 650 -- 650 fineness solder to be used with 16-karat alloys; fineness refers to the gold content

Classification

a. Gold solders-bridges
b. Silver solders-gold-substitute bridges and orthodontic alloys

Structure of gold solders

Composition-lower gold content than of alloys being soldered

Manipulation-solder must melt below melting temperature of alloy

Properties

1. Physical-similar to alloys being joined
2. Chemical-more prone to chemical and electrochemical corrosion
3. Mechanical-similar to alloy  being joined
4. Biologic-similar to alloys being joined