Impression Material

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
 

METALLURGICAL TERMS

a. Cold Working. This is the process of changing the shape of a metal by rolling, pounding, bending, or twisting at normal room temperature.

b. Strain Hardening. This occurs when a metal becomes stiffer and harder because of continued or repeated application of a load or force. At this point, no further slippage of the atoms of the metal can occur without fracture.

c. Heat Softening Treatment (Annealing). This treatment is necessary in order to continue manipulating a metal after strain hardening to prevent it from fracturing. The process of annealing consists of heating the metal to the proper temperature (as indicated by the manufacturer's instructions) and cooling it rapidly by immersing in cold water. Annealing relieves stresses and strains caused by cold working and restores slipped atoms within the metal to their regular arrangement.

d. Heat Hardening Treatment (Tempering). This treatment is necessary to restore to metals properties that are decreased by annealing and cold working. Metals to be heat hardened should first be heat softened (annealed) so that all strain hardening is relieved and the hardening process can be properly controlled. Heat hardening is accomplished in dental gold alloy by heating to 840^o Fahrenheit, allowing it to cool slowly over a 15-minute period to 480^o Fahrenheit, and then immersing it in water.

Stages of manipulation

Definitions of intervals

• Mixing interval-length of time of the mixing stage.
• Working interval-length of time of the working stage
•  Setting interval-length of time of the setting stage

Definitions of times

• Mixing time-the elapsed time from the onset to the completion of mixing
• Working time-the elapsed time from the onset of mixing until the onset of the initial setting time
• Initial setting time-time at which sufficient reaction has occurred  to cause the materials to be resistant to  further manipulation
• Final setting time-time at which the material practically is set as defined by its resistance to indentation

[All water-based materials lose their gloss at the time of setting]

Mouth Protectors

Use - to protect against effects of blows to chin, top of the head, the face, or grinding of the teeth

Types

o    Stock protectors-least desirable because of poor fit
o    Mouth-formed protectors-improved fit compared with stock type
o    Custom-made protectors-preferred because of durability. low  speech impairment, and comfort

I. Components

a. Stock protectors-thermoplastic copolymer of PYA-PE (polyvinyl acetate-polyethylene copolymer)
b. Mouth-formed protectors-thermoplastic copolymer
c. Custom-made protectors- thermoplastic copolymer, rubber. or polyurethane
2. Reaction-physical reaction of hardening during cooling
3. Fabrication

Alginate impression made of maxillary arch. High-strength stone cast poured immediately. Thermoplastic material is heated in hot water and vacuum-molded to cast .

Mouth protector trimmed to within ½ inch of labial fold, clearance provided at the buccal and labial frena, and edges smoothed by flaming. Gagging, taste, irritation. and impairment of speech are minimized with properly fabricated appliances

4. Instructions for use

a. Rinse before and after use with cold water
b. Clean protector occasionally with soap and cool water
c. Store the protector  in a rigid container
d. Protect from heat and pressure during storage
e. Evaluate protector routinely for evidence of deterioration

Properties

1. Physical-thermal insulators
2. Chemical-absorbs after during use
3. Mechanical-tensile strength, modulus, and hardness decrease after  water absorption, but elongation, tear strength, and resilience increase
4. Biologic-nontoxic as long as no bacterial, fungal, or viral growth occurs on surfaces between uses
 

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 .
 

Applications

a. Dentulous impressions for casts for prosthodontics

b. Dentulous impressions for pedodontic appliances

c. Dentulous impressions for study models for orthodontics

d. Edentulous impressions for casts for denture construction