Casting TechnologyDr. Asmaa Abdel-Hakeem Metwally

Lecturer of Dental Materials

Casting procedure is the process of converting the wax pattern into metallic restoration by pouring the molten metal into a mold

Uses:

Inlays, onlays

Post and core.

Crown and bridges.

Removable partial dentures.

Cast metal denture base for complete dentures.

steps of casting

1. Wax pattern construction.

2. Spruing the wax pattern.

3. Investing the wax pattern.

4. Wax elimination.

5. Casting the alloy.

6. Finishing and polishing.

1- Wax pattern construction

Wax pattern is used to rebuild the prepared teeth using blue inlay wax.

Technique:

Direct technique

In mouth on the prepared tooth.

Indirect technique

In the laboratory on a cast or a die.

Indirect and direct technique

The pattern is prepared outside the patient mouth, but the final adjustment is made inside the patient mouth.

2-Spruing the wax pattern.

Functions of Sprue

1. Proper handling of the wax pattern to avoid its distortion.

2. Sprue will create a channel;

To allow the molten wax to escape.

To enable the molten metal or alloy to flow into this mold.

3-Act as a reservoir for the molten alloy, to compensate for solidification shrinkage.

The sprue may be made from, or. .

wax

plastic

metal

sprue

Sprue should be attached to the thickest part

of the wax pattern with 45o degree angle to

the proximal wall surface away from the

margin. Why?

to avoids Turbulent flow of the molten

metal which create air bubbles leading to

porosity

Thickness of sprue depends on:

i) Thickness of the wax pattern

Slightly thicker than the thickest part of the wax pattern because:

- This improves the flow of the molten alloy into the mold.

- This also provides a reservoir during solidification.

ii) Casting technique

In centrifugal casting machine Wider sprue

In air pressure casting machine Slightly narrow

The length of the sprue should be adjusted to keep the mounted wax pattern at

6-8 mm from the end of the casting ring [in case of gold alloy casting]

2-3 mm from the end of the casting ring [in case of base metal alloy casting] due to denser investment

Short sprue Thicker base back pressure porosity.

Long sprue Thinner base fracture of the investment.

according to the size of the wax pattern sprue

may be:

Single, Douple or Multiple

usually made of rubber, which constitutes the base of the

casting ring during investing. Its shape depend on the type of

the casting machine used.

It must has high walls to:

1. Guide the molten metal into the channel & to confine the

expending molten metal especially in air-pressure machine.

2. Allow use of short sprue.

3. Allow the pattern to be positioned near the end of the casting

ring

:Casting ring and liner

The casting ring may be Rubber ring or Metallic

ring lined with wet liner.

The ring liner should be shorter 3mm from both

end of the ring.

The ring liner is made of Asbestos (carcinogenic)

or Celluloid

Function of the liner :

1. Give space to setting expansion of the

investment.

2. Allow setting of investment under water to

give hygroscopic expansion.

3. Regulate heat transfer.

4. Facilitate removal of investment.

Wax pattern should be invested immediately to avoid its distortion

due to release of internal stresses which depends on two factors

namely time and Temperature.

Wax is a hydrophobic material this lead to improper wetting of wax

pattern by investment which lead to irregular surface of the metallic

cast (surface roughness).

This minimized by applying surfactants (surface acting agents) on

the wax surface which allow :

1- increases surface energy of the wax

2- decrease contact angle between it and investment material.

Hand investing

Vacuum investing highly recommended to

obtain casting with minimal surface defect (roughness).

Investing wax pattern can be done by regular single

mix or double mix.

Double mix investing technique is preferred as it

improve the adaptability to the pattern surface and avoid

distortion.

After one hour the investment set and the base former

is removed (also rubber ring and metallic sprue if used)

Heating the investment must be done to achieve the

following :

1- To obtain a mold

2-thermal expansion of the investment by displacive

transformation from α to β form.

bring the furnace to 200º C and hold this temperature for 30

minutes then increase the temperature gradually to the final

burnout temperature (about 650ºC), t

The mold temperature is now ready for casting

Improper heating either :

Too rapid heating which lead to cracking

of the investment or

Overheating which leads to investment

decomposition both can lead to surface

roughness.

a) Melting:

1-Flame

- Blow pipe flame

- Oxyacetylene flame

Use always the middle zone, because it is the hottest and reducing zone.

2-Electric furnace

b) Casting machines [with or without vacuum system]:

- Air pressure

- Centrifugal force

- Centrifugal force and air pressure.

N.B:

The investment should never be allowed to cool before casting.

The metal or alloy should be heated as quickly as possible to a completely molten condition [ 50-100 0C above liquidus temperature]

Finishing and polishing.

When the color of the molten alloy has

changed from red to black the ring should be

quenched in water.

After that devesting, finishing and polishing

of the casting are subsequently performed.

Casting defects:1-Distorted Casting

Improper handling of the wax pattern

Delayed investing of the wax pattern.

2-Dimensionally inaccurate casting

Small or large casting is due to improper selection of :

correct temperature

The correct type of the investment

3-Incomplete casting with rounded margins

Due to resistance of the flow of the molten

alloy by

Using thin sprue causing premature solidification of molten metal

Using short sprue

Insufficient venting of the investment

Insufficient casting pressure to drive the molten alloy into the mold

Incomplete wax elimination

Under heating of the alloy causing premature solidification of molten metal

4-Surface roughness due to

1- Air bubbles on the surface of wax pattern during the

investment procedure due to

* Improper use of wetting agent

* Improper vacuum investing

2- Improper heating:

* Too rapid heating: causes cracking of the investment

* Overheating: leads to investment decomposition.

Types of porosity:

A- shrinkage spot porosity (Suck back porosity) at the point of

attachment to the sprue, it is due to using a thin sprue, which leads to

incomplete feeding of the molten alloy during solidification

B- Back pressure porosity:

due to usage of too short sprue

investment with insufficient porosity or improper venting

C-Pin hole porosity and gas inclusion porosity:

Occurs with some alloys containing Cu, Ag, Pt, Pd or Titanium; as

these metals tend to absorb and occlude gases during melting and then

these gases are expelled leading to pin hole porosity.

D- Porosity due to turbulent flow due to sprues attached at 90o angle

to the wax pattern