The document outlines the casting procedure in dental materials which includes forming a wax pattern, investing it, and casting molten metal into the mold. Key factors influencing casting quality include the sprue size, length, and position, as well as methods for investing and eliminating wax patterns. Common casting defects such as porosity, incomplete casting, and size discrepancies are also discussed.

1. CASTING
Dr. Affan Ahmad
Incharge
Department of Science of Dental Materials
KMDC

2. CASTING PROCEDURE
 The casting procedure consists of:
1. Forming a wax pattern
2. Surrounding it with investment material
3. Heating the investment mould to remove the wax
4. Introduction of molten metal into the mould
 The purpose of dental casting is to provide a metallic
replacement for missing tooth structure

3. COMPONENTS OF INVESTMENT MOULD

4. MOULD CAVITY: Produced by setting of the investment
material around the wax pattern, sprue former and
sprue base, after which the sprue former and base are
removed and wax or resin pattern is eliminated by a
burnout process
 SPRUE: It is a channel embedded in an investment
mould through which molten metal flows
 CASTING RING LINERS

6. INVESTMENT MOLD AFTER DEWAXING

7. CASTING PROCEDURE
 The casting procedure involves the following steps:
1. Formation of wax pattern
2. Spruing of wax pattern
3. Investing of wax pattern
4. Elimination of wax pattern
5. Casting the molten alloy in the mould cavity
6. Removal of the casting and its cleaning

8. FORMATION OF WAX PATTERN

9. 1. DIRECT WAX PATTERN If the wax pattern is prepared
inside the mouth of the patient
2. INDIRECT WAX PATTERN When impression of the cavity
preparation or the prepared tooth is taken and a die is
formed on which the wax pattern is formed

10. SPRUING OF THE WAX PATTERN

11. Sprue wax with reservoir

13. • The factors which influence the quality of the
casting are:
1. Size of the sprue
2. Diameter of the sprue
3. Location of the sprue
4. Length of the sprue
5. Direction of the sprue

14. • SPRUE DIAMETER:
The diameter of the sprue controls the rate of flow of the
molten metal into the mould cavity. It should be larger or
same as the thickest part of the wax pattern.
◦ Large diameter sprue improves the flow of molten metal
into the mould
◦ Less diameter of sprue causes localized shrinkage porosity
• POSITION OF SPRUE:
The position of the sprue should be at the bulkiest portion of
the wax pattern

15. • ATTACHMENT OF SPRUE:
The sprue former should be attached to the portion of the pattern with
the largest cross sectional area so that the molten metal flows from
the thick sections to thin sections minimizing the risk of turbulence
◦ High turbulence causes porosity .
• LENGTH OF SPRUE:
The length of the sprue former should be long enough so that the wax
pattern can be positioned approximately 6mm from the end of
casting ring and should be short enough so that the molten alloy does
not solidify before it fills the mould
◦ Short sprue causes porosity in casting at the junction of sprue and
wax pattern
◦ Long sprue causes solidification of metal in the sprue first leading to
casting shrinkage and incomplete casting

16. • DIRECTION OF SPRUE: The sprue former should be directed
away from any thin or delicate parts of the pattern. To
obtain a satisfactory casting, the sprue former should be at
45° angle to the proximal area

17. INVESTING PROCEDURE FOR WAX PATTERN

18. • There are two methods of investing wax pattern:
1. Hand-Investing Procedure
2. Vacuum-Investing Procedure

19. HAND INVESTING PROCEDURE
 The powder and liquid components of investment material are
mixed properly.
 After being spatulated, the investment is placed on a vibrator to
remove air bubbles.
 The investment is then painted on the wax pattern while holding
the sprue base.
 Then the casting ring is placed over the sprue base and filled with
investment material.
 Left for 45-60 min to allow the investment to set.

20. VACCUM INVESTING PROCEDURE
 The powder and liquid are mixed under vacuum.
 The mixed investment is allowed to flow into the ring
and around the wax pattern in the presence of
vacuum.
 This technique reduces the amount of porosity in the
investment which results in smoother cast surface with
better detail production.

21. After the casting ring is completely filled with the investment
material either by hand or vacuum investing procedure:
• The investment is allowed to harden in air if thermal
expansion technique is used
• If hygroscopic expansion technique is used, the casting
ring should be placed immediately in water bath at 37°C
and allowed to harden under water

22. WAX ELIMINATION

23. • Once the investment material has set, the casting ring is
transferred to the furnace for burnout process.
• Proper and careful heating of the investment material
is necessary because:
1. Complete elimination of wax
2. Correct thermal expansion
3. Prevent overheating of calcium sulfate hemihydrate

24. • For Gypsum-Bonded investment the temperature of the
furnace should be either 500°C for hygroscopic technique
and 700°C for thermal expansion technique
• The range of temperature for phosphate-bonded
investment is 700-1030°C
• Heating of the investment mould should be carried out at a
rate which allows the steam and other gases to be
liberated
• The temperature of the investment should not fall
significantly before casting
• The mould temperature should be high enough to facilitate
the thermal expansion and to prevent premature
crystallization of the molten alloy

25. CASTING MACHINES

26. • When the wax pattern is completely eliminated and the
mould has reached the casting temperature the alloy is
melted by an appropriate method and cast into the
mould cavity.
• There are various types of machines that can be used to
melt the alloy, which have the following three variables:
1. The alloy is melted in the mold sprue base or in a separate
crucible located in the casting machine
2. The alloy is melted by either gas-torch. Oxyacetylene
torch, electrical induction heating or electrical resistance
melting
3. The molten alloy is driven into the mould by gravity, air
pressure, steam pressure or by centrifugal force

27. CASTING RING LINER

28. • Solid casting rings do not permit the investment to
expand laterally, therefore to overcome this lateral
restriction the walls of the casting ring are lined with a
ring liner.
• Ring liner:
1. Allows the setting expansion in the investment by
forming a pliable lining inside the casting ring.
2. Prevents rapid cooling and contraction of the mould
after the burnout process as it has a thermal
insulating property.
3. Causes semi-hygroscopic expansion.

29. CLEANING OF THE CASTING

30. • When the hot molten metal enters the mould cavity, certain
alloys especially gold is contaminated on the surface and the
surface of the casting appears dark with oxides and tarnish.
• PICKLING is a process by which such surface films can be
removed.
• Pickling consists of heating the discolored casting in an acid.
Available pickling solutions are 50% hydrochloric acid solution
or solution of sulfuric acid.
• The casting to be cleaned is placed in a container to which
the pickling solution is added. It is then warmed gently until
surface becomes bright and the casting is then rinsed
thoroughly with water

31. DEFECTS IN CASTING

32. • Defects in casting can be listed under four headings:
1. Finning and bubbling
2. Porosity
3. Incomplete casting
4. Oversized or undersized casting

33. FINNING AND BUBBLING
 FINS are produced due to the flow of molten metal into
the cracks present in the investment due to rapid
heating of the investment in the furnace.
 Bubbling effect of the casting surface is due to the
porosities in the investment caused by entrapment of air
bubbles during mixing.

34. FINNING BUBBLING

35. INCOMPLETE CASTING
 It occurs due to:
1. Improper positioning of the sprue
2. Improper balance between the temperature of the
molten alloy and the temperature of the mould
3. Insufficient thrust created during casting
4. Back pressure effects
5. Cooling shrinkage of the alloy

37. POROSITY
 Porosity may be seen as surface pitting on the casting or
revealed within the cast metal on finishing and polishing.
 Pitting of the surface is due to embedment of broken
pieces of investment or other particles fallen from the
sprue.
 Gaseous porosities are produced due to entrapment of
gases when the alloy become rigid

38. UNDERSIZED OR OVERSIZED CASTINGS
 The casting becomes under or over sized if there in
improper balance between the contractions and
expansions occurring during casting

39. References
• Applied Dental Materials by John. F. McCabe
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