1.
DEPT. OF
CONSERVATIVE DENTISTRY
&
ENDODONTICS 9/26/2013 1

2.
Good Morning…

3.
FAILURES IN CASTING
PROCEDURES
Presented By:- Dr Nishant Khurana
PG Student
Guided By:- Dr M.P. Singh

4.
TOPICS INCLUDED IN SEMINAR
Introduction
Classification
of different
types of
defects
Etiology of
casting
defects
Its
prevention
Conclusion References

5.
INTRODUCTION
• Casting is defined as the act of forming an object in a mold. The object
formed from this procedure is called as casting.
• Any irregularity in the metal casting process that is undesired is called
Casting Defect.
• Casting is one of the most common procedures in dentistry and is mostly
done by dental technicians.
• By understanding the cause of the defect, a dentist as well as technician can
prevent the defect.

6.
CLASSIFICATION OF CASTING
DEFECTS(COOMBE)
CASTING
DEFECTS
DISTORTION INCOMPLETE
OR MISSING
DETAIL
POROSITY
CONTAMINA
TION OF
CASTING
SURFACE
ROUGHNESS
AND
IRREGULARITI
ES

7.
DISTORTION
• Distortion of the casting is usually due to distortion of
wax pattern
Some distortion of wax occurs when the investment
hardens or due to hygroscopic setting expansion. It does not cause
serious problems
Some distortion of wax
occurs during manipulation,
because of the release of
internal stresses

8.
• Minimize distortion by :
1. Manipulation of wax at high temperature
2. Investing pattern within one hour after finishing
3. If storage is necessary, store in refrigerator
DISTORTION

9.
SURFACE ROUGHNESS
• Surface roughness can usually be traced to :
1) Air bubbles on wax pattern cause nodules on the casting.
it can be avoided by :-
- Proper mixing of investment
- vibration of mix or by vacuum
investing
-Application of wetting agents

10.
2) Too rapid heating, it may cracks the investment resulting in fins
it can be avoided by :-
- Heat the ring gradually to
7000
C (in at least 1 hour)
3) Higher W/P ratio gives rougher casting. Larger particle size of
investment has the same effect
it can be avoided by :-
- Using correct W/P ratio &
select investment of correct
particle size

11.
4) Prolonged heating causes disintegration of the mould cavity
it can be avoided by :-
- complete the casting as soon as the ring is heated & ready
- Overheating of gold alloy has the same effect. It
disintegrates the investment.
5) Too high or too low casting pressure
it can be avoided by :-
- Using 15 lbs /sq inch of air pressure or three to four
turns of centrifugal casting machine

12.
Rounded , incomplete margins are evidence
of insufficient casting pressure

13.
6) Composition of the investment.
Proportion of the quartz & binder influences the surface
texture of casting. Coarse silica will give coarse casting
6) Foreign body inclusion shows sharp, well defined
deficiencies. Inclusion of flux shows bright concavities

14.
SURFACE IRREGULARITIES
A)AIR BUBBLES
B)WATER FILMS
C)FOREIGN BODY

15.
POROSITY
• Porosity may be internal or external
• External porosity can cause discoloration of the casting
• Internal porosity weakens the restoration
• Severe porosity at the tooth restoration junction can cause
secondary caries

16.
• Porosities are Classified as
1) Those caused by solidification shrinkage
 Localized shrinkage porosity
 Suck back porosity
 Microporosity
2) Those caused by gas
 Pin hole porosity
 Gas inclusions
 Sub-surface porosity
3) Those caused by air trapped in the mould
Back pressure porosity
Acc. To Phillips

17.
Localized Shrinkage Porosity Or Shrink Spot
• These are large irregular voids usually found near the sprue-casting
junction.
• It occurs when cooling sequence is incorrect & the sprue freezes
before the rest of the casting.
• During a correct cooling sequence, the sprue should freeze last.
• This helps the molten metal to flow into the mould to compensate
for the shrinkage of the casting as it solidifies.

18.
• If the sprue solidifies before the rest of the casting no more molten
metal can be supplied from the sprue.
• The subsequent shrinkage produces voids or pits known as shrink-
spot porosity
it can be avoided by :-
 using sprue of correct thickness
Attach sprue to thickest portion of wax pattern
Flaring the at the point of attachment or placing a reservoir close to
the wax pattern

19.
SEQUENCE OF SOLIDIFICATION OF ALLOY
 Correct Wrong

20.
SUCK BACK POROSITY
• It is a variation of the shrink
spot porosity
• This is an external void usually
seen in the inside of a crown
opposite the sprue
• A hot spot is created by the hot metal impinging on the
mould wall near the sprue

21.
• The hot spot causes this region to freeze last
• The sprue has already solidified, no more molten
material is available & the resulting shrinkage causes a
peculiar type of shrinkage called suck back porosity
• It is avoided by reducing the temperature difference
between the mould & the molten alloy

22.
MICROPOROSITY
• These are fine irregular voids within the casting
• It is seen when the casting freezes too rapidly
• Rapid solidification occurs when the mould or casting
temperature is too low

23.
PIN HOLE POROSITY
• Many metals dissolve gases when molten
• Upon solidification the dissolved gases are expelled causing tiny
voids, e.g.. platinum & palladium absorb hydrogen
• Copper & Silver dissolve oxygen

24.
GAS INCLUSION POROSITY
• These are also spherical voids but are larger than pin hole type
• They may also be due to dissolved gases, but are more likely due to gases
carried in or trapped by the molten metal
• A poorly adjusted blow torch can also occlude gases

25.
BACK PRESSURE POROSITY
• This is caused by inadequate venting (air escape) of the mould
• The sprue pattern length should be adjusted so that there is
not more than 1/4th thickness of the investment between the
bottom of the casting ring & the wax pattern
• When the molten metal enters the mould, the air inside is
pushed out through the porous investment at the bottom

26.
• If the bulk of the investment is too great, the escape of air
becomes difficult causing increased pressure in the mould
• The molten metal will then solidify before the mould is
completely filled resulting in a porous casting with rounded short
margins

27.
• Avoid by
1) Using adequate casting
force
2) Use investment of
adequate porosity
3) Place pattern not more
than 6 to 8 mm away from
the end of the ring
4) Providing vents in large
castings

28.
CASTING WITH GAS BLOW HOLES
• If there is any wax residue remaining in the mould, it gives off a
large volume of gas, as the molten alloy enters the mould cavity
• This gas can cause deficiencies in the casting & blow holes in the
residue button
• To help the elimination of wax completely from the mould, the
burn out should be done with the sprue hole facing
downwards for the wax to run down

29.
INCOMPLETE CASTING
• An incomplete casting may result when
1) Insufficient alloy used
2) Alloy not able to enter thin parts of mould
3) When mould is not heated to casting temperature
4) Premature solidification of alloy
5) Sprues are blocked with foreign bodies
6) Back pressure due to gases in mould cavity
7) Low casting pressure
8) Alloy not sufficiently molten or fluid

30.
INCOMPLETE CASTING
Wax pattern too thin Incomplete wax elimination

31.
To bright and shiny casting with short and rounded margins
• When the wax is not completely eliminated, it combines with oxygen or air in
the mould cavity forming carbon mono-oxide which is a reducing agent. The
gas prevents the oxidation of the surface of the casting gold with the result
that the casting which comes out from the investment is bright and shiny.
The formation of gas in the mould is so rapid that it also has a back pressure
effect.

32.
Incomplete casting

33.
Incomplete casting resulting from incomplete wax
elimination is characterized by rounded margins &
shiny appearance

34.
SMALL CASTING
• If compensation for shrinkage of alloy is not done by adequate
expansion of mould cavity, then a small casting will result.
Another reason is, the shrinkage of the impression material.

35.
CONTAMINATION
1. Due to oxidation when molten alloy is over heated
2. Use of oxidizing zone of the flame
3. Failure to use flux
4. Due to formation of Sulphur compounds
it can be avoid by :-
• Not overheating alloy
• Use reducing zone of the flame
• Use of flux

36.
BLACK CASTING
• Can be due to two reasons
1. Overheating the investment above 7000c causes it
to decompose liberating Sulphur or Sulphur
compounds. They readily combine with the metals in
gold alloy forming a sulphide film. This gives a dark
casting which cannot be cleaned by pickling.

37.
2. A black casting, can be also due to incomplete
elimination of the wax pattern, as a result of
heating the mould at too low temperature. A
carbonized wax remains which sticks to the
surface of the casting. It can be removed by
heating over a flame.

38.
CONCLUSION
• An unsuccessful casting results in considerable trouble
& loss of time
• In almost all instances, defects in castings can be
avoided by strict observance of procedures governed by
certain fundamental rules & principles.
• Seldom is a defect in a casting attributable to factors
other than carelessness or ignorance of the operator
• With present technique the casting failure should be the
exception, not the rule.

39.
REFERENCES
• Fundamentals of fixed prosthodontics-
Herbert T. Shillingburg
• Dental laboratory procedures – FPD
kenneth D.Rudd , Robert M. Morrow
• PHILIPS’ Science of Dental Materials
Eleventh Edition Anusavice
• Essentials of Dental Materials
SH Soratur
• Contemporary fixed Prosthodontics,
Rosenstiel

40.
THANK YOU