DEFECT IN CASTING
Dr Thulfiqar
Dental Material
Block 3
CATEGORIES OF DEFECTS
Defects in casting can be classified under 4
headings:
1) Distortion
2) Surface roughness & irregularities
3) Porosity
4) Incomplete or missing details
Other defects:
5) Fins on casting
6) Contamination of casting
7) Poor fit of castings
1) Distortion
Distortion of the casting is probably related to a
distortion of the wax pattern
Some distortion of the wax pattern occurs as the
investment hardens around it
The setting and hygroscopic expansions of the
investment may produce a non uniform expansion
of the walls of the patterns
There are certain factors that influence distortion
like:
a. Configuration of the pattern
b. The type of wax
c. The thickness
An example for that: the gingival margins are forced
apart by the mold expansion, whereas the solid
occlusal bar of wax resists expansion during the
early stages of setting.
Distortion increases as the thickness of pattern
decreases
The lower the setting expansion of the investment,
the less lower is the distortion
2) Surface Roughness
These are caused by:
Breakage of investment due to overheating of the
mould or the alloy
Air bubbles entrapment on the pattern, due to
i. Inadequate use of detergent (wetting agent) or
ii. Poor vacuum investment
Weak investment surface, due to:
i. Water-film, from too thin a mix of investment
(high L/P ratio)
ii. Use of too great casting force
iii. Use too much detergent (wetting agent)
3) Porosity
It may be seen as a surface defect on the casting
or may be detected within the cast during
finishing & polishing.
It may be due to the following causes:
a. Occluded gases
b. Cooling shrinkage
c. Flux, dirt or investment particles embedded in
the metal
d. Back pressure effect
a. Occluded gases
Alloy containing metals like gold, copper, silver,
platinum and palladium liberate gases when
they solidify (i.e. on cooling)
Gas porosity occurs in the casting due to gas
occlusion and appears as spherical voids that
may affect any area of the casting and may
even involve the entire piece.
This may caused by:
Overheating of the alloy or
Prolonged heating of the alloy
b. Cooling shrinkage
Porosity due to cooling shrinkage (contraction
porosity) appears as irregular voids in the
casting.
It is generally found in the thicker section of
the casting. It caused by:
1) Use of too thin sprues
2) Sprues not placed at the bulkiest section of
the pattern
3) Placing the reservoirs too far from the
pattern to be effective or making it too
small (reservoirs should be twice the bulk of
the nearest portion of the casting. Should
be only 1-2 mm along the sprue)
c. Flux, dirt or investment particles embedded in
the metal
Porosity due to this cause looks like irregular
void within the casting
d. Back pressure effects
This caused by air which cannot escape from
the mould when the molten alloy enters in it.
Porosity due tto back pressure effect gives rise
to rounded margins on the casting or regular
large voids within it.
It may be avoided by:
1) Using investment of adequate porosity
2) Employing correct casting force
3) Ensuring that no residue of wax pattern is
left which might block the pores of the
investment and interfere with the escape of
air or gas
4) Placing the pattern not more than 6 to 8
mm away from the end of the casting ring
(too great a thickness of the investment will
resist the escape of air or gas from the
mould)
4) Incomplete or missing details
This may be due to the following causes
a. Use of insufficient alloy
b. Metal not completely molten or allowed to cool
before the application of casting force
c. Use of too low casting force
d. The mould is too cold causing premature
solidification of the alloy
e. Back pressure of the gases in the mould
f. Blockage of the sprues or of the mould cavity
by foreign matter (eg dirt, broken pieces of
investment material, particles of flux or unburnt
wax)
5) Fins on casting
These are caused by cracks in the investment
during casting. These may occur
a. When the investment heated up too rapidly in
the furnace, or,
b. When 2 different mixes of investment (of
different P/L ratio) are used
6) Contamination of casting
Contamination may occur due to
a. Oxidation that cause by oxidizing zone of
flame, overheating of alloy & failure of using
sufficient flux
b. Sulphur compounds, which are the products
of investment break down due to overheating
7) Poor fit of casting
A casting may be too large or too small
a. Too large due to excessive expansion of mould
which caused by too high temperature or
wrong type of investment material
b. Too small cause by insufficiently heated mould
