CASTING PROCEDURE part 2

1. PRESENTED BY-
DR. SNEHA BASU
PG 1ST YEAR
DEPARTMENT OF PROSTHODONTICS
CASTING PROCEDURE
PART-II

2. Contents
 Casting rings
 Casting ring liners
 Investing procedure
 Burnout
 Casting machines
 Cleaning the casting
 Casting defects

3. CASTING RINGS
 Casting rings are used to confine the fluid investment
around the wax pattern while the investment sets
 They are available as:-
1. complete rings:
a)rigid –metal
--plastic
b)flexible – rubber
2. split rings –metal
--plastic
3. shape – round
--oval

5. SELECTION OF CASTING
RINGS
 The internal diameter of casting ring should be 5-
10 mm greater than the widest measurement of
the pattern and about 6mm higher.
 For single crown/inlay – ring diameter-32mm
 For large FPD – ring diameter- 63mm, shape –
round/oval

6. CASTING RING LINERS
 PURPOSE: to provide a buffer of pliable material
against which the investment can expand to
enlarge the mould.
 If there is no room for expansion outwards, the
expansion forces will be exerted inwards towards
the mould, resulting in distortion of the casting.
 Ring liners:
1. Asbestos liner
2.cellulose (paper) liner
3.aluminosilicate ceramic liner

7.  Asbestos liner – not used anymore because of its
carcinogenic potential.
 Ceramic liners- kaoliner, densply . . Ceramic material does
not readily absorb water except under vaccume . It does
not absorb water like cellulose liner, its network of fibers
can retain water on the surface.
{it has been pointed out, however,that ceramic material
contains fibers in the size range likely to produce lung
tumors in rats and therefore may be no less hazardous
than asbestos}.
 Cellulose liners: absorbs water- becomes thicker and more
compressible. It burns out before the casting is made and
allowing unrestricted thermal expansion and easy escape
of gases from the mould during casting.

8. PROCEDURE
 The liner is cut to fit the inside diameter of the
casting ring with no overlap.
 The dry liner is tacked in position with stickey
wax, and then it is used dry or wet.
 Wet liner technique: the lined ring is immersed in
water for a time, and excess water is shaken
away (squeezing the liner is avoided because this
leads to variable amounts of water removal and
nonuniform expansion).
 This absorbed water causes semi-hygroscopic
expansion as it is drawn into the investment
during setting (for gypsum bonded investments)

10.  Using a thicker liner material or two layers of liner
provides even greater semihygroscopic expansion
and also affords a more unrestricted normal setting
expansion of the investment.
 Thickness of the liner should not be less than
approximately 1mm.
 The expansion of the investment is always greater in
the unrestricted longitudinal direction than in the
radical direction, that is, towards the ring. Therefore it
is desirable to reduce the expansion in the
longitudinal direction.
 Placing the liner somewhat short (3.25mm) of the
ends of the ring tends to produce a more uniform
expansion; thus there is less chance for distortion of

11. RINGLESS CASTING SYSTEM
 The high strength of the phosphate bonded
investment material makes it possible to abandon the
use of casting ring.
 Plastic ring with rubber crucible former are used. The
ring is conical in shape with tapering walls. As the
investment sets the investment is tapped out of ring.
Then burnout is done without casting ring, this causes
greater expansion.
 ADVANTAGES
-allows unrestricted thermal expansion
-smaller marginal discrepancy
-easier technique
-easy to divest
-health hazard of liners are avoided

12. INVESTING PROCEDURE
 wax pattern should be cleaned of any debris,
grease or oils using – a commercially wax pattern
cleaner
--a dilute synthetic detergent.
 Excess liquid is shaken off
 Pattern is left to air-dry while the investment is
being prepared.
 The thin film of cleanser left on pattern
- reduces the surface tension- allows wetting of
the wax pattern by investment material to ensure
complete coverage of the intricate portions of the
pattern.

13. MIXING OF INVESTMENT
 Hand mixing
 Vacuum mixing
 Hand mixing:- appropriate amount of
water(gypsum investment) or colloidal silica
special liquid(phosphate investment)is dispensed
in a clean dry mixing bowl.
-powder is gradually added to the
liquid with caution to avoid air entrapment until all
the powder has been wet
-hand mixing is an option but
vacuum mixing is more common.

14. Vacuum mixing
 Mechanical mixing under vaccum removes air
bubbles better.
 Once mixing is completed, the pattern may be-
-hand invested
-vacuum invested
 Hand investment: entire pattern is painted (inside
out) with a thin layer of investment.the casting
ring in positioned on the crucible former, the
remainder of the investment is vibrated slowly into
the ring.
 Vacuum investment: the same equipment used to

16.  In one study it was found that 95% of vacuum-
invested casting were free of nodules, whereas 17%
of castings made in hand-invested moulds were
entirely free of defects.
 PRECAUTIONS
 Air bubbles may can be entrapped on flat or concave
surfaces (not oriented suitably for air evacuation) :
tilting the ring slightly aids in releasing these bubbles
so that they can rise to the surface.
 Exessive vibration should be avoided: it causes solids
in investment to settle down and free water
accumulation adjacent to the wax pattern, resulting in
surface roughness.
 Exessive vibration may also dislodge small pattern
from the sprue ,resulting in miscast.

17.  HYGROSCOPIC TECHNIQUE: the filled casting
ring is immediately placed in 37 degree water
bath with crucible former side down.
 HIGH HEAT TECHNIQUE: the invested ring is
allowed to bench set undisturbed for the
manufacturer recommended time.

18.  Once the investment is set for an appropriate amount
of time- approximately 1 hr. for most gypsum and
phosphate bonded investment – it is ready for
burnout.
 Crucible former and any metal sprue former are
carefylly removed.
 Any debris from the ingate area (funneled opening at
the end of the ring) is cleaned with a brush.
 If burnout is not carried out immediately ,the
investment is stored in a humidor at 100% humidity.
 Do not let the investment to dry out
 Rehydration of set investment that has been stored
for an extended period may not replenish all of the
lost water.

19. BURNOUT
 It prepares the mould for the molten alloy and allows thermal
expansion to occur.
 It is advisible begin the burnout procedure while the mould is still
wet.
 Water trapped in the pores of the investment reduces the
absorption of wax, and as the water vaporizes, it flushes wax
from the mould.
 This process is facilitated by placing the ring with the sprue hole
down over a slot in a ceramic tray in the burnout furnace.
 When high heat technique is used, the mold temperature
generates enough heat to convert carbon to either carbon
monoxide or carbon dioxide. These gases can then escape
through the pores in the heated investment.
 Gypsum bonded investment: 500˚C (hygroscopic technique) &
700˚ (thermal expansion technique)
 Phosphate bonded investment – 700˚ to 1030˚ C, depending on
the type of alloy selected.

21. Wax elimination and heating
techniques:
 Hygroscopic low heat technique
 High-heat thermal expansion technique
 Hygroscopic low heat technique:
here the compensation expansion is from 3 sources:
1)37 degree Celsius water bath expands the wax pattern
2)Hygroscopic expansion
3)Thermal expansion at 500 degree Celsius.
ADVANTAGES:
-less investment degradation.
-cooler surface for smoother castings
-convenience of placing the mould directly in the 500 degree
Celsius furnace.

22. The mold should remain in the furnace for at least
60 mis , they may be held up to 5 hrs.
Even though the mold is usually held at this
temperature for 60 to 90 mins, sufficient residual
fine carbon may be retained to reduce the venting
of the mold thereby causing backpressure
porosity.
 High-heat thermal expansion technique: uses
high heat burnout to obtain the required thermal
expansion , while at the same time eliminating the
wax pattern.

23. Time allowable for casting
 No more than 30 seconds should be allowed to
elapse between the time the ring is removed from
the oven and the molten alloy is centrifuged into
the mold.
 Any undue delay will cause heat loss and
resultant mold contraction.

24. Casting process and equipment
 Casting machines (based on the method of
casting)
1. centrifugal type
2. air pressure type
CENTRIFUGAL TYPE:
May be spring driven or motor driven
Can cast both large and small castings on the
same machine

25.  AIR PRESSURE TYPE :
Either compressed air or gasses like carbon
dioxideor nitrogen can be used to force the
molten metal into the mould.
Satisfactory to make small castings
Some systems use argon gas to protect the alloy
from oxidation
(specially useful of melting titanium)

26.  ATTACHED VACUUM SYSTEM:
both centrifugal or air presure type are available
with an attached vaccum system
Vacuum creates a negative pressure within the
mould, which helps to draw the alloy into the mold
 CASTING MACHINES CAN BE GROUPED
BASED ON HEATING SYSTEM EMPLOYED
1. Torch melted
2. Induction melted
3. Arc melted

27. Torch melting of noble metal alloy
 Fuel used is a mixture of natural or artificial gas and air. Or
oxygen and acetylene gas (high fusion alloys)
 Parts of the flame:
1. mixing zone- first long cone emanating directly from the
nozzle. Here the air and gas are mixed before
combustion. No heat is present in this zone.
2. Combustion zone-green color, immediately surrounding
the inner cone. Here the gas and air are partially burned.
This zone is oxidizing, should always be kept away from
the molten alloy during fusion.
3. Reducing zone- dimly blue in color. Located just beyond
the tip of the green combustion zone. Hottest part of the
flame. This zone is used for melting alloy.
4. Oxidizing zone-the outer cone. Area in which
combustion occurs with the oxygen in the air. Should not
be used. Oxidises the alloy
The air and gas mixture is adjusted to get a reducing flame,

29. Induction melting
 It is the process of heating an electrically
conducting object (metal) by electromagnetic
induction, where eddy currents are generated
within the metal and resistance leads to Joule
heating of the metal.
 An induction heater consists of an electromagnet,
through which a high frequency alternating
current is passed.
 Induction melting is useful for high fusing alloys
like metal-ceramic and base metal alloys.

30. Arc melting
 Direct current is passed between two electrodes-
a tungsten electrode and the alloy.
 Arc melting produces very high temperatures and
is used to melt high fusion metals like titanium.
 Arc melting may be done under vacuum or an
inert atmosphere like argon.
Numerous combinations of these principles are
employed in different machines.

31. Casting crucibles
 4types:-
1. Clay
2. Carbon
3. Quartz
4. Zirconia-alumina
 Clay crucible is used for high noble and noble
alloys.
 Carbon crucible for high noble crown and bridge
alloys and higher fusing gold based metal ceramic
alloys
 Quartz and zircinia-alunina crucible are
recommended for high fusing alloys of any type.
(specially those which are sensitive to carbon
contamination)

33. CLEANING THE CASTING
 After the casting has solidified, the ring is removed
and quenched in water as soon as the button exhibits
a dull-red glow
 ADVANTAGES
1. the noble metal alloy is left in an annealed condition
for burnishing, polishing and similar procedures
2. when the water contacts the hot investment , a
violent reaction ensues, resulting in a soft, granular
investment that is easily removed.
 Queching is done to achive the best grain structure.
 If it is quenced while it is too hot, the gold will be
softer and weaker.
 If it is allowed to bech cool completely, the grain
structure will be too large

34.  The ring is removed from the water, and the
investment and casting are pushed out of the
ring.
 As much of the investment as possible is broken
off by hand or with old instrument, then the
casting and button are scrubbed with a stiff
brush.the casting should appear smooth, with a
dull, dark oxide layer.
 The oxide layer and any remaining particles of
investment areremoved by lightly air abrading all
surfaces with a 50µm abrasive, taking care not to
abraid thin margins.

35.  Often the surface of the casting appers dark with
oxides and tarnish. This can be removed by a
process known as pickling.
 Pickling consists of heating the discolored casting
in an acid.
 For gypsum bonded investments 50%
hydrochloric acid solution is used. Disadvantage
is the acid fumes are a health hazard.
 Pickling process can be performed ultrasonically
while the prosthesis is sealed in a teflon
container.
 A solution of sulfuric acid may also be more
advantageous in this respect.
 Only gold castings may be cleaned by pickling.

36. CASTING DEFECTS
 any irregularity in the metal casting process that is
undesired is called Casting Defect.
 Casting defects can be classified under four
headings:
1. Distortion
2. Surface roughness and irregularities
3. Porosity
4. Incomplete or missing details.

37. Distortion
 Related to distortion of the wax pattern
 Can be prevented by proper manipulation of the
wax and handling the pattern
 Some distortion of the wax pattern occurs as the
investment around it hardens.
 Distortion increases as the thickness of the
pattern decreases.
 Generally, it is not a serious problem.

38. Surface roughness, irregularities and
discoloration.
 Excessive roughness or irregularities on the outer
surface of the casting necessitate additional
finishing and polishing, whereas irregularities on
the cavity surface prevent proper seating of an
otherwise accurate casting.
 Surface roughness is defined as relatively finely
spaced surface imperfections whose height,
width, and direction establish the predominant
surface pattern.
 Surface irregularities are isolated imperfections,
such as nodules, that are not characteristic of the
entire surface area

39. Types of Surface Roughness
 1. Nodules- Small nodules on the casting are
caused by air bubbles that become attached
to the pattern during or subsequent to the
investing procedure.

40. Types
a) Large nodules: Air trapped during investing
procedure.
b) Multiple nodules:
- Inadequate vacuum during investing
- Improper brush technique
- Lack of surfactant

41. c) Nodules on occlusal surface: due to excessive
vibration
d)Nodules on underside: prolonged vibration after
pouring.

42. Avoided by:
The best method to avoid air bubbles is to
use the vacuum investing technique.
 If manual method is used:
1. Use of mechanical mixer with vibration both
before and after mixing
2. Surfactant may be used. Surfactant should
be applied in thin layer and air – dried
because any excess liquid dilutes the
investment.

43.  When nodules are large
or situated on a margin,
they usually necessitate
remaking of the
restoration.
 When small, they can
often be removed with a
No. ¼ or No. ½ round
bur.
 A slight excess of metal
should be removed to
ensure that the nodule
does not interfere with
complete seating.
Contemporary fixed prosthodontics –
Rosensteil

44. 2. Ridges or Veins
 Wax is repellent to water, if the investment
becomes separated from the wax pattern in
some manner, a water film may form
irregularly over the surface.

45.  This condition occurs:
-If the pattern is slightly moved, jarred or vibrated after
investing.
- If there is no intimate contact of the investment and
the pattern
- Too high a liquid /powder ratio
-Use of surfactant help prevent such irregularities.

46. 3. Fins are caused by cracks in the investment
that have been filled with molten metal.

47. Cause of Fins:
- Weak mix of the investment i.e. high water/
powder ratio.
- Improper positioning of the pattern in investment
i.e. pattern placed too near the edge of the
investment
- Too rapid heating
- Premature heating

48. -Excessive casting force
- Rough handling of the ring after investing
- Liner flushed with the end of the ring.

49. 4. Spines
 Rapid heating of investment results in spines.
Sometime a characteristic surface roughness
may also be evident because of flaking of the
investment when the water/steam pours into
the mold.
 Ideally, 60 min should elapse during the
heating of the investment filled ring from room
temperature to 700°C.

50. 5. Inclusions
Carbon Inclusions: Carbon as from
- A crucible
- An improperly adjusted torch
- Carbon containing investment.

51. Foreign Body Inclusions
Any casting that shows sharp, well defined
deficiencies indicates the presence of some
foreign particles in the mold, such as;
I. Pieces of investment
II. Bits of carbon from flux

52.  A rough crucible former with bits of investment
clinging to it may roughen the investment on its
removal so that small particles of investment are
carried into mold with molten alloy.

53. 6.Porosities
Solidification porosity-
Localized shrinkage porosity
Shrink – Spot Porosity
Suck Back Porosity
Microporosity
 Trapped Gases-
Pin hole porosity
Gas Inclusions
Sub surface porosities
 Residual Air
Back pressure porosity

54. Shrink Spot Porosity
Cause:
 Premature termination of flow of metal during
solidification.
 Linear contraction of noble metal alloys in
changing from liquid to solid is 1.25%. Therefore
continual feeding of molten metal through the
sprue must occur to compensate for casting
shrinkage i. e. shrinkage during solidification.
 If the sprue solidifies before casting it usually
results in localized shrinkage porosity.

57.  It can be avoided by:
- Using sprue of correct thickness
-Attaching the sprue at the thickest portion of the
wax pattern
- Flaring the sprue at the point of attachment
- Placing the reservoir close to the wax – pattern

58. Suck-Back Porosity
 Localized shrinkage may also occur in the interior
of crown near the area of the sprue .
 The entering metal impinges on to the mold
surface at occlusoaxial/ incisoaxial line angle and
creates a higher localized mold temperature at
this region known as “Hot Spot”.
 A hot spot may retain a localized pool of molten
metal after other areas of the casting have
solidified. This in turn creates shrinkage void or
sucks back porosity.

59.  These porosities can be eliminated:
- By flaring the point of sprue attachment
- Reducing the mold melt temperature differential
i.e. lowering the casting temperature by about 30°

60. Left coping was casted at 1370 degree C
and right coping at 1340 degree C

61. Microporosity
 Occurs from solidification shrinkage but is
generally present in fine grain alloy casting
when the solidification is too rapid for the
microvoids to segregate to the liquid pool.
 Microporosity voids are irregular in shape.
This defect is not detectable unless casting is
sectioned.

62. Microporosity, pinhole, and gas inclusion
porosity

63. Pinhole and Gas Inclusion Porosity
 Both these porosities are related to the
entrapment of gas during solidification.
 Both these are characterized by spherical
contour but size is varied i.e. gas inclusion is
larger in size compared to pinhole.
 Many metals dissolve or occlude gases in
their molten state. On solidification, absorbed
gases are expelled resulting in these
porosities.
 These can be minimized by Premelting the
alloy, and by correctly adjusting the torch
flame during melting.

64. Alloy after heat treatment at 725
degree C for 70 min. Pinhole
Grain structure of the alloy

65. Subsurface Porosity
 Caused by simultaneous nucleation of solid
grains and gas bubbles at the first moment
that the alloy freezes at the mold walls.
 Can be minimized by controlling the rate at
which the molten metal enters the mold.

66. Subsurface porosity

67. Back Pressure Porosity
 Also known as entrapped air porosity. It can
produce large concave depression.
 Caused by inability of air to escape through
pores of investment or by pressure gradient
that displaces air towards end of the
investment via molten sprue and button.
 Occasionally found on the outer surface of
casting when the casting or mold temperature
is low that the solidification occurs before the
entrapped air can escape

68. Back pressure porosity

69.  The incidence of entrapped air has increased by
use of:
- Dense modern investment
- Increase in mold density
- By tendency of mold to clog with residual carbon
when low heat technique is used
- Proper burnout, an adequate mold and casting
temperature, a sufficiently high casting pressure
and proper L/P ratio can help to eliminate
entrapped air porosity
- Thickness of investment between the tip of
pattern and end of ring should be not more than

70. Incomplete Casting
 When due to some reason molten alloy is prevented
from completely filling the mold.
Causes:
i. Insufficient venting of mold
ii. Incomplete elimination of wax residue
iii. High viscosity of fused metals
iv. Inadequate metal
v. Cool mold or melt
vi. Wax pattern too thin
vii. If there is marginal discrepancy due to wax
pattern distortion and uneven expansion

71.  If short rounded margins with lumpy/ rounded
button- alloy not hot enough/ inadequate
casting force.
 If short rounded margins with sharp button-
pattern too far from the end of ring.
 If casting is shiny- incomplete burnout.

72. Incomplete casting resulting from
incomplete wax elimination is
characterized by rounded margins
and shiny appearance.

73. References
 Anusavice KJ, Shen C, Rawls HR. Phillips'
science of dental materials. Elsevier Health
Sciences; 2013.
 Shillingburg HT, Sather DA, Wilson EL, Cain JR,
Mitchell DL, Blanco LJ, Kessler JC.
Fundamentals of fixed prosthodontics.
Quintessence Publishing Company; 2012.
 Rosenstiel SF, Land MF. Contemporary fixed
prosthodontics. Elsevier Health Sciences; 2015
Jul 28.
 Morrow RM, Rudd KD. Dental Laboratory
Procedures: Fixed partial dentures. Mosby
Incorporated; 1986.