This document provides an overview of the casting process used in dentistry. It discusses the history of lost wax casting techniques dating back to the Bronze Age. The key steps of the casting procedure are described, including making the wax pattern, adding a sprue, investing in a refractory material, and burning out the wax prior to casting. Common casting defects are also outlined relating to distortion, discoloration, porosities and other issues. Causes of defects are correlated to specific steps in the procedure. Details are provided on factors that influence optimal wax pattern design, sprue design and investment selection and parameters.

1. CASTING
DEFECTS
PRESENTER: -
B A I S H A L I G H O S H
1 S T Y E A R P G T
D E P A R T M E N T O F
P R O S T H O D O N T I C S , C R O W N &
B R I D G E
UNDER THE ABLE
GUIDANCE OF:-
DR.(PROF)JAYANTA BHATTACHARYA
[HOD&PRINCIPAL]
DR.(PROF)SAMIRAN DAS
DR.(PROF)PREETIGOEL
DR.SAYANMAJUMDAR
DR.SUBHABRATAROY

2. Overview
• CASTING
• HISTORY
• CASTING PROCEDURE
• WAX PATTERN
• SPRUE
• CRUCIBLE FORMER
• CASTING RING
• INVESTMENT
• CASTING DEFECT
• CONCLUSION

3. The action of pouring or injecting a flowable material into a
refractory mold.
-GPT-9
CASTING

4. William Harrison Taggart
LOSTWAXTECHNIQUE
• LostwaxcastingtechniquedatesbacktoBronzeAge(approx.
3000-3500 BC).
• Taggart’spresentationtotheNew York OdontologicalGroupin1907
onthefabricationof castinlayrestorationsdevelopedin1905often
hasbeenacknowledgedasthefirstreportedapplicationofthe
lost-waxtechniqueindentistry.
• Taggartdevisedcentrifugalcastingmachineunder theprincipleof
lostwaxtechnique

5. CASTING PROCEDURE:-
DIRECT
WAX
PATTERN
INIDIRECT
WAX
PATTERN

6. CASTING DEFECTS
DISTORTION
DISCOLORATION
SURFACE ROUGHNESS &
IRREGULARITIES
POROSITIES
PROCEDURE
CO-RELATION BETWEEN STEPS OF THE PROCEDURE
& CONCURRENT CASTING DEFECTS.

7. STEPS IN
CASTING
PROCEDURE
I.Tooth/ teeth
preparation
Impression
I.Die
preparation
I.Wax pattern
fabrication
Attachment of
sprue former
I.Ring liner
placement
Assembly of
casting ring
Investing
Burn out or
wax
elimination
Casting
Sand blasting
and recovery
I.Finishing
and polishing

8. DESIARABLE PROPERTY:-
When softened, wax should be uniform.
Color should contrast with the die material.
No Flakiness or similar surface roughness when wax is bent
& molded after softening.
ADASpecificationNo.4requiresthat themeltedwax, whenvaporized at
500˚ C,leavenosolidresidueinexcessof0.10% oftheoriginalweightofthe
specimen.
Wax should not be pulled away with carving instruments
or chip during carving
ACCORDING TO ADA SPECIFICATION NO.
4 FOR DENTAL INLAY CASTING WAX
COVERS TWO TYPES OF INLAY WAX:-
TYPE-I
TYPE-II
USED IN DIRECT
TECHNIQUE.
USED IN INDIRECT
TECHNIQUE.
MEDIUM
WAX
SOFT
WAX
FLOW:- TYPE
OF
WAX
MIN
FLOW
AT 45˚C
MAX
FLOW
AT 45˚C
I 70% 90%
II 70% 90%
If the wax does not have sufficient
plasticity, it will not flow into all of
the areas in the preparation and
reproduce the required detail.
COEFFICIENT OF THERMAL EXPANSION:-
 Inlay wax thermally expands and contracts more per degree of
temperature change than any other dental material.
 Average linear coefficient of thermal expansion is 350 × 10−6/˚ C.
 This inherent disadvantage of wax is more significant when they are used
in the direct technique. This property is less significant when the wax is
used in the indirect technique, because the pattern is not subjected to a
change from mouth to room temperature.
PROPERTIES OF INLAY WAX THAT CAN LEAD
TO A CASTING DEFECT.:-
ELASTIC MEMORY:-
Waxes, like other thermoplastics, tend to return partially to
their original shape after manipulation. The property
responsible for this phenomenon is commonly known as
elastic memory.
A stick of inlay wax is bent into the shape of a
horseshoe and floated on water at room
temperature. After 24 hr the same stick of
wax tends to relax and distortion occurs.
WAX
PATTERN
DESIGNED
BY INLAY
WAX
PHILLIP’S SCIENCE OF DENTAL MATERIALS 11TH EDITION

9. PROVISION FOR ADEQUATE
CEMENT SPACE
ADDING UP WAX IN INCREMENTS
FOLLOWED BY CARVING
INDIRECT WAX PATTERN FABRICATION
P.K.T instruments
no 1 - 5
CONTEMPORARY FIXED PROSTHODONTICS 3RD EDITION, ROSENSTIEL
ASpace should exist betweentheinternal surface
of the casting and theprepared surfaceof the tooth
everywhereexcept immediately adjacent to the
margin.
Theideal dimension for the cement spacehasbeen
suggested at20to 40umfor eachwall whichimplies
that acomplete crown should have aninternal
diameter between 40and 80um larger thanthe
diameter of theprepared tooth.
Diespacer isapplied to provide thisspace leaving a
band of 1mmatthemargin for marginaladaptation.
Pattern fabrication by
incrementing layers
of wax.

10. SPRUE
Sprue is defined as a channel or hole through
which metal is poured or cast into a reservoir and
then into a mold.
- GPT9

11. VARIABLES &
PRINCIPLES
OF OPTIMAL
SPRUE
DESIGN
SPRUE
DIAMETER
SPRUE
POSITION
SPRUE
ATTACHME
NT
SPRUE
DIRECTION
SPRUE
LENGTH

12. BASIC
REQUIREMENT
OF A SPRUE
Sprue must allow molten
wax to escape from the
mold.
Sprue must enable
molten metal to flow into
the mold with as little
turbulence as possible.
Metal within it must
remain molten slightly
longer than the alloy that
has filled the mold.
DIAMETER
Wide diameter= To improve the flow
of molten metal wider diameter of
sprue is preferred specially in
centrifugal casting machine
Narrow diameter=In the air-pressure machines, the
melt is made directly in the depression created by
the crucible former and then forced into the mold
by the sudden change in air pressure. With this
technique a narrow sprue prevents the molten
metal from flowing into the mold prematurely.
FOR MOLAR & METAL CERAMIC
CROWN=2.5mm (10-gauge) sprue is
recommended.
FOR PREMOLAR=2.0mm (12-gauge) is
adequate.
CONTEMPORARY FIXED PROSTHODONTICS 3RD EDITION, ROSENSTIEL

13. Sprue should be long enough to
properly position the pattern in the
casting ring, within 6 mm from the end
of the ring .
 But short enough so that the molten
alloy doesn’t solidify before it fills the
mold.
Length of sprue for,
GYPSUM BONDED INVESTMENT=
6mm
PHOSPHATE BONDED= 3-4 mm
DEFECTS DUE TO SPRUE LENGTH:-
a) SHORT SPRUE= Moves the pattern more
away from the end of the ring and the gasses
cannot be adequately vented.
B) LONG SPRUE= Solidify before the
molten metal reaches the mold leading to
casting shrinkage and incomplete casting.
LENGTH OF
SPRUE
The Sprue should be attached
to the bulkiest part of the
pattern, away from margins
and occlusal contacts.
Normally the largest non-
centric cusp is used.
SPRUE
FORMER
LOCATION
CORRECT
POSITION OF
SPRUE
INCORRECT
POSITION OF
SPRUE
PHILLIP’S SCIENCE OF DENTAL MATERIALS 11TH EDITION

14. SPRUE DIRECTION
 The sprue former is attached at an angle of 45° to
allow the molten metal to flow freely to all the portions
of the mold.
 Turbulence created by a sprue attached at 90° may
cause the following casting defects:-
A) SUCK BACK POROSITIES
B) CASTING FORCES MAY FRACTURE THE
INVESTMENT.
SPRUE ATTACHMENT
 The sprue’s point of attachment to the pattern
should be smooth to minimize TURBULENCE.
Generally it must be FLARED= Facilitates the
entry of the alloy.
Sprueformerprovides adirect
connectionbetweenthepattern
andcrucibleformer.
Aconnectororreservoirbaris
positioned betweenthepattern and
thecrucibleformer.
PHILLIP’S SCIENCE OF DENTAL MATERIALS 11TH EDITION

15. Reservoir is placed at the heat center of ring.
Function:-
a) Prevents localized shrinkage porosity.
b) Molten alloy will remain in the reservoir to
furnish liquid alloy into the mold as it solidifies.
VENTING
Small auxiliary sprues or vents have
been recommended to improve casting
of thin patterns and may help in:-
a) Escape of gases during casting.
b) Compensate for the shrinkage during
solidification.
c) Solidification begins in critical areas
by acting as heat sink.
RESERVOIR

16. According to PHILIPS 11th edition, casting crucibles
can be of following types:_
AccordingtoGPT-9,
Crucible former is the base to which a sprue former is
attached while the wax pattern is being invested in
refractory investment.
Theexactshapeofthecrucibleformerdependson
thetypeofcastingmachineused.
Withmostmodernmachines,thecrucibleformeris
talltoallowuseofashortsprueandallowthepattern
tobepositioned neartheendof thecastingring.
PHILIPS 11th edition
CRUCIBLE is a vessel or container made of any refractory material
(frequently ceramics) used for melting or calcining any substance that
requiresahighdegreeofheat.-GPT-9

17. CASTING RING
METAL
RING
Castingringisametalorsiliconetube
inwhicharefractorymoldismadefor
castingdentalrestorations.-GPT-9
Internaldiameterofcastingring:-
5-10 mmgreaterthanthewidest
measurementofthepattern&6mm
higher.
Forsinglecrown smallrings
of32mmdiameterare used.
For large fixed partial
denture 60mmround/oval
shapedcastingringsareused.
FUNDAMENTALS OF FIXED PROSTHODONTICS 4TH EDITION, SHILLINBERG

18. METAL RING
COMPARATIVEEVALUATION:-
Theverticalmargindiscrepancyoftheringless
groupforthebuccal,thelingual,andtheaxialsites
weresignificantlylessthanthatof the ringgroup.
RINGLESS SYSTEM
Withtheuseof higherstrength,phosphatebonded
investments,theringlesstechniquehasbecome
quitepopular.
Thismethodentailstheuseofapaperorplastic
castingringandisdesignedtoallowunrestricted
expansion.
Usedfor basemetalalloy.

19. CASTING
RING
LINER
Share something memorable
that relates to your topic
PHILIPS 11th edition
TYPES OF CASTING
RING LINERS

20. investing
The process of covering or enveloping ,
entirely or in part, an object such as a
denture, tooth, wax form, crown etc.,
with a suitable investment material
before processing, soldering or casting
(GPT 9)

21. STEP-1
Outer circle represents the peripheral boundary of
the liquid mix and the three round particles, which
are enclosed, represent undissolved particles of the
hemihydrate of calcium sulfate.
STEP-2
In the case of hygroscopic expansion, water lost in
the reaction is compensated for by additionally
supplied water, and no dimensional change is
manifest.
STEP-3
Contraction doesn’t occur due to replenished
water.
STEP-4
In the case of hygroscopic setting, the additionally
supplied water continues to compensate for water
lost in the reaction and provides space for outward
crystal growth.
STEP-5
The expansion ends, with hygroscopic conditions
demonstrating a greater expansion due to
uninhibited crystal growth with longer crystals.
STEP-1
Outer circle represents the peripheral boundary of
the liquid mix and the three round particles, which
are enclosed, represent undissolved particles of the
hemihydrate of calcium sulfate.
STEP-2
Initial crystal growth is represented by the radial lines
extending from the surface of the hemihydrate
particles. At this point, in the case of normal setting,
the utilization of water in the reaction results in the
solid phases being drawn together, and a contraction
is recorded.
STEP-3
The solid phases make contact, and no
further peripheral contraction can occur.
STEP-4
Expansion commences.
STEP-5
Expansion terminates
Normal setting conditions result in shorter crystal.

22. SETTING EXPANSION HYGROSCOPIC
EXPANSION
THERMAL EXPANSION
RANGE 0.4-0.6% 1.2-2% 1-1.6%
RELEVANCE IN
INVESTMENT
PROCEDURE
Compensates for the
solidification shrinkage of alloy.
Hygroscopic expansion
occurs when water is added to
the setting gypsum investment
immediately after the ring has
been filled.
Usually this is accomplished
by submerging the ring in a
water bath at 37° C (100° F) for
up to 1 hour immediately after
investing.
A wet ring liner also
contributes hygroscopic
expansion to that portion of
the mold with which it is in
contact.
As the mold is heated to
eliminate the wax, thermal
expansion occurs.
The silica refractory material
is principally responsible for
this because of solid-state phase
transformations.
Cristobalite changes from a
to B (high-temperature form)
between 200° C (392° F) to
270° C (518° F); quartz
transforms at 573° C (1067° F).
These transitions involve a
change in crystal form,
decreased density, producing a
volume increase in the
refractory components.

23. GYPSUM BONDED
INVESTMENT
PHOSPHATE BONDED
INVESTMENT
ETHYL SILICATE
BONDED INVESTMENT
USE:- For conventional casting of
gold alloy inlays.
Onlays, Crowns and FPD
Type I investment=For metal
ceramic restorations.
Type II investment=Base
metal alloys.
For casting of removable
partial alloy with high fusion
base metal alloy.
PROCEDURE OF
BURNOUT:-
The investment is slowly
heated to 650°c - 700°c in 60
minutes.
It is maintained for 15-
30mins at this temperature
Above 700°c = Sulphur
dioxide contaminates gold
casting and makes them
extremely brittle.
Type 1 investments
Type 2 investments
This type of investment can
be heated between 1090˚ C and
1180˚ C
Investment is set in a furnace
at room temperature, after 1
hour of spatulation.
It is heated to 800°C or higher
over a period of 1 to 2 hours and
held at the upper temperature for
about 30 minutes before casting.
Insertion of a mold, after about 30
minutes of setting time, directly into
a furnace at the burnout temperature
(near 800°C or higher)
DENTAL MATERIALS AND THEIR SELECTION / WILLIAM J. O'BRIEN - 4TH ED

24. TORCH ELECTRICAL
Gas-air Resistance
Gas-oxygen Induction
Oxygen-acetylene
(HOTTEST FLAME)
Electric arc
Air-acetylene
DENTAL MATERIALS AND THEIR SELECTION / WILLIAM J. O'BRIEN - 4TH ED
ZONES OF FLAME
METHODS OF ALLOY MELTING

25. CASTING MACHINES
CENTRIFUGAL CASTING MACHINE:-
It makes use of centrifugal force to thrust the liquid metal into the mold.
Centrifugal casting is a method of casting parts having axial symmetry.
Pouring molten metal into a cylindrical mold spinning about its axis of rotation.
The mold is kept rotating till the metal has solidified.
AIR PRESSURE CASTING MACHINE
Alloy is melted in the hollow left by the crucible former by torch flame and
then air pressure is applied through a piston.
INDUCTION CASTING MACHINE
 The electrical induction furnace is a transformer in which an alternating current flows
through the primary winding coil & generates a variable magnetic field in the location of
the alloy to be melted in a crucible.
The molten metal is then forced into the mold by air pressure.

26. CASTING MACHINES
DIRECT CURRENT ARC MELTING MACHINE:-
Direct current arc is produced between 2 tungsten electrodes.
Temperature exceeds 4000°c, the alloy melts very quickly.
ELECTRICAL RESISTANCE HEATED CASTING MACHINE:-
Current is passed through a resistance heating conductor and automated melting of metal occurs in
graphite crucible.
This is an advantage, especially for alloys such as those used for metal-ceramic prostheses, which are
alloyed with base metals in trace amounts that tend to oxidize on overheating.
VACUUM CASTING MACHINE:-
Vacuum is applied through the base beneath the casting ring and the molten alloy can be
drawn into the mold by negative pressure.

27. Pickling is heating the discolored casting in an acid.
The best pickling solutions for gypsum bonded investments is a 50%
hydrochloric acid solution.
The hydrochloric acid aids in the removal of any residual investment, as well
as of the oxide coating.
The disadvantage of hydrochloric acid is that the fumes from the acid are
likely to corrode laboratory metal furnishings. In addition, these fumes are a
health hazard and should be vented via a fume hood.
 A solution of sulfuric acid may also be more advantageous in this respect.
 Ultrasonics are also useful for cleaning the casting.

28. CLASSIFICATION
OF CASTING
DEFECT
According to Anusavice:
(1) Distortion
(2) Surface roughness
and irregularities
(3) Porosity
(4) Incomplete casting or
missing detail
According to Rosenstiel
 Roughness
 Nodules
 Fins
 Incompleteness
 Voids or porosity
 Marginal discrepancies
 Dimensional inaccuracies

29. General
problems
Problems
with internal
porosity
Problems
with
external
porosity
accuracy
distortion
bubbles
fins
short rounded margins
miscastings
pits
localize shrinkage porosity
subsurface porosity
micro porosity
back
pressure
porosity
According to O’Brien

31. INVESTMENT
MATERIAL
PROPERTIES
OF INLAY
WAX.
MANIPULATION
OF THE WAX
PATTERN
THICKNESS
TYPE
HYGROSCOPIC AND SETTING EXPANSION
of investment materials compensates it.
INVESTMENT MATERIAL
Distortion
increases as
thickness of
pattern decreases.
Gingival margins(thinner than
occlusal bar of wax) are forced apart
by mold expansion whereas the solid
occlusal bar of wax resists expansion
during early stage of setting.
This leads to uneven
movement of walls of
wax pattern when
investment is setting.
In a study
conducted by
Michio Ito et al
4 types of wax with
different softening
temperature were tested
to evaluate their casting
shrinkage.
The study concluded
that wax with highest
softening temperature
yielded greatest casting
shrinkage.

32. TO MINIMIZE
THE DISTORTION
 Not to be subjected to extreme
temperature
Place in increments
Never cool suddenly
Carve with sharp instruments
Patterns should never be left off the die.
Invested as soon as possible after
fabrication.
RATIONALE
Wax has
memory that is it
exhibits some
form of
elasticity unless
it is thoroughly
liquified.
It is overcome
by adding wax in
increments or by
dipping the die
entirely in the
molten wax.
However a serious
problem exists
when the added
wax incorporates
strain within the
pattern as each
increment
solidifies.
The strain tends to be
released with time
and subsequently
distorts the wax
pattern. The rate of
wax change is time
and temperature
dependent.
Because wax has
a relatively high
coefficient of
thermal
expansion.
The pattern will tend to
release its incorporated
strain hence the margins
must be remelted,
readapted, and
resmoothed
immediately before
investing

33. SURFACE
ROUGHNESS AND
IRREGULARITIES.
• According to Phillip’s Science of Dental
materials, 11th edition,
• SURFACE ROUGHNESS:-
• It is defined as relatively, finely spaced
surface imperfections.
• SURFACE IRREGULATIES:-
• They are isolated imperfections such as
nodules that are not characteristic of the
entire surface area.

34. CAUSE APPEARANCE REMEDYPREVENTION
AIR BUBBLES
cause small nodule on a
casting.
 Large nodule:-
• Air trapped during
investing
 Multiple nodules:-
• Improper brushing
technique.
• Inappropriate
application of wetting
agent.
 Nodules on occlusal
surface:-
• Prolonged vibration
after pouring
 The best method to avoid air bubbles is to use the vacuum investing
technique.
Advantages of vacuum mixing-----
1)Remove air bubbles
2) Produce smooth castings
3)Increase tensile strength
 It is important that the wetting agent be applied in a thin layer. It is best to
air dry the wetting agent, because any excess liquid dilutes the investment,
possibly producing surface irregularities on the casting
 The incidence of nodules on casting is more in hand mixing than vacuum
mixing. The best method is vacuum mix and vacuum pour technique. But
most popular method vacuum mix and open pour(hand pour)

35. CAUSE APPEARANCE PRECAUTION
 Wax  water repellent
 Water creates space in
between wax & investment
 Appears as minute ridges or
veins
 Applying wetting agent
 Pattern should not be
moved, jarred or
vibrated after investing.
 Avoid too high liquid :
powder ratio
Rapid heating results in fins or
spines on the casting or may
appear as a characteristic
surface roughness.
The mold should be heated
gradually ; at least 60 min
should elapse during the
heating of the investment-
filled ring from room
temperature to 700˚ C.
The greater the bulk of the
investment, the more slowly
it should be heated
RAPID HEATING
RATES
PHILLIPS SCIENCE OF DENTAL MATERIAL 11TH EDITION

36. CAUSE APPEARANCE PRECAUTION
Under heating leads to:-
 Incomplete elimination of wax
residues may occur if the heating time
is too short or if insufficient air is
available in the furnace.
 The casting may be covered with a
tenacious carbon coating that is
virtually impossible to remove by
pickling
 Voids or porosity may occur in the
casting from the gases formed when
the hot alloy comes in contact with the
carbon residues.
 Heating the investment ring, for
adequate time and temperature so
as to remove carbon residues.
 The burn out should be done with
sprue hole facing downwards( for
the wax to run down). This
facilitates its complete removal.
Prolonged heating leads to:-
 Disintegration of the investment
 Products of decomposition are sulfur
compounds.
 When the thermal expansion
technique is employed. The mold
should be heated to the casting
temperature—never higher.
 The casting should be made
immediately.
UNDER HEATING
PROLONGED HEATING
UNDER HEATING LEADS TO:-

37. If an alloy is heated too high a temperature
before casting, the surface of the investment
is likely to be attacked, and result in surface
roughness.
In all probability, the alloy will not be
overheated with a gas-air torch.
If other fuel is used, special care should be
observed that the color emitted by the molten
gold alloy, for example, is no lighter than a
light orange.
 Too high a pressure during casting can
produce a rough surface on the casting.
A gauge pressure of 0.10 to 0.14 MPa in an
air pressure casting machine
Three to four turns of the spring in an average
type of centrifugal casting machine is
sufficient for small castings.
TEMPERATURE OF ALLOY
CASTING PRESSURE
PHILLIPS SCIENCE OF DENTAL MATERIAL 11TH EDITION

38. Higher the liquid/powder ratio, rougher is the surface of the casting.
However, if too little water is used, the investment may be unmanageably thick and cannot be
properly applied to the pattern.
LIQUID/POWDER
RATIO
A rough crucible former with investment clinging to it may roughen the investment on its
removal so that bits of investment are carried into the mold with the molten alloy.
It creates not only surface roughness but also voids.
Sharp, well defined deficiencies on the casting
indicates presence of foreign particles
FOREIGN BODIES
The ratio of binder to quartz influences the surface texture of casting.
In addition coarse silica causes surface roughness.
COMPOSITION OF
INVESTMENT
Carbon from a crucible or an improperly adjusted torch or a carbon-containing investment,
can be absorbed by the alloy during casting.
These particles may lead to the formation of carbides or even create visible carbon inclusions.
CARBON
INCLUSIONS
PHILLIPS SCIENCE OF DENTAL MATERIAL 11TH EDITION

39. The direction of the sprue
former should be such that the
molten gold alloy does not
strike a weak portion of the
mold surface
Sprues should be placed so that the metal
can flow into the mould without any
major change in direction
Sprues should not be attached to thin
sections of the pattern since the metal may
freeze too quickly
PHILLIPS SCIENCE OF DENTAL MATERIAL 11TH EDITION

40. Pattern
position
 Too many patterns positioned in the same plane
should be avoided,
 They should not be placed too close together.
The expansion of wax is much greater than
that of the investment, causing breakdown or
cracking of the investment if the spacing
between patterns is less than 3 mm.
PHILLIPS SCIENCE OF DENTAL MATERIAL 11TH EDITION

41. I. SOLIDIFICATION
II. TRAPPED GASES
III. RESIDUAL AIR
• Localized shrinkage
porosity
• Suck-back porosity
• Micro-porosity
• Pinhole porosity
• Gas inclusion
• Subsurface porosity
• Back pressure porosity
POROSITY

42. LOCALIZED
SHRINKAGE
POROSITY
 Localized Shrinkage Porosity is caused by
premature termination of the flow of molten
metal during solidification.
 Linear contraction of noble metal alloys in
changing from liquid to solid state is at least
1.4-1.7%.
 Hence continual feeding of molten metal
through sprue must occur to make up for
shrinkage of metal volume during
solidification.
 Localized shrinkage generally occurs near the
sprue-casting junction
 LOCALIZED SHRINKAGE POROSITY
MAINLY TAKES PLACE WHERE
SOLIDIFICATION OCCURS LAST.
Solidifies
first
Solidifies
later
Solidifies
last

43. LOCALIZED SHRINKAGE IN PONTIC
AREA
CAUSE OF POROSITY IN PONTIC AREA:-
It is caused by the ability of the pontic to retain heat because of its bulk and
because it was located in the heat center of the ring.
SOLUTION:-
This problem can be solved simply by attaching one or more small-gauge
sprues (e.g., 18-gauge) at the surface most distant from the main sprue
attachment and extending the sprue(s) laterally within 5 mm of the edge of the
ring. These ensure that solidification begins within the sprues, and they act as
cooling pins to carry heat away from the pontic.

44. CAUSE
Improper sprue length i.e. increased sprue
length.
Constricted sprue pattern junction i.e.
decreased diameter at neck.
Point of attachment of sprue to the pattern
should be flared else it will cause turbulence.
Larger pattern with single sprue.
THE EFFECT OF SPRUE ATTACHMENT
DESIGN ON CASTABILITY AND POROSITY
R.G.VERRET et al did a study on the effect of sprue attachment
design on porosity and castability in 1989.
The study concluded that straight and flared attachments
optimized castability and minimized porosity.
LOCALIZED SHRINKAGE POROSITY

45. Length of sprue should be such that
the end of wax pattern lies at:-
INVESTMENT MATERIAL
6mm from the end of ring Gypsum bonded investment
3-4 mm from the end of ring Phosphate bonded investment
LOCALIZED SHRINKAGE CAN BE AVOIDED BY:-
If the sprue length is large, RESERVOIR should be attached to supply molten metal.
POSITION OF RESERVOIR= Close to the pattern with size larger than the
thickest portion of pattern.
SIZE OF THE SPRUE:- PATTERN
10 gauge sprue Most pattern
12 gauge sprue Small pattern
Point of attachment should be flared.
Sprue attached to the thickest part of the pattern.
If the pattern is large, auxillary sprue should be attached.

46. SUCK BACK
POROSITY
 IF A HOT SPOT HAS BEEN
CREATED BY THE HOT METAL
IMPINGING FROM THE SPRUE
CHANNEL ON A POINT OF THE
MOLD WALL. THIS HOT SPOT
CAUSES THE LOCAL REGION TO
FREEZE LAST AND RESULTS IN
WHAT IS CALLED SUCK-BACK
POROSITY.
 SUCK-BACK POROSITY OFTEN
OCCURS AT AN OCCLUSO-AXIAL
LINE ANGLE OR INCISO-AXIAL
LINE ANGLE THAT IS NOT WELL
ROUNDED.
Hot spot

47. CAUSE
When the sprue is attached at
90°,molten metal impinges on the
mold surface opposite to the point of
entry.
This causes a higher localized mold
temperature called HOT SPOT
It may retain a localized pool of
molten metal after other areas have
solidified creating shrinkage voids.
CAN BE AVOIDED BY
Sprue attachment at gliding angle of
45°
Flaring the point of sprue
attachment.
Reducing the mold-metal temperature
difference;
Lowering the casting temperature by
about 30°
SUCK BACK POROSITY

48. MICROPOROSITY
Microporosity occurs from solidification
shrinkage but is generally present in fine-
grain alloy castings when the solidification is
too rapid for the micro voids to segregate to
the liquid pool.
It can be seen only when the casting is cut.
CAUSE:-
Too low mold
temperature.
Too low casting
temperature.
PRECAUTION:-
Mold and casting
temperature should
not be too low.

49. PIN HOLE
PPOROSITY
Many metals dissolve
or occlude gases when
they are molten.
On solidification,
absorbed gases are
expelled out.
This leads to PIN
HOLE POROSITY.
GAS
INCLUSION
POROSITY
Gas that is mechanically
trapped by molten metal in
the mold or that is
incorporated during casting
procedure leads to GAS
INCLUSION POROSITY
PINHOLE POROSITY/GAS
INCLUSION POROSITY
Displaced air
Molten metal
Sprue
Trapped air
Molten metal
Sprue

50. CAUSE
Use of oxidizing or mixing zones of
flame for melting alloy.
Contaminants of metal during melting.
CAN BE AVOIDED BY
Use of reducing zone of flame.
Use of graphite crucible for heating the alloy.
PIN HOLE POROSITY/GAS
INCLUSION POROSITY

51. • It is caused by the simultaneous nucleation of solid
grains and gas bubbles at the first moment that the
alloy freezes on the mold walls.
• Visible only when casting is polished.
SUBSURFACE POROSITY
PRECAUTION:-
Neck of the sprue shouldn’t be too
narrow or length shouldn’t be too short
else rate of molten metal entering will
be high causing more subsurface
porosity.
It can be diminished by controlling
the rate at which molten metal enters
the mold.

52. • Back pressure porosity is also
referred to as entrapped air
porosity.
• It produces large concave
depressions on inner surface of
casting
BACK PRESSURE
POROSITY

53. CAUSE
• Insufficient casting pressure that
prevents displacement of air
pockets towards the end of
investment.
CASTING
PRESSURE
• Dense investment due to low
liquid/powder ratio.
LIQUID/POWDER
RATIO
• Thickness of investment at the end
of wax pattern is more than 6mm.
• Increased mold density due to
vacuum investing.
INVESTMENT
CAN BE AVOIDED BY:-
BACK PRESSURE POROSITY
• Sufficient casting pressure of 0.10 to 0.14 Mpa (15 to
20 psi).
• Adequate liquid/powder ratio and uniform particle
size.
• Brush technique= After the pattern is painted with
investment; mixed investment is poured in to the ring
by holding at an angle so that the investment flows
down its side, reducing the possibility of air
entrapment.
• Investment between the tip of the pattern and the
end of the ring shouldn’t be more than 6mm.

54. Comparison between various porosities.
Microporosity
& Gas
inclusions
Subsurface
porosity
Localized
Shrinkage
Porosity
TYPE OF
POROSITY
SPRUE
THICKNES
S
SPRUE
LENGTH
MELT
TEMPERA
TURE
MOLD
TEMPERA
TURE
Localized
Shrinkage
Reduced Increased Reduced Reduced
Subsurface
Porosity
Increased Reduced Increased Increased
Microporosity No effect No effect Reduced Reduced

55. INCOMPLETE
CASTING
INSUFFICIENT
VENTING OF
THE MOLD
INCOMPLETE
MELTING OF
ALLOY
TOO LOW
CASTING
FORCE
BLOCKING
OF SPRUE
DUE TO
LOOSE
INVESTMENT
PARTICLES
INCOMPLETE
DEWAXING
INSUFFICIENT
ALLOY
BLOCKING
DUE TO PRE
SOLIDIFICAT
ION OF
ALLOY IN
SPRUE

56. DISCOLORATION
CAUSES
BLACK CASTING:-
Sulphur contamination of casting.
Contamination with copper during
pickling of casting.
Contamination with mercury.
PREVENTION
Avoid overheating of investment.
Avoid using oxidizing zone of torch flame.
Avoid using steel tongs to hold casting
during pickling.
Casting should never be placed with
amalgam dies or kept in a table where
amalgam scrap is present.

57. CONCLUSION

58. REFERENCE
1. Phillip’s science of dental materials-11th edition by Kenneth J.Anusavice
2. Contemporary fixed prosthodontics-4th edition by Rosenstiel.Land.Fujimoto
3. Introduction to metal-ceramic technology-2nd edition by W.Patric Naylor
4. Restorative dental materials -8th edition by Robert G.Craig
5. The science and art of dental ceramics-volume ii bridge esign and laboratory procedures in dental
ceramics by John W.McLean
6. Materials in dentistry –priciples and applications by Jack L.Ferracane
7. Notes on dental materials-6th edition by E C.Combe
8. Applied dental materials-9th edition by John F.McCabe and Angus W.G.Walls
9. Pelopidas Lombardas et al Dimensional accuracy of castings produced with ringless and metal ring
investment systems the journal of prosthetic dentistry, July 2000
10. R. Compagni,et al Effects of sprue design, casting machine, and heat source on casting porosity the journal
of prosthetic dentistry july 1984 volume 52 number 1