Casting procedure

CASTING
PROCEDURE
Dr. Deepak K. Gupta

Definition
 Casting is defined as something that has
been cast in a mold, an object formed by
the solidification of a fluid that has been
poured or injected into a mold.(GPT)
 “Casting is the process by which a wax
pattern of a prepared tooth is fabricated
and converted to its metallic
replica”(Rosenteil)
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Steps in casting
 Preparing the wax pattern
 Spruing the wax pattern
 Attaching the sprue to crucible former
 Investing the pattern in the casting ring
 Burnout of the wax
 Casting
 Recovery
 Finishing and polishing

Steps involved
in the casting

Preparing the wax pattern
Prior to casting
 margin of the wax pattern should be
readapted
 pattern checked for smoothness, finish and
contour
 Sprue should be attached to the thickest
portion of the wax pattern
 wax pattern can be removed from the die
using sprue
 surfactant should be applied on the wax to
obtain wetting of the investment
 invested immediately to prevent distortion

SPRUING OF WAX PATTERN
 Sprue is defined
as “the channel or
hole through
which plastic or
metal poured or
cast into gate or
reservoir and then
into mold”

SPRUE DESIGN
 it must allow the molten wax
to escape from the mold
 enable molten metal to flow
into the mold with minimal
turbulence
 metal within the sprue must
remain molten slightly longer
than the alloy that has filled
the mold this will act as a
reservoir to compensate the
shrinkage

SPRUE MATERIALS

SPRUE MATERIALS
Wax: preferred for most casting because they
melt at the same rate as the pattern and
allow for easy escape for molten wax
Plastic: resist distortion rigid,
 may block the escape of wax,
 hollow plastics are available
Metal: non rusting metal should be used,
 removed before casting

Diameter
 should be larger than the thickest portion
of the wax pattern
 2.6 mm can be used for most patterns
 2.0mm for premolar partial veneer
restoration
 narrow sprue are sufficient for casting to be
done on centrifugal machine

POSITIONING OF THE
SPRUE
 Sprue should be attached to the point
of greatest bulk
 45 angulation near the bulk of the
pattern
 axial wall should have obtuse angle
135
 This prevents air entrapment during
investing and suck back porosity after
casting
 attachment should be flared to
prevent turbulence during metal flow

VENTING
 Small auxiliary sprues or vent should
be placed to improve the casting
 By allowing the gases to escape

CRUCIBLE
FORMER
 The sprue is attached to a crucible
former, usually made of rubber,
which constitutes the base of the
casting ring during investing.
 May be metal , plastic or rubber
 The exact shape of the crucible
former depends on the type of
casting machine used.
 With most modern machines, the
crucible former is tall to allow use of
a short sprue and allow the pattern
to be positioned near the end of the
casting ring.
 also referred to as a sprue former

Diagrammatic
representation of a dental
casting mold
A, Crucible former.
B, Sprue.
C, Cavity formed by wax
pattern after burnout. D,
Investment.
E, Liner.
F, Casting ring.
G, Recommended
maximum investment
thickness of approximately 6
mm between the end of the
mold cavity and the end of
the invested ring to provide
pathways for sufficient gas
escape during casting.

Casting rings
 Casting rings are used to confine the
fluid investment around the wax
pattern while the investment sets.
And …….
 Considerations in selection of casting
rings:

They are available as:
1) Shapes - Round
- Oval
2) Complete rings –
Rigid - Metal
- Plastic
Flexible - Rubber
3) Split rings - Metal
- Plastic

Ring less casting system:
- plastic rings which is conical in shape with
tapering walls are used.
- Used for traditional gold-base alloys.
Flexible rings Split casting rings

Casting ring liner
Materials used are:
- Asbestos liner
- Cellulose liner
- Ceramic liner
- Combination of ceramic
and cellulose liner

Function of casting ring
liner:
 Allow uniform expantion.
 In case of wet liner
technique- hygroscopic
expansion.
 Thickness of the liner
should be less than 1mm.

RINGLESS CASTING TECHNIQUE
 With the use of higher-
strength, phosphate-
bonded investments, the
ringless technique has
become quite popular.
 The method uses a paper or
plastic casting ring and is
designed to allow
unrestricted expansion.

INVESTING
 The process of covering or
enveloping an object such as a
denture, tooth, wax form,
crown, with a suitable
investment material before
processing, or casting
 Investment materials are:
 Gypsum bonded investment
 Phosphate bonded
investment
 Ethyl silicate bonded
investment

-Thin film of cleaner on pattern
reduces surface tension of wax
better “wetting” of wax pattern
by the investment.
-Some of the commercially available
debubblizing agents can be used.
•The wax pattern should
not stand for more than
20-30 min
before being invested.
So, it is best to invest
the wax pattern as soon
as possible

 Investment mixing:
1. Hand mixing
2. Vacuum mixing
 After mixing the
investment is poured in to
the casting ring up to its
rim

Wax elimination or burn out
 It is advisable to begin the burnout procedure while
the mould is still wet.
 Water trapped in the pores of the investment reduces
the absorption of wax.
 As the water vaporizes it flushes wax from the mold.
 Purpose of burn out: to flush out wax pattern and
create a hollow cavity
 Heat application: Heating of ring should be done
slowly.
 Hygroscopic low-heat technique
 High-heat thermal expansion technique

BURNOUT FURNACE

Hygroscopic low-heat technique
 The temperature used in this technique is 500°c for 60-90
mins.
 Obtain compensation expansion from three sources:
 Immersion of investment in 37°c water bath.
 The warm water entering the investment mold from the top
adds some of the expansion.
 The thermal expansion at 500°c
 This technique causes 0.55% of expansion.

HIGH-HEAT THERMAL EXPANSION
TECHNIQUE
 The investment is slowly heated to 650°c - 700°c in
60mins. Then maintained for 15-30 mins at this
temperature.
 Above 700°c sulfur dioxide
- Contaminates gold castings and makes them
extremely brittle

ACCELERATED CASTING METHOD
 To reduce the total time, Alternative Accelerated
casting technique is proposed
 Uses phosphate bonded investment which sets in 15
mins and then 15 min burn out is done at 815°c.
 This method is used for preparing post and core
restorations

HEAT SOURCES
 Torch flame
 Gas air torch: melt conventional
noble metal alloys (used for
inlays, crown and bridge) whose
melting points less than 1000°c
 Gas oxygen torch: Used to melt
metal ceramic alloys of higher
temperature up to 1200°c.
 Oxy acetylene torch: One
volume of acetylene and two
and half volume of oxygen are
needed.
 Hydrogen oxygen generator
 Electricity

Two type of torch tips:
1.Multi-orifice
2.Single-orifice
Zones of the blow torch flame:
Zone 1 - colorless zone /Non combustion zone
Zone 2 – Combustion zone
Zone 3 – Reducing zone
Zone 4 - oxidizing zone

CHANGES SEEN IN METAL DURING
FUSING
 Initially appear spongy
 later small globules of fused metal appear
 molten metal flows assuming a spherical shape
 At proper casting temperature the molten alloy is light orange and
tend to spin or follow.
 At this stage the temperature of molten alloy is 38°c above its liquidus
temperature.
 During melting of the gold alloys flux may be added
 Minimizing porosity
 To increase fusing of metal
 Prevent oxidation
 Commonly used fluxes are fused borax powder ground with boric acid
power or Charcoal www.facebook.com/notesdental

Casting machines
1. Air pressure casting
machines:
Alloy is melted in situ in
crucible hollow of the
ring, followed by applied
air pressure.
2. Centrifugal casting
machine:
Alloy is melted in a
crucible, and forced in to
mold by centrifugal
force.

ELECTRICAL RESISTANCE - HEATED
CASTING MACHINE
 It is used to melt ceramic alloys.
 Here the alloy is automatically
melted in graphite crucible.
Direct-current arc melting machine
 Produce between two electrodes:
the alloys and the water cooled
tungsten electrode.
 > 4000°C – alloy melts very quickly.
 High risk of over heating of the
alloy.

Induction melting
machine
 Metal is melted by an
induction field that
developed with in the
crucible surrounded by
water- cooled metal
tubing.
 It is more commonly used
for melting base metal
alloys
 not been used for noble
alloy casting as much as
other machines

CASTING FORCE
 Force required to overcome the surface tension of alloy +
Resistance offered by gas in the mold.
 This can be done by use of following different type of force
 Vacuum force
 Air or Gas Pressure
 Centrifugal force
 Sufficient mass of alloy must be present to sustain
adequate casting pressure
 6g is typically adequate for premolar and anterior casting
 10g is adequate for molar casting
 12 g is adequate for pontic

Casting Crucible
They are of 3 types:
• Clay Crucibles
• Carbon Crucibles
• Quartz Crucibles
(zircon-alumina)

Cleaning of the casting
 Consider the gold crown & bridge alloys.
 After casting has been completed, ring
is removed & quenched in water.
Advantages:
1. Noble metal is left in an annealed
condition for burnishing & Polishing.
2. When water contacts hot investment,
violent reaction ensues. Investment
becomes soft, granular & casting is
more easily cleaned.

PICKLING:
 Surface of the casting appears dark
with oxides and tarnish. Such a surface
film can be removed by a process called
Pickling.
 Best method for pickling is to place a
casting in a dish & pour acid over it.
 Heat the acid but don't boil it.

 Hydrochloric acid
 Sulfuric acid
 Ultrasonic devices
 Gold and palladium based metal ceramic
alloys and base metals, these alloys are
not generally pickled.

Casting Defect

Introduction
 An unsuccessful casting results in
considerable trouble and loss of time.
 With present techniques, casting
failures should be the exception, not
the rule.
 It can be classified under four headings
 distortion;
 surface roughness and irregularities;
 Porosity
 Incomplete or missing detail

DISTORTION
 It is usually due to distortion of wax
pattern
 minimized or prevented by proper
manipulation of the wax and handling of the
pattern
 Some distortion of the wax pattern
occurs as the investment hardens
around it.
 Setting and hygroscopic expansions of the
investment.
 Produce a non-uniform expansion of the
walls of the patternwww.facebook.com/notesdental

SURFACE ROUGHNESS, IRREGULARITIES,
AND DISCOLORATION
 Accurate reproduction of the surface of the wax pattern
from which it is made.
 Surface roughness and irregularity should not confused,
both are different terms
 These depends on size of investment particles.
 Various other causes
 air voids
 water films
 liquid/powder ratio
 composition of the investment
 foreign bodies
 impact of molten alloy on the mold wall
 pattern position
 carbon inclusions

Porosity
 Porosity or void fraction is a measure
of the void (i.e., "empty") spaces in a
material, and is a fraction of the volume
of voids over the total volume.
 It can be either internal or external
 What it does to the casting
 Weakens
 Discoloration
 Surface roughness
 Plaque accumlation

Classification
 Solidification defects
 Localized shrinkage porosity
 Microporosity
 Suck-back porosity
 Trapped gases
 Pinhole porosity
 Gas inclusions
 Subsurface porosity
 Back pressure porosity

Localized shrinkage porosity
 Caused by premature termination of the
flow of molten metal during solidification.
 Generally occurs near the sprue-casting
junction, but it may occur anywhere

Microporosity
 It also occurs due to solidification shrinkage.
 generally present in fine-grain alloy castings.
 solidification is too rapid for the microvoids to
segregate to the liquid pool.
 causes the formation of small, irregular voids.
 mold or casting temperature is too low.
 Not detectable unless the casting is sectioned
and is not a serious defect

Suck-back porosity
 localized shrinkage porosity,
 interior of a crown near the area of the sprue
 Occlusoaxial line angle or incisoaxial line angle
that is not well rounded.
 entering metal impinges onto the mold surface at
this point – hot spot
 This spot may retain a localized pool of molten
metal after other areas of the casting have
solidified.
 Creates a shrinkage void or suck-back porosity.

Suck-back porosity

Pinhole and gas inclusion
 Entrapment of gas during
solidification
 gas inclusion porosities are usually
much larger than pinhole porosity,
 Many metals dissolve or occlude
gases while they are molten
 Copper and silver: oxygen
 Platinum and palladium: hydrogen as
well as oxygen.

Subsurface porosity
 simultaneous nucleation of solid
grains and gas bubbles.
 diminished by controlling the rate at
which the molten metal enters the
mold

Back-pressure Porosity
 Entrapped air bubbles on the inner
surface of the casting.
 large concave depressions.
 caused by the inability of the air in
the mold to escape through the pores
in the investment

INCOMPLETE CASTING
 Occasionally only a partially complete
casting or perhaps no casting at all is
found.
 The obvious cause is that the molten
alloy has been prevented in some
manner from completely filling the mold.
 insufficient venting of the mold and high
viscosity of the fused metal.
 incomplete elimination of wax residues from
the mold

INCOMPLETE CASTING
 Incomplete venting
 Magnitude of the casting
pressure
 pressure should be
applied for at least
4 seconds.
 lower L/P ratio is
associated with less
porosity of the
investment.
 Incomplete
elimination of wax
 Pores in the investment
may become filled

References
 Phillips' Science of Dental Materials-
Phillip Anusavice_12th
 Basic Dental Materials -2nd.ed Mannapalli
 Clinical Aspects of Dental Materials
Theory, Practice, and Cases, 4th Edition
 Craig's Restorative Dental Material 13th
edition
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Casting procedure

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