Casting procedures / education for a dentist

CASTING PROCEDURES
www.indiandentalacademy.com

CONTENTS
Introduction
The Sprue
Casting ring liners
Investing
Casting
Casting defects
Conclusion
References

Introduction
Matthaeus gottfried purmann (1700) first
mentioned wax models in connection
with prosthetic work.
Historical evidence suggest Philbrook first
described lost wax method.
Taggart is credited to introduction of this
technique to profession in 1906.

Three steps after wax pattern are -
Investing
burn out
Casting

Compensation for solidification shrinkage
1 .hygroscopic expansion of investments
2 .thermal expansion of investment
3 .setting expansion
4 .wax pattern expansion

Die preparation
Die materials – type 1V , V dental stones ,
acrylic, polyester, epoxy resins &
electroformed dies. Inelastic impression
materials like compound amalgam can be
condensed to form dies.
Die stone investment combination
- divestment
- divestment phosphate .

The sprue
Sprue is the channel or hole through which
plastic or metal is poured or cast into a gate
or reservoir & then into a mold.
Wax, plastic or metal former used to form the
channel or channels to allow molten metal to
flow.
Custom made
Pre fabricated
Selection of Sprue pin
1. Gauge selection –5 principles
a. Select the gauge Sprue former with a
diameter that is approximately same as the
thickest area of wax pattern.

b. Sprue former should be attached to the
pattern at the largest cross sectional
areas.
c. length of the Sprue .
d.Type of the Sprue influences the burn
out technique.
e.Patterns may be sprued directly or
indirectly.

2 .Sprue former attachment
3 .Sprue former position
4 .Sprue former direction
5 .Sprue former length
Reservoir

Casting ring & liners
Materials for rings & liners .
Asbestos , aluminum silicate ceramic liner
, cellulose liner.
Placement – 3.25mm short of the ends .
Effects .
Ringless casting systems.

Investing procedure
Cleansing of the pattern
Commercial wax pattern cleaner or
dilute synthetic detergent.

Mixing of the investment
Liquid is taken in a bowl and powder is
added until all the powder has been
wet.
Hand mixing
Vacuum mixing

Investment procedure
Hand investing
Vacuum investing

Compensation for
shrinkage
Liners –no ,
Liquid ;powder ratio of investment.
Hygroscopic expansion
Time of immersion
Temperature of water bath
Burn out temperature
Controlled water added technique

Casting procedures
Casting –is act of forming an object in a mold.
Wax elimination
Gypsum bonded –
468degC –hygroscopic expansion
650 degC– thermal expansion
Phosphate bonded
700 – 870 degC depending on type of alloy .

Hygroscopic low heat
technique
37 deg water bath expands the wax
pattern
The warm water entering the
investment mold from top adds some
hygroscopic expansion .
The thermal expansion at 500 deg
provides the needed expansion .

Added expansion can be obtained by
Increasing water bath temperature.
Use 2 layers of liner
Increasing the burn out temperatures to
600 to 650degC.

High heat thermal
expansion technique
Gypsum investments – mold is heated from
room temp & slowly heated to 650 – 700
deg C in 60 minutes & held for 15 –30
minutes .
Rate of heating
rapid heating – cracking,flaking or spalling of
mold walls, differential thermal expansion &
radial cracks from the interior outwardly.

Temperature – at > 700 deg C
CaSo4 + 4C--- CaS + 4Co
3CaSo4 +CaS-- 4CaO + 4So2
Time of casting – casting should be
made immediately to prevent sulphur
contamination& disintegration .
Rapid burn out procedure
Investments with crystobalite- mold is
placed in a furnace at 350 degC.

Phosphate investments
They expand by –expansion of wax pattern, the
setting expansion, thermal expansion.
The usual burn out temperature ranges from
750 – 900 degC.
Because
Higher temp ensures total elimination of wax
residues
The completion of physical & chemical
changes .
Prevention of premature solidification of high
melting alloy .

Slow heating rate for 350 degC ,then hold at
upper temp for 30 minutes.
Some investments can be subjected to 2
stage heating more rapidly – placed directly
in the furnace at the top temp for 30 min &
cast .
To save time – ring & ring liner are also
eliminated – tapered plastic ring is used so
set investment can be pushed out of the ring ,
when completely set placed directly into the
hot furnace . Here expansion varies & can be
adjusted by varying liquid concentration.

Time allowable for
casting
High heat technique – 1 minute , as the
mold contracts on cooling
Low heat technique –casting should be
done soon after removal from the oven.

Casting machines
The alloy is melted in a separate crucible by a
torch flame & cast into the mold by centrifugal
force.
The alloy is melted electrically by a resistance
or induction furnace & cast into the mold by
centrifugally by motor or spring action.
The alloy is melted by first 2 ways ,but is cast
by air pressure , a vacuum or both .

Centrifugal casting machine
The casting machine spring is first
wound from 2 to 5 times.The metal is
melted by torch flame in a glazed
ceramic crusible attached to broken
arm of the casting machine.
The broken arm feature accelerate the
initial rotational speed of the crusible
and casting ring,increase in the linear
speed of liquid casting alloy as it moves

Once metal reaches casting
temperature and the heated casting ring
is in position.The machine is released
and the spring triggers rotation motion.
As the metal fills the mould there is a
hydrostatic pressure gradient along the
length of the casting.This pressure form
the tip of casting to the bottom surface
is quite sharp and parabolic in form
reaching zero at the button surface.

Pressure gradient at the movement before
solidification reaches about 0.21 to 0.28 Mpa
[30 to 40 psi] at the tip of the casting.
Because of this pressure gradient there is
also gradient in the heat transfer,such that
greatest transfer is at the high pressure end
of the gradient.
Because this end is also is the sharp edge
margin of crown .It is assured that
solidification progresses from thin margin to
the button surface.

Electrical resistance-
Heated casting machine.
Here there is automatic melting of metal in a
graphite crusible with in a furnace rather than
a torch flame.
Is advantageous for alloys like metal ceramic
restorations, which are alloyed with base
metal in trace amounts that tend to oxidize on
over heating and crusible in the furnace is
located flush against the casting ring so metal
button remains molten slightly longer.

Induction melting machine:
Metal is melted by an induction field that
develops with in a crusible surrounded by
an water cooled metal tubing.
When metal reaches casting temperature it
is forced into the mould by the air
pressure,vaccume,or both at the other end
of the ring.
Mostly used for metal base metal alloys.

Casting crusibles:
Clay crusibles: Crown bridge alloys ,
such as high noble and noble types.
Carbon crusibles: Crown bridge alloys ,
and higher fusing ,gold based metal
ceramic alloys.
Quartz crusibles:for high fusing alloys of
any type.

Cleaning the casting
For noble metals
The ring is quenched in water
Advantages are
Noble metal is left in a annealed condition
when water contacts the hot investment a
violent reaction ensures . Investment
becomes soft & granular & casting is more
easily cleaned.

Pickling
50 % HCL – gypsum bonded
50%H2So4
Ultra sonic devices

CASTING DEFECTS

Casting defects
Distortions
Surface roughness ,irregularities &
discolorations
Porosity
Incomplete or missing casting

Distortions
Wax pattern
Investing
Expansion

rface roughness & irregularities
Air bubbles

Water films

Rapid heating rates

Under heating

Liquid ;powder ratio

Prolonged heating

Temperature of the alloy

Casting pressure

Foreign bodies

Impact of molten alloy

Pattern position

Other causes

Porosity
Solidification defects
A. localized shrinkage porosity
B. micro porosity
Trapped gases
A. pin hole porosity
B. gas inclusions
C. subsurface porosity
Residual air

localized shrinkage porosity
By incomplete feeding of molten metal.
Common at Sprue casting junction.
Or any way between dendrites .
Suck back porosity – by hot spot . Can be
eliminated by flaring , reducing casting
temp by 30degC.

Micro porosity
Caused from solidification shrinkage
In fine grain alloy casting when
solidification is too rapid for micro voids
to segregate to liquid pool.
Controlled by increasing the mold melt
temperature.

Pin hole porosity
By entrapment of gas
Molten metals dissolve or occlude
gases, on solidification the gases are
expelled and pin hole porosity results .
Oxygen –copper, silver, platinum
Hydrogen – platinum .

Gas inclusions
Are due to inclusion of gases but are
larger then pin hole porosities .
Caused by occluded gases,
contaminated castings , poorly adjusted
torch flame or use of mixing or oxidizing
zones of flame.
Premelting of the metals & proper
positioning of torch flame reduces this
porosity.

Subsurface porosity
Exact reason not known ,may be
caused by the simultaneous nucleation
of solid grains & gas bubbles at the first
moment that the metal freezes at the
mold walls .
Controlling the rate at which the molten
metal enters the mold reduces this type
of porosity.

Entrapped air porosity or back pressure
porosity –on inner surface of the castings can
produce large concave depressions .
Due to inability of air in the mold to escape
through the pores of investment or pressure
gradient that displaces the air towards the
end of the investment via molten sprue &
button .
Increased density of investments, vacuum
investing, & low heat technique which
decreases venting of the mold .

Incomplete casting
Insufficient venting
High viscosity of the fused metal
Foreign bodies .

Review

Conclusion
Casting procedures are highly
technique sensitive steps which
converts wax pattern to a final
restoration .
Accurate & smooth restorations can be
obtained if operator pays special
attention to each step in the technique.

References
Phillips science of dental materials
-Anusavice
Text book of restorative materials
- Robert .G. Craig

 Fundamentals Of Fixed Prosthodontics
- Herbert T Shillingburg
 Contemporary Fixed Prosthodontics
- Stephen F . Rosenstiel
 Johnston’s Modern Practice In Fixed
Prosthodontics
- Roland W Dykema

Dental casting alloys
Classification of ADA (1984)
High noble >_40wt
%Au & _> 60wt%noble metals
Noble metal
_>25wt% of noble metals .
Predominantly base metal
<25wt% of noble
metals .

According to National institutes of
standards and technology gold casting
alloys are classified as type 1 to IV
based on dental function &hardness.
Type I (soft)
Type II (medium)
Type III (hard)
Type IV (extra hard)

Thank you
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Casting procedures / education for a dentist

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