Casting procedure & defects

Good morning
Good morning

Casting procedures
Casting procedures
AndAnd
Casting defects
Casting defects
Presented by:
Bhanuchandar. D
Dept of Prosthodontics

Contents
• Introduction
• History
• Steps in casting procedure
• Sprue formers
• Crucible formers
• Casting rings and ring liners
• Investing procedure
• Wax burnout
• Casting of alloys into mold
• Casting of titanium alloys
• Cleaning of casting
• Casting defects
• Conclusion

History
• This met iculous procedure of cast ing
was used t o produce j ewelry and
ornament s.
• 3500 B.C. – Egypt ians f irst
manuf act ured ref ract ories which in t he
f orm of glass vessels around a
ref ract ory core of mud, sand, and animal
dung.
• 11t h Cent ury → Theophilus → Described
lost wax t echnique, which was a common
pract ice in j ewelry.
• 1558 → B. Cellini → have
at t empt ed use of wax and clay f or
preparat ion of cast ings.

• 1897 → Phillibrook described a met hod
of cast ing met al f illing
• 1907 – William. H .Taggart int roduced
t he lost wax t echnique in dent ist ry.
• 1949 – Moore and Walt developed
phosphat e bonded invest ment .
• 1959 -Asgar& Peyton st at ed t hat f laring should
occur at t he sprue/ wax pat t ern j unct ion
• 1959 - Strickland et alst at ed t he import ance of
t he t ype, shape locat ion & direct ion
ot her t han t he size of t he sprue
• 1959 - Morrison and Warmick report ed
t he f indings of et hyl silicat e ref ract ory
mat erial f or dent al use.

Steps involved in the casting

Spruing
Sprue former:
A wax, plastic or metal used to form the
channel or channels which allows molten metal
to flow into a mold to make casting.

• Purpose of the sprue former
• Requirements of the sprue former

Advantages of hollow sprue former:
• It increases the contact area.
• It holds less heat than the solid sprue
former.
 “Care must be taken to examine the orifice
for small particles of investment that may
break off while removing the metal sprue
former.”

Sprue former diameter:
• The diameter and length of the sprue
former depends on:
- The type and size of the pattern.
- The type of casting machine to be used.
- The dimensions of the casting rings in
which casting is made.
• Pre fabricated sprue former are available in
a wide variety of gauge from 6 to 18.

• Usually → for molar and metal ceramic
restoration - 10-gauge (2.5mm)
→ Premolars and partial coverage
restoration - 12-gauge (2.0mm)
• A narrow sprue may be useful in air
pressure casting procedure where the
metal is melted in conical depression
formed by crucible former. so narrow sprue
prevention premature metal flow into mold.

Reservoir:
• Reservoir is a small amount of additional
wax added to the sprue former 1mm below
the wax pattern.

Sprue former length:
• The length of the sprue former - it keeps
the wax pattern 6mm from the

Sprue former position:
• The ideal area- point of greatest bulk in the
pattern.
• The point of attachment should permit
stream of metal to be directed to all parts of
the mold without having to flow opposite the
direction of casting force

Sprue former direction:
Attached 45 degrees to the walls
of mold, which decreases the
turbulence of molten alloy.

Attachment morphology:
• The attachment of sprue former to
the wax pattern should be smooth
and do not posses pits or
irregularities.

Spruing
Direct Indirect

Vents
• Vents are the additional sprues placed at thin
or thick wax patterns to improve the quality
of the casting.

Auxillary sprue
• For large casting an additional auxillary
sprue may be placed for filling the mold.
• Usually 14 to 16 gauge sprue are used.

Crucible former
They are available as:
Rubber, Metallic and Plastic
They are of two types:
1) Steep sided cone- to cast metal
using centrifugal casting force
2) Shallow cone- cast metal using
stream or air pressure

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.

Investing
.
Investment materials are:
1. Gypsum bonded investment
2. Phosphate bonded investment
3. Ethyl silicate bonded investment
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

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
Bubble free casting with different
technique-
17% - open investment
95% - vacuum investment
• Advantage of vacuum mixing:

Setting of investment:
• After mixing the investment is poured in to
the casting ring up to its rim.
• For hygroscopic expansion technique
• For controlled water added technique
• For Thermal expansion technique

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:
• Heat application:
• Heating of ring should be done slowly.
- Hygroscopic low-heat technique
- High-heat thermal expansion
technique

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

High-heat thermal expansion
technique
Gypsum bonded Investment :
• 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

Phosphate bonded investment :
• They need higher 2nd stage temperature
for -
- total elimination of wax
- and Prevent premature solidification
of higher melting alloys.
• After initial slow raise of temp to 315°c,
the temperature is rapidly raised to 750-
900°c and maintained for 30 mins.
• The technique cause 1.33-1.58 % of
Thermal expansion

Accelerated casting method
(J Prost dent. 66: 155,1991)
• To reduce the total time, Alternative
Accelerated casting technique is proposed
that 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: two basic modes-
• Torch flame - Gas air torch
- Gas oxygen torch
- Oxy acetylene torch
- hydrogen oxygen generator
• Electricity
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

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

Two types of flame can be obtained
with a casting torch:

The changes seen in metal during fusing
are :
• Initially appear spongy→ later small globules
of fused metal appear → later 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.
• Charcoal

Gas air torch: -
• To 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.

They are:
• Oxidizing flame - 6000°F
• Carburizing flame- 5400°F
• Neutral flame - 5600°F
If distance is reduced to -
7.5 mm → slight porosity
5 mm → increased porosity due to
occluded H2 gas.

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.

3. Electrical resistance - heated
casting machine :-
• It is used to melt ceramic
alloys. Here the alloy is
automatically melted in
graphite crucible.
• The crucible in the furnace
is always against the
casting ring. So the metal
button remain molten
slightly longer and ensures
complete solidification.

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.

4. Induction melting machine:
• Metal is melted by an induction field that
developed with in the crucible surrounded
by water- cooled metal tubing.

• The electric induction furnace is a transformer
in which an alternating current flows through
the primary winding coil and generates a
variable magnetic field in the location of the
alloy to be melted in a crucible
• It is more commonly used for melting base
metal alloys not been used for noble alloy
casting as much as other machines

Casting force:
• Casting force > 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

CASING OF TITANIUM ALLOYS
• Titanium offers lot of advantages and can
be used for casting.
Problems associated are:
• High melting point of titanium of 1671°C
(when other dental casting alloys have
liquidus temperature below 1500° C).
• Tendency for the molten metal absorbs
several gases in molten state.

• 1977 – Walter start :- casted Ti alloy as
crown and bridges and R. P. D. frame
work.
• A new pressure / vacuum casting
machine was developed. With argon –
arc system for melting alloy.
• Molten alloy drawn into the mold by
gravity or vacuum & subjected to
additional pressure to force the alloy into
the mold

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

• Traditionally a wet lining of asbestos sheet
was used on casting crucible. The
moistened asbestos sheet provides a clean
and good surface on which the alloy could
be melted.
• Advantages is, prevent alloy contamination
with oxides and residuals that may be
present in the crucible

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 is both an art and science governed by numerous
rules, or “laws”.
• Based on earlier work of Ingersoll & Wandling (1986),
W. Patrick Naylor formulated an expanded set of 17
separate recommendations for Spruing, investing,
burnout, and melting and casting procedures.
• Collectively these guidelines are referred to as the laws
of casting.

Attach the pattern Sprue former to the thickest part
of the wax pattern
Orient the wax pattern so all the restoration margins will face
the trailing edge when the ring is positioned in the casting machine
Position the wax pattern in a “cold zone” of the investment mold
and the reservoir in the “heat centre” of the casting ring

A reservoir must have sufficient molten alloy to accommodate
the shrinkage that occurs within the restorations
Do not cast a button if a connector (runner) bar,
or other internal reservoir, is used
Turbulence must be minimized, if not totally eliminated

Select a casting ring of sufficient length and diameter
to accommodate the patterns to be invested
Increase the wettability of the wax patterns
Weigh any bulk investment and measure the investment liquid
for a precise powder-liquid ratio

Eliminate the incorporation of air in the casting investment and
remove the ammonia gas by product of phosphate-bonded investments
by mixing under vacuum
Allow the casting investment to set completely before
initiating the burnout procedure
Use a wax elimination (burnout) technique that is specific for
the type patterns involved (wax versus plastic)
and recommended for the particular type of casting alloy selected

When torch casting, use the “reducing zone” of the flame
to melt the alloy and not the oxidizing zone
Provide enough force to cause the liquid alloy
to flow onto the heated mold
Adequate heat must be available to properly melt
and cast the alloy

Cast toward the margins of the wax patterns
Do not quench the ring immediately after casting

Classification
According to Anusavice
• Distortion
• Surface roughness and irregularities
• Porosity
• Incomplete or missing details
Based on location
• Internal
• external

According to Rosensteil
• Roughness
• Nodules
• Fins
• Incompleteness
• Voids or porosity
• Marginal discrepancy
• Dimensional inaccuracies

Distortion
• Distortion of the casting probably related to
distortion of the wax pattern.
Causes:
• Can occur from the time of pattern preparation
to the time of investing due to stress
relaxation.
• Distortion of the wax pattern occurs during the
investment procedure.
Minimized by:
 Application of minimum pressure
 Manipulation of wax at high temperature
 Investing pattern immediately
 If storage is necessary, store in refrigerator

Surface Roughness and
Irregularities
surface roughness
Defined as relatively finely spaced surface
imperfections whose height, width and direction
establish the predominant surface pattern.
Surface irregularities
Isolated imperfections such as nodules that are not
characteristic of the entire surface area
• The surface roughness of the casting is > wax pattern
- the particle size of the investment and its ability to
reproduce the pattern in microscopic detail

Air bubbles:
• Small nodules on the casting
Prevented By:
– Proper investment technique
– Vibration of mix or by vacuum mixing
– Application of wetting agent properly and
correctly – important that it be applied in a
thin layer.

Water films:
• If the Investment becomes separated from the
wax pattern, a water film may form irregularly
over the surface.
• Appears as minute ridges or veins on the
surface.
Prevented By:
1.Use of wetting agent
2.Correct L/P ratio (Too high L/P ratio may
produce these irregularities)

Rapid Heating Rates
Causes:
• Fins or spines
• Flaking of the investment
Prevented by:
• Heat gradually at least 60min from room
temperature to 700°c.
• Greater the bulk – more slowly heated.
Under heating
• Incomplete elimination of wax residues.

Prolonged heating
• Decomposition of the investment
Liquid/Powder Ratio
• The amount of water and powder measure
should be accurate.
Casting pressure
• To high pressure – rough surface of the casting
• To low pressure – incomplete casting
• Average – 0.01 to 0.14 Mpa and 3 to 4 turns of
the spring.

Foreign bodies
• Any casting that shows sharp, well- defined
deficiencies indicates the presence of some
foreign particles in the mold. They may be:
- Pieces of the investment
- Bits of the carbon from the flux
- Sulfur components from – decomposition
of the gypsum investment and high sulfur content
torch flame.
Pattern position
• Should not place too close together
• Should not place many patterns in same plane
Space between the pattern is atleast 3mm

Impact of metal alloy
Cause:
• The direct impact of molten alloy may fracture
or abrade the mold surface regardless of its bulk.
It results ……..
Prevented by:
• This type of surface roughness or irregularities
can be avoided by proper spruing.
Carbon inclusions
• Carbon from- carbon crucible, carbon
containing investment – absorbed by the alloys
during casting results in formation of carbides or
visible carbon inclusion.
Other causes

porosity
Classified as follows:
I. Solidification defects
A. localized shrinkage porosity
B. Micro porosity
II. Trapped gases
A. pin hole porosity
B. gas inclusion porosity
C. sub surface porosity
III. Residual porosity

Localized shrinkage porosity
Cause:
• By premature termination of the molten metal
during solidification.
• Porosity in the pontic is caused by- retain heat
because of its bulk and located in the center of the
rings.

Suck back porosity – Interior of the crown
near the area of the sprue create a hot spot on
the mold wall

Pin hole and Gas inclusion porosity
• Characterized by spherical contour, but gas
inclusion porosities are much larger than pin hole
porosity.
• Occur primarily because most metals dissolve
gases when molten these gases expelled during
solidification..
• Also be caused by gas occluded from a poorly
adjusted torch flame or use of oxidizing zone
rather than reducing zone.
• Casting is usually black, do not clean easily on
pickling

Sub surface porosity
• Caused by simultaneous nucleation of solid
grains and gas bubbles at the first moment that
the alloy freezes at mold walls
• Prevented by controlling the rate at which the
molten metal enters the mold.

Back pressure porosity
• Some times referred to as entrapped-air
porosity.
• The entrapment is frequently found in a “POCKET”
at the cavity surface of a crown or MOD casting.
• Also found on the outer surface of the casting when
the casting or mold temperature is low, that
solidification occurs before the trapped air can
escape.
• Thickness of the investment
• Incomplete elimination of wax residues.

Prevented by:
• Proper burnout
• Sufficiently high casting pressure
• Investment of adequate porosity
• Adequate L/P ratio
• Adequate mold and casting temperature.
• Thickness of investment between tip of pattern
and end of ring is not greater than 6mm.

In complete casting
• Factors that inhibit the mold filling is:
1. In sufficient venting
2. In sufficient casting pressure
3. Incomplete elimination of wax
4. Lower L/p ratio
5. Viscosity of the fused metal

Viscosity of the fused metal:
• In complete casting resulting from too greater
viscosity is due to insufficient heating of the
alloy.
• However, both the surface tension and the
viscosity of the molten alloys are reduced by
increased in temperature higher than its
liquidus temperature

Reference
• K. J. Anusavice, Phillips Science of Dental
Materials, 11th
edition
• Introduction to metal ceramic technology, W.P.
Naylor
• Craig R.G, Restorative Dental Materials;10th
edition
• Rudd and morrow, Dental laboratory procedures-
removable partial denture, 2nd
edition
• Rudd and morrow, Dental laboratory procedure-
fixed partial denture,
• John j. manappallil, basic dental materials, 2nd
edition
• Rosensteil, Contemporary fixed Prosthodontics
• Shillinburg, fundamentals of fixed Prosthodontics
• JPD- 1978; 3, 137-14
• internet

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