Casting & defects

CASTING PROCEDURES
&
DEFECTS
PRESENTED BY:
DR.NABID ANJUM
MDS 1ST YEAR

INTRODUCTION:
ACCORDINGTOGPT(8TH edition):
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.
ACCORDINGTOCRAIG(13thEdition):
Casting is the process by which a wax pattern of a
restoration is converted to a replicate in a dental alloy
ACCORDINGTOWILLIAMJ. OBRIEN(3rdedition): Casting is a process
of forming objects by pouring molten metals in molds that
are cooled to cause solidification.

HISTORY:
 This meticulous procedure of casting was used to produce
jewellery and ornaments.
 11th Century- Theophilus ,described lost wax technique, which
was a common practice in jewellery.
 1558 - B. Cellini have attempted use of wax and clay for
preparation of castings.
 1897 - Phillib rook described a method of casting metal filling
 1949 – Moore and Walt developed phosphate bonded
investment .
 1959 -Morrison and Warmick reported the findings of ethyl
silicate refractory material for dental use

W.H.TAGGART
1907
described a casting process which launched the
cast restoration as it is known today, lost wax
technique.

Sprue
former
his process was
Direct wax
pattern of lost
tooth structure
invest
cast gold into space formed using air pressure

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

 Purpose of a sprue former:
• To create a channel for the elimination of wax
during burnout.
• Forms a channel for entry of molten alloy
during casting.
• Provides a reservoir of molten metal which
compensates for alloy shrinkage during
solidification

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)

Reservoir:
.
Wax is added around the sprue former 1-2 mm from
the pattern in order to create an area in the mold (a
reservoir) with dimensions far exceeding that of the
thickest portion of the pattern

Sprue former length:
6mm (1/4”)
3.25mm
(1/8”)
 The length of the sprue former - it keeps the wax pattern 6mm from the
ring
Major factor governing the sprue length
is the length of the ring.

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.

 The attachment of sprue former to the wax pattern should
be smooth and do not posses pits or irregularities.
Attachment Morphology:
Patterns may be sprued:
directly indirectly

Vents:
• For large casting an additional
auxillary sprue may be placed for
filling the mold.
• Usually 14 to 16 gauge sprue are
used.
Auxillary sprue:
• Vents are the additional sprues
placed at thin or thick wax patterns
to improve the quality of the casting.

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 – to cast metal using steam or air pressure

Casting rings:
• Holds the investment in place during setting
and restricts the expansion of mold with the
use of solid metal rings.
 They are available as :
Shapes – Round , oval
Complete rings – rigid ( metal, plastic) ,
flexible ( rubber)
Split rings – metal and plastic

Ringless casting system:
 This method uses paper or plastic casting ring and is designed to allow
unrestricted expansion (Engelman 1989)
 Used for traditional gold-base alloys.

Casting ring liners:
 The maximum thickness of liner is 1 mm
 Materials used are:
• Asbestos liner
(no longer be used because its carcinogenic potential)
• Cellulose liner (absorbent)
• Ceramic liner ( non absorbent)
• Combination of ceramic and cellulose liner
 The function of a ring liner is:
Allow uniform expansion.
In case of wet liner technique- hygroscopic expansion

 Preparing the wax pattern for investment:
Wax pattern should be cleaned of any debris,grease or oils
Thin films of cleanser left on the pattern reduces the
surface tension of the wax
The distortion of the wax pattern after its removal from the
die is a function of the temperature and time interval
before investing.

Investment materials:
 A heat resistant or a refractory material used to form a mold
into which a metal or alloy is cast .
(OR)
 A molding material that surrounds the pattern & subsequently
hardens & forms the mold after the wax pattern is eliminated.
 The operation of forming the mold is called Investing.
Types – Gypsum bonded investments
Phosphate bonded investments
Ethyl silicate bonded investments

• 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.
• 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 .
 Investment mixing:
1) Hand mixing 2) vacuum mixing
 After mixing the investment, it is poured into the casting ring upto its rim.

Wax elimination or Burn out:
 It is advisable to start the burn out 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 mould.
 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.

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 mould from the
top adds some of the expansion.
 The thermal expansion at 500°C.
 The technique causes 0.55% of expansion.

HIGH –HEAT THERMAL EXPANSION TECHNIQUE
 The investment is slowly heated to 650°C - 700°C in 60 mins.
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
(J Prost dent. 66: 155,1991)
 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.

CASTING MACHINES:
1. Air pressure casting machine:
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.
• > 4000°C – alloy melts very quickly.
• Ensures complete solidification and a high risk of overheating of the
alloy

4. Induction melting machine:
Here, 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 mould.
 This can be done by use of the following different types of force-
 Vacuum force
 Air or gas pressure
 Centrifugal force
 Sufficient mas 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

QUENCHING:
 After the casting has solidified, the ring is removed and quenched
in water as soon as the button exhibits dull red glow.
 Two advantages of quenching are-
 The noble metal alloy is left in an annealed condition for burnishing,
polishing and similar procedures, and
 When the water contacts the hot investment, a violent reaction ensues
resulting in a soft, granular investment that is easily removed.

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

SANDBLASTING:
 The process of altering the surface of a material through the
use of abrasive particles propelled by compressed air or other
gases. (GPT-9)
 It can be done using a high speed sandblaster.
 Most common medium for sandblasting in dentistry is
aluminium oxide.
 The fine aluminium oxides are recycled within the sandblasting
machine and replaced after 30-40 mins of total accumulated
operation time.

CASTING DEFECTS:
 Definition: A casting defect is an irregularity in the metal casting process
that is very undesired. Some defects can be tolerated while others can
be repaired ,otherwise they must be eliminated.
 Errors in the procedure often results in defective castings. The casting in
such a case may not fit or may have poor esthetic and mechanical
properties.
 Casting defects can be avoided by strictly observing the procedures,
which are governed by certain fundamental rules and principles.

Classification:
 ACCORDING TO ANUSAVICE:
 Distortion
 Surface roughness and irregularities
 Porosity
 Incomplete casting or missing details.
 Sulphur compounds, formed when ring is overheated and there
is breakdown of investment.(Discolouration)

DISTORTION:
 Distortion of the casting is due to a distortion of the wax pattern.
Some distortion of the wax pattern
Occurs as investment hardens around
it or due to hygroscopic or setting
expansion.
Some wax distortion occurs
during manipulation ,
because of release of
stresses.
Application of minimum pressure
Manipulation of wax at high temperature
Investing pattern immediately
If storage is necessary, store in refrigerator
 Minimized by:

SURFACE ROUGHNESS & IRREGULARITIES
 Definition of Surface roughness:
It is defined as relatively finely spaced surface
imperfections whose height, width and direction establish the
predominant surface pattern.
(Phillip’s Science of Dental Materials ,11th edn)
 Definition of Surface irregularities:
It is defined as Isolated imperfections such as nodules that
are not characteristic of the entire surface area.
(Phillip’s Science of Dental materials, 11th edn)

 The surface of a dental casting should be an accurate
reproduction of the surface of the wax pattern from which it is
made.
 Even under optimal conditions , the surface roughness of the
casting is invariably somewhat greater than that of the wax
pattern from which it is made.
 Excessive roughness or irregularities on the outer surface of the
casting requires additional finishing and polishing whereas
irregularities on the cavity surface prevents proper seating of the
casting.

 CAUSES OF SURFACE ROUGHNESS AND IRREGULARITIES:
 AIR BUBBLES-
• Air bubbles on wax pattern causes nodules on the casting.
• These air bubbles may occur on the outside or inside of the casting.
• If the voids occur on the outside ,they are theoretically removable , but
requires great expense of time and money. However, if they occur on the
margins of internal surfaces , successful removal is extremely difficult and
often the restoration will have to be recast.
• CAUSES:
Lack of surfactant/wetting agent.
Improper brush technique
Inadequate vibration during mixing.
Inadequate vaccumm during investing.

 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:
Use of wetting agent
Avoiding movement, vibration of pattern after investment.
Correct L/P ratio (Too high L/P ratio may produce these irregularities)

 RAPID HEATING RATES:
• Too rapid a heating rate causes cracking of the investment.
• These cracks produces a casting with fins and spines.
• This condition is specially seen in crystoballite investment.
 Prevented by:
Heat gradually at least 60min from room temperature to 700°C.
Greater the bulk – more slowly heated.

 OVERHEATING:
• Overheating of investment above 700 degree C, disintegrates the
investment liberating sulfur or sulfur compounds.
• These combine in metal in the gold alloy & form sulfide film. This gives a
dark casting (Black Casting) which cannot be cleaned by pickling.
 PROLONGED HEATING:
• Same effect as that of overheating ( disintegration of investment.)

 COMPOSITION OF THE INVESTMENT:
• The ratio of binder to the quartz influences the surface texture
of the casting.
• A coarse silica causes surface roughness.
 CASTING PRESSURE:
• Too high casting 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 or 15lbs/sq inch of air pressure.
• 3-4 turns of centrifugal casting machine is sufficient for small
castings.

 FOREIGN BODIES:
• Identified by sharp, well defined deficiencies.
• Cause : pieces of investment , carelessness in removal of sprue former,
Bits of carbon from flux.
• It may result in : Surface roughness, Surface voids or incomplete areas.
• Prevention : Avoiding carrying pieces of investment loosened during
removal of sprue former and crucible former .
 IMPACT OF MOLTEN ALLOY:
• The direction of sprue former should be such that the molten alloy
does not hit a weak portion of the mould surface.
• Occasionally the molten alloy may fracture the mold surface on
impact, regardless of its bulk.

 TEMPERATURE OF THE ALLOY:
• If an alloy is heated to too high temperature , the surface of the
investment is likely to be attacked, and a surface roughness
may result.
 LIQUID / POWDER RATIO:
• The higher the L:P ratio, the rougher the casting.
• However, if too little water is used, the investment may be
unmanageably thick and cannot be properly applied to the pattern.
• In either conditions, a rough surface on the casting may result.

 CARBON INCLUSIONS:
• Carbon from-carbon crucible, carbon containing investment –
absorbed by the alloys during casting results in formation of
carbides or even create visible carbon inclusion.

POROSITY:
 Porosity may be internal or external.
 External porosity may be a manifestation of internal porosity
and can cause discoloration of the casting.
 Severe porosity at the tooth restoration interface can even
cause secondary caries.
 Internal porosity weakens the restoration.
 Although the porosity in a casting cannot be prevented entirely,
it can be minimized by use of proper techniques.

 Porosities in noble metal alloy castings may be classified as
follows:
I. Solidification defects:
A. Localized shrinkage porosity
B. Suck back porosity
C. Micro porosity
II. Trapped gases:
A. Pin hole porosity
B. Gas inclusion porosity
C. Sub surface porosity
D. Back pressure porosity

 LOCALIZED SHRINKAGE / SHRINK SPOT POROSITY:
Cause:
• By premature termination of the molten metal during solidification.
• Incorrect sequence of cooling . Usually the sprue should freeze at last.
If it freezes before the rest of the casting , it leads to porosity.
(ideally , the molten alloy farthest from the sprue button should
freeze first).
• Metal on solidification shrinks. It generally occurs near the sprue-
casting junction.

Prevented by:
• Use of sprue of correct thickness.
• Decreasing the length of sprue.
• Increasing the melt temperature.
• Attach the sprue to the thickest portion of wax pattern.
• Flare the sprue at the point of attachment.

 SUCK BACK POROSITY:
• It often occurs in occlusoaxial or incisoaxial line angle that is not
well rounded.
• The entering metal impinges onto the mold surface at this point
and creates a higher localized mold temperature in this region that
is called ‘hot spot’.
• It is an external void seen in the interior of a crown near the area
of the sprue.
By reducing the temperature difference
between the mould and molten alloy
(i.e, lowering the casting temperature
by about 30 degree celcius).
Prevented by:

MICROPOROSITY:
• It is generally present in fine grain alloy castings when the solidification
is too rapid.
• The porosity is in the form of small,irregular voids.
• This defect is detected only if casting is sectioned.
Cause : Rapid solidification of the metal or casting temperature is too
low.

PINHOLE & GAS INCLUSION POROSITY:
• Gases are dissolved by some metals when they are in molten state,
during solidification these gases are expelled or released out resulting
in PINHOLE POROSITY.
• For eg : Cu & Ag dissolve oxygen.
Pt & Pd dissolve hydrogen & oxygen.
 GAS INCLUSION POROSITY: Due to entrapment of gas by
molten metal or gas inclusion during casting procedure.
MICROPOROSITY VOIDS: IRREGULAR
PIN HOLE & GAS INCLUSION : SPHERICAL
(GAS INCLUSION: LARGER THAN PINHOLE)

 SUBSURFACE POROSITY:
• They may be caused by simultaneous nucleation of solid grains & gas
bubbles when the alloy freezes at the mold walls.
Prevention:
 By controlling the rate at which the molten metal enters the mold.
 It is reduced by increase in sprue length.
 BACK PRESSURE POROSITY/ RESIDUAL AIR:
• Present on inner side of casting.
• Produces large concave depressions.
• 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.
• Cause : By inadequate venting/air escape of the mould.
• Prevented by: Proper burnout
Adequate casting pressure, temperature and force.
Thickness of investment (should not be greater than
6mm) .

INCOMPLETE CASTING
 Occasionally, only partially complete casting or perhaps no casting at all
is found.
Cause: Molten alloy has been prevented from completely filling the
mould.
 Factors that inhibit the mold
filling are :
• In sufficient venting
• In sufficient casting pressure
• Incomplete elimination of wax
• Lower L/p ratio
• Viscosity of the fused metal

INSUFFICIENT VENTING:
• Insufficient venting is directly related to the back pressure exerted
by the air in the mould.
• So, casting pressure has to be increased to overcome the back pressure
.The pressure should be applied for atleast 4 seconds.
• The mold is filled & metal is solidified in 1 sec or less, yet it is quite soft
during early stages.
• Therefore, the pressure should be maintained for a few seconds beyond
this point.

SURFACE DISCOLOURATION:
 Surface discolouration may be due to:
Over heating
Incomplete elimination of wax
High sulfur contents of torch flame.
Absence of reducing agents in the investment.
 A source of discolouration often overlooked is the surface
contamination of a gold alloy restoration by mercury.
 Thus it is not a good practice to place a new amalgam
restoration adjacent to a high noble metal alloy restoration.

 REFERENCES:
 Phillips’s Science of Dental Materials , Anusavice , 11th Edition
Craig’s Restorative Dental Materials , Powers & Sakaguchi,
12th Edition
Comtemporary Fixed Prosthodontics , Rosenteil, 4th Edition
A textbook of Operative dentistry by Vimal Sikri

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