Dental Casting Guide: Materials, Procedures & Defects

INVESTMENT MATERIALS,
CASTINGS PROCEDURES
AND DEFECTS.
DR. AATIF KHAN

CONTENTS
1. DEFINITIONS
2. INTRODUCTION TO INVESTMENT MATERIALS
3. IDEAL REQUIREMENTS OF INVESTMENT MATERIALS
4. TYPES OF INVESTMENT MATERIALS
5. SETTING EXPANSION OF INVESTMENT MATERIALS
6. INTRODUCTION TO CASTING
7. DIE AND DIE MATERIALS
8. CASTING ARMAMENTARIUM
9. CASTING PROCEDURES
10. REFERENCES

DEFINITIONS
• Casting - 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- 9)
• Casting - is the process by which a wax pattern of a prepared tooth is fabricated
and converted to its metallic replica.”(Rosenteil 5th edition)

DEFINITIONS
• Dental casting investment -
A material consisting primarily of an allotrope of silica and a bonding agent. The bonding
substance may be gypsum or phosphates and silica.(GPT-9)
• Investment –
A ceramic material that is suitable for forming a mold into which a metal or alloy is cast.
The operation of forming a mold is described as investing.(Craig)
• Investing -
The process of covering or enveloping, wholly or in part, an object such as denture, tooth,
wax form, crown, etc. with a suitable investment material before processing, soldering or
casting.(GPT 9)

CASTING SHRINKAGE
• All alloys shrink when they change from liquid to solid.
•Shrinkage affects the fit of the casting. Therefore, it must be controlled and
compensated for in the casting technique.
• Casting shrinkage occurs in 3 stages –
1. Thermal contraction of the liquid metal.
2. Contraction of the metal from changing from liquid to solid state.
3. Thermal contraction of solid metal as it reaches room temperature.

CASTING SHRINKAGE VALUES
ALLOY CASTING SHRINKAGE %
Type 1, Gold based 1.56%
Ni-Cr-Mo 2.3%
The casting shrinkage of gold alloys is (1.25 to 1.65%) less than base metals.

COMPENSATION FOR SHRINKAGE
• INVESTMENT MATERIALS
A. HYGROSCOPIC EXPANSION
B. THERMAL EXPANSION
C. SETTING EXPANSION
• WAX PATTERN EXPANSION
• RING LINERS

• The management of dimensional
changes can be summarized by the
equation:
“Wax shrinkage + metal shrinkage =
wax expansion + normal setting
expansion + hygroscopic
expansion + thermal expansion.”

INTRODUCTION TO INVESTMENT
MATERIALS
An investment is a mixture of three distinct types of materials:
1. Refractory material -
• It is usually a form of silica, such as quartz, tridymite, or cristobalite, or a mixture of
these.
• Sustain exposure to high temperatures without significant degradation.
• Hence they’re needed to compensate for the shrinkage that occurs during casting.

2. Binder material -
• As the refractory materials alone do not form a coherent solid mass, some kind of
binder is needed.
• Common binders used : α-calcium sulfate hemihydrate (GYPSUM), Phosphates, Ethyl-
silicate.
3. Modifiers -
• Chemicals such as sodium chloride, boric acid, potassium sulfate, graphite, copper
powder and magnesium oxide in small quantities.
• Small amounts of boric acid or chlorides enhance thermal expansion of investments
bonded by calcium sulfate.

IDEAL REQUIREMENTS OF INVESTMENT
MATERIALS
Easy
manipulation
Range of
Beneficial casting
temperatures
Sufficient
strength
Stability at higher
temperatures
Smooth surface
Sufficient
expansion
Porosity
InexpensiveEase of
devestment

TYPES OF INVESTMENT MATERIALS
Classification according to the type of Binder used:
1. Gypsum bonded investment material
2. Phosphate bonded investment material
3. Ethyl-Silicate bonded investment material

Classification according to the type of Refractory material (SILICA) used:
1. Quartz investments.
2. Cristobalite investments.
3. Trydimite investments.

Classification according to the Use and Melting range of the Alloy:
A. Gypsum bonded investments -
For conventional casting of gold alloy inlays, onlays, crowns & fixed prosthesis.
ADA specification – 2 divided them further into:
TYPE 1- for casting inlays/crowns
TYPE 2- for casting inlays/onlays/crowns
TYPE 3- for partial dentures with gold alloys

B. Phosphate bonded investments –
For base metal alloys used to produce copings or frameworks for metal-ceramic
prosthesis, pressable ceramics.
ADA specification – 2 divided them further into :
Type 1 – for inlays, crowns and other fixed restorations.
Type 2 – for partial dentures and other cast removable restorations.

C. Ethyl-Silicate bonded investments –
Used principally in casting of RPD with base metal alloys.
D. Brazing or soldering investments - Used for brazing parts of a restoration such as
clasps on RPD.
ADA specification 2 further divided into -
TYPE I: Gypsum bonded dental brazing investment
TYPE II: Phosphate bonded brazing investments.

GYPSUM BONDED
INVESTMENT
MATERIALS

1. GYPSUM BONDED INVESTMENTS
Acc. to ADA specification no-25, there are 3 types (based on type of expansion)–
• Type 1: thermal expansion type; for casting inlays and crowns.
• Type 2 : hygroscopic expansion type; for casting inlays and crowns
• Type 3 : normal expansion type, for casting complete and partial dentures.
• They can withstand temperatures up to 650 0C – 700 0C.
GYPSUM BONDED INVESTMENT MATERIAL

COMPOSITION OF GYPSUM BONDED
INVESTMENT MATERIALS
• A. 55% to 65% - Quartz or Cristobalite (Refractory material).
• B. 25% to 45% - α-Calcium Sulfate Hemihydrate/ Gypsum. (Binder).
• C. 2% to 3% - Carbon, Powdered copper or graphite (Chemical Modifiers).
• These materials are supplied as powders which are mixed with water.

A. SILICA (REFRACTORY COMPONENT)
• Provides a refractory component during the heating of the investment.
• During the heating, the investment is expected to expand thermally to compensate
partially or totally for the casting shrinkage of the gold alloy.
• Regulates thermal expansion.

• Silica exists in four allotropic forms –
• Quartz, Tridymite, Cristobalite, and Fused quartz.
• Quartz and Cristobalite are of great interest in dentistry.
• When quartz or cristobalite is heated, a change in crystalline form occurs at a
transition temperature characteristic of that particular form of silica.
• The β-allotropic forms are stable only above the transition temperature.

The density decreases as the α form changes to the β form, with a resulting increase in
volume that is exhibited by a rapid increase in the linear expansion.
α β

• When quartz is heated, it inverts from a "low" form, α-quartz, to a "high" form, called β-
quartz, at 575o C.
β- quartz
α- quartz

• Cristobalite undergoes transition between 200o C and 270o C from "low" (α-
cristobalite) to "high" (β-cristobalite).
• This greater expansion due to its inversion,
compensates for the normal contraction of
gypsum during heating.

• Quartz, cristobalite, or a combination of the two forms may be used in a dental
investment.
• Both are now available in pure form.
• Based on the type of silica principally employed, dental investments are often
classified as quartz or cristobalite investments.

B. GYPSUM (BINDER)
• α-Calcium sulfate hemihydrate (GYPSUM) to hold the other ingredients together and
to provide rigidity.
• When this material is heated to the temperatures required for complete dehydration
and to ensure complete castings, it shrinks considerably and occasionally fractures.

• All forms of gypsum shrink considerably
after dehydration between 200oC and
400oC.
• A slight expansion takes place between
400oC and 700oC
• and a large contraction then occurs.
DIMENSIONAL CHANGE OF 3 FORMS OF GYPSUM WHEN HEATEDGYPSUM BONDED INVESTMENT MATERIAL

• The shrinkage on heating is due to the dehydration of the set
gypsum –
(CaSO4)2 · H2O → 2CaSO4 + 2H2O + Heat
• Shrinkage is due to the transformation of calcium sulphate
from the hexagonal to the orthorhombic configuration.
Calcium sulphate
hemihydrate
Water Calcium
sulphate
dihydrate
Water
A
B

• This later shrinkage is caused by decomposition and the release of sulfur gases, such
as sulfur dioxide which contaminates the castings.
• Gypsum products should not be heated above 700oC.
• These effects can be minimized by ‘heat soaking’ the mold at 700°C for at least an
hour to allow the reactions to be completed before casting.

C. CHEMICAL MODIFIERS
• Boric acid and sodium chloride (Modifiers)
• Prevent most of the shrinkage of gypsum when it is heated above 400oC.
• Carbon, Powdered copper (Reducing agents) - provide a nonoxidizing atmosphere
in the mold during casting.
• They are used as balancing agents to regulate the setting time and setting
expansion.
• The gypsum products containing carbon should not be heated above 650oC to obtain
proper fit and uncontaminated castings.

SETTING TIME
• Acc. to ADA Specification no. 2 –
• The setting time should not be shorter than 5 mins or longer than 25 mins.
• Usually, the modern inlay investments set initially in 9 to 18 mins.

SETTING EXPANSIONS
• 3 TYPES –
1. NORMAL SETTING EXPANSION
2. HYGROSCOPIC SETTING EXPANSION
3. THERMAL SETTING EXPANSION
• To aid in enlarging the mold to compensate partially or totally for the casting
shrinkage of alloy.

NORMAL SETTING
EXPANSION
Gypsum investment is allowed to set in
contact with air.
• Acc. ADA sp. no 2 :
• Setting expansion for Type 1
investment in air – 0.6%
• For modern investments – 0.4%

FACTORS AFFECTING NORMAL SETTING
EXPANSION
• 1. Gypsum content – Greater the gypsum content of the investment, greater the
exothermic heat transmitted to the wax pattern and greater the mold expansion.
• 2. Type of mix – Thinner mix, lesser expansion.
• 3. Spatulation – Lesser the spatulation, Lesser the expansion.
• 4. Age of investment – More the age of investment, lesser the expansion.
• 5. W/P ratio - Lower the W/P ratio for the investment, greater the exothermic heat
and greater the effective setting expansion.

HYGROSCOPIC
SETTING EXPANSION
• Gypsum investment is allowed
to set in contact with water.
• The hygroscopic setting
expansion maybe 6 or more
times greater than the normal
setting expansion.

HYGROSCOPIC SETTING EXPANSION
Acc. To ADA specification no. 2 –
For Type 2 investments - minimum setting expansion in water of 1.2%
and maximum 2.2%.

FACTORS AFFECTING HYGROSCOPIC SETTING
EXPANSION
1. Composition –
• Finer the particle size of the silica, greater the hygroscopic expansion.
• Higher silica content, greater the hygroscopic expansion.
• The investment should have at least 15% binder to prevent drying shrinkage and to
provide strength after hygroscopic setting expansion of the set investment.

2. W/P Ratio –
• Higher the water content, lesser is the expansion.
3. Temperature –
• Higher the temperature of immersion in water, lesser the surface-tension, hence
greater the expansion.
4. Time of immersion -
• Immersion before the initial set, causes greater expansion.
5. Spatulation –
• The shorter the mixing time, the lesser the hygroscopic expansion.

6. Effect of added water -
•An increase in the amount of water, increases
the hygroscopic setting expansion up to a
certain point, after which further addition of
water does not create any expansion.
RELATIONSHIP OF HYGROSCOPIC SETTING
EXPANSION AND THE AMOUNT OF WATER ADDED.

7. Silica/Binder ratio -
• If this ratio increases, greater the hygroscopic expansion and lesser the strength.
• Because added water can easily diffuse through the silica particles.
8. Shelf life –
• Older the investment, lesser the expansion.
• Should be stored airtight, better to purchase small amounts at a time.

9. Effect of Confinement –
• Both the normal and the hygroscopic setting expansions are confined by
opposing forces, such as the walls of the container in which the investment is
poured or the walls of a wax pattern.
• This confinement can be avoided largely by placing a damp liner on the inner wall of
the ring.

THERMAL
EXPANSION
• It is directly related to the amount
of silica present and the type of
silica employed.
• Quartz transforms at 575°C (1067°F).

THERMAL
EXPANSION
• Cristobalite transforms from the α-form
to the β-form between 200°C and 270°C.
• These transitions cause a change in
crystal form leading to decrease in density
which leads to increase in volume of the
refractory components.
• Because of this greater expansion during
inversion of cristobalite, the normal
contraction of gypsum during heating is
easily eliminated.

THERMAL EXPANSION
• Acc to ADA sp. 2 -
• Type I investments – thermal expansion of not less than 1.0%
• nor greater than 1.6%.
• Maximal thermal expansion to be attained at a temperature of 700°C.
• Thus when a thermal expansion technique is employed, the maximum mold temperature
for the casting of gold alloy should be less than 700°C.

FACTORS AFFECTING
THERMAL EXPANSION
• 1. W/P ratio –
• More the water used,
lesser the thermal
expansion.
• (Thermal expansion is
related to the amount of
solids present).
EFFECT OF W/P RATIO ON THERMAL EXPANSION OF AN
INVESTMENT CONTAINING 20% GYPSUM AND 80% QUARTZ

FACTORS AFFECTING THERMAL
EXPANSION
• 2. Effect of chemical modifiers- Small amounts of NaCl, KCl eliminate the
contraction caused by gypsum and increases expansion.
• 3. Strength –
• ADA specification no. 2, requires a minimum compressive strength not less than 2.4
Mpa, 2 hours after setting.
• 4. Fineness – Increases the expansion and gives smoothness to the casting.

FACTORS AFFECTING THERMAL
EXPANSION
• 5. Porosity – greater the size of gypsum crystals present in the set
investment, lesser is the porosity.
• 6. Particle size - Mixture of coarse and fine particles exhibits less porosity than an
investment composed of uniform particle size.
• 7. Hemihydrate content - Lesser the hemihydrate content, greater the amount of
water and lesser the thermal expansion.

PHOSPHATE BONDED
INVESTMENT
MATERIALS

2. PHOSPHATE BONDED INVESTMENT
MATERIALS
• The rapid growth of metal ceramic prosthesis
has resulted in an increased use of phosphate
bonded investments.
• Base metal alloys and higher melting range
gold alloys require phosphate bonded
investment materials for casting.
PHOSPHATE BONDED INVESTMENT MATERIAL

COMPONENTS OF PHOSPHATE BONDED
INVESTMENT MATERIALS
A. SILICA (REFRACTORY COMPONENT) :
• Silica in the form of cristobalite, quartz or a mixture of the 2, in concentration of
approximately 80%.
• Function – high temperature thermal shock resistance and a high thermal expansion.

B. BINDER
A. POWDER – Magnesium Oxide (Alkaline) and Ammonium Diacid Phosphate (Acidic)
• It is soluble in water, gives strength, provides phosphate ions and reacts with silica at
high temp to increase strength.
B. LIQUID – Colloidal Silica Suspension (instead of water)
• The liquid is a form of silica sol in water, which gives higher thermal expansion.
• These suspensions can freeze and become unusable – stored in frost free environment.

C. MODIFIER
• CARBON is added to the powder. (MODIFIER)
• Produces clean casting and facilitates the devesting of the cast.
• Carbon shouldn’t be used with base metal alloys as it embrittles the alloys.
• The chemical reaction that causes the investment to set and harden is as follows :
NH4H2PO4 + MgO + 5H2O → NH4MgPO4 .6H2O
Ammonium
phosphate
Magnesium Water Magnesium Ammonium
Phosphate

SETTING AND THERMAL EXPANSION
• Upon mixing there is slight expansion which can be increased by using colloidal silica
sol instead of water.
• Early thermal shrinkage occurs due to decomposition of the binder, magnesium
ammonium phosphate and is accompanied by evolution of ammonia.
• Shrinkage is masked because of the expansion of the refractory silica.
• The combined setting and thermal expansion for phosphate investments is around 2%,
if the special silica liquid is used.

WORKING AND SETTING TIME
• Warmer the mix, faster the setting time.
• Setting reaction is exothermic which further accelerates the setting time.
• Increased mixing time gives faster set.
• More efficient the mix, better the casting smoothness and accuracy.
• An increase in liquid/powder ratio increases the working time.

In contrast to gypsum-bonded products, this material is stable at burnout temperatures above
650°C which allows for additional thermal expansion.
SCANNING ELECTRON MICROGRAPHS OF A AND B EACH HEATED TO 700°C.
Gypsum-bonded investment (A) Phosphate-bonded investment

ETHYL-SILICATE
BONDED INVESTMENT
MATERIALS

3. ETHYL SILICATE BONDED INVESTMENT
MATERIALS
• Used in construction of high fusing base metal partial denture alloys.
• Losing popularity because of complicated and time consuming procedures.
• COMPOSITION -
A. SILICA GEL (REFRACTORY COMPONENT) –
• Silica gel that reverts to silica (cristobalite) on heating.
• Several methods are used to produce silica or silicic acid gel binders.
ETHYL-SILICATE BONDED INVESTMENT MATERIAL

• When the pH of sodium silicate is lowered by the addition of an acid or an acid salt
such as monoammonium phosphate, a bonding silicic acid gel forms.
• The addition of Magnesium oxide strengthens the gel.
• An aqueous suspension of colloidal silica can be converted to gel by the addition
of an accelerator, such as ammonium chloride.

METHOD OF OBTAINING SILICA GEL
• A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the presence
of hydrochloric acid, ethyl alcohol and water, as follows:
HCL + Si(OC2H5) + 4H2O → Si (OH)4 + 4C2H5OH
• A colloidal sol of polysilicic acid is formed instead of the simpler silicic acid shown in
the reaction.
Ethyl silicate
(Polymerized)
Water Poly-silicic acid
(Colloidal sol)

• Stage 1 - “HYDROLYSIS”, with the formation of poly-silicic acid sol.
• Stage 2 - “GELATION”, the sol is mixed with quartz or cristobalite to which is added
a small amount of finely powdered MgO to render the mixture alkaline.
• A coherent gel of polysilicic acid then forms accompanied by a slight “setting
shrinkage”.
1Hydrolysis
2
Gelation
3
Drying

• STAGE 3 is called “Drying’’.
• The soft gel is dried to a temperature below 168°C. During drying, the gel loses
alcohol and water to form a concentrated hard gel of silica particles tightly packed
together.
• A volumetric contraction accompanies the drying, which reduces the size of the mold.
• This contraction is known as “green shrinkage” and it occurs in addition to the
setting shrinkage.
• This gelation process is slow and time-consuming.

•Stock solutions of hydrolyzed ethyl-silicate binder may be prepared and stored in dark
bottles.
•The solution gels slowly settle on standing and its viscosity may increase noticeably
after 3-4 weeks when it has to be discarded

• Ethyl-silicate bonded investments being more refractory than phosphate-bonded
investments, can tolerate higher burnout or casting temperatures.
• Temperatures between 1090 °C and 1190°C are employed when the higher fusing
chromium containing alloys are cast.
• Ethyl-silicate investments are less popular because of more complicated and
time consuming procedures.

STEPS IN MAKING A
CAST RESTORATION -
- PART 2 OF THE SEMINAR -

STEPS IN MAKING A CAST RESTORATION -
1. TOOTH /CAVITY PREPARATION
2. IMPRESSION
3. DIE PREPARATION
4. WAX PATTERN FABRICATION
5. SPRUING
6. INVESTING
7. BURNOUT
8. CASTING
9. CLEANING & POLISHING

INTRODUCTION TO CASTING
• Lost-wax castings have been made since ancient times.
• Wax patterns are converted to cast metal patterns.
• Introduced into dentistry in the 19th century by W.H. Taggart.
LOST WAX TECHNIQUE -
• Surrounding the wax pattern with a mold made of heat-resistant investment material,
• Eliminating the wax by heating,
• Introducing molten metal into the mold through a channel called the sprue.
• Resulting casting must be a highly accurate reproduction of the wax pattern both in
surface details and in overall dimensions.

OVERVIEW OF THE LOST WAX TECHNIQUE

OBJECTIVES OF CASTING
• To heat the alloy as quickly as possible to a completely molten condition.
• To prevent oxidation by heating the metal with a well adjusted torch .
• To produce a casting with sharp details by having adequate pressure to the well
melted metal to force into the mold.

DIE
DIE - the positive reproduction of the form of the prepared tooth in any suitable
substance in which inlays, crowns & other restorations are made. (GPT-9)
IDEAL REQUIREMENTS –
• Accurate reproduction of the fine details
• Dimensional accuracy
• Good strength & hardness
• Ease of use
• Abrasion resistance
• Relatively inexpensive

DIE SYSTEMS
A. Working cast with separate die.
B. Working cast with removable die –
1. Conventional dowel pin system
2. Di-lok
3. Pindex
4. DVA
5. Belle de st. Claire
6. Zeiser Accu-trac system
7. Mono trac

DIE MATERIALS
1. Gypsum products - Type IV Dental Stone, Type V Dental stone
2. Die stone - Investment combination
3. Electroformed dies - Silver plated, Copper plated
4. Epoxy resins
5. Polyurethane
• Alternative die materials - Silico-phosphate cement, Amalgam, Metal-sprayed dies,
Flexible die materials, Refractory die materials.

ADA Specification no. 25 –
Type IV Die Stone – should have a setting expansion of 0.1% or less.
Type V Die Stone – setting expansion should be as much as 0.3%
Disadvantage of Type IV Die – susceptibility to abrasion during carving of wax pattern.
Gypsum dies are modified to increase abrasion resistance and refractoriness, change
the dimension or a combination of these.
Means to increase abrasion resistance – Silver plating, coating surface with die
hardener, adding die hardener to gypsum.

CONDITIONING OF THE DIE :
1 - DIE DITCHING 2 - MARKING THE MARGINS 3 -APPLYING DIE HARDENER
4 - APPLYING DIE SPACER AND DIE SEPERATOR
5 -WAX PATTERN
FABRICATION

A. DIE HARDENER
• Applied on the die surface to increase the surface hardness and abrasion resistance.
• Should have low viscosity.
• Commonly used die hardeners:
• Cyanoacrylate resins (Eg: Premabond)
• Acrylic resin lacquer (Eg: Die Prep die hardener)

B. DIE SPACER
• It is applied to the die to increase the cement space between axial walls of the
prepared tooth and the restoration. It is formulated to maintain constant thickness
when painted on the die. However, it should not coat the entire preparation.
• For adequate marginal adaptation, a band of about 0.5 - 1 mm immediately adjacent
to the preparation margin must be left unpainted.
• Use of thickened die spacer can result in an excessive thickness of spacer.
• Common die spacers – Metal oxide powders, Adhesives dispersed in organic solvent
such as ketone.

B. DIE SPACER
• Types -
1. Resins (most commonly used)
2. Model paint
3. Colored nail polish
4. Thermoplastic polymers dissolved in volatile solvents.

OPTIMUM THICKNESS OF DIE SPACER
• Rosenstiel – 20 to 40 µm (4th edition)
• Rudd and Morrow – 10 to 15 µm
• Fusayama et al - Thickness should not be less than 30 µm. (JPD 1964;14;95)
• Jorgensen et al - Variations of thickness from 20-140 µm had only moderate influence
on retention. (JPD 1966:16;740)
• Eames et al – Optimal thickness - 25 µm. (JADA 1978;96;432)

APPLYING DIE SPACER
• The material is available in contrasting colors to facilitate applying the required number of coats.
• Care must be taken to keep the material at least 1 mm from the margin.

C. DIE SEPERATOR / VARNISH
• The lubricants which can be used to prevent wax from adhering to stone.
• They are oils, liquid soap, detergents and other commercially available preparations.
Function –
• Easy removal of wax pattern without damaging margins.
• Applied with brush, excess to be removed with tissue paper.
• Eg: Kleen lube (Kerr), Picosep (Renfert), Dielube wax sep (Dentecon Inc.).

ELECTROFORMED DIES
• The popularity of Copper plated compound dies began in early 1930s, silver
plated dies became more popular in later years.
• Produced by treating the surface of impression material so that it conducts
electricity. This process is known as Electroforming.
• The impression materials which can be electroplated are impression
compound and elastomeric materials. (Polysulfide)

• A thin layer of metal, such as silver powder, is deposited on the surface of
impression material.
• The types of dies produced are - Copper plated dies and Silver plated dies.
• Advantage - high strength, adequate hardness & excellent abrasion
resistance.
• Example : Bronze powder, aqueous suspensions of silver powder, powdered
graphite.

ELECTROFORMING PROCESS
• Electrode Bath :
• Impression - acts as Cathode
• Silver plate - acts as Anode
• Electroplating Bath Solution- Silver Cyanide or Silver
Nitrate solution (better details)
• Electric current - 5-10 mA/cm² is passed for 10 hours.
The impression that contains the electroformed die surface
is then filled with dental stone. When the stone hardens, it
is mechanically locked to the rough interior of the
electroformed metal shell.

ELECTROFORMING PROCESS
• The impression that contains the electroformed die surface is then filled with dental
stone.
• When the stone hardens, it is mechanically locked to the rough interior of the
electroformed metal shell.
• The impression material is then removed to provide a die with greater surface
hardness and resistance to abrasion than that of gypsum.

DIE STONE/ INVESTMENT COMBINATION
• In this, die and Investment medium have a comparable composition.
Divestment is a commercially available gypsum bonded material.
• It is mixed with colloidal silica and a die is made from this mix, then wax pattern is
constructed on it.
• Highly accurate technique for use with gold alloys.
Advantage-
• It eliminates the possibility of distortion of wax pattern while removing it from die.

WAX PATTERN FABRICATION
• It is contouring of wax pattern into desired
shape and form.
• All aspects of final restoration are incorporated
into the wax pattern.
• Shortest time should elapse between the time
the pattern is removed from the die & the time it
is invested.

WAXING TECHNIQUE
DIRECT
• Wax pattern is made directly inside
the mouth, as in inlay case.
• Type 1 medium wax.
• Exceptionally demanding procedure.
INDIRECT
• Wax pattern is made on the die.
• Type 2 hard wax.
• More commonly used technique.

CARVING INSTRUMENTS
PKT (P.K.THOMAS) WAX CARVERS ELECTRICAL CARVING UNITS

ARMAMENTARIUM
FOR WAXING -
•
• Comprises of 5 instruments –
• PKT No. 1 - used for positioning of
functional and non functional cusps.
• The marginal, cusp and triangular ridges
are also added with PKT No. 1.
• PKT No. 2 - used for small increments of
wax and eliminating voids remaining on
the occlusal surface.

• PKT No. 3 - Burnishing
Developmental and supplemental
grooves.
• PKT No. 4 - Smoothening of axial
surfaces.
• PKT No. 5 - used to refine the
ridges.

PKT No. 1 – Large increments of wax.
PKT No. 2 – Small additions of wax.
PKT No. 3 – Burnishing and carving occlusal
surface.
PKT No. 4 – Designed as all purpose carver.
PKT No. 5 – Refine triangular ridges and
occlusal grooves.
1
2
3
4
5

FORMING THE INITIAL WAX COPINGS -

EVALUATION OF WAX PATTERN
•Well-adapted pattern. •Poor adaptation
•Folds and creases indicate that the wax
was not hot enough when applied.

SPRUE AND SPRUE FORMER
SPRUE
The channel or hole through which plastic
or metal is poured or cast into a gate or
reservoir and then into a mold; OR
The cast metal or plastic that connects a
casting to the residual sprue button. (GPT
-9)
SPRUE FORMER
A wax, plastic, or metal pattern used to
form the channel or channels allowing
molten metal to flow into a mold to make a
casting. (GPT – 9)

Types of
Sprues
Metal
Stainless
Steel
Steel
Brass
Non-
Metal
Plastic
Resin
Wax

FUNCTIONS OF SPRUE FORMER
• Used as handle to remove wax pattern.
• To form a mount for the wax pattern & fix the pattern in space so a mold
can be made.
• Creates a channel for elimination of wax during burn out.
• Facilitate flow of molten metal from crucible to mold .
• Stores additional metal in reservoir & prevents shrinkage porosity.
• Compensates for alloy shrinkage during solidification.

FACTORS AFFECTING SPRUE
1. Diameter
2. Position
3. Attachment
4. Direction
5. Length

1. Sprue Diameter –
• Should be approx. the same size as the thickest portion of the pattern.
Recommended sprue diameters –
• 2.5mm (10 gauge) for molar metal ceramic restorations.
• 2.0mm (12 gauge) for premolar partial veneer restoration.
2. Sprue Position –
• Ideally, point of greatest bulk in the pattern to avoid distorting thin areas of wax
during attachment to the pattern.
• To complete flow of the alloy into the mold cavity.

3. Sprue Attachment –
• Should be attached to the portion of the pattern with the largest
cross-sectional area.
• To allow molten metal to flow from the thicker section to
surrounding thin areas (margins) rather than the reverse.
4. Sprue Direction –
• Sprued at 45 degree angle to the bulkiest portion of the wax
pattern.
• Shouldn’t be sprued at 90 degrees to a broad flat surface as it
causes turbulence within the mold cavity leading to severe porosity.

5. Sprue Length –
• Length of the sprue should be long enough to position the pattern properly in the
casting ring within 6mm of the trailing end and yet short enough so that the molten
alloy does not solidify in the mold.

RESERVOIR
Piece of wax attached to the sprue about 1mm away from
the pattern, as a enlarged round mass or a connector bar
between the wax pattern and sprue former.

SPRUING TECHNIQUES
A. DIRECT : sprue former provides direct connection between
pattern area & the sprue-base/crucible former area.
• Disadvantage - potential for suck-back porosity at the
junction of restoration and the sprue.
B. INDIRECT : Connector/ Reservoir bar is positioned between
pattern & the crucible former.
• Advantages - greater reliability & predictability in casting,
enhanced control of solidification shrinkage.

CRUCIBLE FORMERS
• The sprue is attached to a crucible former,
usually made of rubber, plastic or metal,
which constitutes the base of the casting
ring.
• The exact shape of the crucible former depends
on the type of casting machine used.

CRUCIBLES FORMERS
1. Steep-sided cone: used with metal when casted using centrifugal casting force.
2. Shallow cone: used to cast metal using stream/air pressure.

CASTING RING
The casting ring serves as a container for the investment while it sets & restricts
setting expansion of the mold.

CASTING RING
1. Shapes – Round, Oval, Elongated.
2. Complete rings –
A. Rigid – Metal, Plastic
B. Flexible – Rubber
3. Split rings – Metal, Plastic

CASTING RING CONSIDERATIONS
The internal diameter of casting ring should be
5-10mm greater than the widest measurement
of the pattern and about 6 mm higher.
• For single crown/inlay - small rings as used.
Diameter - 32 mm
• For large fixed partial denture – round/oval
shaped casting ring are used.
Diameter - 63 mm

RING LINERS
Types - Asbestos, Cellulose, Ceramic.
Asbestos - carcinogenic potential, biohazard.
Functions of a liner -
•Affords greater normal expansion in the investment.
•The absorbed water causes a semi hygroscopic
expansion.
•Thickness – not less than 1mm.

LINER TECHNIQUES
• Dry Liner – Tacked in position with sticky
wax.
• Wet liner – Immersed in water and excess
water shaken off.
• The liner is cut to fit the inside diameter
of the casting ring with no overlap and
3mm short of the top and bottom of the
ring.
• This serves to lock the investment within
the ring & equalize expansion.

RING LINER CONSIDERATIONS
• Expansion is always greater in unrestricted longitudinal direction than radical
direction.
• It is desirable to reduce longitudinal expansion.
• Placing ring liner somewhat short 3.25mm of the ends of the ring provides a more
uniform expansion,
• Thus less chance of distortion of wax pattern and the mold.

RINGLESS CASTING
• 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.

AFTER SECURING WAX PATTERN
Before investing, it should be cleaned of any debris, grease, oils and separating
medium.

WAX PATTERN – CRUCIBLE FORMER
ATTACHMENT
• Wax pattern is attached to the crucible
former with a sprue ready for investing.
• A ring liner is in place.

SURFACE TREATMENT OF WAX PATTERN
• Cleaning the wax pattern of debris, grease or oil by surfactants-
1. Pattern cleanser.
2. Synthetic detergent/ Acetylene.
3. Debubblizer.

DEBUBBLIZER
A debubblizer is a surface tension reducing agent that is used to reduce the prevalence
of bubbles in industrial processes such as wax casting.
It is also referred to as surfactant or a wetting agent that is sprayed on the set
impression material.
Composition –
• Citric acid
• Sodium 2-phenylpropane-2-sulfonate
• Propylene glycol

INVESTING
•The process of covering or enveloping, wholly 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)

MIXING THE INVESTMENT
• While the wax pattern is air drying, the appropriate
amount of distilled water (Gypsum Bonded
investments)/ colloidal silica special liquid (Phosphate
Bonded investments) is dispensed.
• Powder should be weighed before mixing it with
liquid.

MIXING THE INVESTMENT
The liquid is added to clean dry mixing bowl, and the powder is gradually added to the
liquid using care and caution to minimize air entrapment.

VACUUM INVESTING UNITS
WHIP MIX COMBINATION UNIT MULTIVAC COMPACT

VACCUM MIXING
Advantages –
• The amount of Porosity in the investment is reduced.
• The texture of cast surface is smoother with better detail reproduction.
• The tensile strength is greatly increased.

DISPENSING
Poured into the ring (thin consistency) from a height (20-30 cm), at a slight angle from
bottom to the top of a ring.

PAINT ON TECHNIQUE
• Wet investment material is gently painted
over a complex wax pattern by the use of
fine hair brush, covering it completely.
• No. 6 or No. 8 brush is used to coat the
pattern.
E

SETTING OF INVESTMENT
•(For thermal expansion technique) -
• Allow the investment ring to bench set undisturbed
for the time recommended by manufacturer
(approximately - 1 hour).
•(For hygroscopic technique) –
•Immediately place the filled casting ring in 37 degree
water bath with crucible former-side down.

A - When the investment has set, the “skin” at the top of the
ring is trimmed off.
B - The rubber crucible former is removed, and any loose
particles of investment are blown off.
C - The ring is then placed in the furnace for the
recommended burnout schedule.

BURNOUT
• BURNOUT - WAX ELIMINATION (GPT – 9)
• Elimination of the wax pattern from the mold of set investment is referred to as a
burnout.
• Ring may be placed on a raised object within the oven to completely eliminate the
wax & form a cavity into which the molten metal is cast.

BURNOUT
• Ring is placed in a room temperature furnace and heated to the prescribed max.
temperature.
• For gypsum bonded investments –
• A. Hygroscopic technique – 500 C
• B. Thermal expansion technique – 700 C
• For Phosphate bonded investments –
• Maximum temperature (700 – 1030 C)

BURNOUT OVENS
FIRELITE NEY VULCAN

• The ring should be maintained long enough at the maximum temperature, i.e
“heat-soak” to minimize a sudden drop in temperature upon removal from the
oven.
• Such a drop could result in an incomplete casting because of excessively rapid
solidification of the alloy as it enters the mold.

CASTING
Casting of an alloy into the mold space uses 2 basic requirements:
1. Heat source – to melt the alloy
2. Casting force – to force molten alloy into mold

• Heat Source:
• Different types of materials and method are used as heat source to melt the alloy.
Two basic modes are by using –
1 – Torch flame.
2 – Electricity.

2 types of torch orifices –
1. Multi orifice
2. Single orifice
Zones of the blow torch flame:
1. Zone 1 – Colorless / Non combustion zone
2. Zone 2 – Combustion zone / Oxidizing zone
3. Zone 3 – Reducing zone
4. Zone 4 - Oxidizing zone

ZONES OF COMBUSTION
Zone 1 – Colorless/Non-Combustion zone – First zone emanating from the nozzle, no heat is
present. Air and gas are mixed before combustion.
Zone 2 – Combustion/Oxidizing zone – Green in color, immediately surrounding the inner cone of
flame. Gas and air are partially burned here.
•Definitely oxidizing zone, should always be kept away from the molten alloy during fusion.
1 2

ZONES OF COMBUSTION
Zone 3 – Reducing zone –
• Just beyond the tip of the green combustion zone, dimly blue color.
• Hottest zone, should be kept constantly on the alloy during melting.
Zone 4 – Oxidizing zone –
• Area where combustion occurs with oxygen in the air.
• Shouldn’t be used to melt the alloy because it oxidizes the alloy and its temperature is
lower than that of the Reducing zone.
3 4

1. GAS AIR TORCH
• Used to melt conventional noble metal alloys
whose melting points less than 1000⁰ C (used
for inlays, crown and bridge).

2. GAS OXYTORCH
• Used to melt metal-ceramic alloys of higher temperature
up to 1200⁰C.
• Single orifice/multi-orifice tip.
• Oxygen pressure - 10 - 15 psi.
• The flame is directed onto metal with the nozzle of the
torch about 1.5 cm away from the metal.
• Complete fluid should be obtained within 30 seconds at
the point of which the metal is poured into the mold.

3. OXY ACETYLENE TORCH
• The actual production of flame can be done by adjusting the pressure and flow of
individual gases .
• Advised pressure for acetylene is 3.5 N/cm2 and oxygen is 7-10 N/cm2, i.e one part
acetylene + 2.5 parts oxygen.
• Distance of 10 cm between the face of the nozzle and the base of crucible.
• If distance is reduced to :
- 7.5 mm - slight porosity.
- 5 mm - increased porosity due to occluded H2 gas.

DIFFERENCE BETWEEN TORCH AND
ELECTRICAL MELTING OF ALLOYS
A) TORCH MELTING
• For low temperature metals.
• Mixture of natural/artificial gas,
oxygen/tank, gas–oxyacetylene.
• Slower than electric heating but more
faster than resistance heating.
B) ELECTRICAL MELTING
• For higher temperature metals.
• Electric resistance melting,
induction melting.
• Melts alloy faster and can be
easily over heated.

CASTING MACHINES
1. Air pressure casting machine.
2. Centrifugal casting machine.
3. Electrical resistance–heated casting machine.
4. Induction melting casting machine.
5. Vacuum or pressure assisted casting machine.
6. Direct-current arc melting machine.

1. 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.
▪ Carbon dioxide, carbon monoxide or nitrogen gas can be used.
▪ Pressure - 10-15 psi.

2. CENTRIFUGAL CASTING MACHINE
• This machine makes use of centrifugal force to thrust
the liquid metal into the mold.
• The method involves pouring molten metal into a
cylindrical mold spinning about its axis of symmetry.
• The mold is kept rotating till the metal has
solidified.
KERR BROKEN-ARM MODEL

2. CENTRIFUGAL CASTING MACHINE
Centrifugal casting is carried out as follows:
• The mold wall is coated by a refractory ceramic coating
• Starting rotation of the mold at a predetermined speed.
• Pouring a molten metal directly into the mold
• The mold is stopped after the casting has solidified.
• Extraction of the casting from the mold.
DEGUSSA MODEL TS-1

3. ELECTRICAL RESISTANCE HEATED
CASTING MACHINE
• There is automatic melting of metal in graphite crucible.
• This is advantageous for metal-ceramic restoration in
which trace amount of base metals, which are
prevented from oxidation from torch flame from
overheating.

4. INDUCTION CASTING MACHINE
• Metal is melted by induction field that develops within the crucible surrounded by
water cooled metal tubing. Molten metal is forced in to mold by air pressure or both.

5. VACCUM/PRESSURE ASSISTED
CASTING MACHINE
• Used for Titanium & its alloys.
• Melting point of commercially pure titanium - 1671˚C.
• In such high temperature, either a graphite or water cooled
copper crucible is used.
• To prevent absorption of gases in its molten state, titanium is
cast in the protective atmosphere of argon or in vacuum.

5.VACCUM/PRESSURE ASSISTED
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.
• The material is sucked upwards into
the mold by a vacuum pump.
• The mold in an inverted position from
the usual casting process, is lowered
into the flask with the molten metal.

6.DIRECT CURRENT ARC MELTING
CASTING MACHINE
• The alloy is vacuum melted & cast by pressure in an
argon atmosphere.
• Direct current arc is produced between 2 electrodes
- Alloy & water-cooled tungsten electrode.
• The temperature within the arc exceeds 4000˚c, the
alloy melts very quickly.
• Disadvantage - alloy can become overheated.

CASTING CRUCIBLES
The casting crucible acts as a platform on which heat can be applied to the metal to
facilitate the melting of the alloy.
CERAMIC CRUCIBLEPLASTIC CRUCIBLE

TYPES OF CRUCIBLES
1. Clay - High and noble metal types.
2. Carbon - High noble crown and bridge, alloys also for higher fusing gold-based
metal ceramic alloys.
3. Quartz - Higher fusing, gold based metal ceramic alloy & palladium alloys.
4. Ceramic–alumina - High fusing alloys of any type : specially for alloys that have a
high melting temperature or are sensitive to carbon contamination.

• The crucibles used with noble metal alloys should not be used for melting base
metal alloy.
• Crucible should be discarded if it contains large amount of oxides and
contaminants from previous metals.
• Sufficient mass of alloy must be present to sustain adequate casting pressure -
• 6 gm - adequate for premolar and anterior casting.
• 10 gm - adequate for molar casting.

CASTING FORCE
Casting force must be greater surface tension of alloy + resistance offered by gas in
the mold.
This can be done by use of following different type of forces -
1. Vacuum force
2. Air or Gas Pressure
3. Centrifugal force

DEVESTING
THE RETRIEVAL OF A CASTING OR PROSTHESIS FROM AN
INVESTING MEDIUM. (GPT 9)

RECOVERY OF CASTING
A - Trimming is done from the bottom
end of the ring.
B - Investment is being pushed
out of the casting ring.
C - The mold is broken open.
D - Investment is removed
from the casting.
• Care must be taken to
avoid damaging the margin

QUENCHING
• After the casting has solidified the ring is removed and quenched in water.
• Advantages of quenching -
• When water contacts the hot investment, a violent reaction ensues, resulting in a
soft, granular investment that is easily removed.
•This leaves the cast metal in annealed condition for burnishing polishing.
• Often the surface of casting appears dark with oxides and tarnish, such a surface film
can be removed by process known as “PICKLING”.

PICKLING
• Heating a discolored casting in an acid.
• Masks the dark/tarnished appearance of adherent oxide.
• Solutions used-
1. (50%) HCL
2. (50%) Sulphuric acid
• Others –
• Ultrasonic devices
• Abrasive devices

Ultrasonic pickling can be carried out while the prostheses is sealed in a Teflon
container.

DISADVANTAGES OF HCL
• Health hazard.
• Fumes corrode the clinic and laboratory metal furnishings.
• Dilute HCl shouldn’t be used - unless necessary neutralizing solutions are
immediately at hand.
• Irreversible tissue injury.
• Best method of pickling - the casting is placed in a test tube / dish and acid is
poured. Acid may be heated, not boiled.

• Casting should not be held with steel tongs, this may contaminate the casting.
• Usually the pickling sol. contains small amounts of Copper dissolved from previous
castings.
• When steel tongs contact this copper, a small galvanic cell is created and copper is
deposited on the casting at the point of contact of the tongs.
• The pickling sol. should be renewed frequently, because it is likely to become
contaminated after reusing the sol. several times.

SANDBLASTING
The casting is held in a sandblasting machine to clean the remaining investment from
its surface.
Aluminum oxide particles of size 50 – 200 µ are fired.
SANDBLASTING UNIT

AFTER CLEANING OF CASTING
• Cleaned casting -

TRIMMING AND POLISHING
• The casting is trimmed, shaped and smoothen with suitable burs or stones.
• The sprue is sectioned off with a cutting disc.
• White stone, rubber wheels, rubber disks, and fine-grits are included in the finishing
and polishing agents.

INSPECTION AND FINISHING
• Tiny air bubbles in the investment create very minute nodules on the inner surface,
which interfere with the fitting of the casting.

INSPECTION AND FINISHING
• Inner surface should be carefully
examined under higher magnification &
illumination for any discrepancy.
• Tiny air bubble/ nodules should be
removed by a small round bur.

• Slightly more, rather than less, metal
than the size of the nodule should be
removed to ensure that the casting does
not bind during seating.

REFERENCES -
1. Philip’s Science of Dental Materials – Anusavice , 11th Edition, 2003.
2. Contemporary Fixed Prosthodontics - Rosensteil, 5th Edition, 2015.
3. Craig’s Restorative Dental Materials – Sakaguchi, 13th Edition, 2011.
4. Materials Used in Dentistry - Mahalaxmi, 1st Edition, 2013.

Dental Casting Guide: Materials, Procedures & Defects

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