Casting procedure and casting defects

PROCEDURE AND
CASTING
DEFECTS
Presented by,
Dr. Chaithra Prabhu
1st year post graduate
Department of
Prosthodontics
1

CONTENTS
 Introduction
 Definition
 Steps in casting
 Spruing
 Investing
 Wax elimination
 Casting
 Cleaning
 Finishing and
polishing
 Casting defects
 Alternative methods of
fabricating prosthesis
 Casting of titanium
 Conclusion 2

DEFINITION
• CASTING :An object formed by the solidification of a
fluid that has been cast into a refractory mold
• CASTING :The action of pouring or injecting a flowable
material into a refractory mold
-GPT 9
4

WAX PATTERN
WAX PATTERN METHODS
DIRECT INDIRECT
6
TYPE 1 HARD
INLAY WAX
TYPE 2 SOFT
INLAY WAXShama bhat science of dental materials
with clinical application 3ed
PKT INSTRUMENTS

7
WAX PATTERN
REMOVAL
Contemprorary Fixed Prosthodontics
Rosensteil

SPRUING
SPRUING: Attachment of the wax pattern to a conical base by
short extension of wax, plastic, or metal -Rudd & morrow vol2
• PURPOSE:
Escape of molten wax
Allow flow of molten metal
Act as reservoir
8

TYPES
wax
• Commonly
used
• Most
preferred
• Easy to
manipulate
• Easy to burn
out
• Lacks
rigidity
Plastic
• Has rigidity
of metal
• Minimum
distortion
• May block
the escape of
wax
metal
• Stronger
then wax
sprues
• Cannot be
burnout
removed
with the
crucible
9

VARIABLES & PRINCIPLES OF
OPTIMAL SPRUE DESIGN
FIVE General Principles
1. Sprue diameter
2. Sprue position
3. Sprue attachment
4. Sprue direction
5. Sprue length
10
Phillips science of dental materials 12th
ed

Sprue diameter
• Same size as the thickest area of the wax pattern
11
Wax pattern Sprue diameter result
Small large distortion
large small Suck back
porosity
ed

12
ed

Sprue position
• Can be attached
Occlusal surface
Proximal wall or just below the non-
functional cusp
Greatest bulk in the pattern – IDEAL
AREA
13
ed

Spruing of removable partial denture frame
work
14

Sprue attachment
• The sprue’s point of attachment to the pattern should be
smooth to minimize Turbulence.
• Generally it must be Flared- Facilitating the entry of the
alloy.
15
Indirect
Gate
technique
direct
ed

Sprue direction
The sprue former is attached at
an angle 450 to allow the
molten metal to flow freely to all
the portions of the mold.
Should not be attached at right
angle-
• Turbulence
• Suck back porosity
• Casting forces may fracture the
investment
17
ed

Sprue length
• Depends on the casting ring
• 6mm – Gypsum bonded investments
• 3-4mm –Phosphate bonded investments
• When sprue is to short-------far placed in the investment-----
leads to inadequate venting-----gases cannot be eliminated-----
leads to porosity
18
ed

VENTING
• Small auxiliary sprue/vents are applied to improve casting of
thin patterns. Usually 18- gauges sprues are used.
• Act as heat sink and ensure escape of gases.
20

RESERVOIR
• Small amount of wax added 1 mm away from
the junction of wax pattern and sprue
• Inserted to prevent localized shrinkage porosity.
• Because of its large mass of alloy and position in the heat
center of the ring, the reservoir remains molten to furnish
liquid alloy into the mold as it solidifies.
• The resultant solidification shrinkage occurs in the reservoir
bar and not in the prosthesis.
21

23
AIM : To compare various sprue designs with the conventional
sprue design
CONCLUSION
Within the experimental group, 4mm sprue was effective
compared to 3mm sprue diameter
No statistical significant results were seen among the
conventional and 4mm sprue design

CASTING RINGS
• Serves as a container for the investment while it sets and
restricts the setting expansion of the mold
24

CASTING RINGS
25
Split casting rings or plastic
casting rings are used to get
adequate lateral expansion

RINGLESS SYSTEM
• With the use of higher strength, phosphate bonded
investments, the ring less technique has become quite
popular
• This method entails the use of a paper or plastic casting ring
and is designed to allow unrestricted expansion.
• Used for base metal alloys
26
Contemporary fixed prosthodontics
Rosenstiel 5th ed

27
THE JOURNAL OF PROSTHETIC
DENTISTRY 2000
AIM: This study compared the vertical margin accuracy of lost
wax castings produced with the conventional casting technique
using a metal ring and a technique that uses a ring-less system.
CONCLUSION
The castings of the ring-less technique provided less vertical
margin discrepancy (mean value 181 ± 71 µm) than the castings
produced with the conventional metal ring technique (290 ± 87
µm and 291 ± 88 µm). The difference was significant (P<.001).

CASTING RING LINER
• Lining----Most commonly used technique to provide
investment expansion
28
Ring liner
Asbestos Non asbestos
Allumino-silicate
ceramic liner
cellulose
ed

FUNCTIONS OF LINER
• To allow enough lateral expansion
• To reduce longitudinal expansion
• If wetted, these liners provide some extra water for
hygroscopic expansion mold
• Two or three layers give higher hygroscopic expansion of the
mold laterally
29

• To ensure uniform expansion, the liner has to be cut to fit the
inside diameter of the casting ring with no overlap.
• Liner can be used dry or wet
• If wet lining technique is used
Lined ring immersed in water and excess is shaken away
Squeezing the liner should be avoided [ variable amount of
water removal-----nonuniform expansion ]
30
ed

31
Journal of International Society of Preventive
and Community Dentistry
AIM: To evaluate the effect of varying cellulose casting ring liner
length and its pre-wetting on the marginal adaptation and
dimensional accuracy of full veneer metal castings
CONCLUSION:
The dry cellulose ring liners provided better marginal adaptation in
comparison to prewet cellulose ring liners. Accurate cuspal height
was obtained with shorter ring liner in comparison to full-length
cellulose ring liners.
GROUPS
GROUP I- full length liner pre-wet lining
GROUP II- full length liner dry lining
GROUP III- 2mm short prewet lining
GROUP IV- 2mm short dry lining
GROUP V- 6mm short pre wet lining
GROUP VI-6mm short dry lining

INVESTING PROCEDURE
• INVESTING: The process of covering or enveloping, wholly or
in part, an object such as denture, tooth, wax pattern etc., with a
suitable investment material before processing, soldering or
casting. -GPT 9
Various investment materials
1. Gypsum bonded investment
2. Phosphate bonded investment
3. Ethyl silicate bonded investment
32

WAX ELIMINATION/ BURN OUT
• WAX ELIMINATION : Removal of wax from the mold
usually by heat -GPT 9
34
Gold alloys 450 – 700 deg C
Ni/Cr alloys 700 – 900 deg C
Co/Cr alloys 1000 deg C
Applied dental materials John F
Mccabe 9th ed

POINTS TO CONSIDER
• Invested rings are placed in a room temperature furnace.
• Advisable to begin the procedure when the mold is still wet.
• The ring is placed in the furnace with the sprue hole facing
down.
• The burnout temperature is slowly increased in order to
eliminate the wax and water without cracking the investment.
35
ed

Hygroscopic low heat technique
• Obtains compensation expansion from 3 sources.
1. 37°C water bath expands wax pattern.
2. Warm water entering the investment mold from top adds
some hygroscopic expansion.
3. Thermal expansion at 500°C provides needed thermal
expansion.
36
ed

ADVANTAGES:
1. Less investment degradation.
2. Cooler surface for smoother castings.
3. Convenience of placing molds directly in 500°C
furnace.
DISADVANATAGE:
• Back pressure porosity great hazard in low heat
technique.
37
ed

38
Standardized hygroscopic technique was developed for alloys
with high gold content; the newer noble alloy may require
slightly more expansion. This added expansion may be
obtained by making 1 or more of following changes.
1. Increasing water bath temperature to 40°C.
2. Using two layers of liners.
3. Increasing burnout temperature to a range of 600°C to
650°C.
ed

High-heat thermal expansion technique
• Depend almost entirely on high-heat burnout to obtain the
required expansion, while at the same time eliminating the
wax pattern.
• Additional expansion results from:
1. Slight heating of gypsum investments on setting. Thus
expanding the wax pattern.
2. Water entering from wet liner adds a small amount of
hygroscopic expansion to the normal setting expansion.
39
ed

CASTING
CRUCIBLES
4 types of casting crucibles are available :
1. Clay high noble-alloy.
2. Carbon high noble crown bridge , higher fusing , gold
based metal ceramic.
3. Quartz high melting range alloys and those that are
sensitive to carbon contamination.
4. Zirconia alumina.
40
ed

METHODS OF MELTING THE ALLOY
41
Dental Material And Their Selection
William J O'brien 3rd Ed

TORCH MELTING
• Common for gold alloys
• Gas/ oxygen ------higher fusion temperature of gold alloys
intended for PFM and palladium alloys
• Air/ acetylene and oxygen/acetylene { hottest flame }
mixture generates higher temperature
42

Disadvantages
1. Excessive heat -----distils lower melting component ----
results in change in composition
2. Excessive heat------release of environmental gasses which
dissolves in molten metal and results in porous casting
3. Carbon contamination from acetylene flame---- brittle
castings
4. Excessive oxidation
43

Zones of torch flame
44
ed

ELECTRIC MELTING
• Electric resistance melting, which is suitable for all gold
alloys, as well as
• Induction melting and Electric arc melting, which are capable
of melting cobalt-chromium and titanium alloys.
45

CASTING MACHINE
• Casting machines provide the means for transferring the
molten alloy from the melting crucible to the mold.
46
• A very old method which uses steam
pressure to drive the melt into the mold.
• They all provide good castings when a
porous gypsum investment is used

1. Torch Melting/Centrifugal
casting machine:
• Casting machine spring is first
wound from 2-5 turns
• Alloy is melted by torch flame in a
glazed ceramic crucible attached to
broken arm
• Spring is released
• Mold is casted by centrifugal force
48
ed

• Alloy is melted electrically
by resistance heating.
• It is used to melt ceramic
alloys.
• Here the alloy is
automatically melted in
graphite or ceramic
crucible.
• The crucible in the furnace
is always against the
casting ring. So the alloy
remain molten slightly
longer and ensures
complete solidification.
2. Electrical resistance - heated
casting machine

3. Induction Melting Machine
• The alloy is melted by an induction field that develops within
a crucible surrounded by water-cooled metal tubing.
• Commonly use for base metal alloys.

• Direct current is produce between two electrodes: the alloys and
the water cooled tungsten electrode.
• Temperature between the arc rapidly increases to 4000°C – alloy
melts very quickly.
• High risk of over heating of the alloy.
• Damage may occur even after few seconds of over heating.
4. Direct-current arc melting machine
53
ed

5. Vacuum or Air pressure casting
machines
• The alloy is heated to casting temperature and then drawn
into the evacuated mold by gravity or by vacuum and
subjected to additional pressure to force the alloy into the
mold
• For Titanium and Titanium alloys, vacuum arc heated argon
pressure casting machine is required.
54
ed

CLEANING THE CASTING
After the casting has solidified the ring is removed and
quenched in water as soon as the button exhibits a dull red
glow.
Advantages
• The noble metal is left in annealed condition for burnishing,
polishing and similar procedure
• Aids in separating investment material
55

Clean residual investments on the casting
{ abrasive blasting }
PICKLING:
The surface of casting appears dark with oxide and tarnish.
Such a surface film can be removed by the process called
Pickling, which consist of heating the discolored casting in a
acid.
56

Method of pickling
• Place the casting in a test tube or dish and pour the heated
acid
PRECAUTIONS
• Not to boil
• Avoid using steel tongs ( galvanic cell )
• Avoid using contaminated acids
57

Pickling solutions
• 50% Hydrochloric acid--------gypsum bonded investment
Disadvantage
Fumes from the acid are likely to corrode the laboratory metal
furnishings
Health hazards
• Cold Hydrofluric acid
• 50% Hydrochloric acid or sulfuric acid + potassium
dichromate
58

• Gold based and palladium based metal ceramic and base
metal alloy are bench cooled to room temperature before
casting is removed from investment.
• Acid is NOT used for BASE METAL ALLOYS.
( REACTIVE)
59

FINISHING AND POLISHING
60
contemprorary fixed prosthodontics
Rosenstiel 5th ed

• Rubber, rag, or felt wheels impregnated with abrasives are
used in the initial stages of finishing. Final polishing is
accomplished with various oxides of tin and aluminum used
in conjunction with a small rag or chamois buffing wheel,
followed with an iron oxide rouge.
61

CASTING DEFECTS
CASTING
DEFECTS
DISTORTION
SURFACE
ROUGHNESS &
IRREGULARITIES
POROSITY
INCOMPLETE
OR MISSING
DETAIL
63

DISTORTION
• Marked distortion is related to distortion of wax pattern.
• Both change in shape and size causes misfit of the casting
64
Change of shape
• Careless removal of wax
pattern
• Too much vibration during
investment
• Too thick investment
• Delay in investing
• Inhomogeneous setting and
thermal expansion of
investment
Change of size
• Under or over
compensation of casting
shrinkage
Science of dental materials V Shama
Bhat 2nd ed

SURFACE ROUGHNESS,
IRREGULARITIES AND
DISCOLORATION
• Surface roughness defined as relatively finely spaced
surface imperfections whose height, width, and direction
establish the predominant surface pattern.
• Surface irregularities are isolated imperfections, such as
nodules that are not characteristic of the entire surface
area
65
phillip's science of dental materials
12th ed

Causes of surface roughness
• Rough wax pattern
• Coarse investment powder particles
• High water/powder ratio
• Insufficient wetting of wax pattern
• High casting force
66
Science of dental materials with clinical
application Shama Bhat 3rd ed

FINS
• Feather like thin extensions on the alloy casting surface
CAUSE
• Cracks in the investment
REASONS FOR CRACKS
• Weak investment
• High w/p ratio
• Improper mixing
• Prolonged heating
• Too high casting force
67
Science of dental materials with clinical
application Shama Bhat 3rd ed

POROSITIES
• Most common among the defects
68
Solidification defects Trapped gases
Localized shrinkage
porosity
Pin hole porosity
Microporosity Gas inclusion porosity
Suck back porosity Sub surface porosity
Back pressure porosity
phillip's science of dental materials
12th ed

69
PROBLEM LIKELY CAUSES REMEDIES
Localized shrinkage
porosity
•Premature termination of
flow of molten metal.
•It mainly occurs at sprue-
casting
junction.
• Use of reservoir.
• Increase the sprue
diameter
Suck-back porosity •Hot spot created by hot
metal impinging on point
on mold surface.
•It often occurs at Occluso
axial Or incisoaxial line
angle
•Flare the point of
attachment of sprue.
•Lowering the casting
temperature by about
30°C
Micro Porosity
Small Irregular
•Rapid solidification of the
mold
•Casting temperature is too
low
Increase the mold
or casting
temperature
SOLIDIFICATION DEFECTS

70
TRAPPED GASES
• PROBLEM LIKELY CAUSES REMEDIES
Pin hole Porosity
Small Spherical
•poorly adjusted torch flame, or
the use of the mixing or oxidizing
zones
•Correctly adjusting
and positioning the
torch during melting.
•Prevent oxidation
of alloys-Flux
Gas inclusion
Porosity
Large Spherical
•Gas mechanically trapped
by molten metal in mold.
•Gas incorporated during
casting procedures.
•Absorbed gases are expelled
on solidification.
Sub Surface
Porosity
•Simultaneous nucleation
of solid grains and gas
bubbles
•Rate of entering of
molten metal in mold.

71
• Back Pressure
Porosity
•Inability of gases in mold
to escape.
•Pressure gradient that
Displace air towards
the end of
investment.
•Tendency for mold
to clog with
residual carbon.
•Proper venting.
•Place pattern no more
than 6-8mm from the
end
of the casting ring
•Sufficient casting
pressure.
•Proper burn out
temperature.

72
INCOMPLETE CASTING
• This is due to :
-insufficient alloy .
-alloy not able to enter thin parts of the mold
-when the mold is not heated to adequately
-premature solidification of the alloy
-sprues blocked with foreign bodies
-back pressure of gases
-low casting pressure

ALTERNATIVE TECHNOLOGY FOR
FABRICATING PROSTHESIS
• Sintering of burnished foil
• CAD CAM processing
• Copy milling
• Electroforming
• Three- dimensional printing
73
phillips science of dental materials 12th
ed

ELECTROFORMING
• The dies are duplicated with gypsum product that has high setting
expansion of 0.1% to 0.2%
• After a conductive silver layer is applied to its surface, the die is
connected to a plating head and then to a power source and placed
in a plating solution
• After a sufficient layer of metal is deposited, gypsum is removed
and coping is sandblasted
• Conventional way is used to build the ceramic layer
74
ed

SINTERING OF BURNISHED FOIL
• Commonly used commercial foil system------captek
• The system requires 3 pairs of materials to form composite
metal structure
1. Captek P & Captek G
2. Capcon & capfil
3. Captek repair paste & capfil
Consist of 88.2% of gold
9.0% of platinum group of metal
2.8% of silver
Coping is very thin, so minimum removal of tooth material,
hence better strength.
75
ed

CAD CAM PROCESSING
• CAD CAM became available in 1980s for dental application
• Alternate method that can produce metal, ceramic or
composite restoration in one appointment
76
ed

COPY MILLING
• Based on the principle of tracing the surface resin pattern
• Traced pattern is the replicated on a blank of ceramic,
composite or metal by grinding, cutting or milling by a
rotating wheel whose motion is controlled by a link through
the tracing device.
77
ed

THREE-DIMENSIONAL PRINTING
• It is a form of additive manufacturing technology
• Stereolithography
• Direct laser metal sintering
• Electron beam melting
78
ed

CASTING OF TITANIUM
The high melting point of titanium (1670°C, murray, 1987);
the strong chemical affinity of titanium with gases such as
oxygen, hydrogen, and nitrogen;
The high reducing ability of titanium;
The unusually high solubility of some gases (oxygen, for
example) in titanium; and
The relative low density of titanium.
79

80
By combining the melting and casting techniques, four
different types of machines have been developed.
• Two chambers
• Upper ( Cu Crucible) and
lower
• Arc gas introduced in
both chambers
• Temperature 6000 deg C
• Tilting or split
crucible
• One chamber which
houses parts for
melting and casting
• Before melting, the
chamber is evacuated
and then filled with
argon gas.
• Motor-driven casting
turntable that spins at
approximately 3000
rpm
• A crucible with no
electrical conductivity
should be used.
• The casting force is
created from the
differences in the gas
pressure

REFERENCES
• Phillips science of dental materials 12th ed
• Craig’s dental materials 11th ed
• William J O’brien dental materials and their selection
• Science of dental materials with clinical application Shama
Bhat 3rd ed
• Rudd and Morrow dental laboratory procedure vol 2 & vol 3
82

CROSS REFERENCES
• Evaluation of the influence of the various sprue designs on the surface
porosity and dimensional accuracy of base metal alloy casting
NUJHS VOL 6 NO 4 DECEMBER 2016
• Dimensional accuracy of castings produced with ring-less and metal ring
investment systems
THE JOURNAL OF PROSTHETIC DENTISTRY JULY 2000
• The Effect of Casting Ring Liner Length and Prewetting on the Marginal
Adaptation and Dimensional Accuracy of Full Crown Castings
JOURNAL OF INTERNATIONAL SOCIETY OF PREVENTIVE AND
COMMUNITY DENTISTRY 2017
• The Use of Titanium in Dentistry Toru Okabe Baylor College of Dentistry,
Dallas Hakon Hero Scandinavian Institute of Dental Materials, Haslum
83

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