3. Learning outcomes
1. Define an investment.
2. Describe the three different types
of investment materials.
3. Describe the casting process.
4. Explain about the casting
defects.
6. An investment can
be described as a
ceramic material which is
suitable for forming a mold
into which molten metal or
alloy is cast.
The procedure for forming
the mold is described as
“investing”.
DENTAL CASTING
INVESTMENT MATERIALS
8. PROPERTIES REQUIRED FOR
AN IDEAL INVESTMENT:
1. Easily manipulated
2. Sufficient strength at room
temperature
3. Stability at higher
temperature
4. Sufficient Expansion
10. COMPOSITION
In general, an investment is a mixture of
three types of materials:
refractory material (quartz, tridymite, or
cristobalite)
binder material (calcium sulfate
hemihydrate, phosphate, ethyl silicate)
other chemicals (sodium chloride, boric
acid, potassium sulfate, graphite, copper
powder, or magnesium oxide)
13. The Calcium sulfate-bonded
investment contains 65% to 75%
quartz or cristobalite
25% to 35% of calcium sulfate
hemihydrate;
About 2% to 3% chemical modifiers
COMPOSITION
14. The calcium sulfate–bonded
investment is usually limited to gold
castings and is not heated above
700° C.
The calcium sulfate portion of the
investment decomposes into sulfur
dioxide and sulfur trioxide at
temperatures over 700° C,
contaminating the casting.
15. Properties of Calcium
Sulfate–Bonded Investments
ANSI/ADA Specification No. 126 for
calcium sulfate-bonded casting
investments applies to two different
types of investments:
Type 1: For casting inlays and
crowns
Type 2: For casting complete and
partial denture bases
17. Setting Time
According to ADA/ANSI
specification no. 2 for dental
inlay casting investment, the
setting time should not be < 5
min. or >25 min.
Usually, the investments set
initially in 9 to 18 min.
18. Normal Setting Expansion
The ANSI/ADA Specification no.
126 for Type I investment permits
a maximum setting expansion in
air of only 0.6%.
The setting expansion of modern
investments is 0.4%.
19. Hygroscopic Setting Expansion
Occurs when the
gypsum product is
allowed to set when
placed in contact
with water.
Greater magnitude
than the normal
setting expansion (6
or more times).
Normal setting expansion
Hygroscopic setting expansion
20. The hygroscopic setting
expansion is one of the methods
to expand the casting mold to
compensate for the casting
shrinkage of the gold alloy.
21. Thermal Expansion
It is desirable that the
maximal thermal expansion
be attained at a temperature
not greater than 700°C.
Gold alloys can become
contaminated above this
temperature.
22. Strength
The investment must be
strong enough to prevent
fracture or chipping of the
mold during heating and
casting.
But the compressive strength
should not be very high.
23. Fineness
Fineness of the investment
affects the setting time and
surface roughness of the
casting.
Fine silica particles gives
smoothness to the casting.
24. Porosity
When the molten metal
enters the mold, air must
be forced out ahead of it
or else back pressure
builds up and prevents
the alloy from completely
filling the mold, causing
back pressure porosity.
Pores in the investment
offer adequate venting.
26. One component contains a water-soluble
phosphate ion - monoammonium
phosphate.
The second component reacts with
phosphate ions at room temperature –
magnesia (Mgo).
The third component is a refractory, such
as silica.
COMPOSITION
27. Properties of Phosphate
Bonded Investments
ANSI/ADA specification No. 126 for
phosphate-bonded casting
investments specifies two types of
investments:
Type 1: For inlays, crowns, and
other fixed restorations
Type 2: For removable dental
prostheses
28.
29. Compressive
Strength
The compressive strength at
room temperature shall not be
less than 2.5 MPa for type 1
investments and 3.0 MPa for
type 2 investments.
31. ANSI/ADA SPECIFICATION NO.
IS 126
It is losing popularity
because of the
complicated and time
consuming procedures
involved with its use.
But it is still used in the
construction of high-
fusing base metal partial
denture alloys.
32. Binder → silica acid gel
Refractory fillers → silica
(cristobalite)
Additive → magnesium oxide –
Strengthens the gel
COMPOSITION
33. This type of investment can be
heated between 1090°C to 1180°C
and is compatible with the higher
fusing alloys.
Its low setting expansion minimizes
distortion.
35. INVESTMENT FOR ALL-
CERAMIC
RESTORATIONS
Two types of investment
materials have been
developed recently for
producing all-ceramic
restorations.
Both are phosphate
bonded refractory
materials.
60. Casting is the process by which a
wax pattern of a restoration is
converted to a replicate in a
dental alloy.
61. The casting process is used
to make dental restorations
such as inlays, onlays,
crowns, bridges, and
removable partial dentures.
62. Metal Casting: Lost Wax
Technique (Taggart, 1907)
A crucible former
B sprue
C wax pattern
D investment
material
E ring liner
G thickness of
investment at the top
63. STEPS IN CASTING
1. FORMATION OF THE WAX
PATTERN
2. SPRUNG THE WAX PATTERN
3. INVESTING THE WAX
PATTERN
4. BURNOUT OF THE WAX
PATTERN
64. 1. FORMATION OF THE WAX
PATTERN
Direct Wax Patterns – directly in
the mouth
Indirect Wax Patterns – made on
the die
65. 2. SPRUNG THE WAX
PATTERN
Sprue/ Ingate: The mold
channel through which
molten metal flows into the
mold cavity.
66.
67. For most crowns and inlays, a
single sprue is sufficient.
However, fixed partial dentures
may require multiple sprues.
68. 3. INVESTING THE WAX
PATTERN
Investing is the process by which
the sprued wax pattern is
embedded in a material called an
investment.
69. Investing the wax pattern
The casting ringThe casting ring withwith the wax pattern
72. 4. BURNOUT OF THE WAX
PATTERN
During burnout, the mold is placed
in an oven to completely eliminate
the wax, thereby forming a cavity
into which the molten metal is cast.
73. 5. CASTING
To cast a metal restoration, a
suitable torch or other heating
equipment to melt the alloy is
necessary.
74. The most common method of
heating dental alloys for metal
restorations is by using a gas-air
torch.
76. Casting Defects
Error in the casting procedure
results in defective casting,
these defects are known as
casting defects.
77.
78. DISTORTION
Wax distortion is the most
serious problem that can
occur during the forming
and removal of the pattern
from the mouth or die.
79. SURFACE ROUGHNESS
AND IRREGULARITIES
Surface roughness
Defined as 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.
81. POROSITY
Porosity may be seen as surface
pitting on the casting or may be
revealed within the cast metal on
finishing and polishing.
It can be external
or internal porosity.
82. INCOMPLETE CASTING
Sometimes partially complete
casting or perhaps no casting at
all is found. The obvious cause is
that the molten alloy has been
prevented in some manner from
completely filling the mold.
Ceramic materials are inorganic, non-metallic materials made from compounds of a metal and a non metal
1. not only should it be possible to mix and manipulate the mass readily and to paint the wax pattern easily, but the investment should also harden within a short time.
2.To permit ease in handling and provide enough strength at higher temperature to withstand the impact force of the molten metal.
3. Investment must not decompose to give off gases that could damage the surface of the alloy.
4. Expansion should be enough to compensate for shrinkage of the wax pattern and metal that takes place during the casting procedure.
5. Porous enough to permit the air and other gases in the mold cavity to escape easily during the casting procedure.
6. Fine details and margins on the casting should be preserved.
7. The investment should break away readily from the surface of the metal and should not have reacted chemically with it.
