DR. MAHMOUD N. ALMUGHANY
Wax pattern construction .
Sprue ( Def, Role, Requirements and technique )
Casting ring and liner ( types, mold expansion )
Investment material (requirement, composition and
Investing ( mixing and pouring )
Casting ( Requirement, alloy melting, casting )
Divesting and finishing
The process consists of surrounding the
wax pattern with a mold made of heatresistant investment material. eliminating
the wax by heating and then introducing
molten metal into the mold through a
channel called the sprue.
It’s a channel through
which molten alloy can
reach the mold in an
invested ring after the wax
has been eliminated .
Role of a sprue :
Holds the wax pattern to avoid its distortion.
Creates a channel to allow the molten wax to
escape from the mold.
Enables the molten alloy to flow into the mold
which was previously occupied by the wax
Sprue can made from : Metal , plastic and wax
Requirement of sprue :
In general, a relatively large diameter sprue is
recommended because this improves the flow of
molten metal into the mold.
Molar around 2.5 mm ( 10 gauge), premolar 2 mm (12
Is a small amount of
added to sprue
former 1 mm below
wax pattern .
to compensate for the
of the casting.
The sprue should be attached to the
bulkiest noncritical part of the
pattern. away from margins and
occlusal contacts .
Normally, the largest nonfunctional
cusp is used.
Fuctional cusps :
Buccal cusps of the mandibular posteroir
teeth & Lingual cusps of the Maxillary
posterior teeth .
Should be attached at angle to allow the incoming
molten metal to flow freely to all portions of the
mold, (About 45 degree ).
The length of sprue former should keeps the wax
pattern 6 mm from the casting ring length to
Provides adequate bulk of investment to withstand
Allows gases to escape from end of mold.
The pattern should be placed as possible to the
center of ring
Small auxiliary sprues or vents have
been recommended to improve
casting of thin patterns and may
helps in :
gases escape during casting.
compensate for the shrinkage during
usually made of rubber, which serves as a
base for the casting ring during investing
and the sprue is attached to it .
The pattern is painted with surface tension reducer
and then carefully coated with vacuum-mixed
The flow of molten metal is
straight ( direct from casting
crucible to the pattern.
Using aconnector or (runner
bar) which the wax pattern
sprue is attached .
casting ring serves as a
container for the investment
while it sets and restricts the
setting expansion of the mold.
Types of ring according to
•Rigid : Metal , plastic
Ring Liner :
Creates a space to allow for investment
Allow setting of investment material under water,
to give hygroscopic expansion
Regulate heat transfer through the investment
Facilitate removal of investment from the casting
ring after casting.
Thickness of liner should be around 1 mm not less.
One or two layers.
3.0-mm short of both ends of ring will allow
supporting contact of investment with ring after
liner has burned out.
Material used as liner :
Cellulose liner ( paper )
Ceramic liner ( aluminum silicate )
Combination of ceramic and cellulose liner.
Four mechanisms to produce mold expansion:
Setting Expansion of Investment
Wax Pattern Expansion
Occurs as a result of normal gypsum crystal growth in
About 0.4% but partly restricted by metal investment
Max expansion: immerse investment-filled ring in
water bath at 38°C.
Water in bath replaces water used by hydration
process space between growing crystals is
maintained crystals grow longer outward
expansion of mold.
About 1.2 to 2.2% max with expandable ring.
Wax Pattern Expansion
While investment is still fluid, expansion occurs when
wax is warmed above Temp at which it was
when investment is heated in burnout oven.
Two purposes :
Precise reproduction of wax pattern.
Sufficient strength to withstand burnout
Sufficiently porous to allow escape of
Expand enough to compensate
solidification shrinkage of alloy.
Composition of Investment
Refractory material or filler
Material that resist high temperature
Expand upon heating
Its problem have no cohesion to maintain the
shape of investment when dried
Mixed with filler to give some strength
Gypsum Bonded Investments
use with alloys that melt below 1,000°C.
Used with Type I, II, III gold alloys
Phosphate Bonded Investments
Much stronger and can withstand much higher burnout temps
For alloys with casting temp > 1150°C
Silicate Bonded Investments
base metal alloys such as chrom-cobalt and steel
The reason is that these alloys melt at temperatures ranging
from 1250 - 1400
A silica-bonded investment material can be heated up to
Reduce the amount of porosity in the investment .
Texture of the surface casting is smoother with better
detail reproduction .
Compressive strength of the investment is increased .
1- casting ring is filled up to it’s rim .
2- allow investment to set for about 1 hour .
3- crucible former is carefully removed .
Wax elimination or burnout consists of heating the
investment in a thermostatically controlled
furnace until all traces of the wax are vaporized.
Once the investment is heated during the waxelimination procedure, heating must be
continued, and casting must be completed.
Bring the furnace to 200°C (400°F), and hold this
temperature for 30 minutes. Most of the wax is by
It’s advisable to begin the burn out procedure
while the mould is still wet because water
trapped in the pores of the investment reduces
the absorption of wax .
Melting and casting
heat source : to melt the alloy
Casting force : to drive the alloy into the mould
Molten the alloy
Using electric furnace
Types of torch flame :
Gas air torch
Gas oxygen torch
Oxy- acetylene torch
Hydrogen oxygen generator
Types of torch tips :
Flame zones :
(OXY- ACETYLENE flame)
1- Neutral Flame (Acetylene oxygen in equal
2. Oxidizing Flame (Excess of oxygen)
3. Reducing Flame (Excess of acetylene)
Air pressure casting machine :
Alloy is melted in situ in crucible hollow of the ring ,
followed by applied air pressure on the melt
Centrifugal casting machine:
Alloy is melted in crucible , then forced in the mold
by centrifugal force .
After the red glow has disappeared from
the button, the casting ring is plunged
under running cold water into a large
rubber mixing bowl
Surface of the casting appears dark with oxides and
tarnish. This surface film removed by these process
( pickling ) .
By place the casting in a dish and pour acid over it .
Heat the acid and don’t boil it .
Type of acids :