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Spruing ,
Investing and
casting
DR. MAHMOUD N. ALMUGHANY
Curriculum : Wax pattern construction .
Sprue ( Def, Role, Requirements and technique )
Casting ring and liner ( types, mold expansion )
Investment material (requirement, composition and types )
Investing ( mixing and pouring )
Burn out
Casting ( Requirement, alloy melting, casting )
Quenching
Pickling
Divesting and finishing
Polishing
The process consists of surrounding the
wax pattern with a mold made of heat-
resistant investment material. eliminating
the wax by heating and then introducing
molten metal into the mold through a
channel called the sprue.
Wax Pattern Construction
Spruing
Sprue
Definition:
Its 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
pattern.
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
gauge)
Diameter
Reservoir :
Is a small amount of
additional wax
added to sprue
former 1 mm below
wax pattern .
Function :
to compensate for the
shrinkage occurs
during solidification
of the casting.
Reservoir
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 .
Location
Should be attached at angle to allow the incoming
molten metal to flow freely to all portions of the
mold, (About 45 degree ).
Attachment
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
force.
Allows gases to escape from end of mold.
The pattern should be placed as possible to the
center of ring
Length
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
solidification
Venting
usually made of rubber, which serves as a
base for the casting ring during investing
and the sprue is attached to it .
Crucible Former
The pattern is painted with surface tension reducer
and then carefully coated with vacuum-mixed
investment
Spruing technique
Direct In Direct
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 .
Number of Sprues
Single Double Multiple
Casting
Ring and
Liner
Casting Ring
The 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
shape Complete
ring
Split ring
Round
Oval
Rigid : Metal , plastic
Flexible: Rubber
Metal
plastic
Ring Liner :
Creates a space to allow for investment expansion
Allow setting of investment material under water, to give hygroscopic expansion
Regulate heat transfer through the investment
material.
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 :
Asbestos liner
Cellulose liner ( paper )
Ceramic liner ( aluminum silicate )
Combination of ceramic and cellulose liner.
Four mechanisms to produce mold expansion:
Setting Expansion of Investment
Hygroscopic Expansion
Wax Pattern Expansion
Thermal Expansion
Setting Expansion
Occurs as a result of normal gypsum crystal growth in
air.
About 0.4% but partly restricted by metal investment
ring.
Hygroscopic Expansion
Max expansion: immerse investment-filled ring in
water bath at 38C.
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
formed.
Thermal Expansion
when investment is heated in burnout oven.
Two purposes :
Investment
material
Requirements:
Precise reproduction of wax pattern.
Sufficient strength to withstand burnout
and casting.
Sufficiently porous to allow escape of
gases.
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
Binder
Mixed with filler to give some strength
Chemical modifier
Types:
Gypsum bonded
Phosphate bonded
Silica bonded
Gypsum Bonded Investments
use with alloys that melt below 1,000C.
Used with Type I, II, III gold alloys
Composition:
Phosphate Bonded Investments
Much stronger and can withstand much higher burnout temps
(800C +)
For alloys with casting temp > 1150C
Silicate Bonded Investments
base metal alloys such as chrom-cobalt and steel
A silica-bonded investment material can be heated up to
1200C
The reason is that these alloys melt at temperatures ranging from 1250 - 1400
Investing
Hand mixing Vacuum mixing
Technique
Vacuum mixing
Advantages :
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 .
Investment material
pouring
1- casting ring is filled up to its rim .
2- allow investment to set for about 1 hour .
3- crucible former is carefully removed .
Burn out
Wax Elimination
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 wax-
elimination procedure, heating must be
continued, and casting must be completed.
Bring the furnace to 200C (400F), and hold this
temperature for 30 minutes. Most of the wax is by then eliminated.
Its 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 .
Burnout ovens
manual semiautomatic fully programmable
controls
Casting
Melting and casting
technique
Requires :
heat source : to melt the alloy
Casting force : to drive the alloy into the mould
Molten the alloy
Using Torch Using electric furnace
Types of torch flame :
Gas air torch
Gas oxygen torch
Oxy- acetylene torch
Hydrogen oxygen generator
Types of torch tips :
Multi orifice
Single orifice
Flame zones :
(OXY- ACETYLENE flame)
1- Neutral Flame (Acetylene oxygen in equal
proportions)
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 .
Quenching
After the red glow has disappeared from
the button, the casting ring is plunged
under running cold water into a large
rubber mixing bowl
Pickling :
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 dont boil it .
Type of acids :
Hydrochloric acid
Sulfuric acid
Ultrasonic devices
Divesting and Finishing
Polishing
Thank you