ASSALAMU ALAIKUM WA RAHMATULLAHI WA BARAKATAHU
DR. TAJAMMUL AHMED
a.tajammul@gmail.com
Introduction
Casting is the process by which a wax pattern of a
restoration is converted to a replicate in a dental alloy.
The various steps in casting
procedure are as follows : Preparation of die
Wax pattern
Investing of the pattern
Burnout of the wax
Casting
Cleaning of the casting
Casting defects
Sprue should be attached to the point of
greatest bulk
The length of the sprue former depends on
the length of the casting ring.
Sprue attached at 45 angulations near the
bulk of the pattern.
• This prevents air entrapment during investing and
suck back porosity after casting
• attachment should be flared to prevent turbulence
during metal flow
A reservoir should be added to sprue network
to prevent localized shrinkage porosity
Preparing the wax pattern
Prior to casting, margin of the wax pattern should be
readapted, pattern checked for smoothness, finish
and contour
wax pattern can be removed from the die using sprue
surfactant should be applied on the wax to obtain
wetting of the investment invested immediately to
prevent distortion
Crucible former serves as base for casting ring
Casting ring contains the investment &
restricts setting expansion
Casting ring liner provide investment expansion
Two types of non asbestos ring liner
• - An aluminosilicate ceramic liner
• - A cellulose (paper) liner
Ringless investment technique – phosphate
bonded investments
Plastic or paper casting ring
Useful for higher melting alloys – have a larger
cooling
INVESTMENTS
Investment (K.J. Anusavice) refractory material used to form a
mould casting for cast metals or hot pressed ceramics.
Classification according to Binders used:
Gypsum Bonded
Investment:
alloys that fuse below
1975F (1080c)
Phosphate Bonded
Investments:
- for higher fusing alloys
Ethyl Silicate- Bonded
Investments:
for casting of RPD with
base metal alloys
(cobalt based or nickel
based alloys)
Designed primarily for alloys used to for metal
ceramic prosthesis and for some base metal
alloys. It can also be used for press able ceramics.
IDEAL REQUIREMENTS
An ideal investment should incorporate the following features:
Controllable expansion to compensate precisely for shrinkage of the cast
alloy during cooling
The ability to produce smooth castings with accurate surface reproduction
without nodules
Chemical stability at high casting temperatures
Adequate strength to resist casting forces
Sufficient porosity to allow for gas escape
Easy recovery of the casting
Armamentarium
Vacuum mixer and bowl
Vibrator
Investment powder (gypsum or phosphate bonded)
Water or colloidal silica
Spatula
Brush
Surfactant
Casting ring and liner
Step by step procedure
Brush technique:
In this technique, pattern is first painted
with surface tension reducer; the surface
must be wet completely.
Coat the entire pattern with investment,
pushing the material ahead of the brush
from a single point. Gently vibrate
through out.
the ring is immediately filled by vibrating
the remaining investment out of the bowl.
After the wax pattern has been invested, it should be set
aside until the investment has hardened, usually 45 to 60
minutes.
The removal of wax from the mould usually by heat
melted at 200’c for 30 min
It does not melt but sublimes directly to gases and
escapes through the pores of investment
To burnout remaining traces of wax final burnout 650’c for
45 min
Gypsum beyond 650’c disintegrate investment and form
sulfur dioxide and dis colour the casting
During wax elimination, the investment expands
thermally, which is necessary to compensate for the
casting shrinkage.
Setting up the casting machine
Pre heating the crucible
Weighing the alloy
Casting the alloy
Selecting a casting alloy
Intended use
Physical properties
Type I : simple inlays
Type I : soft
Type II : Complex inlays
Type II : medium
Type III : Crowns and bridges
Type III : hard
Type IV : Partial removable prosthesis
Porcelain : metal ceramic alloys
Type IV : extra hard
Casting crucibles
Generally 4 types of casting crucibles are available:- Clay ,
- Carbon ,
- Quartz, and
- Zirconia - Alumina.
Clay crucibles are appropriate for many of the crown and bridge
alloys, such as the high noble and noble types.
Alloys are melted in one of the four types of casting
machines.
The Alloy is melted in a separate crucible by a torch
flame and is cast into mold by centrifugal force .
The Alloy is melted electrically by a resistance
heating or induction furnace, then cast into the mold
centrifugally by motor or spring action.
The Alloy is melted by induction heating, then cast
into the mold centrifugally by motor or spring action.
The Alloy is vacuum arc melted and cast by pressure
in an argon atmosphere.
In addition to these melting machine the molten
metal may be caste by air pressure , by vacuum , or
both.
Three clock wise turns and locked in position using a pin
Oxy-acetylene gas is used ordinary alloys
Oxygen gas is used for metal ceramic alloys
place alloy on the inner side wall of the crucible,
Heated using open flame from torch
Reducing part of flame is used to prevent oxidation
Flux should be added
If the alloy is ready to cast locking pin is released and casting is completed
Removal of residual investment
Quenching with water gypsum
bonded disintegrates
Using tooth brushes or ultrasonic
cleaners
Phosphate bonded should be
removed using sand blasting
(aluminium oxide)
Often the surface of the casting appears dark with oxides and tarnish.
Such a surface film can be removed by a process known as pickling,
which consists of heating the discolored casting in an acid.
One of the best pickling solutions for gypsum bonded investments is
50% hydrochloric acid solution.
Defects in casting can be classified under four
headings.
(1) Distortion
(2) Surface roughness and irregularities
(3) Porosity
(4) Incomplete or missing detail
Any marked distortion of the casting is probably related to the
distortion of the wax pattern.
Distortion can be minimized or prevented by proper
manipulation of the wax and handling of the pattern
Unquestionably, some distortion of the wax pattern occurs as
the investment hardens around it.
The setting and hygroscopic expansions of the investment may
produce a nonuniform expansion of the walls of the pattern.
Surface roughness should not be confused with surface irregularities.
Surface roughness is 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 .
Porosities in noble metal alloy castings may be classified as follows:
I. Solidification
defects
II. Trapped gases
A. Localized
shrinkage
porosity
Pinhole porosity
B. Microporosity
Gas inclusions
Subsurface
porosity.
III. Residual air.
Due to prevention of molten alloy from
filling the mold space completely
Cause :improper vending of air due to
reduced casting pressure ,
Prevention:adequate casting pressure
,adequate alloy
Finishing the cast restoration
Zone 1 : Internal margin
Zone 2 : Internal surface
Zone 3 : Sprue
Zone 5 : Occlusal surface
Zone 4 : Proximal contacts
Zone 6 : Axial surface
References
Schillinburg Herbert , Fundamentals of fixed
prosthodontics, 3rd edition, Indian edition ,
Quintessence publications, New Delhi , 73- 85
2. Rosensteil Stephen, Land Martin, Fujimoto Junhei,
Contemproray fixed prosthodontics, 3rd edition,
Mosby, New Delhi, 681 - 708, 870 -883
3. Rhoads JE, Rudd K D, Morrow R M, Dental
laboratory procedures – Fixed partial dentures ,
Volume 2, 2 nd edition, Mosby, 213 – 237, 465 - 470