CASTING
PROCESS
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Contents
Introduction
Preparation of a die
Wax Pattern fabrication
Sprue Former
Reservoir
Casting Ring liner
Investing
Burnout
Lost waxTechnique
Crucibles
Casting
Casting machines
Cleaning of casting
Take home message
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The casting method consists of forming a
wax pattern , surrounding it with
investment material , and later heating the
investment mold to remove the wax before
casting the molten metal into the mold .
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Introduction �Casting is the process by which a wax
pattern of a restoration is converted to a
replicate in a dental alloy.
The casting process is used to makedental restorations such as:
Inlays and onlays
crowns, bridges
removable partial dentures.
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Introduction In dentistry the resulting casting must be an accurate reproductionof the wax pattern in both surfacedetails and overall dimension.
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�Reproducing the wax up in metalwith predictable results has alwaysbeen a challenge.
�Small variation in investing or castingcan significantly effect the quality ofthe final restoration.
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Definition
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�“ Casting is the process by which a
wax pattern of a prepared tooth is
fabricated and converted to its
metallic replica”(Rosenteil)
STEPS IN MAKING ARESTORATION
CAST
1. TOOTH PREPARATIONPREPARATION2 . IMPRESSION
/CAVITY
3.4.5.6.
DIE PREPARATIONWAX PATTERN FABRICATIONSPRUINGINVESTING
7.BURNOUT
8.CASTING
9.CLEANING & POLISHING18
Die
Defined as the positive reproduction of the form of the prepared tooth in any suitable substance in which inlays, crowns & other restorations are made.
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IDEAL REQUIREMENTS
Accurate reproduction of the fine detailsDimensional accuracy
Good strength & hardness
Ease of use
Abrasion resistanceRelatively inexpensive
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PREPRATION OF MASTER DIE :
�Commonly used die materials-
Type IV gypsum product - 0.1%(setting expansion)
Type V gypsum productexpansion)
- 0.3% (setting
�Disadvantage- Susceptibility to abrasion
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Means to increase abrasionresistance of die :
-Silver plating
-Coatinghardener
- Adding gypsum
surface with die
die hardener to
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Die spacer
Used to provide relief space for cement.
Applied within 0.5mm of the preparation finish line to provide relief
for the cement luting agent.
Example- resins (most commonly used)
--
-
model paint
colored nail polish
thermoplastic polymers dissolved in volatile solvents.
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Diestone/investment combination
�In this, die material and investment medium have
a comparable composition
Divestment – a commerciallyavailable gypsum bonded material .
�Divestment is mixed with colloidal silica and die is
made from this mix & wax pattern is constructed.
�Advantage- It eliminate possibility ofof wax pattern while removing it from
distortiondie .
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OTHER DIE MATERIALS
Amalgam
Acrylic resins (shrinkage 0.6%)
Polyester resin
Epoxy resins (less shrinkage then
acrylic resins0.1to0.2%)
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Wax pattern fabrication
- It is contouring of wax
pattern into desired shape and
form.
- Shortest time
should elapse
between the time
the pattern is
removed from the
die & the time it is
invested.
Wax Technique
A) DIRECT B) INDIRECT
Wax pattern is made
directly inside the
mouth, as in inlaycase.
Wax pattern is
the die.
made on
Type-1 medium wax. Type -2 soft wax.
Exceptionallydemanding procedure.
More commonly used
technique.
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Sprue & Sprue former
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Sprue
� Part of casting that acts as achannel for the molten metalto flow into the mold cavity after the wax has beeneliminated.
.
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PURPOSE:To form a mount for the wax pattern &fix the pattern in space so a mold can be made.
To create a channel for elimination of wax during burn out.
To form channel for ingress of molten alloy
To compensate for alloy shrinkage
during solidification.
.
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Wax pattern attached to the crucible formerwith a sprue ready for investing. A ring liner isin place.
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Spruing of wax pattern :
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FUNCTION:
�Facilitate flow of molten metal from crucibleto mold.
�Store additional metal & prevent shrinkageporosity.
�May be used as handle to remove waxpattern.
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Types of Sprue
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wax, plastic and metal ------ main sprue
wax or plastic ------accessory sprue only.
Wax sprue :melt at the same rate as the pattern and so
allow an easy escape of the molten wax.
plastic sprue :higher temperature than the wax pattern
which may cause the interruption of the escape of wax
resulting in roughness of the casting. It is mainly used
for casting fixed partial dentures due to its rigidity.
Metal sprue : are often a hollow sprue which provides a
stronger attachment to the pattern and less heat
transfer to wax pattern, so causes less distortion of the
wax pattern.
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Sprue
�DIAMETER�LOCATION�LENGTH�ATTACHMENT�DIRECTION
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Diameter of the sprue former the diameter of the sprue should be larger than the thickest
part of the wax pattern
rate of flow of the molten alloy into the mold cavity is
controlled by:
1- the diameter of the sprue
2- the pressure of the casting machine
3- the density of the molten alloy
This diameter will also ensure a reservoir during solidification
[2.5 mm] sprue --------molar and metal ceramic,
[2.0 mm] sprue -------- premolars and partial coverage
restoration.
Location of the sprue former
The sprue former should be attached to the
bulkiest portion of the wax pattern to minimize
the distortion of the wax pattern because the
sprue is away from the fine margins.
The sprue former should be attached at an
angle to allow the molten alloy to flow freely to
all portions of the mold (fine margins) without
flowing in an opposite direction of the casting
force.
If the sprue is directed at right angle to the mold , a hot
spot may be created at this point . This will lead to the fact
that the alloy adjacent to it molten after the rest of the
casting has been solidified, causing "suck-back porosity ” .
Multiple sprue formers When 2 units or more are being cast together, each is joined to a
runner bar. A single sprue is used to feed the runner bar.
In case of 2 units, they may be cast with a runner bar or each fed
from a separate sprue.
Venting It is recommended to improve casting of thin patterns .
A vent is needed to help gases escape during casting or
to ensure that alloy solidification started by acting as a
heat sink .
This vent is made of small auxiliary sprue formers .
Sprue former
The sprue is attached to a crucible former, usually made of rubber,which constitutes the base of the casting ring during investing.
May be metal , plastic or rubber
The exact shape of the crucible formerdepends on the type of casting machineused.
With most modern machines, the crucible former is tall to allow use of a short sprue and allow the pattern to be positioned near the end of the casting ring.also referred to as a sprue former
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Crucible formers/ Sprue formers are�
basically of 2 types---
a) Steep-sided cone: used with metalwhen casted using centrifugal casting force.
b) Shallow cone: used to cast metalusing stream/air pressure
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RESERVOIR
�Piece of wax attached to the sprue about 1mm awayfrom the pattern ,as a enlarged round mass or aconnector bar between the wax pattern sprue former.
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ImporImporttaancenceImportance :
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SURFACE TREATMENT OFCOMPLETED WAX PATTERN
Before the wax pattern is invested, itshould be cleaned of any debris, grease,oils and separating medium.
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Casting ring & liner
�The casting ring serves as a container
for the investment while it sets & restricts
the mold.setting expansion of
Metal casting
ring
Sprue base or
Crucible former
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Ring liner
Casting Ring
They are available as-
1) Shapes - Round- Oval
2) Complete ringsI) - Rigid
- Metal (stainless steel)
- Plastic
II) Flexible - RubberSplit ringsI) metalII) plastic
3)
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Casting Ring Considerations :
1) The internal diameter of casting ringshould be 5-10mm greater than the widestmeasurement of the patternmm higher.
and about 6
2) For single crown/inlay - small rings asused. Diameter - 32 mm
3) For large fixed partial denture –63mm round/oval shaped casting ring areused
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CASTING RINGLINERS
NON ASBESTOSLINERS
CELLULOSE PAPERAL-SILICATE CERAMIC
ASBESTOSLINERS
Asbestos:- carcinogenic potential makes it a biohazard.
Functions of a linerAffords greater normal expansion in the
investmentThe absorbed water causes a semi hygroscopic
expansionThickness – not less than 1mm
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Liner technique
A)DRYLINERTECHNIQUE - tacked inposition with sticky wax.
B)WETLINERTECHNIQUE - lining ring iswater is shakenimmersed in water & excess
off.
�The liner is cut to fit the inside diameter of
the casting ring with no overlap and 3mm short of the top and bottom of the ring , this serves to lock the investment within the ring& equalize expansion.
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Casting ringliner
Wetdry
Uniformly wet
Minimal th ickness not< "J rnrnAvoid squezing
Helps causesemihygroscopic
expansion
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RINGLESS CASTING TECHNIQUE
�With the use of higher-strength, phosphate-Bonded investments, the ringless technique hasbecome quite popular.
�The method uses a paper or plastic casting ring
unrestricted expansion.and is designed to allow
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The crucible former and plastic ring are removed
before wax elimination, leaving the invested waxpattern.
The systems are designed to achieve expansion that
is unrestricted by a metal ring.
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The position of the pattern in the casting ring
affects expansion:
single crown : should be positioned within the ring
equidistant from its walls.
fixed prostheses : are being cast as one piece,
greater accuracy will be achieved if the pattern is
placed near the center of a large or special oval
ring rather than near the edge of a smaller ring .
103
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