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)

--

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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 .

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