Casting Procedures

• Introduction

• Steps in casting

• Spruing

• Investing patterns

• Burn out procedure

• Casting machines

• Casting techniques

• Casting procedure

• Principles of casting

• Casting defects

• Casting of ceramics

• Casting of titanium

• Casting procedure in RPD

• conclusion

Introduction

Structural faults in dental castings can result from any combination of problems related to spruing, investing, wax elimination, alloy melting, casting & solidification of the casting.

The ability to produce smooth and well fitting castings require strict adherence to certain fundamentals.....

Steps in casting procedure

• Die preparation• Wax pattern• Attachment of sprue• Investing• Burnout, • melting the alloy • Casting, • recovery,• finishing & polishing

SPRUING

“Spruing is an art which is not well

understood…..”

Off all the procedures involving casting work, the spruing technique has a paramount importance in producing a complete & dense casting.

Definitions (GPT-8)

Sprue : the channel or hole through which plastic or metal is poured or cast into a gate or reservoir and then into a mould

Sprue former : a wax, plastic, or a metal pattern used to form the channel or channels allowing molten metal to flow into a mold to make a casting

Sprue button : the material remaining in the reservoir

of the mold after a dental casting

Basic Requirements• To form a mount for the wax pattern

• To create a channel for elimination of wax during burn out

• To form a channel for entry of molten alloy during casting

• Provides a reservoir of molten metal which compensates for alloy shrinkage during solidification

Taggart (1907), introduced the lost wax techniqueAsgar & Peyton (1959), stated that flaring should occur

at the sprue/wax pattern junctionStrickland et al (1959), stated the importance of the

type, shape location & direction other than the size of the sprue

Bruce (1964), tested the different methods of spruing patterns

Rousseau (1964), described a casting method with a constricted sprue

Sprue Formers

• Wax

• Plastic/Resin

• Metal sprues

Factors to be considered

• Size• Length• Shape• Number• Attachment• Location• Angulation

Size • Usually large-diameter sprue is recommended

• The diameter of sprue should be equal to the thickest portion the wax pattern.

• There are various gauges of sprue former gauge 6, 8, 10, 12, 14, 16, 18 (0.4cm - 0.1cm)

• Usually for molar and metal ceramic restoration 10-gauge (2.5mm) Premolars and partial coverage restoration 12-gauge (2.0mm)

Length • The sprue should be long enough to place pattern 6

mm from the trailing end of casting ring usually it should be in the range of 2.5 mm.

• Very short sprue porosity in casting at the junction of sprue and pattern.

• Very long sprue sprue solidifies first leading to casting shrinkage and incomplete casting.

Shape

• The sprue former should be straight to reduce chances of creating turbulence in molten metal entering the mold.

• High turbulence of alloy cause porosity.

Number

• Usually a single sprue is used for small castings.

• When two thick sections of a pattern are connected by thin part of wax, two separate sprues should be attached to each thick portion

Attachment

• Attachment of sprue former and wax pattern: -

1. The connection is generally flared - high density gold

2. It is restricted for low density alloys

Sprue Former Position

• It depends on the shape of the wax pattern

Types Of Spruing

There are 2 types of spruing1. Direct spruing 2. Indirect spruing

• Fig:1

• Fig:2

Direct spruing

• The sprue former provides a direct connection between the pattern area and the sprue base/ crucible former.

• When two thick portions of wax are separated by thin wax, then 2 separate sprues should be attached using direct spruing.

Factors to be considered:

1. Location of attachment

2. Angulation of sprue

3. Attachment morphology

Location of attachment• Pin ledge restoration sprue is located towards the

incisal aspect of lingual surface and angled suitably.

• Full veneer crown sprue is attached to Maxillary buccal and mandibular lingual cusp.

• Partial veneer crown sprue is attached to cusp that encompasses the preparation.

Angle of attachment• The sprue should be attached to pattern such that it

makes 45 degrees to the walls of mold, which decreases the turbulence of molten alloy.

• If the sprue is placed perpendicular to the mold wall, it induces high turbulence in molten alloy, leading to creation of a hot spot on mold wall. This results in localized shrinkage porosity.

• It should be placed away from thin / delicate parts of pattern to prevent fracture of investment.

Attachment morphology

• The transition should be smooth

• Irregularities produces tags of investment which is vulnerable for fracture by molten alloy

• Usually it is flared for high density gold alloys but restricted for low density alloys. Flaring acts as reservoir and facilitates the entry of molted alloy into the mold area.

Indirect spruing

Indirect spruing uses the same basic principles of spruing. But the only difference lies in attachment of 3 running horizontal bars. The whole indirect sprue complex consist of 3 parts:

– Manifold sprue. – Horizontal running bar. – Feeder sprue.

• Once spruing is done the wax pattern and sprue former assembly is carefully removed from the die.

• During removal of pattern no pressure should be applied to prevent its distortion.

Then it is positioned into the crucible former to adjust the distance between the wax pattern and casting ring for INVESTING…..

INVESTING

GPT8 Terminology…..• Investing : The process of covering, enveloping,

wholly (or) in part an object such as denture tooth, wax form, crown, etc with a suitable material before processing, soldering, casting.

• Dental casting Investment: Material consisting

principally of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperature) (or) phosphates and silica (for use in higher casting temperatures).

• Refractory : Difficult to fuse/corrode, capable of enduring high temperatures

• Refractory investment : An investment that can withstand high temperature using a soldering/casting.

Essentials…..

• It should produce mould expansion to compensate for casting shrinkage.

• Should have sufficient strength at room & high temperature.

• Inner surface of the mold should not break at a high temperature.

• Should exhibit sufficient strength, to withstand the force of molten alloy entering the mold.

• Inner surface of mold should be smooth.

• Its history can be traced back around 3000 B.C. But origin of lost wax technique, when viewed makes its presence in the writings of theophilus (11th Century).

• 11th Century Theophilus Described lost wax technique, which was a common practice prevailed in 11th century.

• 1558 Benvenuto Cellini claimed to have attempted, use of wax and clay for preparation of castings.

• 1884 Aguilhon de saran used 24K gold to form Inlay

• 1897 Phillibrook Described a method of casting metal filling.

• 1907 Taggart Deviced a practically useful casting machine

Basic composition…..Refractory Materials : The most commonly used refractory material is silicon dioxide in form

quartz, crystoballite.

Binder : commonly used binders are - calcium sulfate hemihydrateOthers are – Sodium silicate, ethyl silicate, ammonium sulfate, sodium phosphate.

Other chemical modifiersReducing agents – Ex : CharcoalSodium chloride, boric acid – to shrinkage, during dehydration of calcium sulfate dihydrate.Potassium sulfate (accelerator)Copper powder(reducing agent)/magnesium oxide

Expansion…..

This property of investment is needed for compensation of casting shrinkage of alloy.

The expansion occurs because of:-

1. Normal setting expansion 2. Hygroscopic setting expansion 3. Thermal expansion.

Crucible former

• Helps to hold & place the sprue in the center of the casting ring

• Helps to contour the investment surface• They may be preformed or contoured with wax

Casting ring liner

• In a casting ring provision must be made for expansion of the investment

• We can over come this by using split ring or flexible rubber ring

• However, the commonest way of providing space for expansion is by using a liner

• A ring liner is placed on the inner surface of the casting ring

• Should be placed a few mm short of the ring• It can be either

1. asbestos liners (used earlier)2. non-asbestos liners

a) Cellulose (paper), b) Aluminium silicate ceramic

Casting ring

Choice of the ring:

Rubber ring for HSEMetal ring for TE

The diameter and the length of the ring should be selected rightly.

Ringless casting system

Versatile system for accurate & simple spruing, investing & casting with plastic or metal rings

Produces consistent, accurate & predictable castingsFacilitates controlled thermal expansion

Assembling the ring

This step should be coordinated with spruing & choosing the size of the ring

Wetting agent is applied ( to reduce air bubbles) on the wax pattern

Preparation of the investment mix

Mixing of the investment is done by:

Hand mixing (brush technique)

Vacuum mixing

Brush technique

Hand spatulate the mix to incorporate the powder quickly

Coat the wax pattern with the investmentCarefully coat the internal surface & the margin of the

patternFill the ring slowly, starting from the bottom

Vacuum technique

Parts of vacc-u-spat lid with bowl:Pedal, spindle drive nut vacuum outlethole with ringParts of vacc-u-vester:Drive chuckVibratory knobVacuum outlet & vacuum guage

Method

First hand spatulate the mixAttach the vacuum hose & mix accordingly to the

manufacturers recommendationsInvert the bowl & fill the ring under vibrationRemove the vacuum hose before setting of the mixerImmediately clean the bowl & mixing blade under

running water

Setting of investment

It can be:

In open air

hygroscopic technique

controlled water added technique

In open air

• Usually when high heat TE technique is used.

• The investment is allowed to set in open air for 1 hour.

• The setting time is 1 hour for both GBI & PBI.

Hygroscopic technique

Once the casting ring is poured it is immersed into a water bath at 38 degrees temperature immediately

This can be altered by:W:P ratio W:P HSETime of immersion the delay HSEtemp. of water HSE

Controlled water added technique

• The desired amount of expansion is retained by the amount of water added

• A soft flexible rubber ring is employed & invested normally

• A specific amount of water is added on the top of investment & allowed to set at room temperature

Time of placement of investment for burn out

after 20 mins and 24 hrs

after 2 hrs

After 1 hr

Castings were undersized

Castings were satisfactory

Were found to be the best

• In a study by Nakai A, Ogura H when Mg(OH)2 or Ca(OH)2 was added at more than 4.0 mass% to the investment, it was useful in preventing the blackening of the as-cast surfaces of an Ag-Pd-Cu-Au alloy. As for differences in the effects between Mg(OH)2 and Ca(OH)2, they were not found

• In a study by Bandyopadhyay-Ghosh et al both gypsum and phosphate bonded investments could be successfully used for the lost-wax casting of fluorcanasite glasses.

• In a study conducted by Lyon HW et al it was found that vaccum investing eliminated 95% of nodules as compared to 17% of nodule free casting produced from hand mixing.

• Guilin Y et al the type of investment affects the microstructure and microhardness of the alpha-case layer of titanium castings.

• Based on the thickness of the surface reaction layer and the surface microhardness of titanium castings, MgO-based investment materials may be the best choice for casting these materials.

Trouble shooting• Improper mixing• Errors in heating the investment• Errors due to altered W:P ratio

Casting (GPT8)

• something that has been cast in a mold;• an object formed by the solidification of a

fluid that has been poured or injected into a mold

1301 B.C

Gold casting

Three and half ton statue

Crucible

• Carbon crucibles • Clay crucibles • Quartz /zirconia - alumina

Evolution of casting machines

Wax is roughly molded in the cavity of pt mount

• W.H.TAGGART introduced casting machine in 1907.

• Gold melted with a blowpipe was then forced into the plaster mold by means of casting machine, which utilized compressed air

Inlay Mold

BlowpipeCompressed air cylinder

Centrifugal force type

CASTING MACHINES

• Casting machines provide the means for transferring the molten alloy from the crucible to the mold.

1.Pneumatic force (or) Air pressure type.2.Centrifugal force type

PNEUMATIC FORCES

1.Steam pressure2. THE PRESSURE / VACUUM3. THE VACUUM / PRESSURE

Steam pressure

     A very old method

THE PRESSURE / VACUUM CASTING MACHINE

• The molten alloy is pushed and sucked simultaneously into the mould.

THE VACUUM / PRESSURE TYPE OF CASTING

• These machines first evacuate the chamber to reduce oxidation and then apply air pressure to push the metal uniformly.

– Vacuum casting machines

» Melting chamber.» Flask chamber.

– A jet-casting machine, which depends on combustion gas to provide the casting force.

Melting chamber

Flask chamber

Titanium and titanium alloys

• When the melting process has been completed, the casting tilts down and the molten titanium flows into the refractory mold.

CENTRIFUGAL FORCES

spring driven Motor driven

Spring driven

Motor driven

Classification based on How metal melts on the casting machines

1. Torch melting centrifugal casting machine

2. Electrical resistance heated casting machine

3. Induction melting machines

4. Direct current arc melting machines

• The alloy is melted in a separate crucible by a torch flame and is cast into the mold by centrifugal force.

Torch melting/casting machine

ELECTRICAL RESISTANCE HEATED

CASTING MACHINE

• The alloy is melted electrically by a resistance heating , then cast into the mold centrifugally by motor (or) spring action

Resistance heat develops when flow of current was opposed by a opposite power

ep

e

pepe

p

Resistance heating

• Ceramics ,high-palladium alloys, palladium-silver alloy, nickel chromium alloys or cobalt chromium alloy

• The DICOR casting machine used for castable ceramics features a platinum electric resistance-type muffle mounted on a motor driven straight centrifugal casting arm.

INDUCTION MELTING MACHINE

• The alloy is melted by an induction field that develops within a crucible surrounded by water-cooled metal tubing.

• The electric induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic field in the location of the alloy to be melted in a crucible

• It is more commonly used for melting base metal alloys not been used for noble alloy casting as much as other machines

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DIRECT CURRENT ARC MELTING MACHINE

• Arc is produced between two electrodes: The alloy and the water-cooled tungsten electrode generates heat.

anode

cathode

AB

C

D

• The temperature within the arc exceeds 4000oC and the alloy melts very quickly.

• This method has a high risk for over heating the alloy.

Titanium and titanium alloy

Titanium ingot melting-heated to a temp. of 1600-17000 c

• The alloy is vacuum arc melted and cast by pressure in an organ atmosphere.

Vacuum arc melted vacuum-

or pressure assisted casting machine

VACUUM arc melted centrifugal casting machine

Metal receiver

Mold

5

6

Casting techniques

Lost wax technique

Burnout furnaces

BURNOUT (MOLD CONDITIONING)

1. Allow the investment to set (usually 1 hour) and then remove the rubber crucible former

a. If a metal sprue is used, remove it as well

b. Any loose particles of investment should be blown off with compressed air.

c. If burnout is to be delayed – keep in humidor.

• Place the ring with the sprue facing down or on a raised object in the furnace.

Burnout techniques

Mold heating

Preheated furnace Cold furnace

Single stage Two stage

12th law of casting

Rapid or prolonged heating

Removal of mold from furnace

Casting techniques

centrifugal force type Pneumatic force

spring loaded

torch melting machine

Induction melting machine

electrical resistance heated casting machine

Electric arc vacuum casting machine

Casting technique for spring driven centrifugal casting machine

High noble, noble and base metal alloys

Heat treatment (Alloy conditioning )

1. Softening heat treatment

2. Hardening heat treatment

• The casting machine is given three (or) four clock wise turns and locked in position with the pin

Casting procedure

The cradle and counter balance weights are checked for the appropriate size of the casting ring to minimize turbulence for the flow of metal.

• A clay\carbon crucible for the gold alloy being cast is placed in the machine.

• The torch is lit and adjusted

For torch melting The fuel used is a combination of

1. Natural or artificial gas and air

2. Natural or artificial gas and oxygen

3. Air and acetylene4. Oxygen & acetylene

The Flame Has Four Zones

1. Mixing zone 2. Combustion zone3. Reducing zone 4. Oxidizing zone

Reducing zoneReducing zone

• This preheating avoids excessive slag formation during casting

Preheating crucible

• The alloy is heated in the reducing part of the flame until it is ready to cast

• A reducing flux is used in melting the alloy (50% boric acid powder and 50% fused borax ) it increases fluidity and reduces potential for oxidation.

• When reducing zone is in contact-the surface of the gold is bright and mirror like.

• Oxidizing zone in contact-dull film or “dross” development.

• When gold alloy is ready to cast it will be white hot, forming smooth pool.

• The casting machine arm is then released to make the casting.

• Providing enough force to cause the liquid alloy to flow into the mold.

• The machine is allowed to spin until it has slowed enough that it can be stopped by hand, and the ring is removed with casting tongs

Quenching

• After the casting has solidifies, the ring is removed and quenched in water as soon as the button exhibits a dull-red glow.

Casting technique for centrifugal induction casting machine

Predominantly base metal alloys

Counter balance weight checking

weights

Sufficient mass of alloy must be present to sustain adequate casting pressure

• 6g is typically adequate for premolar and anterior casting

• 10g is adequate for molar casting • 12 g is adequate for pontic

Heating base metal alloys

• At casting temp. the base metal ingots should only slump and round over the corners

Casting pressure

• The pressure should be applied for at least 4 sec. the mold is filled and the alloy is solidified in 1sec. or less but it is quite soft during the early stages.

Greater casting pressure are generally required

( Base metal alloys)

• The greater degree of chilling.• Very thin sections have to be cast.• The lower density of base metal alloys.

• Base metal alloys are bench-cooled to room temperature before the casting is removed from the investment

Casting technique for electric resistance heated casting machine

For titanium casting

1. Process of melting and casting takes place in an evacuated two chamber system with a

continuous flow of argon gas 2. Titanium ingot is heated in a

copper crucible

• Prevents overheating & reaction

Laws of casting

• Ingersoll & Walding (1986) formulated an expanded set of 17 separate recommendations for spruing, investing, burnout, melting & casting procedures. Collectively these guidelines are referred to as “the laws of casting”.

1st Law Of Casting

Attach the pattern sprue former to the thickest portion of the wax pattern:

• This provides the molten metal to flow from larger diameter to thinner sections

• Penalty: cold shuts, short margins and incomplete casting

2 nd Law Of Casting

Orient wax patterns so all the restoration margins will face the trailing edge when the ring is positioned in the casting machine:

• Add a wax dot to the crucible so that, it will guide us in placing the ring in casting machine

• Penalty: cold shuts and short margins

3rd Law Of Casting

Position the patterns in the “cold zone” of the investment and reservoir in the “heat center” of the casting ring:

• Adherence to this law causes porosity in the reservoir

• Penalty: shrinkage porosity

4th Law Of Casting

A reservoir must have sufficient molten alloy to accommodate the shrinkage occurring within the restorations:

• Molten metal shrinks and creates a vacuum, for complete casting vacuum must be able to draw additional metal from adjacent source.

• Penalty: shrinkage porosity and/or suck-back porosity

5th law of casting Do not cast a button if a connector bar or another internal

reservoir is used:• With indirect spruing the largest mass of metal should be the

reservoir• A button can draw available molten alloy from the bar, shift

the heat centre and reduce the feed of the metal to the restorations

• Wax patterns should not be larger than the connector bar• Weigh the sprued patterns and use the wax pattern-alloy

conversion chart• Penalty: shrinkage porosity and suck back porosity (potential

distortion during porcelain firing

6th law of casting

Turbulance must be minimized, if not totally eliminated

• Eliminate sharp turns, restrictions, points or impingements that might create turbulance and occlude air in the casting

• Restrictions or constrictions can accelerate the metals flow and abrade the mold surface (mold wash)

• Penalty- voids and /or surface pitting

7th law of casting

Select a casting ring of sufficient length and diameter to accommodate the patterns to be invested

• The casting ring should permit the patterns to be ¼ inch apart and ¼ inch from the top of investment and 3/8 inch of investment between pattern and ring liner

• Penalty- mold fracture, casting fins and shrinkage porosity

8th law of casting

Increase the wettability of wax pattern• Wetting agent should be brushed or stained

on the patterns and dried before investing• Too much wetting can weaken the investment

and produce bubbles or fins on the casting• Penalty- bubbles (due to air entrapment

9th law of casting

Weigh any bulk investment and measure the investment liquid for precise powder liquid ratio

• A thick mix of investment increases investment expansion and produces loose fitting castings

• Thinner mix yields less expansion with tighter fitting castings

• Penalty- ill fitting casting

10th law of castingEliminate the incorporation ofair in the casting

investment and remove the ammonia gas by –product of phosphate bonded investments by mixing under vacuums

• Vacuum mixing removes air and gas providing an uniform mix without large voids

• Entrapped air can affect the expansion at various sites of the investment

• Penalty- small nodules, week mold and distortion of the casting

11th law of casting

Allow the casting investment to set completely before initiating the burn out procedure

• The mold may not withstand steam expansion during burnout if the mold is not set

• Advise to wait till the recommended setting time by the manufacturer

• Penalty- mold cracking/ blowout or fins on the casting

12th law of casting

Use a wax elimination technique that is specific for the type of patterns involved and recommended for the particular type of casting alloy selected

• Plastic sprues should be heated slowly, so they can soften gradually and not exert pressure, so use a two stage burnout

• Recommended atleast a 30 min heat soaking at 800F for the first burnout

• Penalty- cold shuts, short margins, cold welds, mold cracks and/or casting fins

13th law of castingAdequate heat must be available to properly melt

and cast the alloy• Selected heat source should be capable of melting

the alloy to the point of sufficient fluidity• Too much heat or high temperature can burn off

minor alloying elements and /or oxidation (burned metal)

• Penalty- cold shuts, short margins and cold welds (too little heat) or rough castings and investment breakdown (too much heat)

14th law of casting

When torch casting, use the ‘reducing zone’ of the to melt the alloy and not the oxidizing zone

• Melt achieved by the exclusive use of reducing zone minimizes the likelihood of metal oxidation and gas absorption and ensures the proper melt

• Penalty- gas porosity and/ or a change of alloys quotient of thermal expansion

15th law of casting

Provide enough force to cause the liquid alloy to flow into the heated mold

• Low density metal generally needs four winds of a centrifugal casting arm as compared to higher density, gold based alloys

• Do not over wind• Penalty- cold shuts, short margins, cold welds

(insufficient force), or mold fracture and fins (too much force)

16th law of castingCast towards the margins of wax patterns• Place the heated ring in the casting cradle

using the orientation dot, so the pattern margins face the trailing edge

• Penalty- cold shuts, short margins and otherwise incomplete castings

17th law of casting

Do not quench the ring immediately after casting:

• Uneven cooling and shrinkage between alloy and investment can apply tensile forces to the casting dot

• It can reduce strength • Penalty- hot tears

Defects in casting• Defects in casting can be eliminated or minimised by strict

adherence to prescribed procedures.• Defects in casting can generally be divided into:

– Distortion– Poor surface finish

• fins / spines• nodules• veins and ridges• rough surface

– Porosity– Voids– Incomplete casting

Distortion• In most cases distortion of the casting can be

attributed to distortion of the wax pattern • The wax pattern distortion can be minimised by

careful and proper manipulation of the wax and handling of the pattern.

• In addition, some of distortion of the wax pattern can be caused by hardening of the investment around it, whereby setting and hygroscopic expansion could lead to uneven deformation of the walls of the wax pattern.

• This depends, on the other hand, on the type of wax, thickness and configuration of the pattern

Poor surface finish• One of the requirements on the castings is that its surface

should be as accurate reproduction of the wax pattern surface as possible.

• The defects under this category can be further divided in excessive surface roughness, unexpected surface irregularities and inappropriate surface colour

• Surface roughness can be described as finely spaced surface imperfections, while the irregularities relate more to isolated imperfections (such as nodules).

• It should be accepted that the surface roughness of the casting is greater than the roughness of the wax pattern it is made of

• This can be explained by a particle size of the investment which is such that it cannot reproduce the wax pattern in fine enough details

Porosity

• Main causes of the porosity of alloy castings are:1) Solidification defects

• Localized shrinkage porosity• Micro porosity

2) Trapped gases• Pin hole porosity• Gas inclusion• Sub surface porosity

3) Residual air

Solidification Defects• Solidification defects cold lead to two different manifestations

of porosity:

• Localised shrinkage porosity• Microporosity.

Localised shrinkage porosity

• It is caused by insufficient feeding of the alloy during solidification.

• As the alloy solidify, it also shrinks by over 1%, and a sufficient supply of molten alloy is required during this phase

• If the sprue is not properly designed and implemented then it may solidify before the feeding is complete thus preventing a continuous supply of molten alloy

• This type of defect usually occurs close to the sprue-casting junction

Suck-back porosity

• If hot spot is created on in the mould near to the sprue this causes the local region to freeze last and cause porosity

• Generally occurs at occluso-cervical line angle that is not well rounded

• This can be eliminated by flaring the point of sprue attachment and by lowering casting temperature by 30ºC

Microporosity

• It is also caused by solidification shrinkage, but generally happens in fine grain alloys when the solidification is too rapid for the microvoids to segregate.

• This in turn is caused the mould or casting temperature being too low.

Sub-Surface Porosity

• Caused by simultaneous nucleation of solid grains and gas bubbles

• It can be dimnished by controlling the rate at which molten metal enters the mold

Back Pressure Porosity

• Entrapped air porosity• Due to inability of air

to escape through mold

• It is found in cavity surface of a crown or mesio-occlual-distal casting

• In case of low castin/mold temperature it can occur on the outer surface

• Factors to be considered:

1. Dense investments2. Tendency for mold to clog3. Venting4. Proper burn out

Incomplete Casting• If the molten alloy is prevented from fully or partially

filling the mould, insufficient venting of the mould and high viscosity of the metal

• If the air in the mould could not be vented quickly once the molten allow is poured into the mould, its back pressure will prevent compete filling of the mould by the molten alloy

• If the elimination of the wax patter was not fully complete, the pores of the investment might become filled with the combustion products thus preventing venting of the air which can cause similar issue to the above

Castable Ceramics

e.g. DICOR (Corning glass, Dentsply). CERAPEARL (Bioceram, Kyocera)

Mold conditioning

• Two stage technique-1.The invested pattern is placed in a cold

furnace, the temperature is raised to 249ºC, and maintained for 30 min (heat soak).

2.The temperature is raised to 899ºC, and maintained for 2 hrs (heat soak).

Casting

• A 4 gm ceramic ingot is loaded in the ceramic crucible.

• The amount of ceramic material required to make a good casting can be determined when multiplied by a conversion factor of 2.6, the weight of the wax pattern, sprues and button should not exceed 4gm .

Ceramic ingot Ceramic crucible

Front muffle door Rear muffle door

From burnout furnaceCrucible withCeramic ingot

Casting technique

11000 c13600c

Muffle assembly is placed in the casting machine

Cradel

Crucible with casting ring

• The casting ring is removed form the machine and allowed to cool for 45 min before divesting.

Casting Procedure For Removable Partial Denture

• With the duplicaton of the mastercast we get the working cast for making the wax pattern and investing

• Materials used for duplicating the mastercast:– duplicating gels– silicones

• On the surface of the workingcast wax pattern is made from prefabricated wax elements

• After sprueing the working cast is ready for investing

• Investing with flask• Investment materials

used for cobalt-chromium alloys:– Phosphate bonded– Silica bonded– Gypsum bondedInvesting is carried out by

vibration.

deflasking

Finishing of the ready metal frame(sandblasting, trimming, polishing)

Literature review

• In a study by baltag et al, the lowest porosity in titanium circumferential clasp arms was obtained with sprues attached perpendicularly to the minor connectors, regardless of sprue diameter

• Conventional sprue directions produced significantly higher porosity in clasp lingual arms, the amount of porosity increasing with sprue diameter.

In a study by Mohamed et al they compared button and button less

castings• The metal used to cast each framework was either

enough to result in a full button or in no button.

• The use of minimal metal to cast RPDs was equally as successful as using enough for a full button, provided that the appropriate spruing arrangement and metal feeding direction were chosen.

• Introduction

• Steps in casting

• Spruing

• Investing patterns

• Burn out procedure

• Casting machines

• Casting techniques

• Casting procedure

• Principles of casting

• Casting defects

• Casting of ceramics

• Casting of titanium

• Casting procedure in RPD

• conclusion

knowledge.....

thoughtful implementation..... critical appraisal.....

.....”will help to assure the supreme quality of this form of invigorating care”.....

References 1. K. J. Anusavice, Phillips Science of Dental Materials, 11th

edition, W. B. Saunders co 2003, pg: 295-3502. Introduction to metal ceramic technology, W.P. Naylor,

Quintessence publishing co, inc ,1992, pg: 65-823. J.F.MaCabe, applide dental materials, 8th edition, blackwell

science publication, pg:69-714. M.Gladwin, Clinical Aspects of Dental Materials, Lippincott

Williams & Willikins publishing, 1999, pg:115-1265. Craig R.G, Restorative Dental Materials;10th edition; St.Louis:

Mosby, 1997, pg:457-480

6. Baltag I et al: Internal porosity of cast titanium removable partial dentures: influence of sprue direction and diameter on porosity in simplified circumferential clasps. Dent Mater. 2005 Jun;21(6):530-7

7. Mohammed H et al: Button versus buttonless castings for removable partial denture frameworks, J Prosthet Dent. 1994 Oct;72(4):433-44

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Thank you

• Discontinuities in castings that exhibit a size, shape, orientation, or location that makes them detrimental to the useful service life of the casting

• Some casting defects are remedied by minor repair or refurbishing techniques, such as welding

• Other casting defects are cause for rejection of the casting

Casting Defects

(Metallurgy, by B. J. Moniz, American Technical Publishers, Inc., 1994)

• Metallic Projections: fins (flash), swells, and scabs

– Fins are excessive amounts of metal created by solidification into the parting line of the mold

Fins are removed by grinding or sandblasting

– Swells are excessive amounts of metal in the vicinity of gates or beneath the sprue

– Scabs are surface slivers caused by splashing and rapid solidification of the metal when it is first poured and strikes the mold wall

Casting Defects — Metallic Projections

• Blowholes, pinholes, shrinkage cavities, & porosity

– Blowholes and pinholes are holes formed by gas entrapped during solidification

– Shrinkage cavities are cavities that have a rougher shape and sometimes penetrate deep into the casting

Shrinkage cavities are caused by lack of proper feeding or non-progressive solidification

– Porosity is pockets of gas inside the metal caused by micro-shrinkage, e.g. dendritic shrinkage during solidification.

Casting Defects — Cavities

• Cracks in casting and are caused by hot tearing, hot cracking, and lack of fusion (cold shut)

– A hot tear is a fracture formed during solidification because of hindered contraction

– A hot crack is a crack formed during cooling after solidification because of internal stresses developed in the casting

– Lack of fusion is a discontinuity caused when two streams of liquid in the solidifying casting meet but fail to unite

Rounded edges indicate poor contact between various metal streams during filling of the mold

Casting Defects — Discontinuities

• Casting surface irregularities that are caused by incipient freezing from too low a casting temperature

• Wrinkles, depressions and adhering sand particles

Casting Defects — Defective Surfaces

• Particles of foreign material in the metal matrix

• The particles are usually nonmetallic compounds but may be any substance that is not soluble in the matrix

– Slag, dross, and flux inclusions arise from melting slags, products of metal treatment, or fluxes

They are often deep within the casting

– Mold or core inclusions come from sand or mold dressings and are usually found close to the surface

Casting Defects — Inclusions