casting defects

CASTING PROCEDURE FOR

FIXED PARTIAL DENTURE

CONTENTS

INTRODUCTION HISTORY BASIC STEPS OF CASTING

PROCEDURE SPRUE FORMER AND ITS ATTACHMENT CRUCIBLE FORMERS CASTING RINGS AND LINERS INVESTING PROCEDURE

WAX BURNOUT

CASITNG OF ALLOY INTO MOLD CASTING ALLOYS HEAT SOURCE MACHINES TO INDUCE CASTING

FORCE RECOVERY AND CLEANING OF

CASTING

History

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

1558 - Benvenuto Cellini - claimed to have attempted use of wax and clay for preparation of castings

1884 - Agulihon de saran used 24K gold to form Inlay

. 1897 - Phillibrook described a method of casting metal filling

1907 -Taggart -devised a practically useful casting machine.

Apart from this various studies conducted on the properties of investment materials and casting alloys have lead a path for a better, practical and useful processing methods.

CASTING:- Is defined as 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.(GPT-8)

CASTING PROCEDURES FOR FIXED PARTIAL DENTURE

The basic steps involved in the casting of fixed partial denture are

1. Tooth preparation. 2. Impression and die preparation. 3. Wax pattern preparation. 4. Spruing.

5. Investing. 6. Burnout. 7. Casting of molten alloy. 8. Recovery of casting, cleaning. 9. Finishing and polishing

Objectives of casting 1) To heat the alloy as quickly as possible to a

completely molten condition.

2) To prevent oxidation by heating the metal with a well adjusted torch.

3) To produce a casting with sharp details by having adequate pressure to the well melted metal to force into the mold

Wax Pattern

Spruing

The process of attaching a sprue

former/sprue pin to the wax pattern is called as spruing

Purpose:- To provide a channel through which

molten alloy can reach the mold in an invested ring after the wax has been eliminated..

3 Basic requirements of sprue:-

1) Must allow the molten wax to escape from the mould.

2) Sprue must enable the molten metal to flow into the mould with as little turbulence as possible.

3) Metal must remain molten slightly longer than the alloy that has filled the mould

Principles of selecting an

appropriate sprue

Type of sprue

Sprues made of different materials 1. Wax - used for casting of small and

large casting, which use single stage burnout.

2.Plastic/Resin - used for castings of alloys which use 2 stage

burn out with Po4 bonded investment. Their main disadvantage is its softening

temperature, which is higher than wax pattern. And may block escape of wax.

They may be used for casting FPD’s because of their high rigidity, which minimizes distortion. Plastic sprues may be completely solid (or) hallow plastic help in wax elimination.

3. Metal sprues should be a non-rust metal to avoid

contamination of wax.

Hallow metallic sprue increase contact surface area and strengthen the attachment between the sprue and pattern.

They are removed from the investment at the same time as the crucible former.

Care should be taken to examine for any fractured investment material after metal sprue removal

2.Sprue gauge/size (diameter)

Large diameter sprue:this improves the flow of molten metal into the mould. The diameter of sprue should be equal to the thickest portion the wax pattern.

less diameter sprue:causes localized shrinkage porosity

A narrow sprue may be useful in air pressure casting procedure where the metal is melted on crucible former and narrow sprue prevent the premature metal flow into mold.

C) Sprue lengthVery 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

D) Sprue 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

E) Number of sprue:

Usually a single sprue is used for small castings.

When two thick sections of a pattern are connected by thin part of wax, 2 separate sprues should be attached to each thick portion.

The double sprue design is more effective than the single sprue design in decreasing the internal porosity

(jpd vol 78 no 4 oct 1997)

Attachment of sprue former and wax pattern Patterns may be sprued either directly or

indirectly. For direct spruing ;- the sprue former provides a direct

connection between the pattern area and sprue base.

In indirect spruing -

a connector or reservoir bar is positioned between the pattern and crucible former.

It is common to use indirect spruing for multiple single units and fixed partial dentures, although several single units can be sprued with multiple direct sprue formers.

Principles of spruing

a) Location of attachment

b) Angulation of sprue

c) Attachment morphology

a) Location of attachment The ideal location for attachment of sprue

is the thickest portion of wax pattern

The sprue should not be located where it can obliterate centric occulsal contacts and centric cusp tips and margins

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.

If attached to cusp tips near margins of wax pattern, distortion and restriction of flow of molten metal into mold occurs.

b) Angle of sprue attachment

The sprue should be attached to pattern such that it makes 45⁰ 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

c) Attachment morphology

The attachment of sprue former to the wax pattern should be such that the transition is smooth and do not posses pits / irregularities into which investment can flow.

Irregularities produces tags of investment which is prone for fracture by molten alloy leading to casting failure.

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.

Tuccillo and Nielsen - flaring minimize investment debris and aspirated air; allow smooth flow of molten metal.

Nielsen and Shalita - flaring cause the spread of heat over an increased region.

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.

The need for indirect spruing

as the ambient air is colder than molten alloy the button solidifies earlier than molten alloy. So, it can no longer serve as a reservoir to prevent shrink spot porosity.

The use of horizontal runner bar will act as reservoirs of molten alloy, which equalizes the flow to all parts of F.P.D and stabilizes the pattern against distortion during investment.

Reservoir :

Reservoir is a small amount of additional wax which is added to the sprue former near the junction of wax pattern

It prevents localized shrinkage porosity as the alloy in this part solidifies last after the solidification of metal in mold

It is used in direct spruing.

The horizontal running bar of indirect spacing provides the same function; they are used when the distance between the crucible and pattern is high.

The reservoir is present in prefabricated plastic sprues also.

Venting

Small auxiliary sprues/vents are applied to thin wax pattern to improve the quality of casting. Usually 18- gauges sprues are used. It is indicated with extremely thin/thick casting to produce nonporous castings.

They help in escape of gases during casting and ensure beginning of solidification in critical areas by acting as a heat sink.

It is attached to the wax pattern directly opposite to larger sprue former. It terminates either in investment attached to reservoir. As its termination is near the outer wall of investment, it solidifies first and induce solidification in main mold.

Crucible former

The sprue is attached to crucible former which constitutes the base of casting relation with casting ring during investing.

It also helps by holding sprue in desired ring.

Crucible formers are basically of 2 types---

a) Steep-sided core: used with metal when casted using centrifugal casting force. The tall crucible formers allow the use of short sprue.

b) Shallow cone: used to cast metal using stream/air pressure

They are available as----

Rubber crucible former Metallic Crucible former Plastic crucible former

They form a conical depression in investment, which guides flow of molten metal.

It should be clean and petroleum is applied to prevent formation of rough investment tag.

Molten sticky wax is applied on the apex of cone portion of crucible former, which contains a hole (for passing the sprue former)

Then the end of sprue former is passed into the hole and held in position till the molten wax sets.

The attachment area should be smooth and without irregularities to prevent creation of investment tags which are prone to fracture when alloy is forced into mold.

Casting ring

Casting rings are used to confine the fluid investment around the wax pattern while the investment sets.

It also allow the hardened investment to be safely handled during burnout and casting

They are available as---

1) Shapes - Round

- Oval

2)

I) Complete rings - Rigid

- Metal (stainless steel) - Plastic

- Flexible - Rubber

II) Split rings - Metal

- Plastic

3)

I) Cylindrical

II) Conical

Considerations in selection of castings rings: -

1) The internal diameter of casting ring should be 5-10mm greater than the widest measurement of the pattern and about 6 mm higher.

2) For single crown/inlay - small rings as used. Diameter - 32 mm

3) For large fixed partial denture – 63mm round/oval shaped casting ring are used

Ringless casting system

Plastic ring with rubber crucible formers are used. The ring is conical in shape with tapering walls. As the investment sets the investment is tapped out of ring. Then burnout is done with out casting ring, this causes greater expansion

Usually casting rings are rigid in nature. Because of this the mold may become smaller rather than larger due to the reverse pressure resulting from confinement of the setting expansion. To overcome this flexible rings/ split rings are used.

But the most commonly used technique is to provide expansion by lining the ring with a ring liner.

Casting ring liner

They are commonly used to produce expansion of mold. Various materials used as ring liners ----

1. Asbestos liner

2. Cellulose (blotting paper) liner

3. Ceramic ring liner

4. Combination of ceramic and cellulose ring liner

5. Wax crinkled paper

Functions of Ring liner

1. Allow uniform setting expansion of investment by decreasing the confinement of rigid casting ring.

2. In case of wet liner technique ---The absorbed water help in hygroscopic expansion. The water in the setting investment and the

liner form a continuous phase.

The water in liner influences expansion of mold at a distance of at least 25mm. Wetting of the liner prevents absorption of water by liner (if dry) from the setting investment.

3. Thickness of the liner should be < 1mm

4. The amount of expansion depends on the number of liners used. The expansion seen with 2 liners is greater than one liner

1. Asbestos liner: Asbestos is refractory to high temperature, they show a sufficient amount of water absorption. There are 3 types of asbestos---

White asbestos (least toxic) – this type is used in dentistry

Blue asbestos (most toxic) Brown asbestos (Intermediately toxic)

Asbestos is this no longer used in dentistry. As produces 3 types of diseases

1) Asbestosis

2) Bronchogenic lung cancer

3) Mesothelioma – fatal tumour

The carcinogenicity is due to the dimensions and durability of asbestos fibres.

which are longer than 4mm and diameter less than 1.5mm & is known to induce Mesothelioma.

(JPD 1987; 57, 362-369)

2. Cellulose liner

This material shows adequate water absorption.

It is burnt during burnout procedure. So to keep the investment in contact with ring after burnout

The liner is kept 3mm short of ring ends. This also restricts the longitudinal setting and

hygroscopic expansion.

Procedure 9.5 cm long cellulose liner is carefully adopted on the walls of casting ring and is tucked in position with sticky wax.

If wet liner technique is used, the lined ring is immersed in water for some time. Then excess water is shaken away.

squeezing of liner should be avoided.

The liner should end 3mm short of the casting ring end.

Ceramic ring liner

They are basically alumino-silicate fibrous material.

They do not absorb water to, but its network of fibres can retain small amount of water on its surface/wetting agents can be used to increase the water sorption on surface.

They are refractory to high temperature. The binders used in ceramic liner (Ex – neoprene-latex) can contribute to toxicity (stimulate fibrosis/ act as adsorbent surface for carcinogenesis).

They show potential for development of Mesothelioma .They posses fibers of ---

Length 5.3-17.8 mm.

Diameter 0.2- 0.97 mm

Wax crinkled paper

They were used previously.

They are waxed to internal wall of cylinder/held in position by paper clips

INVESTING PROCEDURE

The wax pattern should be cleaned of any debris, grease or oils.

For this we can use either:-

- A commercial wax pattern cleaner, or,

- A diluted synthetic detergent.

The pattern is left to air dry while the investment is being prepared.

The thin film of cleanser left on the pattern reduces the surface tension of the wax & permits better ‘wetting’ of the investment to ensure complete coverage.

Wetting agent reduces the contact angle of a liquid with wax surface.

Lower contact angle indicates that treated wax surface has an affinity for water, which results in investment being able to spread more easily over wax.

MIXING Mixing of investment may be done either by ----

i) Vacuum mixing

ii) Hand mixing

The incidence of bubble free casting with different investing technique

– Open investing - 17%

– Vacuum investing - 95%

The incidence of nodules on casting is more in hand mixing then vacuum mixing. Application of surface tension reducing agent decreased the nodules (Johnston, IJP, 1992, 5; 424-433).

The best method is vacuum mix and

vacuum pour technique. But most popular method vacuum mix and open pour.

I) Vacuum investing technique

1. First, hand spatulate the mix

2. With the crucible former and pattern in place attach the ring to the mixing bowl

3. Attach the vacuum hose and mix according to the manufacturer’s recommendations

4. Invert the bowl and fill the ring under vibration

5. Remove the vacuum hose before setting of the mixer

6. Remove the filled ring and crucible former from the bowl

7. Immediately clean the bowl and mixing blade under running water. This technique is suited for Gypsum-bonded

investment because of their fluidity after mixing. But with phosphate-bonded investments because of greater viscosity it can trap air in sharp corners of the pattern

II) Vacuum mixing and hand investing (Brushing technique)

This technique is used in case of phosphate -

bonded investment to prevent air entrapment and casting defects.

In this technique mixing is done on vac-u-vester (vacuum mixing) then before filling the ring, the wax pattern is painted with investment material using brush.

Then pouring carefully around the pattern fills the ring.

Brush technique: In this technique, pattern is first painted with surface tension reducer; the surface must be wet completely

A finger positioned under the crucible former on the table of the vibrator minimizes the risk of excessive vibration and possible breaking of the pattern from the sprue.

After the pattern has been completely coated, the ring is immediately filled by vibrating the remaining investment out of the bowl.

When the investment reaches the level of the pattern, tilt the ring several times to cover and uncover the pattern, thereby minimizing the possible entrapment of air.

Investing must be performed quickly within the working time of the investment.

If the investment begins to set too soon, rinse it off quickly with cold water.

After the ring is filled to the rim, allow the investment to set.

If the hygroscopic technique is used, the ring is placed in a 37 C (100 F) water ⁰ ⁰

bath for 1 hour

Advantages of vacuum mixing-----

1) Remove air bubbles

2) Produce smooth castings

3) Increase tensile strength of investment

4) 95% of castings free of nodules.

5) Removes all the gaseous by products of chemical reaction of investment material

Small ring: 1 package Large ring: 2 packages Hand mix for 15 seconds Vacuum mix for 60 seconds Working time: 2-3 minutes

Mixing Ratios

General ---

• More investment liquid, less water =more expansion

• Less investment liquid, more water =less expansion

• Begin with a dry bowl Use a maximum of 27ml of liquid

Using more liquid results in a weak mold

• For 100gms of investment:-Crowns/veneers: 22ml liquid,

5ml distilled water

• Inlays/Onlays: 16ml liquid, 11ml distilled water

• Follow instructions on investment packet

INVESTING OF GYPSUM BONDED INVESTMENTS Require very specific W:P ratio’s .

A variation of only 1ml of H2O can significantly alter the setting expansion & the character of the casting surface.

increasing W:P ratio makes investing process easier but investment will lose strength,

cause cracks to occur during heating

surface of casting inferiors.

After the casting ring has been filled with investment material, any excess should be removed before the material sets.

The filled ring is now set aside to allow the investment material to complete its setting reaction & the accompanying setting expansion.

Setting is complete in 30-40min.

Hygroscopic technique is used.

- Freshly filled investment ring is immediately placed into water bath for 30min. & kept at 100ºF(38ºC).

INVESTING OF PHOSPHATE BONDED INVESTMENTS

Expansion of the mold cavity can be increased by--

1) increasing the no. of layers of asbestos or fibrous ceramic lining the casting ring.

2) increasing the special liquid : water ratio.

3) increasing the total L:P ratio.

4) Placing the investment in contact with water during setting.

5) Burning out the mold at a higher temp.

3mm on each end is left as it serves to lock the investment within the ring & equalize radial & axial expansion.

Residual, hardened investment in an unclean mixing bowl will greatly accelerate the set of newly mixed investment

Phosphate investment should not be mixed in an apparatus that has been used for gypsum investment. Residual gypsum will also accelerate the set & will break down at temp. above 2400ºF(1300ºC) liberating sulfurous gases that can be detrimental to the casting

Ammonia gas is given off during mixing, & it is important to hold the mixed investment under the vacuum after mixing ceases to dissipate some of this gas & thereby reduce the incidence of bubbles adhering to the wax pattern ( this additional holding time will vary from 15-45sec).

Initial set of the phosphate bonded investment is generally rapid with the liberation of heat.

If burnout is not carried within 1-2hrs, the ring should be stored in a humidor at 100% humidity, not soaked in water since excessive hygroscopic expansion may result

Rapid steam release from a water saturated ring can fracture the investment.

Carefully grinding or scraping the shiny “skin” off the end of investment just prior to burnout is advisable. This removes a relatively impervious layer, opening the pores of the investment & facilitating gas release as the alloy is cast into the mold.

BURNOUT PROCEDURE Once the investment has set for an appropriate

period 45min. it is ready for burnout.

A crucible former or any metal sprue former are carefully removed.

It is advisable to begin the burnout procedure while the mold is still wet, because water trapped in the pores of investment reduces the absorption of wax & as water vaporizes, it flushes wax from mold.

This burnout after 45min. determines with a gradual increase in temp. with wax elimination & phenomena of crystalline inversion that accounts for volume increase on thermal expansion.

Temp. of investment must be increased in successive stages & be well defined in terms of time.

These time intervals bet. various successive burnout temp. levels must be followed

Purpose of wax elimination. :-

1) To create a mold space – By wax elimination

2) To provide thermal expansion of investment.

3) To remove residual water in investment.

4) Heat soaking of investment - raises the temperature of investment and eliminates the temperature difference between the molten alloy and investment temp difference causes incomplete casting.

For expansion phenomena to take place in the best possible conditions, it is necessary that internal temp. of casting ring gradually reach prescribed level.

The interval between successive temp. level is in dispersible to permit the external heat to reach the internal areas of casting ring

Final burnout temp. of casting ring must satisfy fundamental principles:-

1) Give a degree of expansion that is in harmony with the shrinkage of alloy.

2) Maintain the viscosity of alloy at a level

necessary for complete filling of thinnest area in mold.

3) Permit controlled cooling.

Gypsum Investments:-

These investments are relatively fragile & require the use of metal ring for protection during heating.

So, the mould are usually placed in a furnace at room temp. & slowly heated to 650ºC-700ºC for 60min. & held for 15-30min. at the upper temp.

At 468 C for hygroscopic technique the ⁰investment obtains its compensation expansion from 3 sources:-

1) 37ºC water bath expands the wax pattern

2) Warm water entering the investment mould from top adds some hygroscopic expansion.

3) Thermal expansion at this temp. provides the needed expansion.

Advantages -----

1) Less mold degradation.

2) Cooler surface for smoother castings

3) Convenience of placing molds directly

at 468ºC

Rate of heating has some influence on the smoothness & in some instances on overall dimensions.

Rapid heating can generate steam that can cause flaking of the mould walls.

Too many patterns in the same plane within the investment often cause separation of a whole section of investment because, expanding wax creates excessive pressure over a large area.

Too rapid heating may also cause cracking of the investment. In such case, outside layer of the investment becomes heated before the centre sections.

Outside layer starts to expand thermally, resulting in compressive stress in the outside layer that counteracts tensile stresses in the middle regions of the mold.

Decomposition & alloy contamination is related to a chemical reaction between residual carbon & CaSO4 binder. CaSO4 does not decompose unless it is heated

above 1000ºC.

CaSO4 + 4C CaS + 4CO

3 CaSO4 + CaS 4CaO + 4SO2 This reaction takes place whenever gypsum

investments are heated above 700ºC in the presence of carbon

reduction of CaSO4 by carbon takes place rapidly above 700ºC.

Sulfur dioxide as a product of this reaction contaminates gold castings & makes them extremely brittle.

After casting temp. has reached, the casting should be made immediately.

Maintaining a high temp. for a considerable length of time may result in sulfur contamination, rough surface

Methods for rapid burnout procedure are -----

- Placing the mold in a furnace at 315ºC for 30min. & then rapid heating.

Or

- Directly place into a furnace at the final burnout temp. held for 30min. & cast.

Burnout Procedure For Phosphate bonded Investments

PBI require:-

1) Higher burnout temp. for total elimination of wax patterns.

2) Completion of chemical & physical changes.

3) Prevention of premature solidification of higher melting alloys. Usual burnout temp. range from 750 C-900 C.⁰ ⁰

.

Total expansion of 2% or more is required for porcelain bonding alloys, since gold & base metal alloys require higher melting & solidification temp.

These investments- harder & stronger than GBI.

Heating rate is usually slow to 315ºC & is quite rapid thereafter, reaching completion after a hold at upper temp. for 30min.

Disadvantage:- - Quite brittle & are subject to the same

unequal expansion of adjacent sections as phase changes occur during heating.

TIME ALLOWABLE FOR CASTING---

The investment contracts thermally as it cools.

When high heat technique is used, the investment loses heat after the heated ring is removed from the furnace & the mould contracts.

Because of the liner & low thermal conductivity of the investment, a short period can elapse before the temp. of the mould is appreciably affected.

Under average condition approx. 1min. can pass without a noticeable loss in dimensions.

In low- heat casting technique, temp. gradient between the investment mould & the room is not as great as that employed with high- heat technique.

Casting

Casting of an alloy into the mold space uses 2 basic requirements.

A) Heat source – to melt the alloy

B) Casting force – to force molten alloy into

mould casting force > surface tension of alloy

+ resistance offered by gas in the mold

A) Heat Source: Different types of materials and method are used as heat source to melt alloy. Two basic modes are by using

1) Torch flame--

Gas air

Gas oxygen

Air acetylene

Oxygen acetylene.

Hydrogen oxygen generator

2) Electricity --

Electrical resistance melting

Electrical melting

Electrical\ induction melting

Crucibles : The Melting of alloy requires a crucible to act as a platform on which the heat can be applied to the metal. There are three types of casting crucibles available---

Clay

Carbon

Quartz

Clay crucibles are used with high noble and noble metal alloys used for crown and bridge alloy.

Quartz crucibles are recommended for high-fusing alloys of any type of base metal alloys and palladium alloys

Carbon crucibles –

for high noble crown and bridge and also for higher fusing gold-based metal ceramic alloys.

Carbon crucibles should not be used in melting of high palladium palladium silver alloys (to be melted above 1504 c) ⁰and also with nickel-chromium/cobalt chromium base metal alloys

The crucibles used with noble metal alloys should not be used for melting base metal alloy

Copper –containing gold alloys and non-copper gold alloys for use with porcelain should not be melted in the same crucible

Crucible should be discarded if it contains large amount of oxides and contaminants from previous metals

CLASSIFICATION OF DENTAL CASTING ALLOYS

1. ALLOY TYPES BY FUNCTIONS:

In 1927, the Bureau of Standard established gold casting alloys, type I to type IV according to dental function with hardness increasing from type I to type IV.

Type I (Soft): It is used for fabrication of small inlays,

class III and class V restorations which are not subjected to great stress . These alloys are easily burnishable

Type -II (Medium): These are used for fabrication of inlays

subjected to moderate stress, thick 3/4 crowns, abutments, pontics,

full crowns and soft saddles. Type I and II are usually referred to as

inlay gold

Type -III (Hard): It is used for fabrication of inlays subjected to

high stress, thin 3/4 crowns, thin cast backing abutments,

pontics, full crowns, denture bases and short span FPDs .

Type III alloys can be age hardened.

Type-IV (Extra hard): It is used for fabrication of inlays subjected to

high stress, denture bases, bars and clasps, partial denture frameworks and long span FPDs.

These alloys can be age hardened by heat treatment

Type III and Type IV gold alloys are generally called "Crown and Bridge Alloys", although type IV alloy is used for high stress applications such as RPD framework

Later, in 1960, metal ceramic alloys were introduced and removable partial denture alloys were added in this classification.

Metal ceramic alloys (hard and extra hard)---

It is suitable for veneering with dental porcelain, copings, thin walled crowns, short span FPDs and long span FPDs. These alloy vary greatly in composition and may be gold, palladium, nickel or cobalt based.

Removable partial denture alloys --

It is used for removable partial denture frameworks. Now a days, light weight, strong and less expensive nickel or cobalt based have replaced type IV alloys

2. ALLOY TYPES BY DESCRIPTION:

By description, these alloys are classified into---

A)CROWN AND BRIDGE ALLOYS

This category of alloys include both noble and base metal alloys that have been or potentially could be used in the fabrication of full metal or partial veneers.

1. Noble metal alloys:

i) Gold based alloy - type III and type IV gold alloys , low gold alloys

ii) Non-gold based alloy-Silver -palladium alloy

2. Base metal alloys:

i) Nickel-based alloys

ii) Cobalt based alloys 3. Other alloys:

i) Copper-zinc with Indium and nickel

ii) Silver-indium with palladium

B) METAL CERAMIC ALLOY

1. Noble metal alloys for porcelain bonding: i) Gold-platinum -palladium alloy ii) Gold-palladium-silver alloy iii) Gold-palladium alloy iv) Palladium silver alloy v) High palladium alloy

2. Base metal alloys for porcelain bonding: i) Nickel -chromium alloy ii) Cobalt-chromium alloy

C) REMOVABLE PARTIAL DENTURE ALLOY

Although type-IV noble metal alloy may be used, majority of removable partial framework are made from base metal alloys---

1. Cobalt-chromium alloy 2. Nickel-chromium alloy 3. Cobalt-chromium-nickel alloy 4. Silver-palladium alloy 5. Aluminum -bronze alloy

3.ALLOY TYPE BY NOBILITY

High noble, noble, and predominantly base metal.

Alloy Classification of the American Dental Association (1984)

ALLOY TYPE TOTAL NOBLE METAL CONTENT

High noble metal > 60 wt% of the noble

metal elements

Noble metal > 25 wt % of the noble metal

elements

Predominantly base metal < 25 wt % of the noble metal

elements

Amount of metal needed: -

Usually new gold alloys should be used for castings in case of gold alloys and other alloys if the remounts of castings are used.

They should be cleaned and at least 1/3rd of a new gold pellet by weight must be used for each melting..

Sufficient mass of alloy must be present to sustain adequate casting pressure--- High-density noble metal alloys.

For premolar and anterior castings- 6 grams

For molor castings - 9 grams For pontics - 12 grams

MELTING OF ALLOY

Different types of materials and method are used as heat source to melt alloy. Two basic modes are by using

1) Torch flame–

Gas air Gas oxygen Air acetylene Oxygen acetylene. Hydrogen oxygen generator

2) Electricity -- Electrical resistance melting Electrical melting Electrical\ induction melting

Expansion aids in enlarging the mold to compensate for the casting shrinkage-

For gold alloy- 3 type of expansion may be seen 1)normal setting expansion 2)hygroscopic setting expansion 3)thermal expansion

Hygroscopic low-heat technique

used with gypsum bonded investment which are allowed to set under water.

They are used in casting gold alloys. The temperature used in this technique

is 482 c for 60-90 mins⁰ 0.55% of expansion

High heat thermal expansion technique

.  Gypsum bonded Investment : The investment is slowly heated to 650⁰c - 700⁰c in 60mins. Then maintained for 15-30 mins.

For phosphate bonded: This technique is used when investment

is allowed to set in open.

The temperature of 2nd stage in this technique depends on type of investment material used

After initial slow raise of temp to 315⁰c, the temperature is rapidly raised to 750-900⁰c and maintained for 30 mins.

The technique cause 1.33-1.58 % of Thermal expansion

Normal setting expansion

-0.5% Hygroscopic setting expansion

-minimal-1.2%

- maximum-2.2%

Thermal expansion

-if Hygroscopic setting expansion is used then thermal expansion will be 0.5-0.6%

If normal setting expansion then thermal expansion should be 1-2%

For Phosphate bonded 1)wax pattern expansion;-the heat

during setting allows a significant expansion of the wax pattern

Setting expansion-around 0.7 to 1% Thermal expansion-around 1.33-1.58%

Casting shrinkage occurs in 3 stages 1)thermal contraction of the liquid metal 2)contraction of metal while changing

from liquid to solid state 3)thermal contraction of solid metal as it

cools to room temp.

Casting shrinkage— Type 1- 1.56% Type 2-1.37% Type 3-1.42% Type 4-(ni-cr based)-2.30% Type4-(co-cr based)-2.30%

Melting temp of pure gold –1063⁰c Melting temp of gold alloy-924-960⁰c Melting temp of base metal alloy-1155-

1304⁰c

Fuel gas characteristics flame temp heat content Hydrogen: 2660⁰c 2362kcal/m3 Natural gas:2680⁰c 8898 Propane: 2850⁰c 21221 Acetylene: 3140⁰c 12884

Melting of metal

Gas air torch: -Gas-air torch is used to melt conventional noble metal alloys (used for inlays, crown and bridge) whose melting points less than 1000 c⁰

Gas–oxygen torch:

Used to melt metal ceramic alloys of higher temperature up to 1200⁰c

The tip of torch is available as single orifice/multiorifice.

the oxygen pressure is adjusted to

10-15 psi

The flame is directed onto metal with the nozzle of the torch about 1.5 cm away from the metal.

Complete fluid should be obtained within 30 second at which point the metal is poured into the mould.

Oxy-acetylene torch :

The actual production of flame can be done by adjusting the pressure and flow of individual gases .

commonly advised pressure for acetylene nozzle is 3.5 N/cm2 and oxygen nozzle 7-10 n/cm2

The best results are obtained when flame is used with a distance of 10cm between the face of blow torch nozzle and the base of crucible.

If distance is reduced to - 7.5 mm -slight porosity

- 5 mm -increased porosity due to occluded H2 gas

When the reducing zone is in contact, the surface of the gold alloy is bright and mirror like.

When the oxidizing portion of the flame is in contact with alloy there is a dull film of “dross” developed over the surface

II Electrical source :- A) Electrical resistance heated

casting machine :- It is used to melt ceramic alloys. Here

the alloy is automatically melted in graphite crucible.

provides best means of temperature control. It is quite convenient as compared to blow torch.

B) Electrical arc melting: is used to melt higher fusing alloys.

which is used to create a electrical arc at the end of two electrodes

The apparatus requires a high electrical input (30A)

A current is selected according to which alloy is being melted

One electrode is attached to the negative terminal and the other to the positive terminal. Both electrodes are placed about 5cm apart

A brilliant arc is produced around the end of electrodes.

The arc is directed on to the alloy , the end of electrodes being about 12mm away from the alloy.

High fusing alloys show rounding off of corners and signs of collapsing, at which point they are thrust into the mould.

C) Induction casting :

- It is used to melt base metal alloys of high melting temperature

. The centrifugal casting machine is controlled by electricity.

Principle :- Induction casting is based on electric currents in a metal core caused by induction from a magnetic field.

When high density alternating current (high frequency 1500 KHz) is passed through the copper coil then it will produce electric resistance

Because of electric resistance of coil energy is transferred to thermal energy.

Magnetic field is produced by current oscillating in that particular area.induces an oscillating current in crucible.

Because crucible has some resistance to the current,electric energy of current is continuously transferd to thermal energy

alloy with the capability to have its polarity changed, is placed inside the crucible

the rapid change of polarity produced causes the molecules of the alloy to oscillate and their molecular bonds to break down.

This high intensity of molecular activity produces heat. The effect is that the alloy becomes molten.

1) Water is circulated into the

machine under pressure of 20 psi. This travels through the copper coil to keep it cool during the melting process.

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

In casting machines an electronic eye is present above the crucible.

The starter switch is activated until the metal becomes red in color.

In about 30 seconds the metal will begin to sag and a circular shadow appears to hour over the metal.

Eventually, the shadow will diminish in size towards the center of crucible and then disappear

As the shadow disappears in the direction the induction coil is lowered and the casting machine is activated which usually rotate at a speed of 600 RPM. This causes the flow of molten alloy into the mold.

the electricity supply to the machine switched off and the water supply turned off.

If carbon crucible is used (for gold alloys) the crucible itself becomes hot and transfers heat to the alloy.

CASTING MACHINES

Device for forcing the molten alloy into the mould under pressure after wax has been eliminated

CASTING MACHINES

Type I

Alloy melted in crucible , followed by application of air pressure (10-15 psi) to force the material into the mould Type II

Common

Alloy is melted in separate crucible and the metal is cast into the mould by centrifugal force

Type III Alloy melted electrically by resistance or

induction furnace & cast into the mould by centrifugal force ( INDUCTION CASTING)

Type IV Alloy melted electrically by resistance or

induction furnace, metal cast into mould by air pressure or vaccum

CASTING FORCE

positive force has to be applied..for molten metal

1) Vacuum force

2) Pneumatic (steam/Gas) Pressure

3) Centrifugal force

The casting of alloy is affected by 2 main factors :-

1) Amount of force 2) Time duration within which force is

applied The amount of force can be increased by

increasing the speed of rotation / amount of pressure applied. The time need to fill mold with pneumatic force is greater than centrifugal force.

Casting by vacuum :-

Vacuum is applied to the external surface of the investment mass, drawing out the investment and mold gases, allowing the melt to ingress. It cannot work alone in filling the mold.

So, machines are used in combination centifugal and gas pressure.

B) Gas pressure :- The metal is melted in the investment

crucible. Then gas pressure is applied on the molten metal.

different gases used are carbon dioxide, carbon monoxide / nitrogen.

They apply a pressure of 10-15 Psi

by centrifugal force :-

This is the most feasible and commonly used mode, for casting

. This machine utilize the centrifugal force which is defined as a radical force radiating outward from the center of rotation of a body.

There are various types of machines which use this principle and may be categorized as :-

i) a) Spring operated b) electrically operated ii) a) Horizontally rotating b) Vertically rotating

ADVANTAGE: Both small and large casting on the same machine

a) Spring operated

electrically operated

Horizontally rotating

Vertically rotating

Spring operated centrifugal mechine

It consists two arms

one contains the casting assembly. 1) Cradle to seat the casting ring. 2) Bracket to place the crucible

against the ring. 3) Head plate to prevent

displacement of casting

ring.

The other arm consists of appropriate counter weight for proper rotation.

Loading pin prevents the rotation of arm.

Both arms pivot on this central spindle

Base consists of spring which rotates central

As the metal fills the mould a hydrostatic pressure gradient develops along the length of casting

Ordinarily the pressure gradient at the moment before solidification begins reaches about 0.21 – 0.28 Mpa (30.40 Psi) at the tip of casting.

Because of this pressure gradient, there is also a gradient in the heat transfer rate such that the greatest rate of heat transfer to the mold is at the high pressure end of the gradient (i.e. tip of the casting). Because this end also is frequently the sharp edge of the margin of a crown,

Recovery of a casting

quenched in water as soon as the button exhibits a dull red glow.

Advantages of quenching:- 1) The noble metal alloy is left in an

annealed condition for burnishing, polishing & similar procedures.

2) When water contact with hot investment, a violent reaction ensues. The investment becomes soft & granular & the casting is more easily cleaned

A, Trimming is done from the button end of the ring. B, Investment is being pushed out of the casting ring

C, The mold is broken open. D, Investment is removed from the

casting. Care must be taken to avoid damaging the margin

sandblasting

The casting is held in a sandblasting machine to clean the remaining investment from its surface.

pickling

Surface of casting appears dark with oxides & tarnish. Such surface film can be removed by a process known as pickling

Best method for Pickling is to place the casting in a test tube or dish & pour the acid over it.

May be necessary to heat the acid, but boiling is avoided, because of considerable amount of acid fumes involved.

Pickling solution should be renewed frequently, since it is likely to become contaminated

Precious alloys(Gold-Platinum-

Palladium) can be soaked with hydroflouric acid

Nickel Chromium should never be placed in acid because of high reactivity

TRIMMING

The casting is trimmed , shaped and smooth with a suitable burs or stones.

The sprue is sectioned off with a cutting disc.

POLISHING

Minimum polishing is required if all the procedures from the wax pattern to casting are followed meticulously.

White stone ,rubber wheels, rubber disks, and fine grit are included in the finishing and polishing agents

CASTING DEFECTS

Error in the procedure often results in defective casting

Dimensional inaccuracies &Distortion

Due to distortion of wax

Due to hygroscopic and setting

expansion

Surface roughness and irregularites

Surface roughness –

Cause :silica particle in investment

Inaccurate powder liquid ratio ,too rapid

heating Surface irregularities –nodules or fins

Cause :air bubble attached to pattern ,water film on pattern ,careless removal of pattern Prevention: correct powder liquid ratio,use of

mechanical mixer , use of wetting agent

High W/P ratio . Prolonged heating of the mold cavity . Overheating of the gold alloy . Too high or too low casting pressure . Composition of the investment . Foreign body inclusion

POROSITY May be internal or external . External porosity causes discolouration . Internal porosity weakens the restoration

.

Classification of porosity . I .Those caused by solidification

shrinkage : a) Localised shrinkage porosity . b) Suck back porosity . c) Microporosity . They are usually irregular in shape

. II ) Those caused by gas : a) Pin hole porosity . b) Gas inclusions . c) Subsurface porosity . Usually they are spherical in shape

III ) Those caused by air trapped in the mold :

Back pressure porosity . Localised shrinkage porosity Large irregular voids found near sprue

casting junction.

Suck back porosity It is an external void seen in the inside

of a crown opposite the sprue . Hot spot is created which freezes last . It is avoided by : Reducing the temp difference between

the mold & molten alloy

Microporosity : Fine irregular voids within the casting . Occurs when casting freezes rapidly . Also when mold or casting temp is too

low .

Pin hole porosity : Upon solidification the dissolved gases

are expelled from the metal causing tiny voids .

Pt & Pd absorb Hydrogen . Cu & Ag absorb oxygen

Gas inclusion porosities Larger than pin hole porosities . May be due to dissolved gases or due to

gases Carried in or trapped by molten metal .

A poorly adjusted blow torech can also occlude gases

Back pressure porosity This is caused by inadequate venting of

the mold . This can be prevented by : - using adequate casting force . -use investment of adequate porosity . -place the pattern not more than 6-8 mm

away from tne end of the casting

Incomplete casting This is due to : - insufficient alloy . -Alloy not able to enter thin parts of the

mold . -When the mold is not heated to the

casting temp .

-Premature solidification of the alloy . -sprues blocked with foreign bodies . -Back pressure of gases . -low casting pressure . -Alloy not sufficiently molten

Small casting : occurs when proper expansion is not

obtained & due to the shrinkage of the impression

Contamination of the casting 1) Due to overheating there is oxidation

of metal . 2) Use of oxidising zone of the flame . 3) Failure to use a flux . 4) Due to formation sulfur compounds .

Black casting It is due to : 1) Overheating of the investment . 2) Incomplete elimination of the wax

SUMMARY & CONCLUSION

REVIEW OF LITERATURE

Thomas E.M (1952) conducted studies on hygroscopic and setting expansion of investment and find out that a confined compensating expansion of at least 1.5 % is necessary to compensate for the casting shrinkage of the available inlay gold alloys. Hygroscopic expansion when taking place at 1000º F will compensate for the casting shrinkage and shape of the wax pattern has no influence on the amount of expansion required

Delgado et al (1953) studied hygroscopic expansion of investment and

Stated that: The use of mechanical spatulation or hand

spatulation does not affect the amount of hygroscopic setting expansion, when a water bath at mouth temperature is used.

Mechanical spatulation gives higher expansion values for thick mixes than hand spatulation when a water bath at room temperature is used.

David (1963): investigated the influence of factors on setting expansion and stated that:

Expansion of investment away from the wax pattern is relatively small than the expansion of the investment surrounded by the wax pattern.

Greased and dry asbestos liners tend to decrease effective setting expansion whereas loose and double asbestos tend to increase it.

Use of soft wax results in greater effective setting expansion and Over spatulation and thick mix increase setting expansion.

Robert Neiman and Atul Sarma in 1980 studied the setting and thermal reaction of phosphate investment. They concluded that

The sequence of reaction based on experimental findings were interpreted in terms of chemical and structural presentation.

The simple chemical reaction has been shown to be MgO + NH4 + H2PO4 = NH4MgPO4.6H2O. However the setting reaction is in reality a complex system of multi molecular structure as described.

Alton M. lacy, Hisao Fukui et al in 1983 studied factors affecting investment setting expansion they studied the related effect of mixing rate, ring liner position and storage. Their studies reveled that

-The rate and magnitude of of setting expansion varied directly to the rate of mixing. Although after 24 hrs the rapid mix investment showed reversal of expansion. This was seen with Gypsum bonded investment. No such shrinkage was observed with phosphate bonded investment

Lacy et al (1985) stated that machine mixing under vaccum is more effective than hand mixing in reducing the number of bubbles from investment. They also stated that increasing the mixing time had a little effect on reducing the air bubbles but decrease liquid powder ratio favors reduction of incidence of air bubbles. Debubblizer is effective in reducing air bubble adhering to the surface

Papadopoulos T and Margrette in 1990 studied the heat rate in thermal expansion of phosphate bonded investment. In their study three heating rates were used one too high (15c/min), too low (4c/min), and one in middle (9c/min) from the results it was concluded that rates that were too fast must be avoided in heating procedure. Optimum thermal expansion was found when the heating rate was around 9c per minute

J.E Hutton and G.Marshel in 1995 studied expansion of phosphate bonded investment. The investment were mixed with either distilled water or special liquid and allowed a setting time of 1 or 24 hrs, their study revealed that

1)1 hr is optimum time for achieving complete expansion of investment material after mixing

2) Mixing the material with special liquid increases the setting expansion

REFERENCES ASGAR. K - Further Investigation into the nature

of hygroscopic expansion of dental Casting Investment. J. Prosthet. Dent 1958: 8;678.

DAVID B.M., BRUCEADY - Influence of factors on setting expansion J.Prosthet.Dent 1963; 13:365.

DELGADO V.P. PYTOM F.A. - The hygroscopic Setting Expansion of dental casting Investment. J. Prosthet. Dent 1953; 3-423.

LACY. M. A and MORA. A. incidence of bubbles on sample cast in phosphate bonded investment. J. Prosthet. Dent 1985; 44, 367-369.

THOMAS E.M. - Resume of the expansion required to compensation for casting gold shrinkage. J. Prosthet. Dent 1952; 550-56.

Stephen F. Rosenstiel. Contemporary fixed prosthodontics III Ed. 1995.

Kenneth J. Anusavice. Phillips Science of Dental materials. 11 th Ed. 2003.