CASTING PROCEDURE & DEFECTS

CONTENTS

HISTORY BASIC STEPS OF CASTING PROCEDURE SPRUE FORMER AND ITS ATTACHMENT CRUCIBLE FORMERS CASTING RINGS AND LINERS INVESTING PROCEDURE

WAX BURNOUT CASTING OF ALLOY INTO MOLD 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 for making jewellery

1558 – B. Cellini - claimed to have attempted use of wax and clay for preparation of castings

1884 – A. 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.

1959 - Strickland et al stated the importance of the type, shape location & direction other than the size of the sprue

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)

SUMMARY & CONCLUSION

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

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

1.Wax

uses single stage burnout.

More preferred because it melts at the same rate as the pattern, & allow for easy excape for molten wax.

2.Plastic/Resin -

used for castings of alloys which use 2 stage burn out with Phosphate 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) hollow plastic.

3. Metal sprues should be a non-rust metal to avoid contamination of wax.Hollow 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.

Advantages of hollow sprue former: It increases the contact area. It holds less heat than the solid sprue

former.

“Care must be taken to examine the orifice for small particles of investment that may break off while removing the metal sprue former.”

Sprue former diameter:

The diameter and length of the sprue former depends on:- The type and size of the pattern.- The type of casting machine to be used.- The dimensions of the casting rings in which casting

is made.

Pre fabricated sprue former are available in a wide variety of gauge from 6 to 18.

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

Usually for molar and metal ceramic restoration - 10-gauge (2.5mm)

Premolars and partial coverage restoration - 12-gauge (2.0mm)

A narrow sprue may be useful in air pressure casting procedure where the metal is melted in conical depression formed by crucible former. so narrow sprue prevents premature metal flow into mold.

Large diameter sprue: this improves the flow of molten

metal into the mould.

less diameter sprue:

causes localized shrinkage porosity

Sprue former length:

The length of the sprue former - it keeps the wax pattern 6mm from the end of the ring

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

Sprue former position:

The ideal area- point of greatest bulk in the pattern.

The point of attachment should permit stream of metal to be directed to all parts of the mold without having to flow opposite the direction of casting force

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.

Sprue former direction:

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

Attachment morphology:

The attachment of sprue former to the wax pattern should be smooth and do not posses pits or irregularities.

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

TYPES OF ATTACHMENT

Direct Indirect

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.

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

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)

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.

RESERVOIR & ITS IMPORTANCE

Venting

Small auxiliary sprue/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.

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 cone: used with metal when casted using centrifugal casting force.

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.

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

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) Complete rings I) - Rigid - Metal (stainless steel) - Plastic

II) Flexible - Rubber

3) Split rings I) metal II) plastic

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

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

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.

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

1) Asbestosis 2) Bronchogenic lung cancer 3) Mesothelioma – fatal tumour

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

A long cellulose liner is carefully adapted 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 large extent, but its network of fibres can retain small amount of water on its 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

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.

Thin film of cleaner on pattern a)reduces surface tension of

wax b)better “wetting” of wax pattern by the investment The wax pattern should not stand for

more than 20-30 min before being invested.

So, it is best to invest the wax pattern as soon as possible

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.

Advantages of vacuum mixing-----1) Remove air bubbles2) 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

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

Second part

INVESTING OF GYPSUM BONDED INVESTMENT MATERIAL 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

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 is then 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.

when the alpha form is converted into beta form at inversion temp.

density decreases

volume increases, k/a volumetric expansion

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 such that it permits 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 468⁰C for hygroscopic technique the investment 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 castings3) Convenience of placing molds directly at 468ºC Rapid heating can generate steam

that can cause flaking of the mould walls.

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 the tensile stresses in the middle regions of the mold.

Decomposition & alloy contamination is related to a chemical reaction between residual carbon & CaSO4 binder. 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.

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

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 750⁰C-1030⁰C.

.

PBI obtain their EXPANSION by1)EXPANSION OF THE WAX PATTERN- this is considerable because the setting

reaction raises the mold temp substantially.2)SETTING EXPANSION- this is usually greater than gypsum,

especially because special liquids are used to enhance such expansion.

3)THERMAL EXPANSION- this is greater when taken to temp higher

than those used for gypsum investment

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

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 mold

casting force > surface tension of alloy + resistance offered by gas in the mold This can be done by use of following

different type of force- Vacuum force Air or Gas Pressure Centrifugal force

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

1155-1304⁰c

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 generator2) Electricity --

Two type of torch tips:1. Multi-orifice2. Single-orifice

Zones of the blow torch flame:

Zone 1 - colorless zone

Zone 2 – Combustion

zone Zone 3 - Reducing

zone Zone 4 - oxidizing

zone

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 1000⁰c

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 one part of acetylene + 2 and half part of oxygen

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 developed over the surface

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 Zirconia-alumina

Clay crucibles are used with high noble and noble metal alloys

used for crown and bridges.

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 1504⁰c) and 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

Traditionally a wet lining of asbestos sheet was used on casting crucible. The moistened asbestos sheet provides a clean and good surface on which the alloy could be melted.

Advantages is, prevent alloy contamination with oxides and residuals that may be present in the crucible

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

6gm is typically adequate for premolar and anterior casting

10gm is adequate for molar casting 12 gm is adequate for pontic

Electrical source :-

A) Electrical resistance- 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. It used to create a electrical arc at

the end of two electrodes The apparatus requires a high electrical input (30A)

CASTING MACHINES

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

1)Air pressure casting machines:

Alloy is melted in situ in crucible, followed by applied air pressure.

Pressure of 10-15 psi

2. Centrifugal casting machine:

Alloy is melted in a crucible, and forced in to mold by centrifugal force.

It is used to melt ceramic alloys. Here the alloy is automatically melted in graphite crucible.

The crucible in the furnace is always against the casting ring. So the metal button remain molten slightly longer and ensures complete solidification.

3)Electrical resistance - heated casting machine

Direct current is produce between two electrodes: the alloys and the water cooled tungsten electrode.

Temp between the arc rapidly increases to 4000°C – alloy melts very quickly.

High risk of over heating of the alloy.

Damage may occur even after few seconds of over heating.

Direct-current arc melting machine:-

Metal is melted by an induction field that developed with in the crucible surrounded by water- cooled metal tubing.

4. Induction melting machine:

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

Cleaning of the casting

Consider the gold crown & bridge alloys. After casting has been completed, ring is

removed & quenched in water.

Advantages:1.Noble metal is left in an annealed

condition for burnishing & Polishing.2.When water contacts hot investment,

violent reaction ensues. Investment becomes soft, granular & 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 the casting appears dark with oxides and tarnish. Such a surface film can be removed by a process called Pickling.

Best method for pickling is to place a casting in a dish & pour acid over it.

Heat the acid but don't boil it. 50% Hydrochloric acid Sulfuric acid Ultrasonic devices Gold and palladium based metal ceramic

alloys and base metals, these alloys are not generally pickled.

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 smoothen with 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

Third part

CASTING DEFECTS

Error in the procedure often results in defective casting, these defects are known as casting defects.

Classification

According to philips Distortion Surface roughness and irregularities Porosity Incomplete or missing details

Based on location Internal external

According to Rosensteil Roughness

Nodules

Fins

Incompleteness

Voids or porosity

Marginal discrepancy

Dimensional inaccuracies

Distortion

Distortion of the casting is probably related to distortion of the wax pattern.

Causes: Can occur from the time of wax pattern preparation

to the time of investing due to stress relaxation.

Distortion of the wax pattern occurs during the investment procedure.

Minimized by:

Application of minimum pressure Manipulation of wax at high temperature Investing pattern immediately If storage is necessary, store in refrigerator

Surface Roughness and Irregularities

Surface roughness Defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern.

Surface irregularities Isolated imperfections such as nodules that are not characteristic of the entire surface area

The surface roughness of the casting is greater than the wax pattern from which it is made, because

- the particle size of the investment and -its ability to reproduce the pattern in microscopic

detail

Small nodules on the casting are caused by air bubbles, that become attached to the surface during or subsequent to the investing procedure.

Prevented By: Proper investment technique Vibration of mix or by vacuum mixing Application of wetting agent properly and correctly – important that it be applied in a thin layer.

Air bubbles:

Water films:

Wax is repellent to water, & If the Investment becomes separated from the wax pattern, a water film may form irregularly over the surface.

Appears as minute ridges or veins on the surface.

Prevented By: 1.Use of wetting agent2.Correct L/P ratio (Too high L/P ratio may produce

these irregularities)

Rapid Heating Rates

It produces Fins or spines on the casting

Cause- because of Flaking of the investment

Prevented by: Heat gradually at least 60min from room

temperature to 700°c. Greater the bulk – more slowly heated.

Under heating

Incomplete elimination of wax residues may occur, if the heating time is too short.

This factor is mainly important for low heat technique.

Prolonged heating

During high heat technique, decomposition or disintegration of the investment occurs & the walls of the mold are roughened.

Product of decomposition are sulphorous compounds, which contaminates the casting, this is the reason why the surface of the casting does not respond to pickling sometimes.

Prevented by- when thermal expansion technique is used, the mold should be heated to the casting temperature & NEVER HIGHER.

Liquid/Powder Ratio The amount of water and powder measure should be accurate. Too little water- investment too thick & cannot be applied to

the wax pattern Too much water- making investment easier but reproduces

poor casting.

Casting pressure To high pressure – rough surface of the casting To low pressure – incomplete casting Average – 0.10 to 0.14 Mpa in an air pressure machine and - 3 to 4 turns of the spring in centrifugal casting machine.

Foreign bodies Any casting that shows sharp, well- defined

deficiencies indicates the presence of some foreign particles in the mold. They may be:

- Pieces of the investment - Bits of the carbon from the flux- Sulfur components from – decomposition of the

gypsum investment and high sulfur content torch flame.

Pattern position Should not place too close together Should not place many patterns in same plane Space between the pattern is atleast 3mm

Impact of metal alloy

Cause: The direct impact of molten alloy on the weak

portion of the mold surface, may fracture or abrade the mold surface regardless of its bulk.

Prevented by: This type of surface roughness or irregularities can

be avoided by proper spruing. Placement of sprue at 45 degree

Carbon inclusions Carbon from- carbon crucible, - carbon containing investment, - improperly adjusted torch– can be

absorbed by the alloys during casting results in formation of carbides or visible carbon inclusion.

Porosity

Classified as follows:

I. Solidification defectsA. localized shrinkage porosityB. Micro porosity

II. Trapped gases A. pin hole porosity B. gas inclusion porosity C. sub surface porosity

III. Residual air

Localized shrinkage porosity

It is caused by premature termination of the molten metal during solidification.

It mainly occurs at sprue-casting junction.

Cause:

Diameter is too narrow Length of the sprue is too long Absence of reservoir Direction of sprue at 90 degree

Prevented by- Using sprue of correct thickness Attach sprue to the thickest portion of the

wax pattern Flaring the sprue at the point of

attachment Placing reservoir close to the attachment

A hot spot is created by the hot metal impinging on the mold wall near the sprue.

This hot spot causes this region to FREEZE LAST

Since the sprue is already solidified, NO MORE MOLTEN MATERIAL IS AVAILABLE, resulting in

shrinkage k/a SUCK BACK POROSITY

Suck back porosity

It often occurs at OCCLUSOAXIAL OR INCISOAXIAL LINE ANGLE

PREVENTED BY- Flaring the point of sprue attachment Reducing the temperature between the

mold & molten alloy

Pin hole and Gas inclusion porosity

Characterized by spherical contour, but gas inclusion porosities are much larger than pin hole porosity.

Occur primarily because most metals dissolve gases when molten, these gases expelled during solidification..

Eg- copper & silver dissolves oxygen platinum & palladium dissolves hydrogen Also be caused by gas occluded from a poorly

adjusted torch flame or use of oxidizing zone rather than reducing zone.

Casting is usually black, do not clean easily on pickling

Sub surface porosity

Caused by simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at mold walls

Prevented by controlling the rate at which the molten metal enters the mold.

Back pressure porosity

Some times referred to as entrapped-air porosity.

found on the outer surface of the casting when the casting or mold temperature is low, that solidification occurs before the trapped air can escape.

Causes Inability of the air in the mold to escape through the

pores in the investmentPrevented by: Proper burnout Sufficiently high casting pressure Adequate L/P ratio Thickness of investment between tip of pattern and end of ring

is not greater than 6mm.

Incomplete casting

Factors that inhibit the mold filling is:1. In sufficient venting2. In sufficient casting pressure, pressure should be applied

atleast for 4 sec3. Incomplete elimination of wax4. Lower L/p ratio5. Viscosity of the fused metal

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