CASTING DEFECTS BY: DR Noha Mohammed.

CASTING DEFECTS BY: DR Noha Mohammed

Casting failures I) No casting DEFECTIVE CASTING

I) No casting Possible causes A. Gate ( sprue) obstruction
B. Molten alloy was not directed in the mold cavity C. Premature solidification of alloy during casting

CAUSES OF DEFECTIVE CASTING
CASTING DEFECTS DISTORTION SURFACE ROUGHNESS AND IRREGULARITIES POROSITY SURFACE DISCOLORATION INCOMPLETE OR MISSING DETAILS

DISTORTION

DISTORTION DISTORTION
Wax distortion is the most serious problem that can occur during the forming and removal of the pattern from the mouth or die. DISTORTION DISTORTION

REMEDY Don’t over heat wax. Place in increments. Never cool the pattern suddenly. Avoid occluding air during manipulation. Carve with sharp instruments. Permit it to attain equilibrium. Invest immediately after it is removed from the die. Place it in the centre of the casting ring.

SURFACE ROUGHNESS

SURFACE ROUGHNESS SURFACE ROUGHNESS:
Defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern. NOTE: The surface roughness of the casting is invariably greater than that of the wax pattern from which it is made. The difference is related to particle size of the investment and its ability to reproduce the wax pattern in microscopic detail. SURFACE ROUGHNESS

Ratio of binder/ quartz influences surface texture (A coarse silica causes surface roughness.)
If the investment meets ANSI/ADA specification No.2,the composition not a factor in the surface roughness.

OTHER CAUSES OF SURFACE ROUGHNESS
High L/P ratio Excess wetting agent Premature heating of investment Prolonged heating or over heating of the mold SURFACE ROUGHNESS .

SURFACE ROUGHNESS EXCESS L/P RATIO Excess L/P ratio
Increased amount of excess water Large number of porosities in the investment Rough casting SURFACE ROUGHNESS

PROLONGED HEATING OF THE MOLD
Prolonged heating of the mold cause disintegration of gypsum bonded investment. As a result, walls of the mold are roughened. REMEDY : When thermal expansion technique is employed, the mold should be heated to casting temperature – never higher and the casting should be made immediately. . SURFACE ROUGHNESS Generalized casting roughness may indicate a breakdown of the investment from excessive burnout temperature.

SURFACE IRREGULARITIES

Surface irregularities
These are isolated imperfections such as nodules that are not characteristic of the entire surface area. Nodules Water films Fins Surface irregularities

1. NODULES

TYPES OF NODULES Nodules Small Multiple nodules Large nodules

SMALL NODULES Small air bubbles can become attached to the pattern during or following the investing procedure. During the casting, the bubble is filled with the casting material (alloy) and is manifested as a nodule (small) These nodules if present on the margins or on internal surface might alter the fit of the casting, if removal of these irregularities is attempted. But if they are present in some non-critical area they can be removed easily.

REMEDY: The best method to eliminate the incorporation of air in the casting investment is i) By mixing under vacuum. ii) By using wetting agents SMALL NODULES Wetting agent should be applied in thin layer and air-dried because any excess liquid dilutes the investment, possibly producing surface roughness.

Castings with phosphate bonded investments are more prone to such imperfections.
They can be removed with ¼ or ½ round bur. SMALL NODULES Because of the liberation of ammonia gas as a by product A binocular microscope is extremely helpful in detecting and removing them.

LARGE NODULES MULTIPLE NODULES Inadequate vacuum during investing
Improper brush technique. Lack of surfactant Produced by air trapped during investing procedure

2. WATER FILMS

Wax is repellent to water, if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface. This type of irregularity appears as minute ridges or veins on the surface. Space around the wax pattern Water film

THIS CONDITION OCCURS:-
If the pattern is slightly moved or vibrated after investing Water films If there is no intimate contact of the investment and pattern Too high a liquid / powder ratio REMEDY: Use of Surfactant helps prevent such irregularities

3. FINS

Fins occur when cracks are produced in the investment that radiate out from the surface of the pattern . Molten alloy flows into the cracks forming thin fins on the casting. Significance : Finning increases the time required for to finish the casting and if the defects occur in critical areas (e.g. near the crown shoulder) can result in a need to re-cast.

REASONS THAT PRODUCE FINS
Excessive casting force Premature heating Weak mix of the investment Too rapid heating Cooling of investment prior to casting Improper positioning of the pattern

1.PATTERN POSITION a. Positioning of several patterns too close and in the same plane in the mold lead to formation of fins. Reason : The expansion of the wax is much greater than that of the investment, causing breakdown or cracking of the investment if the spacing between patterns is less than 3mm. FINS These factors tend to slow the venting of gases from the mold during casting.

b. Patterns placed too near the edge of the investment causes fins.
Reason If too little investment covers the wax patterns, the alloy is more likely to break through the mold. c. Too much investment over the wax ups may locate the wax patterns too close to the heat centre of the mold and impair the escape of gases FINS

Remedy: Proper positioning of the wax pattern.
The casting ring should permit the patterns to be 3- 6 mm apart. 6mm from the top of the investment. Minimum 9mm of investment between them and the ring liner.

FINS 2a. RAPID HEATING RATES
Too high heating rate of investment  outside layer becomes hot faster than the inner layer. Outside layer tends to expand more than the inner parts. However, the outside layer is held back by the inner, cooler part. Outside layer is subject to compressive stresses, while the inner part is subject to tensile stresses. Since the investment is a brittle material, it tends to crack under tensile stresses. During casting, these cracks are filled by the casting alloy, manifesting as fins or spines. FINS

2b.Rapid heating rates A characteristic surface roughness may be evident because of the flaking of the investment when the water/ steam pours into the mold. FINS Remedy Ideally, 60 min should elapse during the heating of the investment filled ring from room temperature to 700º C The greater the bulk of the investment, the more slow it should be heated.

3. PREMATURE HEATING If setting is not complete at the time a ring is placed in the oven, the mold may be weak and unable to withstand steam pressure during burnout. Investment could fracture as a consequence. FINS Remedy: Burnout should be initiated only after the recommended setting time.

FINS 4. LIQUID-POWDER RATIO
The higher the liquid/powder ratio, the rougher the casting (the investment becomes weak and develop cracks). If too little water is used the investment unmanageably thick cannot be properly applied to the pattern in vacuum investing and air may not be sufficiently removed leading to back pressure porosity. FINS These factors tend to slow the venting of gases from the mold during casting. REMEDY Correct proportion of powder to liquid and any dilution of the (special) liquid with distilled water should be established for each alloy.

5. CASTING PRESSURE Reason: Too high pressure during casting causes fins. REMEDY Casting should provide enough force to cause the liquid alloy to flow onto the heated mold. Adjust the casting machine to the requirements of each alloy. Lower-density metals generally need four winds of a centrifugal casting arm as compared to higher-density, gold based alloys. Don't over wind. A gauge pressure of 0.10 to 0.14 MPa in an air pressure casting machine is sufficient. FINS

POROSITIES

I. Solidification Localized shrinkage porosity Suck-back porosity Micro-porosity Subsurface porosity II. Trapped gases Pinhole porosity Gas inclusion III. Residual air Back pressure porosity

I.POROSITIES DUE TO METAL
SOLIDIFICATION

1. LOCALIZED SHRINKAGE POROSITY (SHRINK-SPOT POROSITY)
Linear contraction of noble metal alloys in changing from liquid to solid – 1.25% Therefore continued feeding of molten metal through the sprue must occur to compensate for casting shrinkage i.e. shrinkage during solidification.(Insufficient feeding causes porosity) It usually occurs if the sprue solidifies before the casting Cause: Premature termination of the flow of metal during solidification. Liquid state shrinkage  occurs when molten metal is converted into solid metal. It is compensated by continuous flow of the metal from the reservoir till the metal in the casting completely solidifies. Solid state shrinkage occurs after the metal solidifies and during its cooling from mold temperature to room temperature. This is compensated by NSE+HSE+THERMAL expansion.

LOCALIZED SHRINKAGE POROSITY MAINLY OCCURS WHERE SOLIDIFICATION OCCURS LAST.

LOCALIZED SHRINKAGE POROSITY
Remedy: use of reservoir Solidifies last Solidifies first Solidifies later Alloy that fills the restoration will solidify first. As the molten metal solidifies, it shrinks and creates a vacuum Vacuum will draw additional metal from an adjacent source  The reservoir. LOCALIZED SHRINKAGE POROSITY Solidifies first Solidifies later Solidifies last

Occurs usually near the sprue-casting junction. And may occur anywhere between dendrites where the last portion of the casting solidify, mainly the bulkiest portion of the casting, i.e. the sprue pattern junction. LOCALIZED SHRINKAGE POROSITY

REMEDY # 1 LOCALIZED SHRINKAGE POROSITY Attach the large reservoir in the sprue of thickness more than the thickest portion of the pattern and as close as possible to the pattern (1mm) so that the last portion of the liquid, solidifying is in contact with liquid in reservoir, which supplies extra liquid required.

Thickness of sprue Maximum thickness of pattern Thickness of sprue Maximum thickness of pattern Last to solidify

REMEDY # 2 LOCALIZED SHRINKAGE POROSITY Position the wax pattern in a “cold zone” of the investment mold and the reservoir in the “heat centre” of the casting ring.

Sprue base former Sprue Wax pattern Investment Material

The coolest parts of the mold (cold zones) are the end of the ring and along the ring periphery. The hottest portion of the casting ring is located near the centre of the ring (heat centre). Limit the amount of investment covering the patterns to no more than ¼ inch (6mm) & position the reservoir in the heat centre. So the proper length of sprue is important to keep the pattern away from heat centre close to the end of the casting ring. LOCALIZED SHRINKAGE POROSITY

REMEDY # 3. Do not cast a button if a connector (runner) bar, or other internal reservoir, is used with indirect Spruing, the largest mass of metal should be the reservoir. A button is counterproductive because it can draw available molten alloy from the bar, shift the heat centre and reduce the feed of that metal to the restorations. Likewise, the wax pattern should not be larger than the connector bar. LOCALIZED SHRINKAGE POROSITY

2. SUCK - BACK (HOT-SPOT)POROSITY
Localized shrinkage may also occur in the fitting surface of the crown near the area of the sprue. Occurs often on occlusoaxial / incisoaxial line angles that are not well rounded and also when the sprue is attached at right angles to the pattern. Turbulent flow of metal

Hot spot Pathways for the flow of metal should be smooth, gradual, and without impediments. Eliminate sharp turns, restrictions, points, or impingements that might create turbulence and occlude air in the casting. Restrictions, or constrictions, can accelerate the metal's rate of flow and abrade the mold surface (

The entering metal impinges on to the mold surface at this point and creates a higher localized mold temperature at this region known as “Hot Spot”. A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified. This in turn creates a shrinkage void or suck back porosity. Impedance to flow Sprue 90º SUCK - BACK POROSITY

Importance of flared sprue
Maximum impedance to flow occurs when a Sprue former makes an angle of 90º to the pattern. The pattern should be placed at 45º Impedance to flow Sprue former 90º SUCK - BACK POROSITY 45º Sprue former Continuity of flow

3. MICRO-POROSITY Micro-porosity voids are irregular in shape.
These voids occur from rapid solidification if mold or casting temperature is too low. This defect is not detectable unless casting is sectioned. Note : Occurs from solidification shrinkage but is generally present in Fine Grain Alloy Castings

4. SUBSURFACE POROSITY When the molten metal comes in contact with the low temperature mold, the outer layer coming in contact with the mold wall solidifies suddenly and makes a skin of solid metal which is tenaciously adherent to the mold wall. When the inner layer shrinks ,the outer covering of solid metal cannot be dragged along with it. This leads to subsurface porosity.

Hot tears Sulphur contamination from investment materials makes grain boundary weak. So during cooling of casting, due to difference in COTE value between investment and casting alloys, cracks are produced along the grain boundary. These are called hot tears.

II. POROSITIES DUE TO TRAPPED GASES (Gaseous porosity)

Introduction to GASEOUS POROSITY
Gas or air enter within the molten alloy by either chemical reaction or physical mechanism. Chemical entrapment E.g. Gases may be produced by reaction of the liquid metal with volatile substances, such as moisture in the mold. Physical entrapment Mechanically trapped gas. E.g. Air may be entrapped in the casting by the sudden rush of metal during pouring. Since the gases are generally more soluble in liquid metal than in solid, dissolved gases may be liberated during solidification.

GASEOUS POROSITY On cooling the alloys liberate these trapped gases.
But some remain trapped when the alloy becomes rigid. This type of porosity may affect all parts of the casting. Types of gaseous porosity PINHOLE POROSITY GAS INCLUSION POROSITY Gaseous porosity GASEOUS POROSITY Note: Both these porosities are related to the entrapment of gas during solidification and are characterized by spherical contour but size is varied i.e. gas inclusion larger in size compared to pinhole.

GASEOUS POROSITY Remedy 1. Avoid overheating of the alloy
Displaced air Molten metal Sprue 1. Avoid overheating of the alloy 2. Casting in the atmosphere of an inert gas or vacuum. 3. Avoid using large sprues. GASEOUS POROSITY Trapped air Molten metal Sprue

Many metals dissolve or occlude gases in their molten state e. g
Many metals dissolve or occlude gases in their molten state e.g. both copper and silver dissolve O2 in large amount in liquid state. Molten platinum and palladium have a strong affinity for hydrogen as well as oxygen. On solidification of metal absorbed gases are expelled resulting in pinhole porosities. PINHOLE POROSITY

PIN HOLE POROSITY NOTE :
All castings contain certain amount of porosity, but they should be kept minimum as they will adversely affect the physical properties of the casting. Castings that are severely contaminated with gases are usually black when removed and do not clean easily on pickling. PIN HOLE POROSITY

GAS INCLUSION POROSITY
The larger spherical voids are caused by gas that is mechanically trapped by the molten metal in the mold or by gas that is incorporated during the casting procedure. The gas could be occluded from poorly adjusted torch flame or by use of mixing or oxidizing zone of flame rather than reducing zone.

GAS INCLUSION POROSITY
. These can be minimized by pre melting the gold alloy on graphite crucible or a graphite block, if the alloy has been used before.

III. POROSITIES DUE TO RESIDUAL AIR

Introduction Air present in the mold cavity pushed is out by molten metal. Presence of air in the cavity will not allow the metal to flow Air in mold escape by By pressure gradient Escape through pores in the investment

BACK PRESSURE POROSITY
Back pressure affects are caused by an inability of air or other gases within the mould to escape, making a way for the alloy. EXPLANATION : As the liquid enters the mould through the sprue, the air trapped in the mould is compressed at the extremities, which can exert back pressure preventing the alloy liquid to occupy this region.

Note : A casting which has been subjected to back pressure is rounded at the edges and lacking in detail Rounded edges due to back pressure

CAUSES OF RESIDUAL AIR Trapped air Dense modern investment Increased distance between pattern and end of casting ring. Inadequate casting and mould temperature Clogging of mold with residual carbon BACK PRESSURE POROSITY Low mold temp results in metal solidification before the air escapes.

FACTOR # I. DENSE MODERN INVESTMENT BACK PRESSURE POROSITY
Increased density α Inadequate porosity in the investment to vent out trapped air. Investment materials in the increasing order of density and decreasing order of porosity. Density of the material may increase with vacuum mixing and low L/P ratio. Gypsum bonded Silica bonded Phosphate bonded BACK PRESSURE POROSITY Note: Silica bonded and phosphate bonded materials very frequently produce incomplete castings.

REMEDY I BACK PRESSURE POROSITY When using silica bonded or fine grained phosphate bonded investments, a vent sprue former , 0.5 mm in diameter, should be provided to allow escape of gases towards the crucible end of the mold.

REMEDY II The end of the ring should not be completely covered by any part of the casting apparatus. In all cases the plate of metal that supports the end of the ring should be perforated . Dense layer of investment material is often created at the base of the ring, particularly when the base of the ring has been closed temporarily by a sheet of metal or glass. This dense layer should scraped away to facilitate the escape of gases. BACK PRESSURE POROSITY

2. INCREASED DISTANCE BETWEEN PATTERN BACK PRESSURE POROSITY
AND END OF CASTING RING Even though, Gypsum bonded investment is porous, if the thickness of investment at the extremity of pattern is more than 6mm the porosity becomes less effective. BACK PRESSURE POROSITY Remedy : To assist the escape of gases, the investment materials between the casting and end of the ring should be as thin as is consistent with strength

METHOD TO ENHANCE VENTING OF GASES BACK PRESSURE POROSITY

3. 3. CLOGGING OF MOLD WITH RESIDUAL CARBON It is advisable to begin the burnout procedure while the mould is still wet. Water trapped in the pores of the investment reduces the absorption of wax. As the water vaporizes it flushes wax from the mold. BACK PRESSURE POROSITY

INCOMPLETE CASTING It is due to inadequate amount of molten metal entering the mould.

REASONS OF INCOMPLETE CASTING
Incomplete melting of alloy Incomplete dewaxing Poor castability Blocking due to pre solidification in sprue Blocking of sprue due to loose investment particles Insufficient alloy Too low casting force Incomplete casting

INCOMPLETE CASTING REASON 1 INSUFFICIENT VENTING OF MOLD:-
Directly related to back pressure exerted by the air in mold. If insufficient casting pressure is applied the back pressure cannot be overcome, therefore pressure should be applied for 4 seconds. (The mold is filled and the alloy solidifies in 1sec, yet it is quite soft during early stages therefore pressure should be maintained for few seconds beyond this point). INCOMPLETE CASTING

INCOMPLETE CASTING REASON 2
INCOMPLETE ELIMINATION OF WAX RESIDUE:- If too many products of combustion remain in the mold, the pores of the investment become clogged and air cannot be vented properly. Contact of molten metal with wax or moisture produces an explosion that may produce sufficient back pressure to prevent the mold from being filled Castings seen are generally shiny with rounded defects. INCOMPLETE CASTING

INCOMPLETE CASTING REASON 3
HIGH VISCOSITY OF FUSED METALS An incomplete casting resulting from too great a viscosity is attributed to insufficient heating. Temperature of the alloy should be raised higher than its liquid temperature so that its viscosity and surface tension are lowered and so that it does not solidify prematurity as it enters the mold. INCOMPLETE CASTING

INCOMPLETE CASTING OTHER REASONS 4. Inadequate metal.
5. Cool mold or melt 6. Too thin Wax pattern INCOMPLETE CASTING

SURFACE DISCOLORATION

PROLONGED HEATING OF THE MOLD SURFACE DISCOLORATION
When high heat casting technique is used prolonged heating of the mold cause disintegration of gypsum bonded investment and products of decomposition are sulphur compounds that contaminate the alloy to the extent that surface texture is affected. Such contamination do not respond to pickling. SURFACE DISCOLORATION .

Surface discoloration also results from high sulphur content of flame torch. Interaction of molten alloy with sulphur black/Grey layer on the surface of gold alloys that is brittle and doesn't clean readily during pickling. SURFACE DISCOLORATION Remedy : When thermal expansion technique is employed, the mold should be heated to casting temperature – never higher – and the casting should be made immediately

CARBON INCLUSIONS CARBON AS FROM: A crucible An improperly adjusted torch Carbon-containing investment Can be absorbed by the alloy during casting May lead to formation of carbides or even create visible carbon inclusions. SURFACE DISCOLORATION

Conclusion By standardizing technique & paying strict attention to each step, it is often possible to control the location of the solidification shrinkage & minimize the number of actual miscasts. When casting failures occur, we should troubleshoot each miscast to diagnose the cause of the problem so corrective measures may be taken before we make additional casting.

REFERENCES Kenneth J. Anusavice. Phillips’ science of Dental Materials. eleventh edition, Saunders company. John M. Powers, Ronald L. Sakaguchi. Craig’s Restorative Dental Materials. Twelfth edition, mosby.

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CASTING DEFECTS BY: DR Noha Mohammed.

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