CASTING DEFECTS BY DR Noha Mohammed 1 Casting

CASTING DEFECTS BY: DR Noha Mohammed 1

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 DEFECTS DISTORTION SURFACE ROUGHNESS AND IRREGULARITIES POROSITY SURFACE DISCOLORATION INCOMPLETE OR MISSING DETAILS 4

DISTORTION 5

Wax distortion is the most serious problem that can occur during the forming and removal of the pattern from the DISTORTION mouth or die. 6

• 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. 7

SURFACE ROUGHNESS 8

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. 9

SURFACE ROUGHNESS Ratio of binder/ quartz influences surface texture (A coarse silica causes surface roughness. ) 10

SURFACE ROUGHNESS OTHER CAUSES OF SURFACE ROUGHNESS High L/P ratio Prolonged heating or over heating of the mold Surface roughness Premature heating of investment Excess wetting agent 11

SURFACE ROUGHNESS EXCESS L/P RATIO Excess L/P ratio Increased amount of excess water Large number of porosities in the investment Rough casting 12

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. Generalized casting roughness may indicate a breakdown of the. investment from excessive burnout temperature. 13

SURFACE IRREGULARITIES 14

SURFACE IRREGULARITIES: These are isolated imperfections such as nodules that are not characteristic of the entire surface area. Surface irregularities Nodules Water films Fins 15

1. NODULES 16

TYPES OF NODULES Small Nodules Large nodules Multiple nodules 17

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. 18

SMALL NODULES REMEDY: The best method to eliminate the incorporation of air in the casting investment is i) By mixing under vacuum. ii) By using wetting Wetting agent should be appliedagents in thin layer and air-dried because any excess liquid dilutes the investment, possibly producing surface roughness. 19

SMALL NODULES Castings with phosphate bonded investments are more prone to such imperfections. They can be removed with ¼ or ½ round bur. A binocular microscope is extremely helpful in detecting and removing them. 20

LARGE NODULES q Produced by air trapped during investing procedure MULTIPLE NODULES q Inadequate vacuum during investing q Improper brush technique. q Lack of surfactant 21

2. WATER FILMS 22

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. Water film Space around the wax pattern 23

THIS CONDITION OCCURS: - • If the pattern is slightly moved or vibrated after investing • If there is no intimate contact of the investment and pattern • Too high a liquid / powder ratio Water films REMEDY: Use of Surfactant helps prevent such irregularities 24

3. FINS 25

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. 26

REASONS THAT PRODUCE FINS Weak mix of the investment Too rapid heating Improper positioning of the pattern FINS Excessive casting force Premature heating Cooling of investment prior to casting 27

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 FINS the investment, causing breakdown or cracking of the investment if the spacing between patterns is less than 3 mm. 28

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. FINS 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 29

Remedy: Proper positioning of the wax pattern. ØThe casting ring should permit the patterns to be 3 - 6 mm apart. Ø 6 mm from the top of the investment. Ø Minimum 9 mm of investment between them and the ring liner. 30

2 a. RAPID HEATING RATES Too high heating rate of investment outside layer becomes hot faster than the inner layer. FINS 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. 31

2 b. Rapid heating rates A characteristic surface roughness may be FINS evident because of the flaking of the investment when the water/ steam pours into the mold. 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. 32

3. PREMATURE HEATING If setting is not complete at the time a ring FINS is placed in the oven, the mold may be weak and unable to withstand steam pressure during burnout. Investment could fracture as a consequence. Remedy: Burnout should be initiated only after the recommended setting time. 33

4. LIQUID-POWDER RATIO The higher the liquid/powder ratio, the rougher the casting (the investment becomes weak and develop cracks). FINS 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. REMEDY Correct proportion of powder to liquid any dilution of the (special) liquid with distilled water should be established for each 34 alloy.

5. CASTING PRESSURE Reason: Too high pressure during casting causes fins. 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. 35

POROSITIES 36

I. Solidification • Localized shrinkage porosity • Suck-back porosity II. Trapped gases III. Residual air • Pinhole porosity • Back pressure • Gas inclusion porosity Micro-porosity Subsurface porosity 37

I. POROSITIES DUE TO METAL SOLIDIFICATION 38

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. 39

q LOCALIZED SHRINKAGE POROSITY MAINLY OCCURS WHERE SOLIDIFICATION OCCURS LAST. 40

LOCALIZED SHRINKAGE POROSITY Solidifies later Solidifies last Remedy: use of reservoir Alloy that fills the restoration will solidify first. Solidifies first Solidifies last As the molten metal solidifies, it shrinks and creates a vacuum Solidifies later Solidifies first Vacuum will draw additional metal from an 41 adjacent source The reservoir.

LOCALIZED SHRINKAGE POROSITY 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. 42

LOCALIZED SHRINKAGE POROSITY REMEDY # 1 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 (1 mm) 43

Thickness of sprue Maximum thickness of pattern Last to solidify Thickness of sprue Maximum thickness of pattern 44

LOCALIZED SHRINKAGE POROSITY REMEDY # 2 Position the wax pattern in a “cold zone” of the investment mold and the reservoir in the “heat centre” of the casting ring. 45

Sprue base former Sprue Wax pattern Investment Material 46

LOCALIZED SHRINKAGE POROSITY 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 (6 mm) & 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. 47

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. 48

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. 49

Hot spot 50

Impedance to flow 90º Sprue SUCK - BACK POROSITY 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. 51

CK - BACK POROSITY Impedance to flow q. Maximum impedance to Importance of flared sprue 90º flow occurs when a Sprue former makes an angle of 90º to the pattern. Continuity of flow q. The pattern should be placed at 45º Sprue former 52

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 53

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. 54

• 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. 55

II. POROSITIES DUE TO TRAPPED GASES (Gaseous porosity) 56

Introduction to GASEOUS POROSITY Gas or air enter within the molten alloy by either chemical reaction or physical mechanism. q Chemical entrapment – E. g. Gases may be produced by reaction of the liquid metal with volatile substances, such as moisture in the mold. q 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. 57

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 Gaseous porosity PINHOLE POROSITY GAS INCLUSION 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. 58 gas inclusion larger in size compared to pinhole.

GASEOUS POROSITY Remedy Sprue Molten metal 1. Avoid overheating of the alloy 2. Casting in the Displaced air atmosphere of an inert gas or vacuum. 3. Avoid using large sprues. Molten metal Sprue Trapped air 59

Many metals dissolve or occlude gases in their molten state e. g. both copper and silver dissolve O 2 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. 60

NOTE : PIN HOLE POROSITY 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. 61

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. 62

63 GAS INCLUSION POROSITY

III. POROSITIES DUE TO RESIDUAL AIR 64

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 65

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. 66

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 67

BACK PRESSURE POROSITY Dense modern investment Clogging of mold with residual carbon Trapped air Inadequate casting and mould temperature Increased distance between pattern and end of casting ring. 68

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. Gypsum bonded Silica bonded Phosphate bonded • Density of the material may increase with vacuum mixing and low L/P ratio. Note: Silica bonded and phosphate bonded materials very frequently produce incomplete castings. 69

BACK PRESSURE POROSITY REMEDY I 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. 70

BACK PRESSURE POROSITY 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. 71

BACK PRESSURE POROSITY 2. INCREASED DISTANCE BETWEEN PATTERN 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 6 mm the porosity becomes less effective. 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 72

73 BACK PRESSURE POROSITY

BACK PRESSURE POROSITY 3. CLOGGING 3. 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. 74

INCOMPLETE CASTING It is due to inadequate amount of molten metal entering the mould. 75

REASONS OF INCOMPLETE CASTING Incomplete melting of alloy Insufficient alloy Too low casting force Blocking due to pre solidification in sprue Incomplete casting Incomplete dewaxing Poor castability Blocking of sprue due to loose investment particles 76

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 1 sec, yet it is quite soft during early stages therefore pressure should be maintained for few seconds beyond this point). 77

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. 78

INCOMPLETE CASTING REASON 3 HIGH VISCOSITY OF FUSED METALS q An incomplete casting resulting from too great a viscosity is attributed to insufficient heating. q 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. 79

INCOMPLETE CASTING OTHER REASONS 4. Inadequate metal. 5. Cool mold or melt 6. Too thin Wax pattern 80

SURFACE DISCOLORATION 81

SURFACE DISCOLORATION PROLONGED HEATING OF THE MOLD 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. 82

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. Remedy : When thermal expansion technique is employed, the mold should be heated to casting temperature – never higher – and the casting should be made immediately 83

SURFACE DISCOLORATION 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. 84

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. 85

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. 86

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