CASTING DEFECTS Intoduction Casting is defined as something

CASTING DEFECTS

Intoduction Casting is defined as something that has been cast in a mold or an object formed by the solidification of a fluid that has been poured or injected into a mold The precision casting should have the exact shape, size, correct fitting, smooth surface and no internal or external flaws, affecting the properties. There are many chances of introduction of many casting defects in various steps, unless suitable precautions are carefully observed. 2

CLASSIFICATION OF CASTING DEFECTS the various casting defects can be classified as : 1. DISTORTION 2. SURFACE ROUGHNESS, IRREGULARITIES, DISCOLORATION a. Air bubbles b. Water film c. Rapid heating d. Under heating e. L/P ratio. f. Prolonged heating g. Temperature of alloy h. Casting pressure i. Composition of investment j. Foreign bodies k. Impact of molten alloy l. Pattern position m. Carbon inclusions 3

3. POROSITY I. Solidification shrinkage porosities A. Localized shrinkage porosity B. Micro porosity C. hot spot or suck back porosity II. Air inclusion porosities A. Pinhole porosity B. Gas inclusions C. Subsurface porosity 4. INCOMPLETE CASTING 4

1. DISTORTION �Any marked distortion of the casting is related to distortion of the wax pattern � Setting and hygroscopic expansions of the investment may produce a non uniform expansion of the walls of the pattern. �The gingival margins are forced apart by the mold expansion, whereas the solid occlusal bar of wax resists expansion during the early stages of setting. 5

�Configuration of the pattern, the type of wax, and the thickness influence the distortion. �Distortion increases as the thickness of the pattern decreases � Less the setting expansion of investment less the distortion 6

CAUSES : � Wax too hot- Excessive shrinkage results on cooling. � Wax too cool- The pattern undergoes stress release with change in shape. � Insufficient pressure during waxing- The pattern distorts because of thermal shrinkage. �Delayed investment- The sooner the investment is complete, the less distortion � Heating pattern during spruing- Create distortion. � Overheating casting during soldering procedure- This warps or melts the margins. 7

TO AVOID DISTORTION �Appropriate selection of investing material with less setting expansion �Invest the wax pattern as early as possible �Proper handling of the wax pattern 8

2. SURFACE ROUGHNESS, IRREGULARITIES, AND DISCOLORATION �The surface of a dental casting should be an accurate reproduction of the surface of the wax pattern from which it is made. �Excessive roughness or irregularities on the outer surface of the casting necessitate additional finishing and polishing. 9

SURFACE ROUGHNESS DEFINED: Relatively finely spaced surface imperfections whose height, width, and direction establish the predominant surface pattern. PROBLEMS WITH SURFACE ROUGHNESS � Water/powder ratio- A high ratio increases the roughness of the mold. � Excess wetting agent or salivary contamination - This may form a film on the pattern surface and be reproduced on the casting surface. � Prolonged heating or overheating of the mold - may cause investment disintegration, Roughness appears general and feels sharp. � Premature heating of casting investment- Wait a minimum of 45 minutes for burnout. 10

SURFACE IRREGULARITIES �Surface irregularities are isolated imperfections, such as nodules, that are not characteristic of the entire surface area. � Irregularities on the cavity surface prevent a proper seating of an otherwise accurate casting. 11

a) AIR BUBBLES: � Small nodules on a casting are caused by air bubbles that become attached to the pattern during or subsequent to the investing procedure. � Nodules can sometimes be removed not in a critical area. � Nodules on margins or on internal surfaces removal of these irregularities might alter fit of the casting 13

� Inadequate vacuum or ineffective painting procedure. Vacuum must have at least 26 mm mercury for vacuum investing. �Water/powder ratio - Investment is too thick, it will not cover the pattern completely. � Excessive vibration of the ring- Produces small nodules. NODULES 14

TO AVOID AIR BUBBLES �Proper mixing of the investment if manual method is used �Use of a mechanical mixer with vibration both before and after mixing �Vaccum investing technique is the best method. �Use of a wetting agent in a thin layer 15

b) WATER FILMS: � Wax is repellent to water and if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface. � Appears as minute ridges or veins on the surface. �Too high L/P ratios. �Pattern is slightly moved, or vibrated after investing or if the painting procedure does not result in an intimate contact of the investment with the wax pattern. 16

TO AVOID WATER FILMS �Use of a wetting agent in a thin layer �Using Correct water - powder ratio 17

c) RAPID HEATING � Result in formation of fins or spines on the surface of the casting � Due to the flaking of the investment when water or steam pours into the mold � A surge of steam or water may carry certain salts into the mold that are left behind in the walls as the water evaporates 18

FINS ON THE SURFACE OR MARGIN - DUE TO � Prolonged heating- Cracks in the investment that radiate out from the surface of the pattern. � Heating rate is too rapid- Cracks may appear in the investment, caused by nonuniform heating of investment. � Water/powder ratio- A high ratio produces a weak investment that may crack. � Excessive casting pressure- Metal impact may cause investment fracture. �Cooling of the investment prior to casting- Cracks in the investment 19

TO AVOID FINS OR SPINES �Gradual heating of the mold- atleast 60 min should elapse during the heating of the investment- filled ring from room temperature to 700º C. �Greater the bulk of the investment, more slowly it should be heated. 20

d) UNDERHEATING � Incomplete elimination of wax residues may occur if the heating time is too short or if insufficient air is available in the furnace. � Low-temperature investment techniques � Voids or porosity may occur in the casting from the gases formed when the hot alloy comes in contact with the carbon residues. �Casting may be covered with a tenacious carbon coating that is virtually impossible to remove by pickling 21

e)LIQUID/POWDER RATIO � The amount of water and investment should be measured accurately. � The higher the L/P ratio, the rougher the casting. � Too little water Investment thick and cannot be properly applied to the pattern. �In vacuum investing, the air may not be sufficiently removed. TO AVOID �Use the correct W/P ratio according to manufacturer’s instructions. 22

F)PROLONGED HEATING � High-heat casting technique a prolonged heating of mold at the casting temperature disintegration of the gypsum-bonded investment, and the walls of the mold are roughened. �Products of decomposition are sulfur compounds that may contaminate the alloy to the extent that the surface texture is affected. TO AVOID � Thermal expansion technique is employed mold heated to the casting temperature and never higher. �The casting should be made immediately. 23

g) TEMPERATURE OF THE ALLOY � Alloy is heated to too high a temperature before casting, the surface of the investment is likely to be attacked, and a surface roughness may result. � In all probability, the alloy will not be overheated with a gas-air torch when used with the gas supplied in most localities. � If other fuel is used, special care should be observed that the color emitted by the molten gold alloy, for example, is no lighter than a light orange. 24

h) CASTING PRESSURE � Too high a pressure during casting can produce a rough surface on the casting TO AVOID �a gauge pressure of 0. 10 to 0. 14 MPa in an air pressure casting machine (or) � 3 to 4 turns of spring in an average type of centrifugal casting machine is sufficient for small castings. 25

i) COMPOSITION OF THE INVESTMENT � The ratio of the binder to the quartz influences the surface texture of the casting. � A coarse silica causes a surface roughness. � If the investment meets ANSI/ADA specification n 0. 2, the composition is not a factor for surface roughness 26

j) FOREIGN BODIES � When foreign substances get into the mold, a surface roughness may be produced. � Rough crucible former with investment clinging bits of investment are carried into the mold with the molten alloy �Carelessness in the removal of the sprue former �Sharp, well-defined deficiencies pieces of investment and bits of carbon from a flux. 27

�Bright-appearing concavities Flux being carried into the mold with the metal. � Surface discoloration and roughness can result from sulfur contamination � The interaction of the molten alloy with sulfur produces a black or grey layer on the surface of gold alloys that is brittle and does not clean readily during pickling. 28

K) IMPACT OF MOLTEN ALLOY � Direction of sprue former molten gold alloy does not strike a weak portion of the mold surface. � Molten alloy may fracture or abrade the mold surface on impact, regardless of its bulk. TO AVOID � Proper Spruing To prevent the impact of molten metal at an angle of 90 degrees to investment surface. 29

l) PATTERN POSITION � If several patterns are invested in the same ring, it causes breakdown or cracking of the investment if the spacing between the patterns are less than 3 mm. � B/c expansion of wax is much greater than that of the investment. TO AVOID �Do not place several patterns too close together if invested in the same ring �Avoid too many patterns in the same plane in the mold 30

m) CARBON INCLUSIONS � Carbon, as from a crucible, an improperly adjusted torch, or a carbon-containing investment, can be absorbed by the alloy during casting. � May lead to the formation of carbides or even create visible carbon inclusions. 31

3. POROSITY � Porosity may occur both within the interior region of a casting and on the external surface. �The latter is a factor in surface roughness, but also it is generally a manifestation of internal porosity. � Not only does the internal porosity weaken the casting but if it also extends to the surface, it may be a cause for discoloration. �If severe, it can cause plaque accumulation at the toothrestoration interface, and secondary caries may result. �Although the porosity in a casting cannot be prevented entirely, it can be minimized by use of proper techniques. 32

POROSITIES MAY BE CLASSIFIED AS FOLLOWS: I. Solidification defects A. Localized shrinkage porosity B. Microporosity c. hot spot or suck back porosity II. Trapped gases A. Pinhole porosity B. Gas inclusions C. Subsurface porosity III. Residual air 33

LOCALIZED SHRINKAGE POROSITY � CAUSE: Premature termination of the flow of molten metal during solidification. � Linear contraction of noble metal alloys in changing from a liquid to a solid is at least 1. 25%. � SOLUTION: Continual feeding of molten metal through the sprue make up for the shrinkage of metal volume during solidification. �Generally occurs : Near the sprue-casting junction �Alloy or mold temperature is too low -Rapid solidification of the alloy 34

TO AVOID LOCALIZED SHRINKAGE POROSITY �Using sprue of appropriate thickness �Attach the sprue to the thickest portion of the wax pattern �Flare the sprue at the point of attachment or placing a reservoir close to the wax pattern 35

�HOT SPOT: The entering metal impinges onto the mold surface at a point and creates a higher localized mold temperature. � 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. 36

�SUCK -BACK POROSITY: Hot spot causes the local region to freeze last and results suck-back porosity. �Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded. 37

TO AVOID SUCK BACK POROSITY �Flare the sprue at the point of attachment to the wax pattern �Reduce the mold – melt temperature differential, that is lowering the casting temperature by about 30ºC. �With a higher mold temperature, the difference in temperature between the investment located around the sprue and the investment in the area of the pulpal floor of the full crown is decreased. �This decrease helps the molten alloy at the pulpal floor to solidify before the alloy at the sprue 38

MICROPOROSITY � Occurs from solidification shrinkage but is generally present in fine-grain alloy castings when the solidification is too rapid for the micro voids to segregate to the liquid pool. � This premature solidification causes the porosity in the form of small, irregular voids. � Such phenomena can occur from rapid solidification if the mold or casting temperature is too low. 39

PINHOLE AND GAS INCLUSION POROSITIES �Related to the entrapment of gas during solidification. � Both Spherical contour different in size. � The gas inclusion porosities are usually much larger than pinhole porosity. 40

PINHOLE/GAS INCLUSION POROSITY 41

�Many metals dissolve or occlude gases while they are molten. �On solidification, the absorbed gases are expelled and pinhole porosity results. � All castings certain amount of porosity � Porosity should be kept to a minimum adversely affect the physical properties of the casting. �Porosity surface form of small pinpoint holes � Surface is polished, other pinholes appear. � Larger spherical porosities poorly adjusted torch flame, or by use of the mixing or oxidizing zones of the flame. 42

SUBSURFACE POROSITY �Simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at the mold walls. � Short, thick sprue pin- Rapid entry of the alloy causes skin formation; the bulk of alloy pulls away, forming subsurface porosity. � Alloy or mold temperature is too high - The first portion of gold to contact the investment will solidify and form a thin skin. The alloy behind it shrinks during solidification and pulls away, forming small porosities. 43

ENTRAPPED-AIR POROSITY - Occurs on the inner surface of the casting, sometimes referred to as BACK-PRESSURE POROSITY large concave depressions � Caused by the inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces the air pocket toward the end of the investment. 44

� The incidence of entrapped air is increased by - Use of the dense modern investments, - By an increase in mold density produced by vacuum investing, and - By the tendency for the mold to clog with residual carbon when the low-heat technique is used. � Slow the venting of gases from the mold during casting. 45

TO AVOID ENTRAPPED AIR POROSITY �Proper burnout �Adequate mold and casting temperature �High casting pressure �Proper L/P ratio �Thickness of the investment between the tip of the pattern and the end of the ring not greater than 6 mm. 46

4. INCOMPLETE CASTING � Partially complete casting, or perhaps no casting at all, is found. �The obvious cause is that the molten alloy has been prevented, in some manner, from completely filling the mold. �Two factors that may inhibit the ingress of the liquefied alloy are - Insufficient venting of the mold and - High viscosity of the fused metal. 47

48

49

�Insufficient venting, is directly related to the back pressure exerted by the air in the mold. �If the air cannot be vented quickly, the molten alloy does not fill the mold before it solidifies. � In such a case, the magnitude of the casting pressure should be suspected. �If insufficient casting pressure is used, the back pressure cannot be overcome 50

� Furthermore, the pressure should be applied for at least 4 sec. � The mold is filled and the alloy is solidified in 1 sec or less; yet it is quite soft during the early stages. �Therefore the pressure should be maintained for a few seconds beyond this point. �These failures are usually exemplified in rounded, incomplete margins. 51

ROUNDED INCOMPLETE MARGNS 52

�Second common cause for an incomplete casting is incomplete elimination of wax residues from the mold. � If too many products of combustion remain in the mold, the pores in the investment may become filled so that the air cannot be vented completely. � If moisture or particles of wax remain, the contact of the molten alloy with these foreign substances produces an explosion that may produce sufficient back pressure to prevent the mold from being filled. 53