CASTING DEFECTS A dent in your Mercedes, a crack in your mobile, a tear in the brand new Louis Philippe, a scar on a beautiful face, a fracture in a tooth, not really desirable right. Nor are casting defects. Lets delve deeper into this phenomenon under following subheadings: 1. Introduction 2. What is casting. 3. What are casting defects 4. Distortions 5. Surface roughness and irregularities 6. Porosity 7. Incomplete or missing detail 8. Summary and conclusion 9. Bibliography
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INTRODUCTION: A mole on the chin, a dimple on the cheek, imperfections have long been considered signs of beauty and individuality. Not so in the field of dental sciences and specifically casting technology. Castings are an integral and vital component of modern dental practice. They form a superior alternative restorative modality the last resort when all else fails. From inlays to onlays to metal copings to full coverage restoration, cast restorations are a corner stone in the dentists armamentarium. The demand for precision is hence understandable and desirable. Modern advances and breakthroughs in casting materials, techniques and procedures have made casting success more predictable and perfection more achievable. However, inadequacies and errors in the various steps and lack of attention to detail and careless manipulation can and will result in defects and deficiencies. A thorough understanding of the underlying mechanics and principles of what causes these imperfections will not only enable us to recognize and manage such problems but ideally avoid them altogether. As Skinner’s quotes with present techniques, casting failures should be the exception, not the rule.
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Firstly, what is casting: Casting is defined as the act of forming an object in a mould. The object formed from this procedure is called a “Casting�. It is a complex process involving a number of steps wherein the molten alloy is forced into the heated investment mould. What are casting defects: Any imperfections or irregularities that result in unsuccessful casting which interferes with the fit of final restoration or its esthetic and mechanical properties. They are basically classified into 4 categories: 1. Distortion. 2. Surface roughness and irregularities. 3. Porosity. 4. Incomplete or missing detail. Let analyse each section in detail: Distortion: The main cause of distortion is related to a distortion of the wax pattern.
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This type of distortion can be minimized or prevented by proper manipulation of the wax and handling of the pattern. Some distortion of the wax pattern occurs as the investment hardens around it, the setting and hygroscopic expansion of the investment may produce uneven movement of the walls of the pattern. Eames W.B. O`Neal et al (1978) established that die spacing was one of the most suitable methods to compensate for casting variables and it ensured improved marginal adaptation yet increasing retention by 25 percent. This type of distortion occur in part from the uneven outward movement of the proximal walls. The gingival margins are forced apart by the mold expansion. Whereas the solid occlusal bar of wax resist expansion during the early stages of setting. The configuration of the pattern, the type of wax, thickness all influence the distortion that occur. E.g. distortion increases as the thickness of pattern decreases, and the less the setting expansion of investment, the less is distortion. There is probably not a great deal that can be done to control this phenomena. However, Grajower R., Lewinstein (1985) found that shrinkage of wax pattern on dies created marginal gap at shoulders and bevels which was attributed to elastic stress in wax. Remodeling of pattern
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margins by heating marginal wax with spatula was found to improve the adaptation of die. Surface roughness, irregularities and discoloration. 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 necessitates additional finishing and polishing
where as
irregularities on the cavity surface prevent a proper setting of an otherwise accurate casting. Surface roughness is defined as relatively finely spaced surface imperfections whose height width and direction establish the predominant surface pattern. Surface irregularities refer to isolated imperfections such as nodule, that do not characterize the total surface area. The difference in the surface roughness of the casting and the wax pattern from which it is made is probably related to the particle size of investment and its ability to reproduce the wax pattern in microscopic detail. Improper technique can lead to a marked increase in surface roughness as well as to the formation of surface irregularities.
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a) Air Bubbles Air bubbles on the wax cause nodules on the casting. They are presented by: i)
Proper mixing of investment.
ii)
Vibration of mix or by vacuum investing.
iii)
Application of wetting agent properly and converting – important it be applied in a thin layer.
If not in critical areas, nodules can be sometimes removed. However, nodules or margins or internal surfaces might alter the fit of the casting or necessitate recasting. Small nodules on a casting are caused by air bubbles that become attached to the pattern during or subsequent to the investing procedure. Such nodule can sometimes be removed if they are not in a critical area. The best method to avoid air bubbles is to use the vacuum technique. If manual method is used, various precautions can be observed. The use of a mechanical mixture with vibration both before and after mixing should be practiced routinely. A wetting agent may be useful in preventing the collection of air bubbles on the surface of the pattern, but it is by no means a certain remedy. It is important that the wetting agent be applied in
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a thin layer. It is best to air dry the wetting agent because any excess will dilute the investment, producing surface irregularities on the casting. b) Water Films Wax is repellant to water, and if the investment becomes separated from the wax pattern in some manner a water film may form irregularly over the surface. This type of surface irregularity appears as minute ridges or veins on the surface. If the pattern is moved slightly jarred or vibrated after investing or if the painting procedure does not result in an intimate contact of the investment with pattern, such a condition may result. A wetting agent is of aid in the prevention of such irregularities. Too high a W : P ratio may also produce these irregularities. This type of surface irregularity appears as minute ridges or veins on the surface: Avoid by: 1. Use of wetting agents. 2. Correct L/P ratio (too high L/P ratio may produce these irregularities).
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c) Rapid Heating rates Too rapid heating may result in fins or spines on the casting. Also, a characteristic surface roughness may be evident because of flaking of investment when steam or hot water enters the mould. Furthermore, a surge of steam or water may carry some of the salts used as modifiers into the mold and these are left as deposits after the water evaporates. Avoid by: 1. Heat the mold gradually – i.e. at least 60 min. (1hr) should elapse during the heating of investment from room temperature to 700°C. 2. The mold should be heated gradually ; at least 60 minutes should elapse during the heating from room temperature to 700°C. 3. The greater the bulk of the investment the more slowly it should be heated. d) Under Heating : Incomplete elimination of wax residues may occur if the heating time is too short or if insufficient air is available in the furnace. It is particularly important with the low-heat technique. Voids or porosity
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may occur in the casting from the gases formed when the hot alloy comes in contact with the carbonaceous residues. Occasionally, the casting may be covered with a tenacious casting that is virtually impossible to remove by pickling. e) Liquid Powder Ratio: The higher the L:P ratio the rougher the casting. However if too little water is used the investment may be unmanageably thick and cannot be properly applied to the pattern. In vacuum investing the air may not be sufficiently removed. In either instance a rough surface on the casting may result. f) Prolonged Heating : When high heating casting technique is used, prolonged heating is likely to cause disintegration of the investment and the walls of the mold are roughned as a result. Further more the products of decomposition are sulfer compounds that may contaminate the alloy to the extent that the surface texture is affected. Such contamination sometimes doesn’t respond to pickling. When thermal expansion technique is employed the mold should be heated to the casting temperature, never higher than 700°C and the casting should be made immediately. g) Temperature of the Alloy : If an alloy is heated to too high a temperature before casting, the surface of the investment is likely to be attacked and a surface roughness result. Special care should be observed
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that the color emitted by the molten gold alloy, for example is no lighter than a light orange. h) Casting Pressure : Too high a pressure during casting produces a rough surface on the casting. A gauge pressure of 0.10 to 0.14 Mpa [15 to 20 psi] in an air pressure casting machine or three to four turns of the spring in an average type of centrifugal casting machine is sufficient for small castings. i) Composition of the Investment The ratio of the binder to the quartz influence, the surface texture of the casting. In addition, a coarse silica causes a surface roughness. If the investment meets ADA specification no.2 the composition is probably not a factor in the surface roughness. j) Foreign Bodies : When foreign substances get into the mold, a surface roughness may be produced. For example a rough crucible former with investment clinging to it may roughen the investment on its removal so that bits of investment carried into the mold with the molten alloy. Carelessness in the removal of the sprue former may be a similar cause. Any casting that shows sharp well defined deficiences indicates the presence of some foreign
particles in the mold such as pieces of
investment or bits of carbon from a flux. Bright appearing concavities may be the result of flux being carried into the mold with the metal.
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k) Impact of Molten Alloy The molten alloy, should not strike a weak portion of the mold surface. Occasionally the molten alloy may fracture or abrade the mold surface on impact regardless of its bulk. Sometimes the abraded area is smooth so that it can not be detected on the surface of the casting. Such a depression in the mold is reflected as a raised area on the casting, often too slight to be noticed yet sufficiently large to prevent the seating of the casting. This type can be avoided by proper spruing so as to prevent impact at an angle of 90째 to surface. A glancing impact is likely to be less damaging and at the same time an undesirable turbulence is avoided. l) Pattern Position : If several pattern are invested in the same ring they should not be too close together. Likewise too many patterns positioned in the same place in the mold should be avoided, the extension of the wax is much greater than that of the investment, causing breakdown or cracking of the investment if the spacing between pattern is less than 3mm. m) Carbon Inclusions : From a crucible, improperly adjusted torch or a carbon containing investment can be absorbed by the alloy during casting. -
May lead to the formation of carbide or even a visible carbon inclusions.
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n) Other Causes : There are certain surface discolorations and roughness that may not be evident when the casting is completed but that may appear during service various gold alloys, solders, bits of wire and mixture of different casting alloys should never be melted together and reused. The resulting mixture would not possess the proper physical properties form eutectic or similar alloys with low corrosion resistance. Discoloration and corrosion may also occur. A source of discoloration often overlooked is the surface contamination of a gold alloy restoration with mercury. Mercury penetrates rapidly into the alloys and causes a marked loss in ductility and a greater susceptibility to corrosion. Thus it is not a good practice to place a new amalgam restoration adjacent to high noble alloy restoration, it also forms a galvanic circuit leading to the breakdown of the anode i.e., amalgam.
Porosity Porosities can occur both internally and externally on the casting. External porosity leads to surface defects and defective castings while internal porosity weaken the casting and may also cause discoloration. Thus proper techniques should be used to minimize porosity. May occur within the interior region of a casting and on the external surface. The later is a factor in surface roughness but also it is generally a manifestation of internal porosity.
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Internal porosity weaken the casting and extends on the surface it may because for discoloration. If severe it may produce leakage at the tooth restoration interface and 54 secondary caries may result. Although the porosity in a casting cant not be prevented entirely, it can be minimized by use of proper techniques. Porosities in noble metal alloy castings may be classified as follows : I. Solidification defects. a. Localized shrinkage porosity. b. Microporosity. II. Trapped gases. a. Pinhole porosity. b. Gas inclusion porosity. c. Subsurface porosity. III. Residual air : 1) Localised shrinkage porosity : Also called shrink spot porosity. It is generally caused by incomplete feeding of molten metal during solidification. The linear contraction of noble metal alloys in changing from a liquid to a solid is at least 1.25%. therefore there must be continual
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feeding of molten metal through the sprue to make up for the shrinkage of feeding of molten metal through the sprue to make up for shrinkage of metal volume during solidification. If the spure freezes in its cross section before this feeding is completed to the casting proper, a localized shrinkage void will occur in the last portion of the casting that solidifies. The porosity in the pontic area is caused by the ability of the pontic to retain heat because of its bulk and because of it is located in the heat centre of the ring. This problem can be solved by attaching one or more small gauge sprues at the surface most distant from the main sprue attachment and extending the sprue laterally within sprue of the edge of ring. These small (auxiliary) sprues ensures that solidification begins within sprues and they act as cooling pins to carry heat away from the pontic. Localized shrinkage generally occur near sprue casting junction but it may occur any where between dendrites where the last part of the casting that solidified was in the low melting metal that remains as the dendrite branches develop. 2) Suck back porosity: This type of void may also occur externally, usually in the interior of a crown near the area of the sprue. If a hot spot has been created by the
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hot metal impinging from the sprue channel on a point of the mold wall. This hot spot causes the local regions to freeze last and result in what is called suckback porosity. This often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded. The entering metal impinge onto the mold surface at this point and creates a higher localized mold temperature in this region that is called a 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 shrinkage void or suck back porosity. Suck back porosity can be eliminated by flaring the point of the sprue attachment and by reducing the mold melt temperature differential, that is lowering the casting temperature by about 30째C. 3) Microporositiy also occur from solidification shrinkage but is generally present in fine grain alloy castings when the solidification is too rapid for the microvoids to segregate to the liquid pool. This premature solidification causes the porosity in the form of small irregular voids. Such phenomenon can occur from the rapid solidification of the mold or casting temperature is too low. It is unfortunate that this type of defect is not detectable unless the casting is sectioned. In many case it is generally not a serious defect.
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4) Pinhole porosity: Both the pinhole and the gas inclusion porosities are related the entrapment of gas during solidification. Both are characterized by a spherical contour, but they are decidedly different in size. The gas inclusion porosities are usually much larger than pinhole porosity. Many metals dissolve or occlude gases while they are in molten state. For e.g. both copper and silver dissolve oxygen in large amounts in the liquid state, molten platinum and palladium have a strong affinity for H2 as well as oxygen. On solidification the absorbed gases are expelled and the pinhole porosity results. The larger void may also result from the same cause but it seems more logical to assume that such voids may be caused by gas that is mechanically trapped by the molten metal in the mold on that is incorporated during the casting procedure. All castings probably contain a certain amount of porosity. However, the porosity should be kept to a minimum because it may adversely affect the physical properties of the casting. The porosity that extends to the surface is usually in the form of small pinpoint holes, when the surface is polished other pinholes appear.
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5) Gas inclusion Porosity: Larger spherical porosities can be caused by gas occluded from a poorly adjusted torch flame, or the use of the mixing or oxidizing zones of the flame rather than the reducing zone. These types of porosity can be minimized by premelting the gold alloy on a charcoal or a graphite block if the alloy has been used before and by correctly adjusting and positioning the torch flame during melting. 6) Subsurface Porosity : Occurs due to simultaneous nucleation of solid grains and gas bubbles at the first moment that the metal freezes at the mold walls. This type of porosity can be eliminated by controlling the rate at which the molten metal enter mold. 7) Entrapped Air Porosity On the inner surface of the casting. Sometimes referred to as back pressure porosity. Can produce large concave depressions. This is 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 via the molten sprue and button. The entrapment is frequently found in a pocket at the cavity surface of a crown or mesio-occlusal distal casting. Occasionally it is found even on the outside surface of the casting when the casting temperature or mold
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temperature is so low that solidification occurs before the entrapped air can escape. The incidence of entrapped air can be increased by the dense modern investments, an increase in mold density produced by vacuum investing and the tendency for the mold to clog with residual carbon when the low heat technique is used. Each of these factors tends to slow down the venting of gases from the mold during casting. Proper burnout an adequate mold and casting temperature, a sufficiently high casting pressure and proper L:P ratio can help to eliminate this phenomenon. Make sure that the thickness of investment between the tip of the pattern and the end of the ring not be greater than 6mm. 8) Casting with gas blow holes: If any wax residue remains in the mold, it gives off large volumes of gas as the alloy enter the mold. This gas can cause deficiencies and blow hole porosities in the casting. To help eliminate wax completely, adequate burnout with sprue hole facing downwards is done.
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INCOMPLETE CASTING This may result from various causes like: a. Insufficient alloy used. b. Alloy not able to enter thin parts of mold. c. Mold not heated to casting temperature. d. Preventive solidification of alloy. e. Sprues are blocked with foreign bodies. f. Back pressure due to gases in mold cavity. g. Low casting pressure. h. Alloy not sufficiently molten or fluid. This manifests as a partially complete or no casting at all. The obvious cause is that the molten alloy has been prevented from filling the mold. This could be because of insufficient venting or high viscosity of found metal. Occassionally a partially complete or perhaps no casting at all is found because that the molten alloy has been prevented in some manner, from completely filling the mold.
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The two factors responsible are: -
Insufficient venting of the mold and
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High viscosity of the fused metal. 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 doesn’t fill the mold before it solidifies. In such a case the magnitude of the pressure should be suspected. The pressure should be applied at least 4 seconds. The mold is filled and solidified in 1 second or less yet it is quite soft during the early stages. These failures have castings with rounded incomplete margins. A second common cause for an incomplete casting is incomplete elimination of wax residues from the mold if too many products of combusion remains in the mold the pores in the investment may become filled so that the air cant be vented completely. If mixture or particles of air remain, the contact of the molten alloy with these foreign substances produce an explosion that may produce sufficient back pressure to prevent the mold from being filled. The rounded margins are quite shiny in some cases because of the strong reducing atmoshpere created by carbon monoxide left by the residual wax.
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A lower L:P ratio of the investment has been associated with less porosity. An increase in casting pressure during casting solves this problem. Different alloy compositions probably exhibit varying viscosities in the molten state, depending on composition and temperatuere. However, both the surface tension and the viscosity of a molten alloy are decreased with an increase in temperature. An incomplete casting resulting from too great a viscosity of the casting metal can be attributed to insufficient heating. Temperature of the alloy should be raised higher than its liquidus temperature so that its viscosity and surface tension are lowered and its doesn’t solidify prematurely as it enters the mold. Such premature solidification may amount for the greater susceptibility of the white gold alloys to porosity because their liquidus temperature are higher thus they are more difficult to melt with a gas air flame. Assuming that the wax pattern is satisfactory, the procedure techniques become a matter of enlarging the mold uniformly and sufficiently to compensate for the casting shrinkage of the gold alloy. Theoretically, if the shrinkage of the wax and the gold alloy are known, the mold can be expanded an amount equal to such shrinkages and the problem is solved. There are variables in the behaviour of the materials involved, especially the wax that cannot be rigidly controlled.
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SUMMARY AND CONCLUSION “A single rotten mango can spoil the whole basket”. Similarly, a single defect can destroy the whole restoration or casting. The best way to handle casting defects is to prevent or eliminate them altogether. Thus a thorough and indepth knowledge of defects, their etiology and prevention and of correct casting procedures will go a long way in avoiding these undesirable imperfections allowing the fulfillment of the dream of every dentist and desire of every individual – the ideal restoration, and perfect rehabilitation yielding patient function and healing.
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BIBLIOGRAPHY: 1. Phillips Science of Dental Materials. 2. Craig. 3. Basic Dental Materials – John J. Mannappalht. 4. Textbook of Operative Dentistry – Vimal K. Sikri.
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