DEPARTMENT OF CONSERVATIVE &ENDODONTICS
CASTING PROCEDURES & CASTING DEFECTS Seminar Presented By
Dr. Krishna Rao Kilaru Post Graduate Student.
S.D.M.COLLEGEOFDENTALSCIENCES&HOSPITAL, DHARWAD, KARNATAKA, INDIA.
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CASTING PROCEDURES CONTENTS
1. INTROUCTION
2. CASTING PROCEDURE a) Burn out Gypsum b) Expansion of the investment Phosphate c) Placement of the molten metal. 3. CLASSIFICATIONS 4. ALLOYS & ITS DESCRIPTION 5. CASTING MACHINES - Torch casting - Electrical resistance heated casting machine - Induction melting machine - Titanium casting - Direct current arc melting machine 6. DEVESTING 7. FINISHING & POLISHING 8. CASTING DEFECTS 9. CONCLUSION 10. REFERENCES
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Introduction Casting metals by the lost wax process has been known in the art and industry for many years. No record exists to indicate exactly when and where this casting procedure developed. All that was known is the idea of making a wax replica of the item to the cast, surrounding this replica with an investment material allowing this material to harden to a solid mass and preparing the mould into which molten metal was poured. In dentistry, the casting procedure was not popular until 1907, when W.H.Taggart introduced this technique of casting machine to the profession before the New York Odontological group. Modern dental practice involves a variety of casting operations ranging from the simplest inlay to all forms of cast restorations. However the sole objective of the casting procedure is to provide a metallic duplication of missing tooth structure with as great accuracy as possible.
Casting procedure The casting procedure includes the burnout for wax elimination, expansion of the investment and placement of molten alloy into the mould. Improper management of investment material on the alloy of this phase of fabrication can produce defects in casting. Once the investment has set for an appropriate period- approximately one hour for gypsum and phosphate bonded investments it is ready for burnout. The crucible former or any of the metal sprue former is carefully removed. Any debris from the ingot area (Funneled opening at the end of the ring) is cleaned with a brush. If burnout procedure does not follow the investing procedure, the investment should not be permitted to dry out. The invested ring is placed in a humidor at 100 % humidity if possible. (i.e. Plastic vials serve as humidors when damp
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piece of paper towel is placed on the bottom. Rehydration of the set investment that has been stored for an extended period may not replenish all of the lost water.
Wax elimination and Heating: Set the mold in the furnace with the sprue hole placed downwards at first, so most of the wax drains out and is eliminated as a liquid. Porcelain tooth placed in the bottom of the oven provides good support for one edge of the ring, allowing molten wax and gases to escape freely. Burn out ovens available are with manual, automatic or fully programmable controls. Cold temperature or room temperature furnaces are used to start the burnout when mold is wet. Water trapped in the pores of the furnace reduces the absorption of the wax, and as the H20 vaporizes, it flushes wax from the mold. The heating should be gradual as 4000c in 20 min and maintain it for 30 min. Next 30 min, increase the temperature to 700oc and maintain it for 30 min. (1 hr 20 min.) The casting procedure should be completed without permitting the mold to cool. Although the investment will expand when reheated, 1. A significant loss in strength will occur. 2. The crown will become smaller 3. Cooling and reheating of the investment can cause the casting to become inaccurate because the refractory and the binder will not revert to their original form (Hysterisis). The temperature for Gypsum bonded investment can be either 500 0 c for hygroscopic technique or 7000 c for the thermal expansion technique. With Phosphate bonded investments the maximum temperature setting range from 7000c to 10300c, depending on the type of alloy selected. The temperature 4
setting is more critical with Gypsum bonded investment than the phosphate bonded type because the Gypsum investments are more prone to decomposition. The sulfur and chlorine in gypsum investments may be liberated and may contaminate the gold alloy. Rapid heating may produce steam which causes the walls of the model cavity to flake and cause cracks in the investment due to uneven expansion. With a quartz investment, the rate of heating should be carefully controlled, since rapid heating will produce a larger casting than will slow heating. For such investments care must be taken to not initiate heating in a preheated oven on one occasion and then use a cold oven for another casting. The differences in the initial burn out temperature can produce variation in the casting. Apparently this variable is not important in the cristobolite investment. Advantages of good burn out: Good burn out provides a very clean mold cavity, which will greatly reduce the chance of alloy contamination.
Alloy used for Dental Use: Noble metals:- Refer to metals with marked resistance to oxidation and chemical reaction. Base metals:- Refer to metals elements that are chemically reactive to their environment. DESIRABLE PROPERTIES OF CASTING ALLOYS:- Cast metals used in dental laboratories must exhibit the following properties. 1. It should be biocompatible. 2. Easy to melt. 3. Easy to carryout casting, brazing (soldering) and polishing.
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4. Little solidification shrinkage. 5. Minimal reactivity with the mold material. 6. Good wear resistance. 7. High strength and sag resistance (metal ceramic alloys). 8. Excellent tarnish and corrosion resistance.
Functioning of alloying elements. Gold: -Contributes to the color. -Tarnish and corrosion resistance (should be at least 16K.) -Ductility -Malleability To have noble properties the alloy should have gold greater than 65 to 80 %. Copper: - Principal hardener - Reduces melting point and fusion temperature of gold. - It gives reddish color to the alloy. - In greater amounts it reduces resistance to tarnish and corrosion. - Maximum content should not be greater than 16%. Silver: - Principal whitener. - Gives increased strength and hardness - Decreases tarnish resistance in large amounts.
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Platinum: - Strengthens the alloy - Raises the melting point. - Has a whitening effect on the alloy - Reduces the grain size. - It is the best hardener in the alloy superior to copper. Palladium: - Greater than 6% produces whiter gold. - Raises the fusion temperature. - Provides tarnish resistance. - Absorbs H2 gas and other gases. It reduces the cost of the alloy as it is less expensive than platinum. White gold alloys may be porous when cast. Other minor additions: Zinc: It is a scavenger for oxygen. Without zinc, silver absorbs O2 during melting, later during solidification; O2 is rejected causing gas porosities. Gallium: It is added to compensate for the decreased coefficient of thermal expansion that results when silver free alloy are used. Elimination of silver is done to reduce the green stain at the porcelain metal interface. Iridium, tin and iron: Help to harden metal ceramic gold – palladium alloys. Rhuthenium and Rhenium: They decrease the grain size. 7
Composition of NPG Cu 79.35% Al 7.8% Ni 4.8%. Base metal composition Ni 71.85% Cr 12.8% Mo 9% Nb 4% Al 2.5% Si 0.5%.
CLASSIFICATION OF DENTAL CASTING ALLOYS According to ADA specification No.5: 1. Type-I (Soft): For restorations subject to very slight stress such as inlays and are easily burnished. 2. Type-II (Medium): For restorations subject to moderate stress such as onlays. 3. Type-III (Hard): For high-stress situations, including onlays, crowns, thick veneer crowns and short-span fixed partial dentures.
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4. Type-IV (Extra hard): For extremely high stress states, such as endodontic posts and cores, thin veneer crowns, long span fixed partial dentures and removable partial dentures. Type I alloys and Type II alloys are Inlay alloys. Type III alloys and Type IV alloys are Crown and Bridge alloys. The development of modern direct tooth colored filling materials has almost eliminated the use of type I – II alloys. Type I and II are often refined to as inlay alloys. Type III and IV are generally called Crown and Bridge alloys. In 1984, the ADA proposed a simple classification for dental casting alloys. Alloy Type High Noble (HN)
Total noble metal content Contains ≥ 40 wt% Au + 60 wt% of noble metal elements (Au + IS + OS + Pd + Rh + Ru).
Noble metal (N)
Contains ≥ 25 wt% of the noble
Predominantly base metal (PB)
metal elements. Contains < 25 wt% of the noble metal elements.
ACCORDING TO MARZOUK: 1. Class-I: Gold and platinum group based alloys. 2. Class-II: Low gold alloys (gold content < 50%). 3. Class-III: Non-gold palladium based alloys. 4. Class-IV: Nickel-chromium based alloys. 5. Castable moldable ceramics.
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DEWAXING OF WAX PATTERN There are two methods depending upon what type of expansion method is used 1. Hygroscopic low heat technique: In this technique, the ring should not be heated for more than 468 0 C, because the desired expansion is already achieved by hygroscopic means and too much of thermal expansion is not needed. The mold is held at this temperature for 60 to 90 min. But still residual carbon may be retained which reduces the ventilation, because of which back pressure porosity results. Therefore in this technique, the door of the furnace should be opened slightly to permit air to enter and provide enough oxygen for elimination of wax. Airtight furnace prevents complete oxidation of wax residues. Indications: The low heat technique is usually used for Gypsum bonded investments with a high gold content. The noble alloys require slightly more expansion. This is obtained by 1. Increasing the temperature of water bath to 40 deg C. 2. Using 2 layers of liners. 3. Increasing the burnout temperature to a range of 600 to 650 deg C.
2. High heat thermal expansion technique: In this technique, the ring is placed in a furnace at room temperature and heating is continued until a temperature of 7000 c is reached.
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The temperature should be raised gradually because 1. Rapid burnout causes flaking and cracking of the mold producing porosity and fins, 2. And also with the increased temperature there will be contamination of gold alloy with sulfur dioxide, which makes them extremely brittle. Although gas furnace can be used, an electric furnace is more easily controlled. The ring should be kept inverted, which permits some of the wax to drain out of the sprue. After casting temperature has been obtained, the casting should be made immediately, because after the heating ring is removed from the furnace, the investment loses heat and the mold contracts. However under average conditions approximately one min can pass without noticeable loss in dimensions.
Gypsum investment: These investments are fragile and require the use of a metal ring for protection during heating. The mold are placed in a furnace at room temperature, slowly heated to 6500 c to 7000 c in 60 min and held for 15 to 30 min at the upper temperature. The Rapid heating has influence on smoothness and its overall dimensions of the investment. - Generates steam initially, which can cause flaking of the mold walls. - Mechanism of crack formation: In such a case, the outside layer of the investment expands more than the center sections. Consequently the outside layer starts to expand thermally, resulting in compressive stress in the outside layer, which counteracts tensile stress in the middle region of the mold. Such a stress distribution caused the brittle 11
investment to crack outwardly from the interior in the form of radial cracks. These cracks in turn produce casting with fins and spines. This condition is especially likely to be present with a cristobolite investment. The low inversion temperature and rapid rate of expansion during the inversion makes it especially important to heat the investment slowly. Investment decomposition and contamination of the alloy: Is generally related to a chemical reaction between the residual carbon and calcium sulfate binder. Calcium sulfate does not decompose unless it is heated above 10000 c. However the reduction of calcium sulfate by carbon takes place more rapidly above 7000. CaSO4 + 4C
CaS +4CO
3CaSO4 + CaS
4CaO+4S02.
This reaction takes place when gypsum investment is heated above 7000 c. in the presence of carbon. The SO2 as a product of this reaction contaminate gold casting and make them extremely brittle. This can be eliminated by complete elimination of wax and avoiding burn out temperature above 7000 c, particularly if the investment contains carbon. Furthermore sulfur gases are generated when gypsum investment is heated above 700 deg C. After the casting temperature has been reached, the casting should be made immediately. Not withstanding all of these precautions and reasons for using a slow burnout technique, the desire for rapid results has led to improved investment formulations. Few gypsum investments are provided with considerable amount of cristobalite are used for a more rapid burnout procedure. In addition, a few investments may be placed directly at 3150 c for 30 min following which very rapid heating to the final burnout 12
temperature. In addition, a few investments may be placed directly into a furnace at the final burnout temperature held for 30 min and cast. Because of the design of the furnace, the proximity of the mould to the heating element, and the availability of air in the muffle, which may affect the size and the smoothness, it is advisable to examine these factors carefully before the casting is made in this manner.
PHOSPHATE BONDED INVESTMENTS: The phosphate-bonded investments obtain their expansion from the following sources. 1. Expansion of the wax pattern: This is considerable because, the setting reaction raises the mold temperature considerably. 2. Setting expansion: usually higher than in gypsum investment because special liquids are used to enhance such expansion. 3. Thermal expansion: This is greater when taken to temperatures higher than those used for gypsum-bonded investments.
Phosphate bonded investments are usually much harder and stronger than gypsum investments Burnout temperature for phosphate bonded investment range from 7500 c to 10300 c. The heating rate is usually slow till 315 0 c, to prevent fracture. After the temperature reaches 4000 c, the rate of heating can be safely increased. It is thereafter held at the upper temperature for 30 min. Most burnout furnaces are now capable of being programmed for heating rates and holding times. The permeability of Phosphate bonded investments is low compared to
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gypsum-bonded investments. Therefore the required casting pressure should be greater than for the gypsum-bonded investments. Because the entire process is time consuming, demand for time saving changes is strong. Again investment manufacturers have attempted to answer this demand, resulting in the availability of some investments that can be subjected to two stage heating more rapidly, placed directly in the furnace at the top temperature, held for 20 to 30 min, and then cast. To save more time, the manufacturers have eliminated the use of a metal ring and liner, the metal ring being replaced with plastic ring that is tapered so that once the investment has set, it can be pushed out of the ring, held for specific time to ensure complete setting, and then placed directly into the hot furnace. The expansion on setting with this method is different from when a lined ring is used. The required expansion is adjusted by varying the liquid concentration. Recovery and cleaning of the castings are difficult when Phosphate bonded investments are used because phosphate bonded investments do not contain soft gypsum products. It usually contains large grains of quartz. In some instances, such as with gold containing alloys, the investment adheres rather tenaciously, usually requiring cleansing in an ultrasonic cleaner. Either the phosphate binder or the silica refractory is soluble in Hcl or H 2SO4. Cold hydrofluoric acid dissolves the silica refractory quite well, but this must be carefully used with other alloys. In fact even dilute hydrofluoric acid should not be used unless the neutralizing solutions are immediately at hand and the clinician is familiar with the correct handling techniques. Once the tissue injury occurs it cannot be reversed with such solutions. Alternative solution such as No- san can be used with greater safety.
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Base metal alloy require light sandblasting usually with fine alumina. Acid should not be used for cleaning base metal alloys
Casting crucibles: Are of four types: 1. Clay 2. Carbon 3. Quartz (Zirconia- alumina)
Clay crucibles: For crown and bridge alloys such as noble and highly noble alloys Carbon crucibles: For high noble alloys for crown and bridge; and for higher fusing, gold based metal ceramic alloys. Quartz crucibles: For high fusing alloys of any type, especially for alloys having higher melting range and are sensitive to carbon contamination. Eg: 1. High palladium alloys for crown and bridge 2. Palladium silver alloys for metal ceramic copings. 3. Nickel/ Cobalt based alloys.
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CASTING MACHINES: 1. Centrifugal casting machine 2. Electrical resistance heated casting machine 3. Induction casting machine 4. Air pressure type 5. Direct current arc melting machine 6. Titanium casting.
The alloys are melted in one of the following ways depending on the available types of casting machines. 1. The alloy is melted in a separate crucible by a torch flame and is cast into the mold by centrifugal force. 2. The alloy is melted electrically by a resistance heating (on induction furnace), then cast into the mold centrifugally by motor or spring action. 3. The alloy is melted electrically by induction heating then cast into the mold centrifugally by motor or spring action. 4. The alloy is vacuum arc melted and cast by pressure in an argon atmosphere.
Centrifugal casting machine: Here the metal is melted by a torch flame in a glazed ceramic crucible attached to the broken arm of the casting machine. -It is spring or motor driven. 16
-The centrifugal casting machine should be given three clockwise turns and locked in position using a pin. -The crucible for heating the alloy is positioned on the casting machine. -Flame torch is adjusted. Oxy-acetylene gas is used for ordinary alloys. Oxygen gas is used for casting metal ceramic alloys. Mechanism of the centrifugal casting machine: 1.The broken arm feature accelerates the initial rotational speed of the crucible and the casting ring, thus increasing the linear speed of the liquid casting alloy as it moves into and through the mold. 2. Once metal has reached the casting temperature, and the heated ring is in position, machine is released and spring triggers the rotational motion. 3. Pressure Gradient at the moment before solidification reaches about 0.21 to 0.28 Mpa at the tip of the casting. 4. Because of the pressure gradient, the greatest rate of heat transfer to the mold is at high-pressure end of the gradient. Advantages: 1. Simplicity of design and operation 2. Opportunity to cast both small and large castings on the same machine. If gold is cast in centrifugal casting machine at proper temperature and force, button will exhibit flat or concave surface with sharp edges. If gold alloy is cast at temperature that is too cold, button appears rounded with dull edges.
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Electrical resistance heated casting machine: In this device, current is passed through a resistance-heating conductor, and automatic melting of the alloy occurs in a graphite or ceramic crucible than use of a torch flame. Advantages: 1. It is especially used for those alloys used for metal ceramic restoration, which are alloyed with base metals in trace amounts that tend to oxidize on overheating. 2. Metal button remains molten slightly longer ensuring that solidification progresses completely from the tip of the casting to the button surface. Caution: A carbon crucible should not be used in melting of High palladium and Cobalt Chromium base metal alloys.
Induction casting machine: An induction field that develops within a crucible that is surrounded by water-cooled metal tubing melts the alloy. The electric induction furnace is a transformer with 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. Once the alloy reaches casting temperature, it is forced into the mold by air pressure, which could be compressed air like CO2 or Nitrogen or vacuum or both. It is more commonly used for melting base metal alloys.
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Direct current arc melting machine: The direct current arc is produced between two electrodes: The alloy and the water-cooled tungsten electrode. The temperature within the arc exceeds 4000 degrees C. and the alloy melts very quickly. This method has a high risk for overheating the alloy and damage may result only after a few seconds of prolonged heating.
Titanium casting: The casting design for titanium casting is similar to that of other dental alloys. A wax pattern is prepared and sprued. Temperature resistant investment should be used such as phosphate bonded and silica and magnesia investments. Problems associated are: 1.It requires relatively complex and expensive equipment. 2. High melting point of titanium of 1671 0 c, when other dental casting alloys have liquidus temperature below 15000 c. 3. Tendency for the molten metal to become contaminated.
It readily
absorbs several gases in molten state. If H 2, O2, N2, are absorbed, mechanical properties are adversely affected. To prevent absorption of gases, it is cast under Argon in vacuum. The Crucibles used are 1. Graphite 2. Water cooled copper crucibles. The casting system forces the metal into mould by using pressure. Used for:- Crown casting & Partial denture framework.
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Fusion of the Noble metal alloy: 1. Is by blow torch 2. By electrical resistance. The fuel used is a combination of 1. O2/ Propane or O2/ Natural gas mixture. 2. Oxygen and air 3. Acetylene gas (high fusion alloys). Adjust O2 pressure at 35 psi to ensure sufficient O 2, supply. Crucibles: AlO2 or quartz crucible with an open slot. The temperature of gas air flame is greatly influenced by 1. Nature of gas 2. Proportions of gas and air in the mixture. Mixing zone: Dark in color Air and gas are mixed here before combustion No heat is present Combustion zone: Surrounds the inner zone Green in color. Here the gas and air is partially burned. This zone is definitely oxidizing in nature and should always be kept away from the molten alloy during fusion.
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Reducing zone: Dimly blue and located just beyond the tip of the green combustion zone is the reducing bone. This is the hottest part of the flame and it should be kept constantly on the alloy during melting. It does not contaminate the alloy. Oxidizing zone: Is the zone in which final combustion between gas and surrounding air occurs. This zone is not used for fusion. Under no circumstances, should this portion of the flame be used to melt the alloy. Not only is its temperature lower than that of the reducing zone, but it also oxidizes the alloy. When the reducing zone is in contact, the surface of the gold alloy is bright, shiny and mirror like. The alloy first appears spongy, and then small globules of fused alloy appear. Then the molten alloy is elliptic with well-rounded edges, yellow orange in color and appears spinning when the flame is moved slightly. At this point the alloy should be approximately 38 to 66 0 c above its liquidus temperature. The casting should be made immediately when the proper temperature is reached. When the oxidizing portion of the flame is in contact with the alloy there is a dull film developed over the surface and underheating is possible. No more than 30 seconds should be allowed to elapse between the times the ring is removed from the oven and the molten alloy is centrifuged into the mold. Undue delays will cause heat loss and resultant mold contraction. The air and gas mixture is adjusted to get a reducing flame, which is used to melt the alloy.
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When the alloy is molten, the casting ring is shifted to the casting machine, which is already wound 3-4 turns. Place the ring on the casting cradle so that sprue hole adjoins the crucible. Slide the crucible against the ring to avoid spilling of the molten metal. Sprinkle flux over the metal to reduce oxides and increase the fluidity of the casting. - Hold the casting arm so that stop pins drop away. - Release the arm and rotate till it comes to rest. This will create the centrifugal force that will cast the metal into the mold. The ring is allowed to cool for 10 min. - Use of flux for gold alloys: - It is desirable to use flux for gold alloys to 1. Minimize porosity 2. To increase the fluidity of the metal 3. Film of flux formed on the surface of the alloy prevents oxidation. Reducing fluxes: reduce the oxide present to free metal and O 2.They are excellent in cleaning the old alloy. Eg; 1. Powdered charcoal 2.Fused borax powder ground with boric acid powder. Boric acid aids in retaining the borax on the surface of the alloy. Flux is added when the alloy is completely melted. Cleaning the casting: (Devesting) After the casting ring has solidified, the ring is removed and quenched in water as soon as the button exhibits a dull red glow Purpose of quenching:
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1. All the intermediate phases are presumably changed into a disordered solution and tensile strength, proportional limit and hardness are reduced and ductility is increased. 2. When water contacts the hot investment, a violent reaction ensues, resulting in soft granular investment that is easily removed. After casting surface often appears black color indicate that the surface were covered with fine particles of carbon residue, such a surface film is removed by a process pickling. Recovery of the casting: Investment is removed and casting is recovered.
Devesting (or) Sand blasting: The casting is held in a sandblasting machine to clean the investment from the surface. The blasting material that may be used may be: 1. Sand shells. 2. Recycled aluminum oxide (50Âľ) with pressure of 100 psi. 3. Garnet. Cleaning the casting: Casting appears dark with oxides and tarnish. A process called Pickling removes such a surface film. Pickling is the removal of oxide residues of carbon by heating the discolored casting in an acid. Gypsum bonded investments quickly disintegrate and elimination of residue is easily accomplished with a toothbrush. Final traces can be removed ultrasonically. Oxides are removed by pickling in 50 % HCl.
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Phosphate bonded investments do not disintegrate and must be forcibly removed from the casting ring. Phosphate bonded investments do not contain soft gypsum products and also the particles include large grains of quartz. The solution for gypsum-bonded investments is 50% HCl. Earlier 50% H2SO4 was used. The H2SO4 is inorganic acid solution and releases gases on boiling which are poisonous. Hence it is replaced with HCl. Advantages of HCl: Aids in removal of residual investment as well as oxide coating. Disadvantages 1. Likely to corrode laboratory metal furnishings. 2. Fumes are health hazard, which are to be vented through a fume hood. Method of pickling: 1. Place the casting in a test tube or dish and pour acid over it. 2. It may be necessary to heat the acid, but boiling should be avoided because of the considerable amount of fumes involved, which are likely to corrode the metal furnishing. After a few moments of heating, the alloy surface normally becomes bright as oxides are reduced. When the heating is completed, the acid may be poured from the beaker into original storage container and the casting is thoroughly rinsed with water. Periodical changing of solution is necessary to avoid excessive contamination. Tweezers with plastic coated ends are used to handle the casting in acid. In no case, the casting should be held with steel tongs so that both casting and the tongs come in contact with pickling solutions. When steel tongs come in
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contact with the electrolyte, a small galvanic cell is created and copper is deposited on the casting where the tong grips it. This copper deposition extends onto the metal and is a future source of discoloration in the area. Very dark casting are generally indicative of an inadequate burnout time of the mould, the discoloration being removed by sulphuric acid. Stubborn discoloration may be removed by nitric acid. Disadvantage: Delicate margins may be melted in the flame. The casting may be distorted by sudden thermal shock when plunged into the acid. A modern and safer method for removing oxides is by use of an ultrasonic cleaner. The casting is placed in a detergent solution in the ultrasonic bath, and ultrasonic vibrations act upon the surfaces of the metal to form bubbles. The vibratory waves remove the oxides and investment from the casting. Ten minutes in the ultrasonic bath is usually sufficient to clean a casting. Other methods: Heating the casting and then dropping into the pickling solution.
Finishing & polishing: Brown or white AlO2 stones or wheels are used to finish.
Cleaning: Using a hemostat, steam or clean framework.
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CASTING DEFECTS - INTRODUCTION - WHAT IS A CASTING - CASTING DEFECTS - SUMMARY AND CONCLUSION - BIBLIOGRAPHY INTRODUCTION:
Castings are an integral part and cornerstone of modern dentistry through understanding to ideally prevent them altogether skinners “with present techniques, casting defects should be the exceptions, not the rule.” WHAT IS CASTING:
Casting is defined as the act of forming an object in a mold. The object formed from this procedure is called a “CASTING.” It is a complex process involving a number of steps where in the molten alloy (or fused ceramic) is forced into the heated investment mold. WHAT ARE CASTING DEFECTS:
Any impressions or irregularities that result in unsuccessful casting which interfere with the fit of the final restoration or its esthetic and mechanical properties – basically classified into 4 categories. 1) DISTORTION 2) SURFACE ROUGHNESS AND IRREGULARITIES 3) POROSITY 4) INCOMPLETE CASTING OR MISSING DETAIL. 26
1. DISTORTION: The state of being twisted out of normal shape or position. The main cause of distortion is related to the distortion of the wax pattern. Firstly, distortion can occur from the time of pattern preparation to the time of investing due to stress relaxation. Thus, as minimum time interval should be given between these procedures. Next, distortion of the wax pattern occurs during the investment procedure. During hardening of the investment. Setting and hygroscopic expansion of the investment can produce non uniform stresses on the pattern. The configuration of the pattern, type of wax and its thickness influence distortion. Eg: Distortion increases as pattern thickness decreases. Less the setting expansion, less the distortion. Generally not a serious problem, not a great deal can be done to control this. Minimize by: - Manipulation of wax at high temperature - Investing pattern within one hour after finishing - If storage is necessary, store in refrigerator Eames W.B.O Neal et al (1978) established that die spacing was one of the most suitable means to compensate for casting variables and improved marginal adaptation. 2. SURFACE ROUGHNESS, IRREGULARITIES & DISCOLORATION
The surface of a dental casting should be an accurate reproduction of the surface of the wax pattern. Excessive roughness or irregularities necessitate additional finishing and polishing whereas irregularities prevent a proper seating of an accurate casting. 27
Surface roughness is defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern. Surface irregularities are isolated imperfections such as nodules that are not characteristic of the entire surface area. Improper technique leads to a marked increase of these defects. They can usually be treated to the following. 1. Air Bubbles: Air bubbles on the wax cause nodules on the casting Prevented By: 1. Proper mixing of investment 2. Vibration of mix or by vacuum investing 3. Application of wetting agent properly and correctly â&#x20AC;&#x201C; important that it be applied in a thin layer. If not in critical areas, nodules can be sometimes removed. However, nodules on margins or internal surfaces might alter the fit of the casting or necessitate recasting. 2. Water films: Wax repels water and if the investment becomes separated from the wax pattern, a water film may form irregularly over the surface. If the pattern is slightly moved, Jarred or vibrated after investing or if the painting procedure does not result in an intimate contact of the investment such a condition may result. This type of surface irregularity appears as minute ridges or veins on the surface. Prevented By: 1.Use of wetting agent 2.Correct L/P ratio (Too high L/P ratio may produce these irregularities).
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3. Foreign Bodies: Foreign substances can produce surface roughness, Example: Bits of investment due to rough crucible former or careless sprue former removal. These can also cause incomplete areas or surface voids. Sharp well-defined deficiencies or bright appearing concavities indicate foreign particles. Sulfur contamination produces a black or grey layer that is brittle and does not clean readily during pickling. 4. 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 mold. 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. Prevented By: 1.Heat the mold gradually i.e. at least 60 min should during the heating of investment form room temperature to 7000 C. 5. Under heating: Incomplete elimination of wax can occur with under heating (particularly with low temp investment techniques). Voids or porosities may occur in the casting form the gases formed when the hot alloy comes in contact with the carbon residues. Occasionally, the casting may be covered with a tenacious carbon coating virtually impossible to remove by pickling. Prevented by: 1.Using correct burnout times and temperatures.
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6. Liquid \ powder ratio. It should be measured accurately. High liquid \ powder ratio – Rougher casting. Low liquid \ powder ratio – investment too thick, will not wet the pattern. In vacuum investing, the air may not be sufficiently removed. All the above result in rough surface on casting 7. Prolonged Heating. With high heat technique, prolonged heating of the mold at the casting temperature may cause disintegration of gypsum bonded investment resulting in rough walls. Further, decomposition results in sulfur compounds that may contaminate the surface – this does not respond to pickling. Prevented By – When thermal expansion (high heat) technique is used, mold should be heated to casting temperature never higher and cast immediately. 8. Temperature of the alloy: Too high alloy temperature may attack the surface of the investment resulting in surface roughness. Prevented by- Chances of overheating are less with Gas – Air torch normally used. If other fuel is used, care should be taken to check molten alloy’s color – Example. molten gold alloy should be no lighter than a light orange. 9. Casting pressure: Too high a pressure can produce a rough surface. Gauge Pressure of 0.10 to 0.14 MPa in an air pressure casting machines or 3-4 turns in a centrifugal casting machine is sufficient. 10. Composition of the investment: Ratio of binder to quartz influences surface texture of casting. A coarse silica causes surface roughness. If investment meets ANSI/ADA
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specification No.2, the composition is probably not a factor in surface roughness. 11. Impact of molten alloy: Molten alloy may fracture or abrade the mold surface on impact. The direction of sprue formed should be such that the molten alloy does not strike a weak portion of the mold surface. Prevention byProper spruing should prevent impact of molten metal at an angle of 90 0. Hence 450 angle is preferred a glancing impact is less damaging with less turbulence. 12. Pattern position: Several patterns if invested together, should be placed at least 3mm apart and positioned in separate planes to avoid cracking and breakdown of investment. 13. Carbon inclusions: Sources of carbon include. 1)Crucible 2)Improperly adjusted torch 3)Carbon containing investment. These particles may lead to formation of carbides or create visible carbon inclusions. 14. Discoloration and roughness during service. 1. Various gold alloys such as solders, bits of wire, and mixtures of different casting alloys should not be melted together and reused as they may have a higher tendency to corrode. 2. Contamination of gold alloy by mercury may result in galvanic corrosion if amalgam is placed near gold alloys due to EMF differential and failure results.
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3. POROSITY: A) Internal
B) External
Porosities can occur both in internally and externally on the casting. External porosity leads to surface defects and defective casting, while internal porosity weaken the casting and also cause discoloration. Thus proper techniques should be used to minimize porosity. Porosities are basically classified as follows: 1) Those caused by solidification shrinkage or defects. (Irregular in shape). I Localized shrinkage porosity II Suck back porosity III Micro porosity. 2) Those caused by Trapped Gases. (Usually spherical in shape) I Pinhole porosity II Gas inclusions III Subsurface porosity. 3) Those caused by residual air trapped in the mold. I Back pressure porosity II Blow hole porosity.
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1) Those caused by solidification shrinkage or defects. (Irregular in shape). I Localized shrinkage Porosity, also called ‘shrink spot porosity’. It is caused by the premature termination of the flow of molten metal during solidification. These are large irregular voids usually found near the sprue – casting junction but may also occur between dendrites when the cooling sequence is incorrect and the sprue freezes before the rest of the casting. This is because linear contraction of metal alloys ranges from 1.25 to 2.2% requiring continual molten metal feed. If the sprue freezes first (normally it should solidify last), then a localized shrinkage results due to inability of metal to flow producing voids or pits. Prevented by: - Using sprue of correct thickness - Attach sprue to thickest portion of wax pattern - Flaring the sprue at the point of attachment or placing a reservoir close to the wax pattern. II. Suck Back Porosity: A variation of shrink spot porosity, this is an external void usually seen in the inside (fitting surface) of a crown opposite the sprue. If a hot spot has been created by the hot metal impinging from the sprue channel on a point of the mold wall, it causes this region to freeze last with the sprue solidified no more molten metal is available and shrinkage results. Often occurs at occluso axial or inciso axial – line angle that is not well rounded. Prevented by: - Flaring the point of sprue attachment -
Reducing the mold melt temperature differential (By lowering casting temperature by about 300 C). 33
III. Micro porosity: This also occurs from solidification shrinkage as fine irregular voids within the casting especially fine grain alloy castings. It is seen when the casting freezes too rapidly for the micro voids to segregate to the liquid pool. Such porosities occur if the mold or casting temperature is too low. They are not detectable easily and it is generally not a serious defect. 2) Those caused by Trapped Gases. (Usually spherical in shape) I. Pinhole porosity These are related to entrapment of gas during solidification. Characterized by spherical contour. Very small in size, Occur primarily because most metals dissolve gases when molten Example: Cu and Ag dissolve oxygen, while Pt and Pd dissolve (hydrogen)H2 and (oxygen) O2. On solidification these are expelled resulting in porosity. Casting severely contaminated with gases is usually black, do not respond to pickling and show layers of pinpoint holes. II. Gas Inclusion Porosity Also spherical voids related to gas entrapment but larger than pin hole type. They may also be due to dissolved gases but are more likely due to gases carried in or trapped by the molten metal. These can also be caused by gas occluded from a poorly adjusted torch flame or use of oxidizing zone rather than reducing zone. Prevented by: - Pre melting the gold alloy on a graphite crucible or block if the alloy is perused and by correct utilization of the torch flame. III. Sub surface porosity Generally caused by simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at mold walls spherical 34
tiny porosities reason not being completely established may be innocuous or particularly evident. Prevented by controlling the rate at which the molten metal enters the mold. 3) Those caused by residual air trapped in the mold. I. Back pressure porosity Seen on the inner surface of casting, it can produce large concave depressions. It is caused by the inability of air in the mold to escape due to inadequate venting through the pores. The entrapment is frequently found in a “POCKET” at the cavity surface of a crown or MOD casting occasionally may even be found on the outer surface. The pattern is as follows. Normally molten metal enters the mold, air is pushed out through the porous investment. If bulk is too great of the investment (Inadequate venting) escape of air is difficult increased pressure in the mold and premature solidification of the metal this result in porous casting with rounded short margins. Incidence is increased by use of dense modern investments, increase in mold density by vacuum investing and tendency of mold to clog with residual carbon in low heat technique. Prevention: -
Proper burnout sufficiently high casting pressure
- Investment of Adequate porosity and L/P ratio – adequate mold and casting temperature. - Providing vents in large castings – importantly, make sure that thickness of investment between tip of pattern and end of ring is not greater than 6mm (around ¼ inch).
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II. Casting with gas blowholes. If any wax residue remains in the mold, it gives off large volumes of gas as the alloy enters the mold. This gas can cause deficiencies and blow hole porosities in the casting. Prevented by - Elimination of wax completely, - adequately burnout with sprue hole facing downwards is done.
4. Incomplete casting. May result from various causes like. - Insufficient alloy used - Alloy not able to enter thin parts of mold - Mold not heated to casting temperature - Premature solidification of alloy - Sprues are blocked with foreign bodies - Back pressure due to gases in mold cavity - Low casting pressure - Alloy not sufficiently molten or fluid This manifests as a partially complete or rarely as no casting at all. Obvious cause is molten alloy has been prevented from filling the mold. This could be because of insufficient venting or high viscosity of fused metal. Insufficient venting is related to casting pressure (should be sufficient and maintained for at least 4s). Also wax residues as explained earlier. Incorrect L/P ratio also is responsible. Different alloys exhibit varying viscosities. Incomplete or insufficient heating below liquidous will permit premature solidification and thus incomplete casting. 36
Miscellaneous: 1. Too bright and shiny casting with short and rounded margins result when Co is formed from wax residues 2. Inadequate compensation for alloy shrinkage or impression material shrinkage result in small casting. 3. Contamination can be due to use of oxidative flame, overheating of alloy, failure to use flux and formation of sulfur compounds. 4. Black casting result from sulfur compounds as also from carbonized wax residues. CONCLUSION: “ A single rotten mango can spoil the whole basket” BIBLIOGRAPHY: 1. Phillip’s Science Materials 2. Restorative Dental Materials – Robert. G. Craig 3. Basic Dental Materials – John J Mannappallil 4. Textbook of Operative Dentistry – Vimal K Sikri 5. Dental Materials – Ferrero.
Thank you
KILARU.
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