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CASTING PROCEDURE AND DEFECTS  INTRODUCTION  STEPS 1) WAX PATTERN FABRICATION 2) THE SPRUE FORMER • MATERIAL • DIAMETER • LENGTH • NO. OF SPRUES • LOCATION OF SPRUES • ANGULATION OF SPRUE FORMER • SPRUE – WAX PATTERN JOINT • RESERVOIR • VENTING 3) REMOVAL OF WAX PATTERN AND SPRUE FORMER FROM DIE OR TOOTH 4) FORMING THE CRUCIBLE AND ATTACHING THE PATTERN 5) SURFACE TREATMENT OF WAX PATTERN 6) CASTING RING 7) CASTING RING LINER • MATERIAL • DRY LINER Vs WET LINER • NUMBER OF LINERS • LENGTH OF LINER 8) INVESTMENT OF THE WAX PATTERN • INVESTMENT METHODS • PRECAUTIONS TO BE TAKEN • POROSITY OF THE INVESTMENT • REPRODUCTION OF DETAILS • PRINCIPLE OF INVESTING 9) BURN OUT /THERMAL TREATMENT OF THE INVESTMENT • HYGROSCOPIC LOW HEAT TECHNIQUE • THERMAL HIGH HEAT TECHNIQUE 10) CASTING / INJECTION MOLDING • FUSING METAL ALLOY • CARRYING INVESTMENT TO CASTING MACHINE • FORCING METAL ALLOY INTO THE MOLD • CASTING MACHINES  AIR PRESSURE  VACUUM  CENTRIFUGAL 11) SPECIAL CASTING SITUATIONS 12) QUENCHING 13) RECOVERY OF CASTING 14) SAND BLASTING 15) PICKLING AND CLEANING THE CASTING


CASTING PROCEDURE AND DEFECTS INTRODUCTION: The objective of the casting process is to provide a metallic duplication of missing tooth structure, with as much accuracy as possible. The first procedure in the casting of an inlay or crown for the lost wax process is the preparation of a wax pattern. The cavity is prepared in the tooth and the pattern is carved, either directly in the tooth or on a die that is a reproduction of the tooth and the prepared cavity. If the pattern is made within the tooth, it is said to be prepared by the direct technique. If it is prepared within a die, the procedure is called the indirect technique. The wax replica must be perfected in terms of occlusion, contact, contour, anatomy and marginal adaptation. The wax pattern is invested in the appropriate investment for the restorative cast material. The pattern is then evaporated (through burn-out) and the space created by its loss within the investment is completely filled with the wetting cast material. The resulting casting will be the final restoration. Each material has dissimilar components, properties, behaviour and method of manipulation. Despite that, the final product mismatch or discrepancy should not exceed 0.05 – 0.1%. The procedure for the fabrication of the casting will be : 1. Wax pattern fabrication 2. Sprucing and surface treatment of the completed wax pattern. 3. Investment of the wax pattern 4. Burn-Out / Thermal treatment of the investment – wax pattern complex. 5. Casting or injection molding 6. Cleaning the casting from the investment and surface deposits. 1. WAX PATTERN FABRICATION: The A.D.A. Specification No.4 for dental inlay casting wax covers 2 types of inlay wax : Type I is for medium wax employed in direct techniques, and Type II is for soft waxes used in the indirect techniques. Inlay wax is the preferred wax for preparation of inlays or crown, mainly because it does not leave any residue in the walls of the mold during the burn out procedure. It burns out, forming carbon which is later eliminated by oxidation to volatile 2


gases. The other properties which makes it desirable is its colour which contrasts with the die material, or the tooth, smooth texture when it is softened, its rigidity, dimensional stability and ease of carving. However the pattern is prepared, it should be an accurate reproduction of the missing tooth structure. The wax pattern forms the outline of the mold into which the gold alloy is cast. Consequently, the casting can be no more accurate than the wax pattern, regardless of the care observed in subsequent procedures. Therefore, the pattern should be well adapted to the prepared cavity and properly carved, and the distortion should be minimized. Before the adaptation of the wax pattern within a tooth or a die, a separating medium must be used to ensure the complete separation of the wax pattern without distortion. THE SPRUE FORMER: The purpose of a sprue former or sprue pin, is to provide a channel through which molten alloy can reach the mould in an invested ring after the wax has been eliminated. Purpose of Spruing the Wax Pattern: i)

To form a mount for the wax pattern.

ii)

To create a channel for elimination of wax during burnout.

iii)

To form a channel for the ingress of molten alloy during casting

iv)

To compensate for alloy shrinkage during solidification.

1) SPRUE FORMER MATERIAL: The sprue former material should not rust or react with the environment or else the byproducts formed can react with the restorative materials or can act as dislodged pieces or foreign bodies. Most commonly, 3 types of sprue formers are available i.e. resin, wax and metal. a) Wax and Resin Sprue Formers: Advantages: • Easily burnable (melting point is lower or equal to that of wax pattern/. Therefore it can be easily removed). • Low thermal conductivity – They transmit a minimal amount of heat to the pattern with minimal possible distortion before evaporation. Disadvantages: 3


• Lacks strength/ rigidity. b) Metal Sprue Former: • Can be supplied in solid or hollow (tube) forms Advantages: 1) Increased strength and rigidity 2) In case of hollow sprue former, the tube will hold less heat (than solid sprue former), therefore less heat transfer to the wax pattern. Therefore decreased distortion to wax pattern. 3) In case of hollow tubes – Increased retention (in order to increase retention, and decrease thermal conductivity, sticky wax can be placed inside hollow tube). Disadvantages: 1) Must be mechanically removed prior to burnout. Therefore it can cause stress and deformation of the sprue walls or the wax pattern and also loosen some investment.  Sprue former surfaces should be very smooth or else the irregularities present can become incorporated in the stream of melt, leading to obliteration of some details or creation of internal voids.  The type of sprue former selected influences the burnout technique used. If plastic sprue formers are used, a 2-stage burnout technique is carried out to ensure complete carbon elimination, because plastic sprues soften at temperatures above the melting point of inlay waxes. (i.e. MP of plastic higher than MP of wax). 2) SPRUE FORMER DIAMETER:  The sprue former diameter should be approximately the same size as the thickest area of the wax pattern.  A sprue former with larger diameter is preferred than with small dia compared to wax pattern because : Advantages: Acts as a reservoir therefore continuous supply of molten alloy. Disadvantages: Distortion of wax pattern if large sprue dia attached to smaller wax pattern.  The dia of the sprue is the most important factor in determining the speed with which the molten metal enters and fills the mould.  Velocity is directly proportional to the diameter of sprue.  If increased speed is required, then junction between sprue and wax pattern should be flared (larger dia). 4


 If decreased speed is required, the junction should be constricted (decreased dia).  The speed of filling the mould should not be faster than the speed of evacuation of the gases and air from the mould, since this can create a mixture of cast materials and gases in the mould with a subsequent porous or incomplete casting.  Generally the sprue former will be between 8-18 gauge, 8 being 3 mm in dia ,10 gauge being 2.6 mm in dia,12 gauge 2 mm in dia,18 being 0.8 mm in dia. 3) SPRUE FORMER LENGTH  Sprue former length depends on length of the casting ring.  The length of the sprue former should be adjusted such that the distance between the wax pattern and the open end of casting ring is 6 mm for gypsum bonded and 3-4 mm for phosphate bonded investment.  This allows for minimum thickness of the investment that can both withstand melt impact, and also it allows for the escape of the mould gases. If these gases are not completely eliminated, porosity may result.  3-4 mm is sufficient for phosphate bonded investments because of its higher strength and least porosity.  If distance is less than 3-4 mm then molten metal may lead to blast within the investment.  If distance is more than 6 mm then the gases cannot escape rapidly hence leading to casting defects. 4) NUMBER OF SPRUES :  If the wax pattern is designed such that attachment of the sprue former at the bulkiest portion allows the metal to flow uninterrupted from the sprue to the farthest end of the mould, then a single sprue is adequate.  However, if the wax pattern has a thin area between the sprue and the periphery of the pattern, the melt will solidify at the reduced cross-sectional area, preventing the complete filling of the mould. Here 2 sprues can be used.  If multiple sprues are used, they should join together at the crucible former level in a reservoir larger in diameter than all the sprues combined. 5) LOCATION OF SPRUES:  The sprue former should be attached to the bulkiest part of the wax pattern because ; i)

It decreases the effect of released residual stresses by the heat of attaching the sprue. 5


ii)

Continuous supply of the molten metal.

iii)

Flow from thicker to thinner area, therefore thin areas will also be filled completely.

 Sprue formers should be attached to the least anatomical area in the wax pattern i.e. no grooves, cuspal anatomy, fossae or ridges. The proximal surfaces are the ideal location for the sprue attachment. 6) ANGULATION OF THE SPRUE FORMER:  The sprue former should be directed away from any thin or delicate parts of the pattern, because the molten metal may abrade or fracture investment in this area and result in a casting failure.  An angle of 45o to the proximal wall is considered appropriate.  If it is directed at 90o to flat area or broad surface ; i)

The melt may impinge the mould surface and can create a concavity in this area opposite to the sprue. The mould concavity will be reproduced as a convexity in the restoration, preventing its seating and making the restoration rock.

ii)

‘Reverse flow’ / turbulence of the melt, is formed which decreases the melts ability to fill the mould.

7) SPRUE WAX PATTERN JOINT:  The joint should be smooth and uninterrupted.  If high velocity ingress of the melt into the mould is required, the sprue wax pattern joint should be flared, with smooth uninterrupted surfaces. By being flared, there will be greater surface area, thereby insuring a fluid melt and unobstructed flow.  If low velocity ingress is required, a constricted junction should be made. Therefore shear stresses are introduced in the melt passing at the constriction, thereby decreasing the viscosity of the melt and making it more wetting.  To minimize stress release while joining a sprue former to a wax pattern, a drop of sticky wax should be applied to the wax pattern. Then the sprue also with a bead of sticky wax, is brought into contact with the sticky wax on the wax pattern and held immobile until the sticky wax is solidified.  The flare shape of the junction also insures the retention of the sprue in the wax pattern. 8) RESERVOIR :  It is made around 1-2 mm from the pattern. 6


 The purpose of the reservoir is to provide molten metal to prevent localized shrinkage porosity.  When molten metal strikes against the mould wall at 90 o and if the mold metal temperature difference is more, it creates a hot-spot, the molten metal in this area will solidify last. Leading to localized shrinkage porosity, if there is no continuous supply of molten –metal.  The reservoir provides the moltent metal to compensate for this localized shrinkage porosity at the casting sprue junction.  Because of its large mass of alloy and position in centre of ring, it remains molten to furnish liquid alloy into the mould as it solidifies.  If the sprue-former diameter is greater than the area of the cross sectional area of the pattern, there is no need for providing reservoir. The sprue will itself provide the molten metal. 9) VENTING :  In some situations, there is some doubt about the speed with which the mold gases will escape relative to the speed the melt is entering.  The causes could be due to ;  Thickness of the investment walls surrounding the pattern.  A high density of the investment  The use of wax pattern material whose residue may clog the investment pores.  Wax pattern with a lot of minute details or thin cross-sectional areas that are difficult for the melt to wet.  Thus, a vent is prepared i.e. a wax rod is attached to the furthest, or close to the furthest part of the wax pattern, which will stop short of the ring surface (like a sprue former).  Gases that will not escape fast enough ahead of the ingressing melt will be compressed and trapped in these vents.

REMOVAL OF WAX PATTERN AND SPRUE FORMER FROM THE DIE OR THE TOOTH:  Stresses will be present during the removal of wax pattern from the die or tooth.  Therefore to decrease stresses ; i)

Use a sprue former for removing the pattern. 7


ii)

Use 2 fingers i.e. thumb and forefinger on both sides of the pattern and gently remove it.

iii)

A staple, may be attached to the furthest ends of the wax pattern by sticky wax (the staple may be made by wire of the alloy is used for the cast restoration).  This procedure is especially done for MOD cavity preparation.  The staple is slowly removed from its centre ensuring that stresses are applied evenly on both sides of the wax pattern.  Once the wax pattern is removed from the die or tooth, a pair of tweezers is heated and is held over the staple. The transmitted heat will free the staple from the wax.

iv)

small loop of gold zephyr wire is used to remove the wax pattern .loop is created and then inserted into the wax pattern and then wax pattern is removed.

10)FORMING THE CRUCIBLE AND ATTACHING THE PATTERN :  The crucible formers come ready made.  It may be made of rubber, metal or plastic.  It is funnel shaped, bulky flared and smooth.  The wax pattern with its sprue former is attached usually in a special locking area. This area will be covered with wax. It is heated slightly the sprue former is fixed here.  This junction of the crucible former and sprue former is flared and smoothened out. 11)SURFACE TREATMENT OF WAX PATTERN:  Casting waxes are hydrophobic materials. Therefore it repulses water and investment liquids. It cannot be wetted properly.  In order to be wetted properly, these repulsive forces should be counteracted. Therefore soaps, cleansing solutions or diluted synthetic detergent is applied over the wax pattern. So that it decreases the surface tension, and increases the surface – energy or wettability, thus enabling intimate contact of the investment mix to the wax pattern.  The excess liquid is shaken off. Then it is left to dry. CASTING RING:  The metal used in the construction of a ring should be non-corrodible, hard with a thermal expansion similar to the investment used.  Stainless steel is the material of choice because its thermal expansion is 12% at 700 oC which is compatible with the expansion of the investment. 8


 The average diameter of the casting ring is 29 mm in diameter, and 38 mm in height.  It is also available as X, XX, XXX  For 1-2 casting inlays, single crowns, post core

1x

x

3x

(XXX)  3 unit bridge

6x

 Long span bridge

9X

 Long span bridge

12)CASTING RING LINER :  A resilient material placed inside the casting ring which will provide a buffer of pliable material against which the investment can expand to enlarge the mold. Function of Liner: a) For mold expansion: If there is no liner present, the investment is in direct contact with the walls of the mold and will not be able to expand outward (because of the resisting action of the walls of the ring) and so will expand in the direction providing less restriction i.e. towards the centre of the mold thus resulting in distortion of the casting. b) Thermal insulator: When the casting ring is transferred from the furnace to the casting machines, it reduces loss of heat. c) Separating media: Permits easy separation of the investment from the ring after the casting is over. SELECTION OF LINER: A) Material of Liner:  Earlier, asbestos was used. But it had carcinogenic properties, and caused asbestosis, bronchogenic carcinoma etc.  Alternatives to asbestos liners were cellulose and ceramic materials.  Cellulose  Are absorbent materials which absorbs water. Therefore they must be wetted before use.  Disadvantages: It gets burned/charred during burnout.  Ceramic: Are nonabsorbent. Therefore can be used dry.  Studies have shown that fibres from ceramic lining material can dissolve quickly in lung tissues and so have little harmful effect.  Disadvantages: When using ceramic liners, vacuum investing should not be done because in vaccum ceramic absorbs water. B) Dry Liner vs Wet Liner: 9


 Wet liner: Increase in normal setting expansion + Semi-hygroscopic expansion, but it increases W/P Raito, therefore decreases the thermal expansion.  Dry liner: Gives greater expansion than wet liner.  However, for consistency sake, a damp liner is preferred over a dry liner.  After the line has been placed in the ring the ring is dipped in water for 10 seconds and then gently shaken to remove excess water (It should not be squeezed). C) Number of Liners:  2 liners – Greater semi-hygroscopic expansion + Normal setting expansion. 1 liner – semi-hygroscopic expansion No liner – Decreased expansion.  The thickness of the liner should not be less than 1 mm. D) Length of Liner:  The desired length of the liner remains a matter of controversy.  If the length of the liner is shorter than the casting ring itself, the investment cannot expand laterally at the ends of the ring. In the central portion of the ring, it expands laterally to a limited extent. Thus the mold cavity is distorted.  Some feel that liners which are flush with the open end of the ring gives maximum expansion.  But when cellulose liners are used, they burn out before the casting is made. This deprives the investment of support by the ring which could cause cracking of the investment. Therefore 3.25 mm of the casting ring is left unlined at each end to support the investment.  Moreover, some feel that the expansion of the investment is always greater in the unrestricted (longitudinal) direction rather than in the lateral direction i.e. towards the ring itself. Therefore to reduce the expansion in the longitudinal direction, the liner should be placed 3-4 mm short of the end of the ring. Note: The liner should be placed 3-4 mm short of the end of the ring for better uniform expansion and to decrease distortion of the wax pattern. Ringless Casting: Casting ring made of plastic or cellulose (casting ring not used during burnout).  Investment poured into the casting ring. 10


 Once the investment sets, it is pushed out of the ring. Then it is kept for burnout.  This provides better thermal expansion and is usually done for cast partial denture. INVESTMENT OF THE WAX PATTERN:  As mentioned earlier, the wax pattern is cleaned from debris or grease, before it is positioned in the ring by a cleanser or detergent. This will decrease the surface tension and increase the wettability of the wax pattern.

Therefore it ensures the complete

coverage of the wax pattern by the investment. INVESTMENT METHODS: i)

Manual investing

ii) Vacuum investing

i) MANUAL INVESTING:  Water and powder may be incorporated in the appropriate ratio (indicated by manufacturer) in a rubber bowl with a hand spatula : (Water taken in case of gypsum investments and colloidal silica special liquid in case of phosphate investments).  Mixing is done for 1 minute. The mix is placed on a vibrating table to remove any entrapped air.  The investment is then carefully applied to the pattern, using a small brush, and the entire wax pattern is completely covered.  The casting ring is then placed on the crucible and the entire investment is vibrated into the ring. ii) VACUUM INVESTING:  In this technique, the investment is mixed and the pattern is invested under vacuum in vacuum machines.  The investment and water is measured as usual.

The mix is placed in a specially

constructed mixing unit.  The unit is motor driven and the air evacuated by means of a tube attached to the mixing unit from a vacuum pump. During mixing, air is evacuated. Therefore decreased air bubbles formation.  In vacuum investing of pattern  it decreases porosity of the investment. Therefore smoother surface of casting. 11


PRECAUTIONS TO BE TAKEN DURING INVESTING: 1) Air bubbles might remain in the mix, even with vacuum mixing. Therefore slightly tilt the ring. The air bubbles rise to the surface and thus can be eliminated. 2) Avoid excessive vibration of mix. a) Solids in investment will settle down leading to free water accumulation next to wax pattern. Therefore surface roughness. b) Sprue former may get dislodged  can result in miscast. POROSITY OF THE INVESTMENT:  Mold gases should easily leave, then only molten alloy can be cast and can fill the entire mold within the investment. One major factor is the pores of the investment. FACTORS WHICH AFFECT THE POROSITY OF THE INVESTMENT i)

Refractory Silica: If increased refractory silica content  Increased porosity.

ii)

Increased W/P ratio: If increased W/P  Increased number, size and distance between the pores.

iii)

Thickness of mold walls: Increased thickness of mold walls : More distance for gases to escape  Therefore slow escape.

iv)

Depending on type of investment : Gypsum investment : Most porous

v)

Silica investment

: Porous

Phosphate

: Least porous

Excessive vibration : Can increase / decrease porosity of investment.

REPRODUCTION OF DETAILS: Factors which affect the reproduction of details of wax pattern: i)

Investing in vacuum: Increased reproduction of details.

ii)

Investing with vibration: Increased reproduction of details

iii)

Correct viscosity of the investment: Increased reproduction of details.

iv)

Surface treatment of the wax pattern with a wetting agent: Increased reproduction of details.

PRINCIPLES OF INVESTING (As given by Marzouk): 1) Choice of the ring  For hygroscopic expansion  Rubber casting ring is used. 12


 For thermal expansion  A metal ring is used.  The diameter and length of the ring should be selected to allow for the desired dimension of the mold walls.  For moldable ceramics using injection moulding technique A special two part flask of standard size is used. 2) Use of ring liner: For investment expansion.  For investment expansion 3) Assembling the ring and crucible former  The ring is placed within the crucible former in such a way that the wax pattern is in the middle of the casting ring. 4) Preparation of the investment mix  Use water or indicated liquid for the mix.  Vacuum mixing is the most preferred  To decrease voids. 5) Adapting the investment mix to the wax pattern and filling the ring with the investment  Mix is poured gently over the wax pattern under light vibration. 6) Facilitating the setting of investment through hygroscopic expansion.  If maximum hygroscopic expansion in investment is needed  immerse the setting investment in water bath.  If less than maximum expansion needed  specific amount of water added (controlled water – added technique)  Investment left to harden for atleast 45 minutes. 7) Removal of the crucible former and sprue former  It is detached from the set investment. BURN-OUT / THERMAL TREATMENT OF THE INVESTMENT / WAX ELIMINATION AND HEATING:  After investing, and leaving the casting ring aside for 1 hour, burnt out procedure is carried out.  The investment should not completely dry out. To prevent drying, it is placed in humidor at 100% humidity.  It should be slightly wet because the water present within the investment decreases the absorption of wax and when it vaporizes it flushes out the wax. 13


3 Objectives of Burnout: i)

Complete elimination of wax.

ii)

To increase the temperature of mold and investment comparable to fusion temperature of cast alloy.

iii)

For thermal expansion of investment and wax-pattern mold for compensation shrinkage.

 Burn out is done in time – temperature controlled oven.  There should be only gradual heating of the investments because ; 1) Investments are poor conductors of heat  Unequal heating can cause cracks. 2) Residual water in investment  if it vaporizes faster then the escape of gases  causes increase pressure  leading to explosion, cracks etc. 3) For even expansion of mold.  Invested rings placed at room temperature furnace.  For gypsum bonded investments  For hygroscopic (low heat) technique – 468oC  For thermal (high heat) technique – 650o C  Avoid heating gypsum – bonded investments at high temperature, because they are prone to investment decomposition.  For phosphate bonded investments  maxillary temperature range 700oC – 870oC. During Burn Out Procedure: a) Water present in pores of investment, prevents absorption of wax within mold. Also due to high temperature, the water vaporizes and flushes out the wax from the mold. b) Invested ring is held in such a way that the sprue hole faces down so that wax can easily flow down. c) In cases of high heat technique  wax will decompose and form CO or CO 2 and these gases will escape through the pores in the investment. HYGROSCOPIC LOW-HEAT TECHNIQUE:  This technique involves the immersion of the casting ring with the investment in water bath at 37oC.  Compensation of shrinkage is by 3 means. i)

At 37oC water bath, the wax pattern expands. It is kept in water bath for 30 mins and 30-45 mins after the pouring of the investment. 14


ii)

Since placed in water bath, the water enters the investment and provides for hygroscopic expansion.

iii)

During burn out procedure at 468oC  provides for thermal expansion of investment.

Factors Controlling Hygroscopic Expansion: i)

Water / Powder Ratio: Increased water/ powder  Decreased expansion.

ii)

Time of immersion  The longer the delay of immersion in water bath  Decreased expansion.

iii)

Amount of water added  Controlled amount of water added  Increased expansion (controlled water technique).

Controlled Water Technique: If specific amount of water added to the setting investment, a specific amount of expansion results. RECOMMENDED TEMPERATURE: 5000C FOR 60 MINS

Advantages of low heat technique:  Less degradation of mold.  Cooler surface for smoother castings.  Molds can be placed directly in 500oC furnace. Disadvantages of Low Heat Technique:  Allow sufficient burnout time (at least 60 minutes), because the wax is more slowly oxidized and eliminated at low temperature. If sufficient time is not given, residual fine carbon may be retained which reduces the venting of the mold causing BACK PRESSURE POROSITY.  Never keep the furnace so air tight or else it creates a reducing atmosphere, thereby preventing oxidation of wax residues. Therefore keep the door slightly open so that air enters and oxidizes the wax residues. Modification of Hygroscopic Expansion Technique for High Gold Content Alloys:  Increased water bath temperature to > 40oC  43oC for inlays 15


 44 – 45oC for crowns  2 layers of liners  Increased burnout temperature to 600oC to 650oC THERMAL (HIGH HEAT) TECHNIQUE:  This approach depends upon almost entirely on high heat burnout to obtain the required expansion, while at the same time eliminating the wax pattern.  Additional expansion may occur by slightly heating the gypsum investment on its setting, (therefore wax pattern expands), and a small amount of hygroscopic expansion (from the wet liner). A) GYPSUM INVESTMENTS:  They are relatively fragile  High heat technique requires heating the investment slowly to 650oC to 700oC in 60 minutes and holding it for 15-30 minutes at this high temperature.  Rapid heating causes :  Cracking of investments. Here the outside layer of the investment becomes heated before the centre sections. Therefore the outside layer starts to expand thermally, resulting in compressive stresses in the outside layer that counteracts the tensile stresses in the middle regions of the mold. This causes CRACKS in the investment. These cracks in turn, produce a casting with FINS or SPINES.  Chemical Reaction : Between the residual C and CaSO 4 binder. If gypsum investments (which contains carbon) heated above 700oC. : CaSO4 + 4C  CaS + 4CO 3 CaSO4 + CaS  4 CaO + 4SO2 i)

SO2 produced  Contaminates gold castings and makes them very brittle.

ii)

S produced  contaminates gold castings and makes them rough as it disintegrates the investments.

 Modification of gypsum investments. PHOSPHATE INVESTMENTS: Expansion is by 3 sources: i)

Expansion of the wax pattern (during setting reaction of the investment  exothermic heat). 16


ii)

Setting expansion  Because of the special liquid (COLLOIDAL SILICA) that is added.

iii)

Thermal expansion: When it is heated at higher temperature between 750-1050 oC.

Advantages: 1) Phosphate investments are harder and stronger than gypsum investments. Disadvantages: 1) They are brittle investments and have unequal expansion.  Burnout temperature range is 750oC to 900oC Why?  Because for complete elimination of wax.  All chemical and physical reaction will be complete.  To prevent premature solidification of molten alloy.  First slowly heat the investment to 315 oC and then rapidly heat it to 750 oC to 900oC and hold it at this high temperature for 30 minutes. TIME ALLOWABLE FOR CASTING: For low heat technique  Casting should be done quickly. For high heat technique  a short period can elapse that is 1 min or less should be given for casting because if more time is given investment will loose its heat and mould contracts, also there will be the variations between the temperatures of investment and molten metal thus molten metal will solidify faster leading to casting defect. CASTING / INJECTION MOLDING: Objectives: To fill the mold with cast material as completely, efficiently and quickly as possible. 3 basic steps: A) Fusing the metal alloy B) Heated investment should be carried to the casting machine C) Forcing / Casting the metal alloy into the mold. A) Fusing The Metal Alloy : i)

Proper Energy source – Gas fuels, Electric source.

GAS FUELS: 17


a) Natural gas + air  supplies the lowest temperature of all sources. Used for small inlays, and type I and type II alloys. Temperature produced by this fuel is 2680oC. b) Natural gas and O2  supplies higher temperature used for extensive alloys. Eg. alloys used for construction of PFM restorations. Temperature produced by this fuel is 28500C. c) Acetylene and O2 mixture: Supplies the highest temperature that is 31400C.  Too hot for gold alloys  Therefore used for melting base metal alloys like CO-Cr, Ni-Cr alloys  used for RPDs.  Co-Cr, Ni-Cr alloys have high fusion temperature. ELECTRIC SOURCE: By electric melting units, the alloy is melted. Advantages : Less skill is required than that for a torch. Disadvantages : No control over the heating and cannot judge if the alloy is in the proper condition to be cast. Electric Units

 Induction system  Resistance heating system

The temperature to melting alloy is adjusted at less than 1000 oC (for pure Au melting point is 1063oC. If mixed with other metal melting point is < 1000oC). a) Induction System :  Induction heat is the most efficient and popular method of melting an alloy.  The metal is melted by an induction field that develops within a crucible surrounded by water cooled metal tubing.  The molten metal is then forced into the mold by air pressure, vacuum or both. Advantages : Faster melting of alloy than that of torch. Disadvantages : Careful monitor or else overheating of the alloy. Electrical Resistance Heated Casting Machine : Here there is an automatic melting of the metal in a graphite crucible within a furnace rather than use of a torch flame. Disadvantages : Longer time of melting the alloy than that of torch. Advantages :  Prevents oxidation of metals on overheating. 18


 The crucible in the furnace is located flush against the casting ring therefore the metal button remains slightly longer, again ensuring that solidification progresses completely from tip of casting to the button surface. ii) Proper Container :  The container in which the cast metal is fused or softened, should be unaffected by the heat and should not contaminate the cast material. Casting Crucibles used are :  Clay : For crown and bridge alloys Eg. high noble alloys.  Carbon or Graphite : For crown and bridge alloys Eg. high noble alloys. fusing gold based metal ceramic alloys.  Quartz : Crown and bridge alloy having high periodontal content. (Eg. for metal-ceramic copings).  Pd-Ag alloys  Ni-based alloys  Cobalt based alloys iii) Proper Environment for Casting :  Reducing zone of the flame  Applying little amount of fluxes Reducing zone of the flame : The parts of the flame are divided into 4 zones. 1st Zone :

 Cool mixing zone  O2 and gas is mixed here  No heat is present  No color is seen

2nd Zone :

 Partial combustion zone/ combustion zone  O2 and gas is partially burned here.  It can oxidize the molten metal. Therefore in oxidizing nature.  Green colour.

3rd Zone :

 Reducing zone  Hottest part of the flame  All oxides are reduced here therefore no oxide layer 19

For higher


formation.  Therefore the alloy is melted here.  Blue in colour. 4th Zone :

 Oxidising zone Alloy is not melted here because temperature is less than reducing zone and oxides will form on the surface of the alloy.

How do we know whether we are heating the metal in the right zone?  If it is in reducing zone  the alloy is bright and mirror like. It will be light orange in colour.  If it is in oxidising zone  The alloy will have a dull film or a cloudy – surface, which indicates oxidation.  When is the alloy ready to be cast? When it is light orange in colour, spongy and tends to move along with the flame when it is moved. It will be 38oC to 66oC above liquidus temperature. This indicates that the alloy is ready to be cast. Reducing Fluxes :  Powdered charcoal, borax etc is used.  Fluxes should be added to alloy to decrease porosity, increase fluidity of the metal and prevent oxidation. B) Carrying Investment to Casting Machine :  Using forceps, tongs or tweezers as quickly as possible.  The casting ring should be held such that sprue hole faces down (so that if remaining wax, it can easily flow out).  During this time the color of the casting should be cherry red. C) Forcing the Metal Alloy into the Mold :  Resisting forces are present in investment, mold and molten alloy. These should be overcome by air/gas pressure, vacuum forces, centrifugal pressure, piston-plunger forces / for moldable ceramics.  Casting machines – 3 types :  Air pressure  Vacuum casting 20


 Centrifugal Air Pressure :  The alloy is melted in the hollow left by crucible former then air-pressure is applied through a piston.  Pressure of 10-15 psi is applied  CO2 and CO or N2 gas can be used. Vacuum Casting Machine :  Here vaccum is applied through the base beneath the casting ring and the molten alloy can be drawn into the mold by suction.  Combination can be used : • Centrifugal Pressure + Vacuum • Gas pressure and Vacuum • Centrifugal + Gas pressure + Vacuum. Centrifugal Casting Machine :  The machine basically has a strong spring encased in the base of the casting machine.  The spring is wound into tension by rotating the arms with the weights at one end and the casting ring at the other end. In front of the ring is the crucible in which the gold alloy is melted. When the spring is released, the two arms rotate rapidly, and the molten metal is forced into the mold by centrifugal force. Pressure of 30-40 Psi is used. Factors Affecting the Centrifugal Force : Centrifugal force is  Directly proportional to speed  Directly proportional to length of arm of casting machine.  Directly proportional to weight of metal SPECIAL CASTING SITUATIONS : a) PFM Alloys :  PFM alloys have high melting temperature. Therefore special investments and casting facilities are necessary. Investment Preferred : Phosphate bonded investment as it is stronger and denser than gypsum bonded investments. 21


Gas source used : Gas-oxygen torch. Wax pattern design : Bar-type of sprue. Investing : Vacuum investing. Casting Machine : Centrifugal casting machine with adequate casting pressure. Bar type of Spruing :  Here an attempt is made to increase the turbulence so the investment close to the sprue maintains a higher temperature, which keeps the alloy molten for a longer time.  If a bar type of sprue is used with a centrifugal casting machine, the casting ring should be placed in the casting machine so the bar sprue is vertical.  The feeder from the sprue button to bar is attached 1-2 mm below the top of the bar. In this manner, when the alloy at the tip of the bar sprue freezes, the alloy 1-2 mm below the tip remains molten, and feeding from the sprue button to the bar is still possible.  The feeder to bar and from bar to wax-pattern is at 90o. Disadvantages : In this type of bar spruing, an increased amount of alloy is required to make casting. b) Co-Cr and Ni-Cr Alloys : High Melting Temperature. Modifications :  Phosphate investments used.  Painting a die space on die  Using 2 layers of ceramic paper liner in the investing ring to make the setting expansion of the investment more effective.  Refractory cast should be used to make the wax pattern (Refractory cast is prepared by duplicating the master stone model, usually by using the agar duplicating materials).  Since melting point of alloys is higher, special electric or induction melting facilities to melt the alloy.  Gas  Acetylene –O2 torch can be used (but to lesser extent).  Co-Cr and Ni-Cr alloys are generally considered as technique-sensitive.  All elements in this alloy such as Cr, Silicon, Molybdenum, Co and Ni react with carbon to form carbides even fi only a small amount of ceramic is present in the alloy.  These carbides can change the properties of the alloy. Problems encountered during the casting of Co-Cr, Ni-Cr alloy : 22


Trapping of gases I the mold during the casting process. This may be because of higher strength and resistance to thermal shock. Therefore investments will have decreased porosity and thus gases cannot escape  Therefore produces voids and casting defects. To avoid it : 1) Higher temperature of casting alloy. 2) Venting : For rapid elimination of gases. 3) Decrease thickness of investment from end of ring to wax-pattern (3-4 mm for phosphate bonded). Use : Co-Cr and Ni-Cr alloys used for removable partial denture framework. c) Titanium : Technique sensitive Disadvantages : 1) High melting point of titanium  1671oC. 2) Titanium readily absorbs several gases when in the molten state. If H 2, O2 and N2 are absorbed, the mechanical properties are adversely affected.

Therefore to prevent

absorption of these gases, titanium is cast under protective atmosphere of Argon or in a vacuum.

Modification :  Phosphate or silicon or magnesia investments are used.  Casting done under argon gas atmosphere or in vacuum. Melting of alloy in either graphite or water cooled copper crucibles are used. CLEANING THE CASTING : Quenching : After the gold button has lost its glow,remove the casting ring with the tongs and place it into a pan of cold water. The ring is then immersed in a container of water. The water absorbed into the investment pores will undergo immediate vaporization within the hot mass. Steam in large amounts will be produced, cracking the investment into small pieces and most of the time, peeling it off the casting. Time required for quenching It should occur 5 mins after the casting to achieve best grain structure. 23


If quenched earlier gold will be too soft and weak. If it is allowed to bench cool grain structure will be too large. RECOVERY OF CASTING: The investment is peeled off from the casting and can be facilitated by using a brush or sharp explorer. SAND BLASTING: The casting is held in sandblasting machine to clean the investment from its surface. PICKLING:  Often the surface of the casting appears dark because of surface oxides and tarnish.  This surface film can be removed by pickling.  The best pickling solution for gypsum – bonded investments is 50% HCl solution. The disadvantage of this solution is that fumes from the acid are likely to corrode office and laboratory metal furnishings.  A similar solution of sulphuric acid (H2SO4) is more advantageous in this respect. Its action is enhanced by addition of potassium dichromate.  The casting is placed in a test tube or dish and acid is poured over it. The acid may be heated but boiling is avoided because of the excessive fumes produced. After pickling, the acid is poured off and the casting is removed.  The casting should never be held with steel tongs. This is because the pickling solution usually contains small amount of copper dissolved from previous castings. When the steel tongs contact this electrolyte, a galvanic cell is created and copper is deposited on the casting at the point where the tongs grip it.  Another procedure to be avoided is heating the casting and then dropping it in the pickling solution. This may cause delicate margins of the casting to melt or the casting may be distorted by sudden thermal shock when plunged into the acid.  After pickling, the casting should be washed in running water to remove the acid.  Gold based and Pd based metal ceramic alloys and base metal alloys are bench-cooled before it is removed from the investment. Usually they are not pickled. CASTING DEFECTS 1) Distortion 2) Surface roughness, irregularities 24


3) Porosity 4) Incomplete / Missing detail castings 5) Rounded margins 6) Discoloration 1) DISTORTION : Distortion in casting is due to distortion in wax pattern. a) During manipulation, there may be release of stresses. Avoid by ď&#x192;  Proper handling and manipulation of the wax i.e. i)

Manipulation of wax at high temperature. If at low temperature, stresses are introduced.

ii)

Investing the pattern within 1 hour after finishing.

iii)

If storage is necessary, store in refrigerator.

b) Due to uneven movement on the walls of the pattern when the investment is setting. The gingival margins are forced apart by the mold expansion, whereas the solid occlusal bar of wax resists expansion during the early stage of setting. Avoid by : Not much can be done to avoid this. 2) SURFACE ROUGHNESS, IRREGULARITIES : Difference between surface roughness and surface irregularities. Surface roughness : Is defined as relatively finely spaced surface imperfections whose height, width and direction establish the predominant surface pattern. Surface Irregularities : Isolated imperfections, such as nodules that do not characterize the total surface area. Surface roughness, irregularities due to : a) Air bubbles on wax pattern b) Water films causing ridges and veins on surface c) Too rapid heating ď&#x192;  resulting in fins or spines. d) Underheating causing incomplete elimination of wax. e) Inappropriate water / powder ratio f) Prolonged heating g) Temperature of alloy too high h) Casting pressure too high i) Foreign bodies 25


j) Impact of molten alloy k) Pattern position l) Composition of the investment a) Air Bubbles on wax-pattern -

Seen as small nodules on casting

-

May be due to manual investing

-

Excess of wetting agent will dilute the investment, causing irregularities.

Avoid by : -

Vacuum investing technique

-

Vibrate before and after mixing

-

Slightly tilt the casting ring, so that entrapped air bubbles might rise to the surface

-

Use a wetting agent to reduce surface tension of wax pattern so that intimate contact of investment to wax pattern.

-

Air dry the wetting agent to a thin film.

b) Water films -

Wax is repellant to water, and if the investment becomes separated from wax-pattern, a water film may form irregularly over the surface.

-

This may be seen if the wax pattern is moved slightly or vibrated after investing.

Clinical Features : The water films causes minute ridges or veins on the surface. Avoid by :  Avoiding movement, vibration of pattern after investment.  Use of wetting agent and painting investment properly on the pattern to ensure intimate contact. c) Too rapid heating Clinical Features :  Can cause fins / spines on casting.  Due to rapid heating, flaking of investment occurs when steam forms. Also, the steam may carry some of the salts used as modifiers into the mold. Avoid by : Heat the mold gradually for 60 minutes from room temperature to 700oC. 26


d) Under heating :  If heating time is too short or if there is insufficient air in the furnace  can cause incomplete elimination of wax residues. Clinical Features :  Seen as voids / porosity on casting.  Hot alloy comes in contact with these wax residues forming gases to form voids / porosity in the casting.  Seen a black coating because the casting is covered with carbon coating which cannot be eliminated by pickling. Avoid by : Heat the ring for adequate period of time so that the carbonaceous residue is removed. e) Inappropriate water / powder ratio  Both increased water/ powder and decreased water / powder ratio causes rough castings.  Increased water / powder ratio – rougher the casting.  Decreased water/ powder ratio – investment will be too thick. Therefore cannot be applied properly on the pattern.  During vacuum investing, air might not be removed completely. Avoid by: Accurate water powder ratio. f) Prolonged heating :  Especially for gypsum bonded investments.  If gypsum bonded investment is heated above 700 oC, sulphur compounds are formed which will contaminate the alloy.  Also > 700oC – disintegration of the investment. Avoid by : Do not heat the ring > 700oC. g) Temperature of alloy too high.  If alloy temperature is more than temperature of investment, it will attack the surface of the investment causing roughness. Avoid by :  If gas – air fuel is used  temperature of alloy will be apt.  If other fuel is used  light orange colour of alloy shows the alloy is ready to cast. (It should not be lighter than light orange).  The temperature of alloy should be compatible with the temperature of the investment. 27


h) Casting Pressure :  Increase in casting pressure can cause rough surface on casting. Avoid by : Pressure should be adjusted.  0.10 to 0.14 MPa in air pressure casting machine.  3-4 turns of spring in average type of centrifugal casting machine. i) Foreign bodies :  When removing the crucible former, the investment in this area may be loosened. These pieces of investment may be carried into the mold with the molten alloy.  Bits of carbon from the flux could be carried into the mold to cause a bright appearing concavity (carbon reacts with O2 or air in the mold cavity forming CO which is a reducing agent. This CO prevents the oxidation of the surface of the casting gold and thus the cast will be bright and shiny).  ‘Flux’ carried into the mold appears as bright appearing concavities.  ‘Sulpur’ from investment breakdown or from the torch flame having high S content  causes surface discolouration and surface roughness. Avoid by : Heat the mold upside down so that particles fall out of mould. j) Impact of molten alloy :  If sprue former is at 90o - great impact of molten alloy which can strike against the mold and cause concavity opposite to the sprue former which is reflected as raised area in casting. Avoid by : Sprue former should be directed at 45o to decrease the velocity of the flow of molten alloy. k) Pattern position :  If too many wax patterns placed in same ring, there should be a minimum distance of 3 mm between them. If it is less than 3 mm distance (the expansion of wax is more than the expansion of the investment), the expansion of wax can cause breakdown of investment. Avoid by : Avoid placing too many wax pattern close to each other in same ring. Minimum distance of 3 mm among the wax pattern. l) Composition of the investment

28


 Ratio of Binder : Quartz. Decreased binder : Quartz  Increased porosity  Increased surface roughness.  Coarse silica increases surface roughness Avoid by : Investment should follow ADA specification No.2, thus there will be decreased surface roughness. 3) INCOMPLETE CASTINGS : It may be due to : a) Inadequate spruing / sprue former too small. This can cause premature solidification of alloy in the sprue former and thus molten alloy cannot reach all areas of mold. Avoid by :  Use larger size of sprue former.  Sprue former should be placed in thickest area of wax pattern so that molten alloy can flow from thicker to thinner areas. b) Sprues blocked with foreign bodies : Foreign bodies block the flow of molten alloy. Avoid by : Ensure that no debris blocks the sprue ingate. The ring should be held with sprue hole down when removing crucible former and sprue former (if metal). c) Alloy not hot enough or not sufficiently molten or fluid.  Results in viscous alloy which cannot wet all areas of mold cavity. Avoid : Adequate heating of alloy so that it is 57 oC above the fusion / liquidus temperature. d) Mold too cold Avoid by :  Mold should soak heat for approximately 1 hour at burnout temperature.  The temperature of the oven should be checked regularly with a pyrometer.  Mold should be removed from burnout oven and casting completed within 1 minute. e) Insufficient casting force Avoid by : Use adequate amount of force for casting. f) Insufficient gold alloy used Avoid by : Cast enough gold to allow for a good button in the crucible of the ring. 4) ROUNDED MARGINS : 29


a) Incomplete burnout of wax pattern : Can cause carbon residues in the mold to produce shiny rounded margins. Avoid by : Ensure adequate burnout time and temperature to cause the carbon to get converted to CO or CO2 b) Insufficient heating of alloy before casting  can make the alloy too viscous or does not stay in liquid state long enough to force the gases out and reach the marginal areas of the mold. Avoid by : Heat alloy to 57oC above fusion temperature. c) Improper diameter / length of sprue  restricts flow of alloy into the mold. The metal freezes before margins are complete. Avoid by : Average casting should have 10 gauge sprue and approximately 6 mm long. d) Insufficient casting pressure : When the casting machine is underwound and the force on the molten alloy is insufficient to drive the alloy into the mold before the alloy freezes. Avoid by : Use adequate casting pressure. 5) POROSITY:  Porosity may occur both within the interior region of a casting and on the external surface.  External porosity can cause surface roughness.  Internal porosity weakens the casting and if it extends to the surface, it may be a cause for discoloration.

If it is severe, it can produce leakage at the tooth – restoration interface

and secondary caries may result. Classification of Porosities : I.

Those caused by solidification shrinkage. A. Localized shrinkage porosity B. Microporosity

I.

Porosities caused by trapped gases A. Pinhole porosity B. Gas inclusion porosity C. Subsurface porosity

II.

Due to residual air  Back pressure porosity.

I. Solidification defects : 30


1. Localized shrinkage porosity / Shrink spot porosity ď&#x192;° It is caused by the incomplete feeding of molten alloy during solidification. ď&#x192;° When the alloy solidifies from liquid state, a shrinkage of at least 1.25% occurs. Thus during solidification of metal in mold, if additional molten metal is not available to compensate for shrinkage, then porosity occurs. ď&#x192;° It can occur if sprue is thin, then the metal freezes in the sprue before it does in the mold. It generally occurs in the sprue-casting junction. a) Suck-Back Porosity : Shrinkage of alloys takes place in 2 stages : 1) The transformation of the alloy from liquid to solid. 2) The coefficient of expansion of the solid alloy. Suck back porosity due to ; i)

Improper sequence of solidification of metal alloy

ii)

Difference in temperature of investment at sprue area and at pulpal area.

i)

While the molten alloy is cooling, the temperature eventually reaches the solidification range, causing the alloy to change from liquid to solid. This change of state is accompanied by a large shrinkage. As long as the remaining alloy is in the liquid state and the casting machine is

rotating, molten alloy will feed the solidified portion of the casting, thereby compensating for the shrinkage.

The next layer then solidifies, and this process continues until all the

shrinkage resulting from the change of state is compensated for by the available molten alloy in the casting, sprue and sprue button. Solidification takes place in a systematic manner.

Ideally, molten alloy located

furthest from the sprue button should freeze first and the molten alloy in the sprue and the sprue button should feed the rest of the pattern, thus compensating for the shrinkage as a result of the change of state. If solidification does not occur in this systematic manner, and a portion of the alloy freezes before the alloy in the casting, suck back porosity occurs (seen if sprue diameter is thin). ii)

Difference in temperature of investment at sprue area and at pulpal floor area. Investment at the pulpal floor of a full crown pattern heats up because of the higher temperature of the molten alloy, and it keeps the alloy in this area molten somewhat 31


longer than other areas. This is called hot spot. (as molten alloy enters the sprue and his the investment in pulpal floor area, thereby rising its temperature). Thus if alloy in the sprue solidifies before the alloy in the pulpal floor area, the molten alloy in this area would feed the solidifying alloy in the sprue area. When molten alloy in the pulpal floor area solidifies and shrinks because of the change of state, it can not be fed. AS a result, a large suck back porosity in this area occurs. (Investment at pulpal floor area will have higher temperature than that in the sprue area. Therefore molten alloy in sprue will solidify first, whereas in pulpal floor area, it is still molten and this molten alloy can further feed the sprue area. Once it starts the solidification process, there will be no more molten metal to compensate for its shrinkage. Thus suck back porosity formed). Avoid by : -

Using a Y-shaped sprue instead of a single sprue. Here, only half of the molten alloy enters the mold cavity through each leg of the Y-sprue, so the temperature of the investment in the pulpal floor area (occluso-incisal area/ hot spot area) does not rise very high.

-

The arms of the Y must be widely separated to prevent the investment between the arms from overheating, which could also cause suck back porosity.

iii)

Increase the mold temperature from 500oC (which is mostly used with many investments) to 650 – 700oC. 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. Or another method is to lower the casting temperature by about 30oC. `

iv)

Hot spot porosity can be eliminated by flaring the point of sprue attachment.

v)

Using a reservoir.

b) Microporosity: Also due to premature solidification of the metal and is the result of solidification shrinkage.  It occurs due to rapid solidification of the metal due to low temperature of molten alloy.  Too low casting temperature. Clinical Features : It is seen as small irregular voids. 32


Avoid by : Increasing temperature of molten alloy Increasing temperature of casting. II. Porosities due to trapped gases : Due to entrapment of gas during solidification. a) Pinhole Porosity :  Many metals dissolve or occlude gases while they are molten.  For example, copper and silver have great affinity for oxygen. Therefore they dissolve oxygen when they are in liquid state.  Platinum and palladium have a strong affinity for hydrogen as well as oxygen.  On solidification, these absorbed gases are expelled and the pinhole porosity results. Clinical Features : Seen as small spherical voids. b) Gas inclusion Porosities :  These are larger spherical Porosities 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. Avoid by : -

Premelting the gold alloy on a graphite crucible or a graphite block if the alloy has been used before.

-

Correctly adjusting and positioning the torch flame during melting.

C. Subsurface Porosity :  The exact reason for this has not been established.  It may be due to the simultaneous nucleation of solid grains and gas bubbles at the first moment that the metal freezes at the mold walls. Avoid by : Control the rate at which the molten metal enters the mold. III. BACK PRESSURE POROSITY / ENTRAPPED AIR POROSITY :  Caused by the inability of the air in the mold to escape through the pores in the investment. Avoid by : -

Proper burnout so that carbonaceous residues do not decrease porosity of investment.

-

Adequate mold and casting temperature

-

Adequate casting pressure 33


-

Thickness of investment between the tip of the pattern and the casting ring end should not be greater than 6 mm for gypsum bonded investments or 3-4 mm for phosphate bonded investment.

-

Vents can also be used for gases to be collected here.

6) DISCOLORATION i)

Black casting due to sulphur contamination. -

Overheating the investment above 700oC will cause its breakdown and formation of sulphur or sulphur compounds. Avoid by : Do not heat the investment above 700oC.

ii)

Carbon inclusion from the investment, torch or crucible. It can contaminate the alloy during casting. Avoid by : Changing source of heat or crucible etc.

iii)

Copper contamination during pickling Avoided by: Avoid the use of steel tongs to hold casting during pickling. The steel tongs should be covered with rubber or Teflon.

iv)

Contamination with mercury -

If cast gold restoration kept along side with amalgam dies, Hg from amalgam penetrates rapidly into the alloy and decreases the ductility of gold and increases the corrosion.

-

Galvanic cell is formed if cast gold restoration given adjacent to amalgam restoration.

-

Amalgam is the anode and noble metal alloy is the cathode. Avoid by : Castings should never be placed adjacent to amalgam dies/restorations etc.

CONCLUSION: All basic steps should be followed in an orderly manner  like wax pattern fabrication, investing, burn-out followed by casting and recovery.

Proper attention to

cleanliness and also the necessary precautions should be taken to prevent casting defects. Having ensured that, a clean, smooth, dense casting is obtained.



34


Casting procedures/ dental implant courses by Indian dental academy