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Introduction Structural faults in dental castings can result from any combination of problems related to spruing, investing, wax elimination, alloy melting, casting & solidification of the casting. The ability to produce smooth and well fitting castings require strict adherence to certain fundamentals.....
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Introduction
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Principles of casting
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Steps in casting
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Casting defects
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Spruing
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Casting of ceramics
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Investing patterns
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Casting of titanium
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Burn out procedure
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Casting procedure in RPD
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Casting machines
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conclusion
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Casting techniques
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Casting procedure www.indiandentalacademy.com
Steps in casting procedure • • • • • • • • •
Die preparation Wax pattern Attachment of sprue Investing Burnout, melting the alloy Casting, recovery, finishing & polishing www.indiandentalacademy.com
SPRUING Off all the procedures involving casting work, the spruing technique has a paramount importance in producing a complete & dense casting.
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Definitions (GPT-8) Sprue : the channel or hole through which plastic or metal is poured or cast into a gate or reservoir and then into a mould Sprue former : a wax, plastic, or a metal pattern used to form the channel or channels allowing molten metal to flow into a mold to make a casting Sprue button : the material remaining in the reservoir of the mold after a dental casting www.indiandentalacademy.com
Basic Requirements • To form a mount for the wax pattern • To create a channel for elimination of wax during burn out • To form a channel for entry of molten alloy during casting • Provides a reservoir of molten metal which compensates for alloy shrinkage during solidification www.indiandentalacademy.com
Sprue Formers •
Wax
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Plastic/Resin
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Metal sprues www.indiandentalacademy.com
Factors to be considered • • • • • • •
Size Length Shape Number Attachment Location Angulation www.indiandentalacademy.com
Size • Usually large-diameter sprue is recommended • The diameter of sprue should be equal to the thickest portion the wax pattern. • There are various gauges of sprue former → gauge 6, 8, 10, 12, 14, 16, 18 (0.4cm - 0.1cm) • Usually → for molar and metal ceramic restoration → 10-gauge (2.5mm) → Premolars and partial coverage restoration → 12-gauge (2.0mm) www.indiandentalacademy.com
• Sprue former should be attached to the portion of the pattern with the largest cross-sectional area (decreases risk of turbulence
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Length • The sprue should be long enough to place pattern 6 mm from the trailing end of casting ring usually it should be in the range of 2.5 mm.
• Very short sprue → porosity in casting at the junction of sprue and pattern. • Very long sprue → sprue solidifies first leading www.indiandentalacademy.com to casting shrinkage and incomplete casting.
Shape • The sprue former should be straight to reduce chances of creating turbulence in molten metal entering the mold. • High turbulence of alloy cause porosity.
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Number • Usually a single sprue is used for small castings. • When two thick sections of a pattern are connected by thin part of wax, two separate sprues should be attached to each thick portion
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Attachment •
Attachment of sprue former and wax pattern: 1. The connection is generally flared - high density gold 2. It is restricted for low density alloys
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Sprue Former Position • It depends on the shape of the wax pattern
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Types Of Spruing There are 2 types of spruing
• Fig:1
1. Direct spruing 2. Indirect spruing
• Fig:2
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Direct spruing •
The sprue former provides a direct connection between the pattern area and the sprue base/ crucible former.
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When two thick portions of wax are separated by thin wax, then 2 separate sprues should be attached using direct spruing. www.indiandentalacademy.com
Factors to be considered: 1. Location of attachment 2. Angulation of sprue 3. Attachment morphology
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Location of attachment • Pin ledge restoration → sprue is located towards the incisal aspect of lingual surface and angled suitably. • Full veneer crown → sprue is attached to Maxillary buccal and mandibular lingual cusp. • Partial veneer crown → sprue is attached to cusp that encompasses the preparation.
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Angle of attachment • The sprue should be attached to pattern such that it makes 45 degrees to the walls of mold, which decreases the turbulence of molten alloy. • If the sprue is placed perpendicular to the mold wall, it induces high turbulence in molten alloy, leading to creation of a hot spot on mold wall. This results in localized shrinkage porosity. • It should be placed away from thin / delicate parts of pattern to prevent fracture of investment. www.indiandentalacademy.com
Attachment morphology • The transition should be smooth • Irregularities produces tags of investment which is vulnerable for fracture by molten alloy • Usually it is flared for high density gold alloys but restricted for low density alloys. Flaring acts as reservoir and facilitates the entry of molted alloy into the mold area. www.indiandentalacademy.com
Indirect spruing Indirect spruing uses the same basic principles of spruing. But the only difference lies in attachment of 3 running horizontal bars. The whole indirect sprue complex consist of 3 parts: – Manifold sprue. – Horizontal running bar. – Feeder sprue. www.indiandentalacademy.com
• Once spruing is done the wax pattern and sprue former assembly is carefully removed from the die. • During removal of pattern no pressure should be applied to prevent its distortion.
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Then it is positioned into the crucible former to adjust the distance between the wax pattern and casting ring for INVESTING‌..
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INVESTING
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GPT8 Terminology‌.. •
Investing : The process of covering, enveloping, wholly (or) in part an object such as denture tooth, wax form, crown, etc with a suitable material before processing, soldering, casting.
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Dental casting Investment: Material consisting principally of an allotrope of silica and a bonding agent. The bonding substance may be gypsum (for use in lower casting temperature) (or) phosphates and silica (for use in higher casting temperatures). www.indiandentalacademy.com
• Refractory : Difficult to fuse/corrode, capable of enduring high temperatures • Refractory investment : An investment that can withstand high temperature using a soldering/casting.
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Essentials….. • • • • •
It should produce mould expansion to compensate for casting shrinkage. Should have sufficient strength at room & high temperature. Inner surface of the mold should not break at a high temperature. Should exhibit sufficient strength, to withstand the force of molten alloy entering the mold. Inner surface of mold should be smooth. www.indiandentalacademy.com
Basic composition….. Refractory Materials : The most commonly used refractory material is silicon dioxide in form quartz, crystoballite. Binder : commonly used binders are α - calcium sulfate hemihydrate Others are – Sodium silicate, ethyl silicate, ammonium sulfate, sodium phosphate. Other chemical modifiers Reducing agents – Ex : Charcoal Sodium chloride, boric acid – to ↓ shrinkage, during dehydration of calcium sulfate dihydrate. Potassium sulfate (accelerator) Copper powder(reducing agent)/magnesium oxide www.indiandentalacademy.com
Expansion‌.. This property of investment is needed for compensation of casting shrinkage of alloy. The expansion occurs because of:1. Normal setting expansion 2. Hygroscopic setting expansion 3. Thermal expansion.
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Crucible former • Helps to hold & place the sprue in the center of the casting ring • Helps to contour the investment surface • They may be preformed or contoured with wax
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Casting ring liner • In a casting ring provision must be made for expansion of the investment • We can over come this by using split ring or flexible rubber ring • However, the commonest way of providing space for expansion is by using a liner
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A ring liner is placed on the inner surface of the casting ring Should be placed a few mm short of the ring It can be either 1. asbestos liners (used earlier) 2. non-asbestos liners a) Cellulose (paper), b) Aluminium silicate ceramic www.indiandentalacademy.com
Casting ring Choice of the ring:
Rubber ring ďƒ for HSE Metal ring ďƒ for TE
The diameter and the length of the ring should be selected rightly. www.indiandentalacademy.com
Ringless casting system Metallic ring is not used for investing the wax pattern. Flexible tapered plastic ring is used in which wax pattern is invested. Also called powercast ringless system Used for casting of alloys which require more expansion. www.indiandentalacademy.com
Assembling the ring This step should be coordinated with spruing & choosing the size of the ring
Wetting agent is applied ( to reduce air bubbles) on the wax pattern www.indiandentalacademy.com
Preparation of the investment mix Mixing of the investment is done by: Hand mixing (brush technique) Vacuum mixing
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Brush technique Hand spatulate the mix to incorporate the powder quickly Coat the wax pattern with the investment Carefully coat the internal surface & the margin of the pattern Fill the ring slowly, starting from the bottom
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Vacuum technique
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Method First hand spatulate the mix Attach the vacuum hose & mix accordingly to the manufacturers recommendations Invert the bowl & fill the ring under vibration Remove the vacuum hose before setting of the mixer Immediately clean the bowl & mixing blade under running water
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Setting of investment It can be: In open air hygroscopic technique controlled water added technique
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In open air • Usually when high heat TE technique is used. • The investment is allowed to set in open air for 1 hour. • The setting time is 1 hour for both GBI & PBI. www.indiandentalacademy.com
Hygroscopic technique Once the casting ring is poured it is immersed into a water bath at 38 degrees temperature immediately This can be altered by: W:P ratio W:P HSE Time of immersion the delay HSE temp. of water HSE www.indiandentalacademy.com
Controlled water added technique • The desired amount of expansion is retained by the amount of water added • A soft flexible rubber ring is employed & invested normally • A specific amount of water is added on the top of investment & allowed to set at room temperature
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Time of placement of investment for burn out after 20 mins and 24 hrs
Castings were undersized
after 2 hrs
Castings were satisfactory
After 1 hr
Were found to be the best www.indiandentalacademy.com
Burnout furnaces
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BURNOUT (MOLD CONDITIONING) 1. Allow the investment to set (usually 1 hour) and then remove the rubber crucible former a. If a metal sprue is used, remove it as well b. Any loose particles of investment should be blown off with compressed air. c. If burnout is to be delayed – keep in humidor.
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• Place the ring with the sprue facing down or on a raised object in the furnace.
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Burnout techniques Mold heating
Preheated furnace
Cold furnace Single stage
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Two stage
Rapid or prolonged heating
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Removal of mold from furnace
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Casting (GPT8) • something that has been cast in a mold; an object formed by the solidification of a fluid that has been poured or injected into a mold • Casting is the process by which the wax pattern of a restoration is converted to a replicate in a dental alloy.
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1301 B.C
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Gold casting
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Three and half ton statue
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Casting procedures Casting –is act of forming an object in a mold. Wax elimination • Gypsum bonded – 468degC –hygroscopic expansion 650 degC– thermal expansion • Phosphate bonded 700 – 870 degC depending on type of alloy .
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Hygroscopic low heat technique • 37 deg water bath expands the wax pattern • The warm water entering the investment mold from top adds some hygroscopic expansion . • The thermal expansion at 500 deg provides the needed expansion .
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• Added expansion can be obtained by Increasing water bath temperature. Use 2 layers of liner Increasing the burn out temperatures to 600 to 650degC.
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High heat thermal expansion technique • Gypsum investments – mold is heated from room temp & slowly heated to 650 – 700 deg C in 60 minutes & held for 15 –30 minutes . • Rate of heating rapid heating – cracking,flaking or spalling of mold walls, differential thermal expansion & radial cracks from the interior outwardly.
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• Temperature – at > 700 deg C CaSo4 + 4C--- CaS + 4Co 3CaSo4 +CaS-- 4CaO + 4So2 • Time of casting – casting should be made immediately to prevent sulphur contamination& disintegration . • Rapid burn out procedure Investments with crystobalite- mold is placed in a furnace at 350 degC. www.indiandentalacademy.com
Phosphate investments They expand by –expansion of wax pattern, the setting expansion, thermal expansion. The usual burn out temperature ranges from 750 – 1030degC. Because • Higher temp ensures total elimination of wax residues • The completion of physical & chemical changes . • Prevention of premature solidification of high melting alloy . www.indiandentalacademy.com
• Slow heating rate for 350 degC ,then hold at upper temp for 30 minutes. • Some investments can be subjected to 2 stage heating more rapidly – placed directly in the furnace at the top temp for 30 min & cast . • To save time – ring & ring liner are also eliminated – tapered plastic ring is used so set investment can be pushed out of the ring , when completely set placed directly into the hot furnace . Here expansion varies & can be adjusted by varying liquid concentration. www.indiandentalacademy.com
Time allowable for casting • High heat technique – 1 minute , as the mold contracts on cooling • Low heat technique –casting should be done soon after removal from the oven.
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• W.H.TAGGART introduced casting machine in 1907.
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• Gold melted with a blowpipe was then forced into the plaster mold by means of casting machine, which utilized compressed air
Blowpipe
Compressed air cylinder
Inlay Mold www.indiandentalacademy.com
Casting machines • The alloy is melted in a separate crucible by a torch flame & cast into the mold by centrifugal force. • The alloy is melted electrically by a resistance or induction furnace & cast into the mold centrifugally by motor or spring action. • The alloy is melted by induction heating, then cast into the mold centrifugally by motor or spring action. www.indiandentalacademy.com
CENTRIFUGAL FORCES
spring driven
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Motor driven
Centrifugal casting machine The casting machine spring is first wound from 2 to 5 times.The metal is melted by torch flame in a glazed ceramic crusible attached to broken arm of the casting machine. The broken arm feature accelerate the initial rotational speed of the crucible and casting ring,increase in the linear speed of liquid casting alloy as it moves into and through the mould. www.indiandentalacademy.com
Once metal reaches casting temperature and the heated casting ring is in position.The machine is released and the spring triggers rotational motion. As the metal fills the mould there is a hydrostatic pressure gradient along the length of the casting.This pressure form the tip of casting to the bottom surface is quite sharp and parabolic in form reaching zero at the button surface. www.indiandentalacademy.com
Pressure gradient at the movement before solidification reaches about 0.21 to 0.28 Mpa [30 to 40 psi] at the tip of the casting. Because of this pressure gradient there is also gradient in the heat transfer,such that greatest transfer is at the high pressure end of the gradient. Because this end is also is the sharp edge margin of crown .It is assured that solidification progresses from thin margin to the button surface. www.indiandentalacademy.com
ELECTRICAL RESISTANCE HEATED CASTING MACHINE
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The alloy is melted electrically by a resistance heating , then cast into the mold centrifugally by motor (or) spring action
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Electrical resistance- Heated casting machine. Here there is automatic melting of metal in a graphite crucible with in a furnace rather than a torch flame. Is advantageous for alloys like metal ceramic restorations, which are alloyed with base metal in trace amounts that tend to oxidize on over heating and crucible in the furnace is located flush against the casting ring so metal button remains molten slightly longer. www.indiandentalacademy.com
INDUCTION MELTING MACHINE
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The alloy is melted by an induction field that develops within a crucible surrounded by water-cooled metal tubing.
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• The electric induction furnace is a transformer in 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
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It is more commonly used for melting base metal alloys not been used for noble alloy casting as much as other machines
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DIRECT CURRENT ARC MELTING MACHINE
• Arc is produced between two electrodes: The alloy and the water-cooled tungsten electrode generates heat. cathode
D -----------------------------------------
B C
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A
----------------------------------------anode
• The temperature within the arc exceeds 4000oC and the alloy melts very quickly. • This method has a high risk for over heating the alloy.
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Titanium and titanium alloy Titanium ingot melting-heated to a temp. of 1600-17000 c
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Vacuum arc melted vacuumor pressure assisted casting machine
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The alloy is vacuum arc melted and cast by pressure in an argon atmosphere.
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VACUUM arc melted centrifugal casting machine
5 Metal receiver www.indiandentalacademy.com
6 Mold
• When the melting process has been completed, the casting tilts down and the molten titanium flows into the refractory mold.
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Casting crucibles Clay crucibles: Crown bridge alloys, such as high noble and noble types. Carbon crucibles: Crown bridge alloys, and higher fusing, gold based metal ceramic alloys. Quartz crucibles: For high fusing alloys of any type.
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Crucible • Carbon crucibles • Clay crucibles • Quartz /zirconia - alumina
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CASTING MACHINES •
Casting machines provide the means for transferring the molten alloy from the crucible to the mold. 1.Pneumatic force (or) Air pressure type. 2.Centrifugal force type
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THE VACUUM / PRESSURE TYPE OF CASTING
• These machines first evacuate the chamber to reduce oxidation and then apply air pressure to push the metal uniformly.
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– Vacuum casting machines
» Melting chamber. » Flask chamber. – A jet-casting machine, which depends combustion gas to provide the casting force.
on
Melting chamber Flask chamber www.indiandentalacademy.com
Casting techniques
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Lost wax technique
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Casting techniques
centrifugal force type
spring loaded torch melting machine
Induction melting machine
Pneumatic force
electrical resistance heated casting machine
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Electric arc vacuum casting machine
Casting technique for spring driven centrifugal casting machine
High noble, noble and base metal alloys www.indiandentalacademy.com
Heat treatment (Alloy conditioning ) 1. Softening heat treatment 2. Hardening heat treatment
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Casting procedure • The casting machine is given three (or) four clock wise turns and locked in position with the pin
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The cradle and counter balance weights are checked for the appropriate size of the casting ring to minimize turbulence for the flow of metal.
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A clay\carbon crucible for the gold alloy being cast is placed in the machine. • The torch is lit and adjusted
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For torch melting The fuel used is a combination of 1. Natural or artificial gas and air 2. Natural or artificial gas and oxygen 3. Air and acetylene 4. Oxygen & acetylene
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The Flame Has Four Zones 1. 2. 3. 4.
Mixing zone Combustion zone Reducing zone Oxidizing zone
Reducing zone
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Preheating crucible •
This preheating avoids excessive slag formation during casting
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• The alloy is heated in the reducing part of the flame until it is ready to cast • A reducing flux is used in melting the alloy (50% boric acid powder and 50% fused borax ) it increases fluidity and reduces potential for oxidation.
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• When reducing zone is in contact-the surface of the gold is bright and mirror like. • Oxidizing zone in contact-dull film or “dross” development. • When gold alloy is ready to cast it will be white hot, forming smooth pool.
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• The casting machine arm is then released to make the casting. • Providing enough force to cause the liquid alloy to flow into the mold.
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• The machine is allowed to spin until it has slowed enough that it can be stopped by hand, and the ring is removed with casting tongs
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Quenching • After the casting has solidifies, the ring is removed and quenched in water as soon as the button exhibits a dull-red glow.
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Sufficient mass of alloy must be present to sustain adequate casting pressure • 6g is typically adequate for premolar and anterior casting • 10g is adequate for molar casting • 12 g is adequate for pontic
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Heating base metal alloys • At casting temp. the base metal ingots should only slump and round over the corners
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Casting pressure • The pressure should be applied for at least 4 sec. the mold is filled and the alloy is solidified in 1sec. or less but it is quite soft during the early stages.
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Greater casting pressure are generally required ( Base metal alloys) • The greater degree of chilling. • Very thin sections have to be cast. • The lower density of base metal alloys.
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• Base metal alloys are bench-cooled to room temperature before the casting is removed from the investment
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Casting technique for electric resistance heated casting machine
For titanium casting www.indiandentalacademy.com
1.
2. •
Process of melting and casting takes place in an evacuated two chamber system with a continuous flow of argon gas Titanium ingot is heated in a copper crucible Prevents overheating & reaction
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Pickling GBI : 50% Hcl. Sulfuric acid. Ultrasonic devices.
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Finishing & Polishing: 1. carborundum disks 2. tungsten carbide trimmers. 3. rouge with woolen buff. 4.rubber cup polishers.
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Accelerated casting technique PBI : Setting time – 13 to 17 min. wax burnout - for 15 min at 815deg c. GBI : Setting time – 30 min. wax burnout – for 30 min at 700deg c. Adv: Reduced casting time. www.indiandentalacademy.com
Conclusion • Casting procedures are highly technique sensitive steps which converts wax pattern to a final restoration . • Accurate & smooth restorations can be obtained if operator pays special attention to each step in the technique.
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References 1. K. J. Anusavice, Phillips Science of Dental Materials, 11th edition, W. B. Saunders co 2003, pg: 295-350 2. J.F.MaCabe, applide dental materials, 8th edition, blackwell science publication, pg:69-71 3. M.Gladwin, Clinical Aspects of Dental Materials, Lippincott Williams & Willikins publishing, 1999, pg:115-126 4. Craig R.G, Restorative Dental Materials;10th edition; St.Louis: Mosby, 1997, pg:457-480 www.indiandentalacademy.com
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