casting procedure & casting defects seminar
TRANSCRIPT
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BY
Ramesh d, Maharjan
CDT
CONTENTS
INTRODUCTION DEFINITIONS HISTORY STAGES OF CASTING PROCEDURE:
I) CLINICAL STEPS : 1) TOOTH PREPARATION2) MAKING OF THE IMPRESSION
II) LABORATORY STEPS:
3) PREPARATION OF DIE OR CAST4) WAX PATTERN PREPARATION5) SPRUE FORMER & SPRUING6) CASTING RING & RING LINERS7) INVESTING8) WAX ELIMINATION OR BURN OUT 9)CRUCIBLES10) METHODS OF MELTING ALLOYS11) CASTING MACHINES12) CLEANING & FINISHING THE CASTING
CASTING DEFECTS 3
INTRODUCTION
Casting is one the most widely used methods for fabrication of metallic restorations outside the mouth.
The technique of investment casting is both one of the oldest & most advanced of the metallurgical arts.
The lost wax casting technique was 1st described at the end of 19th century as a means of making dental castings.
The process consists of surrounding the wax pattern with a mold made of heat resistant investment material, eliminating the wax by heating and then introducing the molten metal into the mold through a channel called “Sprue”.
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In Dentistry, the resulting casting must be a highly accurate replica of wax pattern with surface details & accurate dimensions.
Small variations in investing or casting can significantly affect the quality of final restoration.
Successful casting depends on accuracy & consistency of technique.
We are going to know the exact influence of each variable in the technique & to make rationale changes to modify the technique according to need.
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DEFINITIONS
1) ACCORDING TO GPT (7th edition): Casting is defined as 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.
2) ACCORDING TO CRAIG (12th Edition): Casting is the process by which a wax pattern of a restoration is converted to a replicate in a dental alloy.
3) ACCORDING TO WILLIAM J. OBRIEN (3rd edition): Casting is a process of forming objects by pouring molten metals in molds that are cooled to cause solidification.
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4) CASTING PROCEDURE: It is a process of obtaining a metallic duplicate of a missing tooth structure by pouring molten metal into a mold of a required form & allowing it to solidify to obtain a metallic duplicate.
5) LOST WAX TECHNIQUE: It is so named because a wax pattern of a restoration is invested in a ceramic material, then the pattern is burned out (“Lost”) to create a space into which molten metal is placed or cast.
6) CASTABILITY: The ability of an alloy to completely fill a mold.
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OBJECTIVE OF CASTING PROCEDURE :
To provide a metallic duplication of the missing tooth structure , with as much accuracy as possible .
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“Lost Wax” Method dates back to atleast 4th millenium B.C.
Dental casting procedures have originated from Black smithery which dates back to the early civilizations of Mesopotamia to Egypt where Utensils, Weapons & Tools were cast in Brass alloys using simple techniques.
Remarkably, civilizations as diverse as China’s Han Dynasty, the Benin kingdom in Africa & the Aztecs of pre-columbian Mexico employed similar techniques.
The use of dental casting machines to fabricate cast metal restoration is credited to the people of the early chinese civilization in the Bronze age.
Dental castings were first seen in the Skulls of people of the Chinese civilization where cast metal restorations were fastened onto the human teeth, with metal wires passing through the space between the teeth.
HISTORY
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11th Century Theophilus Described lost wax technique, which was a common practice prevailed in 11th century.
1558 Benvenuto Cellini Claimed to have attempted, use of wax and clay for preparation of castings.
Cast his bronze masterpiece “Perseus & the Head of Medusa” using the Lost wax process.
1884 Agulihon de saran Used 24K gold to form Inlay
1887 J. R. Knapp Invented Blowpipe
1897 Philbrook Described a method of casting metal filling.
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-The casting procedure by the Lost wax technique was introduced by Dr.William H.Taggart .
-Introduced this technique and the casting machine in 1907 before the New York Odontological group .
1907 Taggart Devised a practically useful casting machine.
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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.
Inlay Mold
BlowpipeCompressed air cylinder
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LANE – Suggested the idea of casting by using investment containing high
percentage of Silica to plaster of paris at 650 degrees c. to compensate for casting shrinkage.
1910 – Von Horn introduced a different method of compensation- wax pattern investmentequal to mouth temperature using high silica content investment.
1932 – SCHEN developed a technique employing hygroscopic expansion of investment to compensate for shrinkage of casting.
1933 – Base Metal Alloys for RPD.
1942 – SONDER recognised that thermal expansion of investment was greatly inhibited by metal casting ring & advocated lining the ring with soft asbestos.
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1950-Development of resin veneers for gold alloys
1959-Porcelain fused to metal technique
1968 – Palladium Alloys for Gold Alloy.
1970 – Ni-Cr Alloys for RPD, FPD.
1971 – The Gold Standard.
1977 - B.G. Waterstrat et al produced the world’s first Ti dental casting .
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1978 - Wilmer B. Eames and John F. MacNamara concluded that:
• Vacuum casting machines produced sharper margins than the centrifugal
casting machines.
1980 – All Ceramic Technologies.
1984 – Classification of Casting Alloys.
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1985 - T.E. Donovan and L.E. White studied the increase in castability of an alloy by increasing the rate of acceleration of the centrifugal casting machine.
Waterstrat designed, split-chamber argon-vacuum casting equipment .
1989 - H. Hamanaka, H. Doi et al devised a new casting machine for dental casting of titanium and Ni-Ti alloys
1990 - Arturo R. Hruska introduced the Titanium Decontaminator, a device which decontaminates the mold before the actual process of casting was carried out.
1999 – Gold alloys for Palladium alloys.
2002 – ADA proposals for mechanical properties
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STAGES OF CASTING PROCEDURE
I) CLINICAL STEPS:
1. TOOTH PREPARATION: Preparation of the tooth structure to
receive the cast restoration.
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TOOTH PREPARATION FOR INLAY CAST RESTORATION
Wall’s proper Facial & lingual walls parallel to the long axis of crown
Occlusal bevel 30-45 degrees to long axis of crown on outer plane of walls.
Flat pulpal floor
Axial wall Flat or slightly rounded in bucco-lingual direction
Gingival floor Flat in bucco-lingual direction
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2. MAKING OF IMPRESSION
- Impression of the prepared tooth structure.
- Indirect technique fabricating cast restorations.
OBJECTIVES OF IMPRESSIONS FOR INDIRECT FABRICATION OF CAST RESTORATIONS:
- Record most finite details
- Duplicate unprepared surface areas beyond peripheries of preparation
- Dulplicate surface anatomy of adjacent & opposing teeth.
- Duplicate surface anatomy of adjacent soft tissues.
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TYPES OF IMPRESSION MATERIALS CHOOSEN
Reversible agar hydrocolloid
Irreversible Alginate hydrocolloid
Polysulfide rubber base
Condensation polymerization silicone rubber base
Additional polymerization silicone rubber base
Polyether rubber bases
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3. PREPARATION OF MASTER DIE OR CAST
A die or cast is formed which is a duplicate of the intra oral structure. (or)
The positive reproduction of the impression involving only the prepared tooth for the processing of inlays or bridge structures.
A) REQUIREMENTS OF AN IDEAL DIE MATERIAL
1.Accuracy & Dimensional stability .
2. Reproducibility of fine detail .
3. Producing a smooth , hard surface .
4. Strength , resistance to shearing force , edge strength & abrasion resistance .
5 . Ease of manipulation . 23
B) MOST COMMONLY USED MATERIALS ARE:
Certified Type IV improved stone with S.E. of 0.1% or less.
Certified Type V improved stone with S.E. of 0.3%.
Electroformed dies
Epoxy resins
ADVANTAGES:
Easy to use
Inexpensive
Compatible with most of the impression materials.
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TYPE IV IMPROVED STONE
TYPE V IMPROVED STONE
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C) METHODS OF ALTERING DIE DIMENSIONS:
- Addition of Accelerator or Retarder can be added to Type IV Stone to reduce its S.E. < 0.1% & to reduce the diameter of the die.
GYPSUM DIES ARE SOMETIMES MODIFIED:
- Make more abrasion resistant.
- Change the dimensions of the dies.
- Increase the refractoriness of the dies.
TO INCREASE THE ABRASION RESISTANCE:
- Silver coating
- Coat the surface with cyanoacrylate &
- Adding the die hardener to the gypsum.
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DIE HARDENERS
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D) DIE LUBRICANTS:
- To avoid sticking of wax pattern to die.
INDICATIONS:
- Easy removal of casting from die.
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E) DIE SPACER:
To produce relief space for cement.
Film thickness- 25microns
Applied in several coats to within 0.5mm of the preparation finish line to provide relief for the cement luting agent.
MATERIALS USED:
• Mostly resins are used.
• Model paint
• Colored nail polish
• Thermoplastic polymers dissolved in
volatile solvents.
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4. WAX PATTERN PREPARATION
• 1st procedure in the casting of an inlay or crown for the lost-wax process is the preparation of a dental wax pattern.
• Direct Wax Technique – Pattern made within the tooth.
Indirect Wax Technique – Pattern prepared within a die.
INLAY WAX – Specialized dental wax applied on the die surface for preparation of direct & indirect patterns - Lost Wax Technique – Casting metals & Hot pressing of ceramics.
American National Standards Institute / American Dental Association Sp No 4:
Type I – Medium Wax – Direct techniques .
Type II – Soft Wax – Indirect techniques . 31
INLAY CASTING WAX
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Accurate reproduction of missing tooth structure.
Forms outline of the mold into which molten metal is poured.
A) COMPOSITION OF INLAY WAX:
Paraffin wax
Gum Dammar
Carnauba wax
Candelillia wax
Ceresin
Bees wax
Colouring agent
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PARAFFIN WAX
-40-60%
• Derived from high boiling fractions of petroleum
• Complex mixture of hydrocarbons of methane series & Amorphous or Microcrystalline phases .
• Type I always – paraffin- high melting point.
DISADVANTAGES:
• Flakes when it is trimmed
• No smooth, glossy surface.
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GUM DAMMAR
• Natural resin
• Improves the smoothness in molding
• Increased resistance to cracking and flaking
• Increases toughness
• Enhances smoothness and lusture of the surface.
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CARNAUBA WAX
• Fine powder leaves of tropical palms .
• Hard
• High melting point
• Decreases the flow at oral temperature
• Agreeable odour
• Contributes to the glossiness of wax surface
Replacements of Carnauba wax :
Complex nitrogen derivatives of higher fatty acids
Esters of acids derived from montan wax . 36
CANDELILLA WAX
Partially or entirely replace carnauba wax.
CERESIN
Modify – Toughness & Carving characteristics of the wax.
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B. DESIRABLE PROPERTIES OF INLAY WAX:
Uniformity
Color contrast
Completely rigid and dimensionally stable at all times until eliminated
No Flakiness when bent and molded after softening
Wax pattern has solidifed – carve wax at margins – conform to surface of the die.
ANSI/ADA Sp.No: 4
Melted wax when vapourised at 500C (932F) shall leave no residue in Excess of 0.10% of the original weight of the specimen .
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C) FLOW
Type I – 37C (98F) is 1%
Type II - 9%
Minimizes distortion of a well Carved pattern as it is withdrawn from an adequately tapered cavity in the tooth .
Min 70%-Max 90% at 45C
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D) THERMAL PROPERTIES
Low thermal conductivity – More time is required to both heat them uniformly and cool them to body temperature .
Thermal expansion data : Maximal TE allowed Between –
0.7% - Expansion with an inc. in temp. of 20 degrees from 37degrees.
0.35% - Contraction - Cooled from 37degrees - 25degrees
Average Linear coeffecient of Thermal expansion :
– 350 x 10-6/degreecentigrade
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Thermal Expansion of Inlay Wax
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INTERPRETATION & CLINICAL IMPORTANCE :
GLASS TRANSITION TEMPERATURE: Temperature at which change in expansion rate occurs .
- Rate of expansion increases abruptly above 35 degree (95 degrees F) .
- On completion of wax pattern , its removal from the tooth cavity and Transfer to the lab bring about a reduction in temperature and subsequent thermal contraction .
“A decrease of 12 degrees to 13degrees in temperature , from mouthtemperature to a room temperature of approx 24 degrees , causes a 0.4% linear contraction of the wax , or about o.04% change for each degree change in temperature . “
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E. MANIPULATION
Methods of softening wax :
1. Best method – Controlled temperature oven .
Why ? Since it’s a poor thermal conductor , a controlled heating device for a prolonged period ensures uniform softening .
2 . Dry heat – FLAME is preferred
- Twirled in the flame ? Always quick heating tends to melt the only the superficial layer . So , this will bring inner surface out Vice versa .
- Repeat till the wax is warm all over .
- Knead it and shape it the prepared cavity .
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3 . Hot water bath – Inclusion of water droplets .
4 . Swaging – The die and the softened wax in the die are mounted into a closed vessel containing water and a piston . When pressed , hydrostatic pressure is evenly applied over the pattern to adapt it to the cavity .
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According to Marzouk:
1. Formulation of a wax pattern by carving – Wax is overfilled & carved.
2. Incremental build-up of the wax pattern
3. Direct wax pattern
4. Anatomic core wax pattern
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F. WAXING INSTRUMENTS
- Dr.Peter K Thomas carvers
1 . Wax addition - PKT No 1 & 2 .
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No 7 or 7 A Waxing spatula – Large increments .
Technique :
- Heat the instrument .
- Touch the wax .
- Quickly reheat the shank 47
G. CARVING
Sharp instruments .
And never heat it
Use light pressure .
Always overfill and carve.
Hold the carvers partly over the remaining tooth structure and complete the carving with this guide .
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PKT No 4
NO 2 WARD CARVERS NO1/2 & 3 HOLLENBACK CARVERS
PKT No 5
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H. BURNISHING
- Slightly warm a blunt instrument and rub the wax , not as hot as it melts .
- Less effective than carving but easier to control and leaves a smoother surface ( esp. near margins ).
-Final polish with a silk cloth .
- In inaccessible areas , cotton wrapped around a toothpick .
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PKT No 3
Darby –Perry Trimmer (DPT ) No 6
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I. REMOVAL OF WAX PATTERN
DIRECT PATTERN:
Sprue former Attached to the pattern Removed directly in line with its path of withdrawl .
- Hook it with an explorer point and rotate it out of the cavity .
- In a MOD use a staple pin . Fasten it and insert a floss and hook it .
-Avoid finger contact it may produce a temperature rise .
INDIRECT PATTERNS:
- Lubricate the die - Sheet of washed rubber dam increases friction & aids removal.- Right hand fingers hold pattern & left hand die die pulled from pattern by
bending fingers of left hand. 52
J.WARPAGE OF WAX PATTERNS
- High tendency to warp or distort .
- Directly related to time of storage and temperature .
- Residual stresses developed in the pattern is associated with forces used to shape the wax originally .
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SOLUTION
- Softening the wax uniformly by heating at 50 degrees for 15 min.
- Warmed carving instruments and a warmed die .
- Adding wax in small amounts .
LOWER STORAGE TEMPERATURE :
- If a wax pattern is allowed to stand uninvested for longer than 30 Min – Keep it in a refrigerator – Distortion is less in lower temperature.
- It should be warmed to room temperature before investing .
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5. SPRUE FORMERMOLD: A negative form in which an object is cast/shaped.
SPRUE: The mold channel through which molten metal flows into the mold.
(or)
A Sprue is a channel in a refractory investment through mold through which molten metal flows.
SPRUED WAX PATTERN: Wax form consisting of the prosthesis pattern
with attached sprue network.
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A) PURPOSE OF SPRUE FORMER:
Create a channel – elimination of wax during burnout.
Channel – ingress of molten alloy during casting.
Compensate for alloy shrinkage during soldification.
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B) SELECTION OF SPRUE FORMER:
Strickland etal (1958) stated the importance of the type, shape, location & direction other than the size of the sprue.
Selection of the diameter & length of sprue former depends on the type & size of wax pattern, type of casting machine & type of flask or casting ring used.
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Wax
Plastic/Resin
Metal sprues
C. TYPE OF SPRUE FORMER
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- Perfectly cylindrical in shape& smooth surfaces.
-Influence the burn out technique
- Wax – Most common
Metal –Hollow metal sprue is more preferred .
-They hold less heat than a solid sprue , this avoids overheating.
- Filled with sticky wax.
Plastic
- Plastic sprues melt at a higher temperature than wax
- Thermal expansion of wax is around 5 times that of plastic 59
D) DIAMETER OF THE SPRUE FORMER
Diameter Same size as the thickest area of wax pattern. Melt velocity is proportional to sprue diameter.
In conjunction with the pressure of the casting machine & density of the molten metal, controls the rate & flow of the molten metal.
Larger diameter of the sprue Distortion. Smaller diameter Solidify before the casting itself &
Localized Shrinkage Porosity.
Solution: Reservoir sprues are used to help overcome this problem.
Range : 10-18 gauge (2.6-1.0mm)
Atleast 1.7 mm unless the pattern is extremely small
2.5 mm – Crowns
Large inlays – 14 gauge & Small inlay -16 gauge 60
E. SPRUE POSITION
Prefer At the occlusal surface (or)
Proximal wall or just below a nonfunctional cusp minimize grinding of occlusal anatomy & contact areas.
Ideal Area: Point of greatest bulk in the pattern to avoid distorting thin areas of wax during attachment to the pattern & permit complete flow of alloy.
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F. SPRUE ATTACHMENT
Asgar & Peyton (1959) Flaring should occur at Sprue/wax pattern junction.
Flared for High density alloys.
Often restricted for lower density alloys.
Best for the molten alloy to flow from thick section to surrounding thin areas (margins) rather than reverse.
Provide smooth flowing entry of gold into the mold and less porosity in the casting minimizes risk for Turbulence.
All attachments , must therefore be TRUMPETED or FILLETED to eliminate all sharp corners , angles and instrument marks .
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- Acts in the same way as a reservoir
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G. THE LENGTH OF THE SPRUE FORMER
- Should be long enough to properly position the pattern in the casting ring within 6 mm of the end of the ring yet short enough so that the molten alloy doesn’t solidify before it fills the mold .
( 6mm – Gypsum bonded investments & 3-4mm –Phosphate bonded )
- Average sprue length – Large inlay – 4-5mm
Small inlay – 3-4mm
- Short sprue – Moves the pattern more away from the end of the ring
The gases cannot be adequately vented
Porosity 65
Long sprues – Solidify before the mold –shrinkage porosity .
WHY 6mm ?
Provides adequate bulk of investment to withstand force of inrushing gold, yet still allows gases to escape from the end of the mold .
More than 6 mm –
The gold will solidify before the entrapped air can escape , resulting in rounded margins , incomplete casting , or mold fracture .
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H. SPRUE DIRECTION
Shd be directed away from any thin or delicate parts of pattern molten metal may abrade or fracture investment in this area.
Shd not be attached at a right angle to a broad flat surface lead of Turbulence within mold cavity & porosity.
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Sprued at 45 degree angle to proximal area satisfactory casting.
- Should always be directed towards the margins Favor the fine margins of the wax patterns .
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I. RESERVOIR
The reservoir is placed approximately 1.5mm from the pattern .
- Diameter should be greater than the bulkiest portion of the sprue .
Function – Prevents localized shrinkage porosity .
- Because of the large mass of alloy and position in the heat centre of the ring , the reservoir will remain molten to furnish liquid alloy into the Mold as it solidifies .
- Resulting shrinkage will occur in the reservoir bar rather than the restorations.
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J. CRUCIBLE FORMER
Funnel shaped.
Rubber, metallic or plastic.
Connected to sprue in same was as sprue attached to mold.
Wax pattern with sprue former is attached to crucible former in a special locking area.
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SPRUING
DIRECT SPRUING
The sprue former provides a direct connection between the pattern area and the sprue base/ crucible former.
When two thick portions of wax are separated by thin wax, then 2 separate sprues should be attached using direct spruing.
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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 spruecomplex consist of 3 parts:
Manifold sprue.
Horizontal running bar.
Feeder sprue.
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6. CASTING RING
-Investment material is poured in the casting ring & allowed to set around wax pattern.
CHOICE OF THE RING:
Rubber ring for HSE
Metal ring for TE
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RING LESS CASTING SYSTEM
-Ringless Casting System is designed to increase productivity by achieving consistently accurate results without the time-consuming steps associated with the use of metal casting rings.
-Utilizes durable, reusable plastic rings that are tapered allowing for unimpeded expansion of investment and easy removal of mold prior to burnout.
-This allows for quick and easy divesting after casting while reducing clean-up chores.
-Investment expansion is easier to control and not limited to the thickness of a ring liner.
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POWERCAST RINGLESS SYSTEM
ROUND RINGLESS CASTING
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CASTING RING LINERS
- The most commonly used technique to provide room for investment expansion.
Triple fold function :
- Freedom to expand which would otherwise be restricted by the ring .- Helps to offset the contraction of the more rapidly cooling ring while the gold alloy is being melted . - In a wet liner certain amount of hygroscopic expansion is afforded
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Traditionally – Asbestos ring liners
- Carcinogenic , biohazard .
NON ASBESTOS RING LINERS :
- Aluminium silicate ceramic liners – Can retain water on the surface .
- Cellulose ( paper ) liners – Better water absorption
Disadvantages: Burned away during casting , Difficult to secure in place.
- Absorb materials much less than asbestos , and theircombination with some gypsum bonded investments will produce fins
- Not compatible with phosphate bonded investments.
-Slightly higher expansion than asbestos liners. 79
Implications : Liner affords greater expansion in the investment
- Absorbed water causes a semihygroscopic expansion as it is drawn into the investment
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- The use of 1 liner increases the normal setting expansion compared with no
liner.
- A thicker liner provides even greater semi hygroscopic expansion and
also afford more unrestricted normal setting expansion .
LENGTH OF THE LINER :
Two concepts –
When maximum expansion is required – Flush with the open end
Smaller casting 3-6 mm (avg. 3.25mm) short of the end 1. Lock for the liner 2 . Uniform expansion
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MANIPULATION OF A CASTING RING LINER
-Cut the liner to fit the inside diameter of the casting ring , no over lap.
- Dry liner technique – Dry liner is tacked in position with sticky wax .
-Wet liner technique –Lined ring is immersed in water and excess water is shaken away . ( Squeezing-uneven water removal and expansion )
- Avoid touching or adapting with fingers reduce the cushioning effect.- Attach the liner firmly to the ring by wax to prevent it from “riding up” during investing and inadvertently affecting the size of the casting .
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7. INVESTING PROCEDURE
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A. WETTING AGENTS or DEBUBBLIZERS
– Wax surfaces are not easily wetted by water
-If not covered by investment will develop surface irregularities in casting .
- Wettax- mild soap solution.
-These agents reduce the surface tension of the wax pattern , promotes better wetting of the surface .
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These agents decrease the contact angle of the liquid with the wax surface.
Fig1 : 98degrees – untreated surface
Fig2 : 61degrees – treated surface more affinity for water - investment being wetted with more ease
Fig3 : 91degrees –Rinsed with tap water and blotted dry
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MANIPULATION
- Paint the debublizing solution on the pattern .
- Gentle air dry
- Don’t soak or rinse in water
- Don’t allow pooling in internal line angles of the pattern .
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B. INVESTMENT MATERIALS
- A heat resistant or a refractory material used to form a
mold into which a metal or alloy is cast .
(OR) A molding material that surrounds the pattern & subsequently hardens & forms the mold after the wax pattern is eliminated.
-The operation of forming the mold is called Investing .
Types – Gypsum bonded investments
Phosphate bonded investments
Ethyl silicate bonded investments 87
C. REQUIREMENTS OF AN IDEAL INVESTMENT MATERIAL:
Easily manipulated.
Sufficient strength at room temperature.
Stability at higher temperature
Sufficient expansion
Beneficial casting temperatures
Porosity
Smooth surface
Ease of divestment
Inexpensive.
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GYPSUM BONDED
INVESTMENT MATERIALS
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D. PROPERTIES OF GYPSUM BONDED INVESTMENT MATERIALS:
i) COMPOSITION:
REFRACTORY MATERIAL: SILICA - 60to 65% – 4 allotropic forms:
Quartz
Cristabolite
Tridymite
Fused quartz
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BINDER: 30 to 35 %
Alpha hemihydrate form of gypsum OR Dental Stone.
MODIFIERS: 5%
Silica- Counterbalances gypsum shrinkage .
Chlorides – Reduces shrinkage below 700C
Sodium , Potassium , Lithium chlorides
BORIC ACID – Disintegrates during heating – Roughened Casting
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ii) CLASSIFICATION:
Acc. To ADA Sp.no: 2- Casting Investments for dental gold alloys.
TYPE I – Inlays & Crowns Thermal expansion
TYPEII – Inlays & Crowns Hygroscopic expansion
TYPE III – Partial dentures with gold alloys.
Can withstand temperature upto 700 degrees C.
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iii) SETTING TIME:
Not <5mins & not > 25mins.
Modern inlay investments set initially in 9 to 18 mins.
iv) SETTING EXPANSION:
PURPOSE: Aid in enlarging the mold to compensate partially for the casting shrinkage .
3 Types: 1. Normal Setting Expansion
2. Hygroscopic Setting Expansion
3. Thermal Expansion.
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1. NORMAL SETTING EXPANSION-
- A mixture of silica and calcinated gypsum (calcium sulphate hemihydrate) results in setting expansion greater than that of the gypsum product used alone.
- Silica particles probably interfere with the intermeshing and interlocking of the cystals as they form thrust of the crystals is outward during growth increase expansion.
ANSI / ADA Specification No.2 for Type I investment permits a maximum setting expansion in air of only 0.6%.
Setting expansion of such modern inlay investments is approx 0.4%.
It can be regulated by retarders and accelerators.
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2. HYGROSCOPIC EXPANSION
When gypsum products are allowed to set in contact with water, it leads to outward growth of crystals expansion which is greater in magnitude than normal setting expansion.
ANSI / ADA Specification No.2 for Type II investments requires
Minimum setting expansion in water of 1.2%;
Maximum expansion permitted is 2.2%.
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FACTORS AFECTING HSE
Effect of composition
Effect of W:P Ratio
Effect of Temperature
Effect of time of immersion
Effect of Spatulation
Effect of Shelf Life of Investment
Effect of Confinement
Effect of the Amount of Added water.
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3. THERMAL EXPANSION:
Achieved by placing the mould in a furnace not greater than 700° C.
ANSI / ADA Specification No.2 requires that the thermal expansion of
- Type II investments be between 0% and 0.6% at 500° C.
For Type I investments, which rely principally on thermal expansion for compensation, the thermal expansion must not be less than 1% nor greater than 1.6%.
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FACTORS AFFECTING THERMAL EXPANSION:
Effect of W:P ratio
Effect of Chemical modifiers
v) STRENGTH:
- Compressive strength for the inlay investments should not be less than 2.4 MPa
when tested 2 hrs after setting.
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vi) THERMAL STABILITY: Gypsum bonded investments decompose above 1200°C.
CaSO4+ SiO2 → CaSiO3 + SO3
CaSO4 + 4C →CaS + 4CO3
CaSO4 + CaS→ 4CaO + 4SO2
Effects can be minimized by ‘heat soaking’ the investment mold at the casting temperature to allow the reaction to be completed before casting commences.
vii) POROSITY:
The more gypsum crystals that are present in the set investment, the less is the porosity.
The more uniform the particle size, the greater its porosity.101
PHOSPHATE BONDED INVESTMENT MATERIALS
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E. PROPERTIES:
i) TYPES:TYPE I – For inlays, crowns and other fixed restorations.TYPE II- For partial dentures and other cast, removable restorations.
ii) COMPOSITION:
POWDER- Ammonium diacid phosphate (NH4 H2 PO4) Silica – Refractory
Magnesium oxide- reacts with phosphate ions.
LIQUID- In the form of silica sol in water.
iii) SETTING REACTION:
NH4 H2 PO4 + MgO→ NH4 MgPO4 + H2O103
iv) SETTING EXPANSION-
With use of full strength liquid, about 0.4% attained.
Hygroscopic technique 0.6%- 0.8% can be realized.
THERMAL EXPANSION-
About 0.8% can be attained with a 50:50 mixture of liquid and water.
1% to 1.2% Use of undiluted liquid.
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v) ADVANTAGES:
Easy to handle without breaking before they are placed in a furnace for the wax burnout process and strong enough withstand the impact and pressure of centrifugally cast molten alloy.
Provide setting and thermal expansions high enough compensate for the thermal contraction of cast metal prostheses or porcelain veneers during cooling.
Ability to withstand the burnout process(~1 to 1.5 hrs)with temperatures that reach 900°C, and they can withstand temperatures upto 1000° C for short periods of time.
105
vi) DISADVANTAGES :
When used with higher melting alloys, those with casting temperatures greater than about 1,375° C, coupled with high mold temperatures result in mold breakdown and rougher surfaces on castings.
High strength of these investments can make divesting ( removal of the casting from the investment) a difficult and tedious process.
When higher expansion is required, more silica liquid is used results more dense and less porous mold incomplete castings if a release for trapped gases is not provided.
106
ETHYL SILICATE BONDED INVESTMENTS
107
F. PROPERTIES:
- Silica is the binder.
SETTING EXPANSION (linear)-
Setting contractions of 0-0.4 %.
THERMAL EXPANSION-
- 1.5% to 1.8% can be attained between room temperature and 1000°C to 1177° C.
108
ADVANTAGES :
Cast high temperature cobalt- chromium and nickel-chromium alloys.
Good surface finishes, low distortion and high thermal expansion (good fit).
Thin sections with fine detail.
109
DISADVANTAGES:
Added processing attention and the extra precaution needed in handling the low strength fired molds.
The low strength and high thermal expansion require a more precise burnout process and firing schedule to avoid cracking and hence, destruction of a mold.
110
G. PREPARATION OF THE INVESTMENT MIX
Mixing of the investment is done by:
Hand mixing
Vacuum mixing
111
HAND INVESTING
- Water is added first slow addition of powder to remove air from powder.
-Hand spatulate the mix to incorporate the powder quickly.
- Cover of the bowlmechanical mixermixed by hand.
- Coat the wax pattern with the investment
- Carefully coat the internal surface & the margin of the pattern
112
-Fill the ring slowly, starting from the bottom .
-Ring completely filled- leveled with top by edge of plaster spatula.
-Phosphate bonded investment – slightly overfilled – top is not levelled off.
-Investment set 45 to 60 min.
113
VACUUM INVESTING
-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
METHOD
114
H.SETTING OF INVESTMENT
It can be:
-In open air
-Hygroscopic technique
-Controlled water added technique
115
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.
116
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
117
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.
118
8. WAX ELIMINATION OR BURN OUT
Elimination of the wax pattern from the mold of set investment material.
Investment setmin. 1 hr.
Ideally kept in oven when mold is wet.
Delayed for several hours kept in humidor.
Rubber crucible former removed.
Place the sprue hole down at first , for easy draining out of wax Eliminated as a liquid .
Invert ring , for the oxygen in the oven atmosphere to circulate more readily , to form gases rather than fine carbon which may interfere with venting .
119
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IDEAL TEMPERATURE RANGES
- 500 degrees for one hour . ( Craig )
-Gypsum bonded investments -468degrees for hygroscopic technique 20mins
650degrees for thermal expansion (or)
-Slow heating to 650-700 degrees in 60 minutes and held for 15 to 30 minutes at the upper temperature .
-Phosphate bonded investments – Room temp. to max. 700 to 1030 degrees C depending on alloy used for 30 mins. 315°C rapid heating held at the upper temperatures for 30 mins.
- General rule – Add 5 minutes to the wax elimination for every extra ring placed in the oven at 500degrees C.
121
HYGROSCOPIC LOW HEAT TECHNIQUE
Sources of compensation expansion
• 37degree water bath
• Hygroscopic expansion by the warm water .
• Thermal expansion in the oven
ADVANTAGES :
- Less mold degradation
- Cooler surface for smoother castings
- Direct placement at 500 degrees – Large laboratories , time factor .
122
HIGH HEAT THERMAL EXPANSION
Compensation expansion factor – Thermal expansion produced by the high heat burn out .
Accessory expansion factors :
- Setting of gypsum products .
- Water entering the wax pattern from liners and hence small amount of Hygroscopic expansion .
123
TIME ALLOWABLE FOR CASTING
Investment contracts thermally as it cools.
When thermal expansion / high heat technique Investment loses heat after heated ring removed from furnace and mold contracts.
124
9. CASTING CRUCIBLES
3 types of casting crucibles are available : Clay, Carbon and Quartz (Zircon alumina).
Clay crucible are – many of crown bridge alloys such as high noble-alloy.
Carbon crucible high noble crown bridge , higher fusing , gold based metal ceramic.
Quartz crucibles High fusing alloys of any type suited for alloys that have a high melting range and are sensitive to carbon contamination.
Palladium silver from metal ceramic copings.
Nickel/cobalt based alloys.125
CARBON CRUCIBLES
126
127
128
10. METHODS OF MELTING THE ALLOY
METHODS :
TORCH ELECTRICAL
Gas/Air (Most Common) Resistance
Gas/Oxygen – PFM,Pd Induction
Air /Acetylene Direct Current Arc
Oxygen /Acetylene
129
TORCH MELTING
- The alloy is melted in a separate crucible by a torch flame and is cast into the
mold by centrifugal force.
-Temperature of gas-air flame is influenced by
i) Nature of the gas &ii) Proportion of gas and air in the mixture.
-Considerable care Obtain a non-luminous brush flame, with combustion zones clearly differentiated for melting the alloy .
130
ZONES OF A FLAME
Zone 1 – Directly from the nozzle
Air and gas are mixed before combustion .
No heat is present
Zone 2 – Combustion zone
Gas & Air are partially burned
Color – GREEN
Oxidizing – KEEP AWAY 131
Zone 3 : Reducing zone
Hottest part of the flame.
Most effective zone for melting and should be kept constantly over the alloy .
Color – Blue
Zone 4 : Oxidizing zone
Combustion occurs with the oxygen in the air .
KEEP AWAY .
132
CRAIG’S METHOD FOR DETERMINING THE EFFECTIVE FLAME :
-Checking & interpreting the flame condition.
Apply the flame to a copper coin – on a soldering block.
Bright & Clean Dark , dull red colour
Oxidation and ineffective heating
Visual scenario practically : Morphology
Spongy Small globules of fused metal appear Spheroidal shape
Color :
- The molten alloy is light orange and tends to spin or follow the flame when it is moved slightly.
133
134
Disadvantages :
Excessive heat may distill lower melting components .
Overheating – gases to dissolve in the casting – porosity
Highly technique sensitive
AIR ACETYLENE & OXYGEN ACETYLENE GAS
- These were designed mainly for Cobalt chromium base alloys higher fusion temperatures
Advantage : Hottest flame hence faster burnout .
135
ELECTRICAL RESISTANCE -- HEATED CASTING MACHINE
PRINCIPLE :
- During Electrical melting of alloys heat energy is produced when electric
current is passed through a conductor depending upon the voltage applied
across it.
- The alloy is melted electrically by a resistance heating .
- Current is passed through a resistance heating conductor, and automatic
melting of the alloy occurs in a graphite or ceramic crucible.
136
- Resistance heat develops when flow of current was opposed by a opposite power
e p
e
pe
p
e
p
RESISTANCE HEATING
137
Advantages:
For metal ceramic prosthesis. Base metals in trace amounts that tend to oxidize on overheating. Crucible located flush against casting ring.
Carbon crucibles should not be used in melting of:
High Pd Pd-Ag Ni-Cr Co-Cr
138
139
INDUCTION MELTING MACHINE
The alloy is melted by an induction field that develops within a crucible surrounded by water-cooled metal tubing.
140
VERTICAL CRUCIBLE POSITOINED WITHIN INDUCTION COIL
WATER COOLED INDUCTON COIL
141
142
The electric induction furnace is a transformer in which an alternating current flows through the primary winding coil and generates a variable magnetic fieldin the location of the alloy to be melted in a crucible
143
Alloy reaches melting temp. forced into mold by air pressure, or by vacuum.
It is more commonly used for melting base metal alloys, more in jewelry .
Not been used for noble alloy casting as much as other machines.
144
--------------------------------------------
--------------------------------------------
DIRECT CURRENT ARC MELTING MACHINE
Arc is produced between two electrodes: The alloy and the water-cooled
tungsten electrode generates heat.
anode
cathode
AB
C
D
145
The temperature within the arc exceeds 4000oC and the alloy melts very quickly.
This method has a high risk for over heating the alloy.
146
- Casting machines provide the means for transferring the molten alloy from the crucible to the mold
11. CASTING MACHINES
147
OBJECTIVES OF CASTING
1) Alloy should be heated as qucikly as possible to a completely molten condition
(above liquidus temp.)
2) Oxidation of alloy prevented heating metal with a well adjusted torch & small amount of flux on metal surface.
3) Adequate force applied to force the well-melted metal into the mold.
148
149
TYPES
CENTRIFUGAL CASTING MACHINE:
The alloy is melted in separate crucible by a torch flame and the metal is cast into
the mold by centrifugal force.
HIGH FREQUENCY CENTRIFUGAL CASTING MACHINE:
The alloy is melted electrically by a resistance or induction furnace, then cast into
the mold centrifugally by motor or spring action.
VACUUM OR PRESSURE ASSISTED CASTING MACHINE:
The alloy is melted by a torch flame or electrically and then cast into mold by air
pressure and / or by vacuum.
.
Acc. To William J. O’ Brien:
1. Centrifugal force type
2. Pneumatic force (or) Air pressure type.
150
CASTING TECHNIQUES
CENTRIFUGAL FORCE TYPE PNEUMATIC FORCE
Spring loaded
torch melting machine
Induction melting machine
Electrical resistance heated casting machine
Electric arc vacuum casting machine
151
ACCORDING TO SH SORATUR
1. Centrifugal casting machine
2. Steam Pressure machine-Solbrig machine
3. Air pressure machine – Hereus
4. Spring wound Electrical resistance-melting furnace-casting machine
5. Induction melting casting machine
152
VACUUM OR PRESSURE ASSISTED CASTING MACHINE
1ST evacuate the melting chamber to reduce oxidation.
Apply air pressure uniformly about the casting ring forcing molten alloy into mold.
Vacuum is applied to the bottom of the mold.
Molten alloy is “PUSHED & SUCKED” into the mold by gravity or vacuum.
Used for titanium and titanium alloys.
153
TITEC F210 M (OROTIG)
COMBILADOR CL-G 77
154
DEGUTRON
HIGH FREQUENCY CENTRIFUGAL CASTING MACHINES
MEGAPULS 3000
155
CENTRIFUGAL CASTING MACHINE
This machine utilize the centrifugal force which is defined as a radial force
radiating outward from the center of rotation of a body, for casting.
156
157
- The machine works on ‘broken-arm’ principle crucible is attached to the
broken-arm, which accelerates the effective initial rotational speed of the
crucible and casting ring thus increasing the linear speed of the liquid casting
alloy as it moves into and through the mold.
- To counter the weight of the molten metal in the crucible and casting ring,
balancing arm is provided with the balancing weights.
158
The casting machine is given three (or) four clock wise turns and locked in position with the pin.
159
A clay\carbon crucible for the gold alloy being cast is placed in the machine.
The torch is lit and adjusted.
160
This preheating avoids excessive slag formation during casting.
PREHEATING CRUCIBLE
161
- Alloy is placed on the inner sidewall of the crucible heated in the reducing part of the flame until it is ready to cast.
- Properly adjusted torch develops adequate temp. 870-1000 degrees C.
-Alloy should be approx. 38°C to 66°C above liquidus temperature.
-The casting should be made when proper temperature is reached
- No more 30 secs should be allowed to elapse between the time the ring is removed from the oven and the molten alloy is centrifuged into the mold.
- When the alloy is molten,slide the crucible against the ring, sprinkle flux over the metal.
162
163
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.
164
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.
165
Hold the casting arm so that the pin drops away
Release the arm and rotate till it comes to rest
Providing enough force to cause the liquid alloy to flow into the mold.
-
166
•Once the metal fills the mold there is a hydrostatic pressure gradient developed along the length of the casting.
• The pressure gradient from the tip of the casting(0.21 to 0.28 Mpa/ 30 to 40 psi) to the button surface is quite sharp and parabolic in function, coming to zero at the button surface.
•Because of this gradient there is also a gradient in the heat transfer rate, such that the greatest rate of heat transfer to the mold is at the high pressure end of the gradient (i.e. the tip of the casting).
•Therefore, solidification progresses from tip to the button side.
•The arm should be stopped with brake lever only when it gets slowed down naturally and the ring is removed with casting tongs.
167
168
QUENCHING
After the casting has solidified, the ring is removed and quenched in water as soon as the button exhibits a dull-red glow.
Ring is quenched sprue open & button upwards 2/3rd length of ring is dipped in water.
169
ADVANTAGES OF QUENCHING
1.Upward direction of sprue compensates casting shrinkage.
2. Metal alloy left in an annealed condition for burnishing, polishing,..
3. When water contacts the hot investment easy removal & soft granular investment.
170
DIVESTING
Removal of Investment / Recovery of Casting.
End of the ring for about ¼ inch
Bulk – finger pressure.
- Sprue is removed from the restoration using an carborundumseparating disk/ abrasive disk mounted in a hand piece.
171
172
12. CLEANING & FINISHING THE CASTING
173
PICKLING
- Removal of oxide residues of carbon by heating the discoloured casting in an acid.
- SOLUTIONS USED: 50% HCL, PHOSPHORIC ACID, HYDROFLUORIC ACID
-Advantages of HCl: Aids in removal of residual investment as well as oxide coating
-Disadvantages:
Likely to corrode laboratory metal furnishings Fumes are health hazard
-Method of cleaning : Place the casting in test tube or dish and pour acid over it
- Other methods: Heating the casting and then dropping into the pickling solution
174
175
SAND BLASTING
Casting is held in an sand blasting machine to clean the investment from the surface.
The blasting materials used are:
Sand shells
Recycled Aluminium oxide with pressure of 100psi
Garnet
Ultrasonic cleaners
Abrasive spray devices
176
Finishing and polishing : Brown or White Al2O3 stones are used.
177
- Rag or felt wheels impregnated with abrasives are used in the initial phase of this stage.
- Final polishing is achieved using various oxides of tin and aluminium used in conjunction with a small rag or chamois buffing wheel, followed with an iron oxide rouge.
- Residual traces of rosin or waxlike matrix from oxides Polishing compound remover followed by a hot, soapy water rinse.
CAUSES OF CASTING DEFECTS
ACCORDING TO PHILLIP’S
1. DISTORTION
2. SURFACE ROUGHNESS, IRREGULARITIES, DISCOLORATION1. a. Air bubbles2. b. Water film3. c. Rapid heating4. d. Under heating5. e. L/P ratio.6. f. Prolonged heating7. g.Temperature of alloy8. h.Casting pressure9. i.Composition of investment10. j. Foreign bodies11. k. Impact of molten alloy12. l. Pattern position13. m. Carbon inclusions 179
3. POROSITY
I. Solidification defects
A. Localized shrinkage porosityB. Microporosity
II. Trapped gases
A. Pinhole porosityB. Gas inclusionsC. Subsurface porosity
III. Residual air
4. INCOMPLETE CASTING
180
An unsuccessful casting results in considerable trouble and loss of time.
Defects in castings can be avoided by strict observance of procedures governed by certain fundamental rules and principles.
181
ACCORDING TO O’BRIEN CASTING PROBLEMS CAN BE CLASSIFIED :
A. GENERAL PROBLEMS
1. Problems with accuracy
2. Problems with distortion
3. Problems with bubbles
4. Problems with surface roughness
5. Problems with fins on the surface or margins
6. Problems with short and rounded margins
7. Problems with miscasting
8. Problems with pits182
B. PROBLEMS WITH INTERNAL POROSITY
9. Problems with localized shrinkage porosity
10. Problems with subsurface porosity
11. Problems with microporosity
C. PROBLEMS WITH EXTERNAL POROSITY
12. Problems with back pressure porosity
183
1. DISTORTION
Any marked distortion of the casting is related to distortion of the wax pattern
Setting and hygroscopic expansions of the investment may produce a nonuniform expansion of the walls of the pattern.
The gingival margins are forced apart by the mold expansion, whereas the solid occlusal bar of wax resists expansion during the early stages of setting.
184
Configuration of the pattern, the type of wax, and the thickness influence the distortion.
Distortion increases as the thickness of the pattern decreases
Less the setting expansion of investment less the distortion
STUDY: Cast post made with an unlined metal casting ring may exhibit anisotropic shrinkage which could result in distortion.
185
Wax too hot- Excessive shrinkage results on cooling.
Wax too cool- The pattern undergoes stress release with change in shape.
Insufficient pressure during waxing- The pattern distorts because of thermal shrinkage.
Delayed investment- The sooner the investment is complete, the less distortion
Heating pattern during spruing- Create distortion.
Overheating casting during soldering procedure- This warps or melts the margins.
186
TO AVOID DISTORTION
Appropriate selection of investing material with less setting expansion
Invest the wax pattern as early as possible
Proper handling of the wax pattern
187
2. SURFACE ROUGHNESS, IRREGULARITIES, AND DISCOLORATION
The surface of a dental casting should be an accurate reproduction of the surface of the wax pattern from which it is made.
Excessive roughness or irregularities on the outer surface of the casting necessitate additional finishing and polishing.
188
SURFACE ROUGHNESS
DEFINED: Relatively finely spaced surface imperfections whose height, width, and direction establish the predominant surface pattern.
PROBLEMS WITH SURFACE ROUGHNESS
Water/powder ratio- A high ratio increases the roughness of the mold.
Excess wetting agent or salivary contamination - This may form a film on the pattern surface and be reproduced on the casting surface.
Prolonged heating or overheating of the mold - may cause investment disintegration, Roughness appears general and feels sharp.
Premature heating of casting investment- Wait a minimum of 45 minutes for burnout.
189
SURFACE IRREGULARITIES
Surface irregularities are isolated imperfections, such as nodules, that are not characteristic of the entire surface area.
Irregularities on the cavity surface prevent a proper seating of an otherwise accurate casting.
190
a) AIR BUBBLES:
Small nodules on a casting are caused by air bubbles that become attached
to the pattern during or subsequent to the investing procedure.
Nodules can sometimes be removed not in a critical area.
Nodules on margins or on internal surfaces removal of these irregularities might alter fit of the casting
192
Inadequate vacuum or ineffective painting procedure- Vacuum must have
at least 26mm mercury for vacuum investing.
Water/powder ratio - Investment is too thick, it will not cover the pattern completely.
Excessive vibration of the ring- Produces small nodules.
NODULES
193
194
TO AVOID AIR BUBBLES
Proper mixing of the investment if manual method is used
Use of a mechanical mixer with vibration both before and after mixing
Vaccum investing technique is the best method.
Use of a wetting agent in a thin layer
195
b) WATER FILMS:
Wax is repellent to water and if the investment becomes separated from the wax pattern in some manner, a water film may form irregularly over the surface.
Appears as minute ridges or veins on the surface.
Too high L/P ratios .
Pattern is slightly moved, or vibrated after investing or if the painting procedure does not result in an intimate contact of the investment with the wax pattern .
196
TO AVOID WATER FILMS
Use of a wetting agent in a thin layer
Using Correct water - powder ratio
197
c) RAPID HEATING
Result in formation of fins or spines on the surface of the casting
Due to the flaking of the investment when water or steam pours into the mold
A surge of steam or water may carry certain salts into the mold that are left behind in the walls as the water evaporates
198
FINS ON THE SURFACE OR MARGIN - DUE TO
Prolonged heating- Cracks in the investment that radiate out from the surface of the pattern.
Heating rate is too rapid- Cracks may appear in the investment, caused by nonuniform heating of investment.
Water/powder ratio- A high ratio produces a weak investment that may crack.
Excessive casting pressure- Metal impact may cause investment fracture.
Cooling of the investment prior to casting- Cracks in the investment
199
TO AVOID FINS OR SPINES
Gradual heating of the mold- atleast 60 min should elapse during the heating of the investment- filled ring from room temperature to 700º C.
Greater the bulk of the investment, more slowly it should be heated.
200
d) UNDERHEATING
Incomplete elimination of wax residues may occur if the heating time is too
short or if insufficient air is available in the furnace.
Low-temperature investment techniques
Voids or porosity may occur in the casting from the gases formed when the hot alloy comes in contact with the carbon residues.
Casting may be covered with a tenacious carbon coating that is virtually impossible to remove by pickling
201
e)LIQUID/POWDER RATIO
The amount of water and investment should be measured accurately.
The higher the L/P ratio, the rougher the casting.
Too little water Investment thick and cannot be properly applied to the pattern.
In vacuum investing, the air may not be sufficiently removed.
TO AVOID
Use the correct W/P ratio according to manufacturer’s instructions.
202
F)PROLONGED HEATING
High-heat casting technique a prolonged heating of mold at the casting temperature disintegration of the gypsum-bonded investment, and the walls of the mold are roughened.
Products of decomposition are sulfur compounds that may contaminate the alloy to the extent that the surface texture is affected.
TO AVOID
Thermal expansion technique is employedmold heated to the casting temperature and never higher.
The casting should be made immediately.
203
g) TEMPERATURE OF THE ALLOY
Alloy is heated to too high a temperature before casting, the surface of the investment is likely to be attacked, and a surface roughness may result.
In all probability, the alloy will not be overheated with a gas-air torch when used with the gas supplied in most localities.
If other fuel is used, special care should be observed that the color emitted by the molten gold alloy, for example, is no lighter than a light orange.
204
h) CASTING PRESSURE
Too high a pressure during casting can produce a rough surface on the casting
TO AVOID
a gauge pressure of 0.10 to 0.14 MPa in an air pressure casting machine (or)
3 to 4 turns of spring in an average type of centrifugal casting machine is sufficient for small castings.
205
i) COMPOSITION OF THE INVESTMENT
The ratio of the binder to the quartz influences the surface texture of the casting.
A coarse silica causes a surface roughness.
If the investment meets ANSI/ADA specification n0.2, the composition is not a factor for surface roughness
206
j) FOREIGN BODIES
When foreign substances get into the mold, a surface roughness may be produced.
Rough crucible former with investment clinging bits of investment are carried into the mold with the molten alloy
Carelessness in the removal of the sprue former
Sharp, well-defined deficiencies pieces of investment and bits of carbon from a flux.
207
Bright-appearing concavities Flux being carried into the mold with the metal.
Surface discoloration and roughness can result from sulfur contamination
The interaction of the molten alloy with sulfur produces a black or grey layer on the surface of gold alloys that is brittle and does not clean readily during pickling.
208
K) IMPACT OF MOLTEN ALLOY
Direction of sprue formermolten gold alloy does not strike a weak portion of the mold surface.
Molten alloy may fracture or abrade the mold surface on impact, regardless of its bulk.
TO AVOID
Proper Spruing To prevent the impact of molten metal at an angle of 90 degrees to investment surface.
209
l) PATTERN POSITION
If several patterns are invested in the same ring, it causes breakdown or
cracking of the investment if the spacing between the patterns are less than 3mm.
B/c expansion of wax is much greater than that of the investment.
TO AVOID
Do not place several patterns too close together if invested in the same ring
Avoid too many patterns in the same plane in the mold
210
m) CARBON INCLUSIONS
Carbon, as from a crucible, an improperly adjusted torch, or a carbon-
containing investment, can be absorbed by the alloy during casting.
May lead to the formation of carbides or even create visible carbon inclusions.
211
3. POROSITY
Porosity may occur both within the interior region of a casting and on the external surface.
The latter is a factor in surface roughness, but also it is generally a manifestation of internal porosity.
Not only does the internal porosity weaken the casting but if it also extends to the surface, it may be a cause for discoloration.
If severe, it can cause plaque accumulation at the tooth-restoration interface, and secondary caries may result.
Although the porosity in a casting cannot be prevented entirely, it can be minimized by use of proper techniques.
212
POROSITIES IN METAL ALLOY CASTINGS MAY BE CLASSIFIED AS FOLLOWS:
I. Solidification defects
A. Localized shrinkage porosityB. Microporosity
II. Trapped gases
A. Pinhole porosityB. Gas inclusionsC. Subsurface porosity
III. Residual air213
LOCALIZED SHRINKAGE POROSITY
CAUSE: Premature termination of the flow of molten metal during solidification.
Linear contraction of noble metal alloys in changing from a liquid to a solid is at least 1.25%.
SOLUTION: Continual feeding of molten metal through the spruemake up for the shrinkage of metal volume during solidification.
Generally occurs : Near the sprue-casting junction
Alloy or mold temperature is too low -Rapid solidification of the alloy
214
TO AVOID LOCALIZED SHRINKAGE POROSITY
Using sprue of appropriate thickness
Attach the sprue to the thickest portion of the wax pattern
Flare the sprue at the point of attachment or placing a reservoir close to the wax pattern
215
HOT SPOT: The entering metal impinges onto the mold surface at a point and creates a higher localized mold temperature.
A hot spot may retain a localized pool of molten metal after other areas of the casting have solidified.
This in turn creates a shrinkage void, or suck-back porosity.
216
SUCK -BACK POROSITY: Hot spot causes the local region to freeze last
and results suck-back porosity.
Suck-back porosity often occurs at an occlusoaxial line angle or incisoaxial line angle that is not well rounded.
217
TO AVOID SUCK BACK POROSITY
Flare the sprue at the point of attachment to the wax pattern
Reduce the mold – melt temperature differential, that is lowering the casting temperature by about 30ºC.
With a higher mold temperature, the difference in temperature between the investment located around the sprue and the investment in the area of the pulpal floor of the full crown is decreased.
This decrease helps the molten alloy at the pulpal floor to solidify before the alloy at the sprue
218
MICROPOROSITY
Occurs from solidification shrinkage but is generally present in fine-grain alloy castings when the solidification is too rapid for the microvoids to segregate to the liquid pool.
This premature solidification causes the porosity in the form of small, irregular voids.
Such phenomena can occur from rapid solidification if the mold or casting temperature is too low.
219
PINHOLE AND GAS INCLUSION POROSITIES
Related to the entrapment of gas during solidification.
Both Spherical contourdifferent in size.
The gas inclusion porosities are usually much larger than pinhole porosity.
220
PINHOLE/GAS INCLUSION POROSITY
221
Many metals dissolve or occlude gases while they are molten.
On solidification, the absorbed gases are expelled and pinhole porosity results.
All castings certain amount of porosity
Porosity should be kept to a minimum adversely affect the physical properties of the casting.
Porosity surface form of small pinpoint holes
Surface is polished, other pinholes appear.
Larger spherical porositiespoorly adjusted torch flame, or by use of the mixing or oxidizing zones of the flame.
222
SUBSURFACE POROSITY
Simultaneous nucleation of solid grains and gas bubbles at the first moment that the alloy freezes at the mold walls.
Short, thick sprue pin- Rapid entry of the alloy causes skin formation; the bulk of alloy pulls away, forming subsurface porosity.
Alloy or mold temperature is too high - The first portion of gold to contact the investment will solidify and form a thin skin. The alloy behind it shrinks during solidification and pulls away, forming small porosities.
Controlled Rate of molten metal that enters the mold
223
ENTRAPPED-AIR POROSITY
- Occurs on the inner surface of the casting, sometimes referred to as
BACK-PRESSURE POROSITY large concave depressions
Caused by the inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces the air pocket toward the end of the investment.
224
The incidence of entrapped increased by
- Use of the dense modern investments,
- By an increase in mold density produced by vacuum investing, and
- By the tendency for the mold to clog with residual carbon when the low-heat technique is used.
Slow the venting of gases from the mold during casting.
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According to William O Brien
Insufficient alloy mass- Air is entrapped in the solidifying alloy.
Insufficient turns on the casting machine - Denser the investment, the greater the force needed to eliminate the gas within the mold chamber.
Pattern is too for away from the end of the ring - Dense investments and lower burnout temperatures
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TO AVOID ENTRAPPED AIR POROSITY
Proper burnout
Adequate mold and casting temperature
High casting pressure
Proper L/P ratio
Thickness of the investment between the tip of the pattern and the end of the ring not greater than 6mm.
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4. INCOMPLETE CASTING
Partially complete casting, or perhaps no casting at all, is found.
The obvious cause is that the molten alloy has been prevented, in some manner, from completely filling the mold.
Two factors that may inhibit the ingress of the liquefied alloy are
- Insufficient venting of the mold and - High viscosity of the fused metal.
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Insufficient venting, is directly related to the back pressure exerted by the air in the mold.
If the air cannot be vented quickly, the molten alloy does not fill the mold before it solidifies.
In such a case, the magnitude of the casting pressure should be suspected.
If insufficient casting pressure is used, the back pressure cannot be overcome
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Furthermore, the pressure should be applied for at least 4 sec.
The mold is filled and the alloy is solidified in 1 sec or less; yet it is quite soft during the early stages.
Therefore the pressure should be maintained for a few seconds beyond this point.
These failures are usually exemplified in rounded, incomplete margins.
ROUNDED, INCOMPLETE MARGINS
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ROUNDED INCOMPLETE MARGNS
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Second common cause for an incomplete casting is incomplete elimination of wax residues from the mold.
If too many products of combustion remain in the mold, the pores in the investment may become filled so that the air cannot be vented completely.
If moisture or particles of wax remain, the contact of the molten alloy with these foreign substances produces an explosion that may produce sufficient back pressure to prevent the mold from being filled.
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According to William O Brien
Casting is nearly or entirely missing - The pattern detached from the spruepin, due to excessive vibration.
Pattern fractured during investing
Gold alloy was too cold during casting
Incomplete burnout
Sprue pin was too small - If the sprue freezes before the alloy fills the mold completely, incomplete casting results.
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CONCLUSION
Thus, these are the various causes for the failure of the castings and methods by which these defects can be avoided, thereby producing a casting of good quality for clinical success.
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REFERENCES
1. PHILLIPS–SCIENCE OF DENTAL MATERIALS-11THEDITION
2. CRAIG’S- RESTORATIVE DENTAL MATERIALS-12TH EDITION
3. OPERAIVE DENTISTRY – MARZOUK
4. DENTAL MATERIALS & THEIR SELECTION –WILLIAM J.O’BRIEN-3RD EDITION
5. CONTEMPORARY FIXED PROSTHODONTICS-ROSENSTIEL-4TH EDITION
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6.NOTES ON DENTAL MATERIALS – VK SUBBARAO
7. SYNOPSIS OF DENTAL MATERIALS – S GOWRI SHANKAR
8. TEXT BOOK OF SCIENCE OF DENTAL MATERIALS
9. ESSENTIALS OF DENTAL MATERIALS - SH. SORATUR
10. PRINCIPLES & PRACTICE OF OPERATIVE DENTISTRY –CHARBENEAU – 3RD EDITION.
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11. JOURNAL OF PROSTHETIC DENTISTRY-1987,57,362-368
12. JOURNAL OF PROSTHETIC DENTISTRY -1989,61,418-424
13. DENTAL MATERIALS JOURNAL -1993 DEC 12 (2) 245-52.
14. DENTAL MATERIALS JOURNAL-2009 MAY 25 (5) 629-33.
15. JOURNAL OF PROSTHETIC DENTISTRY -2009 OCT 102 224-8.
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