Casting of Cobalt Chromium Alloys
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Transcript of Casting of Cobalt Chromium Alloys
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Casting of cobalt chromium
alloys
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Step By Step Operations Of Wax Melting & CentrifugalCasting
The Eight Main Steps
1. Original Pattern Making
2. Rubber Mould Making
3. Wax Pattern Making
4. Investing
5. Burnout6. Casting
7. Cleaning
8. Finishing
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S-U-WAX PATTERNS
Assorted preformed waxpatterns are produced fromcasting wax especially
developed for this purpose.They are distinguished bytheir high flexibility and goodadhesion. A newmanufacturing processensures their easy removalfrom the model. The working
temperature should not fallbelow 68F (20C). Store in acool place.
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-U-FLEXIBLE WAX
& S-U-HOLE
RETENTION
A casting wax insheets for theproduction of Cr-Cocastings. It istransparent, extremelyflexible and stable andbest worked at handtemperature. As it isself-adhesive, this waxis easy to adept, andmaintains constantthickness even inprominent areas.
Available with grained,veined or smooth
surface.
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Why?????
to design the removable partial denture
framework
complete a laboratory work authorization that
communicates the desired design and
authorizes its fabrication
evaluate the quality of the framework
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Duplicating a stone cast
To preserve the original master cast
For fitting a removable partial denture frameworkwithout danger of fracture or altering the surface
of the master cast For processing a temporary prosthesis where wax
relief and blockout on original master cast canallow production of a duplicate cast , to allow for
easier insertion.
To allow formation of of an investment cast forframework fabrication.
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Purpose of Duplication - To produce twocasts:
Refractory cast used for casting RPD frameworks
Heat resistant duplicate of a modified (blocked out andrelieved) master cast
Made from dental casting investment
Acts as the base for forming the RPD framework in wax
and plastic Duplicate master cast used for seating RPD
frameworks
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PREPARING MASTER CASTS FOR
DUPLICATION
Purpose of Blockout and Ledging
Blockout: To eliminate undercut areas on a castwith blockout material
Ledging: To control the amount of undercut theclasp tip engages
Definition of partial denture relief
The adaptation of wax to the edentulous areasof a cast, before duplication, to create a raisedarea on the refractory cast
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Purpose of partial denture relief Acrylic retention
Allows a space for acrylic to flow around the denture
base retention
Internal finish lines Establishes a definite junction for metal framework and acrylic
resin
Located on the tissue side of RPD Used as a guide for placement of external finish line
Relief of lingual bar and approach arms Eliminates the possibility of tissue ulceration due to framework
rubbing during function Lingual bars when used in conjunction with distal extension
defects, usually require relief
Relief is rarely, if ever, indicated under palatal connectors ortoothborne RPDs
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Equipment and Materials
Blockout wax must have the following physicalproperties: Firm - must resist temperatures of at least 10F in
excess of the pouring temperature of the duplicating
material Easy to carve
Sheet casting wax Supplied in various gauges
Pressure sensitive - do not compress the wax uponapplication, it results in an uneven layer of relief
Adhesive - one side is tacky to adapt to cast
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Procedures for Blockout, Ledging and
Relief Wax Adaptation A) Blockout all undercuts
Cast remains on the survey table during the entireprocedure
Application of blockout wax and baseplate wax
Below the survey line around the teeth Below the survey line of soft tissue undercuts within the RPD
design
Blockout gross undercuts outside of framework area, e.g.,(anterior labial)
Remove excess wax Warm the blockout tool slightly
Remove excess blockout wax from the appropriate areas bymoving blockout tool around teeth and tissue
Ensure there is no wax above the survey line
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B) Ledging
Locate desirable undercuts
Expose the terminal 1/3 of the retentive clasp tip
design from the blockout wax
Use a roach carver to trim the wax at a right angle (90)
to the abutment tooth surface; do not scratch the cast
Ledge should be free of debris
Procedures for Blockout, Ledging and
Relief Wax Adaptation
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C) Denture base retention relief
Select the proper gauge wax
Use 24 gauge adhesive wax unless directed otherwise
The lower the gauge numbers, the thicker the wax
Adapt and cut the wax
Ensure the finish line is sharp and accurate
Cut wax at a 90 angle to the tissue
Do not bevel or taper the edge
Cut the wax 1.5mm from the abutment teeth
Procedures for Blockout, Ledging and
Relief Wax Adaptation
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D) Cut the tissue stop Purpose of the tissue stop
To hold the retention area of the framework off
of the tissue while packing forces are beingapplied
Ensure the tissue stop is placed directly over the crest of theridge preventing movement of the tissue stop in a lateraldirection
Cut 2mm by 2mm square
Procedures for Blockout, Ledging and
Relief Wax Adaptation
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Preparation of the cast for duplication
On maxillary casts, check beadline Beadline should be scribe into cast
1 mm wide and .5 mm deep
Used only on maxillary design
Ensure it is void free
Ensure it is free of wax and debris
Soak the master casts Eliminates air from casts and replaces it with water so casts
will not draw moisture from hydrocolloid and adhere to casts
Methods of soaking the casts On their heels or with teeth down
In SDS (Saturated calcium sulfate dihydrate solution) water
Allow to soak in warm hygrobath 90F for 30 mins before removing from water
Casts made from improved stone require a much longer soaking time
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MATERIALS USED FOR DUPLICATION
Hydrocolloid (agar type) Dupli-chrome - Reversible Red Strip investment
Phosphate bound investment
Contains crystobolite which facilitates expansion and enables theinvestment to withstand high heat
Expansion factors: Molten metal shrinks 1.7% as it cools. This must be
compensated by the expansion of the investment material Setting
Expansion - 0.4%
This occurs while the cast is setting up
Hygroscopic
Expansion - 0.3% Occurs as the investment sets- up in the hydrocolloid
Thermal
Expansion - 1.0%
Occurs in the burn-out oven when the investment is subjectedto high heat
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Important Rules
Always use distilled water when mixing
investment
Use the manufacturer's recommendedwater/powder ratio-more water/less
expansion
Be sure to roll the canister to mix allparticles
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Procedures
Autoduplicator: Used to breakdown and store hydrocolloid at a giventemperature
Use manufacturers recommendations for temperature Set low temp at 50 degrees Celsius
Set high temp at 90 degrees Celsius Fill duplicator with hydrocolloid
Fill duplicator at least 1/2 full - material will burn if less than 1/2 full
Start breakdown cycle (cookdown) Push cycle button on
Temp will rise to high temp setting at 2 degrees per minute, then lower to lowtemp setting
In 3 to 5 hours the hydrocolloid will be melted and cooled down to pouringtemperature
Clear the valve -Open the valve to drain off approximately 150 cc of hydrocolloid to clearthe valve of lumps and debris
Safety: Keep your fingers out of the unit when it is running. Use caution when pouringduplicating material, because it is hot
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Select a duplicating flask that allows at least 12mm
clearance around the cast
Remove the cast from the water bath
Check the relief wax to make sure it has not lifted offcast
Blow off excess water from the cast (DO NOT USE AIR
HOSE)
Place cast in duplicating flaskPosition cast on center of base
Place body on base, ensure it is completely closed
Fill flask with hydrocolloid
Center the flask under the valve
Open the valve slowly until the stream of hydrocolloid is
about the diameter of a pencil
Fill flask completely
Close valve
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Cool the duplicating flask
Place in cooling tray.
Ensure water level does not extend higher than top of
the base.
Cast and base cool before the body.
The hydrocolloid solidifies toward cast for more
accurate duplication.Ensure the water remains below room temperature,
but not less than 55 degrees F.
The water should circulate in the tank.
Length of cooling timeSmall flask - 30 minutes
Large flask - 45 minutes
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b.Pour the refractory cast
1.Water/powder ratio for redstripe investment
a.For average size frameworks use 25cc distilled water to 200grams investment
b.For large horseshoe frameworks or cast metal bases uses 24cc
distilled water to 200 grams investment
1.For very large areas of metal, more expansion is needed
2.Less water gives more expansion
2.Retrieve master cast from hydrocolloid
a.Pry off base with a knife
b.Clean off any hydrocolloid covering the bottom of the master cast
c.Using 2 knives, place them against the cast
d.Pry master cast out of hydrocolloid by giving cast a quick upward snap
e.Make sure the relief wax is still on the cast
3.Mix water and investment
Vacuum mix investment for 30 seconds
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Fill the mold with investment
Spot fill; do not flow the investment
around the mold as is done for an alginate
impression
Hydrocolloid contains salts - filling
impression like an alginate will distort the
refractory cast
Insert sprue former - too much pressure can
distort the impression
7-8mm from the major connector design
the sprue provides a channel for the
escape of gases and debris during
burnout procedure
The sprue provides a channel for molten
metal to enter the cavity of investment
mold during casting procedure
Top of sprue cone should be parallel withthe occlusal plane
Fill the mold to the top
Investment will not have proper setting
expansion if it contacts the metal flask
Allow the investment to set for 20- 35
minutes
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Recover refractory cast
Always remove the hydrocolloid from the
cast, never the reverse
Cut the hydrocolloid in 4 areas
Make cut at each cuspid area
Make cut at each heel areaGently peel hydrocolloid away from
refractory cast
Remove sprue former
Gently turn the sprue former until it
comes loose from refractory cast
Make every effort not to touch areaswithin the design on the cast
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Prepare hydrocolloid for reuse
Rinse hydrocolloid under running water
Remove all particles of investment,
modeling clay and wax
Store hydrocolloid in an airtight containeruntil it's ready for cookdown
Replace the hydrocolloid after 200
duplications or as needed
Prepare casts for dehydration
Inspect for voids andbubbles in critical
areasInspect internal finish line for defects
Smooth edge of sprue hole - eliminates
the possibility of investment pick-up
during the casting procedure
Outline cast for trimming- make line
approx. 6 mm away from extremities ofdesign with a wax pencil
Smooth base of refractory cast
Trim heel area at 90 degree angle
Trim the rest of cast at 45 degree angle
Rinse off slurry
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10.Purpose of Dehydration and Wax-dip
a.Dehydration1.Prepares the refractory cast for wax dipping by removing moisture
2.Preheat cast for wax dipping
b.Wax dip
1.Assures a smooth dense cast necessary for waxing procedures
2.Eliminates soaking refractory casts prior to investing
a.Unsealed casts would extract moisture from outer investmentb.Refined Beeswax - used for wax dipping
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Dehydration and Wax-dip
Dehydrating Oven
Purpose: Used to dry refractory casts prior to sealing them with beeswaxSafety: Take precautions for handling hot materials
Beeswax heater
Purpose: To melt refined beeswax and maintain the wax at a temperature
between 280F and 300F
Safety:
Use carrier to immerse casts into the molten waxDo not drop casts into the molten wax, as splashing of hot wax can result in
serious burns
Avoid touching the exterior of the unit during and immediately after its
operation
Place refractory casts into dehydration oven at 180 degree - 200 degrees F for 1 hour
Wax-dip in beeswax at 280 - 300 FBeeswax must be 100 degrees above dehydrating oven
Place cast in hot wax using cast holder
Cool casts after wax-dip
Place wax-dipped casts on a paper towel on their heels
Blow lightly on cast to prevent puddling
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Wirosil duplicating flask with its
stabilizing insert and three replaceable
palate formers of different sizes ensure
extremely low silicone consumption
through flexible positioning.
Wirotop automatic mixing and metering
unit
ensures the two components are mixed
to prevent formation
of bubbles.
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In Wiropress pressure compaction unit,
duplicating silicone is forced into critical
areas under pressure,
thereby reducing number of bubbles.
After hardening under pressure, cast is
detached by blowing compressed air
between cast and mold.
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Spraying with surfactant eliminates any
reaction
with investment material and ensures more
accurate casting
surface.
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WAXING THE REMOVABLE
PARTIAL DENTURE FRAMEWORK
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