Casting of Cobalt Chromium Alloys

8/4/2019 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


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




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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




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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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Documents

Casting of Cobalt Chromium Alloys