Metal ceramic crown lab manual.pptx

Metal substructure

Prepare the die1. Remove die from model & identify margin.

Trim away the soft tissue below the margin level to expose margin

2. Highlight margin with pencil

3. Apply die hardener4. Apply 2 coats of die spacer to allow room for

luting cement. This should finish 1mm above margin to ensure tight marginal fit

5. Once dry, apply separator (ISOLIT) to the die, adjacent & opposing teeth

6. Wax up the die with thin layer of red inlay wax (approx 0.5mm). Check for ease of removal from the die.

7. Apply more Isolit to the die. Allow dry. Place the red wax pattern back onto the die and start waxing up with blue inlay wax to desired tooth morphology

8. After finish, remove the wax pattern & check it under transmitted light for thin spot.

9. Check the margin of wax pattern for over contoured area (over wax), open margin or short margin.

10. Lastly, polish the surface of wax pattern with Isolit.

Investing wax pattern

• WHY do we sprue at the thickest point of wax pattern (usually the cusp) :

1. act as a channel for molten metal to flow through 2. as a reservoir of molten metal that the contacting

casting can draw on during solidification

• The pattern is positioned close to the end of the casting ring to allow air within the mould to escape through porosities in refractory material

Investment material

• Type I – IV gold, gypsum-bonded investment can be used. (they have fine particle size giving good surface detail, and are soft, which makes it easy to de-vest after casting)

• Thermal expansion coefficient (TEC) needs to be closely matched to that of gold alloys. (so that mould can be heated & expanded to compensate for contraction of metal after freezing)

• Use of gypsum is limited to lower melting temperature alloys

-as it breaks down at T above 690°, producing gases that affect the casting -the mould T needs to be high enough to prevent molten metal from freezing

as it enters the mould.

• Ni-Cr (non-precious) alloy used often for MCC has a melting T of ~1400°.

• For this, a mould T of ~900° is required.• SO a phosphate bonded investment material is

required -Disadvantage: they are very hard & difficult to de-vest

-expansion over a greater range,so more difficult to match to metal

-They are mixed with colloidal silica to help control expansion

Investing1. Cut about 5cm length of 3mm spruing wax, soften

one end and carefully attach it to the mesio-palatal (biggest cusp)

2. Ensure the join is smooth. Any dents/nicks will become obstructions in the mould & cause turbulence in flowing metal.

3. Line the casting ring with a ring liner(damp). Measure the length of sprue against the casting ring, adjust the height accordingly

4. Attach sprue to the former. Check that the pattern is about 3-5mm from the top of the mould. Also check that the wax pattern is central in the mould.

5. Spay wax pattern with a surface tension reducer 6. Metal casting investment is usually supplied in

pre-weighed sachets

7. Mix according to manufacturer’s instructions, first hand-mixing in vacuum pot to incorporate all the liquid.

8. Mechanically mix the investment under vacuum (manufacturer will give the mixing, working & setting time in seconds)

9. Pour material into mould with aid of vibration. Ensure no air trapped as the investment reaches the wax pattern.

10. Fill up to the top.11. To ensure that no

air trapped, you may pour the investment back to mixing pot and refill it in again.

12. Leave to bench set according to manufacturer’s instruction (typically 30min)

Casting & de-vesting the pattern1. Remove rubber sprue former from the base

of casting ring.2. Place mould in burnout furnace& adjust

temperature.3. Mould is allowed to heat-soaked for an hour

to allow it to become hot all the way through.

4. Weigh out metal alloy. (larger quantity will be required if use low density metal)

5. Once the mould has heat-soaked, start heating the alloy in crucible of casting machine

6. Observe the alloy melting (use

tinted safety googles). Once alloy has lost form, take the mould from the casting machine.

7. Place the mould on casting arm of casting machine(cone facing the

crucible).Align crucible & mould.Close safety shield and start casting machine.

8. Once cast, retrieve the mould from casting machine using tongs & put aside to cool.

9. Check that the mould has been filled (the “button”of alloy should be complete)

10. Once cooled, the casting is de-vested. Tap casting ring gently on the “button" with small hammer. Remove any remaining investment.

11. Completed casting will have a fine layer of surface oxide

12. For gold alloys, remove surface oxides by immersing into acid (pickling)

13. For non-precious metals, remove the investment by shot-blasting with 50µm aluminium oxide

Finishing the casting1. Gently try to seat the

casting on the die. Do not use force. Remove & inspect the fitting surface.

2. Use small bur remove bubbles. Do not proceed until the coping fully seated on the die.

3. Cut the sprue with carborundrum cut-off discand polish with appropriate bur

Ceramic & metal

Bonding mechanism metal to ceramic• Van der Waals forces -secondary bond –minor attraction btw opposite charges -rely on close proximity, therefore good wetting of surface is important -oxide is required for good wetting

• Mechanical retention -gained from microcopic surface irregularities -this is only minor contributor to overall strength

• Compression bonding -matching TEC of metal &ceramic

• Chemical bonding -the single most significant mechanism, bond using metal oxide -surface oxides dissolve into opaque layer of ceramic that allows atomic contact

with metal surface -both covalent and ionic bond form -this mechanism requires only a molecular layer of oxide; if excessive oxide,

ceramic will not come into contact with metal

Ceramic application: Wash opaque layer (to create bond btw metal & ceramic)

1. Hold the metal substructure with tweezer,by the collar

2. Wet the surface with small amount of modeling fluid.

3. Apply thin coat of opaque ceramic. Drag the ceramic paste out to the margin and metal-ceramic junction

4. Place the framework on firing stand and fire according to manufacturer’s instructions

Ceramic application: opaque layer(thicker layer, ensure metal not show through)

1. Use correct shade, mix opaque ceramic in the palatte with appropriate modelling fluid

2. Apply to substructure with brush.3. Blot the moisture from opaque

ceramic by gently holding the tissue against the side of ceramic (watery, shiny surface will become matte)

4. Vibrate the metal framework by gently dragging the serrations on the shank of the spatula over the tweezer (surface

will become shiny again). Blot with tissue.

5. Any irregularities that occur may be smoothed with brush

6. Repeat the process until the framework completely covered

7. Fire the framework. Repeat the process if any grey shadows visible through the opaque layer

Ceramic buid-up

1. Place framework on the die, make sure fully seated

2. Place ceramic against the substructure taking care to avoid air trap; condense using tissue to blot excess moisture.

3. Buildup ceramic incrementally & condense. Try not to let ceramic dry out completely as it will become difficult to carve

4. Ceramic will shrink on firing, so over-build the restoration by 10%

5. Brush is useful for moving or smoothing the ceramic6. When place translucent enamel, enamel & dentine

ceramic, remember the ceramic must be moist to allow the powders to blend

7. Check palatal aspect where the ceramic meets the metal junction.

8. Remove die from model & shape the interproximal areas

9. Fire the ceramic using the appropriate firing cycle

Trimming and finishing

1. Check seating of the crown2. Trim tight contacts and overcontoured areas3. Ensure margin seats properly4. Trim palatal aspect so that there is no greyish

area on the junction btw ceramic & metal5. If restoration is deficient, ceramic can be

added and fired & adjust accordingly6. Remove glossy surface from ceramic prior to

glazing

Ceramic glazing & staining1. Mix staining powders with glazing

liquid & apply to ceramic surface using fine brush

2. Fire in furnace using appropriate cycle.

3. Surface will have high gloss (if use glazing

powder) or light gloss (if use self-glaze)

4. Polish all metal surface (except fitting surface)

5. Clean the restoration6. Steam clean models 7. Seat the restoration on model