Different fabrication techniques of ceramics

By RADWA EL-DESSOUKY

Conventional powder/slurry technique

Castable ceramics

Pressable ceramics

Infiltrated ceramics

Machinable ceramics

1.Conventional powder /slurry technique

1.Conventional powder /slurry technique:

It is the traditional method of forming ceramic prosthesis and consists of the following steps:

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1.compounding

2.packing

3.Preheating 4.sintering

5.Surface treatments

6. Porcelain cooling

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I. Compounding:

The process of bringing the particles closer and of removing the liquid binder is known as condensation.

Dense packing has the following benefits:Improves substructure of the porcelain & dispense trapped air (Less porosity)increase densityLess shrinkage during firingEnhanced surface texture and strength

2. Packing or condensation:

•Condensation steps:

I. Build-up of Cervical Porcelain

II. Build-up of Body Porcelain

III. Cut-back

IV. Build-up of Enamel Porcelain

•Condensation methods:

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1-MANUAL CONDENSATIO

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2.ULTRASONIC CONDENSATIO

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•Advantages of ultrasonic condensation:

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•Reduces the fluid content of layered ceramics; resulting in denser and more vibrant porcelain mass.

•Enhances translucency and the shade qualities of the fired ceramic.

•Shrinkage can be reduced to below 5%!

•Time-saving as it reduces the number of compensatory firing cycles

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3.Pre-heating (Drying):placing the porcelain object on a tray in front of a preheated furnace at 650C for 5min for low fusing porcelain and at 480C for 8min for high fusing porcelains till reaching the green or leathery state.

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•Significance of pre-heating stage:

Ceramic particles held together in the “green state” after all liquid has been dried off

•Removal of excess water allowing the porcelain object to gain its green strength.

•Preventing sudden production of steam that could result in voids or fractures.

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4. Sintering or firing:

The voids are occupied by the atmosphere of the furnace. As the sintering of the particles begins, the porcelain particles bond at their points of contact.

•The sintered glass gradually flows to fill up the air spaces.•The particles fuse together by sintering forming a continuous mass, this results in a decrease in volume referred to as firing shrinkage

• Mechanism of firing:

•The gaps between particles become porosities.

• The viscosity of the glass is low enough for it to flow due to its own

surface tension.

The result is that the porosity voids will gradually become rounded

as firing proceeds

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The voids slowly rise to free surfaces and disappear

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•Stages of porcelain maturity:

Low bisque Medium bisque High bisque

porcelain surface is quite porous

Pores still exist on the surface of porcelain

smooth porcelain surface

porcelain grains begin to soften and ‘lense’ at their contact points

The flow of glass grains is increased. As aresult, any entrapped furnace atmosphere that could not escape via the grain boundaries becomes trapped and sphere shaped

The flow of glass grains is further increased, thereby completely sealing the surface and presenting smoothness to the porcelain.

minimal Shrinkage A definite shrinkage is evident

the fired porcelain body is extremely weak or friable

The fired porcelain body is strong and any corrections by grinding can be made .

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•Over-firing:Due to:•Firing at above the correct firing temperature Or;

•longer firing time Effect;

•reduce the strength due to formation of undesirable crystal phases at higher temperature

[de-vitrification]

•increases the chances of slumping [eliminate the shape we made and leave a globule of

ceramic].

•under-firing:•Effect; The porcelain object will have a chalky white color overlaying its shade ,Because light is reflected and scattered at boundaries between particles and at the surfaces of porosity

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•The restoration is tried back to working cast, often proximal surfaces need addition (correction).

•Adjustments are made with diamond stones and discs to treat seating problems.

•After reaching the desired contour, occlusion and seating; the restoration becomes ready for surface treatment.

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5.Checking fit on the working cast:

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6. Porcelain surface treatment:

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•Significance of Glazing:

The aim of glazing is to :

seal the open pores in the surface of fired porcelain ; minimizing surface roughness and thus controlling

•Aesthetics

• wear

•mechanical properties

•and plaque accumulation

A. Natural or Auto-Glaze:Normally, the porcelain has the ability to glaze itself by forming a vitrified layer on the surface of dental porcelain ceramic containing a glass phase

B. Applied or Add-on Glaze:

Dental over glazes are composed of clear (colorless) low fusing glass powder, painted on the fired crown surface; and fired again.

Add-on porcelains: are generally similar to glaze porcelains except for the addition of opacifier and color pigments .they're exclusively used for simple corrections of tooth contour or contact points.

C. Polishing:Advantages;

producing smoother less abrasive porcelain surfaces.

Disadvantages;over polishing can Introduce more surface flaws and weaken the material.

D. Custom staining (shade modification):

Stains are generally low fusing colored porcelains used to imitate markings like enamel check lines, calcification spots, fluoresced areas etc.

6. Porcelain cooling:

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•It is necessary to cool the mixture fairly quickly. 

•If it cools too slowly, crystals form within the glass body which will degrade its optical properties, turning if from a clear glass into a cloudy one.  Devitrification.

• if it is cooled too quickly, stresses build up in the glass. •  To reduce the stresses ,it is kept near the glass transition temperature (its solidus) for a long time so that the atoms in the glass can rearrange just enough to relieve the stress. When most of the stress has been eliminated, the finished glass is finally allowed to cool to room temperature [annealing].

• Applications of powder/slurry system:

•These ceramics have a great amount of translucency and are highly esthetic • used mainly as:

1. veneering layers on stronger cores and frameworks 2. or for making PJCs.

• Examples of systems utilizing this technique:

1.IPS e.max Ceram (Ivoclar-Vivadent)2. IPS Eris (Ivoclar-Vivadent)3. Lava Ceram (3M ESPE)4.Cercon ceram kiss (dentsply)

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2. Castable ceramics

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• supplied as solid ceramic ingots, which are heat-treated under controlled conditions (ceramming) using a lost wax and centrifugal-casting technique.  •The best known of these systems is Dicor 

2. Castable ceramics

1. The die is waxed-up , invested then, burnt-out.

2. The ceramic ingot is heated at 1350 c and casted using centrifugal casting machine.

3. Ceramming process: the crown is heat-treated at 1075 c for 10 hours resulting in (controlled crystallization) formation of tiny crystals that are evenly distributed throughout the body of the glass structure enhancing its mechanical properties.

4. The final shade of the crown is applied to the surface of the completed restoration through a complex reaction between the porcelain and surface stains

• fabrication technique:

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•Advantages of DICOR:

•Highly esthetic: as mica crystals that constitute the crystal phase closely match the index of refraction to the surrounding glass phase. •A one-piece restoration made entirely of the same material and no opaque substructure exists to impede light scattering.•A chameleon effect i.e. the restoration acquires a part of the color from adjacent teeth and fillings as well as the underlying cement lute. •Excellent marginal fit.•Simple uncomplicated fabrication technique.

•Disadvantage of DICOR:

The surface stains may be removed after occlusal adjustment.

3. Presssable ceramics

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Technique i. The die is waxed-up , invested then, placed in

specialized mold that has an aluminum plunger.

ii. The ceramic ingot is placed under the plunger, heated to 1150C and the plunger presses the molten ceramic into the mold. iii. The final shade of the crown is adjusted by staining or veneering.

contains 35 - 45 vol % leucite as crystalline Phase• Disadvantage:The presence of about 9% porosity badly affects its flexural strength ,limiting its use to single unit complete-coverage restorations in the anterior region.

 Contain 65 vol % lithium disilicate as the main crystalline phaseADVANTAGE:it has a much higher flexural strength than that of IPS Empress which makes it suitable for the usage for fabrication of 3-unit FPDs in the anteriorregion, and can extend to the second premolar , due to;

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•The final microstructure consists of highly interlocked lithium disilicate crystals contributing to strengthening•About 1% porosity

•Introduced in 2005. •Considered as an enhanced lithium disilicate press-ceramic material when compared to Empress II.•Better physical properties and improved esthetics

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The die is waxed-up to full anatomical contour

After waxing-up to full contour, the incisal third of labial surface is cut-back

The die is waxed-up to minimal thickness just to support the tooth shape

1. Wax contouring

2. sprueing

• Fabrication technique of IPS e.max:

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3. investing

4. Preheating

The investment ring with the waxed-up die is preheated

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5. Pressing

The hot completed investment ring is placed in the hot press

furnace

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6. After cooling and divesting:

7. finshing

I. Staining technique

The stain paste is mixed with the glaze, applied to the framework and then, fired

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II. Cut-back

technique

The veneering technique of frameworks is performed with aid of silicon index

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III. Layering

technique

1. 1st Dentin build-up2. 1st Dentin firing

cycle 3. compensatory Dentin build-up

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technique

• A standard die is scanned using a mechanical scanning device and a computer that turns the shape of the die into digitized data.

•The data is then used to fabricate an oversized die [to compensate for sintering shrinkage ]. •Aluminium oxide powder is compacted onto the enlarged die under high pressure and then sintered at temperatures above 16000C to full density thus shrinking it to the correct size and creating a hard core.

• feldspathic porcelain veneer can be applied

IN-CERAM

4.Infilterated Ceramics

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supplied as one of the three core ceramics:1.In-ceram alumina (70 vol % alumina)2.In-ceram spinnel (contains a mixture of magnesia and alumina)3.In-ceram Zirconia (34 vol % alumina + 33 vol % of ceria-stabilized Zirconia)

•Definition:A porous infrastructure is produced by slip-casting, sintered, and later infiltrated with a lanthanum-based glass, producing two interpenetrating continuous networks, one composed of the glassy phase and the other being the crystalline infrastructure.

•VITA IN-CERAM , Zahnfabrik :

Fabrication technique

1. Die preparation 2. Mixing aluminous powder with water to produce slip

3. The slip is painted onto the die with a brush.

The water is removed by the capillary action of the

porous gypsum, which packs the particles into a

rigid porous network

4. sintering

5. lanthanum alumino-silicate glass is used to fill the pores in

the alumina core.

6. The glass becomes molten and flows into the pores by capillary

diffusion.

7. Removal of excess glass8. Veneering with esthetic

veneer [VITA VM7]

I. Manually Controlled System or Manual-aided Design/Manual-aided Manufacturing (MAD/MAM)

Method:

5.Machinable ceramics

The best-known manual system in the UK is Celay.

Referred to as copy milling, this method is based on the pantographic principle that was used when making duplicate keys

Computer-aided Design/Computer-aided Manufacturing

II. CAD/CAM

They are the systems utilizing a process chain consisting of scanning, designing and milling phases.

•CAD-CAM SYSTEMS:

• Components of CAD-CAM technology :

1.Scanner = Digitizer2. CAD Unit = Soft ware3. CAM Unit = production

A. Scanner:•Function: transform geometry of receptor unit (tooth) into digital data that can be processed by a computer.

1. Mechanical Scanners:

•Technique:contact probe with ruby ball at its end maps entire cast preparation surface line – by - line• Advantage: precise but, Less accurate than newer optical scanning systems.

•Used in: Procera. Scanner, by Nobel Biocare

•Disadvantages:

1. Take long time to scan. 2. expensive.

2.Optical scanners:•Idea: Triangulation Procedures: Source of light (white, colored light or laser) + receptor unit are in a definite angle to one another through this angle, computer can calculate 3D data from image on receptor unit. •Types:1.Chair-side: fabricate restoration at chair.2. Non-chair side: fabricate restoration in Lab.

B. CAD Software:design the restoration to fit the preparation:

margins,connectors,core thickness,cement gap (internal relief).

C. CAM Unit (Production device):

• Function: transform "digital data" to a "product”

•Types: Acc. To No. of milling axes: 3- axis milling device: move in 3- spatial direction (X,Y,Z).

4- axis milling device: more in 3-septial direction + rotatable tension bridge (4 th axis).

5- axis milling device: move in 3- spatial direction + rotatable tension bridge (4th) + rotating milling spindle (5th axis). Acc ton milling environment:

•Dry milling: it is done for zirconium oxide low degree of pre-sintering, to avoid moisture absorption avoid drying time of zirconium before sintering.

•Wet milling it is done for metals, composite, silica-based ceramics, zirconium oxide with high degree of pre-sintering.

• Milling strategies:

I- Subtractive Technique from a Solid Block :•The restoration is cut to contour out of a prefabricated, solid block.•This technique is used in: DCS , Cercon and Lava systems

Disadvantage: can create complete shapes effectively, but at the expense of material being wasted. Approximately 90 percent of a prefabricated block is removed to create a typical dental restoration.

II- Additive Technique by Applying Material on a Die•The framework material is applied on a die of a prepared tooth.•This technique is used in: Procera

iii. Solid free form fabrication•This category includes new technologies originating from the area of rapid prototyping.•This technique is used in:•

•Wax Pro 50 :applies the wax plotter technique, which works according to the ink jet principle.

•Production Concepts(1) Chair side production:

•Advantages: i.Save time only are visit instead of two.

ii.Avoid worrying about improper handling by lab

(2) Laboratory production: Dentist scan preparation and design restoration then send it to laboratory technician who will mill the restoration.