INDEX

SL NO:

CONTENT PAGE NO:

1 Introduction 5

2 Working cast & die 6

3 Die systems Solid cast with individual die Working cast with removable dies

91014

4 wax pattern 26

5 Casting & investment Spruing Investing Burn Out Casting Procedure

3636414646

6 Cleaning of the casting 52

7 Soldering 54

8 Trimming 54

9 Polishing & veneering 5410 conclusion 55

14 References 56

INTRODUCTION

The fabrication of FPD is an indirect laboratory procedure as direct fabrication

in mouth is not possible. This commences with receipt of impression from the

clinician along with necessary instructions for fabrication. The procedures

involve making a master cast on which the wax pattern is fabricated. The

pattern is invested and cast using the lost wax technique. The metal

restoration is then veneered with resin or ceramic as required.

WORKING CASTS AND DIES2

The ease with which a restoration is fabricated, and the accuracy with which it

will fit the mouth, are materially affected by the cast. There are three

requirements for good casts

They must be bubble free, especially along the finish lines of the

prepared teeth.

All portions of the cast must be distortion free.

The casts must be trimmed to insure access for carving wax pattern

margins.

DEFINITIONS

DIE

It is the positive reproduction of the form of a prepared tooth in any

suitable substance

The die is a model of the individual prepared

tooth on which the margins of the wax pattern

are finished.

WORKING CAST

The working cast is the cast that is mounted on an articulator

IDEAL REQUIREMENTS OF A DIE

3

• Reproduce the preparation exactly

• All surfaces should be adequately duplicated

• Avoid voids in the margins

• Adequate access to margin is imperative

• Adequately rigid

IDEAL REQUIREMENTS FOR DIE MATERIAL

Accurate

Dimensionally stable

Setting expansion and contraction, variations in response to change in

temperature need to be minimum

It should reproduce the fine details

Strong and durable

Withstand the carving and finishing procedures

The color should contrast that of the wax to ease the manipulation

Economical

Easy to use

CLASSIFICATION OF DIE MATERIALS

1. Inorganic materials

4

die stone based on calcium sulfate hemihydrate eg: velmix

Dental cement – zinc silicophosphate

Ceramic materials

2. Metallic materials

Dental amalgam

Metal sprayed die

Electroplated die – Cu plated , Ag plated

3. Polymeric materials

Self cure acrylic

Epoxy resin

4. Composite materials

ELECTROPLATED DIES

Electroplated dies are the ones that are produced when an impression

material is electroplated .When a die is made in this manner this process

is known as electroforming.The impression materials which can be

electroplated are impression compound and elastomeric materials

The types of dies are-

- Copper plated dies

- Silver plated dies

ADVANTAGES

No dimensional changes occur during electro deposition of a metal

5

It reproduces the impression accurately

The die is tough and has good strength characteristics

CAST & DIE SYSTEMS

1) Working cast with removable die

i. Straight dowel pin

ii. Curved dowel pin

iii. Di-lok tray

iv. Pindex system

v. Accutrak

2) Working cast with separate die

3) DVA model system

4) Zeiser model system

SOLID CAST WITH INDIVIDUAL DIE

(MULTIPLE POUR TECHNIQUE)

6

Here, two casts are poured from a single impression and one cast is

sectioned and used as a die and the other is not sectioned and is used as the

working cast. The wax pattern is prepared on the die and later transferred to

the working cast.

Advantages

• Ease of fabrication

• Maintains fixed and immovable relationship between the abutments

• Easier to obtain physiologically harmonious restoration contours when

fabricating wax pattern.

Disadvantages

• Wax pattern should be transferred from one to the other.

• Can be used only with elastomeric impressions

IMPRESSION POURING

Use a small instrument to carry stone to the impression of the prepared

tooth Place a small amount of stone on the side

of the impression above the preparation, and

vibrate it until stone reaches the "bottom"

(occlusal surface) of the preparation .Add stone

and vibrate until all the teeth in the arch are

filled. Stone is added to the impression so that

the base of the set stone will be 1 inch. Allow the poured impression to set for

at least 1 hour.

7

DIE PREPARATION

Carefully separate the poured cast from the impression. Wet the cast

thoroughly before trimming excess stone from the working casts on the model

trimmer. Trim the cast from which the die is made on a model trimmer to

remove all excess stone around the prepared tooth. The handle of the die

should be slightly larger in diameter than the preparation and octagonal in

cross section. The handle should parallel the long axis of the tooth. Use a pear-

shaped acrylic bur to trim the die "apical" to the finish line of the preparation.

Begin final trimming of the die with a sharp no. 25 blade.

• The die is smoothened below the finish line with the discoid end of a

tanner carver.

• The prepared finish line on the die should be highlighted with a sharp

Colorbrite red pencil.

• Ditching or trimming the die defines the position of the margin and acts

as a guide to gingival contour when the restoration is being waxed.

8

• Excessive trimming does not give the correct emergence profile and may

lead to an over-contoured or bulky crown.

9

Thickness of die hardener are:

• Cyanoacrylates: 1.0 to 2.5um

• Acrylic lacquers: 4.0 to 10um

• Used to prevent abrasion of die surface by waxing instruments during

the fabrication of the wax pattern.

Luting agent space

• The ideal space for the cement is suggested at 20 - 40μm for each wall.

So the internal diameter of the crown may be 40 - 80 μm larger than the

prepared tooth.

• There needs to be space otherwise the restoration will not seat properly.

DIE SPACER

• Applied to die to increase cement space between axial walls of prepared

tooth and restoration. Formulated to maintain constant thickness.

Available as a paint on or pen type applicators. Die spacer is needed to

provide space for the luting agent (cement) during cementation of the

finished crown. When applying the die spacer over the preparation leave

the area 1mm above the margin line free of spacer. For close adaptation

of the crown and cement (or luting agent). Prevent disintegration and

dissolution of the luting agent at the margin.

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WORKING CAST WITH REMOVABLE DIES

In these the die is an integral part of working cast but can be removed to

facilitate fabrication & finishing of restoration.

REQUIREMENTS OF REMOVABLE DIE

SYSTEM

• Dies must return to their exact

original positions.

• Dies must remain stable, even

inverted.

• Cast containing the dies must be easy to mount on an articulator.

ADVANTAGES

• Wax patterns or copings need not be removed from their respective dies

when they are transferred to the working cast.

• Eliminates discrepancies between a separate die and working cast that

may be caused by impression distortion or deterioration between pours,

or by a cast and die made from separate impressions that are not

identical.

• Eliminates discrepancies that can occur when the die is coated with a

relief agent and the working cast is not, or when they are coated with

different thicknesses.

11

DISADVANTAGES

• Risk of introducing an error in the pattern if the die does not reseat

accurately in the working cast.

DOWEL PIN

A metal pin used in stone casts to remove die sections and replace them

accurately in the original position (GPT8)

TYPES OF DOWEL PINS

Single dowel

Double dowel

Two separate dowels

Curved, single dowel pin

Tapered, flat-sided brass dowel pin

Flat-sided, stainless steel dowel pin

Horizontal contact tracks and vertical ribs

STRAIGHT DOWEL PIN

Brass straight dowel pin is used commonly for many years. Resists

horizontal displacement. Removable die facilitates wax up and ceramic build

up.

No special equipment required. But need technical skill.

12

METHODS OF REPOSITIONING DIE

Several methods can be employed to allow the repositioning of a die in

its working cast. Most of these devices can either be oriented in the impression

before it is poured (prepour technique) or attached to the underside of a cast

that has already been poured (post- pour technique).

TECHNIQUE

The brass dowel pin is one of the most accurate dowel types in terms of

resisting horizontal displacement and the second lowest in vertical

deviation of four types of removable dies.

A dowel pin is positioned over each prepared tooth in the impression.

Place a dowel between the arms of a bobby. Push a straight pin

between the arms of the bobby pin and into the impression material on

both the buccal and the lingual surfaces of each tooth to have a dowel

pin placed over it.

Stabilize the dowel in the bobby pin.

Pour die stone into the impression, filling the impressions of the teeth

and covering the knurled end of the dowel pin.

The pin should parallel the long axis of the preparation.

When the stone is hard and dry, use a saw frame with a thin blade

There should be a cut on the mesial and distal side of each die, and the

cuts should taper toward each other slightly from occlusal to gingival.

13

Take the die from the cast and trim away excess stone gingival to the

finish line.

Complete the trimming of the die with a no.25 blade in the laboratory

knife and then mark the finish line with the red pencil.

Repeat the procedure for each die on the cast.

(This type of dowel also can be cemented into holes drilled into the flat

underside of a cast that has already been poured- Post pour technique.)

CURVED DOWEL PIN

To install pin before pouring the impression, use finger pressure to insert

tip of dowel into large opening of position bar.

Hold the bar faciolingually, so that head of

the dowel is 1-2mm into the proposed area

of impression.

The tail of dowel extends facially; however

if the tooth is linguoversion, turn it towards

lingual side for easy removal.

Insert a straight pin into one of 3 holes into

the facial aspect of bar and into the facial

flange of impression and another pin into

lingual flange through lingual holes of bar.

The dowel should not touch the impression and its head should be

parallel to long axis of prepared tooth or teeth.

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This procedure is repeated for all abutments and pontic areas.

One pin is placed near the center of each segment of unprepared teeth,

this wall help in removal of segment for better access of gingival wax

pattern.

Impression is poured with die stone until it covers heads of dowel. This

will fill the impression about 4 mm above gingival finish line.

The straight pins and positioning bar are removed as soon as stone is

set.

To assist in orienting each large segments of unprepared teeth, cut a

2mm deep hole on either side of each dowel with a large acrylic bur.

Petrolatum is applied on stone and also an exposed part of dowel for

easy separation from the base.

Impression is boxed, allowing the tails of the dowels to extend slightly

through the heat softened wax.

Fill the boxed impression with dental stone. The dowel should be

covered by at least 2mm of stone.

After the stone has hardened, make vertical saw cuts on either side of

each die.

A die is removed by pressing the dowel

TO PLACE THE DOWELS AFTER THE CAST HAS BEEN MADE.

• Pour the impression with die stone to form a horseshoe shaped working

cast.

15

• Trim the bottom of cast flat to level 10mm from the necks of teeth. This

is done because thin cast are easy to saw and short dowels are more

stable than larger ones.

• Drill a 5mm deep hole in the bottom of the cast directly under the center

of each prepared tooth, pontic area, and segment containing

unprepared teeth. This can be made with 2mm diameter drill in hand

piece or drill press.

• Clean the dowel holes, insert the curved dowel pin properly and then

adjusted for proper fit.

• Cement the dowels into holes one at time with placing a drop of

cyanoacrylate cement into each hole.

• The head of a curved dowel is seated into the holes.

• Seat the heads completely with tail pointing facially.

• Then same procedure of pouring base and preparing dies is repeated

PINDEX SYSTEM

(Coltene/Whaledent,Mahwah,NJ

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In the pindex system a reverse drill press is used to create a master cast

with dies that can be removed and replaced repeatedly with great precision.

The impression is poured without positioning and attaching dowel pins

beforehand. The machine accurately drills parallel holes from the underside of

trimmed cast.

• Pour the impression, adding

approximately 20mm of stone.

• Wet the cast prior to trimming

• Then trim the bottom of the cast,

resting the heels on the table of the

trimmer.

• It should sit perfectly flat on a

tabletop, and its thickness must be a

minimum of 15mm.

• Periphery of the cast should be

trimmed.

• Remove any excess stone in the

palate/tongue area with an arbor

band on a lathe.

• The lingual border of the cast should

taper slightly toward the base to

facilitate removal of the dies from the

cast later.

• The faciolingual width -20mm.

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• Use a pencil mark the desired location of

the pins on the occlusal surfaces of the

teeth or preparations.

• Place the prepared cast on the

worktable and align the first pencil

mark with the illuminated dot from the

light beam director.

• Using both hands, exert firm

downwards pressure on the cast with

thumbs.

• Raise the handle bar with slow, even

pressure and the drill assembly moves upwards cutting the pinholes.

• When proper depth is achieved, red

light goes off.

• Same procedure is repeated with each

mark.

• For better results, cast is made slightly

damp to prevent dust formation.

• Compressed air and brush is used to

remove debris from holes.

The pin holes are refined with hand reamer.

• Cyanoacrylate cement is placed on the pins prior to cementing the pin

tips.

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• Shorter pins are placed before the long pins in lingual or palatal holes

and long pins in the facial holes.

• White sleeves are placed on the long pins and gray sleeves on the short

pins.

• The bottom of the cast is lightly coated with the petrolatum.

• Gray sleeves are blocked with small amount of molten wax to prevent

the sleeve from filling with stone when secondary base is added.

• Strip of utility wax along the ends of the long pins to facilitate removal of

the dies later.

• A palatal or tongue filler is made of

boxing wax.

• The filler is seated to the cast.

• Boxing wax is applied around the cast.

• Base is poured with the die stone.

• When stone becomes hard, the cast is seated in the base former.

• Saw cuts are premarked with the pencil.

• Then dies are sectioned from the underside.

• It may also be sectioned from the

occlusal aspect.

• After the die are sectioned, trim them

in a conventional manner.

• Mark the finish line with red pencil.

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• Apply die hardener and die spacer.

• Then place the completed cast on the

articulator and then cast is ready for

fabrication of the wax pattern.

DI-LOK TRAY SYSTEM

A snap apart plastic tray with internal orienting grooves and notches is

used to reassemble working cast and dies.

TECHNIQUE

The cast should be poured in a U shape, with no stone in the center

building it up to 2.5cm. (1.0 inch)

20

A lingual side of the cast base is trimmed with an arbor band.

Horizontal grooves are cut in the base to for retention.

When stone has set for 1 hr, separate it from impression.

Cast is trimmed in horseshoe configuration to fit in Di-lok tray and the

buccal border is tapered towards base with arbor band.

Soak the base of cast with water for 5 min.

Pour the base in tray, until ¾ of it is filled

Seat the cast on tray; in such a way that cervical line of the prepared

teeth should be approximately 4mm above the level of base.

Wipe off the excess stone.

Allow the stone to set until it is hard and dry .

To complete the dies, the cast must be removed from the tray.

Disassemble the tray by lifting the back up, and then slide the buccal

segment forward.

Then with a saw frame and a thin saw blade, cut between the prepared

tooth and the adjacent tooth.

The saw cut should start in the interdental papilla area and extend

downward on a very slight taper.

The occlusal saw cut should extend three-quarters of the way through

the stone base.

Use finger pressure to break the die and attached teeth from the cast.

21

Remove excess stone gingival to the finish line with a pear-shaped

acrylic bur.

Then cast and dies are reassembled in the tray.

Then the cast and tray mounted on the articulator.

When the stone has set, the articulated cast in the Di-lok tray is ready

for the fabrication of the wax pattern.

ACCUTRAK SYSTEM

A removable die system that is a

modification of a plastic tray with internal

orientation grooves and notches. Pour the

impression in die stone, with a minimum base of

20 mm. After the stone has set, remove the cast

from the impression and trim it to a height of 15

mm and a faciolingual width of 10 mm on a model trimmer and arbor band.

The trimmed cast should fit loosely in the tray. Score the base of the die stone.

Mix stone and vibrate it into the assembled tray. Seat the trimmed cast with a

jiggling motion until the cervical areas of the teeth are approximately 5.0 mm

above the edge of the tray. Remove excess stone and allow the stone to set

until it is hard and dry.

Next the tray is disassembled to allow separation of the die(s). Use a saw to

separate the die from the base of the cast to avoid damage to the

interproximal finish lines. The saw cut should extend through the interdental

papillae and stop 1.0 mm short of the interproximal finish line. Use finger

pressure to break the die and attached teeth from the cast by squeezing the

22

two pieces together. Repeat the process to separate the die from the teeth

attached to it. Trim the die and mark the finish line with a red pencil. Apply a

minimum of two coats of cement spacer to the die, staying 1.0 mm away from

the finish line. Then reassemble the die and working cast in the tray.-

DIVESTMENT TECHNIQUE

• Investment material itself is used for making a die.

• Die is directly incorporated into the investment with the pattern.

• This technique is mainly used for patterns that are not removable from

the die.

WAX PATTERNS

A wax form is the positive likeness of an

object to be fabricated

The wax pattern is a precursor of the

finished cast restoration that will be

placed on the prepared tooth.

Careful handling and manipulation of the wax pattern is required to

obtain an accurate casting

23

METHODS OF FABRICATING A WAX PATTERN

There are two accepted ways of fabricating a wax pattern:

1. The direct technique, in which the pattern is waxed on the prepared tooth in

the mouth

2. The indirect technique, in which the pattern is waxed on a stone cast made

from an accurate impression of the prepared tooth

The indirect technique offers the advantage of allowing most of the

procedure to be done away from the chair. It affords an opportunity for

visualization of the restoration and ready access to waxing the

margins .Because it allows a technician to fabricate the pattern, the indirect

technique has become the most popular means of fabricating cast

restorations.

WAX USED IN FABRICATING A WAX PATTERN

Inlay casting wax is used for all wax

patterns. Inlay wax consists of:

Paraffin (40% to 60%).

Dammar resin to reduce flaking

Carnauba resin, ceresin, candelilla wax to raise the melting temperature.

Dyes to provide color contrasts

Type I Wax:

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- Formulated for making intraoral wax patterns

- Medium hardness wax

- Resist flow at mouth temperature

Type II Wax

- Formulated for fabrication of wax patterns extra-orally

- Softer wax; Have a slightly lower melting point

- Resist flow at room temperature

Requirements of a good inlay wax:

1. It must flow readily when heated, without chipping, flaking, or losing

its smoothness.

2. When cooled, it must be rigid.

3. It must be capable of being carved precisely, without chipping,

distorting, or smearing.

WAX PATTERN FABRICATION

Armamentarium

1. PKT (Thomas) waxing instruments (no. 1, no. 2, no. 3, no. 4, and no 5}

2. Beavertail burnisher

3. No. 7 wax spatula

4. Sable brush 5. No. 2 pencil

6. Laboratory knife with no. 25 blade

25

7. Cotton pliers 8. Bunsen burner

9. Inlay casting wax

10. Zinc stearate powder

11. Die lubricant

Coping Fabrication

The first step in making a wax pattern is the

fabrication of a thin coping, or thimble, on the die. The

coping is usually made of wax, but heated resin sheets

also can be used for this purpose. Vacuum-adapted

polystyrene2 and pressure-formed polypropylene3

have been used to make metal-ceramic crown

patterns. This type of coping also can be used with

partial veneer crowns4 and even pin-retained castings.

If the coping is made on a separate die, it then will be transferred to the

articulated working cast, where it will serve as the foundation for the axial

contours and occlusal morphology to be added there. If it is formed on a

removable die, the die is replaced in the master cast.

• Die lubricant applied to die surface

• Flow wax onto die from well heated large waxing instrument or die

dipped in molten wax

• Proximal areas extra bulk for grip and prevent distortion

• Trim excess wax

Axial Contours

26

The proximal contacts and the facial and lingual axial contours of the

wax pattern should be established at this time.

The proximal contacts of posterior teeth

are located in the occlusal third of the

crowns, except for the contacts between

the maxillary first and second molars,

which are located in the middle third.

The contact must be more than just a

point occlusogingivally, but it must not

extend far enough cervically to encroach

on the gingival embrasure.

The axial surface of the crown cervical to the proximal contact should be

flat or slightly concave. There can be no encroachment upon the

interdental papilla.

A flat contour may be the optimum shape because it is easiest to floss.

Overcontouring of the proximal

surfaces apical to the contacts by

making these areas convex will

produce severe inflammation of the

gingiva.

The proximal contacts are located

slightly to the facial aspect of the

middle of the posterior teeth,

except for the contacts between

27

maxillary first and second molars, which are generally centered

faciolingually.

As a result, the lingual embrasures are slightly larger than the facial

embrasures.

Contacts that are too narrow allow fibrous foods to wedge between the

teeth, and contacts that are excessively wide faciolingually do not

adequately deflect food from the gingival tissue.

The height of contour on the facial surface of posterior teeth usually

occurs in the cervical third.

It also occurs in the cervical third on the lingual surface of maxillary pre-

molar and molars; but on mandibular teeth, it occurs in the middle third.

The facial contours of both maxillary and mandibular posterior teeth

extend approximately 0.5 mm beyond the outline of the root at the

cemento-enamel junction.

The amount of lingual prominence differs between maxillary and

mandibular teeth. It is 0.5 mm on maxillary teeth and mandibular first

premolars, about 0.75 mm on mandibular second premolars, and nearly

1.0 mm on mandibular molars.11

Emergence Profile: The part of the axial contour that extends from the

base of the gingival sulcus past the free margin of the gingiva has been

described as the emergence profile by Stein and Kuwata.

Occlusal Morphology

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Waxing of the occlusal surface is deferred until the axial surfaces are

essentially complete.

During centric closure in the normal dentition, the lingual cusps of the

maxillary posterior teeth and the buccal cusps of the mandibular

posterior teeth make contact with the occlusal fossae or the marginal

ridges of the opposing teeth. They grind food like a mortar during

mastication and are called functional cusps.

On the other hand, the buccal cusps of the maxillary molars and the

lingual cusps of the mandibular molars do not contact the opposing

teeth. These cusps act like the rim of a pestle to prevent food from

overflowing, and they protect the buccal mucosa and the tongue by

keeping them away from the functional cusps. Since these cusps do not

make direct contact with opposing teeth, they are called nonfunctional

cusps.

The occlusal scheme:

Classified by the location of the occlusal contact made by the functional

cusp on the opposing tooth in centric relation.

Cusp-Marginal Ridge Arrangement

The cusp-marginal ridge relation is the type of occlusal scheme in which the

functional cusp contacts the opposing

occlusal surfaces on the marginal ridges

of the opposing pair of teeth, or in a

fossa. Therefore, a cusp-marginal ridge

occlusion is basically a one-tooth-to-

two-teeth arrangement. Since the

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majority of adults exhibit the cusp-marginal ridge type of occlusion, it is an

occlusal pattern widely utilized in daily practice.

Cusp-Fossa Arrangement

The cusp-fossa relation is an occlusal pattern in which each functional cusp is

nestled into the occlusal fossa of the opposing tooth. It is a tooth-to-tooth

arrangement. Although considered to be an ideal occlusal pattern, it is rarely

found in its pure form in natural teeth. Each centric cusp should make contact

with the occlusal fossa of the

opposing tooth at three

points. The contact points

are on the mesial and

distal incline and the inner

facing incline of the cusp, producing a tripod contact.

FABRICATION

Placement of cusp wax cones, marginal ridge & triangular ridge.

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• Position and height of functional cusps –occlusal scheme

• Position and height of non-functional cusps- curve of spee & curve of

Wilson

CONE PLACEMENT CUSPAL

RIDGES

SUPERIMPOSED

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CONES, CUSPAL & TRIANGULAR RIDGES SECONDARY AND MARGINAL RIDGES

Margin Finishing

- Remove the pattern from the

working cast and place it back on

the freshly lubricated die.

- Make certain that the red line on

the die finish line is still distinct

Smooth any roughness on the

axial surfaces with a slightly warm beavertail burnisher.

- Remelt the entire marginal periphery with a hot PKT no. 1, making sure

that the wax is melted through to the die.

- This will result in a depression or "trough" 1 to 2 mm wide and extending

along the entire length of the marginal periphery of the wax pattern.

- Eliminate the depression by adding wax with a

hot beavertail burnisher.

- Carve the excess wax almost to the margin

with a PKT no. 4.

- Finish "carving" the margin with a warm

beavertail burnisher.

Polishing of wax pattern

Carefully inspect the margin by turning

the die so that the margin can be viewed

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from a gingival direction. To finish the occlusal grooves, hold a very small

cotton pellet in cotton pliers and dip it in the die lubricant. Run the pellet

carefully through the grooves with the cotton pliers. To finish the axial

surface use a cotton roll, one end of which has been dipped in die lubricant.

Rub this end across the surfaces to be smoothed.

CASTING & INVESTMEMTArriving at a completed casting after fabrication of a wax pattern

involves three steps:

(1) Investing— surrounding the wax pattern with a material that can accurately

duplicate its shape and anatomic features,

(2) Burnout— removal of the wax pattern so that a mold is created into which

the molten alloy can be placed, and

(3) Casting— introducing the molten alloy into the previously prepared mold

SPRUING

The process of attaching a sprue former/sprue pin to the wax pattern is

called as spruing

SPRUE – The channel or hole through which plastic or metal is poured or

cast into a gate or reservoir and then into a mould

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SPRUE FORMER - A wax, plastic or metal pattern used to form the

channel or channels allowing molten metal to flow into a mould to make

a casting.

3 Basic requirements of sprue:-

Must allow the molten wax to escape from the mould.

Sprue must enable the molten metal to flow into the mould with as

little turbulence as possible.

Metal must remain molten slightly longer than the alloy that has

filled the mould

Type of sprue

o Wax

o Metal

o Plastic

Sprue former diameter:

o The diameter of sprue should be equal to/greater than the thickest

portion the wax pattern to prevent shrinkage porosity.

Sprue former length:

o The length of the sprue former - it keeps the wax pattern 6mm from

the end of the ring

o Very short sprue : result in back pressue porosity in casting at the

junction of sprue and pattern.

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o Very long sprue : sprue solidifies first leading to casting shrinkage

and incomplete casting

Sprue former position

o The ideal area- point of greatest bulk in the pattern.

o The point of attachment should permit stream of metal to be

directed to all parts of the mould without having to flow opposite the

direction of casting force

o Always on non-functional cusp

Sprue former direction

o Attached 45 degrees to the walls of mould, which decreases the

turbulence of molten alloy.

Attachment morphology

o The attachment of sprue former to the wax pattern should be smooth

and do not possess pits or irregularities.

o Irregularities produces tags of investment which is prone for fracture

by molten alloy leading to casting failure.

Types of attachment

o Direct: A connector or reservoir bar is positioned between the

pattern and crucible former.

o Indirect: Multiple unit spruing using runner bar

Sprue shape

35

o The sprue former should be straight to reduce chances of creating

turbulence in molten metal entering the mould.

o High turbulence of alloy cause porosity

Number of sprue:

o Usually a single sprue is used for small castings.

o When two thick sections of a pattern are connected by thin part of

wax, 2 separate sprues should be attached to each thick portion.

RESERVOIR

Reservoir is a small amount of additional wax which is added to the

sprue former near the junction of wax pattern

It is used in direct spruing.

It prevents localized shrinkage porosity as the alloy in this part solidifies

last after the solidification of metal in mould

VENTING

Small auxiliary sprue /vents are applied to thin wax pattern to improve

the quality of casting. Usually 18- gauges sprues are used. It is indicated

with extremely thin/thick casting to produce nonporous castings.

They help in escape of gases during casting and ensure beginning of

solidification in critical areas by acting as a heat sink.

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It is attached to the wax pattern directly opposite to larger sprue former.

CRUCIBLE FORMER

• The base to which a sprue former is

attached while the wax pattern is being

invested in refractory investment; a

convex rubber, plastic or metal base that

forms a concave depression or crucible in

the refractory investment.(GPT8)

CASTING RING

Casting ring is a metal tube in which a refractory mould is made for

casting dental restorations.(GPT8)

Casting rings are used to confine the fluid investment around the wax

pattern while the investment sets.

It also allow the hardened investment to be safely handled during

burnout and casting

RINGLESS CASTING SYSTEM

Plastic ring with rubber crucible formers are used. The ring is conical in

shape with tapering walls. As the investment sets the investment is tapped out

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of ring. Then burnout is done without casting ring, this causes greater

expansion to compensate for metal shrinkage

CASTING RING LINER

They are commonly used to produce expansion of mould. Various

materials used as ring liners ----

1. Asbestos liner

2. Cellulose (blotting paper) liner

3. Ceramic ring liner

4. Combination of ceramic and cellulose ring liner

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INVESTING PROCEDURE

Surrounding the wax pattern with a material that can accurately

duplicate its shape and anatomic features

Investment material

Requirements:

Precise reproduction of wax pattern.

Sufficient strength to withstand burnout and casting.

Sufficiently porous to allow escape of gases.

Expand enough to compensate solidification shrinkage of alloy.

Shrinkage Compensation

• Four mechanisms to produce mould expansion:

1. Setting Expansion of Investment

2. Hygroscopic Expansion

3. Wax Pattern Expansion

4. Thermal Expansion

1. Setting Expansion

Occurs as a result of normal gypsum crystal growth in air. About 0.4%

but partly restricted by metal investment ring.

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2. Hygroscopic Expansion :

1. Immerse investment-filled ring in water bath at 38°C.

2. Water in bath replaces water used by hydration process

3. space between growing crystals is maintained

4. crystals grow longer

5. Outward expansion of mould.

6. About 1.2 to 2.2% max with expandable ring.

3. Wax Pattern Expansion

While investment is still fluid, expansion occurs when wax is warmed above

temperature at which it was formed.

4. Thermal Expansion

When investment is heated in burnout oven.

Types

Gypsum bonded

Phosphate bonded

Silica bonded

Gypsum Bonded Investments

• Use with alloys that melt below 1,000°C.

• Used with Type I, II, III gold alloys

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Phosphate Bonded Investments

• Much stronger and can withstand much higher burnout temps For alloys

with casting temp > 1150°C

Silicate Bonded Investments

• Base metal alloys such as chrom-cobalt and steel

• A silica-bonded investment material can be heated up to 1200C

• The reason is that these alloys melt at temperatures ranging from 1250

– 1400

MIXING

Vacuum mixing

Hand mixing

The best method is vacuum mix and vacuum pour technique.

Advantages of vacuum mixing-----

1) Remove air bubbles & produce smooth castings

2) Increase tensile strength of investment

3)95% of castings free of nodules.

4) Removes all the gaseous by products of chemical reaction of investment

material

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INVESTING OF GYPSUM BONDED INVESTMENT MATERIAL

Require very specific Water: Powder ratio.

A variation of only 1ml of H2O can significantly alter the setting

expansion & the character of the casting surface.

Increasing Water: Powder ratio makes investing process easier but

investment will lose strength, cause cracks to occur during heating

surface of casting inferiors.

After the casting ring has been filled with investment material, any

excess should be removed before the material sets.

The filled ring is now set aside to allow the investment material to

complete its setting reaction & the accompanying setting expansion.

Setting is complete in 30-40min.

Hygroscopic technique is used.

Freshly filled investment ring is immediately placed into water bath for

30min. & kept at 100ºF (38ºC).

INVESTING OF PHOSPHATE BONDED INVESTMENTS

Expansion of the mould cavity can be increased by--

1) Increasing the no. of layers of asbestos or fibrous ceramic lining the casting

ring.

2) Increasing the special liquid: water ratio.

3) Increasing the total L: P ratio.

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4) Placing the investment in contact with water during setting.

5) Burning out the mould at a higher temp.

Residual, hardened investment in an unclean mixing bowl will greatly

accelerate the set of newly mixed investment

Phosphate investment should not be mixed in an apparatus that has

been used for gypsum investment. Residual gypsum will also accelerate

the set & will break down at temp. above 2400ºF(1300ºC) liberating

sulfurous gases that can be detrimental to the casting

Ammonia gas is given off during mixing, & it is important to hold the

mixed investment under the vacuum after mixing ceases to dissipate

some of this gas & thereby reduce the incidence of bubbles adhering to

the wax pattern (this additional holding time will vary from 15-45sec).

Initial set of the phosphate bonded investment is generally rapid with

the liberation of heat.

If burnout is not carried within 1-2hrs, the ring should be stored in a

humidor at 100% humidity, not soaked in water since excessive

hygroscopic expansion may result

Carefully grinding or scraping the shiny “skin” off the end of investment

just prior to burnout is advisable. This removes a relatively impervious

layer, opening the pores of the investment & facilitating gas release as

the alloy is cast into the mould.

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BURNOUT PROCEDURE

Removal of the wax pattern so that a mould is created into which the

molten alloy can be placed

Once the investment has set for an appropriate period 45min. it is ready

for burnout.

A crucible former is then carefully removed.

It is advisable to begin the burnout procedure while the mould is still

wet, because water trapped in the pores of investment reduces the

absorption of wax & as water vaporizes, it flushes wax from mould.

CASTING PROCEDURE

Introducing the molten alloy into the previously prepared mold.

Casting of an alloy into the mould space uses 2 basic requirements.

Heat source – to melt the alloy

Casting force – to force molten alloy into mould

This can be done by use of following different type of force-

o Vacuum force

o Air or Gas Pressure

o Centrifugal force

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Classification System for Noble Dental Alloy

High-noble alloys have a noble metal content of 60% or greater. At least

40% of the alloy must be gold.

Noble alloys must be at least 25% noble metal.

Predominantly base alloys have a noble metal content of less than 25%.

Melting temp of pure gold –1063⁰c

Melting temp of gold alloy-924-960⁰c

Melting temp of base metal alloy-1155-1304⁰c

HEAT SOURCE:

Different types of materials and method are used as heat source to melt

alloy. Two basic modes are by using

1) Torch flame--

o Gas air

o Gas oxygen

o Air acetylene

o Oxygen acetylene.

o Hydrogen oxygen generator

2) Electricity --

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Zones of the blow torch flame:

1. Gas air torch:

Gas-air torch is used to melt conventional noble metal alloys

(used for inlays, crown and bridge) whose melting points less

than 1000⁰c

2. Gas–oxygen torch:

Used to melt metal ceramic alloys of higher temperature up to

1200⁰c

The tip of torch is available as single orifice/multi orifice.

The oxygen pressure is adjusted to

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10-15 psi

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3. Oxy-acetylene torch :

The actual production of flame can be done by adjusting the pressure

and flow of individual gases.

commonly advised pressure for acetylene nozzle is 3.5 N/cm2 and

oxygen nozzle 7-10 N/cm2

one part of acetylene + 2 and half part of oxygen

For base metal alloy

CRUCIBLES

The Melting of alloy requires a crucible to act as a platform on which the

heat can be applied to the metal. There are three types of casting

crucibles available---

o Clay

o Carbon

o Quartz

o Zirconia-alumina

The crucibles used with noble metal alloys should not be used for

melting base metal alloy

Clay crucibles are used with high noble and noble metal alloys used for

crown and bridges.

Quartz crucibles are recommended for high-fusing alloys of any type of

base metal alloys and palladium alloys

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Carbon crucibles – for high noble crown and bridge and also for higher

fusing gold-based metal ceramic alloys.

Traditionally a wet lining of asbestos sheet was used on casting

crucible. The moistened asbestos sheet provides a clean and good

surface on which the alloy could be melted.

Advantages is, prevent alloy contamination with oxides and residuals

that may be present in the crucible

Sufficient mass of alloy must be present to sustain adequate casting

pressure---

o 6gm is typically adequate for premolar and anterior casting

o 10gm is adequate for molar casting

o 12 gm is adequate for pontic

ELECTRICAL SOURCE

A) Electrical resistance-

• It is used to melt ceramic alloys. Here the alloy is automatically melted in

graphite crucible.

• Provides best means of temperature control. It is quite convenient as

compared to blow torch.

B) Electrical arc melting:

Is used to melt higher fusing alloys. It used to create an electrical arc at the

end of two electrodes. The apparatus requires a high electrical input (30A).

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CASTING MACHINES

Device for forcing the molten alloy into the mould under pressure after wax

has been eliminated

1. Air pressure casting machines

• Alloy is melted in situ in crucible, followed by applied air pressure.

• Pressure of 10-15 psi

2. Centrifugal casting machine

• Alloy is melted in a crucible, and forced in to

mould by centrifugal force

3. Electrical resistance - heated casting machine

• It is used to melt ceramic alloys. Here the alloy is automatically

melted in graphite crucible.

• The crucible in the furnace is always against the casting ring. So the

metal button remain molten slightly longer and ensures complete

solidification.

4. Direct-current arc melting machine

• Direct current is produce between two electrodes: the alloys and the

water cooled tungsten electrode.

• Temp between the arc rapidly increases to 4000°C – alloy melts very

quickly.

• High risk of overheating of the alloy.

• Damage may occur even after few seconds of overheating.

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5. Induction melting machine

• Metal is melted by an induction field that developed with in the crucible

surrounded by water- cooled metal tubing.

• The electric induction furnace is a transformer in which an alternating

current flows through the primary winding coil and generates a variable

magnetic field in the location of the alloy to be melted in a crucible

• It is more commonly used for melting base metal alloys not been used

for noble alloy casting as much as other machines

CLEANING OF THE CASTING

For gold crown & bridge alloys.

After casting has been completed, ring is removed & quenched in water.

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Advantages:

• Noble metal is left in an annealed condition for burnishing &

Polishing.

• When water contacts hot investment, violent reaction ensues.

Investment becomes soft, granular & casting is more easily

cleaned.

Trimming is done from the button end of the ring.

Investment is being pushed out of the casting ring. The mould is broken

open. Investment is removed from the casting. Care must be taken to avoid

damaging the margin

Sandblasting: The casting is held in a sandblasting machine to clean the

remaining investment from its surface.

Pickling: Surface of the casting appears dark with oxides and tarnish. Such

a surface film can be removed by a process called Pickling. Best method for

pickling is to place a casting in a dish & pour acid over it. Heat the acid but

don't boil it.

o 50% Hydrochloric acid

o Sulfuric acid

o Ultrasonic devices

Gold and palladium based metal ceramic alloys and base metals, these alloys

are not generally pickled. Pickling solution should be renewed frequently, since

it is likely to become contaminated Precious alloys (Gold-Platinum-Palladium)

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can be soaked with hydroflouric acid. Nickel Chromium should never be placed

in acid because of high reactivity.

SOLDERING

Act of uniting two pieces of metal/alloy by a filler metal whose fusion

temperature is lower than the metal/alloy to be soldered

Use

• Joining individually casted retainer and pontic -fabricating

connectors

• Add proximal contact

• Repair casting voids

• Repair single piece FPD with poor seating

TRIMMING

The casting is trimmed, shaped and

smoothen with suitable burs or

stones.

The sprue is sectioned off with a cutting disc.

POLISHING & VENEERING

Minimum polishing is required if all the procedures from the wax pattern

to casting are followed meticulously.

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White stone ,rubber wheels, rubber disks, and fine grit are included in

the finishing and polishing agents

Done with ceramic or resin for aesthetics

CONCLUSION

The lab procedures and the technicians play a major role in the

success of a prosthodontics procedure. The lab procedure starts

from pouring the model till finishing the restoration. All precautions

should be taken to make sure that the procedures are done

according to the prescribed methods, because any mistake in the

first step will only have a snowball effect with a poor restoration at

end.

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REFERENCES

• Fundamentals of Fixed Prosthodontics

3rd edition Shillingburg

• Contemporary fixed prosthodontics

4th edition Rosentiel Land Fujimoto

• Phillips’ Science of Dental Materials

• Glossary of prosthodontic terms

• Textbook of prosthodontics - V Rangarajan

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