Model and Die Materials Final 1

Model and Die Materials

Definitions: (GPT-7)

Model: A facsimile used for display purposes, a miniature

representation of something, an example for imitation or emulation

Die: The positive reproduction of the form of a prepared tooth in

any suitable substance.

Cast: A life size likeness of some desired form. It is formed within

or is a material poured into a matrix or impression of a desired

form.

Types of Model & Die materials

1. Gypsum products:

Model plaster-Beta hemi hydrate (type II)

Dental stone -Alpha hemi hydrate (type III)

Dental stone with high strength (type IV)

Dental stone with high strength& setting expansion (type V)

2.Metals: Electroformed or Electroplated, Sprayed metals, Amalgam

3 .Polymers :Polyurethane, Epoxy resins.

4 .Cements: Silicophosphate or poly acrylic acid bonded cements.

5. Divestment

Selection factors

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Model & Cast

Die


1 Material used for impression

2 Purpose of use of the die or cast.

Impression material Die material

Impression compound Plaster/ stone/ Electroformed copper

Alginate/ Agar Plaster/Stone/ investment

Rubber base materials Gypsum/ Electroformed/ Epoxy die

Desirable Qualities of cast and die material

1. accurate should reproduce fine details

2. remain dimensionally stable

3. strong ,durable ,resist abrasion

4. should be available in a colour that contrast with wax so that

preparation margin can be seen

5. wettable easily by wax

6. should be compatible with impression material

7. economical

Gypsum products

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Manufacture-They are obtained from gypsum by the process of

Calcination.

1. Commercial gypsum is ground& heated to 110 to 120 *c. to

drive off a part of the water of crystallization & form calcium

sulfate hemihydrate.

2. On further heating, the hemihydrate forms anhydrite due to the

removal of the remaining water.

Comparison of model plaster & dental stone

Criteria Model plaster Dental stone

Manufacture Heating gypsum in an open

Kettle at a temp of 110 -

120 C.

Calcining gypsum under Steam

pressure, in an autoclave at

120- 130 C.

Shape of crystals Spongy & irregular. Dense& prismatic.

Compressive strength 90kg/cm 1300 psi 210kg/cm 3000psi

W: P ratio 0.45-0.5 0.28-0.3

Max.Setting expansion,

at 2hr.

0.3% 0.2%

Setting time(min) 12+/-4 12+/-4

Other names Beta- hemi hydrate Alpha hemi hydrate

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DENTAL STONE DENTAL PLASTER

Type IV & V dental stone

Manufacture

Calcining gypsum by boiling in 30% Cacl2 solution. The

chlorides are then washed away or autoclaved in the presence of

sodium succinateO.5%. The hemihydrate is then ground to the

desired fineness. Microscopically cuboidal in shape.

Advantages

Good strength

Minimal shrinkage

Easy manipulation

Sets quickly

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Compatible with impression materials

Smooth, hard surface.

Easy trimming

Color contrast

Economical

Disadvantages

Brittle Edges& occlusal surface may be rubbed off.

Comparison of type IV & type V stone

Criteria Type IV stone Type V stone

W: P ratio 0.22-0.24 0.18-0.22

Max. Setting

expansion, at 2hr.

0.1% 0.3%

Setting time(min) 12±4 12±4

Compressive strength 350kg/cm 5000 psi 490kg/cm 7000 psi

Setting reaction:

The hemihydrate reacts with water to form dihydrate with the

release of energy.

The reaction is the same for all gypsum products.

CaSO4.1/2H 2O + 3H 2O → 2CaSo4.2H 2O+ 3900cal/gmol

This reaction is exothermic.

This reaction is explained on the basis of difference in the

solubility of the hemihydrate and the dehydrate, Hemihydrate is 4

times more soluble than the dihydrate.

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1. On mixing hemihydrate with water a suspension is formed

which is fluid and workable.

2. Hemihydrate dissolves in water to form a saturated solution.

3. The saturated hemihydrate gets supersaturated with the

dihydrate which precipitates.

4. Since the soln. is no longer saturated with hemihydrate, it

continues to dissolve and new crystals are formed and further

growth of crystals occurs.

Induction period: Initial stage when there is little reaction and little

rise in temperature.

Spherulite: The needle like crystals formed when the mass thickens

and hardens.

Microstructure of set gypsum

Set material consists of entangled aggregate of gypsum

crystals having lengths of 5-10microns. Micro porosity is due to

residual unreacted water or due to the growth of the gypsum

crystals.

Water: Powder ratio

It is the amount of water and powder gauged by weight.

Determines the physical and chemical properties of the set

mass.

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Higher the W: P ratio, longer the setting time& weaker is the

product.

Properties

1. Mixing time-Time: elapsed from the addition of powder to water,

till the mixing is completed. Mechanical mixing-20-30sec.

Hand mixing-1 min

2. Working time: It is the time available to use a workable mix, one

that maintains an even consistency, which may be manipulated to

perform one or more tasks. Working time-3min.

3. Setting time: Time taken from the beginning of mixing till the

material hardens. Initial setting time -The time at which the mass is

viscous enough & can no longer be poured. It is rigid, but not hard.

Final setting time - The time at which the material can be separated

from the impression without distortion or fracture.

Measurement of setting time

Loss of gloss test- from the start of mixing till the gloss is

lost. The mix can't be removed from the mould since it does

not have sufficient compressible strength.

Exothermic heat- Rise in temperature from the mix can also

be used to measure the setting time.

Penetration tests

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1. Vicat Needle

2. Gillmore Needle- small and large.

Factors affecting setting time

1. Manufacturing process (Impurities) - If calcination is incomplete

& considerable gypsum dihydrate is left in the final product, the mix

sets faster. Finer the particle size, faster the mix hardens.

2. Mixing- Longer & more rapid the mixing, faster the plaster sets.

3. W: P ratio- Increase in the ratio increases the setting time.

4. Temperature- The effect is erratic.

I. 0-50*C- Little change

II. >50*C- Increasing retardation

III. At 100*C-No reaction

IV. Higher temp- Reversal of reaction.

5. Modifiers - Accelerators and Retarders-Most effective&

practical method of controlling setting time.

i. Accelerators:

a. Terra alba

b. 2% NaCl

c. 3.4%Na2SO4

d. 2% K2SO4

ii. Retarders

a. >2% NaCl

b. Citrates acetates, borates

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c. Colloids like Glue, gelatin, agar and coagulated blood

d. Borax- 1-2% - most effective.

Setting expansion

Normal setting expansion-this is due to the outward thrust of

the growing gypsum crystals during setting. This effect is desired in

investment materials, since it helps to compensate for the casting

shrinkage. The expansion is minimized by the addition of potassium

sulfate.

Hygroscopic setting expansion

This is due to the free growth of the crystals in a continuous

aqueous phase.

Strength

It increases as the material sets. Factors affecting the strength-

Free water content

Wet strength-when excess water is present.

Dry strength- when excess water is lost by evaporation. Two

times or more greater.

Temperature-Gypsum is stable only below 40*C.

Water powder ratio

Timeofspatulation

Accelerators or retarders.

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Manipulation-Proportioning

Premeasured water& powder according to the W:P ratio.

Flexible bowl & stiff bladed spatula.

Clean equipment

No air entrapment during mixing

Procedure

1. Take a measured quantity of water in the bowl & sift a

weighed amount of powder into it .

2. Spatulate with rapid stirring motion with frequent jarring &

squeezing.

3. Do not change the W: P ratio at this stage.

4. Vibrate the mix using a vibrator to get rid of air bubbles &

pour into the Impression

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METHOD’S FOR MANIPULATION

A. VACCUM MIXER

B. BOWL AND SPATULA

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Disinfection-Models can be disinfected with a spray of iodophor, or

by immersion in 1:10 dilution of sodium hypochlorite solution for

30min.

Care for the cast - Safest method for soaking the cast is to place it

in plaster slurry, which provides a saturated solution of calcium

sulfate.

ELECTRO FORMED/ ELECTROPLATED CASTS AND DIES:

Electro deposition of copper or silver on the impression gives

a hard metallic surface to the cast.

Advantages

1. Dimensional accuracy.

2. Hard and abrasion resistant.

3. Better marginal definition.

4. Does not absorb oil or water.

5. Prevents cupsal wear due to repeated contact with opposing

cast.

Disadvantages

1. Difficult to trim.

2. Silver bath - health hazard.

3. Not compatible with all impression materials.

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4. Color contrast not as good as die stone.

5. Adaptation of wax not as good as the die stone. Pattern tends

to lift from margins.

Potential inaccuracies are associated with electroformed dies

do not occur with properly made stone dies. Plating process of

sufficient duration can cause dimensional changes in the impression.

Method of electro forming of impression for dies and casts

It is also known as electroplating and electrodeposition.

Metalizing: the impression surface is made conductive so that it can

conduct electric current.

The metalizing agents are:

1. Aqueous suspension of silver powder.

2. Powdered graphite.

Metals used for electro forming are:

1. Copper

2. Silver

Plating can be done for:

A) Individual tooth impression.

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B) Full arch impression.

Plating is done on:

Compound impression (usually copper plated)

Polysulphide impression (usually silver plated)

Components of electroplating apparatus:

1. Impression to be coated is made the cathode.

2. Anode is the metal to be deposited- copper or silver.

3. Anode holder and cathode holder.

4. Electrolyte is the solution through which electric current is

passed. The ions are deposited from the anode to the cathode -

silver cyanide or copper sulphate.

5. Ammeter - registers the current in milliamperes. The current

passed is of 10 milliamperes per tooth area, for 12 hours.

6. Plating tank - Glass or hard rubber with wall fitting cover to

prevent evaporation.

Composition of electroplating bath

Copper Silver

Copper sulphate 200gms silver cyanide 36gms

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crystals

Sulphuric acid

concentrate

30ml potassium

cyanide

60gms

Phenol sulphonate 2ml potassium

carbonate

45gms

Water distilled 1000ml Water distilled 1000ml

Temperature: 77° to 80° F (room temperature).

With a lower current - deposit is slow but hard.

With a higher current - deposit is fast but soft.

Procedure

1. Wash and dry the impression.

2. Attach the impression to the cathode holder. An insulated

wire is used for this.

3. The impression material is a non conductor of electricity. It is

made conductive by applying a metalizing solution with a

brush. The excess powder is blown off.

Colloidal graphite - for copper plating.

Silver powder - for silver plating.

4. The surface of the copper ring or impression tray is covered

by wax 2mm. beyond the margin of the impression, so the ring

is not plated.

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5. With the dropper the impression is filled with the electrolyte.

Avoid air bubbles.

Silver cyanide - for silver - plating.

Copper sulphate - for copper - plating.

6. The electrode is attached to the cathode and the impression is

immersed in the electrolyte bath. Distance between the

cathode and the anode should be at least 4inches.

7. The electric current is turned on - it should not exceed

50milliamperes. The current is adjusted 10mA per tooth. If

high current is used the surface will be granular, rough and

weak. With low currents the deposit is smooth and hard.

8. Run the current for 12 to 15 hours, to get a deposit of 0.5mm.

9. The current is disconnected. The impression is washed. The

die is completed by pouring resin on dental stone to support

the surface of the cast and to form the base. Then the copper

ring is gently warmed over the flame and removed from the

die. The die is trimmed.

EPOXY RESIN DIE MATERIALS

They are used to overcome the low strength, low abrasion

resistance of die stone. Most available resin die material is epoxy,

but polyurethane also used.

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Are compatible with

- Polyether

- Silicon impression materials.

Advantages: Tougher and more abrasion resistant than die stone.

Disadvantage: Not as accurate and dimensionally stable, as it

shrinks. Polymerization shrinkage of 0.1 to 0.3 is seen.

Available as: 2 components - resin and hardner.

DIE STONE: INVESTMENT COMBINATION (DIVESTMENT)

A gypsum-bonded material called divestment is mixed with a

colloidal silica liquid.

A die is prepared from the mix and a wax pattern is

constructed. The possibility of distortion of Wax pattern removal

from the die or during setting of the investment is minimized.

The setting expansion of divestment is 0.9% and thermal

expansion is 0.6%, when heated to 677°c.

Advantage

It is a highly accurate technique for conventional gold alloys

especially for extra - coronal preparations.

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Divestment phosphate or DVP is a phosphate - bonded

investment that is used in the same manner as divestment and is

suitable for use with high fusing alloys.

Silicophosphate cement

This is similar to filling and cementing material.

Advantage: Harder than die stone.

Disadvantage: shrinkage on setting, loss of water on standing.

Amalgam:

This is similar to restorative material.

Advantage

Produces a hard die, reproduces fine details and sharp

margins.

Disadvantages

Can only be packed into a rigid impression; long time to

reach maximum hardness; high thermal conductivity, so can cool a

wax pattern rapidly, which may lead to distortion of the pattern-

this can be overcome by warming the die; a separating agent is

needed as with stone dies.

Metal sprayed dies

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A bismuth-tin alloy, which melts at 138°c, can be sprayed

directly on to an impression to form a metal shell, which can then be

filled with the dental stone.

Advantage: A metal coated die can be obtained rapidly from

elastomeric impression materials.

Disadvantage: The alloy is rather soft; care is needed to prevent

abrasion of the die.

Ceramic die materials

Two ceramic die materials are available.

a) A material for the production of dies on which porcelain

restorations are to be fabricated, without the use of a platinum

foil matrix. To form the dies heating to over 1000°c is

necessary.

b) A ceramic material, supplied as a powder and liquid, and

mixed to putty -like consistency. After one 1 hour the material

is removed from the impression and fired at 600°c for 8

minutes to produce a hard strong die.

Advantages Disadvantages Recommended Precautions

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Use

ADA Type IV

stone

Epoxy resin

Electroplating

Dimensional accuracy

Straightforward

technique Low cost

Straightforward in-

office procedure

High strength Good

abrasion resistance

High strength Good

abrasion resistance

Will be damaged if

not handled

carefully

Lower abrasion

resistance

Polymerization

shrinkage Time-

consuming complex

procedure

Time-consuming

Special equipment

needed

Most

situations

Complete

ceramic

crowns

Complete

ceramic

crowns

Accurate

proportioning

essential

Vacuum mix

recommended

Not compatible

with polysulfide

and

hydrocolloid

Silver uses

toxic cyanide

Incompatible

with many

impression

Materials

Recent developments

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Two techniques are being investigated to produce dental

stones with improvement in 'abrasion resistance and other

mechanical properties:

c) Impregnation of the gypsum by a polymer - polyester,

polystyrene, acrylics and epoxy resin have been suggested for

this purpose.

d) Incorporation of wetting agents such a lingnosulphonates -

such additives can reduce the water requirement of a stone,

and enable the production of a harder, stronger and more

dense set gypsum.

In addition, these additives retard the setting time and

increase the setting expansion. Both of these effects can be

overcome by the incorporation of potassium sulphate.

References

1. Craig RG, Powers JM, Wataha JC: Dental Materials,

Properties and Manipulation. Seventh Edition, India 2001;

Harcourt private limited.

1. Anusavice: Philips science of Dental materials. Eleventh

Edition, India 2003; Saunders Publication.

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Documents

Model and Die Materials Final 1