By

Dr. RASHID HASSAN

MODEL & DIE MATERIALS

1) Gypsum products2) Epoxy products

MODEL & DIE MATERIALS

Dental CastA positive and accurate reproduction of a mandibular or maxillary arch or a portion, made from an impression of that arch.

Model A positive replica of several teeth and surrounding structure or an edentulous arch, which is prepared from an impression.

DieA positive replica of a single tooth, prepared from an impression.

MODEL & DIE MATERIALS

MODEL PLASTER:Weakest and most porous form of gypsum product used in dentistry.

DENTAL STONE:A stronger and less porous form of gypsum product used in dentistry.

DIE STONE:The densest form of gypsum product used in dentistry.

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MODEL & DIE MATERIALSDIAGNOSTIC CASTS:

The casts generally used for patient education, treatment planning or tracking the progress of treatment. (e.g. orthodontic models)

WORKING CAST:The cast that is strong enough to resist the stresses of fabricating an indirect restoration or prosthesis.

POURING:The process of introducing the flowable gypsum product into the impression.

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MODEL & DIE MATERIALS

GYPSUM DENTAL INVESTMENT:A refractory material, consisting of silica and gypsum as a binder. Used for taking moulds for metal casting process.HYGROSCOPIC SETTING EXPANSION:The amount of setting expansion that occurs when the gypsum bonded casting investment is immersed in water.NORMAL SETTING EXPANSION:The amount of setting expansion that occurs when a gypsum bonded casting investment is allowed to set in air.

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Requirements

Dimensional Accuracy• Accuracy of fit of

restoration or appliance depends on the accuracy of the replica.

• Small dimensional changes during setting may be compensated.

Dimensional Stability• Changes during storage

are more serious problems.

• Mechanical properties:

• Strong enough to avoid accidental fracture.

• Hard enough to resist abrasion during carving wax patterns.

Other requirements of dental cast materials

Should ideally be fluid at the time it is poured into the impression.

Set material should be sufficiently strong to resist accidental fracture.

Set material must be hard enough to resist the abrassion during carving of wax pattern.

Should be compatible with all the materials it comes in contact.

Should have good colour contrast with various waxes.

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1) GYPSUM PRODUCTS

INTRODUCTION TO GYPSUM PRODUCTS

• Gypsum is a white powdered mineral widely found in nature.

• Used for making dental casts since 1756.

• Chemical name Calcium sulphate dihydrate.(CaSO.2H2O)

• In dentistry, gypsum is used in the form of Calcium sulphate hemihydrate. (CaSO.½H2O)

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TYPES OFGYPSUM PRODUCTS

Current I.S.O standard for dental gypsum products identifies 5 types .

Type 1: Dental plaster, Impression.

Type 2: Dental plaster, model.Type 3: Dental stone, die and model.Type 4: Dental stone, die, high strength,

low expansion.Type 5: Dental stone, die, high strength,

high expansion.

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All 5 types are chemically identical but differ in

their physical form depending upon the method used for manufacturing.

Gypsum products

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Plaster of Paris Dental stone

Die stone

GYPSUM PRODUCTS

TYPE 1 (DENTAL PLASTER, IMPRESSION)

• Earliest of impression materials in dentistry.

• Rigid (non elastic) impression material.

• Fractures on removal from undercut areas.

USES:• For complete denture

impressions.• Impressions for Maxillofacial

prosthesis.• Bite registration material.

TYPE 2 (DENTAL PLASTER, MODEL)

• Also called as model plaster/laboratory plaster.

• Contains beta hemihydrate & modifiers.

USES:

• For making study cast & models.

• For making molds for curing dentures.

• For mounting casts on articulator.

GYPSUM PRODUCTS

TYPE 3 (DENTAL STONE, MODEL)

• Also called as class I stone/Hydrocal.

• Alpha hemihydrate with potassium sulphate(accelerator) & borax (retarder)

USES:

• For preparing master cast.

• For making molds.

TYPE 4 (ENTAL STONE, DIE , HIGH STRENGTH, LOE EXPANSION)

• Also called as class II stone,/die stone/Densite/Improved stone)

• Cast should be separated 1 hour after pouring.

• Type 4 gains max hardness after 20 hours.

USES:• For dies used for inlays wax

pattern.• For crown & bridge wax

pattern.

GYPSUM PRODUCTS

TYPE 5 (DENTAL STONE, DIE, HIGH STRENGTH, HIGH EXPANSION)

• Most recent gypsum product.

• Has highest compressive strength.

• Setting expansion increased to maximum 0.10% to 0.30%.

USES:

• To prepare dies with increased expansion.

Composition

• Gypsum products used in dentistry are formed by driving off part of the water of crystallization from gypsum to form calcium sulphate hemihydrate.

Gypsum Gypsum product + water

2CaSO4.2H2O (CaSO4)2.H2O + 3H2O

Calcium sulphate dihydrate Calcium sulphate hemihydrate

• Application of gypsum product in dentistry involves the reverse of the above reaction.

(CaSO4)2.H2O + 3H2O 2CaSO4.2H2O

Production of Calcium Sulphate Hemihydrate

CaSO4 + 2H2O CaSO4 + ½H2O CaSO4 + ½H2O CaSO4

CaSO4 CaSO4

Gypsum Calcium sulphate hemihydrate

Calcium sulphate Hexagonal hemihydrate anhydrate

Hexagonal anhydrate Orthorhombic anhydrate

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110 - 130ºC

130 - 200ºC

200 - 1000ºC

110 - 130ºC

130 - 200ºC

200 - 1000ºC

Method of manufacturingModel & die materials

DENTAL PLASTER: (Plaster of Paris/ Soft plaster)1) Produced by dry

calcination.2) Grounded gypsum is

heated in an open vessel/kettle at 110-130 °C.

3) β hemihydrate crystals are formed.

DENTAL STONE:1) Calcination is done

under steam pressure in an autoclave at 120-130°C at 17lbs/sq.inchfor 5-7 hours.

2) α hemihydrate crystals are formed.

HIGH STRENGTH (αMODIFIED) STONE:1) Wet Calcination is done

by boiling gypsum in 30% calcium chloride solution.

ManipulationWater to powder ratios

Gypsum Product Water (ml) Powder (g) W/P ratio (ml/g)

Plaster 50 - 60 100 0.55

Stone 20 – 35 100 0.30

Theoretical

ratio

18.6 100 0.186

Theoretical ratio (0.186) is enough to satisfy

chemical reaction.

Excess water is absorbed by porosities.

Manipulation

HAND MIXING:• Instruments required are

rubber bowl & stiff spatula• Water & plaster are taken

according to recommended ratio.

• Water is taken first in bowl.• Mixing is done vigorously

with the help of stiff spatula.

• Continue spatulation till a smooth creamy mixture is formed.

• Vibrate the mix to avoid air entrapment.

Manipulation

MECHANICAL MIXING:

Requires expansive equipment.

Mixing is done under vacuum.

Gives stronger & denser cast.

SETTING REACTION

• Setting starts rapidly after mixing P/W.

• Water becomes saturated with hemihydrate.

• Dissolved hemihydrate is then converted into dihydrate.

• Crystals of dihydrategrow form specific sites called nuclei of crystals (crystals are ultimate units of the substance from which it was formed)

• These may be small particles of impurities, unconverted gypsum crystals (calcium sulphate dihydrate) within the hemidyrate.

SETTING REACTION• If more water is

added, centers of nucleation are more widely dispersed by the dilution effect.

• With greater spaces between crystals leading to a significant reduction in strength

• Two stages can be identified during setting.

• Initial set: During this stage the material becomes rigid but not hard and can be carved but not moulded (8-16 minutes).

• Final set: The material becomes hard and strong, however hydration reaction is not necessarily completed.(30- 40 minutes).

Exothermic Reaction

• The hydration of the hemihydrate is exothermic, so heat is given out during setting.

• Temperature is maximum during the stage of final setting.

• Temperature rise is negligible at the time of initial set.

• The magnitude of the temperature rise depends on the bulk of the material used.

Measurement of Setting of Gypsum

TESTS FOR MEASURING SETTING

• Loss of gloss test for initial set.

• Gillmore test for initial setting.

• Gillmore test for final setting.

Measurement of Setting of Gypsum

GILLMORE APPARATUS

VICATPENETOMETER

READY-FOR-USE CRITERION:

Ready-for-use criterion is a measure of time at which the set material can be handled safely.

Ready-for-use criterion is not judged by any specific test.

Comes with experience.

Technically set material is considered ready for use when the compressive strength is 80%. (attained in 1hr)

Modern products reach ready-for-use stage in 30 min.

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PROPERTIES OF GYPSUM PRODUCTS

1) SETTING EXPANSION

2) STRENGTH.

3) HARDNESS & ABRASSION RESISTANCE.

4) REPRODUCTION OF DETAILS.

1) SETTING EXPANSION

I. Normal setting expansion.

II. Hygroscopic setting expansion.

Show linear expansion during setting (0.05-0.5%)

May be desirable (investment materials) or undesirable (impression plaster, dental plaster, stone)

NORMAL SETTING EXPANSION

PROPERTIES OF GYPSUM PRODUCTS

Control of normal setting expansion

1) Mechanical mixing.

2) Increase in W/P ratio.

3) Use of modifiers.

4) 4% solution of Potassium sulphate.

5) Sodium chloride.

6) Borax.

HYGROSCOPIC SETTING EXPANSION

• Occurs when placed under water before initial set stage.

• Expansion is greater than normal setting expansion.

• Used to expand gypsum bonded investments.

• Reduced W/P ratio increases the hygroscopic setting expansion.

• Increased spatulationincreases the hygroscopic setting expansion

PROPERTIES OF GYPSUM PRODUCTS

2) STRENGTH• Strength increases rapidly as

material hardens.• Greater the water = lesser the

strength.1) Wet strength:Strength of gypsum when water is more than necessary for reaction.2) Dry strength:Strength of gypsum when excess free water is lost due to evaporation.

FACTORS AFFECTING STRENGTH

1) Temperature.

2) W/P ratio.

3) Spatulation.

4) Addition of accelerators & retarders.

PROPERTIES OF GYPSUM PRODUCTS

3) HARDNESS & ABRASION RESISTANCE

• Good surface hardness & abrasion resistance is essential.

• Hardness is related to compressive strength.

• However surface hardness & abrasion resistance are not related.

• Commercial hardening solutions are available.

4) FLOW

• Depends upon W/P ratio.

• Correctly proportioned mix has sufficient flow.

• Vibrating the mix improves flow of gypsum.

• Flow reduces as gypsum reaches initial set.

• Produce accurate details.

• Important in case of duplicating casts

5) REPRODUCTION OF DETAILS

APPLICATIONS OF GYPSUM PRODUCTS

1. Study models.

2. Working casts.

3. Dies for crown & bridge work.

4. Models for denture processing.

5. Mounting cast to the articulator.

6. Dental impression. (only type 1 impression plater)

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2) EPOXY PRODUCTS

EPOXY PRODUCTS

• 2 component systems

I. Resin component. (viscous material called as monomer)

II. Hardener. (Polyamine, causes resin monomer to polymerize)

• Slow setting reaction. (may take upto several hours)

• Hardener is toxic & allergic.

PROPERTIES OF EPOXY PRODUCTS

1) Working time = 15 minutes.

2) Setting time = 1 to 12 hours.

3) Have superior compressive strength & abrasion resistance.

4) Not as hard as gypsum dies.

5) Shrinkage during setting = 0.03% to 0.3%

6) High chances of porosities.

EPOXY PRODUCTS

USES• Used in the fabrication of:

a) Crown.

b) Bridge.

c) Inlays.

All Elastomeric impression materials.

COMPATIBLE WITH

Compatibility comparison of Model & Die Materials with Impression Materials

MODEL & DIE MATERIAL COMPATIBLE WITH

Gypsum ProductsAgar

Alginate

All elastomers

Epoxy resins

Polyether

Addition Silicone (Some require a separator)

Condensation Silicone (Some require a separator)

Polysulphide (with separator)

Comparison of Properties of Model & Die Materials

PROPERTIES GYPSUM PRODUCTS EPOXY PRODUCTS

Abrasion resistance Poor Good

Ease of Use Easy Moderately difficult

Time required Minimal Several hours

Equipment required Minimal Minimal

Harmful chemicals None Some

Dimensional change Slight expansion Slight contraction

Accuracy Good Good

Compatibility with impression materials

Excellent Limited