L2 - Model and Die Materials

8/4/2019 L2 - Model and Die Materials

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Model and Die Materials

Dr. Arti


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Introduction

Replicas of the hard and soft tissues.

Models, casts or dies

Modelsused for observation of the patients

oral structures (eg. Study models) Cast working model

Dies extremely accurate replicas of a single

tooth Made from impression and therefore accuracy is

dependent on accurate impression


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Desirable Qualities

Accurate

Dimensional stability

Ability to reproduce fine detail

Strength and resistance to abrasion

Ease of adaptation to the impresssion

Color

Safety

Is compatible with many impression materials


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MATERIALS

1. Gypsum

Model plaster

Dental stone

Die stone

2. Epoxy


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Model Plaster

used as a general purpose material mainly forbases and models as it is cheap and easy to useand shape.

Used for study models (Does not require abrasionresistance)

Orthodontic plaster (Dental plaster)

Impression plaster
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Dental Stone

Stronger and durable compared to plaster

Used for models of the mouth

More resistant to abrasion

Used for casts (working model)


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Die Stone

for individual tooth models

Require high abrasion resistance and strength

Made of high strength dental stone


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Chemical and Physical Nature

Physical form of plaster, stone and die differ butare made of the same chemical calcium sulfate.

Dihydrate form of calcium sulfate gypsum

Mineral gypsum - source of plaster of paris

Gypsum products are obtained from the gypsumrock.


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Composition - Gypsum

Gypsum products are heated to a temperature to drive off somewater and therefore in the form of calcium sulphate hemihydrate

Higher temperatures the anhydrite is formed

The three basic raw materials (plasters, hydrocal, densite) arederived from the partial dehydration of the gypsum rock, dependingon the dehydration process.

Calcium sulphate hemihydrate produces versions of gypsum with

different properties and hence different applications.

The plaster, stone and die are chemically identical differing only inform and structural detail.


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Manufacture of Plaster

Gypsum mineral is heated in an open kettle at atemperature of approx 115 deg

Hemihydrate powder produced is called -calcium hemihydrate

Has irregular shape

Porous in nature


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Manufacture of Dental Stone

Produced by dehydrating gypsum under

pressure and in the presence of steam at 125

deg

Product is called hydrocal calcium sulfate

hemihydrate

Crystals are more uniform in shape, larger and

denser than crystals of plaster.


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Manufacture of High Strength Dental

Stone

Made by boiling gypsum rock in a

30%^solution of calcium chloride

Chloride is washed away with water (100 deg)

Material is ground to the desired fineness


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Setting Process

Some calcium sulphate hemihydrate dissolves in the water

The dissolved calcium sulphate hemihydrate reacts with the water and revert to

form calcium sulphate dihydrate

This reaction gives of heat and thus is exothermic

Amount of water needed to mix needs to be controlled.

For instance 19mL of water is sufficient to react with all hemihydrate particles in

100g of hemihydrate to form dihydrate.

However this does not give a suitable consistency for it to be manipilated and for it

to flow into an impression.


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Setting Process

Hence excess water must be added to form a workable consistency.

Too thin a mix is also discouraged as this will produce model or cast of inferiorand weaker quality

Due to the different crystalline size and porosity amongst plaster, stone andhigh strength stone, different amounts of excess water is needed.

High strength stone requires the least excess water whereas plaster needs themost.

Presence of excess water has an effect on physical properties.

Upon setting of the material, water evapourates and leaves voids which makesthe density of the material lesser.

Hence plaster is the least dense where as high density stone is most dense.


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Accelerators and Retarders

Rate of gypsum reaction can be altered by

chemicals

Accelerators make the dihydrate less soluble

than hemihydrate, so the reaction is moved

towards the dihydrate forming the solid

rapidly. Example potasium sulfate

Retarder make the hemihydrate only slightly

less soluble than the dih drate which makes


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Accelerators and Retarders

Other acclerators and retarders:

Terra alba (accelerator)

Colloidal particles such as blood, saliva and

unset alginate retard by binding to

hemihydrate particles and interfering with

water being added to it.

This produces a structure which is soft and a


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Hardening Solutions

Commercial solutions available which can be

mixed in place of water which hardens the

gypsum and also increases abrasion

resistance.

Solutions composed of water, 30% colloidal

silica, and other chemical modifiers.


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Temperature and Humidity

Setting of gypsum is affected by the

temperature of the water.

From room temperature to body temperature,

setting time decreases.

Above 37.5 deg, setting time will increase.

At 100deg, hemihydrate will not set at all.


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Working and Setting Times

The material must be mixed and poured

before it reaches the end of its working time.

Vary from product to product and are chosen

to suit the particular application.


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Properties

Dimensional stability

Once the material has set, there is little or no

dimensional change.

Compressive strength affected by P/L ratio.

Reduction of water improves compressive

strength.

Tensile Strength

Plaster is very low when wet. Dental stone has

twice the tensile strength of plaster.


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Properties

Setting Time

Divided into initial and final setting time

Initial when powder and water is mixed and mix canno longer be poured into the impression

Observed when material loses its gloss. Indication

hemihydrate has used up the water and surface waterhas been drawn into the material.

At this stage material is still weak.


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Properties

Final Setting Time

When conversion of hemihydrate to dihydrateis complete.

Detected by the dissipation of the heat ofreaction.


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Properties

Reproduction of Detail

Measured by the ability of the gypsum

product to flow into and register fine details

For die materials it is extremely important as

precision casting will be fabricated on gypsum

die.

To capture the most details the model and die


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Properties

Gypsum is a water based material and thus wetswater based impression materials such as alginatewell.

As for non water based impression materials suchas addition silicones, wetting is reduced.

This limits the ability of the gypsum product toflow into all details of the impression andincreases the risk of bubble formation.


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Properties

Strength

Ability of the material to resist fracture

Compressive strength:

Plaster has the lowest as it has the most

excess water High strength die stone is four times stronger

in compression.


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Properties

Tensile Strength

Brittle materials and tend to fracture during

bending forces.

Higher tensile strength in high strength stone

and model stone than in model plaster.

Strength of model and die is half when wet

compared to when it is dry


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Properties

Hardness and Abrasion Resistance

Important as practitioner requires little or no

loss of shape to occur on the model during its

manipulation.

Directly related to compressive strength

Improved in dry conditions and use of

hardening solutions


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Properties

Dimensional Stability

Ideally expansion and contraction is notfavoured.

However gypsum materials expand slightlyupon setting.

Model plaster expands the most whereas highstrength die stone expands the least.


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Manipulation

Powder and water combination

Spatulation

Pouring the model


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The End!!!!!!!

Reference:

Craig. R.G, Powers. J.M, Wataha. J.C, (2000),

Dental Materials Properties and

Manipulation, Mosby, Missouri.

Craig. R.G., Powers. J.M., (2002), Restorative

Dental Materials, Mosby, Missouri

L2 - Model and Die Materials

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