l2 - model and die materials
TRANSCRIPT
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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