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1. DIE MATERIALS
INTRODUCTION
Once the tooth preparation is completed, it is necessary that it be
replicated so that a wax pattern can be developed. Although it is possible to
make the wax pattern directly on the prepared tooth, such techniques are
difficult to master. Also direct wax patterns are difficult to make if the
margins of the finished cavity preparation extend below the gingival crest
or if visibility is limited. Furthermore, the temperature of the oral cavity
tends to make the wax pattern more susceptible to deformation. Also
instrumentation for direct wax pattern is difficult. Such problems can
eliminated if the wax pattern is fabricated on a removable die.
Definition of Die
It is the positive reproduction of the form of a prepared tooth in
suitable hard substance, usually in metal or specially prepared dental stone.
Materials used for fabrication of a die:
a) Gypsum products  Type IV stone / high strength stone / Densite.
 Type V stone / die stone / high strength high
expansion stone.
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2. b) Electroformed dies
 Electroformed copper dies.
 Electroformed silver dies.
c) Epoxy resin dies.
d) Amalgam dies.
e) Silicate cement dies.
f) Acrylic resin dies.
g) Metal sprayed dies / low fusing metal alloy dies.
h) Ceramic dies.
i) Refractory dies.
The selection of any of the materials is determined by the following:
a) The impression material in use.
b) The purpose for which the die is to be used.
Ideal requirements of die materials:
a) Accuracy of surface reproduction. One should be able to see all the fine
details and sharp margins.
b) Dimensional accuracy and stability.
c) Mechanical properties.
 High strength to be able to withstand accidental breakage.
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3.  Surface hardness and abrasion resistance so that the die can
withstand the manipulative procedures during carving of wax pattern.
d) Compatibility with impression materials.
e) Good colour contrast with other materials being used e.g., inlay casting
wax.
f) Economical.
g) Easy to use.
a) Gypsum products – The most commonly used materials for fabrication
of a die are Type IV and Type V gypsum products.
Advantages
i Generally compatible with all impression materials.
ii Have the ability to reproduce fine detail and sharp margins.
iii Dimensionally accurate and stable.
iv Easy to use.
Disadvantages
i Poor surface hardness make them susceptible to abrasion during
carving of wax pattern on the die.
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4. Manufacture of Type IV and Type V gypsum products
Gypsum products in dentistry are formed by driving off part of the water of
crystallization from calcium sulphate dihydrate to form calcium sulphate
hemihydrate.
CaSO4, 2H2O
(Dihydrate)
110°C-130°C CaSO4 ½ H2O
Loss of 1.5g moles to 2g moles of water of crystallization
Die materials are based on autoclaved calcium sulphate
hemihydrate, plus additives to adjust the setting time, control the setting
expansion and pigments for colour contrast.
Calcium sulphate dihydrate is boiled in 30% calcium chloride or
magnesium chloride. Densite or Type IV stone is obtained which is an α-
hemihydrate with cuboidal shaped particles.
This can be pulverized into a fine particle size with the addition of
modifiers to obtain a high strength high expansion stone (die stone).
Setting Reaction
When α-calcium sulphate hemihydrate in the form of high strength
stone is mixed with water, a chemical reaction takes place and the
hemihydrate is converted to the dihydrate form with the evolution of heat.
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5. CaSO4
.
½ H2O + 1 ½ H2O  CaSO4
.
2H2O + 3900 cal/g mole
The first stage in the process of setting is that the water becomes
saturated with hemihydrate which has a solubility of around 0.9%* at room
temperature. The dissolved hemihydrate is then rapidly converted to
dihydrate which has a solubility of 0.2%*. Since the solubility limit of
dihydrate is exceeded it being to crystallize out of solution. This forms the
second stage of the reaction.
Crystals of dihydrate are needle like clusters called spherulites
which grow from specific sites called nuclei of crystallization. These nuclei
may be small particles of impurity such as the unconverted gypsum
crystals within the hemihydrate powder.
Diffusion of calcium and sulphate ions into these nuclei seem to
play a role in the setting process. As the dihydrate crystallizes, more
hemihydrate dissolves and the process continues.
Physical changes in the setting process:
Initially the mix of hemihydrate and water can be poured. Next the
material becomes rigid but not hard. This is called initial set of the material
and at this stage it can be carved but not moulded. Also there is very little
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6. reaction with little or no rise in temperature. This period is known as the
“Induction Period”.
The final set follows when the mix becomes hard and strong.
However at this stage the hydration reaction is not necessarily complete,
nor has optimum strength and hardness necessarily been achieved. The
reaction is exothermic. Dimensional changes also take place. A setting
expansion of 0.05 to 0.3% is observed due to the outward thrust of the
growing crystals of dihydrate. This is called normal setting expansion.
If the material is placed under water at the initial set stage, a greater
expansion occurs known as hygroscopic setting expansion.
Manipulation of gypsum products:
1) Storage : They should be stored in air tight containers to prevent
reaction with moisture from the atmosphere which can cause formation
of dihydrate. These dihydrate crystals behave as new nuclei of
crystallization and accelerate the setting reaction.
2) Water/powder ratio: To attain maximum strength, surface hardness and
a well controlled setting expansion, it is necessary to gauge the amount
of water and powder as recommended by the manufacturer.
Type IV – 0.22-0.24; Type V – 0.18 – 0.22
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7. This would mean that 22ml and 18 ml of water are required for
100g of powder of Type IV and Type V stone respectively.
The W:P ratio is a very important factor in determining the physical
and chemical properties of the set gypsum product. For example, higher the
water/powder ratio, the longer will be setting time and weaker will be the
gypsum product. This is because there is more water per unit volume and
less nuclei of crystallization per unit volume.
3) Spatulation: Take the measured amount of water in a flexible rubber
mixing bowl. The powder is then dispersed into the water and allowed
to settle for 30 seconds. This minimizes the air incorporated in the mix
during the initial spatulation. A spatula with a stiff blade is used.
Spatulation is carried out by stirring the mixture vigorously and at the
same time wiping the inside surface of the bowl with the spatula to be
sure that all the powder is wet and mixed uniform with water. Mixing
time of one minute for hand spatulation and 30 seconds for mechanical
spatulation is usually sufficient to give a smooth lump free mix.
Use of mechanical means reduces air entrapment during mixing.
Use of an automatic vibrator helps the mix to flow well into the impression
and helps to eliminate air bubbles. Over vibration should be avoided as this
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8. may cause distortion of impression materials. The time and rate of
spatulation have a definite effect on the setting time and setting expansion.
An increase in the amount of spatulation, i.e. mixing time breaks the
already formed dihydrate crystals providing more nuclei of crystallization
and thereby accelerating the setting reaction causing a decrease in setting
time. An increase in the rate of spatulation increases the setting expansion.
Properties:
1) Setting time : The time that elapses from the beginning of mixing until
the material hardens is known as setting time. The initial setting time is
also called the working time during which the material can be mixed
and poured into the impression. As the chemical reaction proceeds
more and more dihydrate crystals are formed. The viscosity of the
reacting mass increases rapidly and can no longer flow into the fine
details of the impression. At this point the materials should not be
forcefully manipulated. Initial setting time can be detected clinically by
a phenomenon called as Loss of Gloss (LG). The initial setting time is
measured by Gillmore needle which should not longer leave an
impression when lowered onto the mix and should occur within 8 to 13
minutes from the start of the mix.
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9. The final setting time is the time at which the material can be
separated from the impression without distortion or fracture. The chemical
reaction at this stage is practically completed.
This is usually measured as the time taken for the material to
become sufficiently rigid to withstand the penetration of a needle of known
diameter under a known load. Here the Gillmore needle should leave a
barely perceptible mark on the surface. Vicat needles have also been used.
Factors affecting setting time:
a) Factors under the control of manufacturer
 Concentration of nucleating agents in the hemihydrate powder. An
increase in the concentration decreases the setting time e.g. dihydrate
particles.
 Accelerates and Retarders
Accelerators used are:
i. Potassium sulphate (K2SO4) – less than 2% to 3% in solution. The
setting time decreases from 10 to 4 minutes. The reaction product is
called Syngenite which crystallizes rapidly.
ii. Calcium sulphate (CaSO4) – it is ground and added to the powder
and it provides nuclei for growth. The set gypsum is called Terra Alba
and the concentration used is 0.5 to 1.0%.
iii. Sodium chloride (NaCl) – less than 2% is used.
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10. iv. Sodium sulphate (Na2SO4) – 3-4% is used.
v. Slurry water
vi. Retarders used are:
a. 2% borax.
b. Potassium citrate.
c. Organic materials like gum, glucate.
d. Increased addition of inorganic salts : e.g., NaCl > 2%, Na2SO4 >
4%
 Fineness of particle size
Pulverization of the manufactured product into a fine particle size
accelerates the setting reaction. Grinding increases the surface area of the
particles exposed to water which dissolve rapidly. It also increases the
nuclei of crystallization. The rate of solubility increases and decreases the
setting time.
b) Factors under the control of operator
 W/P ratio
An increase in W/P ratio retards the setting reaction.
 Mixing time
An increase in the mixing time accelerates the reaction and
decreases the setting time.
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11.  Colloidal films such as mucin, saliva, blood retard the setting reaction.
Therefore thoroughly rinse the impression in running cold water prior to
pouring the cast.
 Temperature
Temperature variation has little effect on the setting time. An
increase in the temperature from 20°C to 37°C causes a slight increase in
the rate of reaction. As the temperature is raised further, the rate of the
reaction decreases and the lengthens the setting time.
2) Reproduction of Surface Detail
Gypsum dies produce an adequate surface detail but not as accurate
as electropated dies. This is because the surface of the set gypsum is porous
on a microscopic level. The porosity causes the surface to be rough. The
use of surface hardners during mixing can produce a smooth surface.
Incompatibility with some impression materials can result in air inclusions
and surface voids.
3) Compressive strength
The strength of gypsum products is directly related to the density of
the set mass. The wet strength is the strength when the water in excess of
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12. that required for the hydration of the hemihydrate is left in the test
specimen. When the specimen has been dried free of the excess water, the
strength obtained is the dry strength. The one hour compressive strength of
densite is 5000 psi and that of die stone is 7000 psi.
4) Tensile strength
It is 330 psi. It is a brittle material and is considerably weaker in
tension than in compression.
5) Surface hardness and abrasion resistance
The surface hardness of gypsum die material is three times that of
an epoxy die but half that of an electroplated die. The use of hardening
solutions increase the resistance to abrasion.
a) Internal hardners such as 30% colloidal silica can be used instead of
water during mixing of the stone. These surface active modifiers allow
the powder particles to be more easily wetted by water.
Incorporation of wetting agents such as lignosulphonates derived
from lignin can reduce the water requirement and enable the production of
a harder, stronger, dense set gypsum.
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13. b) External hardners include polymers such as polyester, polystyrene,
acrylic or epoxy resin. A solution of 10% polystyrene in amyl acetate is
painted on to the surface of the die, the excess blown off and allowed to
dry for 5 minutes. Surface hardness of Type IV stone is 92 RHN.
6) Setting expansion and dimensional accuracy
All gypsum products show a measurable linear expansion on setting.
High strength stone has a setting expansion of 0.0 to 0.1%, that of high
strength high expansion stone being 0.1 to 0.3%. Expansion occurs due to
the outward thrust of the growing nuclei of crystallization.
Factors affecting expansion:
i. An increase in spatulation increases the setting expansion.
ii. An increase in W/P ratio decreases the setting expansion and vice
versa.
Hygroscopic setting expansion occurs under water with almost
double the normal expansion, due to the replenishment of water of
hydration. Setting expansion to an extent compensates for the casting
shrinkage of the metal.
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14. SYNTHETIC GYPSUM
They are made from the by-products or waste products of the
manufacture of phosphoric acid and remains a trade secret. They are highly
expensive and exhibit superior properties.
DIE STONE INVESTMENT COMBINATION
Here the die material and the investing medium have a comparable
composition. A commercial gypsum bonded material, called Divestment is
mixed with a colloidal silica liquid. The die is made from this mix and the
wax pattern constructed on it. Then the entire assembly (die + pattern) is
invested in the Divestment, thereby eliminating the possibility of distortion
of the pattern on removal from the die or during the setting of investment.
The setting expansion of the material is 0.9% and thermal expansion is
0.6% when heated to 677°C. Since Divestment is a gypsum bonded
material, it can be used only for gold alloys.
Divestment phosphate or DVP is a phosphate bonded investment
that is used in the same manner for high fusing metal ceramic alloys.
a) ELECTROFORMED DIES
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15. Electroforming refers to the electrodeposition of metal on a metallic
or non-metallic silicon object thus building up the counterpart of the object
by the passage of electrical current through the electrolyte. The art of
electroformng is called as Galvanoplasty. Jacobe in 1934 first used it and
Wajna in 1937 applied it in dentistry. Electroforming compound
impressions require copper and elastomeric impressions require silver.
Advantages:
i. Dimensionally accurate with absolutely no dimensional change
unless, the impression material shrinks before the initial plating is
deposited.
ii. Superior surface reproduction and sheen with accuracy of marginal
definition.
A line 4µm or less in width is readily reproducible.
iii. Higher strength, surface hardness, abrasion resistance.
iv. Easy to carve and recarve pattern on the die.
v. High points of occlusion can be determined with great accuracy.
vi. Allows satisfactory finishing and polishig of metal restorations on
the die.
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16. Disadvantages
i. Time consuming.
ii. Special equipment is needed.
iii. Expensive.
iv. Not compatible with all impression materials.
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17. Electroforming unit consists of the following:
a) A container which is of hard rubber or of glass.
b) Electrical current from a battery or from the mains through a rectifier.
c) Quantum of current which is measured in terms of amperes with an
ammeter.
The normal current required for a tube impression is 5 milliamperes,
silver plating requires smaller units of current.
d) Anode terminal (+ve) is pure copper or pure silver of 99.9% purity.
The cathode terminal (-ve) is the impression to be electroformed i.e.
low fusing compound for Cu and mercaptan or silicone rubber for Ag.
Surfaces which don’t require deposition such as wire terminal and copper
tube should be coated with wax. The minimum distance between the
terminals should be 4 ½ - 5 inches.
e) Electrolyte – solution is of one of the metals that needs to deposited.
For copper electroforming an acidulated solution of copper sulphate is
used. It contains ethanol, phenol, hydrochloric acid and distilled water
in addition to CuSO4. The solution has to be used after 48 hours of
standing for maturing. If not allowed to mature, a rough surface is
obtained. Acid allows the passage of electric current. Ethanol and
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18. phenol improve the throwing power or ionic penetration power of
copper ions for deposition on the cathode terminal. Distilled water is
used as a vehicle.
For silver electroforming a basic solution of silver cyanide is used.
It contains silver cyanide, potassium cyanide for increased penetration,
potassium carbonate and distilled water.
The solutions of the two systems have to be kept away from each
other to prevent the formation of cyanide vapour which are extremely
lethal.
f) Metallizer / metallizing agent
It is that part which is employed to make the surface of compound
or rubber base conductive to the passage of electric current.
For copper electroforming the following are used:
i. “Aqua-dag” : It is a suspension of powdered graphite. It is supplied
in collapsible tubes. A couple of mm of paste is mixed with distilled
water with a brush and applied on the surface of the impression.
ii. Suspension of bronzing powder in oil of bitter almonds.
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19. For silver electroforming the following are used:
i. Silverizer / Flash – It is an alcoholic solution of finely ground silver.
ii. Finely ground silver powder.
The impression is washed to eliminate streaks of blood, saliva,
mucin, air dried, and the agent is applied in the form of strokes 
Burnishing technique. A very thin layer is applied and allowed to air dry.
The solution of electrolyte is poured into the impression and static charges
are started. After 9-11 hours, an even layer of metal of 100µ thickness is
obtained. The solutions are poured back into the container. The void is
filled with dental stone. When the stone hardens it is mechanically locked
to the rough interior of the electroformed metal shell. The surface coating
can be altered by altering the composition, time and distance.
b) EPOXY RESIN DIES
They are either self-curing acrylic materials e.g. epoxy resins,
polyester and epimines or polymeric materials with metallic or ceramic
fillers.
Advantages:
i. Adequate surface hardness and abrasion resistance.
ii. Less brittle than die stone.
iii. Can be cured at room temperature.
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20. Disadvantages:
i. Shrinkage on polymerization leads to dimensional inaccuracy.
ii. Expensive – Epoxy die materials can be used with polyether,
polysulphide or silicone rubber impression materials.
Composition:
Epoxy die materials are 2-component systems that include a resin
and a hardness. The viscous resin may be a difunctional epoxy to which
filler may be added.
CH2 – CH – R – CH – CH2
O O
The harder is a polyamine that when mixed with the resin for about
a minute causes polymerization. The hardner is toxic and should not come
into contact with the skin during mixing and manipulation of the unset
material.
Properties :
i. Working time  15 minutes.
ii. Setting time  1-12 hours depending on the product.
iii. Hardness  25 KHN.
iv. Compressive strength after 7 days is 16,000 psi.
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21. v. Superior abrasion resistance.
vi. Dimensional changes between 0.03 to 0.3% and continues to occur
for up to 3 days.
vii. High viscosity can result in surface voids.
viii. Most epoxy dies should not be used until 16 hours after pouring
since they harden slowly.
ix. Cannot be used with agar and alginate because water retards the
polymerization of the resin. They are compatible with polyether,
polysulfide or silicone impression materials.
c) AMALGAM DIES
They are made by packing amalgam into impression made of
impression compound. Dies exhibit superior strength and reproduce fine
details.
Although a material of choice for a number of years, it has been
replaced by electroplated dies because of the following limitations.
i. It can be packed only into a rigid impression material.
ii. It is technique sensitive and may result in varied dimensional
accuracy.
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22. iii. Time required for fabrication is lengthy. Although the die packing
procedure may take only 30 minutes, amalgam requires 12-24 hours for
hardening.
iv. It has high thermal conductivity and can cool a wax pattern rapidly
which may lead to distortion of the pattern. This can be overcome by
warming the die.
v. Residual mercury presents a health hazard.
vi. Dimensional changes due to delayed expansion
d) SILICATE CEMENT DIES
It is similar to the filling and cementing material.
Advantages:
Initial strength and surface hardness is superior to that of die stone.
Disadvantages:
i. The cement contracts during setting and may be dimensionally
inaccurate.
ii. There is loss of water on standing, causing a rough and dehydrated
surface.
iii. High viscosity predisposes to surface voids.
e) ACRYLIC RESIN DIES
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23. They have adequate surface hardness and abrasion resistance but
undergo shrinkage on polymerization. PMMA resins are used.
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24. f) METAL SPRAYED DIES/ LOW FUSING METAL ALLOY DIES
A bismuth-tin alloy, which melts at 138°C is sprayed directly on to
an impression to form a metal shell, which can then be filled with dental
stone. A metal coated die can be obtained rapidly from elastomeric
impression materials.
The disadvantage is that the alloy is soft and does not fulfill the
mechanical requirements of a die.
g) CERAMIC DIE MATERIALS
i. A material for the production of dies on which porcelain restorations
are to be fabricated, without the use of a platinum foil matrix. T form
the dies high temperatures of 1000°C is required.
ii. A ceramic material supplied as a powder and liquid and mixed to a
putty like consistency. After 1 hour the material is removed from the
impression and fired at 600°C for 8 minutes to produce a hard strong
die.
h) REFRACTORY DIE MATERIALS
They are made from refractory materials and are heat stable.
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25. Conclusion
Type IV and Type V stones appear to be the most successful die
materials available. With care, abrasion during pattern carving can be
avoided. In case of high-fusing metal ceramic restorations, resin or metal
electroplated dies can be used.
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