Dental tissues and their replacements/ oral surgery courses
Die materials and technique of fabrication
1. DIE MATERIALS AND TECHNIQUE OF
FABRICATION
CONTENTS:
INTRODUCTION
MATERIALS USED FOR FABRICATION OF DIE
BASIC REQUIREMENTS OF DIE MATERIALS
GYPSUM PRODUCTS
ELECTROPLATED DIES
SILICO PHOSPHATE CEMENT
EPOXY RESIN
METAL SPRAYED DIES
CERAMIC DIE MATERIALS
TECHNIQUE OF FABRICATION OF STONE DIE
CONCLUSION
REFERENCES
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2. 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 in the prepared tooth. Such techniques are
difficult to master. Also direct wax patterns are difficult to make it the margins
of the finished cavity preparation extended below the gingival crest or it
visibility is limited. Further more the temperature of the oral cavity tends to
make the wax pattern more susceptible to determination. Also instrumentation
for direct wax pattern is difficult such problems can be eliminated it the wax
pattern is fabricated on a removable die with the removable die finish line
margin of the wax pattern can be carved better.
Defination
A die is a working replica of a single tooth or several teeth.
Materials used for fabrication of Die:
1. Gypsum products
2. Electroformed dies
-
Electroplated copper
-
Electroplated silver
3. Epoxy resins
4. Silicophosphate cement
5. Amalgam dies
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3. 6. Ceramic die materials
7. Metal sprayed dies
The selection of one of this is determined by the particular impression
material in use and by the purpose for which the die is to be used.
Basic requirements of die materials
1. Ability to reproduce fine detail and sharp margins.
2. Dimensional accuracy and stability – should show little dimensional
change on setting and should remain stable.
3. Mechanical properties
a) High strength to reduce the likelihood of accidental breakage.
b) Abrasion resistance so that the die can withstand the manipulative
procedures during carving of wax pattern.
4. Compatibility with impression materials: There should be no intraction
between surface of impression and die.
5. Good colour contrast with other materials being used for ex. Inlay wax or
porcelain.
6. Economical
7. Easy to use
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4. 1. Gypsum Products
The most commonly used die materials are Type IV (dental stone, high
strength) and Type V (dental stone, high strength) improved stones.
Advantages
1. Generally compatible with all impression materials.
2. Have the ability to reproduce fine detail and sharp margins.
3. Dimensional accuracy and stability are good.
4. Produces consistent results.
5. Easy to use.
Disadvantages
Susceptibility to abrasion during carrying of the wax pattern especially
with Type IV Gypsum die.
Manufacture of Type IV and V Gypsum materials
Die materials are based on outoclaved calcium sulphate hemihydrate
plus additives to adjust the setting time and pigments for colour contrast.
To manufacture gypsum die material, calcium sulphate dehydrate is
boiled in 30% solution of calcium chloride or magnesium chloride. The
hemihydrate particles thus obtained are least porous.
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5. Gypsum products used in dentistry are tuned by driving off part of the
H2O of crystallization from calcium sulphate dehydrate to form calcium
sulphate hemihydrate.
2 CaSO4 . 2H2O (CaSO4)2 H2O + 3H2O
dehydrate
heat
hemihydrate
Setting reaction
When calcium sulphate hemihydrate in the form of high strength stone
is mixed with water a chemical reaction takes place and the hemihydrate is
converted back to the dehydrate form of calcium sulphate. This is an
exothermic reaction.
CaSO4 . ½ H2O ½ H2O CaSO4 . 2H2O + Heat
The 1st stage in the process is that the H2O becomes saturated with
hemihydrate which has a solubility of around 0.8% at room temperature. The
dissolved hemihydrate is then rapidly converted to dighydrate which has a
solubility of 0.2% since the solubility limit of dehydrate is immediately exceed
it begins to crystallize out of solution the process continues until most of the
hemihydrate is converted to dehydrate.
The crystals of dehydrate are spherilite in nature and grow from specific
sities called nuclei of crystallization. These may be small, particles of impurity
such as unconverted gypsum crystals with in the hemihydrate powder.
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6. Diffusion of the Ca2+ and SO42- ions in to these nuclei also appears to be
important.
As the dehydrate crystallizes more hemihydrate dissolves and the
process continues.
Manipulation
a) Storage: In closed containers to prevent reaction with moisture from the
atmosphere which can cause formation of the dehydrate which can
accelerate the setting time.
b) Correct water / powder ratio
To attain maximum strength and resistance to abrasion it is necessary to
use the current H2O to powder ratio when preparing dies made of gypsum
products. Reducing or increasing w:p ratios, the powder to liquid ratio below
that recommended by the manufactures result in not only reduced strength and
abrasion resistance but also a deviation from the expected setting expansion.
The w/p ratio for gypsum die materials is 0.22 to 0.24 i.e. 100 gm of
material is mixed with 22 ml of water.
c) Hardening solutions
Commercial hardening solutions composed of H2O, 30% collided silica
and modifiers may be wed in place of H 2O. The amount of solution is less if
6
7. H2O were used alone because surface active modifiers in the hardener allow
the powder particles to be more easily wetted by H2O.
Use of hardening solutions affects the hardness and setting expansion of
gypsum die increase in the hardness of high strength stone dies poured against
impressinos are 20% for poly silicons 20% for polysulphide, 70% for agar and
110% for polyether. High strength stones mixed with hardner show a slightly
higher setting expansion of 0.07% as compared with 0.05% for mixes with
H2O alone scraping resistance is also improved high strength stones mixed
with hardener.
Spatulation: Measured amounts of water and powder are added to a flexible
rubber mixing bowel. The water is dispensed in the bowl first the powder is
added and allowed to settle in to the water for approximately 30 sec. This
minimizes the amount of air incorporated in the mix during the initial
spatulation. A spatulate with a stiff blade is used. Spatulation is carried on 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
uniformly with H2O mixing time of one minute is usually sufficient to give a
smooth lamp free slurry.
Use of an automatic vibrator helps the slurry to flow well into the
impression and helps to eliminate the air bubbles over vibration should be
avoided as this may cause distortion of some impression materials.
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8. The time and rate of spatulation have a definite effect on the setting
time and expansion with in practical limites increase in the amount of
spatulation will shorten the setting time. The setting expansion is also
increased by increase in the rate of spatulation.
Setting process: Initially a mix of hemihydrate and H2O can be poured.
Next the material becomes rigid but not hard this is called initial
setting. At this stage the material can be carved but not moulded.
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.
Heat is given out during setting since the hydration of the
hemihydrate is exothermic.
Dimensional changes also takes place. A setting expansion of 0.05 –
0.3% is observed for dental stones. This is caused by the outward thrust
of the growing crystals of dehydrate. If the material is placed under water
at the initial set stage a greater expansion on setting occurs. This is
hygroscopic expansion.
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9. Properties
1. Initial and final setting time
The initial setting time is also called the working time. During the
working time the material can be mixed and poured in to the impression.
As the chemical reaction proceeds more and more dehydrate crystals
form. The viscosity of the reacting mass increase rapidly and the mass no
longer flows into the fine details of the impression. At this point the material
has reached the initial setting time and should no longer be manipulated.
Initial setting time can be defected clinicaly by a phenomenon known as
loss of gloss.
The initial setting time must occur with in 8-16 minutes from the start
of the mix. The final setting time is defined as the time at which the material
can be seperated from the impression without distortion or fracture. The time
at which the chemical reaction is practically completed. This is usually
measured as the time taken for the setting material to become sufficiently rigid
to withstand the penetration of a needle of known diameter under a lesser load.
Two such pieces of apparatus or known as vicat and gillmore needles.
i) Control of setting time factors under the control of manufactures
Concentration of nucleating agents in the hemihydrate powder
increase nucleating agents decreasing setting time. Ex. Dehydrate particles.
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10.
Addition of accelerators and retarderes accelerators used are K 5SO4
and (CaSO4) H2O crystals. Retarders – 2% Borax.
Grinding of gypsum product during manufacture accelerates the
setting (grinding increase the surface area of the hemihydrate exposed to
water. These increases the rate of solubility of hemihydrate).
ii) Factors under the control of operator
Water / powder ratio.
Increase w/p ratio retards the setting by decreasing the concentration of
nuclei of crystallization.
Mixing time: An increase in the mixing time an accelerates the set.
Mixing can break up some of the formed dehydrate crystals these forming
more nuclei of crystallization.
Colloidal septems such as blood, saliva can retard setting time.
Temperature
Temperature variation has little effect on the setting time on increase
from a room temperature of 20°C to a body temperature of 37°C. The rate of
the reaction increase slightly and the setting time is shortened. As the
temperature is raised above 37°C the rate of reaction decrease and the setting
time is lengthened.
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11. 2. Reproduction of detail
Gypsum dies do not reproduce surface detail as well as electroplated or
epoxy dies because the surface of the set gypsum is porous on a microscopic
level. The porosity of the set gypsum causes the surface to be rough compared
with other die materials.
The use of a hardener solution instead of water during mixing may
reduce surface roughness. Air bubbles frequently are tuned at the interface of
the impression and stone because the freshly mixed gypsum does not wet some
impression materials well.
3. Compressive strength
The strength of gypsum material is directly related to the density of the
set mass because high strength dental stone is mixed with the least amount of
H2O it is the densest of the gypsum materials and the strongest. The 1 hour
compressive strength of high strength dental stone is 4980 psi.
4. Tensile strength
It is 330 psi it is a brittle material and is considerably weaker in tension
than in compression.
5. Hardness and abrasion resistance
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12. The surface hardness is related to the compressive strength. The higher
the compressive strength of the hardened mass the higher the surface hardness.
The hardness of gypsum die material is 3 times that at an epoxy die but
hart that of an electroplated die. Though it is the most resistant of the gypsum
materials to abrasion.
The use of a hardening solution in place of water may increase hardness
and improve abrasion resistance as a result of a smooth surface.
6. Dimensional accuracy
All gypsum materials show a measurabe liner expansion on setting. The
expansion result from the growth of the CaSO4 2H2O (dehydrate) crystals and
teir impingement on one another. High strength stone has a setting expansion
of about 0.01% to 0.08%.
This expansion of the die material compensates for the casting
shrinkage of the metal.
Recent developments
Two techniques have been investigated to produce dental stone with
improvement in abrasion resistance and other mechanical properties.
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13. a) Impression of the gypsum by a polymer like polyether, polystyrene, acrylic
or epoxy resin. A solution of 10% polystyrene in amyl acetate can be
painted on to the surface of the die the excess blown off and then allowed
to dry for about 5 min mineral oils like Derusil can also be used.
b) Incorporation of setting agents such as lignosulphonates can reduce the
H2O requirements of a stone and enable the production of a harder,
stronger and more dense set gypsum.
These aditives retarded the setting time and increase the setting
expansion (Both of these effects can be overcome by the incorporation of
K5SO4).
Die stone- Investment combination
In this the die material and the investing medium have a comparable
composition. A commercial gypsum bonded material called divestment is
mixed with colloidal silica liquid. The die is made from this mix and wax
pattern constructed on it then the entire assembly (Die + Pattern) is invested in
a mixture of divestment and water, thereby eliminating the possibility of
distoration 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. because divestment is a gypsum
bonded material it is not recommended for high fusing alloys like metal
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14. ceramic restorations. It is highly accurate technique for conventional gold
alloys especially intracoronal preparations.
Divestment phosphate is a phosphate bonded investment that is used in
the same manner as divestment and its suitable for use with high fusing alloys.
II. Electroplated dies
Metal dies can be made by copper plating compound impression or
silver plating rubber base impression when a die is made in this manner the
process is referred to as electroplating.
Advantages of electroplated dies
1. With materials such as gypsum products dimensional change may occur as
the die material sets. No such expansion or contraction occur with
electroformed dies unless the impression material shrinks before the initial
plating is deposited.
2. Electroformed dies have higher strength hardness and abrasion resistance.
3. Allows satisfactory finishing and polishing of metal restoration on the die.
Disadvantages
1. Time consuming
2. Special equipment is needed
3. Not compatible with all impression materials.
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15. Copper plating
Copper plated dies are most commonly made from compound or
addition silicone rubber impressions.
The popularity of copper plated compound dies began in the early
1930’s.
The first step in the procedure is to treat the surface of the impression
material so that it conducts electrically. This process is referred to as
metallizing.
The surface of the impression is rendered conductive by coating it
with fine particles of copper or graphite.
The coated impression is made the cathode (-ve electrode) and
electrolytically pure copper plate is attached at the anode. Both anode and
cathode are immersed in an electrolytic solution continuing an acidic
solution of copper sulphate (about 250 gIL) together with organic
constituents like alcohol or phenol. Which are believed to increase the
hardness of the deposited metal.
A current is passed of 15 miliampher/ cm2 of cathode surface for
approximately 10 hours. This cause slow dissolution of the anode and
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16. movement of copper ions from anode to cathode this plating the
impression.
The impression that contains the electrotuned die surface is then
filled with dental stone. When the stone hardens it is mechanically locked
to the rough interior of the electroformed metal shell. The impression
material is then removed to provide a die with greater surface hardness and
resistance to abrasion than that of gypsum.
Silver plating
Indicated for polysulphide polyether, and silicon rubber impression
materials.
The process of silver plating is similar to that of copper plating but a
smaller current of 5 miliamphes is sufficient.
The impression is coated with silver or graphite powder is made the
cathode.
Anode is silver plate.
The electrolyte is an alkaline solution of silver cyanide (30 gm) with
other constituents like potassium (60 gm) cynide and potassium carborate
(45 gm) in distilled water (1000 ml).
Precaution: care must be taken to avoid the addition of acids to the cyanide
solution. Which can cause the release of cyanide vapor a death chamber gas.
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17. Copper plating should not be done in the same area. In which silver
plating is done because the risk of contamination the silver plating solution
with acidic copper plating solution.
Amalgam Dies
They are made by packing amalgam into impression made of
impression compound.
Advantages
Dies made of amalgam exhibit superior strength resistance to abrasion
and reproduce fine details and sharp margins.
Although a material of choice a number of years ago it has been largely
replaced by electroplated dies. Which are also resistant to abrasion the
property of amalgam dies has declined for a number of reasons.
1. It can be packed only into a rigid impression like that of impression
compound.
2. (Because of the tech necessary to produce a sound die) dimensional
accuracy may vary from one die to the next.
3. Time required for fabricating an amalgam die is lengthy. Although the die
packing procedure may take only 30 minutes amalgam requires 12 to 24
hours of hardening before it can be manipulated as a die.
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18. 4. It has high thermal conductivity and so can cool a wax pattern rapidly
which may lead to distraction of the pattern. This can be overcome by
warming the die.
IV. Silico phosphate cement
It is similar to the filling and cementing material. The powder is a
mixture of silicate powder and zinc oxide liquid contains phospheric acid.
Advantages: Strength and surface hardness are superior to those of die stone.
Disadvantages: This material contracts during setting and may be
dimensionally inaccurate. There is loss of water on standing since the viscosity
of these material is relatively high. Presence of surface voids can occur.
V. Epoxy resin (polymers)
These are either self curing acrylic materials for Eg. Epoxy resins, poly
yesters and Epimines or polymeric materials with fillers (either metallic or
ceramic fillers).
Advantages
1. More abrasion resistance.
2. Less brittle than die stones.
3. Can be carved at room temperature.
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19. Disadvantages
1. Shrinkage on polymerization
2. Less dimensional stasility
3. Expensive.
Epoxy die material can be used with polyether, polysulphide or silicone
epoxy to which filler may be added.
CH2 – CH – R – CH – CH2
O
O
The hardner is a polyamine that when mixed with the resin for about a
minute causes polymerization. The hardness is toxic and should not come into
contact with the skin during mixing and manipulation of the unset material.
Properties
1. Working time – 15 min.
2. Setting time – 1 to 12 hours depending on the product.
3. Knoop hardness number is 25 KHN ±15 less than that of high strength
stone (77 KHN).
4. Compressive strength after 7 days is 16,000 psi.
5. Abrasion resistance is superior to stone dies.
6. Dimensional change due to shrinkage during polymerization is between
0.03% and 0.3% and continues to occur for upto 3 days.
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20. 7. Epoxy materials are very viscous when poured hence porosity can occur.
8. Epoxy resin cannot be used with water containing agar and alginate
materials because water retards the polymerization of the resin. they are
compatible with polyether, polysulphide or silicon impression materials.
VI. Metal sprayed dies
A bismuth – tin alloy which melts at 138°C can be sprayed directly on
to an impression to form a metal shell which can than be filled with dental
stone.
Advantage: A metal coated die can be obtained rapidly from elestomeric
impression materials.
Disadvantages: The alloy is soft, care is helded to prevent abrasion of the die.
VII. Ceramic die materials
Two ceramic die materials are available
1. 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.
2. 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 min to produce a hard stone die.
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21. Technique of fabrication of stone die
After the impression has been removed from the patients mouth it is
washed under running tap water blown dry inspected and disinfected.
The dowel pin should be positioned correctly over the prepared tooth
with the help of pins and sticky wax. Their correct location and orientation is
important. For example placing the head of a dowel too deep in the impression
may weaken the die positioning the dowel at an incorrect angle may make die
removal impossible.
1. Using the right w/p ratio mix Type IV or V stone with water.
2. Pick up a small amount of stone with a suitable brush or instrument and
place it in the most critical area.
Usually the occlusal aspect of narrow preparations or immediately
adjust to the sulcus area. Bubbles will be trapped it to much stone is added
abruptly.
During powering the tray should be held on a vibrator.
3. Slowly release the stone into the preparation along the axil walls by tilting
the impression and guiding the material with the instrument. Be absolutely
sure that the stone flows onto the margins of the preparation without
trapping any air bubbles.
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22. 4. Place a second amount of stone on top of the first and continue with a third
and so forth until the preparation is completely filled the rest of the
impression is then filled and the head of the dowel must be covered with
stone.
5. Place retentive devices in areas where there are no dowels so the two layers
of stone will not separate in the wrong places.
6. Allow the stone to set.
7. Inspect the area where separation is required. Smooth it as necessary and
cool it with a separating medium by 10% sodium silicate. Then pour
another layer to act as a base and retain the dowel. This second layer
should not cover the tip of the dowel to facilitate its retrieval later.
8. When the cast is separated from the impression it must be inspected for
voids. It found to be satisfactory it is ready for sectioning and trimming.
9. Trim the buccal and lingual sulcus area adjacent to the removable section
first so the die will separate cleanly.
10. Mark the position of each saw cut which should be parallel to the dowel
with a pencil.
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23. 11. Carefully insert the saw blade between the preparation and the adjacent
tooth being sure that neither the margin nor the proximal contact is
damaged.
The cut must pass completely through the first layer of stone once the
saw cuts are made the dies can be separated out and are ready for trimming for
waxing.
Upon completion of die trimming the dies are repositioned in the
master cast and it is verified that they can be repositioned accurately and
precisely.
Conclusion
All factors considered the high strength stones (Type IV and V)
appear to be the most successful die materials available with care abrasion
during carving of the wax pattern can be avoided. In case of metal ceramic
restorations gypsum dies can be damaged. Hence a resin or metal die may be
prepared
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24. REFERENCES:
Fundamentals
of
fixed
Contemporary fixed Prosthodontics
Prosthodontics:Shillingberg.
:
o Stephen F.Rosenstiel
Philip’s science of dental materials
:Anusavice
Dental material -properties and manipulation :
o Craig ,powers
Notes on dental materials- E C Combe
Restorative dental materials- Robert G Craig
Philip Duke et al ;Physical properties of type IV gypsum, resin
containing and epoxy die materials JPD April 2000 vol 83, no. 4
p-466-73.
Jacinthe M et al in 2000 dimensional accuracy of an epoxy resin
die material using two setting methods. JPD March 2000 vol 83
no3 p 301-305
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