DIE MATERIALS AND TECHNIQUE OF FABRICATION
CONTENTS:
o INTRODUCTION
o MATERIALS USED FOR FABRICATION OF DIE
o BASIC REQUIREMENTS OF DIE MATERIALS
o GYPSUM PRODUCTS
o ELECTROPLATED DIES
o SILICO PHOSPHATE CEMENT
o EPOXY RESIN
o METAL SPRAYED DIES
o CERAMIC DIE MATERIALS
o TECHNIQUE OF FABRICATION OF STONE DIE
o CONCLUSION
o REFERENCES
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
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
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.
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 à (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 à 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. 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 H2O. The amount of solution is less
if 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.
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.
Ø Next
the material becomes rigid but not hard this is called initial setting. At this
stage the material can be carved but not moulded.
Ø 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.
Ø Heat
is given out during setting since the hydration of the hemihydrate is
exothermic.
Ø 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.
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
Ø Concentration
of nucleating agents in the hemihydrate powder increase nucleating agents
decreasing setting time. Ex. Dehydrate particles.
Ø Addition
of accelerators and retarderes accelerators used are K5SO4
and (CaSO4) H2O crystals. Retarders – 2% Borax.
Ø 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
Ø
Water / powder ratio.
Increase
w/p ratio retards the setting by decreasing the concentration of nuclei of
crystallization.
Ø 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.
Ø 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.
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
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.
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 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.
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.
Ø The
surface of the impression is rendered conductive by coating it with fine
particles of copper or graphite.
Ø 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.
Ø A
current is passed of 15 miliampher/ cm2 of cathode surface for approximately 10
hours. This cause slow dissolution of the anode and movement of copper ions
from anode to cathode this plating the impression.
Ø 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.
Ø The
impression is coated with silver or graphite powder is made the cathode.
Ø Anode
is silver plate.
Ø 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.
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.
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.
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.
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.
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.
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.
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
REFERENCES:
Ø Fundamentals
of fixed Prosthodontics:Shillingberg. Contemporary fixed
Prosthodontics :
o
Stephen F.Rosenstiel
Ø Philip’s
science of dental materials :Anusavice
Ø Dental
material -properties and manipulation :
o
Craig ,powers
Ø Notes on
dental materials- E C Combe
Ø Restorative
dental materials- Robert G Craig
Ø 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.
Ø 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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