DIE MATERIALS AND DIE SYSTEMS Indirect fabrication of wax pattern and casting by lost-wax technique is still continuing as the most common technique for construction of cast restorations. For this, it is necessary to obtain an accurate reproduction of the prepared tooth, the surrounding tissues and the adjacent and opposing teeth. For an accurate reproduction of the teeth and associated structres, a thorough knowledge of the materials and technique for making impression and preparing cast and dies are essential. DEFINITIONS: Cast:- A dimensionally accurate reproduction of a portion of the oral cavity and extra oral facial structures produced in a durable hard material and used as a base for construction of orthodontic and prosthetic appliances. Die:- The positive reproduction of the form of a prepared tooth in any suitable hard substance, usually in metal or specially prepared artificial stone. 50 Dr. Vinni T.K
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DIE MATERIALS AND DIE SYSTEMS
Indirect fabrication of wax pattern and casting by lost-wax technique is still
continuing as the most common technique for construction of cast restorations.
For this, it is necessary to obtain an accurate reproduction of the prepared tooth,
the surrounding tissues and the adjacent and opposing teeth. For an accurate
reproduction of the teeth and associated structres, a thorough knowledge of the
materials and technique for making impression and preparing cast and dies are
essential.
DEFINITIONS:
Cast:- A dimensionally accurate reproduction of a portion of the oral cavity and
extra oral facial structures produced in a durable hard material and used as a base
for construction of orthodontic and prosthetic appliances.
Die:- The positive reproduction of the form of a prepared tooth in any suitable
hard substance, usually in metal or specially prepared artificial stone.
Model:- A facsimile used for display purposes; a miniature representation of
something; an example for imitation or emulation.
PREREQUISITES:
Cast:-
It should be free of defects.
It must reproduce both prepared and unprepared surfaces.
Unprepared tooth immediately adjacent to the preparation must be free
of voids.
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Teeth involved in anterior guidance and occlusal surface of unprepared
teeth must allow for precise articulation.
All relevant soft tissues should be reproduced in the working cast. i.e.
edentulous spaces and residual ridge contours.
Die:-
It must reproduce the prepared tooth accurately without any bubbles or
voids.
The remaining unprepared tooth structure immediately cervical to the
finish line should be easily discernible on the die, ideally with 0.5 – 1
mm visible.
Adequate access to the margin is imperative.
MATERIAL SCIENCE
The materials available for making dies are numerous and range from stone
to metal, resin and cement. The successful production of dies is to a considerable
extent dependent upon choosing a die material that is suitable for a particular
impression material.
REQUISITES:
1. Dimensional Stability:-
They should not expand and contract or distort after pouring into the
impression under normal storage and use.
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2. Accuracy:-
All the surfaces of the impression must be accurately duplicated. No voids
or bubbles should be present. It must have a good flow to reveal the minute details
of tooth and supporting tissues.
3. Surface Hardness and Abrasion Resistance:-
Should be made of a material that produce smooth hard surface. It should
be strong enough to withstand subsequent manipulation procedures without
abrasion of the surface or fracture of the die.
4. Compatibility:-
Should not react with impression materials and should be compatible with
the lubricating agents used to separate wax.
5. Colour:-
It should be available in a colour that contrasts with the wax used, so that
the preparation margin can be seen.
6. It should be easily wettable by wax.
7. It should be easily sectionable and easy to trim with the routinely available
equipments.
8. Production of die in a short time.
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CLASSIFICATION OF DIE MATERIALS:
1. Dental stone IV and V.
2. Electroplated silver and copper.
3. Epoxy resins.
4. Divestments.
5. Silver amalgam.
6. Flexible die materials
Selection of one of these materials is determined by the particular
impression material in use and the purpose for which the die is to be used.
1. DIE STONE (TYPE IV & V)
The hemihydrate is manufactured by heating the dihydrate under controlled
conditions to drive off some of the water of crystallization. The improved physical
properties of die stone is due to the fact that less water is needed to obtain a
sufficiently fluid mix. W /P ratio is 18.6 gm /100 ml.
Mechanical mixing under vacuum is preferred to hand mixing. Newly
poured cast should be left undisturbed for at least 30 minutes. The surface
reproduction is acceptable with Type IV gypsum. The material is capable of
reproducing a 20µm wide line as prescribed by ADA Specification No. 19.
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If soaking is required, it should be done in water saturated with plaster
slurry and only enough to achieve the desired degree of wetting.
Advantages:
1. Dimensional stability.
2. Dimensional accuracy – expansion 0.1 % or less.
3. Surface reproduction is adequate.
4. Relatively inexpensive.
5. Compatibility with impression material – generally compatible with
most impression materials and adapt easily to them.
6. Easy to use and ready for use in approximately 1 hour.
7. Colour contrast.
Disadvantages:
The greatest disadvantage of gypsum is its relatively poor resistance to
abrasion.
Gypsum dies are some times modified to,
i) make them abrasion resistant
ii) change the die dimension.
iii) increase the refractoriness.
iv) combination of these effects.
i. Improving abrasion resistance:
a) ‘Gypsum hardeners’ (colloidal silica) have relatively little effect on the
hardness of the stone, but they do improve abrasion resistance as much as
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100 %. Their use is accompanied by a slight expansion, which is not
clinically significant.
b) Low viscosity resins such as cyanoacrylate.
Care is needed in selecting and applying resin so resin film has no
significant thickness.
ii. Changing die dimension :
a) To reduce the dimension, additional accelerators and retarders can be
added.
b) Apply die spacers – polymers dissolved in volatile solvents are applied in
several coats to within 0.5 mm of the preparation finish line to provide
relief for the luting cement.
Die lubricants (oil, liquid soap, commercial preparations) has to be
applied to prevent the wax from adhering to die spacer during construction of wax
pattern.
iii. Increase refractoriness:
Die material and investment medium have a comparable composition. A
commercial gypsum bonded material, called Divestment (Whipmix Corp. USA) is
mixed with a colloidal silica liquid. The die is made from this mix and the wax
pattern is constructed on it. The entire assembly (die pattern) is invested in a
mixture of Divestment and water, thereby eliminating the possibility of distortion
of the pattern on removal from the die or during the setting of the investment. The
setting expansion is 0.9 % and thermal expansion is 0.6 % when it is heated to
677˚C. Divestment is not suitable for casting base metals and metal-ceramic
alloys.
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Divestment phosphate is a phosphate-bonded investment that is used in the
same manner as Divestment and is suitable for use with high-fusing alloys.
Disadvantages:
Application of die spacer is difficult.
Sending divestment cast for interocclusal records may be
problematic.
The die will be destroyed, hence, second cast & die material must be
made to permit finishing and polishing.
Dental stone, high strength, high expansion (Type V):
This is a recent gypsum product, having even higher compressive strength
than type IV. The improved strength is attained by making it possible to lower the
water: powder ratio. Setting expansion is increased to 0.3 %. Higher expansion is
required in the stone used for the die to aid in compensating for higher casting
shrinkage of base metal alloys.
2. RESIN BASED DIE MATERIALS
Resins are used as a die material to overcome the low strength and abrasion
resistance of die stone. Resins used are acrylic, polyester, epoxy resins and
polyurethane. These materials are limited in their compatibility with non-aqueous
elastomers. Abrasion resistance is many times greater than gypsum. It is more
expensive than gypsum and has longer setting time. Autopolymerizing acrylic
resin cannot be used to produce accurate die since their casting shrinkage is high
(0.6 %). The same is true for polyester resins.
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Epoxy resins are reliable with respect to the 0.1% – 0.2% shrinkage on
polymerization. They exhibit more shrinkage in axial direction. Polymerization
shrinkage is less of a problem with polyurethane resins.
In the last few years, gypsum die stones have been compounded with resin
to provide the advantage of each material. These modified die stones maintain the
low expansion of stone and toughness and abrasion resistance of resin. E.g.
ResinRock ( Whipmix Corporation). They have high strength and and low
expansion, and are particularly suitable for casts for implant restorations.
3. ELECTROFORMED DIES
Electroplating can be defined as the process of covering the surface of an
object with a thin coating of metal by means of electrolytes.
Review of literature:
The German scientist Jacobi laid the groundwork in 1837 for industrial and
dental electroforming. Electroplating activity in fixed prosthodontics and
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restorative dentistry started in the early 1930s with the introduction of modelling
compound copper plated dies. Hydrocolloid was the first elastic impression
material to be used as a substrate for obtaining electroformed copper dies in the
1950’s. However it proved to be unstable and hence its usage declined. The initial
enthusiasm for copper as an electroplating metal soon faded and silver became the
metal of choice with the introduction of mercaptan rubber. But soon after the
dimethyl siloxane impression material appeared both copper and silver were
reported as acceptable die materials.
Procedure:
The basic system consists of an electrolytic solution of CuSO4, or AgCN
and a source of direct current. The anode is either pure Cu or pure Ag. The surface
to be electroplated is the cathode. Generally, Cu plating is used for compound
impression and Ag plating for poly sulphide impression. Silicone impression
materials by virtue of their low surface energy are difficult to electroplate.
1. Clean and dry the impression and coat the surface to be electroplated with
metallising powder like Ag powder or graphite.
2. Direct the cathode wire in the border of the impression in the metallised areas but
not too close to preparations.
3. Fill the preparations and teeth with electrolytic solution with eyedropper to
prevent air bubbles and voids and completely immerse the impression in the
solution. The Ag anode immersed in the solution should be at least equal in size to
the area of the surface to be plated.
4. Proceed with the plating initially at approximately 5 mA per tooth for 1 hour and
examine the impression after 1 hour to check for deposition of metal and to make
certain there are no voids. Resubmerge the impression in the plating solution,
plate for12 hour at 10mA per tooth.
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5. After completion of electroplating, wash the impression and use dental stone as a
backing material.
Advantages:
i. More surface hardness.
ii. Superior surface detail, which exceeds most of the die materials.
Disadvantages:
i. Incompatibility with some impression materials – condensation
silicone impression materials are difficult to electroplate evenly due to
their low surface energy. Polyether and hydrocolloid imbibe water and
become distorted. Therefore cannot be electroplated accurately.
ii. Time consuming and expensive
iii. Extreme toxicity of AgCN solution – when it contacts acid, it produces
fumes of extremely toxic hydrogen cyanide.
iv. Variable degrees of distortion (limited control of die accuracy).
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Electroplating unit for silver and copper plating of impressions This electroplating unit allows
you to electroplate impressions in as little as 3 to 6 hours. The system ensures an even and
homogenous deposition of silver or copper and offers high stability whilst providing ideal
coverage on both large and small surface areas.
4. AMALGAM
Amalgam dies are used with copper band modelling plastic impression.
The impression has to be wrapped in wax or tape and embedded in a rigid matrix
such as plaster to resist the pressure necessary to condense the amalgam
effectively.
Disadvantage:
Time consuming – 12 hours to set.
5. FLEXIBLE DIE MATERIALS:
Flexible die materials are similar to heavy-bodied silicone or polyether
impression materials and have been used to make interim restorations or indirect
composite resin inlays or onlays chairside. The advantages of the flexible material
over a stone die include more rapid setting and the ease of removal of the interim
restoration or inlay. When choosing materials for flexible dies, the dentist must be
sure to select a compatible combination of impression and die materials that
provide good surface details. One study revealed that the best detail reproduction
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was obtained when Impregum F die material (3M ESPE) was combined with
Extrude Light impression material.
OTHER DIE MATERIALS:
Few other die materials were discussed in the review of literature but not
popular nowadays.
1) Bismuth-Tin alloy:
Described by Stallwood (1964) and Friend & Barrell (1965). The alloy
which melts at 138˚C is sprayed directly on to the impression, forming an an
accurate metal shell which is subsequently backed up with self-cure acrylic or
stone. Suitable for polysulfides and silicones. Difficult to spray in narrow
confined areas. The fine spray is injurious to health.
2) Ceramic Die:
Diamond die, Ceramco die
Supplied in powder and liquid. When mixed it has a putty like consistency.
The initial set is reached after 45 minutes, when care is needed while removing
from impression as the material is weak at this stage. To obtain maximum
hardness it heated in a oven at 600˚C for 8 minutes and **** in light mineral oil.
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The material is suitable for use with flexible impression materials such as
polysulfide or silicones. Dies with well-defined margins can be made quickly with
experience.
3) Cement Dies:
Model Kryptex
It is supplied as a powder and liquid. The mixing and working properties
are similar to those of dental cements. It can be used successfully with any
impression material.
Sharp well-defined margins are reproduced. Flow properties are good when
assisted by vibration. Dies can be produced in an hour. If the material is allowed
to dry out, it becomes brittle. Die when not in use can be placed in to liquid
paraffin or light mineral oil.
METHODS FOR MAKING SEPARABLE DIES:
In fabrication of wax pattern, working cast provides articulation; it
normally should represent the entire arch and is used to establish interproximal
contacts, buccal and lingual contacts and occlusion with opposing tooth.
The die is a model of the individual prepared tooth on which the margins of
the wax pattern are finished. With the help of die, proper proximal contacts can be
established. There are two working cast and die systems. – a working cast with a
separate die and a working cast with a removable die.
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Working cast with a separate die:
The working cast with a separate die is the simplest means of fabricating a
working cast and die. On ethanol advantage is that the gingival tissue and other
landmarks are intact in the cast. One of the disadvantage encountered is that the
wax pattern must be transferred from one to other which can cause distortion of
pattern or can destroy some of the internal adaptation. Technique utilizes double
pouring. Double pouring possible only with elastomeric impression materials.
Procedure:
Impression is poured in Type IV stone in the area of preparation(s) only.
When set, it is separated and a second pour is made of the entire arch.
The first pour is trimmed into a die with a handle of sufficient length
(similar to a tooth root). The complete arch cast is mounted on an articulator. The
wax pattern is started on the initial pour (the die) and is then transferred to the
articulated cast for refinement of axial contour and occlusal anatomy. When
completed, this pattern is returned to the die so the margins can be readapted
immediately before investing.
Die Preparation:
The handle of the die should be slightly larger in diameter than the
preparation and octagonal in cross-section. Its side ought to be parallel or slightly
tapered toward the base. Handle should be parallel to the long axis of the tooth
preparation. The handle should be approximately 1 inch long.
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A pear shaped acrylic bur to trim the die apical to the finish line of the
preparation. Begin final trimming of the die with a sharp no.25 blade. The area
apical to the finish line should be smoothed and made free of ridges with the
discoid end of a Tanner carver.
The contour of the die apical to the finish line should approximate that of
the root to facilitate good axial contours in the finished restoration.
Sharply undercutting or ditching the die below the finish line is not
advised. Because the instrument used for finishing the margin of the wax pattern
will rest against this portion of die, acute angulations can be exaggerated by
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undercuts. This will result in a thick gingival area on the restoration and an axial
contour that is not conducive the good gingival health.
After the die has been trimmed, the finish line should be highlighted with a
sharp colorbrite red pencil. Do not use excessive pressure when marking the finish
line as it may be rounded over. A black graphite pencil should not be used for this
purpose. When used with usual blue or green wax, a finish line outlined in black
does not become more visible, but instead makes every wax pattern margin
appear unsealed or open. In addition, remnants of clay binder used in graphite
pencil could contaminate the margin of the casting.
Relief should be applied to the
preparation area of the die to produce
space for cement. Enamels and laquer
have been used for this purpose. A relief
of 20 – 40 µm is desired. The tooth
preparation on the die is painted to
within 0.5 mm of the finish lines.
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A die hardening agent cyanoacrylate or acrylic resin laquer can be applied
to the finish line area of the die to prevent abrasion by waxing instruments during
fabrication of wax pattern. Coating material must have a low viscosity and it must
be applied lightly. Thickness for cyanoacrylate – 1 to 25 µm , acrylic – 4 to 10
µm.
Working cast with removable die:
1. Straight Dowel Pin:
This means of orienting dies have been in use for a number of years and
most of the dowel systems are modifications of it. A dowel pin is positioned over
each prepared tooth in the impression. The accurate placement of dowels can be a
problem. If the dowel pins are positioned inaccurately, they may impinge on the
margins, weaken the die, or prevent the die from being easily removed from the
cast. There are devices made specifically for precise positioning of dowels before
the pouring of impression.
1 – Utilized putty on a movable table to hold the impression in an exact,
repeatable position while pins are suspended above the impression from magnets
on a larger immovable table.
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2 – Wire clasps that can be stack in to the periphery of impression can be
purchased or they can be fashioned from orthodontic wire.
3 – Other items like paper clips, bobby pins etc.
Procedure:
1. The position of the dowel pin is located and marked in the impression.
2. Dowel pins are positioned over the impression with bobby pin /wire and
stabilize the bobby pin to the impression with straight pin and sticky wax.
3. Pour die stone in the impression, filling the impression of the teeth and
covering the knurled end of the dowel pin. Pin should parallel the log-axis
of the preparation and it must not touch impression.
4. Paper clips can be added to non-removable parts of the unset first pour to
provide retention for the second pour of stone.
5. When the stone has set, remove the straight pins and bobby pins from the
impression. Place a small ball of soft utility wax on the tip of each dowel.
Cut a v-shaped buccolingual orientation groove or a round dimple on each
die to aid in reseating the die completely and accurately during use.
Lubricate the stone around each dowel with a thin coat of petrolatum.
Remove excess lubricant.
6. Seal the open lingual space with wax or paper towel. Box the impression
and pour the stone.
7. When stone sets, remove the cast from impression and trim the excess with
a model trimmer.
8. Wax at the ends of the dowel pins are located and removed. Allow the
stone to harden for 24 hours.
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9. When the stone is hard and dry use a saw to cut through the layer of die
stone. There should be a cut on the mesial and distal side of each die and
the cut should taper towards each other from occlusal to gingival. Gently
tap on the end of the dowel with an instrument handle to loosen the die.
10. Die trimming should be done as discussed previously.
11. Reseat the dies to make certain that they will seat completely and will be
stable.
12. Place utility wax back into the wells around the tips of the dowel to protect
from plaster contamination. Soak the cast in water and mount it on the
articulator using mounting stone. When the stone has se, remove the wax
covering the tips of the dowels. Make certain that no chips of stone or wax
are left in the wells.
Types of dowel pins:
1. Flat-sided single dowel.
2. Single curved dowel
3. Double straight dowel with a common head
4. Two separate parallel dowels
5. Parallel dowel with plastic sleeve. e.g.Cross pin.
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Flat-sided single dowel Single curved dowel Double straight dowel Two parallel dowels