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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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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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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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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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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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

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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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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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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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Addition of accelerators and retarderes accelerators used are K5SO4

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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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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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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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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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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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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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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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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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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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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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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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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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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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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REFERENCES:

Fundamentals of fixed Prosthodontics:Shillingberg.

Contemporary fixed Prosthodontics :

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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A SEMINAR ON

DIE MATERIALS AND TECHNIQUE OF

FABRICATION

Presented by

Dr.U.Varalakshmi SIGNATURE OF H.O.D

P.G Student

Date:

Dept. Of Prosthodontics

SIBAR Institute of Dental Sciences

GUNTUR

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