Model and die materials/ dental implant courses by Indian dental academy

MODEL AND DIE MATERIALS A model is a replica of the teeth and the associated supporting bony tissue of a jaw, which is prepared from an impression. Die is a positive reproduction of the form of a prepared tooth in any suitable hard substance usually in metal or specially prepared artificial stone.

Type of Model/casts 1. Study cast 2. Diagnostic cast 3. Pre-operative cast 4. Refractory cast 5. Investment cast 6. Master cast 7. Final cast 1, 2 & 3 – cast made for the purpose of study and treatment planning.

4 & 5 – cast made of a material which will tolerate the temperature used in casting and soldering.

6 & 7 – are the replica of the prepared tooth surfaces, residual ridges used to fabricate a dental prosthesis or restoration.

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Model and cast are same, but model is used for exhibition. An object formed and poured in a matrix with any desired material to create positive likeness for exhibition.

Requirements of Model and Die materials 1. Should have accuracy and dimensional stability.

2. Should have a smooth, hard surface which should not easily abrade.

3. Should be compatible with impression material.

4. Should have high strength.

5. Should have good colour contrast.

6. Should be cheap.

7. Should be able to manipulate easily and fast.

8. Should be non-injurious to health.

Materials used for models and dies 1. Gypsum products Model plaster Dental stone Improved stone Gypsum bonded investment Phosphate bonded investment

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2. Metals Amalgam Electroplated copper Electroplated silver Metal sprayed dies 3. Cements Silico phosphate cements 4. Polymers and filled polymers Self curing acrylics Epoxy resins Polyesters and epiamines 5. Ceramics 6. Divestments Selection of model and die material depends on: 1. The impression material used.

2. Purpose for which the cast or die is used.

3. Time factor involved.

4. Materials that will be manipulated on the cast or die.

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Compatibility of impression materials with model and die materials: 1. Gypsum products Impression compound Zinc oxide eugenol Hydrocolloids Impression plaster coated with separating medium Polysulphide rubber base Poly ether rubber base 2. Electroplated copper Impression compound 3. Electroplated silver Polysulphide rubber base Silicone rubber base (addition type) Poly ether rubber base 4. Epoxy Resin Silicone rubber base Poly sulphide rubber base coated with separator Poly ether rubber base

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5. Amalgam Impression compound 6. Low fusing alloy Poly sulphide rubber base Silicone rubber base 7. Ceramic Poly sulphide rubber base Silicone rubber base Any non-aqueous impression material 8. Silicophosphate cement Impression compound 9. Acrylic or polyester amine Silicone rubber base Model Materials The most commonly used model materials are gypsum products.

Gypsum is a mineral found as white to milky-yellowish mass in nature. It is calcium sulphate di-hydrate (CaSO4-2H2O). From this (CaSO4)2.H2O i.e. calcium sulphate hemihydrate is prepared.

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Types of hemihydrates used in dentistry 1. Dental plaster or plaster of paris.

(Calcined hemihydrate or beta hemihydrate) 2. Stone plaster (Autoclaved hemihydrate or alpha hemihydrate) 3. Improved stone (Autoclaved hemihydrate) – harder variety.

As per American Dental Association specification No.25 gypsum products are classified as follows: 1. Type I Plaster (Impression plaster).

2. Type II Plaster (Model plaster).

3. Type III stone (Dental stone) (Hydrocal) – Class I dental stone.

4. Type IV stone (Improved stone) or die stone (Densite).

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Manufacture of Gypsum Products Hydrates of Calcium Sulphate Mineral source

By product of other industries Calcium sulphate dihydrate, (CaSO4.2H2O)

Heat in open vessel 110°C-130°C

Heat in autoclave under steam pressure, 120-130°C

Heat ground gypsum in water with small quantity of organic acid or salt in an autoclave, 140°C

Calcined hemihydrate (beta hemihydrate) (CaSO4)2. H2O

Autoclaved hemihydrate (alpha hemihydrate or hydrocal) (CaSO4)2.H2O)

Autoclaved hemihydrate (alpha hemihydrate) (CaSO4)2.H2O

Heat in 30% solution calcium or magnesium chloride Calcium hemihydrate (Densite) (CaSO4)2·H2O

Heat 130-200°C Hexagonal calcium sulphate (soluble anhydrite) CaSO 4 Heat 200-1000°C Orthorhombic calcium sulphate (insoluble anhydrite) CaSO 4

In short the reaction is: 2CaSO4.2H2O

heat

(CaSO4) 2H2O + 3H2O

Plaster of paris or beta hemihydrate crystals are spongy, porous and irregular in shape. Stone plaster or alpha hemihydrate crystals are more dense and have a prismatic (rods and prisms) shape. Improved stone (Densite) has cubic or rectangular shaped crystals and are the densest hemihydrates.

Setting of Gypsum products When alpha hemihydrate is mixed with water, a reaction which is the reverse of the one occurring during its formation takes place and this results in a much stronger and harder product than obtained when the beta hemihydrate is

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mixed with water. This is mainly because the alpha variety requires much less gauging water/mixing than the beta variety.

Factors determining the gauging water a) Particle size and particle size distribution – smaller the particles less the gauging water. After manufacture, the hemihydrates are ground to smaller particles. b) Adhesion between the particles if more, less is the gauging water surface active materials like gum Arabic plus lime increase adhesion.

Setting reaction of gypsum products The setting reaction of plaster and stone is exothermic and the heat evolved is equivalent to the heat used originally in calcination. 100gm of hemihydrate requires 18.6 ml water for complete hydration. But the water requirement is more if a workable mix which can be poured is to be obtained.

There are two theories regarding the setting reaction: 1. Crystalline theory.

2. Colloidal (gel) theory.

The crystalline theory was originated in 1887 by Henry Louis Le Chatelier and it received fullsupport of Jacobus Hendricus Van’t Hoff in 1907.

Accordingly the following steps constitute the reaction:

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1. When hemihydrate is mixed with water, a suspension of the hemihydrate is formed. The mixture appears quite fluid at this stage.

2. Some hemihydrate then dissolves, the solubility being 0.8% at room temperature.

3. The dissolved calcium sulphate hemihydrate reacts with water and forms calcium sulphate dihydrate.

4. The solubility of dihydrate is much less than that of the hemihydrate at room temperature (0.4%) and therefore the formed dihydrate is a supersaturated solution.

5. Since all supersaturated solutions represent an unstable condition, the calcium sulphate dihydrate crystals precipitate out of the solution to bring the solution to saturation, which is a stable form. The precipitation or crystallization occurs on nuclei of crystallization.

6. As the dihydrate precipitates out of the solution, more calcium sulphate hemihydrate dissolves and follows the same stages until all of the hemihydrate is converted to dihydrate.

It is the difference in solubilities of the dihydrate and hemihydrate which causes the setting of plaster.

This theory cannot be accepted as such. It is now known that neither the hemihydrate nor the dihydrate can exist in a dissolved state. Only Ca++ and SO4—ions and probably various complexes between these substances and water can exist. Also, in a setting mass of plaster, two centres are recognized

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dissolution centres around the hemihydrate and precipitation centres around the dihydrate. The calcium sulphate ions travel in solution by diffusion from the dissolution centres to precipitation centres.

The gel theory or colloidal theory was originated in 1893 by M.Michaelis. According to this, calcium sulphate dihydrate exists initially as a dispersed phase of colloidal gel, out of which the gypsum crystals grow.

The crystalline theory is more acceptable.

The reaction rate can be followed by the exothermic heat evolved or the rise in temperature as a function of the elapsed time. Owing to the relatively low thermal conductivity of plaster, the observed temperature rise is likely to lag the actual reaction time to some extent. The elapsed time before the exothermic heat becomes evident and the temperature time curve begins to rise is known as the ‘induction period’. The crystals formed are characteristically needle-like clusters called ‘spherulites’. The final rigid structure of the set gypsum is the result of intermeshing and entangling of the crystals. The spaces between the crystals is initially occupied by the excess water used in mixing, containing calcium sulphate in solution.

Water/Powder (W/P) Ratio Water and hemihydrate should be measured by weight rather than by volume. Higher the W/P ratio, longer will be the setting time and weaker the set gypsum. The recommended W/P ratios are as follows:

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Model plaster

0.45 to 0.55 (45 to 55ml water to 100 gm of powder)

Class I stone

0.30 to 0.35

Class II stone

0.20 to 0.25

These ratios give a workable mix that can be poured into a mould without traping air bubbles.

As the porous crystals of plaster soak up more water, the amount of gauging water required is more than that of stone.

The test for the consistency of the gypsum products is called ‘slump test’. It involves mixing 100 gm of powder with water, removing the cylinder and allowing the mix to spread over a glass slab. By changing the W/P ratio one can alter the diameter of the slumped mass until it falls within the range allowed by A.D.A. specification No.25.

Setting time The time elapsing between the beginning of mixing until the material hardens is known as the setting time.

The dentist should be able to control this e.g. an impression plaster should not take too long to set. Setting time is measured by ‘penetration tests’.

1. Using Gillmore needles; the smaller needle weighs ¼ lb and has a point 1/12” in diameter. Larger needle weighs 1 lb and has a point 1/24” in diameter. Time elapsing from start of mixing until the point of ¼ lb needle no longer penetrates the surface on being gently lowered on to it, is the initial setting time or working time. Time elapsing from start of

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mixing until the point of the 1 lb needle no longer penetrates the surface of the set plaster is the final setting time.

2. Using Vicat needles: this is used to measure the setting time which is the time elapsing from the start of the mixing until the needle no longer penetrates to the bottom of the plaster. This measures only the intial setting time.

3. Setting time is also determined clinically by noting the time at which the gloss disappears (loss of gloss) from the surface of the setting plaster.

The interstices of the crystals withdraw the surface water film by capillary action to replace water of hydration, when loss of gloss occurs.

This also coincides with the end of the induction period and the initial setting time. When the initial setting has occurred, the dentist can begin working with the cast without crumbling or distorting it. The material becomes rigid but not hard; it can be carved, but not moulded.

Control of setting time Theoretically this is affected by: 1. Increasing or decreasing the solubility of the hemihydrate. When solubility increases, supersaturation of calcium sulphate is greater, rate of crystalline deposition is increased and setting time lowered.

2. Altering the number of nuclei of crystallization. Greater the number of nuclei faster the setting.

3. Altering the rate of crystal growth.

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4. Practically, setting time is controlled by the physical or chemical composition of gypsum or by the method of manipulation by the dentist.

These factors include: 1)

Impurities Gypsum can remain if calcination is incomplete or if the manufacturer

adds gypsum; the nuclei of crystallization increases in number and setting time is lowered.

Hexagonal anhydrite lowers the induction period. Orthorhombic anhydrite increases the induction period.

2)

Fineness Finer the particle size, faster the setting. The hemihydrate may be

ground during manufacture to give more solubility due to increased surface area of plaster and more nuclei of crystallization.

3)

W/P ratio More the water used, fewer the nuclei per unit volume and longer the

setting time.

4)

Mixing Longer the mixing time, and more rapidly it is mixed within practical

limits, faster the setting. The initial crystals formed get broken up by spatulation and get distributed throughout, increasing the number of nuclei.

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Effect of W/P ratio and mixing time on the setting time of plaster of paris W/P ratio

Mixing time (min.)

Setting time (min.)

0.45

0.5

5.25

0.45

1.0

3.25

0.60

1.0

7.25

0.60

2.0

4.50

0.80

1.0

10.50

0.80

2.0

7.75

0.80

3.0

5.75

(from Gibson and Johnson J. Soc. Chem. Ind. 1932) 5)

Temperature Little change occurs between 0°C and 50°C. If the temperature of the

plaster water mix exceeds 50°C, a retardation of setting reaction occurs. At nearly 100°C, no reaction takes place as the solubilities of hemihydrate and dihydrate are equal. At higher temperatures, a reverse hydrate. (Worner H.K. 1944, ‘The effect of Temperature on the Rate of setting of plaster of paris’.

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Accelerators and retarders The most effective and practical method to control the setting time is the

addition of chemical modifiers to plaster or dental stone. If the chemical added decreases the setting time, it is an accelerator. If it increases the setting time, it is a retarder. Gelatin, blue, agar, gum arabic etc. absorb onto the surface of the hemihydrate particles and reduce their dissolution. Most other salts accelerate the reaction.

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7)

Humidity Hemihydrate is hygroscopic and if stored in open containers, absorb

water and form dihydrate which in small concentrations is an accelerator.

Above 0.5-1% the dihydrate increases the setting time.

Setting Expansion An expansion of the plaster mass can be observed during the change from hemihydrate to dihydrate. The expansion depends on the composition of the gypsum product and varies from 0.06% linear to 0.5% linear. Theoretically however, a contraction or reduction in volume of –7.11% can be calculated.

The crystals growing from the nuclei not only intermesh but also intercept each other during growth. This impingement of crystals causes stress and the crystals move in an outward direction to relieve the stress. Thus an apparent or observed expansion takes place even though the true volume of crystals alone is less. This crystal impingement and movement results in the production of micropores. As external volume is greater and crystalline volume less, the set material is porous, the set plaster of paris being more porous than the set stone plaster.

On drying, the excess water filling the interspaces of crystals is lost and the total empty spaces increase. Greater the W/P ratio, greater the porosity.

Only that part of the setting expansion which occurs after the initial set is of interest. Any expansion before this is compensated by the fluidity of the mix. This setting expansion can cause warpage at the palatal vault region of impression plaster, as the tray flanges do not allow lateral expansion. The

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A.D.A. specification No.25 lists maximum permissible values of setting expansion of 0.3%, 0.2% and 0.1% for model plaster, stone and improved stone, respectively.

Control of Setting Expansion Setting expansion is useful in some cases like in investment materials to compensate for the casting shrinkage by mould expansion.

Lesser the W/P ratio and longer the mixing time, greater the setting expansion. Addition of chemicals can control the setting expansion.

Anti Expansion solution (A.E. Solution) Used to decrease setting expansion of impression plaster.

Composition: Potassium Sulphate 4% Borax

0.4-1%

Alizarin red

0.04% (Pigment)

Flavouring agent Effect of W/P ratio and mixing time on setting expansion of plaster of paris W/P ratio

Mixing time (Min.)

Setting expansion (%)

0.45

0.5

0.41

0.45

1.0

0.51

0.60

1.0

0.29

0.60

2.0

0.41

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Theory of Accelerators and Retarders They generally decrease the setting expansion. Sodium chloride in concentrations up to 5% is an accelerator but in concentrations of 20% or above, is a retarder. The NaCl crystals which form initially deposit on the nuclei of crystallization thus retarding further crystallization. This is called ‘nuclei poisoning’.

Sodium sulphate is also an accelerator in lower concentrations and a retarder in concentrations above 12%.

Potassium sulphate is the most commonly used accelerator which decreases the setting time in any concentration. In concentrations of 2-3% or above, the reaction product is ‘syngenite’ K2Ca(SO4)2.H2O, which crystallizes very rapidly. A 2% solution of K2SO4 in water is an excellent accelerator. This reduces the setting time of plaster from approximately 10 minutes to about 4 minutes.

Borax is a retarder in any concentration, as it forms insoluble calcium borate which causes poisoning of nuclei.

Powdered

gypsum

and

hexagonal

anhydrite

are

accelerators.

Orthorhombic anhydrite, agar, blue, gum arabic etc. are retarders.

Reduction of setting expansion noticed with modifiers is effected either by a change in crystalline form of the dihydrate or an initial rate of crystallization so rapid the subsequent growth is resisted by early formation of a rigid frame work.

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Dried blood, saliva and most colloidal gels like alginate citrates, acetates and borates are retarders. Impressions should be thoroughly rinsed in cold running water to remove traces of blood and saliva, before the impression is poured.

Hygroscopic Setting Expansion If setting is allowed to occur under water, setting expansion may be more than doubled in magnitude. Under the water, additional room for crystal growth is permitted. In normal setting in air, water around the hemihydrate particles is reduced by hydration and the particles are drawn more closely together by the surface tension of water. But such limitation of crystal growth is not there in hygroscopic setting as the water is again replenished from the normal setting expansion as well as the hygroscopic setting expansion.

Hygroscopic setting expansion affects the accuracy of dentures fabricated on such casts, but this expansion is utilized in the fabrication of accurate cast restorations with type II gypsum investments.

Strength Compressive strength is more important than the tensile strength.

Strength increases rapidly as mateial hardens after initial setting.

Two strengths are recognized – wet strength and dry strength. Wet strength or ‘green’ strength is the strength when excess water is left in the set plaster. Dry strength is the strength when excess water is lost by drying. Dry strength is 2 or more times the wet strength. Compressive strength is inversely proportional to W/P ratio, but tensile strength is directly proportional to it.

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ADA specification No.25 recommends the 1 hour compressive strength as follows: Plaster

1280psi

Dental stone

2980psi

Improved stone

4980psi

Dry compressive strength increases with mixing time up to one minute and then decreases as the gypsum crystals are broke up.

Accelerators and retarders lower both wet and dry strengths. For optimum properties, plaster or stone should be left to hydrate for one hour and then dried to constant weight at 45째C. Surface Hardens Higher the compressive strength, greater the surface hardness. Hardness increases from the time of mixing until hydration is complete. From then it remains constant till most of the excess water is lost by evaporation, after which it increases again. Soaking the gypsum dies or casts in water, glycerin or oils do not improve the surface hardness, but make the surface so that the wax carver will not cut the stone as it glides over the surface. Surface hardness of set plaster can be improved by dipping the cast in 2% borax solution for several hours. This treatment is not effective for dental stone.

Abrasion Resistance High abrasion resistance is desirable in dies and models. Improved stones have the highest abrasion resistance and dental plaster the least. Oils give smoothness as mentioned previously.

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Gypsum Products – Technical Considerations Dental stone contains added colouring matter to distinguish from impression plaster and plaster of paris. Potassium sulphate an accelerator and sodium citrate, a retarder, are added to make the stone ‘balanced’, to provide a setting time of 5-8 mins setting expansion of stone is limited to 0.06-0.12%.

Class I stone is Hydrocal, and Class II stone is Densite or Improved stone.

Class I stones are used for making casts and Class II, for dies (hence called ‘die stones’).

Proportioning A smooth mix that can be easily poured should be made. The W/P ratio is adjusted accordingly by weighing.

Mixing A flexible rubber or plastic bowl which has a parabolic shaped innerwall and a stiff-bladed spatula with rounded tip is used. Air bubbles weaken the cast and should be avoided in the mix. The powder is sifted into the water and allowed to stand for 30 seconds. Brisk spatulation is done for 1-2 mins. Until a uniform slurry consistency is obtained. Air bubbles are removed by placing on a vibrator or tapping the bowl on table top, or by mixing in vacuum using a Vac-U-Vester. A mechanical spatulator can also be used.

A laboratory plaster spatula can be modified so that it can be attached to the vibrating handle of an electric tooth brush, to provide a bubble free mix.

(Macro A.B. Pontual, J. Pros. Dent; Jan. 1985).

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Construction of the cast 3 methods are there: 1. A beading wax is placed 1/2" below the periphery on the outer side of the impression. A longitudinally cut modeling wax strip is adapted around the impression and fused to the beading. This procedure is called ‘boxing’. Now the impression is poured without trapping air bubbles.

2. The impression is poured as usual. After the initial set, some plaster is mixed and piled onto a glass plate and the cast inverted onto it to form a base.

3. The impression filled with stone is invested and placed in a rubber mould called base former into which freshly mixed plaster has been vibrated.

The cast is removed after 45 to 60 minutes. When setting is complete.

Storage Gypsum products are stored in air tight containers to ward off humidity.

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Examples of currently available gypsum model and die materials Manufacturer Modern materials

Model Plaster Model plaster orthodontic

Dental stone Denstone Snow

Improved stone Die- stone

manufacturing Co.

plaster lab plaster

white stone

Die-Keen

Labstone

Tru-Stone

Rapid stone

Vel-Mix

Castone

Duroc Glastone

Orthodontic stone

Super-Cal

Coecal buff Lab

Super-Die

stone

Silky Rock

Kerr corporation

Snow white plaster No.1

Ranson and Randolph Co.

Model plaster

Coe Laboratories Model plaster

Inc. Whip-Mix

Microstone A Orthodontic grade

orthodontic stone

plaster Laboratory plaster

Die Materials 1.

Die stone This is type IV gypsum or Class II stone or Densite and is most

commonly used.

Advantages a. Relatively inexpensive. b. Easy to use.

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c. Compatible with most impression materials. d. Good reproduction of surface details. e. Dimensional accuracy – a line less than 20 microns in width can be reproduced accurately.

Disadvantages a. Poor abrasion resistance and strength. b. Poor refractoriness of the dies.

Abrasion resistance can be increased by nearly 100% by using gypsum hardeners like colloidal silica instead of water while mixing. Hardness is not increased significantly by this. (Toreskog S., Philips R.W. et al, ‘Properties of Die Materials- A Comparative Study’).

Impregnation of gypsum by a polymer – polyester, polystyrene acrylics and epoxy resins can increase the abrasion resistance. Incorporation of wetting agents such as lignosulphonates reduces the amount of gauging water, increasing the hardness and strength.

When the surface is treated with resins without proper precautions, an increase in thickness of 10 microns or more results leading to inaccuracy of the stone die.

Methods of altering die dimensions If less setting expansion is desired, potassium sulphate (accelerator) and borax (retarder) are added to the gauging water; this limits the setting expansion to 0.01%.

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Sometimes a die slightly larger than the prepared tooth is desired to compensate for casting shrinkage of metallic crowns etc; and to provide a uniform space for the luting cement between the prepared tooth and the cast restoration. Here colloidal silica may be used in the gauging liquid, or surface application of resins is done.

A larger die is also obtained by dipping the die first in benzoyl peroxide and then in BISGMA/TEGMA (Bis-GMA : Bisphenol A Glycidyl Methacrylate) monomer containing tertiary amine. Under standardized conditions the polymerized resin coats the die surface to a depth of 30 microns.

(Jorgensen K.D. and Fiwger W. – ‘Die Spacing Technique By Diffusion Precipitation’).

Die spacing prevents the layer of luting cement from interfering with the complete seating of an otherwise precisely fitting casting. This is usually achieved by coating the gypsum die with resin, paint- on liquids, model paint, coloured nail polish or thermoplastic volatile solvents to about 0.5 mm of the margin of the die; the margin is not coated.

Oils, liquid soap, detergents etc. are used as separators which facilitate removal of wax pattern from the stone die.

2. Amalgam Dies Model amalgam is similar to silver amalgam used for fillings. Model amalgam is used to make hard metal dies which reproduce fine details and sharp margins from compound impression of prepared teeth. Amalgam dies cannot be made in hydrocolloid impressions as they will not withstand the

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condensation pressure. After packing the impression with amalgam the die can be removed after a minimum of 12 hours, by gently warming the compound.

As amalgam and other metal dies are good conductors of heat, softened wax applied to them cools rapidly. This may produce internal stresses which may distort the wax pattern after removal from the die. Sudden cooling of wax may also result in contraction of the wax away from the die. These are avoided by warming the metal die to mouth temperature. A separating agent is needed as with stone dies.

3.

Silicophosphate cements Zinc silicophosphate cements are used in compound impressions. These

dies are harder than stone dies. Powder contains silica and zinc oxide and the liquid is orthophosphoric acid.

Disadvantages Air bubbles are easily trapped and so a centrifuging device is required to fill the impression. It shrinks on setting and losses water on standing, and so are not accurate; the surface becomes fragile. Dies can be removed in 1 hour from the impression.

4.

Dies formed by Electrodeposition of metal Some impression materials can be electroplated e.g. impression

composition can be copper plated and elastomeric materials can be silver plated.

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a)

Copper plated dies When a model or die is prepared by electroplating, the process is

referred to as ‘electroforming’. Copper dies reproduce fine detail. No dimensional change occurs in electro-deposition, unlike in dental stone, amalgam or cement. Electro-deposited die is tough and has good strength; metal restorations can be finished and polished accurately on these dies.

A copper plating apparatus for dental use consists of a transformer and rectifier to reduce the line voltage and to convert AC to DC. Composition of Electrolyte Copper sulphate crystals

-

200gm

Concentrated sulphuric acid

-

30ml

Phenol sulphonic acid

-

2ml

-

1000ml

Distilled water Technique

The surface of the impression is rendered conductive by coating it with fine particles of copper or graphite. This process is called ‘metallization’. The coated impression is made the cathode (-ve electrode) of the plating bath, with an anode (+ve electrode) of copper. The copper sulphate in the electrolyte is the source of copper, sulphuric acid increases the conductivity, phenol sulphonic acid assists penetration of Cu ions to the deeper parts of the impression. To start with, a current of 15ma is suitable. The current causes dissolution of the anode and movement of Cu ions from the anode to the cathode. Once a thin

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layer of copper is deposited, the current can be increased to 50ma. The plating time varies but is usually about 10 hours. Dental stone is then poured into the plated impression; when it has set, the metal covered die can be removed from the impression.

This technique is not suitable for elastomeric impression materials as they are not dimensionally stable in an acid solution. b)

Silver plated dies Polysulphide and silicone impression materials can be silver plated by the same technique as above, except: i.

The impression is coated with silver or graphite powder.

ii.

The anode is pure silver.

iii.

The electrolyte is an alkaline solution of silver cyanide, with the following composition: Silver cyanide 36 gm Potassium cyanide 60 gm Potassium carbonate 45 gm Distilled water

1000 ml

Alkaline silver baths soften the surface of impression compound and hence are not compatible with it.

The silver electrolyte is poisonous and extreme care must be taken not to contaminate the hands, workbench and clothing with it. Accidental contact of solution with any acid (pickling acid or acid copper sulphate solution) produces hydrogen cyanide, and extremely poisonous gas. The bath should be well covered and the room well ventilated. Electro-deposition should be done for 12 to 15 hours.

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Problems in silver plating a) Faulty conduction Ammeter may show the reading but the impression does not plate, or plates irregularly or very slowly. This may be due to short circuiting of the wires in the solution. b) Exhausted solution Plating is very slow and the deposit is discoloured. Replace with fresh solution. c)

A ceramic material, supplied as powder and liquid and mixed into a putty-like consistency. After 1 hour the material is removed from the impression and fixed at 600째C for 8 mins. To produce a hard strong die.

Dies with well defined margins can be produced quickly. The material is

strong and abrasion resistant but expensive, and the short working time may be detrimental.

8)

Divestment It is a die stone-investment combination in which the die material and

investing medium have a comparable composition. Divestment is a commercial gypsum-bonded material; it is mixed with colloidal silica liquid. The die is made from this mix and the wax pattern constructed on this. Then the entire assembly (die and pattern) is invested in the divestment, thereby eliminating the possibility of distortion of the pattern upon removal from the die or during

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the setting of the investment. The setting expansion of the material is 0.9% and the thermal expansion is 0.6% when heated to 677°C. Since Divestment is a gypsum bonded material, it is not recommended for high fusing alloys, as used with metal ceramic restoration. However, it is a highly accurate technique for use with conventional gold alloys, especially for extra-coronal preparations.

References: 1. Worner H.K. : The effect of temperature on the rate of setting of plaster of paris, 1944.

2. Macro A.B. Pontual; J.P.D., 1985.

3. Toreskoy S., Philips R.W. et al : Properties of Die materials – A comparative study.

4. Skinner’s Science of Dental Materials.

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