Sunday, August 18, 2013
DIE MATERIALS AND TECHNIQUE OF FABRICATION
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
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.
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.
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.
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.
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.
Susceptibility to abrasion during carrying of the wax pattern especially with Type IV Gypsum die.
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
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.
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.
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 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.
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).
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.
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.
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.
1. Time consuming
2. Special equipment is needed
3. Not compatible with all impression materials.
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.
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.
They are made by packing amalgam into impression made of impression compound.
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.
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).
1. More abrasion resistance.
2. Less brittle than die stones.
3. Can be carved at room temperature.
1. Shrinkage on polymerization
2. Less dimensional stasility
Epoxy die material can be used with polyether, polysulphide or silicone epoxy to which filler may be added.
CH2 – CH – R – CH – CH2
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.
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.
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.
Disadvantages: The alloy is soft, care is helded to prevent abrasion of the die.
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.
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.
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
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