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Die materials and technique of fabrication (2)


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Die materials and technique of fabrication (2)

  1. 1. DIE MATERIALS 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 on the prepared tooth, such techniques are difficult to master. Also direct wax patterns are difficult to make if the margins of the finished cavity preparation extend below the gingival crest or if visibility is limited. Furthermore, the temperature of the oral cavity tends to make the wax pattern more susceptible to deformation. Also instrumentation for direct wax pattern is difficult. Such problems can eliminated if the wax pattern is fabricated on a removable die. Definition of Die It is the positive reproduction of the form of a prepared tooth in suitable hard substance, usually in metal or specially prepared dental stone. Materials used for fabrication of a die: a) Gypsum products  Type IV stone / high strength stone / Densite.  Type V stone / die stone / high strength high expansion stone. 1
  2. 2. b) Electroformed dies  Electroformed copper dies.  Electroformed silver dies. c) Epoxy resin dies. d) Amalgam dies. e) Silicate cement dies. f) Acrylic resin dies. g) Metal sprayed dies / low fusing metal alloy dies. h) Ceramic dies. i) Refractory dies. The selection of any of the materials is determined by the following: a) The impression material in use. b) The purpose for which the die is to be used. Ideal requirements of die materials: a) Accuracy of surface reproduction. One should be able to see all the fine details and sharp margins. b) Dimensional accuracy and stability. c) Mechanical properties.  High strength to be able to withstand accidental breakage. 2
  3. 3.  Surface hardness and abrasion resistance so that the die can withstand the manipulative procedures during carving of wax pattern. d) Compatibility with impression materials. e) Good colour contrast with other materials being used e.g., inlay casting wax. f) Economical. g) Easy to use. a) Gypsum products – The most commonly used materials for fabrication of a die are Type IV and Type V gypsum products. Advantages i Generally compatible with all impression materials. ii Have the ability to reproduce fine detail and sharp margins. iii Dimensionally accurate and stable. iv Easy to use. Disadvantages i Poor surface hardness make them susceptible to abrasion during carving of wax pattern on the die. 3
  4. 4. Manufacture of Type IV and Type V gypsum products Gypsum products in dentistry are formed by driving off part of the water of crystallization from calcium sulphate dihydrate to form calcium sulphate hemihydrate. CaSO4, 2H2O (Dihydrate) 110°C-130°C CaSO4 ½ H2O Loss of 1.5g moles to 2g moles of water of crystallization Die materials are based on autoclaved calcium sulphate hemihydrate, plus additives to adjust the setting time, control the setting expansion and pigments for colour contrast. Calcium sulphate dihydrate is boiled in 30% calcium chloride or magnesium chloride. Densite or Type IV stone is obtained which is an α- hemihydrate with cuboidal shaped particles. This can be pulverized into a fine particle size with the addition of modifiers to obtain a high strength high expansion stone (die stone). Setting Reaction When α-calcium sulphate hemihydrate in the form of high strength stone is mixed with water, a chemical reaction takes place and the hemihydrate is converted to the dihydrate form with the evolution of heat. 4
  5. 5. CaSO4 . ½ H2O + 1 ½ H2O  CaSO4 . 2H2O + 3900 cal/g mole The first stage in the process of setting is that the water becomes saturated with hemihydrate which has a solubility of around 0.9%* at room temperature. The dissolved hemihydrate is then rapidly converted to dihydrate which has a solubility of 0.2%*. Since the solubility limit of dihydrate is exceeded it being to crystallize out of solution. This forms the second stage of the reaction. Crystals of dihydrate are needle like clusters called spherulites which grow from specific sites called nuclei of crystallization. These nuclei may be small particles of impurity such as the unconverted gypsum crystals within the hemihydrate powder. Diffusion of calcium and sulphate ions into these nuclei seem to play a role in the setting process. As the dihydrate crystallizes, more hemihydrate dissolves and the process continues. Physical changes in the setting process: Initially the mix of hemihydrate and water can be poured. Next the material becomes rigid but not hard. This is called initial set of the material and at this stage it can be carved but not moulded. Also there is very little 5
  6. 6. reaction with little or no rise in temperature. This period is known as the “Induction Period”. 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. The reaction is exothermic. Dimensional changes also take place. A setting expansion of 0.05 to 0.3% is observed due to the outward thrust of the growing crystals of dihydrate. This is called normal setting expansion. If the material is placed under water at the initial set stage, a greater expansion occurs known as hygroscopic setting expansion. Manipulation of gypsum products: 1) Storage : They should be stored in air tight containers to prevent reaction with moisture from the atmosphere which can cause formation of dihydrate. These dihydrate crystals behave as new nuclei of crystallization and accelerate the setting reaction. 2) Water/powder ratio: To attain maximum strength, surface hardness and a well controlled setting expansion, it is necessary to gauge the amount of water and powder as recommended by the manufacturer. Type IV – 0.22-0.24; Type V – 0.18 – 0.22 6
  7. 7. This would mean that 22ml and 18 ml of water are required for 100g of powder of Type IV and Type V stone respectively. The W:P ratio is a very important factor in determining the physical and chemical properties of the set gypsum product. For example, higher the water/powder ratio, the longer will be setting time and weaker will be the gypsum product. This is because there is more water per unit volume and less nuclei of crystallization per unit volume. 3) Spatulation: Take the measured amount of water in a flexible rubber mixing bowl. The powder is then dispersed into the water and allowed to settle for 30 seconds. This minimizes the air incorporated in the mix during the initial spatulation. A spatula with a stiff blade is used. Spatulation is carried out 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 uniform with water. Mixing time of one minute for hand spatulation and 30 seconds for mechanical spatulation is usually sufficient to give a smooth lump free mix. Use of mechanical means reduces air entrapment during mixing. Use of an automatic vibrator helps the mix to flow well into the impression and helps to eliminate air bubbles. Over vibration should be avoided as this 7
  8. 8. may cause distortion of impression materials. The time and rate of spatulation have a definite effect on the setting time and setting expansion. An increase in the amount of spatulation, i.e. mixing time breaks the already formed dihydrate crystals providing more nuclei of crystallization and thereby accelerating the setting reaction causing a decrease in setting time. An increase in the rate of spatulation increases the setting expansion. Properties: 1) Setting time : The time that elapses from the beginning of mixing until the material hardens is known as setting time. The initial setting time is also called the working time during which the material can be mixed and poured into the impression. As the chemical reaction proceeds more and more dihydrate crystals are formed. The viscosity of the reacting mass increases rapidly and can no longer flow into the fine details of the impression. At this point the materials should not be forcefully manipulated. Initial setting time can be detected clinically by a phenomenon called as Loss of Gloss (LG). The initial setting time is measured by Gillmore needle which should not longer leave an impression when lowered onto the mix and should occur within 8 to 13 minutes from the start of the mix. 8
  9. 9. The final setting time is the time at which the material can be separated from the impression without distortion or fracture. The chemical reaction at this stage is practically completed. This is usually measured as the time taken for the material to become sufficiently rigid to withstand the penetration of a needle of known diameter under a known load. Here the Gillmore needle should leave a barely perceptible mark on the surface. Vicat needles have also been used. Factors affecting setting time: a) Factors under the control of manufacturer  Concentration of nucleating agents in the hemihydrate powder. An increase in the concentration decreases the setting time e.g. dihydrate particles.  Accelerates and Retarders Accelerators used are: i. Potassium sulphate (K2SO4) – less than 2% to 3% in solution. The setting time decreases from 10 to 4 minutes. The reaction product is called Syngenite which crystallizes rapidly. ii. Calcium sulphate (CaSO4) – it is ground and added to the powder and it provides nuclei for growth. The set gypsum is called Terra Alba and the concentration used is 0.5 to 1.0%. iii. Sodium chloride (NaCl) – less than 2% is used. 9
  10. 10. iv. Sodium sulphate (Na2SO4) – 3-4% is used. v. Slurry water vi. Retarders used are: a. 2% borax. b. Potassium citrate. c. Organic materials like gum, glucate. d. Increased addition of inorganic salts : e.g., NaCl > 2%, Na2SO4 > 4%  Fineness of particle size Pulverization of the manufactured product into a fine particle size accelerates the setting reaction. Grinding increases the surface area of the particles exposed to water which dissolve rapidly. It also increases the nuclei of crystallization. The rate of solubility increases and decreases the setting time. b) Factors under the control of operator  W/P ratio An increase in W/P ratio retards the setting reaction.  Mixing time An increase in the mixing time accelerates the reaction and decreases the setting time. 10
  11. 11.  Colloidal films such as mucin, saliva, blood retard the setting reaction. Therefore thoroughly rinse the impression in running cold water prior to pouring the cast.  Temperature Temperature variation has little effect on the setting time. An increase in the temperature from 20°C to 37°C causes a slight increase in the rate of reaction. As the temperature is raised further, the rate of the reaction decreases and the lengthens the setting time. 2) Reproduction of Surface Detail Gypsum dies produce an adequate surface detail but not as accurate as electropated dies. This is because the surface of the set gypsum is porous on a microscopic level. The porosity causes the surface to be rough. The use of surface hardners during mixing can produce a smooth surface. Incompatibility with some impression materials can result in air inclusions and surface voids. 3) Compressive strength The strength of gypsum products is directly related to the density of the set mass. The wet strength is the strength when the water in excess of 11
  12. 12. that required for the hydration of the hemihydrate is left in the test specimen. When the specimen has been dried free of the excess water, the strength obtained is the dry strength. The one hour compressive strength of densite is 5000 psi and that of die stone is 7000 psi. 4) Tensile strength It is 330 psi. It is a brittle material and is considerably weaker in tension than in compression. 5) Surface hardness and abrasion resistance The surface hardness of gypsum die material is three times that of an epoxy die but half that of an electroplated die. The use of hardening solutions increase the resistance to abrasion. a) Internal hardners such as 30% colloidal silica can be used instead of water during mixing of the stone. These surface active modifiers allow the powder particles to be more easily wetted by water. Incorporation of wetting agents such as lignosulphonates derived from lignin can reduce the water requirement and enable the production of a harder, stronger, dense set gypsum. 12
  13. 13. b) External hardners include polymers such as polyester, polystyrene, acrylic or epoxy resin. A solution of 10% polystyrene in amyl acetate is painted on to the surface of the die, the excess blown off and allowed to dry for 5 minutes. Surface hardness of Type IV stone is 92 RHN. 6) Setting expansion and dimensional accuracy All gypsum products show a measurable linear expansion on setting. High strength stone has a setting expansion of 0.0 to 0.1%, that of high strength high expansion stone being 0.1 to 0.3%. Expansion occurs due to the outward thrust of the growing nuclei of crystallization. Factors affecting expansion: i. An increase in spatulation increases the setting expansion. ii. An increase in W/P ratio decreases the setting expansion and vice versa. Hygroscopic setting expansion occurs under water with almost double the normal expansion, due to the replenishment of water of hydration. Setting expansion to an extent compensates for the casting shrinkage of the metal. 13
  14. 14. SYNTHETIC GYPSUM They are made from the by-products or waste products of the manufacture of phosphoric acid and remains a trade secret. They are highly expensive and exhibit superior properties. DIE STONE INVESTMENT COMBINATION Here the die material and the investing medium have a comparable composition. A commercial gypsum bonded material, called Divestment is mixed with a colloidal silica liquid. The die is made from this mix and the wax pattern constructed on it. Then the entire assembly (die + pattern) is invested in the Divestment, thereby eliminating the possibility of distortion 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. Since Divestment is a gypsum bonded material, it can be used only for gold alloys. Divestment phosphate or DVP is a phosphate bonded investment that is used in the same manner for high fusing metal ceramic alloys. a) ELECTROFORMED DIES 14
  15. 15. Electroforming refers to the electrodeposition of metal on a metallic or non-metallic silicon object thus building up the counterpart of the object by the passage of electrical current through the electrolyte. The art of electroformng is called as Galvanoplasty. Jacobe in 1934 first used it and Wajna in 1937 applied it in dentistry. Electroforming compound impressions require copper and elastomeric impressions require silver. Advantages: i. Dimensionally accurate with absolutely no dimensional change unless, the impression material shrinks before the initial plating is deposited. ii. Superior surface reproduction and sheen with accuracy of marginal definition. A line 4µm or less in width is readily reproducible. iii. Higher strength, surface hardness, abrasion resistance. iv. Easy to carve and recarve pattern on the die. v. High points of occlusion can be determined with great accuracy. vi. Allows satisfactory finishing and polishig of metal restorations on the die. 15
  16. 16. Disadvantages i. Time consuming. ii. Special equipment is needed. iii. Expensive. iv. Not compatible with all impression materials. 16
  17. 17. Electroforming unit consists of the following: a) A container which is of hard rubber or of glass. b) Electrical current from a battery or from the mains through a rectifier. c) Quantum of current which is measured in terms of amperes with an ammeter. The normal current required for a tube impression is 5 milliamperes, silver plating requires smaller units of current. d) Anode terminal (+ve) is pure copper or pure silver of 99.9% purity. The cathode terminal (-ve) is the impression to be electroformed i.e. low fusing compound for Cu and mercaptan or silicone rubber for Ag. Surfaces which don’t require deposition such as wire terminal and copper tube should be coated with wax. The minimum distance between the terminals should be 4 ½ - 5 inches. e) Electrolyte – solution is of one of the metals that needs to deposited. For copper electroforming an acidulated solution of copper sulphate is used. It contains ethanol, phenol, hydrochloric acid and distilled water in addition to CuSO4. The solution has to be used after 48 hours of standing for maturing. If not allowed to mature, a rough surface is obtained. Acid allows the passage of electric current. Ethanol and 17
  18. 18. phenol improve the throwing power or ionic penetration power of copper ions for deposition on the cathode terminal. Distilled water is used as a vehicle. For silver electroforming a basic solution of silver cyanide is used. It contains silver cyanide, potassium cyanide for increased penetration, potassium carbonate and distilled water. The solutions of the two systems have to be kept away from each other to prevent the formation of cyanide vapour which are extremely lethal. f) Metallizer / metallizing agent It is that part which is employed to make the surface of compound or rubber base conductive to the passage of electric current. For copper electroforming the following are used: i. “Aqua-dag” : It is a suspension of powdered graphite. It is supplied in collapsible tubes. A couple of mm of paste is mixed with distilled water with a brush and applied on the surface of the impression. ii. Suspension of bronzing powder in oil of bitter almonds. 18
  19. 19. For silver electroforming the following are used: i. Silverizer / Flash – It is an alcoholic solution of finely ground silver. ii. Finely ground silver powder. The impression is washed to eliminate streaks of blood, saliva, mucin, air dried, and the agent is applied in the form of strokes  Burnishing technique. A very thin layer is applied and allowed to air dry. The solution of electrolyte is poured into the impression and static charges are started. After 9-11 hours, an even layer of metal of 100µ thickness is obtained. The solutions are poured back into the container. The void is filled with dental stone. When the stone hardens it is mechanically locked to the rough interior of the electroformed metal shell. The surface coating can be altered by altering the composition, time and distance. b) EPOXY RESIN DIES They are either self-curing acrylic materials e.g. epoxy resins, polyester and epimines or polymeric materials with metallic or ceramic fillers. Advantages: i. Adequate surface hardness and abrasion resistance. ii. Less brittle than die stone. iii. Can be cured at room temperature. 19
  20. 20. Disadvantages: i. Shrinkage on polymerization leads to dimensional inaccuracy. ii. Expensive – Epoxy die materials can be used with polyether, polysulphide or silicone rubber impression materials. Composition: Epoxy die materials are 2-component systems that include a resin and a hardness. The viscous resin may be a difunctional epoxy to which filler may be added. CH2 – CH – R – CH – CH2 O O The harder is a polyamine that when mixed with the resin for about a minute causes polymerization. The hardner is toxic and should not come into contact with the skin during mixing and manipulation of the unset material. Properties : i. Working time  15 minutes. ii. Setting time  1-12 hours depending on the product. iii. Hardness  25 KHN. iv. Compressive strength after 7 days is 16,000 psi. 20
  21. 21. v. Superior abrasion resistance. vi. Dimensional changes between 0.03 to 0.3% and continues to occur for up to 3 days. vii. High viscosity can result in surface voids. viii. Most epoxy dies should not be used until 16 hours after pouring since they harden slowly. ix. Cannot be used with agar and alginate because water retards the polymerization of the resin. They are compatible with polyether, polysulfide or silicone impression materials. c) AMALGAM DIES They are made by packing amalgam into impression made of impression compound. Dies exhibit superior strength and reproduce fine details. Although a material of choice for a number of years, it has been replaced by electroplated dies because of the following limitations. i. It can be packed only into a rigid impression material. ii. It is technique sensitive and may result in varied dimensional accuracy. 21
  22. 22. iii. Time required for fabrication is lengthy. Although the die packing procedure may take only 30 minutes, amalgam requires 12-24 hours for hardening. iv. It has high thermal conductivity and can cool a wax pattern rapidly which may lead to distortion of the pattern. This can be overcome by warming the die. v. Residual mercury presents a health hazard. vi. Dimensional changes due to delayed expansion d) SILICATE CEMENT DIES It is similar to the filling and cementing material. Advantages: Initial strength and surface hardness is superior to that of die stone. Disadvantages: i. The cement contracts during setting and may be dimensionally inaccurate. ii. There is loss of water on standing, causing a rough and dehydrated surface. iii. High viscosity predisposes to surface voids. e) ACRYLIC RESIN DIES 22
  23. 23. They have adequate surface hardness and abrasion resistance but undergo shrinkage on polymerization. PMMA resins are used. 23
  24. 24. f) METAL SPRAYED DIES/ LOW FUSING METAL ALLOY DIES A bismuth-tin alloy, which melts at 138°C is sprayed directly on to an impression to form a metal shell, which can then be filled with dental stone. A metal coated die can be obtained rapidly from elastomeric impression materials. The disadvantage is that the alloy is soft and does not fulfill the mechanical requirements of a die. g) CERAMIC DIE MATERIALS i. A material for the production of dies on which porcelain restorations are to be fabricated, without the use of a platinum foil matrix. T form the dies high temperatures of 1000°C is required. ii. 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 minutes to produce a hard strong die. h) REFRACTORY DIE MATERIALS They are made from refractory materials and are heat stable. 24
  25. 25. Conclusion Type IV and Type V stones appear to be the most successful die materials available. With care, abrasion during pattern carving can be avoided. In case of high-fusing metal ceramic restorations, resin or metal electroplated dies can be used. 25