Die, Cast, and Model Materials PDF

DIE, CAST, AND MODEL MATERIALS Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry Primary Impression & Primary casts Secondary impression & Master casts Dental stones, plaster, epoxy resin, and refractory materials are some of the materials used to make c...

DIE, CAST, AND MODEL MATERIALS Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry Primary Impression & Primary casts Secondary impression & Master casts Dental stones, plaster, epoxy resin, and refractory materials are some of the materials used to make casts or dies from dental impressions. Factors affecting the selection of materials Impression material in use Purpose for which the die or cast is to be used. Example: Impressions in alginate hydrocolloid can be used only with a gypsum material, such as plaster, stone, or casting investment. But elastomeric impression materials can be used to prepare gypsum or epoxy dies.  Desirable Qualities of a Cast or Die Material Reproduce an impression accurately Dimensionally stable under normal conditions of use and storage. Reproduce fine detail Have a smooth, hard surface. Qualities of strength, resistance to shearing forces or edge strength, and abrasion resistance The color of a cast or die can facilitate manipulative procedures Easy to use Dental Plaster and Stone (Uses) Gypsum materials :to make casts and dies from dental impressions with any impression material. Stone casts: stronger and resist abrasion better than plaster casts, used whenever a restoration or appliance is to be made on the cast. Plaster may be used for study casts (record purposes only). Uses Impression plaster is used to make impressions of edentulous mouths or to mount casts, whereas dental stone is used to form a die that duplicates the oral anatomy when poured into any type of impression. Gypsum products are also used as a binder for silica in gold alloy casting investment, soldering investment, and investment for low-melting-point nickel-chromium alloys. These products are also used as a mold material for processing complete dentures.  High-strength dental stones make excellent casts or dies, readily reproduce the fine detail of a dental impression, and are ready for use after 1 hour.  The resulting cast is dimensionally stable over long periods and withstands most of the manipulative procedures involved in the production of appliances and restorations. Epoxy Die Materials Epoxy materials were supplied in the form of a paste to which a liquid activator (amine) was added to initiate hardening.  Advantages of Epoxy Die 1. The hardened resin is more resistant to abrasion 2. Stronger than a high-strength stone die.  Disadvantages of Epoxy Dies 1. The activators are toxic, they should not come into contact with the skin during manipulation of the unset material. 2. Shrinkage of 0.1% has occurred during hardening, which may take up to 24 hours. 3. Because water retards the polymerization of resin, epoxy resins cannot be used with water-containing agar and alginate impression materials, and thus are limited to use with elastomeric impression materials.  GYPSUM PRODUCTS Dental plaster, stone, high-strength/high- expansion stone, and casting investment constitute this group of closely related products. With slight modification, gypsum products are used for several different purposes. The dihydrate form of calcium sulfate, called gypsum, usually appears white to milky yellowish and is found in a compact mass in nature. The mineral gypsum has commercial importance as a source of plaster of Paris. The term plaster of Paris was given this product because it was obtained by burning the gypsum from deposits near Paris, France. Manufacture of Dental Plaster, Stone, and High-Strength Stone Three types of base raw materials are derived from partial dehydration of gypsum rock, depending on the nature of the dehydration process. 1. Plasters are fluffy, porous, and least dense 2. The hydrocal variety has a higher density and is more crystal-Line (dental stone) 3. Densite is the densest of the raw materials. These three types of raw materials are used to formulate the four types of relatively pure gypsum products used in dentistry ( high strength dental stone)  Types of dental stone Classification Name Properties Uses Type I Impression Plaster Contraction on setting Impression in edentulous ( Plaster of Paris) ridge in denture fabrication ( Now rarely use) Type II Dental Plaster Weakest and least Model and Laboratory use expensive (Mounting, flasking,.) ) Type III Dental Stone Low to moderate strength Diagnostic Casts Type IV Improved dental stone High strength and Low Master casts and Dies expansion Type V High expansion improved High strength and high Master casts and Dies dental stone expansion The reason for the differences among the recommended amounts of mixing water for model plaster, dental stone, and high-strength dental stones. Some calcium sulfate hemihydrate crystals are comparatively irregular in shape and porous in nature (crystals in model plaster) Crystals of dental stone and the two high-strength stones are dense and more regular in shape oAll gypsum products have the same chemical formula, and that the chemical nature of the masses produced by mixing them with water is also identical; the differences among them are their physical properties. Factors affecting setting time: 1. Spatulation The mixing process, called spatulation, has a definite effect on the setting time and setting expansion of the material. An increase in the amount of spatulation (either speed of spatulation or time or both) shortens the setting time. 2. The Temperature Effect of Temperature The temperature of the water used for mixing, as well as the temperature of the environment The setting time is affected more by a change in temperature than by any other physical property. The first effect of increasing temperature is a change in the relative solubilities of calcium sulfate hemihydrate and calcium sulfate dehydrate(alters the rate of the reaction). As the temperature increases, the solubility ratios decrease, As the ratio of the solubilities becomes lower, the reaction is slowed, and the setting time is increased.  The second effect is the change in ion mobility with temperature. As the temperature increases, the mobility of the calcium and sulfate ions increases, which tends to increase the rate of the reaction and shorten the setting time. as the temperature is raised over 37° C, the rate of the reaction decreases, and the setting time is lengthened. At 100° C the solubilities of dihydrate and hemihydrate are equal, in which case no reaction occurs, and plaster does not set. 3. Humidity The initial result is acceleration of setting. Further contamination by moisture can reduce the amount of hemihydrate remaining to form gypsum and retardation of setting will occur. 4. Colloidal Systems and pH Colloidal systems such as agar and alginate retard the setting of gypsum products. Liquids with low pH, such as saliva, retard the setting reaction. Liquids with high pH accelerate setting.  Setting Time The time required for the reaction to be completed is called the final setting time. If the rate of the reaction is too fast or the material has a short setting time, the mixed mass may harden before the operator can manipulate it properly.  Proper setting time is one of the most important characteristics of gypsum materials. The chemical reaction is initiated at the moment the powder is mixed with water,  The viscosity of the mixed mass increases, and the mass can no longer flow easily into the fine details of the mold. This time is called the working time.  The final setting time is defined as the time at which the material can be separated from the impression without distortion or fracture. The initial setting time is the time required for gypsum products to reach a certain arbitrary stage of firmness in their setting process 5. Powder / water ratio Water-powder (W/P) ratio can also affect setting time; using more water in the mix can prolong the setting time The W/P ratio has a pronounced effect on the setting time. The more water in the mix of model plaster, dental stone, or high-strength dental stone, the longer the setting time 6.Addition of chemicals The easiest and most reliable way to change the setting time is to add different chemicals. Potassium sulfate, K2SO4, is known as an effective accelerator, and the use of a 2% aqueous solution of this salt rather than water reduces the setting time of model plaster from approximately 10 minutes to about 4 minutes.  Sodium citrate is a dependable retarder. The use of a 2% aqueous solution of borax to mix with the powder may prolong the setting time of some gypsum products to a few hours.  Compressive Strength When set, gypsum products show relatively high values of compressive strength. The compressive strength is inversely related to the W/P ratio of the mix. The more water used to make the mix, the lower the compressive strength. Model plaster has the greatest quantity of excess water and high- strength dental stone contains the least excess water. At 1 or 2 hours after the final setting time, the hardened gypsum material appears dry and seems to have reached its maximum strength. Contamination of the impression in which the gypsum die is poured by saliva or blood can also affect the detail reproduction. Rinsing the impression and blowing away excess water can improve the detail recorded by the gypsum die material.  Setting Expansion The setting expansion may be controlled by different manipulative conditions and by the addition of some chemicals. Mechanical mixing decreases setting expansion.  Vacuum-mixed high-strength stone expands less at 2 hours than when mixed by hand. The W/P ratio of the mix also has an effect, with an increase in the ratio reducing the setting expansion. Manipulation Thank You  Dr. Malak Bamigdad BDS KAU, MSc Medical education, Saudi Board in Prosthodontic dentistry  Purpose of Impression Materials  Desirable Qualities  Types of Impression Materials  Alginate Hydrocolloids  Elastomeric Impression Materials  PURPOSE OF IMPRESSION MATERIALS Replica or mold of the hard and soft oral tissues a negative reproduction of the tissues by filling the impression with dental stone or other model material, a positive cast is made. to duplicate a cast or model that has been formed (duplicating materials )  DESIRABLE QUALITIES 1. A pleasant odor, taste, and acceptable color 2. No toxic or irritant constituents 3. Adequate shelf life for requirements of storage and distribution 4. Economically commensurate with the results obtained  DESIRABLE QUALITIES 5. Easy to use with the minimum of equipment 6. Setting characteristics that meet clinical requirements 7. Satisfactory consistency and texture 8. Readily wets oral tissues 9. Elastic properties that allow easy removal of the set material from the mouth and good elastic recovery  DESIRABLE QUALITIES 10. Adequate strength to avoid breaking or tearing upon removal from the mouth 11. Dimensional stability over temperature and humidity ranges. 12. Compatibility with cast and die materials 13. Accuracy in clinical use 14. Readily disinfected without loss of accuracy 15. No release of gas or other byproducts ALGINATE HYDROCOLLOIDS (Irreversible) Composition and Chemistry  Potassium and sodium salts of alginic acid have properties that make them suitable for compounding a dental impression material.  Alginic acid from a marine plant has high-molecular-weight block copolymer ALGINATE HYDROCOLLOIDS (Irreversible)  ALGINATE HYDROCOLLOIDS (Irreversible) Powder: ( calcium Sulfate dehydrate + Soluble alginate + Sodium Phosphate) + Water 1. Insoluble calcium phosphate (retarder) Provide working time for mixing 2. Insoluble calcium alginate with water form ( irreversible calcium alginate gel) Topic / Subject Impressions should be disinfected with a spray solution after removal from the mouth and before pouring with a casting material. Proportioning and Mixing  The proportioning of the powder and water before mixing is critical to obtaining consistent results.  Changes in the water/powder ratio will alter the consistency and setting times of the mixed material and the strength and quality of the impression.  Usually, the manufacturers provide suitable containers for proportioning the powder and water.  The mixing time for regular alginate is 1 minute undermixing and overmixing are detrimental to the strength of the set impression. Fast-set alginates should be mixed with water for 45 seconds. best mixed vigorously in a flexible rubber bowl with an alginate spatula Mechanical mixing devices are also available. Working Time: 45 seconds for the fast-set types, 30 to 75 seconds of working time remain before the impression needs to be completely set regular-set materials, a mixing time of 60 seconds leaves 2 to 3.5 minutes of working time color change is a pH-related change of a dye Setting Time: Setting times range from 1 to 5 minutes (at least 15 seconds longer than the stated working time) Lengthening the setting time is better accomplished by reducing the temperature of the water used with the mix than by reducing the proportion of powder. Reducing the ratio of powder to water reduces the strength and accuracy of the alginate. Selecting an alginate with a different setting time The setting reaction is a typical chemical reaction, and the rate can be approximately doubled by a temperature increase of 10° C. using water that is cooler than 18° C or warmer than 24° C is not advisable.  The impression should be left in place 2 to 3 minutes because the tear strength and elastic recovery (recovery from deformation) increase significantly during this period. Elastic Recovery A typical alginate impression is compressed about 10% in areas of undercuts during removal. Note that permanent deformation is a time- dependent property  Clinically these factors translate into requirements for a reasonable bulk of alginate between the tray and the teeth, appropriate retention of the alginate in the tray, and a rapid removal of the impression from the mouth. Disinfect the impression and produce a gypsum model provide adequate time for any recovery that might occur. Flexibility: most alginates have a typical value of 14%. some of the hard-set materials have values from 5% to 8%. A reasonable amount of flexibility is required for ease of removal of the impression. Strength: Compressive strengths range from 0.5 to 0.9 MPa. Tear strengths vary from 0.4 to 0.7 kN/m, and this property is probably more important than the compressive strength. The compressive and tear strengths of alginates are time dependent properties The tear strength is a measure of the force/thickness ratio needed to initiate and continue tearing. Tearing occurs in the thin sections of the impression, and the probability of tearing decreases with increasing rates of removal. Compatibility with Gypsum:  The impression must be rinsed well in cold water to remove saliva and any blood, and then disinfected. Next, all free surface water should be removed before preparing a gypsum model.  Saliva and blood interfere with the setting of gypsum, and if free water accumulates, it tends to collect in the deeper parts of the impression and dilute the gypsum model material, yielding a soft, chalky surface.  If the alginate impression is stored for 30 minutes or more before preparing the model: it should be rinsed with cool water to remove any exudate on the surface caused by syneresis of the alginate gel; exudate will retard the setting of the gypsum. it should be wrapped loosely in a moist paper towel and sealed in a plastic bag to avoid moisture loss. Topic / Subject Dimensional Stability: Alginate impressions lose water by evaporation and shrink when standing in air. Impressions left on the bench for as short a time as 30 minutes may become inaccurate enough to require remaking the impression. For maximum accuracy, the model material should be poured into the alginate impression as soon as possible. If for some reason the models can not be prepared directly, the impressions should be stored in 100% relative humidity in a plastic bag or wrapped in a damp paper towel. Disinfection:  Viruses may be transferred to gypsum models and present a risk to dental laboratory and operating personnel Topic / Subject The most common form of disinfection is spraying, but studies have shown that such impressions can be immersed in disinfectant also. The disinfectant solutions are 1% sodium hypochlorite or 2% potentiated glutaraldehyde solutions ELASTOMERIC IMPRESSION MATERIALS  Polysulfides  condensation silicones  Addition silicones(polyvinylsiloxanes)  Polyethers Topic / Subject Polysulfides were the first synthetic elastomeric impression material introduced (1950). Consistencies Addition silicones are available: Extra-low, low(syringe or wash), medium (regular), monophase, high (tray), and putty (extra-high) consistencies. Polyether impression materials are now available in low, medium, and high consistencies. Mixing Systems Two types of systems are available to mix the catalyst and base thoroughly before taking the impression: static automixing and dynamic mechanical mixing Topic / Subject Topic / Subject latex gloves may interfere with setting of addition silicone impression materials Impression Techniques  Three common methods for impression for fixed restorations 1. Simultaneous, dual- viscosity technique 2. Single-viscosity or monophase technique 3. Putty-wash technique. The simultaneous, dual-viscosity technique  low-consistency material is injected with a syringe into critical areas and the high-consistency material is mixed and placed in an impression tray.  After injecting the low-viscosity material, the tray containing the higher-viscosity material is placed in the mouth Because they are both mixed at nearly the same time, the materials join, bond, and set together. After the materials have set, the tray and the impression are removed. In the single-viscosity or monophase technique The medium viscosity material is forced through an impression syringe, the viscosity is reduced, whereas the viscosity of the same material residing in the tray is unaffected. Same materials can be used for syringing and for trays The putty-wash technique  two-step impression, a preliminary impression is taken in high- or putty- consistency material before the cavity preparation is made.  Space is provided for a low-consistency material by a variety of techniques, and after cavity preparation, a low-consistency material is syringed into the area and the preliminary impression reinserted.  The low- and high-consistency materials bond, and after the low-consistency material sets, the impression is removed. This procedure is sometimes called a wash technique.  Addition Silicone  Available in extra low, low, medium, heavy, and very heavy (putty) consistencies.  Addition Silicone The accelerator (catalyst) and the base paste contain a dimethylsiloxane polymer with vinyl terminal groups, plus filler.  it is recommended that one wait at least 30 minutes for the setting reaction to be completed before the gypsum models and dies are poured.  Epoxy dies should not be poured until the impression has stood overnight.  Addition Silicone  Latex gloves have been shown to adversely affect the setting of addition silicone impressions.  Sulfur compounds that are used in the vulcanization of latex rubber gloves can migrate to the surface of stored gloves.  Residual monomer in acrylic provisional restorations and resin composite cores has a similar inhibiting effect on the set of addition silicone materials.  Addition Silicone Washing of the gloves with water just before mixing sometimes minimizes this effect. some brands of gloves interfere with the setting more than others. Vinyl and nitrile gloves do not have such an effect. Residual monomer in acrylic provisional restorations and resin composite cores has a similar inhibiting effect on the set of addition silicone materials. The preparation and adjacent soft tissues can also be cleaned with 2% chlorhexidine to remove contaminants.  Polyether Polyethers are supplied in low-, medium-, and heavy- body consistency. The base paste consists of a long-chain polyether copolymer with alternating oxygen atoms and methylene groups (O—[CH2]n) and reactive terminal group. Also,a silica filler, compatible plasticizers of a nonphthalate type, and triglycerides.  Viscosity Viscosity is a function of time after the start of mixing. A shearing force can affect the viscosity of poly ether and addition silicone impression materials, as This effect is called shear thinning or pseudo- plasticity. For impression materials possessing this characteristic, the viscosity of the unset material diminishes with an increasing outside force or shearing speed.  When the influence is discontinued, the viscosity immediately increases. This property is very important for the use of monophase impression Materials. In the case of polyether, shear-thinning properties are influenced by a weak network of triglyceride crystals. The crystals align when the impression material is sheared, as occurs when mixed or flowing through a syringe tip. The microcrystalline triglyceride network ensures that the polyether remains viscous in the tray or on the tooth but flows under pressure. This allows a single or monophase material to be used as a low- and medium-consistency material. Cooling of the pastes results in substantial viscosity increase. Before using, pastes have to be brought to room temperature. Working and Setting Times In general, for a given class of elastomeric impression materials by a specific manufacturer, the working and setting times decrease as the viscosity increases from low to high. Polyethers show a clearly defined working time with a sharp transition into the setting phase. This behavior is often called snap-set. Topic / Subject Note that the working and setting times of the elastomeric impression materials are shortened by increases in temperature and humidity; on hot, humid days this effect should be considered in the clinical application of these materials. Dimensional Change on Setting The impression material undergoes a dimensional change on setting. The major factor for contraction during setting is cross-linking and rearrangement of bonds within and between polymer chains. Impressions can expand if water sorption takes place and an impression can be distorted if seated after the material has set to any degree. Finally, distortion or creep will occur if the material does not recover elastically when the set impression is removed from undercuts Mechanical Properties Addition silicone and polyether impression materials undergo shrinkage due to polymerization. The addition silicones have the smallest change, about −0.15%, followed by the polyethers at about 0.2%. The contraction is low for these two products because there is no loss of byproducts. In general, about half the shrinkage observed at 24 hours occurs during the first hour after removal; for greatest accuracy, therefore, the models and dies should be prepared promptly, although in air the elastomeric impression materials are much more stable than hydrocolloid products. Elastic Recovery addition silicones have the best elastic recovery during removal from the mouth, followed by polyethers. Strain in Compression The strain in compression under a stress of 0.1 MPa is a measure of the flexibility of the material. Polyethers are generally the stiffest, followed by addition silicones.  Flow Flow is measured on a cylindrical specimen 1 hour old, and the percent flow is determined 15 minutes after a load of 1 N is applied. Silicones and polyethers have low values of flow. Hardness Hardness increases from low to high consistency. The low-, medium-, and high-viscosity addition silicones do not change hardness significantly with time, whereas the hardness of polyether does increase with time. Tear Strength The hardness and strain in compression affect the force necessary to remove the impression from the mouth. Low flexibility and high hardness can be compensated for clinically by producing more space for the impression material between the tray and the teeth. This can be accomplished with additional block-out for custom trays or by selecting a larger tray when using disposable trays.  Tear strength is important because it indicates the ability of a material to withstand tearing in thin interproximal areas and margins of periodontally involved teeth. As the consistency of the impression type increases, tear strength undergoes a small increase.  Polysulfide is the most flexible and polyether is the least flexible. addition silicones and polyethers have the best elastic recovery. Wettability and Hydrophilization of Elastomeric Impression Materials Wettability may be assessed by measuring the advancing contact angle of water on the surface on the set impression material or by using a tensiometer to measure forces as the material is immersed and removed (Wilhelmy technique). Traditional addition silicone is not as wettable as polyether. When mixes of gypsum products are poured into hydrophobic addition silicone, high contact angles are formed, making the preparation of bubble-free models difficult. From a clinical standpoint, most impression materials produced acceptable detail under wet and dry conditions. Polyethers produced slightly better detail than did addition silicones, and were generally unaffected by the presence of moisture, whereas detail decreased for addition silicones under wet conditions, Relationship of Properties and Clinical Application Accuracy, the ability to record detail, ease of handling, and setting characteristics are of prime importance in dental impressions. The time of placement of an elastomeric impression material is critical, because viscosity increases rapidly with time and the polymerization reaction ( an inaccurate impression ) Thorough mixing is essential; otherwise portions of the mix could contain insufficient accelerator to polymerize thoroughly or may not set at the same rate as other portions of the impression (an inaccurate impression). Auto mixing and mechanical mixing systems produce mixes with fewer bubbles than hand mixing, save time in mixing, and result in a more bubble free The manufacturer usually recommends a minimum time for leaving the impression in the mouth Dimensional changes on setting can be compensated for by use of a double-impression or putty-wash technique.  The double-impression technique is suitable for use with a stock impression tray, because the preliminary impression serves as a custom tray. With the monophase and simultaneous dual-viscosity technique, a slight improvement in accuracy results when a custom-made tray is used because it provides a uniform thickness of impression material. Clinical studies have shown that the viscosity of the impression material is the most important factor in producing impressions and dies with minimal bubbles and maximum detail. The accuracy of the impression may be affected when the percentage of deformation and the time involved in removing Second pours of gypsum products into elastomeric impressions produce dies that are not quite as accurate as the first, Because elastomeric impressions recover from deformation for a period after their removal, some increase in accuracy can be expected during this time. Insignificant elastic recovery occurs after 20 to 30 minutes; therefore, dies should be prepared promptly after that time for greatest accuracy. Addition silicones that release hydrogen are an exception to this guideline.  IMPRESSION TRAYS  Stock tray ( prefabricated ) ready made , metal or platic  Custom impression trays provide a nearly constant distance between the tray and the tissues allowing a more even distribution of the impression material during the impression procedure, and improved accuracy.  IMPRESSION TRAYS  Light-activated and vacuum formed polymers are now used more frequently than chemically accelerated acrylic  Tray adhesive Tray Adhesive is used to enhance the adhesion of vinyl polysiloxane (VPS) impression materials to metal or plastic, custom acrylic and double-arch type impression trays. Use of the adhesive helps ensure that the completed impression remains firmly attached to the tray upon removal from the mouth. Thank You Investment materials Dr. Malak Bamigdad BDS KAU, Saudi Board in Prosthodontic dentistry  CASTING INVESTMENTS An investment can be described as a ceramic material that is suitable for forming a mold into which a metal or alloy is cast. Casting operations: (1) a wax pattern of the object to be reproduced (2) a suitable mold material investment placed around the pattern and permitted to harden ( 3) Suitable furnaces for burning out the wax patterns and heating the investment mold (4)Proper facilities to melt and cast the alloy. The operation of forming the mold is described as investing Ceramic Metal Crowns  Properties Required of an Investment 1. Easily manipulated 2. Sufficient strength at room temperature 3. Stability at higher temperatures 4. Sufficient expansion: expand enough to compensate for shrinkage of the wax pattern and metal that takes place during the casting procedure. 5. Beneficial casting temperatures 6. Porosity: It should be porous enough to permit the air or other gases in the mold cavity to escape easily during the casting procedure. 7. Smooth surface: Fine detail and margins on the casting should be preserved. 8. Easy to use 9. Inexpensive  Types of Dental Investment Based on the nature of binder 1. Gypsum bonded investments ( calcium Sulfate): used for casting gold alloys, withstand temperature up to 700°C 2. Phosphate bonded investments: for metal ceramic and cobalt- chromium alloys, withstand higher temperatures. 3. Silica bonded investments: alternative to phosphate bonded investments for high temperature casting.  CALCIUM SULFATE–BONDED INVESTMENTS The calcium sulfate–bonded investment is usually limited to gold castings and is not heated above 700° C. The calcium sulfate type of binder is usually not used in investments for making castings of high-melting-point metals such as palladium or base metal alloys. Composition: mixture of three distinct types of materials: refractory material, binder material, and other chemicals. For example, small amounts of chlorides or boric acid enhance the thermal expansion of investments bonded by calcium sulfate.  Properties of Calcium Sulfate–Bonded Investments Casting dental restorations of gold alloys: Type 1: For casting inlays and crowns Type 2: For casting complete denture and partial removable dental prosthesis bases  Setting Expansion of Calcium Sulfate–Bonded Investment Have both setting and thermal expansion. The sum of these two expansions results in a total dimensional change that is an essential property of dental casting investments because it provides compensation for the casting shrinkage of the casting alloys. If the investment is setting surrounded by air, the expansion is referred to as normal setting expansion. If the mixed investment is setting in contact with water, the expansion is greater and is called hygroscopic setting expansion Contact with water can be achieved in the commonly used casting techniques of : (1) Placing a wet liner inside the casting ring in which the investment is poured, or (2) After investing, the casting ring is placed in a water bath. The particle size of calcium sulfate hemihydrate has little effect on hygroscopic expansion, whereas the particle size of silica has a significant effect. Finer silica produces higher setting.  Silica/Binder Ratio If the silica/stone ratio is increased, the hygroscopic expansion of the investment also increases, but the strength of the investment decreases.  Water/Powder Ratio As with the setting expansion of gypsum products, the more water in the mix (the thinner the mix or the higher the W/P ratio), the less the normal and hygroscopic setting expansions. Less thermal expansion is also with a thinner mix  Spatulation The effect of spatulation on the setting and hygroscopic expansion of the investment is similar to that on the setting expansion of all gypsum products.  Age of Investment Investments that are 2 or 3 years old do not expand as much as freshly prepared investments. For this reason, the containers of investment must be kept closed as much as possible, especially if the investment is stored in a humid atmosphere. Water-Bath Temperature For the water-bath immersion technique, the temperature of the water bath has a measurable effect on the wax pattern. At higher water-bath temperatures, the wax pattern expands, requiring less expansion of the investment to compensate for the total casting shrinkage. The net effect is higher expansion of the mold with higher water-bath temperatures. THERMAL AND HYGROSCOPIC CASTING INVESTMENT Casting techniques classified in thermal or hygroscopic techniques. 1. The thermal technique directs placing the invested ring after setting into the burnout oven set for a relatively high temperature (649° C) 2. the hygroscopic technique directs immersing the invested ring before setting in a water bath and then, after setting, placing the ring into the burnout oven set for relatively low temperature (482° C). Perform the clinical steps needed for complete denture construction including obtaining primary and secondary impressions using adjustable trays , border molding, recording jaw relation, performing the necessary steps during try-in and delivery of the final complete denture  Phosphate-Bonded Investment The most common type of investment for casting high-melting point alloys. Three different components. 1. a water-soluble phosphate ion. 2. Reacts with phosphate ions at room temperature. 3.The third component is a refractory, such as silica. The binding system of a typical phosphate-bonded investment undergoes an acid-base reaction  Phosphate-Bonded Investment Type 1: For inlays, crowns, and other fixed restorations Type 2: For removable dental prostheses  Silica-Bonded Investment Another type of binding material for investments used with casting high-melting-point alloys  Ethyl silicate has the disadvantage of giving off flammable components during processing, and the method is expensive  Brazing Investment When brazing (soldering) the parts of a restoration, such as clasps on a removable dental prosthesis, the parts must be surrounded with a suitable ceramic or investment material before the heating operation. The assembled parts are temporarily held together with sticky wax until they are surrounded with the appropriate investment material, after which the wax is softened and removed.  Type 1: Gypsum-bonded dental brazing investments Type 2: Phosphate-bonded dental brazing investments It has lower setting and thermal expansions than casting investment  INVESTMENT FOR ALL-CERAMIC RESTORATIONS Two types of investment materials: The first type is used for the cast glass technique and provided by the manufacturer of the glass casting equipment and is composed of phosphate bonded refractories. The second type of investment for making all-ceramic restorations is the refractory die type of material, which is used for all-ceramic veneers, inlays, and crowns. Refractory dies are made by pouring the investment into impressions. When the investment is set, the die is removed, and is heated to remove gases that may be detrimental to the ceramic (degassing).  A refractory die spacer may be added to the surface. Next, porcelain or other ceramic powders are added to the die surface and fired. These materials are also phosphate-bonded, and they generally contain fine-grained refractory fillers to allow accurate reproduction of detail. Thank You