DIE, CAST, AND MODEL MATERIALS. final.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