Model And Die Materials PDF

Model And Die Materials By: Eman Mohamed Mohamady Lecturer Of Dental Biomaterials  Model And Die Materials Model or cast: Is the positive replica of the teeth and associated supporting tissue of the jaw which is prepared from an impression. Die: Is a model of a sing...

Model And Die Materials By: Eman Mohamed Mohamady Lecturer Of Dental Biomaterials  Model And Die Materials Model or cast: Is the positive replica of the teeth and associated supporting tissue of the jaw which is prepared from an impression. Die: Is a model of a single tooth.  Requirements Of Model And Die Materials 1-Have sufficient mechanical properties. -Sufficient strength to resist breakage. -Sufficient hardness to resist scratching and damage. 2- Able to Produce fine details and sharp margin of the impression.  Requirements Of Model And Die Materials 3-Little dimensional changes in setting and remain unchanged during storage. 4-Compatible with the impression (not interact with the impression). 5-Good color contrast with other materials. 6- Ease of use. 7-Cheap.  Most commonly used materials for model and die are : gypsum products. Gypsum Present in nature as Calcium Sulphate Dihydrate (CaSO₄.2H₂O) (CaSO₄.2H₂O) heating (CaSO₄.1/2H₂O) gypsum gypsum products calcium sulphate calcium sulphate dihydrate hemihydrate  Types of Gypsum products 1.Impression plaster..(type I) β calcium sulphate (Plaster) hemihydrate 2. Model plaster ……..(type II). (Plaster) (Stone) 3. Dental stone ……….(type III). α calcium sulphate 4. High-strength stone… (type IV). hemihydrate High strength 5. High strength high expansion dental stone ……………….(type V). All are chemically the same CaSO₄.1/2H₂O  Gypsum products All are chemically the same CaSO₄.1/2H₂O but differ in : Method of manufacture Particle size , shape and form. Water /powder ratio. Physical and mechanical properties uses Manufacturing of the different types of gypsum products Calcination Reaction heating Calcination reaction (dehydration) : is a high temperature reaction to drive off part of water (one and half molecule ) of gypsum to produce gypsum products. CaSo4.2H2O heating CaSo4.1/2H2O+ 1 1/2H2O Calcination reaction (dehydration)  Model Plaster Dental Stone , High-strength Stone, Calcined Gypsum, Hydrocal Improved Stone. Type II Type III Type IV Β- Hemihydrate ᾁ- Hemihydrate Densite. Heating in open air at Heating in autoclave at Boiling in 30% calcium 120°C 120-130°C chloride Powder is irregular Powder is more Powder is most large and porous regular smaller and regular, smallest and less porous dense. least porous dense the denset.  Setting reaction Gypsum products are supplied in dentistry as powder (calcium sulphate hemihydrate) to be mixed with water.  Setting reaction (Crystallization) Ca SO₄. ½ H₂O + 1 1/2H₂O Ca SO₄. 2 H₂O + Heat gypsum product gypsum A Chemical reaction named crystallization Exothermic reaction  Water/powder ratio Chemically each 100 g of CaSO₄.1/2 H₂O require only 18,61 g of water for the CaSO₄.2 H₂O to be formed. The water/powder ratio for each type is: Gypsum product W/P ratio (g/ml) Dental plaster 50- 60 /100 Dental stone 30 /100 Improved stone 20/100  Water/powder ratio Excess water is needed to produce a homogenous workable mix. (give reason) Excess water will be present in the final product as free water. Then evaporate leaving the set material porous, less hard and weaker. Therefore minimum amount of water is needed to produce a homogenous workable mix.  Setting time It is the time elapsed from the beginning of mixing, until the material hardens.  Stages of Setting time Setting time is divided into various stages: setting time Mixing time Working time Initial setting time Final setting time  Stages of Setting time 1. Mixing time: Is the time taken from the addition of powder to the water until a homogenous mix is obtained. 1 minute for hand miing and 30 sec for mech mixing Mechanical mixing  2-Working time Is the time available for mixing and pouring the gypsum product into the impression (pouring a cast) before the initial setting. Taking 3 minutes from beginning of mixing  3-Initial setting time It is the time elapsed from the beginning of mixing until partial setting occurs. The material did not completely set. Weak. It takes 12 minutes to reach to Not flow. this stage Can carve away the excess material with a knife. Resist penetration of Vicate and Gillomore needles  4- Final setting time It is the time elapsed from the beginning of mixing until the reaction is completed. Strong and hard. Resist penetration of Gillomore needles (heavier and thinner than those used for measuring initial setting time). It takes several hours to reach to this stage  Ready to use stage It is the time at which the set material may be safely handled without fear of fracture. After 30 minutes.  Measuring of setting time 1. Loss of gloss. 2. Temperature rise. 3. Penetration tests: Resist of the set gypsum material for penetration by certain needles, which have a specific Weight and definite tip diameter. a- Vicate test measure the initial setting time only. b- Gillmore test measure initial and final setting time. Factor controlling the setting time Three mechanisms for controlling the setting time: 1. Solubility of hemihydrate. 2. Nuclei of crystallization. 3. The rate of crystal growth.  Factor controlling the setting time 1-Factors controlled by manufacture 2-Factors controlled by the operator  1-Factors controlled by manufacture: 1. Fineness of the powder: Smaller particles of hemihydrate the faster the hemihydrate dissolution which accelerates the setting time or decreases the setting time. 2. impurities: A small amount of impurities ( calcium sulfate dihydrate) will accelerate the setting time by increasing the nuclei of crystallization. Give a reason why gypsum product powder should be protected from moisture contamination during storage. 3. Chemicals (retardation or acceleration): a- Retarders (increase the setting time): Form a coating that decreases the solubility of hemihydrate. e.g. borax b-Accelerators (decrease the setting time): -Increase the solubility of hemihydrate. -Adding more nuclei of crystallization. e.g potassium sulphate K₂SO₄  2- Factor controlled by the operator: 1- Water/ powder ratio: W/P ratio (thin mix) retardation W/P ratio (thick mix) acceleration 2- mixing (spatulation) time and rate: mixing time and rate accelerate the setting time by breaking up some of the already formed dehydrated provide more nuclei od crystallization 3-Temperature: The rate of chemical reaction can be changed by changing the temperature: From 20°C to 50°C , acceleration of setting time. At 50°C, no acceleration. Above 50°C, retardation of the setting reaction. At 100°C, no setting occurs because the solubility of hemihydrate= the solubility of dihydrate.  Properties of gypsum products 1- Dimensional expansion: ( Give reason) Setting expansion due to outward thrusting action of the growing crystals from the nuclei of crystallization (the crystals push each other apart when they are growing). There are a lots of microspores containing excess water require for mixing.  Outward Thrusting Action Expansion of model plaster Expansion of improved stone  Setting expansion Gypsum product linear expansion porosity Plaster of Paris 0.2 % 45% Dental stone 0.08% 15% High strength stone 0.05% 10% Factors controlling setting expansion 1-W/P ratio: W/P ratio setting expansion. More space between the growing crystal , the less crystal growing and the less thrusting. 2- mixing time and rate: mixing time and rate setting expansion Increase the formation of the nuclei of crystallization so increase the thrusting action. Factors controlling setting expansion 3- Chemicals “retarders and accelerators”: Chemicals setting expansion. By regulating the shape of the crystals so decrease the thrusting action. Factors deceasing the setting expansions Chemicals Setting expansion w/p ratio Setting expansion Mixing time and rate Setting expansion  Hygroscopic expansion It is expansion of gypsum Occurs when gypsum is allowed to set under water. It is double the setting expansion. Water provide additional space for crystal to grow. Hygroscopic expansion can be controlled by: Immersion time Hygroscopic expansion Amount of water added Hygroscopic expansion W/P ratio of the original mix Hygroscopic expansion 2- Strength: Gypsum is a brittle materials. The strength increase rapidly as the materials hardens. Excess water of the final product affect the strength. Two types of strength can be recognized: 1- wet strength congaing excess water 2- dry strength “7day strength”. Dried and lost the excess water, is double the wet strength. Factors affecting the strength: 1) Type of gypsum products. 2) Chemicals decrease the strength. 3) Dryness. 4) W/P ratio strength…..excess water. 5) Mixing time strength but over mixing decease the strength. 6) Addition of the wetting agent strength. 3- surface hardness and abrasion resistance Have low hardness. Addition of resin improve the hardness and abrasion resistance.  Uses 1. Impression plaster “Type I”: Impression materials 2- Model plaster “Type II”: Primary cast. Fill the flask in denture construction. 3- Dental stone Type III: Secondary cast 4- Improved stone “Type IV”: Construction of harder and stranger models. 5- High strength high expansion “Type V”: When high expansion die is needed.  Advantages 1-Dimensional accuracy. 2-Pigmented and good color contrast. 3-Inexpensive and easy to use.  Disadvantages Mechanical properties are not ideal ,low strength and abrasion resistance Some impression materials causes slight softening of the stone surface Poor surface detail reproduction.  Recent advances impregnation by polymer Hardness Reduce the Incorporation of wetting required water agent strength  Give reason: 1. Setting expansion of gypsum. 2. gypsum products powder should be protected from moisture contamination during storage. 3. Excess water is needed for gypsum mixing. 4. Different type of gypsum. 5. Addition of resin and wetting agent in gypsum products. 6. Addition of dihydrate to gypsum products. Gypsum is subjected to a process called …………to produce gypsum products. Gypsum product is mixed with ……….and the setting reaction is called…………..