Model and Die Materials PDF

Mission :”. Provide learner-focused dental education, cutting-edge scientific research, patient-centered care, and community engagement all within positive organizational culture”. : ‫الرسالة‬ ‫عل المتعلم والبحث العلمر المتطور روالرعاية الصحية‬ ‫وفير ت...

Mission :”. Provide learner-focused dental education, cutting-edge scientific research, patient-centered care, and community engagement all within positive organizational culture”. : ‫الرسالة‬ ‫عل المتعلم والبحث العلمر المتطور روالرعاية الصحية‬ ‫وفير تعليم طب االسنان الذي يرتكز ر‬ ‫كل ذلك فر اطار ثقافة تنظيمية إيجابية‬, ‫عل المريض والمشاركة المجتمعية‬ ‫المرتكزة ر‬. Core Values Leadership. Excellence. Innovation. Collaboration. Respect. Integrity :‫القيم الجوهرية‬ ‫ ز ز‬. ‫االحتام‬ ‫التاهة‬ ‫ ر‬. ‫التعاون‬. ‫االبتكار‬. ‫التفوق‬. ‫القيادة‬ Model & Die Materials MODEL & DIE MATERIALS MODEL AND DIE MATERIALS Cast: a positive replica of teeth and/or the associated supporting soft and hard tissues of the jaw prepared from an impression. MODEL AND DIE MATERIALS Die: is a model of a single tooth,also prepared from an impression.  Edentulous Orthodontic Working Cast Model Cast Working Cast with Removabl Removable e Dies Die with Waxed Inlay  MANIPULATION of Gypsum Products Proportion P and L Microstone Bulk P Pre-packaged P Transfer to impression  Requirements for model and die materials: 1- Mechanical properties: Should have high strength to resist breakage during use. Should be hard to resist scratching during use. Requirements for model and die materials: 2- It should be able to reproduce fine details of the impression.  Requirements for model and die materials: 3- It should have little dimensional change on setting, and should remain dimensionally stable during storage. Requirements for model and die materials: 4- Compatibility with impression materials. There should be no interaction between the surface of the impression and the model or die  Requirements for model and die materials: 5- Good color contrast Ease of use Cheap  TYPES OF MODEL AND DIE MATERIALS 1. Gypsum products.  TYPES OF MODEL AND DIE MATERIALS 2. Alternatives to gypsum products:- Silicophosphate cement, amalgam, polymers and filled polymers, metal sprayed dies, electroplated dies, and ceramic dies.  GYPSUM = calcium sulfate = naturally occuring as dihydrate Heat removes water and converts dihydrate to hemihydrate. [Gypsum Powder] + [H2O] ← → [Gypsum] + [Heat] Calcium Sulfate Hemihydrate Calcium Sulfate Dihydrate  Effect of heat a≠b≠c α=β= 90° ≠ γ CaSO4. 2H2O 110 -130°C Monoclinic dihydrate CaSO4. ½ H2O a=b=c 130 -200°C α=β=γ≠90° Rhombohedral hemihydrate CaSO4 + H2O 200°C - 1000°C a=b≠c Hexagonal soluble anhydrate α=β=90°γ = 120 CaSO4 1200°C Orthorhombic natural anhydrite Ca + S + O a≠b≠c α=β=γ=90° Types of gypsum products According to ADA specification No. 25, pure gypsum products, used in dentistry, are classified as 1. Plaster 2. Dental stone 3. High-strength stone Types of gypsum products All forms of gypsum products are chemically the same → calcium sulphate hemihydrate “CaSO4. ½ H2O” However, gypsum products differ in each other in: 1-The method of manufacturing. 2-This leads to difference in particle size, shape, and form However, gypsum products differ in each other in: 3-This further leads to different W/P ratio 4-Physical properties 5-Their use in dentistry Manufacturing of Different Types of Gypsum Products All the gypsum products are made from gypsum mineral (CaSO4.2H2O), The main difference being in the manner of driving off (eliminating) water of crystallization from the dihydrate. Manufacturing of Different Types of Gypsum Products Plaster An open vessel at 120°C Dental stone Autoclave under steam pressure at 120°C High strength stone Boiling in 30% CaCl2solution All have the same chemical formula of CaSO4.1/2 H2O  PLASTER STONE Improved STONE Chemical Name: β-calcium sulfate α-calcium sulfate α-calcium sulfate Formula: CaSO4-(1/2)H2O CaSO4-(1/2)H2O CaSO4-(1/2)H2O Powder Shape Irregular Uniform Uniform Density Porous Moderately Dense Dense Production Heat to 120°C in Heat to 125°C with Heat to 100°C Steps: air steam pressure in CaCl2 solution : : Dental Products: Plaster Stone Improved stone Common names Plaster of Paris Hydrocal Densite Plaster Dental stone Improved stone Particle Size and Shape of Gypsum Products Plaster of Paris larger in size & irregular shape & porous 45% Particle Size and Shape of Gypsum Product Dental stone intermediate size & more regular in shape & less porous 15 % **i.e. dense Particle Size and Shape of Gypsum Product High strength stone smallest in size & most regular in shape, prismatic in shape”,and the least porous (10%) i.e. densest Setting Reaction of Gypsum It is the reverse of manufacturing process: ❏Calcium sulfate hemihydrate + Water ⇒ Calcium sulfate dihydrate + Heat ❏(CaSO4) 2. H2O + 3 H2O ⇒ 2 CaSO4. 2H2O + Heat Mechanism of Setting Crystalline. theory Depending on the difference in the solubility of calcium sulphate dihydrate and hemihydrate causes the setting of these materials. Mechanism of Setting Crystalline theory On mixing hemihydrate with water the following will occur: I- Some of the hemihydrate dissolves in water, giving Ca++ and SO4- ions, which in turn forms the dihydrate in the solution. These are considered nuclei of crystallization. *** The solubility of the hemihydrate in water is much higher than the formed dihydrate. Mechanism of Setting Crystalline theory On mixing hemihydrate with water the following will occur: II- Concentration of the dihydrate increases rapidly to render the solution super saturated with dihydrate. Mechanism of Setting Crystalline theory On mixing hemihydrate with water the following will occur III- More dihydrate will be precipitated around the nuclei of crystallization, leading to crystal growth. until all the hemihydrate is transformed into dihydrate. Water/Powder Ratio (W/P) Theoretically: (W/P) for all types is: 100-g of hemihydrate requires 18.6 ml water to give complete hydration This water is water of crystallization or bonded water. Water/Powder Ratio (W/P) However hemihydrates will not produces a smooth workable mix, when mixed with this water ratio. Therefore The actual ratios used :- Plaster: 50-60 ml/100 g. Stone: 30 ml/ 100 g. Improved stone: 22-24 ml / 100 g. NB: This excess water will be present in the final product as free water. It will evaporate leaving the set material porous. It may take 7 days to lose the excess water. Setting time Time elapsed from the beginning of mixing, until hardening occurs. Setting time *** Divided into: 1) Mixing time: From addition of powder to the water until a homogeneous mix (not friable) Setting time 2)Working time (3 minutes): Time available for mixing, and its use before initial setting. Setting time 3)Initial setting time (12 minutes): Beginning of the mixing until partial setting occurs. During this stage, the material will not flow, it is rigid but not hard. **It is possible to carve away the excess material. Setting time 4) Final setting time (Several hours): Beginning of mixing until complete setting takes place. The model or die will be strong and hard.  Measuring Setting time: 1- Loss of gloss: - To judge proper working time. - It indicates partial setting 2- Temperature rise: Since the reaction is exothermic, it indicates setting.  Measuring Setting time: 3- Penetration tests: depend on resistance of the set gypsum material for penetration by needles, which have a specific weight and definite tip diameter.  Measuring Setting time: 3- Penetration tests: *Vicat needle for measuring initial setting time *Gillmore needles for measuring initial and final setting times  Control of the Setting Time *** According to the crystalline theory → 1- The solubility of the hemihydrate 2- The number of nuclei of crystallization 3- The rate of crystal growth Control of the Setting Time A) Operator: B) Manufacturer: 1- W/P ratio 1- Fineness of the 2- Mixing time and powder rate 2- Impurities 3- Temperature 3- Chemicals “Retarders and accelerators”  Factors controlled by the Operator 1) W/P ratio: High W / P = thin mix → ** less nuclei of crystallization ** few CaSO4.2H2O crystals will be formed ** slow growth rate Retardation i.e. longer S.T.  Factors controlled by the Operator 2) Mixing time and rate: Increasing mixing time and rate → ** more nuclei of crystallization in a given volume ** more growing CaSO4.2H2O crystals ** rapid growth rate  Factors controlled by the Operator 3) Temperature: ** 20-50 °C Acceleration of setting time. (heat accelerate any chemical reaction) ** Above 50 °C Retardation of the setting time ** At 100 °C No reaction takes place (solubility of hemihydrate = the solubility of dehydrate)  Factors controlled by the Manufacturer: 1) Fineness of the powder: Smaller particles → ** more wetting ** faster rate of dissolution of the hemihydrate ** increases number of nuclei of crystallization ** increases growing CaSO4.2H2O Acceleration or shorter setting time will be obtained.  Factors controlled by the Manufacturer: 2) Impurities: Small amount of Terra Alba (as nucleating agents) (Calcium sulphate dihydrate 0.5-1% ) ** increase the number of nuclei of crystallization ** more rapid growth rate Acceleration (decrease) the setting time  Factors controlled by the Manufacturer: 3) Chemicals “Retarders and accelerators”: a) Accelerators (e.g. 2% K2SO4): ** faster rate of dissolution of the hemihydrate ** increases number of nuclei of crystallization ** increases growing CaSO4.2H2O Acceleration or shorter setting time will be obtained.  Factors controlled by the Manufacturer: 3) Chemicals “Retarders and accelerators”: b) Retarders (Borax, blood, saliva, or hydrocolloid): 1. By coating hemihydrate particles thus reducing their solubility. Or 2. By coating the growing crystals thus inhibiting the growth. → ** Slow growth rate Retard the S.T. i.e. longer S.T. or increased S.T. Give Reason: The impression should be washed before pouring the model ???  Properties of gypsum products I. Dimensional changes: A) By calculations, a volumetric contraction should occur during the setting reaction (7%).  Properties of gypsum products B) Setting expansion is actually observed ????? *** on the basis of crystalline theory due to the outward thrusting action of the growing crystals  Properties of gypsum products *** Give Reason: The set gypsum material is porous. Because the set gypsum is greater in external volume, than its crystalline volume The final structure is composed of interlocking dihydrate crystals between which pores containing the excess water.  Factors affecting the setting expansion: i.W/P ratio: High W / P = thin mix → **less nuclei of crystallization **few CaSO4.2H2O crystals will be formed in giv volume ** their outward growing thrust is decreas ** less setting expansion is reduced or decrease Factors affecting the setting expansion: ii.Mixing time and rate: Increasing mixing time and rate → ** more nuclei of crystallization in a given volume ** more growing CaSO4.2H2O crystals in this volume ** increasing the outward thrust of the growing crystals ** increased setting expansion (S.E.) Factors affecting the setting expansion: iii.Chemicals: Chemicals regulate the shape of the growing crystals thus decreasing their thrusting action, thus decreasing the setting expansion  C. Hygroscopic expansion: -It is the expansion of gypsum when it is allowed to set under water during initial stage of setting. This additional water provides more room for crystal growth. C. Hygroscopic expansion: i.e. Hygroscopic expansion is a physical reaction. -Hygroscopic expansion may be more than double the normal setting expansion in air. II. Strength: The strength of the gypsum products increases rapidly, as the material hardens after the initial setting time. Two types of strength of gypsum products: 1) Wet strength (green strength or 1 hour strength): Strength of the set of gypsum containing the excess water II. Strength: 2) Dry strength: Strength obtained after the gypsum has been dried and lost the excess water. ** Dry strength is usually double the wet strength.  Factors affecting strength: 1.W/P ratio: Higher the W/P ratio → more excess water will remain, eventually vaporize leaving more pores and weaker product.  Factors affecting strength: 2.Mixing time and rate: Increasing the mixing time and rate within limits will provide more nuclei of crystallization in a given volume.  Factors affecting strength: 2.Mixing time and rate: more growing calcium sulphate dihydrate crystals will be present in this volume, increasing the crystalline interlocking,strength will be increased  Factors affecting strength: 2.Mixing time and rate: *** But over-mixing results in decreasing the strength, because the formed gypsum crystals will be broken up and less crystalline interlocking will be obtained.  Factors affecting strength: 3.Chemicals: The chemicals regulate the shape of the growing CaSO4.2H2O crystals thus reducing the intercrystalline cohesion and decreasing the strength.  Factors affecting strength: 4.Type of gypsum products: Strength of Improved stone > Strength of dental stone > Strength of model plaster. N.B; Improved stone can be weak as model plaster if it is mixed with excess water than the required W/P ratio. 5.Dryness: Dry strength is higher than the wet strength  Factors affecting strength: 6. Addition of wetting agent (lignosulfonate) allows the use of less water therefore producing less porosity and resulting in increased strength III. Surface hardness and abrasion resistance:: -Generally, the surface hardness is low, so the material is easily susceptible to scratching -Addition of resin [polymer] to the surface of the set gypsum will increase hardness and abrasion resistance. IV. Reproduction of detail: They do not reproduce surface details accurately *** Because: ** Surface of the set gypsum is porous on microscopic level ** Air bubbles are formed at the interface of certain impressions and gypsum cast.