Dental Cast and Dies PDF

Cast and Die Cast and Die 1  Cast and Die Definition of cast & die Cast: It is a positive replica of the teeth and adjacent structures used primarily for observation and diagnosis. Die: It is a positive repl...

Cast and Die Cast and Die 1  Cast and Die Definition of cast & die Cast: It is a positive replica of the teeth and adjacent structures used primarily for observation and diagnosis. Die: It is a positive replica of the individual prepared tooth on which the inlays and crowns are made. Gypsum Metals Polymers Cements Ceramics - Type II dental plaster -Electroformed/Electroplated. -Acrylic -Silicophosphate ________ - Type III dental stone - Low fusing metals. - Epoxy cement -Type IV dental stone, - Amalgam. high strength -Type V dental stone, high strength, high expansion Ideal properties of model and die materials 1. Dimensionally accurate. 2. High abrasion resistance to resist scratching during carving of wax pattern. 3. Good strength and smooth surface. 4. Ability to reproduce all fine details in the impression. 5. Compatibility with all impression materials. 6. Color contrast with wax pattern for easy distinguishment. 7. Easy & quick manipulation and rapid fabrication. 8. Non-injurious to health by touch or inhalation. 9. Economics. 10. Reasonable setting time. 2  Cast and Die Gypsum products ✓ Gypsum is a mineral naturally found in nature. ✓ The mineral gypsum is a dihydrate (2 molecules of H2O) of calcium sulfate known as ALABASTER. ✓ When it is ground and heated up (to change it into a powder), it loses water and becomes hemihydrates of calcium sulfate. Dental applications o Impression materials. o Constructing casts and dies. o Making gypsum bonded investment when mixed with filler as silica. Production of gypsum products o Gypsum products are produced by Calcination process where: o In which Ca sulfate dihydrate is heated resulting in water loss producing Ca sulfate hemihydrate o Gypsum products are chemically the same (CaSO4. 1 /2 H2O) but they differ in: 1. Method of manufacturing. 2. Particle size and shape. 3. Water/powder ratio. 4. Physical and mechanical properties. 5. Uses. Manufacturing Heating in open air at 120°C gives powder particles which are large , porous and irregular (ß-hemihydrate, plaster). Heating at 120°-130°in an autoclave under steam pressure giving powder particles which are smaller, more regular and less porous (α-hemihydrate or dental stone). Heating gypsum in boiling 30% solution of CaCl2 or MgCl2 results in smallest, most denser stone, high-strength or die stone. 3  Cast and Die Products It is used as a final impression for edentulous patient. Impression plaster (Type I) Rarely used because it is rigid. It is used for making diagnostic cast Articulation of stone cast. Model plaster (Type II) Flasking procedure for acrylic dentures (cast end). The mix produces a weak cast compared to dental stone. The W/P ratio is 0.5 (50 ml water to 100g powder). It is used for making: ✓ Full or partial denture master casts. ✓ Orthodontic models. Dental stone (Hydrocal) ✓ Flasking procedure for acrylic dentures (teeth end). (Type III) It requires less water. It is stronger than plaster. The W/P ratio is 0.35(35ml water to 100g powder). Die stone is used in Master Casts for fabricating wax patterns of cast Dental stone, high restorations (crown &bridge). strength (Densite) (Type IV) The W/P ratio is 0.22(22 ml water to 100g powder). It is almost 2 times stronger than type III stone. Used for construction of dies on which base metal alloys will be High strength, high casted. expansion dental stone Lowest W/P ratio and higher compressive strength. (Type V) The need for higher expansion (0.1%) was to compensate for the solidification shrinkage of some alloys used as base metals used for 4  Cast and Die Comparison between different gypsum products‫كتاب‬ Type W/P Porosity Compressive Abrasion Setting Particle size ratio strength resistance expansion and shape 0.50 Low High Large, Plaster High Low irregular 0.35 Moderate Moderate Moderate Moderate Smaller, Stone regular more High strength 0.22 Low High High Low Smallest, stone most regular High strength 0.20 Low High High High Smallest, High expansion most regular stone Setting reaction When hemihydrate is mixed with water, the reaction is reversed and the dihydrate is formed by crystallization reaction. The reaction is called crystallization reaction. The reaction is an exothermic reaction. Steps of the reaction: 1. Solubility Hemihydrate dissolve in water giving Ca &SO4 ions which react with water forming CaSO4 dihydrate. (Solubility difference) (Hemihydrate is water soluble while dihydrate is water insoluble.) 2. Precipitation As reaction proceed, conc. Of dihydrate increase and start to precipitate in the form of Nuclei Of Crystallization 3. Crystal growth Continuous precipitation around nuclei of crystallization lead to Growth. This growth continues until all hemihydrate is transformed into 4. Crystal dihydrate Crystals continue to grow then they intermesh and interlock and the intermeshing material changes from soft to hard consistency. 5  Cast and Die Dimensional -Setting expansion : Results from crystal growth caused by outward thrusting of changes growing crystals of dihydrates during setting. -Hygroscopic expansion : If gypsum is soaked in water during setting, water fills the pores leading to additional crystal growth. It is double the setting expansion -Working time: it is the time available to use a workable mix with even consistency.(3 min) Setting Time -Initial setting time: it is the time from the beginning of mixing until patrial setting occurs.(loss of gloss)(12 min) -Final setting time: it is time elapsed from mixing until complete setting take place (reaction is completed). -Final setting takes several hours however it may be safe to handle material after (30 min to) 1. W/P ratio : Increasing w/p ratio (watery mix), decreases nuclei of crystallization and retards the setting time. ( decrease setting expansion) 2. Spatulation :Rapid and prolonged spatulation accelerates setting time and setting expansion by breaking up some of dihydrate which increases the nuclei of crystallization. 3. Temperature : Increasing water temperature to a certain level will accelerate setting. Factors affecting 4. Impurities :Impurities will accelerate the setting time by providing nuclei of Setting Time crystallization. 5. Fineness : The finer the particle size, the faster the mix will set by increasing nuclei of crystallization 6. Accelerators and retarders : Accelerators: Such as potassium sulfate or set gypsum ✓Act as sites for crystallization or Increasing the rate of solubility of hemihydrates. Retarders: Such as Borax, blood, saliva, alginate. ✓Act as a retarder by forming a coat around the hemihydrate particles. (decreases the rate of solubility.) 6  Cast and Die Manipulation 1. Powder is sifted into water in the mixing bowl to ensure good wetting and avoid air bubbles. 2. Metal spatula is used for hand mixing for 1 minute, against the wall of the rubber bowl. 3. Mechanical mixing can be done under vacuum to reduce the entrapped air. 4. Vibration is done immediately after mixing and during pouring minimizes air bubbles in the set mass. 5. The mix is made to flow on the surface of the impression. 6. The gypsum material is allowed to set for 45 to 60 minutes. 7. After separation, the model should be disinfected. Advantages Disadvantages 1. Inexpensive and easy to use. 1. The mechanical properties are not ideal (low abrasion 2. The accuracy and dimensional resistance). stability are good. 2. The brittle nature of gypsum occasionally leads to fracture. 3. Reproduction of fine detail from 3. Problems when gypsum model and die materials are used with the impression. alginate impression. The surface of the model may remain relatively soft due to an apparent retarding effect which hydrocolloids have on the setting of gypsum products. I. Electroplated dies Electroplating involves the deposition of a coat of pure silver or copper on the impression. The impression is placed in an electroplating bath. A layer of pure metal is deposited on the impression and is supported with type IV stone or resin. The apparatus is composed of anode (metal bar), cathode (impression), source of direct current and a bath of electrolytic solution. Preparation of the impression ‫كتاب‬ o Areas of the impression to be electroplated should be covered with metallic powder to make it conductor for electricity. 7  Cast and Die o Unwanted areas and the tray should be coated with wax to avoid precipitation of metal over them. Electroplating mechanism‫كتاب‬ o When the current is passed, it causes dissolution of the electrolyte into +ve ions which are directed towards the cathode and precipitates on the impression, and –ve ions which are directed to the anode and react with it to form a new salt that dissolve again in the electrolytic solution. o This cycle is repeated until a layer of metal is formed over the impression surface. o Then, dental stone is poured into the plated impression and when it sets, it is removed from the impression. Different electroplated dies Difference Copper plated die Silver plated die Cathode Cmpound or silicone Polysulfide or silicone Anode Copper bar Silver bar Impression surface coating Copper powder or graphite Silver powder Electrolytic solution Acidic solution of copper sulfate Alkaline solution of silver cyanide Electric current 15-35mA 5-15mA Time required 12-15hrs 12-15hrs - Advantages ✓ Hard metallic smooth surface. ✓ Reproduce fine details. 8  Cast and Die - Disadvantages ✓ Expensive. ✓ Waste time and sensitive technique. II. Amalgam dies ✓ The material is similar in composition to amalgam filling materials (Ag-Sn alloy mixed with mercury to form plastic mass which can be condensed into impression). Advantages Disadvantages ✓ Its use is restricted to rigid impression material (compound) ✓ High surface hardness. to withstand forces of condensation. ✓ They have a long setting time (24 hrs. to reach complete setting). ✓ Reproduce fine details and sharp ✓ Separating medium is required margins. ✓ The material has good thermal conductivity which affects the wax pattern constructed on it. III. Low fusing metal dies These alloys have low melting temperature and can be melted and sprayed on the surface of the impression to give a metal shell that is filled with dental stone to form the body of the die. Bismuth-tin alloy is an example of a die material which melts at 138° C. IV. Acrylic resin dies Self-curing acrylic resin with fillers can be used for die construction. They are used with water free impression materials because water retards the polymerization reaction of the resin. Advantages Disadvantages ✓ More abrasion resistance than ✓ Polymerization shrinkage (fillers reduce this shrinkage). stone dies. 9  Cast and Die V. Epoxy die materials The material is a two component system 1. Paste containing the resin. 2. Liquid hardener or activator called polyamine. Setting occurs by polymerization reaction which is very slow and may take hours to reach the setting (1- 12h) (overnight setting). The liquid is toxic and should not contact the skin because it causes allergic reactions. They can be used with rubber impression materials. The materials cannot be used with agar or alginate impressions because the water on the impression retards the polymerization reaction. They are so vicious. So, occurrence of porosity is possible during impression pouring. Fast-setting epoxy resin (S.T. 30 min.) is supplied in the auto mixing system. The epoxy resin is in one cartridge and the catalyst in the other cartridge in an auto mixing system to minimize the percentage of porosity. Properties of epoxy dies ✓ The materials show more resistance to abrasion and are stronger than high strength dental stone. (adv) ✓ They do not reproduce the fine details as dental stone because of the high viscosity of epoxy resin paste. (disadv) ✓ They shrink by about 0.1% during hardening. (disadv) ✓ Expensive. (disadv) 10  Cast and Die VI. Ceramic dies A special type of ceramic is supplied as a powder and liquid that are mixed and fired according to the manufacture’s instruction (fired after the condensed porcelain is removed from the impression). Advantages Disadvantages ✓ Hard and strong die. ✓ The material shows shrinkage on firing. ✓ Expensive and sensitive fabrication technique. VII. Cement dies Silicophosphate cement may be used with all impression materials. It has vibrated into the impression and is ready for use within an hour. Advantages Disadvantages ✓ Harder than stone die. ✓ Brittle and fractured in thin sections. ✓ The material shows setting contraction. 11