MODEL AND DIE MATERIAL

Definition of model or cast It a positive replica of the teeth and adjacent structures. Definition of die It is a positive replica of the individual prepared tooth on which the inlays and crowns are made.

Requirements for model and die materials 1. Should have high strength to resist breakage  during use. 2. Should be hard to resist scratching during use. 3. It should be able to reproduce fine details of the impression. 4. It should have little dimensional change on  setting and should remain dimensionally stable during storage.

5.Compatibility with impression materials.  There should be no interaction between the surface of impression and the model and die. 6. Good color contrast with other  materials being used. 7. Ease of use and cheap.

Ideal requirements of die system 1. Easy to remove and replace in its original position. 2. Stable when placed in the cast. 3. Easy to mount in the articulator.

Types of model & die materials Metallic dies Non-metallic casts Electroplated Dies Copper plated Silver plated Amalgam used die material Low fusing metal die Gypsum Type I-Impression plaster Type II-Model plaster Type III-Hard stone Type IV-Extra hard stone Type V –Extra hard, high expansions stone Resin Epoxy Resin Polyurathane Flexible die materials Ceramic dies Cement dies Types of model & die materials

Gypsum products Most gypsum products are obtained from natural gypsum rock. Because gypsum is the dihydrate form of calcium sulfate (CaSO4 · 2H2O), on heating, it loses1.5 g mol of its 2 g mol of H2O and is converted to calcium sulfate hemihydrate (CaSO4· ½H2O). When calcium sulfate hemihydrate is mixed with water, the reverse reaction takes place, and the calcium sulfate hemihydrate is converted back to calcium sulfate dihydrate.

Gypsum products are chemically the same (CaSO4 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.

Extra hard stone IV, V Hard stone III Model Plaster I, II Difference CaSO4 · 2H2O Source Boiling of CaSO4 · 2H2O in 30% CaCl2 Autoclaving of CaSO4 · 2H2O At 125 ᵒC Heating of CaSO4 · 2H2O in open air At 115 ᵒC Manufacturing CaSO4 · ½H2O Most regular Least porous Most homogenous α-CaSO4 · ½H2O More regular Less porous More homogenous β-CaSO4 · ½H2O Irregular Porous Non homogenous Particles 100 gm ̸25-30 mL 100 gm ̸35-40 mL 100 gm ̸50-60 mL P/L ratio The strongest More strong The weakest Strength The hardest More hard The least hard Surface hardness 15 min 15-45 min Setting time Working cast for crown, bridge & metallic denture base Secondary cast In flasking Primary cast Mounting of cast Uses

high-strength stone die

Setting reaction When hemihydrate is mixed with water, the reaction is reversed and the dihydrate is formed. The reaction is exothermic and depends on: 1. Solubility differences Where hemihydrates are more soluble in water than the formed dihydrates.

2. Nuclei of crystallization Where the reaction proceeds leading to precipitation of excess dehydrates in the form of nuclei of crystallization. 3. Crystal growth During which more dihydrates will be precipitated around the nuclei of crystallization leading to crystal growth. The reaction continues till all the hemihydrates are transformed into dihydrates.

Factors affecting setting time W/P ratio Increasing water /powder ratio, decreases nuclei of crystallization and retards the setting time. Spatulation Rapid and prolonged spatulation accelerates setting and also increases setting expansion by breaking up some of dihydrate which increases the nuclei of crystallization. Temperature Increasing water temperature to a certain level will accelerate setting.

Impurities Impurities will accelerate the setting time by providing nuclei of crystallization. Fineness The finer the particle size, the faster the mix will set by increasing nuclei of crystallization. Accelerators and retarders Accelerators Such as potassium sulfate or set gypsum where they act as sites for crystallization and increasing the rate of solubility of hemihydrates. Retarders Such as blood, saliva, alginate. Borax as a retarder forms a coat around the hemihydrate particles and this decreases the rate of solubility or around dihydrates (nucleating sites).

Manipulation Water is dispensed into a mixing bowl. The powder of gypsum products is added and mixed with water to obtain a smooth mix. Spatulation by hand using a spatula with a stiff blade with the bowl on a vibrator or a power- driven mechanical spatulator.

Metallic materials: Electroplated dies Electroplated dies are made by electrodeposition of metal ions into the fitting surface of an impression material. It produces a die with a very hard and smooth surface registration of fine details

Copper electroplating

Silver-plated die Copper-plated die Difference Polysulfide imp. Silicone imp. Polyether imp. Compound imp. Cathode Silver bar Copper bar Anode Silver cyanide Potassium cyanide Potassium carbonate water Copper sulfate Sulfuric acid Sulphonic acid Electrolytic solution 5-10 amp 15-35 amp Electric current 12-15 hours 10-15 hours Time required

Dental waxes

Classification of dental waxes According to their use and application 1. pattern waxes Inlay Casting Baseplate 2. Processing waxes Boxing Utility Sticky 3.Impression waxes Corrective Bite Bite wax

Inlay pattern wax

Casting wax

Baseplate wax

Boxing wax or carding wax

Utility wax

Sticky wax

Write short notes on Requirements and types of model and die materials. Differences between types of gypsum products.