Model and Die Materials Dr. Waseem Bahjat Mushtaha Specialized in prosthodontics

Definitions 1) A model or cast: is a replica of the hard or soft oral tissues or both. It is poured from an impression and is then used to construct an appliance, such as a full or a partial denture. 2) A die: is a model of a single tooth, generally used in the construction of crowns and bridges.

Desirable qualities 1) High strength, to reduce accidental breakage, and hard to resist scratching during carving of the wax pattern. 2) Produce fine detail of the impression 3) Should show little dimensional change on setting, and remain dimensionally stable. 4) Compatible with impression materials ( no interaction between the impression surface and the model surface) 5) Good color contrast with the impression material used. 6) Cheap and easily used.

Gypsum products a) chemistry of gypsum Gypsum is naturally occurring mineral, chemically it is calcium sulphate dihydrate, CaSO4. 2H2O. When the gypsum is heated to a temperature sufficiently high to drive off some of the water, it is converted into calcium sulphate hemihydrate, CaSO4. ½H2O, which some times written (CaSO4)2. H2O. The production of calcium sulphate hemihydrate can be undertaken in one of three ways, producing 3-different products with different properties and hence different applications.

It should be noted that the three products are chemically the same, they are calcium sulphate hemihydrate (CaSO4. ½H2O) but they differe in 1)the methode of manufacture 2) Particle size and shape 3) Water-powder ratio 4) The physical and mechanical properties of set material 5) Their use.

1) Plaster Calcium sulphate dihydrate is heated in an open vessel. Water is driven off, and the dihydrate is converted into hemihydrate known as calcined calcium sulphate or β-hemihydrate. The resultant material consists of large irregular porous particles. It needs to be mixed with a large amount of water to obtain a mix suitable for dental use, as much of the water is absorbed into the pores of the particles. The usual mix is 50% ml of water to 100 gm of powder.

2) Dental stone If the calcium sulphate dihydrate is heated in an autoclave (under steam pressure), the hemihydrate that is produced – known as α-hemihydrate or hydrocal- consist of small, more regular particles which are less porous. Due to its low porous and regular structure of the particles, it can be mixed using less water. The mix is 30ml water to 100 gm of powder.

3) Dentist (improved stone) If the dihydrate is boild in the presence of 30% calcium chloride or magnisium choloride, the hemihydrate particles that are produced are much more regular and less porous than those of the plaster and dental stone. The dentist is mixed in the ratio of 100gm of powder to 22ml of water.

b) The Setting process When calcium sulphate hemihydrate is mixed with water, a chemical reaction takes place, and the hemihydrate is converted back to calcium sulphate dihydrate. This chemical reaction, which is exothermic, is written as follows: CaSO4 ½H2O + 1½H2O CaSO4. 2H2O+Heat Plaster Water Gypsum

C. Mechanism of setting (Crystalline theory) On mixing hemihydrate with water the following are believed to occur: 1) Some calcium sulphate hemihydrate dissolve in water. 2) The dissolved hemihydrate reacts with water and forms calcium sulphate dihydrate. 3) The solubility of calcium sulphate dihydrate is very low (0.2%) i.e.stable. 4) This stable calcium sulphate dihydrate crystal precipitate. 5) As the stable calcium sulphate dihydrate precipitates out of the solution, more calcium sulphate hemihydrate is dissolved and this continues until all the hemihydrate has dissolved and changed into calcium sulphate dihydrate.

The following factors can be observed during the setting reaction a) The material becomes rigid, but not hard (initial set) at this stage can be carved but not shaped. b) The so-called (final set) follows, when the material becomes hard and strong. c) Heat is given out during setting, since the reaction is exothermic. d) Dimensional change also take place (setting expansion).

Properties of gypsum products 1- Setting time 2-Expansion 3-Strength

1- Setting time Def : it is time required for the reaction to be completed. Types : a) Initial setting time (working time): It is the time since the beginning of mixing until the Gillmore needle (¼pound wt and 1/12 inch diameter) does not penetrate its surface.

During the working time the material can be mixed and poured into the impression. Although the chemical reaction is initiated at the moment the powder is mixed with water, only a small portion of the hemihydrate is converted to gypsum at this early stage. As the reaction proceeds, more hemihydrate crystals react to form dihydrate crystals. The viscosity of the mass begins to increase rapidly, and the mass can not flow into the fine detail of the impression. At this point the material reaches the initial setting time and should no longer be manipulated.

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