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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 impression3 )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 productsa) 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.
b) Final setting time
It is the time since the beginning of mixing till the ( 1-pound wt and 1/24 inch diameter) Gillmore needle does not penetrate its surface
Measuring the setting time
1 )Loss of the surface gloss: indicates the initial setting time. Excess water on the surface is absorbed into the mass and at this time the surface does not reflect light and appears dull.
2 )Penetration : the victa needle is used to measure the initial setting time. It consist of a rod weighing 300gm with a needle of 1mm diameter. The Gillmore needle has two weights and diameters. The initial setting time is measured by the rod weighting ¼ pound and the needle1/12 inch in diameter, while the final setting time is measured by the
rod weighing 1 pound an the needle 1/24 inch in diameter .
Factors affecting the setting time
a) Factors controlled by the operator:
1 -water powder ratio: the higher the w/p used for mixing, the less the nuclei of crystallization will be per unit volume. Consequently the setting time will be prolonged.
2 )Mixing time and rate: an increase in the mixing time and/or rate accelerates the setting time because mixing can break up some of the formed dihydrate crystals, thus forming more nuclei of crystallization.
3 )Water temperature: the temperature of the water used for mixing, as well as the temperature of the enviroment, have an effect on the setting time of gypsum materials. The longest setting time at 0ºC, however it cold be accelerated by the increase of temperature up to 50ºC. From 50ºC to 80ºC the setting time is more or less constant. At higher temperatures retardation occurs and at 100ºC there is no setting (same rate of dissolution of hemihydrate and dihydrate)
b) Factors controlled by the manufacture
a) Finenesses of the powder: grinding of the manufactured hemihydrate increases the surface area of the hemihydrate exposed to water, thus, increasing the rate of solution of the hemihydrate. This accelerates the setting.
b) Impurities in the powder
Gypsum initially present as an impurity will accelerate the setting by providing more nuclei crystallization.
c) Addition of accelerators and retarders: nearly all acids and salts when added to amix of plaster or stone, affect the setting. Potassium sulphate is an accelerator as it increases the rate of dissolution of CaSO4 1/2H2O. Borax is a retarder because the it reduces the rate of dissolution of CaSO4 1/2H2O. It is adsorbed on the nuclei of crystallization thus poisoning these nuclei
making them ineffective .
2 )Expansion
a) Setting expansion:Theoretically, by mathematical calculations, the volume of the final dihydrate should be about 7% less than the total volume of the hemihydrate plus the water necessary for the reaction. However, actually gypsum products develop a linear expansion during hardening ,i.e. linear expansion is observed to accompany the change from the hemihydrate to the dihydrate. This linear expansion is caused by “the outward thrust of the
growing crystals of the dihydrate. ”
The linear expansion of gypsum products during setting can be reduced by increasing the amount of water in the mix, or, by adding certain salts that regulate the shape of the crystals formed during setting e.g. K2SO4, NaCl . The increase in the mixing time and rate will lead to an increase in the setting expansion because more nuclei of crystallization will be formed that will interfere with growing leading to the
outward thrust .
b) Hygroscopic Expansion
If mixed plaster is placed under water at the initial set stage a greater expansion occurs; this is known as hygroscopic expansion. It is about double the normal setting expansion. Therefore, immersion of plaster or stone casts in water during setting should be avoided because of the hygroscopic expansion (disadvantage). However it is used to expand gypsum bonded investment materials to compensate for the casting shrinkage (advantage)
3 )Strength
The strength of gypsum products depends not only on 1) the type of the product 2) dryness off the set material, but also on 3) the water/powder ratio.
Gypsum products are brittle materials i.e. they have high compressive strength and low tensile strength.
The products have two strengths:
a) Wet strength: it is the strength of gypsum products when excess water is still present in the specimen.
b) Dry strength : it is the strength of gypsum products when excess water evaporates.
-The dry compressive strength is usually double the wet strength.
-The dry strength is reached at normal room conditions after 7 days .
Manipulation
a) Storage : closed containers to prevent reaction with moisture from the atmosphere which can cause formation of the dihydrate that can accelerate the setting.
b) Correct water/powder ratio should be used. If stone is mixed with too much water, the set material may be as weak as plaster.
c) Incorporation of the air in the mix should be avoided . The powder is to be sifted on to the water in the rubber bowel and allowed to stay into the water for 30 seconds. This is technique minimizes the amount of the air incorporation into the mix during spatulation. To obtain a smooth mix, the powder and water should be mixed slightly with a spatula having a stiff blade then vigorously and at the same time wiping the inside surfaces of the bowel with the spatula to be sure that all powder is wet and mixed uniformly with water.
d) Vibration helps the mix to flow well into impression and helps to eliminate air bubbles (increasing strength).
N.B: Mechanical mixers under vacuum are available and produce gypsum with superior properties.
Recent developments
Because the mechanical properties are not ideal, fracture of teeth from models can occur with careless handling.
Two techniques are tried to produce dental stones with improvement in abrasion resistance and sterngth.
1 )Impregnation of the gypsum by a polymer such as polyester, polysterne, acrylics and epoxy resin.
2 )Addition of wetting agent such as lignosulfonates. Such additives reduce the water/powder ratio and enable the production of a harder, stronger and more dense set gypsum.
