After the choice of casting alloy has been made, the investment material can be selected.
Ideal Properties. An ideal investment should incorporate the following features:
Controllable expansion to compensate precisely for shrinkage of the cast alloy during cooling The ability to produce smooth castings with accurate surface reproduction without nodules Chemical stability at high casting temperatures Adequate strength to resist casting forces Sufficient porosity to allow for gas escape Easy recovery of the casting
Gypsum-bonded Investments. Gypsum-bonded investments satisfy most of the requirements for an ideal material, although they are not suitable for casting metal-ceramic alloys because the gypsum is unstable at the high temperatures required. Additionally, with some materials, obtaining adequate expansion may be difficult. This can be critical when casting complete crowns. A casting that is slightly oversized (in a controlled manner) is advantageous for accurate seating (see Chapter 7). Factors that increase expansion17 of gypsum-bonded investments include the following:
1. Use of a full-width ring liner
2. Prolonged spatulation
3. Storage at 100% humidity
4. Lower water-powder ratio
5. Use of a dry liner
6. Use of two ring liners
7. Hygroscopic technique with the pattern in the upper part of the ring28
Phosphate-bonded Investments. Phosphate-bonded investment materials offer certain advantages over gypsum-bonded investments. They are more stable at high temperatures and thus are the material of choice for casting metal-ceramic alloys. They expand rapidly at the temperatures used for casting alloys, and their size can be conveniently controlled. The increased expansion that they exhibit results from a combination of the following factors: 1. Heat from the setting reaction softens the wax and allows freer setting expansion. The increased strength of the material at high temperatures restricts shrinkage of the alloy as it cools.
3. The powder mixed with colloidal silica reduces the surface roughness of the castings and also increases expansion. Thus expansion can be conveniently controlled by slightly diluting the colloidal silica with distilled water. However, castings made with phosphate-bonded investments are rougher than those made with gypsum-bonded investments29 and are more difficult to remove from the investment 30 Because phosphate-bonded investments have lower porosity, 31 complete mold filling becomes more difficult. Castings also are more likely to have surface nodules, which must be removed. (Vacuum mixing and a careful investing technique help reduce but do not eliminate the occurrence of nodules.)
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