The aim of dental precision casting is to produce defect-free castings with a fine surface finish and which match the dimensions of the pattern (e.g. crown, inlay, bridge, partial denture framework).
Dental phosphate-bonded casting investments are a powder mixture of a refractory system and a binding system specially designed for casting dental alloys.
The refractory filler system usually consists of refractory oxides such as silica whereas the binding system usually consists mainly of an acidic phosphate, such as ammonium dehydrogen phosphate (ΝΗ4Η2PO4), together with a basic oxide, such as dead-burned magnesium oxide. When the powder is mixed with an appropriate liquid, it forms a paste that hardens to form an investment mould suitable for casting dental alloys. The appropriate liquid is either water, special liquid or special liquid mixed with water. The reaction that takes place is the following:
MgO + NH4H2PO4 + 5H2O → MgNH4PO4.6H2O
A setting expansion accompanies the reaction.
The product of the reaction, struvite, is the connecting phase of the investment. The reaction is very fast and such investments usually set in less than 30 min.
The pattern for dental casting is frequently formed in inlay wax, or casting wax, on a model. The pattern in positioned in a “casting ring” (steel or plastic) and surrounded with the investment material (dental cement) to form the mould. A heating cycle follows, which causes softening and thermal decomposition of the wax to yield the mould space. The molten alloy is then forced to the mould to produce the casting.
The properties required for dental cement suitable for precision casting are:
- Higher melting temperature than the melting range of the alloy
- No chemical reactions between the alloy and the investment
- Should yield a fine surface finish on the casting
- Permeable to gases
- Adequate strength to withstand the incoming alloy
- Should be expandable to compensate for cooling shrinkage of the alloy.