Natural mineral or synthetic granulates or sands.
When referring to sands, this means mineral substances in a grain diameter range of 0.02 to 2.0mm. The basic mold material is a bulk grain material with certain granulometric properties. It constitutes the mold skeleton and also serves as a filler.
The main requirements are favorable granulometric properties, high thermal stability (sintering temperature > 1350°C), low thermal expansion (if possible, without any anomalies in expansion), sufficient resistance to temperature changes and a favorable chemical behavior.
Currently, the most important basic mold material is quartz sand. It is inexpensive and can be used for all known binder systems with only few limitations. The main component is the mineral quartz, a crystalline form of silicon dioxide (SiO2). Different SiO2 modifications are resistant depending on temperature and pressure, the transition between the individual SiO2 modifications being connected to volume changes which may influence the expansion behavior of the quartz sands (see Sand expansion) and thus also mold sand-based casting defects (see also Sand expansion defects).
An example shows table 1 on silica sand GS 23 of the Strobel Quarzsand GmbH.
In practice, a number of other basic mold materials, in particular exhibiting higher sintering points and low expansion values, are used for highly-stressed molds and mold sections These include, among others, the minerals fused silica (see Fused-silica sand), zircon (see Zircon sand), olivine, chromite, chome-magnesite granulate and fireclay.
Moreover, corundum (Al2O3) and fused mullite (3Al2O3 · SiO2) are used as basic mold materials for special molding processes (see also Special sand).