Material moisture

Something which is particularly important for casting mold materials is that the setting and maintenance of a basic water content which is adjusted to suit the type of compaction and compaction intensity is a basic requirement for a high mold material and mold quality. The specification of the metrological value is done in percent and is the specification of the mass from the wet material to the dry material (Eq. 1).

Eq. 1:

Typical moisture values for used sand after unpacking are between 0.5 and 1.5%. For freshly conditioned synthetic mold sand, the mold sand moisture values range between 3 and 4%. For natural sand, they are usually around 1 to 2% above those for synthetic mold material.

The total water in the mold sand consists of the free water, the surface moisture (surface water which is held to the grain surface by adhesion), the capillary water (inner, hygroscopically-bound water) and the water of crystallization (s. Moisture).

The surface water does not have any strong binding forces and can be removed most quickly using mechanical forces (sand aerator behind the mixer).

The capillary water is bound very strongly by the close molecular forces (Van der Waals forces) (similar to the swelling of the montmorillonite in bentonite) and can no longer be removed by mechanical forces. This residual (“secondary”) moisture can only be removed by drying.

The water of crystallization is trapped in the crystal lattice of a mineral or a chemical substance and can only be removed by powerfully heating (500 - 600 °C), i.e. destroying, the crystal. The water of crystallization content can affect the physical and chemical behavior of the material.

How the water separates and where it is currently located is a dynamic process in mold material systems which changes constantly. Aside from possible compaction processes, it moves through the stratification (aging) of the mold material always in the direction of the montmorillonite and is deposited in the intermediate lattice. By doing so, it forces the desired swelling of the bentonite and therefore the construction of its bonding property. Upon initial visual inspection after several hours, the sand appears drier, however, this is not determined after a thermal-gravimetric moisture test in the laboratory. With the exception of losses of compaction, the moisture content has hardly changed.

Measuring devices which are standard in the market are not able to determine the different locations of the water. Only an overall signal is identified.

The moisture of a material cannot be calibrated; measuring devices which measure it cannot be calibrated.

Additional references:
Mold sand testing
Mold material management
Mold material recovery
Mold material hardening
Mold material compaction
Compaction testing