Mold sand-based casting defect

 

When casting molds made of bentonite- or chemically bonded mold material systems, this results in intensive interactions between the melt, the solidifying casting and the mold during the pouring and solidification stage.

Especially the metal/mold interface is subject to high mechanical and thermal stresses, and the heating in the mold wall due to the molten casting material causes non-stationary heat and material flows. This results in site- and time-dependent changes in temperature and humidity, causing changes in gas pressure and strength.

All of these changes can be the cause of different defects on castings (Figs. 1 to 4). The behavior of molds during casting is directly related to the maximum temperature properties of the mold material. In particular, the unfavorable expansion characteristics of quartz sand and the associated expansion behavior of the mold material mixture (sand expansion) are causes of typical mold sand-based casting defects (see Scabs, Veining, Mold erosion, Roughness/Penetration, Burnt-on sand, mold material-relatedGas blisters, Mold wall movement).

In order to determine the behavior at high temperatures under realistic conditions and obtain relations to the casting behavior of mold materials, so-called casting specimens were developed for some essential casting defects (e.g. scabbing test) which overstate the susceptibility to defects to obtain reproducible information on the impact of certain influencing factors. Based on logic diagrams, changes in important mold material properties (water content, specimen weight, permeability to gas, strength) can then be used to define technological measures and prevent casting defects.

Additional references:
Mold sand
Mold sand testing
Sand expansion defects
 

  • Fig. 1: Extensive pinholes on a GJL casting(source: Handbuch der Gussfehler, S&B Industrial Minerals GmbH, Marl)
  • Fig. 2: Extensive lustrous carbon inclusions with rippled appearance (source: Handbuch der Gussfehler, S&B Industrial Minerals GmbH, Marl)
  • Fig. 3: Scab at the top of a casting (source: Handbuch der Gussfehler, S&B Industrial Minerals GmbH, Marl)
  • Fig. 4: Sand layer firmly adhering to a thin-walled GJL casting (source: Handbuch der Gussfehler, S&B Industrial Minerals GmbH, Marl)
Back to list