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Scabs result from the formation of a shell and appear as irregular metallic crusts on the surface of the cast piece that are only a few millimeters thick, usually have sharp edges and are firmly attached to the cast piece. There are top scabs, i.e. those that form on the upper horizontal wall of the mold, and bottom scabs, which occur below the flow of casting metal on the floor of the mold.
This casting defect occurs in all cast pieces made of all casting materials that are produced in bentonite-bonded green molds. Scabbing is likely to occur on the surface of the upper side of the cast part (top wall of the cavity), but also on the bottom of the cavity. Highly compressed parts in the mold are particularly at risk. Scabbing is often accompanied by veining (primarily on the core components) and rat-tails. All these defects are visible to the naked eye and generally lead to rejections.
Pronounced scabbing on a section of a cast part made of cast iron with nodular graphite (the wet tensile strength of the mold material is 0.16 N/cm²).
Areas of sand expansion, in this case scabbing and rat-tails, can be combated successfully by increasing the wet tensile strength. Defects are clearly prevented by increasing the wet tensile strength (N/cm²) (see values below the figures).
Veining also forms as a result of the formation of a shell; however, this shell does not break open but grows into the mold cavities as a result of the silica expanding, which causes the casting metal to fill in the resulting gap and form a vein-like crust. Veining is very frequently observed in synthetic resin-bonded cores (also commonly referred to as scabbing).
The influence of heat (radiation, thermal conduction) from the casting metal causes a thermal expansion of the mold material. Considerable differences in stress occur as a result of the different temperatures in the individual zones of the mold. This stress influences the surface layers of the mold or the walls of the mold.
The expansion of silica, especially as a result of the reversible silica conversion at 573°C, causes compression stress in the surface layers of the mold and this leads to the formation of a shell. Tensile stress occurs at the boundary layer between the highly heated shell and the sand inset core behind it that is still relatively cool and this stress must be absorbed by the mold material if a separation is not intended. The tensile strength of green sand molds, i.e. the wet tensile strength, determines whether the shell bulges or is separated and therefore leads to the formation of veining, rat-tails or scabbing.
As a rule of thumb, the quotient of compression stress and wet tensile strength can be adopted as a way to gauge the tendency to form scabbing.
The compression stress is counteracted by the moldability of the wall of the mold (the mold material) and the strength of adhesion to the surface, i.e. the height of the wet tensile strength in the layers of mold material that lie behind. These casting defects, which are shown schematically and described here, form if the mechanical/thermal loads (stress) exceed the moldability and the strength in the load cross-section.
Top scabs thus develop from the condensation zone in the mold surface as a result of the compression stress that occurs there and that pushes up the wall of the mold and lifts it off (a). This process is generally referred to as shell formation. Veining is also caused by the formation of a shell (b). However, the shell does not break open, so that a burr-like elevated seam becomes visible on the cast part. This veining can be removed by means of increased cleaning work, and the cast part can possibly be saved. Bottom scabs form in the mold cavity below a flow of casting metal and thus on the bottom of the mold. The condensation zone that effects the formation of a shell is the cause here, too. If the sand shell remains attached to the bottom of the mold, the protruding edges of the shell cause either scabbing or rat-tails (c). In contrast, if the sand shell bulges and breaks apart under the weight of the metal that flows across it, a typical bottom scab forms at the underside of the cast piece (d).