Characteristic features
Irregular small to large-area solid metallic thickening, frequently several millimetres thick. They often run parallel to the
casting surface, their surfaces being rough. The edges frequently exhibit so-called "undercuts". Castings with solid
scabbing often exhibit sand crust inclusions.
Incidence of the defect
Scabs may appear on all casting surfaces when pouring into green sand moulds. They appear more frequently on the
top and bottom areas of the mould cavity, highly compacted parts of the mould cavity also being at risk. Scabs can be
created when the mould wall is heated by radiant heat, but may also be caused by the direct flow of heat from the
molten metal into the mould wall.
Explanation
During pouring, parts of the sand mould near the surface are heated up rapidly. A few millimetres thick dry sand crust
forms on an over-wetted stratum (condensation zone). As quartz tends to strongly expand at 300 oC - 575 oC, this
causes a high compressive stress in the sand crusts which, in the case of poor wet tensile strength in the condensation
zone, can lead to them being separated from the stratum.
Possible causes
Clay-bonded sand
Too little bentonite or poor bentonite quality in the moulding sand.
Proportion of fine quartz in the sand too high
Moulding sand too fine.
Insufficient conditioning of bentonite
Degree of chamottization of the sand too low .
Too much salt in the sand.
Moulding plant
Too high or irregular compaction
Gating and pouring practice
Sand heated up too severely by inflowing metal and too long exposure to radiated heat.