8/18/2019 Erosion scab.doc

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Erosion scab

An erosion scab is an expansion defect in which the loosened sand has been eroded

away by the montion of metal, such as boiling and has left a solid junction between

the casting and defect. As such, the defect can be removed by cold chisel, usually

leaving a solid surface. However, it should be noted that the erosion scab result in

sand holes or sand inclusions in some other part of the casting. Figure 8. shows a

typical erosion scab.

 !he erosion scab is similar in appearance to a cut or wash. "t sometimes loo#s li#e a

rat$tail or drop. !he cure, however, may be entirely di%erent because an erosion

scab is essentially an expansion problem rather than a low strength &hot or dry'

problem. "t is best understood by visuali(ing an expansion ta#ing place in or on the

mold surface before the metal touches the surface at that point. )uch expansion

loosens or wea#ens the mold surface to a predetermined depth.

*hen molten metal +ow over such an expanded surface, it can readily cut or wash

the sand grains away, leaving a growth on the casting. As implied by the word

erosion,- it is a cut, but the word scab- implies that the cut or wash would not

occur if the sand had not tended to scab or buc#le ahead of the metal +ow.

bviously, this defect is caused by the tendency of the sand to scab and of the

metal to erode or cut. !herefore, both metal +ow and sand &or core' properties must

be evaluated.