Characteristic features
Large-area penetration occurs in regions of the mould which have
been filled with metal too quickly.
Incidence of the defect
With molten metal flowing into a sand mould, explosive water
evaporation often occurs at positions where streams of metal
flow together or flow faster. This “water vapour explosion”
imparts a dynamic jolt to the metal which forces it into the sand
pores over a large area. In contrast to pure penetration, this
phenomenon does not just occur at under-compacted areas or hot
spots. Its incidence is closely coupled with the path taken by the
flowing metal and is often observed in the last part of the mould
cavity to be filled. A uniformly thick penetrative crust often
forms.
Explanations
The defect has occurred more frequently with the introduction of
high-pressure moulding and, in particular, with the introduction of
core-making processes which have led to a high degree of replenishment
of the moulding sand. Levelink1
was the first to
investigate these phenomena and found the reason to be the
explosive expansion of water which has condensed on the mould
surface. A fast flow rate of molten metal causes the condensed
water to evaporate explosively when the metal comes into contact
with it. The resultant pressure increase then forces the molten
metal into the sand pores.