This defect is often present in the casted parts. On this points the molten metal reach a limit value of solid fraction. We’re talking about 30% (blue color), over this value the flow stops leaving unfilled zones. Even over the 10% (yellow color) the metal flow slow down and the result is the presence of ripples on the surface. Those defects are present on the points that fill last and are amplified by the excessive cooling of the melt due to excessive cooling of the mold or too long filling time and when the wall to fill is too thin.