Why Solidification Shrinkage Produces a Core Cavity Rather Than Deforming the Part: 23-Ind-B2 Manufacturing Processes, December 2019, Q3

Question 3: Why Solidification Shrinkage Produces a Core Cavity Rather Than Deforming the Part (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Solidification of a casting proceeds directionally, from the outside in: the molten metal touching the (much cooler) mold wall freezes first, forming a thin, rigid solid skin that takes on the exact shape of the mold cavity almost immediately. As cooling continues, that skin thickens inward while the interior — the last region to reach freezing temperature, usually near the geometric core or thermal centre of the part — stays liquid the longest.

Because the outer skin has already solidified and become rigid, it locks in the part’s external dimensions; it cannot move inward or collapse to compensate for the shrinkage still occurring in the liquid interior (both ordinary thermal contraction of the remaining liquid and the volumetric shrinkage that occurs as liquid metal converts to solid, which for most metals is a net contraction of a few percent). The liquid core keeps shrinking as it cools and continues to freeze, but it is now sealed off by the already-rigid shell and increasingly starved of any further liquid metal supply to fill the volume it is losing (unless a riser is specifically positioned to feed it, which this question’s scenario does not include). With nowhere else for that missing volume to come from, a void — a shrinkage cavity, or scattered micro-porosity if the freezing front is more mushy/dispersed — opens up in the last-to-freeze core region.

The part’s external geometry is therefore preserved (it was fixed by the rigid outer shell early in solidification, and the rigid shell resists any external collapse), while the missing volume is instead absorbed internally, invisibly, as a cavity in the core — exactly the pattern the question describes.

Topic. Directional solidification and the mechanism by which casting shrinkage produces an internal cavity instead of external deformation.

Key relations. Solidification proceeds from the mold-wall inward; solid metal is dimensionally rigid once formed (skin fixes external shape early); shrinkage volume that cannot be fed by liquid metal (e.g. via a riser) must open up as a void at the last point to freeze — typically the part's thermal/geometric core.

Why this works. The part's outer shape is "frozen in" (literally) long before the whole casting has finished solidifying, so any further volume loss from the still-liquid interior has nowhere to be absorbed except as an internal void, since the already-rigid exterior cannot deform inward to close the gap.

Common pitfall. Describing shrinkage as if the whole part shrank uniformly and simply got smaller — in reality the OUTSIDE stops shrinking (dimensionally) once it solidifies, and it is only the still-liquid inside that keeps contracting, which is exactly why the defect appears as an internal cavity rather than a smaller/deformed part. This directional-solidification effect is also why risers (extra reservoirs of liquid metal) are deliberately placed on castings — to feed exactly this core shrinkage before it can open into a void.

Source. Groover, Fundamentals of Modern Manufacturing, 6th ed., Ch. 10–11 (solidification of metals; shrinkage; riser design).

Question 3 of 7: Why Solidification Shrinkage Produces a Core Cavity Rather Than Deforming the Part

Question 3: Why Solidification Shrinkage Produces a Core Cavity Rather Than Deforming the Part (20 marks)