Casting defects · shrinkage

Shrinkage porosity in castings

Shrinkage porosity is spongy, interdendritic voiding at the thermal centres of a casting. It is a feeding problem: the last metal to freeze was not fed, so it left voids behind.

By Versatile Equipments Pvt Ltd · equipment manufacturer & calibration provider · updated 2026-07-15

What it is

As metal solidifies it contracts, and the liquid that remains must be fed from a reservoir (a feeder or riser). Where a heavy section or a junction stays molten after everything around it has frozen - a thermal hot spot - and no feed metal can reach it, the metal pulls apart into spongy, rough-walled voids. That is shrinkage porosity; a larger single void is a shrinkage cavity.

Shrinkage or gas?

Gas porosity is rounded and smooth-walled and can appear anywhere; shrinkage is jagged, dendritic and located at hot spots - under feeders, at bosses and section junctions. If the voids trace the thickest, last-to-freeze regions, it is shrinkage. See gas defects for the contrast.

Root causes

• Inadequate feeding. Feeders too small, too far, or frozen before the section they feed.
• Hot spots by design. Heavy junctions and bosses that solidify last with no feed path.
• Gating and temperature. Poor thermal gradients, or pouring so hot that shrinkage volume rises.

Prevention checklist

• Feed the hot spots: size and site feeders to stay liquid until the section they serve has frozen.
• Use chills and padding to steer solidification toward the feeder (directional solidification).
• Soften junctions in the method; avoid isolated heavy masses where you can.
• Control pouring temperature; hotter is not better for shrinkage.
• Track which parts and which features shrink, so the method change targets the real hot spot.

Frequently asked questions