Trapped volume: A trapped volume is a closed cavity inside a part from which unused powder or liquid resin cannot escape after printing, adding weight and cost or causing the part to crack.
Powder and resin printers build parts inside a bath of material. Any space the part fully encloses fills with that material and stays full when the part is removed. In resin printing it can also trap uncured liquid that later leaks or expands and cracks the part. In powder printing it traps loose powder that adds weight and is lost from the reuse cycle.
Trapped volumes appear most often when a designer hollows a solid model to save material without adding a way out. They can also form in suction-cup shapes during resin printing, where a hollow cup facing the build plate creates a pressure difference with each layer and pulls the part off.
Example: a 120 mm resin figurine is hollowed to 2 mm walls to save resin. Without an opening, the cavity inside stays full of liquid resin, so the print uses nearly as much material as a solid one and may crack weeks later as the resin cures. With two drain holes near the base, the resin can be poured back and the interior washed.
The common confusion is assuming the slicer will deal with it. Most slicers will print a sealed hollow quite happily. The fix is in the design: add drain or escape holes, or leave the part solid. A DFM check that looks for closed internal cavities can catch this before printing. Read design for 3D printing for related rules.
Go deeper: Design for 3D printing: walls, overhangs, holes, clearances and orientation