In an example implementation, a method of cooling a 3D build volume includes receiving a 3D object model that represents a 3D part to be formed in a build volume, and determining a placeholder position for forming a placeholder part within the build volume. Based on the placeholder position, a 3D part position is determined for forming the 3D part within the build volume. The method also includes controlling components of a 3D printing system to form the placeholder part in the placeholder position within the build volume and to form the 3D part in the 3D part position within the build volume.