Abstract: Inter-layer bonding in objects manufactured by 3D printing techniques is improved by one or more targeted heat sources (THSs) that preheat a targeted portion of existing object material before additional material is added to the object. 3D Printing is also improved, optimized or calibrated by pre- or post-heating a targeted area. THS elements may be fixed, mobile, or a combination thereof to apply heat to targeted areas. A THS may be integrated in or as an add-on to an existing 3D printer. A THS controller may use 3D printer information, such as a current direction or a future direction of printing, to select one or more THSs and to perform other targeted heat operations, such as setting energy levels, aiming, moving, etc. Properties, e.g., strength, of 3D printed objects with improved interlayer bonding may, for example, be several multiples better than the same object printed without improved interlayer bonding.