An apparatus and device for creating a vertical strengthening feature within a 3D printed article of manufacture for improving mechanical performance in the Z-direction. Fill material is deposited in voids vertically crossing multiple layers during the build of 3D printing. The device includes a penetrating extension that fits within the void to create a vertical strengthening feature via heat and/or extruded fill material. The size and/or movement of the heated extension can impact the void side walls to reflow the build material and blend the layers together within the void side walls.