High Feed Rate Wire Heating and Embedding for Large Area Additive Manufacturing of Parts Containing Embedded Electronic Functionality
The introduction of large area thermoplastic extrusion additive manufacturing (AM) for direct fabrication of large end-use parts (such as prototype vehicle components) or tooling for indirect fabrication of large carbon fiber parts has seen much attention. While challenges related to interlayer bonding, thermomechanical simulations, and development of new materials are being addressed by other research efforts, the work described here is focused on the development of a wire heating and embedding technology for use with large area thermoplastic extrusion AM. The automated inclusion of wires within large thermoplastic 3D printed parts promises to augment a part’s structural function by, for example, introducing electronic functionality that can aid in instrumenting parts with embedded sensors for health monitoring or embedding heating elements to increase the temperatures of carbon fiber layup tooling. ^ Considering the requirements of relatively high wire feed rates (on the order of 5,000 mm/min), the use of relatively large copper wire diameters (~1 mm), and desired temperature increases of at least 250°C, thermal analytical models were identified in this work and used to compare two wire heating methods: resistance heating (also known as Joule or I2R heating) method and arc heating method. Of the considered methods, the use of a discharge arc (similar to what is used in tungsten inert gas welding) was found to induce a higher temperature increase with smaller required currents. This is mainly due to resistance heating being well suited for materials with high resistivity. When considering copper, whose resistivity is low, the resistance heating method is not suitable, especially for large wire diameters. As such, an experimental benchtop stand was designed and constructed to drive wire across a discharge electrical arc, akin to arcs produced by the tungsten inert gas welding process, so that temperature measurements were allowed during wire driving and heating using the direct current electrode negative configuration. ^ The use of infrared thermography allowed for making temperature measurements on stationary and driven bare copper wire. When making observations on stationary wire, arc blowing and arc narrowing were observed. When making observations on drive copper wire, two primary temperature regions were identified: a region of temperature ramp and a region of relatively constant temperature. In the region of temperature increase, the rate of temperature increase was the same regardless of the arc current used. However, it was noted that the time required to reach the relatively constant temperature during the ramp increased with increasing arc current. In the constant temperature region, the wire temperature was a function of the supplied electrical arc current. Wire temperatures were in the range of 200°C to 300°C and, as previously mentioned, the temperature was a function of the electrical arc current with good fit to a linear trend. ^ After being driven and exposed to different arc currents, the processed wires were measured and it was determined that the diameters of arc-exposed wires had a statistical significant difference when compared to the diameters of non-exposed wires. A decrease in wire diameter, when compared to vendor supplied dimension, was noted in the range of 5–6%, where 1% error was noted in the as-received wire diameter. Even though a change in diameter was noted because of the arc exposure, the volume and weight resistivity was the same for all wires, regardless of arc current or exposure. As an alternative to copper material where heating elements are desired, nickel chromium was also evaluated for use with the arc heating method. Through testing and temperature observations, it was determined that the arc heating method was applicable for nickel chromium. For experiments performed using 10 A arc current, 1.6 mm wire diameter, and 4,792 mm/min wire feed rate, the nickel chromium average temperature was recorded as 140°C. For the same experimental setup, copper’s average temperature was recorded as 201°C versus. The difference in temperature was driven by the materials difference in volumetric heat capacity (3.61 J˙°C-1˙mm-3 for nickel chromium and 3.39 J˙°C-1˙mm-3 for copper), where average temperature increase had an inverse relationship with volumetric heat capacity. ^ Future applications of the proposed technology (i.e., large area material extrusion AM with integrated wire embedding capabilities) can include the 3D printing of form-in-place gasket tooling for aircraft production. Through information provided by Lockheed Martin, it was determined that lead time for the conventional manufacturing of such tooling is approximately 8 weeks... ^
Espalin, David, "High Feed Rate Wire Heating and Embedding for Large Area Additive Manufacturing of Parts Containing Embedded Electronic Functionality" (2017). ETD Collection for University of Texas, El Paso. AAI10825126.