Investigation of nonisothermal fusion bonding for extrusion additive manufacturing of large structural parts

Abstract

Fusion bonding is understood to be the decisive mechanism for the interlayer strength in extrusion additive manufacturing. This study links the characteristic temperatures of semicrystalline thermoplastics, namely, PA6 with 40% carbon fibers, to the bond formation in respect to real-world processing conditions. Based on theoretical investigations, a process window is proposed for bonding to occur without polymer degradation. This range from the glass transition to the initial degradation temperature was determined through differential scanning calorimetry and thermogravimetric analysis. A second process window for optimal bonding is proposed from the extrapolated onset crystallization temperature, T eic , to the melt temperature, T m . The validation of these process windows was conducted by bending tests according to DIN EN 178. T m was confirmed as the upper limit, with the part's geometric integrity compromised at higher temperatures. T eic had to be refuted as lower limit as no discrete reduction in bond strength was determined in alignment with T eic . Authors suggest the lower limit is defined by the lowest substrate temperature to lead to interface temperatures above the onset of melt temperature. By utilizing thermal analyses, less time and resources are required to detemine a suitable process window for extrusion additive manufacturing.

Foundational Investigation on the Characterization of Porosity and Fiber Orientation Using XCT in Large-Scale Extrusion Additive Manufacturing

“…Both destructive and non-destructive methods have confirmed this anisotropy with reduced mechanical strength and an increased number and volume of voids in the build direction [ 8 ]. As expected [ 9 , 10 ], previous flexural and tensile testing of EAM specimens showed a high mechanical anisotropy due to reduced mechanical properties along the layer interfaces, requiring further investigation and optimization [ 11 , 12 ].…”

“…Due to their complex semi-crystalline structure and the strong dependence of crystallization on the thermal conditions during processing, the final part properties and the mechanical strength of the weld line strongly depend on the processing parameters selected [ 15 , 18 , 19 ]. Previous investigations correlated this high dependency of the resulting mechanical properties to the characteristic properties of the polymer [ 11 ].…”

“…The aim was to analyze the fundamental influence of processing temperatures, layer heights, and the air gap on fiber orientation, pore content, and pore distribution, as well as the overall quality of the interface. Thorough material investigations and mechanical testing [ 11 ] had previously identified an ideal processing window for the substrate temperature at the deposition of the subsequent layer. This processing window ranged from the melt temperature to the onset of crystallization, with greatly reduced mechanical properties below the lower limit at 166 °C.…”

Polymer composites: Additive manufacturing of composites

“…Several other papers in this issue are also dedicated to the FFF process. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] The work of FFF spans the spectrum of new biomaterials, [16,19,20] to the hybrid method of solution casting coupled with FFF as demonstrated by Subbarayan, [15] to the bio-tissue work of Sivagnanamani. [14] Han [2,3] manufactured liquid crystalline polymer [2] and polyamide [3] strands filled with nanotubes and demonstrated the improvement the mechanical behavior of the printed parts made with the filled resins.…”

“…Included in this special issue is a series of papers that seek to extend the work in filament-based systems of FFF to large area 3D printing, sometimes termed fused granulate/pellet fabrication (FGF), of short fiber reinforced parts. [6,7,21,22] Several authors highlight the importance of controlling and understanding the orientation of discontinuous fiber reinforced beads through the guiding of the printing process resulting in structures with improved properties. [7,21] Such controlled directional reinforcement can be improved even further when using continuous fibers.…”