Impact of thermal bonding and temperature on mechanical properties of additively manufactured polymer parts

Nowadays there are seven categories of additive manufacturing, including material extrusion and powder bed fusion. These two are suitable for thermoplastic polymers and are now industrially applied. Both technologies are already well-established, but some aspects – such as the influence of material combination on the mechanical properties or the effects of temperature during and after processing – are not yet fully elucidated. Therefore, the focus of the investigations for this thesis was on two aspects: the influence of different material combinations on bending loads for material extrusion-based parts and the effects of temperature on the powder and part for powder bed fusion printed components.
The former, the impact of material combinations, was investigated with cyclic and quasi-static bending loads, which were partially mismatched from a mechanical point of view. Besides a combination of (un-)filled thermoplastics and thermoplastic elastomers, a metal-polymer bond was studied. The latter was produced by depositing electrically conductive copper particles onto the material extrusion-based surface of adapted bending specimens. These specimens were tested in cyclic bending load and showed a failure mechanism originating from the metal layer. To exclude any other origin of failure, different conditions of the specimens, such as uncoated or cleaned samples, were investigated as well.
The second study focused on the quality of bonding of two polymers using a special geometry (single-leg bending) for different processing parameters. For this, a powder bed fusion-based plate was imprinted with fibre-reinforced polyamides and thermoplastic polyurethane. The studied parameters included nozzle temperature, thickness ratio of the two materials, and orientation of the bonding layer. One important finding was the limitation of the testing methodology for insufficient bending stiffness values of the overprinted layers.
The second aspect of this thesis, the impact of temperature on the selected properties, was studied with a focus on powder bed-based specimens. The effects of powder ageing during multiple re-uses of the powder were examined in detail for two polyamides. Contrary to the commonly used refreshing of the powder with virgin material, for keeping the properties as constant and reproducible as possible, these studies skipped the refreshing completely. These studies aimed to understand which ageing mechanisms occur and, thus, find possibilities to reduce or avoid the disposal of the aged powder. Each build job was investigated mechanically, rheologically, thermally, and optically to document the ongoing ageing process. Overall, three effects and their mechanisms can be named: decreasing bulk density due to the formation of agglomerates, increasing melt viscosity because of post-condensation reactions, and increasing melting temperature (onset and peak) due to the cumulative annealing during the print jobs.
While a non-stop print job without any interruption is desired, it does not always happen. The impact of such an interruption, but without oxygen leakage, was studied by purposely stopping the build job in the middle of the print for three minutes and continuing afterwards. Overall it showed enhanced quasi-static properties, but a critical deterioration for impact loads. Thus, depending on the later application, the build job does not necessarily need to be discarded.
Besides the production itself, also the later application conditions affect the performance of a polymeric component essentially. One critical factor is the strong temperature-dependence of polymers. To gain more knowledge about the tensile properties of powder bed fusion-based samples, tests were done in a broad temperature range from -20 °C up to 140 °C for three printing orientations (flat, edgewise flat, and vertical). Continuously printed specimens behave similarly to injection moulded ones for the investigated temperature range, with a decrease in stiffness and strength for elevated temperatures, while the maximum deformation is increasing.