The assessment of the extent of degradation of polymer molecules during processing via conventional (e.g., extrusion and injection molding) and emerging (e.g., additive manufacturing; AM) techniques is important for both the final polymer material performance with respect to technical specifications and the material circularity. In this contribution, the most relevant (thermal, thermo-mechanical, thermal-oxidative, hydrolysis) degradation mechanisms of polymer materials during processing are discussed, addressing conventional extrusion-based manufacturing, including mechanical recycling, and AM. An overview is given of the most important experimental characterization techniques, and it is explained how these can be connected with modeling tools. Case studies are incorporated, dealing with polyesters, styrene-based materials, and polyolefins, as well as the typical AM polymers. Guidelines are formulated in view of a better molecular scale driven degradation control.

Acrylonitrile butadiene styrene (ABS) based polymeric composites consisting of mono‐ and hybrid nano‐compounds, that is, graphene nanoplatelets (GNP's), multi‐walled carbon nanotubes (CNT's), and titanium dioxide (TiO2), are studied for fused filament fabrication (FFF). Rheological analysis in a screening step reveals that nanocomposites containing CNT result in a better nano‐filler dispersion within the matrix and enhanced matrix interaction. The addition of GNP and TiO2 leads to a better coalescence between the deposited filaments. For the actual FFF specimens, emphasis is on the tensile, flexural and impact properties as well as the void content. It is shown that the joint addition of GNP, CNT, and TiO2 gives rise to a remarkable synergistic effect, leading to an improved dispersion and an increased tensile modulus and strength of 3D printed ABS by 16 and 20%. Decreasing the layer thickness increases the mechanical properties of the materials, while the printing temperature does not lead to major variations of the mechanical properties, due to a dominant effect of the addition of nanoparticles. It is also shown that for well‐designed composites the slower sintering and higher void content is overruled by the reinforcement effect.

An important polymer processing technique is additive manufacturing (AM), which enables shape-free design of complex final parts with limited waste during the development change, at least if the impact of molecular degradation reactions is minimized. In the present work, polystyrene (PS) and acrylonitrile butadiene styrene (ABS) polymer have been processed via: (i) fused filament fabrication (FFF), separately accounting for the prior single screw extrusion (SSE) filament production; and (ii) pellet-based additive manufacturing (PBAM), which are two important AM techniques. The influence of printing temperature, layer thickness, printing velocity, and printing technique on the degradation of both polymeric materials is studied by means of thermogravimetric analysis (TGA), size exclusion chromatography (SEC), small amplitude oscillatory shearing tests (SAOS), Fourier-transform infrared spectroscopy (FTIR), and yellowness index (YI) measurements. For ABS, SSE-FF leads to more fission (higher mechanical loading) whereas PBAM results in more cross-linking (more thermal loading). For PS, fission is always dominant and this more evident under FFF conditions. ABS also exhibits yellowing upon processing, indicating thermo-oxidative degradation although below the FTIR sensitivity limit. The selected PBAM conditions with PS are already delivering printed specimens with good mechanical properties and lower degradation. For ABS, a further PBAM optimization is still desired compared to the FFF countercase, taking into account layer-by-layer adhesion.