Raphael Freund

Additive manufacturing provides new possibilities in product design compared to traditional manufacturing processes. Particularly additive material extrusion offers the freedom to combine multiple materials in a single component without additional steps. However, combining multiple materials often leads to reduced adhesion, which can hinder the creation of high-strength designs. This issue can be largely mitigated using the geometric freedom of additive manufacturing to produce interlocking structures. This publication investigates the use of lattice structures as interlocking bonds in multi-material applications. The aim is to aid the design of suitable lattice structures by collecting geometric freedoms of lattices, application requirements, and manufacturing constraints, for this information to be used in suitable designs in the future. Initially, the general design freedoms of lattice structures are compiled and explained. Subsequently, these design freedoms are narrowed down based on the specific requirements for interlocking bonds and the limitations imposed by geometry and material combinations during manufacturing. The publication concludes with design recommendations that can be used as the basis for interlock bonds. Suitable lattice designs should aim for high interconnectivity, interconnected porosity, and a high number of similar strut structures, all the while maintaining low dimensions in the interface direction.

The complexity offered by additive material extrusion (MEX) presents new opportunities for novel design, especially with regard to multi-material components. However, this design freedom has heretofore only been scarcely used thus stifling innovation. One reason for this development is the complicated nature of adhesion at the interface of multi-material parts. Hence, this publication aims to investigate the process-dependent influences in multi-material MEX by conducting tensile tests on ABS-PLA multi-material specimens. By implementing a distance gauge into the tool change procedure, positional fluctuations of the nozzle will be eliminated and the effects of extrusion temperature, line placement, and over- or under-extrusion on composite strength can be determined more precisely. In addition, thermal imaging is conducted to give an informed estimate of the effects of build chamber or build plate temperature on diffusion at the material interface. The results show a clear influence of extrusion temperature and over-extrusion on composite strength, while the effect of line placement is determined to only be minor. The build chamber temperature is predicted to have no meaningful effect on composite strength. Overall, the results suggest that deviations in printer calibration, by as little as 0.04 mm, can have a significant influence on composite strength.

Additive manufacturing (AM) typically is more costly than traditional manufacturing processes, but it opens new opportunities for design that are often not fully utilized because of its price. Bonding complex additively manufactured structures to conventional parts is a way of reducing cost, while maintaining design freedom. However, not all polymers used in AM are easily bondable. These materials are typically pre-treated with plasma in order to increase adhesion at the interface. By utilizing the design freedom of AM, it becomes possible to omit this pre-treatment step by producing undercut areas that can be filled by the adhesive during bonding. Instead of adhesive bonding, forces are transmitted by positive fit of the adhesive and the AM part. By producing complex lattice structures with AM that are surrounded completely by adhesive, the strength of these bonds can further be increased. The design of such structures is highly complex, however. This publication shows that AM lattice structures can greatly improve bonding strength of low adhesive polymers and suggest a way of characterizing such structures in lap shear tests in order to separate design influence on such bonds. It could be shown that resulting shear strength is comparable to bonding after plasma pre-treatment.

Material composition complexity offered by material extrusion additive manufacturing offers new opportunities for function-driven part design. Nevertheless, since influencing factors on the interface strength between different materials are not well understood, this complexity is only used infrequently, in part, in design thereby restraining innovation. This paper proposes a systematical approach for identification and quantification of relevant adhesion phenomena that influence interface strength. For this reason, suited test specimen, which utilize the geometric freedom offered by additive manufacturing, are developed for roll peeling tests and peeling resistance of several combinations of rigid and flexible materials is determined. The results show that material choice especially regarding polarity as well as mechanical interlocking in regards to surface roughness and design features have high influence on the interface strength of multi-material parts manufactured by material extrusion. These results are explained through the relevant adhesion mechanisms that determine the interface strength in additively manufactured parts. Finally, criteria that predominantly affect interface strength are deduced and design recommendations for creating functional parts with ill-fitting material combinations are formulated.