A Feature-Specific Local Cooling System to Control Tensile Strength and Dimensional Accuracy in Fused Filament Fabrication.

There is in-depth understanding of the effects and interactions of various process parameters on the mechanical properties and dimensional accuracy of parts produced through fused filament fabrication (FFF). Surprisingly, local cooling in FFF has been largely overlooked and is only rudimentarily implemented. It is, however, a decisive element of the thermal conditions governing the FFF process and of particular importance when processing high-temperature polymers such as polyether ether ketone (PEEK). This study, therefore, proposes an innovative local cooling strategy, which allows for feature-specific local cooling (FLoC). This is enabled by a newly developed hardware in combination with a G-code postprocessing script. The system was implemented on a commercially available FFF printer and its potential was demonstrated by addressing typical drawbacks of the FFF process. Specifically, with FLoC, the conflicting requirements for optimal tensile strength versus optimal dimensional accuracy could be balanced. Indeed, feature-specific (i.e., perimeter vs. infill) control of thermal conditions resulted in a significant increase in ultimate tensile strength and in strain at failure in upright printed PEEK tensile bars compared with those manufactured with constant local cooling-without sacrificing the dimensional accuracy. Furthermore, to improve the surface quality of downward-facing structures the controlled introduction of predetermined breaking points at feature-specific part/support interfaces was demonstrated. The findings of this study prove the importance and capabilities of the new advanced local cooling system in high-temperature FFF and provide further directions on the process development of FFF in general.

Mesh manipulation for local structural property tailoring of medical warp-knitted textiles.

Medical meshes are used as structural reinforcement both in clinical surgery and tissue engineering. However, complex loading conditions often found in such applications result in a non-homogenous stress distribution, for which the uniform reinforcement provided by the meshes is not optimal. This study aims to design a textile reinforcement with a spatially heterogeneous, load-tailored fiber architecture. To this end, we developed a simple method of manipulating a standard uniform mesh by stretching in warp and weft directions to various extents in order to control fiber orientation and fiber volume fraction. Subsequent thermal treatment locked the manipulated configurations allowing further use of the meshes. Firstly, samples in five uniform configurations (two manipulated longitudinally (warp direction), two manipulated transversely (weft direction), one non-manipulated) were obtained and analyzed regarding their morphology as well as their mechanical properties under cyclic uniaxial loading. Significant effects of the manipulation on key characteristics of the pores such as angles, side lengths, aspect ratios, and fiber volume fraction were shown. Tensile testing demonstrated the range of tensile properties achievable with the simple manipulation of the mesh, not only in magnitude but also in the shape of the stress-strain response curve. Finally, local manipulation combining different mesh configurations was exemplarily applied to create a spatially heterogeneous load-tailored reinforcement to match local strain directions in tissue-engineered tubular heart valves. The proposed method enables the use of well-established uniform medical meshes to produce load-tailored non-uniform mesh reinforcement for many applications in an easy-to-implement manner.