Institute of Fundamental Technological Research

This work demonstrates how a well-known malfunction that frequently occurs in material extrusion technologies, known as filament stringing or oozing, can be used to increase the acoustic performance of 3D printed sound absorbing materials. The purpose is first achieved with conventional slicer software by deliberately setting some printing parameters 'wrong' to provoke filament stringing. Acoustic materials based on the same original design of narrow slits are 3D printed with retraction enabled or disabled, or using a higher than required printing temperature. The uncontrolled filament stringing that occurs in this way creates fibres in the slits, which ultimately affects the sound absorption measured for these materials. This cannot be ignored in modelling if accurate sound absorption predictions are to be obtained. However, inspired by the uncontrolled stringing, we developed a new concept to print parts with deliberate parametrically adjustable micro-fibre substructures. These are achieved by directly designing innovative toolpaths with recently developed design software (FullControl GCODE Designer), which has never been used previously for sound absorption purposes. The method permits low-cost 3D printers to produce tailored complex acoustic materials with enhanced viscous dissipation effects and improved sound absorption properties. This behaviour is correctly captured by the proposed, experimentally verified, mathematical model of such acoustic composites. The examples presented in the article are also used to discuss some aspects of the reproducibility of acoustic materials 3D printed by extrusion.

Prototyping acoustic composites, Exploiting 3D printing imperfections, Fully controlled G-code, Filament stringing, Fibrous fillers, Sound absorption

In this work, slotted acoustic materials based on a space filling curve called the Peano-Gosper curve are proposed and investigated. The slits in such materials form a complex pattern because they are divided by walls built along lines generated by the Gosper curve algorithm. The pattern can be twisted around an axis normal to its surface to increase the tortuosity inside the material, and therefore, modify its acoustic properties, which can be controlled by the turning angle or pitch of the twist. A highly efficient semi-analytical model has been developed to accurately predict the acoustic properties, in particular the sound absorption of such materials. It only requires a representative part of the pattern, or better, scanning the surface of the fabricated material so that the actual geometry and dimensions (in particular slit widths) are well reproduced in a two-dimensional finite element mesh generated on a representative fluid domain. The mesh is used to solve a dedicated Poisson problem and determine a few key parameters, and the rest of the modelling is based on analytical formulas. Material samples with straight and twisted slit patterns were 3D printed and then measured in an impedance tube to confirm semi-analytical sound absorption predictions.