Injection moulding of resonators for locally resonant metamaterial production

In the field of noise and vibration control, locally resonant metamaterials have come forth as promising lightweight and compact noise and vibration solutions. By the inclusion or addition of small resonant structures in or on a host structure, targeted frequency ranges of strong vibration attenuation can be created, referred to as stop bands. Presently, locally resonant metamaterials are predominantly still created as academic demonstrators as their manufacturing is not straightforward. For example, an add-on approach allows to manually glue or screw resonators produced with e.g. laser cutting, rubber milling or additive manufacturing to the host structure. Additionally, more geometrically intricate integrated structures can be made by additive manufacturing or the combination of multiple processes such as milling the resonators out of a thermoformed host structure. However, these manufacturing approaches are expensive and have an inadequate throughput in view of mass production. Moreover, as the stop bands of metamaterials are driven by the tuning of the resonant structures, the vibro-acoustic metamaterial performance is susceptible to deviations in resonator geometry, mass and material properties which can arise from manufacturing inaccuracies. For this reason, the use of these manufacturing approaches has been found to lead to off-design metamaterial performance in many of the presented academic demonstrators. As a result, multiple design iterations are typically still required to achieve the targeted vibro-acoustic performance. In view of overcoming these current limitations in metamaterial manufacturing, in this work, the use of injection moulding is investigated for the manufacturing of resonators. This manufacturing process is highly appealing for the mass-production of metamaterials as it features several benefits such as low cycle times and low cost for mass production. Three resonators are designed for the injection moulding process after which the manufacturability is assessed with CAMECAD software. For each resonator, a corresponding mould insert has been manufactured and resonators have been produced in ABS and PP material. Based on the characterization of the mass and resonance frequency of the produced samples, the injection moulding process is found to be highly repeatable in terms of dynamic product behaviour. In addition, a comparison with additive manufacturing also demonstrates the potentially beneficial nature of injection moulding in terms of product repeatability and thus robustness.