## 1 Introduction
Moreover, they realized standard quasi-static tensile tests on ABS under room (23 \({}^{\circ}\)C) and the maximum operating (80 \({}^{\circ}\)C) temperature. With increasing temperature, the behaviour of the ABS changed from brittle to ductile. When comparing the 3D printed and injection-moulded specimens, the second had a 24% higher value of tensile strength. The effect of post-heat treatment on the mechanical properties of the specimen made of PLA with a 100% fill density is described in [34]. The authors concluded that this process considerably affects the fracture limit of the structure, while the effect on the tensile strength is minimal. Moreover, the Young's modulus obtained for the heat treatment above the glass transition is much higher than those obtained below the glass transition. [PERSON] et al. [35] investigated the effect of freeze-thaw cycles on the tensile, bending, and fracture resistance of specimens made of PLA material. They found that the infill ratio of PLA together with the number of freeze-thaw cycles reduced the tensile strength, flexural strength, and fracture resistance. However, all the structures related to the above-mentioned fields are subjected to frequent vibrations or dynamic loading and it is therefore important to know their dynamic behaviour. To date, the modal and vibration response testing of AM structures has been rarely carried out. In 2017, [PERSON] [36] studied the natural frequencies of structures prepared by the FDM method. He analysed the results obtained from one experimental analysis, whereby the excitation was ensured by a modal shaker and the response was measured in one location using an accelerometer. Natural frequency values comparable to those from the numerical analysis were obtained for only a few modes. An experimental modal analysis (EMA) of several PLA beams with free-free boundary conditions was provided by [PERSON] et al. [37]. According to their conclusions, significant changes in natural frequencies and damping ratios were obtained, relating to the variations in the fill density and infill pattern. The use of EMA as an alternative non-destructive approach for testing the mechanical properties of 3D printed beams with different infill patternsand orientations was provided by [PERSON] [38]. The effect of the adhesion type, printing direction, and layer thickness on the vibration response and parameters obtained from EMA was analysed in [39]. The results obtained from the analysis of eight groups of ABS cantilevers showed that the adhesion type has the most significant impact. Moreover, according to the authors, neither the layer thickness nor the printing direction has a major impact on the measured dynamic modulus, natural frequencies, or damping coefficients. The study presented in this paper describes the process of the estimation of the modal parameters of 3D printed circular plates. Unlike most of the previously published works in the field of 3D printing, the authors do not focus on comparing the results obtained under different 3D printing conditions (as mentioned in the literature review above), but instead pay attention to the aspects that can affect the accuracy of the modal parameters, i.e., mode shapes, natural frequencies, and damping ratios. For this reason, comparative measurements have been carried out using equipment specialized for modal testing, the main purpose of which was to obtain information on the optimum choice of the number of excitation points, the number and position of the response measurement locations, and the frequency resolution of the measurement. The obtained information was then used to carry out experimental modal analysis and to compare the modal parameters of PLA specimens produced with three different infill patterns (honeycomb, stars, and concentric).