## 4 Discussion  
The experimental studies conducted in the present paper show that the noncontact ultrasonic method using fundamental antisymmetric Lamb wave mode (A0) is a very sensitive tool for detecting and identifying inconsistencies and defects in polymer-based plate materials with a homogeneous structure. The measurement method allowed for the excitation and recording of this specific Lamb wave mode over the entire width of the tested materials (distance of approximately 1 m). Defects were identified by analyzing the recorded signals' amplitudes as a function of the probe position. Analysis of these amplitudes for reference materials (without defects) shows that in the middle part of the board, the amplitudes of the signals have an almost constant value (with slight oscillations). On the contrary, significant signal amplitude fluctuations are observed close to both boards' edges (at the beginning and the end of the scanning length-approximately 200 mm from the edges). This observation, also visible in the B-scans of the boards, is caused by the interference of the propagating Lamb wave with various wave modes reflected/refracted from the edge of the board. To the authors' knowledge, this issue has not been addressed previously in the ultrasonic studies of Lamb waves in such materials. The other studies in the literature show results concerning defect detection for polymers or composites, typically in the middle parts of the boards where the overlapping \"boundary\" effects with material inconsistencies do not occur [17; 42; 43; 44]. By introducing artificial defects in the form of cracks or delaminations close to the edge of the tested materials (perpendicular or parallel to the surface of the board), the registered Lamb wave signals are affected both by the defect presence and interference of propagating Lamb wave with reflected/refracted from the edge various wave modes. Despite significant amplitude oscillations in these areas, the method is still very sensitive to the presence of defects. A decrease in the signal amplitude was observed even for defects of a few millimeters in length. Thus, the method is very sensitive to the slightest changes in the material structure. This is particularly important because most defects (like cracks) resulting from inadequate transportation or storage conditions appear close to the board edges. The presence of a discontinuity in the tested material and the length of the defect affect the fading of the signal amplitude (including edge fluctuations). The signal amplitude measured in the polyethylene was almost twice as high as that in the polyamide. The higher signal amplitude resulted from its intrinsic material properties. Polyamide shows more significant signal attenuation compared to polyethylene. The experimental tests show the sensitivity of the method to defects in the form of (i) delamination from the edge of the material, (ii) cracks from the edge of the material, and (iii) material discontinuity in the middle part of the boards. The results of the studies performed using air-borne ultrasound, presented here, also illustrate that it is necessary to maintain the temperature of the material surface and environmental conditions during all measurements as similar as possible. In the studies, the tests were performed in laboratory conditions within a few hours, assuming the temperature was constant. Within the present studies, the boards were tested along their entire length and at the beginning and end of the scanned boards, significant variations in the amplitudes of the recorded signals were observed. An explanation of the observed effect was not a subject of the present paper and is planned in future studies using computer/numerical simulations of Lamb wave propagation in plate-like materials. The second objective of this study was to improve automated quality control and defect detection systems based on machine learning for Industry 4.0 and 5.0, with a focus on high-precision manufacturing.