key: cord-0893797-8crh1yyp authors: Mohamed, Esam T. Ahmed; Zhai, Min; Schneider, G.; Kalmar, R.; Fendler, M.; Locquet, Alexandre; Citrin, D. S.; Declercq, N. F. title: Scanning acoustic microscopy investigation of weld lines in injection-molded parts manufactured from industrial thermoplastic polymer date: 2020-08-16 journal: Micron DOI: 10.1016/j.micron.2020.102925 sha: c7c4dd9a52bd5fb6e27f0d9cb72e9ceed5f40c85 doc_id: 893797 cord_uid: 8crh1yyp Abstract Scanning acoustic microscopy (SAM) is used to characterize welds in a trade thermoplastic polymer (ABS) manufactured by injection-molding, particularly at the locations of weld-lines known to form as unavoidable significant defects. Acoustic micrographs obtained at 420 MHz clearly resolve the weld lines with morphological deformations and microelastic heterogenity. This is also where terahertz (THz) measurements, carried out in support of the SAM study, reveal enhanced birefringence corresponding to the location of these lines enabling verification of the SAM results. Rayleigh surface acoustic waves (RSAW), quantified by V(z) curves (with defocusing distance of 85 µm), are found to propagate slower in regions close to the weld lines than in regions distant from these lines. The discrepancy of about 100 m/s in the velocity of RSAW indicates a large variation in the micro-elastic properties between areas close to and distant from the weld lines. The spatial variations in velocity (VR ) of RSAWs indicate anisotropic propagation of the differently polarized ultrasonic waves. Ulrasound and terahertz (THz) imaging provide access to the mechanical and optical properties of materials, thus giving complementary information. The current study applies ultrasound, specifically scanning acoustic microscopy (SAM) to obtain quantitative information concerning mechanical properties related to injection-mold weld lines in a commercial ABS electrical receptical plate, while THz imaging is applied to map the birefringence and is used to verify the SAM results. Injection molding is one of the most widely used mass-production of manufactured polymers. It is an attractive process because of its versatility in forming various shapes with high degree of complexities [1] [2] . In injection molding, the plastic polymer is subjected to a multi-phase process including heating, plasticizing, pressurizing, and infusion to the mold. Many parameters associated with the various process steps are involved [2] , including melt processing temperature, mold temperature, injection flow rate, and holding pressure [3] [4] . The polymer is therefore subjected to a set of thermomechanical conditions, which result in modification of its intrinsic properties [5] [6] . As a result of the multi-step process, changes in the strength and consequently morphological and structural variations take place [7] . The large pressure variations, for instance, were reported to be substantially influential on the shrinkage and warpage of the product [8] . Ultrasound tomography investigations indicate fast solidification and isotropic or anisotropic shrinkage accompanied by warpage, depending on conditions. The solidification and changes in volume and structure were attributed to the large difference between melt temperature and mould temperature followed by a freezing phase, leading to the occurrence of internal (residual) stresses [7] , [9] . Changes in morphology as manifested by formation of crystalline phases were found not to have a significant effect on the elastic properties [10] . However, during the process, line welds (or knit lines) unavoidably form during the recombination, when two or more flow fronts meet in the cavity during injection moulding [11] . Welds comprise points of weak mechanical properties [12] . They are also known to have an altered optical refractive index [13] . The sample investigated here, is a commercially purchased electrical receptacle plate made up by injection molding of the thermoplastic polymer Acrylonitrile butadiene styrene (ABS). ABS are thermos-copolymers with styrene and acrylonitrile constituting the matrix and polybutadiene or a butadiene elastomers dispersed as a grafted particulate phase, known to be highly adaptable and suitable for injection molding [14] . While SAM is sensitive to the variations in the mechanical and visco-elastic variations at microscopic scales, THz imaging is sensitive to the optical properties at wavelengths between a few tens to hunders of microns. SAM has been employed in the evaluation of molded plastic electronic components to detect defects such as voids, delaminations, and cracks, where water ingress (displacement of air that fills defect-generated cavities by water molecules) was studied [15] . SAM also faciltated the inspection of the microstructural detailed analysis of weld joints and the detection of defects, such as nugget failures. Tittmann et al. [16] elaborate the use of a combination of various SAM-based techniques and report on the high efficiency of such approaches for these purposes. THz imaging was carried out to verify the SAM results. We use a pulsed, broadband THz time-domain system (TeraView Ltd, TPS Spectra 3000) [17] . THz imaging is performed in near normal incidence reflection mode. The sample is raster-scanned by a set of motorized stages moving in the x-and y-directions with transverse step size of 200 m which is also roughly the transverse resolution of the system. We first carry out reflective experiments disregarding polarization. The arrival time at the detector of the THz pulse reflected from the top air/ABS interface provides a way to image the surface morphology on the tens-of-microns scale. Our second set of experiments employed polarization-resolved imaging. A knowledge of the sample thickness (measured by calipers) and the time delay between the reflected THz pulse from the top air/ABS interface and from the bottom ABS/air interface provides the refractive index for the given polarization. Reflection SAM utilizes a focusing piezoelectric transducer that receives a short RF pulse (duration ~ 10-30 ns) and converts it into mechanical (acoustic) waves. The acoustic waves propagate through the lens body ( Fig. 1 ) and are focused sharply by the spherical cavity of the lens into a diffraction limited point at the interface between a coupling fluid and the sample. Reflection occurs when a mismatch in the acoustic impedance is realized at interfaces [18] . For normal incidence, it is the product of the mechanical density and velocity of ultrasound. The reflected signal is received by the transducer and is converted into an electric signal. An image is then formed by scanning in two dimensions and processing the reflected signal into a greyscale map. The bulk and shear moduli of elasticity are related to the velocity of propagation of longitudinal and shear ultrasonic waves, respectively. The velocity of surface acoustic waves (SAW) is related to both compression and shear moduli of rigidity [19] . The attenuation of the acoustic signal passing through the inspected object is related to the viscous properties of the sample in addition to scattering. The spatial variations in the mico-viso-elastic properties, thus, comprise a major contribution to the contrast in SAM. The contrasts are also considerably influenced by the topography, thickness, interferences, and the variations of the reflected signal with the defocus distance of the transducer. A specific feature of SAM is that imaging contrast varies considerably with the distance between the acoustic lens and the specimen. This enables the visualization of subsurface regions depending on the acoustic properties of the specimen [20] [21] and the numerical aperture (opening angle) of the acoustic lens. The ray model illustrated in Fig. 1 provides a simplified view of the excitation and detection of RSAW by defocusing the acoustic lens towards the sample. Acoustic waves incident at the Rayleigh critical angle (ray 1), excite J o u r n a l P r e -p r o o f Rayleigh surface acoustic waves (rays 3) that propagate along a path symmetrical to the exciting rays to the transducer. Rayleigh waves can be excited and detected by appropriate defocusing utilizing an acoustic lens with a sufficiently large aperture that exceeds the critical angle for exciting Rayleigh waves. RSAWs propagate along the surface of the sample and leak energy back into the couplant and thus, excite a compressional wave in the fluid under the Rayleigh angle. The echoes of these compressional waves interfere with the specularly reflected waves (ray 2). The voltage value of the resulting (rectified and integrated) complex signal depends sensitively on the lens-to-sample distance (z) and the wavelengths in the couplant. As the axial spacing between the acoustic lens and the sample surface z changes, the relative phases of the interfering waves vary, so that the superposition alternates between constructive and destructive interference. This effect considerably contributes to the contrast in the acoustic micrographs. A plot over the defocus results in an interference pattern referred to as the V(z) curve containing minima and maxima according to the interference of the two wave modes [22] . The period of the resulting oscillations Δz, in the V(z) curve, can be calculated from the phase relations and forms the basis for calculating the velocity of the RSAWs in the sample surface. Equation (1) gives V(z) in terms of the sample and system parameters as [23] . where P( ) is the pupil function that expresses the angular emission and detection properties of the acoustic lens and is the incidence angle. The integral above considers the overall geometrical incident waves with incidence angles that range from 0° to the half opening of the semi-aperture SA, R( ) is the reflectance function describing the amplitude and the phase of reflected waves as a function of the incident angle , and k is the propagation constant, = 2 , with f the acoustic frequency and v the velocity of propagation of acoustic waves in the coupling fluid. In addition, we have where, Δz is the periodicity of the oscillations in the V(z) curve, 0 is the wavelength of the acoustic waves during propagation in the coupling fluid, and R is the Rayleigh critical angle. The velocity of propagation of the Rayleigh surface acoustic waves VR can be expressed in terms of the ratio to the critical angle R as J o u r n a l P r e -p r o o f The value of VR is related to shear wave velocity of the material VS as [24] ( − ) where, = ( ) 2 /( ) 2 and is Possion's ratio. To inspect the micro-elastic features of the welded sample, high-resolution acoustic micrographs were taken at various positions on the surface of the sample and subsurface imaging was performed for regions close to the weld lines. The SAM used (ELSAM, Ernst-Leitz Scanning Acoustic Microscope; PVA TePla Analytical Systems, GMBH, Westhausen, Germany) allows imaging at frequencies f that range from 400 MHz to 1.2 GHz. A focusing transducer operating at a center frequency of 420 MHz was utilized. Figure 2 shows an optical camera image of the investigated plate, with the specified area A, on the front side. This is a commercially produced ABS electrical receptical plate. The weld line runs horizonatally through the plate and its location is indicated by the red arrows. The weld line is not visible to the eye. We present results for region A shown in the figure. The acoustic micrograph in Fig. 3 is acquired in region A shown in Fig. 1 . Figure 3 (a) shows a weld line forming a trough-like structure in an elevated region (of about 120 µm above the surface of the sample) at the location of the weld. Figure 3 (b) shows the adjacent flat part of the sample, 1500 µm a part of the weld line. The confocal nature of SAM allows the pointwise insonifying and detection of reflection of ultrasound at interfaces between media of different acoustic impedances. Images are formed from the magnitude of the detected signal in a C-scan and represented as a greyscale. The contrast of focused images (performed with the focal distance adjusted to the surface of the sample) originates from the variations in the acousto-mechanical properties, mainly the mechanical density and the micro-elasticity, density, interferences and from wave mode conversions. Acoustic attenuation (viscous losses and scattering) also contribute to image contrast. A considerable source of contrast is, however, in addition obtained by defocusing, during subsurface imaging. A closer look into the microstructure was performed by imaging a smaller field of view (500 µm 500 µm), 2 mm right and 3.5 mm left to the weld line, as shown in Fig. 4a and Fig. 4b , respectively. While the morphological and topographical variations are clearly obvious in the images in Fig. 3 , the strong contrast in Fig. 4 indicates considerable variations in the microelastic properties. The spatially variable strength of the reflected signal indicates variations in the acoustic impedances. The variations in the acoustic impedances may be attributed to corresponding variations in the microelastic moduli, resulting in a spatially varying acoustic propagation velocity and mode conversion.Variations in the mechanical density could also contribute to the mismatch in the acoustic impedance, in addition to viscous losses, as the received echoes traverse regions over which the extinction coefficients vary [25] . A stack of images was taken by scanning in C-mode (in two dimensions), while translating the acoustic lens in the axial (orthogonal) direction (regarded as z-direction). Scanning was started slightly further from the focal plane and moving stepwise, passing through the focal region toward the sample (z in Fig. 1 ). More steps of scanning were performed in the z-direction (subsurface imaging), where the corresponding V(z) curves carry the information about the surface acoustic waves (SAW) and their interferences. The stack of images in each measurement lasts about 8 minutes with the current technology. The V(z) curves generated for the areas within the sample as shown in Fig. 3(a) , are depicted in Fig. 3(b) . The spacing distances Δz were calculated and the Rayleigh angle as well as the velocity of propagation of RSAW, VR were computed as indicated in Table 1 . The closest area to the weld line that is flat permitting acquisition of V(z) curves was 2 mm from the line, as shown in Fig. 3(a) . The average velocities of Rayleigh surface acoustic waves (VR) were computed using Eqs. (2) and (3) to be 1535±27 m/s. VR corresponding to V(z) curves generated for the flat area 3.5 mm to the left of the weld line have an average value of 1624±29 m/s. VR is related to the velocity of the shear and compressional waves propagating in isotropic samples for the same Possion's ratio [24] [26] . Therefore slower VR in regions closer to the weld lines indicate lower compression and shear moduli and consequently lower Young's modulus than regions located within further distances [27] . These findings are consistent with reported low rigidity near weld lines. Areas of weld lines were reported in the literature to have reduced mechanical properties, as investigated for polymers with different filler nanoparticles using x-ray diffraction and scanning calorimetry [28] . A significant decrease in the tensile strength and impact properties in weld lines was also realized in scanning electron microscopy measurements, and was related to unfavorable fiber orientation beside the weld lines [29] . The flow induced molecular orientations were found to be different close to the weld line than they are away from the line [30] . High likely the orientation at areas close to the weld lines is not parallel to the residual stress leading to lower stifness [31] . The residual stresses known to be anisotropic [32] , were reported to have a complicated spatial distribution in welds and accordingly the velocity of surface acoustic waves varies close to a weld line than a way from it (acoustoelastic effect) [33] . Average 1535±27 1624±29 Table 1 . VR computed from the V(z) curves generated for the regions located at a distance of 2 mm right to the wild line as in Fig. 5a and for the regions located at a distance of 3.5 mm right to the wild line as in Fig. 6a . We next carried out THz imaging to verify the SAM results. Figure 7 shows a THz C-scan in which the contrast mechanism is the arrival time of the echo from the top air/ABS interface, thus providing a map of the surface morphology. Later arrival time indicates the V-shaped groove associated with the weld evident in the image. The width is 400 m and the depth 10 m. The width is close to the value obtained by SAM considering that the transverse resolution of the THz image is ~ 200 m. In Fig. 8 is plotted the birefringence n along a section perpendicular to the weld obtained from polarization-resolved reflective measurements. A knowledge of the plate thickness (measured with calipers) and the time delay between the pulse reflected from the top air/ABS interface and from the back ABS/air interface as a function of polarization provides n. Note the birefringence falls of on the 1mm scale from the weld line. This is somewhat larger than the width of the surface morphological feature (V-shaped groove) and indicates that frozen-in strain and/or molecular orientation anisotropy extends somewhat further away from the weld than the morphological feature. The deployment of many promising materials has been hampered by poor strength at certain locations. Specifically, injection-mold lines are often associated with poor mechanical strength. In this study, we apply the technique of SAW to explore the acoustic properties near injection-weld lines by comparing the velocity of (VR) in locations close to and locations distant from the weld lines. We realize a considerable discrepancy in the VR, indicating that the elastic moduli are much lower close to the weldline than distant from the weldlines and indicate anisotropic propagation. Beside the mechanical weakness close to weldlines, the microstructures visualized based on the micro-elastic contrasts indicate heterogeneity of the surface. Comparisons were also made with THz imaging. Time-of-flight measurements indicate a Vshaped groove of width 400 m and depth 10 m. This width is in good agreement with the SAM measurements. We then characterized THz birefringence as a function of distance from the weld and find enhanced birefringence at the weld falling off to a background value over the 1-mm scale from the weld. This indicates that frozen-in strain and/or molecular anisotropy extends further from the weld than is indicated by the width of the V-shaped groove. In conclusion, SAM, in conjunction with THz birefringence imaging, may serve as a useful quality controle combination to characterize local mechanical properties in a range of materials conducive to these techniques. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The raw/processed data required to reproduce these findings cannot be shared at this time due to technical limitations related to our Corona-virus lock-down. 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