Tomaž Kek

Assessment of Laser-Cut Quality on the Basis of Acoustic Emission Signals Captured with a PZT Sensor

The paper describes an analysis of the AE signals captured with PZT sensor during and after laser cutting. In the course of cutting the continuous AE signals are treated in a defined time interval. After cutting, bursts of acoustic emission will occur. They are due to solidification of the molten material and cooling of the laser-cut surface. The results shown refer to cutting of steel plate DC04 and of austenitic stainless steel plate X5CrNi18-10. Both plate types are frequently used in automobile industry, where, because of mass production, optimisation of the cutting process is desired. Numerous laser-cutting conditions assuring different quality levels of the cut made were chosen. It was proved that the laser cutting conditions had a significant influence on the amplitude value and intensity of the continuous signal and AE activity after the termination of laser cutting. Separately it was also confirmed that there was a relation between the measured signal and laser-cut quality, which is shown, to a large extent, by the presence of dross at the lower edge of the cut.

Detection of Damaged Tool in Injection Molding Process with Acoustic Emission

ABSTRACT The measurement of acoustic emission (AE) signals during injection molding of polypropylene with new and damaged mold is presented. The damaged injection mold was fitted with a steel insert with cracks induced by laser surface heat treatment. Two resonant piezoelectric AE sensors were attached to the mold via AE waveguides. To improve the mold integrity prediction with smaller defects, AE signal frequency characteristics and a measure of AE signal amplitude probability distribution are implemented. A 5-dimensional feature vector with real-valued explanatory variables is proposed, providing the defining points in an appropriate multidimensional space to characterize the state of injection molding tool. Feature vectors are classified with neural network pattern recognition. The results confirm that presented AE technique offers characterizing the integrity of molds also with resonant sensors.

Use of acoustic emission testing in injection moulding process

This paper presents the experimental results regarding acoustic emission signals measured during the injection moulding of those standard test specimens commonly used for examining the shrinkage behaviour of various thermoplastic materials. The acoustic emission was measured on a new injection mould and injection mould with the visible cracks on the cavitys surface. Two resonant 150 kHz piezoelectric AE sensors were attached to tool steel inserts via AE waveguides. The results of acoustic emission testing on the crack defected tool steel insert revealed higher acoustic emission activity compared with that captured on the brand new engraving insert under same processing conditions. Specimens produced with a new and defected tool were scanned with an optical 3D digitiser. Results of the experiments confirm that acoustic emission measuring during injection moulding of polymer materials is promising technique to characterise integrity of injection moulding tool regarding occurrence of cracks.

AE signal measurements during laser cutting of structural steel sheet and deep-drawn parts

The study treated is focused on quality assessment of a laser cut on the basis of the AE signals captured with PZT sensors. The paper describes an analysis of the AE signals measured during and after laser cutting. In the course of cutting, the continuous AE signals are treated in a defined time interval. After cutting, AE bursts will occur. They are due to solidification of the molten material and cooling of the laser-cut surface. The results shown refer to cutting of steel plate DC04 that is frequently used in automobile industry, where optimisation of the cutting process is desired. Numerous laser-cutting conditions assuring different quality levels of the cut made were chosen. It was proved that the laser cutting conditions had a significant influence on the continuous and burst AE. Separately, it was also confirmed that there was a relation between the measured signal and laser-cut quality.

Choosing a Laser-Cutting Technology to be Applied to a Deep-Drawn Product

Residual stresses are an inevitable result of mechanical and heat treatments of steel. The stress field generated at the surface and through the cross section of a product is usually non-uniform, and there can be important stress gradients. In general residual stresses can be defined as self-balanced internal stresses existing in a free body on which no external forces are acting. In the present study the focus was on the residual stresses occurring in a steel deep-drawn product of a complex form. The residual stresses occur mainly because of different degrees of plastic deformation at different locations. The residual stresses occurring in the product as a result of the manufacturing process of deep drawing can be comparatively strong. In slitting or cutting a useful product out of a deepdrawn part by laser cutting, a portion of the residual stresses will get relieved and produce distortion of the deep-drawn product. The studies made showed that the distortion of the deep-drawn product depended on the mode of cutting-out. Also the state of the residual stresses after slitting or cuttingout of the deep-drawn product will depend on the mode of cutting-out, i.e., on the sequence of individual cuts chosen. The product used in the analysis made was a deep-drawn steel mud-guard, which makes a component part of a working machine. The various modes of cutting-out a useful product of 1260x800x370 mm in size were performed with laser cutting using cutting oxygen. The residual stresses in the mud-guard were determined with a semi-destructive hole drilling strain gage method.