Studio teorico-sperimentale sulla asportazione mediante laser di polimeri acrilici al fine della realizzazione di prototipi

SOMMARIO: Questo lavoro ha come oggetto l’analisi di una tecnica innovativa di asportazione di materiale su di un polimero con adeguate caratteristiche (il polimetilmetacrilato PMMA), attraverso un fascio laser a CO2, con lo scopo di realizzare cavità tridimensionali da utilizzare come stampi di colata per la produzione in serie di prototipi. Nella prima fase di questo studio, si esamina la tecnica della vaporizzazione laser della superficie del polimero fornendo un semplice modello in grado di prevedere la profondità e la larghezza delle tracce ottenute. In particolare viene esaminata l’influenza della sequenza di taglio, del numero di passate, della velocità di scansione e della potenza associata alla radiazione laser. Viene presentato, inoltre, un metodo pratico per variare il profilo delle tracce usando più scansioni sovrapposte. Nella seconda fase è stata realizzata l’asportazione selettiva di uno strato di materiale attraverso l’affiancamento di più tracce rettilinee e variando il parametro del passo di scansione. Nella terza parte di questo studio, sono stati esaminati gli effetti della vaporizzazione di strati successivi sulla rugosità della superficie e sulla profondità di asportazione. Si evidenzia che, con la tecnica sviluppata, la profondità di asportazione varia proporzionalmente con il numero degli strati mentre la rugosità tende ad assumere un valore costante (Rz(DIN)=[80-90]μm). Tutte le prove sono state ripetute due volte. Infine, utilizzando i risultati sperimentali, sono stati realizzati diversi prototipi colando resina siliconica all’interno delle complesse impronte, ricavate per vaporizzazione laser nel PMMA, che non sarebbero potute essere ottenute mediante tecniche convenzionali di asportazione di materiale. Theoretical and experimental analysis of a laser machining technique on acrylic polymers for prototypes fabrication Keywords: laser, vaporisation, polymethylmethacrilate, prototypes ABSTRACT: This work focuses on an innovative technique of complex CO2 laser machining in order to create, by vaporisation of a suitable polymeric material (polymethylmethacrilate PMMA), three-dimensional cavities to be used as molds for rapidly producing of prototypes. At the beginning the cutting process with the laser beam is analized through theoretical models for estimating the shape depth and width of micro-slots, obtained by vaporisation of the polymeric surface. In particular, the influence of the cutting sequence, the number of passes, the laser beam speed and the radiant flux are examinated. A method of changing the channel profile using multiple passes in the same position is also presented. In the second step of this work, microablated surfaces are generated using multiple overlapping grooves and varying the scan spacing parameter. In the third step the effects of machining layer by layer on surface roughness and removal depth are investigated; with the developed technique it’s proved that removal depth varies proportionally with number of layers machined while surface roughness tends to assume a costant value(Rz(DIN)=[80-90]μm). All tests have been repeated twice. Using all the acquired data, several prototypes have been finally carried out casting silicone resin into three-dimensional PMMA molds manufactured by the proposed technique and not obtainable by conventional removal processes

ADVANCED MONITORING METHODS AND CONTROL OF PRODUCTION PROCESSES

In global competitive environment companies have experienced that even in crisis periods their success depends on the diversification of products, on the conquest of new market niches and at the same time on the reduction of production costs. In this context the strong need to increase production performance, maintaining high quality standards, has motivated the development of devices for monitoring and controlling the production systems. In the meanwhile, efforts to streamline the production process have been made by replacing, where possible, metals and alloys with unconventional materials such as engineering plastics and composites. These materials offer comparable mechanical properties combined to a very considerable saving in weight and processing time, but require, at the same time, developments of new tools, appropriate processing parameters, alternative and cheaper production technologies. Strong inputs to research are therefore imposed by such difficult-to-machine materials. These materials also increase the interest of companies also for manufacturing technologies that are mostly retained non-conventional and need to be widely investigated. As it happens so often in the field of production, the introduction of an innovative technology or analysis of the workability of a particular material requires a substantial effort to consolidate, through numerous tests, the actual potential. Research in production engineering is therefore aimed at understanding the physical phenomena that govern the mechanism by which matter is transformed and determine the optimal values of the main processing parameters in order to achieve a predetermined goal. However it may happen that often the complexity of a process makes it difficult to analytically describe the physical reality: an example of such problem is given by the machining of materials which significantly differ from the assumptions of homogeneity and isotropy and by material removal techniques in which the interaction with the workpiece is not mechanical. In these cases the theoretical and experimental approach afore mentioned, can be further supported by the use of monitoring systems. These systems help to highlight the complex relationships between process parameters and the effects they produce on the workpieces

Experimental study on the development of a micro-drilling cycle using ultrashort laser pulses

Microholes for the production of high precision devices were obtained by ultrashort pulsed laser machining of martensitic stainless steels. A micro-drilling cycle based on the sequence of a drilling through phase, an enlargement and finishing phase is proposed in order to solve the trade-off between process time and quality of the ablated surfaces without making use of complex design of experiments. The three phases were studied taking into account the evolution of the microhole shape as a function of the main process parameters (number of passes per phase, incidence angle and radius of the beam trajectory respect to the hole's axis). Experiments testified that the drilling strategy was able to produce cylindrical holes with diameter of 180±2 μm on a 350 μm thick plate in total absence of burrs and debris within a drilling time of 3.75 s. Repeatability tests showed a process capability of nearly 99%. SEM inspection of the inner surface of the microholes showed the presence of elongated and periodic ripples whose size and inclination can be controlled adjusting the incidence angle of the beam over the tapered surface before the ultimate finishing phase

Fatigue analysis of adhesive joints with laser treated substrates

Abstract Recent literature works focused on the analysis of laser irradiation on the strength of adhesive joints under quasi-static loading conditions. It has been demonstrated that laser surface preparation allows to remove impurity and weak boundary layers from the mating substrates and, depending on the energy density, it is also able to modify surface morphology promoting mechanical interlocking. In previous works, the authors assessed the effect of Yb-fiber laser ablation over the quasi-static strength and toughness, of aluminum and stainless steel adhesively bonded joints. The experimental results demonstrated the ability of laser irradiation to improve the mechanical properties of the joints. The aim of this work is to extend the scope of previous investigations to fatigue loading. Double Cantilever Beam (DCB) samples with laser treated aluminum substrates have been bonded with a two component epoxy adhesive. For comparison standard degreasing and grit blasting have been also deployed for samples preparation. The results have been compared in terms of cycles to failure and the fracture surfaces have been analyzed by means of Scanning Electron Microscopy (SEM) in order to investigate the mechanism of failure

Experimental characterization of the inner surface in micro-drilling of spray holes: A comparison between ultrashort pulsed laser and EDM

In this research, the inner surface characteristics of micro-drilled holes of fuel injector nozzles were analyzed by Shear Force Microscopy (SHFM). The surface texture was characterized by maximum peak-to-valley distance and periodicity whose dimensions were related to the adopted energy. 180 μm diameter holes were drilled using ultrashort pulsed laser process using pulse energies within the range of 10-50 μJ. Laser ablated surfaces in the tested energy range offer a smooth texture with a peculiar periodic structure with a variation in height between 60 and 90 nm and almost constant periodicity. The Scanning Electron Microscopy (SEM) photograph of the Laser Induced Periodic Surface Structure (LIPSS) showed the co-existence of Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). A comparative analysis was carried out between the highest laser pulse energy in the tested range energy laser drilling which enables the shortest machining time and micro-Electrical Discharge Machining (μ-EDM). On the contrary, results showed that surfaces obtained by electro-erosion are characterized by a random distribution of craters with a total excursion up to 1.5 μm with a periodicity of 10 μm. The mean-squared surface roughness (Rq) derived from the scanned maps ranges between 220 and 560 nm for μ-EDM, and between 50 and 100 nm for fs-pulses laser drilling

Multi-response Optimization of Laser Welding of Stainless Steels in a Constrained Fillet Joint Configuration Using RSM

This paper presents experimental design approach to process parameter optimization for CW Nd/YAG laser welding of ferritic/austenitic stainless steels in a constrained fillet configuration. To determine the optimal welding parameters, response surface methodology was used to develop a set of mathematical models relating the welding parameters to each of the weld characteristics. The quality criteria considered to determine the optimal settings were the maximization of weld resistance length and shearing force, and the minimization of weld radial penetration. Laser power, welding speed, and incident angle are the factors that affect the weld bead characteristics significantly. A rapid decrease in weld shape factor and increase in shearing force with the line energy input in the range of 15-17 kJ/m depicts the establishment of a keyhole regime. A focused beam with laser power and welding speed respectively in the range of 860-875 W and 3.4-4.0 m/min and an incident angle of around 12° were identified as the optimal set of laser welding parameters to obtain stronger and better welds

Metal micro drilling combining high power femtosecond laser and trepanning head

Trepanning heads are well known to be efficient in high aspect drilling and to provide a precise control of the hole geometry. Secondly, femtosecond lasers enable to minimize the heat effects and the recast layer on sidewalls but are typically used on thin sheet. The combination of both present a high potential for industrial applications such as injector or cooling holes where the bore sidewall topology has a major influence on the dynamics of the gas flow. In this paper we present results using this combination. The effect of pulse energy, repetition rate and revolution speed of the head on both geometry and roughness are discussed. The quality of the sidewall is checked by roughness measurement and by metallographic analysis (SEM; chemical etching, micro hardness)

Laser beam welding of dissimilar stainless steels in a fillet joint configuration

This paper investigates laser beam welding of dissimilar AISI 304L and AISI 430 stainless steels. Experimental studies were focused on effects of laser power, welding speed, defocus distance, beam incident angle, and line energy on weld bead geometry and shearing force. Metallurgical analysis was conducted on a selected weld only to show various microstructures typically formed at different zones and consequent change in microhardness. Laser power and welding speed were the most significant factors affecting weld geometry and shearing force. All the bead characteristics but radial penetration depth decreased with increased beam incident angle. The focused beam allowed selecting lower laser power and faster welding speed to obtain the same weld geometry. Weld shape factor increased rapidly due to keyhole formation for line energy input ranging from 15 kJ/m to 17 kJ/m. Fusion zone microstructures contained a variety of complex austenite-ferrite structures. Local microhardness of fusion zone was greater than that of both base metals

direct laser interference patterning of stainless steel by ultrashort pulses for antibacterial surfaces

Abstract Direct Laser Interference Patterning (DLIP) with ultrashort pulses was exploited to produce tailored periodic sub-micrometer structures on stainless steel surfaces to reduce bacterial attachment and retention. Laser pulses with wavelength 1030 nm and duration 8 ps were employed to form a two-beam line interference pattern that was applied in a two-pass strategy to produce fine cross-wise surface structures with a period of ~ 850 nm and a depth of ~ 500 nm. The laser setup and process parameters were selected based on a simple theoretical model of the resulting interference pattern and ablation depth to limit the number of contact points available for bacterial cells with dimensions 500–2000 nm. Periodic 'cones' and 'holes' were produced covering areas of 250 mm2 with the same interference pattern by exploiting the dependence of laser-induced periodic surface structures on polarization. Cones and holes yielded reductions in E. coli retention of 99.8% and 99.4%, respectively, and S. aureus retention of 70.6% and 79.1%, respectively, after two hours immersion in bacterial solution compared to reference samples. Such reductions achieved over large surface areas suggests that this approach is appropriate for upscaling and high throughput production of antibacterial metallic surfaces in the food and healthcare industries