Chair of Optodynamics and Laser

Applications

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Introductory notes

CCOONNTTEENNTTSS ........................................................................................................ 1 INTRODUCTION

...................................................................................................... 3 FUNDAMENTALSUsing short laser pulses.......................................................................................................... 3 Interaction laser beam / workpiece material – the ablative processing regime...................... 5 Beam quality .......................................................................................................................... 5 Mark formation – deflection system and image conversion .................................................. 6 Processing parameters ............................................................................................................ 7

........................................................................................ 8 EXPERIMENTAL SYSTEM........................................................10 EVALUATION OF THE PROCESS OUTCOME

Visual appearance (Aesthetics) ............................................................................................ 10 Contrast measurement .......................................................................................................... 10 Workpiece examples and explanation of main drawbacks .................................................. 11

............................................................................................................12 LITERATURE

Laser Marking LASTED.NET

Introduction Laser marking is a method for labeling various kinds of objects using a laser [1, 2]. The basic principle of laser marking is that a laser beam somehow modifies the optical appearance of a surface it hits. This can occur through a variety of mechanisms:

• localized melting or vaporization (ablation - laser engraving and deep engraving); sometimes removing some if the colored surface layer (most commonly used mechanism),

• localized change of surface microstructure (annealing),

• slight burning (carbonization) e.g. of paper, cardboard, wood or polymers,

• transformation (e.g. bleaching) of pigments (industrial laser additives) in a plastic material,

• expansion (foaming and microcracking) of a polymer, if e.g. some additive is evaporated,

• generation of surface structures such as small bubbles.

Laser marking can be performed on wide palette of materials where efficiency of the method depends on the absorption of the laser light in the material (Figures 1 and 2). Plastic materials, wood, cardboard, paper, leather and acrylic are often marked with relatively low power CO2 lasers operating at the 10.64 µm wavelength (Figure 2). For metallic surfaces lamp- or diode-pumped solid-state lasers (typically Q-switched) or fiber lasers are more appropriate (Figure 1). They typically operate at one of the following wavelengths: 1.064 µm (fundamental), 0.532 µm (frequency doubled), 0.355 µm (frequency triplet). Highly transparent materials such as glasses can be marked using lasers that emit UV light (e.g. excimer lasers).

Figure 1: Absorption of metals at different wavelengths

Figure 2: Absorption of nonmetals (1-polymers, 2-pigment, 3-fillers) at different wavelengths

To generate a required mark, the laser beam should be someway projected on the workpiece surface. Various beam delivery methods have been developed to produce laser markings [2]. The mark may be formed by illuminating a mask, or reticule, that contains the desired pattern. This is a very fast method if the mask does not need to be changed. Other fast beam projection techniques are laser array marking and raster marking with multi-faceted mirror. A widely used method is based on beam deflection using two orthogonally movable mirrors which are controlled by a computer. Software is available which allows marking with high flexibility

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Laser Marking LASTED.NET

and speed. Beam deflection marking systems can quickly write letters, symbols, bar codes, and other graphics. Mark generation can be applied even to moving workpieces (»mark on the fly«).

Laser marking has a huge variety of applications:

• adding part numbers, "use by" dates and alike on food packages, bottles, etc.

• adding traceable information for quality control

• marking printed circuit boards (PCBs), electronic components, and cables

• producing logos, bar codes and other information on products,…

Laser marks on polymer materials.

Laser marks on metals.

Laser mark on a mirror (metallic coating on glass)

Laser mark with grayscale tones on a stainless steel plate

Laser mark on leather. Laser mark on artificial leather.

Figure 3: Examples of laser marks on different materials

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Laser Marking LASTED.NET

Compared with other marking technologies, such as ink jet printing or mechanical marking, laser marking has a number of advantages:

• very high processing speeds,

• low operation cost (no use of consumables),

• constant high quality and durability of results (laser markings are resistant to abrasives, heat, UV light, chemicals,…),

• process cleanness (no paints, inks or acids are used, which could contaminate the product),

• non-contact processing (no tool wear, no workpiece distortion),

• the possibility to mark fragile, soft and hard surfaces,

• very high flexibility (markings are changed through the changes of input data to the control software),

• the possibility of automation and integration into high volume, high speed process lines.

The main disadvantage of laser marking is the high initial investment cost.

Laser marking is widely used in industrial practice. According to a recent market survey by Optech Consulting, Tägerwilen, Switzerland, newly installed laser marking systems accounted for about 14% of the total world market of processing laser systems by sales revenue (Figure 4).

Figure 4: World market for laser materials processing systems by application (Source:

Optech Consulting)

Fundamentals

Using short laser pulses In this experiment you will use a pulsed solid-state laser emitting at the 1.064 µm wavelength to produce laser markings (engravings) on a stainless steel plate. At this wavelength, steel absorbs light reasonably well in comparison with the color metals (Figure 1). This fact will allow you to produce clear markings even on a shiny stainless steel surface.

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Laser Marking LASTED.NET

The fundamental marking mechanism employed in this case is laser engraving, which involves localized melting and vaporization of the surface. Visibility of the marking is provided by different optical properties between the marked (resolidified/vaporized) and non-marked parts of surface.

Localized melting and vaporization of the surface shall be achieved using a laser beam that consists of a train of short bursts of light (laser pulses). Figure 5 illustrates schematically this situation. For the purpose of gaining a basic understanding of the order of magnitude of quantities involved, we shall assume that pulses are perfectly rectangular: the laser emits light only during a time interval Tp (pulse duration) and during this interval the emitted power Pp (peak power) of the laser beam is constant. This assumption will allow us a significant simplification of the required calculations. In our experiment, Tp will be about 35 ns.

Figure 5: Simplified temporal history of the laser output power during laser marking employing the engraving mechanism.

Using such short laser pulses we do not allow the heat, which is generated by means of light absorption, to diffuse into the surrounding material. Therefore, we compress the available pulse energy into a smaller volume and thus achieve higher temperature increase in that volume. This is important because we want to raise the temperature of a small amount of material absorbing the laser light above the temperature of melting /vaporization.

The other reason why we want to use short laser pulses is the fact that this allows us to increase the peak power Pp of the pulse. Typical parameter values used in laser marking are:

Pulse duration (Tp): 30 to 300 ns,

Repetition period (Tr): 50 μs to 2 ms,

Average power (Pa): 10 W to 100 W,

Pulse energy (E): 1 mJ to 10 mJ.

Suppose we have E = 1mJ, Tp = 35 ns, and Tr = 100 μs. Using formulas from Figure 5 we can calculate the average power Pa = E/Tr = 1 mJ/100 μs = 10 W and the peak power Pp = E/Tp = 1 mJ/35 ns = 29 kW. From this figures we realize that the peak laser power is 2900 times larger than the average laser power. Laser sources used for this type of marking generally include a device called “Q-switch” which enables energy accumulation within the active (lasing) medium with no light output during most of the pulse repetition period and a quick release of the accumulated energy in a very short time interval.

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Laser Marking LASTED.NET

Interaction laser beam / workpiece material – the ablative processing regime Figure 6 illustrates the situation when laser beam pulse (pulse length > 100 ps) hits the workpiece surface. A part of the incident light energy is absorbed in the material (figures 1 and 2) and transformed into heat. Depending on material conductivity and thermal capacity the bulk material temperature rises. When temperature reaches a certain threshold value, melting and vaporization appear. Parts of the heated material that reach such high temperatures are removed by means of vaporization and vapor-caused eject of melt. A round crater with diameter approximately equal to focus beam diameter is formed. Solidified melt forms a recast layer on the crater wall (the sides and bottom of a laser mark) and condensed vapor is deposited as debris on the surface around the interaction region. These phenomena (debris and recast layer) may adversely influence the laser mark visibility so they should be minimized by proper choice of processing parameters. Generally, shorter pulses are more efficient (heat concentration at interaction point) and produce less unwanted depositions near the processing area. In addition, at a given beam energy E the peak beam intensity (power / area) is higher at shorter beam pulse, so material removal threshold values are attainable.

Figure 6: Illustration of the phenomena that accompany laser beam/material interaction.

Beam quality Laser beam quality is a figure of merit for the ability to concentrate the beam to a very small diameter. Laser beam quality is a property of a laser source and can not be improved by any optical system outside the source without considerable loss of power/energy. Practical laser sources differ significantly in the quality of their beams. Similarly, various laser manufacturing processes exhibit different beam quality requirements. Figure 7 shows a comparison of the major processing parameters (beam quality and power) required by different laser manufacturing processes. Marking requires rather high quality laser beam of medium average power. The reason for that is the fact that we usually require very fine features to be visible in laser marks.

Laser beam quality is closely related to the transverse shape of the beam. High quality beams have the maximum intensity on the beam axis and gradually and monotonously decreasing intensity in directions away from the beam axis. The highest quality beams exhibit transverse intensity profiles that have that shape of the Gaussian (bell shaped) curve and are thus called Gaussian beams. The beam of the laser employed in the experiment is nearly Gaussian.

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Laser Marking LASTED.NET

Figure 7: Comparison of major processing parameters required by different laser manufacturing processes. Marking requires rather high quality laser beam of medium average power.

Mark formation – deflection system and image conversion A single laser pulse produces only a single crater. In order to produce a mark, we guide the processing beam in two orthogonal directions by a system (called “scan head”) that uses two mirrors (one mirror for each direction) which are tilted (oscillatory rotated) under computer numerical control. By rapid movement of the mirrors and synchronized turning on and off of the laser output we produce a series of craters which give the appearance of the laser mark. Mirror tilt is realized by two »galvo motors« (tilt is proportional to the voltage applied to the motor) which are driven and position feedback controlled by a special PC interface board (Figure 8). Mirror tilting speed defines »marking speed« i.e. speed of translation of the beam incident point over the workpiece surface.

The task of the flat-field lens, located after the two mirrors, is to focus laser beam on the workpiece surface and to keep focus on the flat surface when the beam is deflected from its central position (focus position lies on the sphere when ordinary lens is used). The flat-field lens is sometimes called F-theta lens.

Figure 8: Basic configuration of the 2-D deflection system using orthogonal mirrors.

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Laser Marking LASTED.NET

With 2-D deflection system two main modes of mark generation can be performed:

• raster mode, where beam scans the area (like electron beam of CRT) and mark is generated by switching ON/OFF (pulsing) the laser beam. The image is converted into a matrix of dots in rows. For »black dot« the beam pulse is initiated. Raster mode is generally used for photograph images marking.

• vector mode, where laser beam is guided through a series of equally displaced locations on the mark contour (laser beam acts as a pen). The image is converted into a series of coordinates to direct the laser beam which is switched on and consequent laser pulses are generated with certain frequency respectively while mark is processing. Exception is the beam jump from one to another contour, when the laser beam is switched off. Vector mode is generally used for text, barcode and similar images marking where contour must be precisely defined. For marking of »black areas« image must be transformed into crowd of parallel vectors (series of coordinates) which »fill« the area.

Processing parameters In general, four process parameters define the process outcome: laser output power, marking speed, pulse repetition frequency and fill distance (and inclination). All of them are adjustable in the »settings panel« of the user interface – marking software according to instruction in »Experiment conduction«.

Laser output power Intensity of a laser beam directly influences the intensity of ablation process. Higher beam intensity increases the amount of melted and vaporized material so thus formed crater is deeper and consequently debris and recast layer is more noticeable. The increase of beam intensity usually increases mark contrast, however this correlation can exhibit material dependence and should be practically tested. Laser beam intensity depends on the size of a beam on the workpiece, which is kept constant, and on laser beam power which is adjustable.

Marking speed and pulse repetition frequency A great role on mark contrast has a so-called »overlapping« of laser pulses. Consecutive interactive dark spots on workpiece surface will overlap which depends on pulse repetition and marking speed. Overlapping area acts as an area where doubled (multiplied) number of pulses interacts with a material. In general, the lower the marking speed and the higher the repetition, the more intensive is ablation effect and the higher is mark contrast. Even some other mark properties can be regulated by ablation intensity (gray scale hues). Furthermore, at higher overlapping the mark precision is greater because of lower mark edge jag. Conditions are shown in Figure 9 (constant beam intensity over the whole spot area is assumed). However, if a marking speed is decreased, the whole marking process is prolonged and such situation should be avoided when marking is used as an industry application. Generally, if laser pulse repetition frequency is increased we obtain longer laser pulse durations and consequently a decrease in pulse peak power – decrease of ablation. Adjustment should be a compromise between process speed and the quality of a mark.

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Laser Marking LASTED.NET

Figure 9: Mark properties dependance on overlap rate

Filling areas Overlapping effect must be already taken into account at vectorisation of image’s black areas where distance between two adjacent vectors (fill distance) has the same role as distance d (between two spots). The quality of the filled area is also slightly dependent on the orientation of the parallel vector lines, especially if the filled area is generated by multiple beam passes.

Experiment

The goal The aim of this remote experiment is to give the student an opportunity to grasp a basic understanding of the process and to gain »hands-on« experience in operation of a laser marking system. Specifically, the student will learn how to prepare a computer generated mark and import it into the laser marking system. In the next step, the student will select the processing parameters and set up the marking system exercising a full remote control of the marking system. The progress of the laser processing can be monitored via a remotely controlled video camera. The video stream may be captured on the remote computer for a reference and replayed later with the aim to analyze the progress of the process. After the experiment, the student may order and download high resolution color photographs of the produced marks for the immediate evaluation of the outcome. The marked workpieces shall be mailed to the student upon request allowing detailed analysis at a later date. The experiment requires no special software to be preloaded on the remote operator’s computer other than a standard version of the Windows XP operating system. A wide-band internet connection is necessary.

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Laser Marking LASTED.NET

The set-up

Figure 10: Experimental set-up:

1 … laser source

2 … scanning head (galvo motors, mirrors, lens, controller),

2a … flat-field lens

3 … workpiece

4 … positioning platform,

4a … fine positioning

4b … coarse positioning

5 … camera

Not shown: laser power supply, personal computer.

Laser marking system LPKF MarkLine 8V • Diode pumped Nd:YVO4 (Neodymium vanadate) laser 1064 nm,

• Pulse repetition frequency (PRF): 0-100 kHz

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Laser Marking LASTED.NET

• Pulse duration: 35 ns @ 30 kHz

• Average output power: > 8W@10 kHz

• Working area: 100x100 mm

• Engraving speed: 0-6000 mm/s

• Focal length: 160 mm

• Laser beam diameter in focus: 25 µm

• 2-D scanning head: SCANLAB SCANgine & RTC3 PC interface board

• User interface: marking software SCAPS SAM Light

Evaluation of the process outcome Evaluation can include contrast estimation, mark edge jag measurement and fill quality evaluation considering mark process duration.

Visual appearance (Aesthetics)

Visual appearance is performed by visual inspection and includes uniformity of mark black regions, jag and resolution of thin dark regions, visual appearance of marked lines (mark outline). Appearance should be assessed with numbers from 0 (very bad looking mark) to 10 (perfect mark). For the purpose of inspection the macro- and micrographs of the workpiece will be sent to the student-operator.

Contrast measurement Contrast should be measured by the mean of gray-scale intensity (colour) of the mark regarding to mark’s surrounding. Ordinary 8 bit gray hue of a bitmap (BMP) image is represented as a number between 0 (black) and 255 (white). Contrast should be expressed in the terms of the visibility V:

sm

ms

IIII

V+−

= ,

where Im is intensity of mark’s dark region and Is is intensity of mark’s surrounding. Visibility range is between 0 (mark can not be recognized ) and 1 (the darkest mark). For the purpose of a contrast measurement a color photograph of the workpiece should be first converted into a gray-scale image. An area should be chosen where the region of the mark and unmarked region have equal surfaces. The average gray-scale hue is calculated for both regions (we recommend MATLAB or similar tool). Colour represents area’s intensity so calculated average values can be used for calculation of visibility (equation above).

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Laser Marking LASTED.NET

Workpiece examples and explanation of main drawbacks For example, a marking galvanized mild steel workpiece was used which is not the perfect material. Much better results can be obtained by choosing INOX steel or eloxated aluminum as workpiece material. The following photographs show the influence of the processing parameters on the process results. The parameters are given beside each figure and should be used as the initial ones.

This figure shows the marking process outcome when the proper parameters were used (laser power, marking speed, overlapping and fill distance). In this case, the mark contrast is satisfactory and there are also no visible jags on the mark edges. Power ........................... 70 % Marking speed............ 500 mm/s Repetition frequency.... 45 kHz

Fill distance.................. 25 μm

The mark surface is burned because the entering energy per surface unit was too high. An increased amount of debris on the mark surface and its surroundings is noticed. On the other hand, the contrast is greater than in the previous case and debris can be removed by physical cleaning – brushing. The appearance of debris is especially noticeable when INOX material is marked. Power ........................... 70 %

Marking speed............. 200 mm/s Repetition frequency..... 45 kHz Fill distance................... 25 μm

When the fill density (distance between lines) is too large, separate lines can be seen in the filled regions of image. Contrast is lower, however overall process speed is higher. When the consummation of time for the whole marking is more important than the mark contrast, such process parameters adjustment can be used. In this case the orientation of the fill vector lines could be an important factor. Power ............................. 70 %

Marking speed............ 1049 mm/s Repetition frequency...... 30 kHz Fill distance................... 0,1 mm

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Laser Marking LASTED.NET

Too high processing speed and too large distance between lines reflect on a spotty mark surface. This happens due to the poor overlapping. Contrast is low and mark edges are jagged. This is a typical result when the need for the speedy mark completion overcomes the need for the quality of the mark. Power ............................. 95 % Marking speed............ 3000 mm/s

Repetition frequency...... 30 kHz Fill distance................... 0,2 mm

Literature

[1] J.T. Luxon, Industrial Lasers and their Applications, Prentice Hall, Inc., Englewood Cliffs, New Jersey, 1985.

[2]. J.C. Ion, Laser Processing of Engineering Materials: Principles, Procedure and Industrial Application, Chapter 15: Marking, Elsevier Butterworth-Heinemann, Oxford, 2005.

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