ANALYSIS OF THE LASER MARKING TECHNOLOGIES - Revtn.ro

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Nonconventional Technologies Review – no.4/ 2008Table 1 Laser marking technologiesProcess Schema MechanismVaporizationMaterial removal mainlyby vaporizationThin layersmaterialremovalCarbonisationor colorationSurface modifyby materialfusionThin layers materialremoval by controlledvaporisation, changingthe colour of the underlayersColour changing byphoto-thermal or photochemicalreactionsDestruction of materialsby pyrolyze reactionthat leads to irregularstructures into thematerialIn mask marking (fig 1.a), a stencil of thedesired mark is projected onto the workpiece.The picture of the mask on the object is madeusing a lens. Employed in the mask markingare often the pulsed lasers [1], [2], [3].In the beam deflected method (fig. 1.b), thelaser is directed via two galvanometer mirrorsand a lens system to the object to be marked.Using special software, a computer controlsthe galvanometer mirrors. The marking ismade by directing the beam in directions xand y. The beam deflection method is veryflexible and can transmit a high density ofinformation. The lasers used in this methodare often continuous wave lasers.Comparing the two marking methods, it isconcluded that• marking speed: mask marking has highermarking speed up to a few tens piecesper second. Because the laser pulseduration is in the range of micro-second tonanosecond, the workpiece to be markeddoes not need to stop.• marking area: beam deflected markinghas a bigger marking area. The markingarea by mask marking is very smallbecause of limited beam spot size andenergy per pulse.• flexibility: in mask making, the patternsrequired are produced by masks. Toproduce a mask is time-consuming.Therefore, mask marking is more suitablefor high-volume production without anychange on the patterns. In beam deflectedmarking, the patterns are produced bysoftware. Thus, it is highly flexible tochange patterns.• cost of investment: in general, the cost ofbeam deflected marking is higherbecause the scanning system is moreexpensive.18
Nonconventional Technologies Review – no.4/ 2008a) The mask marking b) The beam deflected methodFig. 1 Principe of laser marking4. <strong>LASER</strong> FOR <strong>THE</strong> <strong>MARKING</strong> PROCESSThe most popular systems are lamp-pumpedNd:YAG lasers, which produce near infraredlight at a wavelength of 1,064 µm, and CO 2lasers, which produce light at the 10,64 µmwavelength.Because of their wavelength, Nd: YAG lasersare more capable than CO 2 lasers—but theyare also substantially higher in cost. The1,064-µm wavelength is more easilyabsorbed by a vast range of materials. CO 2lasers are an economical marking solution,as long as the wavelength and materialmatch.The excimer lasers are used in the lasermarking too [4]. Among all laser types, theyassure the highest resolution of the mark.But, these lasers are rarely used because ofthe low productivity of the process and of thevery expensive cost of the laser equipment.Recently, the diode-pumped Nd:YAG lasersare used for the marking process. Theselasers offer high beam quality, excellentpulse-to-pulse stability, and longmaintenance intervals (typically every 12,000to 15,000 hours of use).When selecting a laser marking system for aparticular application there are many factorsto consider [1], [5], [6]:• power density• thermal thermal conductivity19 heat capacity melting point heat of vaporisation• reflectivity material wavelength temperaturePower density is determined by the amount of(peak) power generated by the laser dividedby the area of focused beam. Wavelength,beam divergence and quality of opticsbecome important factors in determining howsmall a beam can be produced.The amount of time the laser power isfocused onto the material also has impact onthe ease of marking and depth of penetration.Sometimes, the pulse duration is a key factorin determining which laser can be used.Reflectivity or absorptivity is effected by thetype of material, surface condition (i.e.smooth or rough, polish or oxidised),wavelength and surface temperature. Ingeneral, metals absorb a greater percentageof Nd:YAG laser energy than that of carbondioxide laser energy. On the other hand,white paper and most transparent materialsabsorb a great amount of carbon dioxidelaser energy. Some materials such as siliconabsorb the same percentage of energy fromeither laser.For the evaluation of available laser markingsystems in the market for different
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ANALYSIS OF THE LASER MARKING TECHNOLOGIES - Revtn.ro

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