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j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425

journa l homepage: www.e lsev ier .com/ locate / jmatprotec

ffects of feed speed ratio and laser power on engravedepth and color difference of Moso bamboo lamina

heng-Jung Lina, Yi-Chung Wangb, Lang-Dong Linc,hyi-Rong Chioud, Ya-Nan Wangd, Ming-Jer Tsaid,∗

Division of Forest Utilization, Taiwan Forestry Research Institute, 53 Nan-Hai Road, Taipei 100, TaiwanDepartment of Forestry, Chinese Culture University, 55 Hwa-Kang Road, Yang-Ming-Shan, Taipei 11114, TaiwanDepartment of Forest Products Science, National Chiayi University, Chiayi, TaiwanSchool of Forestry and Resource Conservation, College of Bio-Resource and Agriculture, National Taiwan University,o.1, Sec. 4, Roosevelt Road, Taipei 10608, Taiwan

r t i c l e i n f o

rticle history:

eceived 3 April 2007

eceived in revised form

5 May 2007

ccepted 15 July 2007

eywords:

aser engraving

a b s t r a c t

In this study, Moso bamboo (Phyllosachys edulis) lamina was engraved using various laser

output power levels in conjunction with various feed speed ratios in order to understand

the effects of feed speed ratio and laser output power on engraved depth and color differ-

ence. The bamboo culm was sliced into strips and then the strips were planed for obtaining

smooth surfaces. Two kinds of Moso bamboo laminae, including without and with steam

treatment were investigated. The results showed that the engraved depth became deeper

for either higher laser power or a lower feed speed ratio. Moreover, the color difference

values increased under a lower feed speed ratio and higher power, and resulted in a brown-

ish color in the engraved zone. The average engraved depth and color difference values

ngraved depth

olor difference

oso bamboo

were 0.69–0.86 mm and 46.9–51.9 pixels by different engraving parameters, respectively. The

engraved depth and color difference values could be predicted and estimated by regression

analyses. Because of various desired engraving depths and color differences of product, we

suggested that the fitting both laser’s speed and power is important for valuable engraving

and cost effective.

that in the laser cutting process, the process parameters can

. Introduction

n addition to plantation wood, bamboo is also an impor-ant material due to its fast growth and a shortage ofood supplies in Taiwan. Moso bamboo (Phyllostachys edulis)

s a multipurpose species grown in Taiwan for fuel wood,ood (bamboo shoot), construction materials, handicrafts,

at boards, pressboards, and several other uses. In order to

evelop an innovative processing system which significantly

ncrease the value of the utilization of bamboo, manufactur-rs have engaged in sequential studies on Moso bamboo, in an

∗ Corresponding author. Tel.: +886 2 33664641; fax: +886 2 23686335.E-mail addresses: d88625002@yahoo.com.tw (C.-J. Lin), tmj@ntu.edu

924-0136/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2007.07.020

© 2007 Elsevier B.V. All rights reserved.

attempt to utilize it as stock for quality products or valuablehandcrafts. For example, laser engraving has gained increas-ing interest in the bamboo handcraft industry and is wellsuited for high-volume automated manufacturing owing tothe high processing speed, low waste, precision of operation,and high quality of engraved products.

Barnekov et al. (1986, 1989) and Yilbas (2001) indicated

.tw (M.-J. Tsai).

be adjusted and tuned to achieve the quality of cut desired.However, if a different workpiece material is used for cut-ting, all of these parameters may require re-adjustment, which

n g t

420 j o u r n a l o f m a t e r i a l s p r o c e s s i

consumes a considerable amount of time and effort. Theseparameters include the laser power, energy coupling factor,and cutting speed. Black (1998) reported that the laser machin-ing of any material is a complex process involving manydifferent parameters, all of which need to work is concernedto produce a quality machining operation. Straight-line testingwas used to evaluate the laser parameters for acceptable full-through cutting. This test combines the examination of twoseparate parameters in one test. Laser beam power (W) andcutting speed (mm/min) are the most important laser parame-ters, as they dictate the amount of energy input per unit lengthof cut. Therefore, they were paired for the test runs.

However, the applicability of laser engraving for creatingvaluable and quality wood materials has been investigated(Su et al., 2005; Wang et al., 2005). No report has been pub-lished that details the effects of feed speed ratio (%) and laserpower (W) on the engraving depth and color difference of Mosobamboo engraved by laser machining. Therefore, it is impor-tant to develop such information to understand the effect ofthe two parameters on the engraved results of bamboo whenestimating engraving performance using lasers.

In the light of the above reasons, the present studywas carried out to investigate the effect of engravingspeed ratio and laser power (two important parameters)on the engraved depth and color difference. Furthermore,mathematical models based on the two parameters wereemployed to formulate relationships between the engraveddepth and color difference. Thus, the process parameters(feed speed ratio and laser power) can be adjusted andtuned to achieve the quality (engraved depth and colordifference) of engraving desired. The results can provide infor-mation for estimating utilization of Moso bamboo in laserengraving.

2. Materials and methods

2.1. Preparation of test samples

Moso bamboo culm was sliced into strips and then thestrips were planed for obtaining smooth surfaces. Two dif-ferent bamboo laminae, including internode material withoutsteam treatment (laminae N) and internode material withsteam treatment (laminae S) were selected. The bamboolaminae were boiled in a solution (H2O2, 100 ◦C, 6–8 h) toreduce the starch and sugar contents that would otherwiseattract termites or beetles, and then half the laminae (lam-inae S) were steamed (carbonized) under heat and pressure(3.5 kg/cm2, 145 ◦C, 90 min) to darken the color. Twenty lami-nae were prepared from each type of material for each set ofexperiments.

2.2. Experimental materials

Two kinds of bamboo laminae, including laminae N andlaminae S were investigated. The size of each specimen

of bamboo lamina was 200 (longitudinal) × 25 (tangen-tial) × 7.5 mm (radial). Specimens were conditioned in acontrolled-environment room at 20 ◦C and 65% relativehumidity (moisture content of 12%).

e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425

The density value of lamina was calculated from the fol-lowing formula:

� = W

V(1)

where � (g/cm3) is the density, V (cm3) the volume, and W (g)is the weight of lamina at moisture content of 12%.

2.3. Laser engraving method

A nominal 100-W EPILOG, a commercially designed carbon-dioxide laser coupled to a precision computer-controlled X–Ytable, was used in the study. The laser engraving tests wereconducted for two kinds of bamboo specimens, using a lasermachine (Epilog Radius Model 4000, Denver, CO, USA). Thestandard of the laser focusing lens was 2 in. (5.1 cm). The scan-ning model factor was used to set the laser engraving (in theraster mode). The scanning resolution of the operation soft-ware was 600 dpi (dots per inch).

Image to engraving in following processes: (1) start by con-necting laser system to computer, (2) import the engravingimage into CorelDRAW software, (3) convert the image tograyscale, (4) configure the laser’s speed and power, and then(5) send the print job to the laser system for engraving. Twofactors (speed and power) were considered in this practicalexperiment: (1) nominal engraving speed ratios (S) (set 10%[780 mm/min], 20% [1560 mm/min], 30% [2340 mm/min], 40%[3120 mm/min], 50% [3900 mm/min], 60% [4680 mm/min], 70%[5460 mm/min], 80% [6240 mm/min], 90% [7020 mm/min], and100 %, with the fastest feed speed at 7800 mm/min); (2) laseroutput power (P) (set 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 W).Other laser-engraving factors were held constant at the defaultsettings.

The X–Y table was computer-programmed to engrave an8-mm2 area on the surface of the specimens. The engrav-ing positions and sequences on the surface of the specimensare shown in Fig. 1. The engraving feed speed ratio and laserpower (S × P) were paired for the test runs. The same S × Ptreatments were repeated 20 times. Therefore, the data setconsisted of 20 replicates × 10 feed speed ratios × 10 laserpower levels × 2 kinds of bamboo specimens. In total, 4000engraved areas were investigated for engraved depth and colordifference.

After engraving, the average engraved depth of the differenttreatments (S × P) was measured in specimens with a caliper(with an accuracy of 0.001 mm). The engraved color difference(grayscale intensity differences) was calculated using Eq. (2):

Color difference = (pixels before engraving)

− (pixels after engraving) (2)

Specimens were scanned (using a MICROTEK scanner,China), and the average number of pixels (ranging from 0[black] to 255 [white]) was measured using Adobe Photoshopsoftware (version 7.0.1).

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j o u r n a l o f m a t e r i a l s p r o c e s s i n g

.4. Analysis

n analysis of variance (multifactor ANOVA by the SPSS soft-are) was used to determine if the feed speed ratio (%) and

ig. 1 – Diagram of Moso bamboo specimen with different laser fngraving machining.

h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425 421

laser power (W) levels significantly affected the engraved

depth and color difference. F values were computed to testfor the significance of different treatments. A stepwise mul-tiple regression was applied to deduce the most appropriate

eed speed ratios and output power levels using laser

422 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425

Table 1 – Average engraved depth and color difference of Moso bamboo

Specimen Density (g/cm3) Engraved depth (mm) Color difference (no. of pixels)

With steam treatment 0.710 0.855 46.890.689 51.88

al. (2005) reported that the engraved depth became deeper foreither higher laser power or lower feed speed ratio. The totalwork per unit length was found to have increased either withan increase in laser output power or with a decrease in feed

Without treatment 0.832

Means within each column significantly differ at p < 0.05.

regression equations for engraved depth and color differenceto provide a better understanding of their interrelationships.

3. Results and discussion

3.1. Density

The mean densities (�) of the internode material with steamtreatment (laminae S) and internode material without steamtreatment (laminae N) are summarized in Table 1.

A statistical t-test indicated that the mean � values of spec-imens S and N significantly differed (p < 0.05). It was found thatthe average values of � in the bamboo laminae obtained fromspecimens without steam treatment were significantly higherthan those from specimens with steam treatment.

It is speculated that heating the bamboo strips during thecarbonization process may have altered the chemical compo-nents (heat and pressure). This treatment reduces the starchand sugar contents of bamboos so that have less mass or den-sity than those without treatment. Lin et al. (2006) indicatedthat the density of bamboo was reduced after carbonizingtreatment.

3.2. Engraved depth

The average engraved depths of specimens S and N were 0.855and 0.689 mm (Table 1) which significantly differed (p < 0.05).The specimen S had a deeper engraved depth than specimenN. Wang et al. (2005) and Arai et al. (1976) indicated that thereis a negative relationship between wood density and engraveddepth. Therefore, our result showed that a deeper engraveddepth in specimens S occurred with engraving owing to the

decrease in bamboo density.

The effects of feed speed ratio, laser power, and feedspeed ratio plus laser power interaction regimens on theengraved depth were significant by ANOVA. A comparison of

Fig. 2 – Average engraved depth with various feed speedratios.

Fig. 3 – Average engraved depth with various laser powerlevels.

the engraved depth for various feed speed ratio and laserpower regimens are plotted in Figs. 2 and 3. As shown inFig. 2, the engraved depth values decreased with an increasein the feed speed ratio. However, the engraved depth valuesincreased with an increase in laser power (Fig. 3).

The engraved depth under various feed speed ratio pluslaser output power interaction regimens for the two kinds ofMoso bamboo specimens are shown in Figs. 4 and 5. Resultsindicated that a lower feed speed ratio and higher laser powerproduced deeper engraved depths than did a higher feed speedratio and lower laser power. Li and Mazumder (1991) and Su et

Fig. 4 – Engraved depth under various feed speed ratiosand laser output power levels for Moso bamboo with steamtreatment.

j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425 423

Fig. 5 – Engraved depth under various feed speed ratiosas

sis

3

Taesbbodt

rfdmdri

Fr

Fig. 7 – Average color difference with various laser powerlevels.

Fig. 8 – Color difference under various feed speed ratios

nd laser output power levels for Moso bamboo withoutteam treatment.

peed ratio. Therefore, the engraved depth increased with anncrease in the total power, with but a decrease in the feedpeed.

.3. Color difference

he average color differences of specimens S and N were 46.89nd 51.88 pixels (Table 1). Specimen S had a lower color differ-nce than specimen N. According to the statistical analysis,ignificant differences (p < 0.05) existed for the color differenceetween specimens S and N. Specimen S was steamed (car-onized) under heat and pressure which caused a darkeningf the color. Therefore, our result showed that a lower colorifference in specimen S occurred after engraving owing tohe already darkened color.

The effects of feed speed ratio, laser power, and feed speedatio plus laser power interaction regimens on the color dif-erence were significant by ANOVA. A comparison of the colorifference with various feed speed ratio and laser power regi-

ens are plotted in Figs. 6 and 7. As shown in Fig. 6, the color

ifference values decreased with an increase in the feed speedatio. However, the color difference values increased with anncrease in laser power (Fig. 7).

ig. 6 – Average color difference with various feed speedatios.

and laser output power levels for Moso bamboo with steamtreatment.

The color difference under various feed speed ratio pluslaser output power interaction regimens for the two kinds ofMoso bamboo specimens are shown in Figs. 8 and 9. Resultsindicated that a lower feed speed ratio and higher laser powerproduced larger color differences than did a higher feed speedratio and lower laser power. Su et al. (2005) reported that colordifference values increased under higher power and a lowerfeed rate in wood materials. Li and Mazumder (1991) indicatedthat the process of cutting wood with a laser is always accom-panied by an exothermic reaction of distillation of the woodmaterial. Therefore, the color difference values increase withan increase in the laser output power.

3.4. Relationship between density and engraved depth

The values of engraved depth under different feed speedratio plus laser power interaction regimens increased with a

decrease in the bamboo density, and the relationships could berepresented by negative linear regression formulas. Relation-ships between engraved depth and density under feed speedratio of 10% plus laser power of 10–100 W interaction regimens

424 j o u r n a l o f m a t e r i a l s p r o c e s s i n g t

Fig. 9 – Color difference under various feed speed ratios

and laser output power levels for Moso bamboo withoutsteam treatment.

had R2 values of 0.44–0.70; the coefficients of determinationobtained under the other conditions in this study were lower(R2 = 0.01–0.50).

Su et al. (2005) indicated that the engraved depth was shal-lower for higher-specific-gravity materials, because of the highratio of wood substance and the accompanying greater ther-mal conductivity.

3.5. Relationship between engraved depth and colordifference

Values of color difference for all bamboo specimens increasedwith an increase in engraved depth under different feed speedratio plus laser power interaction regimens, and the rela-tionship could be represented by the following second-orderpolynomial regression formula:

engraved depth = 0.001 color difference2

− 0.006 color difference + 0.12, R2 = 0.75,

F = 6061∗∗

3.6. Engraved depth and color difference estimation byregression analysis

Values of engraved depth increased with an increase in laserpower; however, there was a decrease in the feed speed ratio.Their relationships could be represented by the following lin-ear and polynomial regression formulas:

engraved depth = 0.015 laser power − 0.037, R2 = 0.15,

F = 679.6∗∗ and engraved depth

= −0.208 speed ratio + 0.003 speed ratio2

− 1.549 speed ratio3 + 4.72, R2 = 0.58,

F = 1846∗∗

e c h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425

Moreover, values of color difference increased with anincrease in laser power, but with a decrease in the feed speedratio. Their relationships could be represented by the followinglinear and polynomial regression formulas:

color difference = 0.59 laser power + 17.0, R2 = 0.54,

F = 1664∗∗ and color difference

= − 0.21 speed ratio + 0.012 speed ratio2

+ 114.5, R2 = 0.50, F = 2030∗∗.

This suggests that engraved depth and color differencewere greatly affected by the two engraving treatments offeed speed ratio and laser output power. There were pos-itive relationships for engraved depth and color differencewith laser power, but negative ones with the feed speedratio.

Furthermore, for a better understanding of the relation-ships between these engraved parameters (feed speed ratioand laser power) and two engraved performances (depth andcolor difference), the resulting data were fitted to curves bymultivariable models. Then, a stepwise regression procedurewas used to acquire the most-suitable multiple regression topredict various engraved depths and color differences. The fol-lowing regression equations were obtained using the F-valuetest:

engraved depth = −0.54−0.005 speed ratio+0.006 speed ratio2

− 0.001 speed ratio3 + 0.0015 laser power,

R2 = 0.73, = 2648∗∗ and

color difference = 7.21 − 0.712 speed ratio+0.012 speed ratio2

+ 0.59 laser power, R2 = 0.80, F = 5258∗∗

Some causes for the variation in results may have been dueto radial variation, anatomic characteristics (vascular bun-dles), and steam treatment. Lin et al. (2006) indicated thatthe bamboo cavity layer-to-bamboo skin radial variation pat-terns of density (profiled in the radial direction) increase fromthe inner bamboo cavity layer outward to the outer bam-boo skin. Moreover, the tissue structure of bamboo differsfrom that of wood. For example, vascular bundles of lepto-morph rhizome species possess a central vascular strand only.In addition, the properties of bamboo are affected by var-ious levels of steam processes (carbonizing). Owing to theabove reasons, the engraved depth and color difference val-ues may have been influenced by the engraving methodsused.

4. Conclusions

The effects of different feed speed ratios and laser outputpower levels on the engraved depth and color difference ofMoso bamboo laminae were investigated, with the followingresults.

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laser-engraving technique on native woods of Taiwan. Taiwan

j o u r n a l o f m a t e r i a l s p r o c e s s i n g

. The laser engraved depth became deeper for either higherlaser power or a lower feed speed ratio.

. Color difference values increased under a lower feed speedratio and higher power, and resulted in a brownish color inthe engraved zone.

. Effects of the feed speed ratio by laser power interactionregimens on the engraved depth and color difference weresignificant. Therefore, values of the engraved depth andcolor difference increased with an increase in laser outputpower; however, there was a decrease in the feed speedratio.

. The engraved depth and color difference values of Mosobamboo could be predicted and estimated by regressionanalyses. This prediction of two engraving performancescan help laser engraving achieve varied requests andapplied to the fields of decoration and gift industry. Wesuggested that the fitting both laser’s speed and power isimportant for valuable engraving and cost effective thatdesired engraved depth and color difference.

cknowledgement

he authors wish to thank the financial support of the Taiwanorestry Research Institute.

h n o l o g y 1 9 8 ( 2 0 0 8 ) 419–425 425

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Barnekov, V.G., Huber, H.A., McMillin, C.W., 1989. Laser machiningwood composites. For. Prod. J. 39, 76–78.

Black, I., 1998. Laser cutting decorative glass, ceramic tile. Am.Ceram. Soc. Bull., Acad. Res. Library 77, 53–57.

Li, L., Mazumder, J., 1991. A study of the mechanism of lasercutting of wood. For. Prod. J. 41, 53–59.

Lin, C.J., Tsai, M.J., Wang, S.Y., 2006. Nondestructive evaluationtechniques for assessing dynamic modulus of elasticity ofMoso bamboo (Phyllosachys edulis) lamina. J. Wood Sci. 52,342–347.

Su, W.C., Tsai, M.H., Yang, C.M., Wang, Y., 2005. Factors affectingthe characteristics of wood engraved by laser. Taiwan For.Prod. Ind. 24, 45–55.

Wang, Y.S., Lin, Y.J., Lee, M.C., 2005. Test of treatments of a

J. For. Sci. 18, 401–408.Yilbas, B.S., 2001. Effect of process parameters on the kerf width

during the laser cutting process. Proceedings of the Institutionof Mechanical Engineers. ProQuest Sci. J. 215, 1357–1365.