Journal of Porous Materials 7, 483–490 (2000)c© 2000 Kluwer Academic Publishers. Manufactured in The Netherlands.

Influence of Laser Beam Irradiation Conditions on the Machinabilityof Medium Density Fiberboard Impregnated with Phenolic Resin

KEISUKE HATAChiba Polytechnic College, Chiba 260-0025, Japan

KIYOTAKA SHIBATAIbaragi Polytechnic College, Mito 310-0005, Japan

TOSHIHIRO OKABE AND KOUJI SAITOIndustrial Research Institute of Aomori Prefecture, Hirosaki 036-8363, Japan

MASAHISA OTSUKAShibaura Institute of Technology, Faculty of Engineering, Tokyo 108-0023, Japan

Received May 20, 1998; Revised April 23, 1999

Abstract. The applicability of CO2 laser beam machining (cutting) to resin impregnated woody products has beeninvestigated with special reference to the effect of laser beam machining parameters such as laser power, feed speedof workpiece, pulse duty, frequency, assist gas pressure and defocus distance. The optimum laser beam machiningconditions are as follows: Laser power: 1500 W; output wave form: pulse oscillation wave (pulse duty: 40%);frequency: optional (0 to 1000 Hz); feed speed of workpiece: 900 mm/min; assist gas: nitrogen gas; assist gaspressure: optional (0.1 to 0.5 MPa); and defocus distance: (0 to 1 mm).

A carbonized layer of 1 to 2 mm thickness is always formed near the cut surface. In other words, materials notthermoformed after impregnation with phenol resin are influenced by heat to some extent while being machined bythe laser beam.

Keywords: woodceramics, porous carbon, laser beam machining, burning temperature, carbon content

1. Introduction

Woodceramics (WCS) are porous carbon materials ob-tained by impregnating wood (or woody materials)with phenol resin and thermoforming them at 300 to2800◦C [1]. They have the following features: 1) Theyhave a porous structure similar to that of wood and arelight (0.6 to 1.1 g/cm3); 2) They can be manufacturedat low cost; 3) They are made by carbonizing nat-ural materials and so do not cause environmental

pollution when they are disposed of .; 4) They canbe manufactured from thinned wood and waste wood;5) Waste (wood vinegar liquid) produced during themanufacturing process can be used as a soil improvingagent; and 6) They can be reused as activated carbonwhen powdered and retreated [2].

WCS also have superior heat resistance, wearresistance, and corrosion resistance, and thereforehave excellent potential for application as electromag-netic shielding materials, friction materials, and heat

484 Hata et al.

insulation materials. However, to use porous brittlematerials as structural or functional materials, it is im-portant to establish a method of processing them intoa specified form of specified dimensions.

Cutting, grinding, water jet, laser beam and elec-tric discharge machining are now feasible. Laser beammachining offers certain advantages, such as higherprocessing accuracy, higher processing efficiency, andless dust generation. Therefore, we have investigatedthe applicability of laser beam machining to WCS [3].

Figure 1 shows the general WCS manufacturing pro-cess when medium-density fiberboard (MDF) made ofthe needle-leaf tree pinus radiata is used as the raw ma-terial. Laser beam machining can be used in each stageof the manufacturing, but it is reasonable to use it toprocess thermoformed WCS because it allows accurateprocessing to the specified dimensions. Therefore, weinvestigated laser beam machining of thermoformedWCS. However, the performance of processing afterthermoforming may be adversely influenced by thebrittleness and superior heat resistance that are char-acteristic of porous ceramics [4]. Therefore, we con-sider that laser beam machining may be useful forthe processing stages before thermoforming, whenthe materials are relatively soft and pyrolyze at lowtemperatures.

The aim of this paper is to investigate the applica-bility of laser beam machining to MDF of pinus radi-ata which is impregnated with phenol resin and thendried.

2. Experimental Methods

2.1. Specimen

MDF of the needle-leaf tree pinus radiata (air-drieddensity 0.66, moisture content 8.5%) were impreg-nated with phenol resin. PX-1600 were of Honen

Figure 1. Manufacturing process of woodceramics. Laser beam cutting was performed at either stage denoted by arrows.

Table 1. Characteristics of phenolic resin used in this study.

Properties Conditions Typical values

Non-volatile matter (Mass%, 1 hr/135◦C) 46.0

Specific gravity (Density 25/4) 1.119

Viscosity (Poise/25◦C) 0.16

Gelation time (min/135◦C) 11.0

pH (25◦C) 8.5

Misciblity with water (times/25◦C) 2.8

Corporation make, at room temperature and under at-mospheric pressure.

2.2. Laser Beam Machining Conditions and CriticalProcessing Speed

We used a commercially available CO2 laser beam ma-chine (NLF-404, Niigata Manufacturing Co. Ltd., ratedoutput 1.5 kW, peak output 3 kW) and scanning electronmicroscope, equipped with energy dispersive X-rayanalyzing system.

A colorimeter was also used in order to determinehow the machining parameters influence the propertiesof carbonized layers formed on the specimens.

Three machining parameters were used: 1) Beamoutput: 1000 W and 1500 W; 2) Oscillation mode:CW oscillation (CW) mode and pulse oscillation (PW)mode; and 3) Processing speed: 0 to 1500 mm/min.The maximum processing speed at which a laser beamcan penetrate a specimen was defined for each com-bination of beam output and oscillation mode as thecritical processing speed.

The ratio of pulse duration to the time of one cycle inPW mode was defined a pulse duty (unit: %) and usedfor the analysis. The defocus distance of laser beamwas+1 mm, assist gas was nitrogen gas (pressure:0.5 MPa), nozzle diameter 2.1 mm, nozzle height 1 mm,and frequency 500 Hz unless stated otherwise.

Machinability of Medium Density Fiberboard 485

2.3. Measurement of Thickness of Carbonized Layer

The thickness of carbonized layers is estimated asfollows:

1) Microscopic observation: a small cube contain-ing a laser cut out of the specimen, covered withpolyester resin, and then sliced into a sample of7 mm thickness. The thickness of carbonized layerwas measured through microscopic observation ofthe sample.

2) SEM observation: The thickness of carbonizedlayer was separately measured through SEM ob-servation of a cut surface.

3) EDS analysis: the distribution of carbon concentra-tion in the direction perpendicular to the cut surfacewas measured by SEM/EDS method to determinethe thickness of the carbonized layer.

4) Evaluation using colorimeter: a Minolta make col-orimeter (CR-221) was used to measure the colordifference between carbonized and uncarbonizedpart, from which the thickness of the carbonizedlayer was estimated [5].

A non-contact type of surface thermometer (TA-0510bF, Minolta Ltd.) was also used to measure thetemperatures of points near the cut surface. The widthof heat affected zone as evaluated from temperaturedistribution in the vicinity of cut surface which was de-termined using non-contact type surface thermometer.

3. Results and Discussion

3.1. Cutting Performance

The maximum processing speed is proportional to thebeam output, and the relationship is given by the ex-pressionVmax= 0.7P as shown in Fig. 2.

This figure also shows that the MDF not thermo-formed after impregnation with phenol resin is diffi-cult to process. Therefore, we examined the resultsobtained at the maximum laser power of 1500 W (ratedoutput). For comparison, the results for 1000 W arealso shown in Fig. 3. Figure 3 shows the relation be-tween the maximum feed speed of workpiece and pulseduty for 1000 W and 1500 W, where pulse duty 10% for1000 W is not shown because the specimen could notbe cut with this pulse duty and laser power. As shownin this figure, the feed speed of workpiece increases asthe pulse duty increases and laser power increases.

Figure 2. Relation between laser power and maximum feed speed.

Figure 3. Relation between pulse duty and maximum feed speed.

Figure 4 shows how the pulse duty influences thethickness of the carbonized layer. The carbonized layerthickness remains at about 1 mm for pulse duty vary-ing from 30 to 80%. For example, when the pulse dutyis reduced to 10%, the carbonized layer thickness in-creases to 2 mm. This is because the feed speed ofworkpiece must be lowered and heat supply must beincreased in order to penetrate the specimen. As a

486 Hata et al.

Figure 4. Carbonized layer thickness as a function of pulse dutyand laser power.

result, the input thermal energy increases and a thickerlayer is carbonized. This finding can also be explainedby the fact that the amount of input heat increases asthe irradiation time increases, even if the pulse duty islarge.

In general, as the pulse duty increases, the heat sup-ply increases, irradiation energy per unit length in-creases, cutting performance increases, and the maxi-mum usable feed speed of workpiece increases. Whenthe feed speed of workpiece is high, the influence ofheat on the cut surface is small, even if a large amountof heat is supplied, because the heat acts on the cut sur-face for only a short time. However, the feed speed ofworkpiece of this material is limited, and a speed thatis sufficiently high to suppress the influence of heatcannot be used for large pulse duty. Therefore, we caninfer that excessive heat leads to a thicker carbonizedlayer.

Figure 5 shows macroscopic view of laser cut speci-mens. Figure 6 shows the influence of assist gas pres-sure on the thickness of the carbonized layer. As isevident in the figure, the assist gas pressure hardly in-fluences the thickness. Figure 7 shows macroscopicview of MDF laser cut at various frequencies. Figure 8shows that the thickness of carbonized layer does notdepend on the frequency.

Figure 9 shows that the cutting performance is re-duced as the defocus distance shifts outside or insidethe specimen surface. We consider that the thermal

Figure 5. Macroscopic views of MDF laser cut at various gaspressures.

Figure 6. Carbonized layer thickness as a function of assist gaspressure.

effect is small when the shift is between−1 mm and+1 mm. However, the degree of taper is small whenthe shift is between 0 mm and 1 mm.

From the above results, the optimum machiningconditions of laser beam machining of MDF impreg-nated with 70 mass% phenol resin are summarized asfollows:

Machinability of Medium Density Fiberboard 487

Figure 7. Macroscopic views of MDF laser cut at various frequen-cies.

Figure 8. Carbonized layer thickness as a function of frequencyand laser power.

Laser power: 1500 WOutput wave form: PW (pulse duty: 40%)Frequency: optional (0 to 1000 Hz)Feed speed of workpiece: 900 mm/minAssist gas: nitrogen gasAssist gas pressure: optional (0.1 to 0.5 MPa)Defocus distance: 0 to 1 mm

Figure 9. Macroscopic views of a specimen laser cut at variousdefocus distances.

A high energy is necessary for laser beam machiningof MDF impregnated with phenol resin, which is akind of composite material. We postulate that this isbecause the phenol resin is a thermosetting resin withheat resistance and flame resistance, and is not softened(because of the three-dimensional bridge structure) butis carbonized when it is burned [6–9].

3.2. SEM Observation of Cut Surface

We used SEM to observe carbonized surfaces (cut sur-faces) formed under the above machining conditions.Figure 10 shows SEM photograph as an example. Forcomparison, Figs. 11 and 12 show, respectively, SEMmicrographs of an uncut surface and a surface cut

Figure 10. SEM Micrograph showing the surface morphology of aspecimen laser cut under an optimum condition (see text).

488 Hata et al.

Figure 11. SEM Micrograph showing the surface morphology ofas resin impregnated MDF.

Figure 12. SEM Micrograph showing the surface morphology ofresin impregnated MDF which was machined with circular sawingmachine.

by the circular wood sawing machine. From Fig. 11we can see that phenol resin is impregnated betweenwood fibers and in tubes and from Fig. 12 that thewood fibers and hardened phenol resin are separatedby the saw teeth rotating at high speed, and as a result,there are innumerable cut fibers and resin fragments onthe cut surface.

In laser beam machining, phenol resin impregnatedin and between wood fibers (straw-type tracheids)composing the material is affected by the heat oflaser beam and forms a carbonized layer as shownin Fig. 10. We postulate that carbon is produced be-cause benzene rings are connected to one another, dis-charging CO2 and CO as a result of dehydrogenation

[6–9]. Innumerable vacant spaces arranged reticularlycan also be observed on the carbonized surface.

We consider that bubbles are formed when volatilegas (CO2, CO) is emitted during the burning of phenolresin, and the volume reduces by 50% at and above800◦C to form holes. As a result, the laser irradiatedportion becomes extremely brittle.

3.3. SEM/EDS Analysis of Carbonized Layer

In macroscopic observation, most of the part influ-enced by the laser beam can be distinguished visually.In microscopic observation, the part subjected to thesecondary influence of heat (contamination) can alsobe recognized inside the heat-affected zone that canbe clearly distinguished visually. We believe that thesecondary influence should also be considered in dis-cussing the degree of heat influence in laser beam ma-chining. Therefore, we used an SEM/EDS analyzer(energy dispersion type) to measure the size of the partsubjected to the secondary influence to assess the de-gree of carbonization of the cut specimen. Figure 13shows the result of analysis.

The figure shows that the size of the whole heat af-fected zone has a thickness of approximately 2.4 mm.The scatter of data at different measurement points

Figure 13. Distribution of carbon in laser cut specimen measuredalong two different lines, a and b.

Machinability of Medium Density Fiberboard 489

seems to becaused by the difference in heat conduc-tivity and the nonuniform distribution of resin whichare brought about by nonuniform fiber structure.

The laser beam machining of this material is charac-terized by high laser beam absorptivity and removalcutting by vaporization. The cut surface is tapered.There is a difference of 0.01 to 0.05 mm in the cutgroove width between the part struck by the laser beamand the part having the thickest carbonized layer. Inview of this, heat affects a range of up to about 2.4 mmfrom the point of laser beam incidence.

3.4. Color Difference Analysis of Carbonized Layer

The color of wood changes when it is influenced byburning. Therefore, we estimated the degree of heatinfluence by progressively removing the cut surface,and at each stage comparing the surface color to thatbefore removal. Figure 14 shows the result. We cansee that with increase in the depth from cut surface, thecolor difference between a checked surface and the un-carbonized part decreases and finally diminishes whena total layer thickness of 2.4 mm is removed. In otherwords, heat affects a range of about up to 2.4 mm fromthe cut surface. This size coincides with the thicknessof the carbonized layer obtained in the above SEM/EDSanalysis. We thus consider that 2.4 mm is the depth of

Figure 14. Distribution of color difference in a laser cut specimen.

layer that is carbonized under the machining conditionsused in this experiment.

3.5. Temperature of Cut Surface

Laser beam irradiation with higher energy is neces-sary to process MDF which is not thermoformed afterimpregnation with phenol resin and a thicker layer iscarbonized in the MDF cut using laser beam machiningwhen compared to thermoformed MDF. The depth ofthe carbonized layer depends on heat supply and thecarbonization degree depends on the temperature onthe cut surface.

Therefore, we obtained temperatures during cutting(surface temperatures) at whcih a layer is carbonizedby measuring peak temperatures at some points dur-ing laser beam irradiation. As shown in Fig. 15, thetemperature at a point which is near the center ofthe carbonized layer and 1 mm distance from cut sur-face reaches 1050◦C or higher.

The temperature at the boundary between the car-bonized layer and uncarbonized part reaches about130◦C and that in the uncarbonized part (at a distance

Figure 15. Distribution of surface temperature along a line perpen-dicular to feed direction.

490 Hata et al.

of 7 mm or 10 mm from the irradiation point) reachesclose to 50◦C.

It was reported that phenol resin is carbonized atand above 500◦C. The fact that the temperature at adistance of 1.6 mm from the irradiation point reachedabout 464◦C suggests that the temperature at a dis-tance of 1.5 mm from the irradiation point reaches atleast 500◦C. Though a carbonized layer of thicknessof about 1 mm could be recognized visually, from theabove analysis, we estimate that a layer of thicknessof about 1.5 mm was carbonized. The depth of lay-ers carbonized under the same machining conditionsmay vary because the carbonization may be influencedby structural nonuniformity that causes thermal con-ductivity variations in the porous structure and phenolresin impregnation nonuniformity which results in thenonuniformity of resin concentration. Further investi-gation on this carbonization dispersion is necessary.

4. Conclusions

1) The following conditions are optimum for laserbeam machining of MDF impregnated with70 mass% phenol resin.

Laser power: 1500 WOutput wave form: PW (pulse duty: 40%)Feed speed of workpiece: 900 mm/min

Frequency: optional (0 to 1000 Hz)Assist gas: nitrogen gasAssist gas pressure: optional (0.1 to 0.5 MPa)Defocus distance: 0 to 1 mm

2) The fact that a carbonized layer of thickness 1 to2 mm is always formed near the surface must betaken into account when applying laser beam ma-chining to MDF.

Acknowledgments

We sincerely thank Tohoku Chemicals Corporation forsupplying the materials for this research.

References

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Mater.5, 1 (1998).4. R.J. Wallace, M. Bass, and S.M. Copley, J. Appl. Phys.59, 3555

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