Study On The Mechanism Of Medium Density Fibreborad Cutting

The production and application of Medium Density Fiberboard (MDF) were developing sofast that it has already taken a significant position in the man-made board industry. But thefurther processing of the MDF boards, cutting for example, causes a series of problems such asbad processing quality, serious dust pollution and so on. This paper was an important part of theproject named "Study on the dust flow control of the wood and wood based materials in a highspeed cutting condition", which was subsidized by Natural Science Foundation of China(NSFC). And this paper placed an emphasis on the study of chip forming mechanism, cuttingforce, dust characteristics of milling and the dust flow control.The MDF chip forming mechanism was investigated by testing the MDF orthogonalcutting with the technology of high speed camera and micro-analysis of the materials. The basiccharacteristics of the chip formation process and the influence of the cutting parameters such ascutting thickness, rake angle of the tool and cutting speed to the chip formation process havebeen studied. And the relationship between cutting parameters and the ratio of the surfacedamage had been analyzed. The result showed that: with the increasing of the cutting thickness,the dominated style of MDF chips changed from dust chip to granular chip and thenquasi-continuous chip; by increasing the rake angle or the cutting speed under the condition oflarge cutting thickness, the main chip style of the MDF changed from quasi-continuous chip togranular chip. While the cutting thickness was small, the leading chip type of MDF changedfrom granular chip to dust chip with the decreasing of the rake angle or the increasing of thecutting speed. The calculated ratio of surface damage based on the image processing technologycan not only be used as the evaluation index of the quality of processed surface, but also used asthe main approach, which was reasonably used in this paper, to study the cutting mechanismand cutting force of MDF systematicly.The characters of the time domain wave of cutting force during the chip forming processand the influence of the cutting parameters on the variation of MDF tangential force and radialforce were studied in the aspect of MDF cutting force research, which were leaded to anestablishment of experience formula for calculation tangential force and radial force of MDFcutting. It was showed that: approximate periodic fluctuations of Cutting Force-Time curve ofthe quasi-continuous chip forming process are found, and the variation of the tangential forceand radial force were obvious. During the formation of the granular chip formation, the Cutting Force-Time curve showed a regular serrated waveform, which corresponding to the differentsizes of MDF granular chip caused by the extrusion. When the cutting thickness was small, theaverage tangential force was comparatively small, more or less than42N, and the wave of theCutting Force-Time curve is comparatively smooth. The cutting thickness had obviousinfluence on the cutting force, and with the increasing of the cutting thickness the tangentialforce increased sharply, while the radial force decreased slowly. The rake angle of the tool alsohad some influence on the cutting force, with the increasing of the rake angle the tangentialforce decreased quickly while the radial force decreased slowly. The cutting speed also hadsome influence on the cutting force, with the increasing of the cutting speed the tangential forcedecreased gradually while the trend of variation of radial force had no significant impact.By not only testing the particle size distribution of the dust, typical particle sizemicrograph and the processed surface roughness with different average milling thickness butanalyzing the influence of different particle sizes on the formaldehyde emission, thecharacteristics of the dust produced during the MDF milling had been researched. The studyresult showed that: The distribution of the particle size was mainly ranging from0.5to450mand the particles of the dust chip can be classified into long-narrow fiber bundle, micelle orsmall fiber bundle. And with the increasing of the average milling thickness, the averageparticle size {D (4,3), D (3,2)} and the median-particle-size D (50) all tended to increase. Andthe cutting speed had more significant influence than the feed rate. However, with theincreasing of the cutting thickness, the processed surface quality decreased, in this case, thecutting speed also had more significant influence than the feed rate. Based on this result, properincreasing of the cutting speed can be considered as a helpful method to improve the processedsurface quality.Based on the high speed video technology and the image identifying and processingtechnology, the transient images of the chip flow during the wood-based composites' millingprocess were recorded. The formation mechanism and the variation law of the velocity of flowfield and the diffusion angle were studied as a main part of the research on MDF chip flow. Thestudy result showed that: The forming process of the chip flow can be classified into two stages;the first stage was ranging from the uncut chip leaving away from the work piece to the chipcontacting with the gullet, and the second stage was that the chips left away from the gullet anddiffused into the air in the end. The velocity of the flow field can also be classified into twostages; the first one was the velocity of the flow field, got at the beginning of the chip flowformation, started to decrease, and the second one was the velocity of flow field increasedsuddenly to a peak and then decreased. While milling the wood-based composites with high speed, when the cutter tooth started to cut into the work piece the milling speed tended todecrease and at the moment of the cutter tooth leaving the work piece the linear velocity of thecutter tooth recovered quickly to the rated speed and then kept steady. In the same time, thevelocity of the chip in the middle of the chip flow was the fastest, the second location was theplace near the cutter tooth and the location away from the cutter tooth had the lowest velocity.With the increasing of the milling speed, the diffusion angle of the chip flow decreased. Amongthe wood-based composites, the MDF had the largest diffusion angle during the milling process,ranging from53.3to66.5°.Some significant theoretical and application value were showed in this paper which notonly filled in a gap on the study of the mechanism of MDF cutting at home and abroad, but alsoa theoretical basis for the dust pollution control during the MDF cutting process was provided.