Oil palm (Elaeis guineensis) is one of the most important plantation tree crops in Malaysia and it covers a land area of almost 3.5 million hectares (Khalil et al., 2006). Cultivated primarily for the production of palm oil, the tree crop is an important source of alternative fibers for the manufacture of value-added products. Approximately, 30 million tons of usable fibers from the trunk, frond and empty fruit-bunch of the oil palm is produced annually in the country. The empty fruit-bunches produces approximately 4 million tons of fibers per annum (Khalil et al., 2006). This fiber source is highly cellulosic and exhibits good mechanical properties (Sreekala et al., 1997).
The feasibility of manufacturing OPEFB particleboard in Malaysia is proven (Chew and Ong, 1985) and industrial production is estimated to be 25,000 m3 per annum (Khairiah and Khairul, 2006). Despite the industrial success, the OPEFB particleboard has limited market acceptance, due to the accelerated wear of cutting tools during machining processes and the resultant high tooling cost (Ratnasingam and Scholz, 2006). Although the OPEFB particleboard has a lower production cost and comparable mechanical properties compared to the conventional wood-based particleboard, its excessive tooling cost may offset these advantages. Therefore, a study was undertaken to evaluate the tool wearing and tooling cost incurred when machining OPEFB particleboard, in order to provide information on the process economics.
MATERIALS AND METHODS
OPEFB particleboards were obtained from a local manufacturer in Malaysia. The boards of the dimension 18x2000x2000 mm were conditioned in a controlled environment at a temperature of 20°C and 70% relative humidity for a week prior to experimentation.
The machining experiments were carried out using an ANDERSON-810 Computer-Numerical
Control (CNC) router. Commercially available single-fluted router bits, 12 mm
in diameter, with a 15° rake angle, were used as the cutting tool. The bit
was operated at 18,000 revolutions per minute (rpm), while the feed speed and
depth of cut were fixed at 4.5 m min-1 and 1.5 mm, respectively.
The cutting tool traveled along the length of the experimental board and retracted
automatically to the starting point before resuming the next cutting operation.
Due to the high silica content in the oil palm EFB fiber (Sreekala et al.,
1997), tungsten carbide cutting tools were selected on its high wear resistance
(Bayoumi and Bailey,1985).
||Composition of tungsten carbide tools
In this study, however, three different compositions of tungsten carbide was
used, in order to establish the optimal grade of tungsten carbide for cutting
OPEFB particleboard (Table 1). The extent of cutting tool
wear during the machining operation was measured using the cutting edge-recession
technique as described by Ratnasingam and Perkins (1998). Since, cutting edge-recession
has a direct link to tool life, it allow the cost of tooling to be determined
when machining the experimental boards. Microscopic examinations of the cutting
edges were done at the end of the experimentation to characterize the mode of
failure in the cutting tools. The power consumption during the machining operation
was also measured on the basis of the changes in the torque of the drive-motor
of the CNC router, as described by Ratnasingam and Perkins (1998). A parallel
study was conducted on wood-based particleboards under similar experimental
conditions for comparison purposes.
Extent of tool wear and power consumption: The extent of wear experienced by the tungsten-carbide tools of the three different compositions is shown in Table 2. The best performance, in terms of tool edge recession and distance of cut before failure, was obtained from the grade C tool, with the highest chromium content in the matrix and smallest carbide particles. Complete cutting tool failure for the three grades of cutting tools occurred after cutting distances of 2150, 2380 and 2890 m, respectively. This is in line with the study by Bayoumi and Bailey (1985), who found that higher chromium content in the tool matrix and smaller carbide particles improved the wear resistance of tungsten carbide tools. In this context, machining OPEFB particleboard requires the use of highly wear resistant cutting tools, in order to optimize the process.
When the power consumption curves during the machining operations were compared, a similar result was obtained. Since power consumption during the machining operation is closely related to the cutting tool wear, the power consumed was the least with the high chromium content tungsten carbide tools (Fig. 1). The improved wear resistance of the grade C cutting tool ensured lower power consumption during the machining operation, which in turn improved the process economics.
||Comparative power consumption of different tool materials
||Extent of wear of the various grades of cutting tool
Upon microscopic examination of the tool cutting edge, it was found that wear occurred primarily as a result of indentation, micro-fracture and mechanical abrasion. This wear mechanism has been suggested by Bayoumi and Bailey (1985), who reported that the presence of silica and other impurities in material resulted in the removal of the binder followed by the loss of carbide grains from the cutting tool. In fact, the tools cutting OPEFB particleboard suffered greater mechanical wear compared to those cutting wood-based particleboard, due to the high silica content in the EFB fiber.
Comparative wear characteristics: When compared to wood-based particleboard, it is apparent that the OPEFB is almost two times more abrasive on the cutting tool (Fig. 2). Further, the initial wear rate is significantly higher in the OPEFB particleboard compared to that of the wood-based particleboard, possibly due to the inherent density gradient in the boards (Klamecki, 1979, 1980). Coupled with its high silica content, the OPEFB particleboard is highly abrasive on the cutting tools.
Industrial implications: On the basis of the tool wear experiments that
were conducted, Table 3 provides a comparative tooling cost
for machining OPEFB particleboard and the conventional wood-based particleboard.
It is apparent that the OPEFB particleboard is much more abrasive and therefore,
results in a higher tooling cost during its machining processes.
||Comparative tool wear characteristics
||Comparative tooling cost
On this account, the acceptance of OPEFB particleboard in the marketplace as
a substitute for the wood-based particleboard may be hampered due to the higher
tooling cost and hence, the unfavorable process economics. Further, the study
also indicates that highly wear resistant tools need to be developed, in order
to improve the process economics of machining OPEFB particleboard.
Although the production of OPEFB particleboard is competitive, it acceptance in the marketplace as a substitute for wood-based particleboard is constrained by its high tooling cost incurred during its machining. The high silica content in the EFB fibers accelerates the wear of cutting tools, which in turn has a negative effect of the process economics.