Original Article

A study on cutting temperature for wood–plastic composite

Journal of Thermoplastic Composite Materials 2016, Vol. 29(12) 1627–1640 ª The Author(s) 2015 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0892705715570988 journals.sagepub.com/home/jtc

Zhijian Pei1,3, Nanfeng Zhu1 and Yu Gong2

Abstract Wood–plastic composite (WPC) material has been developed rapidly and used widely to replace wood production in recent years. The cutting process of WPC material is the key to directly affect the efficiency of utilization and processing. The infrared thermal imaging system and numerical control machine tool were used in this article to analyze the cutting temperature under different cutting parameters, which was further compared with massoniana wood cutting procedure to provide theoretical basis for WPC material processing. Under certain conditions, the cutting depth was the most important factor on the cutting temperature, followed by spindle speed, while cutting width was the least affected. In the cases of similar processing parameters, although cutting temperature for massoniana wood is always higher than WPCs, the change trends of their cutting temperature are similar. Besides, shear heat moderately affected the cutting temperature during cutting. Keywords Wood–plastic composite material, wood, cutting temperature, cutting parameters

Introduction Wood–plastic composites (WPCs), a new type of composite materials based on wood panel production processing and manufactured by mixing wood materials and recycled plastics with further hot pressing, have been developed rapidly in recent years. Inside this 1

College of Wood Science and Technology, Nanjing Forestry University, Nanjing, People’s Republic of China Department of Art Design, Wuxi City College of Vocational Technology, Wuxi, People’s Republic of China 3 Department of Mechanical and Electrical Engineering, Changzhou College of Information and Technology, Changzhou, People’s Republic of China 2

Corresponding author: Nanfeng Zhu, College of Wood Science and Technology, Nanjing Forestry University, No.159 Lonpan Road, Nanjing 210037, People’s Republic of China. Email: [email protected]

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material, plastics became not only the modifier but also the traditional adhesive of wood panel with cost and performance advantages from both wood and plastic.1 The cost effectiveness of WPCs besides their role in preserving natural sources, such as forests, their sufficient mechanical and physical properties, and lack of hazardous chemicals2 have made them favorably accepted as ‘‘green materials’’ in recent years.3,4 Since cellulosic fibrils have low decomposition temperature, they should be processed with a thermoplastic material that melts below 200 C.5 There are signs that polypropylene (PP) is an appropriate option for this purpose. Recently, properties of WPCs such as PP filled with different kinds of wood sawdust have been studied, and it has been agreed that natural fillers decline thermal properties of the composite.6 Therefore, it is particularly important to investigate the effects of processing parameters including cutting speed, cutting width, and cutting depth on cutting temperature in the high-speed cutting process. However, there is a lack of sufficient data on the effects from several cutting parameters of WPC materials on cutting temperature until now. The main purpose of the study is to investigate dynamic variation of cutting temperature in cutting WPC material and provide theoretical basis for WPC processing.

Experimental methods and principle Experimental equipments Wood CNC machining center. In this study, smart 5-axis computer numerical control (CNC) machining center (PAOLINO BACCI, Italy) was used, which had the capability of cutting independently in the direction of X, Y, or Z axis and also moving simultaneously with two axes around X and/or Y axis rotation.

ThermoVision A20-M infrared imaging system. ThermoVision™ A20-M (FLIR Systems, Inc, USA) can be utilized to detect subtle temperature changes of 0.10 C between 20 C and þ900 C to generate clear thermal images with high resolution (160  120 pixels) with the refresh rate of 50 Hz.

Cutting tool. As shown in Figure 1, 24-mm end mill for high-speed milling was purchased from Leitz (Germany) in this study, and related technical data for this cutter are shown in Table 1.

Experimental materials. WPC materials used in the experiments are mainly composed of high-density polyethylene (about 60% by weight), silane coupling agent (about 2% by weight), and massoniana wood powder (about 38% by weight) with a density of 0.936 g/ cm3. WPC samples made through melt compounding and injection molding were purchased from Anhui Guofeng WPC Company (China). Massoniana wood, in comparison with WPCs, has a moisture content of 15% and density of 0.495 g/cm3.

Cutting parameters. When the temperature is higher than 125 C, the surface of WPCs will start to soften.7 In order to study the effect of processing parameters on the extreme

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Figure 1. Twenty-four millimeter stalk cutter. Table 1. Cutter technical data. Cutter diameter (mm)

24

Cutter length (mm) Cutter edge length (mm) Cutter shank diameter (mm)

70 25 10

Table 2. Cutting parameters. Spindle speed (r/min)

Cutting depth (mm)

Cutting width (mm)

Feeding speed (m/min)

10,000

5

12

15,000

5

12,000

8 12 16 5

8 12 16 10

3 6 10 6

10,000 13,000 16,000

10

8

6

temperature (125 C + 10%), we chose higher spindle speed, feed rate, and cutting width than usual. Cutting parameters in the study are listed in Table 2.

Cutting temperature measurement Cutting processing was filmed by ThermoVision A20-M infrared imaging system to obtain video information for real-time cutting temperature curve. Temperature was measured between 0 C and 150 C with a frequency of 50.0 Hz.

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Figure 2. The cutting path.

Figure 3. Experimental devices.

CNC machining cutting method The volume size machinable by CNC machining of workpiece is 250  140  26 mm3. The cutting path is shown in Figure 2.

Experimental processing and data acquisition Experimental devices are designed as shown in Figure 3. The movement of workpiece in the direction of X or Y axis was completed by shifting feed table, which ensures that the scope of cutter operation was detectable by the infrared imaging system. Temperature acquisition area is shown in Figure 4.

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Figure 4. Temperature acquisition area.

Experimental results and data analysis Cutting temperature analysis for cutting WPC material Effects of feed rate on cutting temperature. As shown in Figure 5(a) to (c), the curves showed relation between cutting temperature and feed rate during cutting with a spindle speed of 10,000 r/min, depth of 5 mm, and width of 12 mm. In general, the cutting temperature was slightly reduced as feed rate increased from 3 m/min to 10 m/min. On one hand, shear heat would be produced more for burden of each tooth and was heavier resulting from increased feed rate. On the other hand, the friction heat was reduced significantly between rake face/major flank and wood chip/workpiece surfaces. Finally, along with raised feed rate, more cutting heat was taken away, due to which the heat supposed to be accumulated in the cutter was dissipated, the cutting temperature became a little lower eventually (Figure 6). It would be possible to improve productivity by increasing the feed rate moderately. However, it is not always good to increase the feed rate, which actually brings about more burden, easier hurt of major flank, and lower quality of cutting surface.

Effects of spindle speed on cutting temperature. The curves shown in Figure 7(a) to (c) were drawn for real-time cutting temperature during cutting, while the spindle speeds varied at 10,000, 13,000, and 16,000 r/min with a groove depth of 5 mm, width of 12 mm, and feed rate of 6 m/min. It showed that the cutting temperature would augment as spindle speed went up. Although the burden of each tooth was reduced to keep shear heat down when the spindle speed increased, the friction between rake face/major flank and wood chip/ workpiece surfaces increased, during which friction heat was far more than the loss of

Figure 5. Relation between cutting temperature and feed speed.

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Figure 6. Heat conducting flow in cutting process.

shear heat resulting from reduced burden and then cutting temperature rose. High spindle speed would decrease burden to improve the quality of cutting surfaces, but cutting temperature cannot be higher than softening point. Hence, spindle speed needs to be controlled within a certain range.

Effects of cutting depth on cutting temperature. The curves in Figure 8(a) to (c) show the relation between cutting temperature and cutting depth when keeping the spindle speed, feed rate, and cutting width at 10,000 r/min, 6 m/min, and 12 mm, respectively, which revealed that cutting temperature would augment dramatically as cutting depth increased. The burden of each tooth became heavier following the increase of cutting depth, which resulted in the augmentation of shear heat. Meanwhile, the shear area between rake face/major flank and wood chip/workpiece surfaces became larger, which raised the temperature markedly. Therefore, cutting with over depth should be avoided when cutting WPC material to obtain a large groove depth, and multiple cutting could be preferred then.

Effects of cutting width on cutting temperature. As seen from Figure 9, the curves show real-time temperature changes during cutting under three different cutting widths at spindle speed of 10,000 r/min, cutting depth of 5 mm, and feed rate of 6 m/min. These curves demonstrated that cutting temperature increased slightly following the augment of cutting width. When the cutting width increased both the burden of each tooth and the shear area between rake face/major flank and wood chip/workpiece surfaces would rise, leading to the augments of shear heat, and then cutting temperature was elevated through small scale, which suggested that it could be one way to improve productivity by increasing cutting width.

Figure 7. Relation between cutting temperature and spindle speed.

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Figure 8. Relation between cutting temperature and cutting depth.

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Figure 9. Relation between cutting width (16 mm) and cutting temperature.

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Figure 10. Relation between cutting temperature and feed rate.

Comparison of cutting temperatures between WPC material and massoniana Experimental parameters between cutting temperature and varying feed rates were 10,000 r/min spindle speed, 5 mm cutting depth, 12 mm cutting width, and 3, 6, and 10 m/min feed rates, respectively. Experimental parameters between cutting temperature and varying spindle speed were 5 mm cutting depth, 12 mm cutting width, 6 m/min feed rate, and 10,000, 13,000, and 16,000 r/min spindle speeds, respectively. Experimental parameters between cutting temperature and varying cutting depths were 12 mm cutting width, 10,000 r/min spindle speed, 6 m/min feed rate, and 5, 10, and 15 mm cutting depths, respectively. Experimental parameters between cutting temperature and varying cutting width were 10,000 r/min spindle speed, 5 mm cutting depth, 6 m/min feed rate, and 8, 12, and 16 mm cutting widths, respectively. As seen in Figures 10 to 13, under the same cutting conditions, there were similar trends of temperature changes in cutting massoniana wood and WPC, but the temperatures for cutting massoniana wood were usually higher. It is generally attributed to friction roles from the following points: first, massoniana wood has higher fiber content, which is also more brittle compared with WPC and then its friction coefficient would be larger. Second, many small air holes are formed in natural wood from which the heat taken away was harder for massoniana wood. Third, WPC is isotropic with a uniform texture; in contrast, massoniana wood has different densities due to its unequal texture such as knots and spring or summer wood, which would generate more heat during cutting. For all the above-mentioned reasons, the cutting temperature for massoniana wood is always higher than WPC.

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Figure 11. Relation between cutting temperature and spindle speed.

Figure 12. Relation between cutting temperature and cutting depth.

Conclusions 1. During cutting, cutting temperature rose when cutting depth, spindle speed, and cutting width increased. Simply put, cutting depth was the most important factor on the cutting temperature, followed by spindle speed, while the cutting width was the least affected.. Once the feed rate went up, cutting temperature decreased on a small scale. When the cutting operation was performed repeatedly with

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Figure 13. Relation between cutting temperature and cutting width.

small cutting depth, small cutting width, appropriate low spindle speed, and relatively high feed rate, we could keep the cutting temperature lower than the WPC softening point and, finally, make good quality of cutting surface for WPC material. 2. Under the same conditions, friction coefficient of massoniana wood is larger than that of WPC material as wood has far more fiber content than WPC. Moreover, the heat conduction of WPC chip is also harder than massoniana chip for its uneven texture, which finally leads to heavier spindle work. Thus, cutting temperature of massoniana wood was higher than WPC material. However, there were similar trends on cutting temperature changes between them. 3. Shear heat moderately affected the cutting temperature during cutting. In the cutting process, chip flowed through the rake face, through which most of the shear heat generated in the shear zone was taken away. The rest of the shear heat was transferred to workpiece, tool, and adjacent medium. The temperature of cutting tools would increase mainly from friction heat from rake face and the major flank of tools. Declaration of Conflicting Interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article. Funding The author(s) received no financial support for the research, authorship, and/or publication of this article.

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Reference 1. Jayamol G, Sreekala M and Thomas S. A review on interface modification and characterization of natural fiber reinforced plastics composites. Polym Eng Sci 2001; 41(9): 1471–1485. 2. Adhikary KB, Pang S and Staiger MP. Dimensional stability and mechanical behavior of woodplastic composites based on recycled and virgin high-density polyethylene (HDPE). Composites B 2008; 39: 807–815. 3. Vu Thi T, Vu-Khanh and Lara J. Effect of friction on cut resistance of polymers. Journal of Thermoplastic Composite Materials 2005; 18(1): 23–35. 4. Takashi U, Mahfudz AH, Keiji Y, et al. Temperature measurement of CBN tool in turning of high hardness steel. Ann CIRP 1999; 48(1): 63–66. 5. Stark NM and Matuana LM. Characterization of weathered wood-plastic composite surfaces using FTIR spectroscopy, contact angle, and XPS. Polym Degrad Stabil 2007; 92: 1883–1890. 6. Cabrera D, Fuentes I and Khamlichi A. Multi-criteria optimization using Taguchi and grey relational analysis in CNC turning of PEEK CF30. J Thermoplast Compos Mater 2012; 25(1): 101–114. 7. Thi B, Vu-Khanh T and Lara J. Effect of friction on cut resistance of polymers. J Thermoplast Compos Mater 2005; 18(1): 23–35.