CHARACTERISTICS OF SCREWDRIVING TORQUES IN WOOD-PLASTIC COMPOSITES
The characteristics of torque magnitudes for driving screws into wood-plastic composites (WPCs) were investigated through evaluating effects of different factors on seating torque (SET) and stripping torque (STT) commonly used to characterize the process of driving screws into amaterial. The factors were embedded screw orientation in WPCs, pilot-hole diameter, screw penetration depth, and screwdriver air pressure. Recorded torque-time curves of driving screws into WPCs evaluated in this study indicated that the complete screw driving process can be described as a three-phase process of thread forming and screw seating, clamping, and screw stripping WPCs. Mean SET values for driving screws into WPCs can range from 0.47 to 1.83 N-m, STT values from 1.54 to 4.87 N-m, and their corresponding STT-to-SET ratios from 1.0 to 4.4. Statistical analyses indicated that mean SET and STT values of driving screws into WPCs increased as screw penetration depth increased from 12.7 to 25.4 in increments of 6.35mm. The significance of pilot-hole diameter effects on mean SET and STT values is influenced by screw penetration depth. Mean SET values at 0.45 MPa air pressure level were higher than those at 0.62MPa, whereas the significance of screwdriver air pressure effects on mean STT values is influenced by pilot-hole diameter and also screw penetration depth. The significance of embedded screw orientation effects on mean SET values is influenced by screw penetration depth and pilot-hole diameter and on mean STT values by screw penetration depth, pilot-hole diameter, and screwdriver air pressure.
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