The effects of different variables on the direct screw withdrawal strength in plywood

Aim of study: The purpose of this study was to investigate the effect of the pilot-hole, screw type, screwing direction, water soak and adhesive on the screw withdrawal strength in plywood. Material and Methods: Testing blocks were cut from 18-mm-thick beech plywood and were sized to 50 mm x 50 mm according to TS EN 13446 standards. Polyvinyl acetate (PVAc) and polyurethane (PU) as adhesives were injected into predrilled holes (80% of the screw major diameter). Two types of screws (3.5 mm and 4 mm in diameter) were inserted in perpendicular (face) and parallel (edge) to the surface of blocks. Half of the blocks were immersed in water for 2 hours before the screw withdrawal test. The results were analyzed and interpreted by statistical analysis. Main results: The results indicated that the screw withdrawal strength perpendicular to the surface is higher than that the one parallel to the surface. The testing blocks with PU specimens in pilot-hole showed higher screw withdrawal strength than the blocks with PVAc. The screw withdrawal strength was lower when blocks were immersed in water. It was also found that the screw withdrawal strength increases, as the screw major diameter increases. Research highlights: It was concluded that applying PU in pilot-holes before driving screw into plywood significantly affected screw withdrawal performance.