Effect of extrusion drawing and twist-orientation on mechanical properties of self-reinforced poly(lactic acid) screws

In order to improve torsional strength of a bioabsorbable poly(lactic acid) (PLA) screw, twist-orientation method was developed in this study. PLA screws were prepared through a series of routes including compression molding, extrusion method, twist-orientation, and forging. Shear and torsional strength were measured as mechanical properties of screw. As a result, twist-orientation improves torsional strength of PLA screw without the decrease in shear strength. The maximum torsional strength was obtained at helix angle of 45° and extrusion ratio of 8. This is because fibrous crystals were oriented close to the direction of principal tensile stress which occurred during torsion test. Since this principal stress occurs in the 45° direction on the surface of specimen, the maximum torsional strength is achieved by twist-orientation at this helix angle.