Analysis of orientation behavior in poly(lactic acid) billets during extrusion using a finite element method and chain network model

Poly(lactic acid) (PLA) has attracted much attention for use in bone fixation devices, which are used for fracture treatment, because it degrades to nontoxic lactic acid through nonenzymatic hydrolytic degradation. However, the application of such device is limited to relatively low-load regions, because mechanical properties of PLA is poor than that of metal materials. The process of drawing PLA fibers has attracted much attention as an approach for improving the mechanical properties of PLA bone fixation devices. The mechanical properties and molecular orientation of drawn PLA fibers and films have been investigated. However, the optimization of drawing conditions in bulk processes in relation to bone fixation devices is difficult because a large number of potential drawing conditions exist. The purpose of this study was to analytically clarify molecular orientation behavior in drawn PLA billets. The molecular orientation obtained by extrusion in PLA billets was investigated using a combination of the finite element method (FEM) and a chain network model. As a result of the FEM, first principal plastic strain was generated, mainly in the tapered components during extrusion; this strain was in the axial direction of the rod. Radius strain distributions showed that strain in the center region was almost uniform, whereas strain peaked near the surface. The orientation function increased with extrusion ratio (ER), and the peak orientation function in the surface region increased with taper angle. Thus, the magnitude and distribution of the orientation function in extruded PLA billets may be controllable by ER and taper angle, respectively.