Injection molding is an economical and effective method for manufacturing polymer parts with nanostructures and residual stress in the parts is an important factor affecting the quality of molding. In this paper, taking the injection molding of polymethyl methacrylate (PMMA) polymer in a nano-cavity with an aspect ratio of 2.0 as an example, the formation mechanism of residual stresses in the injection molding process was studied, using a molecular dynamics simulation. The changes in dynamic stress in the process were compared and analyzed, and the morphological and structural evolution of molecular chains in the process of flow were observed and explained. The effects of different aspect ratios of nano-cavities on the stress distribution and deformation in the nanostructures were studied. The potential energy, radius of gyration and elastic recovery percentage of the polymer was calculated. The results showed that the essence of stress formation was that the molecular chains compressed and entangled under the flow pressure and the restriction of the cavity wall. In addition, the orientation of molecular chains changed from isotropic to anisotropic, resulting in the stress concentration. At the same time, with the increase in aspect ratio, the overall stress and deformation of the nanostructures after demolding also increased.
Keywords: PMMA; aspect ratio; injection molding; molecular dynamics simulation; residual stress.