Formation Mechanism of Residual Stresses in Micro-Injection Molding of PMMA: A Molecular Dynamics Simulation.

Can Weng,Jiezhen Liu,Mingyong Zhou,Hao Wang,Tao Ding

doi:10.3390/polym12061368

Abstract

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.

Highlights

Functional surfaces with micro/nano-structures are widely used in microfluidic chips [1,2], superhydrophobic surfaces [3], optical conversion surfaces [4] and biomimetic surfaces [5] due to their excellent optical, electrochemical and biological properties
Yang et al [28] studied the nano-imprint of polymethyl methacrylate (PMMA) using molecular dynamics simulation, and the results showed that the potential energies at the beginning and end of the imprinting were different to some extent; they were not completely released, resulting in residual stress
The filling and packing stages could not be completely divided in the simulation, and these two stages could only be distinguished according to the PMMA layer completely entering the mold cavity to form a clear nanostructure profile

Summary

Introduction

Functional surfaces with micro/nano-structures are widely used in microfluidic chips [1,2], superhydrophobic surfaces [3], optical conversion surfaces [4] and biomimetic surfaces [5] due to their excellent optical, electrochemical and biological properties. Micro/nano manufacturing technology has become the forefront of modern science and technology. Micro/nano size and high precision requirements pose great challenges to processing and manufacturing. Micro-injection molding is an ideal means for manufacturing various shapes and sizes of micro/nano structured parts, which is economic, effective and has the potential for mass production [6]. Many scholars have been eager to resolve the residual stresses of surface micro/nano structured parts via injection molding

Methods

Results

Conclusion