Residual Stress Formation Mechanism And Optical Property Of Injection Compression-Molded Product

Injection compression molding (ICM) is one effective process for molding precise plastics products including optical parts. However, its complicated procedure is not easy to be optimized due to involving many factors influencing product properties. Besides, conventional to dimensional simulation software badly predicts the residual stress and optical properties especially for thick products. The present thesis conducted three dimensional (3D) simulation for exploring the residual stress formation mechanism and analyzing the effects of different factors on optical properties in ICM. The main results are as follows.1. The molding process was3D-simulated by Moldex3D software for systematical study of effects of processing parameters on residual stress and optical properties. In the analyses, considered were compression flow in cavity thickness direction, melt compressibility and White-Metzner constitutive equation including shear-induced heat, the variation in viscoelasticity and relaxation by pressure rise and temperature deviation. The melt flow frontier was captured using deformable network system and volume fraction whereas thermal stress was calculated from the viewpoint of thermal deformation and time-temperature equivalence. The simulation for2mm plate showed that the flow residual stress drops in S shape while thermal one in W shape from gate along central line. The former is larger by one to two orders than the latter resulting their summation, i.e., the total residual stress drops in S shape. It is favorable to reduce flow residual stress in molding at higher melt temperature, lower compression rate, larger compression gap and properly higher injection speed. In comparison, the effects are slight for higher mold temperature and longer cooling time. There little variation in product thermal residual stress from different injection-compression stage. The tendency in birefringence is similar in terms of stress-birefringence correspondence. The experimental residual stress data are in fair agreement with predicted values.2. Numerical simulation shows that gating system greatly influences flow residual stress by varying melt-filling rate, duration and shear rate against slight thermal residual stress. The residual stress is lowest in product molded using tab gate among four kinds of gates. Variation in the location of melt inlet and the gate size bring about residual stress in small variation in central domain, especially in high stress domain. Hot-runner slightly reduces thermal residual stress in central domain. The larger flow stress really results in higher total stress.3. The molding analyses of products with different thicknesses show that ICM process can reduce flow rate in thickness and shear rate as well as the resultant flow residual stress. Compression reduces melt flow distance, distribution difference in thermal course and cavity melt pressure leading to homogeneous distribution in density and thermal residual stress. At the same molding conditions, smaller cavity pressure, larger flow rate in thickness and temperature gradient were observed in thicker products with smaller average density, flow stress and total stress against. Thin PS product exhibits lower density than thick PC products even negative birefringence near gate. Compression causes higher shear rate and orientation when compression gap is close to product thickness. Thus, compression gap should not be too great with relative to thickness.4. ICM is also beneficial for optical properties to len with variable thickness. Compression induces the variation of flow rate at different positions with time. Lower compression rate or larger gap corresponds to gate pressure, residual stress and the caused flow-induced optical retardation and warpage.5. The prepared samples were tested for optical angular deviation and optical distortion by optical multiplication for analyzing the effects of material, product shape, process conditions and annealing on the optical properties of flat plate and cone-shaped product. It was found to be worse when choosing material with larger refringence, of larger curvature and with larger residual stress. Thick product is apt to exhibit larger optical angular deviation and optical distortion regardless of improvement by adopting ICM instead of common injection. Annealing at higher temperature in longer duration relaxed the residual stress for smaller angular deviation while resultant warpage increases optical distortion.Consequently, ICM combines the advantages of compression and injection improving the product properties. The3D simulation well predicted the variation rules of stress optimizing process conditions for higher properties.