Constitutive viscoplastic behavior of amorphous PET during plane-strain tensile stretching

Optimization of the Thickness of PET Bottles during Stretch Blow Molding by Using a Mesh-free (Numerical) Method

The stretch-blow molding process of polyethylene terephthalate (PET) bottles generates some important modifications in the mechanical properties of the material. Considering, the process temperature (T > Tg) that is usually used, the material exhibits a very high viscosity and involves a strain hardening effect associated with the microstructure evolution. An anisotropic viscoplastic model coupled with induced properties, identified from experimental results of uniaxial and biaxial tensile tests previously published by Chevalier and Marco (2006), is presented in a first part of the paper. Secondly, we perform a numerical simulation to simulate the free inflation of a preform under an internal pressure with different parameters. Because the final strains are up to 300 to 400%, it generates important distortion of node distribution and we chose to use the mesh-free Constrained Natural Elements Method (C-NEM) for numerical simulation. The final goal is to use these simulations in order to fit the best parameter set leading to a quasi-homogeneous distribution of the thickness along the bottle. Homogeneous thickness implies homogeneous biaxial stretching and more uniform induced properties for the final bottle and this is an important industrial goal.

Modelling Behaviour of PET for Stretch and Micro-Blow Moulding Applications Using an Elasto-Visco-Plastic Material Model

Polyethylene terephthalate (PET) has been widely used in the stretch blow moulding (SBM) process for packaging applications. Finite element analysis has become extensively useful for assessing container designs and enabling the designers to perform analyses earlier in the design cycle to determine the best material and the best structure. However, there are several challenging issues due to various processing parameters and complex material behaviour, which is both temperature and strain-rate dependent. In this paper, we generalize the G'Sell-Jonas law in the three-dimensional (3D) case to model and simulate the elasto-visco-plastic (EVP) behaviour of PET, taking into account strain-hardening and strain-softening. In addition, it is observed that the internal pressure (inside the preform) is significantly different from the nominal pressure (imposed in the blowing device upstream) since the internal pressure and the enclosed volume of the preform are fully coupled. In order to accurately simulate this phenomenon, a thermodynamic model was used to characterize the pressure-volume relationship (PVR). The predicted pressure evolution is therefore more realistic when imposing only the machine power of the blowing device (air compressor or vacuum pump). Mechanical and temperature equilibrium equations are fully nonlinear and solved separately with implicit schemes on the current deformed configuration, which is updated at each time step. Biaxial characterization tests were used to determine the model parameters in order to simulate the SBM process using the PVR. Three industrial case studies, comparing simulated thickness predictions to experimental measurements, will be presented in order to illustrate the applicability of the proposed model.

Mechanical Behavior of a Series of Copolyester Blends near the Glass Transition: Monotonic and Load-Hold Behavior in Compression

Monotonic loading tests were conducted on five commercial blends of poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) at temperatures of 90°C and 100°C and strain rates of 0.1/s, 0.05/s, and 0.005/s in uniaxial and plane strain compression. On comparing the mechanical behavior of the five materials, it was found that the behavior of the low-PCT content materials was different from the high-PCT content materials only at conditions that favored strain-induced crystallization, particularly in plane strain compression. Load-hold tests were also conducted on three of the blends with similar results to the monotonic tests. Material differences were only pronounced at certain conditions, and in these cases the low-PCT content materials showed increased strain hardening after the hold period while the high-PCT content material did not. Therefore, it was found that the addition of a hold period was not exclusively required to observe differences in the crystallizable materials over the noncrystallizing blends. The increased strain hardening likely associated with crystallization in PET was only observed when the following conditions were met: (i) strain rates of 0.1/s and above, (ii) temperatures of 90°C–100°C, (iii) plane strain compression, and (iv) after a certain level of deformation.