Computer Simulation and Experimental Validation of Complex Thermoforming Processes

Under a recent great demand for thermoformed products with complicated and highly draped designs, more precise computer simulation of this process has been required. For establishing the appropriate numerical model, understanding of the effects of material behavior and the processing parameters is important. This paper comprehensively studied the behavior of extruded Acrylic-modified PVC (AM-PVC) sheet during both the drape forming and the air slip forming using an axis symmetric columnar male mold. The rate of extension ratio of 0.05-0.13 s^(-1) and the variation of sheet temperature profile of 170 ± 5℃ were measured as the forming conditions. Both thickness distributions of the sheet and material slip sizes over the mold surface were measured after each forming stage, which consists of pre-blowing with different blowing heights, mold stretching, and vacuuming. The mechanisms of characteristic patterns in the extruded sheet thickness distributions with the effect of anisotropic residual strain were explained. An equi-biaxial extension examination of the high-temperature and large-strain behavior of AM-PVC sheet as a function of the forming conditions was performed by lubricated compression testing. The stress-strain curves showed the tendency of strain hardening and the dependences on the strain rate and especially on the temperature of the specimen. This finding shows that the temperature-dependent viscoelastic material model must be employed to simulate accurately this forming process.