Simulations of thermoforming and blow-molding manufacturing processes for load-bearing structural components.

摘要

Thermoforming and blow molding are two related processes that can be characterized as inflating a pre-heated polymeric membrane into a mold. The most important issue of these manufacturing processes is to control the distribution of thickness of final product. Utilizing differential heating in thermoforming and varying the initial thickness distribution of parison in blow molding, it provides a way to control the final thickness distribution of a formed part. However, both control techniques will require some predication of the final product to determine the control parameters that are required at the beginning of these processes. Historically, these control parameters are obtained by trial and error. As a result, the development of new mold designs and the thermal process parameters has been inefficient and expensive.; The purpose of present study is to investigate the final thickness and temperature distributions of a formed part that will be used to optimize the control parameters at the beginning of the manufacturing processes. By plotting the final thickness distribution at final deformed state on the initial configuration, one can obtain the information of final thickness for every material point. This information can be used to determine the control parameters of differential heating by specifying the heating temperature on individual region of the polymeric sheet in thermoforming to obtain the desired material properties at the specified temperature. In blow molding, it can be used to determine the initial thickness distribution of extruded parison.; In the proposed numerical model, first the parison is inflated freely without any contact with the mold cavity along with the convection heat transfer. Then the polymer hits the mold surface and the heat is transferred from the membrane to the mold surface. Finally, all parts of the membrane contact the mold and the membrane is cooled. Using membrane analogy, the behavior of a parison sheet for thermoforming or a tube for blow molding is analyzed by finite element analysis. In addition to deformation analysis for both axisymmetric and three-dimensional cases, a one-dimensional cooling analysis is also considered to account for the heat transfer from membrane to the air or mold.; This analysis is first verified by theoretical solutions. A set of experimental data from previous literature is also applied to show the agreement. It is found that the thinnest part of the final product is always at the material point that has longest travel path from the undeformed configuration to deformed configuration. To obtain more uniform thickness distribution predicted by the program, one can adjust the initial thickness distribution of the membrane by setting the initial thickness of the corresponding finite elements, re-run the program a couple times until the desired final thickness distribution is obtained. Following this analysis, the stresses and strains obtained by the program can be transferred to a standard structure analysis program to perform residual analysis as well as to determine load-bearing capability to accelerate the design cycle.