Simulation-Based Approach for Probing Rheology-Processing-Structure Relationships in Foam Blow Molding

The broad objective of this work was to demonstrate a modelling and simulation framework for foam blow molding using commercially available simulation software. The simulation framework would have to account for the initial morphology of the foam, the relationship between the morphology and the rheological and deformation characteristics of the foam at high temperatures and high strains that are typically encountered during blow molding, and correlate the strains developed during blow molding to the morphological aspects in the resulting blow molded part. These aspects are addressed in this paper using simulations of uniaxial tensile deformation of a virtual representative volume element of a foam microstructure (rendered in DIGIMAT-FE) to derive the nonlinear tensile response of the foam at high temperatures (using ABAQUS). The resulting simulated stress-strain curve is employed to parameterize a nonlinear rheological constitutive equation. These parameters are then employed for the homogenized representation of the foam in the blow molding simulation carried out in B-SIM, a commercially available simulation software for blow molding. The regions where the simulated parison has undergone primarily uniaxial elongation are then mapped back to the expected local foam morphology using the transfer functions derived from the RVE simulations. These steps result in a preliminary and simple demonstration of the simulation framework, and offer a template that can be detailed further with experimental rheological information on actual foamed parisons, and more detailed post-processing algorithms to correlate multiaxial elongations with microstructure.

Effects of dynamic mold temperature control on melt pressure, cellular structure, and mechanical properties of microcellular injection-molded parts: An experimental study

In this work, the effects of dynamic mold temperature control (DMTC) on melt pressure, cellular structure, and mechanical properties of microcellular injection molding (MIM)-molded parts are investigated experimentally. It is found that with the increase of the mold temperature, the duration of foaming pressure in the cooling stage increases. Meanwhile, the average cell diameter and cell diameter dispersion increases as well as the cell density decreases in MIM molded parts. The turning point of mold temperature after which the foaming pressure in the cooling stage and the cellular structure in MIM molded parts generate a significant change is around the glass transition temperature of the used plastic material. Under DMTC conditions, with the increase of mold temperature, the tensile strength, flexural strength, and impact strength of MIM molded specimens of single gate without weld line change a little, while the tensile strength, flexural strength of MIM molded specimens of double gates with weld line increase obviously. When the mold temperature increases to 120°C and over, the tensile strength, flexural strength of MIM molded specimens of double gates with weld line reach an equivalent level of specimens of single gate without weld line.

Extensional Rheology and Processing of Polymeric Materials

Many processing operations of polymeric materials are dominated by extensional flow. In this paper the question is addressed in which way rheological data obtained from elongational laboratory experiments can be used to describe the material behavior in various processes. Particularly discussed is the uniformity of geometrical features of items with respect to the strain hardening of the polymeric materials, because uniformity is important for many end-use properties. Convincing correlations with extensional rheology could not be established for thermoforming experiments on polypropylene. However, for blown and drawn films it was found that the uniformity of thickness improved, if strain hardening was introduced. The bubble stability could be related to the melt strength of the sample. Furthermore, the influences of elasticity and strain hardening on the neck-in during drawing of films were elucidated. Moreover, it was shown in which way extensional rheology can be used to assess foaming.