Philipp Bratfisch

Investigation of continuously produced thermoplastic honeycomb processing - Part II: Fusion bonding

Following paper Part I, this article investigates the last step of ThermHex process namely fusion bonding. The critical point of the fusion bonding step is to prevent catastrophic deformation (collapsing) of cell walls under heat and pressure. This demands a rapid heating rate and short heating time to keep sufficient stiffness for the inner part of cell walls against the pressure applied on the honeycomb. Hence, in this work the transient heat transfer behavior during the fusion bonding is investigated by using a finite element model for the detailed hexagonal unit cell. Furthermore, a mathematical model is developed to analyze the deformation of molten cell walls and their gradual contact to the skin strips. The experiments based on the talc-filled polypropylene material have been performed to validate the analytical model. Consequently, the optimal fusion bonding conditions can be determined to ensure good quality for the final products.

Investigation of Continuously Produced Thermoplastic Honeycomb Processing — Part I: Thermoforming

A continuous process of thermoplastic honeycomb core, ThermHex, has been patented and is under development at K. U. Leuven. This new concept of thermoplastic honeycomb provides more affordable choices for structural applications than the previous expensive honeycombs thanks to the automated and continuous processing technology. The ThermHex process starts from one continuous thermoplastic sheet and consists of three main processing steps: thermoforming of half-hexagonal webs, folding of webs to honeycomb geometry, and internal fusion bonding. In this article, the polymeric material behavior during the thermoforming step is investigated in detail. First the material is characterized by a series of polymer tests to obtain basic data for the next numerical calculation. Then a 1D viscoelastic model and a 2D finite element (FE) model are used to analyze the polymer behavior during the thermoforming step, to acquire the correct understanding and hence to minimize the effect of the anelastic strain recovery (ASR) during the production. It is found that the material stress relaxation is one of the key features of the final process quality.