PhD Defence at DTU Mechanical Engineering

PhD Defence 2nd February: Numerical Modelling of Hot-Wire and Hot-Blade Cutting Processes

Tuesday 31 Jan 17

Contact

Jesper Henri Hattel
Head of Section, Professor
DTU Mechanical Engineering
+4545 25 47 10

Kiril Petkov from DTU Mechanical Engineering defends his PhD, "Numerical Modelling of Hot-Wire and Hot-Blade Cutting Processes", Thursday 2nd February 2017, at 13:00. The defence takes place in Building 101, room S01 at the Technical University of Denmark. Professor Jesper Hattel is supervisor.

Abstract
The present thesis deals with the challenges of numerically modeling the processes of Hot-Wire and Hot-Blade cutting of expanded polystyrene foam. The technologies under investigation are studied and furtherly developed during the time-span of the project and are commercially Applied in the building industry for production of ruled and double-curved moulds for casting of concrete. The proposed technologies present a cost-efficient method for production of complexcurved shapes and structures in the building-related design and architecture, thus aiming to increase the possibilities of the architectural expressions and aesthetics currently available. The geometrical complexity calls for higher tolerances in the production facilities and fabricated parts, and the process-modelling tool is used to answer some of the questions related to accuracy and in-dept understanding of the process.

The numerical study presents thermo-electro-mechanical models for Hot-Wire and Hot-Blade cutting of polystyrene foam. The models are solving a system of non-linear differential equations (including the heat diffusion equation, the electrical diffusion equation and the static equilibrium equation) with temperature dependent material properties in order to find the cutting tool’s internal parameters during cutting procedures- temperature, voltage distribution, energy generated due to resistance heating, thermal expansion, longitudinal strain and stresses. The main parameter responsible for the accuracy of the produced foam surface- the kerfwidth (i.e.gap space after material removal) is predicted based on a new formulation relating the temperature of cutting, the foams melting energy and the cutting speed, thus accounting for the transient behaviour of the kerfwidth. Additional considerations for the variations of the kerfwidth with regard to cutting with inclinations from the horizontal plane are discussed and
analyzed. The models describe the process dynamics between the different parts of the cutting tool- those involved in the cutting of foam and those surrounded by air, with reference to the parameters responsible for the kerfwidth prediction. To increase the accuracy of the model, investigation on the relevant natural convection heat transfer coefficients was conducted and the most suitable correlations describing the heat transfer at the parts of the blade not-involved in the cutting were selected and used.

The numerical models were calibrated with respect to experimental studies for Hot-Wire and Hot-Blade cutting carried out in collaboration with external project partners. The challenges  with conducting the experimental investigations, together with the applied robotic procedures, Infra-red and kerfwidth measurements were described with reference to the numerical models. The complicated phenomena of phase transition of the cellular foam between the solid, liquid and gas, was studied and incorporated in the model based on empirical relations obtained during the experimental studies. Based on the empirical investigations, the numerical models were found to be in a good agreement with the experimental results and represent the cutting processes well.

Finally, the thesis presents the development and construction of the Hot-Blade cutting tool used for single-pass cutting of large-scale double-curved surfaces from polystyrene foam. The specifications and requirements of the tool- in terms of flexibility, stiffness, mechanical and electrical material properties, are discussed and the process of developing a fully functional blade is presented.