Plastic Foam Cutting Mechanics for Rapid Prototyping and Manufacturing Purposes

Abstract

Development of foam cutting machines for rapid prototyping and manufacturing purposes began shortly after the first additive manufacturing machines became commercialised in the late 1980s. Increased computer power, the development and adoption of CAD/CAM software and rising demand for customisation has caused the rapid prototyping industry to grow swiftly in recent decades. While conventional rapid prototyping technologies are continuing to improve in speed and accuracy the ability to produce large (> 1m³) prototypes, moulds or parts it is still expensive, time consuming and often impossible. Foam cutting rapid prototyping and manufacturing machines are ideally suited to fulfil this niche because of their high speed, large working volumes and inexpensive working materials. Few foam cutting rapid prototyping machines have been commercialised to-date leaving significant opportunities for research and development in this area.

Thermal plastic foam cutting is the material removal process most commonly used in foam cutting rapid prototyping to shape or sculpt the plastic foam into desired shapes and sizes. The process is achieved by introducing a heat source (generally a wire or ribbon) which alters the physical properties of the plastic foam and allows low cutting forces to be achieved. In thermal plastic foam cutting the heat source is generated via Joule (electrical) heating. This study investigates the plastic foam cutting process using experimental cutting trials and finite element analysis.

The first part of this thesis presents an introduction to conventional foam cutting machines and rapid prototyping machines. It is suggested that a market opportunity lies out of reach of both of these groups of machines. By combining attributes from each, foam cutting rapid prototyping machines can be developed to fill the gap.

The second part of this thesis introduces the state-of-the-art in foam cutting rapid prototyping and investigates previous research into plastic foam cutting mechanics. The third part of this thesis describes cutting trials used to determine important factors which influence plastic foam cutting. Collectively over 800 individual cutting tests were made. The cutting trials included two main material sets, expanded polystyrene and extruded polystyrene, three different wire diameters, two hot-ribbon configurations and a wide range of feed rates and power inputs. For each cut the cutting force, wire temperature and kerf width was measured as well as observations of the surface texture. The data was then analysed and empirical relationships were identified. An excel spreadsheet is established which allows the calculation of important outcomes, such as kerf width, based on chosen inputs.

Quantitative measurements of the surface roughness and form, of cuts made with hot-tools, will not be addressed in this thesis. This body of work is currently under investigation by a colleague within the FAST group.

The fourth part of this thesis describes the formation of a nonlinear transient two-dimensional heat transfer finite element model, which is developed for plastic foam cutting simulations.

The conclusion is that the cutting trials contributed to a better understanding of plastic foam cutting mechanics. A new parameter was identified called the mass specific effective heat input, which is a function of the foam material and the cutting tool, it allows the prediction of cutting conditions with given cutting parameters and hence provides the necessary relationships needed for adaptive automated foam sculpting. Simulation results were validated by comparison with experimental data and provide a strong base for further developments including optimisation processes with adaptive control for kerf width (cut error) minimization. This study has added considerably to the pool of knowledge for foam cutting with a hot-tool. In general, much of the work reported herein has not been previously published. This work provides the most advanced study of foam sculpting work available to date.