Hollowing and Reinforcing 3D CAD Models and Representing Multiple Material Objects for Rapid Prototyping

This dissertation, "Hollowing and Reinforcing 3D CAD Models and Representing Multiple Material Objects for Rapid Prototyping" by Wai-kei, Chiu, 趙偉奇, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Hollowing and Reinforcing 3D CAD Models and Representing Multiple Material Objects for Rapid Prototyping submitted by CHIU, Wai Kei for the degree of Doctor of Philosophy at the University of Hong Kong in July 2000 Abstract Rapid Prototyping technologies (RP) can be used to produce prototypes in a shorter time as compared to the traditional manufacturing processes. The prototypes can be used for testing of design ideas, verifying the form, fit and functionality of the designs. If the technologies are carried out properly, the lead time in the product development cycle can be effectively reduced. It is an advantage if the process performance in terms of the material usage and the build time can be further improved. To improve the process performance, one of the methods is to build a hollowed prototype. To do this, the solid CAD model of an object must be hollowed out. In this research, a simple hollowing algorithm is proposed. By the algorithm, voxels which are completely inside a solid are found. When Boolean subtraction is performed between these voxels and the solid, a hollowed model with its minimum wall thickness equal to a prescribed value is created and there are no constraints on the wall thickness requirement of the model. A hollowed prototype may not be able to satisfy strength considerations in the subsequent process of the product development cycle. To reinforce its strength, internal structures should be added into its interior, resulting in the formation of a reinforced prototype. To generate the CAD model of this reinforced prototype, a method is proposed and Boolean operations are employed as the basis of this method. The centre of gravity of a hollowed model (or a reinforced model) is usually different from the centre of gravity of its solid counterpart. In some cases, the centre of gravity of a model needs to be unchanged. To cope with this requirement, an algorithm is proposed. In this algorithm, volumes are added into the hollowed portion of a hollowed model in a controlled manner so that the centre of gravity of the hollowed model is made invariant from the centre of gravity of the original solid model. When a different material is added into the hollowed portion of a hollowed model or when the internal structures are made with different materials, a multiple material prototype is produced. To build the prototype, its CAD model must be represented right at the outset. The representations of contemporary CAD systems do not provide or contain any information with regard to the materials of the objects. With the advent of RP techniques, which allow multiple material objects to be produced, this arrangement is inadequate. In this research, a scheme for representing multiple material objects is proposed. A material tree of the object is established within the CAD system′s data structure. By extracting information from this material tree, and outputting a modified version of a STL file format to the RP machines, multiple material objects could be fabricated. To illustrate the results of the research, a number of example parts are created and their prototypes are built by the SLS process and the FDM process. DOI: 10.5353/th_b2976847 Subjects: Prototypes, Engineering Computer-aided design