Introduction
Background
Recently, the trends of manufacturing industry are to seek new and innovative methods to accelerate new product development and to manufacture the product rapidly. These issues have developed into global competition among manufacturing companies to improve their business process. To help do this, rapid manufacturing has appeared as an option.
Rapid manufacturing has been developed as an advanced manufacturing technology to shorten time to market. The technologies are based on an additive approach, where a component is built gradually by adding material layer by layer to create a geometry of components in two-dimensional profiles, controlled directly from 3D computer-aided design (CAD) solid models system. This additive manufacturing approach has several advantages. The technologies are able to fabricate parts rapidly without the use of any additional tooling or moulds for any complexity of geometry [Kruth et al, 1998; Pham, 2002; Levy et al, 2002]. In addition, it allows reduced design and production lead-time and cost.
There are many different commercial additive fabrication technologies available, such as stereolithography (SLA), selective laser sintering (SLS), laminated object manufacturing (LOM), fused deposition modelling (FDM), solid ground curing (SGC) and three-dimensional printing (3DP). All technologies have limitation on the type and properties of materials that can be applied [Gibson, 1997; Pham, 2002; Debasish, 2001].
For Selective laser sintering (SLS), which was developed in the late 1980's as a technique for Rapid Prototyping (RP), there is a wide range of materials, including metals (steel, titanium, alloy mixtures and composites), polymers (nylon, also glass-filled or with other fillers, and polystyrene), and foundry sand [Levy et al, 2003]. Hence, SLS is frequently used for prototyping, tooling and manufacturing purposes.
In order to manufacture products, acceptable material characteristics must available. Material properties, mainly mechanical properties of product that involve the strength and stiffness are critical issues regarding service loading and its operation [Caufield et al, 2007].
In conventional manufacturing process, the repeatability of mechanical properties is known. Whereas for layer manufacture, the material properties are less well understood and depend on the fabrication orientation [Gibson, 1997], with the tensile strength and density of specimens produced in different orientations with different fabrication parameters varying. In addition, Hague et al [Hague, 2004] and Hou [Hou, 2005] in their studies conclude that SLS material can be considered as anisotropic material, showing different strengths and modulus in different orientations.
There is another issue in rapid manufacturing field, regarding mechanical properties and improvement. Zarringhalam, et al [Zarringhalam, 2006] investigated effects of various processing factors on the microstructure and properties of SLS Nylon 12. They analysed different specimens of SLS Nylon 12 powder and parts, produced from different thermal histories, using different machines which were configured with contrasting parameters. It showed that a significant raise in elongation at a break for refreshed and used powder compared with virgin powder, for parts makes on different machines. It also demonstrated that there are small variations in ultimate strength between virgin and refreshed material, with increases in tensile strength for used powder. The values of Young's modulus also varied, even though the same machine and material was used to fabricate the specimen.
It has been recognised that material properties are influenced by microstructure, and the forming of material microstructure is depend on temperature and time when it processed. In addition, the properties of a structure can be affected by different temperatures, times and heating/cooling rates of heat treatment processes.
There are several research works that has been investigated heat treatment of polymers materials. Among them, Zarringhalam and Hopkinson [Zarringhalam, 2003] reported that using heat treatment (conventional heating chamber) as post processing can increase the tensile strength and has impact strength of SLS parts made from Nylon 12. In addition, Scobbo, et al [Scobo, 1994] showed that when annealing on Poly Phenylene Sulfide material the modulus increased as a result of an increase in crystallinity.
Ramazani, et al [Ramazani, 2005] investigated vacuum annealing effects on the mechanical properties of thermoplastic materials, polystyrene and Nylon 6,6. They studied heat treatment process at different vacuum levels and for different time-temperature settings. They showed that the heat treatment in a vacuum condition has a positive impact on the mechanical properties of polymeric parts.
In this study the mechanical properties of Nylon SLS parts will be investigated and methods for improving and making more consistent the mechanical properties using heat treatment will be evaluated.
Structure Of The Thesis
This thesis consists of seven chapters. The second chapter is a literature review that considers fundamentals of layer manufacturing technology and its applications, definition of rapid manufacturing, selective laser sintering (SLS), materials, and post processing. At the end of this chapter a summary of the literature review is presented.
The third chapter outlines the aim and objectives of research work, describes implementation of rapid manufacturing, especially selective laser sintering in product design step, and the engineering context of this research work.
Chapter four details experimental studies and procedures, that involves standards, test methods and analysis, and mechanical testing equipments that have been used in this study. In the next chapter experimental results and discussion are presented regarding the benchmarking of mechanical properties characterisation.
In chapter six describes heat treatment studies, that shows details of heat treatment regimes which were conducted for Selective Laser Sintered Nylon 12 and their input on the mechanical properties. In addition, it shows the comparison between heat treated and non heat treated specimen's characterisation results and discussion.
Chapter seven consist of general discussion the work, considering applicability and outlines conclusions and recommendation for future work.
