Study On Technology And Equipment Of Removable Partial Denture Framework By Laser Rapid Prototyping

The removable partial denture framework is very complex in shape with many curved surfaces, which is a very important assistant tool in prosthodontics. Generally, frameworks are fabricated by single piece basing on mould by traditional cast technology, which has many shortages of multiple steps, time cost and defects such as sand adheres, inner holes, cracks and segregation etc. Note that it is difficult for this method to be used to fabricate frameworks of titanium alloys due to their high activity. In order to achieve the proper fabrication environment, the cost of casting is very high. Hence, it is significant to find a novel manufacturing technology to overcome shortages of the traditional dental framework fabrication method.As a novel method, the laser rapid prototyping technology (LRPT) has the advantages of short manufacturing cycle, high precision and flexibility, simple fabrication arts, which can be used to replace the conventional cast technology in the areas of prosthodontics. Whereas, the experimental and theoretical researches on the fabrication of removable partial denture frameworks by LRPT are very few up to now, which retard the applications of this technology in prosthodontics. In this Ph.D. dissertation, the technologies of direct laser fabrication and selective laser melting were compared to investigate the possibility of fabricating the removable partial denture frameworks. The equipment design, software development, basic fabrication parameters, processing efficiency and precision, microstructure and mechanical properties of final parts are investigated to realize the formation of removable partial denture frameworks. The following are the main results:Firstly, a DLF (Direct Laser Fabrication) setup composed of a Nd: YAG laser, a CNC stable and a powder feeder was built up. The effects of DLF parameters on the structure and mechanical properties of parts were analyzed systemically on the basis of the DLF setup above mentioned were investigated. The optimized parameter ranges are as follows: laser power 200～500W, scan velocity 700～1400mm/min, powder feed rate 4.2～6.8g/min. The phase of the fabricated parts is austenite, which is the same as that of the starting powder material. The fabricated sample has a tensile strength over 912MPa and an average micro-hardness of HV261. But the open-loop control and coaxial powder nozzle decide the DLF technology is not suitable for denture framework fabrication with high precision (smaller than 2.5% in dimension) and high efficiency (1.06cm~3/h) despite good microstructure and mechanical properties can be obtained.Accordingly, the selective laser melting technology was used to fabricate removable partial denture frameworks. In our project, a SLM system was designed and developed by ourselves, which is consisted of a Yb-fiber laser, an optical scanner, a powder coating device, a gas purification system and a control system. By using above SLM system, the process parameters were investigated systematically and optimized. The optimized parameter ranges are: laser power 150～200W, scan velocity 5～25m/min, slicing layer thickness 0.02～0.04mm. The experimental results show that several key steps should be noticed during SLM process to obtain metal parts with better performance: (1) the substrates should be milled to Ra0.8μm and free of water before sand blasting; (2) The thickness of slice should be adjusted easily and controlled in the range of 0.01～1.0mm. (3) Proper laser power density should be adopted to prevent the emergence of balling or splash.The parts formed by optimized parameters have a density above 96%, a tensile strength of 635MPa and an average micro hardness of HV307 for stainless steel powder. It is interesting to find that the phase of the built parts is also austenite, which is the same as that of the starting material. The measurements also show that the average dimension error is about 1.1% and the manufacturing efficiency of SLM is 1.77cm~3/h. The surface roughness is decided by slicing thickness, laser process parameters and the incline angle. The smaller slicing thickness or bigger incline angle leads to low roughness surface. Importantly, the overlap is a key parameter to control the roughness, the surface with lowest roughness can be obtained at an overlap of 30% when other parameters are kept the same. Hence, the microstructure and mechanical properties of fabricated parts is better, but the manufacturing precision and efficiency is high by SLM in comparison with that of DLF technology.The manufacturing efficiency of SLM was studied systemically by using this system. The results indicate the main ways to increase the manufacturing efficiency on the basis of ensuring the precision, microstructure and mechanical properties are proper scanning path layout, higher powder coating velocity, less time of slicing and filling path calculation. In this dissertation, a partition scanning arithmetic called dichotomy was used to design the scanning paths on the basis of reducing the laser skipping time. In addition, multithread calculation was introduced to make the slicing, scanning filling calculation and laser process run synchronous, which saves 50% of all manufacturing time and increases the manufacturing efficiency.We finally fabricated removable partial denture frameworks with stainless steel and titanium materials successfully by using the SLM system under optimized parameters. The tests showed that the frameworks have an average dimension precision of±0.172mm. The wearing experiments on the plaster mouth molds demonstrated the removable partial denture frameworks fabricated by SLM can be fully applied to clinic after some postprocessing procedures.