The Powder Injection Molding Of ZrO₂ Samll Parts And The Dimension Precision Control

Powder injection molding (PIM) is a new type of near-net parts forming technology. In this paper, the forming technologies of ZrO2 ceramic small pieces were investigated, and the method to control the dimension precision of the final products was also discussed.The Powder employed in the feedstock was 3Y - ZrO2 ceramic powder with an average particle size of 0.2μm, while the binder was the PW-PP-SA thermoplastic system. A reasonable mixing process was developed after several times of tries. The feedstock prepared under the mixing process, with 40~45vol% zirconia powder, has uniform texture and good liquidity, therefore, was suitable for the requirements of PIM.Though the investigation, it was found that, under inappropriate processing parameters, for the test samples formed in the experiments, the main defects were insufficient filling, holes and flying edges. After repeated experiments, more reasonable injection molding process parameters are found: the plastics room temperature: 220℃, melt temperature of 210℃, injection temperature: 200℃, mold temperature: 30 ~ 50℃; Injection pressure: 40 ~ 100Mpa; the colding time according to the mold temperatures, is 5~9s. Under those process parameters, the chance for defects to occur was very low.The method of the degreasing of green parts was thermal degreasing, while alumina powder was used to embed those parts. Based on thermal properties of binder and the TGA curve of the feedstock, a suitable degreasing processing curve was developed.The defatted green parts were then sintered in the molybdenum bar furnace, in air atmosphere. According to the phase transformation ZrO2 principle, identified in the tetragonal phase sintering stable region of 1,350~1,450℃. Investigate of the sintered samples'cross-section under the SEM, it was found that the internal structure of the sintered samples were compact and sizes of the particles ranged from 0.3 to 0.7μm. The density measuring of those samples showed that with the increase of sintering temperature, the extension sintering time density of the samples increased.In order to the control of the precision of the final products, a five parameter three level orthogonal experiments was designed to analyze how processing parameters affect the small parts cylinder radial dimension. The result of the experiments indicated that with the increasing of the powder loading, the injection pressure, the shrinkage of the parts decreased, the final size of the parts increased; and as the mold temperature, sintering temperature increased and the extension of the sintering time the shrinkage of the parts increased, and the size of the final parts decreased.At last, based on orthogonal experiments, this paper presents a new method using neural networks to predict and control the dimension of the parts formed by PIM. The comparison between the results from neural network and the experiments indicated that using neural network to predict and control the dimension of PIM products was reasonable.