Effect of porosity on mechanical properties and dimensional accuracy of metal injection moulded (MIM) parts

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Effect of porosity on mechanical properties and dimensional accuracy of metal injection moulded (MIM) parts

 

Author: Chan, Kai-chung Gabriel
Title: Effect of porosity on mechanical properties and dimensional accuracy of metal injection moulded (MIM) parts
Degree: M.Sc.
Year: 1999
Subject: Injection molding of metals
Porosity
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Mechanical Engineering
Pages: xiv, 121, [4] leaves : ill. ; 31 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1479177
URI: http://theses.lib.polyu.edu.hk/handle/200/1525
Abstract: Abstract of dissertation entitled "Effects of porosity on mechanical properties and dimensional accuracy of metal injection moulded (MIM) parts" submitted by Chan Kai Chung Gabriel for MSc in Precision Engineering at The Hong Kong Polytechnic University in May 1999. Throughout the development of metal injection moulding (MIM) technology, users of MIM components were primarily concerned with the effect of porosity on the mechanical properties which affected the product performance. In view of those requirements, a need was arisen to study the effect of porosity on the mechanical properties of MIM parts. The objectives of this project were set to determine the effects of porosity on selective mechanical properties, including tensile, compression, hardness, surface roughness, Young's modulus, as well as dimensional accuracy of MIM products in order to produce a product with satisfactory performance. Through the confirmation of the validity of both MIM and powder metallurgy (PM) theories with regard to mechanical properties, the process conditions and optimization of the MIM process parameters settings were obtained. A literature review of past experimental work regarding MIM theories and the studies in mechanical properties of MIM was used to gather the related information/knowledge and shows that no body has established a systematic approach to investigate the relationship among porosity, mechanical properties and dimensional accuracy. Taguchi Method was used to design the experiment for injection moulding of MIM parts so that the highest green density could be achieved. The specimens used including stainless steel AISI 316L watch buckle and tensile rod machined from sprue. The injection parameters including injection pressure, injection speed and screw speed were the primary injection conditions to be optimized. A series of experiment study was performed to find out the optimum debinding time by debinding groups of specimens in an incremental time step of 30 minutes ranged from 30 to 180 minutes. The optimization of sintering conditions was performed under temperatures from 1250 C to 1370 C in steps of 10 C and time from 30 minutes to 180 minutes in steps of 30 minutes. The results showed that by increasing green and product density, hardness and tensile strength of final product also increase and so did the dimensional accuracy of MIM products. The PM's equation regarding sintering time found by Coble in the year 1981 was proved also valid for MIM application. It was also found that the tensile strength and dimensional change equation developed by Randall German (1996) and the equation for Young's Modulus developed by Macadams were valid. An analysis of the linear accuracy was performed through the use of the least square fitting for fourth order polynomials. It was revealed that the trend of the linear accuracy was affected by the degree of porosity. Curves were fitted to the experimental results of the debinding time against weight loss and sintering density against time and temperature to correlate the trend of their effects on density. The relationship was used to find out the optimized debinding time and sintering temperature. Finally, the optimized injection pressure, injection speed and screw speed setting were found to be 110 bar, 70 cm3/s and 120 rpm respectively. The optimum debinding and sintering time were found to be 3 hours and 2 hours respectively whilst the optimum sintering temperature was 1360 C. The results obtained in this project could be used to predict performance of MIM parts that could meet the users' requirement. Based on this methodology, further work could be carried out to establish processing guidelines for other material and products having different geometry and sizes.

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