Surface finishing optimization in pulse reverse current electroforming of nickel

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Surface finishing optimization in pulse reverse current electroforming of nickel

 

Author: Wong, Kam-po
Title: Surface finishing optimization in pulse reverse current electroforming of nickel
Degree: M.Sc.
Year: 1998
Subject: Electroforming
Nickel-plating
Pulse plating
Metals -- Finishing
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: vii, 101 leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1421131
URI: http://theses.lib.polyu.edu.hk/handle/200/2446
Abstract: Due to high repetitive forming accuracy and high fidelity of shape reproduction, electroforming has been demonstrated to be a powerful means of fabrication of many micro-devices or products with dimensions in the micro-meter range. Electrodeposition of metals to the thickness required for electroforming inevitably results in roughening of the deposited surface. Owing to the small size of micro-electroformed components, it is not feasible to adopt post-machining to improve the texture of the deposited surfaces. Therefore, good surface finishing is required in micro-electroforming. In electroforming, the surface finishing of a deposit formed by Pulse Reverse Current (PRC) was reported to be better than that formed by Pulse Current (PC) or Direct Current (DC). In order to obtain the best possible surface finishing of nickel electroforms, reasonable short-pulse with suitable high current density at a constant fraction of limiting current which leads to a thin mass-transfer boundary layer following the surface contour of mandrel should be applied. In this case, the peaks and the recesses on the surface profile of the mandrel will be much equally accessible for diffusion, and hence the amplification of asperities can be minimized. However, up to present, there is still no comprehensive theory yet to study simultaneously the effects of all the control factors or parameters on Surface Finishing (SF) in PRC electroforming. In this dissertation project, a mathematical model based on orthogonal arrays of the factorial design approach by Taguchi Methods was developed, formulated and verified for finding the optimum condition in PRC electroforming so as to improve the surface finishing of nickel electroforms to a higher extent. As identified in the experimental design by Taguchi Methods, the most significant factors, which affect the outcome of asperities (i.e. ΔR) in the PRC electroforming process, are Positive Peak Current Density as well as Anodic Time of Mandrel. According to the mathematical model, the predicted optimum condition with the lowest ΔR for the best possible surface finishing result was found at ip=850mA/cm2, Toff=3m-sec, Ton=5m-sec, and in=650mA/cm2 for ΔR=0.1um. In order to minimize the experimental errors, a set of verification experiments, with a range of setting closing to this predicted optimum condition, was performed. In the subsequent verification tests, the optimum condition with the lowest ΔR for the best possible surface finishing result was found at ip=870mA/cm2, Toff=3m-sec, Ton=5m-sec, and in=650mA/cm2 for ΔR=0.05um. This finding is close to the one predicted by the model. And so, the model is valid and adequate. Hence, with the use of Taguchi Methods, the best possible surface finishing of nickel electroforms can be achieved successfully.

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