Micro-Fabrication of PMMA Using Electrochemical Discharges

Abstract

The Poly (methyl methacrylate)-PMMA material also recognized as acrylic is an significant material for the different microsystem’s technologies. PMMA properties such as optical transparency, high hydrophilicity, biocompatibility, chemical inertness and lower autofluorescence, makes it a suitable candidate for the microfluidics devices These devices consist of the various micro-features i.e., micro-holes as well as the micro-channels. In current times, fabrication methods such as laser machining, hot embossing, injection molding, and mechanical cutting are predominantly used for the microfabrication in PMMA material. Nonetheless, the major processes such as laser machining and mechanical cutting exhibit some limitations due to issues like higher processing+setup costs, higher thermal damage, edge burr formations etc. Recently, a method i.e. machining with electrochemical discharges emerged as the potential alternative to the existing methods to machine the PMMA substrates. This method also called ‘electrochemical discharge machining (ECDM)’ considered to be the hybrid combination of microfabrication based on electric-discharge and electro-chemical processes. ECDM is a relatively simple, economical process which, in principle, can perform micro-fabrication in various materials irrespective of its electrical-conductivity, hardness as well as brittleness. In ECDM heat energy generated by electrochemical (EC) discharges in an electro-chemical cell (consists of a micro tool rod/electrode, a big-sized counter electrode and an aqueous alkaline electrolyte).In ECDM process, when voltage (>2 V) is provided between the smaller size tool (cathode) and bigger counter electrode (anode) which are kept in the aqueous KOH. The applied voltage results in the electrolysis of aqueous KOH electrolyte and produces gas bubbles of hydrogen (H2) at cathode point and of oxygen at anode point respectively. At the point of cathode (tool electrode) gas bubble production is very higher due to higher current density at the smaller tool surface. The subsequent increase in the applied voltage starts the process of merger of bubble which forms an envelope of gas film around the smaller tool electrode. At this stage, tool electrode gets complete isolation from surrounding electrolyte. Breakage of this gas film envelope happens at elevated voltage (> 24±0.1 V) and discharges are generated which produces heat energy. The material removal from the PMMA workpiece happens which is kept below the discharge producing tool. The material removal is result of melting, evaporation and etching due to heat energy.The most of research on ECDM is demonstrated for the glass and alumina ceramics in past decade which are electrically nonconductive workpieces. Nevertheless, the ECDM process has not been completely explored for the polymers, such as PMMA. Bhargav et al. have recently explored this process for the micro-channel formation in PMMA using NaOH electrolyte with 0.7 mm titanium tool electrode having cylindrical shapes. It is very much clear from the known literature that work related to PMMA micro-structuring using ECDM is scarce. Furthermore, the experimental analysis was directed on the micro-channel (> 700 µm) formation only. Consequently, there is a wide research gap for micro-structuring of PMMA for formation miniaturized micro-holes (> 300 µm) of different shapes such as circular, slotted. Analysing EC discharge behaviour, tool electrode condition, and micro-hole geometric features are the key tasks which are performed.The present work is performed to investigate the EC discharge behaviour and identification of the appropriate applied voltage on the micro-hole formation as well as the geometric characteristics such as hole size, depth, heat affected zone (HAZ) and circularity. Also, the tool condition after usage and its effect on the EC discharges and micro-hole quality have also been explored. The identification of Vc i.e. critical voltage has been carried out. The EC discharges stability for the range of applied voltage has been analyzed. It has been found that the applied voltage needed for the appropriate machining quality with uniform EC discharges should be in the range of Vc+4 to Vc+6. The lower values of applied voltage Vc+2 indicated the region of the unstable and non-uniform EC discharges affecting the micro-hole quality. The worn-out tool also affects the hole quality and EC discharge stability. Figure 1