Page 25 - PCMI Journal December 2020

Manufacturing Bespoke Aesthetic and Functional Surfaces via Photochemical Machining, Electrolytic Photoetching and Electrolytic Photopolishing Presented by: David M. Allen, Emeritus Professor of Microengineering, Cranfield University, UK
current density over the area to be etched and the etch rate was 50% slower than that achieved by spraying with ferric chloride [8].
Electrolytic photopolishing
What happens if a specific etched surface finish is required on AISI 304 stainless steel
that PCM cannot achieve, such as in the edge filter element (Figure 11) where the half-etch needs to be extremely smooth and polished at a depth of 9.5 ± 0.5 μm with Ra ≤ 0.08 μm?
Figure 11. Single element of an edge filter assembly to filter out particles ≥ 20 μm in diameter. The fabrication was achieved by a combination of two separate etching processes employing two sets of phototools [9].
The first process used positive-working photoresist (for ease of removal) and electrolytic photopolishing in an electrolyte comprising 90% orthophosphoric acid (300 ml), glycerol (530 ml) and water (90 ml) at a temperature of 105±5°C. This process was slow; a big advantage when trying to achieve the very tight tolerance on the depth of etch but easily achieved as, by Faraday’s Laws of Electrolysis, etch rate was controlled solely by current density. The resultant Ra ≈ 0.1 μm was reduced to ≤ 0.08 μm by adding a surfactant to the electrolyte, thereby reducing the size of evolved gas bubbles clinging to the surface being etched. The fast, second process used ferric chloride spray-etching to profile the part [9].
A similar electrolytic process has been used for a microengineering application where exceptionally smooth channels are required in microfluidic circuits [10]. The technique, referred to as “Through-Mask Electrochemical MicroMachining” (TMEMM) has been used to etch an aluminium micromixer (Figure 12) using an electrolyte comprised 85% orthophosphoric acid and a surfactant, resulting in a channel Ra = 0.028 ± 0.006 μm.
Surface textures
In addition to the above surface finishes, some product applications require specific
functional surface textures. Examples include abrasive structures and tribological surfaces.
Issue 136 December 2020 PCMI Journal 25