Page 29 - 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
Conclusions
By reviewing five applications where the etched surface finish forms an important
parameter in the part technical specification, it has been demonstrated that a wide range of surface finishes can be obtained by PCM, electrolytic photoetching and electrolytic photopolishing. In addition, it should be noted that, in all cases, the surface finish is non- directional and uniform. Table 1 shows the typical results obtained from using individual and combined PCM, electrolytic photoetching and electrolytic photopolishing techniques.
The PCM production of specific surface textures required in abrasive and tribological (bearings) applications is summarised in Table 2.
Table 1. Etch parameters for bespoke surface finishes.
Metal / Application
Etch process
Resist
Etchant
Temp (°C)
Etch rate
Surface finish (Ra)
Stainless steel / Visual contrast of microtext
PCM
Negative
40°Bé ferric chloride
50
20-25 μm/min (by spraying)
0.35 μm (minimum)
Stainless steel / Gyroscope with optical beam reflection contrast
PCM
Positive
39°Bé ferric chloride
30
3 μm/min
(by immersion)
≈ 2 μm
Stainless steel / Visual contrast
Electrolytic photoetching
Negative
10% (v/v) hydrochloric acid
25
10 μm/min at 50 Adm-2
1.35 μm
Stainless steel / Fluid edge filter
Electrolytic photopolishing (followed by PCM)
Positive (followed by negative)
Orthophosphoric acid, glycerol, water and surfactant (followed by ferric chloride)
105
0.7 μm/min
at 25 Adm-2 (electropolish)
≈0.065 μm
Aluminium/ Microfluidic channels
Electrolytic photopolishing (TMEMM)
Positive
Orthophosphoric acid, water and surfactant
75
2.6 μm/min at 1.47 V for
a cumulative charge of 10 C
≈0.040 μm
Issue 136 December 2020 PCMI Journal 29