Microscale shaping and rounding of ridge arrays and star pattern features on nickel mould via electrochemical polishing

Abstract

High quality micro mould tools are critical for ensuring defect-free production of micro injection moulded products. The demoulding stage of the micro injection moulding can adversely affect the surface integrity due to friction, adhesion and thermal stresses between the metallic mould and polymeric replicated part. In the present work, we propose the use of precision electropolishing (EP) as a shaping and polishing process to control the draft angle and fillet radius of micro features in order to ease demoulding. Typical defects that occur in replicated polymer parts include cracks, burrs and distorted features. A nickel mould having multiple linear ridges and star shape patterns was designed for the present investigation to have characteristic dimensions ranging from 10 μm to 150 μm and with various aspect ratios to study the effect of electropolishing on modifying the shape of micro features and surface morphology. A transient 2D computational analysis has been conducted to anticipate the effect of shaping on the Ni mould after electrochemical polishing with non-uniform material removal rates, based on the distribution of current density. The experimental results indicate that after shaping using EP, the draft angle of star-patterns and linear patterns can be effectively increased by approximately \(3.6^\circ\), while the fillet radius increases by up to 5.0 μm. By controlling the electropolishing process, the surface roughness can be maintained under 50 nm. This work uses a green and environmental friendly nickel sulfamate electrolyte which can be effective for shaping of nickel micro features without causing any surface deposition.