Anodizing & Electroplating Surface Area & 720 Rule Prepress Guide

1. The 720 Rule of Anodizing

The fundamental empirical relationship in sulfuric acid anodizing is the 720 Rule:

This means delivering 720 Amp-minutes of DC electrical current to one square foot of aluminum will convert the metal surface into exactly $1.0\text{ mil}$ ($25.4\,\mu\text{m}$) of aluminum oxide ($ ext{Al}_2 ext{O}_3$).

2. Current Density Standards

• MIL-A-8625 Type II (Sulfuric Acid Decorative / Colored): Run at $12 - 18\text{ Amps/sq ft (ASF)}$ ($1.3 - 1.9\text{ A/dm}^2$) in a $20^\circ\text{C}$ ($68^\circ\text{F}$) electrolyte bath. Typical thickness: $10 - 25\,\mu\text{m}$.
• MIL-A-8625 Type III (Hardcoat / Wear Resistant): Run at $24 - 36\text{ ASF}$ ($2.6 - 3.9\text{ A/dm}^2$) in a chilled $0 - 4^\circ\text{C}$ ($32 - 40^\circ\text{F}$) bath at higher voltages ($35 - 75\text{ V}$). Typical thickness: $50\,\mu\text{m}$ ($2.0\text{ mils}$).

3. Calculating True Wetted Area from 2D Vector CAD Profiles

Laser cut and CNC milled plates have substantial edge surface area that is frequently overlooked:

4. The 50/50 Growth & Penetration Rule for Precision Tolerancing

Anodizing is not a coating deposited on top of the metal; rather, the aluminum substrate is converted into oxide. The resulting layer grows 50% into the substrate and 50% outward from the original dimension:

• For a $50\,\mu\text{m}$ hardcoat: the outside dimensions of a part expand by $+25\,\mu\text{m}$ per surface ($+50\,\mu\text{m}$ on total width).
• Inside hole diameters shrink by the full coating thickness ($-50\,\mu\text{m}$). Vector DXF drawings must be pre-adjusted by these offset values prior to machining.