Applying scratch-resistant hard coats to camera lenses and glass prisms is challenging. Unlike flat glass, pulling curved or angled optical parts out of a liquid at a constant speed always results in an uneven film—thin at the top, thick at the bottom. This article explains how gravity and surface tension cause these defects, and provides a solution using “linear withdrawal” (variable speed control) tailored to curved surfaces.
1. Why Curves Ruin Film Thickness
Changing Angles and Gravity
Imagine pulling a round lens out of a dip tank. As it rises, the angle of the glass relative to the liquid surface constantly changes.
On a steep, near-vertical angle, the liquid drains down quickly. On a gentle, flat curve, the liquid tends to sit and pool. Because the slope is always changing, pulling the lens at one constant speed means gravity pulls the liquid unevenly. This ruins your thickness uniformity.
2. Liquid Pooling on Prisms and Lens Edges
Surface Tension Causes Optical Distortion
To make matters worse, sharp corners on prisms and the bottom edge of a lens act like magnets for liquid. Surface tension strongly holds the liquid there. If a thick pool (edge effect) forms at the bottom of an optical component, it distorts the light. The lens fails inspection, dropping your production yield significantly.
3. [Solution] “Linear Withdrawal” Tailored to the Shape
Perfect Hard Coats with Seamless Speed Profiles
The only way to achieve a uniform coat on a curved or 3D surface is to dynamically change your pulling speed to match the curve of the lens.
You must pull faster on steep angles and slow down smoothly as you reach the bottom edge to drain the excess liquid. SDI’s Linear Dip Coater allows you to program this exact, seamless speed profile. By speeding up and slowing down in perfect sync with the shape of the part, you counteract gravity and surface tension, guaranteeing a perfectly uniform hard coat on any complex optic.
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