The Meniscus Effect

Question:

I have painted some graphics for an overmolded automotive control button. When backlighted, the edges appear brighter and pass more light that the center areas of the plastic. The customer wants a consistent glow. Is this the meniscus effect? How can I alleviate this? M. S.

Answer:

Unless you are painting the inside of the button, I have never heard the phenomenon you described called the meniscus effect. I’ll bet that neither Euler nor Bernoulli heard of it either when they were laying their foundations of hydrodynamics, but that’s another story. Merriam-Webster’s Collegiate Dictionary defines meniscus as “The curved upper surface of a column of liquid.” This is due to capillarity, and, if you are painting the inside, it is the meniscus effect.

If you are painting the outside, the difference in light transmission could be caused by the edge effect, where the liquid pulls away from the edges causing a decrease of film thickness in those areas. Adjusting the rheology of the coating can minimize this effect. Since a thixotropic material is less susceptible to the edge effect, its use will minimize, but not eliminate, the problem. Another option is to back up the button with grounded metal while applying the coating electrostatically. This may cause a higher film build at the edges. Other than the aforementioned, I’m out of suggestions.