Two-layer Slot Coating Frequency Response for Active Control

Precise, rapid coating two layers simultaneously with a dual slot die is an established technology (e.g. Ishiwata et al. 1971, Sartor et al. 1996). However, interlayer diffusion can be deleterious; interlayer mixing by microvortices disruptive. Barring nonuniformities of unacceptable magnitude can be produced by back-up roll runout, feed pump ripple, air pressure (“vacuum”) fluctuations, and substrate transport cogging that are impracticable to eliminate. Steady-state modeling by computational fluid mechanics reveals mixing, and is the precursor to frequency response analysis, an aid to understanding the nonuniformities and to designing active control to reduce them, as we reported about single-layer slot die coating at ISCST 2004. Here we report subsequent research on two-layer coating. 2. Interlayer treatment A key issue is interdiffusion in the interlayer zone, which is not an interface (cf. Taylor & Hrymak 1997) yet has been approximated as one, though devoid of interfacial tension (e.g