Stability of inkjet printing lines on inclined surfaces

Conformal inkjet printing on curved surfaces is essential for high-precision manufacturing of complex shapes. Since inkjet droplets are typically much smaller than the millimeter-scale curvature of most surfaces, substrate inclination becomes the dominant geometric factor. However, the inclination increases the risk of droplet sliding, which can lead to undesired line morphologies, such as bulging. Therefore, the stability of inkjet-printed circuits is critical to their practicality and reliability, but existing models, which are developed largely for planar substrates, cannot be directly applied to inclined surfaces. This limitation necessitates the present investigation into droplet spreading behavior and line formation on inclined surfaces, with a focus on the effects of inclination angle and surface wettability. By considering droplet spreading dynamics and gravitational pressure differences, a new predictive model for morphology transition boundaries is developed. To parameterize the model, key coefficients are determined through measurements of the droplet spreading ratio, which is subsequently fitted to a curve. Within the studied parameter range, two distinct morphological types are observed, and surface inclination is shown to help maintain line uniformity. Further analysis confirms that the proposed model is applicable and accurate within the normal wettability range. Beyond this range, however, morphologies on super-hydrophilic and hydrophobic surfaces fall outside the model's current predictive scope. Overall, this study advances the understanding of inkjet printing on non-planar substrates and provides a practical strategy for improving line quality in conformal manufacturing processes.