May 2026 Volume 8

AUTOMATION

• Insufficient momentum – Droplets land with low kinetic energy, forming thicker local pools and rivulets rather than homogeneous thin films. The liquid may partially evaporate before achieving uniform distribution. External atomization makes it possible to adjust droplet size, velocity, and spray angle to reach this ideal momentum condition. By tuning atomizing air pressure and liquid flow independently, the system can deliver droplets that neither rebound nor stagnate but instead spread efficiently into a thin, continuous film.

Figure 3: Course vs. Fine Atomization Coverage

Figure 3., illustrates how droplet size determines surface coverage and wetting capacity for a fixed liquid volume. A given volume of liquid (0.45 ml in the schematic) can be atomized into: • 32 drops with approximately 1.4 mm³ volume each and 3.2 mm² wetting surface, resulting in ~100 mm² surface coverage. • Or 256 drops with approximately 0.175 mm³ volume each and 0.78 mm² wetting surface, resulting in ~200 mm² surface coverage. Thus, by transitioning from coarse to fine atomization, the wetted area can be doubled using the same liquid volume, effectively providing 100 % more wetting capacity. For closed‑die forging, this translates into: • More uniform lubricant and cooling coverage across complex die geometries. • Improved release performance with less lubricant. • Reduced media consumption despite equal or better coverage. These benefits directly depend on achieving small, uniformly distributed droplets, which are more readily attained with external‑mixing atomization. Another consideration regarding spray atomization in closed die metal forming is droplet film formation and its behavior on hot die surfaces. Figure 4. demonstrates the impact of varying momentum conditions. • Too high momentum (spray too strong) – Droplets impact the surface with excessive kinetic energy and are deflected away, reducing surface contact time and effective cooling. A large portion of the lubricant may be removed before forming a stable film. • Ideal momentum – Approximately ≥ 50 % of the droplet surface contacts the hot die, maximizing heat transfer and film formation. Droplets spread adequately without splashing excessively.

Figure 4: Droplet Size and Film Formation

Upon impact, droplets spread and evaporate according to local die temperature. With good external atomization the film thickness is small and uniform, promoting rapid and nearly complete evaporation of water or volatile carrier components. The residual solid lubricant (e.g., graphite or synthetic boundary additives) remains as a thin, adherent layer that supports metal flow and die release. Excess pooling and run‑off are minimized, reducing smoke, contamination, and surface defects. Leidenfrost Phenomenon The “Leidenfrost effect” describes the behavior of a liquid droplet levitating on a surface that is much hotter than the liquid’s boiling point, riding on a self-generated vapor layer instead of making direct contact. When we consider lubricant and cooling spray for metal forging we must take this into account. Proper spray atomization can drastically impact the presence of the “Leidenfrost effect”. At low lubricant or cooling spray operating pressures, relatively large droplets contact the hot surface directly. Heat transfer is dominated by nucleate boiling and convection, but coverage may be uneven. As pressure increases, droplets become smaller and spread better, improving wetting and cooling. At very high die temperatures without sufficient atomization quality, a steam barrier (Leidenfrost layer) forms between the droplet and surface. The droplet levitates on its own vapor, drastically reducing heat transfer and preventing direct contact with the hot die surface. A steam barrier is generated and prevents the liquid from contacting the hot die surface at inappropriate conditions. The Leidenfrost

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