May 2026 Volume 8

AUTOMATION

FUNDAMENTALS OF LUBRICANT SPRAY ATOMIZATION IN CLOSED-DIE METAL FORGING By Mackay Wilson P.Eng.

C losed‑die metal forging relies on precise control of die temperature and surface lubrication to achieve high part quality, long die life, and stable process capability. Spray systems are the primary means of delivering cooling water and lubricants to the hot die surfaces between forging blows. Traditionally, many systems have used poorly controlled internal‑mixing nozzles that generate coarse, non‑uniform sprays with limited adjustability. These limitations can result in high media consumption, long spray times, excessive or insufficient die cooling, and unstable part quality. External atomization of cooling and lubricant spray, using well‑controlled external‑mixing nozzles and optimized spray parameters, offers significant advantages over conventional internal‑mixing concepts. The following technical analysis summarizes several key aspects: • Poor versus good atomization. • Influence of nozzle type on droplet size. • Influence of particle size on surface coverage. • External versus internal‑mixing nozzle. • Droplet film formation and impact behavior on die surfaces. • Leidenfrost effect and its mitigation. • Die temperature evolution during spray application. • Heat reduction by evaporation. In many conventional spray applications, the atomization of the lubricant or cooling medium is poor. Most of the mixture drains off and goes to waste. There is little or no control of the distribution and momentum of the atomized spray. The resulting spray is characterized by large droplets with low surface‑area‑to‑volume ratio. When such droplets impinge on the hot die surface, only a small fraction of the liquid evaporates. A large portion of the fluid forms rivulets and runs off under gravity, providing limited cooling and lubrication while increasing wastewater volume and housekeeping issues. The non‑uniform droplet distribution also leads to spatially inhomogeneous die temperatures and lubricant films. With optimized external atomization, there is a possibility of intervention of atomization quality: Most of the water from the lubricant mixture evaporates. Spray jet, distribution, and intensity can be well controlled. Fine droplets with narrow size distribution and appropriate velocities are produced. These droplets can be directed precisely onto specific die regions, forming a uniform, thin liquid film that evaporates largely on the hot surface. The high surface‑area‑to‑volume ratio enhances convective and evaporative heat transfer, enabling efficient die cooling with reduced media consumption.

Figure 1 Internal mixing: The lubricant/water mixture and atomizing air are combined within the nozzle before exiting the orifice. The mixing process is strongly coupled to nozzle geometry and pressure conditions, leaving little flexibility for independent adjustment of droplet size and spray momentum. Larger droplets often result, and the risk of clogging can be higher because liquid and air share internal passages.

Figure 2 External mixing: Liquid and atomizing air are fed separately and meet outside the nozzle. The liquid emerges as a coherent jet or sheet that is subsequently fragmented by the surrounding high‑velocity air. External mixing permits finer control of both atomizing air pressure and liquid flow, enabling optimization of droplet size distribution and spray shape for each die area.

FIA MAGAZINE | MAY 2026 34