Step 3 of 5
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Lubrication application: spray, swab, roller, drip, and why coverage beats volume
Set up a spray bar so coverage is uniform and dwell matches the cycle, tell a wet die from a lubricated die, and read the four classic application failures off the die face.
Step 3 of 5Spray-bar setup, in numbers
A spray bar over a hot forge die is engineered, not estimated. The starting numbers below are typical for closed-die hot forge with water-based graphite; check the lube vendor's application sheet for the specific product.
Nozzle count. One nozzle per 50-100 mm of die face along the longest dimension, more on impressions with depth or geometric complexity. A 400 mm die face wants 4-8 nozzles at minimum, with extra nozzles aimed into deep cavities or sharp transitions.
Nozzle spacing. Even spacing along the bar, with the fan pattern of adjacent nozzles overlapping by 15-30% at the die face. Less overlap and you get coverage gaps between fans. More overlap and you waste lube in the double-covered zone and starve the ends of the bar.
Nozzle angle. Aim each nozzle so the fan center hits the working surface at the spot it is supposed to lubricate. A nozzle aimed perpendicular to a flat die face puts the fan on the face. A nozzle aimed at a sloped wall needs to be angled so the fan center lands on the wall, not on the floor of the impression behind it. Deep cavities need nozzles angled steeply enough that the fan reaches the cavity floor without being shadowed by the cavity wall.
Fan width. 60-90° fans are common for general die coverage. Wider fans cover more area at low pressure with thinner film; narrower fans concentrate lube on a target zone. Mixing fan widths on the same bar is normal: wide fans for the open face, narrower fans aimed into specific features.
Pressure. 30-80 psi at the nozzle is the working range for water-based graphite. Below 30 psi the atomization breaks down and the spray becomes a stream. Above 80 psi the carrier velocity is high enough to blow lube off the die before it deposits, and the overspray cooling effect (covered in Step 5) gets aggressive. Vendor sheets specify the right pressure for their product; the bar's regulator and the gauge on it are the way the operator confirms the spec is being held.
Witness card. Pull a piece of card stock or thin sheet aluminum the size of the die face, hold it where the die would be, and fire the spray bar through one cycle at production pressure. The card shows the actual coverage pattern. Compare it to the engineered map. A coverage check on a witness card every shift catches drift before the die sees it.
Quick check
A spray bar is set up with 6 nozzles on a 480 mm die face at 80 mm spacing, 75° fans, 50 psi. The witness card after one cycle shows a uniform pattern across the face but a 60 mm dry band running down the centerline. What changed and how do you fix it?