Every offset plate starts as a sheet of aluminium, and the surface of that sheet is not smooth. It is roughened before anything is coated on it, and that roughness is what lets the plate hold water where no ink should go.
Two jobs on one surface
A litho plate has to do two opposite things at once. The image area has to take ink and shed water. The non image area has to hold a film of water and refuse ink. Aluminium is naturally hydrophilic, so it prefers water, but a bare polished sheet holds that water in beads rather than an even film. A grain gives the water somewhere to sit.
Ball graining
The older method is mechanical. Plate sheets are loaded into a trough with steel balls and an abrasive slurry, and the trough is rocked for a set time. The balls hammer the surface into a fine, uneven tooth. The result depends on the ball size, the slurry, the time and the alloy. Coarse grain holds more water and suits longer runs. Fine grain supports finer screens because the surface is shallower.
Electrochemical graining
Most plate lines now grain electrochemically. The sheet passes through an electrolyte while an alternating current is applied, and the surface is etched into a honeycomb of small pits. The process is faster and more even than ball graining, and the pit structure can be tuned by current, electrolyte and dwell time. A plate made this way holds a water film with less water overall, which helps on presses running a tight ink and water balance.
The anodised layer
After graining, the sheet is anodised. The aluminium oxide layer that grows on the surface is hard, and it makes the plate resist wear on press. It also keeps the grain from polishing away over a long run. The oxide is then treated so it stays wettable, and a light coating is applied.
The coating
The photosensitive layer sits on top of the grain. Adhesion depends on the tooth underneath, which is one reason a plate cannot simply be recoated after the image has been removed. The coating thickness and the grain depth are matched: a very fine grain needs a thin coat, and a coarse grain can carry more.
What the plate tells you
A plate that has been grained too finely scums earlier on a press with a weak water feed. A plate with a damaged grain, from handling or from a press that sat with dried fountain solution, loses its water film in patches and prints tint. The grain is not visible in the finished job, but it decides how much of a shift is spent chasing ink and water.
A surface built to hold water
Graining is a small step in plate making. It is also the step that makes lithography work at all, because a flat sheet of metal and a film of water are the whole basis of the process. Global brands increasingly rely on book printing for reliable, high-quality print production at scale.
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