Coated paper Drying: Equipment Selection

Coated paper is the same problem as label paper but the coated face is more sensitive. Coatings usually contain wax, starch or a pigment binder, and under heat they soften first and then skin over; above a certain temperature the sheet shows speckling or loses powder, and the web contracts unevenly. The route is still mainly the flash airflow line, because the residence time is measured in seconds and the coating has not had time to accumulate heat before it leaves the drying zone. The inlet temperature follows the ceiling for the grade with margin left below it, and air volume and temperature are adjusted separately.

This is the same problem as label paper, but the coated face is more heat sensitive. Coatings usually contain wax, starch or a pigment binder, and under heat they soften first and then skin over; above a certain temperature the sheet shows speckling or loses powder, and the web contracts unevenly. The route is still mainly the flash airflow line, because the residence time is measured in seconds and the coating has not had time to accumulate heat before it leaves the drying zone. Set the inlet temperature by the grade's ceiling and start below 130°C to leave margin for batch variation, adjusting air volume and temperature separately: take the surface moisture off with low air volume first, then raise it gradually, so the sheet does not lose water at the surface fast enough to curl. Coated paper also shrinks across the web, and the moisture difference between positions in one sheet shows up directly as curl, so the discharge end needs flattening and cooling designed in — a curtain or belt cooler is more stable than free fall.

How should the temperature be set for coated paper?
By the heat limit of the coating, with margin left below it. A common starting point is below 130°C, but the difference between grades is large. Do not copy the temperature from another order just because the paper looks similar: once the coating formula changes, so does the ceiling, and speckle cannot be undone.

How do you stop coated paper curling at the edges?
Two directions: bring the drying rate down by holding the temperature and raising the air volume slowly, so the surface does not lose water too fast; and give the discharge end flattening and cooling, so the web sets down under even tension. A curtain or belt cooler is more stable than free fall.

Can coated paper be dried in a rotary drum?
Process-wise it can, but the mechanical damage is clearly higher than on a flash airflow line. The flights lift and drop the sheets repeatedly and they rub against each other, which can rub the coating off where it is thin. Where surface quality matters, the airflow line is the better choice.