nip

In papermaking and printing, the nip is the gap or contact zone between two rotating rollers where material is pressed, dried, or finished. The width and pressure of this zone determine how the material is treated. A typical nip might span 10 to 50 millimeters depending on the machine, with pressures ranging from a few hundred to several thousand pounds per linear inch.

The nip performs different jobs depending on where it sits in the mill. In a calender at the paper machine tail end, hard rollers close to zero clearance compress the sheet to smooth its surface and control thickness. In a press section, a soft roller backed by hard metal forms a nip that squeezes water from the wet web without damaging fibers. A sizing press applies chemical coatings. Each application demands different roller hardness, temperature, and gap settings.

Pressure and clearance in the nip must be held tight tolerances. If the gap widens, the sheet passes through incompletely processed; if it narrows too far, the material jams or the rollers score and mark the surface. Temperature matters especially in calenders: heat softens the sheet fibers and helps set surface properties, but too much causes sticking or cockle. Wear of the rollers gradually changes nip geometry, forcing periodic grinding or replacement.

Common failure modes

The nip can generate heat through friction that damages sensitive coatings or synthetics. Sheet breaks occur when material enters at an angle or when a hard object lodges in the gap. Roller deflection under heavy load can cause the nip to close unevenly across the width, producing streaks in the finished product. In mills running multiple materials, residue buildup in the nip from one grade can contaminate the next.

The term originates from the verb nip, meaning to pinch or squeeze. In textile and rubber manufacturing, similar terminology applies to the contact zones in mixing rolls, calenders, and presses. Paper workers use nip pressure, nip temperature, and nip clearance as fundamental machine parameters, alongside dwell time, the brief moment a fiber spends in contact with the roller.