Electrostatic Sticking of Sheets Fed from a Stack

摘要

In sheet feeding applications such as printing, sheets are typically fed from the top of an input stack. In one common failure mode, the first few sheets feed reliably from a fresh stack. However, as feeding continues and the input stack grows shorter, sheets fed from the middle of the original stack begin to stick to the top of the now shorter stack. A theoretical analysis identifies a mechanism to explain this feeding failure. The electric fields in the air gaps between the sheets are predicted to increase with depth into the stack. When these air gap fields exceed the Paschen limit, air ionization occurs that results in charge separation across the air gaps. This charge separation causes strong electrostatic attraction between the sheets, which results in sticking. To prevent this feeding failure, the electrostatic surface charge density on the sheets must be kept below a critical value when the sheets are stacked during converting and packaging phase of their manufacture. The theoretical maximum allowable surface charge density ρ_(S, MAX) is on the order of 3μC/m~2 for a stack of 100 sheets. To allow a margin of robustness, the specification limit for maximum charge on a sheet should be 10X below this theoretical value; 2 ρ_(S, LIMIT) < 0.3μC/m~2. For stacks with more sheets, charge density ρ_(S, MAX) is lower. Charge density ρ_(S, MAX) varies with the air gap between the sheets, which is determined by the surface roughness of the sheet material and by the flatness of the sheets. Smooth, flat sheets can tolerate a somewhat higher surface charge density.