Monroe Electronics Electrostatic Fieldmeter - Static Monitor - model 177A - Manual (Page 58)

APNE-0015 57
Webs and rollers exchange charge because of differences in material properties. The
factors that we saw above translate into the following factors in a coating, converting, or
printing machine:
Roller bulk conductivity and surface conductivity
Roller surface chemistry (material and contamination)
Roller surface roughness, texture, and venting
Roller/Web alignment and tracking
Roller/Web frictional drag (bearing failure)
Roller/Web intimacy of contact (surface hardness or compliance)
Roller/Web wrap angle
Web bulk conductivity and front/back surface conductivity
Web slip and weave
Web speed and tension
Web surface chemistry (material, antistatic agents, surfactants and contamination)
Web surface roughness and texture
Any other object that the web touches, such as turning bars, splice board, and the
operator's hand, can also cause charge transfer.
High electric fields that occur at the separation of surfaces, such as at the nip of unwinding
rolls or roller/web contacts, can result in corona discharges (glow). Corona discharges
produce ions of both polarities that tend to reduce the overall web charge. Static control
devices (passive or active ionizers) ionize air in order to control charges on the web by
attracting opposite polarity air ions to neutralize the charges on the web.
Strong electric fields on insulating surfaces in the converting machine can charge
ungrounded conductors (including people) by induction, especially at wind-up and delivery
areas. These charged conductors could then release dangerous sparks to any other nearby
conductor.