Static Electricity 2000 Edition

77
–33
2000 Edition
FIGURE 9.2.5.3
Typical locations for static control.
High-speed operations might require a second static neu-
tralizer at a single location, but providing more than two usu-
ally gains no additional benefit. Neutralizers should extend
across the full width of the web. For fast-running presses,
inductive neutralizers, such as tinsel wrapped around a bar or
grounded needle points, spaced 12 mm to 25 mm apart and
placed at the inlet and delivery side of each impression roller,
have been found to be effective.
As with all neutralizers, positioning is important and effec-
tiveness of individual installations should be confirmed by
field measurement of residual charge or voltage. An area of
web as small as 0.01 m
2
or an area of 115 mm in diameter
might be capable of releasing an incendive discharge. The
neutralizer should be installed as far as possible from
grounded metal machine parts or areas where the web is sup-
ported by rollers. The recommended location of the initial
inductive ionizer is 100 mm to 175 mm from the roller tangent
(i.e., web exit) point and 6 mm to 25 mm from the web. Thus,
the first neutralizer after the roller might have to be placed so
close to the roller that it is partially suppressed from ionizing
but can minimize the area of high charge. An additional ion-
izer or two downstream from the roller might also be neces-
sary to reduce the charge to the desired extent. Final location
near critical rollers should be determined by adjustment and
measurement to minimize web charge, as indicated by appar-
ent voltage.
9.2.5.4
Tinsel or needle points remain functional only as long
as they are clean and sharp. Accumulation of contaminants
(e.g., ink, coating solution, or paper dust) and corrosion prod-
ucts should be controlled by an effective maintenance program.
Visual inspection and performance verification of the neutraliz-
ers should be done at intervals that will depend on how process
conditions affect performance of the neutralizers.
9.2.5.5
Humidification has been used in the past to decrease
static electric charge on materials in the environment, such as
in garments and floor coverings. However, with modern high-
speed operations, nonconductors, such as paper and plastic
films, cannot adsorb enough moisture from the atmosphere
during the brief time that they are exposed to humid air to
increase their surface conductivity. Plastics generally do not
become sufficiently conductive, even in humid atmospheres,
due to lack of ionized molecules in an acquired surface layer
of moisture. Therefore, humidification is not a recommended
method of control of static electricity in such processes.