Control for High Speed Converting
charges on paper, film, fabric, and other non-conductive materials
reduce processing efficiency and creates
Inc. (www.electrostatics.com) a leader in
static control and manufacturer of a variety of static control
or static eliminator products and web cleaning equipment for
the high speed converting industry has prepared this booklet. It
will help familiarize you with the causes of and problems relating
to static electricity in the converting environment and how these
problems can be solved.
The first portion of this booklet will help clarify
what static electricity is, as well as define some of the terminology
important to understanding and controlling static electricity in the
converting industry today.
The final sections of the booklet are dedicated to specific
static related problems commonly found within the converting environment,
as well as specific product recommendations for your individual needs.
HIGH SPEED CONVERTING
is static electricity? Static electricity denotes or pertains to
electricity at rest.
How simple and yet inadequate this definition is of a phenomenon that
creates defects which costs industry millions of dollars per year.
A clearer understanding of static electricity can be gained by explaining
lightning. Static electricity is generated by the transfer of charge
from cloud to cloud as warm air is caused to rise at a rapid rate.
The friction between air masses transfers charges from cloud to cloud,
increasing an electrical potential voltage between clouds. Since
the clouds are isolated and have no connection to the out-side world,
the voltage between the clouds rises to the point where it is high
enough to cause the air to ionize between the clouds, in this case
air, becomes conductive because ionized air will conduct electricity
when a sufficient voltage difference is reached. Then, a lightning
discharge is created to equalize the voltage potential between clouds.
For a brief instant the lightning flashes, static electricity in no
longer at rest and tremendous energies are discharged along the shortest
What causes static electricity? How do we eliminate or at least control
it? Let's examine it further, as it applies to sheet and web handling,
especially high speed converting.
Static electricity is generated by disturbing the electrical balance
of relativity non-conductive insulators such as plastic and paper.
All matter is composed of atoms. A balanced atom contains positive
charges that are present in the nucleus of the atom. An equal amount
of negative charges orbits this nucleus in the form of electrons.
Normally both charges are equal. Therefore the overall charge of
a balanced atom is zero.
However, should this configuration be altered and several electrons
removed from this atom, there will be a positive charge in the nucleus
and a deficiency of electrons, which gives an overall charge in the
positive direction. Conversely, if extra electrons are added, an
overall negative charge will result because there will be an excess
of electrons and the net charge is now negative.
Recognizing the above, when two surfaces are rubbed together, a transfer
of charges takes place between these two surfaces. The friction causes
electroncs to flow from one surface to the
other, simultaneously creating one positive surface and one negative
conductivity of material is extremely important when working with
can be charged by induction by exposure to an electrostatic field
that is present on a surface charged with static electricity. This
is extremely important in areas where small charges of static electricity
can create havoc, such as in the handling of plastic or paper sheet
or webs. The ability of semi-conductive materials, such as paper,
or non-conductive plastics to surrender electrons is relatively easy.
Transfer of charges can occur by friction, the action of separation,
heat or pressure.
can also be charged by induction, if it is isolated. For example,
laying a piece of copper or aluminum on top of a piece of Plexiglas
creates an isolated conductor. Rubbing the bottom surface of the
Plexiglas under the copper will generate an extremely high voltage
on the conductor through induction. It is impossible to charge a conductor
which is grounded. The charges absorbed by the conductor will drain
same condition will also charge an insulator that is isolated. It
is useless to attach a ground to an insulator. Induction will cause
a voltage on the insulator which cannot be drained to ground because
electrons cannot move over an isolative surface. Thus, if processing
non-conductive materials, static electricity cannot be drained from
them simply by grounding them. This means that as non-conductive
material passes over grounded rollers, static electricity cannot be
drained from it.
of plant machinery and related equipment is critical. Besides the
safety factor, a grounded machine will help drain off extremely high
charges of static electricity from partial conductors. However, grounding
is only an aid to reducing problems with static electricity. It is
not the total solution. Non conductive materials can not be grounded.
example, grounding a web handling roller will not drain off static
electricity from the isolative material. The conductivity of paper,
cloth and many plastics is so low the electricity cannot flow to ground,
therefore, static electricity remains. To solve this problem, ionization
must be utilized.
practical method of controlling static electricity build up is by
ionization. It is important to understand that static electricity
cannot be entirely eliminated. In fact, the terminology, "static
eliminators", is definitely misleading.
eliminators are really ionizing units that produce both positive and
negative ions to be attracted by the unbalanced material so that neutralization
does occur. (For example, a charged piece of ionization
equipment (static neutralizer).) However, it does not permanently
eliminate the static electricity because, if the material is again
rubbed after being neutralized, static electricity will be generated.
order to gain the most benefit from static neutralizing equipment,
it is important to understand how they operate and how they accomplish
the means of neutralization. Most electrical neutralizers are constructed
by placing a high voltage on a sharp point in close proximity to a
grounded shield or casing. As the high voltage alternating current
pulses through the 50 or 60 cycle operation, the air immediately at
the sharp points is actually broken down by corona discharge and,
therefore, both positive and negative ions are generated. Half of
the cycle is utilized to generate negative ions. When operation occurs
at 60 hertz, the polarity of ionization changes every 1/120 of a second.
the material being neutralized is charged positive, it will immediately
absorb negative ions from the static neutralizer and repel the positive
ions into space. When the material becomes neutralized, there is
no longer electrostatic attraction and the material will cease to
absorb ions. Conversely, if the material being neutralized is charged
negative, it will absorb the positive ions being generated by the
neutralizer and repel the negative ions. Again, once neutralization
is accomplished, the material will no longer attract ions.
control static electricity in high speed converting, it I extremely
important to understand the electrostatic field present on non-conductors
charged with static electricity and its effect. Basically, this electrostatic
field is mother nature's antenna, sending
a signal to the world that the material is unbalanced and is seeking
anything which will neutralize it. Therefore, if a grounded roller
is present or a grounded member of any machine is present, the electrostatic
field will attract the non-conductor to the surface. When this happens,
machines can jam, webs can wrap around conductive
rollers, and create general havoc. Likewise, the same electrostatic
field sends the signal to the ions being generated by any static neutralizing
system, causing ions of opposite polarity to be attracted to the surface.
is important to appreciate that material must be placed in "free space"
in order for a static neutralizers to do its job. Likewise, the electrostatic
field also causes the non-contacting static locator meters to operate.
This electrostatic field sets up the energy necessary to unbalance
the electronic circuit and hence the field strength can be measured
on the static meters.
the non-conductive, charged material is placed against any surface,
the electrostatic field is collapsed and, hence, the material appears
neutral. For example, as the web of charges material passes over
a grounded roller, the electrostatic field will look as follows:
can be seen from the above sketch, placing the static neutralizing
bar over top of the grounded roller, will do no good in controlling static electricity,
because there is no electrostatic field present at that location.
The static neutralizing bars must be placed against moving webs and
sheets so that there is no grounded backing present when utilizing
Finally, unless the material is extremely thick, it is only necessary
to apply static neutralizers on one surface only. This is due to
the fact that the electrostatic field present on both surfaces is
a mirror image and is identical in both strength and polarity.
must also be understood that when neutralizing multi-webs or multi-sheets,
a static neutralizer will only neutralize both outside surfaces.
Hence, as in the case of sheeting, if four stacked webs are processed
at one time, four sheets are converted simultaneously. The application
of static neutralizers on the "clip", consisting of four sheets, will
only neutralize the static electricity on both outside surfaces and
will not neutralize the static which is present between the multi-layers.
this phenomenon, it can easily be seen that controlling static electricity
in high speed sheeting can be accomplished, due to the fact that each
"clip" must go into a pile easily and hence the converting is accomplished.
However, the "locked in" static electricity that is present in between
the sheets of the "clip" will cause problems in the future. For example,
when the sheets of converted paper reach the user, a printer, for
example, it will be difficult to get single sheets to enter his printing
press. In this case, the printer must also utilize static control
equipment at the proper location.
Keeping in mind that the conventional, alternating current, neutralizing
systems can be "over-run" when running at extremely high speeds because
we are changing polarities every 1/120 of a second. It is important
to recognize that if the speed of the material being processed to
a point above about 800-1000 feet per minute, a condition will arise
when, for example, positively charges material passes under the static
neutralizer during the positive ionization generating cycle. At this
point in time no neutralization takes place. Since like charges repel
each other, the same polarity of ions as the static charge on the
material will cause a repulsion or the ions.
That is why no neutralization occurs. The faster the speed goes above
600 feet a minute, the wider the "striping" occurs. This means that
as the speed increases further, there will be portions of the above
phenomenon. Increasing the speed to, say. 1200 feet a minute will cause
the "stripe effect" to increase in size. A larger portion of the
processed material will remain charged, even though it is passed under
a static neutralizer.
operating at speeds above 800 feet per minute, it is necessary to
use what is commonly known as dual-phasing. Normally, static neutralizing
bars are placed on one side of the web. To overcome the "striping",
phasing can be accomplished by utilizing two static neutralizing systems
as per the following diagram.
As can be seen from the above, the primary side of the transformer,
labeled A and B can be reversed so that the top static neutralizing
bar is generating positive ions, while the bottom static neutralizing
bar is making negative ions. Therefore, there occurs a better flow
of both polarities, ionization during the AC cycling of these systems.
Yet the stripe effect may occur at very high speeds because of the
pulsing ionization out put of alternating current.
next step up the cost / performance ladder is to go to a resistance
coupled static bar. This technology offers better static control performance
especially at a greater distance and higher web speeds using conventional
AC power suppliers.
the best way to manage controlling static electricity in high speed
operations is to utilize a double DC unit. Basically, a double DC
unit is two static neutralizing bars, each connected to its' own DC
source. Since direct current only goes in one direction, one row
of ionizing points places negative ions in the area at steady state
and the second row of ionizing points generates positive ions and
hence, both polarities are steady. This equipment will serve to extremely
factor to consider when utilizing at high speed is that a high speed
moving web will always carry an air curtain with it,
hence, steps must be taken to overcome this phenomenon.
the problem with this air current is the fact that it becomes difficult
for the ions to penetrate this air current and, hence, the ions will
travel across the top of the web and on top of this air curtain as
overcome this, it is necessary to either "peel" this air current from
the area by utilizing a non-conductive shield as per the following
alternate is to use air boost on the static neutralizing system so
that the compressed air can pass through the use of high pressure
air, penetrate this air curtain, causing the ions to reach the surface
of the highly charged, moving web.
Static electricity can
only be neutralized and not eliminated. To eliminate static electricity,
something physically has to be done to the conductivity in the material
being processed. Therefore, if you are having problems with static
electricity at the delivery end of your equipment, it will serve no
purpose to add static neutralizers at the feed end of this equipment.
The static neutralizing system must be applied directly ahead of the
problem in any processing equipment.
a non-conductor, such as a ground wire floating across the top of
the web, will not neutralize static electricity, due to the fact that
electricity cannot flow in insulators and hence, the term "static
equipment must be applied to the material being neutralized only in
locations where the material is in "free space" and not against any
grounded backing surface.
static neutralizers are only effective up to running speeds about
600 feet per minute. Beyond that speed, "striping" will take effect
whereby a certain stripe area is neutralized and the next area is
charged. Above speeds of 600 feet per minute, phase shifting must
be applied or a double DC system must be utilized.