Crating & Shipping

Glossary

William E. Hawkins, Contributing Editor

There are many terms used to describe similar things in the converting industry. These vocabulary differences can present communication problems around the globe. This column is the first installment of a working glossary that is my interpretation of the terms I use when I describe technology connected with web handling.

Air Entrapment:
The process of capturing boundary air between web wraps during the winding process.
Alignment:
Making the axis of all rolls in the machine parallel to one reference roll.
Asperity:
The roughness of the film surface, usually expressed in microns (one micron is one millionth of a meter) or micro inches (one micro inch is one millionth of an inch).
Baggy edges:
Webs with longer film length on each edge than in the middle for any span.
Boundary air:
Atmospheric air that stays on the film web surface until it is displaced, either mechanically or by a vacuum pump.
Bowed roll:
A roll that has a curved axis. The roll covering is flexible and stretches during one half of a revolution and compresses during the other half. This type of roll is more difficult to turn than a straight roll, because energy is required to compress or stretch the covering during each revolution. The energy to turn the roll must be supplied by the film web when the bowed roll is not driven by another means.
Caliper variation:
Thickness variation from point to point in the web. These variations may be orientated so that the thicker areas form machine-direction (MD) bands in the wound roll. Lanes of thicker web are often called transverse-direction (TD) gauge or standing gauge bands, because they are observed to stay generally at the same axial area while the roll is being wound. The thickness variation may also be oriented transversely across the web and form lanes. Thicker lanes in this direction often are called MD gauge variations because of the alternating thickness that is observed in the axial direction as the web is wound on a roll.
Chicken tracks, snail trails, etc.:
Wrinkles that are seen in a winding roll or that develop after the roll is doffed. These wrinkles are usually caused by non-uniform web tensions and entrapped boundary air. When the entrapped boundary air escapes, the wrinkles orient themselves along the lines of forces produced by the non-uniform tensions in that area of the roll.
Core strength:
The capability of the core to withstand the radial pressure of the film wraps that are wound under tension.
Conductor:
A material that can carry electrical current and/or transmit heat energy by conduction to another surface.
Contact roll:
Referred to as a lay-on roll in certain configurations. This roll is usually used during winding to limit the amount of boundary air that is entrapped in the roll. This roll is also used to tighten the wraps on the roll. The roll may be stationary in the machine frame while the winding roll pivots away to accommodate roll buildup, or it may pivot into the winding roll when the winding roll axis is stationary.
Constant tension:
The film tension does not change in the span between the last roll of the winder stand and the windup roll as the roll builds from core to full roll when winding in this mode. When thin, stretchable webs are wound in this mode, excessive radial pressure often builds as the roll diameter increases, to the point where the core compression strength is overcome and the core fails toward the axis.
Constant torque:
The film tension reduces in the span between the last roll of the winder stand and the windup roll as the roll diameter builds when winding in this mode. This mode is sometimes called constant current winding when the winder chuck is driven by a separate electric motor, because the motor drive amperage remains the same as the roll diameter builds from core to full roll. Sometimes the outside wraps become loose and telescope (called "telescoping") before the required footage is wound on the roll when winding in this mode.
Counterbalance pressure:
Fluid pressure applied to actuators used to offset the gravity force acting on a dancer roll, nip roll, or contact roll.
Creases:
Film web fold-over wrinkles ironed into the web, permanent web defects that usually render the web reject for production.
Dancer roll:
A roll that usually is mounted near the unwind stand and changes its position as the web tension increases or decreases. Its running position determines the amount of reverse web tension (brake pressure) to apply so that the web payoff tension is constant. Also, dancer rolls are used in special situations to control the windup tension. When a dancer roll is used to control the winding tension, another signal in addition to roll running position must be sent to the winding motor control panel. This signal must indicate winding roll diameter so that taper may be applied to the winding tension control program. The greatest asset of the dancer roll is its ability to adjust to web path length changes that may occur due to eccentricity of the web payoff roll.
Dead band:
An area of a sensor that does not promote change in the signal being supplied to the control station. A wider dead band in the sensor allows more web deviation from the desired set point. The sensor may be sensing alignment of the web path, web tension, transparentness of web, or web continuity. Dead band width does not increase or decrease machine response time (gain control), but a wider one can be used to reduce overcontrol of web steering devices by not disturbing the steering device with minor web edge flutter.
Deflection:
Usually used to describe the maximum amount of bending that takes place when a force such as a nipping roll is applied to another roll surface. Two examples where such bending occurs are tension isolation nip rolls and lay-on (contact or rider) rolls. This term sometimes is used when describing the total amount of bending that a mandrel under- goes during winding.
Dielectric:
A material that does not conduct electrical current through its matrix body. However, most of these materials will give up or add surface electrons easily when an incentive presents itself. Thus, significant static charge can occur on a dielectric material, especially one that is running over rolls in a coating and drying process.
Driven rolls:
Rolls that are driven by either the machine's main drive train or a separate auxiliary drive motor. Rolls that are driven by surface contact only are not considered driven rolls.
Eccentricity:
The out-of-roundness of a roll, core, or mandrel. Eccentricity usually is expressed as TIR (total indicated runout) in mils. When eccentricity is present, points on the roll surface do not rotate in the same axial circle.
Edge sensor:
A device used to monitor the film edge as it moves through the machine. It signals the steering roll(s) control system the web is not on the desired web path centerline. The control system uses this signal to move the web back to the machine centerline.
Elastic limit:
The point of elongation that a web material obtains under tension where it will not return to the original length when the tension is removed. Permanent web deformation takes place when the elastic limit has been reached or exceeded.
Elastomer:
A material that behaves like rubber but is made from synthetic polymers and is superior to rubber in several mechanical or chemical properties. Elastomeric roll covers can be tailor made to fit many web processes that are beyond the capabilities of natural rubber.
Electrostatic charge:
Electrical charges that are trapped on the web surface. The polarity of these charges may be either positive or negative. Electrostatic charges may collect on dielectric materials through the exchange of electrons across the surface interface with either conducting or non-conducting materials. Thus, electrostatic charge may build up on a web when it passes over a roller with either a conducting or non-conducting surface.
Encoder:
A device that measures and signals a control system how far the subject part has moved relative to a fixed reference point. This device is sometimes used to measure the pivot rotation of roll arms, linear position of sliding components, or the rotational position of rolls. The most significant feature of this device is the ability to signal location in a very short time or distance interval.

For example, it can transmit a roll's angular position to a motor control panel more than 1,000x/rev. This allows the motor to compare the actual speed with that of the desired speed and make the proper corrections more than 1,000x/sec. The encoder is one of the essential components responsible for the vast improvements in precision speed control of today's motor drive systems.
Feedback:
A signal that is used in control logic to tell the control system whether or not to act on the process it is controlling. Usually a sensor(s) is used to monitor the process parameter that is most sensitive to change when control changes are made to the upstream process(es).
Field strength:
The amount of electrical force that is generated between electrical charges on dielectric webs and any other body that has or can be induced to have an unlike charge. There are two variables that define the field strength: The field intensity varies directly with the amount of charge and inversely with the square of the distance between the two charges.

There are also two kinds of electric fields: uniform and non-uniform. The electrical field converges to a point in a non-uniform field. The field strength becomes much stronger near the point. When the strength has increased sufficiently to remove or add electrons to the boundary air molecules surrounding the point, ions are produced from the air molecules. When there are many electrons exchanging at some distance from the surface, there is a high probability of electron/molecule collisions. Other free electrons are produced, and that often produces a cascading breakdown of the dielectric capacity of the atmosphere. When this situation occurs, an arc discharges the electrical field.
Flash wrinkles:
Foldover wrinkles in the film web that come and go quickly. Usually, the faster the machine speed, the quicker the wrinkles appear and disappear. When these wrinkles are present, there is nonalignment between the web resistance forces and the roll tracking forces. Because the wrinkles are coming and going, the web stiffness is almost able to overcome the nonaligned resistance and tracking forces.
Foldover wrinkles:
Wrinkles in the web that stay in the folded position for a long duration. When these wrinkles are present, the tracking and resistance web forces are so badly misaligned in that area that the web stiffness is completely overcome. Machine alignment must be regained before the process can continue without this defect.
Gauge:
A common name for transverse-direction (TD) web thickness variation across the width of the web. There is also machine-direction (MD) web thickness variation along the length of the web. Standing TD gauge variation is more harmful to winding smooth rolls than MD gauge because of the diameter buildup differences across the roll width.
Idler rolls:
Rolls that are driven only by the surface friction of the running web.
Ions:
Molecules of any material that have an electrical charge. The molecules of the material have either gained (negative ion) or lost an electron (positive ion). Air is composed of gases that can be ionized in strong electric fields.
Lateral Shifting:
Sidewise movement of the web as it moves through the converting machine. If the web-shifting movements are not corrected, the web tracks off center of the winding roll(s) during that period. A shifted web is usually the result of tension changes in the moving web. These changes can result from speed changes in the machine, or they can be due to ambient temperature changes that cause the web tension members to grow at a non-uniform rate side to side. Lateral shifting also may be due to uneven drying and/or shrinkage forces acting on the web surface. Web steering is essential to maintain web tracking on the machine centerline when ambient temperature is causing the lateral shifting.
Lay-on Roll:
The contact roll that is used to limit the amount of boundary air that is entrapped between the wraps on the roll as it is being wound. This roll is sometimes referred to as a rider roll or simply a contact roll. The lay-on roll also is used to spread the web when there is substantial wrap around the roll. And the roll is used to tighten the wraps of the winding roll by utilizing the stack compression available in the winding wraps. The magnitude of contact area pressure that is exerted in the lay-on roll nip determines the amount of boundary air entrapped and the winding roll hardness.
Load Cell Roll:
A web guide roll that has strain gauge or other type sensors that sense the amount of force (web tension) that the web is exerting on the roll. Usually there is a sensor for each end of the sensor roll. The signals from each end normally are combined and compared with a reference signal in the control panel of the device that maintains web tension in the area of the machine that is being controlled. When there is a difference between the reference signal and the signals from the load cell roll, the controlling program of the panel will signal the tension-producing control panel to change the web tension in the direction to null (or match) the reference and load cell signals.
MD wrinkles:
Ridges in wound rolls that resemble the strengthening, undulating bends in a tin can. They often are referred to as tin can wrinkles. They usually are formed under the surface of the winding roll during the winding process although you may be able to see these wrinkles as the roll is being wound. When the MD wrinkles form under the surface, they will appear on the roll surface after some lag time has elapsed, because the entrapped boundary air that has been supporting the web bleeds from between the wraps out the roll ends. MD wrinkles usually are formed when a lay-on roll is used, and they always appear between two standing gauge bands. There may be several standing gauge bands, and MD wrinkles may appear between all of these bands, or they may span between the most prominent ones that may include some bands of lesser diameter. MD wrinkles may be reduced significantly by randomizing the gauge variation across the web width, reducing gauge variation, and exerting lateral stress on the web as it is laid onto the winding roll.
Modulus of elasticity:
The value of the amount of stress divided by the amount of strain in the elastic region of the web material. You calculate this ratio by dividing the amount of force acting on a cross-sectional area of that web by the amount the web has elongated. This ratio is very helpful in calculating the correct tensions to apply in any zone of the machine that prevents permanent deformation of the web.
Nip rolls:
Normally a set of two rolls (the set may be made with more) that have parallel axes, with at least one roll mounted on movable supports so that the roll faces can be pressed together (nipped) in order to give the rolls sufficient traction to pull the web through the machine at the desired web speed. One roll should be covered with an elastomer, and the other should have a metal surface. The elastomer-covered roll must be crowned in order to produce a uniform nip footprint across the full roll width. Sometimes these rolls are used to isolate web tensions in the two adjacent zones. There are many web handling processes in which there is no satisfactory substitute for nip rolls. These include applications such as collapsing the bubble on blown film lines, some types of cast web stretchers, web laminators, squeegee applications for liquid removal, etc.
Neck-in:
The amount of width reduction that takes place in the film web when that web is subjected to tension in the machine. Permanent width reduction takes place when tension exceeds the yield strength of the web material. Permanent width reduction distorts the web thickness profile by making the edges thicker than the rest of the web. This leads to winding problems as the thicker edges build diameter much faster than the rest of the web. Web tension must be controlled carefully in any process zone when the ambient temperature is elevated, since the yield strength tends to be very temperature-sensitive.
Parallel roll alignment:
The mechanical adjustment of each of the machine rolls so that the axis of each roll is aligned with the axis of a selected master roll on the machine. Parallel alignment is done so that the web may be pulled evenly over the process rolls as it moves through the machine.
Permanent deformation:
The condition of an area of film web that has been stretched beyond its elastic limit and remains deformed after the tension has been removed. Permanent deformation may occur in very small areas, such as the damaged area surrounding slip pimples, or larger areas, such as when MD or TD wrinkles occur in the wound roll, or even larger areas, such as described in "neck-in" item above.
Roll profiles:
The possible shapes of the surfaces of film guide rolls. There are two acceptable shapes for film web handling: the straight cylinder and the concave surface cylinder. The crowned roll is a special case involving nip rolls where the crowned roll surface is designed to conform to the deflection of the straight cylinder roll that is resisting the nipping load. Crowned rolls should never be used as web guide rolls.
Runout:
The axial turning eccentricity of the circumference of the roll surface from the point on the roll axis where it is being measured. During runout, all points on the roll surface do not rotate in the same axial circle. Total indicated runout (TIR) is the largest value of the eccentricity variation when the measurement instrument has been incrementally traversed the total length of the roll face. Runout is present to some degree in all turning rolls. An acceptable magnitude of runout depends on the web process.
"S" wrapped rolls:
Rolls that are usually installed in pairs so that the thread path forms an "S" when each roll is wrapped 180 deg. They are installed this way to provide more pulling or braking traction on the web. Sometimes several pairs of these type of rolls are installed to provide the amount of tension isolation needed in the process.
Shaftless unwind/rewind stands:
Basic pieces of equipment used in holding the supply roll(s) and production roll(s) on the converting machine. These stands usually are equipped with at least one fixed (braking or drive side) rotary chuck and one horizontal sliding (idler side) rotary chuck that will interface with the mandrel ends on cores that are dressed with expandable mandrels or that will interface directly with the roll core ends. The chuck faces usually are conical so that the mounted core or roll is centered when the sliding chuck is end-loaded against the core or mandrel ends.

During high-speed operation (above 200 fpm), one of the most important features that these stands can possess is the ability to accurately center the roll on the turning axis. Another important feature for the rewind stand during high-speed operation is rigidity. Rewind vibration due to roll eccentricity is often the cause of excessive boundary air inclusion during rewind at high speeds.
Stack compression:
What happens to the wraps on the rewind roll when a contact (lay-on or rider) roll is used. Stack compression of film webs is made possible by two non-related phenomena. One is the height of the web surface asperity, and the other is the amount of entrapped boundary air between the wraps of the roll.

Surface asperity promotes limited stack compression by permitting adjacent web surfaces to deform around the asperity in such a way that the centerlines of the adjacent webs are closer together under contact roll pressure than they would be if no contact roll were used.

Entrapped boundary air is compressed by the pressure of the contact roll within the footprint of that roll. This compression results in closer wrap centerlines than if no contact roll were used.

One asset of stack compression is that it reduces the radius differences on the rewind roll during the rewind process and works to improve roll formation.
Static charge and static charge reduction:
Related terms in the converter field. The amount of electrostatic charges on the web surface that can be measured with a static meter is usually referred to as static charge. These charges are sometimes called "triboelectric" charges. They result when a dielectric surface is separated from another surface quickly.

The charge buildup will occur whether the other surface is conductive (grounded or not) or is another dielectric material. Static charge may be removed by producing clouds of both positive and negative ions (charged air molecules) close to the web surface.

These ions flow from the point of generation to the film surface by the force of the electric fields generated by the static charges. The air molecules give up or absorb electrons at the web surface. This action reduces the static charge on the web surface. The optimum frequency for generating the charge-reducing ions depends on the web speed, with higher-speed processes requiring higher frequencies. You can use 60 Hz as a frequency benchmark for a web handling process running at 700 fpm when a 6-mil piano wire is used as the generating source.

Many processes reduce static charge in their machines with grounded tinsel or "static string" because of simplicity and economics. These methods are not as effective as powered ion generators mentioned above. They depend on ion generation between the electric field on the web and the very small grounded points on the tinsel or static string. As a result, static charges may not be entirely removed from the web using these methods.

One point to remember when using tinsel or static string: For optimum efficiency, the charge removal device should be very close but not touch the running web.