Troubleshooting roll defects - causes and corrective actions

This article describes roll defects and presents causes and possible corrective actions.

Troubleshooting roll defects

Symptom

• Possible problem: Recommended action

Body damage

Refers to the body of a roll. Such defects as gouges and cuts are not common, however, they do occur. The damage will always be observable when the roll wrapper has been removed and often before.

• Transit damage: Better packaging and/or load preparation
• Mishandling of rolls: Tractors and conveyors are the first items to check. The list of possible remedies is dependent on handling methods.

Crushed core

When the core within a roll of the paper has collapsed.

• Excessive compressive pressure from the web: Softer start
• Sharp outside blow or other mishandling: Avoid mishandling of rolls.
• Core is not strong enough: Use cores of adequate strength.

Loose core

When the core rotates at a different rate than the surrounding roll. Rotational displacement may cause the web to tear free from the core and can cause tension and draw control problems in subsequent processing.

• Start of wound roll too soft with respect to remainder of the roll: Wind harder at start of roll.

Offset core

A core that is not evenly aligned with the edge of the roll.

• Core not properly aligned with edge of roll at the winder: Align cores prior to starting the winder

Poor start

When there an obvious difference in appearance between the paper near the core and the remainder of the roll.

• Web wrinkling during start of the roll: Reset sheet spreading equipment.
• Web slack during start of the roll: Fasten web to core securely prior to winder startup.
• Web weaving during start of the roll: Reduce web cross-deckle caliper variation.
• Winding too soft near the core: Wind tighter near the core.
• Poor quality cores: Change to better cores.

Blister cut

A cut in the web, usually at an angle to the machine direction. It is normally a result of excess paper accumulating as a "blister" at the entrance of a nip. When the excess suddenly carries through, the force applied in the nip cuts the sheet. The term "calender cut" refers to the same defect that has been caused at a machine calender or supercalender. The are adjacent to the cut usually appears translucent when viewed by transmitted light.

• Excess paper accumulating in front of a nip. May be across entire deckle or only in localized area: Tighten draw
• Corrugations in wire or wet felt: This and the following items have numerous remedies. Once the cause of the problem is determined, the remedy is often obvious.
• Non-uniform basis weight: See above.
• Non-uniform moisture: See above.
• Non-uniform nip pressure: See above.

Crushed roll

Sometimes called a "baggy sock roll" and is the result of stacking rolls on end in an excessively high pile which in turn causes the lower rolls to fail in an axial direction.

• Excessive axial pressure: Stack rolls to a more conservative height.
• Winding too soft: Wind rolls harder.

Dished roll

A progressive roll edge misalignment that can be concave or convex. Observed immediately upon unwrapping.

• Non-uniform cross-deckle caliper: Reduce cross-deckle caliper variation.
• Winding too soft at same radius of the roll with respect to other radius: Control hardness of wind throughout the problem area.
• Cores not stationary with respect to web while roll is being wound: Keep cores from shifting.

Telescoped roll

A progressive roll edge misalignment that can be concave or convex. Observed only after the roll has started to be unwound.

• Non-uniform cross-deckle caliper: Reduce cross-deckle caliper variation.
• Winding too soft at same radius of the roll with respect to other radius: Control hardness of wind throughout the problem area.
• Excessive web tension while winding: Reduce web tension.

End damage

Defects which are apparent when the roll ends are examined closely; often more serious than body damage. Examples: gouges from handling equipment; core indentations on the edge caused by an offset core in the roll stacked on top of the damaged end.

• The cause is normally obvious and the remedy apparent such as a gouge from foreign material on the car floor: Protect end of roll sufficiently.

Offset

A defect that manifests itself as an abrupt change in position of the edge of the sheet.

• Abrupt change in tension or web draw control at winder: Make necessary tension changes gradually.
• Moisture growth during splicing at a winder: Make splices more rapidly.
• Webs not aligned when spliced: Maintain proper web tension while making splice.
• Abrupt change in rider roll loading: Better control of rider roll loading.

Out-of-round roll

A defect that can be identified because of a flat section on the outside of the roll. The core is not in the center of the roll.

• Roll wound too soft: Wind roll tighter.
• Severe impact: Provide for proper handling, storage and transport of roll.
• A combination of the above...
• Non-uniform roll structure: Pay attention to the TNT's of winding with respect to modifying controls and winding recipes.

Slitter rings

Excessive slitter run-out.

• Excessive slitter run-out: Repair or replace slitters.

Starred end

Identified by observing the "star" pattern on the end of the roll. Often the star is symmetrical, but frequently one or more rays are missing.

• Tightly wound paper above paper that is loosely wound: Wind tight at the start of the roll or gradually soften as the diameter increases.
• Thin web thickness at the edge of the roll causing a soft end: Keep cross-deckle caliper variation to a minimum.
• Severe impact: Provide for proper handling, storage and transport of roll.
• Any combination of the above...

Cracked edge

Cracked edge refers to a broken edge of the web. It usually extends in only a short distance and is sometimes termed "edge tear".

• Slitter equipment in poor working order: Get slitters in good working condition. (See Slitter Management)
• Slitters improperly set: Set slitters properly. (See Slitter Management)
• High sheet caliper at edge: Often heavy coating or size application is the cause. Maintain uniform sheet cliper at the edge.
• Rolls stored on end with single or multiple sheet run-out: Eliminate sheet run-out particularly at splices.

Poor slitting

Poor slitting results in a rough or fuzzy roll edge appearance. When the web edge is examined closely, it is not straight. The edge often appears to be nicked or dented. Wiping the roll with a black cloth results in a discolored cloth due to pieces of fiber and fines termed slitter “dust”.

• Dull slitters: Sharpen slitters (See Slitter Management)
• Slitters not adjusted or set properly: Set slitter at proper shear angle, depth, etc. (See Slitter Management)
• Speed differential between web and slitters too great: Slitters should move at the same speed as the web or faster, not slower. (See Slitter Management)

Slitter edge curl

Slitter edge curl results in a wavy edge at the slit which give the roll edge a wavy or shaggy appearance.

• Edge of web stretched because bottom or top slitter is too far into the sheet: Set slitters at proper depth. Reduce web tensions through slitter section. (See Slitter Management)

Baggy paper

Refers to a web with non-uniform draw. The web width does not uniformly support web tension. There are slack and tight sections across the width of the sheet. If severe enough, the defect results in web tension difficulty in subsequent operations. Other terms used for this defect are: "slack areas", "baggy end", "slack end", "baggy center", or "slack center". These terms refer to the position of the defect across the face of the roll.

• Non-uniform cross-deckle web thickness: Normally the thick area results in a baggy area. Reduce cross-deckle caliper variation to a minimum.
• Winding too tight: wind as soft as practical.
• A combination of the above...
• Tight winding in a local area across the web or incorrect setting of the web spreading device: Adjust spreading device properly.

Air shear burst

An irregular separation or rupture of the web. Normally the defect extends for some length in the machine direction and is commonly located 10 – 25 cm from the edge of the roll. Air trapped in the winding roll lubricates the internal layers of paper to the extent that some sections relieve themselves more than others. This produces machine direction shear deformations. Such situations arise when:

• Excessive cross-deckle web caliper variation: Minimize cross-machine sheet caliper variation.
• Winder drums are not properly grooved: Experiment with various drum surface patterns (single and double grooved) to find an optimum style.
• Winding is too tight: Wind softer.

Caliper shear burst

An irregular separation or rupture of the web, generally occurring between an area of relatively high and low caliper and normally extending some distance in the machine direction. Non-uniform nip velocities between hard and soft section of the roll may result in machine direction shear deformations which exceed the strength of the web.

• Non-uniform nip velocities: Minimize cross-machine sheet caliper variations. Until this can be accomplished, run soft rolls if possible.

Cross-machine tension burst

An irregular separation or rupture of the web. While the defect is normally in the machine direction, the fiber separation is in the cross machine direction. Normally this defect is not visible at the edge of the roll.

• An abrupt change in cross-machine caliper: Minimize cross-machine sheet caliper variation.
• Winding too tight: Wind softer.
• A combination of the above...

Full machine-direction tension burst

A complete separation or rupture of the tension to web. While the defect is normally in the cross machine direction, the fiber separation is in the machine direction. It is often visible at the edge of the roll, appearing as a stickout or offset.

• Winding too tight: This causes wound-in tension to exceed the ultimate strength of the paper. Soften winding

Partial machine-direction tension burst

Partial machine direction tension burst refers to a partial, irregular separation or rupture of the web. While the defect is normally in the cross-machine direction, the fiber separation is in the machine direction. It may be visible to a trained eye if the defect occurs at the edge of a roll appearing as a stickout or offset.

Internal stress of the reel has locally exceeded the ultimate strength of the paper. This is caused by:

• Non-uniform cross-deckle caliper variation: Reduce cross-deckle caliper variation to a minimum.
• Winding too tight: Soften winding.
• A combination of the above...

Rope marks

Bands of relatively uniform width around the roll at a constant distance from the edge. Usually uniform diagonal markings are present within the band. Other terms for this defect include: "chain marks" and "corrugations".

• Non-uniform cross-deckle web caliper: Rope marks usually show in the thinner areas. Minimize cross-machine sheet caliper variations, basis weight, moisture, etc.
• Tight winding: Wind softer.
• A combination of the above...

Trim between paper layers

A defect caused by winding slitter trim shavings into a roll.

• Neglecting to turn trim removal fan on: Turn fan on.
• Trim is too narrow for removal system to handle at times: There is a minimum trim below which it is dangerous to go.
• Trim removal system of marginal capacity for speed of winder: Slow winder down and/or increase trim removal system capacity.

Dust

Very fine, loose material located on the web surface.

• Poor slitting: Improve slitting. (See Slitter Management)
• Processing web over a stationary member: Consult with winding expert about replacing stationary member with rotating member or adding dust removal capability.

Crepe wrinkle

A web defect that looks like an accordion. Usually there are several wrinkles gathered in or folded immediately next to each other at an angle to the axis of the roll. Often crepe wrinkles are associated with caliper shear bursts and generally with a poorly structured roll (excessive caliper variation).

• Roll wound too soft at or near core: Wind harder near core.
• Abrupt change in hardness of the wind: Change hardness of wind slowly as roll diameter increases.

Core is not in the center of the shipping roll when the reel is ready

• Too hard winding near the core: Wind softer near the core.
• Ends of cores are not straight: Saw the ends of the cores straight.

For more information on avoiding or remedying wound roll defects, consult your Valmet representative or request customized training. TAPPI offers an excellent comprehensive book on the subject, entitled "The Ultimate Roll and Web Defect Troubleshooting Guide."