6.4.Slitting Methods

Paper , film , and Laminates are processed in wider widths.We have to cut them into smaller widths as per customerrequirements.It’s called as Slitting Flexible Packaging a diverse range ofmaterials-Paper, Films , Foil, and the laminates of various types , withthickness ranging from 25 microns and above, need to slit it.the challenge is to chose a slitting methodthat does a Clean and Precise job , easily, and safely.We must choose the slitting method for each particular productto be slit. It is a three-step procedure

Identify the characteristics of each slittingmethod.

Identify the characteristics of the materialto be slit.

Match the material to the best slittingmethod.

The three most common slitting methods usedare shear, crush (score), and razor .

Each method separates web materials using distinctly differentprinciples.We must understand these principles before determiningwhich method is preferred for a given material.

The other associated Problems areMaterial ConditionsGauge VariationGauge BandSlack zonesSurface Air As a lubricant EntrapmentMATERIAL CONDITIONAll films, coated materials and laminations are subject to someor all of the following variations:Gauge VariationA gradual uneven thickness across the web width from edge toedge.Gauge BandAn abrupt change in thickness in a small area.Gauge bands run parallel to the main axis of the web and canexist for the length of the web,

or only for short lengths.

Slack ZonesSlack Zones are caused by the uneven production of theoriginal material. In this condition, the gauge can be even, butif the web were laid out on a flat surface and cut into stripsparallel to the machine direction, the slack zone strips wouldbe longer than the normal or tight zone strips (Figure 1-1).

Surface AirAll materials carry a surface layer of airWhen films are slit and rewound, the air is carried to therewound roll.At low speeds. this layer of air can escape at the ends of theslit rolls without causing problems in winding. At high speeds,air is wound into the roll.This can result in the air acting as a lubricant , or it is trapped

between the layers of material.When air acts as a lubricant it allows shifting of the second,third, and lower layers

see (Figures 1-2 and 1-3).

Air wound between layers showing the relative shifting of thematerial

01 October

2016By Shrikant AthavaleContact : psatechno@gmail.comCell : 00 91 9850242282

When air is trapped between the layers of material and iswound into the roll, it forms , balloons or bubbles, causingdeformation and occupying volume as a secondary layer(Figure 1-4).This causes poor rollformation and deformation of the web.

Now coming back to Slitting methodsThe three most common slitting methods used are

Razor , Shear , and Crush (score)

Razer Cutting Method

The Advantages

lower equipment cost, cost of knives, and speed of set-up.Set-up of razor blade slitting in air is faster, InexpensiveReplacement cost

The Disadvantages

a loss of accuracy in slitting. cannot be set to hold closeslitting tolerances, BecauseThe web tends to stretch or shift while slitting in air betweentwo rolls. not capable of cutting through heavy rigid, orabrasive materials.

The problem solution isThe use of grooved back up rollAdvantagesgrooved back up roll with the web wrapped around the roll,provides for a cleaner, smoother and more accurate cutbecause

the roll gives very close support to the web .It cannot stretch or shift the web.DisadvantagesIf the web runs tangential to the grooved roll rather thanwrapping around it, the benefits of the grooves are lost.The grooved roll requires more set-up timebecause

the blade must be centered in the groovenot touching either side or the bottom.

Contact between the razor blade and the grooved roll quicklydestroys the blade and tends to damage the groove.See Figures II-1 and II-2

At nominal running speeds, say through 150 M/min , theblades also tend to burnish the film as it is being slit giving apolished edge. Over these speeds heat is generated. If enoughheat is generated to start melting the film, a bead can begenerated at the slit edge. This bead can be very detrimentalto achieving good winding of the slit strips. The razor blade isthe most economical of the knives used for slitting. Set-up iseasily accomplished just by clamping a blade in place along abar so that the blade penetrates through the web. Hence razorblade slitting can be recommended for quality and for fast set-up on any light, non-abrasive materials , especially Films.

Shear Cut Slittinguses two rotary knives for the same cutting effect as obtainedby using a scissors.

This is exactly what happens with a pair of rotary knives.

If the web is running slower than the knives, effectively theknife is closing in on the web or cutting it the way scissors cut.If the web is running faster than the knives, the effect is thesame as pushing the material through the scissors.

This is why material can be effectively slit at a knife speedwhich is either slower than synchronous, synchronous with theweb, or faster than synchronous. The angle of junctionbetween the knife cutting edges is the critical factor.

The web is, therefore, torn rather than cut.Knives and spacers must be assembled on the shaft to form orto simulate a solid supporting shaft.Unless the web is supported by a shaft solidly stacked withknives and spacers, the web will pull or sag into the voids andirregular slit widths will result.

The second arrangement is tangential or kiss slitting. In thisarrangement the web only kisses or touches the outsidediameter of the female knife, it doesn't wrap around it.See Figure II-9. If the web hits the male knife before it hits thefemale, a cutting angle is not achievedThis is the same as taking scissors and trying to cut with onlyone blade. The web is ruptured, not cut.The advantagekiss slittingfast setup of knives.The knives repositioning without knife shaft removal.Wrap slitting requires removal of the shaftIn kiss slitting,the knife is loosened, shifted to the next cutting position, andtightened again.It can be accomplished in a minimum of time in the machine

without removing any parts from the machine.

The disadvantage

the quality of cut achieved.The cut is not as good as that obtained by wrap slitting.it is seen that when shear slitting a web, on the cutting side themale knife comes in and cuts clean and square.On the other side, the knife has pushed the web aside anddown in the cutting operation. Consequently, the web,depending upon the condition or type of material being slit,can be permanently or temporarily deformed.The edge of foil, for example, takes a permanent deformationon the edge, and as it winds up layer after layer, builds a bevelon the edge of the rewound roll which eventually causes thesheet to tear.

Wrap slitting requires the removal of the shaft from themachine, taking off the knives and spacers and then puttingthem back in rearranged order. the web wraps around thefemale knife.The male knife penetrates to the ideal depth and gives a goodclean cut. In kiss slitting, the web comes in tangential to thefemale knife.

Looking at Figure II-11, you will notice that a good portion ofthe male knife now is underneath the web that has been cut.When the web and the male knife come in contact at this samepoint the cut is clean. To achieve this geometrical arrangementthe male knife must be offset toward the exiting side of theweb.

As the knife rotates out of the web, the web rubs both on thecutting side and the backside of the male knife, creating africtional drag and filing the material that passes through.This is a decided disadvantage because it causes fuzz and dust.

In foil, this deformation occurs with both kiss and wrapslitting.There is a second problem that arises in kiss slitting.

When the web comes straight out, there is a greaterinterference angle between the web and the male knife. As themale knife rotates out of the web, it has a rough grind on theback edge; it will tend to file the edge of the slit strip. Theresult on a hard material is filing action which causes littleparticles to fall onto the edge of the slit strip. These particleswill create a buildup similar to that mentioned above withidentical results. Kiss slitting is good for fast setups. Thequality of the cut is not as good as in wrap slitting because ofthe greater interference between the male knife and the exitingweb In wrap shear slitting, accurate slit widths and closetolerances are possible through the use of spacers formeasuring and positioning the knives.This combination of knives and spacers, stacked on the shaft,provide a complete support for the web, thereby reducing, to aminimum, the possibility of wrinkling, bagging or sagging.This results in excellent slit width accuracy. In kiss slittingbecause there are voids between the knives, permitting the webto sag and wrinkle, slit width accuracy is more difficult toachieve.conclusion, shear cut slitting offers many advantages and canbe used in several different ways.It is standard practice to incorporate driven rolls in all types ofmachines to maintain control of the web as it passes throughthe knives and to isolate the slitting tension from the unwindand rewind tensions. All driven rolls also serve to keep aconstant tension in the slitting area, thereby maintaining theslit width.A slight overdrive in web speed is maintained fromdriven roll to driven roll to hold tension constant. For speeds inexcess of 40 M/min.and film thickness below 25 micron, it isnecessary to positively drive all rolls since changes in web

speed, due to un driven roll inertias, tend to distort the web andaffect the quality of the rewound rolls. This is especially truewhen narrow webs of light plastic films are run on widemachines (i.e. webs less than 70 per cent of the maximum webwidth capacity of the machine).

Score Knife Slitting

Score or 'crush cutting' is commonly used for adhesives andcertain papers. The score knife is a pneumatically loadedcircular blade that sits against a hardened steel anvil. The knifethen crushes the web against the anvil. Generally, slit edgesare not of the highest quality, but makes up for it in quick andeasy setup procedures.Score Cut, also known as Crush Cut,slitting is the oldest form of cutting It is crushing of thematerial to sever one part from another.The "knife" cannot bedead or razor sharp, because a very narrow knife edge will dulland chip almost immediately upon contact with the surface itis crushed against.Hence, the knife used in score cut slitting is one that is dulled(rounded) to the ideal condition before we start trying to slitwith it.When slitting a continuous web,a round backup orplaten roll and round slitting knife must be used.the knifepressed against the web with sufficient pressure to cut throughit, we achieve "slitting". the web, the platen roll and the knifemust all be moving at the same speed.The method requires enough pressure to cause the web beingslit to separate completely,it is again obvious that after a short running time, the platen

roll will groove and the knife edge abrade.At this point, there is intermittent cutting only.To avoid this, we make both the platen roll and the knife ofsteel that can be hardened.Further, the platen roll is ground to a very smooth finish.

The score knife itself after hardening is not only ground butthe cutting edge is radiused and then burnished to a very highpolish.The smoother the platen roll and the knife edge, the cleaner thecut and the longer the life of the blade and roll.score cut slitting is very quick to set up, thus offering a verydistinct advantage.It is, however, losing favor in many areas.As explanation, let's examine this cutting method first onpaper.When we crush the fibers of the paper web, a very fine powderof crushed paper fibers is created.If the knife is "sharp", that is smooth and highly polished, theslit edge of the paper will appear smooth, but there will also bea deposit of paper dust. The amount of paper that the knifedisplaces, turns to dust. Years ago, this dust was quiteacceptable, but in today's high speed printing presses andautomatic packaging machines, dust accumulates faster andcauses equipment malfunctions. For this reason, score cutslitting is becoming less acceptable.In addition to "dust", score cut slitting paper may beundesirable in another way.As soon as the edge of the knife becomes slightly rough, theslit edge is no longer smooth. Instead, it has slight feathers orthreads where the paper has not been completely pulverized.These feathers or threads tend to accumulate on the edge of aslit roll making it feel and look very rough. The normaltendency when this occurs is to increase the slitting pressureon the knives. This is usually successful, but only for a shorttime. The increased pressure causes further deterioration ofthe knife resulting in a still poorer slit edge.However, as soon as any defect appears in either surface, thisdefect creates a void and the film is squeezed into the void andis not separated. At first, these strings or ties tear to giveseparation but they soon reach a point where adjacent sectionsof film strech and deform causing rejects On hard plastic filmssuch cellophane, acetate and oriented polystyrene, the result issomewhat different. As the knife crushes its way through thematerial, it not only severs the web as desired but causesminute cracks perpendicular to the slit web.These cracks when put under slight strain tend to propagatefurther into the web.With wide webs, this is not too critical. On narrow webs, theonly too frequent result is a tear completely across the web.

This is medium duty shafted surface driven slitter winder withtop rider roll designed to slit and surface wind news print on aroll to roll basis. The method of slitting is score cutting on ahardened entry anvil bedroll. The unwind stand is a shaftedfloor pick type eliminating the need for a roll handling hoist.

BASIC WINDING TECHNIQUESCenter WindingDifferential RewindingCores and AdaptorsIndividual Top Riding RollsPreventing Gauge Band Build-UpSurface-Center WindingSurface Winding

Center surface winding is used primarily for rewinding largediameter rolls on extended web width applications.Center surface winders today, are available in several modelsincluding lift out mandrel, cantilevered and with multipleindividual shaft less rewinds.On a center surface slitter, the web is pulled by the primarysurface roller.

Finished roll density is then developed independently from theweb tension.This is done through controlled lay-on nip pressure as well astorque from the primary surface roller.Centre WindingOn some materials contact pressure cannot be applied becausethe materials could become blocked or damaged. Here centerwinding becomes necessary. Today center winding is the mostprevalent type of winding. and the basic principles remain thesame whether the operation takes place in a vacuum or inordinary atmosphere or in a pressurized chamber.In center winding (Figure 111-1) the winding force is derivedsolely from the rewind shafts and is transmitted to the windingweb through the core and layers of material that have alreadybeen rewound. The winding force is generated by a D.C.motor. a fluid motor. or a slip clutch.

Fig. 111-1. Center Winding - (Duplex Winder)

Fig III-2. Reduction in Tension as web diameter increasesduring winding.

The graph in Figure 111-2 shows the effect of constant torqueinput. This results when a slip clutch is used and noadjustments are made as the roll grows in diameter.As the roll builds up in diameter, the tension drops off.This puts a limit on the maximum diameter to somewherebetween 12" to 14" if initial winding was started on a 3" I.D.core. Keeping a 3" core, the only way to get larger rewinddiameters is by increasing the input power or torque as the 'rolldiameter builds up.The bottom line in Figure 111-3 shows the basic constanttorque curve. As the roll diameter builds up. the web tensiondecreases. Normally after slitting there are two rewindpositions, when center winding, generally located one abovethe other, to ensure positive separation of the rolls. Adjacentcuts are wound alternately on the upper and lower rewindmandrels. Therefore, in center winding it is easiest to wind atconstant torque. When it is necessary to wind to largerdiameters on a given core size, programmed torque is used. Itis seldom advisable to attempt constant tension winding in acenter wind operation. The bottom line in Figure 111-3 showsthe basic constant torque decreasing tension curve. The upperline is the constant tension curve. This id one form ofprogrammed torque.The ideal arrangement for building a good roll is Somewherein between as is shown in the programmed torque curve. Theexact ideal curve varies with the material and windingconditions and is usually best found by experimenting withvarying programs until ideal performance is arrived at. Theabove conditions hold true in all center winding operations.

Fig III-3 Effects of Different Tensions in Centre Winding.

Differential Rewinding - Core/Spacer/Core

In slitter-rewinders, the most common type of center windingis the duplex type winder.The duplex center winder is an application of center windingutilizing differential rewinding.Differential rewinding is an arrangement whereby eachindividual rewinding roll is completely isolated from allothers. This is essential to satisfactorily rewind materials ofnon-uniform gauge. Without differential action. the heavygauge areas of the material would wind tightly while lightergauge areas would wind loosely. The former condition mayresult in stretching of the material and/or collapsing of thecore. The latter condition may cause the roll to telescopeand/or fall apart in handling. High quality rewinding.therefore. depends directly on differential windingDifferential rewinding is accomplished on duplex centerwinding machines through a system of alternating cores,andspacers on each mandrel. The spacers are keyed so that theyare free to slide axially on the mandrel, while being restrainedfrom rotating. The latter condition may cause the roll totelescope and/or fall apart in handling. High quality rewinding.therefore. depends directly on differential windingDifferential rewinding is accomplished on duplex centerwinding machines through a system of alternating cores,andspacers on each mandrel. The spacers are keyed so that theyare free to slide axially on the mandrel, while being restrainedfrom rotating. Thus, the cores are held back by the slit stripsand are forced to slip with respect to the mandrel and thekeyed spacers. The adjoining faces of the cores and spacersact as slip faces of slip clutches. providing a driving torque oneach core proportional to the axial pressure exerted. Each coretherefore has the equivalent of its own individual slip Clutchdrive.

The tension of all slit strips is the same since all cores areacted on by the same axial force acting on identical slippingsurfaces.Each core is its own slip clutch and, therefore, each slit strip isfree to wind at its own speed and tension regardless of the sizeor condition of the rolls being rewound on either side of it.For maximum versatility, four sets of rewind tension controls,one at each end of each mandrel, are installed as standardequipment on this type of machine.Thus, by utilizing each of the four different tension. controlzonesit is possible to slit and rewind up to four different slit widthsconcurrently from a single web and maintain the correcttension for each individual slit width.Cores & AdaptersCores are an integral part of differential rewinding and theyare a critical factor in obtaining a good rewound roll. For bestresults, the ends , of the cores must be smooth and square tothe inside diameter.Any burrs or turned over edges on the inside diameter of thecore will cause binding between the core and the mandrel.Burrs catch in the mandrel keyway or act as a point contactwith the rewind spacer.. These conditions cause an erraticslipping action resulting in erratic tensions on the rewoundweb strip. Since the cores are stacked together with the rewindspacers, any cumulative size variation in the individualelements will cause successive increases in misalignmentbetween the cores and the slit webs.Techniques here, therefore, necessitate holding close toleranceon the length or width of the core used. Where commerciallycut narrow-width cores are used and erratic operation existsdue to poor core edges and/ or excessive friction between thecore I.D. and the mandrel body. core adapters have been

developed. Core adapters are used to eliminate contactbetween the core body, the rewind mandrel, and the rewindspacer(Figure 111-5).All core adapters serve to control the friction between themandrel. rewind spacer and the core.By placing the core on the core adapter and utilizing a lowfriction insert in the core adapter as the slipping surface on themandrel and against the rewind spacer, very fine tensions canbe maintained on all operations. especially when thin. stretchyfilms are being rewound.Careful attention must be given when winding at high tensiondirectly on mandrels of the same diameter as the core I.D.-Even a slight compression of the core due to tight windingcould cause a locking of the core to the mandrel. eliminatingthe slipping and differential winding action.High torque adapters are used to obtain the higher rewindtensions while eliminating the problem of the core collapsingand seizing. Here the core is supported by the pilot of theadapters and not by the mandrel. All types of core adapters canbe used for reminding on largesize cores where mounting and handling make it impractical orexpensive to use a mandrel of matching diameter.The type of core adapter selected depends on the intended use.Differential Rewinding - Slip ChucksThe core/spacer/core method requires increased set up timebecause of the need to alternate spacers and cores, generatesheat and core end profile can have an effect on the windingtension. Individual slipping chucks have been developedwhich standardize the forces exerted on the core and thereforeincrease winding quality.Each chuck consists of an inner drive tube which is keyed ontoand driven by the rewind shaft, and an outer segmented tubewhich is designed to expand and grip the inside of the core.Between the inner drive tube and the segmented core tubethere are some clutch pads which are actuated by a rubber baginflated bycompressed air.When only a small air pressure is applied to the chuck, the bagonly inflates slightly and therefore the drive from the rewindshaft via the inner drive tube to the segments (and thereforethe core) is of very low power.If the core is stopped from rotating (ie by the web) then theouter tube is slipping relative to the inner drive.If the air pressure is increased to the bag then more force isexerted by the clutch pads and more power is fed to the core,

therefore more tension wiII be created by the core into theweb.Chucks are positioned on the rewind shaft such that twochucks are used per core, these should be mounted near toeach end of the core. If the core is less than 150 mm long thenone chuck can be used but it must be mounted central to thecore length.Plastic spacers are used to position the chucks and thesespacers have asealing face to prevent air leaking out from the groove whichis machined down the full length of the rewind shaft.This groove is used like a pipe to feed the air pressure to thechucks and then inflate the bags which cause the tension rise.The air pressure value supplied to the chucks is dependant onthe tension setting control and the diameter of the rewind reel,at core diameter the pressure will be low, but as the rewinddiameterincreases the pressure will be increased gradually by themachine control system to enable a constant tension to bemaintained at the increasing radius point of the web.

A variety of Differential Core Chucks.

Individual Top Riding RollsBeing constantly aware of the obligation to provide equipmentcapable of satisfactorily winding at high speeds, the individual

top riding roll for multiple roll slitting was developed as stillanother refinement to an already production proven andversatile piece of equipmentThe development of the independent top riding roll permittedsatisfactory high speed winding of films and other off calibermaterials because these rolls would positively maintainconstant contact with the rewinding roll; thus preventing airfrom entering between the layers of film.The incoming web is guided precisely to and brought intocontact with the rewinding roll so that side shift and weavingare eliminated, and individual rolls are not affected by off-gauge conditions of materials in adjacent rolls.Individual top riding rolls are used to obtain straight, smoothsided rewound rolls, at high and low speed, when winding off-gauge, high slip - light tension "problem" materials.Of all the progress which has been made with regard to theslitting and rewinding of these materials, the development ofthe individual top riding roll for each individual rewound rollis probably the most outstanding engineering advance.Efficient subsequent Operations. to a large extent, depend onthe smooth edged, even density rolls of packaging materialswhich can be produced on the slitting-rewinding equipment.Four major factors must be compensated for in order toproduce the desired slit and rewound roll. These factors are:gauge variation, gauge bands. slack zones and entrainedsurface air. which acts as a lubricant when an excessiveamount is trapped and wound within the rewound roll.

Preventing Gauge Band Build-UpIn addition to eliminating air from between the layers of therewound rolls, individual top riding rolls can be utilized toovercome the problems caused by the build-up of gauge bands.For example, a roll with a gauge band build-up being woundwithout top riding rolls will develop tension concentration in

the gauge band area resulting in uneven distribution of webtension across the full slit width.Hence, for a 10 in. wide roll winding at one pound per inch ofweb width (10 lb. total) all 10 lb. will be concentrated at thegauge band.Thus. if the gauge band is 2 in. wide we will have aconcentration of 5 lb. per inch instead of the desired 1 lb. Thisexcess tension will cause distortion of most plastic films.causing further build-up of the gauge band. This in , turncauses wrinkles to form in the web coming into the roll.Refer to Figure 111-12.

When individual top riding rolls are used, this tensionconcentration is immediately redistributed to the full webwidth as the web wraps around the top riding roll.Further, if the individual top riding roll is held parallel to therewind mandrel, contact will be maintained only on thegauge.band, thus permitting air to be wound into the rewindingroll at all of the other points.The resulting roll will be tightly wound (without stretch) overthe gauge band and soft throughout the remainder of the roll.This roll can then be handled easily without telescoping ordamage and will feed into the next machine operation withoutany problems.

ConclusionBecause top riding rolls are maintained in constant positivecontact with the rewound rolls. the air which would be trappedbetween the layers of film is ironed out.Top riding rolls are -sometimes referred to as "lay on rolls" or"ironing rolls". They also serve to distribute rewind tensionevenly across the slit width. Because parallelism between thetop riding roll and the rewind mandrel is maintained. unevenweb tension due to build-up on the rewound roll is eliminated.This action produces a wrinkle free rewound roll; and the rollsarenot affected by off-gauge material conditions in the adjacentrewinding roll.Slitter Rewinder OverviewCantilevered Slitters by definition are machines with rewindshafts that are supported on one end. Deacro's cantileveredslitter rewinder machines are actually supported during therun-time for optimum performance.Slitter Rewinder OverviewCantilevered Slitters by definition are machines with rewindshafts that are supported on one end. Deacro's cantileveredslitter rewinder machines are actually supported during therun-time for optimum performance.

Salvage Rewinder Overview

Salvage rewinders allow operators to correct finished rollprofiles, tensions, splice out defects and take edge trim.salvage rewinder lineup is even capable of basic slitting.

The high-performance reversible salvage rewinders that areideal for inspection of printed webs and can reduce rewindtimes by 50-90%.A reversible machine allows operators to maintain the printdirection without running the roll more than once.