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II. BASIC SLITTING TECHNIQUES |
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| The type and thickness of
material determines whether shear cut, razor blade or score cut slitting should
be employed. For example, soft films such as polyethylene, vinyl and
polypropylene are cut quite easily by razor blades, as are the light gauges of
polyester. Heavier gauges of polyester, starting at .002" thick should be shear
cut, as should paper, foil, and most laminates of the above. Score cut slitting
is usually employed where speed of set-up is more important than the quality of
the slit edge or the material is too abrasive for shear or razor blade
slitting.
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| II.1 RAZOR BLADE SLITTING | |
| If original
equipment cost, cost of knives, and speed of set-up alone were used as a
criterion, only razor blade slitting would ever be used. But razor blades
cannot be set to hold close slitting tolerances, nor are they capable of
cutting through heavy rigid, or abrasive materials.
The use of the grooved back up roll with the web wrapped around the roll, provides for a cleaner, smoother and more accurate cut because the roll gives very close support to the web as it is being cut. It cannot stretch or shift under the blade as it can when slitting in air between two rolls. If the web runs tangential to the grooved roll rather than wrapping around it, the benefits of the grooves are lost. Set-up of razor blade slitting in air is faster, but also results in a loss of accuracy in slitting. The grooved roll requires more set-up time because the blade must be centered in the groove not touching either side or the bottom. Contact between the razor blade and the grooved roll quickly destroys the blade and tends to damage the groove. Figures II-1 and II-2 also show the most commonly used method of having the blade penetrate the web. However, recent tests have shown that much clearer cuts are obtained by using the method shown in Figure II-3. |
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| When hollow
ground blades are used there is less interference between the blade and the
film being cut.
At nominal running speeds, say through 1500 FPM, the blades also tend to burnish the film as it is being slit giving a polished edge. Over these speeds heat is generated. If enough heat is generated to start melting the film, a bead can be generated at the slit edge. This bead can be very detrimental to achieving good winding of the slit strips. The razor blade is the most economical of the knives used for slitting. Set-up is easily accomplished just by clamping a blade in place along a bar so that the blade penetrates through the web. Hence razor blade slitting can be recommended for quality and for fast set-up on any light, non-abrasive materials. |
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II.2 SHEAR SLITTING |
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| Shear cut
slitting uses two rotary knives to produce the same cutting effect as obtained
by using a pair of scissors. Scissors present a condition with two sharp blade
edges crossing at an angle as is shown in Figure II-4. To cut the material it
is necessary to apply force on the handle of the scissors. The cutting takes
place at the junction of the two sharp edges, and as the scissors close, the
angle of the junction gets smaller. As the angle changes, the cutting effect
changes. There is an ideal angle at which a material will cut. We have all
tried taking scissors, holding them open, and pushing them through a sheet of
paper. At a certain angle they cut cleanly and very easily. Change the angle
slightly, and the paper tears. This is exactly what happens with a pair of
rotary knives. If the web is running slower than the knives, effectively the
knife 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 the same as pushing the
material through the scissors.
This is why material can be effectively slit at a knife speed which is either slower than synchronous, synchronous with the web, or faster than synchronous. The angle of junction between the knife cutting edges is the critical factor. |
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| Changing our analysis from scissors to two rotary knives, there are two circles that overlap and form a junction angle (Figure II-5) similar to that formed by the scissors. Squeezing the blades together creates a reaction force which pushes the blades apart. This is the same force that is used to squeeze the handle of the scissors. When a large number of cuts are being made, these reaction forces can bend a knife shaft. This is a factor that must be considered in all slitting applications. No table of the shearing forces required for various materials is available. | |
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| At the
present, we are using a figure of approximately a pound, to a pound and a half
per cut, per mil of material thickness. Also uncertain, is the effect that the
relative speed between the material and the knives has on the shearing force.
We now feel that when the knives are running faster, the unit shearing force is
less, because less material is being cut per knife revolution than would be at
synchronous speed. Hence higher knife speed results in lower shearing forces.
There is some indication that faster speeds are better for foil. To date most
plastic films and paper are slit at synchronous speed with excellent results.
On most materials, problems exist at speeds below synchronous. These are
generalities that can be made about cutting effects and the relationship
between scissors and shear knives.
Next, consider the configuration of the web and the knives in the machine. There are two primary arrangements: "Wrap Slitting" and "Kiss Slitting". The preferred arrangement is "Wrap Slitting". Here the web actually wraps around the female knife and is positively supported while being cut as shown in Figure II-6. |
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| The arc of contact between the web and the female knife must be greater than the arc of contact between the male knife and the female knife. See Figure II-7. If the arc of web contact is smaller than the arc of knife contact, the male knife hits the unsupported section of the web. | |
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| The web is, therefore, torn rather than cut. This condition is illustrated in Figure II-8. Knives and spacers must be assembled on the shaft to form or to simulate a solid supporting shaft. Unless the web is supported by a shaft solidly stacked with knives and spacers, the web will pull or sag into the voids and irregular slit widths will result. | |
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| The second arrangement is tangential or kiss slitting. In this arrangement the web only kisses or touches the outside diameter of the female knife, it doesn't wrap around it. See Figure II-9. If the web hits the male knife before it hits the female, a cutting angle is not achieved. This is the same as taking scissors and trying to cut with only one blade. The web is ruptured, not cut. | |
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| Conversely,
if the web hits the female knife first, nothing happens, but, when it leaves
the female and hits the male, again the web is ruptured. A clean cut is not
achieved. It is essential in kiss slitting that the female knife, the web and
the male knife contact each other at the critical point. This critical point
will change as the male knife overlap is increased; therefore, you cannot
increase the overlap beyond an ideal condition without creating problems. This
ideal condition is minimum overlap and point contact. Increasing overlap
increases the arc of contact between the two knives and the male knife hits the
web before the female.
Using a male knife of a smaller diameter will give a greater overlap before reaching the critical point. See Figure II-10. The large diameter male knife has a larger arc of contact for the same overlap. Therefore, the smaller the diameter of the male knife, the greater the permissible amount of overlap. The greater the diameter of the male knife, the smaller the permissible overlap. The large radius will give a bigger arc as you increase the overlap. To this degree, the individual knives, permitting fine adjustment of each individual knife, give better control than multiple knives mounted on a single shaft where additive tolerances must be overcome by increasing the overlap. |
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| The advantage of kiss slitting is fast setup of individual female knives. These knives can be repositioned on the shaft without removing the entire knife shaft. Wrap slitting requires the removal of the shaft from the machine, taking off the knives and spacers and then putting them back in rearranged order. In kiss slitting, the knife is loosened, shifted to the next cutting position, and tightened again. It can be accomplished in a minimum of time in the machine without removing any parts from the machine. The disadvantage lies in the quality of cut achieved. The cut is not as good as that obtained by wrap slitting. In wrap slitting, the web wraps around the female knife. The male knife penetrates to the ideal depth and gives a good clean cut. In kiss slitting, the web comes in tangential to the female knife. When the web and the male knife come in contact at this same point the cut is clean. To achieve this geometrical arrangement the male knife must be offset toward the exiting side of the web. Looking at Figure II-11, you will notice that a good portion of the male knife now is underneath the web that has been cut. As the knife rotates out of the web, the web rubs both on the cutting side and the backside of the male knife, creating a frictional drag and filing the material that passes through. This is a decided disadvantage because it causes fuzz and dust. | |
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| Looking at Figure II-12 it is seen that when shear slitting a web, on the cutting side the male knife comes in and cuts clean and square. On the other side, the knife has pushed the web aside and down 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 deformation on the edge, and as it winds up layer after layer, builds a bevel on the edge of the rewound roll which eventually causes the sheet to tear. | |
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| In foil,
this deformation occurs with both kiss and wrap slitting. There is a second
problem that arises in kiss slitting. When the web comes straight out, there is
a greater interference angle between the web and the male knife. As the male
knife rotates out of the web, it has a rough grind on the back edge; it will
tend to file the edge of the slit strip. The result on a hard material is
filing action which causes little particles to fall onto the edge of the slit
strip. These particles will create a buildup similar to that mentioned above
with identical results. Kiss slitting is good for fast setups. The quality of
the cut is not as good as in wrap slitting because of the greater interference
between the male knife and the exiting web.
In wrap shear slitting, accurate slit widths and close tolerances are possible through the use of spacers for measuring and positioning the knives. This combination of knives and spacers, stacked on the shaft, provide a complete support for the web, thereby reducing, to a minimum, the possibility of wrinkling, bagging or sagging. This results in excellent slit width accuracy. In kiss slitting because there are voids between the knives, permitting the web to sag and wrinkle, slit width accuracy is more difficult to achieve. In conclusion, we can say that shear cut slitting offers many advantages and can be used in several different ways. Careful consideration must be given to results that are desired or required in selecting the best arrangement for each application. It is standard practice to incorporate driven rolls in all types of machines to maintain control of the web as it passes through the knives and to isolate the slitting tension from the unwind and rewind tensions. All driven rolls also serve to keep a constant tension in the slitting area, thereby maintaining the slit width. A slight overdrive in web speed is maintained from driven roll to driven roll to hold tension constant. For speeds in excess of 500 FPM and film thickness below .001, it is necessary to positively drive all rolls since changes in web speed, due to undriven roll inertias, tend to distort the web and affect the quality of the rewound rolls. This is especially true when narrow webs of light plastic films are run on wide machines (i.e. webs less than 70 per cent of the maximum web width capacity of the machine).
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| II.3 SCORE CUT SLITTING | |
| Score Cut,
also known as Crush Cut, slitting is the oldest form of cutting known to man,
and the first employed in slitting machinery. As the name implies, it is
crushing of the material to sever one part from another.
Since this method of slitting relies on the crushing of the material, it follows that the "knife" cannot be dead or razor sharp, because a very narrow knife edge will dull and chip almost immediately upon contact with the surface it is 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 slit with it. Figure II-13a shows a fully sharp knife. Figure II-13b shows the proper edge configuration of a good score cut knife. Figure II-13c is an enlarged view of the cutting edge of the good score cut knife. Figure II-14 shows these respective knives penetrating through a material and against the "bed" or backup cutting surface. This illustrates our original premise that pressure on the 'Dead Sharp" knife will dull this knife almost immediately, whereas the rounded edge of the other knife will not break down as easily. |
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| When
slitting a continuous web, it is obvious that a round backup or platen roll and
round slitting knife must be used. With the knife pressed against the web with
sufficient pressure to cut through it, we achieve "slitting". For best results,
the web, the platen roll and the knife must all be moving at the same speed.
Since this method requires enough pressure to cause the web of material being slit 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 of steel 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 but the cutting edge is radiused and then burnished to a very high polish. The smoother the platen roll and the knife edge, the cleaner the cut and the longer the life of the blade and roll. The oldest method, score cut slitting is very quick to set up, thus offering a very distinct advantage. It is, however, losing favor in many areas. As explanation, let's examine this cutting method first on paper. When we crush the fibers of the paper web, a very fine powder of crushed paper fibers is created. If the knife is "sharp", that is smooth and highly polished, the slit edge of the paper will appear smooth, but there will also be a deposit of paper dust. The amount of paper that the knife displaces, as shown in Figure II-15, turns to dust. Years ago, this dust was quite acceptable, but in today's high speed printing presses and automatic packaging machines, dust accumulates faster and causes equipment malfunctions. For this reason, score cut slitting is becoming less acceptable. |
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| In addition
to "dust", score cut slitting paper may be undesirable in another way. As soon
as the edge of the knife becomes slightly rough, the slit edge is no longer
smooth. Instead, it has slight feathers or threads where the paper has not been
completely pulverized. These feathers or threads tend to accumulate on the edge
of a slit roll making it feel and look very rough. The normal tendency when
this occurs is to increase the slitting pressure on the knives. This is usually
successful, but only for a short time. The increased pressure causes further
deterioration of the knife resulting in a still poorer slit edge.
Applying score cut slitting to plastic films and sheet, we have similar situations. On extensible, soft plastic films like polyethylene, a good smooth knife and platen roll yield excellent results. However, as soon as any defect appears in either surface, this defect creates a void and the film is squeezed into the void and is not separated. At first, these strings or ties tear to give separation but they soon reach a point where adjacent sections of film strech and deform causing rejects.(Figure II-16 shows a good score cut slitting operation). On hard plastic films such cellophane, acetate and oriented polystyrene, the result is somewhat different. As the knife crushes its way through the material, it not only severs the web as desired but causes minute cracks perpendicular to the slit web. As shown in Figure II-17 these cracks when put under slight strain tend to propagate further into the web. With wide webs, this is not too critical. On narrow webs, the only too frequent result is a tear completely across the web. |
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| This brings
us to the area where score cut slitting really shines: the slitting of paper
pressure sensitive tapes. The nature of the material is ideally suited to this
slitting method. Figure II-18 shows a paper coated with pressure sensitive
adhesive being slit into narrow tape. You will notice in this illustration the
desirable separation of the paper web. In addition to this, you can see the
adhesive is forced away from the edge of the paper. Also, the adhesive forms a
small ridge on each edge.
These conditions give us a perfect result. When we wind up the strip, we have a roll that has a "dry" non-sticky edge. This is seen in Figure II-19 where the adhesive is away from the edge of the roll. Score cut slitting is ideal where quick and easy set-up is required. It can be used to some degree of satisfaction on most materials being slit today. However, before making a final decision on score, shear or razor blade slitting, a careful analysis of the desired results should be made. |
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