Optimizing the die-cutting process

These elements are indicated in the diagram, Figure 3.1. The correct setting and adjustment for all these elements of the cutting unit, both on their own and in relation to each other, are ideally required for the optimum results.  Things can still go wrong, though, and not provide maximum cutting performance.

In order to make an accurate and successful cutting separation of the converted material during its process through the die cutting unit, there is a range of parameters that need to be considered as influencing the process. Some of the most important of these parameters are examined here, namely:

Fortunately, there are test methods and procedures that can be used to assess the performance of many of these parameters. The necessary steps can then be taken to eliminate or minimize their effects.

One simple test procedure is the use of an ink stain on the liner material using a broad tipped permanent marker. A full cylinder repeat of the liner material is typically ink tested.

For automatic label dispensing it should be possible to see an outline of the shape in the front or back of the release liner – depicted as a light liner impression or liner strike (Figure 3.3). However, no fluid or ink should be absorbed by the paper fibers themselves. This can be observed by viewing the liner from the back side.

Labels that are semi-automatic or hand applied can have some slightly visible ink penetration when visually inspecting the liner.

Official test procedures have also been drawn up by some of the main label industry associations. In particular, the following Finat Test Methods are available:

• Finat Test Method FTM 23a Die strike for paper

This test allows the converter to assess the degree and consistency of the die strike and die cutting during the conversion process. The method can be used during press makeready to assess the condition and settings of cutters, to prevent label dispensing failures or web breaks during high speed dispensing. The test is applicable to paper based liners

• Finat Test Method FTM 23b. Die strike on clear filmic liners

This test is used for the evaluation of backing damage or marking to the liner that may be caused due to kiss cutting via a die.

Using the basis of these test methods and recommendations provided by cutter and tooling manufactures enables guidelines to be established for the key parameters. These are outlined below.

WEB TENSION

Web tension issues are most commonly found when filmic materials are being converted. The filmic materials, for either mono-web or laminate structures, will exhibit elastic behaviours under tension.  Increasing tension will eventually reach a point where these films will deform irreversibly and can even cause them to break. Web tensions in pressure-sensitive filmic labeling however, will not generally get to this point. If the liner can be tensioned successfully without breaking, the issue of die strike should not be a problem. The evaluation of any backing damage or marking to the liner due to die strike can be assessed using Finat Test Method FTM23b: Kiss cutting of filmic liner.

It should be noted that web tension alone in reel fed printing and converting does not mean anything on its own unless this is in relation to the elasticity of the material being processed. For a specific ‘pull’ on a web, both the width and the calliper (thickness) of the web material has an influence on how elastic and ‘stretched’ the material may become.

For example, if the web width is doubled for a given brake force, the relative web tension will be half. The same goes in theory for the thickness of the material if this is uniform. However, this is not necessarily the case for pressure sensitive material since it consists of at least three separate layers: release liner, adhesive and face material.

In effect, the way web tension has an influence on a specific web material is that when it is being stretched due to the brake force, both the width and the thickness of the material will theoretically decrease, since the volume is constant. This behaviour is rather like how a rubber band acts under tension, only the web material is much less elastic than the rubber band.

Since die-cutting is usually performed on a web under tension, the cut-out label will shrink slightly in length and grow in width when the material is in a relaxed state again. By how much depends on the elasticity of the material being converted and the tension of the material during the die-cutting process.

SPEED

Web speed can have an impact on the die-cutting performance, particularly if the die cutting force is not uniform in the longitudinal direction.

For example, in a web with a number of square labels and a small gap around each label, the cross-cutting section of the die requires a sudden drastic increase in cutting pressure or force (see also under Pressure/Force and under Image Configuration). This phenomenon is commonly manifesting itself as bounce and is caused by the impact energy, where speed is playing a major role.

MATERIAL/SUBSTRATE BEING DIE-CUT

As previously discussed, pressure-sensitive materials are a laminate construction consisting of at least three layers: the face material, adhesive and the backing or release liner. If a die should cut into the liner excessively this could reduce the tensile strength of the web sufficiently to cause a web break. As such, damage in the transverse direction (TD) across the web can be more of an issue than in the machine.

Figure 3.5 - The die-cutter needs to cut through the label face material and adhesive, but not the silicone coating of the backing liner direction (MD) along the web. To produce the correct tooling, the die manufacturer needs to know what type of material is going to be converted, in particular, the thickness or caliper of the release liner and the type of liner being used, e.g. glassine, kraft, PET, PP.     

During the ‘kiss cutting’ process, the cutter edge needs to cut through the face material and adhesive onto the liner, but without damaging the actual silicone layer of the liner (Figure 3.5). This means that the tooling supplier will have to determine how deep the cut must be in order to cut through the specific material being converted, but not cut into and damage the silicone coated liner (die-strike). 

Die-strike through filmic liners can be influenced by several parameters. It is recommended that the following parameters are carefully checked if die-strike problems occur:

• the solidity and robustness of the die-cutting unit

• the tool diameter should be adjusted according to the width of the printing equipment

• the tolerance between magnetic and anvil cylinder

• the settings of the flexible die (wear-ness - height profile of the die)

• the consistency of the liner thickness

• the temperature at which the die-cutting operation take place (influence of UV light on the film and adhesive softness).

To eliminate or minimize these problems it may also be feasible to adjust the cutting angle to improve the die-cutting operation, verify the tension of the web (avoid too high a tension), and strip the matrix immediately after the die-cutting operation to avoid recovery of the adhesive between the die-cutting and the stripping steps. Consider cooling down the laminate before the die-cutting operation.

Depending on the cutting angle, the sharpness of the cutting edge, and the specific material properties of the face material, a certain force will be acting on the release liner at the precise moment of cutting. The release liner is compressible like any other material and will compress from the force applied by the cutting tool. How much the release liner will compress depends on the thickness and the properties of the liner.

FRICTION/TEMPERATURE

It is not always realized that increasing or fluctuating temperature can have an effect on die cutting tools and more specifically on the temperature of the contact points between the anvil cylinder and die-cutting cylinder. Temperature should therefore be regularly monitored.

The force applied to the cylinders in order to keep them together during a die-cutting process leads to both friction and compression. This takes place in the contact points between the anvil and the cutting cylinder when the cylinders are rotating.

In most cases, the force from the lead screws is transmitted via the bearers of the cutting cylinders.  Because of the compression that is caused by the force holding the cylinders together and the friction that is caused by the rotation of the cylinders under load, heat will build up in these contact points.

How much heat is generated depends on a variety of factors such as the load on the cylinders, the width of the bearers, the diameter of the cylinders, how fast they are rotating and how well they are lubricated. If the build-up of heat cannot escape as quickly as it is generated, then the temperature will increase on the bearers of the cutting cylinder and locally on the anvil cylinder.

Any heating-up of the bearers of the cutting cylinder will ultimately cause the bearers to grow in size due to heat expansion. As long as the die-cutting frame support is capable of giving sufficient resistance against the increase in force, the increase in diameter of the bearers will result in an increase in force between the cutting cylinder and anvil roller. Over-pressuring the cutting die can increase the wear to the blades and shorten the die life. This heat and friction can cause premature wear on the bearers of the cutting die as well. It may eventually cause problems with maintaining the correct gap between the two cylinders.

For this reason, it is important not to apply more force than necessary through the lead screws and also to keep the contact area between the anvil cylinder and the cutting cylinder clean and lubricated, otherwise problems such as accelerated wear of bearer rings, press rollers and anvil roller, or even losing control over the die-cutting process, may occur. To avoid such issues, the pressure should be reduced down to the necessary level at the right time so as to prevent excessive heat build-up and cutting into the liner.

SOLIDITY AND STABILITY OF THE CUTTING UNIT

One of the key parameters in the die-cutting process is the actual die-cutting unit itself (see Figure 3.6), which supplies the required frame for suspension and support of the cylinders during the cutting process and to also provide the necessary stability and resistance to the cutting forces. In essence the cutting unit is responsible for supplying sufficient resistance and stability against all the internal forces created as a result of the cutting process.

The station frame and side panels of a cutting unit are there to keep all components involved in the die-cutting process in their specified positions. They should be able to absorb all occurring forces safely and vibration-free. Both the side panels and the (pressure) bridge should be adequately dimensioned, as otherwise this could lead to instability in the cutting unit and contribute to potential cutting problems.

Optimized cutting systems feature a force transfer from the bridge through pressure adjustment jacks or gauges with sturdily dimensioned threaded rods.  Pressure is transferred from the intermediary pressure truck via the fitted pairs of roller bearings to the bearers of the cutting die or magnetic cylinder.

The pre-load pressure attained when setting up and the running pressure must be greater than the material resistance when cutting across the web.

Therefore, the level of pre-load will depend on the material being cut, the working width of the machine, the label shape and the design features of the cutting lines.

The cutting cylinder (a solid or flexible tool on a magnetic base) is pressed onto the anvil cylinder by pneumatically or mechanically operated pressure systems. Pneumatic force should be used only to control an adjuster cam or toggle lever. It should never be exerted on the press unit, magnetic cylinder and anvil roller, as otherwise vibration may occur.

Mechanically operated systems are divided into systems which are able to apply pressure to the shaft of a magnetic cylinder or die, which may lead in extreme cases to bending or flexing of the magnetic cylinder (Figure 3.7) and in extreme cases to cylinder shaft breakage. Flexing may also occur when using small diameter dies.

Commonly, presses today apply pressure to bearers on the die. Force transmission is via the pressure bridge.

There are also systems which operate without bearer rings, which means no pre-load is permitted between the cutting die and the anvil roller. The magnetic cylinder or anvil roller is mounted in conically adjustable bearings or is pressed against the resistance by springs. These systems strongly depend on the perfect condition of the bearings that must not have any play, and an adequate dimensioning of the shafts. Such a system is not recommended for the converting of pressure sensitive labels.

Anvil rollers also play an important role in the die-cutting process. Anvil rollers that are too small in diameter and are mounted in the machine side panels without benefit of a support roller underneath pose a risk of deflecting due to the necessary pressing force exerted by the cutting cylinder. See Figure 3.8.

The consequence is less pronounced cutting towards the middle of the web.

As was earlier seen in Figure 3.6, modern die-cutting units are likely to contain a built under or support roller which is positioned under the anvil roller. This roller allows the perpendicular transmission of force from the pressure bridge to the support roller with the aim of preventing bending of the anvil roller due to pre-loading of the magnetic cylinder.

Inadequately dimensioned (too small diameter) anvil rollers may possibly still deflect despite the extra support from the support rollers especially at the point of cutting a horizontal line.

PRESSURE/FORCE

This refers to the force applied through the two lead screws mounted in the die-cutting unit for both semi- and full rotary die-cutting units.

In flatbed die cutting, the force is not usually something that can be altered. Only the stroke length is adjustable in order to accommodate for the desired cutting depth.

The force required to perform the die-cutting process depends on many factors, some of which have already been discussed, such as the substrate, the web running speed, anvil size and the various elements of the die-cutting unit. Other factors are:

• the geometry and sharpness of the cutting edges on the cutting tool

• the weight of the cylinders and other component parts if stacked on top of each other in gravitational direction

• the bearer width

• the cutting length in contact with the web

• the cylinder diameter

All of the factors mentioned have an influence on the necessary force that will be required to perform the actual die-cutting. Wear of the cutting tool will over time dull the cutting edges, in turn increasing the required cutting force as well.

The pressure required for die-cutting depends upon the amount of blade that penetrates the material in the same instant. The pressure serves to prevent the die from bouncing, by keeping the bearers in constant contact with the anvil roll surface. When the pressure is inadequate the die will bounce. When the pressure is excessive, it will accelerate the wear of the die, the anvil roll and other components of the station.

When the pressure, required to cut cleanly, exceeds the recommended pressure by 400 lbs. It is recommended the die be retooled. Don’t wait until the die stops cutting before sending it to be re-sharpened. This practice results in costly emergencies and can reduce the number of re-sharps that can be achieved.

The force applied through the two lead screws needs to exceed the maximum cutting force required by the cutting pattern, where maximum die line is in contact with the web on a rotation of the die-cutting tool. Two close lines across the web direction will require a much higher cutting force than a single line in the web direction because of both the sudden interference with the web and the actual total length of the cutting line in contact with the web. This will be discussed further when looking at die-cutting image patterns.

Certainly there can be big fluctuations in the required cutting force during a revolution of a cutting tool, again depending on factors such as the cutting pattern. However, it should be remembered that any internal bending of cylinders cannot be prevented by simply increasing the force holding the die-cutting and anvil cylinders together. The force applied through the lead screws must only be applied in order to counter for the maximum force required for the cutting edge(s) to penetrate the substrate.

IMAGE CONFIGURATION

In the die-cutting process there are die line/cutter layouts that are deemed to be more preferable than others. Certainly, the most demanding cut in any rotary die-cutting operation is always going to be that of cross cutting, as shown in Figure 3.9. 

In most cases, labels are arranged symmetrically to save space and keep the consumption of materials to a minimum. However, this configuration is less beneficial for the rotary die-cutting process because vertical lines tend to cut more forcefully than horizontal lines and high contact pressure damage the liner material and result in wear and lasting damage to all components in the cutting unit. 

Poor image configuration may lead to insufficient separation of the web material in the areas where the higher cutting force is required. If the combined rigidity of the framework of the die-cutting unit and the rigidity of the anvil and die cutting cylinder is not sufficient to deliver the resistance against this sudden increase in cutting force, the elements will deflect away from the resulted force and thus momentarily increase the distance between the anvil cylinder and the die-cutting cylinder, causing a potential flaw in separation of the processed web material. As discussed, Figure 3.9 shows a wide line getting in contact with the material very suddenly.

Figure 3.9 - Cross cutting across a label web

A way to counteract this negative effect is by vertically shifting or staggering both the impressions and cutting in the machine's running direction, which reduces the number of horizontal lines and evenly distributes cutting pressure. See Figure 3.10. This possibility is available with, say, Esko’s ‘staggered cut’ software.Staggered cutting will reduce the pressure required, help to improve waste stripping and prevent die and anvil deflection. If in-mold material, folding cartons, or tag production is being carried out, then staggered cutting will facilitate stacking and more uniform cutting.

This type of solution is particularly advantageous when cutting shapes with many horizontal lines. It optimizes die-cutting results and reduces liner damage. The dies last longer as well as various other machine parts. The waste stripping is stabilized preventing web breaks and reducing down time.

It is usually not a problem applying sufficient force through the two lead screws, but the internal deflection of the anvil roller and the cutting cylinder may exceed the desired maximum variation in gap due to the peak in force generated by a wide cross cut.

The force necessary to cut a line across the web is proportional to the width of the line being cut. The deflection of both the anvil and the cutting cylinder will depend on both diameter and length of each cylinder. Since both diameter and length are affecting the maximum deflection to the power of four, it is important that the diameter/length ratio is sufficient to withstand the force from the die cutting process.

CUTTING ANGLE/DIE HEIGHT/CLEARANCE

The geometry of a cutter will vary depending on the material being converted. Based on the construction and thickness of the material being cut , adhesive, liner thickness and type of liner . The three most important die parameters, cutting angle, die height and clearance (drop), can be determined for flexible dies. For rotary dies, both the cutting angle and the ´drop´ (distance between tip of cutting line and bearer) are the most important parameters.

The Die Clearance or drop is the difference between the height of the cutting blades and the height of the bearers. It sets the depth of the cut. This is shown in Figure 3.11. To determine the proper clearance, the exact thickness or caliper of the liner material is required.

In terms of the cutting angle, this will normally vary between 50° - 75°. The height of the cutting line is measured from inside the pocket to the tip of the cutting edge and will normally vary between 0.38mm – 0.80mm (0.015” – 0.030”) for flexible dies and will be 1mm (0.039”) or more for solid rotary dies and rule dies. In general, thicker materials require a sharper cutting angle just for the fact that the cutting edge has to travel further through the material as well as for plastic materials such as PE, PET, PP etc.

A narrow cutting angle will, in general, require less force to penetrate the substrate simply because less area is being pushed through the material. As mentioned earlier, less force also has a positive impact on the deflection of the cylinders, which in turn means the potential for a more even die strike.

The harder types of material, like most plastic filmic materials, generally tend to require a higher cutting force than softer materials like paper and therefore it makes sense to try and compensate for this by making the cutting edge sharper (narrowing the cutting angle). See Figure 3.12. On the downside, a narrow cutting angle also tends to wear out more easily simply because the cutting load is being exposed to a smaller surface area.

The profile of the cutting blade should be fine-tuned to compensate for differences in the thicknesses and elasticity of the substrate being cut. Elastic materials such as polyethylene will require a steeper blade profile with a keener edge than, say, a more rigid material such as paper. Steeper, keener blades also have a lower resistance to wear than thicker blades. This is the reason why there is no such thing as a multi-substrate die.

Using film dies to cut paper may diminish the die life for cutting film. Wear to the blades may not leave the blades with the edge required to cut the film they were ordered for. This could create a costly emergency in production that would erase any cost savings achieved by using one die for several materials.

As indicated in Figure 3.12, softer materials like the different paper substrates will normally benefit from a wider cutting angle because the material is easier to die-cut and the wider cutting angle will generally expand the life of the cutting edge.

The cutting angle is determined mostly by the properties of the face material to be cut. As explained earlier, face materials with high tensile strength like most plastic materials are usually cut with sharper cutting angles in order to decrease the cutting force and thereby also the compression of the release liner during the cutting process. Thicker types of materials often also require sharper cutting angles in order to accommodate for sufficient space inside the die cavity during the cutting process.

For environmental, economic and technological reasons, label stock suppliers have significantly changed the specifications and use of their release liners, such as:

• moving from kraft to glassine liner

• reduced caliper glassine liner

• trending towards thin PET liner such as 36 micron to 19 micron liners (.0015” - .00075”) or even thinner 

The use of thinner liners will reduce the tolerance in variation for cutting force simply because there is much less compression to work within. Thinner liners require narrower tolerances in tool making in order to get consistent performance and maximum life from the cutting tool.

When looking at the overall thickness of pressure-sensitive materials, the tooling manufacturer needs to determine if the inside pocket clearance (the blade height) is sufficient. For thick materials, the cutting angle might also be affected by this as thicker materials may require sharper cutting in order to preserve the label edge.

MAGNETIC CYLINDER DIFFERENCE

The difference (diff) or undercut of a magnetic cylinder is the difference between the diameters of the bearers and the diameter of the body of the cylinder.

This measurement is crucial to calculate the proper flexible die height.

The most commonly used difference is 0.038” /0.965 mm. This would be a clearance on each side (also called an ‘air gap’) of .019” / 0.48 mm (Figure 3.13). This size however can vary. Cylinder manufacturers can adjust the diff based on the requirements of the converter. A larger ‘diff’ may be used when a taller plate height is required (such as for thicker materials). If the flexible die manufacturer did not manufacture the magnetic cylinder though, determining this diff is a necessary part of manufacturing the flexible die. The converter can supply this information as provided to them by their magnetic cylinder supplier. It can be measured also.  

Guessing at the diff though can risk possible errors and reordering of the flexible dies.

A number of magnetic cylinder suppliers, such as RotoMetrics, keep a database of the differences for all of the magnetic cylinders they manufacture. Each flexible die is custom manufactured to fit a specific cylinder for the most precise cutting results.

The bearers of the magnetic cylinder will wear over time and should be measured every three or four years to determine how much the diameters have changed. In many cases, the magnetic cylinders can be ground to re-establish the original ‘diff’.

CORNER SHAPES/RADII

A square cornered label or waste matrix does not release as easily from a liner as a rounded corner will.  A corner radius is used to make it easier for automatic label application as well. It can adhere better to the final product. The term ‘corner radius’ itself refers to the radius of the circle created by extending the corner arc to form a complete circle. Figure 3.14 shows a corner radius guide.

Square corners may increase the cost of the die and reduce productivity and die life. It is recommended that a corner radius of at least 3 mm is provided. The minimum recommended radius is 1 mm (.0394”). There may be a supplementary charge per corner for radii less than 0.793mm (.0312”) due to the added difficulty of manufacturing them.

CUTTING DEPTH FOR LABELS AUTOMATICALLY APPLIED

For automatically applied labels, the cutting blade should burst the face stock and adhesive without penetrating through the silicon coating on the liner.

However, die life can be shorter because the die may stop cutting after minimal wear. As mentioned previously, this bursting process is difficult to achieve when cutting very elastic synthetic face stocks or when cutting to a soft thick liner. If the die cuts too deep, though, it can cause the liner to break in the label applicator or the de-lamination of the silicon layer along with the label.

For hand applied labels the blade should burst through the face stock, adhesive and slightly penetrate the liner.

In all cases, always make sure to tell the die supplier how the label will be applied!

CYLINDER DEFLECTION

Mention has already been made in preceding pages to the deflection of die-cutting cylinders or anvil cylinder under pressure and in certain conditions.

Such deflection can critically affect die-cutting results. It is not technically possible to apply pressure across a cylinder suspended on two outer points or bearers without the cylinder bending or deflecting to some degree under the pressure load. And as previously discussed, the internal deflection of the cutting cylinders can be critical to the cutting result. Fortunately, it is possible to keep the deflection to a working tolerance if various issues can be addressed as follows:

The combined deflection between both the anvil and the cutting cylinder is what determines an increasing deflection gap. In other words, deflection is not just a phenomenon attributed to the cutting cylinder alone but additionally has a component coming from the anvil roller. During the rotation of the cutting cylinder, the cutting pressure will vary with the amount of cutting edge engaged in cutting. To overcome this, the pressure applied to keep the die-cutting cylinder and the anvil roller together during the cutting operation must exceed the peak in pressure caused by the maximum cutting edge engaged. If not, the two cylinders will make a relative move away from each other.

However, deflection is not the same all across the full width of the cylinders. Since the applied pressure is transferred through the bearers of the two cylinders there will be no deflection (lift) in these contact points. The maximum deflection will occur along the cylinder widths where the distance from the contact points is the greatest for an even distributed cutting force - typically in the middle of the cylinder’s body width. See figure 3.15. 

Factors which will impact on the amount of deflection in the cutting and/or anvil cylinders are the distance between the bearers, the diameter of the body, the cutting force, and the type of steel used for producing the cylinders.

With the maximum deflection experienced at the center of the cylinder, any deflection will cause a variation in the gap size over the whole width of the cylinder. If any variation in gap exceeds the tolerance for cutting the material, insufficient or poor separation will be the result. (see Figure 3.15)

Since the lowest cutting height inclusive tool wear and deflection will determine when a cutting tool is no longer useful, the D/L (Diameter/Length) ratio can have a dramatic influence on the cutting tool life. For this reason, cutting tools potentially last longer on bigger diameter cylinders than smaller especially if compared to D/L ratios in the critical range.

CYLINDER CONCENTRICITY

Concentricity can be said to occur when two or more objects share the same center or axis. For example, all the features shown below can be said to be concentric.

Usually, designs require that a feature be round as well as concentric like example A below (Figure 3.16).

A better geometric control is usually circular run-out. Circular run-out controls circularity (roundness) as well as concentricity.

Accuracy in tool making has improved significantly over the years. The range in which a cutting tool will actually cut a material is only a fraction of the thickness of the material. This leaves the tool maker only a very small tolerance to manufacture the cutting tool.

Any impurity trapped between the surface of a magnetic cylinder and the backside of the flexible cutting tool will influence the cutting performance, any impurity between the bearers of the cutting cylinder and the anvil cylinder will have an influence on the cutting performance, deflection of cylinders will decrease the working range for the cutting tool.

Besides supplying sufficient resistance against the cutting forces, the cutting unit is also responsible for the critical aligning of the axes between the cutting tool and the anvil cylinder. If those two axes are not parallel, a heavier cutting impression will be dominant in certain areas of the width of the cutting. This tendency is more critical for cylinders with a smaller D/L ratio (see deflection).

Imagine two cylinders on top of each other perfectly parallel. The contact between these two cylinders will be a line with no possibility to make any rocking motion. The same two cylinders with non-parallel axes, will have a single point of contact with the possibility of rocking. See Figure 3.17.

STRIPPING ROLL

The matrix stripping roll should be available in various diameters and facilitate easy removal of matrix waste. Larger diameter stripping rollers are suitable for stripping materials that can easily tear, such as paper. Smaller diameter stripping rollers are more suitable for converting plastic materials and small labels.

The speed of a machine can usually be increased using blunt stripping knives instead of stripping rolls. Maximum flexibility is needed with regard to the positioning of the stripping rolls in order to facilitate the waste stripping.

The matrix must be guided by way of a closely controllable pull roller to the rewind spindle or to the extraction hopper. The correct web tension is a key aspect of the matrix separation process.

SLIP

This is the relative movement of an object or surface (web) and a solid surface (rollers/cylinders). In web handling this refers to rollers/cylinders losing traction if conditions are not within tolerance. This phenomenon may have an impact on the control of the web-tension during the production process and can even lead to problems with the measurements of a finished product.