Indeed, precision cutting, foiling, and embossing of labels and packaging can be considered as virtually impossible today without the use of pressure jacks (shown in Figure 8.1) or regulator gauges showing the amount of contact pressure being applied. Ideally, the aim should always be to apply the minimum cutting and pressure force so as to preserve tool life and bearings as much as possible.
Figure 8.1 - A die-cutting or foiling unit with pressure measuring gauges/jacks
Although experienced operators will undoubtedly have a good feel for how much torque/pressure is required for any specific tool to operate correctly and accurately, and exact measurement using pressure gauges will be the most consistent method for set up and longest tooling life possible. The printer can die-cut with optimum preload avoiding uneven cutting results. Documenting this pressure history can improve set up time for the next use of the tool as well as help to predict the life expectancy of the tooling.
Figure 8.2 - Shows the clearance between the magnetic cylinder and the anvil roller which can be measured with a feeler gauge
For flexible die plates, the gap measurement between the base of the tool (surface at the magnets) is a critical specification for the manufacturing of the flexible die. This gap, also called ‘air gap’ or ‘clearance’ can be measured using a feeler gauge. This easy-to-use, precision tapered, measuring instrument is very accurate. Using a feeler gauge is quite easy; firstly, clean the magnetic cylinder and anvil roller carefully. Next, insert the magnetic cylinder into the cutting unit and apply standard pressure onto the magnetic cylinder. Finally, use the feeler gauge to measure the gap between anvil roller and magnetic cylinder to the left, in the center, and to the right to prove its parallelism. Then repeat the action after each third-revolution of the magnetic cylinder. These new measuring points not only indicate the real/actual slit dimension but also give information about the running accuracy and provide an accurate picture of the parallelism and gap between the magnetic cylinder and the anvil roller.
If the feeler gauge is used at regular intervals it will provide the operator with vital information about the condition of the die-cutting unit, pre-empting potential die-cutting problems. Using the information, a die can be made to suite the gap and type/thickness of the carrier.
However, when using rotary die-cutting systems, the basic requirement for perfect cutting is sufficient pre-tension between the magnetic/rotary cylinder and the anvil. Excessive or poor pre-tension may often lead to insufficient cutting results as well as unnecessary wear and tear with the printing machine. Different printing widths as well as different printing materials also require individual cutting pressures. In these cases more sophisticated pressure gauge systems, regulators and monitoring devices may be used. Some of the most commonly used gauges and devices are described below.
Pressure adjusting and load cell monitoring equipment today . depending on the degree of sophistication . can enable press operators to set, monitor, adjust and detect or improve some or all of the following functions:
Set a certain pressure from the start
Ensure that the same and an even pressure is applied on both sides of the cutting unit (operating and drive side)
Monitor the force required for rotary die-cutting or hot stamping
Reduce the applied pressure to the minimum as soon as the machine reaches its optimum operational temperature
Predict and prevent die failure
Detect if cutting/foiling unit components (e.g. bearings) are failing
Improve product quality and increase die life
The compacting of load cells to allow easy installation
Some pressure gauge systems may also enable data to be downloaded, while other can include an alarm output with an adjustable setting point.
When assessing the technological aspects of pressure gauges, there are two key ways of monitoring the force or pressure being applied. These two methods are:
Conventional hydraulic pressure gauges
Electronic pressure gauges
Essentially, both types of gauge do the same job.
HYDRAULIC PRESSURE GAUGES/JACKS
The simplest types of hydraulic pressure gauge systems enable machine operators to easily monitor and regulate the force being applied to cutting and other dies using easy to read gauges. They are therefore an ideal tool with which to help decrease downtime and extend the working life of cylinders, anvils and dies by reducing the amount of wear and damage caused by the application of excess pressure.
Pressure gauge systems may be already built into a new press, or they can be retrofitted to the majority of die-cutting, foiling or embossing stations at a later stage.
In use, the hydraulic indicator shows the internal pressure of the hydraulic fluid for a given load to the lead screw, thereby detecting the force being directly applied from the load to the die.
Figure 8.3 - RotoMetrics hydraulic pressure gauge system
In the example shown in Figure 8.3 the load cells and gauges are part of the screws, which eliminates the need for separate wiring or mechanical connection.
They only require simple retrofitting of the locking device to the existing bridge in most cases. Screws can move freely up and down until the cam or clip lock is engaged, then pressure can be adjusted and monitored. Easy to read clock dials or gauges, calibrated in pounds of force are certified to national standards.
Figure 8.4 - Universal pressure gauge. Source- Kocher+Beck
Another similar example of a hydraulic pressure gauge that replaces the exiting screw can be seen in Figure 8.4. This universal pressure gauge system is suitable for all rotary presses. The setting unit is mounted on a special, hardened threaded spindle, which is used in place of the original spindle the same as the one previously mentioned in Figure 8.3. It does not have to be laboriously screwed in and out as it is held in place by an ingenious rapid clamping system.
Yet another style, shown in the load cells illustration in Figure 8.5, is universal and not specific to a press or unit. These are placed onto the pressure loading truck – they are a light-duty version. Also they may not be as easy to read as the gauges are not on top of the bridge as the others mentioned.
For all of these styles, pressure is applied straight to the cutting tool. Cutting pressure can be adjusted with the minimum of effort by means of a hand-wheel.
Figure 8.5 - Load cells (shown on truck). Source- RotomMetrics
ELECTRONIC PRESSURE GAUGES
Electronic pressure gauges come in a variety of formats which offer active pressure control to maximize tool life and guarantee higher productivity.
In the active electronically controlled cutting pressure regulator show in Figure 8.6 a die-cutting head is mounted onto a stepper unit and is driven into position against an anvil cylinder by a servo motor to generate a specific pre-calculated cutting pressure.
Figure 8.6 - Schober electronically-controlled pressure cutting regulator
Feedback signals from the servo motor are constantly monitored to ensure that the cutting force between die cutting and anvil cylinder remains constant. This means that the cutting quality will not be affected by fluctuation in the speed of the web. Due to the precisely controlled cutting pressure, tool wear can be reduced to a minimum and consequently lifetime of tooling is increased. In addition to the increased lifetime other advantages are: no scrap generated from bringing the machine down and back up, increased productivity, reproducible setting data and fast change-over.
Another device for measuring rotary cutting pressure is that shown in Figure 8.7, which monitors the left and right side bearer rings separately to provide an exact measurement of actual cutting pressure, measured in kilo Newton (kN), and is designed to maximize tool life and increase cutting efficiency. The measuring gauge is equipped with an audible and visual alert feature to ensure the perfect cutting pressure. With its data interface option, this electronic power check can be linked into any operational data system and can offer superior cutting results and a means to avoid damage to magnetic cylinders and anvil rollers. It can be positioned on the pressure bridge or underneath the anvil roller.
Figure 8.7 - Electronic pressure gauge
As already mentioned, some electronic pressure gauge indicators will enable the analysis of real-time data showing the actual applied force at any given time in a revolution including the dynamic effects (see Figure 8.8.).
With all pressure gauges, regular calibration to National Standards is vital to ensure that instrumentation is operating at its peak performance and guaranteeing that the accuracy necessary is being delivered. Calibrating equipment periodically also has the added advantage of helping spot potential problems before they arise as instruments can lose accuracy for many reasons.
Figure 8.8 - Analysis of actual applied force (real time data)
LATERAL ADJUSTMENT JOURNAL BLOCKS
Lateral adjustment journal blocks (Figure 8.9) are used to replace standard journal blocks on the operator side of the press. They were designed with the press operator in mind by offering three key benefits.
Figure 8.9 - Lateral adjustment journal blocks. Source- RotoMetrics
Firstly, to reduce the need to position the tool with fiber washers. Secondly, to eliminate the need to keep the die level as it is lowered into place. Thirdly — and perhaps most significantly — to allow the press operator to move a die in the across web direction with the turn of a knob. The blocks fit most presses without modification, and feature adjustment travel up to .250" / 6.35 mm. (Note: Washers are still needed to position the die gear from being adjusted over the anvil body.)
ADJUSTABLE ANVIL ROLLERS
Instead of a fixed connection between the cutting cylinder and the anvil roller in which the bearers on the cutting cylinder are resting on the anvil under pressure, it is also possible to have an ‘adjustable’ anvil roller. See Figures 8.10, 8.11 and 8.12. This approach changes the effective undercut in order to compensate for variations or deliberate changes in thickness/calliper of the liner and to some degree, the wear of the cutting edges.
Figure 8.10 - This shows the RotoMetrics AccuStrike adjustable anvil roller
Figure 8.11 - A RotoMetrics adjustable anvil shown set up in the frame
Figure 8.12 - Shows a Kocher+Beck adjustable anvil roller installed in a cutting unit
In these systems, the bearers of the cutting cylinder are resting on separate bearers on the anvil. The bearers on the anvil are no longer connected to the anvil body, but the anvil body is instead spinning freely on a cam.
When the cam is turned, the gap between the cutting cylinder and the anvil body can therefore be adjusted in small increments to suit either the wear on the cutting tool or variations in material.
Some of the anvil cylinders that are adjustable are no longer solid so risk of deflection increases slightly. This is typically compensated by an increase in diameter. It is also crucial that the operator remembers to reset the gap when a new cutting tool is placed in the cutting unit or it might be damaged straight away. The adjustments can be made during operation and do not require the cutting process to stop. Some of the designs available are easy ‘drop-in’ ready and require little or no maintenance.
There are also less sophisticated adjustment possibilities. One possibility is a ‘compressible’ bearer. These bearers are made with a deflection zone that allows the pressure applied by the lead screws to ‘compress’ the bearers.
The compression range is however limited compared to the mechanical systems.
It is also possible to choose a more traditional method of exchanging the ‘zero’ anvil cylinder with a ‘plus’ or ‘minus’ anvil cylinder. A ‘plus’ anvil cylinder is an anvil cylinder with a bigger diameter in the cutting zone than that in which the bearers of the cutting cylinder are resting, thereby closing the gap between the cutting surface and the blades on the cutting die.
Alternatively, a ‘minus’ anvil cylinder will have a smaller diameter than the bearers in the cutting zone of the anvil cylinder, putting the cutting blades farther away from the material (reducing the cutting depth). Using an anvil with a step in the body (a plus or a minus step) will require the cutting unit be at a standstill during the change out.
Figure 8.13 - Examples of correction tools. Source- Electro Optic
Special tools have been developed for label converters to be able to adjust the height of the cutting edge and to repair minor damages to the cutting lines/blades during or after the use of flexible magnetic dies. A plus tool is used to increase/lift the height of the cutting edge, while the minus tool enables the cutting line to be re-sharpened to the original cutting angle.