Technology explained

Flat die cutter, flatbed die cutter

Flat die-cutting is the oldest method of die-cutting shapes out of a (sheet-like) material. Hole die-cutting was already used in ancient Greece and before.

Today's flat die-cutters are machines in which a die-cutting tool (die-cutting plate, steel rule cut, die carrier plate) usually penetrates the material to be die-cut via a vertical stroke, separates it and then moves out again. Very high die-cutting forces can be achieved with the appropriate mechanics. The process is discontinuous, the material to be die-cutted must be fixed in position for the work step. The feed is usually carried out by driven rubber rollers or a feed frame (feed unit, gripper feed). Different processes are used depending on the required register accuracy. The cutting contour is created with cutting lines or perforation lines. Creasing and embossing can also be integrated into the die-cutting tool. The separation of the panels and the residual skeleton winding is usually integrated into the die-cutting machines and/or die-cutting tool.

The die-cuts can be very exact and precise with well-aligned machines. Kiss-cut die-cutting (i.e. only part of the material is die-cutted) of labels, for example, can be realized particularly well. However, hard and very stable products such as thin sheets, PMMA, textiles or leather can also be die-cutted. The stroke die-cut is usually driven by a hydraulic die-cut or an eccentric.

Friction shafts for flexible working

Friction shafts consist of individual segments (friction rings) that can be twisted in relation to each other. This means that rolls with different diameters, for example, can be wound onto the same shaft even though they rotate at different speeds.

Friction linings inside the shaft are pressed against the friction rings using compressed air. The transmitted torque can be adjusted by varying the air pressure.

In order to achieve uniform winding of the rolls, it is recommended to maintain a lead of the friction shaft of up to 5% compared to the web speed.

Stroke die-cutting machines

Stroke die-cutting machines are discontinuously running die-cutting machines. The die-cutting tool is clamped in a tool holder (die-cutting plate). Die-cutting is performed by lowering the tool onto the material underneath. A distinction is made between kiss-cutting and die-cutting of the die-cutting material. After the die-cutting process, the tool holder moves upwards again. Now the next material is fed into the die-cutting area by a feed (this "feed" can also be a manual insertion). Inaccuracies in the feed result in repeat differences. Stroke die-cutting is also relatively slow due to the discontinuous die-cutting process. On the other hand, the tools are usually much cheaper than for rotary die-cutting.

Depending on the production and budget requirements, mechanical and hydraulic die-cutting presses can be considered. While mechanical die-cutters with an articulated die drive can work comparatively quickly, hydraulic die-cut drives build up considerably more force, which is advantageous for thick materials and/or large die-cutting  formats with many die-cutting lines.

Kiss-cut and die-cut

In die-cutting technology, a rough distinction is made between two processes: complete die-cutting out of a material web and partial die-cutting to a predefined die-cutting  depth.

The reason for the two different die-cutting processes is that sealing rings made from rubber mats, for example, are required as loose parts. In contrast, roll labels must be on a carrier material so that they can be used in an automatic labeling machine, for example. There are therefore different applications that each require adapted die-cutting processes.

With die-cutting, the problem often arises of transporting the die-cutted-out material out of the tool area. There are various methods for this, which we will be happy to explain to you.

Kiss-cut labels and die-cut parts are usually self-adhesive. The die-cutting knives only penetrate through the cover material (e.g. printed PE film) and the adhesive, but not through the silicone paper underneath. As thin materials are usually processed with this design, the die-cutting process must be carried out with micrometer precision. If you die-cut too deeply and damage the siliconization of the carrier, the adhesive film can bond with the non-siliconized material underneath. If you do not die-cut deep enough, you will not cut through the adhesive layer or even the cover material. Both errors lead to processing problems.

Laminating

In the laminating unit or on larger laminators, non-adhesive films (paper, textiles, rubber) are often brought together with a self-adhesive film or a transfer adhesive (pure adhesive layer without carrier).

During this process, it is important to work without distortion and bubble-free, which is achieved through web tension and plane-parallel alignment of the laminating rollers.

Laminating with double-sided adhesive tapes or linerless transfer adhesive tapes is a very common method of making printed label films self-adhesive, for example.

However, protective films can also be laminated on, for example, or two products such as RFID tags on a roll and matching rigid film carriers can be combined.

Magnetic particle brake

Magnetic particle brakes are used instead of eddy current brakes or mechanical-pneumatic disk brakes. Their main advantage is the very even torque curve. Unlike eddy current brakes, the braking effect is largely independent of the speed. This makes it easy to realize soft starts. The main application of magnetic particle brakes in CMC Maschinenbau machines is sensitive web tension control.

Thanks to the clever technology, no great control power is required.

In principle, a magnetic particle brake consists of a rotor and a stator in which a magnetic coil is embedded. Magnetic powder is embedded in the gap between the stationary and rotating components. This magnetic powder generates a linearly increasing frictional connection between the stator and rotor depending on the current flow. The two components do not touch each other (as is the case with a disk brake, for example), which enables low-wear continuous operation.

In the magnetic particle clutch, the stator is also mounted on a rotating bearing and can therefore transmit torque forces to another shaft continuously.

Mechanical and pneumatic expansion chucks

Larger rolls of material (bales) are not supported by a continuous shaft, but are fixed between two expansion chucks. The transmission of force to the core of the wrapping material is particularly important here.

Expansion chucks have conical wedges that are pressed outwards either mechanically or pneumatically. They are self-locking in the paper or plastic core and can transmit the required torque.

With pneumatic clamping heads, compressed air can be supplied either via the shaft or radially on the clamping head.

Expansion chucks are selected depending on the machine used and the maximum roller weight. In many cases, an aluminum or steel version is available.

Upper and lower knives for circular knives

The interaction between the plate-shaped upper knives (plate knives) and the ring-shaped lower knives (cutting bushes, grooving knives) results in a continuous shear cut.

While the sharp blade sits in the upper knife, it lies on the sharp cutting edge of the lower knife. By pressing the upper blade against the lower blade, a cutting angle close to 90° is achieved. This enables very precise and straight cuts to be made in paper and film. So-called intermediate rings allow the cutting width to be adjusted in 1/10 mm increments)

As these products are subject to constant wear, the upper blades in particular must be reground and/or replaced regularly. Damage to the cutting edge can lead to considerable problems with the sliced product (e.g. risk of tearing). Such blades should be replaced immediately.

Upper knife holder

With the rotating shear cut of the circular knives, it is important that the plate-shaped upper knives are fixed exactly to the lower knives. To achieve a good cutting result, the plate-shaped upper knives are pressed against the lower knives. The upper knife holders take on the task of mechanically connecting the knives to the knife shaft. Incorporated spring elements create a uniform pressure of the upper knife on the lower cutting sleeve.

A distinction is made between top knife holders with an eccentric fastening, which are mechanically tensioned with a key. There are also fastenings using set screws, inner ring fastenings and freely engaging top knife holders. Spacers can be used between the blade holders to determine the cutting width. Alternatively, automatically moving electromechanical or pneumatic knife holders can also be used, which move to their intended position by programming and are fixed there.

Pneumatic expansion shafts

In contrast to a mechanically more complex friction shaft, expansion shafts are particularly suitable for thin materials with low thickness fluctuations.

A rough distinction is made between expansion shafts with continuous bars or individual clamping elements.

In both cases, the clamping elements are pressed outwards against the sleeve by filling the internal flat hoses with compressed air. The clamping elements can be made of different materials and have longitudinal or transverse grooves. When the air is released, the clamping elements slide back into their original position thanks to the integrated spring elements.

The maximum load capacity is important to prevent the shaft from bending. Precise concentricity of the paper or film rolls can be optimized using pre-centring bars.

Crushing knife

In contrast to continuous shear cutting with top and bottom knives, only one cutting edge is used in the crush cut knife. It is pressed with high pressure through the material to be cut onto a counter shaft.

The main advantage of crush cutters is the very short set-up time. You can change over from one job to the next very quickly, thus reducing downtimes.

Due to the cutting technique, however, the use of crush cut knives is limited to certain materials (cleanliness of the cut edge). Typical applications include cutting textiles, emery, fleece, tissue, rubber and adhesive tape.

In addition to continuous cutting blades, perforating crush cutters and profile crush cutters can also be used. Material can also be creased using crush cutters that do not go all the way through to the lower shaft. Scoring is also possible with the appropriate tool holders. This separates the self-adhesive label material, for example, but not the siliconized backing paper underneath.

The counter-pressure shaft is usually made of chrome-alloyed tool steels for a long service life. Further special surface hardening is available from the relevant manufacturers.

Rotary die-cutting

Rotary die-cutting is a variant of stroke die-cutting. The main advantage is the continuous operation, which results from the production process.

In principle, a rotary die-cutter consists of two cylinders. The upper cylinder (die) is equipped with cutting lines. The counter-pressure cylinder (anvil), which holds the die-cut material at the appropriate distance, is arranged below it. Both cylinders maintain a defined distance (clearance, gap) by means of high-precision bearing rings (Schmitz rings).

The bearer rings determine how deep the cutting lines of the upper die-cutting cylinder (solid or as a magnetic cylinder with cutting plate) penetrate the material. When die-cutting labels, the knife penetrates the label material and the adhesive, but should not damage the siliconized paper or film underneath (kiss-cut). Rotary die-cutting is therefore accurate to a gap of a few µm. With die-cutting, on the other hand, the die-cutting lines run metal on metal and cut through the die-cut material completely.

Solid die-cutting cylinders or magnetic cylinders are used. Thin sheet metal films are mounted on magnetic cylinders, which are provided with the order-specific die-cutting lines. The advantage: they can be easily replaced.

As the material to be die-cutted is transported continuously through the rotating cylinders, a high level of accuracy is possible for the die-cutted parts, but also for the repeat (distance from die-cut to die-cut). The speed can be considerably higher than with stroke die-cut.

Thermal expansion, unfortunate cutting line arrangement (long cross-cutting lines), excessive pressure or incorrect blade design in relation to the material to be die-cutted are potential sources of error.

Razor blade" cutting tool

In combination with the corresponding razor blade holders, very narrow cutting widths are possible in some cases.

As the blades are narrow and sharp, razor blades are particularly good for cutting wafer-thin plastic and metal films. In addition to standard carbide blades, various manufacturers also offer ceramic-coated blades with a particularly long service life.

Depending on the machine configuration, it is easy to swap between disc blades (scissor cut) and razor blades.

Comparison of stroke die-cutting vers. Rotary die-cutting vers. Laser die-cutting

All three processes have their specific advantages and disadvantages. It therefore depends on what objective you are pursuing with the purchase of a corresponding die-cutting machine. Although all three processes offer a high degree of freedom in terms of the materials to be processed, they are nevertheless quite different. In the case of mechanical die-cutting, the options for adapting to the material to be processed are primarily made possible by the different die-cutting tools. However, accuracy and speed are largely determined by the die-cutting technology used.

The rather general statements made here refer to films, papers and other thin materials such as leather, rubber or thin metal films.

Advantages of rotary die-cutting

• High productivity thanks to high production speed
• High part accuracy achievable
• High accuracy of the repeat distance
• Long service life of the tool
• Comparatively quiet and low-vibration production process
• Suitable for large to very large orders

Disadvantages of rotary die-cutting

• Expensive tools, especially cutting cylinders
• Complex storage and maintenance of cutting cylinders, cutting plates are              mechanically sensitive
• Extremely sensitive technology for thickness changes (gap distance is fixed)
• Strong dependence on available die-cutting cylinder diameters (otherwise            e.g. increased waste; remedy: semi-rotary die-cutting)

Advantages of stroke die-cutting

• Favorable tool costs (steel rule cutting)
• Quick tool change, short set-up times
• Enormously high die-cutting pressures possible for thick or hard materials
• Thickness fluctuations can be compensated for by readjustment

Disadvantages of stroke die-cutting

• Discontinuous production process, relatively slow
• Manufacturing process can generate noise and vibration
• Low repeat accuracy

Advantages of laser die-cutting

• No tools required
• purely digital change necessary for shape change
• Very fast job changeover
• Processing of very thin films possible
• Extremely fine structures can be produced that cannot be produced using            conventional die-cutting techniques
• non-contact die-cutting process
• Very large die-cutted part dimensions possible depending on the design of          the laser punch

Disadvantages of laser die-cutting

• Thermal energy input can lead to undesirable die-cutting edges (e.g.                      charring)
• Relatively slow process even with high-power lasers, as multiple panels                  cannot be die-cutted at the same time
• Technically demanding process (digitization, workplace equipment,                        operation)

Semi-rotary die-cutting

One disadvantage of rotary die-cutting is its dependence on the die-cutting cylinder diameter. This cannot be freely selected. The size of the die-cutted part and the circumference of the die-cutting cylinder are set in relation to each other and then result in a certain number of die-cutted parts and a certain distance between the molded parts.

If the size of the die-cut part in relation to the available circumference is such that there would be a large waste (for example: roll circumference 350 mm, 5x label 75 mm -> 4.6 die-cut parts), you can switch to semi-rotary die-cutting. In this way, the repeat between the individual parts can be reduced to the minimum possible dimension without having to pay attention to the appropriate pitch.

Semi-rotary die-cutting involves working with a die-cutting plate with a maximum circumference of 270°. Once the last cut has been made, the die-cutting material is moved so that it is ready for the first cut of the next rotation. It is therefore slowed down to the right extent, pulled back slightly and accelerated again. The advantage is the significantly reduced waste compared to the full-circumference die-cutting tool if the pitch is not suitable.

A semi-rotary die-cutting machine is therefore much more versatile when it comes to die-cutting molded parts. In addition, this process (die-cutting plate instead of cylinder change) is also suitable for smaller printing and die-cutting jobs, which makes it interesting for niche suppliers.

Web edge control

Web edge control is very important in order to be able to produce precise and high-quality end products with cutting and die-cutting machines. In many cases, the web edge is the starting point for correct punching or maintaining tolerances during cutting.

The web edge control system includes

a. Sensors

b. Control

c. Actuators

Light barriers or ultrasonic sensors can be used as sensors. Ultrasonic sensors are superior to light sensors (usually infrared) for transparent films. However, light sensors are more common.

The control electronics are integrated into the system technology in most machines with web edge control from CMC Maschinenbau.

The actuators are either guide shafts that are moved by a linear drive (e.g. the unwinding shaft) or rotating frames. Rotating frames are usually square tables over which the material is guided. Two shafts, which are aligned parallel to each other, can be rotated by a few degrees to the running direction and thus prevent the material from drifting in one direction or the other. Simple machines are equipped with the cheaper linear drive-driven shafts. The drive moves the material roll back and forth parallel to the following transport and deflection rollers.

The aim of web edge control is, for example, to keep the cutting edge as constant as possible when splitting rolls, even if the starting roll is not wound with a straight edge. During die-cutting, web edge control can be used to support the precise die-cutting and separation of label blanks.

Other options include the detection of fiducial marks using camera technology (image processing), which is, however, considerably more expensive and is only used if the above-mentioned technologies do not achieve sufficient results.

Web tension control

As the name suggests, web tension control is used to keep the tensile forces within the material constant during processing.

Optimum slitting and die-cutting results can be achieved for different products by using the right web tension. Excessive web tension can, for example, lead to dimensional inaccuracies in rotary die-cutting (a circle is stretched into an ellipse) and can be responsible for rolls that are wound too loosely or too tightly during the cutting process. Rigid films are more likely to be wound with high tensile stress, such as nonwovens, which tend to require lower material tensile stress. A typical sign of incorrectly set tensile tension is telescoping (lateral escape of the material from the roll plane) when rolls are wound too tightly.

A web tension control system consists of the following components:

- Sensor, e.g. load cell or measuring roller

- Control regulation

- Braked or driven shafts, if applicable

Load cells register the force that the material web exerts on a shaft. From the continuously measured force, the control system calculates how strongly, for example, a brake on the unwind or the clutch on the drive axle needs to be adjusted. The magnetic particle brakes and clutches from CMC Maschinenbau are particularly advantageous here, as they can be controlled linearly over a wide range and with minimal power consumption.

In this way, the web tension is kept constant.

Another option for keeping the material tension constant is dancer shafts (dancer controls). This technology is only used in our systems in a few exceptional cases (e.g. in conjunction with a material accumulator)