Technique explained

The cmc Maschinenbau Lexicon

Steel rule cutting tools

The steel rule die is a very old means of punching out shapes. However, it is still an inexpensive way of die-cutting labels, moulded die-cuts and, for example, seals, thanks to its comparatively simple design and rapid manufacturability. Steel rule die-cutting tools are used in stroke die-cutters.

In principle, slots are milled or lasered into a stable base plate (plastic, wood, Pertinax). Knife lines made of strip steel are then inserted into these slots. They are perpendicular to the cutting plate, can have different cutting angles and can be adapted to different products (cutting height, hardness). In addition to the strip steel cutting lines, there are usually ejector rubbers that prevent the punched material from getting jammed between the strip steel cutting lines.

It is true that steel rule cutting tools are not quite as precise as e.g. punching cylinders and also have a shorter service life. On the other hand, they are unbeatably inexpensive, which has a positive effect on unit costs, especially for smaller quantities. These tools are often used in flat die-cutting aggregates of label printing machines, but can also be used in other areas for punching, perforating and creasing.

Aufbau eines Bandstahlschnitt-Stanzwerkzeugs:

Mittels Säge oder Laser werden in eine Grundplatte (häufig hochwertige Sperrholz-Verbundplatten) Längsschlitze eingebracht. In diese Schlitze werden standardisierte Stahlbänder mit einseitig einer Schneidkante eingebracht. Die Holzplatte dient dabei zur Ausrichtung der Schneiden, während der Bandstahl selbst die Stanzkraft überträgt, mit dem Produkte wie beispielsweise Folien, Gewebe, Karton oder Dichtungen gestanzt werden können.

Auswerfer-Gummis rund um die Schneiden oder sogar über die ganze Fläche sorgen dafür, dass das Stanzgut sich nicht zwischen den Schneiden verklemmt.

+ Vorteile von Bandschnitt-Werkzeugen

  • Kostengünstig, keine hohen Initialkosten

  • Kurze Lieferzeiten, flexibel bei Änderungen

  • Geeignet für kleine und mittlere Fertigungsmengen

  • Für die meisten Anwendungen ausreichend genau

- Nachteile von Bandstahl-Schnittwerkzeugen

  • Nur mittlere Genauigkeit

  • Keine ausreichende Standzeit bei großer Auflage (Nachbemesserung ist jedoch möglich)

  • Diskontinuierliche Stanztechnik, daher langsamer wie Rotationsstanzen

  • Rapport-Genauigkeit nicht immer ausreichend (mit entsprechenden Maßnahmen jedoch deutlich zu steigern)

Flat punch

Flat punching is the oldest method of punching shapes out of a web-shaped material. Punching was already used in ancient Greece and before.

Today's flat punches are machines in which a punching tool (punching plate, steel strip cut) enters the material to be punched per stroke and then moves out again. Due to the corresponding mechanics, very high punching forces can be achieved. The process is discontinuous, the material to be punched must be fixed in position for the work step. The feed is usually effected by driven rubber rollers or a feed frame (feed unit, gripper feed). Different processes are used depending on the required register accuracy.

The die-cuts can be very exact and precise with well-aligned machines. In particular, kiss-cut punching (i.e. only part of the material is punched) of e.g. labels can be realised very well. But also hard and very stable products like sheet metal, PMMA, textiles or leather can be punched. The stroke punch is usually driven by a hydraulic punch 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 allows rolls with e.g. different diameters to be wound on the same shaft, although they rotate at different speeds.

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

In order to achieve an even winding of the rolls, it is recommended to keep an advance of the friction shaft of up to 5% compared to the web speed. 

Lifting punch presses

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

Depending on the production and budget requirements, mechanical and hydraulic punch presses come into question. Whereas mechanical lifting punches with an articulated head drive can work comparatively quickly, hydraulic punch drives build up considerably more power, which is advantageous for thick materials and/or large punching formats with many punching lines.

Kiss-cut and die-cut (punching, punching through)

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

The reason for the two different die-cutting processes is that, for example, sealing rings made of rubber mats need to be loose parts. In contrast, roll labels must be on a backing material so that they can be used in an automatic labeller, for example. So there are different applications that each require adapted die-cutting processes.

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

Punched (kiss-cut) labels and shaped die-cuts are mostly self-adhesive. The punching knives only penetrate the cover material (e.g. printed PE film) and the adhesive, but not the silicone paper underneath. Since this type of construction usually involves rather thin materials, the punching process must be carried out with micrometre precision. If you punch too deep and damage the siliconisation of the backing, the adhesive film can bond with the non-siliconised material underneath. If you do not punch deep enough, you will not cut through the adhesive layer or even the cover material. Both mistakes lead to processing problems.

Laminate

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).

The important thing in this process is to work without distortion and without bubbles, 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 to make 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 the roll and matching rigid film carriers can be combined.

Magnetic powder brake (magnetic powder clutch)

Magnetic particle brakes are used instead of eddy current brakes or mechanical-pneumatic disc 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 realise soft starts. The main application of magnetic particle brakes on CMC Maschinenbau machines is sensitive web tension control.

The clever technology means that 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 the rotor depending on the current flow. The two components do not touch each other (as is the case with a disc brake, for example), which enables low-wear continuous operation.

In the magnetic particle clutch, the stator is also rotatably mounted and can thus continuously transmit torque forces to another shaft.

Mechanical and pneumatic expansion chucks

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

Expansion chucks have conical expansion wedges that are pressed outwards either mechanically or pneumatically. In the process, they tighten by self-locking in the paper or plastic core and can transmit the required torque.

The compressed air supply for pneumatic expansion chucks can be either via the shaft or radially on the expansion chuck.

Expansion chucks are selected depending on the machine used and the maximum roll weight that occurs. In many cases, a version in aluminium or steel is available.

Upper and lower knives for circular knives

The plate-shaped upper knives (disc knives) and the ring-shaped lower knives (cutting bushes) interact to produce a continuous shear cut.

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

As these products are subject to permanent wear, the upper knives in particular must be reground and/or replaced regularly. Damage to the cutting edge can lead to considerable problems with the material being cut (e.g. risk of tearing). Such knives 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 applied to the lower knives with pressure. The upper knife holders take over the task of mechanically connecting the knives firmly to the cutter head. Incorporated spring elements create an even pressure of the upper knife on the lower cutting sleeve.

A distinction is made between top knife holders with eccentric fastening, which are mechanically clamped with a key. There are also fixings by means of set screws, inner ring fixings and freely engaging upper knife holders. Spacers can be used between the knife holders to define the cutting width. Alternatively, automatically moving electromechanical or pneumatic knife holders can 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 well suited for thin materials with small variations in thickness.

Roughly speaking, a distinction is made between expansion shafts with continuous strips or individual clamping elements.

In both cases, filling the inner flat hoses with compressed air presses the clamping elements outwards against the sleeve. The clamping elements can be made of different materials and have longitudinal or transverse grooves. When deflated, the clamping elements slide back into their original position due to the integrated spring elements. 

The maximum load is important to avoid bending of the shaft. Precise concentricity of the paper or film rolls can be optimised by pre-centring bars.

Crushing knife

In contrast to the continuous shear cut with upper and lower knives, only one cutting edge is used in the crush 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 and thus reduce downtimes.

Due to the cutting technique, however, the use of crush cutters is limited to certain materials (cleanliness of the cutting edge). Typical applications are cutting textiles, emery, fleece, tissue, rubber and also adhesive tapes.

In addition to continuous cutting knives, perforating squeezing knives and profile squeezing knives can also be used. It is also possible to score material by using squeezing knives that do not go all the way to the bottom shaft. With appropriate tool holders, scoring is also possible. In this case, for example, the self-adhesive label material is separated, but the underlying siliconised backing paper is not. 

The counter-pressure shaft is usually made of chrome-alloyed tool steel for a long service life. Further special surface hardenings are available from the corresponding 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. Below this is the impression cylinder (anvil), which holds the material to be punched at the appropriate distance. Both cylinders maintain a defined distance (gap dimension, clearance, gap) by means of high-precision bearing rings (bearer rings).

The bearer rings determine how deep the cutting lines of the upper punching cylinder (solid or as a magnetic cylinder with cutting plate) plunge into the material. Thus, during label die-cutting, the knife penetrates the label material and the adhesive, but should not damage the underlying siliconised paper or film (kiss-cut). For this reason, rotary die-cutters are accurate to within a few µm of the gap. In die-cutting, on the other hand, the cutting lines run metal on metal and cut through the die-cut material completely.

Solid punching cylinders or magnetic cylinders are used. Thin sheet metal foils are mounted on magnetic cylinders, which are provided with the order-specific punching lines. Advantage: they can be easily exchanged.

As the material to be cut is continuously transported through the rotating cylinders, a high degree of accuracy is possible for the cutting parts on the one hand, but also for the repeat (distance from cutting to cutting). The speed can be considerably higher than with stroke punching.

Thermal expansion, unfortunate cutting line arrangement (long cross cutting lines), too high pressure or incorrect design of the blades in relation to the material to be cut are potential sources of error.

Cutting toll "razor blade"

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

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

Depending on the machine configuration, a simple exchange between disc knives (scissor cut) and razor blades is possible. 

Punch printing (punching technique) - Punch plate

During die cutting, the material is put under pressure by the cutting edge to such an extent that it breaks. So it is not a smooth penetration and displacement of the material by the cutting edge, but a pressure-induced material failure. 

This behaviour is of course very much dependent on the pressure applied. The punching pressure required for successful punching depends on several parameters, such as for example 

  • Material to be punched

  • Material thickness

  • Material hardness

  • Contour and number of punch lines

  • Punched area

and some other influencing factors. Mostly, however, the required punching pressure is specified by the toolmaker (e.g. steel rule cutting, rotary punching).

Materials such as Pertinax hard paper or non-ferrous metals such as aluminium or brass are used as counterpressure plates (cutting pads) for stroke punching. They guarantee a good punching result with the least possible damage to the sensitive cutting edge. In the case of adhesive products (labels, die-cut parts), care must also be taken to use a covering material suitable for die-cutting (silicone paper with sufficient strength and insensitivity to pressure, e.g. Glassine paper).