What is Punching?
Punching is a separating technique, mainly used to process holes into flat materials like paper, plastic film or sheet metals. Normally, the punching equipment consists of two tool parts – one support for the punches and one for the dies. While the punches enter the dies, the material is being shear cut.
Punching is mainly used to process round or profile holes into flat material, but it is also suitable for applications where waste removal can cause problems.
The following materials can be processed: Thin metal or plastic films and paper, but also thicker materials like packing materials, compounds, leather and metal sheets used in the packaging, food, hygienic, medical and pharmaceutical industry.
Over the last years, punching also became very important for the processing of metal foils for technically sophisticating products, e.g. for aerospace or electronics industry.
Another application is the punching of profiles into metal sheets for the metal and construction industry.
What final products contain punched holes?
Tea bags, tea labels & tea sticks, price tags, self-adhesive tags and injection holes for inmould labels, parking tickets, boarding passes, fruit wrappings, straw holes and spouts in liquid packaging, packaging materials, security features on documents, tear-off perforation for post stamps or continuous forms, synthetic leather, pharmaceutical products like blood sugar test strips, films with transport holes, fuel cells, batteries, filters, cigarette packaging, connection profiles, technical films, and much more.
What different punching techniques exist?
In general, there are two different punching techniques, the conventional flat-bed punching and the rotary punching. These two processes will be described in detail below.
A flat-bed punching unit consists of a press with an integrated punching tool. The punching tool usually contains an upper and a lower tool. The tools are mounted parallel in linear guides and adjusted precisely, so that the punches enter the dies correctly. The material web runs between the vertically moving tools. When the punches enter the dies, the whole punching pressure acts on the material at once (surface load). The punched waste is pushed into the dies and falls down vertically.
The material is fed into the punching unit intermittent, which means the feed motion of the material is interrupted for the punching. If the punched material needs further processing in future converting steps, the material feed needs to work precisely without any slippage or clearance. Otherwise, inaccuracy and register problems can occur.
The punching tools of a rotary punching unit are mounted either directly in high precision bearings or adjustable on two cylinders. The material web runs continuously between the two rotating tools. Depending on the application, the entire punching unit can either be integrated into a machine frame or into a separate unit.
The tools rotate continuously when punching; therefore, the punch enters the die punctually. The punctual force requires a lower punching force than the flat-bed punching. The punched waste is removed by a hollow die cylinder or suctioned by a vacuum hub.
For flat-bed punching, the production output in linear meters per minute depends on the punching tool length in web direction, i.e. the feed motion, and the number of strokes per minute.
High performance punching machines can achieve up to 350 strokes per minute when punching thin films. Depending on the material web width, feed motions can range from 50 mm up to 1000 mm.
Working widths for rotary punching currently range from 10 mm up to 2000 mm. Depending on the application, speeds between 1 m/min and 800 m/min can be achieved. The punching diameter can range from 0.5 mm up to approx. 50 mm.
Advantages of each technique
In some cases tool costs are lower than for rotary punching. Advantageous for frequently changing punching contours.
Higher productivity, as continuous material feed enables to work with very high working speeds. The accurate, controlled web tension leads to a high punching quality and a precise placement of the punches. Since the material is sheared punctually and progressively (line load), the punching forces are considerably lower, which leads to longer tool life, reduced energy consumption as well as lower noise and vibration generation.
What should be considered when deciding for a punching technique?
Both punching techniques have their specific advantages and disadvantages. Accordingly, you should decide for the most suitable technique depending on the application.
The investing costs for the press of a flat-bed punching unit are generally higher than for a rotary punching unit. However, the tool costs for flat-bed punching unit are often lower than for rotary punching. Therefore, flat-bed punching units are primarily advantageous for small production volumes and frequently changing punching contours.
If further processing of the products requires high accuracy or high production speeds, rotary punching is most often preferable. Since the required punching force is lower, rotary punching also generates considerably less noise, which can be an important point if certain noise requirements have to be met.
Additionally, upstream processes (like the printing or the laminating of the material) as well as the amount of available space should be considered.