Ultrasonic Cutting

The fundamental principles of cutting with ultrasonically excited blade were explored already in the mid-eighties at the Fraunhofer Institute for Production Technology in Aix-la-Chapelle (Aachen).

As manufacturers of multi-axes trimming machines for the trimming of plastic parts, in the past we have been confronted time and again with components and materials that were difficult to process or not at all. 

Examples are carpets, fabrics and fleeces, artificial leather and excess lengths of artificial leather on laminated parts, extremely thin foils, rubber parts or similar.


Method of operation

Ultrasound means sound frequencies beyond the limit of hearing. The limit is assumed to be about 20 kHz. For cutting, frequencies in the range between 20 kHz and 40 kHz are used.

Currently, to generate ultrasound, the inverse piezoelectric effect - the electrostriction - is used nearly exclusively. This means that an alternating voltage produced by a high-frequency generator is connected in a so-called converter to a ceramic or quartz crystal plate. The plate thereby generates oscillations with corresponding frequency which is particularly powerful with resonance. The amplitude of the oscillation is about 6 to 8 µm. The larger amplitudes necessary for welding, cleaning or cutting, as in the case of GEISS, can be generated by amplifier trunks, also called transformation pieces or, mostly, sonotrodes. The sonotrodes for cutting are nearly exclusively manufactured of titanium and are designed for durability.

The oscillations thus produced are mechanical longitudinal waves that are reflected at the end of the sonotrode. To utilize the oscillation energy, a standing wave with a wave loop must be produced at the end of the sonotrode. To achieve this it is necessary to determine the length of the sonotrode in such a way that it is a multiple of half the wave length of the oscillation.

This simple physical condition for a mechanical standing wave has an unavoidable unpleasant consequence for cutting. It means that converter, blade and sonotrode must be exactly tuned to each other and that it is not possible to use arbitrary blades for cutting.

All generators now in the market do have an automatic frequuency readjustment in case something is changed in the system for example by sonotrode wear, yet this frequency readjustment is possible only to a limited extent since the resonance range of the converter must be retained.

The reason for the reduction of the cutting power when cutting with ultrasound is in the increase of the cutting speed and the longitudinal oscillation of the blade. When cutting with a knife without oscillations, the cutting speed is equal to the feed speed. With an excited blade, the average cutting speed just by the oscillation of the blade is:
V = 4 * Xmax * f

 (Xmax = Amplitude of oscillation; f = Frequency of ultrasound)

Hence, with an amplitude of 12 mm (0.000012 m) and a frequency of 
35 kHz, the resulting cutting speed will be

 4 * 0,000012m * 35000Hz = 1,68m/s.

Generally it can be said that units with a lower frequency become bigger and heavier and have a higher power output. The amplitudes that can be reached are also higher at lower frequencies.

Units with 20 kHz are therefore more suitable for cutting thicker and harder materials. A drawback of the 20 kHz units is that possibly noise reduction measures must be taken since the frequency is very near the audible range.

Units with 35 kHz are more suitable for thinner materials such as foils, artificial leather and textiles. Due to their smaller size they are also more suitable for use in multi-axes cutting heads for the processing of components with complex shape. Noise reduction is not necessary.

The power of the units is generally sufficient for cutting since they were designed for ultrasonic cutting where much higher power is required. The power consumption when cutting 3 mm thick PP at 10 m/min feed for example is approximately 70 W. For 2 mm thick PP and 20 m/min, the power consumption is also abt. 70 W.

As is known from other technologies (e.g. trimming), not only the cutting force decreases with increasing cutting speed, but also the wear of the blades increases. Therefore, for ultrasonic cutting, only hard metal blades are recommended. Only when trimming thin and very soft foils it is possible to use also commercial blades of high-speed steel. Hard metal blades can reach cutting lengths of 20.000 m and more.

When compared to conventional methods, ultrasonic cutting offers substantial benefits. The cutting forces are reduced by abt. 75 % and the productivity of the cutting process is greatly increased.

Actually the ultrasonic cutting unit for some applications is the technology of the future since it is partially far superior to other processes in terms of precision and cleanliness. 

Unlike trimming it produces neither swarf nor noise and unlike laser cutting there are no burnt edges or flue gases. When compared to water jet cutting, there is no moisture penetration of the material. Yet above all, in terms of cost, it is an alternative to laser and water jet cutting.

Due to the geometry of the knife, the limits of the process are in the thickness and the hardness of the material.

Applications include trimming of PVC floor tiles via weakening of slush skins up to cutting of invisible airbags. The skin of these airbags is incised on the back of the dashboard so that in the event of a crash it will deploy at exactly the right place. The cutting line must not be visible on the front side. Our cutting lines are the most precise and "most invisible" worldwide!