ROTARY SONOTRODE FOR ULTRASONIC CUTTING

DESCRIPTION

The present invention relates to a rotary sonotrode for ultrasonic cutting according to the preamble of claim 1.

Furthermore, the invention also relates to an ultrasonic cutting machine using said sonotrode. Sonotrodes using ultrasound for cutting textile products, foodstaffs, caoutchouc or rubber based products, etc. are commonly used in many technical fields.

At present, commonly used solutions employ "fixed" sonotrodes fitted with a blade which is vibrated while the product to be cut is carried on a belt under the blade.

However, these solutions do not allow to cut thick materials, because veiy sharp cutting profiles make the blade very fragile and tending to break under the effect of ultrasonic vibrations.

These solutions also suffer from the drawback that, if the sonotrode breaks, it will become an obstacle preventing the product from advancing any farther, thus stopping the production line and causing damage to the product itself. Cutting rotary sonotrodes are known from patent application EP 1466709 and patent

EP0665083.

The sonotrodes disclosed by these documents have a body with a substantially discoidal portion having a peripheral cutting edge. The sonotrode is also provided with an arm through which it is rotated and vibrated. Rotary sonotrodes cause less damage to the product in the event of breakage, since the rotating sonotrode follows the movement of the product transported by the belt and cannot become a stationary obstacle countering the forward motion of the product.

However, the rotary sonotrodes known from these patents do not allow to cut thick materials due to the thinning of their discoidal portion (necessary for cutting), which makes them even more fragile than "fixed" sonotrodes.

Therefore, these sonotrodes only allow to cut thin materials.

The object of the present invention is to solve the problems of prior-art ultrasonic cutting sonotrodes.

In particular, the present invention aims at providing rotary sonotrodes which can also cut

veiy thick materials.

These objects are achieved through a sonotrode incorporating the features set out in the appended claims, which are intended as an integral part of the present description. The general idea at the basis of the present invention consists in providing a rotary sonotrode having a body characterized by a thickness that decreases in the radial plane about the axis of rotation according to a strictly decreasing monotonic law from the arm to a cutting edge.

The dimensions being equal, such a body allows to obtain sonotrodes with a cutting edge which are less fragile than prior-art solutions, because it provides better energy distribution within the sonotrode.

Further objects and advantages of the present invention will become apparent from the following description and from the annexed drawings, wherein:

- Fig. 1 shows an ultrasonic cutting machine according to the present invention;

- Fig. 2 shows a rotary sonotrode according to an embodiment of the present invention; - Fig. 3 shows the sonotrode of Fig. 2 as viewed from the axis of rotation thereof;

- Fig. 4 shows a second embodiment of the body of a sonotrode according to the present invention;

- Fig. 5 shows a third embodiment of the body of a sonotrode according to the present invention; - Fig. 6 shows a second embodiment of an ultrasonic cutting machine according to the present invention.

With reference to Fig. 1, there is shown an ultrasonic cutting machine according to a preferred embodiment of the present invention.

The machine comprises, in a known manner, an actuator 1 adapted to generate ultrasonic vibrations and to rotate a shaft 2 (i.e. a drive member), which is coupled to sonotrode 5 by means of suitable bearings 3 for the puipose of turning it and transmitting to it the vibration generated by actuator 1.

In the preferred embodiment shown in Fig. 1, the sonotrode consists of a body 50 which is radially symmetrical about the axis of rotation AA. Body 50 has a pair of opposite faces (51 and 51 ') which are joined together into a cutting edge 52, thus forming a blade intended for cutting materials.

At the centre of both faces 51 and 51\ two arms 4 and 4' extend along the axis of rotation

AA.

Arm 4 is connected to shaft 2 through bearings 3, whereas arm 4' is connected to a suitable counterweight 6 acting as a balancing member.

In the example of Fig. 1 , sonotrode 5 (body and arms) shows further symmetry about the cutting plane (designated BB) of cutting edge 52.

Cutting plane BB is orthogonal to axis of rotation AA.

As known, sonotrodes need to be frequency-activated: in this case, this implies that the sonotrode is such that a longitudinal vibration applied to one end of arm 4 causes a vibration having the same frequency on cutting edge 52. For this purpose, in a preferred embodiment sonotrode 5 has a longitudinal extension between the ends of the two arms which is equal to an integer multiple of a wavelength of said longitudinal vibration.

This causes the vibration wave to propagate longitudinally in the sonotrode, with nodes at the ends of the arms and an antinode at cutting edge 52. Products 7 to be cut are placed under cutting edge 52; the product is then carried under cutting edge 52 by a conveyer belt.

The sonotrode may be either one machined piece or a modular unit as shown in Fig. 2.

In a modular sonotrode, arms 4 of Fig. 1 consist of two sections 41 and 42 which can be assembled together through removable fastening means, e.g. beads and threads. In Fig. 2, the body 50 of the sonotrode has aims 41 fitted with beads 53 to be secured to arms 42 having matching threads 53 \

Fig. 3 shows body 50 of the sonotrode of Fig. 2 as viewed from axis of rotation AA.

From this perspective, body 50 looks like a circle with arm 41 and bead 53 at its centre.

By sectioning the sonotrode according to a radial plane (r) about axis of rotation AA, body 50 has a thickness (designated by letter 's' and hereafter referred to as "radial thickness'") which decreases according to a strictly decreasing mono tonic law from junction E of arm

41 (i.e. the point where the ami projects from the face) to cutting edge 52.

A body having this characteristic may be obtained by shaping faces 51 and 51 ' in several manners. In the examples of Figs. 1 and 2, faces 51 and 51' have a truncated-cone shape, i.e. a section thereof in a radial plane about the axis of rotation has a profile (hereafter referred to as "radial profile") consisting of a single straight line connecting the arm junction E to

cutting edge 52.

Angle θ comprised between faces 51 and cutting plane BB of cutting edge 52 is preferably between 2° and 60°, higher values of this angle being particularly suitable for simultaneously cutting and welding a material. In the example of Fig. 4, faces 51 and 51 ' have each a radial profile with two straight portions (CD and FE) joined together by a bevelled profile (portion DF). In the example of Fig. 4, two portions CD and FE lie on two distinct parallel straight lines (p, q) forming with cutting plane BB an angle θ which is comprised, in a preferred embodiment, between 2° and 10°, thus creating a veiy elongated sonotrode body which is useful for cutting very thick materials, such as foodstuffs having a thickness of 3-4 cm.

In the example of Fig. 5, faces 51 and 51' have a radial profile comprising only one straight portion CD. Portion DE, which connects the straight portion to arm junction E, has an exponential profile. This profile provides a less sharp sonotrode body, which is useful for cutting and welding materials. To this end, portion CD lies on a straight line forming an angle θ of 40° to 65° with cutting plane BB.

It is now apparent that faces 51 and 51' may have many different profiles while still allowing to obtain a body having a radial thickness which decreases according to a strictly decreasing monotonic law in the radial direction from arm junction E to cutting edge 52. For example, faces 51 and 51 ' may have a radial profile with two straight portions lying on non-parallel straight lines forming different angles with cutting plane BB. For example, in order to obtain cutting and welding sonotrodes, faces 51 and 51' may be machined in such a way that the straight portion which is closer to cutting edge 52 forms a smaller angle with the cutting plane than the one formed with the same axis by the straight line on which a second straight portion lies. In particular, the first portion may form a first angle (e.g. 40-60°), while the second portion may form a different angle which may be either smaller (e.g. 2-10°) or greater depending on the desired cutting area and on the sonotrode dimensions. Of course, combinations of straight portions and exponential portions are also possible, provided that the radial thickness of the sonotrode decreases from the arm to the cutting edge. Although the embodiment examples described with reference to Figs 1-5 show a sonotrode

having identical faces 51 and 51 ', it is apparent that the invention may also be applied to sonotrodes not having a symmetry plane BB orthogonal to the axis of rotation. This is the case, for example, of bell-shaped sonotrodes of the type disclosed by patent EP 1466709, wherein only one sonotrode face has an arm through which the sonotrode is rotated and vibrated.

The angles specified in the embodiment examples of Figs. 1-5 are merely exemplificative and provide preferred and advantageous values and ranges; it is however clear that different angle ranges and values may also be used in accordance with the present invention. Furthermore, the radial profiles of the faces may also comprise portions being different from the straight or exponential portions described herein.

With reference to Fig. 6, there is shown an ultrasonic cutting machine which allows to cany out several cutting operations simultaneously.

The machine comprises a plurality of sonotrodes connected together by an arm, the body of said sonotrodes having a radial thickness which decreases according to a strictly decreasing monotonic law from the arm to cutting edge 52.