There are known in the art rotating hot-wire cutting apparatus for plasticators in which the cutting speed is changed so that the blade can cut the plasticized sheets by moving at the same speed as the latter. To this purpose, such apparatus provides a computer in which operating speed and length of the sheets are freely regulated, and the movement of the cutting blade is calculated according to the actual operating times and as a function of the desired length of the sheets.

A disadvantage of such known cutting apparatus and methods is due to the fact that the unavoidable calculation approximations involve necessarily deviations that accumulate as the machining proceeds even if they are individually very small, and cutting position errors may occur so that the reset of the machine and the new positioning of the laminate to be cut is required.

A first object of the present invention is to overcome such disadvantages by providing a control system for

the superimposition or"overlap"point between two adjacent sheets which are plasticized so that the positioning error accumulation is fully avoided.

Another purpose of the invention is to improve the cutting operation by providing in combination: a cutting assembly in which the rotating blade is replaced by a cutting wire which is heated as an electric resistance by a potentiometer that regulates the needed current, means for keeping said wire under traction as the operating temperatures and its length change, and means for rotating said cutting assembly by an electronic cam drive so that the laminate is cut by a blade moving upon cutting at the same speed as the film used to plasticize the sheet.

Still another object of the invention is to improve the performance of the cutting operation by preventing the hot wire from coming in touch with the coating film by covering the hot wire with an adhesive Teflon layer which performs a double action of insulating and cleaning the cutting wire.

A better understanding of the invention will result from the following description with reference to the accompanying drawings that show a preferred embodiment thereof only by way of a not limiting example.

In the drawings: Fig. 1 is a block diagram of the invention; Fig. 2 is an elevation longitudinal side view of the

cutting assembly and the overlap sensor according to the invention; Figs. 3 and 4 are simplified diagrams relative to a first and a second embodiment of the invention, respectively; Fig. 5 shows schematically the operating principle of a sensor of the optical type.

Referring particularly to the diagram of Fig. 1, the cutting apparatus according to the invention essentially includes: A. A sensor SRS for detecting the overlap of the plasticized material (laminate); B. A rotating hot blade LC; C. A motor MB, preferably of the brushless type, causing said blade to rotate; D. A device (AM) for driving the motor (MB) ; E. A programmable card LCP of the control logic controlling the operating device AM.

As known, during the operation of the plasticator, the paper or paperboard sheets are coupled with a film of plastic material such as polyethylene, polyester or the like. This causes an overlap to be formed, i. e. a light superimposition of the sides of adjacent sheets.

Starting from this fact, the present invention provides that the just plasticized material moves along a plane where a sensor detecting automatically the presence of an overlap during the movement of the

plasticized material is positioned at a predetermined distance from the position in which the cut is performed.

According to a peculiar feature of the invention, when an overlap passes by sensor SRS a signal is sent to the operating device AM which causes the blade to follow a predetermined cam profile.

In the described embodiment said cam is of the electric type and is designed so that a position of the hot blade (slave) LC corresponds to every position of the coupler (master) of the plasticator. Thus it is guaranteed that blade LC always moves in absolute synchronism with the coupler, thus setting safely its position and its speed with respect to the approaching overlap In this case illustrated by way of example, a precise positioning of the overlap with respect to the cutting point equal to two tenths of millimetre is guaranteed.

Advantageously, the profile of said electric cam is such as to ensure that upon cutting blade LC moves at the same speed as the material to be cut, thus avoiding any relative movement which can give rise to friction and/or problems of material pulling.

It should be noted (Fig. 1) that, according to the invention, in order to ensure the maximum synchronism in time between the cutting movement and the feed of the plasticized material to be cut, the operating and control device AM of motor MB for driving blade LC receives signals from sensor SRS detecting the overlap, encoder E detecting the position of motor MB

(and then blade LC), and a further encoder EA detecting the position of the coupler of the plasticator, respectively.

In other words, this means that sensor SRS detects the passage of any overlap and the two encoders E, EA detect the position of the sheets to be cut with respect to the position of the blade in real time so that the control logic LCP controlling the rotating cutting blade LC according to said information calculates exactly the cutting time as a function of the (fixed) distance between the sensor and the cutting point.

According to the invention, sensor SRS is located upstream of the cutting assembly provided with the rotating hot blade LC with respect to the feeding direction of the sheets to be cut. As it is shown in the embodiment of Fig. 2, sensor SRS includes a commercially available mechanical precision microswitch 1 operated by an oscillating arm 2 which is driven by a tracer end roller 3.

As can be clearly seen from the above figure the sheets to be cut pass between tracer roller 3 and a corresponding counterroller 4 rotating about a fixed axis in their feeding movement to the cutting point (indicated by the arrow direction). Thus, any change in the thickness of the sheets passing through said two rollers causes a shift of the tracer roller 3 that moves arm 2 and operates microswitch 1.

Such detection of the overlap causes the rotating cutting apparatus to operate without accumulation of

systematic errors just due to the fact that the operation device of blade LC is controlled in real time as a function of the detection signals from sensor SRS and the signals from encoders E, EA controlling continuously the speed of the sheets to be cut.

It should be noted that the operating stroke of microswitch 1 is advantageously a function of the length of oscillating arm 2 and the distance of microswitch 1 from the swing centre about which arm 2 rotates.

Still another advantage of the invention is that the programmable card LCP for the control logic receives operating parameter inputs from an outside control interface CE so that it can be easily programmed again according to the requirements without modifying the whole cutting apparatus.

According to a first embodiment of the cutting apparatus schematically shown in Fig. 3, an idle roller 8 which is fitted elastically into a support 5 is placed at the cutting point under the sheets and is able to facilitate the cutting action of the hot-wire blade by keeping the sheets in the correct position and holding them perfectly aligned with the cutting plane of blade LC. Said support 5 is parallel to the blade and moves perpendicular to the plane of movement of the sheets.

In particular there are provided springs 10 preferably made of steel which keeps the rotating hot-wire blade LC steadily taut on its support 9 preferably made of

Teflon.

As far as the elastic support 5 holding the sheets to be cut at the cutting point is concerned, it is preferably made of polyzene or other high-strenght plastic material and is supported by springs 6 preferably made of steel.

Fig. 3 shows a second embodiment of the invention which is simplified over the preceding embodiment in which the elastic support 5 of the sheets to be cut has no idle roller 8. In this case the movable sheets glide directly on the elastic support 5.

In the embodiments shown the blade is formed by a hot wire preferably made of tungsten steel.

It should be appreciated that, according to the present invention, sensor SRS for detecting the overlap may also be of the non-mechanical type but use another operating principle even if it performs the same function. For example, a commercially available sensor of the optical type could be used, the operating principle of which is schematically shown in Fig. 5, with its output signal changing when sudden variations in the thickness of the sheets gliding in its field of action occur.

In this case, there is no contact between optical sensor SRS and the sheets traversing to the cutting point so that all of the problems of wear and maintenance of the embodiments described above are advantageously overcome.

In fact photoreceiver FR of the optical sensor receives light signals originated by the reflection of

the signals of the transmitters on the right and left sides TD and TS, respectively. The intensity of said signals received changes as a function of the fact that the reflecting surface is either continuous or changes in thickness such as a step due to the presence of an overlap.

In particular, the optical sensor shown operates on the basis of the shadow area ZO projected by any surface unevenness or any step on the surface of the sheets which is illuminated alternately by right and left transmitters TD and TS, respectively. In the illustrative example, the light power of right transmitters TD is adjusted at a higher level than that of the left transmitters TS. Thus, photoreceiver FR receives a greater light intensity when the right transmitters are switched on and a lower light intensity when the left transmitters are switched on.

Such a difference in the intensity is constant until the surface of the sheets remains smooth.

In fact, in case there are shadow areas for right transmitters TD, as shown in Fig. 5, the difference in the light intensity received by the photoreceiver reduces or even is reversed, thus causing the receiver to detect an overlap. What mentioned above is valid for surfaces having any reflection factor.

It is self-evident from the foregoing that the optical sensor of the type described so far should be oriented correctly with respect to the direction of movement of the sheets which must be such that, as shown in the figure, the overlaps generate shadow areas only when

the transmitters with greater light power are switched on. Such transmitters TD and TS are preferably of the infrared type.

Finally, it should be appreciated that the invention can also be used without essential modifications in apparatus in which the cutting operation is not performed by a hot blade.

The present invention has been described and illustrated according to a preferred embodiment and some variations thereof, however, it should be understood that those skilled in the art can make equivalent technical modifications and/or replacements without departing from the scope of the present industrial invention.