ES1070247U - Horizontal packaging machine (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Horizontal packaging machine, of the type comprising: A work area (50) with successive operating stations (31-37) including at least reception, opening, dosing, closing and delivery of envelopes; An envelope conveyor (30) comprising a plurality of pairs of front and rear grippers (1, 2) transported by means of transportation along a closed circuit having a working section (3) through said stations operational (31-37) and a return section (4); An envelope supply means (55) empty to said envelope conveyor (30); Means for closing the front and rear clamps (1, 2) of each pair at the beginning of said working section (3) in order to grip an envelope (5) transferred from said envelope supply means (55); Means for bringing the front and rear clamps (1, 2) closer and farther apart in order to open or close the mouth of the envelope in accordance with the requirements of said operating stations (31-37) as they are traversed ; and means for opening the front and rear clamps (1, 2) of each pair at the end of the working section (3) in order to release the envelope (5) and transfer it to an exit conveyor (38); Characterized in that each pair of front and rear clamps (1, 2) is installed on a carriage (6) arranged to move freely along a guide device (7) defining said closed circuit, first traction means being arranged configured to be coupled with each carriage (6) at the beginning of the working section (3), to drag the carriage (6) along the working section (3) with a first type of movement and to uncouple from the carriage (6) at the end of the working section (3) and second entraining means configured to engage with each carriage (6) at the beginning of said return section (4), to drag the carriage (6) along the return section (4) with a second type of movement and uncoupling from the carriage (6) at the end of the return section (4), said second type of movement being faster than the first type of movement. (Machine-translation by Google Translate, not legally binding)

Description

Horizontal packing machine.

Technical field

The present invention concerns machine horizontal packing machine provided with a work area where find several operational filling and sealing stations of envelopes and along which some envelopes, suitable for containing by food example, are transported by a transporter. He conveyor is able to move the envelopes supported in a vertical position, with the mouth on the top, and to facilitate the opening and closing of the mouth of each envelope in accordance with the requirements of successive operating stations in the machine packing machine

Background of the invention

It is usual to provide products in general, and especially food products, whether powdered, granulated, pasty or liquid, packed in sealed envelopes. The Said envelopes are usually formed of a sheet material waterproof, heat-welded, heat sealed four sides, and may optionally include a double fold in the bottom to provide a widened base. To open one of such envelopes just cut a top corner of it, although optionally the envelopes can have a built-in mouthpiece that once opened can be closed again.

A type of packaging machine is known horizontal envelope comprising a material feeder heat sealable sheet in the form of a continuous continuous band of a coil, including a unwinding device for unwind the coil band. Then a device bender is arranged to bend the web sheet in "V" or "W", which then passes so folded by a vertical heat welding device to form seams separate verticals in the folded band that will constitute the edges side of the envelopes and by a heat-sealing device lower horizontal to form or reinforce the fold or folds bottom of the folded band that will constitute the bottom of the envelopes. Next, a disposed cutting device is provided to cut the folded and heat welded band in the middle of the vertical seams to form individual envelopes, which They are provided with a closed upper opening. Then a transfer device is arranged to transfer each individual envelope to a corresponding pair of tweezers that will transfer envelopes along a work area where they are arranged consecutively several operational filling and sealing stations, among which are, for example, one or more devices filling and a superior horizontal heat-welding device for seal the envelope. The clamps of each pair are operated to zoom in and out of each other during their journey along the work area in order to open or close the mouth of the envelope according to the requirements of the successive stations Operational work area. Finally there is a unloading device to unload filled and sealed envelopes to, for example, an exit conveyor belt.

The patent ES-A-2117929 describes one such packaging machines that includes an envelope conveyor associated with a work zone. The mentioned conveyor it comprises an endless roller chain guided by corresponding toothed pulleys along a closed circuit which has a working section through the successive stations operations of the work area and a return segment. In the roller chain are fixed a plurality of tweezers front and rear calipers alternated forming pairs of calipers front and rear that move in carousel along said closed circuit.

EP-0999137 describes an envelope conveyor associated with a packaging machine comprising a plurality of front and rear calipers fixed to independent, based, endless transport devices in roller chains, guided along a closed circuit with a working section and a return section, where the envelopes are singing displaced.

A disadvantage of the cited patents ES-A-2117929 and EP-0999137 is that, if you want to minimize the length of the work section through the work area, there are to arrange the pairs of front and rear calipers the most close to each other possible, and this implies a large number of couples of clamps fixed to the chain or chains, with a very high number of pairs of idle clamps in the return section, which implies considerable manufacturing and maintenance costs.

Patents ES-A-2012200 and US-A-5058364 describe respective envelope conveyors associated with a work area of a packaging machine, in which the envelopes are transferred flat, that is, with one of its wide faces ahead, instead of edge using two endless roller chain carousels with respective parallel and respective parallel work sections return sections.

From the aforementioned background it follows that known carousel type envelope conveyors perform the transfer of envelopes using tweezers dragged by chain rollers, both in the work area where the operational stations as in the return segment. By consequently, they have a single type of movement, which can be intermittent or continuous, throughout the closed circuit, but not they can perform these two types of movement in different areas of The same machine. For this reason there are packaging machines horizontal with continuous movement carousel and machines horizontal packing machines with movement carousel intermittent.

An objective of the present invention is to provide a horizontal packing machine equipped with conveyor Carousel type envelopes that are able to have a larger number of pairs of tweezers in the work section and a smaller number of pairs of clamps on the return leg, and moving the pairs of clamps at different speeds and / or with movement types different in the work section and in the return section.

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Exhibition of the invention

The above and other objectives are achieved by according to the present invention providing a packaging machine horizontal with a new envelope conveyor that uses two independent drag means to drag the pairs of clamps along a closed circuit in the form of a carousel. Some first drag means are dedicated to moving the pairs of tweezers, each carrying a corresponding envelope, along of a working section of the horizontal packaging machine where they find different operative stations of packaging, sealing, etc., and a few second drag means are dedicated to moving the pairs of clamps along a return section, being the pairs of clamps transferred from one of the means of drag to the beginning and end of the work segment.

Unlike the machines of the technique previous, where the drag of the pairs of tweezers is carried out by a roller chain, in the machine of the present invention the first drive means include, for example, a drum cam capable of transporting the envelopes of a operating station to another with different types of movement, by example, intermittent or continuous movement. This is achieved varying the profile of the drum cam adapting it to the different  movements that you want to carry out to transport the envelopes of One operating station to another. The second drag means they comprise a roller chain, so that the movement of the pairs of clamps along the return leg is a continuous movement, which can be performed at a higher speed high than the average speed provided by the cam of drum in the working section, so that the number of couples of tweezers in the return leg can be significantly smaller, for example, up to half, than in the section of job.

Thus, the horizontal packaging machine of the The present invention includes a work area where the different operating stations, the number and type of which may vary depending on the applications, usually included envelope reception stations, envelope opening, dosed, stretched and sealed superior by welding, and the conveyor of envelopes ensures that all these workstations are occupied by cars with clamps that carry the envelopes. For him on the contrary, in the return zone there is only a limited number, for example half of the cars with clamps that are in the area of job. This is possible because the return chain to the work area goes at a higher speed, for example twice as much speed, that the drum cam that moves the envelopes of a operating station to another in the work area.

The present horizontal packaging machine invention is of the type comprising a work zone with successive operating stations including at least reception, opening, dosing, closing and delivery of envelopes; a conveyor of envelopes comprising a plurality of pairs of tweezers front and rear transported by means of transport to along a closed circuit that has a working section to through said operating stations and a return segment; some means for supplying empty envelopes to said conveyor of envelopes; means to close the front and rear clamps of each couple at the beginning of said work section in order to grab an envelope transferred from said means of supply of envelopes; means to zoom in and out the front calipers and rear of each other in order to open or close the mouth of the envelope according to the requirements of the mentioned stations operational as they are crossed; and a means to open the front and rear clamps of each pair at the end of the section of work in order to drop the envelope and transfer it to a outbound conveyor The machine of the present invention is characterized in that each pair of front and rear calipers is installed on a car arranged to move freely to along a guide device defining said closed circuit, being arranged first drag means configured to engage with each car at the beginning of the work section, drag the car along the work section with a first type of movement and disengage the car at the end of the section of work and a few second drag means configured to engage with each car at the start of said return leg, drag the car along the return leg with a second type of movement and disengage the car at the end of the section of return, said second type of movement being faster than the First type of movement.

This construction provides several advantages. with respect to prior art devices. In first place, the fact of being each pair of tweezers installed on a corresponding car instead of being each front and rear caliper individually attached to one or more drag chains provides greater precision in the positions of the front calipers and rear one in relation to the other. In addition, some parts of the clamps and the means to zoom in or out of the pliers of each other can also be installed on the car, which provides greater precision in the operation of mechanisms The guidance of the cars, being independent of the drag means, can be carried out in a precise way and stable, for example by equipping each car with spinning wheels free coupled to a rigid rail arranged along the circuit. This allows to eliminate the negative effect that the possible vibrations and buckling of the roller chain or other means of drag have on the stability of the tweezers in the prior art devices. Moreover, the fact of have two independent drag means for the section of work and for the return leg it allows to provide a higher drag speed on the return leg, which offers the possibility of having the cars closest to each other possible in the work section and only a few cars in the section of return, with a noticeable decrease in the total number of cars, that is, of pairs of clamps, in the circuit.

Brief description of the drawings

The above and other features and advantages will be more fully understood from the following detailed description of some examples of practical realization with reference to the attached drawings, in which:

Fig. 1 is a perspective view of a envelope conveyor that is part of a packaging machine horizontal according to an embodiment example of the present invention;

Fig. 2 is a plan view of the conveyor of Fig. 1;

Fig. 3 is a front perspective view of one of the cars in relation to a guiding device and a cam slot;

Fig. 4 is a front perspective view of one of the cars in relation to the guiding device and a flexible traction element;

Fig. 5 is a schematic side view, partially sectioned, of one of the cars in relation to the guiding device, cam groove, traction element flexible and a drum cam at the beginning of a working section;

Fig. 6 is a schematic representation of a horizontal packaging machine according to a first example of embodiment of the present invention; Y

Fig. 7 is a schematic representation of a horizontal packaging machine according to a second example of embodiment of the present invention.

Detailed description of some embodiments

In relation to Figs. 6 and 7 are described in First, two different embodiments of the machine horizontal packing machine of the present invention, which have in common a working area 50 with successive operating stations 31-37, an envelope conveyor 30 in the form of carousel comprising a plurality of pairs of tweezers front and rear 1, 2 transported by means of transport along a closed circuit with a working section 3 that runs through said operating stations 31-37 in work zone 50 and a stretch of return 4. In Figs. 6 and 7 the aforementioned closed circuit is represented only symbolically by a dashed line around which the pair of pliers are shown front and rear 1, 2 of envelope conveyor 30. The configuration and operation of the envelope conveyor 30 they will be described in detail below in relation to Figs. 1 a 5. At the entrance of work area 50 there are some means envelope supply 55 to supply empty envelopes without open to said envelope conveyor 30, and at the exit of the zone working 50 is an output conveyor 38 for remove envelopes 5 once filled and sealed.

The first of the operating stations at the work area entrance 50 is a collection station for envelopes 31, where the tweezers of the corresponding pair of tweezers front and rear 1, 2 are in the open position waiting for a transfer device 43, 62 transfer envelopes 5 empty and unopened from the line of the mentioned means of supply of envelopes 55 to the line of the carousel envelope conveyor 30. Once an envelope 5 is received, the front and rear clamps 1, 2 will be they close to hold the envelope and the envelope is transported to the next operating station, which is an opening station 32, in which envelopes 5 are opened by one of several known mechanisms, such as, for example, opening suction cups by vacuum, or upper blow nozzle, in combination with a mutual approach movement of the first and second pliers 1, 2 holding the envelope 5. The envelope 5 empty and with the opening Open top is moved by envelope conveyor 30 to The next operating station. In the embodiments illustrated, after opening station 32 there are two Dosing or filling stations 33 consecutive, configured and willing to supply different products inside Envelopes 5. Next, envelopes 5 filled and with the opening upper open are driven to a closed or stretched station 34 of envelopes, where the envelope conveyor 30, by means of a movement of the first and second clamps 1, 2 opposite the previously performed at said opening station 32, stretches the top of envelopes 5 to close is top opening. TO then the 5 filled and closed envelopes are transferred to some first and second upper welding stations 35, 36 for seal the upper opening by means of bars application of pressure and temperature to the sheet material heat-sealable envelope 5 to effect the hermetic sealing of the top opening of the envelope. Finally, the 5 envelopes filled and sealed are transferred to an exit station 37, in which the heat welding is cooled by a cooling device and the first and second pliers 1, 2 of the pair of pliers are opened by a mechanism to release the envelope and transfer it to said exit conveyor 38, which may be in the shape of a conveyor belt or other alternative device. Inspection means (not shown) are arranged to inspect envelopes 5 once filled and sealed, and if any fault or defect was detected in one of them, this would be released by exit station 37 on a rejection ramp 39 instead of being transferred to the mentioned exit conveyor 38.

One skilled in the art will understand that the type and the number of operating stations, as well as the order in the that these are arranged, may vary depending on the application provided for the horizontal packaging machine without leaving the Scope of the present invention.

In the first embodiment shown in Fig. 6, the envelope delivery means 55 of the machine horizontal packing machine comprise an envelope forming device 40 configured to form envelopes 5 from a continuous band 41 of heat-weldable sheet material from a coil 42 and a transfer device 43 for transferring envelopes 5 formed to the envelope conveyor 30. The aforementioned device envelope maker 40 includes a unwinding device 44 for unwind the band 41 of the coil 42, and a bender device 45 arranged to bend the web of "V" sheet material or "W" The band 42 thus folded then passes through a device of horizontal weld 46a to reinforce the bending or lower folds that will constitute the bottom of the envelopes and by a  vertical heat sealing device 46b to form seams separate verticals in the folded band 41 that will constitute the side edges of envelopes. Once heat welded, band 42 passes through a cooling device 47 arranged to cool the welds Next is a forming device notch 48 to make one or more tear notches in a upper part of the band 41 bent and heat welded. Finally I know find a cutting device 49 to cut the band 41 folded and heat welded in the middle of the vertical seams with in order to form the individual envelopes 5, which are provided with a closed upper opening. Envelopes 5 individual are then changed line and transferred by the transfer device 43 to envelope conveyor 30 in the envelope collection station 31 of work area 50. Current downstream of the notch forming device 48 and current a cutting device 49 is arranged above the cutting device drag 52 based, for example, on passing the band 41 between the pinch of drag rollers facing each other. Generally the envelope forming device 40 includes one or more devices of inspection 51, such as one or more photocells, for inspect the folded and heat welded band 41 before being cut.

In the second embodiment shown in Fig. 7, the envelope delivery means 55 of the machine horizontal packing machine comprise a feeder device of envelopes 60 associated with a storage container 61 in which a quantity of 5 pre-packaged envelopes are stored provided with a closed upper opening. Said envelope feeding device 60 comprises, for example, a conveyor belt 63 to feed envelopes 5 from the storage container 61 to a transfer device 62 "pick and place" type arranged to change envelopes 5 line and transfer them to envelope conveyor 30 in the envelope collection station 31 of work area 50.

With reference now to Figs. 1 to 5 se describes in detail an envelope conveyor 30 according to a exemplary embodiment of the present invention, which is applicable to any of the embodiments of the packaging machine horizontal described above in relation to Figs. 6 and 7. The envelope conveyor 30 comprises a guide device 7 in the shape of a rail arranged in a geometric plane substantially horizontal and defining the closed circuit mentioned above, with a work section 3 and a section of return 4. Work section 3 runs through successive operating stations (not shown in Figs. 1 to 5) in the area 50. A plurality of cars 6 are arranged to be freely displaced along said guide device 7. For this, each carriage 6 has freewheel wheels 14 (better shown in Figs. 3, 4 and 5) coupled to said lane. He conveyor comprises first drag means configured to drag cars 6 along the stretch of work 3 of the guide device 7 with a first type of movement and a few second drag means configured to drag the carriages 6 along the return section 4 of the guiding device 7 with a second type of faster movement than the first mentioned type of movement. Even though that him return section 4 is longer than work section 3, the cars 6 take a short time to travel the return section 4 that the one they use to travel the work section 3 and, in consequently, in return section 4 there is only a small number of cars 6 significantly distanced while in the section of work there are a large number of cars 6 very close to each other. The first and second drag means will be explained in greater detail below.

In each car 6 a pair of front and rear calipers 1, 2 (best shown in Figs. 3, 4 and 5) capable of grabbing an envelope 5. Envelopes 5 are formed to from a double layer of a sheet material and have edges , sealed side and bottom edge, and a mouth or opening in the top The mentioned front and rear calipers 1, 2 of each couple are willing to grab each envelope 5 by the upper ends of their opposite side edges, next to the top opening, and envelopes 5 grabbed by the pairs of front and rear clamps 1, 2 are transported from edge and in vertical position. The conveyor includes means to close the front and rear clamps 1, 2 of each pair at the beginning of the working section 3 in order to grab an envelope 5 which is transferred from a transfer device 43, 62 or input device, means to zoom in and out of front and rear pliers 1, 2 of each other in order to open or close the mouth of envelope 5 according to the requirements of the mentioned operating stations 31-37 in the area of work 50 as they are being traversed, and means to open the front and rear clamps 1, 2 of each pair to end of work section 3 in order to release said envelope 5 and transfer it, for example, to the output conveyor 38 or to the rejection ramp 39. The means to open, close and move the Tweezers will be explained in greater detail below.

Work section 3 of guide device 7 it is substantially rectilinear and the first means of drag they are in the form of a drum cam 8 comprising a cylindrical surface of length substantially equal to the section of  work 3, arranged to rotate about an axis substantially parallel to working section 3, and a groove of helical cam 13 or similar to helical formed around and at along said cylindrical surface. Each car 6 carries a follower of cam 9 (best shown in Figs. 3, 4 and 5) in the form of a roller protruding from a rear part of the carriage 6, and arranged to rotate about an axis substantially perpendicular to the drum cam shaft 8. Cam follower 9 is configured and  arranged to be inserted in said cam slot 13 at the beginning of the working section 3, run through cam slot 13 while the carriage 6 is displaced along work section 3 a consequence of the rotation of the drum cam 8 and out of the groove of cam 13 at the end of working section 3. To do this, the groove of cam 13 has an entry opening 13a at the initial base of the cylinder that configures drum cam 8 and an outlet opening 13b at the final base of the cylinder that configures the drum cam 8.

In Figs. 1 and 2 drum cam shown 8 with a cam slot 13 of uniform helical shape, which provides a continuous type of movement at a uniform speed for cars 6 when drum cam 8 rotates at speed uniform. Intermittent movement can be achieved through a intermittent rotation of drum cam 8. In an example of alternative embodiment (not shown), cam slot 13 of the drum cam 8 has a helical shape of non-uniform pitch or with specific settings to vary the speed and / or type of movement of cars from one operating station to another at length of working section 3. This non-uniform helical groove it can also be combined with an intermittent turn of the cam of drum 8.

The second drag means comprise a flexible tensile element 10 endless in the form, for example, of a roller chain, which is provided with prominent drag configurations 11 distributed over distances regular throughout it. In the embodiment example illustrated, said drag configurations 11 comprise some stubs (best shown in Figs. 4 and 5) that protrude towards from the flexible traction element 10. Each carriage 6 carries  a coupling configuration 12 (best shown in Figs. 4 and 5) in the form of a fork protruding down from the rear of the car 6. The aforementioned fork has a pair of arms configured to house one of said lugs between both, so that the coupling configuration 12 can engage with one of said drag configurations 11 at the start of return section 4, stay coupled with the configuration of drag 11 along return section 4 and disengage from the drag configuration 11 at the end of return section 4.

As can be seen in Fig. 1, the endless flexible traction element 10, that is, the roller chain that forms part of the second drive means, is guided by a set of pulleys and guide elements (not shown) for moving in a substantially horizontal geometric plane located at a lower level than the guide device 7 and, as shown in Fig. 2, runs parallel to said guide device 7 along the return section 4 and moves away of the guiding device 7 in the working section 3. For this, at the beginning of the working section 3 the flexible traction element 10 has an uncoupling section 10a which moves away from the guiding device 7 crossing under the drum cam 8 and at the end of the working section 3 the flexible tensile element 10 has a coupling section 10b that converges towards the guide device 7 again crossing under the drum cam 8. Between the decoupling section 10a and the coupling section 10b, the flexible tensile element 10 comprises an inactive section 10c remote from the guide device 7 and substantially parallel to the axis of the drum cam 8. Preferably, the decoupling section 10a forms an angle with the axis of the drum cam 8 substantially equal to the angle that forms a tangent to the cam groove 13 with the axis of the drum cam 8 at the lowest point thereof and the coupling section 10b forms an angle with the axis of the drum cam 8 substantially equal to the angle
which forms a tangent to the cam groove 13 with the axis of the drum cam 8 at the highest point thereof.

Drive means are arranged to synchronously activate the movements of the element of flexible traction 10 and drum cam 8 so that when each carriage dragged by the second drag means to along the return section 4 in the direction of the arrows arrives at the beginning of work section 3, the coupling configuration 12 of carriage 6 is decoupled from the corresponding configuration of drag 11 of the flexible tensile element 10 substantially at the same time as cam follower 9 of the same carriage 6 is couples with cam slot 13 of drum cam 8, and when a carriage 6 that is dragged by the first means of drag to along the work section 3 reaches the end of work section 3, the cam follower 9 of the carriage 6 is disengaged from the cam slot 13 of the drum cam 8 substantially at the same time as the coupling configuration 12 of the same carriage 6 is coupled from again with one of the drag configurations 11 of the element of flexible traction 10. The cam groove 13 of the drum cam 8 It has a step selected for cars 6 coupled to the cam slot 13 of drum cam 8 mutually arrange adjacent along work section 3 and configurations towing 11 of the flexible tensile element 10 are separated mutually selected distances for cars coupled to the flexible tensile element 10 are arranged spaced along the return leg 4.

Making specific reference to Figs. 3, 4 and 5, each front or rear clamp 1, 2 comprises a jaw fixed 19 and a movable jaw 20 mounted on a corresponding front or rear pivot arm 15a, 15b. The mentioned gag mobile 20 can pivot about an axis 27 between a position open and a closed position, and a spring 21 (Fig. 5) is arranged to permanently push the movable jaw 20 towards the closed position (shown by solid lines in Fig. 5). In the closed position, the movable jaw 20 presses against said fixed jaw 19 to grab an edge of envelope 5. The movable jaw 20 is attached to a lever 22 that can be pressed by a trampler (not shown) to place the mobile jaw 20 in the open position (shown by dashed lines in Fig. 5) in order to admit an envelope 5 at the beginning of work section 3 or drop an envelope at the end of work section 3. Although in Fig. 5 spring 21 is shown as a traction coil spring arranged between an appendix of the movable jaw 20 and the corresponding pivot arm 15a, 15b, alternatively could be a torsional helical spring arranged around axis 27 or any other elastic means arranged to push the jaw mobile 20 towards the closed position.

Each of the pivoting arms 15a, 15b is mounted so that they can pivot individually with respect to a vertical axis supported by upper fork parts e bottom 23, 24 in each carriage 6, and is rigidly attached to a respective cam lever 16 which has at its free end a roller 17 inserted to run in a cam slot 18 arranged along the closed circuit. The aforementioned cam slot 18 define lateral displacements along its path of work section 3 in order to cause a movement of the cam levers 16 which in turn causes a pivot of the pivoting arms 15a, 15b and an approach or distance from the front and rear calipers 1, 2 from each other in order to open or close the mouth of the bag 5 grabbed by the tweezers front and rear 1, 2 according to the requirements of the operating stations 31-37 in the work area 50. Although cam slot 18 could be arranged only to along working section 3, it is preferred that it be arranged to throughout the closed circuit to ensure that the arms pivots 15a, 15b do not make unwanted movements during your displacement along the return section 4. In Fig. 5, show protective screens 25, 26 which are intended protect the conveyor mechanisms from possible splashes along work area 50.

Although not represented in the figures, the Conveyor of the present invention is installed in a structure that supports the guiding device, drum cam 8, the guiding members of the flexible tensile element 10, the cam slot 18, the drive means of the first and second drag means, the media traders of opening / closing of the clamps, the protective screens 25, 26, etc., as well as the various operating stations 31-37 of work area 50. The conveyor of the present invention is applicable, for example, to a machine horizontal envelope packer.

A person skilled in the art will be able to perform modifications and variations from the examples of embodiment shown and described without departing from the scope of the present invention as defined in the claims attached.

Claims (17)

1. Horizontal packing machine, of the type that understands: a work area (50) with successive operational stations (31-37) including at least reception, opening, dosing, closing and delivery of envelopes; an envelope conveyor (30) comprising a plurality of pairs of front and rear calipers (1, 2) transported by means of transport along a closed circuit that has a working section (3) through said operating stations (31-37) and a section of return (4); means for supplying empty envelopes (55) to said envelope conveyor (30); means to close the front clamps and rear (1, 2) of each couple at the beginning of said work section (3) in order to grab an envelope (5) transferred from said envelope supply means (55); means to zoom in and out of the tweezers front and rear (1, 2) of each other in order to open or close the mouth of the envelope according to the requirements of the mentioned operational stations (31-37) tailored that are crossed; Y means to open the front clamps and rear (1, 2) of each couple at the end of the work section (3) with in order to release the envelope (5) and transfer it to a conveyor of exit (38); characterized in that each pair of front and rear calipers (1, 2) is installed on a carriage (6) arranged to move freely along a guide device (7) defining said closed circuit, first dragging means being arranged configured to engage with each carriage (6) at the beginning of the work section (3), drag the carriage (6) along the work section (3) with a first type of movement and disengage the carriage (6) at the end of the working section (3) and a few second drag means configured to engage with each carriage (6) at the start of said return section (4), drag the carriage (6) along the return section (4) with a second type of movement and disengage the carriage (6) at the end of the return section (4), said second type of movement being faster than the first type of movement. 2. Machine, according to claim 1, characterized in that said first drive means comprise a drum cam (8) and said second drive means comprise a flexible tension element (10). 3. Machine, according to claim 2, characterized in that the working section (3) is substantially rectilinear and said drum cam (8) comprises a cylindrical surface of length substantially equal to the working section (3) arranged to rotate around of an axis substantially parallel to the working section (3) and a cam groove (13) formed around and along said cylindrical surface. Machine according to claim 3, characterized in that said cam follower (9) protrudes from a rear part of the carriage (6) and comprises a roller arranged to rotate about an axis substantially perpendicular to the axis of the drum cam (8) and configured to be inserted in said cam slot (13) at the beginning of the working section (3), run through the cam slot (13) along the working section (3) and exit the groove of cam (13) at the end of the work section (3). 5. Machine according to claim 2, 3 or 4, characterized in that said second drive means comprise a flexible traction element (10) provided with prominent drive configurations (11) distributed at regular distances and each carriage (6 ) carries a coupling configuration (12) configured to engage with one of said drag configurations (11) at the start of the return section (4), to remain coupled with the drag configuration (11) along the return section ( 4) and disengage from the drag configuration (11) at the end of the return section (4). Machine according to claim 5, characterized in that said flexible traction element (10) is an endless flexible traction element and is guided and actuated to move parallel to said guide device (7) along the section of return (4) and to move away from the guide device (7) in the working section (3). Machine according to claim 6, characterized in that the guiding device (7) is arranged in a substantially horizontal geometric plane and said flexible traction element (10) moves in another substantially horizontal geometric plane at a lower level than the guiding device (7). Machine according to claim 7, characterized in that said drag configurations (11) comprise lugs that protrude upwards from the flexible traction element (10) and the coupling configurations (12) comprise forks that protrude towards down from the carriages (6), each fork having a pair of arms configured to house one of said lugs between them. Machine, according to any one of claims 5 to 8, characterized in that the flexible traction element (10) is a roller chain. Machine according to claim 6, characterized in that at the beginning of the working section (3) the flexible traction element (10) has an uncoupling section (10a) that crosses below the drum cam (8) forming an angle with the axis of the drum cam (8) substantially equal to the angle that forms a tangent to the cam groove (13) with the axis of the drum cam (8) at the lowest point thereof and At the end of the working section (3) the flexible traction element (10) has a coupling section (10b) that crosses below the drum cam (8) at an angle to the axis of the drum cam (8). ) substantially equal to the angle formed by a tangent to the cam groove (13) with the axis of the drum cam (8) at the highest point thereof. Machine according to claim 10, characterized in that the flexible tension element (10) comprises an inactive section (10c) away from the guide device (7) between the decoupling section (10a) and the coupling section ( 10b). 12. Machine according to claim 10, characterized in that the flexible traction element (10) and the drum cam (8) are synchronously actuated such that the coupling configuration (12) of each carriage (6) is decoupling the corresponding drive configuration (11) of the flexible traction element (10) substantially at the same time as the cam follower (9) of the same carriage (6) is coupled with the cam groove (13) of the cam drum (8) at the beginning of the working section (3), and the cam follower (9) of each carriage (6) is disengaged from the cam groove (13) of the drum cam (8) substantially at the same time that the coupling configuration (12) of the same carriage (6) is coupled with a drag configuration (11) of the flexible traction element (10) at the end of the working section (3). 13. Machine according to claim 12, characterized in that the cam groove (13) of the drum cam (8) has a selected passage for the carriages (6) coupled to the cam groove (13) of the Drum cam (8) are arranged mutually adjacent along the working section (3). 14. Machine, according to any one of claims 1 to 7, characterized in that the guide device (7) comprises a rail and each carriage (6) has freewheel wheels (14) coupled to said rail. 15. Machine according to any one of claims 1 to 7, characterized in that the front and rear calipers (1, 2) are mounted on respective pivoting arms (15a, 15b) mounted so that they can pivot individually on each carriage (6), where each of said pivoting arms (15a, 15b) is rigidly attached to a respective cam lever (16) having at its free end a roller (17) inserted to run in a cam slot (18) arranged along at least the working section (3), said cam groove (18) defining lateral displacements in its path to cause an approach or distance of the front and rear calipers (1, 2) one of a the other. Machine according to any one of the preceding claims, characterized in that the envelope supply means (55) comprise an envelope forming device (40) for forming envelopes (5) from a continuous band (41) of heat sealable sheet material from a coil (42) and a transfer device (43) to transfer the formed envelopes (5) to the envelope conveyor (30). 17. Machine according to any one of claims 1 to 15, characterized in that the envelope supply means (55) comprise an envelope feeder device (60) for feeding envelopes (5) from a storage container (61) and a transfer device (62) for transferring the envelopes (5) fed to the envelope conveyor (30).