Stretching machine for stretching thermoplastic films

Abstract

The machine (1) includes at least one assistance subassembly (10) having: an endless chain or belt (16); a plurality of thrust members (20) connected to the chain or belt (16) in succession and projecting from the chain or belt (16), and a drive device (21) of this chain or belt (16), several thrust members (20) are brought into the path (3) of the sliders (8) connected to the grippers (4) for gripping the film (F), so that at least one of the thrust members (20) comes to bear against at least one of these sliders (8) and exerts a thrust on this slider to assist the displacement of this slider along the rail (5e) on which this slider moves.

Claims

1. A machine for stretching films made of thermoplastic material comprising: a heating tunnel for heating a plastic material of the film to be stretched; two endless paths for the circulation of grippers capable of gripping the lateral edges of the film located on either side of the path of the film in the heating tunnel; each endless path has an inner longitudinal portion extending partly inside the heating tunnel, and an outer longitudinal portion extending outside the heating tunnel, and comprising an inner guide rail closer to the edge of the film and an outer guide rail further from the edge of this film; the grippers for the lateral edges of the film are mounted on first sliders running along the inner guide rail; pairs of connecting rods connecting the first sliders to second sliders running along the outer guide rail; at least one assistance subassembly located in the heating tunnel, comprising: an endless chain or belt; a plurality of thrust members connected to the chain or belt in succession and projecting from this chain or belt, and a drive device of this chain or belt, the assistance subassembly being arranged that a thrust part of the chain or belt is located close to the second sliders, and the drive device driving the chain or belt so that said thrust part of the chain or belt moves in the vicinity of the second sliders in the same direction as the direction of travel of the second sliders, several thrust members carried by said thrust part of the chain or belt are thus brought into the path of the second sliders, so that at least one of the thrust members comes to bear against at least one of the second sliders and exerts a thrust on the second slider to assist the displacement of the second slider along the rail on which the second slider moves.

2. The machine according to claim 1, wherein each thrust member comprises a free end part which is movable relative to a base part connected to the chain or belt; the mobility is effective between an extension position of the free end part relative to the base part, in which the thrust member is capable to exert thrust against one second slider, and a retracted position of the free end part with respect to the base part, in which the free end part is removed with respect to the second slider; a resilient device act on each free end part, in order to normally hold the free end part in the extended position.

3. The machine according to claim 1, wherein the machine comprises a plurality of assistance subassemblies, wherein each assistance subassembly has a length less than the length of the zone of the inner longitudinal portion where stretching of the film may be effected, in particular a length substantially less than a quarter of the length of such a zone.

4. The machine according to claim 3, wherein the assistance subassemblies face each other in the transverse direction to the forward direction of the film.

5. The machine according to claim 1, wherein the portion of one second slider against which a thrust member is adapted to exert thrust is in the form of a roller.

6. The machine according to claim 1, wherein each assistance subassembly comprises electronic devices for limiting the driving torque exerted by the driving means on the chain or belt.

7. The machine according to claim 1, wherein each assistance subassembly comprises a support structure against which the thrust part of the chain or belt may bear.

8. The machine according to claim 1, wherein each assistance subassembly comprises a drive motor for the chain or belt, wherein the motor is placed in a thermal protection enclosure.

9. The machine according to claim 8, wherein the thermal protection enclosure is cooled.

10. The machine according to claim 1, wherein each assistance subassembly comprises devices for varying the speed of the drive motor and a detection devices for detecting the speed of movement of the sliders upstream and downstream of the this assistance subassembly, wherein the detection devices are connected to a central control unit of the device for variation of the motor speed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be better understood, while other features and advantages thereof will become apparent by reference to the accompanying schematic drawing, this drawing showing, by way of a non-limiting example, a preferred embodiment of the mechanism concerned; in the drawing:

(2) FIG. 1 shows a top view of a known machine, shown schematically and in a very simplified manner;

(3) FIG. 2 shows a top view, also schematic and very simplified, of a portion of this machine on an enlarged scale;

(4) FIG. 3 shows a perspective view of a film gripping and stretching assembly of this known machine;

(5) FIG. 4 shows a view of the machine according to the invention, similar to FIG. 1;

(6) FIG. 5 shows a view similar to FIG. 2, particularly showing one of the assistance subassemblies of this machine;

(7) FIG. 6 shows a view similar to FIG. 3;

(8) FIG. 7 shows a sectional view along the line VII-VII of FIG. 5;

(9) FIG. 8 shows a view similar to FIG. 5, on an enlarged scale; and

(10) FIGS. 9 to 11 shows views of different engagement situations of thrust members with sliders of the machine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(11) FIG. 4 shows the machine 1 according to the invention, which has the same basic general structure as the machine 1 described with reference to FIGS. 1 to 3, but which comprises subassemblies 10 to assist the sliding of the sliders 6, 8 on the rails 5i, 5e; the elements or parts already described, which, as they are identical, will not be described again but will simply be designated by the same reference numbers.

(12) In the example shown, the machine 1 comprises eight subassemblies 10, each of which is located in the heating tunnel 2 and arranged on a base 9 near the outer rail 5e. As may be seen, the subassemblies 10 face each other in a direction transverse to the forward direction of the film. Each of them has a length much shorter than the length L of the zone of an internal longitudinal portion 3a where stretching of the film is to be effected, wherein the length of each subassembly 10 is, in the example shown, of the order of one tenth of the length L of this zone.

(13) With reference to FIG. 5, it appears that an assistance subassembly 10 comprises:

(14) an endless chain 16 traveling on two wheels 17 carried by a frame 18;

(15) a plurality of thrust members 20, connected to the chain 16 in succession and protruding from this chain, on the outside thereof;

(16) a geared motor 21 for driving one of the wheels 17, located in a thermal protection enclosure 22;

(17) a support structure 23;

(18) a central control unit 24 for the geared motor 21, located outside the heating tunnel 2; and

(19) two sensors 25 to detect the speed of travel of the sliders 8.

(20) As shown, the frame 18 is in two parts and extends longitudinally in a direction parallel to that of the rail 5e. One of the parts of the frame 18 comprises the geared motor 21, and one of the two wheels 17 is directly connected to this geared motor and is driven by it; the other part of the frame 18 comprises the other wheel 17. These two parts are connected to one another telescopically and a spring 26 is interposed between them to exert a thrust on these two parts that tends to move them apart from one another and ensure permanent tensioning of the chain 16.

(21) The chain 16 comprises the thrust members 20 that are attached to it. The subassembly 10 is arranged beyond the rail 5e but in the immediate vicinity thereof, so that an part of the chain 16 lies parallel to the rail 5e and is located near the sliders 8; it is also shown that the protrusion of each thrust member 20 with respect to the chain 16 enables at least one thrust member 20 to engage with rollers 28 which comprise the sliders 8, at said part of the chain. FIG. 6 shows more particularly such a roller 28.

(22) As shown in FIGS. 5, 7 and 8, each thrust member 20 comprises a base part 30 connected to the chain 16 and a free end part 31 telescopically connected to the base part 30. Each base part 30 forms a smooth lower portion allowing sliding of the thrust member 20 against the bearing structure 23. Each free end part 31 has a slidably-engaged rod in the base part 30, and an end part having a shape arranged to bear against the corresponding roller 28 of a slider 8; this shape is that of an L in the example shown. Each thrust member 20 further comprises a spring 32 interposed between the base part 30 and the free end part 31 in order to exert pressure on the base part 30 and the free end part 31 and normally maintain the free end part in an extended position relative to the base part 30.

(23) It may be seen in FIG. 8 that each free end part 31 is arranged to be brought into a retracted position by sliding of the rod in the base part 30 against the elastic force of the spring 32 in the event of a conflict between a roller 28 of a slider 8 and the free end part 31.

(24) FIG. 7 also shows the various rollers 35 which allow the rolling of the slides 6 and 8 along the rails 5i and 5e.

(25) The geared motor 21 is of the conventional type that is well known per se, and therefore does not have to be described in detail. It comprises a speed variator.

(26) The thermal protection enclosure 22 has insulating linings and is connected to a ventilation system (not shown) with air inlet and outlet openings.

(27) The control unit 24 of the geared motor 21 comprises electronic means for processing the signals transmitted by the sensors 25, electronic means for controlling the speed variator as a function of the speed detected by the sensors 25, and electronic means for limiting the torque exerted by the geared motor 21 on the wheel 17.

(28) In practice, the measurements made by the two sensors 25 allow the central unit 24 to determine whether the current stretching is positive or negative stretching, and to adapt the speed of the gear motor 21 accordingly. The speed of the latter, and thus the speed of travel of the thrust members 20, is thus adapted in real time to the displacement of the sliders 8. If it is determined that the current stretching is negative stretching, the central unit 24 acts on the speed the geared motor 21 so that the speed of movement of the thrust members 20 at the active part of the chain 16 is slightly greater than that of the sliders 8 at said part of the chain. This slightly higher speed causes at least one of the thrust members 20 on the chain active part 16 to bear against the roller 28 of a slider 8 and to exert a thrust on the slider via this roller as shown in FIG. 9. This thrust assists the movement of the sliders 8, and thus also the sliders 6 connected to them, along the rails 5i and 5e, thus relieving the main drive chain of the slider assemblies 6, 8 and connecting rods 7. This assistance allows the sliders 6, 8 to move appropriately along the rails 5i, 5e even under strong forces, particularly when stretching films at relatively low temperatures, or when it is necessary to simultaneously implement transverse stretching and negative longitudinal stretching, and, therefore, the sliders 6 and 8 must move along on the rails 5i, 5e which diverge and which deflect outwards.

(29) The bearing structure 23 opposes any bowing of the chain 16 during the application of this thrust, thus eliminating the risk, in normal situation, of thrust members 20 escaping from the rollers 28 and the risk of damage of these thrust members in the event of the thrust surface being too small.

(30) If a thrust member 20 comes into conflict with a roller 28, as may be seen in FIG. 10 or 11, the possibility of retraction of the free end part 31 of the thrust member 20 makes it possible to avoid any damage to the latter; the reduction of the intervals between the sliders 8 as the sliders move along the rail 5e causes at least one member 20 located in front of the member 20 subject to the conflict to achieve a corresponding situation of thrusting of a roller 28 along the said active part of the chain 16.

(31) In the event of excessive force being exerted on the thrust members 20, the escape of these thrust members 20, and thus disengagement of the assistance subassembly, is also made possible, thus avoiding the risk of damage to the machine 1, this disengagement being favored by the rotation of the roller 28.

(32) The electronic means for limiting the torque exerted by the geared motor 21 on the wheel 17 which comprises the central unit 24, makes it possible to prevent the assistance subassembly 10 from jolting when a member 20 passes beyond a corresponding slider 8, or in the event of the escape of this member 20 relative to the slide 8. Such jolting would be detrimental to the quality of the stretching obtained and the mechanical strength of the assistance subassembly 10.

(33) If the sensors 25 determine that the current stretching is positive stretching, the speed of the geared motor 21 will also be adapted to provide the required assistance, if necessary.

(34) The invention thus provides a machine having the decisive advantage, compared with machines according to the prior art, of allowing significant negative stretching rates, without the risk of generating inaccuracy in the positioning of the grippers, wherein this machine also has the decided advantage of not requiring a mechanical intervention for adaptation to the different gripper interval values that are required according to the different types of films to be stretched.

(35) The invention has been described above with reference to an embodiment provided by way of an example. It goes without saying that the invention is not limited to these embodiments, and the scope of protection is defined by the appended claims.