Method for controlling the stretching means of a blowing station

Abstract

The invention relates to a method for controlling a stretching means (57) of a container blowing station comprising: - a mold consisting of at least two half-molds and a mold base, - a blowing nozzle moving between a so-called rest position and a so-called working position where said nozzle covers the mold when it is closed, - a stretching means (57) comprising at least one actuator (58) coupled to an elongation rod (59), said elongation rod sliding through the nozzle and the preform positioned in the mold;Characterized in that it comprises at least the following steps: - pressing said blow nozzle against the mold which is in the closed position, - moving said elongation rod between said rest position and said extended position, - measuring at least one parameter representative of the operation of the stretching means (57) during all or part of said movement of said elongation rod (59), - comparing said measured parameter with a determined reference value, - emitting an alert signal if a critical deviation is detected between said measured parameter and the reference value. Figure for the abstract: Figure 3.;

Description

Title of the invention: Method for controlling the stretching means of a blowing station TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to the manufacture of plastic containers from preforms by forming using a fluid.

[0002] The invention relates more particularly to a method for manufacturing containers made of plastic material, such as PET, by blowing or stretch blowing preforms, such as preforms, with a pressurized fluid, in particular air.

[0003] To manufacture a container with such a method, a preform is first heated to a temperature higher than the glass transition temperature of the plastic material of the preform used. The preform is then introduced into a mold to be formed there. TECHNICAL BACKGROUND OF THE INVENTION

[0004] The forming of a container is carried out in a container production facility from a preform previously heated in a thermal conditioning unit to a temperature above the glass transition temperature of the material constituting the preform (which is approximately 80°C for PET). The machine comprises a plurality of forming stations, each equipped with a mold, an elongation rod and a nozzle connected to a source of pressurized fluid and through which the fluid is injected into a blank placed in the mold.

[0005] Ordinarily, the preform is stretched longitudinally with the elongation rod and simultaneously, the fluid is injected into the preform in two stages: a first stage called pre-blowing which consists of putting the blank in communication with a source of so-called pre-blowing pressure, relatively low (less than 10 bars), followed by a second stage called actual blowing, which consists of putting the blank in communication with a source of so-called blowing pressure significantly higher (greater than 15 bars, and generally greater than 20 bars). The container is then depressurized prior to its removal from the mold.

[0006] The forming of containers involves a large number of actuators which may fail. These failures may be perceived directly on the production installation by installation shutdowns linked to the failure of an actuator, or indirectly on the containers, which have manufacturing defects. The identification of a failure linked to the actuators therefore requires on the part of the operator an in-depth knowledge of the correlations which may exist between a malfunction of the system and a defect on a container taken at random from production.

[0007] Assuming that such a failure is identified, it remains to identify the faulty forming station, which requires stopping the machine (and therefore production). BRIEF SUMMARY OF THE INVENTION

[0008] The invention proposes a method for controlling a stretching means of a container blowing station equipping an installation for manufacturing containers obtained by stretch blowing from a preform made of thermoplastic material, said blowing station comprising: - a mold consisting of at least two half-molds and a mold base, said mold being capable of receiving a preform to be stretched and blown after closing the mold, - a blowing nozzle moves between a so-called rest position and a so-called working position where said nozzle covers the mold when it is closed, - the stretching means comprising at least one actuator coupled to an elongation rod, said elongation rod sliding through the nozzle and the preform positioned in the mold between a so-called rest position and a so-called extended position corresponding to the free end of the elongation rod touching said mold bottom; Characterized in that it includes at least the following steps: - place said blowing nozzle against the mold which is in the closed position, -move said elongation rod between said rest position and said extended position, -measure at least one parameter representative of the operation of said stretching means during all or part of said movement of said stretching rod, - compare the said measured parameter to a determined reference value, - issue an alert signal if a critical deviation is detected between the said measured parameter and the reference value.

[0009] This process makes it possible to ensure that the stretching means is functioning correctly for the next production of containers and if this is not the case, this control process makes it possible to be alerted.

[0010] According to another characteristic of the control method carried out according to the teachings of the invention, the steps are carried out in the absence of preform in the mold before the start of the production of containers or after having carried out the production of containers.

[0011] According to another characteristic of the control method carried out according to the teachings of the invention, the steps are carried out with a so-called degraded speed of the blowing station, that is to say with a rate lower than the production rate.

[0012] This method makes it possible to acquire the parameters allowing the implementation of this process because at the production rate, this is not possible.

[0013] According to another characteristic of the control method carried out according to the teachings of the invention, the actuator of the stretching means is a mechanical or pneumatic actuator such as a jack.

[0014] According to another characteristic of the control method carried out according to the teachings of the invention, the actuator of the stretching means is an electric actuator.

[0015] According to another characteristic of the control method carried out according to the teachings of the invention, the electric actuator is a linear motor.

[0016] According to another characteristic of the control method carried out according to the teachings of the invention, the measured parameter corresponds to a torque generated during all or part of the movement of the elongation rod.

[0017] According to another characteristic of the control method carried out according to the teachings of the invention, the measured parameter corresponds to the intensity of the operating current of said electric actuator.

[0018] According to another characteristic of the control method carried out according to the teachings of the invention, the measured parameter is acquired by means of a dedicated sensor.

[0019] According to another characteristic of the control method carried out according to the teachings of the invention, it comprises a step of constructing a signal characteristic of the intensity of the measured current representative of the stretching means of each blowing station.

[0020] According to another characteristic of the control method carried out according to the teachings of the invention, it comprises a step of sampling the signal to detect similar sequences repeating at constant sensitive intervals (T).

[0021] According to another characteristic of the control method carried out according to the teachings of the invention, when a deviation is detected between one of said sequences and a reference sequence, then an operation of determining an anomaly is implemented to identify the faulty element of said stretching means, each anomaly corresponding to a predetermined deviation.

[0022] According to another characteristic of the control method carried out according to the teachings of the invention, the alert signal comprises the display of the anomaly of said stretching means. BRIEF DESCRIPTION OF THE FIGURES

[0023] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which one will refer to the appended drawings briefly described below.

[0024] [Fig.l] is a schematic general view of a container production installation seen from above;

[0025] [Fig.2] is a detail view of the medallion of [Fig.l] representing a preform;

[0026] [Fig.3] is a schematic sectional view partially showing a unit of molding within the container production facility of [Fig.l] whose elongation rod is in a so-called rest position;

[0027] [Fig.4] is a schematic sectional view partially showing a unit of molding within the container production facility of [Fig.l] whose elongation rod is in a so-called extended position;

[0028] [Fig.5] is a schematic sectional view partially showing a unit of molding within the container production facility of [Fig.l] equipped with a sensor;

[0029] [Fig.6] is a diagram on which a reference curve is plotted showing the variations in the intensity of the current of the drawing means during all or part of the movement of the elongation rod as a function of time, for a given blowing station;

[0030] [Fig.7] is a diagram on which a curve is plotted showing the variations current intensity as a function of the time for which a fault was detected; the reference curve in [Fig.6] is shown in dotted lines;

[0031] [Fig.8] is a diagram on which a curve is plotted showing the variations current intensity as a function of the time for which another fault was detected; the reference curve in [Fig.6] is shown in dotted lines. DETAILED DESCRIPTION OF THE FIGURES

[0032] In the remainder of the description, elements having an identical structure or similar functions will be designated by the same references.

[0033] The longitudinal, vertical and transverse orientations will be adopted without limitation with reference to the trihedron (L, V, T) shown in the figures.

[0034] By convention, the longitudinal and transverse directions are determined in a fixed manner relative to the molding devices so that the open or closed position occupied has no effect on said orientations.

[0035] The terms "front" and "rear" will also be used, without limitation, in reference to the longitudinal orientation, as well as "upper" and "lower" in reference to the vertical orientation and finally "left" or "right" and "inner" or "outer" in reference to the transverse orientation.

[0036] [Fig.l] shows schematically an installation 1 for the mass production of containers 2 made of thermoplastic material from preforms 4.

[0037] In the remainder of the description, the preforms 4 and the containers 2 move in the production installation along a production path from upstream to downstream. The preforms 4 are moved in line along a production path T by conveying means which will be detailed later.

[0038] In a non-limiting manner, the containers here are bottles. The thermoplastic material is for example here formed by polyethylene terephthalate, hereinafter referred to by its acronym "PET".

[0039] Such a preform 4, with reference to [Fig.2], has a main axis "X" shown vertically in the figure. It has a tubular cylindrical body 6 closed at one of its axial ends by a bottom 8, and which is open at its other end by a neck 10, also tubular. The neck 10 is delimited downwards by a collar 12 and upwards by an upper end edge called a drinking mouth 14.

[0040] The neck 10 generally has its final shape while the body 6 of the preform is intended to undergo a relatively significant deformation to form the final container 2 during a forming step.

[0041] As shown in [Fig.l], the container manufacturing installation 1 comprises at least one thermal conditioning unit 16, one forming unit 18.

[0042] The thermal conditioning unit 16, also called an oven, makes it possible to heat a succession of preforms to a reference temperature. The reference temperature is chosen so that the body of each preform at the outlet of the thermal conditioning unit 16 is in a malleable state allowing deformation of the body 6 of the heated preform in order to form the container 2 in the forming unit. The reference temperature is between the glass transition temperature and the crystallization temperature of the plastic material of the preform. In the case of PET, the reference temperature is, for example, close to 110°. The value of the reference temperature may vary depending on the product with which the container will be filled or depending on the container filling technique. Thus, the reference temperature is different for hot filling or for a carbonated product, for example.

[0043] According to the embodiment shown in [Fig.l], the thermal conditioning unit 16 is a scroll oven, in which the preforms 4 are transported to be exposed to a plurality of heating radiation sources 22.

[0044] For this purpose, the thermal conditioning unit 16 comprises a means for conveying the preforms circulating along a frame 23 through the thermal conditioning unit along a heating path extending between an inlet 25 and an outlet 27 of the thermal conditioning unit defining a trajectory T.

[0045] The conveying means comprises a plurality of conveying devices 26 capable of receiving a preform by fitting its neck.

[0046] The oven also comprises a heating cavity which comprises two side walls 36 facing each other and at least one of these walls being the one which supports several radiation sources 22, arranged one above the other and one next to the other facing the preforms.

[0047] In other words, the thermal conditioning unit 16 comprises a plurality of radiation sources 22 distributed along the path of the trajectory T and at a height corresponding substantially to the height of the preforms so that the entire height of the body of each preform is exposed to the radiation sources on the path of the preform in the thermal conditioning unit. By rotating the preforms around their main axis X, the conveying devices 26 make it possible to uniformly expose the entire body 6 of the preforms to the radiation sources 22. In this particular embodiment, the radiation sources 22 are distributed on only one side of this path, and a reflective wall 38 is arranged on the other side of the heating path to reflect the heat towards the preforms.

[0048] In another embodiment not shown, the radiation sources 22 can be distributed on either side of the heating path without departing from the scope of the invention.

[0049] It should also be noted that the radiation sources 22 are arranged, where appropriate, so as not to subject the neck 10 to the heat emitted by the radiation sources. Indeed, as indicated previously, only the body 6 of the preform 4 is heated to produce the container. Consequently, the neck 10 must not be deformed during forming and must not be heated. To avoid heating the neck 10, the furnace may comprise a ventilation device, not shown in the figures, positioned in line with the necks 10 to evacuate the heat likely to be absorbed by said necks 10.

[0050] Each radiation source 22 is formed by an incandescent lamp emitting infrared radiation.

[0051] In another embodiment, each radiation source 22 is a laser diode emitting infrared radiation.

[0052] In other words, each radiation source 22 is a laser (for example laser diodes) emitting in the infrared and organized by juxtaposition and / or superposition to form one or more matrices.

[0053] In another embodiment, each radiation source 22 is a microwave generator.

[0054] It is quite obvious that the radiation sources 22 may consist of any radiation source well known to those skilled in the art, or a combination of these radiation sources without departing from the scope of the invention.

[0055] Then, once the preform 4 has been thermally conditioned in the thermal conditioning unit 16, it is transferred to the forming unit 18 to be formed there.

[0056] The unit 18 for forming containers 2 from preforms 4 consists of a forming wheel 42 rotating a plurality of blowing stations 44 of a inlet to an outlet, at which a succession of containers are formed from the preforms and then extracted, as shown in [Fig.l]. The axis of rotation of the forming wheel is, for example, substantially parallel to the main axis X of the preforms as they are transported by the forming wheel.

[0057] Each blowing station 44 comprises a mold 46 provided with walls forming a molding cavity having the shape of the container to be formed and arranged to receive a preform so that the body of the preform extends into the molding cavity.

[0058] As shown in [Fig.3], each blowing station 44 further comprises:

[0059] -the mold 46 comprising two half-molds 48, 50 articulated around a hinge and a mold bottom 52;

[0060] - a housing 54 defining a nozzle 56 which, during the manufacture of the container 2, covers the neck 10 of the preform 4. The nozzle comprises a drinking support called a stabilizer 55 which presses the preform against the upper surface of the mold during the descent of the nozzle;

[0061] -a stretching means 57 comprising at least one actuator 58 coupled to an elongation rod 59. The elongation rod 59 slides through the nozzle 56 and the preform 4 positioned in the mold along a main axis A (generally of revolution) thereof, between a so-called rest position and a so-called extended position where, at the end of the longitudinal stretching of the preform 4, the elongation rod 59 reaches the mold bottom 52, pressing the bottom 8 of the preform there.

[0062] According to a first embodiment, the actuator 58 of the stretching means 57 is an electric actuator, and more particularly, a linear motor.

[0063] In principle, the linear motor consists of a rotor that is movable relative to a stator, both being "flattened". The stator is generally composed of electromagnets supplied with current, fixed to the frame (not shown) of the blowing station and the rotor is a permanent magnet on which one of the ends of the elongation rod 59 is fixed. The current supply to the stator allows the movement and in particular the translation of the rotor relative to the stator.

[0064] Alternatively, according to the general knowledge of those skilled in the art, there are different types of linear motors: the cylindrical linear motor, the U-channel linear motor, or the flat linear motor.

[0065] According to another embodiment, the actuator 58 of the stretching means 57 is a mechanical actuator as described in the applicant's European patent EP1725389.

[0066] According to another embodiment, the actuator of the stretching means 57 is a pneumatic actuator such as a single or double-acting pneumatic cylinder.

[0067] Furthermore, according to a first embodiment, the movement of the nozzle is controlled by a pneumatic cylinder. This is independent of the movement of the stretching means 57.

[0068] In an alternative embodiment, the movement of the nozzle 56 is coupled to the movement of the stretching means 57 and more particularly to that of the elongation rod 59. This embodiment is described in French patent FR2943941.

[0069] The blowing station 44 also further comprises several fluid circuits opening into the nozzle 56 via the housing 54, namely:

[0070] -a medium pressure pre-blowing air circuit 60 (between 5 and 16 bars), this circuit 60 comprising a source 62 of pre-blowing air and a conduit 64 (which can be formed at least partially in the housing 54) connecting this source 62 to the nozzle 56 with the interposition of a first solenoid valve 66, called the pre-blowing solenoid valve,

[0071] - a high pressure blowing air circuit (between 30 and 40 bars), comprising a source 70 of blowing air and a conduit 72 (which can be formed at least partially in the housing 54) connecting this source 70 to the nozzle 56 with the interposition of a second solenoid valve 74, called the blowing solenoid valve,

[0072] - a degassing circuit 76 comprising a vent 78 and a conduit 80 connecting the nozzle 56 to this vent 78 with the interposition of a third solenoid valve 82, called the degassing solenoid valve.

[0073] The stretching means 57 and the solenoid valves 66, 74, 82 are electrically connected to a control unit 84 which controls the movement of the rod 59 and the opening and closing of the solenoid valves.

[0074] This control unit 84 notably comprises a calculator 86 (or processor), a database 88, a console (or graphical interface) for interaction with an operator.

[0075] The preform 4 is first introduced into the oven where it is heated under the conditions described above. Upon leaving the oven, the preform 4 is gripped by a clamp of a transfer wheel and introduced into a previously opened mold. In this position, the elongation rod 59 is in the rest position.

[0076] As the carousel rotates, the mold closes over the preform 4 and begins a forming cycle.

[0077] The forming cycle is repeated for each blowing station 44. It comprises a step of lowering the nozzle 56 to cover the preform positioned in the mold and at least one step of injecting a pressurized fluid into the preform 4, followed by a depressurization step.

[0078] More specifically, the forming cycle comprises:

[0079] -a first phase, called pre-blowing, consisting of injecting a fluid into the preform under a pre-blowing pressure, by placing the nozzle 56 in communication with the source 62 of air at the pre-blowing pressure to radially stretch the body of the preform. For this purpose, the control unit commands, via its actuator 58, the opening of the pre-blowing solenoid valve 66, at a pre-blowing start time, with a pre-blowing flow rate, a pre-blowing duration, and a pre-blowing pressure, i.e. part of the blowing parameters;

[0080] -substantially simultaneously with this first phase, a stretching phase during which a free end of the elongation rod 59 is inserted into the body of the preform by its neck 10 to the bottom of the mold to axially stretch the body of the preform to arrive at a position called the heard of the elongation rod 59, as can be seen in [Fig.4].

[0081] At the end of the pre-blowing, the container 2 is not completely formed but the material has reached the walls of the mold without closely matching the reliefs;

[0082] - a second phase, called blowing, consisting of injecting into the unfinished container 2, a fluid under a blowing pressure, by placing the nozzle 56 in communication with the source 70 of air at the blowing pressure to press the wall of the preform against the mold 46. For this purpose, the control unit 84 controls, via their respective actuators, the opening of the blowing solenoid valve 74 and the closing of the pre-blowing solenoid valve 66. At the end of the second phase, the container 2 is completely formed, the material intimately matching the reliefs of the wall of the mold 46;

[0083] - a third phase, called depressurization, which optionally comprises a step called recovery consisting of depressurizing the container 2 by placing the nozzle 56 in communication with a recovery tank (not shown) and / or a degassing step consisting of completely depressurizing the container 2, in this case by placing the nozzle 56 in communication with the atmosphere; to this end, the control unit 84 controls, via its actuator, the closing of the recovery solenoid valve 82 and the opening of a solenoid valve (not shown) for venting to the atmosphere.

[0084] The forming cycle is repeated successively in each forming station. At each instant, the forming stations are in different states. Each state is nevertheless reached by the forming stations with a periodicity Tp proportional to the angular speed of rotation of the wheel. It is moreover with this same periodicity Tp that the elongation rod 59 is moved. As a result, the movement of the elongation rod 59 is affected by periodic variations which, as illustrated in [Fig. 6], give it a periodic appearance of period Tp. The signal characteristic of the variations in the movement of the elongation rod 59 is constructed by the control unit 84 from at least one parameter representative of the operation of the stretching means 57 at the actuator 58.

[0085] As shown in [Fig.5], the characteristic signal of the variations of the movement of the elongation rod 59 is constructed by the control unit 84 from information obtained from a displacement sensor 90 of the movement of the elongation rod 59 or a sensor for measuring the force required to move the elongation rod 59.

[0086] At the start or end of container production, the container manufacturing facility implements a method for controlling the stretching means 57 of each blowing station to check its operating condition. Alternatively, this control method can be implemented during container production.

[0087] This method of controlling the stretching means 57 consists of: - place the blowing nozzle 56 against the mold, at the level of its upper surface, which is in the closed position, -move the elongation rod between the rest position and the extended position where one of the free ends of the rod is in contact with the bottom of the mold, -measure at least one parameter representative of the operation of the stretching means 57 during all or part of the movement of the elongation rod, - compare the measured parameter to a determined reference value, - issue an alert signal if a critical deviation is detected between the measured parameter and the reference value.

[0088] To detect a deviation, the control unit 84 is programmed to compare at each instant the measured parameter with a predetermined reference value corresponding to a normal forming process.

[0089] A deviation E is said to be critical when the measured parameter deviates from a predetermined reference value or when it is higher than a threshold predetermined by the operator or by the configuration of the control unit, in particular during the running-in of the installation.

[0090] The critical deviation E may also correspond to a modification of the shape of the curve compared to the shape of the reference curve or the critical deviation may also correspond to a modification of certain sequences of the operation of the drawing means 57 (such as a start-up sequence, a sequence of the descent of the rod and the nozzle, etc.) compared to the different reference sequences constituting the curve during a period Tp.

[0091] Figures 6 to 8 are curves representing the current intensity as a function of time. It is collected at the level of the drawing means.

[0092] [Fig. 6] shows a reference curve representative of the operation of the electric stretching means 57, in particular at least one linear motor. In this case, the parameter collected is the intensity of the current which supplies the drive of the linear motor as a function of time. It is known to those skilled in the art that for this type of actuator 58, the intensity of the current is representative of the movement of the elongation rod during the forming cycle.

[0093] As can be seen in this figure, the signal (or curve) of the variations in the intensity of the current therefore comprises a succession of similar sequences repeating at substantially constant intervals (of period Tp).

[0094] Each sequence, called reference (i.e. in normal operation) of period Tp comprises:

[0095] - a first starting sequence 94 corresponding to a growth phase from a minimum value to a maximum value, passing through a peak of the curve;

[0096] - a second sequence 96 of descent of the rod corresponding to a phase of decrease, followed by a phase of stagnation of the curve corresponding to the movement of the descent of the elongation rod 59,

[0097] - a third sequence 98 where the end of the rod touches the bottom of the mold corresponding on the curve to a new phase of increasing up to a new peak;

[0098] - a fourth sequence 100 of raising the rod corresponding to a new phase of stagnation of the curve.

[0099] Signal processing by the control unit 84 makes it possible, by sampling, to detect these different sequences in the signal (or the curve) representing in this figure the variations in the intensity of the current as a function of time. These different sequences make it possible to determine at least one reference intensity or one reference sequence of the intensity which will be used in the method of controlling the drawing means 57.

[0100] As shown in Figures 7 and 8, a number of characteristic anomalies have been identified and allow us to identify a present or future failure, by extrapolation. The detection of these characteristic anomalies will possibly require a subsequent maintenance operation on the stretching means.

[0101] As can be seen in [Fig.7], a critical deviation El is visible between the reference curve (dotted line) and the measured curve (solid line). This deviation El corresponds to a translation of the reference curve upwards and to an increase in the peak. In other words, the amplitude of the start sequence is greater than the reference sequence. This deviation is characteristic of a twisted rod 59.

[0102] As can be seen in [Fig.8], a critical E2 deviation is visible between the reference curve (dotted line) and the measured curve (solid line). This E2 deviation corresponds to a translation of the reference curve to the left. In other words, the start-up sequence was anticipated. This deviation is characteristic of an anomaly of the nozzle 56 and more particularly of the stabilizer 55 located inside the nozzle.

[0103] The number of characteristic anomalies is not limited to the two anomalies stated above.

[0104] These characteristic anomalies are identified by comparing the deviations E with experiments, for example on a test bench or by continuous monitoring carried out on the drawing means 57 during running-in or during the production of containers in the installation. Then, they are stored in the database 88 of the control unit 84.

[0105] On the other hand, momentary deviations of a non-characteristic anomaly affecting one of the forming stations may indeed occur. In this case, the deviations between the measured parameter and the corresponding reference value are less than a predetermined threshold and are not taken into account.

[0106] As soon as a critical deviation E is detected, the control unit 84 generates an alert. This alert is in the form of a signal which can: -be displayed directly on a console to be read by an operator responsible for maintaining the manufacturing facility, or - be operated by the manufacturing facility itself in order to modify its own operating parameters, -or even cause the installation to automatically shut down to allow subsequent maintenance operations.

[0107] This method of controlling the stretching means 57 implements these steps in the absence of a preform in the mold. In other words, the manufacturing installation comprising the thermal conditioning unit and the forming unit operates empty.

[0108] These steps of the method for controlling each stretching means 57 are carried out with a so-called degraded speed of the blowing station, that is to say with a rate lower than the production rate to allow the acquisition by the control unit of at least one parameter representative of the operation of the stretching means 57 during all or part of the movement of the elongation rod 59.

Claims

Claims

1. Method for controlling a stretching means (57) of a container blowing station equipping an installation for manufacturing containers obtained by stretch blowing from a preform made of thermoplastic material, said blowing station comprising: - a mold consisting of at least two half-molds and a mold base, said mold being capable of receiving a preform to be stretched and blown after closing the mold, - a blowing nozzle (56) moves between a so-called rest position and a so-called working position where said nozzle (56) covers the mold when it is closed, - the stretching means (57) comprising at least one actuator 58 coupled to an elongation rod (59), said elongation rod (59) sliding through the nozzle (56) and the preform positioned in the mold between a so-called rest position and a so-called extended position corresponding to the free end of the elongation rod (59) touching said mold base;Characterized in that it comprises at least the following steps: -placing said blowing nozzle (56) against the mold which is in the closed position, -moving said elongation rod (59) between said rest position and said extended position, -measuring at least one parameter representative of the operation of said stretching means (57) during all or part of said movement of said elongation rod (59), -comparing said measured parameter with a determined reference value, -emitting an alert signal if a critical deviation is detected between said measured parameter and the reference value.;

2. Control method according to the preceding claim, characterized in that said steps are carried out in the absence of preform in the mold before the start of the production of containers or after having carried out the production of containers.

3. Control method according to claim 2 characterized in that said steps are carried out with a so-called degraded speed of the blowing station, that is to say with a rate lower than the production rate.

4. A control method according to any one of the preceding claims- characterized in that the actuator (58) of the stretching means (57) is a mechanical or pneumatic actuator such as a jack.

5. Control method according to any one of claims 1 to 3 characterized in that the actuator (58) of the stretching means (57) is an electric actuator.

6. Control method according to the preceding claim 5 characterized in that the electric actuator (58) is a linear motor.

7. Control method according to any one of the preceding claims, characterized in that the measured parameter corresponds to a torque generated during all or part of the movement of the elongation rod (59).

8. Control method according to claim 5 characterized in that the measured parameter corresponds to the intensity of the operating current of said electric actuator (58).

9. Control method according to any one of the preceding claims, characterized in that said measured parameter is acquired by means of a dedicated sensor.

10. Control method according to claim 8 characterized in that it comprises a step of constructing a signal characteristic of the intensity of the measured current representative of the stretching means (57) of each blowing station.

11. Control method according to claim 10 characterized in that it comprises a step of sampling the signal to detect similar sequences repeating at constant sensitive intervals (T).

12. Control method according to claim 11 characterized in that, when a deviation is detected between one of said sequences and a reference sequence, then an operation of determining an anomaly is implemented to identify the faulty element of said stretching means (57), each anomaly corresponding to a predetermined deviation.

13. Control method according to any one of the preceding claims, characterized in that the alert signal comprises the display of the anomaly of said stretching means (57).

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