WO2026008368A1 - Method for controlling a facility for producing containers - Google Patents

Abstract

The present invention relates to a method for controlling the cyclic production of containers via stretch-blow-moulding from preforms made of plastic material in a machine comprising one or more stretch-blow-moulding stations, the machine being positioned downstream of a device for heat-treating the preforms, the preform being blown into the mould, with a pre-blowing step, the steps of heating the preforms, pre-blowing and blowing being controlled by a control system on the basis of different parameters referred to as control parameters; the method being characterised in that it comprises at least the following steps: measuring at least one pre-blowing and/or blowing curve in at least one stretch-blow-moulding station during a production cycle; recording the pre-blowing and/or blowing curve in a memory; determining at least one reference position of a first characteristic point of the pre-blowing and/or blowing curve and a position of a second characteristic point of the pre-blowing curve; determining at least one variable characterising a variation in the natural draw ratio, referred to as the NDR, with the reference time and/or the reference pressure of the first characteristic point and/or the second characteristic point; modifying at least one of the control parameters if the variation in the variable is greater than or less than a predetermined value.

Description

Method for regulating a container production plant

The field of the invention is that of the design and manufacture of plastic containers. The present invention relates more particularly to a method of regulating an installation for the production of containers obtained by stretch blow molding from plastic blanks, such as polyethylene terephthalate (PET) and/or recycled polyethylene terephthalate (rPET), the term blank covering both a preform obtained by injection and an intermediate container which has undergone one or more temporary forming operations.

State of the art

The forming of a container is carried out by blow molding or blow-stretching from a blank that has undergone a preliminary heating operation. The hot blank is introduced into a mold with the impression of the container; a fluid (particularly a gas such as air) under pressure is then injected into the blank to give it the shape of the container by means of a counter-impression in the mold. The blank may also be stretched using a sliding rod to minimize misalignment and to ensure uniform material distribution.

Industrial-scale container forming requires extremely short cycle times. For a typical modern production rate (around 50,000 containers per hour), the cycle time, measured from the introduction of the blank into the mold to the removal of the formed container, is only between 1 and 2 seconds. Current production rates can reach almost 100,000 containers per hour, and at these rates, the individual output of each mold is several thousand containers per hour.

Increasing production speed can be a contributing factor to the development of shape defects affecting containers. Among the most frequent shape defects are poor mold formation and uneven material distribution, which are often correlated. It is known that these defects can be linked to various machine parameters, including the heating temperature of the blanks, fluid pressure and flow rate, and the drawing speed.

Manually adjusting these parameters requires operators to have a thorough understanding of the correlations between variations in a given parameter and their impact on correcting form defects. Faced with observed errors and the slowness of the process, which leaves the operator free to make immediate adjustments to the machine, several manufacturers have committed to fully automating machine control.

The applicant has already contributed to this trend by systematizing the analysis of blow curves to detect singular points likely to indicate the conformity (or, on the contrary, the non-conformity) of the container to a predefined model, the modification of machine parameters being able to be ordered in the event of non-conformity, see in particular documents WO 2008/081107 and WO 2012/035260.

Document WO 2008/081107 describes a process for manufacturing a container by blowing into a mold from a plastic blank, which includes the operations of measuring the pressure inside the blank, detecting a time t _A , called the actual time of the start of pre-blowing, when the pressure in the blank begins to increase, comparing this time t _A with a theoretical time of the start of pre-blowing; if the actual time t _A of the start of pre-blowing is later than the theoretical time of the start of pre-blowing, advancing the top pre-blowing t _P ; if the actual time t _A of the start of pre-blowing is earlier than the theoretical time of the start of pre-blowing, delaying the top pre-blowing t _P.

As for document WO 2012/035260, the latter describes a process for manufacturing a container by stretch blow molding from a plastic blank, which includes the operations of measuring the pressure inside the blank during a pre-blow phase; detecting an instant corresponding to a local minimum pressure in the blank; recording the instant at which this minimum pressure occurs and the corresponding pressure in the blank; comparing the instant and pressure of the detected minimum with, respectively, a predetermined instant and pressure of a theoretical minimum pressure; if the measured minimum and the theoretical minimum are not coincident, ordering a modification of at least one of the following parameters: pre-blow pressure, pre-blow flow rate, stretch speed, heating temperature.

The cyclic production of containers by stretch blow molding typically involves heating a plastic preform to the glass transition temperature of the preform, then introducing the heated preform into a mold, injecting pressurized fluid into the preform to form a container in the mold, and removing the container from the mold.

The injection of fluid under pressure itself comprises several successive steps. The first step, called pre-blowing, consists of injecting a fluid under reduced pressure, generally between 5 and 16 bar, into the preform to form a bubble while a stretching rod stretches the preform. The stretching rod causes mechanical stretching along the longitudinal direction of the preform, while the injection of fluid under pressure causes stretching along a transverse direction, perpendicular to the longitudinal direction. The stretching is thus bidirectional, ensuring a homogeneous distribution of the material and proper orientation of the molecular chains.

A second operation, called blow molding, involves injecting a fluid under high pressure, generally between 20 and 30 bar. The blow molding operation forces the bubble formed by the plastic preform against the walls of the mold, thus creating a container with a desired and well-defined shape.

The pre-blowing phase is crucial in the development of the plastic preform and in the orientation of the molecular chains. A pre-blowing defect leads to poor material distribution, resulting in defective containers that do not conform to one or more specifications, such as resistance to vertical load (also known as "top load"). In a high-speed industrial production environment, pre-blowing must be controlled to guarantee the quality of containers formed from plastic preforms. It is known from WO2013/178903 to regulate the cyclic production of containers on a machine comprising stretch-blowing stations based on pressure reference points within the preform, according to the time recorded during pre-blowing at a designated reference station. Pressure-as-time reference points are used for machine regulation, in order to modify the settings of a station where the pressure measured at a given time, or the time to reach a given pressure, would be outside a tolerance zone around the reference pressure or reference time for that point.

We are also familiar with document US2010/176528 which describes a process and a machine in which the start of pre-blowing is regulated.

Sometimes an operator requires a change in one or more operating parameters of the machine, for example when new preforms are used which have different characteristics in terms of composition, or absorption coefficient (thermal).

However, changing a machine operating parameter during production causes the control system to stop, and the cyclical production of containers continues without regulation. It is then necessary to manually reset the reference points for the control system to resume.

This results in a drift in the pre-blowing phase, as well as a decrease in productivity.

To overcome this drawback, a method has already been devised for regulating the cyclic production of containers by stretch-blowing from plastic preforms in a machine comprising one or more stretch-blowing stations, each designed to produce one container during a production cycle, and each equipped with a pre-blowing solenoid valve fluidly connecting the preform to a pressurized fluid source providing a pre-blowing flow during a pre-blowing phase. The regulation method comprises:

a) an initialization phase including a step of setting up by an operator and memorizing a plurality of machine operating parameters during the production cycle, and control parameters including at least one reference position (Ac, Bc, PCab, Fc) of at least one characteristic point (A, B, Pab, F) of a pre-blowing curve corresponding to the pressure prevailing inside the preform during at least part of the pre-blowing, said reference position (Ac, Bc, PCab, Fc) being determined by a reference instant and/or a reference pressure,

b) a regulated production phase during which, for each production cycle, at least one preform is stretched and blown by injecting a pressurized fluid into each station and during which:

b1) for at least one reference station of the machine, the pre-blowing curve including the characteristic point is measured and stored, and a real time and/or a real pressure is calculated or determined, corresponding to said characteristic point (A, B, Pab, F) for the measured pre-blowing curve, and

b2) a new value of at least one machine operating parameter is calculated and stored as a function of a difference between the actual time and the reference time and/or between the actual pressure and the reference pressure.

The regulated production phase includes a monitoring step (b3) of any change imposed by the operator to the value of a machine operating parameter,

The said control method further includes an automatic update phase for the control parameter(s), which is implemented in the event of a required change to a machine operating parameter, including:

c1) a stabilization step during which production is continued from the imposed parameter for a predetermined stabilization period, and for each production cycle actions b1) are executed, and actions b2) are suspended, and

c2) a correction step of the reference time and/or reference pressure according to the actual time and/or actual pressure values memorized during the stabilization step, in order to continue the regulated production phase.

However, one of the causes of process deviations concerns changes in preform characteristics; these can be related to the material itself, to IVs, to humidity (Rh), or to the stress induced during injection.

A change in material, if accompanied by a change in IR absorption, can be detected by the camera at the furnace outlet. However, factors IV, Rh, and stress are completely or almost completely transparent to heating.

To date, there is no way to detect changes in these characteristics online and in a non-destructive manner.

In the near future, IV factors are likely to be subject to more variation than they are currently, as the massive introduction of rPET into production will amplify this type of problem.

Changes in IV, Rh or stress will directly impact the mass distribution of the bottle because they will increase or decrease the resistance of PET to inflation.

There is therefore a need to improve the operation of the machine in order to guarantee a constant production quality regardless of variations in IV, humidity (Rh), or the stress induced during the injection of preforms.

Disclosure of the invention

One of the aims of the invention is therefore to remedy these drawbacks by proposing a simple and inexpensive design process that allows for the detection of a change in the characteristics of the preforms (IV and/or humidity (Rh) and/or stress) by correlating with other controlled factors such as the temperature of the preforms, to identify a change in the resistance of the elastic modulus of the preforms and to modify the various so-called control parameters of the container manufacturing installation such as the power of the heating elements of the furnace, the blowing pressure in the mold and/or the pre-blowing pressure and/or the pre-blowing flow rate and/or the speed of the stretching rod for example to act accordingly to realign as quickly as possible the expected wall thicknesses of the containers.

To this end, and in accordance with the invention, a method is proposed for regulating the cyclic production of containers by stretch blow molding from plastic preforms in a machine comprising one or more stretch blow molding stations, said machine being positioned downstream of a thermal conditioning device for said preforms, each stretch blow molding station being designed to produce one container during a production cycle and being equipped with a pre-blow and/or blow solenoid valve of a blow circuit fluidly connecting the preform to a pressurized fluid source providing a pre-blow and/or blow flow during a pre-blow and/or blow phase, each stretch blow molding station comprising a mold consisting of two half-molds delimiting a molding cavity, said preform being blown into said mold, with a pre-blow step, said steps of heating the preforms, pre-blow and blowing operations being controlled by a control system based on various parameters known as control parameters, such as the power of the furnace heating elements, the blowing pressure in the mold and/or the pre-blowing pressure and/or the pre-blowing flow rate and/or the speed of the drawing rod, for example, characterized in that it comprises at least the following steps:

measurement of at least one pre-blowing and/or blowing curve in at least one stretch-blowing station during a production cycle;

recording of said pre-blowing and/or blowing curve in a memory;

determination of at least one reference position of a first characteristic point of the pre-blowing and/or blowing curve corresponding to the pressure prevailing inside the preform or the blowing circuit during at least part of the pre-blowing and/or blowing, said reference position of the first characteristic point being determined by a reference instant and/or a reference pressure, and determination of a position of a second characteristic point of the pre-blowing curve corresponding to the pressure prevailing inside the preform or the blowing circuit during at least part of the pre-blowing, said reference position of the second characteristic point being determined by a reference instant and/or a reference pressure;

determination of at least one quantity characterizing a variation of the natural draw ratio, known as NDR according to the Anglo-Saxon acronym "Natural Draw Ratio", of the preform as a function of the reference time and/or the reference pressure of the first characteristic point and/or the second characteristic point;

modification of at least one of the control parameters if the variation of the quantity characterizing a variation of the natural NDR stretch rate of the preform is greater or less than a predetermined value.

The said first characteristic point corresponds, preferably, to the pressure peak of the pre-blowing stage.

The said second characteristic point corresponds, preferably, to the so-called positive inflection point of the pre-blowing pressure curve.

Furthermore, the process according to the invention includes a step of determining at least one reference position of a third characteristic point of the pre-blowing and/or blowing curve, said third characteristic point corresponding to the reference position of the end of pre-blowing at a predetermined time t ₁ .

The said process also includes a step of determining at least one reference position of a fourth characteristic point of the pre-blowing and/or blowing curve, said fourth characteristic point corresponding to 90% of the pre-blowing and/or blowing pressure setpoint.

The said step of determining at least one quantity characterizing a variation of the natural draw rate (NDR) of the preform consists of calculating the difference between the reference time of the second characteristic point and the reference time of the first characteristic point. .

According to a first embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating the difference between the reference pressure of the second characteristic point and the reference pressure of the first characteristic point. .

According to a second embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating the difference between the reference time of the third characteristic point and the reference time of the second characteristic point. .

According to a third embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating the difference between the reference time of the fourth characteristic point and the reference time of the third characteristic point. .

According to a fourth step in determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform, it consists of calculating the difference between the reference pressure of the third characteristic point and the reference pressure of the second characteristic point. .

According to a fifth embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating the difference between the reference pressure of the fourth characteristic point and the reference pressure of the third characteristic point. .

According to a sixth embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the second characteristic point and the reference pressure of the first characteristic point by the difference between the reference time of the second characteristic point and the reference time of the first characteristic point. .

According to a seventh embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the third characteristic point and the reference pressure of the second characteristic point by the difference between the reference time of the third characteristic point and the reference time of the second characteristic point. .

According to a final embodiment, the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the fourth characteristic point and the reference pressure of the third characteristic point by the difference between the reference time of the fourth characteristic point and the reference time of the third characteristic point. .

Furthermore, if the variation in the quantity characterizing a variation in the natural NDR stretch rate of the preform is greater or less than a predetermined value, a visual or audible drift alert from the reference station is issued.

Furthermore, if the variation in the quantity characterizing a variation in the natural NDR stretch rate of the preform is greater or less than a predetermined value, at least one of the control parameters is modified automatically.

Another object of the invention relates to a machine for manufacturing containers by stretch blow molding from plastic preforms, comprising at least one fluid source at a pre-blow and/or blow pressure, one or more stretch blow stations, each station comprising a mold having a cavity for receiving a preform, a solenoid valve for a pre-blow and/or blow circuit designed to connect the interior of the preform, received in the cavity, with said fluid source at a predetermined pre-blow and/or blow flow rate, a control device for opening and closing the solenoid valve, at least one sensor capable of measuring the pressure inside the preform or inside the blow circuit, said machine being positioned downstream of a thermal conditioning device for said preforms, and a memory unit in which various control parameters are recorded, such as the oven heating temperature, the pressure of blowing into the mold and/or pre-blowing pressure and/or pre-blowing flow rate and/or the speed of the drawing rod for example, and a control system including means for controlling the preform heating, pre-blowing and blowing stages from said different control parameters; said machine is remarkable in that it includes means capable of implementing the process according to the invention.

A third object of the invention relates to a computer program product comprising a sequence of instructions which, when the program is executed by a computer, leads the computer to implement the steps of the process according to the invention.

A final object of the invention relates to a computer-readable recording medium comprising instructions which, when executed by the control unit of a suitable stretch blow molding machine, lead the machine to carry out the steps of the process according to the invention.

Other advantages and features will become clearer from the following description of several embodiments, given by way of non-limiting examples, of the process according to the invention, with reference to the attached drawings in which:

is a schematic representation of a container manufacturing installation, comprising a forming unit and a heating unit according to the invention,

is a schematic representation illustrating in more detail the architecture of the container manufacturing machine implementing the process according to the invention, and more particularly a stretch blow molding station and a thermal conditioning device,

is a representation of a curve illustrating the variations in pressure inside the preform or blowing circuit during pre-blowing,

is a representation of two curves illustrating the variations in pressure within two preforms with different characteristics or of the blowing circuit during pre-blowing.

Method of embodying the invention

Figures 1 and 2 schematically represent an installation 1 for manufacturing containers 2 from blanks 3, also called preforms, made of thermoplastic material, for example PET (polyethylene terephthalate) and/or rPET (recycled polyethylene terephthalate).

Installation 1 includes at least two units 4, 5 for processing containers 2 or preforms 3. For simplicity, it is assumed in the following that the preforms 3 are preforms made of PET (polyethylene terephthalate) and/or rPET (recycled polyethylene terephthalate) and obtained by injection for example.

The said installation 1 includes a heating unit 4 or furnace, also a heat treatment device, which includes a series of heating modules 6, each having a radiant wall 7 equipped with superimposed infrared radiation sources 8 and a reflective wall 9 placed opposite the radiant wall 7 to reflect the portion of radiation not absorbed by the preforms 3, and a blow molding or stretch blow molding unit 5, equipped with at least one forming station 10 (and in this case a series of stations), also called a stretch blow molding station, the forming station or stations 10 being equipped with a mold 11 with the imprint of a container 2.

Typically, the preforms 3, initially at room temperature, are introduced into the furnace 4 through an inlet, for example by means of a wheel or a feed conveyor, not shown in the figures. The preforms 3 are then heated in the furnace 4 to a temperature above the glass transition temperature of the material (the final temperature of the blanks is around 120°C for PET, whose glass transition temperature is approximately 80°C).

In the oven 4, the preforms 3 are, for example, mounted on pivoting supports 12 or turntables. Each turntable 12 is mounted on a chain running on a drive wheel 13, which is rotated by a motor 14. The turntable 12 is equipped with a pinion 15 that meshes with a rack 16 to rotate the turntable 12 as it moves through the oven 4, thus exposing the surface of each preform 3 to radiation.

To help distribute energy throughout the thickness of the preform walls, the oven 4 can be equipped with a ventilation system including, for example, a fan 17 driven by a motor 18 and positioned at the necks of the preforms 3.

Furthermore, the power of the radiation emitted by the radiating wall 6 can be modulated by means of a power dimmer 19, as in the embodiment illustrated on the .

The thermal profile of the preforms 3 is preferably controlled, either directly in the furnace 4 or at its outlet, by means of a thermal sensor 20. According to an embodiment illustrated in the , the thermal sensor 20 is a thermal camera pointing towards the preforms 3.

At the exit of the oven 4, the preforms 3 thus heated are transferred to the forming unit 5 via a transfer unit, such as a transfer wheel for example, not shown in the figures, to be blown or stretched blown individually into a mold 11. The preforms 3 are introduced into the forming unit 5 at a so-called loading point.

After forming, the containers 2 are removed from the molds 11 at a designated discharge point for immediate filling and labeling, or temporarily stored for later filling and labeling. Once filled and labeled, the containers 2 are grouped and packaged, for example, within a shrink-wrapping unit (not shown in the figures) that wraps each group of containers with heat-shrink film.

As can also be seen in Figures 1 and 2, the forming unit 5 includes a pivoting wheel 21 on which the forming stations 10 are mounted, and a sensor 22 of the instantaneous angular position of the wheel 21, in the form for example of an encoder (i.e., in practice, an instrumented bearing).

Each forming station 10 is equipped with a nozzle 23 through which a fluid (in particular a gas such as air) is injected into the mold 11. Each forming station 10 is also equipped with an injection device comprising a block 24 of actuators connected to the nozzle 23 to control the fluid injection. In addition, each forming station 10 is equipped with a device 25 for measuring the pressure prevailing in the vessel being formed. In the illustrated example, the measuring device 25 includes a pressure sensor mounted at the nozzle 23, in which the pressure being formed is identical to the pressure prevailing in the vessel 2.

According to an embodiment corresponding to a stretch blow forming process, each forming station 10 further comprises a movable stretching rod 26, attached to a carriage 27 mounted in translation relative to a support 28.

The movement of the rod 26 can be controlled electromagnetically. For this purpose, the support 28 includes an electromagnetic track connected to a motor 29, and the carriage 27 is itself magnetic. The sign and strength of the current flowing through the track allow the rod 26 to be moved along a predetermined movement profile, including a direction and a speed of movement.

As illustrated on the , the forming stations 10 describe a path (in this case circular) which includes a so-called forming sector extending from the preform loading point 3 to the discharge point of the formed containers 2, and a so-called buffer sector, complementary to said forming sector and extending from the discharge point to the loading point.

Furthermore, the installation 1 is equipped with a control system 30 comprising a central control unit 31 for the installation 1 and, for each processing unit 4, 5, a dedicated control system 32, 33 which automatically controls the operation of the respective unit 4, 5 in order to implement the regulation process according to the invention as detailed below.

The said regulation method according to the invention comprises a preform heating step at the oven 4, a pre-blowing step and then a blowing step of the preforms at the forming unit 5, said preform heating, pre-blowing and blowing steps being controlled by the control unit 31 from various so-called control parameters such as the power of the oven heating elements, the blowing pressure in the mold and/or the pre-blowing pressure and/or the pre-blowing flow rate and/or the speed of the drawing rod for example.

With reference to Figures 2 and 3, the method according to the invention includes a step of measuring at least one pre-blowing and/or blowing curve in at least one stretch-blowing station during a production cycle, by means of said pressure measuring device 25; a step of recording said pre-blowing and/or blowing curve in a memory, such as the memory of the central control unit 31 of the installation 1 for example; a step of determining at least one reference position of a first characteristic point of the pre-blowing and/or blowing curve corresponding to the pressure prevailing inside the preform or the blowing circuit during at least part of the pre-blowing and/or blowing, said reference position of the first characteristic point being determined by a reference instant and/or a reference pressure, and of determining a position of a second characteristic point of the pre-blowing curve corresponding to the pressure prevailing inside the preform or the blowing circuit during at least part of the pre-blowing, said reference position of the second characteristic point being determined by a reference instant and/or a reference pressure; a step of determining at least one quantity characterizing a variation of the natural draw ratio (NDR) of the preform as a function of the reference time and/or the reference pressure of the first characteristic point and/or the second characteristic point; and finally a step of modifying at least one of the control parameters if the variation of the quantity characterizing a variation of the natural draw ratio (NDR) of the preform is greater or less than a predetermined value.

In this particular example of implementation, with reference to figures 3 and 4, the first characteristic point (A) corresponds to the pressure peak of the pre-blowing stage and the second characteristic point (B) (also called NDR) corresponds to the so-called positive inflection point of the pre-blowing pressure curve.

Advantageously, the process according to the invention may include a step of determining at least one reference position of a third characteristic point (C) of the pre-blowing and/or blowing curve, said third characteristic point (C) then corresponding to the reference position of the end of pre-blowing at a predetermined time t1.

Furthermore, the process according to the invention may include a step of determining at least one reference position of a fourth characteristic point (D) of the pre-blowing and/or blowing curve, said fourth characteristic point D corresponding to 90% of the pre-blowing and/or blowing pressure setpoint.

Furthermore, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating the difference between the reference time of the second characteristic point (B) and the reference time of the first characteristic point (A).

According to one embodiment variant, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating the difference between the reference pressure of the second characteristic point (B) and the reference pressure of the first characteristic point (A).

According to a second variant of execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating the difference between the reference time of the third characteristic point (C) and the reference time of the second characteristic point (B).

According to a third variant of execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating the difference between the reference time of the fourth characteristic point (D) and the reference time of the third characteristic point (C).

According to a fourth variant of the execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating the difference between the reference pressure of the third characteristic point (C) and the reference pressure of the second characteristic point (B).

According to a fifth variant of the execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating the difference between the reference pressure of the fourth characteristic point (D) and the reference pressure of the third characteristic point (C).

According to a sixth variant of the execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the second characteristic point (B) and the reference pressure of the first characteristic point (A) by the difference between the reference instant of the second characteristic point (B) and the reference instant of the first characteristic point (A).

According to a seventh variant of the execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the third characteristic point (C) and the reference pressure of the second characteristic point (B) by the difference between the reference time of the third characteristic point (C) and the reference time of the second characteristic point (B).

According to a final variant of the execution, the step of determining at least one quantity characterizing a variation of the natural NDR stretch rate of the preform consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the fourth characteristic point (D) and the reference pressure of the third characteristic point (C) by the difference between the reference instant of the fourth characteristic point (D) and the reference instant of the third characteristic point (C).

Regardless of the execution variant of the step of determining at least one quantity characterizing a variation of the natural NDR of the preform, if the variation of the quantity characterizing a variation of the natural NDR of the preform is greater or less than a predetermined value, a visual or audible drift alert of the reference station is issued.

Furthermore, if the variation in the quantity characterizing a variation in the natural draw rate (NDR) of the preform is greater or less than a predetermined value, at least one of the control parameters is automatically modified. These control parameters consist of the power of the furnace heating elements, the blowing pressure in the mold and/or the pre-blowing pressure and/or the pre-blowing flow rate and/or the speed of the drawing rod and/or any other control parameter well known to those skilled in the art.

It is understood that the process according to the invention is implemented by a computer program comprising a sequence of instructions which, when the program is executed by a computer, leads the computer to carry out the steps of the process according to the invention. Installation 1 thus comprises a data processing device and a computer-readable recording medium containing instructions which, when executed by a computer, lead the computer to carry out the steps of the process according to the invention.

Finally, it is quite clear that the examples we have just given are only particular illustrations and in no way limiting as to the fields of application of the invention.

Claims (19)

A method for regulating the cyclic production of containers by stretch blow molding from plastic preforms (3) in a machine comprising one or more stretch blow molding stations (10), said machine being positioned downstream of a thermal conditioning device (4) for said preforms (3), each stretch blow molding station (10) being designed to produce one container (2) during a production cycle and being equipped with a pre-blow and/or blow solenoid valve of a blow circuit fluidly connecting the preform (3) to a pressurized fluid source providing a pre-blow and/or blow flow during a pre-blow and/or blow phase, each stretch blow molding station (10) comprising a mold (11) consisting of two half-molds defining a molding cavity, said preform (3) being blown into said mold (11), with a step of pre-blowing, said preform heating (3), pre-blowing and blowing stages being controlled by a control system (30) based on various so-called control parameters such as the power of the furnace heating elements, the blowing pressure in the mold and/or the pre-blowing pressure and/or the pre-blowing flow rate and/or the speed of the drawing rod for example, characterized in that it comprises at least the following stages of:
a) measurement of at least one pre-blowing and/or blowing curve in at least one stretch-blowing station (10) during a production cycle;
b) recording of said pre-blowing and/or blowing curve in a memory;
(c) determination of at least one reference position of a first characteristic point of the pre-blowing and/or blowing curve corresponding to the pressure prevailing inside the preform (3) or the blowing circuit during at least part of the pre-blowing and/or blowing, said reference position of the first characteristic point being determined by a reference instant and/or a reference pressure, and determination of a position of a second characteristic point of the pre-blowing curve corresponding to the pressure prevailing inside the preform (3) or the blowing circuit during at least part of the pre-blowing, said reference position of the second characteristic point being determined by a reference instant and/or a reference pressure;
d) determination of at least one quantity characterizing a variation of the natural draw ratio (NDR) of the preform as a function of the reference time and/or the reference pressure of the first characteristic point and/or the second characteristic point;
e) modification of at least one of the control parameters if the variation of the quantity characterizing a variation of the natural NDR stretch rate of the preform (3) is greater or less than a predetermined value. Method according to claim 1 characterized in that the first characteristic point corresponds to the pressure peak of the pre-blowing step. A method according to any one of claims 1 or 2 characterized in that the second characteristic point corresponds to the so-called positive inflection point of the pre-blowing pressure curve. A method according to any one of claims 1 to 3 characterized in that it comprises a step of determining at least one reference position of a third characteristic point of the pre-blowing and/or blowing curve, said third characteristic point corresponding to the reference position of the end of pre-blowing at a predetermined time t ₁ . A method according to any one of claims 1 to 4 characterized in that it comprises a step of determining at least one reference position of a fourth characteristic point of the pre-blowing and/or blowing curve, said fourth characteristic point corresponding to 90% of the pre-blowing and/or blowing pressure setpoint. A method according to any one of claims 1 to 5, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating the difference between the reference time of the second characteristic point and the reference time of the first characteristic point. . A method according to any one of claims 1 to 6, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform consists of calculating the difference between the reference pressure of the second characteristic point and the reference pressure of the first characteristic point. . A method according to any one of claims 4 to 7, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating the difference between the reference time of the third characteristic point and the reference time of the second characteristic point. . A method according to any one of claims 5 to 8, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating the difference between the reference time of the fourth characteristic point and the reference time of the third characteristic point. . A method according to any one of claims 4 to 9, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating the difference between the reference pressure of the third characteristic point and the reference pressure of the second characteristic point. . A method according to any one of claims 5 to 10, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating the difference between the reference pressure of the fourth characteristic point and the reference pressure of the third characteristic point. . A method according to any one of claims 1 to 11, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the second characteristic point and the reference pressure of the first characteristic point by the difference between the reference time of the second characteristic point and the reference time of the first characteristic point. . A method according to any one of claims 4 to 12, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the third characteristic point and the reference pressure of the second characteristic point by the difference between the reference time of the third characteristic point and the reference time of the second characteristic point. . A method according to any one of claims 5 to 13, characterized in that the step of determining at least one quantity characterizing a variation in the natural draw rate (NDR) of the preform (3) consists of calculating a coefficient corresponding to the division of the difference between the reference pressure of the fourth characteristic point and the reference pressure of the third characteristic point by the difference between the reference time of the fourth characteristic point and the reference time of the third characteristic point. . A method according to any one of claims 1 to 14 characterized in that, if the variation of the quantity characterizing a variation of the natural NDR stretch rate of the preform (3) is greater or less than a predetermined value, a visual or audible drift alert from the reference station is issued. A method according to any one of claims 1 to 7 characterized in that, if the variation of the quantity characterizing a variation of the natural NDR stretch rate of the preform (3) is greater or less than a predetermined value, at least one of the control parameters is modified automatically. A machine for manufacturing containers (2) by stretch blow molding from plastic preforms (3), comprising at least one fluid source at a pre-blow and/or blow pressure, one or more stretch blow stations (10), each station (10) comprising a mold (11) having a cavity for receiving a preform (3), a solenoid valve of a pre-blow and/or blow circuit for connecting the interior of the preform (3), received in the cavity, with said fluid source at a predetermined pre-blow and/or blow flow rate, a control device for opening and closing the solenoid valve, at least one sensor (25) capable of measuring the pressure inside the preform (3) or inside the blow circuit, said machine being positioned downstream of a thermal conditioning device (4) for said preforms (3), and a memory unit in which are recorded various so-called control parameters such as the heating temperature of the oven, the blowing pressure in the mold and/or the pre-blowing pressure and/or the pre-blowing flow rate and/or the speed of the drawing rod for example, and a control system (30) comprising means for controlling the heating steps of the preforms (3), pre-blowing and blowing from said various control parameters, characterized in that it comprises means capable of implementing the process according to any one of claims 1 to 16. Product computer program comprising a sequence of instructions which, when the program is executed by a computer, causes the computer to carry out the steps of the process according to any one of claims 1 to 16. Computer-readable recording medium comprising instructions which, when executed by the control unit of a suitable stretch blow molding machine, cause the machine to carry out the steps of the process according to any one of claims 1 to 16.