Method for manufacturing containers from preforms, allowing a systematic check on the conformity of the preforms

Abstract

Method of manufacturing containers by blow-molding preforms, comprising a phase of heating the preforms (3) followed by a phase of forming the containers (this phase comprising a pre-blowing step followed by a blowing step), this method comprising the operations consisting in: detecting a pressure spike during the pre-blowing and comparing it against a reference spike, if the pressure in the pressure spike is below the reference pressure, checking an event history to determine whether a heating instruction and/or a forming instruction has been modified; if the result of this check is positive, commanding at least one of the following actions: o generating an alert alarm, o stopping the supply of preforms to the oven, o ejecting the containers derived from the affected preforms.

Claims

1. A process for manufacturing containers (2) by blow molding from preforms (3) made of thermoplastic material, comprising: a cyclic phase for heating preforms (3) in a stream within a furnace (4), at a heating temperature and a speed of advance according to programmed heating instructions (CC), followed by a shaping phase for shaping preforms (3) within a shaping unit (6) with a plurality of molding stations (14), each of said molding stations equipped with a mold (15) bearing an imprint of a container (2), the shaping phase being cyclic and comprising a pre-blow-molding step that includes injecting into the preforms (3) a gas at a pre-blow-molding pressure (P) and a pre-blow-molding flow rate according to programmed shaping instructions (CF), followed by a blow-molding step that includes injecting into the preforms (3) a gas at a blow-molding pressure that exceeds the pre-blow-molding pressure, the process also comprising operations of: measuring, at every instant, a real value of the pressure prevailing in the preforms (3) within each mold (15), at least during the pre-blow-molding step; storing the real pressure values thus measured and associated measurement times; detecting among said real pressure values a maximum real value of the pressure; comparing said maximum real value of the pressure and the associated measurement time with, respectively, a predetermined maximum reference value of the pressure (P.sub.B1) and an associated reference time (t.sub.B1); verifying whether the following necessary conditions are fulfilled together over a period that is greater than or equal to a length of time of a heating cycle and for all of the molding stations (14): the measurement time associated with the maximum real value of the pressure coincides with the reference time (t.sub.B1), and the maximum real value of the pressure is less than the maximum reference value of the pressure (P.sub.B1), where the maximum real value of the pressure is determined to be less than the maximum reference value of the pressure (P.sub.B1) if the difference between the maximum real value of the pressure and the maximum reference value of the pressure (P.sub.B1) is greater than several hundred millibars; in the event said necessary conditions are fulfilled together, verifying in a chronology of events whether the heating instructions (CC) and the shaping instructions (CF) have undergone any modifications during a predetermined time interval preceding said measuring; and in the event the result of said verifying indicates that neither of the heating instructions (CC) nor the shaping instructions (CF) has undergone a modification during said predetermined time interval, determining the preforms (3) to be sub-standard and carrying out at least one of the following actions: generating an alert, stopping the supply of the furnace (4) with preforms (3), and ejecting the containers (2) produced from sub-standard preforms (3).

2. The process according to claim 1, wherein the measurement time associated with the maximum real value is determined to be simultaneous with the reference time if the difference between them is less than or equal to approximately 10 ms.

3. The process according to claim 2, wherein the measurement time associated with the maximum real value is determined to be simultaneous with the reference time if the difference between them is less than or equal to approximately 5 ms.

4. The process according to claim 1, wherein the generation of an alert includes displaying a message on a graphic interface.

5. The process according to claim 1, wherein with the furnace (4) and the heating unit (6) being directed respectively by dedicated slave control units (37, 38), both attached to a master control unit (35), the verification operations are carried out by the master control unit (35).

6. An installation (1) for manufacturing containers (2) by blow molding from preforms (3) made of thermoplastic material, comprising: a furnace (4) in which the preforms (3) undergo a cyclic heating phase at a heating temperature and a speed of advance according to heating instructions (CC); a system (5) for supplying the furnace (4) with preforms (3); a shaping unit (6) comprising a number of molding stations (14), each of said molding stations equipped with a mold (15) bearing an imprint of a container (2), for the shaping of containers (2) by blow molding or stretch blow molding of the preforms (3) exiting the furnace (4), following a cyclic shaping phase comprising a pre-blow-molding step that includes injecting into the preforms (3) a gas at a pre-blow-molding pressure (P) and a pre-blow-molding flow rate according to shaping instructions (CF), followed by a blow-molding step consisting in injecting into the preforms (3) a gas at a blow-molding pressure that exceeds the pre-blow-molding pressure; a command system (34) including at least one memory (39) in which a chronology of events is stored, said chronology of events including at least the heating instructions (CC) and the shaping instructions (CF), together with a time from which either of a new heating instruction or a new shaping instruction is made active, with said system (34) being programmed for: measuring, at every instant, a real value of the pressure prevailing in the preforms (3) within each mold (15), at least during the pre-blow-molding step, storing the real pressure values thus measured and associated measurement times, detecting among said real pressure values a maximum real value of the pressure, and comparing said maximum real value of the pressure and the associated measurement time with, respectively, a predetermined maximum reference value of the pressure (P.sub.B1) and an associated reference time (t.sub.B1), wherein the command system (34) is programmed for: verifying if the following necessary conditions are fulfilled together over a period that is greater than or equal to a length of time of a heating cycle and for all of the molding stations (14): the measurement time associated with the maximum real value of the pressure coincides with the reference time (t.sub.B1), and the maximum real value of the pressure is less than the maximum reference value of the pressure (P.sub.B1), where the maximum real value of the pressure is determined to be less than the maximum reference value of the pressure (P.sub.B1) if the difference between the maximum real value of the pressure and the maximum reference value of the pressure (P.sub.m) is greater than several hundred millibars; and in the event that said necessary conditions are fulfilled together, verifying in the chronology of events that no modification of the heating instructions (CC) and no modification of the shaping instructions (CF) has occurred during a predetermined time interval preceding said measuring, wherein the command system is further programmed to determine that the preforms (3) are sub-standard in the event that the verifying indicates that neither of the heating instructions (CC) nor the shaping instructions (CF) has undergone a modification during said predetermined time interval, wherein the installation further comprises an actuator controlled by the command system and structured for carrying out at least one of the following actions: generating an alert, stopping the supply of the furnace (4) with preforms (3), and ejecting the containers (2) produced from said sub-standard preforms (3).

7. A non-transitory computer-readable recording medium having recorded thereon a computer program executable by a computer processing unit integrated in a command system (34) of a container manufacturing installation (2), where said installation includes a furnace (4) in which the preforms (3) undergo a cyclic heating phase at a heating temperature and a speed of advance according to heating instructions (CC), a system (5) for supplying the furnace (4) with preforms (3), and a shaping unit (6) comprising a number of molding stations (14), each equipped with a mold (15) bearing the imprint of a container (2), for the shaping of containers (2) by blow molding or stretch blow molding of the preforms (3) exiting the furnace (4), said recording medium having recorded thereon heating instructions (CC) and shaping instructions (CF), defining a cyclic shaping phase with a pre-blow-molding step that includes injecting into the preforms (3) a gas at a pre-blow-molding pressure (P) and a pre-blow-molding flow rate according to shaping instructions (CF), followed by a blow-molding step that includes injecting into the preforms (3) a gas at a blow-molding pressure that exceeds the pre-blow-molding pressure, and said computer program being configured to, upon execution by the processing unit of the command system, cause the command system to: measure, at every instant, a real value of the pressure prevailing in the preforms (3) within each mold (15), at least during the pre-blow-molding step; store the real pressure values thus measured and associated measurement times; detect among said real pressure values a maximum real value of the pressure; and compare said maximum real value of the pressure and the associated measurement time with, respectively, a predetermined maximum reference value of the pressure (P.sub.B1) and an associated reference time (t.sub.B1), verify if the following necessary conditions are fulfilled together over a period that is greater than or equal to a length of time of a heating cycle and for all of the molding stations (14): the measurement time associated with the maximum real value of the pressure coincides with the reference time (t.sub.B1), and the maximum real value of the pressure is less than the maximum reference value of the pressure (P.sub.B1), where the maximum real value of the pressure is determined to be less than the maximum reference value of the pressure (P.sub.B1) if the difference between the maximum real value of the pressure and the maximum reference value of the pressure (P.sub.B1) is greater than several hundred millibars; in the event that said necessary conditions are fulfilled together, carry out a verification in the chronology of events that no modification of the heating instructions (CC) and no modification of the shaping instructions (CF) has occurred during a predetermined time interval preceding said measuring; and in the event that the verification indicates that neither of the heating instructions (CC) nor the shaping instructions (CF) has undergone a modification during said predetermined time interval, determine that the preforms (3) are sub-standard and cause an actuator controlled by the command system to carry out at least one of the following actions: generate an alert, stop the supply of the furnace (4) with preforms (3), and eject the containers (2) produced from said sub-standard preforms (3).

8. The process according to claim 2, wherein the generation of an alert includes displaying a message on a graphic interface.

9. The process according to claim 2, wherein with the furnace (4) and the heating unit (6) being directed respectively by dedicated slave control units (37, 38), both attached to a master control unit (35), the verification operations are carried out by the master control unit (35).

10. The process according to claim 3, wherein the generation of an alert includes displaying a message on a graphic interface.

11. The process according to claim 3, wherein with the furnace (4) and the heating unit (6) being directed respectively by dedicated slave control units (37, 38), both attached to a master control unit (35), the verification operations are carried out by the master control unit (35).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other objects and advantages of the invention will become evident from the description of an embodiment, given below with reference to the accompanying drawings in which:

(2) FIG. 1 is a diagrammatic view showing a container manufacturing installation, comprising a furnace and a blow-molding machine;

(3) FIG. 2 is a diagrammatic view illustrating in more detail the structure of the device that makes it possible to perform the measurement of pressure within preforms;

(4) FIG. 3 is a diagram in which are plotted two curves illustrating variations of the pressure based on the time during the pre-blow-molding phase, for two preforms having different moisture levels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(5) Shown diagrammatically in FIGS. 1 and 2 is an installation 1 for manufacturing containers 2 from preforms 3 made of thermoplastic material, in particular PET (polyethylene terephthalate).

(6) The installation 1 comprises three successive handling units connected to one another, namely a heating unit 4 or furnace, a system 5 for supplying the furnace with preforms 3, and a unit 6 for shaping containers 2 by blow molding or stretch blow molding preforms 3 exiting the furnace 4.

(7) The furnace 4 comprises a series of heating modules 7, each having a radiating wall 8 provided with superposed sources 9 of infrared radiation and preferably (as illustrated) a reflecting wall 10 placed facing the radiating wall 8 for reflecting the portion of radiation not absorbed by the preforms 3.

(8) The supply system 5 comprises: A hopper 11, in which the preforms 3 are stored in bulk by being discharged therein uniformly in lots (either manually or in an automated manner, from rolling stocks), A lift 12 that continuously removes preforms 3 from the hopper 11, A tunnel 13 for alignment and disentangling, into which the preforms 3 removed by the lift are discharged.

(9) Such a supply system 5 is well known, in particular from the French patent FR 2 864 050 or its U.S. equivalent U.S. Pat. No. 7,556,137, and will therefore not be described in more detail.

(10) The shaping unit 6 is equipped with a series of shaping stations 14, each provided with a mold 15 bearing the imprint of a container 2.

(11) In FIG. 1, the installation 1 is shown in a very diagrammatic manner. In particular, this drawing is not to scale, for the sake of clarity and compactness. It thus is seen that in an artificial manner, the supply system 5 is undersized in relation to the furnace 4, which indicates the smaller size of the preforms 3.

(12) In a conventional manner, the preforms 3 at ambient temperature coming from the supply system 5 are, optionally after returning to be oriented neck down, inserted into the furnace 4 via an inlet of the latter. The preforms 3 are then heated in a stream in the furnace 4 at a temperature that is higher than the glass transition temperature of the material (approximately 80 C. for PET). The heating temperature of the preforms 3 made of PET is typically approximately 120 C.

(13) According to a particular embodiment illustrated in FIGS. 1 and 2, the preforms 3 are, in the furnace 4, mounted on pivoting supports 16 referred to as spinners. Each spinner 16 is mounted on a chain that circulates on a drive wheel 17 driven in rotation by a motor 18. Each spinner 16 is provided with a pinion 19 that engages a rack 20 for driving the spinner 16 in rotation during its travel into the furnace 4 and thus for exposing the surface of each preform 3 to the radiation coming from the sources 9.

(14) To evacuate at least a portion of the excess heat produced by the radiating wall 8, the furnace 4 can be equipped with an extraction system comprising, for example, a fan 21 driven by a motor 22 and positioned at right angles to the necks of the preforms 3.

(15) In addition, the power of the radiation emitted by the radiating wall 8 can be modulated by means of a power variable-speed drive unit 23, as in the embodiment illustrated in FIG. 2.

(16) The thermal profile of the preforms 3 is preferably controlled, either directly in the furnace 4, or at the outlet of the latter, by means of a heat sensor 24. According to an embodiment illustrated in FIG. 2, the heat sensor 24 is a thermal camera pointing toward the preforms 3.

(17) At the outlet of the furnace 4, the thus heated preforms 3 are transferred (optionally by being returned to be oriented neck up) toward the shaping unit 6 via a transfer unit (such as a transfer wheel) each to be shaped into a container by blow molding or stretch blow molding in an individual mold 15.

(18) At the end of the shaping, the containers 2 are evacuated from the molds 15 for the purpose of being filled and labeled immediately, or stored temporarily for the purpose of being filled and labeled subsequently. Once filled and labeled, the containers 2 are grouped and packaged, for example, within a plastic-wrapping unit that envelops each group of containers in a heat-shrinkable film.

(19) As is also seen in FIG. 1, the shaping unit 6 comprises a pivoting carrousel 25, commonly called a wheel, at the periphery of which are mounted the shaping stations 14 and a sensor 26 of the instantaneous angular position of the wheel 25, in the form of, for example, a coder (i.e., in practice, an instrument-equipped bearing device).

(20) Each shaping station 14 is equipped with a nozzle 27, by which a fluid (in particular a gas such as air) is injected into the mold 15. Each shaping station 14 is also equipped with an injection device comprising an actuator block 28 connected to the nozzle 27 for controlling the injection of the fluid.

(21) The actuator block 28 comprises, for example, solenoid valves, by means of which the nozzle 27 is brought into communication with pressurized fluid sources, at least one pressure variable-speed drive unit for adjusting the pressure of the fluid according to predetermined pressure instructions, and at least one flow rate restrictor for adjusting the fluid flow rate according to predetermined flow rate instructions.

(22) In addition, each shaping station 14 is provided with a device 29 for measuring the pressure (denoted P) prevailing in the preform 3i.e., in the container 2 during shaping. In the illustrated example, the measuring device 29 comprises a pressure sensor mounted in the area of the nozzle 27, in which the pressure is, during shaping, identical to the pressure prevailing in the latter.

(23) According to an embodiment corresponding to a process for shaping by stretch blow molding, each shaping station 14 also comprises a movable stretching rod 30, integral with a carriage 31 mounted in translation in relation to a support 32.

(24) The movement of the rod 30 is controlled in an electromagnetic manner. For this purpose, the support 32 comprises an electromagnetic track connected to a motor 33, and the carriage 31 is itself magnetic. The sign and the power of the current passing through the track make it possible to move the rod 30 along a predetermined movement profile, comprising a direction and a speed of motion, also referred to as a stretching rate.

(25) The installation 1 is equipped with a command system 34 that comprises a master control unit 35 of the installation 1 and multiple slave control units 36, 37, 38 (in this case, three in number) slaved to the master control unit 35 and respectively directing the supply system 5, the furnace 4, and the shaping unit 6.

(26) The master control unit 35 is computerized and comprises: A memory 39 into which programs for directing the furnace 4, the supply system 5, and the shaping unit 6 are written, A processor 40 connected to the memory 39 for applying the instructions of the programs, and A communication interface 41 connected to the processor 40 for communication with the slave control units 36, 37, 38.

(27) The control unit 37 of the furnace 4 comprises: A memory 42 into which programs for directing heating modules 7 are written, A processor 43 connected to the memory 42 for applying the instructions of the program, A communication interface 44 connected to the processor 43 for communication with the master control unit 35 via the interface 41 for communication with the latter, An input interface 45 connected, on the one hand, to the processor 43, and, on the other hand, to the heat sensor 24, An output interface 46 connected, on the one hand, to the processor 43 and, on the other hand, to the power variable-speed drive unit 23 and to the motors 18, 22 of the drive wheel 17 and the fan 22.

(28) The control unit 37 of the furnace 4 is programmed to carry out in particular the following operations: Directing the or each heating module 7 with which it is associated, according to heating instructions CC that are programmed (i.e., written) into the memory 42; Based on the measurement of temperature (denoted T) obtained from the heat sensor 20, plotting the instantaneous thermal profile of each preform 3, which can come in the form of an average temperature measured for the entire preform 3, a set of multiple temperature values at different heights on the body of the preforms 3, or a curve providing the temperature T based on the height (denoted h) on the preform 3; Communicating the thus plotted thermal profile to the master control unit 35.

(29) The heating instructions CC are part of the directing program of the furnace 4; in this connection, they are initially programmed into the memory 39 of the master control unit 35, which communicates them to the control unit 37 of the furnace 4. The instructions CC comprise at least one, and preferably all, of the following parameters: A reference temperature value or a reference temperature profile, or else a reference power value or a reference power profile delivered by the variable-speed drive unit 23, A reference speed of rotation of the motor 22 of the fan 21, A speed of rotation of the drive wheel 17 (and therefore, consequently, a speed of advance of the preforms 3in other words, the production rate of the furnace 4).

(30) The heating instructions CC are able to undergo variations, for example, at the initiative of an operator. In this case, the latter can make the correction that he believes necessary by a reprogramming of the master control unit 35. The corrected instructions CC are then relayed to the control unit 37 of the furnace 4 and written by the processor 43 in the memory 42.

(31) The variations of the heating instructions CC are written in a chronology that is kept up to date in the memory 39 of the master control unit 35.

(32) Conversely, the control unit 35 uses thermal profiles of the preforms and can be programmed to perform an analysis of them for the purpose of detecting variations therein in relation to a reference thermal profile. These variations can be written in the chronology.

(33) The slave control unit 38 associated with the shaping unit 6 comprises: A memory 47 into which programs for directing shaping stations 14 are written, A processor 48 connected to the memory 47 for applying the instructions of the program, A communication interface 49 connected to the processor 48 for the communication with the master control unit 35, An input interface 50 connected, on the one hand, to the processor 48 and, on the other hand, to the device 29 for measuring pressure, denoted P, prevailing in the mold 15, An output interface 51 connected, on the one hand, to the processor 48 and, on the other hand, to the block 28 of actuators, and, if necessary, to the control motor 33 of the stretching rod 30.

(34) As a variant, the input and output interfaces 50, 51 can be reassembled within a single input/output interface.

(35) The shaping is cyclic, i.e., each preform 3 successively undergoes the same sequence of operations. The shaping comprises two steps: a first pre-blow-molding step, during which the fluid is injected into the preform 3 at a relatively low pre-blow-molding pressure (typically on the order of 7 bars), at a certain pre-blow-molding flow rate, followed by a blow-molding step during which the fluid is injected into the preform 3 at a comparatively higher blow-molding pressure (typically on the order of 25 bars), at a certain blow-molding flow rate. The general trend in the variations in pressure in the preform 3 during shaping is known, cf. for example, the pressure curve of FIG. 3 of the French patent application FR 2 909 305 or its U.S. equivalent US 2010/201013.

(36) The control unit 37 of the shaping unit 6 is programmed to carry out in particular the following operations: Directing each shaping station 14 for the fulfillment of a complete shaping cycle, from the loading of a preform 3 to the unloading of the shaped container 2, according to programmed shaping instructions CF (i.e., written) in the memory 47; Ordering a measurement, at every instant, of a real value of the pressure prevailing in the preforms within each mold 15. The pressure measurement is carried out continuously by the pressure sensor 29 continuously or in a sequential and regular manner, at predetermined intervals (for example, 5 ms) and communicated to the processor 48 via the input interface 50; Storing the real pressure values thus measured and the associated measurement times, Based on these measurements, plotting during the shaping cycle a blow-molding curve describing the change in the fluid pressure P in the mold 15 at every instant, denoted t (in practice, at times measured by the internal clock of the processor 48, corresponding to the angular positions provided by the angular sensor 26), this curve (diagrammatically visible in FIG. 2) being plotted by the processor 48 and stored during the cycle.

(37) The shaping instructions CF are part of the program for directing the shaping unit 6; in this regard, it is initially programmed in the memory 39 of the master control unit 35, which communicates it to the control unit 38 of the shaping unit 6. The instructions CF comprise at least one, and preferably all, of the following parameters: The pre-blow-molding pressure to be delivered by the block 28 of actuators, The pre-blow-molding flow rate to be delivered by the block 28 of actuators, The blow-molding pressure to be delivered by the block 28 of actuators, The blow-molding flow rate to be delivered by the block 28 of actuators, The stretching rate of the rod 30.

(38) The shaping instructions CF are able to undergo variations, for example at the initiative of an operator. In this case, the latter can make the correction that he believes necessary by a reprogramming of the master control unit 35. The corrected instructions CF are then relayed to the control unit 38 of the shaping unit 6 and written by the processor 48 in the memory 47.

(39) The variations of the shaping instructions CF are written in the chronology that is kept up to date in the memory 39 of the master control unit 35.

(40) The instructions CF are applied by the processor 48, which consequently directs the block 28 of actuators via the output interface 51; the stretching rate can be converted by the processor 48 of the controller 38 into power to be delivered by the motor 33.

(41) The control unit 38 of the shaping unit 6 is also programmed for carrying out the following operations: Analyzing the blow-molding curve and extracting from it the coordinates (real value, associated measurement time) of a local pressure peak during the pre-blow-molding step, referred to as point B, as defined in the application FR 2 909 305 (or in its U.S. equivalent US 2010/201013); Communicating from the end of the cycle the coordinates of point B to the master control unit 35.

(42) The processor 40 of the master control unit 35 is then programmed for: Taking into account the real points B communicated by the control unit 38 of the shaping unit 6 over a period that is greater than or equal to the length of a heating cycle and for all of the shaping stations 14, Comparing each real point B with a reference point (defined by a value of pressure, called reference pressure, and an associated time, called reference time) written in advance in the memory 39 of the master control unit 35 and corresponding to a model container.

(43) More specifically, the processor 40 is programmed to compare the pressure and the time of each point B with, respectively, the pressure and the reference time.

(44) The objective is to detect a variation in the moisture level in the preforms 3 from the drift of point B. The processor 40 is consequently programmed to act only if the following conditions, so-called necessary conditions, are fulfilled together over a period that is greater than or equal to the length of a heating cycle and for all of the shaping stations 14: The time of the real point B coincides (with a predetermined tolerance, typically on the order of several milliseconds) with the reference time; The pressure at the real point B is less than the maximum reference value.

(45) Experience shows that actually two preforms 3 having different moisture levels have, under the same heating and shaping conditions, pressure peaks during the pre-blow molding (points B) that differ only by the value of the pressure (on the ordinate in FIG. 3), with the associated times being combined. In other words, a variation in the moisture level in the preform does not mean that there is a temporal offset of point B.

(46) Consequently, a temporal drift of point B is analyzed as having other causes than a variation in the moisture level in the preforms.

(47) In addition, the inventors observed that the pressure at the point B varies inversely to the moisture level in the preform. Thus, when the pressure at the point B is greater than the reference pressure, this means that the moisture level of the preform from which the container is produced is less than that of the model container and consequently does not justify any particular action.

(48) By contrast, the fact that the above-mentioned necessary conditions are fulfilled is a sign that an at least temporary variation in the moisture level affects the preforms. The fact that these conditions are fulfilled over a period that is greater than or equal to the length of a heating cycle and for all of the shaping stations shows that such a defect, if it transpires, affects all of the preforms, from which it is possible to deduce that a complete lot is possibly affected by a variation in the moisture level.

(49) These conditions are not sufficient, however, to justify the above-mentioned conclusions. Actually, a defect affecting at least one of the machine parameters (heating temperature or heating profile, speed of advance, pre-blow-molding pressure, pre-blow-molding flow rate, and optionally stretching rate) can be at the origin of a variation in point B.

(50) This is why the central control unit 35 is programmed for carrying out a verification that none of these parameters has undergone for modification of instructions CC or CF during a predetermined time interval that has preceded the measuring sequence of the points B having a drift that is greater than or equal to the length of the heating cycle. Actually, any modification of the instructions CC or CF induces a modification of the operating conditions to which the preforms are subjected. In particular, any modification of the heating profile or heating temperature requires a stabilization of the furnace 4, the preforms 3 that are present in the latter during the modification having properties that are not stabilized due to the thermal inertia of the furnace 4.

(51) The control unit 35 is programmed to act only if the result of this verification is positive (i.e., none of these parameters has undergone a variation).

(52) In this case, it has turned out that a defect of the moisture level (too high) is the cause of the drift of point B, since the causes internal to the installation (variation in the heating temperature, of the speed of advance, of the pre-blow-molding pressure or the pre-blow-molding flow rate) have been eliminated. The preforms in question are then declared to be sub-standard.

(53) This situation is illustrated in FIG. 3, which shows a graph on which are plotted the blow-molding curves (centered on the pre-blow-molding step) of two preforms that are distinguished from one another by their moisture levels. The first preform, which has a moisture level corresponding to the reference level of a model container, and whose blow-molding curve is plotted in bold lines, exhibits, during the pre-blow molding, a pressure peak at a point B1, whose pressure value is denoted P.sub.B1 (corresponding to the reference pressure stored in the memory 39) and whose corresponding measurement time is denoted t.sub.B1 (corresponding to the reference time stored in the memory 39). The second preform, which has a moisture level that is essentially greater than the reference level of the model container and whose blow-molding curve is plotted in broken lines, has during pre-blow molding a pressure peak of a point B2, whose pressure value is denoted P.sub.B2 and whose corresponding measurement time is denoted t.sub.B2. It is noted that the two points B1 and B2 have simultaneous measurement times:
t.sub.B1t.sub.B2

(54) In practice, the master control unit 35 is programmed to declare simultaneous any time t.sub.B2 for measuring a pressure peak with the reference time t.sub.B1 as soon as the difference between these times t.sub.B1 and t.sub.B2 is less than or equal to several milliseconds (at the very most about ten milliseconds, preferably less than approximately 5 milliseconds).

(55) By contrast, it is noted that the real pressure P.sub.B2 at the point B2 is essentially less than the reference pressure P.sub.B1 at the point B1:
P.sub.B2<P.sub.B1

(56) In practice, the master control unit 35 is programmed to declare any real pressure P.sub.B2 to be different from the reference pressure P.sub.B1 as soon as the difference between these values is greater than or equal to several tens, and even several hundreds, of millibars.

(57) The control unit 35 is programmed then to control at least one of the following actions: Generating an alert, which can be displayed on a control screen, directed at the operator responsible for monitoring the installation; this alert comprises, for example, a message inviting the operator to check the moisture level of the preforms, to replace the preforms 3 of the hopper by another lot, or else to perform checks on the containers 2 produced; Stopping the supply of the furnace 4 with preforms, for example by ordering the shutdown of the lift 12 of the supply system 5; Ejecting the containers 2 produced from sub-standard preforms 3; this operation can be carried out thanks to the traceability of the containers 2 produced by the angular position information of the shaping stations 14, coming from the angular sensor 26. The container 2 coming from a preform declared to be sub-standard can then be ejected upon its discharge from the mold 15.

(58) The process that was just described offers several advantages.

(59) First, it makes it possible to distinguish, when a drift is noted at the point B (showing a poor quality of the containers produced), between causes for the internal drift in the machine and defects affecting the preforms themselves. This makes it possible to prevent a worsening of the drift at the point B by improper correction of the heating or shaping instructions.

(60) Second, this process makes it possible to initiate a continuous check of the conformity of the preforms with regard to their moisture level, thanks to the detection of the point B and the verification of its similarity with a reference point corresponding to a model container.

(61) Thanks to the verification carried out on the heating and/or shaping instructions, this conformity check is reliable.