Patent Publication Number: US-8532814-B2

Title: Method for producing a vessel from a preform, with feedback depending on the expansion end point of the preform

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a National Stage of International Application No. PCT/FR2007/001992 filed Dec. 4, 2007, claiming priority based on French Patent Application No. 0610618, filed Dec. 5, 2006, the contents of all of which are incorporated herein by reference in their entirety. 
     FIELD OF INVENTION 
     The invention relates to producing vessels by blow-moulding or stretch blow-moulding, from preforms made of plastic material. 
     BACKGROUND 
     In order to manufacture a vessel according to the blowing technique, we begin by heating a preform (whether entailing a preform or an intermediary vessel that has undergone a first blowing operation from a preform) at a temperature greater than the glass transition temperature of the material of the preform. The preform is then introduced into a mould, then the blowing of the preform is carried out by injecting therein a gas (such as air) under high pressure (generally greater than 30 bars). The stretch blow-moulding technique consists, other than the blowing, in stretching the preform using a sliding rod, in order in particular to minimise the offset of the vessel and to render uniform as much as possible the distribution of the material. 
     In order to prevent the preform from bursting during the blowing operation, prior to the blowing, a pre-blowing operation is generally provided, during which a gas under reduced pressure (generally between 5 and 16 bars) is injected into the preform. It is during the pre-blowing operation that is carried out the stretching of the preform, with the blowing intervening only afterwards. 
     In a context of industrial production, where manufacturing rates reach several tens of thousands of vessels per hour and per machine (a machine generally comprises several moulds mounted on a rotating carrousel), the duration of the method for producing a vessel, between the moment the preform is introduced into the mould and the moment the formed vessel is ejected, is of a few seconds. The duration of the pre-blowing is only of a few tenths of a second. 
     The constant desire of industrialists to reduce the quantity of material used for each vessel, combined with the velocity of the method, makes the manufacture delicate and can lead to substantial scrap rates. Among the main defects observed on the vessels is a poor distribution of the material. The difficulty that exists in combining saving material and the increase in the rates has led the industrialists to become more interested in the unfolding of the method by monitoring the variations in certain parameters over time, the main one being the pressure inside the preform, combined with the stretching speed. 
     Certain entities, such as Du Pont De Nemours in its American patent U.S. Pat. No. 4,042,657, suggest that there is a correlation between the pressure variations in the preform and the final form of the vessel, and recommend rejecting the vessels of which the pressure curve does not comply with a standard curve. If the proposal to reject the non-compliant vessels makes it possible to improve the overall quality of production, it puts a strain however on productivity and, in the hypothesis where the rejected vessels would not be recycled, leads to a wasting of material although it is desired to save it. 
     CERTAIN OBJECTS OF THE INVENTION 
     The invention in particular aims to overcome these disadvantages, by proposing a method making it possible to improve the quality of the vessels produced, while still limiting scrapping, and by maintaining—and even by increasing—the production rates. 
     To this effect, the invention proposes, according to a first object, a method for producing a vessel by blowing in a mould from a preform made of plastic material, comprising the following steps:
         introducing the preform into the mould;   at a predetermined moment, called pre-blowing cue, controlling the opening of an electrovalve for establishing a communication between the inside of the preform and a source of gas at a predetermined pre-blowing pressure;   measuring the pressure inside the preform;   at a predetermined moment, called the blowing cue, following the pre-blowing cue, controlling the opening of an electrovalve for establishing a communication between the inside of the preform and a source of gas at a blowing pressure that is greater than the pre-blowing pressure;   memorising the variations of the pressure inside the preform at least in the interval between the pre-blowing cue and the blowing cue;   detecting in this interval the so-called expansion end moment, from which the pressure in the preform increases in a linear manner;   advancing the blowing cue according to the moment of expansion end.       

     It has been observed that not only the vessels as such produced have, in relation to the vessels obtained by the conventional methods, a quality that on the average is higher thanks to a better distribution of the material, but also that this quality is more constant over time. 
     Additional operations can be considered consisting in:
         measuring the delay separating the moment of expansion end from a real moment of the beginning of blowing, after the moment of expansion end, where the pressure sags;   advancing the blowing cue by a value equal to this delay.       

     Alternatively, the following operations are provided:
         measuring the delay separating the moment of expansion end from the blowing cue;   advancing the blowing cue by a value equal to this delay, increased by a response time, predetermined, of the blowing electrovalve.       

     The invention proposes, according to a second object, a machine for manufacturing vessels from preforms made of a plastic material, comprising:
         a mould having a cavity intended to receive a preform;   a source of gas at a predetermined pre-blowing pressure;   an electrovalve able to establish a communication between the inside of the preform, received in the cavity, and the source of pre-blowing gas;   means for controlling the opening and the closing of the pre-blowing electrovalve;   a source of gas at a predetermined blowing pressure;   an electrovalve able to establish a communication between the inside of the preform, received in the cavity, and said source blowing of gas;   means for controlling the opening of the blowing electrovalve at a predetermined moment, called blowing cue;   a sensor able to measure the pressure inside the preform;   means for detecting the so-called moment of expansion end, from which the pressure in the preform increases in a linear manner;   means for adjusting the blowing cue according to the moment of expansion end.       

     According to an embodiment, the machine further comprises means for comparing the moment of expansion end with a so-called moment of beginning of blowing, following the moment of expansion end, from which the pressure in the preform sags. 
     The invention also proposes, according to a third object, a computer programme product intended to be implemented on a machine for manufacturing vessels such as described hereinabove, comprising instructions for:
         controlling the opening and the closing of the blowing electrovalve;   taking into account the moment of expansion end;   adjusting the opening moment of the blowing electrovalve according to the moment of expansion end.       

    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Other objects and advantages of the invention shall appear when reading the description provided hereinafter in reference to the annexed drawings wherein: 
         FIG. 1  is a schematic view showing a machine for manufacturing vessels; 
         FIG. 2  is a schematic cross-section view showing partially a moulding unit in the machine in  FIG. 1 ; 
         FIG. 3  is a diagram whereon is plotted a curve showing the variations in the pressure inside a preform during the manufacture of a vessel; 
         FIG. 4  is a diagram whereon is plotted a curve showing the variations in the pressure inside a preform during the pre-blowing operation; 
         FIGS. 5A to 5H  are cross-section view showing the preform in the mould, at different stages during the pre-blowing; 
         FIG. 6  is a diagram showing the superposition of two pressure curves corresponding respectively to two different moments of controlling the opening of a pre-blowing electrovalve; 
         FIG. 7  is a diagram showing the superposition of two pressure curves corresponding respectively to two distinct values of the pressure or of the pre-blowing flow rate; 
         FIG. 8  is a diagram showing the superposition of two pressure curves corresponding respectively to two distinct values of the stretching speed; 
         FIG. 9  is a diagram showing the superposition of two pressure curves corresponding respectively to two distinct values of the heating temperature of the preform; and 
         FIG. 10  is a diagram showing the superposition of two pressure curves corresponding respectively to two moments of controlling the opening of a blowing electrovalve. 
     
    
    
     DETAILED DESCRIPTION OF NON-LIMITED EMBODIMENTS 
       FIG. 1  shows a machine  1  for the manufacturing of vessels  2  from preforms  3  made of plastic material such as PET (polyethylene terephtalate). According to a preferred embodiment, the preforms  3  are preforms from which the final vessels  2  are directly obtained, without passing by an intermediary vessel. Each preform  3  comprises a neck  4  (which does not undergo any deformation during the formation of the vessel  2 ), and a cylindrical body  5  terminated by a hemispheric base  6 . 
     This machine  1  comprises an oven  7  of the tunnel type, wherein the preforms  3  are continuously heated to a temperature greater than the glass transition temperature of their material. In the case of PET, of which the glass transition temperature is in the neighbourhood of 80° C., the heating temperature is more preferably between 100° C. and 140° C. 
     The machine  1  further comprises a plurality of moulding units  8  mounted on a rotating carrousel  9  arranged at the outlet of the oven  7  with interposition of a transfer wheel  10  providing the synchronisation of the heating and moulding operations of the preforms  3 . 
     Each moulding unit  8  comprises a mould  11 , made of steel or aluminium alloy, comprising two half-moulds  12 ,  13  and a mould base  14  which together define an internal cavity  15 , intended to receive a preform  3  coming from the oven  7 . 
     Each moulding unit  8  further comprises:
         a stretch rod  16  mounted sliding in relation to mould  11  along a main X axis (generally of revolution) of the latter, between a high position ( FIG. 2 ) making it possible to introduce the preform  3  and a low position ( FIG. 5H ) where, at the end of the stretching of the preform  3 , the rod  16  reaches the mould base  14  pushing therein the base  6  of the latter,   a case  17  defining a nozzle  18  wherein slides the rod  16  and which, during the manufacture of the vessel  2 , interacts with the neck  4  of the preform  3 .       

     The moulding unit  8  further comprises several fluid circuits exiting into the nozzle  18  via the case  17 , i.e.:
         a medium pressure pre-blowing air circuit  19  (between 5 and 16 bars), this circuit  19  comprising a source  20  of pre-blowing air and a duct  21  (which can be formed at least partially in the case  17 ) connecting this source  20  to the nozzle  18  with interposition of a first so-called pre-blowing electrovalve  22 ,   a high pressure blowing air circuit  23  (between 30 and 40 bars), comprising a source  24  of blowing air and a duct  25  (which can be formed at least partially in the case  17 ) connecting this source  24  to the nozzle  18  with interposition of a second so-called blowing electrovalve  26 ,   a degassing circuit  27  comprising an outlet for venting  28  and a duct  29  connecting the nozzle  18  to this outlet  28  with interposition of a third so-called degassing electrovalve  30         

     The electrovalves  22 ,  26 ,  30  are connected electrically to a control unit  31  which controls the opening and the closing of it (duly taking their response times into account). These electrovalves  22 ,  26 ,  30  can be arranged at a distance from the case  17 ; however, for more compactness, they are more preferably at least partially included in the latter, in accordance with the embodiment described in the French patent application published under number FR 2 872 082 or in the equivalent international application published under the number WO 2006/008380, both in the name of the applicant and to which those skilled in the art may refer. 
     The moulding unit  8  is moreover provided with a pressure sensor  32  with which is measured the pressure inside the preform  3  during the manufacture of the vessel  2 . To this effect, the sensor  32 , connected to the control unit  31  that memorises the pressure measurements taken, is provided with a substantial portion  33  exiting into the nozzle  18  (where the pressure is identical to that inside the preform  3 ). 
     The manufacture of a vessel  2  from a preform  3  is carried out as follows. 
     The preform  3 , mounted on a conveyor, is first of all introduced in the oven  7  where it is heated, neck  4  downwards, in the conditions described hereinabove. At the outlet of the oven  7 , the preform  3  is grasped by a clamp of the transfer wheel  10  and introduced, after having been turned over to have the neck  4  directed upwards, into a mould  11  opened beforehand. In this position, the stretch rod  16  is in high position. 
     As the carrousel  9  rotates, the mould  11  closes on the preform  3 . Once the mould  11  is closed ( FIG. 5A ), begins, at a predetermined moment called starting cue, the descending of the stretch rod  16  into the moulding cavity  15  ( FIG. 5B ). The starting cue constitutes the origin of the axis of time (abscissas) in  FIGS. 3 ,  4  and  6  to  10 . Starting from this moment the pressure P inside the preform is measured continuously, each value being memorised in the control unit  31 . The term “continuously” means that the period during which the pressure measurements are taken is low in light of the time needed for the pressure variations in the preform  3 . To this effect, a high-performance sensor  32  will be chosen, making it possible to take pressure measurements at a period of less than or equal to 5 ms (millisecond), more preferably less than or equal to 2 ms, with the ideal being to be able to take the measurements at a period of 1 ms. 
     To starting cue corresponds a predefined angular position of the mould  11  on the circumference of the carrousel  9 . According to a first embodiment, the controlling of the displacement of the stretch rod  16  is purely mechanical, carried out using a cam against which rolls or slides a cam follower integral with the stretch rod  16 . In this case, the moment when the stretch rod  16  begins its descent is confounded with the starting cue, which corresponds to the moment when the cam follower enters into contact with the cam. 
     According to a second embodiment, the displacement of the stretch rod  16  is provided by an electromechanical system controlled by the control unit  31 . In this case, the moment when the descent of the stretch rod  16  begins is shifted in relation to starting cue by an offset value equal to the response time of the electromechanical system. This offset value, provided by the manufacturer of the system, is taken into account in the adjustment of the starting cue so that the descent of the stretch rod  16  starts as soon as possible after the closing of the mould  11 . V E  denotes the displacement speed of the stretch rod  16 ; this speed is also called stretching speed. 
     At a predetermined moment, called pre-blowing cue t P , the control unit  31  controls the opening of the pre-blowing electrovalve  22  for establishing a communication between the inside of the preform  3  and the source  20  of pre-blowing air. 
     The moment, called real moment of the beginning of the pre-blowing and noted as t A , from which the pressure P in the preform  3  begins to increase, registers a delay in relation to the pre-blowing cue t P , equal to the response time of the pre-blowing electrovalve  22 . 
     This delay, either provided by the manufacturer of the electrovalve  22 , or measured, is taken into account in the adjustment of the pre-blowing cue t P , so that the real moment t A  of the beginning of pre-blowing begins after the moment when the stretch rod  16  reaches the base of the preform  3 , with this moment able to be calculated simply from the descending speed (known) of the stretch rod  16  and from the length (also known) of the preform  3 . 
     In  FIGS. 4 and 6 , the point of the curve having for abscissa the real moment t A  of the beginning of the pre-blowing is noted as A. To this point corresponds a pressure (relative) substantially nil, the pressure inside the preform  3  during its introduction into the mould  11  being indeed substantially equal to the atmospheric pressure. 
     From point A begins an axial expansion phase of the preform  3 . As can be seen in  FIGS. 4 and 6 , the slope of the pressure curve at point A is nil on the left, and positive on the right. From point A, the pressure in the preform  3  increase as it is stretched, the flow of air introduced in the preform  3  being higher than the increasing of the internal volume of the preform  3  (which undergoes no radial expansion, Cf.  FIG. 5C ), up to a moment, called development moment and noted as t B , when the plastic flow threshold of the preform  3  has been reached. At this moment t B  a pressure peak occurs, the corresponding point, called development point, being noted as B in  FIGS. 4 and 7  to  9 . At point B the slope of the blowing curve is nil, this slope reversing from left (where it is positive) to right (where it is negative) of point B. The corresponding pressure, called development pressure, is noted as P B . 
     Starting from the development moment t B  begins an axial and radial expansion phase of the preform. At the beginning of this phase, immediately after the development point, a momentary drop ΔP in the pressure P is noted inside the preform ( FIG. 5D ). This drop ΔP in pressure is due to the sudden beginning of the radial expansion of the preform  3 , which, with its plastic flow threshold crossed, thus undergoes a plastic deformation. 
     After having dropped by a value ΔP of approximately 1 bar, the pressure P is maintained, with the increase in the volume of the preform  3  offsetting the flow of air injected in the latter ( FIGS. 5E ,  5 F). This radial and axial expansion phase ends when the stretching ends, the stretch rod  16  having reached its low position where it pushes the base  6  of the preform  3  against the mould base  14  ( FIG. 5G ). 
     The combination of a radial deformation and of an axial deformation of the preform  3  leads to a bi-orientation (axial and radial) of the macromolecules of the material, which in particular results in increasing the mechanical strength of the vessel  2  and in decreasing its permeability to gases and liquids. For further information, refer to the work of Rosato and DiMattia, “Blow Molding Handbook” (2 nd  edition, Hanser, 2004). 
     After the end of the stretching, the rod  16  being maintained in its low position, takes place a radial expansion phase of the preform  3  during which no inflection of the pressure P is noted, which continues to be stable. That is why at the moment, noted as t E , when the stretching ends, no significant point corresponds on the pressure curve, this moment t E  being however known as it is predetermined by the value (which may be adjustable) of the stretching speed V E  and the height of the vessel  2 . This radial expansion phase ends at a moment, called moment of expansion end, noted as t C , when the preform  3  has reached the surface of the cavity  15  over substantially all of its height, in other words when the internal volume of the preform  3  is almost equal to the final volume of the vessel  2 , if localised zones are accepted where the contact between the preform  3  and the cavity  15  is not complete, for example in zones wherein are arranged ribs, grooves or other recesses aiming to confer upon the vessel  2  better strength or a certain aesthetics ( FIG. 5H ). To the moment t C  of expansion end corresponds on the pressure curve a significant point, called expansion end point and noted as C in  FIGS. 4 and 10 , where the curve sags and has an increase in its slope. The pressure at the point C, called expansion end pressure, is noted as P C . 
     From the expansion end point C begins a growth phase in the pressure P in the preform  3 , at a constant volume. The rate, noted as D P  and the pressure, noted as P P , of pre-blowing air being maintained constant, the pressure P in the preform  3  increases in a linear manner up to a moment, noted as t D , of the beginning of blowing from which starts a blowing phase: communication of the inside of the preform  3  is established with the source  24  of blowing air, the pressure P in the preform  3  increasing suddenly up to the value of the blowing pressure. In light of the response time of the blowing electrovalve  26 , the moment t D  of the beginning of blowing registers a slight delay in relation to a so-called blowing cue and noted as t S , to which the control unit  31  controls simultaneously the closing of the pre-blowing electrovalve  22  and the opening of the blowing electrovalve  26 . To the moment t D  of beginning of blowing corresponds on the pressure curve a significant point, called blowing starting point and noted as D, when the curve sags and has a substantial increase in its slope. 
     The inventors feel that the pre-blowing is a determining factor in the quality of the final vessel. Controlling pre-blowing is carried out by the intermediary of the plot of the corresponding pressure curve. Ideally, all the vessels are identical. In reality, the repeatability of the method for producing is not provided due to variations in physical parameters (pressure, temperature) and mechanical parameters (electrovalve response time, obstruction of the pneumatic circuits, leaks in the pre-blowing and blowing circuits, etc.) which can have an impact on the different moulding units  8  of the machine  1  during the production of vessels  2 . That is why substantial differences are noted between the vessels  2 . In order to provide the constancy of the quality of the vessels obtained, in order to place them in conformity as much as is possible with a pre-established standard, the inventors propose to provide a continuous adjustment of the pre-blowing operations through feedback on at least one of the following parameters:
         heating temperature T of the preforms;   pre-blowing pressure P P ;   pre-blowing rate D P ;   stretching speed V E ;   pre-blowing cue t P ;   blowing cue t S .       

     Through hypothesis, we have a theoretical pre-blowing curve, (i.e. the portion of the pressure curve between the starting cue, taken as the origin of time, and the point D) for which the quality of the vessel obtained appears to be compliant with pre-established criteria including for example the mass of the base of the vessel, assumed to lie within a range of values, the uniformity of the thickness of the wall, the coaxiality of the base and of the neck, etc. 
     The inventors have considered that it is illusory to attempt to obtain for each vessel produces a curve that is strictly identical to the theoretical pre-blowing curve and consequently propose a simpler method for controlling while still remaining effective. According to this method, at least one characteristic point is selected on the theoretical curve, defined by a time/pressure ordered pair, deemed characteristic of the proper unfolding of the pre-blowing operations, and a tolerance zone is defined around this point, the combination of a time tolerance (along the x axis) and of a pressure tolerance (along the y axis), in order to take into account in particular the error margins on the measurements taken. 
     For the or each characteristic point, the real point, such as results from the measurements taken during the production, is compared with the corresponding theoretical characteristic point. In practice, it is verified that the real point, such as is measured, is located within the tolerance zone defined around the theoretical characteristic point by checking that the moment of the real point is located in the time tolerance and that its pressure is located within the pressure tolerance. In the hypothesis where the real point is effectively located in the tolerance zone, the real and theoretical points are declared confounded. On the contrary, when the real point is located outside of the tolerance zone, the real and theoretical points are declared distinct, in which case, as we shall see hereinafter, feedback measurements can be taken in order to modify at least one of the parameters listed hereinabove in order to cause to converge, on the pressure curve of the following vessel, the real point towards the theoretical point. 
     In practice, points A, B, C and D are controlled (and are possibly acted upon), alone or in combination. We shall now describe how to control and act on each of these points. 
     Point A 
     Recall that the point A corresponds to the moment t A  from which the pressure in the preform  3  begins to increase, after the control unit  31  has controlled the opening of the pre-blowing electrovalve  22  (at the pre-blowing cue t P ) in order to establish communication between the inside of the preform  3  and the source  20  of pre-blowing air. 
     It is important that the position of the point A be substantially identical for all of the moulding units  8 . The occurrence of the point A is therefore detected, i.e. the moment t A  is detected where the pressure P in the preform  3  begins to increase. (This measurement can be taken either during manufacture of the vessel  2 , or a posteriori, directly on the pre-blowing curve.) 
     The real point A is then compared with the theoretical point A. Where the pressure at the point A is nil, in practice the moment t A  of the real point A is compared with the moment of the theoretical point A, i.e. the theoretical pre-blowing beginning moment, in order to check if the moment t A  of the real point A is located within the tolerances defined for the point A. 
     In the hypothesis where the real moment t A  of the beginning of pre-blowing follows (taking the tolerance into account) the theoretical pre-blowing beginning moment, the pre-blowing cue t P  is advanced. 
     In the hypothesis where on the contrary the real moment t A  of the beginning of pre-blowing precedes (taking the tolerance into account) the theoretical moment of the beginning of the pre-blowing, the pre-blowing cue t P  is delayed. 
     The two curves plotted in solid lines in  FIG. 6  show two different settings of the pre-blowing cue t P . The left curve shows an advancing of the pre-blowing cue t P ; the right curve shows on the contrary a delaying of the pre-blowing cue t P . The theoretical curve is, between the latter, plotted in dotted lines. 
     More precisely, it is preferable to measure the difference separating the real moment t A  of the beginning of pre-blowing of the theoretical pre-blowing beginning moment. And, if the real moment t A  of the beginning of pre-blowing follows—respectively precedes—the theoretical pre-blowing beginning moment, the pre-blowing cue t P  is advanced—respectively delayed—by the value of the difference. 
     An example with numbers is provided in the table hereinafter for the purposes of illustrating the above. The measurements shown correspond to the pressure curve in  FIG. 4 . 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Value 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Parameter 
                   
                   
               
               
                   
                 Type of vessel 
                 Bottle 1.5 
                 l 
               
            
           
           
               
               
               
            
               
                   
                 Material 
                 PET 
               
            
           
           
               
               
               
               
            
               
                   
                 Heating temperature 
                 120-140° 
                 C. 
               
               
                   
                 Pre-blowing pressure 
                 10 
                 bars 
               
               
                   
                 Stretching speed 
                 2000 
                 mm/s 
               
               
                   
                 Pre-blowing cue 
                 0.03 
                 s 
               
               
                   
                 Response time of the pre-blowing 
                 0.01 
                 s 
               
               
                   
                 electrovalve 
                   
                   
               
               
                   
                 Theoretical pre-blowing beginning 
                 ±0.005 
                 s 
               
               
                   
                 moment tolerance 
                   
                   
               
               
                   
                 Measurement 
                   
                   
               
               
                   
                 Theoretical pre-blowing beginning 
                 0.04 
                 s 
               
               
                   
                 moment 
               
               
                   
               
            
           
         
       
     
     In order to allow for the adjustment of the point A, the machine  1  comprises:
         means for detecting the real moment t A  of the beginning of pre-blowing;   means for comparing this moment t A  with the theoretical pre-blowing beginning moment; and   means for adjusting the pre-blowing cue t P  according to the result of this comparison.       

     The machine  1  may also comprise means for memorising a plurality of points A (for example approximately ten) detected on a series of manufactured vessels  2 , in particular for the purposes of statistical study. 
     These means can have the form of instructions of a computer programme product implemented in the control unit  31  of the machine  1 . 
     Point B 
     Recall that the point B corresponds to the plastic flow threshold of the preform  3 , from which starts its radial expansion. 
     Adjusting the development moment t B  (abscissa of the point B) makes it possible to control the radial expansion of the preform  3 . A premature occurrence, or on the contrary a late one, of the development point B can in the same way indicate a poor distribution of the material, with as a consequence a defect in the strength of the vessel  2 . 
     The occurrence of the point B is therefore detected, i.e. the moment t B  when a pressure peak occurs in the preform  3  after the pre-blowing cue t P  (but before the blowing cue t S ) is detected. As with the point A, this measurement can be taken either during manufacture of the vessel  2 , or more preferably a posteriori, directly on the pre-blowing curve. 
     The real point B is then compared with the theoretical point B. In practice, the real development moment t B  is compared with the theoretical moment of development on the one hand and, on the other hand, the real development pressure P B  is compared with the theoretical development pressure in order to check that both lie within the tolerances defined for the point B. 
     The real point B is declared confounded with the theoretical point B when it is located within the tolerance zone defined around the theoretical point B. 
     When on the contrary the real point B is not confounded with the theoretical point B, i.e. it is located outside of the tolerance zone, by feedback a modification is carried out on at least one of the following parameters:
         pre-blowing cue t P ,   pre-blowing pressure P P ,   pre-blowing flow rate D P ,   stretching speed V E ,   heating temperature T of the preforms.       

     Several cases can occur, according to whether the real development moment t B  precedes, is confounded (i.e. is within the tolerance zone) or follows the theoretical development moment, and according to whether the real development pressure P B  is less than, equal (i.e. within the tolerance zone) or greater than the theoretical development pressure. 
     We have seen that modifying the pre-blowing cue t P  produces a shift in the point A, towards the right on the axis of time when the pre-blowing cue t P  is delayed, or on the contrary towards the left when the pre-blowing cue t P  is advanced. In fact, this modification has an impact on the entire pre-blowing curve, which is then shifted in the same direction as the point A. The modification of the pre-blowing cue t P  thus impacts the development moment t B , i.e. the abscissa of the point B. In the hypothesis where the real development moment t B  is, either before or after the theoretical development moment, a simple measurement could consist in delaying or, respectively, advancing the pre-blowing cue t P . However, the real moment t A  of the beginning of pre-blowing (abscissa of the point A) would then be affected needlessly. That is why it appears preferable that feedback on the pre-blowing cue t P  in order to modify the development moment t B  be carried out only when the modification of the other parameters is not enough to return the development point B to the tolerance zone. 
     The influence of the other parameters is shown in  FIGS. 7 ,  8  and  9 . 
       FIG. 7  shows the influence of the pressure P P  or of the rate D P  of pre-blowing on the unfolding of the pre-blowing. It has been observed that the influence of the pressure P P  is similar to that of the rate D. The joint influence of these two parameters can therefore be expressed as the influence of their product P P ×D P : the increase in this product can result from an increase in the pre-blowing pressure P P  at a constant rate D P , from an increase in the pre-blowing rate D P  at a constant pressure P P , or from a simultaneous increase in the both D P  and P P ; on the contrary, a decrease in the product P P ×D P  can result from a decrease in the pre-blowing pressure P P  at a constant rate D P , a decrease in the pre-blowing rate D P  at a constant pressure P P , or a simultaneous decrease in the pressure D P  and in the rate D P , being understood that increasing one while decreasing the other does not appear to be useful. 
     In  FIG. 7 , the left curve corresponds to a value of the product P P ×D P  greater than that corresponding to the right curve: it is observed that an increase in the product P P ×D P  results in an increase in the slope of the pressure curve at the point A (i.e. at the real moment t A  of the beginning of pre-blowing), an advancing of the development moment t B  and an increase in the development pressure P B  (on the curve, the point B is therefore displaced towards the top and towards the left). On the contrary, a decrease in the product P P ×D P  results in a decrease in the slope of the pressure curve at the point A, a delaying of the development moment t B  and a decrease in the development pressure P B  (on the curve, the point B is therefore displaced towards the bottom and towards the right). 
       FIG. 8  shows the influence of the stretching speed V E  on the unfolding of the pre-blowing. To the lower curve corresponds a stretching speed V E  greater than that corresponding to the upper curve. It is observed that the increase in the stretching speed V E  results in an advancing of the development moment t B  and a decrease in the development pressure P B  (on the curve, the point B is therefore displaced towards the bottom and towards the left), while the decrease in the stretching speed V E  results in a delaying of the development moment t B  and an increase in the development pressure P B  (on the curve, the point B is therefore displaced towards the top and towards the right). No influence of the stretching speed V E  on the slope of the pressure curve between the point A and the point B is however noted. 
       FIG. 9  shows the influence of the heating temperature T on the course of the pre-blowing. To the lower curve corresponds a heating temperature T greater than that corresponding to the upper curve. It is observed that the increase in the heating temperature T has an impact that is similar to the increase in the stretching speed V E : advancing of the development moment t B  and a decrease in the development pressure P B , while the decrease in the heating temperature T has an influence similar to the decrease of the stretching speed V E : delaying of the development moment t B  and an increase in the development pressure P B . 
     We shall present hereinafter the recommended measures to return, in each possible case, the development point B to its tolerance zone, according to the observed influence of the parameters listed hereinabove and of the priority given to certain parameters over others. In practice, priority is given to the pressure P P  or to the rate D P  over the heating temperature T. In light of its influence on the entire curve, and in particular on the real point A of the beginning of pre-blowing, the pre-blowing cue t P  is, in practice, modified only when modifications of the pressure P P , in the rate D P  or in the heating temperature T have not been enough to return the development point B to its tolerance zone. As for the stretching speed V E , this is modified only in order to refine the positioning of the development point B (and with the condition that the machine  1  be provided with means of adjusting the stretching speed V E ). 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                 Development 
                 Development 
                 Modification of the 
               
               
                   
                 moment 
                 pressure 
                 parameters 
               
               
                   
               
             
            
               
                   
                 Before 
                 Lower 
                 Decrease T 
               
               
                   
                   
                 In the tolerance 
                 Decrease D P  or P P   
               
               
                   
                   
                 zone 
                 Decrease T 
               
               
                   
                   
                 Higher 
                 Decrease D P  or P P   
               
               
                   
                 In the 
                 Lower 
                 Decrease T 
               
               
                   
                 tolerance 
                   
                 Increase D P  or P P   
               
               
                   
                 zone 
                 In the tolerance 
                   
               
               
                   
                   
                 zone 
                   
               
               
                   
                   
                 Higher 
                 Increase T 
               
               
                   
                   
                   
                 Decrease D P  or P P   
               
               
                   
                 After 
                 Lower 
                 Increase D P  or P P   
               
               
                   
                   
                 In the tolerance 
                 Increase D P  or P P   
               
               
                   
                   
                 zone 
                 Increase T 
               
               
                   
                   
                 Higher 
                 Increase T 
               
               
                   
                   
                   
                 Where applicable: 
               
               
                   
                   
                   
                 Decrease D P  or P P , 
               
               
                   
                   
                   
                 Advance pre-blowing cue 
               
               
                   
               
            
           
         
       
     
     An example with numbers is provided in the table hereinafter, in order to show the values of the moment and of the development pressure. The measurements shown correspond to the pressure curve in  FIG. 4 . 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Value 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Parameter 
                   
                   
               
               
                   
                 Type of vessel 
                 Bottle 1.5 
                 l 
               
            
           
           
               
               
               
            
               
                   
                 Material 
                 PET 
               
            
           
           
               
               
               
               
            
               
                   
                 Heating temperature 
                 120-140° 
                 C. 
               
               
                   
                 Pre-blowing pressure 
                 10 
                 bars 
               
               
                   
                 Pre-blowing flow rate* 
                 40 
                 l/s 
               
               
                   
                 Stretching speed 
                 2000 
                 mm/s 
               
               
                   
                 Pre-blowing cue 
                 0.03 
                 s 
               
               
                   
                 Development moment tolerance 
                 ±0.005 
                 s 
               
               
                   
                 Development pressure tolerance 
                 0.5 
                 bar 
               
               
                   
                 Measurement 
                   
                   
               
               
                   
                 Development moment (point B) 
                 0.07 
                 s 
               
               
                   
                 Development pressure (point B) 
                 5 
                 bars 
               
               
                   
               
               
                 *given in normoliters per second (equivalent of the liter per second at atmospheric pressure) 
               
            
           
         
       
     
     Note that the displacement of the development point B (especially its abscissa, the development moment t B ) has an incidence on the moment t C  of expansion end (abscissa of the point C). The modifications of parameters that affect the position of the point B can thus result in having to modify the position of the point D (i.e., according to the case, in advancing or in delaying the blowing cue t S ) which can indeed be located shifted in relation to point C; the corresponding adjustments are presented hereinafter. 
     In order to allow for the adjustment of the point B, the machine  1  comprises:
         means for detecting the development moment t B ;   means for comparing the development moment t B  and the development pressure P B  with, respectively, the theoretical development moment and pressure;   means for adjusting the pre-blowing rate D P  according to this comparison.       

     These means can take the form of instructions of a computer programme product implemented in the control unit  31  of the machine  1 . 
     Furthermore, in order to effectively proceed in adjusting of the pre-blowing rate D P , the machine  1  comprises for example a flow regulator  34  mounted on the pre-blowing electrovalve and controlled by the control unit  31  (cf.  FIG. 2 ). 
     The machine  1  can, according to the parameters that are chosen to adjust, include, for example in the form of additional instructions from the computer programme:
         means for adjusting the pre-blowing pressure P P  according to the comparison between the real point B and the theoretical point B;   means for adjusting the heating temperature T according to the comparison between the real point B and the theoretical point B;   means for adjusting the pre-blowing cue t P  according to the comparison between the real point B and the theoretical point B;   means for adjusting the stretching speed V E  according to the comparison between the real point B and the theoretical point B.       

     Alternatively, it can be desired to adjust the development moment t B  independently of the development pressure P B , or reciprocally adjust the development pressure P B  independently of the development moment t B . 
     In the first case, the pressure peak is detected and action is taken on at least one of the following parameters: pre-blowing pressure P P , pre-blowing flow rate D P , pre-blowing cue t P , stretching speed V E , heating temperature T in order to advance or, on the contrary, delay the development moment t B . 
     In the second case, the pressure peak is detected and action is taken on at least one of the following parameters: pre-blowing pressure P P , pre-blowing flow rate D P , pre-blowing cue t P , stretching speed V E , heating temperature T in order to increase or, on the contrary, decrease the development pressure P B . 
     The machine  1  may also comprise means for memorising a plurality of points B (for example approximately ten) detected on a series of manufactured vessels  2 , in particular for the purposes of statistical study. 
     Points C, D 
     Recall that the point C corresponds to the end of the expansion of the preform  3 , when the latter has reached the surface of the cavity  15  over substantially all of its height, i.e. when the internal volume of the preform  3  has almost reached the final volume of the vessel  2 . At the point C, the pressure curve sags and has an increase in its slope. 
     Once the expansion of the preform  3  is terminated, it is useless to continue to inject pre-blowing air therein, of which the pressure P P  is not enough to allow the material (which continues to cool and is therefore less and less deformable) to perfectly hug the surface of the cavity  15 : such an operation therefore constitutes a loss of time that puts a strain on the productivity with no incidence on the quality of the vessel  2 . It therefore appears desirable to start the blowing as soon as the expansion of the preform  3  has completed. In other words, it is desirable to take measurements in order to converge, on the pressure curve, the point D (beginning of the blowing) towards the point C (expansion end of the preform  3 ). 
     The occurrence of the point C is therefore detected, i.e. the moment t C  from which the pressure in the preform  3  begins to increase in a linear manner (moment of expansion end). Then, in the hypothesis where the blowing cue t P  follows the moment of expansion end, the blowing cue is advanced in order to converge the point D towards the point C. 
     More precisely, the delay separating the moment t C  of expansion end from the blowing cue t S  is measured, and the blowing cue t S  is advanced by a value equal to this delay. In reality, the response time of the blowing electrovalve  26  must be taken into account, due to which the real moment t D  of the beginning of blowing (abscissa of the point D) is slightly after the blowing cue t S . 
     In other words, either the delay separating the moment t C  of expansion end from the moment t D  of the beginning of blowing is measured and the blowing cue t S  is advanced by a value equal to this delay, or the delay separating the moment t C  of expansion end from the blowing cue t S  is measured and the blowing cue t S  is advanced by a value equal to this delay, increased by the response time of the blowing electrovalve  26 . 
     As with points A and B, a certain tolerance is accepted for the point C. In other words, the points C and D are assumed to be confounded when the delay separating the moment t C  of expansion end from the moment t D  of the beginning of blowing (or the delay separating the moment t C  of expansion end from the blowing cue t S  increased by the response time of the blowing electrovalve  26 ) is less than a predetermined value. 
     In  FIG. 10 , two curves are shown: the one on the right corresponds to the presence of a difference between the moment t C  of the expansion end and the moment t D  of the beginning of blowing. The left curve, in dotted lines, corresponds to an adjustment carried out through feedback on the base of the right curve: the blowing cue t S  has been advanced by the value of the difference separating on the right curve the moment t C  from the moment t D . 
     An example with numbers is provided in the table hereinafter for the purposes of illustrating the above. The measurements shown correspond to the pressure curve in  FIG. 4 . 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Value 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Parameter 
                   
                   
               
               
                   
                 Type of vessel 
                 Bottle 1.5 
                 l 
               
            
           
           
               
               
               
            
               
                   
                 Material 
                 PET 
               
            
           
           
               
               
               
               
            
               
                   
                 Heating temperature 
                 120-140° 
                 C. 
               
               
                   
                 Pre-blowing pressure 
                 10 
                 bars 
               
               
                   
                 Stretching speed 
                 2000 
                 mm/s 
               
               
                   
                 Pre-blowing cue 
                 0.03 
                 s 
               
               
                   
                 Response time of the pre-blowing 
                 0.01 
                 s 
               
               
                   
                 electrovalve 
                   
                   
               
               
                   
                 Response time of the blowing 
                 0.01 
                 s 
               
               
                   
                 electrovalve 
                   
                   
               
               
                   
                 Point C tolerance 
                 ±0.005 
                 s 
               
               
                   
                 Measurement 
                   
                   
               
               
                   
                 Moment of the beginning of pre-blowing 
                 0.04 
                 s 
               
               
                   
                 (point A) 
                   
                   
               
               
                   
                 Development moment (point B) 
                 0.07 
                 s 
               
               
                   
                 Moment of stretching end 
                 0.15 
                 s 
               
               
                   
                 Moment of expansion end (point C) 
                 0.18 
                 s 
               
               
                   
               
            
           
         
       
     
     In order to allow for the adjustment of the point D, the machine  1  comprises:
         means for detecting the moment t C  of the expansion end; and   means for adjusting the blowing cue t S  according to the moment t C  of the expansion end.       

     More precisely, the machine  1  can include:
         means for detecting the moment t D  of the beginning of blowing; and   means for comparing the moment t C  of the expansion end with the moment t D  of the beginning of blowing, the means for adjusting being provided in order to advance the blowing cue t S  by the value of the difference between them.       

     The machine  1  may also comprise means for memorising a plurality of points C and/or D (for example approximately ten) detected on a series of manufactured vessels  2 , in particular for the purposes of statistical study. 
     Alternatively (or as a complement), the machine  1  can include means for comparing the moment t C  of the expansion end with the pre-blowing cue t S , the means for adjusting being provided in order to advance the blowing cue t S  by the value of the difference between them, increased by the response time of the blowing electrovalve  26 . 
     These means can have the form of instructions of a computer programme product implemented in the control unit  31 .