# Patent

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

## Abstract

A method for producing a vessel (2), involving: placing a preform (3) into the mold; at the pre-blowing cue (tp), controlling the opening of an electrovalve (22) for establishing communication between the preform (3) and a source (20) of gas at a predetermined pre-blowing pressure (Pp); measuring the pressure (P) inside the preform (3); at the blowing cue (tS), controlling the opening of an electrovalve (26) for establishing a communication between the preform (3) and a source (24) of a gas at a blowing pressure (PS) higher than the pre-blowing pressure (Pp); memorising the variations of the pressure (P) inside the preform (3) at least during the interval between the pre-blowing cue (tp) and the blowing cue (tS); detecting the expansion end moment (tC) from which the pressure in the preform (3) increases in a linear manner; and advancing the blowing cue (tS) according to the expansion end moment (tC).

## Background
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. VE denotes the displacement speed of the stretch rod 16; this speed is also called stretching speed.
At a predetermined moment, called pre-blowing cue tP, 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 tA, from which the pressure P in the preform 3 begins to increase, registers a delay in relation to the pre-blowing cue tP, 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 tP, so that the real moment tA 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 tA 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 tB, when the plastic flow threshold of the preform 3 has been reached. At this moment tB 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 PB.
Starting from the development moment tB 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, 5F). 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” (2nd 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 tE, when the stretching ends, no significant point corresponds on the pressure curve, this moment tE being however known as it is predetermined by the value (which may be adjustable) of the stretching speed VE and the height of the vessel 2. This radial expansion phase ends at a moment, called moment of expansion end, noted as tC, 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 tC 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 PC.
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 DP and the pressure, noted as PP, 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 tD, 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 tD of the beginning of blowing registers a slight delay in relation to a so-called blowing cue and noted as tS, 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 tD 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 PP;
        pre-blowing rate DP;
        stretching speed VE;
        pre-blowing cue tP;
        blowing cue tS.
    
    


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 tA 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 tP) 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 tA 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 tA 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 tA of the real point A is located within the tolerances defined for the point A.
In the hypothesis where the real moment tA of the beginning of pre-blowing follows (taking the tolerance into account) the theoretical pre-blowing beginning moment, the pre-blowing cue tP is advanced.
In the hypothesis where on the contrary the real moment tA 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 tP is delayed.
The two curves plotted in solid lines in FIG. 6 show two different settings of the pre-blowing cue tP. The left curve shows an advancing of the pre-blowing cue tP; the right curve shows on the contrary a delaying of the pre-blowing cue tP. The theoretical curve is, between the latter, plotted in dotted lines.
More precisely, it is preferable to measure the difference separating the real moment tA of the beginning of pre-blowing of the theoretical pre-blowing beginning moment. And, if the real moment tA of the beginning of pre-blowing follows—respectively precedes—the theoretical pre-blowing beginning moment, the pre-blowing cue tP 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 tA of the beginning of pre-blowing;
        means for comparing this moment tA with the theoretical pre-blowing beginning moment; and
        means for adjusting the pre-blowing cue tP 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 tB (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 tB when a pressure peak occurs in the preform 3 after the pre-blowing cue tP (but before the blowing cue tS) 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 tB is compared with the theoretical moment of development on the one hand and, on the other hand, the real development pressure PB 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 tP,
        pre-blowing pressure PP,
        pre-blowing flow rate DP,
        stretching speed VE,
        heating temperature T of the preforms.
    
    


Several cases can occur, according to whether the real development moment tB precedes, is confounded (i.e. is within the tolerance zone) or follows the theoretical development moment, and according to whether the real development pressure PB 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 tP produces a shift in the point A, towards the right on the axis of time when the pre-blowing cue tP is delayed, or on the contrary towards the left when the pre-blowing cue tP 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 tP thus impacts the development moment tB, i.e. the abscissa of the point B. In the hypothesis where the real development moment tB is, either before or after the theoretical development moment, a simple measurement could consist in delaying or, respectively, advancing the pre-blowing cue tP. However, the real moment tA 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 tP in order to modify the development moment tB 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 PP or of the rate DP of pre-blowing on the unfolding of the pre-blowing. It has been observed that the influence of the pressure PP 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 PP×DP: the increase in this product can result from an increase in the pre-blowing pressure PP at a constant rate DP, from an increase in the pre-blowing rate DP at a constant pressure PP, or from a simultaneous increase in the both DP and PP; on the contrary, a decrease in the product PP×DP can result from a decrease in the pre-blowing pressure PP at a constant rate DP, a decrease in the pre-blowing rate DP at a constant pressure PP, or a simultaneous decrease in the pressure DP and in the rate DP, 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 PP×DP greater than that corresponding to the right curve: it is observed that an increase in the product PP×DP results in an increase in the slope of the pressure curve at the point A (i.e. at the real moment tA of the beginning of pre-blowing), an advancing of the development moment tB and an increase in the development pressure PB (on the curve, the point B is therefore displaced towards the top and towards the left). On the contrary, a decrease in the product PP×DP results in a decrease in the slope of the pressure curve at the point A, a delaying of the development moment tB and a decrease in the development pressure PB (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 VE on the unfolding of the pre-blowing. To the lower curve corresponds a stretching speed VE greater than that corresponding to the upper curve. It is observed that the increase in the stretching speed VE results in an advancing of the development moment tB and a decrease in the development pressure PB (on the curve, the point B is therefore displaced towards the bottom and towards the left), while the decrease in the stretching speed VE results in a delaying of the development moment tB and an increase in the development pressure PB (on the curve, the point B is therefore displaced towards the top and towards the right). No influence of the stretching speed VE 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 VE: advancing of the development moment tB and a decrease in the development pressure PB, while the decrease in the heating temperature T has an influence similar to the decrease of the stretching speed VE: delaying of the development moment tB and an increase in the development pressure PB.
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 PP or to the rate DP 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 tP is, in practice, modified only when modifications of the pressure PP, in the rate DP 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 VE, 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 VE).














Development
Development
Modification of the



moment
pressure
parameters








Before
Lower
Decrease T




In the tolerance
Decrease DP or PP




zone
Decrease T




Higher
Decrease DP or PP



In the
Lower
Decrease T



tolerance

Increase DP or PP



zone
In the tolerance





zone





Higher
Increase T





Decrease DP or PP



After
Lower
Increase DP or PP




In the tolerance
Increase DP or PP




zone
Increase T




Higher
Increase T





Where applicable:





Decrease DP or PP,





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 tB) has an incidence on the moment tC 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 tS) 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 tB;
        means for comparing the development moment tB and the development pressure PB with, respectively, the theoretical development moment and pressure;
        means for adjusting the pre-blowing rate DP 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 DP, 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 PP 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 tP according to the comparison between the real point B and the theoretical point B;
        means for adjusting the stretching speed VE according to the comparison between the real point B and the theoretical point B.
    
    


Alternatively, it can be desired to adjust the development moment tB independently of the development pressure PB, or reciprocally adjust the development pressure PB independently of the development moment tB.
In the first case, the pressure peak is detected and action is taken on at least one of the following parameters: pre-blowing pressure PP, pre-blowing flow rate DP, pre-blowing cue tP, stretching speed VE, heating temperature T in order to advance or, on the contrary, delay the development moment tB.
In the second case, the pressure peak is detected and action is taken on at least one of the following parameters: pre-blowing pressure PP, pre-blowing flow rate DP, pre-blowing cue tP, stretching speed VE, heating temperature T in order to increase or, on the contrary, decrease the development pressure PB.
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 PP 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 tC 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 tP 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 tC of expansion end from the blowing cue tS is measured, and the blowing cue tS 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 tD of the beginning of blowing (abscissa of the point D) is slightly after the blowing cue tS.
In other words, either the delay separating the moment tC of expansion end from the moment tD of the beginning of blowing is measured and the blowing cue tS is advanced by a value equal to this delay, or the delay separating the moment tC of expansion end from the blowing cue tS is measured and the blowing cue tS 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 tC of expansion end from the moment tD of the beginning of blowing (or the delay separating the moment tC of expansion end from the blowing cue tS 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 tC of the expansion end and the moment tD 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 tS has been advanced by the value of the difference separating on the right curve the moment tC from the moment tD.
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 tC of the expansion end; and
        means for adjusting the blowing cue tS according to the moment tC of the expansion end.
    
    


More precisely, the machine 1 can include:

    
    
        means for detecting the moment tD of the beginning of blowing; and
        means for comparing the moment tC of the expansion end with the moment tD of the beginning of blowing, the means for adjusting being provided in order to advance the blowing cue tS 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 tC of the expansion end with the pre-blowing cue tS, the means for adjusting being provided in order to advance the blowing cue tS 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.

## Claims

1. Method for producing a vessel (2) by blow-moulding in a mould (11) from a preform (3) made of plastic material, comprising the following steps:
introducing the preform (3) into the mould (11);
at a predetermined moment, called pre-blowing cue (tP), controlling the opening of an electrovalve (22) for establishing a communication between the inside of the preform (3) and a source (20) of gas at a predetermined pre-blowing pressure (PP);
measuring the pressure (P) inside the preform (3);
at a predetermined moment, called blowing cue (tS), following the pre-blowing cue (tP), controlling the opening of an electrovalve (26) for establishing a communication between the inside of the preform (3) with a source (24) of gas at a blowing pressure (PS) that is greater than the pre-blowing pressure (PP);
memorizing the variations in the pressure (P) inside the preform (3) at least in the interval between the pre-blowing cue (tP) and the blowing cue (tS);
detecting in this interval the expansion end moment (tC), from which the pressure (P) in the preform (3) increases in a linear manner;
advancing the blowing cue (tS) according to the moment of expansion end (tC).

2. Method set forth in claim 1, comprising the following steps:
measuring the delay separating the moment (tC) of expansion end from a real moment (tD) of the beginning of blowing, following the moment (tC) of expansion end, where the pressure (P) in the preform sags;
advancing the blowing cue (tS) by a value equal to this delay.

3. Method set forth in claim 1, comprising the following steps:
measuring the delay separating the moment (tC) of expansion end from the blowing cue (tS);
advancing the blowing cue (tS) by a value equal to this delay, increased by a response time, predetermined, of the blowing electrovalve (26).

4. Machine (1) for manufacturing vessels (2) from preforms (3) made of plastic material, comprising:
a mould (11) having a cavity (15) intended to receive a preform (3);
a source (20) of gas at a predetermined pre-blowing pressure (PP);
an electrovalve (22) able to establish a communication between the inside of the preform (3), received in the cavity (15), and the source (20) of pre-blowing gas;
means (31) for controlling the opening and the closing of the pre-blowing electrovalve (22);
source (24) of gas at a predetermined blowing pressure (PS):
an electrovalve (26) able to establish a communication between the inside of the preform (3), received in the cavity (15), and said source (24) of blowing gas;
means (31) for controlling the opening of the blowing electrovalve (26) at a predetermined moment (tS), called blowing cue:
a sensor able to measure the pressure (P) inside the preform (3);
means for detecting the moment of expansion end (tC), from which the pressure (P) in the preform (3) increases in a linear manner;
means for adjusting the blowing cue (tS) according to the moment (tC) of expansion end.

5. Machine (1) set forth in claim 4, comprising means for comparing the moment (tC) of expansion end with the moment (tD) of beginning of blowing, following the moment (tC) of expansion end, from which the pressure (P) in the preform sags.

6. Computer program product configured to be implemented on a machine (1) for manufacturing vessels (2) set forth in claim 4, comprising:
instructions on at least one non-transitory computer readable storage medium for:
controlling the opening and the closing of the blowing electrovalve (26);
taking into account the moment of expansion end (tC);
adjusting the opening moment of the blowing electrovalve (26) according to the moment (tC) of expansion end.

7. Computer program product configured to be implemented on a machine (1) for manufacturing vessels (2) set forth in claim 5, comprising:
instructions on at least one non-transitory computer readable storage medium for:
controlling the opening and the closing of the blowing electrovalve (26);
taking into account the moment of expansion end (tC);
adjusting the opening moment of the blowing electrovalve (26) according to the moment (tC) of expansion end.