Control method for controlling a plastic container forming method

Abstract

The invention relates to a method of controlling a method of forming a thermoplastic preform (12), the method of forming comprising: an operation of heating the preform (12); an operation of introducing the preform (12) into the mould (36); a stretching operation in which the stretching rod (46) is moved, the stretching rod (46) passing through a position (PO) at which the stretching rod (46) strikes the bottom (18) of the preform (12); the control method is characterized in that it comprises: a step (E1) of measuring the length (L) between the neck (16) and the bottom (18) of the preform (12), which is carried out after the heating operation and before the operation of introducing the mould (36); and a step (E2) of adjusting the striking position (PO) of the tension rod (46) on the basis of the measurement of the length (L) in the measurement step (E1).

Description

Technical Field

[0001] The present invention relates to the manufacture of containers such as bottles or jars obtained by stretch blow moulding of preforms made of thermoplastic material, for example polyethylene terephthalate (PET), recycled polyethylene terephthalate (rPET).

[0002] The invention relates more particularly to a method for producing containers made of plastic material, such as PET, by stretch blow moulding a preform in a mould with a pressurised forming fluid, in particular air. Background Art

[0003] Such a manufacturing plant generally comprises a control system, a heating station and a blow molding station. The blow molding station comprises at least one blow molding station, which comprises a mold into which the preforms are introduced from the heating station, and each preform is subjected to a forming operation for forming a container. The forming operation comprises a stretching step (also called a lengthening step) of stretching the preforms using a stretching rod associated with the mold and controlled to slide toward the bottom of the mold.

[0004] Stretching and blowing the body of the preform requires heating this body to a temperature above the glass transition temperature of the material. Therefore, the preform is first thermally conditioned by passing it through a heating station. The heating station comprises heating means formed, for example, by infrared lamps. The preform is moved in the heating station by a conveyor.

[0005] The heated preform is then introduced into the mold and stretched using a sliding rod (called a stretching rod or elongation rod). Pressurized gas is also introduced into the preform in order to transform it into a container by blow molding.

[0006] In all cases, the introduction of the pressurized gas includes a strict blow molding step, which consists in introducing a high pressure (usually between 18 and 40 bars) forming fluid (usually air) into the preform. The blow molding step is usually preceded by a first step called the pre-blowing step, which consists in introducing a gas at a lower pressure (between 8 and 15 bars), while the stretching rod reaching the bottom of the preform causes the longitudinal stretching of the preform. The stretching, pre-blowing and blowing steps (alternatively, the stretching and blowing steps) are carried out according to a sequence pre-established during the parameterization of the manufacturing equipment, which takes into account the shape of the preform used and the container to be obtained. Stretching and blowing (or pre-blowing and blowing) allow the constituent materials of the preform to undergo molecular bi-orientation, thereby giving the final container specific mechanical properties. Pre-blowing begins to deform the preform in order to transform it into a container, while blowing allows optimal shaping in the mold in order to clearly mark the container details.

[0007] In the following of this specification, unless otherwise specified, the expression "blow molding process" will be used indiscriminately to denote a sequence comprising a pre-blowing step and then a blowing step, or to denote a process comprising only a blowing step. Thus, "starting the blow molding process" will mean starting the pre-blowing step (if such a step exists) by injecting a pre-blowing gas, or directly starting the blowing step by injecting a blowing gas.

[0008] After the plastic material has been in contact with the mould for a certain time, during a degassing step, the pressure in the container is returned to atmospheric pressure before the final container is removed from the mould.

[0009] The preform is usually obtained by injecting the material into a special injection mold. The preform has a tubular cylindrical body, which is closed at one axial end and open at the other end by a neck, also tubular. The neck of the preform is usually injection molded to already have the shape of the neck of the final container, while the body of the preform needs to undergo a relatively large deformation after the blow molding operation to form the body of the final container. The neck of the preform usually has a supporting hoop, which is intended to keep the preform on the upper edge of the mold when forming the container.

[0010] The container comprises a side wall (also referred to as a body), a neck extending from an upper end of the body, and a bottom extending from a lower end of the body opposite to the neck. The bottom of the container generally defines a base at a junction with the body, and the container can be placed on a flat surface (such as a table) by the base.

[0011] The mould comprises walls defining a mould cavity intended to give its shape to the body of the container. The mould cavity is closed at the lower end by a mould bottom intended to give its shape to the bottom of the container.

[0012] Nowadays, in order to parameterize the equipment to produce a given type of container, when the equipment stops, the preform of the injection molding output corresponding to the preform for this type of container (so it is cold and not heated) is placed in the equipment mold, and then the stretch rod position when the stretch rod reaches the bottom of the preform is determined. The position where the end of the stretch rod reaches the bottom of the preform is usually called the stretching zero point ("point 0") by the applicant. From point 0, the continued movement of the stretch rod leads to stretching the preform. When parameterizing, the stretch rod position at which the blow molding process must start is also established, which can correspond to point 0 or be located outside this point. In other words, the theoretical stretching length between point 0 and the start of the blow molding process is determined. In fact, once the preform begins to be stretched, if nothing else is done, the constituent material of the preform will tighten on the stretch rod. The first consequence is the risk of overcooling the preform area in contact with the stretch rod (which is usually cold), resulting in poor container quality, because then these areas become impossible to deform well due to cooling. Another consequence is the risk that the base of the preform will be damaged by the stretching rod, in particular pierced by it.

[0013] Other operations are further parameterized between the moment when the stretch rod reaches the bottom of the preform and the moment when the bottom of the preform driven by the stretch rod reaches the mold bottom (called point 10). Other operations are further performed after reaching point 10.

[0014] This mode of operation has various disadvantages. In fact, when the equipment is at rest, the determination of point 0 is carried out on the cold preform coming out of the injection molding machine.

[0015] However, it has been found that after heating in the heating station, the length of the body of some preforms may vary, and the difference in length from one preform to another may reach 5 to 10 mm. In particular, it has been found that for small-thickness preforms used to produce small-format bottles (such as 0.5 liter bottles) and small-walled bottles, the length of such preforms when leaving the heating station may be more than 10 mm shorter than their length when entering the heating station, in other words, than when leaving the injection molding machine. In the case of preforms intended to be transformed into small-format bottles (usually 0.5 liter lightweight water bottles), this reduction in length, also called shrinkage or "shrinkage" by those skilled in the art, corresponds to a longitudinal variation of about 15%. This reduction is also accompanied by an increase in the diameter of the preform. For the same type of bottle, an increase in diameter of about 7% has been noted. On the other hand, for the same preform production, there may also be a difference of several millimeters between the maximum shrinkage and the minimum shrinkage.

[0016] The presence of shrinkage when the preform is heated is caused by stresses in the preform when it is made. These stresses are caused by the pressure or temperature to which the material is subjected when it is injected. During heating, some of the stresses are released, resulting in a decrease in length and an increase in diameter of the preform. The release of stresses is sometimes called "relaxation".

[0017] Furthermore, the shrinkage of the preforms also depends on the temperature to which they are heated in the heating station. Thus, the temperature difference between a preform entering the heating station just after it is started and a preform entering the heating station after one hour of operation can be significant enough that the shrinkage is significantly higher on the hotter preform.

[0018] Furthermore, since some preforms become shorter than others before they are introduced into the mould, the actual stretching will start before reaching the point 0 determined during parameterisation. However, the stretched length of the preform before the start of pre-blowing has a great influence on the distribution of the packaging material (for some containers, the stretching must be zero). Furthermore, if stretching starts too early with respect to pre-blowing, or if stretching occurs when it should not, and if the shrinkage is different for different preforms, a different material distribution will occur on the container, and the risk of material tightening on the stretch rod will also be higher for containers manufactured with high stretching rates.

[0019] Current equipment is increasingly automated, which makes it possible to correct various time drifts of the blow-molding process. It is known (see document WO2008 / 081107 in the name of the applicant) to associate the singular points of the actual blow-molding curve with the parameters of the equipment (in particular the flow or pressure of the pre-blowing) and to apply parameter corrections according to the differences observed at these singular points. However, automation does not allow correction of some of the defects mentioned above. In fact, one or more drifts caused by changes in the physical properties of the preform cannot be corrected by the regulation method described in the above-mentioned document WO2008 / 081107.

[0020] A first object is to overcome these drawbacks by proposing a method which makes it possible to improve the shaping of the containers produced while limiting the rejection rate and maintaining or even increasing the productivity.

[0021] Another object is a method which can be implemented, if necessary, in an automated production plant as described in the above-mentioned document WO 2008 / 081107.

[0022] In the following of this description, "forming" is understood more particularly to mean the distribution of the material. Summary of the invention

[0023] The invention proposes a control method for controlling a forming method for forming containers by stretch-blow molding preforms of thermoplastic material, in particular PET, each preform comprising a body open at one end by a neck and closed at the other opposite end by a bottom, the forming method comprising, for each preform moving along a production path:

[0024] - a heating operation in which the preform is heated to a predetermined temperature in a heating zone;

[0025] - an introduction operation of introducing the heated preform into the mold in an introduction zone of the production path;

[0026] - A stretching operation in which a stretching rod is moved to stretch the preform, the stretching rod passing through a touch position at the moment of contact with the bottom of the preform during the movement.

[0027] The control method implemented according to the teaching of the present invention is characterized in that it comprises:

[0028] at least one measuring step for measuring the length between a reference point and the bottom of the neck of the preform, the measuring step being carried out after a heating operation of heating the preform and before an introduction operation of introducing the preform into the mold;

[0029] An adjustment step for adjusting the contact position of the stretching rod based on at least one length measurement performed during at least one measuring step.

[0030] According to a further aspect of the control method implemented according to the teaching of the invention, the reference point of the neck is formed by a lower support surface of a collar of the neck or by a lip of the neck.

[0031] According to another aspect of the control method implemented according to the teachings of the invention, the forming method includes a blow molding process of injecting a pressurized forming fluid into the preform, the start of the blow molding process being determined by the contact position of the stretch rod adjusted during the adjustment step of the control method.

[0032] According to another aspect of the control method implemented according to the teaching of the invention, the measuring step for measuring the length is performed in a measuring zone through which the production path passes, the measuring zone being located between the heating zone and the introduction zone for introducing the preform into the mold.

[0033] According to another aspect of the control method implemented according to the teachings of the invention, the heating operation is carried out in a heating station, the heating station comprising a conveyor, which transports the preforms through the heating zone, and the measuring zone is arranged on the section of the production path along which the preforms are transported by the conveyor.

[0034] According to another aspect of the control method implemented according to the teachings of the present invention, the measuring step is performed using a non-contact measuring device that can measure the length of the preform in real time as the preform continuously moves along the production path.

[0035] According to another aspect of the control method implemented according to the teachings of the invention, the measuring device comprises an image capture device for capturing an image of the preform passing through the measuring zone, and the measuring step comprises a first substep for capturing an image of the preform.

[0036] According to another aspect of the control method implemented according to the teaching of the invention, in a first sub-step for capturing an image, the preform is illuminated by a light source which is located on the opposite side of the production path 27 to the position of the image capturing device.

[0037] According to another aspect of the control method implemented according to the teachings of the present invention, the measuring step includes a second sub-step, which is used to process the image captured in the first sub-step for capturing the image using an electronic processing unit, including: detecting at least one measuring point of the image of the bottom of the preform; then measuring the distance between a reference point on the image and the measuring point along the direction of the axis of the preform on the image, the position of the reference point relative to the neck of the preform being known; and then converting the distance into an actual distance.

[0038] According to a further aspect of the control method implemented in accordance with the teaching of the present invention, the reference point is formed by an optical axis of the image capture device.

[0039] According to another aspect of the control method implemented according to the teaching of the present invention, the conversion of the distance measured on the image into the actual distance is performed after the calibration substep.

[0040] According to another aspect of the control method implemented according to the teachings of the invention, a calibration substep is performed at each iteration of the preform length determination sequence, the calibration substep consisting in measuring a known dimension of the preform, such as the outer diameter, to determine the ratio between the distance on the image and the actual distance.

[0041] According to another aspect of the control method implemented according to the teaching of the present invention, the adjustment step for adjusting the contact position is performed based on the length measured at the end of each iteration of the measuring step.

[0042] According to another aspect of the control method implemented according to the teaching of the invention, the adjustment step for adjusting the contact position is performed based on a sliding average of the lengths measured at the end of a plurality of iterations of the measuring step performed successively on a plurality of preforms.

[0043] According to a further aspect of the control method implemented according to the teaching of the invention, the measuring step is performed with a certain frequency, for example for each preform passing through the measuring zone or for one preform out of a certain number of preforms. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Other features and advantages of the present invention will become apparent upon reading the following detailed description, for which reference should be made to the accompanying drawings which are briefly described below.

[0045] Figure 1 is a top view schematically illustrating a container manufacturing apparatus implemented according to the teachings of the present invention.

[0046] Figure 2 is intended to provide Figure 1 Side view of the preform of the device.

[0047] Figure 3 It is schematically shown Figure 1Axial section through the blow moulding station of the machine.

[0048] Figure 4 is a schematic side view showing a preform height measuring device implemented according to the teachings of the present invention, the measuring device being equipped with Figure 1 equipment.

[0049] Figure 5 is a front view showing the use of Figure 4 An image obtained by an image capture device which is part of the measuring device shown.

[0050] Figure 6 is a block diagram schematically illustrating a control method implemented according to the teachings of the present invention, the control method is intended to be Figure 1 Equipment implementation. DETAILED DESCRIPTION

[0051] In the following of this description, for the sake of clarity, the terms "higher", "lower" and the derivative terms "upper", "lower" are used with reference to the directions of the drawings, but this does not have any limiting effect.

[0052] Figure 1 Schematically shown in FIG. 1 is a production device 10 for producing containers 11 from preforms 12 made of thermoplastic material, in particular PET (polyethylene terephthalate) or rPET (recycled polyethylene terephthalate).

[0053] like Figure 2 As shown, each preform 12 comprises a body 14 of cylindrical shape with an axis "A". The upper end of the body 14 is open by a neck 16, which has the final shape of the neck of the container 11 to be obtained (the neck will generally not undergo any deformation during the manufacture of the container). The body 14 comprises a bottom 18 closing its lower end, the shape of the bottom being generally hemispherical. The neck 16 comprises a hoop 20 arranged at its junction with the body 14. The lower surface of the hoop 20 is intended to form a bearing surface 22 to allow the preform 12 to be supported during its molding, as will be described below.

[0054] At the end of the injection molding, the preform 12 is suddenly cooled to give the thermoplastic material an amorphous state. The thermoplastic material can then be made ductile again by heating above the glass transition temperature.

[0055] The bottom 18 of the preform 12 has, at its intersection with the axis “A” of the body 14 , an injection point 23 , which corresponds to the injection point of the thermoplastic material when it is injection-molded.

[0056] Reference again Figure 1The manufacturing apparatus 10 includes a heating station 24 and a forming station 26. The preforms 12 move in a row along a production path 27 that passes through the heating station 24 and the forming station 26. The moving direction of the preforms 12 is Figure 1 Indicated by the arrow “ F1 ”. When the manufacturing device 10 is operating normally, the preforms 12 are constantly moved along the production path 27 .

[0057] The function of the heating station 24 is to heat the body 14 of the preform 12 to a temperature above or equal to the glass transition temperature of the constituent material, for example above 70° C. when the material is PET. The heating station 24 comprises a conveyor 28 (shown schematically) for conveying the preform 12 while rotating the preform about itself.

[0058] The conveyor 28 generally comprises a mandrel 30 which engages with the neck 16 to convey the preform 12. The mandrel 30 moves along a closed loop. The mandrel 30 is carried, for example, by links of a chain or by a separate shuttle moving along a track.

[0059] Here, the loop comprises two parallel straight sections connected by a 180° turning section. The conveyor 28 also comprises two guide wheels 31A, 31B for guiding the mandrel 30 in the turning section of the closed loop.

[0060] The heating station 24 also comprises heating means 32 for heating the preforms 12. For example, these are lamps facing reflectors or laser sources emitting electromagnetic radiation in the infrared range. As a variant of the invention not shown, the heating means emit electromagnetic radiation in the microwave range.

[0061] The heating device 32 is arranged in a heating zone 33 along the production path 27 of the preforms. Figure 1 In the example shown, the heating station 24 comprises a heating zone 33 which is divided into two parts which are arranged upstream and downstream of the turning section guided by the guide wheel 31B.

[0062] The preforms 12 enter the heating station 24 and are mounted on a conveyor 28, where they form a U-shaped section of the production path 27 that passes through a heating zone 33. They are heated as they travel in a line by heating devices 32, which are arranged, if necessary, on one or both sides of the preforms relative to the travel direction of the preforms 12. After passing through the heating zone 33, the hot preforms 12 are removed from the heating station 24 and conveyed to the molds of the forming station 26 by means of a first conveying device 34, such as a conveying wheel, interposed between the heating station 24 and the forming station 26.

[0063] According to a non-limiting embodiment, the preforms 12 are heated with the necks 16 facing downward and are flipped between when they leave the heating zone 33 and before leaving the conveyor 28 so that the necks 16 face upward.

[0064] The transfer wheel comprises arms (not shown, as they are known per se) which successively grasp the preforms 12 at their neck 16 as they leave the heating station 24, so as to introduce each preform in turn into a mold 36 of the forming station 26. The forming station 26 comprises a rotating turntable 38, at the periphery of which a plurality of blowing stations 40 are arranged.

[0065] Each blowing station 40 comprises at least one mould 36 which is generally composed of three parts, namely two mould halves 36A, 36B and a mould base 36C, which define a mould cavity 42 having the shape of the container 11 .

[0066] Each hot preform 12 leaving the heating station 24 is introduced into a mold 36 of the blow molding station 40 so as to be blown therein and transformed into a container 11. Once completed, the container 11 is removed from the blow molding station 40 by a second conveyor 44, which is similar to the first conveyor 34 and is well known to those skilled in the art. The mold cavity 42 of the mold 36 includes an upper axial channel to allow the neck 16 of the preform 12 to extend from the mold cavity 42. Thus, the body 14 is accommodated in the mold cavity 42, while the neck 16 remains above the mold cavity 42, and the preform 12 is held in place by the contact between the support surface 22 of its hoop 20 and the circumference of the channel. Therefore, the position of the support surface 22 of the hoop 20 constitutes a constant fixing point for any preform 12 introduced into the relevant mold 36.

[0067] exist Figure 3 4 shows a blow-moulding station 40 in detail. The blow-moulding station comprises a mould 36, for example made of steel or aluminium alloy, consisting of two mould halves 36A, 36B and a mould base 36C, which define a mould cavity 42, the profile of which has the shape of the final container 11 to be produced. The mould 36 is intended to successively receive the hot preforms 12 coming from the heating station 24.

[0068] The blow molding station 40 also comprises a stretching rod 46 mounted so as to be movable along the main axis X of the mold 36 between a high position allowing the introduction of the preform 12 into the mold 36 when the mold is open, and a low position in which the end of the stretching rod 46 has stretched the preform 12 until its bottom 18 is in contact with the mold bottom 36C, indicated by the reference numeral "P10". The stretching rod 46 is displaced from the high position to the low position so as to axially stretch the material of the preform 12 along the axis X of the mold, which axis X is here coaxial with the axis "A" of the preform 12. As previously mentioned, between its high position and its low position P10, the stretching rod 46 passes through a contact point P0 at the moment when it comes into contact with the inner surface of the bottom 18 of the preform 12.

[0069] The blow-moulding station 40 also comprises a blow nozzle 48 mounted so as to be movable between a high position when the preform 12 is introduced and a low position in which the lower end of the blow nozzle 48 will cover the preform 12 in a sealed manner in order to convey a shaping fluid, in particular air, into the preform 12 through its neck 16 in order to press the plastic material against the wall of the mould 36. The stretching rod 46 slides in the blow nozzle 48.

[0070] The blow-moulding station 40 also comprises a pressure sensor 50 which opens into the blow nozzle 48 in order to measure the pressure therein during the forming operation of the container 11 in the closed volume constituted by the blow nozzle 48 and the preform 12 .

[0071] The blow molding station 40 comprises a circuit 52 of a pre-blow molding forming fluid at a pre-blow molding pressure between 3 and 15 bar. This circuit 52 comprises: a low-pressure source 54 and a pipe 56 allowing the forming fluid to be conveyed from the low-pressure source 54 to the blow nozzle 48 to form the container 11; and a solenoid valve EV1, called a pre-blow molding solenoid valve, allowing the low-pressure source 54 to communicate with the interior of the preform 12 through the blow nozzle 48.

[0072] The solenoid valve EV1 is arranged on the conduit 56 between the low-pressure source 54 and the blowing nozzle 48. The circuit 52 of pre-blowing fluid also comprises a non-return valve 58 which prevents fluid coming from another source or contained in the preform 12 (or container) from being introduced therein.

[0073] The blow molding station 40 also includes a circuit 60 for a blowing fluid at a blowing pressure between 15 bar and 40 bar, which circuit 60 includes: a high-pressure source 62 and a pipe 64 allowing the fluid to be transported from the high-pressure source 62 to the blow nozzle 48 to form the container 11; and an electromagnetic valve EV2, called a blow molding solenoid valve, which controls the connection between the high-pressure source 62 and the preform 12.

[0074] The solenoid valve EV2 is arranged on the conduit 64 between the high pressure source 62 and the blowing nozzle 48. The circuit 60 of the blowing fluid also comprises a non-return valve 66 which prevents fluid coming from another source or contained in the preform 12 (or container) from being introduced therein.

[0075] The blow molding station 40 also includes a circuit 68 for recovering the forming fluid, which circuit 68 includes: a pipeline 70, which allows the forming fluid contained in the container to be transported to a recovery device 72, such as a recovery circuit or a recovery storage device, after the container is made; and a solenoid valve EV3, called a recovery solenoid valve, which controls the connection between the container and the recovery device.

[0076] The solenoid valve EV3 is disposed on the pipe 70 of the recovery loop 68 .

[0077] The blow molding station 40 includes a degassing circuit 74, which allows the interior of the container to communicate with the outside so as to restore to atmospheric pressure before the blow nozzle 48 rises to a high position. The degassing circuit 74 includes: a loop to the atmosphere, one end of which is connected to a muffler 76 to avoid any noise interference, and the other end is connected to a pipe 78 connected to the blow nozzle 48; and a solenoid valve EV4, called a degassing solenoid valve, which controls the connection of the forming fluid contained in the container 11 with the atmosphere.

[0078] The blow molding station 40 comprises an electronic control unit 80, which is in particular in the form of an industrial programmable logic controller (API). The electronic control unit 80 is electrically connected to the pre-blow molding control solenoid valve EV1 via an actuator R1. Thus, the start of the pre-blow molding phase by injecting a forming fluid under a pre-blow molding pressure is controlled by the electronic control unit 80.

[0079] The electronic control unit 80 is also electrically connected to the pressure sensor 50 and to the solenoid valves EV2, EV3, EV4 via respective actuators R2, R3, R4, the function of which is to ensure that the solenoid valves EV2, EV3, EV4 are opened and / or closed.

[0080] exist Figure 3 In the example shown, the electronic control unit 80 includes, in a non-limiting manner: a processor 82; an analog input module 84, connected to the pressure sensor 50 to collect its measurements and convert the measurements into digital signals for processing by the processor 82; a memory 86, connected to the processor to store data from the pressure sensor 50 (after conversion); an analog output module 88, which is controlled by the processor 82 and controls the solenoid valves EV1, EV2, EV3, EV4 through actuators R1, R2, R3, R4 to adjust the opening and closing to change the flow rate of the fluid supplied to the mouthpiece 48, and it also controls the axial movement control device of the stretch rod 46; a communication interface 90, which is used to execute a program in the electronic control unit 80 to control its operation.

[0081] In one embodiment, the electronic control unit 80 is also electrically connected to a driving device 92 for driving the stretching rod 46 to control its movement. The driving device 92 is constituted by a motor. The analog output module 88 also receives the position data of the stretching rod 46 from the motor.

[0082] The operation of the manufacturing equipment 10 is similar to that of the known equipment, but is different from the known equipment in that: the manufacturing equipment 10 implements a control method, according to which the theoretical touch position P0tha of the stretching rod 46 is continuously adjusted during its operation, and the adjusted theoretical touch position P0tha is closer to the actual touch position P0r than the initial theoretical touch position P0thi, and even coincides with the actual touch position P0r.

[0083] Thus, the method for manufacturing the container 11 comprises a heating operation of heating the body 14 of the preform 12 to a predetermined temperature, in particular above the glass transition temperature, by passing the preform 12 through a heating zone 33 .

[0084] At the end of the heating operation, the method comprises an introduction operation of introducing the heated preform 12 into the mold 36, which takes place as follows Figure 1 In the relevant area 93 shown.

[0085] After this introduction operation, the method comprises a stretching operation consisting in stretching the preform 12 introduced into the mold 36 by moving the stretching rod 46 , which passes through the actual contact position P0r at the moment of contacting the bottom 18 of the preform 12 when moving from its high position to its low position P10 .

[0086] The forming method also comprises a pre-blowing operation consisting in injecting a forming fluid under pressure, in particular under a pre-blowing pressure, into the preform 12 .

[0087] The blow molding process start command is issued based on the adjusted contact position P0tha, rather than based on the initial theoretical contact position P0thi determined based on the cold preform 12, as described earlier in the description.

[0088] The preform 12 is shown in dashed lines. Figure 3 In FIG. 1 , the actual contact position (P0r) of the stretch rod 46 before stretching has begun is indicated. Assuming that the preform 12 has undergone shrinkage while being heated by the heating device 32 in the heating station 24, the initial theoretical contact point (P0thi) used in the previous method will be slightly further in the travel of the stretch rod 46, such as Figure 3 shown.

[0089] The adjustment of the theoretical contact position (P0th) of the stretch rod 46 can be performed by measuring the length of the at least one preform 12 after the at least one preform is heated and before it is introduced into the mold 36. More specifically, the length "L" of the preform 12 between the reference point of the neck 16 and the bottom 18 is measured.

[0090] During heating, only the body 14 of the preform 12 expands significantly. The neck 16 of the preform 12 remains cool enough so that its dimensions do not change. In particular, the height "h" of the neck 16 section taken axially between the bearing surface 22 of the hoop 20 and the lip remains substantially constant during heating. Therefore, once the length "L" of the preform 12 between a reference point of the neck 16 and the bottom 18 is obtained, the axial distance between any point of the neck 16 and the bottom 18 is easily obtained.

[0091] The reference point of the neck 16 is formed, for example, by the bearing surface 22 which forms a fixed reference point relative to the mold 36 .

[0092] As a variant, the reference point of the neck 16 is formed by the upper edge of the neck 16 , also called the lip.

[0093] Thus, the control method for controlling the forming operation comprises at least one measuring step "E1" for measuring the length "L" of the preform 12. Figure 2 As shown, the length “L” is taken parallel to the axis “A” of the preform 12 between the bearing surface 22 of the hoop 20 and the bottom 18 of the body of the preform 12 .

[0094] Advantageously, but not necessarily, for a more accurate measurement, the length "L" extends axially from the support surface 22 to a point on the outer surface of the bottom 18 that is immediately adjacent to the injection point 23. In reality, the size of the injection point 23 is random from one preform 12 to another, which can cause a slight deviation in the length measured between the injection point 23 and the support surface 22.

[0095] The measuring step “ E1 ” is carried out after the heating operation of heating the preform 12 and before the introduction operation of the preform 12 into the mold 36 .

[0096] The step "E1" of measuring the length "L" is more specifically carried out in a measuring zone "Z" through which the production path 27 of the preform 12 passes. Figure 1 As shown, the measuring zone “Z” is located between the exit of the heating zone 33 for heating the preform 12 and the beginning of the introduction operation of the preform 12 into the mold 36 .

[0097] according to Figure 1 In the example shown, the measuring zone "Z" is arranged on the section of the production path 27 along which the preforms are conveyed by the conveyor 28 of the heating station 24. Thus, the measuring zone "Z" is arranged downstream of the heating zone 33 and upstream of the first conveying device 34.

[0098] In the case where the preforms 12 are heated with the neck 16 facing downward and then turned over with the neck 16 facing upward before being conveyed to the first conveyor device 34, the measurement zone "Z" is advantageously arranged downstream of the point at which they are turned over with the neck facing upward so that the preforms 12 pass through the heating zone 33 with the neck 16 facing downward.

[0099] The measurement step “ E1 ” is performed using a contactless measuring device 94 capable of measuring the length “L” of the preform 12 in real time, that is to say during its continuous movement along the production path 27 .

[0100] The measuring device 94 comprises an image capturing device 96 for capturing an image 98 of the preform 12 passing through the measuring zone "Z". The image capturing device 96 is more specifically arranged to capture an image 98 of at least one lower end section of the body 14 (including the bottom 18), such as Figure 5 shown.

[0101] The image capture device 96 is arranged fixedly relative to the ground on which the manufacturing apparatus 10 rests.

[0102] The image capture device 96 has an optical axis “O” arranged orthogonally to the axis “A” of the preform 12. The optical axis “O” is arranged at a known axial distance “D” from the bearing surface 22 of the hoop 20 of the preform 12. This is particularly possible because the mandrel 30 carrying the preform 12 comprises a stop surface 100 against which the lip of the neck 16 of the preform abuts when the mandrel 30 engages with the neck 16. Therefore, the distance “D” also corresponds to the distance between the stop surface 100 of the mandrel 30 and the optical axis “O”. The stop surfaces 100 of all the mandrels 30 of the conveyor 28 are of course arranged at the same height, so that the distance “D” remains the same for each mandrel 30.

[0103] The optical axis “O” is arranged here essentially orthogonally to the direction of movement “ F1 ” of the preform 12 in the measuring zone “Z”.

[0104] To improve the quality of the image 98 of the preform 12 and thus allow for a more accurate processing of the image 98, the measuring device 94 further comprises a light source 102 which is arranged opposite the image capture device 96 with respect to the production path 27 in the measuring zone "Z". The light source 102 is activated during the image capture by the image capture device 96 so that a light beam emitted by the light source 102 passes through the preform 12 and the light beam subsequently reaches the image capture device 96. This allows a higher contrast, clearer image 98 to be obtained while also reducing the time required for the capture, for example to less than 1 / 100 of a second.

[0105] The measuring device 94 also comprises an electronic processing unit 104 comprising the structural components and software required for processing the images 98 obtained by the image capture device 96 . The electronic processing unit 104 is electrically connected to the electronic control unit 80 of the forming station 40 .

[0106] The measuring step “ E1 ” comprises a first sub-step “ E1 - 1 ” for capturing an image 98 of the preform 12 passing through the measuring zone “Z”.

[0107] Then, the measurement step “E1” comprises a second sub-step “E1-2” for processing the image 98 captured during the first capture sub-step “E1-1”. This second processing sub-step “E1-2” is performed using an electronic processing unit 104 that receives the image 98 sent by the image capture device 96.

[0108] The second processing sub-step "E1-2" includes: detecting at least one measuring point "M" of the image 98 of the bottom 18 of the preform 12; then measuring the distance "d" between a reference point on the image and the measuring point "M" along the axis "A" of the preform 12 on the image, wherein the position of the reference point relative to the neck 16 of the preform 12 is known; and then multiplying the distance "d" by a determined ratio to obtain an estimate of the actual distance "D".

[0109] The reference point is here formed by the optical axis "O" of the image capture device 96, which is here represented by a cross on the image 98. The position of the optical axis "O" is practically always located at the same position on the captured image 98. This information is data recorded in the electronic processing unit 104.

[0110] The position of the measuring point "M" is determined by moving upward along an axial measuring line 106, which is arranged by default at a certain distance from the axis "A" of the preform 12, for example 4 mm from the axis "A" of the preform 12. The line 106 is deliberately extended below the initial theoretical position of the bottom 18 so that at the bottom of this line 106, the pixels appear white under the action of the light source 102. When the pixels of the image become much darker along this line 106, the measuring point "M" is detected.

[0111] The location of the axis "A" of the preform 12 is determined, for example, by analyzing the curved shape of the bottom 18, or by measuring the outer diameter of the body 14 and then taking the center of that diameter.

[0112] As a variant, if the capture of the image 98 is always carried out precisely at the same position of the measurement zone “Z”, the position of the axis “A” of the preform 12 can also constitute the data recorded in the electronic processing unit 104 .

[0113] The conversion of the distance measured on the image into the actual distance is performed after the calibration sub-step "E1-3".

[0114] The calibration sub-step “E1-3” is here performed at each iteration of the first measurement step “E1”. The calibration sub-step “E1-3” consists, for example, in measuring known dimensions of the preform 12 to determine the ratio “r” between the distance on the image 98 and the actual distance.

[0115] For example, the dimension is the outer diameter "Δ" of the body 14. In fact, the heating of the preform 12 has no significant effect on the outer diameter "Δ" of the body 14 of the preform 12. Therefore, the outer diameter "Δ" measured on the cold preform 12 is substantially equal to the outer diameter "Δ" of the body 14 on the heated preform 12. For example, the outer diameter "Δ" is measured by detecting the contrast changes of the pixels of the image 98 along the transverse line 108 through the body 14. The actual measured outer diameter "Δ" on the cold preform on the image is then divided by the outer diameter "Δ" measured on the image to form a ratio "r".

[0116] As a variant of the invention not shown, the calibration sub-step "E1-3" is performed only once before the production device 10 is put into operation. The ratio "r" is therefore a data recorded in the electronic processing unit 104. This is particularly achievable when the capture of the image 98 is always carried out precisely at the same position of the measurement zone "Z", so that the distance between the image capture device 96 and the preform 12 remains constant at each measurement. Therefore, for each captured image 98, the ratio "r" always remains the same.

[0117] Therefore, the length "L" of the preform 12 is calculated according to the following formula:

[0118] L=D-(dxr)-h

[0119] Wherein, L is the measured length of the preform 12, D is the actual distance between the support surface 22 and the optical axis "O", d is the distance between the measuring point M measured on the image 98 and the optical axis "O", r is the multiplication ratio used to obtain the actual distance based on the distance measured on the image 98, and "h" is the axial height of the neck 18 between the lip and the support surface 22.

[0120] At the end of one or more first measurement steps "E1", an adjustment step "E2" is initiated for adjusting the touch position "P0th" of the stretching rod 46. This second step "E2" consists in replacing the previous value of the theoretical touch position "P0th" based on at least one length measurement carried out during at least one measurement step "E1".

[0121] This therefore allows the start of the blow-moulding process to be optimally adjusted by taking into account the shrinkage of the preforms 12 after heating. This involves a continuous adjustment which also allows taking into account the fact that the heating station 24 may heat the preforms 12 more or less during its continuous operation.

[0122] To this end, the electronic processing unit 104 sends the measured value of the length “L” of the preform 12 to the electronic control unit 80 , which allows it to calculate a new adjusted theoretical point “P0tha” by taking into account, for example, the thickness of the bottom 18 of the preform 12 .

[0123] The adjustment step "E2" for adjusting the contact position is performed, for example, based on the length "L" measured at the end of each iteration of the measurement step "E1". However, this may result in too large a variation of the contact position, since extreme values ​​for the measured length "L" are taken into account.

[0124] Preferably, the adjustment step "E2" for adjusting the touchdown position is therefore carried out on the basis of the average value of the lengths "L" measured at the end of a plurality of iterations (for example ten iterations) of the measurement step "E1" carried out successively on a plurality of preforms 12. This thus allows smoothing the adjustment signal of the touchdown position, avoiding excessive variations that could produce counterproductive effects.

[0125] This involves, for example, a sliding average value which allows the theoretical contact position “P0tha” to be continuously adjusted at each iteration of the measuring step “ E1 ”.

[0126] Each measuring step "E1" is performed with a determined frequency.

[0127] The iteration of the first measurement step "E1" is usually very fast, for example of the order of 20 milliseconds. This allows the measurement step "E1" to be performed on all preforms 12 that successively pass through the measurement zone "Z". The frequency of the iteration of the measurement step "E1" therefore corresponds to the frequency with which the preforms 12 pass through the measurement zone "Z".

[0128] As a variant, the measurement step "E1" is not performed for each preform 12. For example, the first measurement step "E1" is iterated for the passage of one preform out of a certain number of preforms 12, for example one preform out of every two preforms or one preform out of every three preforms. The iteration frequency of the measurement step "E1" is therefore a divisor of the frequency with which the preforms 12 pass through the measurement zone "Z".

[0129] The invention is particularly suitable for lightweight preforms with a small thickness, for which the orientation rate is high and therefore the applied heating is also high.

[0130] In fact, the small preform thickness of the order of 1.5 mm that is increasingly encountered is particularly conducive to the presence of a large amount of stress during injection molding. When heating the preform, the bi-orientation rate of the material is high and the heat applied in the heating station should be at a high temperature (125°C-135°C). This high temperature, combined with the stress present in the preform, will produce the effect of shrinking the preform along its length and slightly expanding along its diameter.

[0131] Significant shrinkage mainly concerns small thickness preforms intended for the production of common water bottles of small volume (usually less than 0.5 liters). These preforms are less than 90 mm in length (usually between 60 and 70 mm). The shrinkage after heating on this type of preform can easily reach 10 mm, which corresponds to a shrinkage of nearly 15% and a diameter expansion of about 7% on a 70 mm preform. The same preform with the same resin but reduced stress level will have a shrinkage of about 5 mm after heating (i.e. a shrinkage of about 7% and a diameter expansion of about 3%).

[0132] The invention is also particularly applicable to preforms made of rPET, for which the shrinkage values ​​may vary significantly depending on the composition of the recycled material from which they are composed.

Claims

1. A control method for controlling a forming method for forming containers by stretch-blow molding preforms (12) of thermoplastic material, in particular PET, each preform (12) comprising a body (14) open at one end by a neck (16) and closed at the other opposite end by a bottom (18), the forming method comprising for each preform (12) moving along a production path (27): - a heating operation of heating the preform (12) to a predetermined temperature in a heating zone (33); - an introduction operation of introducing the heated preform (12) into the mold (36) in the introduction zone (93) of the production path (27); - a stretching operation of moving the stretching rod (46) to stretch the preform (12), the stretching rod (46) passing through the contact position (P0) at the moment of contacting the bottom (18) of the preform (12) during the movement; Characterized in that the control method comprises: at least one measuring step (E1) for measuring the length (L) between a reference point and the bottom (18) of the neck (16) of the preform (12), the measuring step being carried out after a heating operation of heating the preform (12) and before an introduction operation of the preform (12) into the mold (36); An adjustment step (E2) for adjusting the contact position (P0) of the stretching rod (46) based on at least one length (L) measurement performed during at least one measuring step (E1).

2. The control method according to claim 1, characterized in that: The forming method includes a blow molding process of injecting a pressurized forming fluid into a preform (12), the start of the blow molding process being determined based on the contact position (P0tha) of the stretch rod (46) adjusted during the adjustment step (E2) of the control method.

3. A control method according to any one of the preceding claims, characterized in that The measuring step (E1) for measuring the length (L) is carried out in a measuring zone (Z) through which the production path (27) passes, the measuring zone (Z) being located between the heating zone (33) and the introduction zone (93) for introducing the preform (12) into the mold (36).

4. The control method according to claim 3, characterized in that: The heating operation is carried out in a heating station (24) comprising a conveyor (28) which transports the preforms through a heating zone (33), the measuring zone (Z) being arranged on the section of the production path (27) along which the preforms (12) are transported by the conveyor (28).

5. A control method according to any one of the preceding claims, characterized in that The measuring step (E1) is performed using a non-contact measuring device (94) capable of measuring the length (L) of the preform (12) in real time while the preform is continuously moving along the production path (27).

6. The control method according to claim 5, characterized in that: The measuring device (94) comprises an image capturing device (96) for capturing an image of the preform (12) passing through the measuring zone (Z), and the measuring step (E1) comprises a first sub-step (E1-1) for capturing an image (98) of the preform (12).

7. The control method according to claim 6, characterized in that: During a first sub-step (E1-1) for capturing an image, the preform (12) is illuminated by a light source (102) which is located on the opposite side of the position of the image capture device (96) relative to the production path (27).

8. The control method according to claim 7, characterized in that: The measuring step (E1) includes a second sub-step (E1-2), which is used to process the image (98) captured in the first sub-step (E1-1) for capturing the image using an electronic processing unit (104), including: detecting at least one measuring point (M) of the image (98) of the bottom (18) of the preform (12); then measuring the distance (d) between a reference point (O) on the image (98) and the measuring point (M) along the axis (A) of the preform on the image (98), wherein the position of the reference point on the image relative to the neck (16) of the preform (12) is known; and then converting the distance (d) into an actual distance (d×r).

9. The control method according to claim 8, characterized in that: The reference point on the image is formed by the optical axis (O) of the image capture device (96).

10. The control method according to claim 8 or 9, characterized in that: The conversion of the distance (d) measured on the image (98) into the actual distance is performed after the calibration sub-step (E1-3).

11. The control method according to claim 10, characterized in that: The calibration sub-step (E1-3) is performed at each iteration of the preform (12) length determination sequence, and consists in measuring a known dimension of the preform, such as the outer diameter, in order to determine the ratio (r) between the distance on the image (98) and the actual distance.

12. A control method according to any one of the preceding claims, characterised in that The adjustment step (E2) for adjusting the touchdown position (P0tha) is performed based on the length (L) measured at the end of each iteration of the measurement step (E1).

13. The control method according to any one of claims 1 to 11, characterized in that: The adjustment step (E2) for adjusting the contact position (P0tha) is performed based on a sliding average of the lengths (L) measured at the end of a plurality of iterations of the measurement step (E1) performed successively on a plurality of preforms (12).

14. A control method according to any one of the preceding claims, characterised in that The measuring step ( E1 ) is carried out with a determined frequency, for example for each preform passing through the measuring zone or for one preform out of a determined number of preforms.

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