METHOD FOR CONTROLLING A CONTAINER MANUFACTURING PLANT

Abstract

The invention relates to a method for controlling a hollow body manufacturing installation comprising: - a control unit (56); - a molding cavity (13); - a blow nozzle (14); - a rod (16) for drawing the hollow body (12); - a pre-blow source (18); - a pre-blow pipe (20); - a controlled flow limiter (26) interposed in the pre-blow pipe (20), and capable of varying the cross-section (S) of the forming fluid; characterized in that, during a pre-blow step (E1), the cross-section (S) of the forming fluid at the pre-blow pressure (Pf1) is automatically controlled according to a value representative of the current pre-blow pressure (Pf1) so that the forming fluid flows with a flow rate (Q) conforming to a setpoint value (Cq). Figure for the abbreviation: Figure 1

Description

Title of the invention: METHOD FOR CONTROLLING A CONTAINER MANUFACTURING PLANT Technical field of the invention

[0001] The invention relates to a method of controlling an installation for manufacturing hollow bodies in the form of a container made of thermoplastic material, in particular PET, by stretch-blowing hollow bodies in the preform state during successive manufacturing cycles, comprising: - a control unit; - at least one molding cavity suitable for receiving a hollow body in the preform state; - at least one blow nozzle capable of being hermetically connected with said hollow body enclosed in the molding cavity;

[0002] - at least one drawing rod which is slidably controlled in the cavity of molding in order to stretch the hollow body; - at least one pre-blowing source delivering forming fluid at a time-varying pre-blowing pressure, the value of which is automatically controlled by the electronic control unit;

[0003] - at least one pre-blowing line which connects the pre-blowing source to the blow nozzle; - A flow restrictor, which is inserted in the pre-blow pipe, and which is capable of varying the cross-sectional area of ​​the forming fluid at the pre-blow pressure. Technical background

[0004] It is known to produce containers from thermoplastic material, such as polyethylene terephthalate (PET), by a stretch-blowing process of preforms.

[0005] In general, a preform has an axisymmetric shape. The preform includes a neck that already has its final shape, while a body of the preform is intended to be deformed during the forming process. The principal axis of the preform passes through the center of the neck. The bottom of the preform generally has a hemispherical wall centered on the principal axis of the preform.

[0006] To allow its deformation, the body of the preform is heated above a glass transition temperature, making the body wall malleable by significantly reducing its elastic limit. Conversely, the neck is maintained at a temperature below the glass transition temperature to prevent its deformation.

[0007] In order to produce containers with a wall of substantially constant thickness, it is known to perform a so-called "biaxial" drawing of the material constituting the The preform wall is plastically deformed. The forming process includes a first pre-blowing stage during which forming fluid is injected at a pre-blowing pressure. During this pre-blowing stage, a sliding stretching rod is inserted coaxially into the neck of the preform, pushing the bottom of the preform to stretch the body wall in an axial direction. The injection of forming fluid at the first pre-blowing pressure, simultaneously with the preform stretching, ensures proper distribution of the material constituting the preform body walls.

[0008] At the end of this pre-blowing step, a blowing step is triggered, during which a compressed forming fluid is injected at a blowing pressure, higher than the pre-blowing pressure, into the body of the preform so as to allow a stretching of the material properly distributed in a globally circumferential direction of the wall of the body of the preform to "inflate" the preform until it reaches its final shape.

[0009] Generally, this forming process is carried out in a molding cavity that has an impression conforming to the final container to be obtained. During the blow molding step, the wall of the preform body is pressed against the wall of the impression under the pressure of the forming fluid to give the container its final shape.

[0010] When it begins to deform, the preform becomes an "intermediate container" before reaching its final form as a "final container".

[0011] In the rest of the description and in the claims, the term "hollow body" will be used to refer indifferently to a preform, an intermediate container or a finished container.

[0012] Typically, the operator in charge of controlling the manufacturing installation can adjust the pre-blowing pressure to obtain high-quality containers, particularly bottles. Such a process relies heavily on the operator's experience to adapt to the various operating constraints of the installation.

[0013] Furthermore, the device can be equipped with a flow limiter that allows the cross-sectional area of ​​the forming fluid delivered during the pre-blowing stage to be adjusted. In the prior art, the cross-sectional area is fixed before the manufacturing installation is put into production and remains fixed throughout its production period.

[0014] Furthermore, with the aim of saving energy, it has become important to recycle the forming fluid at the end of the forming operation. Indeed, at the end of this operation, the finished container is filled with high-pressure forming fluid. Before demolding the container, it is imperative to drain the forming fluid under Pressure. Rather than venting the pressurized forming fluid into the atmosphere, it has been proposed to vent a portion of the pressurized forming fluid to recovery tanks. Advantageously, the pre-blowing source is formed by such a recovery tank.

[0015] To make the best use of the recycled fluid, it is preferable to be able to actively adjust the pre-blowing pressure during production. The pre-blowing pressure is, for example, automatically determined by an electronic control unit that applies a recycling process designed to maximize the amount of fluid recycled throughout the process.

[0016] If such a recycling process makes it possible to satisfactorily reduce the energy consumed during the production of containers, it interferes with the possibility of freely controlling the pre-blowing pressure to obtain good quality containers.

[0017] There is therefore a need to be able to obtain good quality containers while minimizing the energy expended to produce the containers. Summary of the invention

[0018] The invention proposes a method for controlling an installation for manufacturing hollow bodies in the form of a container made of thermoplastic material, in particular PET, by stretch-blowing hollow bodies in the preform state during successive manufacturing cycles, comprising: - a control unit; - at least one molding cavity suitable for receiving a hollow body in the preform state; - at least one blow nozzle capable of being hermetically connected with said hollow body enclosed in the molding cavity; - at least one drawing rod which is controlled by sliding in the molding cavity in order to draw the hollow body; - at least one pre-blowing source delivering forming fluid at a time-varying pre-blowing pressure, the value of which is automatically controlled by the electronic control unit;

[0019] - at least one pre-blowing line that connects the pre-blowing source to the blow nozzle; - a controlled flow limiter which is interposed in the pre-blowing line, and which is capable of varying the passage cross-section of the forming fluid at the pre-blowing pressure; characterized in that, during a pre-blowing stage which begins when the pre-blowing line supplies the nozzle with forming fluid at the pre-blowing pressure and which ends at the end of the drawing of the hollow body by the drawing rod,

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[0031] the form fluid passage section at the pre-blowing pressure being automatically controlled according to a representative value of the current pre-blowing pressure so that the form fluid flows through said passage section with a flow rate conforming to a setpoint value. According to another feature of the process carried out according to the teachings of the invention, the setpoint value of the flow rate is a constant. According to another feature of the process carried out according to the teachings of the invention, the setpoint value of the flow rate is controlled by an operator. According to another feature of the process carried out according to the teachings of the invention, the passage section of the flow limiter is modified exclusively between two successive pre-blowing stages. According to another feature of the process carried out according to the teachings of the invention, the current value of the pre-blowing pressure is measured on a previous manufacturing cycle. According to another feature of the process carried out according to the teachings of the invention, the current value of the pre-blowing pressure corresponds to a setpoint value determined automatically by the electronic control unit. According to another feature of the method carried out according to the teachings of the invention, the passage section of the flow limiter is further determined as a function of the setpoint value of at least one determined parameter. According to another feature of the process carried out according to the teachings of the invention, a parameter determined among the at least one parameter determined is formed by a value representative of the volume of the hollow body at the end of the pre-blowing step. According to another feature of the process carried out according to the teachings of the invention, a parameter determined among the at least one parameter determined is formed by a value representative of the duration of the pre-blowing step. According to another feature of the process carried out according to the teachings of the invention, a parameter determined among the at least one parameter determined is formed by a value representative of the pressure in the hollow body at the end of the pre-blowing step. According to another feature of the process carried out according to the teachings of the invention, a parameter determined from among the at least one parameter determined is formed by the function g(P' l,P'b) according to the following relation: [Mathl] P'b) = in which: b is a characteristic parameter of the conduction; P' 1 is a representative value of the pre-blowing pressure;

[0032] P'b is a representative value of the pressure in the hollow body at the end of the pre-blowing step.

[0033] According to another feature of the process carried out according to the teachings of the invention, the representative value of the pressure in the hollow body at the end of the pre-blowing step is an average of the pressure value measured by a pressure sensor mounted on the nozzle, during previous manufacturing cycles.

[0034] According to another feature of the process carried out according to the teachings of the invention, a portion of the pressurized forming fluid is recovered in the pre-blowing source after the hollow body has been formed into the final container state, the value of the pre-blowing pressure fluctuating according to the quantity and pressure of the formed fluid recovered.

[0035] According to another feature of the process carried out according to the teachings of the invention, the value of the pre-blowing pressure is automatically controlled by the electronic control unit in order to maximize the quantity of forming fluid recovered.

[0036] The invention also proposes an installation for manufacturing hollow bodies in the form of a container made of thermoplastic material, in particular PET, by stretch-blowing hollow bodies in the form of a preform during successive manufacturing cycles, the installation being capable of implementing the process according to the teachings of the invention, the installation comprising: - a control unit; - at least one molding cavity suitable for receiving a hollow body in the preform state; - at least one blow nozzle capable of being hermetically connected with said hollow body enclosed in the molding cavity; - at least one drawing rod which is controlled by sliding in the molding cavity in order to draw the hollow body; - at least one pre-blowing source delivering forming fluid at a time-varying pre-blowing pressure, the value of which is automatically controlled by the electronic control unit; - at least one pre-blowing duct that connects the pre-blowing source to the blowing nozzle; characterized in that it includes a controlled flow limiter which is interposed in the pre-blowing line, and which is capable of varying the passage cross-section of the forming fluid at the pre-blowing pressure. Brief description of the figures

[0037] Other features and advantages of the invention will become apparent during the reading of the detailed description which follows, for the understanding of which reference should be made to the attached drawings.

[0038] The [Fig. 1] is an axial cross-sectional view which schematically represents a blowing station for a hollow body suitable for implementing the process according to the teachings of the invention.

[0039] Fig. 2 is a diagram that represents the evolution of the forming fluid pressure in the hollow body as a function of time in the blowing station of Fig. 1 during the manufacture of a hollow body. Detailed description of the invention

[0040] In the rest of the description, similar or identical elements will be designated by the same references.

[0041] A blowing station 10 belonging to a mass production container manufacturing installation has been illustrated in [Fig.1].

[0042] The blow molding station 10 for stretch-blowing molding of a hollow body 12, initially in the form of a thermoplastic preform, comprises a mold forming a two-part molding cavity 13 that can be separated to release the hollow body 12 in the final container state. The blow molding station 10 further comprises a blow nozzle 14 equipped with a vertically movable stretching rod 16 between a retracted position, shown in solid lines in [Fig. 1], and an extended position shown in dashed lines in [Fig. 1].

[0043] The blow nozzle 14 is connected to at least one pre-blow source 18 delivering forming fluid at a first pre-blow pressure Pfl.

[0044] The pre-blowing source 18 is connected to the blowing nozzle 14 via a pre-blowing duct 20.

[0045] The circulation of forming fluid between the pre-blowing source 18 and the nozzle 14 is controlled here by a first pre-blowing valve 22 which is interposed in the pre-blowing duct 20. The first pre-blowing valve 22 is controlled between a fully open state in which it supplies the blowing nozzle 14 with forming fluid at maximum flow rate and a fully closed state in which the passage of forming fluid is prohibited.

[0046] The pre-blowing duct 20 includes at least one flow-limited section 24. In this section, the pre-blowing duct 20 includes at least one flow limiter 26.

[0047] The flow limiter 26 is here interposed in the pre-blowing pipe 20 between the pre-blowing source 18 and the pre-blowing valve 22.

[0048] The flow limiter 26 is controlled to vary a limited flow rate "Q" of forming fluid in the pre-blowing line 20. To this end, the flow limiter 26 is capable of varying the cross-sectional area "S" through which the forming fluid passes at the pre-blowing pressure Pfl through the pre-blowing line 20 between a minimum and a maximum cross-sectional area. The value of the limited flow rate "Q" circulating through the pre-blowing line 20 depends in particular on said cross-sectional area "S" and the pre-blowing pressure "Pfl". For a given pre-blowing pressure "Pfl", the flow rate "Q" is generally proportional to the cross-sectional area "S".

[0049] By way of non-limitation, the pre-blowing source 18 here comprises a forming fluid storage tank 28. The storage tank 28 contains forming fluid stored at a fluctuating pressure "Ps" greater than or equal to the pre-blowing pressure Pfl.

[0050] The pressure "Ps" in the storage tank 28 is preferably higher by at least 0.5 bar compared to the pre-blowing pressure Pfl to ensure a good limited flow rate "Q" of forming fluid during the pre-blowing stage.

[0051] To deliver the forming fluid at the pre-blowing pressure Pfl, the pre-blowing source 18 includes a pressure regulator 30 arranged downstream of the forming fluid storage tank 28 to bring the forming fluid to the pre-blowing pressure Pfl. The pressure regulator 30 is arranged upstream of the flow-limiting section 24.

[0052] In an unrepresented variant of the invention, the pre-blowing source 18 does not include a pressure regulator and the pre-blowing pressure is equal to the pressure in the reservoir.

[0053] The blow nozzle 14 is also connected to the storage tank 28 via a recovery line 32 in which a recovery valve 34 is interposed. This allows a portion of the pressurized forming fluid to be reused in the forming of a subsequent hollow body 12 into the final container. This reduces the overall energy expenditure for producing a final container.

[0054] Part of the pressurized forming fluid contained in the hollow body 12 in the final container state at the end of forming is thus recovered in the pre-blowing source 18 at the end of the forming of the preform into the container, the value of the pre-blowing pressure Pfl fluctuating according to the quantity and pressure of the forming fluid recovered.

[0055] In the embodiment shown in [Fig.1], a bypass duct 36 is arranged in parallel with the flow-limited section 24 to connect the forming fluid source 18 at the pre-blowing pressure Pfl with the blowing nozzle 14.

[0056] The bypass pipe 36 has two ends. An upstream end is connected to the pre-blowing pipe 20 upstream of the flow-limited section 24. A downstream end is connected to the pre-blowing pipe 20 downstream of the pre-blowing valve 22.

[0057] The bypass pipe 36 is sized to have a cross-sectional area for the forming fluid greater than the cross-sectional area for the flow restrictor 26, so that the forming fluid flows at a higher rate than that passing through the flow-limited section 24. A bypass valve 38 is interposed in the bypass pipe 36. The bypass valve 38 is controlled between a fully closed and a fully open state.

[0058] Thus, when the bypass valve 38 is open, the pre-blow valve 22 must also be open to allow the blow nozzle 14 to be supplied with forming fluid at the higher flow rate. The blow nozzle 14 is thus supplied with the sum of the flow passing through the flow restrictor 26 and the additional flow passing through the bypass line 36.

[0059] In an unrepresented variant of the invention, the device does not include a bypass conduit 38.

[0060] The blow nozzle 14 is also connected to a blow source 40 delivering forming fluid at a blow pressure Pf2, for example approximately 40 bar. The blow source 40 is connected to the blow nozzle 14 via a blow line 42 in which a blow valve 44 is interposed.

[0061] Generally, the blowing station 10 is arranged in a forming unit (not shown) comprising several identical blowing stations 10. In this case, the storage tank 28 may be common to several blowing stations 10.

[0062] The forming fluid is here formed by air.

[0063] The blowing pressure Pf2 is, for example, on the order of 40 bar. The pre-blowing pressure Pfl is lower than the blowing pressure Pf2. It is, for example, between 6 bar and 20 bar.

[0064] The blowing nozzle 14 is also connected to atmospheric pressure via an exhaust pipe 46 equipped with a silencer 48. An exhaust valve 50 is interposed in the exhaust pipe 46.

[0065] A pressure sensor (not shown) can be arranged in the blow nozzle 14 so as to measure the instantaneous pressure prevailing in the hollow body 12.

[0066] The hollow body 12 in the preform state comprises a neck 52 and a body 54. The body 54 is preheated before being introduced into the molding cavity 13. During stretch blow molding, the molding cavity 13 is closed around the body 54, the nozzle 14 of blowing connects to the neck 52 and the sensor can then measure the pressure in the body 54.

[0067] An electronic control unit 56 allows the valves 22, 34, 44, and 50 to be controlled, for example, according to a predetermined timing and / or based on data communicated by the pressure sensor. The different forming fluid pressures inside the hollow body 12 are controlled by adjusting the opening time of the valves 22, 34, 44, and 50.

[0068] The electronic control unit 56 also allows the flow limiter 26 to be controlled to vary its passage section "S" and thus control the limited flow "Q" of forming fluid circulating through the pre-blowing pipe 20.

[0069] When the forming unit is in production, the blow molding station 10 successively manufactures several final containers during successive manufacturing cycles. During each manufacturing cycle, a new hollow body 12 in the final container state is manufactured from said hollow body 12 in the preform state.

[0070] When the blowing station 10 is in normal operation, the storage pressure Ps in the storage tank 28 is likely to vary, but it remains greater than or equal to the pre-blowing pressure Pfl.

[0071] Advantageously, to optimize the recycling of the pressure forming fluid, a setpoint "Cpf 1" for the pre-blowing pressure "Pfl" is automatically determined by the electronic control unit 56. The electronic control unit 56 is free to vary this setpoint "Cpfl" during production to maximize the recycling of the pressure forming fluid. This means that the operator in charge of running the blow molding station 10 cannot adjust this parameter to obtain a final hollow body 12 of the desired quality.

[0072] As shown in a [Fig.2], each manufacturing cycle comprises successive stages of increasing pressure on the hollow body 12. The hollow body 12 is successively subjected to a first stage "E1" of pre-blowing, during which the hollow body 12 in the preform state is stretched axially completely and circumferentially partially, then a second stage "E2" of blowing, during which the wall of the hollow body 12 is pressed against the mold cavity until it takes its complete shape.

[0073] The first pre-blowing step 11 comprises a stretching operation of the hollow body 12. The hollow body 12 is then in a preform state. The stretching rod 16 descends into the body 54 to stretch it axially from a point of contact of the stretching rod 16 with the bottom of the hollow body 12, until it reaches a maximum stretching position indicated by the point at which the bottom of the hollow body 12 is located. proximity to the bottom of the molding cavity 13, as shown in dashed lines in [Fig.1].

[0074] During this first pre-blowing step El, forming fluid at the pre-blowing pressure Pfl is injected into the body 54 with a limited flow rate "Q" passing exclusively through the flow limiter 26. The pressure in the body 54 increases due to the injection of the pressurized forming fluid.

[0075] The partial circumferential stretching of the material under the effect of the injection of the forming fluid at the pre-blowing pressure, along with its axial stretching under the effect of the sliding of the stretching rod 16, allows the material to be properly distributed within the wall of the hollow body 12. This distribution makes it possible to obtain a substantially constant thickness in the wall of the finished container.

[0076] More specifically, the pre-blowing step “El”, as defined in the invention, begins when the pre-blowing line 20 supplies the nozzle 14 with forming fluid at the pre-blowing pressure “Pfl”. The pre-blowing step “El” begins here when the pre-blowing valve 22 is opened. Furthermore, the pre-blowing step “El” ends at the completion of the preform drawing operation by the drawing rod 16.

[0077] During the entire "El" pre-blowing stage, the bypass valve 38 remains closed.

[0078] Pressurization is achieved by opening the pre-blowing valve 22, while all other valves are closed. The forming fluid is thus injected at the first pre-blowing pressure Pfl and with the flow rate "Q" limited by the storage tank 28 via the flow limiter 26.

[0079] The limited flow rate "Q" prevents the body 54 from being stretched radially too quickly, thus allowing for better axial material distribution. As the volume of the body 54 gradually increases under the effect of stretching, the forming fluid pressure inside the body 54 initially increases to a fluctuating pressure lower than the pre-blow pressure Pfl, before reaching a final stretch pressure "Pb". The final stretch pressure "Pb" is lower than the Pfl pressure.

[0080] After the drawing process is complete, the material constituting the walls of the hollow body 12 is properly distributed. It is therefore possible to increase the pressure in the body 54 more rapidly to finalize the drawing of the walls in a circumferential direction.

[0081] As explained previously, contrary to what was practiced in the prior art, it is not possible to freely play on the value of the pre-blowing pressure "Pfl" to obtain a desired material distribution at the end of the pre-blowing step "El" because this would risk reducing the amount of recycled pressure forming fluid.

[0082] To enable good distribution of material in the hollow body 12 at the end of the "El" pre-blowing step while continuing to maximize the quantity of recycled pressurized forming fluid, the invention proposes that, during the "El" pre-blowing step, the "S" section of the formering fluid passage at the "Pf 1" pre-blowing pressure is automatically controlled according to the current value of the "Pfl" pre-blowing pressure so that the formering fluid flows through said "S" passage section with a setpoint value "Cq" representative of the fluid flow rate "Q".

[0083] Preferably, the setpoint value “Cq” representing the fluid flow rate “Q” is controlled by an operator. This is, in fact, the value that the operator can adjust to improve the distribution of matter in the hollow body 12.

[0084] The invention proposes to achieve a flow rate “Q” of forming fluid in the pre-blowing line 20 that tends towards the setpoint value “Cq” without altering the pre-blowing pressure “Pfl” determined by the electronic control unit 56, in order to maximize the quantity of pressurized forming fluid recycled. To achieve this, the electronic control unit 56 controls the limited flow rate “Q” of forming fluid in the pre-blowing line 20 by acting on the cross-sectional area “S” of the flow limiter 26.

[0085] More specifically, the electronic control unit 56 determines a target value "Cs" for the passage section "S" as a function of the flow setpoint value "Cq".

[0086] This “Cq” flow setpoint value may optionally be limited to a range of permissible values ​​for the proper functioning of the blowing station 10.

[0087] Alternatively, the setpoint value is a constant determined experimentally based on the hollow body 12 treated by the blowing station 10. When the pressure "Pfl" varies to maximize recirculation, the electronic control unit 56 of the flow limiter 26 passage section "S" maintains the limited flow rate "Q" constantly equal to the setpoint value.

[0088] Preferably, the cross-sectional area "S" of the flow limiter 26 is modified exclusively between two successive pre-blowing stages "El" while the blowing station 10 is still in production. This ensures that the cross-sectional area "S" remains constant during each pre-blowing stage "El". In particular, this allows for precise control of the limited flow rate "Q" of forming fluid during the pre-blowing stage "El".

[0089] Furthermore, due to the response time of the flow limiter 26, the operation of modifying the passage section "S" is likely to last longer than the pre-blowing step "El" itself. The duration between two successive "El" steps the pre-blowing during production is, however, greater than or equal to the said response time.

[0090] The value of the pre-blowing pressure “Pfl” used to determine the target value “Cs” of the passage section “S” adapted to obtain a limited flow rate “Q” equal to the setpoint value “Cq” corresponds to the setpoint pressure “Cpfl” determined automatically by the electronic control unit 56.

[0091] Alternatively, the current value of the pre-blowing pressure "Pfl" is measured, for example by means of a pressure sensor located in the pre-blowing line 20. In this case, the pressure value measured during a previous manufacturing cycle is used to update the flow section "S" for the next cycle.

[0092] The target value "Cs" of the "S" passage section is further determined as a function of the setpoint value of at least one parameter determined in addition to the pre-blowing pressure "Pfl".

[0093] In a non-exhaustive manner, at least one specified parameter is formed from one or a combination of the parameters from the following list:

[0094] A representative value of the volume “Vp” of the hollow body 12 at the end of the pre-blowing step “El”;

[0095] The duration “Tps” of the pre-blowing step “El”;

[0096] A representative value of the pressure “Pb” in the hollow body 12 at the end of the pre-blowing step “El”.

[0097] More specifically, the determined parameter can be formed by a value representing the volume "Vp" of the hollow body 12 at the end of the pre-blowing step "El". This value is determined, for example, as a function of the pressure "Pb" prevailing inside the hollow body 12 at the end of the pre-blowing step "El" and as a function of the quantity of forming fluid injected into the hollow body 12 during the pre-blowing step "El".

[0098] A specific parameter among the at least one specified parameter may also be formed by a value representing the duration "Tps" of the pre-blowing step El. The representative value of the duration of the pre-blowing step El is, for example, the preform stretching time applied by the electronic control unit 56. This is a parameter set by default or a parameter updated regularly by the electronic control unit 56.

[0099] A determined parameter among the at least one determined parameter is for example formed by a representative value of the pressure "Pb" in the hollow body 12 at the end at least of the previous pre-blowing step El.

[0100] The representative value of the pressure “Pb” in the hollow body 12 at the end of the pre-blowing step El is, for example, an average of the pressure values ​​measured by a pressure sensor mounted on the nozzle 14, during several cycles of

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[0115] Previous manufacturing cycles. The average is, for example, a moving average calculated over a specified number of previous manufacturing cycles. For example, the "S" section of the passage is controlled according to the value obtained by the following function g(P'l,P'b). [Math2] t P'b) = 1- [--- / in which: b is a characteristic parameter of the pre-blowing line 20; P' 1 is a representative value of the pre-blowing pressure (Pfl); P'b is the representative value of the pressure "Pb" in the hollow body 12 at the end of the pre-blowing stage El. According to a non-limiting example, the target value "Cs" of the "S" section of the passage is calculated by an equation of the following form: [Math3] \ V Pb+kl Cs - 1 + | p,l T ,p, + k0 in which: k0, k1, k2 and k3 are constants V' is a representative value of the volume "Vp" of the hollow body 12 at the end of the pre-blowing stage "El" P'b is a representative value of the pressure in the hollow body 12 at the end of the "El" pre-blowing stage P' 1 is a representative value of the pre-blowing pressure T'pl is a representative value of the duration of the "El" pre-blowing stage. Then comes a blowing step E2 which transforms the hollow body 12 into the final container. During this blowing step E2, the body 54 continues to expand until it is pressed against the wall of the molding cavity 13. During this blowing step E2, shown in [Fig. 2], the pressure in the hollow body 12 increases from pressure Pb to a blowing pressure Pf2. The blowing pressure Pf2 is thus higher than the pre-blowing pressure Pfl. At the end of the "El" pre-blowing stage, the pre-blowing valve 22 remains open and the bypass valve 38 is open in order to make the best use of the recycled forming fluid. Once the pressure in the hollow body 12 reaches approximately the pre-blowing pressure "Pfl", the In an unrepresented variant of the invention, at the end of the "El" pre-blowing step, the pre-blowing valve 22 is closed directly.

[0116] The electronic control unit 56 can, if necessary, control the flow limiter 26 in masked time from the closing of the pre-blowing valve 22 during the end of the manufacturing cycle.

[0117] Then the pressure increase to the blow pressure "Pf2" is carried out by opening the blow valve 44, while all other valves are closed. The hollow body 12 is thus supplied with forming fluid by the blow source 40.

[0118] Following this blowing step E2, the hollow body 12, in its final container state, is maintained at the aforementioned blowing pressure Pf2 during a holding step E3. Maintaining the final blowing pressure Pf2 allows the hollow body 12 to conform to all the shape details imposed by the molding cavity 13 in order to be shaped into the final container.

[0119] Each manufacturing cycle then includes successive stages of venting the forming fluid until the hollow body 12 in the final container state is at atmospheric pressure.

[0120] Thus, the holding step E3 is followed by a forming fluid recovery step E4. During this recovery step E4, the pressure in the hollow body 12 decreases from the blowing pressure Pf2 to a recovery pressure Pr. The recovery pressure Pr is therefore lower than the blowing pressure Pf2. The forming fluid is vented by opening the recovery valve 34, while all other valves are closed. The forming fluid is thus discharged into the storage tank 28.

[0121] The first recovery pressure Pr is controlled to an equilibrium value with the storage pressure Ps.

[0122] The equilibrium value of the first recovery pressure Pr is equal to the first storage pressure Ps plus a determined constant K, according to the following equation: Pr = Ps + K

[0123] The constant K determined is preferably strictly greater than 0 bar and is preferably less than or equal to 1 bar. The constant K is for example equal to 0.5 bar.

[0124] Although the hollow body 12 is in its final container state, the mold cannot be opened directly to retrieve the hollow body 12 because the air inside is still at a pressure higher than atmospheric pressure. The pressure inside the hollow body 12 must be reduced before opening the mold.

[0125] Finally, during a final exhaust stage E5, the remaining pressurized forming fluid still present in the hollow body 12 is vented to the atmosphere through the exhaust line 46. During this exhaust stage E5, the exhaust valve 50 is open while all other valves are closed.

[0126] In an unrepresented variant of the invention, the blowing step can be carried out in several stages involving additional recovery tanks to improve the overall efficiency of the process.

[0127] The pre-blowing pressure Pfl is advantageously determined by the electronic control unit 56 so that it falls within a range of values ​​enabling the production of a good quality final hollow body 12. However, the pre-blowing pressure Pfl must also have a value that allows for optimal filling of the storage tank 28.

[0128] Indeed, if the pre-blowing pressure Pfl is too high, the pressure in the storage tank 28 may quickly fall below the value required for the recycled forming fluid to be delivered at the required pre-blowing pressure Pfl. In this case, the pre-blowing step El must be carried out, at least in part, with forming fluid at the blowing pressure that is expanded to the pre-blowing pressure. This therefore results in an increase in the energy expenditure required to obtain a finished hollow body 12.

[0129] On the contrary, by allowing the electronic control unit 56 to decide the value of the pre-blowing pressure Pfl, it can choose the highest value that also maintains sufficient pressure in the storage tank 28. This ensures that the pre-blowing step El is carried out with the maximum amount of recycled forming fluid.

[0130] The invention thus makes it possible to obtain a container of good quality while maximizing the use of recycled forming fluid.

Claims

Demands

1. A method for controlling an installation for manufacturing hollow bodies in the container state of thermoplastic material, in particular PET, by stretch-blowing hollow bodies in the preform state during successive manufacturing cycles, comprising: - a control unit (56); at least one molding cavity (13) adapted to receive a hollow body (12) in the preform state; - at least one blow nozzle (14) adapted to be hermetically connected with said hollow body (12) enclosed in the molding cavity (13); - at least one stretching rod (16) which is controlled by sliding in the molding cavity (13) in order to stretch the hollow body (12); - at least one pre-blowing source (18) delivering forming fluid at a time-varying pre-blowing pressure (Pfl) whose value is automatically controlled by the electronic control unit (56);- at least one pre-blowing line (20) which connects the pre-blowing source (18) to the blowing nozzle (14); - a controlled flow limiter (26) which is interposed in the pre-blowing line (20), and which is capable of varying the cross-section (S) of passage of the forming fluid at the pre-blowing pressure (Pfl); characterized in that, during a pre-blowing step (El) which begins when the pre-blowing line (20) supplies the nozzle (14) with forming fluid at the pre-blowing pressure (Pfl) and which ends at the end of the drawing of the hollow body (12) by the drawing rod (16), the passage section (S) of the forming fluid at the pre-blowing pressure (Pfl) is automatically controlled according to a representative value of the current pre-blowing pressure (Pfl) so that the forming fluid flows through said passage section with a flow rate (Q) conforming to a setpoint value (Cq).

2. Method according to the preceding claim, characterized in that the setpoint value (Cq) of the flow rate is a constant.

3. Method according to claim 1, characterized in that the setpoint value (Cq) of the flow rate is controlled by an operator.

4. A method according to any one of the preceding claims, characterized in that the passage section (S) of the flow limiter (26) is modified exclusively between two successive pre-blowing stages (El).

5. Method according to the preceding claim, characterized in that the current value of the pre-blow pressure (Pfl) is measured on a previous manufacturing cycle.

6. A method according to any one of claims 1 to 4, characterized in that the actual value of the pre-blowing pressure (Pfl) corresponds to a setpoint value (Cpfl) determined automatically by the electronic control unit (56).

7. A method according to any one of the preceding claims, characterized in that the passage section (S) of the flow limiter (26) is further determined as a function of the setpoint value of at least one determined parameter.

8. Method according to the preceding claim, characterized in that a determined parameter among the at least one determined parameter is formed by a value representative of the volume (Vp) of the hollow body (12) at the end of the pre-blowing step (El).

9. A method according to any one of claims 7 or 8, characterized in that a determined parameter among the at least one determined parameter is formed by a value representative of the duration (Tps) of the pre-blowing step.

10. A method according to any one of claims 7 to 9, characterized in that a determined parameter among the at least one determined parameter is formed by a value (P'b) representative of the pressure (Pb) in the hollow body at the end of the pre-blowing step (El).

11. A method according to any one of claims 7 to 10, characterized in that a determined parameter among the at least one determined parameter is formed by the function g(P' l,P'b) according to the following relation: / DM nr|.\ 11 / V g(Pt P'b) 17- ' in which: b is a characteristic parameter of the pipe; P' l is a representative value of the pre-blowing pressure (Pfl); P'b is a representative value of the pressure (Pb) in the hollow body at the end of the pre-blowing step (El).

12. Method according to the preceding claim, characterized in that the value (P'b) representative of the pressure (Pb) in the hollow body (12) at the end of the pre-blowing step (El) is an average of pressure value measured by a pressure sensor mounted on the nozzle (14), during previous manufacturing cycles.

13. A method according to any one of the preceding claims, characterized in that a portion of the pressurized forming fluid is recovered in the pre-blowing source (18) after the forming of the hollow body (12) into the final container state, the value of the pre-blowing pressure (Pfl) fluctuating according to the quantity and pressure of the formed fluid recovered.

14. Method according to the preceding claim, characterized in that the value of the pre-blowing pressure (Pfl) is automatically controlled by the electronic control unit (56) in order to maximize the amount of forming fluid recovered.

15. An installation for manufacturing hollow bodies in the form of a container made of thermoplastic material, in particular PET, by stretch-blowing hollow bodies in the preform state during successive manufacturing cycles, the installation being capable of implementing the process according to any one of the preceding claims, the installation comprising: - a control unit (56); - at least one molding cavity (13) capable of receiving a hollow body (12) in the preform state; - at least one blow nozzle (14) capable of being hermetically connected with said hollow body (12) enclosed in the molding cavity (13); - at least one stretching rod (16) which is slidably controlled in the molding cavity (13) in order to stretch the hollow body (12);- at least one pre-blow source (18) delivering forming fluid at a time-varying pre-blow pressure (Pfl) whose value is automatically controlled by the electronic control unit (56); - at least one pre-blow line (20) which connects the pre-blow source (18) to the blow nozzle (14); characterized in that it comprises a controlled flow limiter (26) which is interposed in the pre-blowing pipe (20), and which is capable of varying the section (S) of passage of the forming fluid at the pre-blowing pressure (Pfl).

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