Method for controlling a container manufacturing facility, and corresponding facility

Abstract

The invention relates to a method for controlling a hollow-body manufacturing facility comprising: - a control unit (56); - a moulding cavity (13); - a blowing nozzle (14); - a stretch rod (16) for stretching the hollow body (12); - a pre-blowing source (18); - a pre-blowing duct (20); - a controlled flow limiter (26) inserted in the pre-blowing duct (20) and capable of varying the cross section (S) of flow of the forming fluid; characterized in that, during a pre-blowing step (E1), the cross section (S) of flow of the forming fluid at the pre-blowing pressure (Pf1) is controlled automatically as a function of a value representative of the current pre-blowing pressure (Pf1) such that the forming fluid flows with a flow rate (Q) in accordance with a set value (Cq).

Description

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 suitable for being connected in a hermetic manner with said hollow body enclosed in the molding cavity;

[0002] - 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;

[0003] - at least one pre-blowing line which connects the pre-blowing source to the blow nozzle; - a 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. 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 the 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 typically has a hemispherical wall centered on the principal axis of the preform.

[0006] To allow its deformation, the preform body 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] To produce containers with a wall thickness of approximately constant thickness, a known technique is to perform a "bi-axial" stretching of the material constituting the preform wall to plastically deform it. The forming process includes an initial 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 initial pre-blowing pressure, simultaneously with the preform stretching, ensures the proper distribution of the material constituting the walls of the preform body.

[0008] At the end of this pre-blowing stage, a blowing stage 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 the material to be stretched correctly distributed in a generally 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 produced. During the blow molding stage, the wall of the preform body is pressed against the wall of the cavity by 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 remainder 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 equipment can adjust the pre-blowing pressure to produce high-quality containers, particularly bottles. This process relies heavily on the operator's experience to adapt to the various operating constraints of the equipment.

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

[0014] Furthermore, to conserve energy, it has become important to recycle the forming fluid at the end of the forming process. Indeed, at the end of this process, the finished container is filled with high-pressure forming fluid. Before demolding the container, it is essential to remove the pressurized forming fluid. Rather than releasing the pressurized forming fluid into the atmosphere, it has been proposed to transfer a portion of it to recovery tanks. Advantageously, such a recovery tank acts as the pre-blowing source.

[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 can, for example, be automatically determined by an electronic control unit that applies a recycling process designed to maximize the amount of fluid recycled throughout the process.

[0016] While 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] Therefore, there is a need to be able to obtain good quality containers while minimizing the energy expended to produce the containers.

[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 suitable for being connected in a hermetic manner with said hollow body enclosed in the molding cavity; - at least one stretching rod which is controlled by sliding in the molding cavity in order to stretch the hollow body; - at least one pre-blowing source delivering forming fluid at a pre-blowing pressure that varies over time and whose value is automatically controlled by the electronic control unit;

[0019] - at least one pre-blowing line which 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 step 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, the passage cross-section of the forming fluid at the pre-blowing pressure is automatically controlled according to a value representative of the current pre-blowing pressure so that the forming fluid flows through said passage cross-section with a flow rate conforming to a setpoint value.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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'1,P'b) according to the following relation: [Math1]

[0030]

[0031] in which: b is a characteristic parameter of the pipe; 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 stage.

[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 stage 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 capable of receiving a hollow body in the form of a preform; - at least one blow nozzle capable of being connected in a hermetic manner with said hollow body enclosed in the molding cavity; - at least one stretching rod which is controlled by sliding in the molding cavity in order to stretch the hollow body; - at least one pre-blowing source delivering forming fluid at a pre-blowing pressure that varies over time and whose value is automatically controlled by the electronic control unit;- at least one pre-blowing line which connects the pre-blowing source to the blow 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 that follows, for the understanding of which reference should be made to the attached drawings.

[0038] This 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] This is a diagram that represents the evolution of the forming fluid pressure in the hollow body as a function of time in the blow molding station 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] We have illustrated in, a blowing station 10 belonging to a mass production container manufacturing installation.

[0042] The blow molding station 10 for stretch-blowing a hollow body 12, initially in a thermoplastic preform state, comprises a mold forming a two-part molding cavity 13 that can be separated to release the hollow body 12 in its 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 Figure 1, and an extended position shown in dashed lines in Figure 2.

[0043] The blow nozzle 14 is connected to at least one pre-blow source 18 delivering forming fluid at a first pre-blow pressure Pf1.

[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 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. Within 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 achieve this, the flow limiter 26 is capable of varying the cross-sectional area "S" through which the forming fluid passes at the pre-blowing pressure Pf1 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 this cross-sectional area "S" and the pre-blowing pressure "Pf1". For a given pre-blowing pressure "Pf1", the flow rate "Q" is generally proportional to the cross-sectional area "S".

[0049] By way of example, the pre-blowing source 18 here includes 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 Pf1.

[0050] The pressure "Ps" in the storage tank 28 is preferably higher by at least 0.5 bar compared to the pre-blowing pressure Pf1 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 Pf1, the pre-blowing source 18 includes a pressure regulator 30 located downstream of the forming fluid storage tank 28 to bring the forming fluid to the pre-blowing pressure Pf1. The pressure regulator 30 is located 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 Pf1 fluctuating according to the quantity and pressure of the forming fluid recovered.

[0055] In the embodiment shown in the figure, a bypass conduit 36 ​​is arranged in parallel with the limited flow section 24 to connect the forming fluid source 18 at the pre-blowing pressure Pf1 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 larger cross-sectional area for the forming fluid than the cross-sectional area of ​​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 therefore 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 blow molding station 10 is arranged in a forming unit (not shown) comprising several identical blow molding stations 10. In this case, the storage tank 28 may be shared by several blow molding stations 10.

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

[0063] The blowing pressure Pf2 is, for example, around 40 bar. The pre-blowing pressure Pf1 is lower than the blowing pressure Pf2. For example, it is between 6 bar and 20 bar.

[0064] The blow 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 in the hollow body 12.

[0066] The hollow body 12 in its 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 blow nozzle 14 mates onto 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 transmitted by the pressure sensor. The various forming fluid pressures inside the hollow body 12 are controlled by adjusting the opening duration of 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 Pf1.

[0071] Advantageously, to optimize the recycling of the pressure forming fluid, a pre-blow pressure setpoint "Cpf1" is automatically determined by the electronic control unit 56. This electronic control unit 56 is free to vary this setpoint "Cpf1" 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, each manufacturing cycle includes 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 E1 involves a stretching operation of the hollow body 12. The hollow body 12 is then in the 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 in which the bottom of the hollow body 12 is located near the bottom of the molding cavity 13, as shown in dashed lines in the figure.

[0074] During this first pre-blowing stage E1, forming fluid at the pre-blowing pressure Pf1 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 for the proper distribution of the material within the wall of the hollow body 12. This distribution results in a substantially constant thickness in the wall of the finished container.

[0076] More specifically, the pre-blowing step "E1", as defined in the invention, begins when the pre-blowing line 20 supplies the forming nozzle 14 with forming fluid at the pre-blowing pressure "Pf1". The pre-blowing step "E1" begins when the pre-blowing valve 22 is opened. Furthermore, the pre-blowing step "E1" ends at the completion of the preform drawing operation by the drawing rod 16.

[0077] Throughout the entire "E1" 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 Pf1 and with the flow rate "Q" limited by the storage tank 28 via the flow restrictor 26.

[0079] The limited flow rate "Q" prevents the radially stretched body 54 from being stretched 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 rises to a fluctuating pressure lower than the pre-blow pressure Pf1, before reaching a final stretch pressure "Pb". The final stretch pressure "Pb" is lower than the pressure Pf1.

[0080] After the drawing process is complete, the material forming 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 "Pf1" to obtain a desired material distribution at the end of the pre-blowing step "E1" because this would risk reducing the amount of recycled pressure forming fluid.

[0082] To enable good material distribution in the hollow body 12 at the end of the "E1" pre-blowing step while continuing to maximize the amount of recycled pressurized forming fluid, the invention proposes that, during the "E1" pre-blowing step, the "S" section of the forming fluid passage at the "Pf1" pre-blowing pressure is automatically controlled according to the current value of the "Pf1" pre-blowing pressure so that the forming 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 the value that the operator can adjust to improve the distribution of material within the hollow body 12.

[0084] The invention proposes to achieve a flow rate "Q" of forming fluid in the pre-blowing line 20 that approaches the setpoint value "Cq" without altering the pre-blowing pressure "Pf1" determined by the electronic control unit 56, thereby maximizing the amount of pressurized forming fluid recycled. To this end, the electronic control unit 56 regulates the limited flow rate "Q" of forming fluid in the pre-blowing line 20 by adjusting 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 "S" passage section as a function of the flow setpoint value "Cq".

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

[0087] Alternatively, the setpoint value is a constant determined experimentally based on the hollow body 12 processed by the blowing station 10. When the pressure "Pf1" 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 "E1" while the blowing station 10 is still in production. This ensures that the cross-sectional area "S" remains constant during each pre-blowing stage "E1". In particular, this allows for precise control of the limited flow rate "Q" of forming fluid during the pre-blowing stage "E1".

[0089] Furthermore, due to the response time of the flow limiter 26, the operation to modify the "S" section of the passage is likely to take longer than the "E1" pre-blowing step itself. However, the time between two successive "E1" pre-blowing steps during production is greater than or equal to this response time.

[0090] The pre-blowing pressure value “Pf1” 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 “Cpf1” automatically determined by the electronic control unit 56.

[0091] Alternatively, the current value of the pre-blowing pressure "Pf1" is measured, for example, using 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 "Pf1".

[0093] In a non-exhaustive manner, at least one specific 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 "E1" pre-blowing step;

[0095] The duration "Tps" of the "E1" pre-blowing step;

[0096] A representative value of the pressure “Pb” in the hollow body 12 at the end of the “E1” pre-blowing stage.

[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 "E1". This value is determined, for example, based on the pressure "Pb" prevailing inside the hollow body 12 at the end of the pre-blowing step "E1" and based on the quantity of forming fluid injected into the hollow body 12 during the pre-blowing step "E1".

[0098] A specific parameter, chosen from among at least one other parameter, can also be a representative value of the duration "Tps" of the pre-blowing step E1. The representative value of the duration of the pre-blowing step E1 is, for example, the preform stretching time applied by the electronic control unit 56. This is either a default parameter or a parameter regularly updated 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 E1.

[0100] The representative value of the pressure "Pb" in the hollow body 12 at the end of the pre-blowing step E1 is, for example, an average of the pressure values ​​measured by a pressure sensor mounted on the nozzle 14 during several previous manufacturing cycles. The average is, for example, a moving average calculated over a specified number of previous manufacturing cycles.

[0101] For example, the "S" section of the passage is controlled according to the value obtained by the following function g(P'1,P'b). [Math2]

[0102]

[0103] in which: b is a characteristic parameter of the pre-blowing pipe 20; P'1 is a representative value of the pre-blowing pressure (Pf1); P'b is the representative value of the pressure "Pb" in the hollow body 12 at the end of the pre-blowing stage E1.

[0104] 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]

[0105]

[0106] where: k0, k1, k2 and k3 are constants

[0107] V' is a representative value of the volume "Vp" of the hollow body 12 at the end of the pre-blowing step "E1".

[0108] P'b is a representative value of the pressure in the hollow body 12 at the end of the "E1" pre-blowing stage.

[0109] P'1 is a representative value of the pre-blowing pressure

[0110] T'p1 is a representative value of the duration of the "E1" pre-blowing stage.

[0111] 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.

[0112] During this blowing step E2, indicated in the figure, 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 Pf1.

[0113] At the end of the "E1" 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.

[0114] Once the pressure in the hollow body 12 reaches approximately the pre-blowing pressure "Pf1", the

[0115] In an unrepresented variant of the invention, at the end of the "E1" 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 blow-molding step E2, the hollow body 12, in its final container state, is held at the aforementioned blow-molding pressure Pf2 during a holding step E3. Maintaining the final blow-molding pressure Pf2 allows the hollow body 12 to conform to all the shape details imposed by the molding cavity 13, thus being 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 released by opening the recovery valve 34, while all other valves are closed. The forming fluid is then 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 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 the last 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 Pf1 is advantageously determined by the electronic control unit 56 so that it falls within a range of values ​​that allows for a final hollow body 12 of good quality. However, the pre-blowing pressure Pf1 must also have a value that allows for optimal filling of the storage tank 28.

[0128] Indeed, if the pre-blowing pressure Pf1 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 necessary pre-blowing pressure Pf1. In this case, the pre-blowing step E1 must be carried out, at least partially, 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] Conversely, by allowing the electronic control unit 56 to determine the pre-blowing pressure value Pf1, it can select the highest value that also maintains sufficient pressure in the storage tank 28. This ensures that the pre-blowing stage E1 is carried out with the maximum amount of recycled forming fluid.

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

Claims

A method for controlling a manufacturing installation for hollow bodies in the container state, made of thermoplastic material, particularly 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 sealed with said hollow body (12) enclosed in the molding cavity (13); - at least one stretching rod (16) which is slidably controlled within 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 (Pf1) whose value is automatically controlled by the electronic control unit (56); - at least one conduit (20) pre-blowing 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 the forming fluid at the pre-blowing pressure (Pf1); characterized in that, during a pre-blowing step (E1) which begins when the pre-blowing line (20) supplies the nozzle (14) with forming fluid at the pre-blowing pressure (Pf1) and which ends at the end of the drawing of the hollow body (12) by the drawing rod (16), the cross-section (S) of the forming fluid at the pre-blowing pressure (Pf1) is automatically controlled according to a value representative of the current pre-blowing pressure (Pf1) so that the forming fluid flows through said cross-section with a flow rate (Q) conforming to a setpoint value (Cq). Method according to the preceding claim, characterized in that the setpoint value (Cq) of the flow rate is a constant. Method according to claim 1, characterized in that the setpoint value (Cq) of the flow rate is controlled by an operator. 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 (E1). Method according to the preceding claim, characterized in that the current value of the pre-blowing pressure (Pf1) is measured on a previous manufacturing cycle. Method according to any one of claims 1 to 4, characterized in that the actual value of the pre-blowing pressure (Pf1) corresponds to a setpoint value (Cpf1) determined automatically by the electronic control unit (56). 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. Method according to the preceding claim, characterized in that a determined parameter among the at least one determined parameter is formed by a representative value of the volume (Vp) of the hollow body (12) at the end of the pre-blowing step (E1). 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. 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 (E1). 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'1,P'b) according to the following relation: in which: b is a characteristic parameter of the pipe; P'1 is a representative value of the pre-blowing pressure (Pf1); P'b is a representative value of the pressure (Pb) in the hollow body at the end of the pre-blowing step (E1). 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 (E1) is an average of pressure value measured by a pressure sensor mounted on the nozzle (14), during previous manufacturing cycles. 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 (Pf1) fluctuating according to the quantity and pressure of the recovered forming fluid. Method according to the preceding claim, characterized in that the value of the pre-blowing pressure (Pf1) is automatically controlled by the electronic control unit (56) in order to maximize the amount of forming fluid recovered. An installation for manufacturing hollow bodies in the form of thermoplastic containers, particularly 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 sealed with said hollow body (12) enclosed in the molding cavity (13); - at least one stretching rod (16) which is slidably controlled within 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 (Pf1) 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); characterized in that it comprises 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 the forming fluid passage at the pre-blowing pressure (Pf1).

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