Method for manufacturing hollow bodies with recirculation of the forming fluid

Abstract

The invention relates to a method for manufacturing hollow bodies (12) by stretch-blow moulding, the method comprising: - a pre-blowing phase (S1) for pre-blowing the hollow body (12) by injecting forming fluid at a supply pressure (P1); - an intermediate blowing phase (S2) for blowing the hollow body (12) at a second intermediate blowing pressure (Pf2) controlled so as to maintain equilibrium with a storage pressure (R1) of a first tank (22); - a first phase (S5) of recovering the forming fluid; - a second phase (S6) of recovering the forming fluid via a second storage tank (24) having a storage pressure (R2); characterised in that, when the difference between the second storage pressure (R2) and the first supply pressure (P1) becomes lower than a threshold (Th1), the second blowing pressure (Pf2) is lowered below its equilibrium value.

Description

Description Title of the invention: METHOD FOR MANUFACTURING HOLLOW BODIES WITH RECYCLING OF THE FORMING FLUID Technical field of the invention

[0001] The invention relates to a method for manufacturing hollow bodies made of thermoplastic material in the state of final containers by stretch-blow molding of the hollow body in the preform state comprising a succession of manufacturing cycles during each of which a new hollow body is manufactured, each manufacturing cycle comprising successive phases of increasing pressurization of the hollow body and successive phases of exhausting the forming fluid, a portion of the forming fluid being recycled via storage tanks. Technical background

[0002] To manufacture a container by stretch blow molding, a preform or intermediate container made of thermoplastic material, such as PET, is heated to a temperature above the glass transition temperature of the material and then inserted into a molding unit. A forming fluid, such as air, under high pressure, for example of the order of 40 bars, is injected into the preform, the wall of which expands until it is pressed against an impression of the molding unit to be shaped into the final container. Then, the pressure in the container is released to open the molding unit and evacuate the container to atmospheric pressure.

[0003] When it begins to deform, the preform becomes an “intermediate container” before reaching its final shape as a “final container”.

[0004] In the remainder of the description and in the claims, the term "hollow body" will be used to designate indifferently a preform, an intermediate container or a finished container.

[0005] The invention relates in particular to methods where the pressure build-up is carried out in stages by connecting the hollow body during manufacture, first to a first forming fluid reservoir at an intermediate pressure, then to a second forming fluid reservoir at a higher pressure, and so on. This makes it possible to use cheaper forming fluid sources at intermediate pressures rather than exclusively using a forming fluid source at maximum pressure.

[0006] In addition, the pressure drop can be done in stages in said tanks in order to be brought to atmospheric pressure. This allows the forming fluid of the final container compressed to the high pressure to empty successively into intermediate pressure tanks and allows part of the energy to be recovered.

[0007] The disadvantage of pressure-step processes is that the pressure rise and fall in the hollow body are slower. For the same molding rate, this reduces the time required to press the final container against the mold walls at high pressure. This makes it more difficult to obtain small shape details in the final container.

[0008] As a result, to reduce this negative impact it is necessary to optimize the settings of the process and each of its stages, which significantly complicates the task of the production operator compared to a conventional process.

[0009] Furthermore, during production, or when the type of product changes, physical balances can change depending on ambient parameters, or parameters linked to the hollow body in the preform state, or in the final container state, or even depending on recovery pressures, which implies a readjustment of the process to remain optimal.

[0010] The technician is generally required to observe a pressure curve of an article being formed, its various successive phases of pressure increase, its various successive phases of depressurization, and to make an interpretation allowing him to shorten or lengthen each of the phases in time without compromising industrial efficiency. He will also have to check on the other forming units of the machine that his intervention is compatible with any possible dispersions.

[0011] Document EP-2.911.860-B1 has already proposed improving such a process by automating the supply of intermediate pressure forming fluid as well as the recovery of the compressed forming fluid.

[0012] This document proposes in particular that during each phase of pressure build-up by injection of forming fluid into the hollow body from a reservoir, said phase lasts until the pressure in the hollow body is equal to the pressure of the reservoir minus a determined constant, for example 0.5 bar. This makes it possible in particular to guarantee a rapid pressure build-up phase.

[0013] Similarly, during a phase of recovery of compressed forming fluid in the hollow body towards a reservoir, said phase lasts until the pressure in the hollow body is equal to the reservoir pressure increased by a predetermined constant, for example 0.5 bar. This makes it possible in particular to guarantee a rapid pressure increase phase.

[0014] It has been found that such a process can ensure a very high recycling rate of forming fluid. The recycling rate is determined as the ratio of the amount of forming fluid recovered to the amount of forming fluid used during an entire manufacturing cycle.

[0015] Such a process is described in more detail in paragraphs

[0036] has

[0043] of aforementioned document EP-2.911.860-B 1.

[0016] However, in certain circumstances, the recycling rate has been found to drop considerably.

[0017] Indeed, during a first pressure build-up phase, called the pre-blowing phase, the deformation of the preform is initiated by pressure build-up up to a determined pre-blowing pressure. To achieve good distribution of the material constituting the walls of the hollow body, it is known to inject the forming fluid at a feed pressure higher than the pre-blowing pressure. To do this, the forming fluid contained in the associated reservoir must have a pressure higher than said feed pressure. Then this compressed forming fluid is expanded to the injection pressure before being injected into the preform. Collaterally, the feed pressure also influences the duration of the pre-blowing phase.

[0018] However, when the feed pressure has a very high value, the recovery phase of the forming fluid in the reservoir stops prematurely before enough forming fluid can be recovered to guarantee a high recycling rate. Summary of the invention

[0019] The invention proposes a method for manufacturing hollow bodies made of thermoplastic material in the state of final containers by stretch-blow molding of the hollow body in the preform state comprising a succession of manufacturing cycles during each of which a new hollow body is manufactured, each manufacturing cycle comprising at least the following successive phases of increasing pressure on the hollow body:

[0020] - a phase of pre-blowing the hollow body in the preform state at a first pre-blowing pressure by connection to a source of forming fluid at a first pre-blowing supply pressure;

[0021] - at least one intermediate blowing phase of the hollow body at a second fluctuating intermediate blowing pressure by connection to a first forming fluid storage tank at a first fluctuating storage pressure;

[0022] - a final blowing phase of the hollow body at a third final blowing pressure by connection to a source of compressed forming fluid at a pressure greater than or equal to said final blowing pressure;

[0023] each cycle then comprising the following successive forming fluid exhaust phases:

[0024] - a first phase of recovery of the forming fluid by connecting the hollow body to the first storage tank until the forming fluid present in the hollow body presents a first fluctuating recovery pressure;

[0025] - at least a second phase of recovery of the forming fluid by connecting the hollow body to a second recovery tank until the forming fluid present in the hollow body has a second fluctuating recovery pressure;

[0026] the second intermediate blowing pressure and the first recovery pressure each being controlled to an equilibrium value with the first storage pressure,

[0027] the second recovery pressure being controlled at an equilibrium value with a second fluctuating fluid storage pressure in the second recovery tank,

[0028] during the pre-blowing phase, the hollow body in the preform state being supplied with the forming fluid stored in the second storage tank expanded from the second storage pressure to the supply pressure,

[0029] characterized in that, when during operation a value representative of the difference between the second storage pressure and the first supply pressure becomes lower than a predetermined threshold, the second blowing pressure is lowered to a target value below its equilibrium value during the following cycles.

[0030] According to another aspect of the method carried out according to the teachings of the invention, the equilibrium value of the second intermediate blowing pressure is equal to the first storage pressure reduced by a first determined constant, for example 0.5.

[0031] According to another aspect of the method carried out according to the teachings of the invention, the equilibrium value of the first recovery pressure is equal to the first storage pressure increased by a second determined constant, for example 0.5 bar.

[0032] According to another aspect of the method carried out according to the teachings of the invention, the equilibrium value of the second recovery pressure is equal to the second storage pressure increased by a third determined constant, for example 0.5 bar.

[0033] According to another aspect of the method carried out according to the teachings of the invention, when during operation, the value representative of the difference between the second storage pressure and the first supply pressure becomes greater than or equal to the first predetermined threshold, the second blowing pressure is controlled towards its equilibrium value during the following cycles.

[0034] According to another aspect of the method carried out according to the teachings of the invention, the value representative of the difference between the second storage pressure and the first supply pressure is calculated from the differences between the second storage pressure and the first supply pressure calculated on the previous cycle.

[0035] According to another aspect of the method carried out according to the teachings of the invention, the value representative of the difference between the second storage pressure and the first supply pressure is equal to the average of the differences between the second storage pressure and the first supply pressure calculated over all the cycles since the start of production.

[0036] According to another aspect of the method carried out according to the teachings of the invention, the first predetermined threshold is equal to 1 bar.

[0037] According to another aspect of the method carried out according to the teachings of the invention, the second intermediate blowing pressure has a floor value below which it cannot be controlled.

[0038] According to another aspect of the method carried out according to the teachings of the invention, the second blowing pressure is controlled by controlling the opening time of a valve connecting the hollow body with the first storage tank.

[0039] According to another aspect of the method carried out according to the teachings of the invention, when during operation, the value representative of the difference between the second storage pressure and the first supply pressure becomes less than or equal to a third predetermined threshold lower than the first predetermined threshold, the second recovery phase is carried out chronologically before the first recovery phase.

[0040] According to another aspect of the method carried out according to the teachings of the invention, when during operation, the value representative of the difference between the second storage pressure and the first supply pressure becomes greater than or equal to a fourth predetermined threshold greater than the third predetermined threshold, the second blowing pressure is controlled towards its target value during the following cycles. Brief description of the figures

[0041] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the attached drawings.

[0042] [Fig. 1] is an axial sectional view which schematically represents a unit for molding a hollow body capable of implementing the method according to the teachings of the invention.

[0043] [Fig.2] is a diagram which represents the evolution of the forming fluid pressure in the hollow body as a function of time in the molding unit of [Fig.l] during the implementation of the method carried out according to the teachings of the invention.

[0044] [Fig.3] is a block diagram which represents an example of implementation of the method according to the teachings of the invention. Detailed description of the invention

[0045] In the remainder of the description, similar or identical elements will be designated by the same references.

[0046] As illustrated in [Fig. 1], a molding unit 10 for stretch-blow molding a hollow body 12, initially in the state of a thermoplastic preform, comprises a mold forming a two-part molding cavity 14 which can move apart to release the hollow body 12 in the final container state. The molding unit 10 further comprises a blowing nozzle 16 equipped with a vertically movable elongation rod 18.

[0047] The blowing nozzle 16 is connected to:

[0048] - a first reservoir 22 for storing forming fluid at a first pressure Fluctuating storage RI,

[0049] - a second reservoir 24 for storing forming fluid at a second fluctuating storage pressure R2;

[0050] - a source 26 of forming fluid at maximum pressure R3, for example approximately 40 bars.

[0051] For the remainder of the description and in the claims, the term “determined pressure” means that the pressure is controlled to tend towards a fixed set pressure, while the term “fluctuating pressure” means that the pressure is capable of varying freely during the process depending on various parameters as will be explained later.

[0052] The first storage tank 22 is connected to the nozzle 16 via a supply line in which a first valve 20A is interposed. The nozzle 16 is also connected to the first storage tank 22 via a recovery line in which a second valve 21A is interposed.

[0053] The second storage tank 24 is connected to the nozzle 16 via a supply line in which a third valve 20B is interposed. The nozzle 16 is also connected to the second storage tank 24 via a recovery line in which a fourth valve 21B is interposed.

[0054] In a variant not shown, the first storage tank 22 can be connected to the nozzle 16 by a single pipe through which the forming fluid can circulate in one direction or the other depending on the pressure prevailing in the hollow body. In this case, a single valve is interposed in the single pipe. This valve alternately plays the role of the first valve 20A and the second valve 21A.

[0055] Furthermore, at least one pressure control member 27 is interposed in the supply line of the second storage tank 24, here upstream of the third valve 20B. The pressure control member 27 is controlled so that the pressure and the flow rate of forming fluid supplying the nozzle 16 via the third valve 20B can vary.

[0056] Said pressure control member 27 is here formed by a controlled restriction of passage of the forming fluid.

[0057] In a variant of the invention not shown, the control member 27 also comprises a pressure reducer which helps to bring the forming fluid to the supply pressure.

[0058] Generally, the molding unit 10 is arranged in a forming station (not shown) comprising several identical molding units 10. In this case, the storage tanks 22, 24 may be common to several molding units 10.

[0059] The maximum pressure blowing source 26 is connected to the nozzle 16 via a supply line in which a fifth valve 20C is interposed.

[0060] The forming fluid here is formed by air.

[0061] The maximum pressure is, for example, around 40 bars.

[0062] The blowing nozzle 16 is also connected to atmospheric pressure via an exhaust pipe equipped with a silencer 28. A sixth valve 21C is interposed in the exhaust pipe.

[0063] A first pressure sensor 30 is arranged in the nozzle 16 so as to measure the instantaneous pressure prevailing in the hollow body 12.

[0064] A second pressure sensor 32 is arranged in the first reservoir 22 to measure the instantaneous pressure of the forming fluid it contains.

[0065] A third pressure sensor 34 is arranged in the second reservoir 24 to measure the instantaneous pressure of the forming fluid it contains.

[0066] The hollow body 12 in the preform state comprises a neck 36 and a body 38 which is preheated before being introduced into the cavity 14. During stretch-blow molding, the cavity 14 is closed around the body 38, the blow nozzle 16 couples to the neck 36 and the sensor 30 can then measure the pressure in the body 38.

[0067] An electronic control unit 40 makes it possible to control all of the valves 20A, 20B, 20C, 21A, 21B, 21C. The electronic control unit 40 is here in communication with the pressure sensors 30, 32, 34 so that the latter can transmit the result of the pressure measurements to it. Subsequently, the different forming fluid pressures inside the hollow body 12 are controlled by controlling the opening time of the valves 20A, 20B, 20C, 21A, 21B, 21C.

[0068] The electronic control unit 40 also makes it possible to control the organ 27 of pressure control to control the supply pressure.

[0069] When implementing the manufacturing method, the molding unit 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 molding unit 10 is in operation, the storage pressure RI, R2 in the storage tanks 22, 24 is between atmospheric pressure and the maximum pressure R3. The storage pressure RI in the first storage tank 22 is higher than the storage pressure R2 in the second storage tank 24. The method operates in a so-called "normal" mode, as illustrated in [Fig. 3].

[0071] In addition, the setpoints for the supply pressure PI, the pre-blowing pressure Pfl and the final blowing pressure Pf3 are given to the electronic control unit 40 prior to the start of the process, as indicated by box “E” of the block diagram in [Fig.3].

[0072] Each manufacturing cycle comprising successive phases of increasing pressure on the hollow body 12.

[0073] During a first pre-blowing phase SI of the hollow body 12, between times t0 and tl in the diagram shown in [Fig. 2], the hollow body 12, in the preform state, is pressurized to a first pre-blowing pressure Pfl. The pressurization is carried out by opening the third valve 20B, while all the other valves are closed. The forming fluid is thus injected at the first supply pressure PI by the second storage tank 24 via the pressure control member 27. Thus, during this first pre-blowing phase SI, the storage pressure R2 in the second storage tank 24 gradually decreases.

[0074] The third valve 20B is controlled by the electronic control unit 40. In general, the opening time of the third valve 20B is determined by calculations or by tests depending on the geometry of the hollow body in the preform state in order to be able to reach the pre-blowing pressure Pfl.

[0075] The pre-blowing pressure Pfl is generally a pressure determined according to the geometry of the hollow body 12 in the preform state, either by calculations or by laboratory tests. The pre-blowing pressure Pfl setpoint can possibly be adjusted manually by an operator.

[0076] During this first pre-blowing phase SI, the preheated body 38 begins to expand and the pressure in the body 38 increases under the effect of the injection of the pressurized forming fluid. During this first phase SI, the elongation rod 18 descends into the body 38 to stretch it axially. This thins the wall of the body 38 and rapidly increases the internal volume of the body 38 until the body has the length of the cavity 14.

[0077] The first feed pressure PI is a determined pressure. It can be either predetermined according to the geometry of the hollow body 12 and the time required for the duration of the pre-blowing phase SI. This feed pressure PI is generally determined to obtain a good distribution of the material constituting the walls of the hollow body during the pre-blowing phase in order to obtain a final container of good quality.

[0078] The higher the feed PI pressure, the shorter the pre-blowing SI phase will be. This feed PI pressure can optionally be adjusted manually by an operator, for example if the operator wishes to shorten the duration of each manufacturing cycle.

[0079] Then comes a global blowing step which makes it possible to transform the hollow body 12 into the final container. During this step, the body 38 continues to expand until it is pressed against the wall of the cavity 14. This blowing step comprises two following phases S2, S3.

[0080] The pre-blowing phase S1 is followed by at least one phase S2 of intermediate blowing of the hollow body 12, in the intermediate container state, between times t1 and t2. During this phase, the pressure in the hollow body 12 is increased from the first pre-blowing pressure Pfl to a second fluctuating intermediate blowing pressure Pf2. The second fluctuating intermediate blowing pressure Pf2 is thus greater than the pre-blowing pressure Pfl. Pressurization is carried out by opening the first valve 20A, while all the other valves are closed. The hollow body 12 is thus supplied with forming fluid by the first forming fluid storage tank 22 at the first fluctuating storage pressure RI.

[0081] The second fluctuating intermediate blow-off pressure Pf2 is automatically controlled to an equilibrium value "VI" with the first fluctuating storage pressure RI. The fluctuating storage pressure RI decreases during the first pre-blowing phase SI until the fluctuating pressure Pf2 reaches its equilibrium value "VI".

[0082] Since the second intermediate blowing pressure Pf2 fluctuates from one cycle to the next, the duration of the intermediate blowing phase S2 also fluctuates. Thus, the instant t2 is likely to vary depending on the equilibrium value “VI” of the intermediate blowing pressure Pf2.

[0083] The equilibrium value “VI” of the second fluctuating intermediate blowing pressure Pf2 is equal to the first storage pressure RI reduced by a constant Kl determined, according to the following equation:

[0084] Pf2 = RI - Kl

[0085] The opening of the first valve 20A is thus controlled by the electronic control unit 40 as a function of the pressure measured by the pressure sensor 32 associated with the first storage tank 22 and as a function of the pressure measured by the pressure sensor 30 associated with the nozzle 16. The pressure is monitored by said pressure sensors 30, 32 throughout the pre-blowing phase SI to allow the drop in the first storage pressure RI to be taken into account.

[0086] The determined constant Kl is preferably strictly greater than 0 bar and is preferably less than or equal to 1 bar.

[0087] Indeed, if the constant Kl were equal to 0, the duration of the intermediate blowing phase S2 would risk being too long. Furthermore, a constant Kl of zero or negative value risks causing resonance phenomena and undesirable variation of the pressure in the hollow body 12.

[0088] Similarly, if the constant Kl were greater than 1 bar, the recycling rate of the forming fluid would be negatively affected.

[0089] The constant Kl is for example equal to 0.5 bar.

[0090] Furthermore, to ensure proper operation of the molding unit 10, the second intermediate blowing pressure Pf2 has a floor value below which it cannot be controlled. This floor value corresponds to a minimum opening time of the first valve 20A, for example 40 ms. This minimum time corresponds to the minimum response time of the first valve 20A.

[0091] The blowing phase S2 is followed by a final blowing phase S3 of the hollow body 12, in the intermediate container state, between times t1 and t3. During this phase, the pressure in the hollow body 12 increases from the second intermediate blowing pressure Pf2 to a third final blowing pressure Pf3. The third final blowing pressure Pf3 is thus greater than the intermediate blowing pressure Pf2. Pressurization is carried out by opening the fifth valve 20C, while all the other valves are closed. The hollow body 12 is thus supplied with forming fluid by the source of forming fluid compressed to the maximum pressure R3. The fifth valve 20C is controlled by the electronic control unit 40 in particular as a function of the pressure measured by the pressure sensor 30 of the nozzle 16.

[0092] The maximum pressure R3 is a determined pressure which is greater than or equal to said final blowing pressure Pf3.

[0093] The final blowing pressure Pf3 is also a pressure determined in particular as a function of the geometry of the hollow body 12 and as a function of the geometry of the final container, either by calculations or by laboratory tests. The pressure setpoint Pf3 final blowing can optionally be adjusted manually by an operator.

[0094] At the end of this final blowing phase S3, the hollow body 12 in the final container state is maintained at said final blowing pressure Pf3 between times t3 and t4 during a holding phase S4.

[0095] Each cycle then comprises successive phases of exhausting the forming fluid until the hollow body 12 in the final container state is at atmospheric pressure. Maintaining the final blowing pressure Pf3 allows the container thus produced to fit all the shape details imposed by the cavity 14.

[0096] Although the container is finished, the mold cannot be opened directly to retrieve the container because the air in it is still at 40 bars. The pressure in the container must be lowered before opening the mold.

[0097] Thus, the holding phase S4 is followed by a first phase S5 of recovery of the forming fluid between times t4 and t5. During this first recovery phase S5, the pressure in the hollow body 12 decreases from the final blowing pressure Pf3 to a first fluctuating recovery pressure Prl. The first fluctuating recovery pressure Prl is thus lower than the final blowing pressure Pf3. The forming fluid is released by opening the second valve 21A, while all the other valves are closed. The forming fluid is thus discharged to the first storage tank 22.

[0098] The first recovery pressure Prl is automatically controlled to an equilibrium value with the first storage pressure RI. The storage pressure RI increases during the first recovery phase S5 until the recovery pressure Prl reaches its equilibrium value.

[0099] Since the first recovery pressure Prl fluctuates from one cycle to another, the duration of the first recovery phase S5 also fluctuates. Thus, the instant t5 is likely to vary depending on the equilibrium value of the first recovery pressure Prl.

[0100] The opening of the second valve 21 A is thus controlled by the electronic control unit 40 as a function of the pressure measured by the pressure sensor 32 associated with the first storage tank 22 and as a function of the pressure measured by the pressure sensor 30 associated with the nozzle 16. The pressure is monitored by said pressure sensors 30, 32 throughout the first recovery phase S5 to take into account the increase in the first storage pressure RI.

[0101] The equilibrium value of the first fluctuating recovery pressure Prl is equal to the first storage pressure RI increased by a constant K2 determined, according to the following equation:

[0102] Prl = RI + K2

[0103] The determined constant K2 is preferably strictly greater than 0 bar and it is preferably less than or equal to 1 bar.

[0104] Indeed, if the constant K2 were equal to 0, the duration of the first recovery phase S5 would risk being too long. In addition, a constant K2 of zero or negative value risks causing resonance phenomena and undesirable variation of the pressure in the hollow body 12.

[0105] Similarly, if the K2 constant were greater than 1 bar, the forming fluid recycling rate would be negatively affected.

[0106] The constant K2 is for example equal to 0.5 bar.

[0107] Furthermore, to ensure proper operation of the molding unit 10, the first intermediate blowing pressure Prl has a ceiling value above which it cannot be controlled. This ceiling value corresponds to a minimum opening time of the second valve 21A, for example 40 ms. This minimum time corresponds to the minimum response time of the second valve 21A.

[0108] The first recovery phase S5 is followed by at least a second recovery phase S6 of the forming fluid between times t5 and t6. During this second recovery phase S6, the pressure in the hollow body 12 decreases from the first recovery pressure Prl to a second fluctuating recovery pressure Pr2. The second fluctuating recovery pressure Pr2 is thus lower than the first recovery pressure Prl. The forming fluid is released by opening the fourth valve 21B, while all the other valves are closed. The forming fluid is thus discharged to the second storage tank 24.

[0109] The second recovery pressure Pr2 is automatically controlled to an equilibrium value with the fluctuating second pressure R2 for storing the fluid in the second recovery tank 24. The second storage pressure R2 increases during the second recovery phase S6 until the second recovery pressure Pr2 reaches its equilibrium value.

[0110] Since the first recovery pressure Prl and the second recovery pressure Pr2 fluctuate from one cycle to another, the duration of the second recovery phase S6 also fluctuates. Thus, the instants t5 and t6 are likely to vary depending on the equilibrium value of the first recovery pressure Prl and depending on the equilibrium value of the second recovery pressure Pr2.

[0111] The opening of the fourth valve 21B is thus controlled by the electronic control unit 40 as a function of the pressure measured by the pressure sensor 34 associated with the second storage tank 24 and as a function of the pressure measured by the pressure sensor 30 associated with the nozzle 16. The pressure is monitored by said pressure sensors 30, 34 throughout the second recovery phase S6 to take into account the increase in the second storage pressure R2.

[0112] The equilibrium value of the second recovery pressure Pr2 is equal to the second storage pressure R2 increased by a constant K3 determined according to the following equation:

[0113] Pr2 = R2 + K3

[0114] The determined constant K3 is preferably strictly greater than 0 bar and is preferably less than or equal to 1 bar.

[0115] Indeed, if the constant K3 were equal to 0, the duration of the second recovery phase S6 would risk being too long. In addition, a constant K3 of too low a value risks causing resonance phenomena and undesirable variation of the pressure in the hollow body 12.

[0116] Similarly, if the K3 constant were greater than 1 bar, the recycling rate of the forming fluid would be negatively affected.

[0117] The constant K3 is for example equal to 0.5 bar.

[0118] Furthermore, the second recovery pressure Pr2 depends on a minimum opening time of the fourth valve 21B, for example 40 ms. This minimum time corresponds to the minimum response time of the fourth valve 21B.

[0119] Finally, the second recovery phase S6 is followed by a final phase S7 of exhausting the forming fluid between times t6 and t7. During this final exhaust phase S7, the pressure in the hollow body 12 decreases from the second recovery pressure Pr2 to atmospheric pressure. The second fluctuating recovery pressure Pr2 is thus greater than atmospheric pressure. The exhaust of the forming fluid is carried out by opening the sixth valve 21C, while all the other valves are closed. The forming fluid is thus discharged to the atmosphere via the silencer 28.

[0120] Finally, at time t7, determined by the speed of the stretch-blow molding machine, the mold opens. That is to say, the blow nozzle 16 rises and the cavity 14 moves apart to release the finished container.

[0121] The molding unit 10 comprises two storage tanks 22, 24 for recycling the forming fluid. It is understood that the molding unit may comprise more than two storage tanks. The blowing cycle will comprise successive intermediate blowing phases and just as many recovery phases.

[0122] In such a process, the storage pressures RI, R2 in the tanks 22, 24 fluctuate freely, which makes it possible to obtain a very efficient recycling rate of the forming fluid and an optimized cycle time.

[0123] However, it may happen that the supply pressure PI during the first pre-blowing phase S 1 is controlled to be very high. This may be a consequence of different parameters inherent to the container to be manufactured, such as the geometry of the hollow body in the preform state, the geometry of the hollow body in the final. The PI feed pressure setpoint can also be increased manually by an operator in order to shorten manufacturing cycle times to meet productivity constraints.

[0124] When the pressure PI becomes too high, it risks approaching the second storage pressure R2 in the second storage tank 24. As a result, during the first pre-blowing phase SI, the supply of forming fluid to the hollow body 12 by the second tank 24 via the pressure control member 27 risks being less efficient, slower, or even impossible. The molding unit 10 would then have to operate without recycling the forming fluid from the second tank 24, the hollow body 12 being supplied by a source of non-recycled compressed forming fluid. The forming fluid recycling rate would therefore be significantly reduced.

[0125] To avoid this situation and maintain an optimal recycling rate, the invention proposes that the process switches to a so-called “alternative” mode, illustrated by [Fig.3].

[0126] To do this, we calculate a criterion A which is equal to the difference between: - The smallest pressure among the first storage pressure RI and the second storage pressure R2, and - The first PI power pressure.

[0127] During normal operation, the second pressure R2 is the lowest.

[0128] During operation, the criterion A representing the difference between the second storage pressure R2 and the first supply pressure PI becomes lower than a first predetermined threshold “Thl”, the second blowing pressure Pf2 is lowered below its equilibrium value “VI” during the following cycles.

[0129] The first predetermined threshold “Thl” is preferably strictly greater than 0 and less than 4 bars. More preferably, it is between 0.5 bar and 1.5 bar to guarantee an optimal recycling rate. For example, it is equal to 1 bar.

[0130] Criterion A is calculated by the electronic control unit 40 from the measurement provided by the pressure sensor 34 associated with the second storage tank 24 and from the set value of the supply pressure PI.

[0131] Preferably, criterion A is directly constituted by the difference between the second storage pressure R2 and the supply pressure PI, according to the equation:

[0132] A = R2 - P1

[0133] When the criterion A thus calculated is lower than the first threshold Thl on a cycle, the second blowing pressure Pf2 is lowered to a new target value “V2” lower than its equilibrium value “VI” during the following cycle.

[0134] As a variant of the invention, criterion A represents the difference between the second storage pressure R2 and first supply pressure PI is calculated from the differences between the second storage pressure R2 and the first supply pressure PI calculated over several consecutive cycles. For example, it is equal to the average of the differences between the second storage pressure R2 and the first supply pressure PI calculated over all cycles since the start of production.

[0135] The new target value “V2” is, for example, the floor value of the intermediate blowing pressure Pf2.

[0136] Alternatively, the new target value “V2” is the equilibrium value “VI” from which a new constant K4 is subtracted according to the equation:

[0137] Pf2 = RI - Kl - K4

[0138] This new constant K4 is for example equal to 0.5 bar.

[0139] Alternatively, the constant K4 is calculated according to the following equation:

[0140] K4 = RI - Pfl - Thl - PI

[0141] By lowering the intermediate blowing pressure Pf2, the duration of the intermediate blowing phase S2 is reduced and less forming fluid is thus injected from the first reservoir RI to the hollow body 12. As a result, the first storage pressure RI in the first reservoir 22 decreases less during the intermediate blowing phase S2. As a result, the first recovery pressure Prl is higher, and the duration of the first recovery phase S5 is reduced. As a result, the second recovery phase S6 starts earlier, which makes it possible to recover a greater quantity of forming fluid in the second reservoir 24, the storage pressure R2 of which can increase more quickly so that the criterion A representative of the difference between the second storage pressure R2 and the first supply pressure PI can increase.

[0142] As long as criterion A does not become greater than or equal to said first predetermined threshold Thl, the process continues to operate with this new target value “V2” for the intermediate blowing pressure Pf2.

[0143] When during operation the criterion A becomes greater than or equal to the first predetermined threshold Thl, the second blowing pressure Pf2 is again commanded towards its equilibrium value “VI” during the following cycles.

[0144] In a variant of the invention not shown, the second blowing pressure Pf2 is again controlled towards its equilibrium value “VI” when the criterion A becomes greater than or equal to a second threshold “Th2” greater than the first predetermined threshold Thl, for example greater than or equal to 1.25 bar.

[0145] If, despite everything, criterion A continues to fall and becomes lower than a third predetermined threshold “Th3”, the method according to the invention proposes to operate in an “inverted” mode in which the first recovery phase S5 and the second recovery phase S6 are reversed chronologically. This solution allows the forming fluid to continue to be recycled in the two storage tanks 22, 24, thus limiting the drop in the recycling rate.

[0146] In this "reverse" mode, the first storage pressure RI becomes lower than the second storage pressure R2. In this case, criterion A is representative of the difference between the first storage pressure RI and the first supply pressure PI. It is directly constituted by the difference between the first storage pressure RI and the supply pressure PI, according to the equation:

[0147] A = RI - PI

[0148] In this "reverse" mode, the intermediate blowing phase S2 may not take place. In this case, the final blowing phase S3 directly follows the pre-blowing phase SI depending on the storage pressures RI and R2.

[0149] The third predetermined threshold “Th3” is lower than the first predetermined threshold “Thl” and is preferably strictly greater than 0. The third predetermined threshold Th3 is for example equal to 0.75 bar.

[0150] The second blowing pressure Pf2 is again commanded towards its target value “V2” when the criterion A representing the difference between the first storage pressure RI and the supply pressure PI becomes greater than or equal to a fourth threshold “Th4” greater than the third predetermined threshold “Th3”, for example greater than or equal to 1 bar.

[0151] When during operation the value A representative of the difference between the second storage pressure R2 and the first supply pressure PI becomes greater than or equal to the first predetermined threshold Thl, the second blowing pressure Pf2 is again commanded towards its equilibrium value “VI” during the following cycles.

[0152] The method of the invention thus allows an alternative operating mode making it possible to maintain a fairly high recycling rate even in the case of a supply pressure PI which approaches the second storage pressure R2.

Claims

Claims

1. Method for manufacturing hollow bodies (12) made of thermoplastic material in the state of final containers by stretch-blow molding of the hollow body (12) in the preform state comprising a succession of manufacturing cycles during each of which a new hollow body (12) is manufactured, each manufacturing cycle comprising at least the following successive phases of increasing pressure on the hollow body (12): - a phase (SI) of pre-blowing the hollow body (12) in the preform state at a first pre-blowing pressure (Pfl) by connection to a source of forming fluid at a first pre-blowing supply pressure (PI); - at least one phase (S2) of intermediate blowing of the hollow body (12) at a second fluctuating intermediate blowing pressure (Pf2) by connection to a first reservoir (22) for storing forming fluid at a first fluctuating storage pressure (RI); - a phase (S3) of final blowing of the hollow body at a third final blowing pressure (Pf3) by connection to a source (R3) of compressed forming fluid at a pressure greater than or equal to said final blowing pressure (Pf3); each cycle then comprising the following successive forming fluid exhaust phases: - a first phase (S 5) of recovering the forming fluid by connecting the hollow body (12) to the first storage tank (22) until the forming fluid present in the hollow body has a first fluctuating recovery pressure (Prl); - at least a second phase (S6) of recovering the forming fluid by connecting the hollow body (12) to a second recovery tank (24) until the forming fluid present in the hollow body (12) has a second fluctuating recovery pressure (Pr2); the second intermediate blowing pressure (Pf2) and the first recovery pressure (Prl) each being controlled to an equilibrium value with the first storage pressure (RI), the second recovery pressure (Pr2) being controlled to an equilibrium value with a second fluctuating storage pressure (R2) of the fluid in the second recovery tank (24), during the pre-blowing phase (SI), the hollow body (12) in the state of preform being supplied with the forming fluid stored in the second storage tank (24) expanded from the second storage pressure (R2) to the supply pressure (PI), characterized in that, when during operation a value (A) representative of the difference between the second storage pressure (R2) and the first supply pressure (PI) becomes lower than a predetermined threshold (Thl), the second blowing pressure (Pf2) is lowered to a target value (V2) below its equilibrium value (VI) during the following cycles.

2. Method according to the preceding claim, characterized in that the equilibrium value (VI) of the second intermediate blowing pressure (Pf2) is equal to the first storage pressure (RI) reduced by a first determined constant (Kl), for example 0.

5.

3. Method according to any one of the preceding claims, characterized in that the equilibrium value of the first recovery pressure (Prl) is equal to the first storage pressure (RI) increased by a second determined constant (K2), for example 0.5 bar.

4. Method according to any one of the preceding claims, characterized in that the equilibrium value of the second recovery pressure (Pr2) is equal to the second storage pressure (R2) increased by a third determined constant (K3), for example 0.5 bar.

5. Method according to any one of the preceding claims, characterized in that, during operation, the value (A) representative of the difference between the second storage pressure (R2) and the first supply pressure (PI) becomes greater than or equal to the first predetermined threshold (Thl), the second blowing pressure (Pf2) is controlled towards its equilibrium value (VI) during the following cycles.

6. Method according to any one of the preceding claims, characterized in that the value (A) representative of the difference between the second storage pressure (R2) and the first supply pressure (PI) is calculated from the differences between the second storage pressure (R2) and the first supply pressure (PI) calculated on the previous cycle.

7. Method according to any one of claims 1 to 5, characterized in that the value (A) representative of the difference between the second storage pressure (R2) and the first supply pressure (PI) is equal to the average of the differences between the second storage pressure (R2) and the first supply pressure (PI) calculated over all cycles since the start of production.

8. Method according to any one of the preceding claims, characterized in that the first predetermined threshold (Thl) is equal to 1 bar.

9. Method according to any one of the preceding claims, characterized in that the second intermediate blowing pressure (Pf2) has a floor value below which it cannot be controlled.

10. Method according to any one of the preceding claims, characterized in that the second blowing pressure (Pf2) is controlled by controlling the opening time of a valve (20A) connecting the hollow body (12) with the first storage tank (22).

11. Method according to any one of the preceding claims, characterized in that when during operation, the value (A) representative of the difference between the second storage pressure (R2) and the first supply pressure (PI) becomes less than or equal to a third predetermined threshold (Th3) lower than the first predetermined threshold (Th1), the second recovery phase (S6) is carried out chronologically before the first recovery phase (S5).

12. Method according to the preceding claim, characterized in that, during operation, the value (A) representative of the difference between the second storage pressure (R2) and the first supply pressure (PI) becomes greater than or equal to a fourth predetermined threshold (Th4) greater than the third predetermined threshold (Th3), the second blowing pressure (Pf2) is controlled towards its target value (V2) during the following cycles.

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