Method for stretch-blow moulding a container, including measuring the movement of the stretch rod during a boxing operation

Disclosed is a method for manufacturing a container (2) from a blank (3) made of plastic material, in a stretch-blow molding unit (7) including: a mold (11) provided with a wall (13) and a mold base (14) which is axially movable relative to the wall (13) between a low position and a raised position; a stretch rod (22) which is axially movable relative to the mold (11) between a high position and a low position; the method including: a phase of inserting the blank (3) in the mold (11); a phase of stretch-blow molding; a phase of boxing; a phase of determining the start and end of the boxing phase, respectively, by detecting the movement of the rod (22) from the low position thereof and the stopping of the rod (22) at the raised position thereof.

The invention relates to producing containers by stretch-blow-molding from blanks of plastic material such as polyethylene terephthalate (PET).

Whether it involves a preform or an intermediate container that has already undergone a preforming operation, a blank comprises a body, generally rotationally symmetrical, a neck, which constitutes the rim of the container being formed, and a base that closes the body opposite the neck.

The standard production technique consists in introducing the blank, previously heated to a temperature that is higher than the glass transition temperature of the material (about 80° C. in the case of PET), in a mold provided with a wall that defines a cavity having the shape of the container, and in injecting into the blank, through the neck, a fluid, such as a gas (generally air), under pressure to press the material against the wall of the mold.

Under the effect of the pressure, the material that has been softened by heat forms a bubble that swells and expands both in an axial direction that is parallel to the main axis of the mold and in a radial direction that is perpendicular to the axis of the mold.

To avoid any off-centering of the container and to ensure a good equidistribution of the material, the axial stretching of the blank is forced by means of a rod that can be moved axially in the mold, this rod comprising a distal end that pushes the base of the blank until pressing it against a mold base having the shape of the base of the container.

Certain applications require the use of containers known as HR (Heat Resistant) that have the feature of being able to be hot-filled (i.e., with contents, typically a tea or a pasteurized fruit juice, at a temperature that is higher than or equal to about 90° C.) without major deformation, and in particular without making the side wall (or body) of the container oval.

It is known to provide the body of the container with deformable panels intended to absorb the variations in volume that accompany the retraction of the internal volume during cooling. This technique is widely used but is not without drawbacks. Actually, the flexibility of the body is considered to be a quality defect by some users who prefer that the body of the container not become deformed when gripped.

For this reason, it has been proposed to stiffen the body while providing the base of the container with a flexible membrane able to be deformed both during filling (because of the hydrostatic pressure and the temperature of the contents) and during cooling of the contents (accompanied by a retraction of it). Such a technique is shown by the European patent EP 2 173 637 and its U.S. equivalent US 2010/219152 (SIDEL).

The stiffness of the side wall (or body) of the container is generally obtained by means of heat, consisting in keeping the body in contact with the wall of the mold that is heated to a predetermined temperature (generally greater than 100° C.). This operation, known as heat-setting, leads to an increase of the crystallinity of the material, which causes an increase in its stiffness.

For the formation of the base, use is generally made of the technique known as “boxing.” In this technique, normally use is made of a molding unit equipped with a mobile mold base that is initially retracted and deployed during forming to push the material in the area of the base, which ensures an over-stretching of the material and thus will allow the formation of complex shapes, particularly deep arches. To illustrate this technique, reference can be made to the European patent application EP 2 349 678 and to its American equivalent US 2012/0031916 (SIDEL).

This technique is satisfactory, but its implementation is, however, difficult. Actually, a compromise must be found between the desire to stretch the material beyond its final shape to increase the taking of the shape of the base and the necessity of preventing the material from setting before having acquired its final shape. Also, because of the mobility of the mold base, it happens that the material is pinched between the wall of the mold and the mold base, which leads to the formation of seams that are unsightly and that consume material at the expense of the base.

The final performances of the container consequently rest on the abilities of the technicians to finely adjust the parameters of the machine, in particular with regard to the time at which the boxing must be conducted.

Generalities are expressed on this subject in the U.S. Pat. No. 6,277,321 (SCHMALBACH-LUBECA). This document, however, does not say much about the means to be used to perform a proper boxing, even though (as is recalled in the document EP 2 349 678 mentioned above) the blow molding has uncertainties as to the manner in which the air bubble expands within the blank, which makes the performing of a proper boxing especially difficult.

The following objectives have driven the inventors who have wished to propose solutions that eliminate the above-mentioned drawbacks:to improve the reliability of the method for producing containers with a boxed base (intended in particular for HR applications);to improve the repeatability of such a method, i.e., the ability of this method to produce containers of constant quality;to maintain or increase the rates of production;to facilitate the automation of the production process;ultimately, to improve the quality of the containers produced.

For this purpose, there is first proposed a method for producing, from a blank of plastic material, a container having a body and a base, in a stretch-blow-molding unit comprising:a mold provided with a wall having the shape of the body of the container, and with a mold base having the shape of the base of the container, the mold base being movable axially relative to the wall between a low position and a raised position,a stretch rod that is movable axially relative to the mold between a high position in which the rod is outside of the mold, and a low position in which the rod locally presses the blank against the mold base in its low position;
this method comprising:a phase for introduction of the blank into the mold;a stretch-blow-molding phase, during which a fluid under pressure is injected into the blank and the stretch rod is moved from its high position to its low position;a boxing phase, during which the mold base, initially in low position, is moved to its raised position, and during which the stretch rod, accompanying the mold base, is moved from its low position to a raised position corresponding to the raised position of the mold base;
this method further comprising the determination of the start of the mold base from its low position by detection of the movement of the rod from its low position, and the determination of the arrival of the mold base at its raised position by detection of the stopping of the rod at its raised position.

The knowledge of the actual start and of the actual end of the boxing phase makes it possible to facilitate the adjusting of the machine using reliable and objective data.

To perform the measurements on the rod avoids having to equip the mold base with instrumentation, the environment of which is not very conducive to such instrumentation (presence of vibrations, available space).

Various additional characteristics can be foreseen, alone or in combination:determining the instantaneous position of the mold base between its low position and its raised position by detecting the instantaneous position of the rod between the low position and its raised position is foreseen;an adjustment operation for a movement order for the mold base, when the start of the mold base does not correspond to a predetermined theoretical start, is foreseen;an operation for adjustment of the flow rate of a solenoid valve connecting an actuator carrying the mold base to a source of fluid under pressure, when the actual arrival of the mold base does not correspond to a predetermined theoretical arrival, is foreseen.

Secondly, a stretch-blow-molding unit is proposed for producing, from a blank of plastic material, a container having a body and a base, this stretch-blow-molding unit comprising:a mold provided with a wall having the shape of the body of the container, and with a mold base having the shape of the base of the container, the mold base being movable axially relative to the wall between a low position and a raised position;a stretch rod that is movable axially relative to the mold between a high position in which the rod is outside of the mold, and a low position in which the rod locally presses the blank against the mold base in its low position;means for detecting at least movement of the rod from its low position, and stopping of the rod at a raised position corresponding to the raised position of the mold base, anda central processing unit designed to deduce the start of the mold base from its low position, and the arrival of the mold base at its raised position.

Various additional characteristics can be foreseen, alone or in combination:the detection means appear in the form of an optical sensor, such as a laser sensor;with the stretch-blow-molding unit comprising a frame and the stretch rod being mounted on a carriage that is mobile relative to the frame, the sensor is attached to the frame and points toward the carriage.

Partially depicted inFIG. 1is a machine1for forming containers2by stretch blow molding from blanks3of plastic material (such as PET).

Although a blank3can be an intermediate container coming from a first forming operation, in the following it is considered that the blank3is a raw injection preform.

The container2to be formed comprises a body or side wall4, a neck5already formed on the preform3, and a base6that extends into the extension of the body4opposite the neck5.

The machine1is equipped with at least one stretch-blow-molding unit7(and in this particular case with a series of stretch-blow-molding units7), also called a forming station. According to one embodiment, shown inFIG. 1, the machine1comprises a wheel8(also called a carrousel) driven in rotation around a central axis and on which the forming stations7are mounted, as well as a sensor9of the instantaneous angular position of the wheel8, in the form of, for example, an encoder (i.e., in practice, an instrumented bearing).

Machine1comprises a control system that operates its functioning automatically, in the form of a computerized central processing unit10, and controllers (for example of PLC—Programmable Logic Controller—type) equipped with actuators that individually operate each forming station7.

Each forming station7is equipped with a mold11having the shape of the container2and with a stretching device12.

The mold11is, for example, of the wallet type and comprises two half-molds hinged around a common hinge and that open to enable, successively, the evacuation of a formed container2and the introduction of a preform3that has been previously heated in a heating unit (not shown).

The mold11has a wall13that defines a cavity having the shape of the body4of the container, extending along a main axis X, and a mold base14provided with an upper surface15having the shape of the base6of the container2.

Each forming station7is equipped with a pipe (not shown) through which a fluid (particularly a gas, such as air) is injected into the mold11. Each forming station7is also equipped with an injection device comprising a block of actuators connected to the pipe to control the injection of the fluid.

More specifically, the block of actuators comprises one or more solenoid valves that are arranged to put the pipe in communication with, respectively, a source of fluid at a pre-blow-molding pressure (at a value generally between 5 and 10 bar), a source of fluid at a blow-molding pressure (at a value generally between 15 and 40 bar), and the open air. This block of actuators is operated by the central processing unit10.

Furthermore, each forming station7is provided with a device for measuring the pressure prevailing in the container being formed, connected to the central processing unit10. This measuring device comprises, for example, a pressure sensor mounted at the level of the pipe, in which the pressure in the course of forming is identical to the pressure prevailing in the container2. The central processing unit10can be programmed to establish a curve of the variations of the fluid pressure (denoted P) prevailing in the preform3during forming, as depicted inFIG. 5.

The wall13exhibits, in a lower part, an opening16that defines a passage for the mold base14, which is mounted to be able to move axially relative to the wall13between:a low position (FIG. 2), in which the mold base14is separated from the opening16, anda raised position (FIG. 4), in which the mold base14blocks the opening16, and in which the upper surface15thus completes the shape of the container2.

The mobility of the mold base14makes it possible to initiate an over-stretching of the base6of the container, during a boxing operation that will be described later. The movement of the mold base14is, for example, assured by an actuator17on which the mold base14is mounted, this actuator17being connected to a source18of fluid (for example, the source of fluid at the blow-molding pressure) by means of a solenoid valve19operated by the central processing unit10.

The stretching device12comprises a frame20, attached to the wheel8of the machine1and that extends vertically approximately directly above the mold11.

The stretching device12comprises a mobile element that includes a carriage21that slides on a rail that is integral with the frame20, as well as a stretch rod22attached to the carriage21by an upper end.

The stretching device is, for example, of the magnetic type, and therefore comprises:a pair of electromagnets or motors that are integral with the frame20and operated by the central processing unit10;a pair of magnetic tracks that are integral with the carriage21, and each formed with a series of permanent magnets with alternate polarity, placed opposite and at a slight distance from each of the motors.

By means of the carriage21, the rod22is thus mounted mobile relative to the frame20, between:a high position (FIG. 2) in which the rod22is fully removed from the mold11, a lower (free) end of the rod22that is found at a distance from the mold11that is sufficient to enable the evacuation of a formed container2and the introduction of a preform3to be formed.a low position (FIG. 3), in which the stretch rod22is received into the mold11while coming to the immediate vicinity of the mold base14(which in this case is in low position), the material of the container2being formed being locally sandwiched between the rod22and the mold base14.

In this position, an injection sprue, present in projection at a lower end of the preform, is lodged in a hollow recess made for this purpose in the center of the upper surface15of the mold base14. Keeping the sprue in the recess guarantees a good distribution of the material during the movement of the mold base14from its low position to its raised position. This is why the rod22and the mold base14are held together during the movement of the mold base14.

On the machine, the low position of the stretch rod22can be pre-adjusted so that an interspace is made between the open end of the rod22in low position and the upper surface15of the mold base14in low position. The value of this interspace, which is predetermined, is less than or equal to the local thickness of material at the center of the base6of the container2. In practice, the interspace is preferably between 0.2 and 1 mm (and, for example, about 0.5 mm).

The production of a container2is achieved under the control of the central processing unit10.

In the example illustrated, where the path of the forming stations7is circular and where, in stationary operating conditions of the machine1, the angular speed of the wheel8is approximately constant, the relative angular position of each forming station7(deduced by the central processing unit10from angular data furnished by the sensor9of the wheel8) and the relative time (appropriate to each forming station7) can be considered equivalent.

The angular (or time) origin at the beginning of the cycle is assigned arbitrarily, embodied by the point—denoted A inFIG. 1—(or the time) when the preform3is introduced into the mold11. The end of the cycle is embodied by the point—denoted H inFIG. 1—(or the time) when the formed container2is ejected from the mold11.

The start operation consists in introducing preform3, previously heated to a temperature that is higher than the glass transition temperature of the material (about 80° C. in the case of PET), into the mold11(point A). Once the preform3is in position and the mold11is closed, the central processing unit10causes the unlocking then the movement of the carriage21(and therefore of the stretch rod22) from the high position to the low position, in which the free end of the rod22comes in contact with the base of the preform3.

There follows a phase, known as pre-blow-molding (between the points B and C inFIG. 1), which consists in injecting into the preform3the fluid at the pre-blow-molding pressure, while moving the stretch rod22from its high position to its low position while the mold base14is held in its low position.

At the end of this pre-blow-molding phase:the container2is not completely formed, several areas of the container2still not being in contact with the wall13of the mold11because of the insufficiency of the pressure;the stretch rod22, in its low position, locally presses the base of the preform3being formed against the upper surface15of the mold base14. In other words, the base of the preform3is sandwiched between the free end of the stretch rod22and the upper surface15of the mold base14.

At the end of the pre-blow-molding phase (in practice, as long as the rod22has reached its low position), a blow-molding phase of the container2is ordered, consisting in injecting into the container2being formed the fluid at the blow-molding pressure. As is seen on the lower curve inFIG. 5, the pressure undergoes an abrupt increase from the pre-blow-molding pressure until reaching the blow-molding pressure. The blow-molding phase includes a stabilization step, consisting in maintaining for a predetermined period (corresponding in practice to a predetermined angle, in this particular case between the points D and F inFIG. 1) the blow-molding pressure in the container2so as to properly press the material against the wall13of the mold11(and thus to facilitate the taking of the shape of the container2while ensuring a heat-setting of the material).

During the blow-molding phase (and optionally straddling the pre-blow-molding phase), an operation is ordered (by the central processing unit10) for boxing the base6of the container2being formed, which consists in moving the mold base14from its low position to its raised position by actuating the actuator17.

So as to make possible the raising of the mold base14, the stretch rod22is preferably disengaged as soon as the movement of the mold base14is ordered. The rod22accompanies in this way the mold base14during the period of the boxing.

After the blow-molding phase, a final phase of degassing is ordered (between the points F and G), consisting in cutting the supply of fluid under blow-molding pressure while maintaining the venting to the open air. The effect of this is a complete depressurization of the container2, whose inner pressure decreases to the atmospheric pressure.

The container2thus formed is then removed from the mold11(point H).

The quality of the base6of the container2depends in large part on the boxing. A boxing that is initiated (or finalized) too early can prove useless, the material being insufficiently stretched. The effect of this is then a poor taking of shape and/or an insufficient stiffness. Initiated (or finalized) too late, the boxing can lead to a pinching of the material between the wall13and the mold base14and the appearance of beads that are unsightly and that consume material, indeed a local slice of the material. It is understood therefore that it is necessary to properly adjust, relative to all of the other forming operations, at least the beginning and the end of the boxing.

Taking into account the response times of the solenoid valves, which can be estimated thanks to the manufacturer's data but without this data being guaranteed, it is not sufficient to be based on the order times of opening and closing the solenoid valve19for controlling the actuator17that carries the mold base14. Further, since the raised position of the mold base14corresponds to an end-of-travel stop, the time (or the angular position) at which the mold base14reaches raised position cannot be deduced by any machine parameter.

This is why it is desired to determine at least the actual start of the mold base14from its low position and the actual arrival of the mold base14at its raised position. In theory, the boxing is initiated at the same time as (or preferably in a manner slightly prior to) the blow-molding phase (at the point C) and is ended during the stabilization step (between the points D and F).

By “to determine,” an occurrence is meant to locate this occurrence during the forming, either as a function of time, or as a function of the position of the forming station7on its path.

It is theoretically possible to equip the mold base14directly with instruments to obtain the data that makes it possible to deduce at least the start of the mold base14from its low position and the arrival of the mold base14at its raised position. However, in numerous machine configurations, the environment of the mold base14is not very conducive to such instrumentation, because in particular of vibrations by which the mold base14and its actuator17are affected, of unfavorable heat conditions (high or fluctuating temperature), and of possible moisture resulting from leaks of fluid (even minor) at the connections between the mold11and supply circuits of heat-transfer fluid intended to heat the wall13and/or the mold base14.

This is why, according to the invention, it is intended to equip with instruments the stretch rod22, which remains coupled to the mold base14during its raising, and to obtain the above-mentioned data concerning the mold base14by means of the stretch rod22.

Thus:the start of the mold base14from its low position is determined by detecting the movement of the rod22from its low position, which corresponds (apart from the material thickness) to the low position of the mold base14;the arrival of the mold base14at its raised position is determined by detecting the stopping of the rod22at a raised position corresponding (apart from the material thickness) to the raised position of the mold base14.

In practice, as soon as the movement of the rod22from its low position is detected, the central processing unit10stores the time, denoted t1inFIG. 5, (or the angular position of the forming station7in question) at which this movement is detected. InFIG. 5, H represents, as the ordinate of the upper curve, the position of the mold base14.

Also, as soon as the stopping of the rod22at its raised position is detected, the central processing unit10stores the time, denoted t2inFIG. 5, (or the angular position of the forming station7in question) at which this stopping is detected.

According to a particular embodiment, these two times (or these two angular positions) are considered sufficient to permit, by feedback and reprogramming of the central processing unit10, an adjustment of the order for opening the solenoid valve19of the actuator17(on which the actual start of the mold base14depends) or of the flow rate of the solenoid valve19(on which the speed of movement of the mold base14depends, and therefore the actual arrival of the mold base14at its raised position).

Such an adjustment can be made when the actual start of the mold base14from its low position (or the actual arrival of the mold base14at its raised position) does not correspond to a predetermined theoretical start (or a predetermined theoretical arrival), for which it is determined that the boxing is performed satisfactorily.

It is conceivable to be satisfied with these values (time or angular position) for actual beginning and for actual ending of the boxing while ignoring the behavior of the material during the boxing as long as its beginning and its ending are determined to be correctly situated during the forming.

As a variant, it is conceivable to use these values to reconstruct a movement curve of the mold base14, assumed to be linear as a function of time. This curve is represented by the dotted line between the times t1and t2inFIG. 5.

According to another embodiment, the position of the mold base14between its low position and its high position is determined in a systematic and repeated way (instantaneously if it is looked at on a time basis, and by angle unit if it is looked at on an angular basis). This position is deduced from a systematic and repeated measurement of the position of the rod22between its low position and its raised position. Then, the actual curve of the movements of the mold base14between the start from its low position (time t1) and the arrival at its raised position (time t2) can be deduced from it.

This curve is not necessarily linear, because of the variations of pressure inside the container2being formed. This is the case particularly when the boxing is initiated at the beginning of the blow-molding phase (or slightly before it). In this case, the abrupt increase of pressure in the container2results in an increasing strength on raising of the mold base14. If there is no change in the flow rate from the solenoid valve19supplying the actuator17, a reduction in the speed of movement of the mold base14then occurs gradually from its movement (as shown by the solid line between the times t1and t2on the upper curve ofFIG. 5) and accordingly a delay in the arrival of the mold base14at its raised position.

By providing instantaneous position data of the mold base14(via the measurement of the instantaneous position of the rod22), it is possible, by feedback from one forming cycle to the next cycle, to cause the flow rate of the solenoid valve19to vary in order to adjust the speed of movement of the mold base14(for example following a linear model) and thus to adjust the arrival time (or point) of the mold base14at its raised position.

Specifically, the instrumentation of the rod22can be of the capacitance type and can comprise, for example, a low-position presence detector of the rod22, and a high-position presence detector of the rod22.

However, according to a preferred embodiment, the instrumentation of the rod is of the optical type.

In this case, the instrumentation comprises an optical sensor23for measuring distance, for example of the laser telemetric type. This type of sensor is, in particular, marketed by the Micro Epsilon company under the trade name Opto NCDT. The sensor23is connected to the central processing unit10with which it communicates its measurements.

The sensor23is integral with the frame20and points to a lower face24of the carriage21that carries the stretch rod22, so as to detect any movement (or to measure the position) of the carriage21, and therefore of the stretch rod22.

The sensor23detects at least the movement of the rod22from its low position and its stopping at its high position. It can also detect instantaneously (i.e., continuously) the positions of the rod22between its low position and its high position.

From the measurements coming from the sensor23, the central processing unit10deduces at least the actual start of the mold base14from its low position and its actual arrival at its raised position. As a variant, and depending on the type of sensor, the central processing unit10can determine the actual instantaneous position of the mold base14between its low position and its high position, i.e., during the entire boxing period.

On the basis of these measurements, the central processing unit10can perform, by feedback, a modification of the machine parameters (particularly the time or the angle for control of the solenoid valve19, and optionally its flow rate).

A certain number of advantages result from the structure and the method presented above.

Firstly, the determination of the actual beginning (corresponding to the actual start of the mold base14from its low position) and of the actual ending (corresponding to the actual arrival of the mold base at its raised position) of the boxing facilitates the automation of the machine1by making possible, by feedback, an automatic adjustment at least of the order for opening the solenoid valve19.

Secondly, since this adjustment can be conducted systematically, the effect is an improvement of the reliability and of the repeatability of the process for manufacturing containers with a boxed base.

Thirdly, by limiting the machine stops necessary for a manual reprogramming, the rates of production are increased.

Fourthly, the optimization of the forming process makes it possible to improve the quality of the containers produced.