Method for controlling a heating unit in a plant for the production of containers

Description

[0001] Technical field of the invention : The invention relates to the field of manufacturing containers (in particular bottles, flasks) by forming from plastic preforms, such as for example polyethylene terephthalate (PET).

[0002] The manufacture of such containers is carried out in an installation comprising a heating unit and a forming unit, equipped with a succession of molds with the imprint of the container model to be formed and corresponding injection devices.

[0003] The manufacturing comprises two main phases, namely a preform heating phase, during which a succession of preforms is heated to a reference temperature at which the preforms are in a malleable state in which they can be formed, and a forming phase, during which the heated preforms are each transferred into a mold of the blowing station 8 and a pressurized fluid is injected into each preform by the injection device also called a corresponding nozzle to give the preform the final shape of the container. The pressurized fluid is for example a gas, such as air. The forming generally includes a stretching phase carried out by means of a movable stretching rod 13 arranged to apply a stretching force to the bottom of a preform in a mold in order to stretch the preform along its axis, which helps to keep the preform centered relative to the mold.

[0004] Furthermore, a production installation generally includes a control console from which many parameters can be manually adjusted in the heating unit, for example: heating temperature, machine speed which modifies the speed of movement of the preforms, power of a ventilation ensuring the evacuation of part of the heat;

[0005] This operation is tedious and time-consuming to implement and requires the presence of an operator with good knowledge of the production installation and the preform models likely to be introduced into the installation in order to obtain a container with good material distribution over its entire height corresponding to the customer's specifications.

[0006] Technical background: In order to overcome this drawback, European patent EP1998950 (KHS) describes a production installation which includes a method for regulating the heating parameters of the furnace, in particular for regulating the variations in the electrical power of the radiation sources, as a function of the thickness of the wall of the container formed.

[0007] However, this change in the furnace heating parameters can lead to a change in the thermal conditioning of the preform during production and thus lead to non-compliance of the formed container with the customer's specifications. Furthermore, non-compliance of the formed container increases manufacturing costs and may require the plant to be shut down, further increasing manufacturing costs.

[0008] Summary of the invention: The invention aims to provide a real practical solution to the problem set out above, by proposing a method making it possible to modify the thermal conditioning of the preforms, on the one hand, without having to stop the production installation, and, on the other hand, by maintaining the quality of the containers produced during the production of the containers.

[0009] For this purpose, there is proposed, firstly, a method for producing containers made of thermoplastic material from preforms, which defines a circulation path for the preforms and containers, which comprises: a unit for forming containers with a vertical axis comprising at least one blowing station each including a mold with the imprint of a container and a device for injecting a pressurized fluid into the preform; a preform heating unit located on the circulation path upstream of the forming unit relative to the direction of circulation of the preforms comprising: a means for conveying the preforms comprising a plurality of individual supports for the preforms defining a preform heating path; a heating cavity comprising two side walls facing each other and spaced apart from each other and at least one of these walls being the one which supports several radiation sources arranged one above the other and one next to the other facing the preforms thus defining a line of radiation sources relative to the preform heating path;a power supply providing each radiation source with electrical power; a control unit connected to the power supply, this control unit having a memory in which are recorded: at least one electrical power setpoint for at least one line of radiation sources; at least one thickness setpoint at at least one point of the container; the control unit implements at least one measurement of the thickness of a wall of the container at at least one point of said container by means of at least one sensor; ;

[0010] The control unit compares the thickness measurement to the recorded thickness setpoint, if the measured thickness is different from the thickness setpoint then the control unit modifies the electrical power of at least two lines of radiation sources while maintaining the same sum of the electrical powers supplying all the lines of radiation sources.

[0011] According to other steps of the production method according to the invention, the control unit: modifies said electrical power of at least two lines of radiation sources in a homogeneous manner; increases the electrical power on one line by a determined value then the control unit decreases the electrical power on the remaining lines by the determined value, divided by the number of remaining line(s); decreases the electrical power on one line by a determined value then the control unit increases the electrical power on the remaining lines by the determined value, divided by the number of remaining line(s);

[0012] On a line of radiation sources, if at least one radiation source is operating at its maximum electrical power and if at least one radiation source is switched off on the same line, then the control unit switches on one of the radiation sources which was switched off and decreases the electrical power of each radiation source in the line while maintaining the sum of the electrical powers supplying the line of radiation sources constant;

[0013] If at least one radiation source is operating at a "minimum effective" electrical power, then the control unit turns off one of the radiation sources and increases the electrical power of each radiation source in the line while maintaining the sum of the electrical powers supplying the line of radiation sources constant;

[0014] According to other characteristics of the production method according to the invention, the radiation sources are: halogen lamps emitting in the IR; laser diodes emitting in the IR;

[0015] Secondly, a computer program product is provided comprising a sequence of instructions which, when the program is executed by a computer, causes the computer to implement the steps of the method.

[0016] Brief description of the figures : Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which one will refer to the appended drawings which we describe briefly below. [ Fig. 1 ] there Figure 1 is a general top view of a production facility [ Fig.2 ] there Figure 2 is a schematic side view of a heating unit and a blowing station 8; [ Fig. 3 ] there Figure 3 is a schematic configuration of a heating unit showing the status of the radiation sources.

[0017] Detailed description of the invention : In the remainder of the description, elements having an identical structure or similar functions will be designated by the same reference.

[0018] The figures schematically show installations for the mass production of containers 4 made of thermoplastic material from preforms 2.

[0019] In the remainder of the description, the preforms 2 and the containers 4 move in the production facility along a circulation path from upstream to downstream. The preforms 2 are moved in line along a heating path by conveying means which will be detailed later.

[0020] In a non-limiting manner, the containers 4 are bottles here. The thermoplastic material is for example formed here by polyethylene terephthalate, hereinafter referred to by its acronym "PET".

[0021] An example of such a preform 2 is shown in the figure. The preform 2 has a main axis "X" shown vertically in the figure. It has a cylindrical body 5 with a tubular wall closed at one of its axial ends by a bottom 3, and which is open at its other end by a neck 7, also tubular. At its base, the neck 7 here has a collar 9 used for its transport in the production installation.

[0022] As shown in detail in the Figure 1 , the wall is delimited by an external face and by an internal face. The neck 7 generally has its final shape while the body 5 of the preform 2 is intended to undergo a relatively significant deformation to form the final container 4 during a forming step.

[0023] As shown in the Figure 1, the production installation comprises at least one forming unit 6, a heating unit 11 and a control unit 28 of the production installation, which notably comprises a computer (or processor), a memory 30 and a console (or graphical interface), not shown, for interaction with an operator.

[0024] The forming unit 6 of containers 4 is formed by a forming wheel rotating a plurality of forming stations from an inlet to an outlet, at which a succession of containers 4 formed from the preforms 2 is extracted, as shown in the figure. 1 . The axis of rotation of the forming wheel is, for example, substantially parallel to the main axis X of the preforms as they are transported by the forming wheel.

[0025] Each blowing station 8 comprises a mold 10 forming a molding cavity having the shape of the container 4 to be formed and arranged to receive a preform 2 so that the body 5 of the preform extends into the molding cavity, as shown in the Figure. 2 . Each mold 10 is formed of several parts movable between an open position, in which the mold 10 can receive a preform and release a container 4, respectively at the inlet and at the outlet, and a closed position, in which the molding cavity is closed on the preform between the inlet and the outlet. When a preform is received in the mold 10, the body 5 extends into the molding cavity and the neck 7 extends projecting from the mold 10 outside the molding cavity, as shown in the figure. 2 .

[0026] Each blowing station 8 further comprises an injection device 12 arranged to inject a fluid into the internal volume of the preform placed in the mold 10 of the blowing station 8 so that the fluid deforms the preform and the latter acquires the shape of the molding cavity, that is to say that the preform 2 is formed into a container 4 under the action of the fluid. According to one embodiment, the fluid is for example a pressurized gas, for example pressurized air. In this case, the injection device 12 is formed by a nozzle and one or more valves making it possible to control the injection of the fluid into the preform 2. The pressure at which the fluid is injected is adjustable.By way of example and in a known manner, the forming installation comprises a pressurized air tank at a first pressure and another pressurized air tank at a second pressure, the valves making it possible to selectively inject the air at the first pressure or at the second pressure.

[0027] According to one embodiment, shown in the figure. 2, each blowing station 8 further comprises a stretching rod 13 movable relative to the mold 10 in a direction coincident with the main axis X of the preform 2 when it is placed in the mold 10. The stretching rod 13 is arranged to exert pressure on the bottom 3 of the preform in order to extend the latter in the axial direction. More particularly, the stretching rod 13 extends the preform until the bottom 3 of the preform comes into contact with the bottom of the molding cavity. The movement of the stretching rod 13 is controlled by an actuator either mechanical of the jack type controlled by a cam and roller system or by an electric actuator of the linear motor type.

[0028] The forming unit 6 is automatically controlled by a control unit 28.

[0029] The heating unit 11, also called an oven, makes it possible to heat a succession of preforms 2 to a reference temperature, after its configuration, as will be described later. The reference temperature is chosen so that the body 5 of each preform 2 at the outlet of the heating unit 11 is in a malleable state allowing deformation of the body 5 of the heated preform in order to form the container 4 in the forming unit 6. The reference temperature is between the glass transition temperature and the crystallization temperature of the plastic material of the preform 2. In the case of PET, the reference temperature is for example close to 110°. The value of the reference temperature may vary depending on the product with which the container 4 will be filled or depending on the container filling technique. Thus, the reference temperature is different for hot filling or for a carbonated product for example.It will be noted that the reference temperature corresponds to the temperature at a reference location of the preform 2, that is to say in a localized zone. Indeed, the preform is not necessarily heated to a uniform temperature but can have a temperature profile causing the temperature of the preform to vary over its height depending on the material distribution desired at the level of the container 4.

[0030] According to the embodiment shown in the figure. 1 , the heating unit 11 is a scroll oven, in which the preforms are transported to be exposed to a plurality of heating radiation sources 22. For this purpose, the heating unit 11 comprises a conveying means 14 of the preforms 2 through the heating unit 11 along a heating path extending between an inlet and an outlet of the heating unit 11.

[0031] As represented in the figures.1 And 2, the conveying means 14 comprises a succession of supports 16, each being capable of supporting a preform 2, mounted on a chain, moving along the heating path in the heating unit 11.

[0032] In another embodiment not shown, the conveying means 14 comprises for example a succession of supports 16, each being capable of supporting a preform, mounted on a linear motor type shuttle circulating on a closed magnetic loop. The movement of each of these shuttles being controlled independently of each other by the control unit 28.

[0033] Each support 16 is for example capable of receiving a preform 2 by fitting the neck onto the support 16, as shown in the figure. 2. Each support 16 is for example movable in rotation relative to the chain around an axis of rotation coinciding with the main axis X of a preform 2 when the latter is supported by the support 16. The heating radiation sources 22 are distributed in a matrix manner (as shown in the Figure 3 ) along the heating path and in a direction transverse to the path, the transverse direction being substantially parallel to the main axis X of the preforms when they are supported by the supports 16.

[0034] The heating unit 11 also comprises a heating cavity 18 which comprises two lateral walls 20 facing each other and distant from each other and at least one of these walls 20 being the one which supports several radiation sources 22 arranged one above the other and one next to the other facing the preforms, thus defining a line 24 of radiation sources 22 relative to the heating path of the preforms.

[0035] In other words, the heating unit 11 comprises a plurality of radiation sources 22 distributed along the heating path and according to a preform height such that the entire height of the body 5 of each preform is exposed to the radiation sources 22 on the path of the preform in the heating unit 11. By rotating the preforms around their main axis X, the supports 16 make it possible to uniformly expose the entire body 5 of the preforms to the radiation sources 22. The radiation sources 22 may be distributed on only one side of this path, in which case a reflective wall 21 may be arranged on the other side of the heating path to reflect the heat towards the preforms.

[0036] In another embodiment not shown, the radiation sources 22 can be distributed on either side of the heating path.

[0037] It should be noted that radiation sources 22 are arranged, where appropriate, to subject the body 5 of the preform 2 to radiation, thus defining a suitable temperature profile across the wall of the preform from the outside to the inside. It can basically be assumed that areas of the preform with a lower temperature lead to thicker walls of the molded container 4, while hotter areas of the preform are stretched more during the blowing operation and thus lead to a thinner wall of the container 4.

[0038] It should also be noted that the radiation sources 22 are arranged, where appropriate, so as not to subject the neck 7 and the ring to the heat emitted by the radiation sources 22. Indeed, as indicated previously, only the body 5 of the preform is formed to produce the container 4. Consequently, the neck 7 and the ring must not be deformed during forming and must not be heated. To avoid heating the neck 7 and the ring, the heating unit 11 may comprise a ventilation device positioned in line with the necks and the rings to evacuate the heat likely to be absorbed by the necks and the rings. The ventilation device comprises for example at least one fan 15 controlled by the control unit 28.

[0039] Each radiation source 22 is formed by an incandescent lamp emitting infrared radiation.

[0040] In another embodiment, each radiation source 22 is a laser diode emitting infrared radiation.

[0041] In other words, each radiation source is a laser (e.g. laser diodes) emitting in the infrared and organized by juxtaposition and superposition to form one or more matrices.

[0042] In this case, each array may be an array of vertical cavity surface emitting laser (VCSEL) diodes, each diode emitting, for example, a laser beam with a unit power of the order of a Watt at a wavelength of approximately 1 µm.

[0043] These radiation sources are radiant, that is to say that the emitted radiation is transmitted to the preforms 2 without the air serving as a transmission vector.

[0044] It goes without saying that each radiation source 22 may consist of any other radiation source without departing from the scope of the invention.

[0045] The heating unit 11 also comprises a power supply 27 supplying each radiation source 22 with electrical power. Each radiation source 22 thus converts the electrical power supplied to them into radiation heating the preforms.

[0046] The power supply 27 makes it possible to switch on (or power up) or switch off each of the radiation sources 22 individually so that it is possible to control each of the radiation sources 22 and thus decide which one or ones will be used along the path to heat the preforms moving in the heating unit 11. The distribution of the radiation sources switched on or off along the path and in the transverse direction is generally referred to as "mapping" of the radiation sources as will be described in the remainder of the description.

[0047] The electrical power supplied to the radiation sources 22 can also be variable between a maximum electrical power and a minimum effective electrical power of the radiation source 22. “Maximum electrical power” means the maximum electrical power to which the radiation source 22 can be subjected without being degraded and “Minimum effective electrical power” means the minimum electrical power from which the radiation source 22 emits radiation making it possible to heat a preform 2.

[0048] Electrical power is expressed directly in power, that is, in watts or as a percentage of the maximum electrical power of the radiation source 22.

[0049] As represented in the figures.1 And 2 , the heating unit 11 comprises a control unit 28 connected to the power supply 27.

[0050] The control unit 28 controls the power supply 27 via power variators 26.

[0051] The power variators 26 make it possible to vary the electrical power at the input of the radiation source 22 between the maximum electrical power and the minimum effective electrical power. These variators 26 can be analog or electronic;

[0052] The control unit 28 can advantageously control the power supply(s) 27 and / or the dimmer(s) 26 of each radiation source 22.

[0053] As represented in the figures.1 And 2 , the control unit 28 comprises a memory 30 in which are recorded at least one electrical power setpoint for at least one line 24 of radiation sources 22 and at least one thickness setpoint at at least one point of the container 4.

[0054] Said instructions can be recorded in the memory 30 of the control unit 28 by an operator.

[0055] The instructions are, for example, determined empirically on the basis of tests or from databases of previous preforms 2 heated in order to obtain a container 4.

[0056] The control unit 28 implements at least one measurement of the thickness of a wall of the container 4 at at least one point by means of at least one sensor 32.

[0057] The thickness measuring sensor 32 is placed at the outlet of the forming unit 6 to measure the thickness of the container 4 which has just been formed. In this embodiment, the sensor 32 is of the optical type. In another embodiment not shown, the thickness measuring sensor can be placed in the mold 10. In this case, the sensor can be of the capacitive type.

[0058] To get a good idea of ​​the material distribution of the container 4 formed, it is advantageous to position several sensors 32 on the height of the measured container.

[0059] For each container 4, the control unit 28 compares the thickness measurement obtained via a sensor with the corresponding recorded thickness setpoint at the same height: If the thickness measurement and the thickness setpoint are identical then the control unit 28 does not modify the electrical powers supplying the lines 24 of radiation sources.

[0060] On the other hand, if the measured thickness is different from the thickness setpoint then the control unit 28 modifies the electrical power of at least two lines 24 of radiation sources while maintaining the same sum of the electrical powers supplying all the lines of radiation sources.

[0061] In other words, the electrical power is modified on at least two lines 24 in order to be able to modify an electrical power on one line and adjust the electrical power on at least one other line, this makes it possible to maintain the sum of the electrical powers of all the lines identical.

[0062] In short, the lines 24 of radiation sources 22 are corrected and compensated so as to have the sum of the electrical powers of the lines 24 identical before and after the modification.

[0063] In fact, if the electrical power is only modified on one line 24 then the control unit 28 will be unable to maintain the sum of the electrical powers of all the lines identical.

[0064] Advantageously, the control unit 28 modifies the electrical power of at least two lines 24 of radiation sources 22 in a homogeneous manner. The term "homogeneous" is understood to mean, for example, increasing the electrical power of one line by a certain value and decreasing the electrical power of another line 24 by this same value and vice versa.

[0065] Advantageously, when the control unit 28 increases the electrical power on a line by a determined value, then the control unit decreases the electrical power on the remaining lines by the determined value, distributed over the remaining line(s).

[0066] Advantageously, when the control unit 28 decreases the electrical power on a line 24 by a determined value, then the control unit increases the electrical power on the remaining lines by the determined value, divided by the number of remaining line(s) 24.

[0067] In addition, these modifications in electrical power are made progressively during the production of containers 4 to avoid a shutdown of the installation due to a non-compliant container.

[0068] There Figure 3 represents a schematic configuration of a heating unit 11 of preforms 3.

[0069] The schematic configuration shows a distribution of a number of rectangles symbolizing radiation sources 22 arranged next to each other, i.e. in row 24, and above each other, i.e. in column 34, and thus defining a matrix.

[0070] As shown in the Figure 3 , in the matrix, the rectangles can be of three different colors depending on the operating status of the radiation sources 22: a white rectangle 36 means that the radiation source 22 is off, i.e., the radiation source is not supplied with electrical power. Therefore, this radiation source is not regulated; a black rectangle 38 means that the radiation source 22 is on, i.e., the radiation source is supplied with 100% of the maximum electrical power for this type of radiation source. Therefore, this radiation source is not regulated; a gray rectangle 40 means that the radiation source 22 is on, i.e., the radiation source is supplied between a maximum electrical power and a minimum effective electrical power. Therefore, this radiation source 22 is regulated in electrical power.

[0071] In the particular case shown in the Figure 3, the radiation source matrix 22 comprises 12 columns 34 and 8 rows 24 with a number of radiation sources switched off, switched on and regulated.

[0072] There Figure 3 shows a column of numbers on the left 42 of the matrix, corresponding to the sum of the electrical powers (expressed as a percentage of the maximum power) supplying the regulated and unregulated radiation sources for a given row.

[0073] There Figure 3 also shows a column of numbers to the right 44 of the matrix, corresponding to the electrical power (expressed as a percentage of the maximum power) applied to each of the regulated radiation sources for the given row.

[0074] For example, for line “8” shown in Figure 3 , all sources of radiation 22 are switched off.

[0075] Another example, for line “1” shown in Figure 3, the radiation sources 22 located at columns 34: 1, 2, 3, 4 and 12 are off, 5, 6 are on at maximum electrical power, 7, 8, 9, 10, 11 are regulated, each at 80% of the maximum power to which a general percentage of 46 of heating is applied (this general percentage of heating makes it possible to maintain a constant preform temperature despite external disturbances, for example variations in the workshop temperature). The sum of the electrical powers of the radiation sources constituting the line is 45%.

[0076] Advantageously, on a line 24 of radiation sources 22, if at least one radiation source operates at its maximum electrical power 44, if the control unit requires an increase in power and if at least one radiation source 22 is switched off on the same line 24, then the control unit 28 switches on one of the radiation sources 22 which was switched off and reduces the electrical power of each regulated radiation source in the line while maintaining the sum of the electrical powers supplying the line 24 of radiation sources constant.

[0077] Advantageously, on a line 24 of radiation sources 22, if at least one radiation source operates at a “minimum effective” electrical power, if the control unit requires a reduction in power then the control unit 28 turns off one of the regulated radiation sources and increases the electrical power of each radiation source 22 of the line 24 while maintaining the sum of the electrical powers supplying the line 24 of radiation sources constant.

Claims

1. Method for producing containers made of thermoplastic material from preforms, which defines a path for circulating the preforms (2) and the containers (4), comprises: - a moulding unit (6) for moulding containers having a vertical axis A comprising at least one blowing station (8), each including a mould (10) with a cavity in the shape of a container (4) and a device (12) for injecting a pressurized fluid into the preform (2); - a heating unit (11) for heating preforms which is located on the circulation path upstream of the moulding unit (6) relative to the direction of circulation of the preforms, comprising: - a means (14) for conveying the preforms comprising a plurality of individual supports (16) for the preforms defining a path for heating the preforms; - a heating cavity (18) comprising two lateral walls (20) that face one another and are at a distance from one another, at least one of said walls (20) being that which bears a plurality of radiation sources (22) arranged one above the other and side by side facing the preforms (2), thereby forming a row (24) of radiation sources (22) relative to the path for heating the preforms (2); - an electrical power supply (27), supplying each radiation source (22) with electrical power; - a control unit (28) connected to the electrical power supply (27), said control unit (28) having a memory (30) for storing: - at least one electrical power setpoint for at least one row (24) of radiation sources (22); - at least one thickness setpoint in at least one point of the container (4); - said control unit (28) carrying out at least one measurement of the thickness (e) of a wall of the container (4) at at least one point of said container (4) by means of at least one sensor; characterized in that the control unit (28): - compares said thickness measurement (e) with said stored thickness setpoint; if said measured thickness (e) is different from the thickness setpoint, then the control unit (28) modifies said electrical power of at least two rows (24) of radiation sources (22) while keeping the sum of the electrical powers being supplied to all of the rows (24) of radiation sources (22) the same.

2. Method for producing containers according to Claim 1, characterized in that the control unit (28) modifies said electrical power of at least two rows (24) of radiation sources (22) in a homogeneous manner.

3. Method for producing containers according to Claim 2, characterized in that when the control unit (28) increases the electrical power on a row (24) by a specific value, then the control unit (28) reduces the electrical power on the remaining rows (24) by the specific value, divided by the number of remaining row(s).

4. Method for producing containers according to Claim 2, characterized in that when the control unit (28) reduces the electrical power on a row (24) by a specific value, then the control unit (28) increases the electrical power on the remaining rows (24) by the specific value, divided by the number of remaining rows (24).

5. Method for producing containers according to Claim 1, characterized in that on one row (24) of radiation sources (22), if at least one radiation source (22) operates at its maximum electrical power and if at least one radiation source (22) is switched off on the same row, then the control unit (28) switches on one of the radiation sources (22) which has been switched off and reduces the electrical power of each radiation source (22) of the row (24), while keeping the sum of the electrical powers being supplied to the row of radiation sources (22) constant.

6. Method for producing containers according to Claim 1, characterized in that on one row (24) of radiation sources (22), if at least one radiation source (22) operates at a "minimum effective" electrical power, then the control unit (28) switches off one of the radiation sources (22) and increases the electrical power of each radiation source (22) of the row (24), while keeping the sum of the electrical powers being supplied the row of radiation sources (22) constant.

7. Method for producing containers according to any one of Claims 1 to 6, characterized in that said radiation sources (22) are halogen lamps emitting in the infrared range.

8. Method for producing containers according to any one of Claims 1 to 6, characterized in that said radiation sources (22) are laser diodes emitting in the infrared range.

9. Computer program product comprising a sequence of instructions which, when the program is executed by a computer, results in the computer implementing the steps of the method according to any one of Claims 1 to 8.