Device for measuring the absorption spectrum of a preform

Abstract

The invention relates to a device (56) for on-the-fly measurement of the absorption spectrum (σ(λ1)) in the visible light range of at least one wall (16) of preforms (12), comprising: - means (36) for conveying preforms (12), - a measurement light source (58) which emits a measurement light beam (55) according to a continuous measurement spectrum (σm), - a spectrometer (62) measuring the intensity (I) and the wavelength (λ1) of the first light beam (55) after having passed through the preform (12), characterised in that the measurement light source (58) comprises at least two primary light sources (58A, 58B, 58C, 58D) which each emit a primary light beam according to an associated continuous spectrum (σA, σB, σC, σD), the primary continuous spectra (σA, σB, σC, σD) of the primary sources (58A, 58B, 58C, 58D) being different.

Description

DEVICE FOR MEASURING THE ABSORPTION SPECTRUM OF A PREFORM Technical field of the invention

[0001] The invention relates to a device for on-the-fly measurement of the absorption spectrum in the visible light range of at least one wall of preforms intended to be transformed into final containers by forming, in particular by stretch blow molding. The absorption spectrum measurement device comprises: - means for conveying preforms in a line along a production path passing through a measurement zone, - a measurement light source which emits a measurement light beam according to a continuous measurement spectrum which extends at least into the visible range, - a spectrometer measuring the intensity and wavelength of the first light beam after passing through at least one thickness of the preform wall during its passage through the associated measurement zone. Technical background

[0002] It is known to manufacture containers by forming, particularly by stretch blow molding, preforms made of thermoplastic material. The material forming the preforms is generally in an amorphous state that is not suitable for cold forming. Prior to the forming operation, the preforms are therefore heated to a temperature equal to or greater than their glass transition temperature, which allows them to be shaped into the final container.

[0003] More specifically, preforms generally have a substantially cylindrical body of revolution with thick tubular walls, closed at one of its axial ends by a thick-walled base, and extended at its other end by a neck, also tubular. The neck is formed to its final shape and dimensions, while the body of the preform is intended to undergo relatively significant deformation to form it into a container during a forming step.

[0004] For this reason it is preferable that only the body of the preform be heated to a set temperature which is higher than the glass transition temperature, the neck remaining at a temperature lower than said glass transition temperature to avoid its deformation during the manufacture of the container.

[0005] Furthermore, the set temperature must not exceed a crystallization temperature that is higher than the glass transition temperature. Above this crystallization temperature, the thermoplastic material crystallizes and no longer possesses the mechanical properties required for high-quality forming.

[0006] The mass production of containers is carried out in a production facility where preforms move along a predetermined production path. Each preform is handled by conveyors. Such conveyors can be rail conveyors along which the preforms are free to come into contact with each other, or conveyors equipped with gripping devices for each preform individually.

[0007] The production facility includes a heating station which, during a heating step, makes the preform body malleable by heating it to the set temperature. During this heating step, each preform is exposed to radiant heat as it moves. The power of the radiant heat is controlled to ensure that the preform body is heated to the set temperature.

[0008] The production plant also includes a forming station, which is arranged downstream of the heating station according to the direction of preform flow within the production plant. During the forming stage, the hot preform is placed in a forming unit, for example, in a mold of the forming station that has a molding impression corresponding to the desired container. A pressurized fluid, such as air, is then injected into the malleable body of the preform to press its wall against the mold impression. Generally, the injection of pressurized fluid is preceded and / or accompanied by axial stretching of the preform, notably by means of a stretching rod inserted into the preform. As is known, the body is thus subjected to biaxial stretching.

[0009] During its passage through the heating station, each preform is typically exposed to continuous-spectrum heating radiation, which heats the thermoplastic material. The temperature to which the preforms are heated generally depends on the preform's absorption coefficient (A) for the wavelengths of the heating radiation. The absorption coefficient (A) is sometimes referred to by its English name, "absorptance."

[0010] The absorption factor (A) is defined as the ratio of absorbed heating radiation flux to incident heating radiation flux. The absorbed heating radiation flux causes a temperature rise in the thermoplastic material composing the preform. The absorption factor (A) of the preform thus allows us to determine the temperature rise of a preform as a function of the intensity of the infrared radiation and the duration of exposure to that radiation. Less energy is required to heat a preform with a high absorption factor (A) compared to a preform with a lower absorption factor (A).

[0011] The absorption factor (A) can be deduced from two other parameters called the transmission factor (T) of the preform and its reflection factor (R).

[0012] The transmission factor (T), sometimes called "transmittance", is defined as the ratio between the heat radiation flux transmitted through at least one wall of the preform and the incident heat radiation flux.

[0013] The reflectance factor (R), also called reflectance, is defined as the ratio of the heat radiation flux reflected by the preform to the incident heat radiation flux. Reflectance (R) is calculated, for example, as a function of the refractive index (n) of the thermoplastic material using the following formula:

[0014] R = [(n-1) / (n+1)]²

[0015] in which the value "1" corresponds to the refractive index of air.

[0016] These three factors depend on the wavelength of the radiation in question.

[0017] The heating radiation extends at least partially into the near-infrared range, as thermoplastic materials, such as PET, are known to absorb this type of radiation very readily. Therefore, the wavelength used to define the preform's transmission factor is generally taken from the near-infrared range.

[0018] These three factors are linked by the following equation:

[0019] A=1-RT

[0020] The transmission factor depends on the wavelength of the radiation passing through it. Thus, a perfectly transparent preform exhibits a transmission factor close to 100% for wavelengths in the visible light range, while it exhibits a significantly lower transmission factor for radiation with wavelengths in the near-infrared range. This reflects the fact that the preform can be heated by radiation in the near-infrared range, whereas radiation in the visible light range will have only a negligible influence on the preform's temperature, even at high power levels.

[0021] Sometimes the preforms are made of a material with a color.

[0022] This tint can be intentionally obtained by adding dye to color the preform in a desired color in order to obtain a colored container.

[0023] This discoloration can also occur unintentionally and uncontrollably, for example, when the preform is made of a plastic material composed at least partially of recycled plastic. This is the case, for instance, when the preform contains mechanically recycled polyethylene terephthalate (rPET). In this case, the preform will have a more or less dark tint depending on the quality and quantity of recycled material incorporated into the composition of the thermoplastic material from which it is made.

[0024] Such a preform comprises at least a portion of rPET. To produce such a preform, rPET granules are, for example, mixed with virgin PET granules. The mixed granules are melted together before being injected into a mold to obtain the preform. Thus, the rPET as defined in the invention is melted and used directly to form preforms, possibly mixed with virgin PET. The recycled rPET granules are obtained mechanically by grinding previously used PET objects, which have been sorted by color and cleaned, without chemically decomposing them. The virgin PET granules are obtained chemically by monomer esterification. The ratio between the weight of rPET and the weight of virgin PET determines the rPET content in the preforms.

[0025] The inventor observed that in both cases, the color of the plastic material is likely to influence the temperature of the preform wall when exposed to heating radiation.

[0026] Indeed, the heating radiation has a spectrum that extends at least partially into the visible light range. However, when the preform has a color, and especially a dark color, this part of the heating radiation spectrum is likely to significantly heat the thermoplastic material.

[0027] It is important to be able to correctly define the absorption lines in the visible range, in particular to be able to adjust the heating power, but also to be able to determine the composition of the material forming the preform.

[0028] To achieve this, the light source used to determine the preform's color must closely resemble a standard illuminant. Ideally, this illuminant is D65, as defined by ISO 11664-2:2022. This reference illuminant is a colorimetric standard that corresponds to natural daylight in a temperate zone. It is a cool white.

[0029] To produce such an illuminant, it is known to use incandescent light sources, such as a xenon lamp. However, such a source is difficult to use for on-the-fly measurements in a high-volume production environment, i.e., when the preforms are in motion. Among the various drawbacks of such a light source, it is bulky, requires the use of filters to obtain a spectrum close to that of the reference illuminant, produces a significant amount of heat, is unstable over time, and requires a long exposure time.

[0030] Therefore, there is a need to correctly determine the color of a preform without resorting to such technology.

[0031] The invention proposes a device for on-the-fly measurement of the absorption spectrum in the visible light range of at least one wall of preforms intended to be transformed into final containers by forming, in particular by stretch blow molding; the absorption spectrum measurement device comprising:

[0032] - means of conveying preforms in a line along a production route passing through a measurement zone,

[0033] - a measuring light source that emits a measuring light beam according to a continuous measuring spectrum that extends at least into the visible range,

[0034] - a spectrometer measuring the intensity and wavelength of the first light beam after passing through at least one thickness of the preform wall as it passes through the associated measurement zone,

[0035] characterized in that the measurement light source comprises at least two primary light sources, each emitting a primary light beam according to an associated continuous spectrum extending at least into the visible range, the primary continuous spectra of said primary sources being different and their light beams being merged to form said measurement light beam.

[0036] According to another feature of the measuring device made according to the teachings of the invention, the addition of the primary continuous spectra of said primary sources forms the continuous measurement spectrum, the primary sources being selected so that the continuous measurement spectrum has a profile that is substantially closer to the reference spectrum of a reference illuminant than each of the primary spectra taken alone.

[0037] According to another feature of the measuring device made according to the teachings of the invention, the reference illuminant is the D65 illuminant as defined by ISO 11664-2:2022.

[0038] According to another feature of the measuring device made according to the teachings of the invention, the measuring light source comprises more than two primary light sources, all exhibiting different light spectra.

[0039] According to another feature of the measuring device made according to the teachings of the invention, the measuring light source comprises at least two primary light sources which have the same primary light spectrum so as to increase the intensity of the measuring light beam for the wavelengths of said light spectrum so that the measuring spectrum is closer to the reference spectrum of the reference illuminant.

[0040] According to another feature of the measuring device made according to the teachings of the invention, each primary light source is formed by a luminescent semiconductor component.

[0041] According to another feature of the measuring device made according to the teachings of the invention, the primary light beams are fused by means of at least one optical coupler.

[0042] According to another feature of the measuring device made according to the teachings of the invention, it comprises a primary light source which emits at least partly in the near-infrared range.

[0043] According to another feature of the measuring device made according to the teachings of the invention, it includes at least one electronic unit for controlling the light intensity emitted by each primary light source.

[0044] According to another feature of the measuring device made according to the teachings of the invention, the preforms are made of a thermoplastic material, in particular PET.

[0045] The invention also relates to an installation comprising means for conveying preforms in a line along a predetermined production path passing successively through a heating zone of a body of the preforms and through a preform forming station into a container, in particular by stretch blow molding, the installation comprising the measuring device made according to the teachings of the invention. Brief description of the figures

[0046] Other features and advantages of the invention will become apparent during the reading of the detailed description which follows, for the understanding of which reference should be made to the attached drawings briefly described below.

[0047] This is a top view which schematically represents a container manufacturing installation made according to the invention comprising a heating station.

[0048] This is a side view that represents a preform intended to be processed by the installation of the.

[0049] Laest is a vertical cross-sectional view along the cutting plane 3-3 of laqui which represents a device for measuring an absorption spectrum in the visible light range of the walls of a preform circulating in the installation made according to a first embodiment.

[0050] Laest is a diagram representing wavelengths on the x-axis and light intensities on the y-axis, schematically representing the light spectrum of a reference illuminant in solid line and the light spectrum of a white LED in dotted line.

[0051] This is a diagram similar to that of the one that schematically represents the light spectrum of the reference illuminant in solid line, as well as the measurement light spectrum of a measurement light source represented by crosses, and the measurement light spectrum being obtained by adding the spectra of a white LED in dotted line, and the spectra of three primary light sources represented respectively by diamonds, by an axis line and by points.

[0052] This is a diagram similar to that of the one which represents the measurement spectrum of the la in dotted line and a transmission spectrum in solid line. Detailed description of the invention

[0053] In the following description, the terms "top", "bottom", and the derived terms "high", "low", are used for clarity in reference to the orientation of the figures without this having any limiting scope.

[0054] In the following description, the wavelengths of light radiation will be expressed in nanometers, indicated by the abbreviation "nm".

[0055] In theory, a monochromatic source is an ideal source emitting a sinusoidal wave of a single frequency. In other words, its frequency spectrum consists of a single line of zero spectral width (Dirac delta function).

[0056] In practice, such a source does not exist; a real source has a frequency emission spectrum that extends over a band of small but non-zero spectral width, for example a few tens of nanometers, centered on a principal frequency where the intensity of the radiation is maximum.

[0057] In the following description, such a real source is considered to be monochromatic.

[0058] Subsequently, an absorption spectrum is defined as a curve that represents the value of the intensity of a continuous spectrum light beam as a function of the wavelength after its passage through a medium to be analyzed, in particular the wall of a preform.

[0059] Figure 10 schematically illustrates an installation for manufacturing containers from thermoplastic preforms, particularly PET (polyethylene terephthalate) or rPET (recycled polyethylene terephthalate). The thermoplastic material may, but does not necessarily, contain additives that artificially increase the preform's absorption coefficient, or a dye to change its color.

[0060] As shown in Figure 1, each preform 12 comprises a cylindrical body 14 with axis "X". The body 14 has a transparent lateral wall 16 that delimits an internal volume. The wall 16 has a shape of revolution about the axis "X" such that, at a given height, the wall 16 has a constant thickness. However, the thickness of the wall 16 is likely to vary depending on the height. The wall 16 is delimited by an external face 18 that faces outwards from the preform 12 and by an internal face 20 that faces into the internal volume.

[0061] An upper end of the body 14 opens through a neck 22. The neck 22 has the final shape of the neck of the container 11 to be produced. Consequently, the neck 22 does not undergo any deformation during the manufacture of the container 11. The body 14 has a base 24 that closes its lower end and is generally hemispherical in shape. The neck 22 has a collar 26 arranged at its junction with the body 14. The lower face of the collar 26 is designed to form a support surface 28 to support the preform 12 during its molding and / or transport.

[0062] At the end of their injection molding process, the preforms 12 are rapidly cooled to give the thermoplastic material an amorphous state. This makes it possible to render the thermoplastic material malleable again by heating it above a glass transition temperature.

[0063] Referring again to Figure 10, the manufacturing installation includes a heating station 30 and a forming station 32. The preforms 12 move in a line along a production path 34 that passes through the heating station 30 and the forming station 32. The direction of movement of the preforms 12 is indicated by the arrows "F1" in Figure 10. During normal operation of the manufacturing installation 10, the preforms 12 are in constant motion along the production path 34.

[0064] The heating station 30 is designed to heat the body 14 of the preforms 12 to a setpoint temperature that is greater than or equal to the glass transition temperature of the constituent material, for example, greater than 70°C when this material is PET and / or rPET. The heating station 30 includes a conveyor 36 (shown schematically) for transporting the preforms 12 while rotating them.

[0065] The conveyor 36 typically includes mandrels (not shown) which are fitted to the neck 22 to transport the preforms 12. The mandrels move along a closed circuit. For example, the mandrels are carried by the links of a chain or by independent shuttles moving along a rail.

[0066] The circuit here comprises two parallel straight sections connected by 180° turning sections. The conveyor 36 also includes two wheels 38A, 38B for guiding the mandrels in the turning sections of the closed circuit.

[0067] The heating station 30 also includes heating means 40 for heating the preforms 12. These include, for example, lamps facing reflectors that emit continuous spectrum heating electromagnetic radiation.

[0068] The heating radiation extends, at least in part, into the near-infrared range, which spans wavelengths from 780 nm to 3000 nm. The intensity of the heating electromagnetic radiation is particularly high in the higher intensity wavelength range between 780 nm and 1600 nm.

[0069] The heating radiation also extends, at least in part, into the visible light spectrum, which is generally between 380 nm and 780 nm.

[0070] The spectrum of heating radiation extends, for example, between 300 nm and 3000 nm.

[0071] The heating means 40 are arranged along a heating zone 42 of the preform production path 34. In the example shown in the figure, the heating station 30 comprises a heating zone 42 divided into two parts arranged upstream and downstream of the bend section guided by the guide wheel 38B.

[0072] The preforms 12 entering the heating station 30 are individually handled by the conveyor 36, on which they travel a U-shaped section of their production path 34, passing through the heating zone 42. They are heated as they pass by the heating means 40, which, where appropriate, are positioned to one side or on either side of the preforms 12 relative to their direction of travel. The hot preforms 12 are extracted from the heating station 30 after passing through the heating zone 42 and transferred into molds in the forming station 32 by a first transfer device 44, such as a transfer wheel, interposed between the heating station 30 and the forming station 32.

[0073] The transfer wheel includes arms (not shown, as they are known in themselves) which successively grasp the preforms 12, as they exit the heating station 30, at the level of their neck 22, to introduce each one in turn into a mold 46 of the forming station 32. The forming station 32 includes a rotating carousel 48 around the periphery of which several blowing stations 50 are arranged.

[0074] Each blowing station 50 includes at least one mold 46 which defines a cavity having the imprint of the container 11. Each blowing station 50 includes means (not shown) for forming / deforming the body 14 of the preform 12 and pressing it against the imprint of the mold 46, for example by stretch-blowing.

[0075] Each hot preform 12 coming out of the heating station 30 is introduced into a mold 46 of the blowing station 50 to be blown and transformed into a container 11. Once completed, the container 11 is extracted from the blowing station 50 by a second transfer device 52, similar to the first transfer device 34.

[0076] Generally, the heating means 40 emit heating radiation at a power determined solely by the absorption factor of the preforms 12 in the near-infrared range in order to reach the setpoint temperature. This power is calculated assuming that the preform 12 has perfectly transparent walls 16 in the visible light range.

[0077] It is useful to know the absorption spectrum of the wall 16 of the preform 12, for example to allow the preforms 12 to be heated as close as possible to the set temperature or to be able to determine the composition of the material constituting the preforms 12.

[0078] To this end, the invention proposes to measure an absorption spectrum “σ(λ1)” of the wall 16 of the preform 12 in the visible light range, for wavelengths generally between 380 nm and 780 nm, in a measurement zone “Z” crossed by the production path 34 upstream of the heating zone 42.

[0079] This measurement can be used to control the power of the heating electromagnetic radiation according to the measured absorption spectrum "σ(λ1)" or to determine the composition of preforms. This process is, for example, implemented automatically by an electronic control unit 54, visible in particular at the.

[0080] The measurement is carried out when the preform 12 is transported by a conveyor along the production path 34 in the container manufacturing installation 10. Preferably, the preform 12 is in constant motion during the measurement. Such a measurement is said to be "on the fly".

[0081] In the examples shown in the figures, the measurement is carried out when the preforms 12 are transported by conveyor 36 from the heating station 30. This conveyor 36 transports the preforms 12 through to the heating zone 42.

[0082] In an unrepresented variant of the invention, the measurement can be carried out when the preforms are transported by another conveyor, for example a transfer wheel.

[0083] The measurement is preferably carried out on the preforms 12 before they are heated, therefore upstream of the heating zone 42.

[0084] In an unrepresented variant of the invention, the measurement is carried out downstream of the heating zone.

[0085] The measurement of the absorption spectrum “σ(λ1)” in the visible range is carried out here by measuring the attenuation of the intensity “I” of a light beam 55, called measurement beam 55, presenting a continuous spectrum “σm”, called measurement spectrum “σm”, in the visible range for each of the wavelengths of the spectrum “σm”.

[0086] The measurement beam 55 is a beam of so-called "white" light emitted in the visible light range, that is, generally between 380 nm and 780 nm. For example, the spectrum "σm" of the measurement beam 55 extends over a range of wavelengths "λ1" that extends between a lower bound "λ1-min", for example equal to 380 nm, and an upper bound "λ1-max", for example equal to 780 nm. The spectrum "σm" of the measurement beam 55 is represented by a dashed curve in Figures 5 and 6.

[0087] For this purpose, the installation 10 includes a device 56 for measuring the absorption spectrum "σ(λ1)" in the visible light range. As shown in Figure 1 by a solid line curve, this device 56 is arranged to measure the absorption spectrum "σ(λ1)" of a preform 12 passing through the measurement zone "Z" shown in Figure 1.

[0088] As shown in the figure, the device 56 for measuring the absorption spectrum "σ(λ1)" in the visible light range includes a measurement light source 58 which emits the measurement light beam 55 with a continuous spectrum "σm".

[0089] To obtain a measurement device 56 suitable for use on the installation 10 in production mode, it is preferable to use luminescent semiconductor components to produce the measurement beam 55. Such a luminescent semiconductor component is, for example, a light-emitting diode (LED), an organic light-emitting diode (OLED), a superluminescent diode (SLED), or a laser diode. This type of component therefore excludes incandescent light sources such as lamps. Such a component is particularly advantageous because it is compact, generates little heat, and is very responsive upon activation.

[0090] Furthermore, to obtain a correct measurement of the absorption spectrum, it is preferable that the measurement spectrum σm of the measurement light source 58 has a profile as close as possible to the reference spectrum σr of a reference illuminant.

[0091] An example of such a reference illuminant is illuminant D65 as defined by ISO 11664-2:2022. Such a spectrum σr is schematically represented with solid lines. It can be seen that the σr spectrum exhibits a nearly constant intensity across the entire visible light range. In particular, it shows relatively subtle dips and peaks.

[0092] However, to date, no luminescent semiconductor component is known that emits light with a spectrum sufficiently close to that of the reference illuminant.

[0093] For example, so-called "white" light-emitting diodes (white LEDs) exhibit a σA spectrum, as schematically illustrated with dashed lines, which shows a lack of intensity in certain areas of the visible spectrum. Specifically, the σA spectrum has a very pronounced "P1" peak around 420 nm, this peak being flanked by two very distinct dips around 405 nm and 450 nm. Furthermore, the intensity of the radiation drops very rapidly for wavelengths above 600 nm. Therefore, the use of a white LED alone is insufficient to obtain a reliable measurement of the absorption spectrum of preform 12.

[0094] To solve this problem, the invention proposes that the measuring light source 58 comprise at least two primary light sources 58A, 58B, 58C, 58D, 58E. Each primary light source 58A, 58B, 58C, 58D, 58E thus emits a primary light beam. Each primary light beam is, for example, guided by an associated optical fiber 60A, 60B, 60C, 60D, 60E.

[0095] The primary light beams are fused to form the said measurement light beam 55. The primary light beams are fused, for example, by means of at least one optical coupler, here a single optical coupler 61.

[0096] As shown in Figure 58, each primary light source 58A, 58B, 58C, 58D, 58E emits a primary light beam with an associated primary continuous spectrum σA, σB, σC, σD, σE that extends at least into the visible range. The primary continuous spectra σA, σB, σC, σD, σE of at least two of said primary sources 58A, 58B, 58C, 58D, 58E are different from each other.

[0097] As shown in Figure 1, the summation of the primary continuous spectra σA, σB, σC, σD, σE of the primary sources 58A, 58B, 58C, 58D, 58E forms the continuous measurement spectrum σm. The primary sources 58A, 58B, 58C, 58D, 58E are selected such that the continuous measurement spectrum σm exhibits a profile that is substantially closer to the reference spectrum σr of the reference illuminant than each of the individual primary spectra σA, σB, σC, σD, σE taken individually.

[0098] Thus, one of the primary light sources is, for example, a white LED which has a spectrum σA such as represented in the.

[0099] A second primary light source is, for example, formed by a primary light source 58B whose spectrum σB has a peak at a wavelength that corresponds to a dip in the spectrum σA of the first primary light source 58A. In the case of a white LED, the dips appear around 405 nm, before the peak, at 450 nm and around 680 nm. Thus, the second primary light source 58B preferably has an emission peak near at least one of these wavelengths.

[0100] For example, the second primary light source 58B has an emission peak around 405 nm. The second primary light source 58B is, for example, an LED, as shown in the figure.

[0101] To best approximate the reference σr spectrum of the reference illuminant, the measurement light source 58 preferably comprises more than two primary light sources with different light spectra.

[0102] Preferably, the measurement light source 58 thus includes a third primary light source 58C which has an emission peak around 450 nm. The third primary light source 58C is, for example, an LED.

[0103] Preferably, the measurement light source 58 also includes a fourth primary light source 58D which has an emission peak around 680 nm. The fourth primary light source 58D is, for example, an LED.

[0104] To balance the intensities of the different wavelengths in the measurement spectrum σm, the device 56 includes at least one electronic control unit 59 for the light intensity emitted by each primary light source 58A, 58B, 58C, 58D, 58E. The electronic control unit 59, for example, controls the electrical power supplied to each primary light source 58A, 58B, 58C, 58D, 58E so that it emits a light beam at an intensity that allows the measurement spectrum σm to approximate the reference spectrum σr as closely as possible.

[0105] When the maximum intensity provided by a primary light source 58A, 58B, 58C, 58D, 58E is insufficient to accurately approximate the reference spectrum σr, the measurement light source 58 may also include at least two primary light sources 58A, 58E that exhibit the same primary light spectrum σA, σE. This allows the intensity of the measurement light beam 55 to be increased for the wavelengths of said primary light spectrum σE so that the measurement spectrum σm is closer to the reference spectrum σr of the reference illuminant.

[0106] In the embodiment shown in the figure, the measuring light source 58 thus includes a second white LED.

[0107] As shown in the figure, the superposition of the different spectra σA, σB, σC, σD, σE of the primary light sources 58A, 58B, 58C, 58D, 58E allows us to obtain a measurement beam 55 presenting a measurement spectrum σm whose profile approaches the reference spectrum σr of the reference illuminant.

[0108] Depending on the requirements, it is possible to extend the measurement spectrum σm of the measurement light source 58 beyond the visible range, for example into the near-infrared range, located between 780 nm and 1600 nm, by adding an additional primary light source whose light spectrum extends at least partly into the near-infrared range.

[0109] As shown in the figure, the measuring light beam 55 is emitted perpendicularly to the wall 16 of the preform 12. The measuring light beam 55 is emitted so as to pass through the preform 12 by passing through its main "X" axis.

[0110] The measurement light beam 55 is emitted more particularly along an axis which is perpendicular to a tangent to the production path 34 of the preform 12 in the measurement “Z” zone.

[0111] The measuring light beam 55 passes through the wall 16 at a determined height "h".

[0112] The 55 light measurement beam can be guided in the correct direction by an optical guiding means such as an optical fiber 60.

[0113] The device 56 for measuring the absorption spectrum "σ(λ1)" in the visible light range further includes a spectrometer 62 which measures the intensity "I" as a function of the wavelength "λ1" of the spectrum "σ0" of the measurement light beam 55 after passing at least once through the wall 16 of the body 14 of the preform 12, as shown in the figure.

[0114] The spectrometer 62 is arranged here so as to measure the intensity "I" of the measurement light beam 55 on the other side of the preform 12 with respect to the axis "X" of the preform 12 after it has passed twice through the wall 16 at two diametrically opposite points, at the same height "h" of the preform 12.

[0115] In an alternative (not shown) of the invention, the spectrometer is arranged to measure the intensity of the measurement light beam after it has passed through the preform wall only once. In this case, a light beam interceptor is introduced into the preform, the interceptor guiding the intercepted light beam to the spectrometer.

[0116] Spectrometer 62 thus allows obtaining the measured absorption spectrum “σ(λ1)” shown in the figure.

[0117] According to one example of the use of the measurement performed, the power of the heating electromagnetic radiation is modified according to the absorption spectrum "σ(λ1)" measured by the electronic control unit 54.

[0118] The measured absorption spectrum “σ(λ1)” is used for example to calculate the color parameters of the wall 16 of the preform 12 in a determined color space.

[0119] As a non-limiting example, such a color space is here formed by the CIELAB color space as defined by the ISO 11664-4:2019 standard, sometimes referred to as "L*a*b* CIE 1976".

[0120] In an unrepresented variant of the invention, the determined color space may be another normalized color space equivalent to the CIELAB color space, such as the CIE RGB color space, defined by ISO 61966-2-1:2003, or the CIE XYZ color space, defined by ISO 11664-3:2019, or the CIE 1976 L*u*v* color space, defined by ISO 1164-5:2016, or even the CIE U'V'W' color space. A color space is "equivalent" to the CIELAB color space when there exists a bijection that allows easy conversion from one space to the other.

[0121] Three parameters characterize colors in the CIELAB color space. Lightness (L*) is derived from luminance. The two parameters a* and b* express the difference between the color and that of a gray surface of the same lightness. A gray, uncolored, achromatic surface is illuminated by a reference light, which is defined here as standardized daylight D65 according to ISO 23603:2005.

[0122] The parameters of the L*a*b* system are as follows:

[0123] - the brightness L* which takes values ​​between 0, corresponding to black, and 100, corresponding to white;

[0124] - a* represents the value on an axis ranging from green to red;

[0125] - b* represents the value on an axis ranging from blue to yellow.

[0126] According to ISO 11664-4:2019, the different parameters are calculated based on the transmission factor T(λ1) for each associated wavelength.

[0127] In particular, the luminous intensity "I0" at which the measurement beam 55 is emitted as a function of the wavelength "λ1" is a known value. The luminous intensity "I0" at which the measurement beam 55 is emitted can also be measured directly by the spectrometer 62 when no preform 12 passes through the measurement zone "Z", as represented by the dashed curve in the figure.

[0128] After passing twice through the wall 16 of the preform 12, the luminous intensity of the measurement beam 55 is attenuated. The measurement beam 55 then exhibits a luminous intensity called the "transmitted luminous intensity "I1", as represented by the solid line curve in the figure.

[0129] Thus, it is easy to deduce the transmission factor of the wall 16 of the preform as a function of the wavelength in the visible light range by calculating the ratio between the light intensity "I0" of emission and the light intensity "I1" transmitted.

[0130] Each parameter in the determined chromatic space, such as the CIELAB space, is then calculated as a function of the transmission factor T(λ1) and the associated wavelength "λ1".

Claims

1. Device (56) for on-the-fly measurement of the absorption spectrum (σ(λ1)) in the visible light range of at least one wall (16) of preforms (12) intended to be transformed into final containers by forming, in particular by stretch blow molding. The device (56) for measuring the absorption spectrum (σ(λ1)) comprises: - means (36) for conveying preforms (12) in a line along a production path (34) passing through a measurement zone (Z), - a measurement light source (58) that emits a measurement light beam (55) according to a continuous measurement spectrum (σm) that extends at least into the visible range, - a spectrometer (62) measuring the intensity (I) and the wavelength (λ1) of the first light beam (55) after passing through at least one thickness of the wall (16) of the preform (12) during its passage through the associated measurement zone (Z), characterized in that the measurement light source (58) comprises at least two sources (58A, 58B, 58C,58D) primary light sources, each emitting a primary light beam according to an associated continuous spectrum (σA, σB, σC, σD) extending at least into the visible range, the primary continuous spectra (σA, σB, σC, σD) of said primary sources (58A, 58B, 58C, 58D) being different and their light beams being merged to form said measurement light beam (55).

2. Device (56) according to the preceding claim, characterized in that the summation of the primary continuous spectra (σA, σB, σC, σD) of said primary sources (58A, 58B, 58C, 58D) forms the continuous measurement spectrum (σm), the primary sources (58A, 58B, 58C, 58D) being selected so that the continuous measurement spectrum (σm) has a profile that is substantially closer to the reference spectrum (σr) of a reference illuminant than each of the primary spectra (σA, σB, σC, σD) taken alone.

3. Device (56) according to the preceding claim, characterized in that the reference illuminant is illuminant D65 as defined by ISO 11664-2:2022.

4. Device (56) according to any one of the preceding claims, characterized in that the measuring light source (58) comprises more than two primary light sources (58A, 58B, 58C, 58D) all having different light spectra (σA, σB, σC, σD).

5. Device (56) according to any one of the preceding claims, characterized in that the measuring light source (58) comprises at least two primary light sources (58A, 58E) which have the same primary light spectrum (σA, σE) so as to increase the intensity of the measuring light beam (55) for the wavelengths of said light spectrum (σA, σE) so that the measuring spectrum (σm) is closer to the reference spectrum (σr) of the reference illuminant.

6. Device (56) according to any one of the preceding claims, characterized in that each primary light source (58A, 58B, 58C, 58D, 58E) is formed by a luminescent semiconductor component.

7. Device (56) according to any one of the preceding claims, characterized in that the primary light beams are fused by means of at least one optical coupler (61).

8. Device (56) according to any one of the preceding claims, characterized in that it comprises a primary light source which emits at least partly in the near-infrared range.

9. Device (56) according to any one of the preceding claims, characterized in that it comprises at least one electronic unit (59) for controlling the light intensity emitted by each primary light source (58A, 58B, 58C, 58D, 58E).

10. Device (56) according to any one of the preceding claims, characterized in that the preforms (12) are made of a thermoplastic material, in particular PET.

11. Installation (10) comprising means (36) for conveying preforms (12) in a line along a predetermined production path (34) passing successively through a heating zone (42) for a body (14) of the preforms (12) and through a station (32) for forming the preforms (12) into a container (11), in particular by stretch blow molding, the installation (10) comprising the device (56) according to any one of the preceding claims.

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