Device and method for heating plastic containers, in particular PET preforms, using spectrally narrowband light sources

Abstract

Device for heating plastic containers (101) for the production of bottles, in a heating section comprising a control unit (105) for controlling the heating and one, preferably several, heating stations (190), wherein each heating station (190) comprises at least one, preferably a plurality of spectrally narrowband light sources (261), characterized in that: each spectrally narrowband light source (261) is suitable for emitting light with a wavelength in a range in which between 30% and 50% of the incident energy can be absorbed in the first wall of the plastic container, wherein the at least one narrowband light source is capable of emitting narrowband light with a half-width of less than Δλ = 50nm.

Description

[0001] The invention relates to a device for heating plastic containers using spectrally narrowband light. background

[0002] Devices for partially heating preforms using laser light are already known from the prior art.

[0003] EP 2 253 453 A1 shows a belt-like heating of the preform material directly below the support ring.

[0004] Furthermore, WO 2008 / 145331 A1 discloses a device and a method for heating preforms using laser light, in particular using a laser with further laser optics for shaping and / or focusing the laser beam within the wall material of the plastic container in order to achieve the most uniform heating possible.

[0005] Other devices and methods for heating preforms using laser light are also known from documents US 2012 / 0160822 A1, DE 10 2009 047 540 A1, WO 2012 / 066459 A1 and US 2011 / 0006462 A1.

[0006] However, previous methods had the disadvantage that lasers were used which have a high absorption rate in the respective plastic and therefore complete heating through the wall of the preform was not possible or only a thin layer of the preform was burned, while the interior of the preform remained cold. Task

[0007] The invention is therefore based on the objective of improving and simplifying existing methods for heating plastic containers with narrowband light.

[0008] This problem is solved according to the invention by the device according to claim 1 and the method according to claim 10. Advantageous embodiments of the present invention are covered by dependent claims 2-9 and 11-14.

[0009] The device for heating plastic containers for bottle production is characterized by the fact that the spectrally narrowband light sources used (hereinafter also referred to simply as narrowband light sources) are suitable for emitting light with a wavelength in a range where the plastic container can absorb at most 30 to 50% of the incident energy in its first wall. Although only a relatively small proportion of the energy emitted by the narrowband light source is deposited in the plastic container, this amount of deposited energy allows for continuous, uniform heating of the container through its entire wall thickness without the use of complex focusing optics. At the same time, excessive heating of the plastic container's surface, which would lead to white crystallization and even melting of the plastic surface, is prevented.In the following, spectrally narrowband radiation refers to electromagnetic radiation with a half-width of less than Δλ = 50 nm. A narrowband light source, in this context, is a light source capable of emitting narrowband light with a half-width of less than Δλ = 50 nm.

[0010] In one embodiment, the device is characterized in that the narrowband light source is a laser. Lasers are generally very well suited for generating spectrally narrowband light because, due to their design, they have a very small spectral half-width, thus avoiding the emission of energy in a wavelength range not used for heating the plastic container and consequently leading to a more economical process.

[0011] In another embodiment, the device is characterized in that the spectrally narrowband light sources are semiconductor lasers. Semiconductor lasers prove to be very effective due to their high efficiency of up to 50% and can therefore, provided they are available in the suitable wavelength range, lead to energy savings, as they convert a high percentage of the supplied electrical energy into narrowband light. A further advantage of semiconductor lasers is that it is possible to change their wavelength by altering their chemical composition and / or doping. This makes it much easier to obtain an optimal wavelength that best matches the absorption characteristics of the respective plastic container.

[0012] In one embodiment, the device is characterized in that each heating station, which includes one or more narrowband light sources, is equipped with motors for changing the relative position of the narrowband light sources to the plastic container. These motors can be controlled by a control unit during the heating process. This allows the heating of the plastic container to be controlled in a targeted manner, and in particular, it is possible to heat only specific areas of the plastic container.

[0013] In a further embodiment, the device is characterized in that the control unit is suitable for controlling the heating of the plastic container according to a predetermined temperature gradient. This ensures, for example, that the heating of the plastic container can be optimized according to its intended use. This increases the device's flexibility.

[0014] In a further embodiment, the device is characterized in that the narrowband light sources in each heating station are arranged such that they emit light orthogonally to the axis of symmetry of the plastic container located in the heating station. This allows for uniform illumination of the plastic container to be heated.

[0015] In a further embodiment, the device is characterized in that the narrowband light sources of a heating station are arranged symmetrically to the axis of symmetry of the plastic container located in the heating station. This ensures that the distance between the plastic container to be heated and each narrowband light source is always the same, so that the area illuminated by each narrowband light source is heated to the same degree as another area illuminated by a different narrowband light source. This allows for very uniform heating and prevents uneven heating due to absorption in the ambient air.

[0016] In another embodiment, the radiation emitted by the narrowband light sources has a wavelength of 1.5 µm to 2.3 µm, particularly preferably 1.6 µm to 1.9 µm.

[0017] Using, for example, this device, a method for heating plastic containers for bottle production can be implemented, the method being characterized in that the narrowband light sources emit light in a range where a first wall of the plastic container absorbs 30 to 50% of the incident energy. This prevents the crystallization of layers in the plastic container to be heated and ensures uniform heating that penetrates the entire wall of the plastic container.Absorption in a wall means that the plastic container is completely permeated by the spectrally narrowband radiation. The emitted radiation passes through the wall facing the light source (first wall) of the plastic container, where it is partially absorbed, and is then transmitted, again with partial absorption, through the wall facing away from the narrowband light source (second wall). Between 30% and 50% of the incident light is absorbed when passing through the first wall. Upon passing through the second wall, another 30% to 50% of the light transmitted through the first wall and reaching the second wall is absorbed.

[0018] In one embodiment, the method is characterized in that the emitted light strikes the irradiated plastic container orthogonally to its axis of symmetry. This prevents light, and thus energy, from penetrating parts of the plastic container that are not intended to be heated or that are to be heated in a different way at a later time.

[0019] In a further embodiment, the method is characterized in that an inhomogeneous heating profile is generated by changing the relative position of the narrowband light sources to the plastic container during heating, which is controlled by the control unit. This allows for the targeted heating of specific areas of the plastic container and therefore ensures significantly greater flexibility compared to the use of infrared radiation for heating plastic containers.

[0020] In a further embodiment, the method is characterized in that the achieved heating profile of the plastic container corresponds to a temperature gradient specified in the control unit. This ensures that certain necessary temperature differences between different areas of the plastic container, which are required, for example, for subsequent stretching, can be precisely set.

[0021] In a further embodiment, the method is characterized in that the plastic container is irradiated with light in the range of 1.5µm to 2.3µm, particularly preferably from 1.6µm to 1.9µm, by the narrowband light sources. Brief description of the characters Fig. 1 Schematic representation of a device according to the invention. Fig. 2 Schematic representation of a heating station according to the invention. Fig. 3 Schematic representation of an emitter element when using lasers. Fig. 4 Schematic representation of the adjustment device of a heating station. Fig. 5 Schematic representation of a possible embodiment of a heating station. Fig. 6 Schematic representation of another possible embodiment of a heating station. Detailed description

[0022] Fig. Figure 1 shows a device 100 according to the invention for heating plastic containers 101. For this purpose, the plastic containers 101 are moved from a previous process section 140 into an oven 130 on supports 102, which preferably hold the plastic containers 101 at the support ring by means of provided holding elements 120. This oven 130 comprises several heating stations 190, each of which has a heating element 106. This heating element 106 can emit narrowband radiation 104 into the plastic container 101. This heats the plastic container 101 along the length L of the oven 130. Additionally, a control unit 105 is provided, which controls the movement of the plastic containers 101 on the supports 102 through the oven 130 as well as the control of the heating stations 190 and, in particular, the heating elements 106.

[0023] Fig. Figure 2 shows a schematic representation of a heating element 206. The heating element 206 preferably comprises, in addition to a holder 262, heating segments 260. The heating segments 260 in turn comprise one, preferably several, emitters 261. Narrowband radiation 204 is emitted from these emitters 261. The heating segments 260 are preferably arranged modularly within the heating element 206, meaning that they can be individually controlled and, in the event of a malfunction, individually removed. The emitters 261 of each heating segment 260 contain the necessary means for generating the narrowband radiation 204.

[0024] Fig. Figure 3 shows possible realization forms of the heating segments 360 within the heating elements 360. In one embodiment according to Fig. The emitter 361 comprises lasers 363 suitable for generating narrowband light. Preferably, the lasers 363 generate narrowband light with a wavelength between 1500 and 2200 nm, particularly preferably between 1680 nm and 2100 nm, which is absorbed by the first wall to a degree of 30 to 50%. To increase the efficiency of the heating process, it is advantageous if the lasers 363 are semiconductor lasers for the corresponding wavelength range, since these lasers allow for an efficient conversion of the supplied electrical energy into emitted radiation. The emitter 361 can further comprise optics not shown here. These optics are preferably located at the output of the emitter 361 and allow for any necessary focusing or scattering of the narrowband light, so that the plastic container to be heated is irradiated with as much of the narrowband light as possible and thus heated.

[0025] It should be noted here that while there are currently no sufficiently efficient LEDs in the required spectral range that exhibit yields comparable to semiconductor lasers, it is certainly conceivable that LEDs could be used for heating with appropriate technological developments. Since LEDs have a significantly wider beam angle than conventional semiconductor lasers, additional focusing devices, such as mirrors or lenses, would be necessary to achieve effective and intensive irradiation of the preforms. The depicted embodiments of the emitter 361 are by no means to be considered limiting. In particular, the distribution of the lasers 363 can be modified. For example, it is conceivable to use several layers of lasers 363 stacked on top of each other in an emitter 361 in order to irradiate not only a narrow strip of the container to be heated, but also a very large area simultaneously.It is also conceivable to combine different emitters 361, which can include not only different sources for the narrowband radiation but also different arrangements of these sources. For example, if it is intended that a certain area of ​​the plastic container is to be heated completely, whereas another area is only to be heated line by line, a corresponding arrangement of the narrowband light sources in the emitters 361 can be provided in advance to minimize the control effort.

[0026] Determining the wavelength or narrowband light source to be used depends significantly on the percentage of energy to be deposited in the first wall, and thus on the wall thickness and, in particular, the material properties (color, absorption behavior, additions of IR absorbers (such as carbon black), etc.) of the plastic preform. Assuming that the absorption spectrum of the material used for the plastic preform is known (for example, in the form of corresponding graphs), it is possible to determine the wavelength to be used from the intended wall thickness and the intended percentage of energy to be deposited. This can be approximated using the absorption law, which for homogeneous media is N(x) = N(0)e -µ·xThe equation is given by , where µ is the absorption coefficient, × is the path length traveled through the material by the light emitted by the narrowband light sources, and N(0) is the number of incident photons. This is, of course, only a first approximation in this context, as this law only applies to homogeneous media. For different material layers, more accurate models will be required. The given mathematical relationship therefore serves only as an illustration and should not be interpreted as a limitation of the method. The relative fraction I of absorbed photons, based on the number of incident photons, is thus given by I = e -µ·xThe absorption coefficient µ depends at least on the wavelength of the incident radiation and material properties (absorber, color, etc.). If all parameters are known—wall thickness (and thus the path length x traveled by the photons in the material), the relative fraction I of the energy to be absorbed in the first wall, and the material properties influencing the absorption coefficient—the wavelength required for a given material can be directly determined from the corresponding absorption behavior. For example, a PET preform with a wall thickness of 3 mm, in whose first wall 50% of the incident energy is to be absorbed, must be irradiated with light from a narrowband light source with a wavelength of 1.76 µm. For the same material, but with a wall thickness of 1 mm, the ideal wavelength for 40% absorption is 1.67 µm.

[0027] Since it is possible to vary the wavelength of semiconductor lasers within a certain range by changing the properties of the active medium (doping, operating temperature, etc.), a corresponding narrowband light source can be provided if the wavelength to be used is known.

[0028] In this context, real-time adjustment of the narrowband radiation used is also possible, although this adjustment can only be based on changes in the operating parameters of the narrowband light sources and therefore can only be very small. However, it may prove advantageous to use oven 130 in Fig. 1. Upstream sensors are used to register minor changes in the surface, wall thickness, or color of the plastic preforms and, insofar as the operating parameters of the narrowband light sources allow, to make adjustments. Such adjustments are particularly advantageous for each individual narrowband light source, as this allows changes or discontinuities (irregular wall thickness, distribution of absorbers, color differences, etc.) in the material of the plastic preform to be compensated for very precisely.

[0029] Fig. Figure 4 shows a possible implementation of a movable heating element 406. The heating element 406 is connected to its mounting 462 via a rail 466, which is mounted in a guide 467. A suitable motor can be provided in the guide 467, enabling the heating element 406 to move up and down. This has the advantage over a movable mounting of the emitters or narrowband light sources that the angle of incidence of the radiation on a plastic container to be heated is always the same, thus preventing uneven heating due to different angles of incidence in different areas of the container.

[0030] However, if a movable mounting of the emitters is advantageous, particularly for rotating the beam path by a predetermined angle, this can be achieved through appropriate design of the optics or the emitters. Combinations of movable emitters and a movable heating element 406 are also conceivable.

[0031] Fig. Figure 5 shows a possible embodiment of a ring-shaped heating element 506. The heating element 506 can be mounted to allow movement in the vertical direction. In this configuration, it is lowered concentrically around the plastic container 501 in the holder 502, and the emitter 561, which completely surrounds the plastic container 501, ensures continuous, all-around heating of the container.

[0032] Fig. Figure 6 shows another alternative for implementing the heating elements 606. Here, several heating elements 606 are also arranged concentrically around the plastic container 601 in the holder 602. These do not necessarily have to be the four heating elements 606 shown here; any other number of heating elements 606 can also be provided. Since the heating elements 606 together do not completely enclose the plastic container 601, it is advantageously provided that at least some of the heating elements 606, preferably all of them, are movably mounted so that they can completely encircle the plastic container 601. Furthermore, with a suitable design of the emitters 661, this allows some of the heating elements 606 to be used for heating a large area of ​​the plastic container 601, while other heating elements 606 are used for the targeted heating of specific regions, such as...The area directly below the support ring or the application of specific heating profiles are suitable. The simultaneous vertical mobility of the heating elements 606 allows for highly flexible heating of the plastic containers 601, enabling the implementation of a variety of heating profiles. The heating profiles are preferably specified to the control unit via a corresponding temperature gradient, which then controls the heating elements 606 so that a corresponding heating profile is imprinted onto the plastic container 601.

[0033] Alternatively, the heating elements 606 can be fixed to the walls of the oven instead of being movable. The plastic container 601 can then be moved past them and rotated for complete heating. The control unit can preferably also apply a heating profile by appropriately switching the light sources.

[0034] It can also be provided that the heating of the plastic container 601 is supported by a mandrel inserted into the plastic container 601, which is not shown here for the sake of clarity. This mandrel can, for example, be inserted into the opening of the plastic container 601 through the holder 602 and reflects incident light, at least in the wavelength range used, with a reflectance of at least 0.8, preferably 0.9, and particularly preferably between 0.95 and 1. Thus, when the plastic container 601 is irradiated from all sides, radiation transmitted through the walls of the plastic container 601 can preferably be reflected back in the direction of incidence, thereby achieving heating from the inside of the wall.

[0035] In this context, it would also be possible to insert heating elements into the plastic container 601 instead of the mandrel, thus heating the container either in addition to the externally mounted heating elements 606 or solely through the opening of the inserted heating element. This offers particular advantages if the externally directed radiation and the necessary heating elements 606 require too much space. If inserting heating elements through the opening is not easily achievable, the interior of the container 601 can also be heated by heating elements mounted above the opening, for example, on the bracket 602.

[0036] The latter implementations can be compared with those that are, for example, in Fig. The 6 items shown can be combined or used individually.

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