Abstract:
The invention relates to a method and a device for heating preforms of a thermoplastic material, the preforms, after having been heated, being subjected to a reshaping operation, and microwaves being applied to the preforms, at least during a portion of the period of heating, in a resonator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is filed under 35 U.S.C. 371 as a U.S. national phase application of PCT/EP2007/004154, having an international filing date of May 10, 2007, which claims the benefit of DE 10 2006 022 207.5 having a filing date of Mar. 11, 2006, the contents of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The disclosure relates generally to methods and devices for heating preforms and an arrangement for producing containers that include these methods and devices. More particularly, the disclosure relates to methods and devices for heating preforms via microwaves and an arrangement for producing containers including the same. 
     BACKGROUND 
     Frequently, in the production of plastic hollow bodies, a method is used in which preforms are first heated, and then shaped. A particular domain of application of this technique is, for example, the production of foodstuffs containers, in particular plastic bottles, which are produced from preforms. For this purpose, these preforms are heated, and subsequently subjected to final shaping, to form containers. The heating operation is usually performed using infrared radiation or near-infrared radiation. A disadvantage of these heating devices with infrared is that the efficiency of the heating operations is only very low, at approximately 20%. 
     It is therefore the object of the invention to create a method, a device and an arrangement by means of which it is possible to heat plastic preforms in an energy-saving manner. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the disclosure is directed to a method for heating preforms of a thermoplastic material that comprise a region to be heated and a region not to be heated, wherein the preforms are to be subjected to a reshaping operation after heating. The method may comprise moving at least one preform that is to be heated into a resonator, and applying microwaves, at least during a portion of a period of heating, to the region of the at least one preform that is to be heated in the resonator. 
     The method serves to heat preforms, preforms to be understood to be bodies that are composed mainly of plastic and are reshaped, in particular blow-moulded or stretch-blow moulded, after heating. Preferably, preforms in this case are plastic preforms from which containers for the foodstuffs industry, in particular bottles for the beverages industry, are produced. The plastic is usually a thermoplastic plastic, preferably PET. However, it is also possible to use other materials, such as, for example, PET derivatives/copolyesters, other polyesters, polyamides, polyacryls, polycarbonates, polyvinyls (such as PVC; EVOH; PVA) or polyolefins. In the selection of plastics in this case, a good excitation capability must be the primary objective. A measure of such good excitation capability is a high dielectric loss factor. This means it is possible to heat even plastics that, although they do not have a high dielectric loss factor themselves, nevertheless acquire the latter through addition of additives or through purposeful modifications made to the plastic. 
     These plastic preforms consist of an opening region, a body region and a base region, the opening region preferably having a screw thread or a plurality of beads, at least one bead preferably being realized as a support ring. In most cases, it is not desirable for the upper part of the preform, i.e. the opening region, to be heated as well, since this region is not intended ultimately to be reshaped. Preforms to be shaped are therefore frequently divided into different regions, which are either not to be heated at all, or are to be heated only slightly. Alternatively, a region that is to be heated slightly, or that is not to be heated, can also be a region that is very thin, and which therefore does not require a large amount of shaping. 
     It is self-evident that the regions of the preforms that are to be heated can be heated completely in one step, as hitherto, by means of the new heating method, a preferred development of the invention consisting in that the regions of the preforms that are to be heated are heated in a stepwise manner or portionally. Stepwise in this case means that the region of the preforms that is to be heated is heated step-by-step in temporal succession. 
     The preform in this case is divided into a plurality of sub-regions—preferably 3 to 9—which are heated successively. 
     Portional heating is understood to be that whereby, although the region to be heated is heated simultaneously, it is nevertheless heated differentially in respect of a height or thickness profile. In addition, portional heating can also be understood to be differential heating in respect of the circumference of the preforms. Circumferentially differential heating makes it possible for differing shapes of containers, such as, for example, oval containers or containers of other contours, to be produced more easily. 
     According to a preferred development of the invention, the heating of the preforms is performed in a resonator. A resonator is to be understood to be a component into which electromagnetic radiation is introduced and in which the latter, the radiation, is held for a defined period of time by continuous reflection. It is sought to generate in the resonator an appropriate microwave field having an appropriate field-strength distribution. The field-strength distribution in this case depends on the resonator geometry and the other components (e.g. microwave compact head). Appropriate means that it is as uniform as possible for isotropic heating, while a non-uniform field-strength distribution is sought for anisotropic heating. In order to achieve an optimum field-strength distribution, it is also possible to overlay a plurality of microwaves, the microwaves being able to have both the same and differing frequencies. 
     The resonator is so designed that the resonator height, and thereby also the extent of the microwave field, preferably corresponds to only a portion of the height of the preforms to be heated. Preferably, the height is 0.01 cm to 20 cm, particularly preferably 0.5 cm to 1 cm. Furthermore, the resonator is an open, annular or disc-shaped system, in which the preforms can be inserted. For this purpose, it has a preferably round opening, the diameter of which corresponds approximately to 1.1 to 2 times the diameter of the preform and thus encompasses the full circumference of the preform, at least portionally in respect of its height. In this way, the preform can be moved into and out of the resonator. The resonator width is between 1 cm and 15 cm, preferably approximately 4 cm. 
     The opening of the resonator in this case is so selected that the diameter is smaller than the wavelength of the radiation that is to heat the preform, in order to prevent the radiation from emerging from the resonator. Such a geometric configuration of the resonator makes it possible to construct a microwave-based heating device for preforms that does not have to fulfil very strict requirements in respect of shielding. Depending on the geometry of the resonator, protective devices that prevent the escape of leakage radiation can be mounted above and/or below the resonator. Preferred protective devices are hollow cylinders whose geometry is matched to the resonator geometry, the wavelength, etc. 
     The opening of the resonator is preferably so mounted that the preform is moved in the direction of its longitudinal axis into or through the resonator. It is also possible, however, for the resonator to have an opening that makes it possible for the preform to be moved into the resonator, transversely in relation to the longitudinal axis of the preform. 
     According to a preferred development of the invention, a resonator is assigned to each preform in the heating equipment. An embodiment of the invention consists in that the preforms are transported along a path such as that currently found frequently in the prior art, namely, along two straight lines respectively connected to one another at their ends by reversing regions. 
     A particularly preferred development of the invention consists, however, in that the resonators revolve in a circular motion about a fixed machine axis. The preforms thus describe a circular path in respect of a plane perpendicular to the machine axis. This variant has the advantage that very high machine speeds can be realized. Preferably, this heating equipment has 10 to 80 resonators, particularly preferably 20 to 40 resonators. 
     The heating of the regions to be heated is preferably performed such that the preform is moved, in one direction of its longitudinal axis, and subsequently in the opposite direction, through the resonator. It is also conceivable, however, for the preforms to be passed, not merely once or twice, but several times, through the resonator. A straight-line movement, such as, for example, movement twice, four times or six times through the resonator, is particularly appropriate in this case. 
     It is also possible, however, for the purpose of heating the preforms, to use a resonator cavity into which the region of the preforms that is to be heated is inserted fully. The process of heating can thus take place without movement of the preforms in respect of their longitudinal axis, namely, in that the preforms are inserted, for example radially, in the cavity. Also conceivable, however, is that the preforms be moved into and/or taken out of the cavity vertically. 
     Instead of the use of a cavity, it is also possible to use a resonator stack, in which a differing number of resonators are stacked over one another, in order thereby to produce a “cavity” into which a preform can be inserted in its entirety, or almost in its entirety, in respect of its length. 
     A particularly preferred development of the method consists in that the temperature of the preform is measured at least once before and/or during and/or after heating. The method thereby acquires the advantage that a particularly accurate temperature regulation or temperature profiling of the preform is possible, it being immaterial whether the temperature is measured from inside and/or from outside. The measurement is effected by means of an appropriate sensor, such as, for example, a pyrometer or a glass fibre-optic sensor, the latter having the advantage that it is not affected by microwaves and that the microwaves are not affected by the sensor. The pyrometer, however, has the advantage that it operates very rapidly and in a contactless manner. 
     The measured temperature values are preferably forwarded to an open-loop and closed-loop control device in order that temperature control of the preforms can be effected as precisely as possible, a particularly preferred development of the invention consisting in that, in the case of multiple passage of the preforms through the resonator, the temperature is measured after the first passage, such that an adaptation of the heating operation can be performed even at the stage of a second through-movement of the preform. In this way, it is possible, on the one hand, to produce a very accurate temperature profile and, on the other hand, for external circumstances, such as, for example, the moisture of the preforms, to be taken into account in a particularly satisfactory manner. 
     As a control variable for the entire operation, it is possible to use not only the temperature of the preform, but also the reflected power of the microwave. 
     A particular development of the heating operation, including the temperature measurement, consists in that, in a first step, the preform is moved in the direction of its longitudinal axis through the resonator, in a second step the temperature of the preforms is measured while they are still within the zone of influence of the microwave field, the actual temperature profile of the preforms is compared with a specified temperature profile in a third step, and the microwave power/the field-strength distribution in the resonator is immediately adapted in a fourth step, such that an adapted radiation power is already being applied to the preforms during the further movement through the zone of influence of the microwave field. The advantage of this method variant consists in that a very rapid reaction to external circumstances, such as, for example, the moisture content of the preform, is possible. Consequently, the minimum amount of time can be used for the heating operation and, possibly, fewer in and out cycles of passage through the resonator have to be performed. 
     A further method variant for heating consists in that, in a first step, the preform is moved in the direction of its longitudinal axis through the resonator, in a second step the movement of the preform in the direction of its longitudinal axis is stopped, in a third step determination of a temperature profile of the preform is effected by means of vertically movable temperature sensors, in a fourth step the actual temperature profile of the preform is compared with a specified temperature profile, in a penultimate step the resonator is adapted in respect of its radiation power/field-strength distribution, and in a final step the preform is moved in the opposite direction through the adapted resonator. This method variant has the advantage, inter alia, that, prior to the temperature measurement, the preform retains an equalizing period in which the applied heat can become better distributed. As a result, more precise values are obtained in respect of the wanted temperature profile of the preform. 
     Clearly, it is also possible to heat preforms, to determine the attained temperature profile at the end of the heating operation, and to determine therefrom the settings for the next heating operations of the preforms. This variant has the advantage of not requiring any powerful or very rapid feedback control algorithms or feedback control structures. 
     A further possible heating operation consists in that the preforms are put into the resonator when the microwave has been switched off, and are taken out of the resonator when the microwave has been switched on. This enables simple (not strongly profiled) heating operations to be realized rapidly, in an energy-saving manner. 
     According to a particularly preferred development of the invention, the preforms are rotated about their own longitudinal axis during the heating operation. On the one hand, the rotation can be effected uniformly, which—with a uniform field-strength distribution—results in a uniform temperature profile in respect of the circumference, but it can also be effected in a non-uniform manner, which then results in temperature profiling in respect of the circumference of the preforms. The second method variant can be used, for example, in the production of shaped containers, such as, for example, oval containers. The production of shaped containers can be supported in that an anisotropic field-strength distribution is generated in the resonator or in the cavity, whereby differential heating is effected even if the preforms are not rotated or moved. A further possibility for producing differing temperature regions in preforms is that of differential retention time in the microwave field. 
     According to the invention, a microwave heating unit has at least one microwave generator, a microwave conductor and a resonator. The microwave generator can be, for example, a magnetron, a klystron or a gyrotron, the waves being able to be generated in any manner. The microwave conductors are preferably hollow conductors, round or rectangular cross-sections being particularly preferable. Use of coaxial conductors is not precluded. 
     According to a preferred development of the invention, the microwave generator is located in a microwave compact head, the latter further comprising at least a water load, a circulator, a microwave conductor and a heater transformer, as well as terminal leads for the microwave generator and the heater transformer. The water load in the microwave compact had is required for absorbing excess microwave energy and rendering it harmless. This water load is preferably a water-filled silicone or plastic tube, the water circulating in a circuit in order that sufficient residual energy can be absorbed continuously. 
     Additionally provided is in circulator, which preferably consists of three crossed coaxial conductors, and the function of which is to route microwaves into the compact head, in the correct direction in each case. The circulator is necessary because, inter alia, microwaves that are routed from the site of their generation to the resonator and back again must not flow back into the magnetron, since otherwise there is a risk of the latter being destroyed. In this case, therefore, forwarding into the water load must be performed. For this purpose, the circulator has integrated ferrites, which forward the microwaves to the respectively correct output. 
     Additionally provided, preferably, is a microwave tuner, which enables the power to be set in the resonator. It is equally possible to use a manual tuner, rather than an automatic tuner, in this case. 
     According to an embodiment of the invention, a heating oven for preforms has a central microwave generator, which forwards the generated microwaves to the respective generators. This solution has the advantage that the technology to be made available, such as, for example, the microwave generator, is required only once. 
     Another development of the invention makes provision for a plurality of microwave generators, which respectively supply microwaves to several resonators. It is thus possible, for example, for four microwave generators to be available for the resonators. This solution has the advantage that, in the event of a failure of one microwave generator, at least three-quarters of the heating course can nevertheless continue to be operated. According to a particularly preferred development of the invention, each resonator has its own microwave compact head with a microwave generator. This characteristics enables each preform to receive the most individual treatment possible. 
     A further advantageous development of the invention consists in that the resonators, with their assigned microwave generators, are located on a carousel that revolves about a central axis. This carousel arrangement has the advantage that very high machine capacities, and thereby preform throughputs, can be realized. 
     It is also conceivable, however, for the resonators to be fastened to carries that describe, not a circular path, but an at least partially rectilinear path, such as, for example, in linear ovens. For this purpose, it is conceivable for the resonators to be fastened to carriers and for the latter to be moved along a line with the aid of a chain. 
     The output power of the at least one microwave generator is preferably in the range of between 1 kW and 10 kW, particularly good results being achieved with a generator output power of 2 kW to 3 kW. 
     The frequency range of the microwaves to be used for heating is between 300 MHz and 300 GHz, particularly preferred frequency ranges being 915 MHz, 2.45 GHz and 5.8 GHz. 
     According to an advantageous development of the invention, each resonator has a receiving unit for preforms. This receiving unit can preferably be rotated, such that the preform moves about its own axis. A movement along the longitudinal axis of the preform is also necessary, however, in order to move it into and out from the resonator. Preferably, a movement unit has a combined lifting/lowering/rotary drive. Although the movements of the preforms to and in the resonator are preferably effected in the direction of or about the longitudinal axis of the preforms, a movement about other axes is nevertheless also possible. It is thus conceivable, for example, for the preform not to be rotated about its longitudinal axis, but for the preform to execute an oscillating movement about an axis through the resonator. This development has the advantage that the microwaves effect a more random heat distribution in the preform. 
     Alternatively, however, the resonator can also be moved relative to the preform. 
     A particularly preferred embodiment consists in that a microwave heating unit is assigned to each preform. Also conceivable are other arrangements, in which there are fewer heating units than preforms that can be accommodated in the device. 
     In order to increase the capacity of such a heating device, it is also conceivable for a plurality of heating levels to be located above one another. These heating levels can either be loaded independently of one another or also conceivable is a pass in which the preforms are brought into the device on one level while being taken out of the device on the other level. 
     A further advantageous heating variant consists in that the preforms do not undergo any movement in respect of their longitudinal axis, but the resonators are moved along the preforms. 
     The heating can be set in an absolutely individual manner for individual or for various groups of preforms. Parameters that can be set independently of one another are, for example, lift, lift speed, rotation speed, microwave power, microwave tuning settings, field-strength distributions in the resonator or impedance matching in dependence on a measured dielectric constant. Individual settings can be necessary for various reasons, such as, for example, because of the differing moisture contents of the preforms. 
     The device is preferably a heating device for preforms from which PET bottles are produced. It is therefore appropriate for such a heating device to be placed in a machine arrangement comprising, for example, a stretch-blow moulding machine, a filler, a closer, a labelling machine, or the like. 
     A further advantageous embodiment consists in that the heating device has a slipring joint for high voltage, such that energy can be transmitted from stationary parts into the moving parts. 
     According to a preferred development of the invention, it is advantageous if not only microwaves, but also infrared radiation, are applied to the preforms. It is thus also conceivable, for example, for a basic heat profile to be imparted to the preform by the infrared radiation and for the exact temperature profiling then to be performed only by the microwave. The inverse of this process is also conceivable, however, i.e., such that a basic temperature profile is imparted only by means of the microwaves, such that a more precise temperature profiling can then be produced in the preform by means of infrared radiation. It is also conceivable in this case, however, for infrared radiators to be additionally mounted in a microwave oven, such that the two heating methods can be combined with one another in any sequence, in any zones and in any intensity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An actual embodiment of the invention is described more fully with reference to the following drawings, wherein: 
         FIG. 1  shows an isometric view of a heating device, 
         FIG. 2  shows a portion of the heating device according to  FIG. 1 , 
         FIG. 3  shows a detail view of a heating device, 
         FIG. 4  shows a microwave compact head, 
         FIGS. 5   a  to  5   e  show a schematic operational sequence for the heating of preforms, 
         FIG. 6  shows a top view of an installation for producing containers, 
         FIG. 7  shows a side view of a resonator, 
         FIG. 8  shows a side view of a transfer situation, from a resonator and to a resonator, 
         FIG. 9  shows an isometric view of a realization of a heating device, 
         FIG. 10  shows a portion from the representation according to  FIG. 9 , and 
         FIG. 11  shows a further embodiment of a microwave conductor with resonator. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a circular heating device for preforms  1 , the latter being moved on a circular path, according to the circumference of the heating device, in the course of the heating operation. The heating device has a carrier  4 , which, in this case, simultaneously constitutes a rectangular hollow conductor. Fastened to this carrier  4  are various structural units, for instance eight microwave compact heads  20  and forty microwave heating units  3 . These units, fastened to the carrier  4 , revolve jointly about the machine axis  5 . The transfer from an upstream unit to the oven  40  is effected by means of a star, such as, for example, a sawtooth star or a clip star. 
       FIG. 2  shows a portion of the oven  40  according to  FIG. 1 , the microwave heating unit  3  being better explained here. Seen here, likewise, are the microwave compact heads  20 , which generate the microwaves and which are directly connected to the carrier  4 , which, in this case, constitutes a hollow conductor  22 . Mounted in the direction from the carrier  4  towards the microwave heating unit  3  is an injecting element  8 , which injects into the microwave heating unit  3 , from the carrier  4 , the microwaves generated by the microwave compact head  20 . 
     The microwave heating unit  3  consists of a rectangular microwave hollow conductor, bent in an S shape, the first end of which is fastened to the carrier  4  and to the second end of which a resonator  11  is fastened. The resonator  11  is a disc-shaped/plate-shaped, internally hollow element, in the centre of which there is a circular hole. The dimensions of the hole are so selected that the respective preforms  1  to be heated can be guided through without difficulty, the resonator  11  being of a height that corresponds to only a portion of the height of the preforms. The hollow resonator  11  constitutes an extension of the microwave heating unit  3  and—like the hollow conductor—has microwaves flowing through it. Fastened to the microwave heating unit  3 , in the region of the resonator  11 , is a temperature sensor  24 , which measures the temperature of preforms  1  that are being lowered into or raised out of the resonator  11  or are guided out of the latter. 
     Additionally fastened to the microwave heating unit  3  is a microwave tuner  23 , by means of which it is possible to influence the microwaves by altering the conductor space of the microwave heating unit—i.e., to so optimize the field-strength distribution, a preform having been inserted, that the quantity of energy that is reflected, and not absorbed by the preform  1 , is minimized—and thereby also to effect open-loop or closed-loop control of the operation of heating the preforms  1 . 
     Located radially outwards before the microwave heating unit  3  there is a receiving unit  25 , the basic function of which is to receive the preform  1  and to impart to it a movement that renders possible effective heating. The receiving unit  25  consists of a preform holding unit  26  and a movement unit  27 . Here, the preform holding unit  26  is a rod, which goes into the opening of the preform  1  and thereby holds the latter. Preferably, at least a portion of the preform holding unit is to be made of a suitable, non-metallic material, since otherwise leakage radiation might possibly emerge from the cavity. Preferred materials are plastics having a low dielectric loss factor, such as, for example, Teflon. 
     Here, however, it is conceivable for holding to be effected not only by an internal gripper, but also by an external gripper. 
     The movement unit  27  is preferably a multifunction drive, with either differing drives being combined to constitute the movement unit  27  or the movement unit  27  being constituted by a drive that fulfils all movement requirements. On the one hand, a lowering movement is required, which inserts the preform  1  in the resonator  11  from above, along the longitudinal axis A of the preform. Also required, on the other hand, is a lifting movement, which takes the preform  1  back out of the resonator  11 , along the longitudinal axis of the preform. A further movement, which renders the heating process very much more flexible, is a rotary movement, which allows the preform  1  to rotate about its longitudinal axis A. 
       FIG. 3  has another embodiment of the heating unit for preforms  1 . The essential differences consist in that each microwave heating unit  3  has its own assigned microwave compact head  20 . For this purpose, the microwave heating unit  3 , which, here likewise, is again realized as a rectangular microwave hollow conductor, is bent radially outwards in a C shape. In this case, the one end opens in the microwave compact head  20 , while a resonator  11  is again fastened at the other end. Furthermore, the microwave heating unit  3  again has a microwave tuner  23 , which performs the same function as that according to  FIG. 2 . Here, likewise, the preform  1  is held by a receiving unit  25 , which comprises a preform holding unit  26  and a movement unit  27 . 
       FIG. 4  shows a section through the microwave compact head  20 . Located therein is a microwave generator  21 , such as, for example, a magnetron or a klystron, by which microwaves are generated and routed, by means of a microwave conductor  22 , to the circulator  29  and to the applicator output  30  of the latter. The microwave compact head  20  has two terminations  32 , one termination being the plug-in termination for the heating voltage and the other termination being the termination for the high voltage for the microwave generator  21 . 
     Microwaves that are routed from the applicator output  30  into the microwave heating unit  3 , not shown in  FIG. 4 , are reflected continuously, and thus also return back to the circulator  29  of the microwave compact head  20 . In order to prevent the microwaves from penetrating the microwave generator  21 , the circulator  29  is capable of selectively forwarding reflected radiation in the direction of the water load  28 . Here, the water load  28  is constituted by a U-shaped silicone tube that has cooling water flowing through it. This microwave compact head  20  enables the generation of microwaves for heating the preforms  1  to be effected in a very compact, restricted space. 
       FIG. 5  shows, in five steps, various positions of the preform  1  in the course of its heating in the heating unit. The preform  1  in this case has a region  2  that is to be heated, a region  10  that is not to be heated, and a support ring  9  located in the region  10 . The preform  1  is placed concentrically on a preform holding unit  26 . By means of the movement unit  27  described in  FIGS. 2 and 3 , it is now possible to move the preform  1  through the resonator  11  in the direction of the arrow  17  and  18 . The black bar  33  in this case represents the microwave zone of influence  23  of the resonator  11 , not shown here. In a first step, according to  FIG. 5   a , the preform  1  starts to be moved, along its longitudinal axis A, in the direction of the arrow  17 , into the microwave zone of influence  23 . 
       FIG. 5   b , already, shows a position in which the preform  1  has been moved to an extent through the microwave zone of influence  33 . 
     The temperature sensor  24 , which is mounted a short distance beneath the resonator plane TE, has in this case registered from the outside the temperature of the preform  1 , which has already been altered by the microwaves. 
     Also fundamentally conceivable is a temperature measurement from within the preform  1 . 
       FIG. 5   c  shows a position in which the entire region of the preform  1  that is to be heated has already been or is in the microwave zone of influence  33 . This is also the reversal point, following attainment of which the preform  1  is guided out of the microwave zone of influence  33 , along the longitudinal axis A, in the direction of the arrow  18 . 
       FIG. 5   d  shows a position corresponding to that of  FIG. 5   b , the preform  1  here being moved along its longitudinal axis A in the direction of the arrow  18 . Upon commencement of the removal of the preform  1  from the microwave zone of influence  33 , adaptation of the heating by means of microwaves is also performed, such that the specified temperature of the preform  1  can be achieved exactly. Through this combination of temperature sensor  24  and adaptation of the microwaves, a temperature profile that is as exact as possible is to be imparted to the preform  1 . The preform holding unit  26  can be provided with a hole, not shown here, through which, for example, a temperature sensor extends into the interior of the preform  1  to be heated, or through which particular media, e.g. a cooling medium for the purpose of temperature equalization, can enter. 
       FIG. 5   e  shows a position corresponding to that from  FIG. 5   a , the heating operation having already been completed here. 
     There may now follow, for example, a rest phase, although a second or third heating operation, by passage through the microwave zone of influence  33 , can also be performed. 
       FIG. 6  shows a device for producing containers. This device has a preform storage means  34 , into which the preforms  1  are put, without having been sorted. 
     A roller sorter  35  assumes the function of separating and sorting the preforms  1 , which are then guided to a feed chute  36  that feeds them to the oven  40 . 
     In the oven  40 , the preforms  1  are heated as described. After heating, they are transferred into the stretch-blow moulding machine  14 , which produces finished containers. Following production of the containers, they are transferred to the filling machine  15  and into a closing and/or labelling machine, which are not shown further here. In this way, a fully filled and closed container, such as, for example, a beverage bottle, is produced. Mounted between the oven  40  and the stretch-blow moulding machine  14  there are preferably at least two transport stars, which effect active and/or passive cooling of the heated preforms  1 . At this point in the production process, it can be necessary for the heated preforms to be provided with an equalization period, in order for the imparted energy to become uniformly distributed. This equalization period can be provided, for example, via integrated transport stars. An equalization or cooling period can also be necessary again after the stretch-blow moulding operation. Here, once again, it is possible to provide active and/or passive cooling of the containers. Active cooling can be effected, for example, by means of water, air, nitrogen or other media, from the inside or from the outside. Passive cooling can be effected through the provision of a transport course between the stretch-blow moulding machine  14  and the downstream machine. 
       FIG. 7  shows a resonator  11 , which has a hollow cylinder  11   a  on its upper side and lower side, respectively. This hollow cylinder acts mainly to shield against microwaves. Here, this protective device is cylindrical in form, but it is within the scope of action of persons skilled in the art that this protective device also be of a different design, thus, for example, having an angular cross-section. In the resonator  11  there is a reflector element  19 , within the zone of action of the microwave. The function of this reflector element  19  is to additionally heat the tip of the preform  1 . The reflector element  19  is so realized that the microwave radiation is focussed in the direction of the tip of the preform  1 . 
     Advantageously, the reflector element  19  is so realized that, during the movement of the preform  1  through the resonator  11 , at least in the zone of action of the microwave, it is always at the same distance from the tip of the preform  1 . This can be realized, for example, in that the reflector element  19  is moved concomitantly through the resonator  11  in the direction of the longitudinal axis A of the preform  1 . It is also conceivable, however, for a plurality of reflector elements  19  to be mounted within the zone of action of the microwave, these reflector elements also being conceivable at various heights. It is then possible for the reflector elements  19  to be swivelled, respectively, into the path of movement of the preforms  1 . The reflector element  19  is realized to be easily exchangeable. This has the advantage that various preform geometries can be optimally processed in the resonator  11  in each case. 
       FIG. 8  shows a preferred transfer situation, of preforms  1  to the device, and, at the same time, a takeover situation, of preforms  1  from the resonator  11 , before they are transported further to the next machine. The transport of the preforms  1  to the resonator  11  and away from the resonator  11  is performed here by grippers  50   a ,  50   b . The grippers are preferably part of a transfer and takeover star, which are indicated only in schematic form here by the grippers  50   a  and  50   b  and by the central column  37 . The gripper  50   a  in this case takes over a preform  1  from an upstream machine, such as, for example, a preform separating device, and then transfers this preform to a preform holding element  26 , which then performs the treatment operation described above, with the preform  1  being guided at least once through the resonator  11 , along the longitudinal axis A of the preform. When, after its treatment, the preform emerges on the lower side of the resonator  11 , it can be gripped, as by the gripper  50   a , by a gripper  50   b  that is operatively connected to the same central column  37 , and transferred to a machine located downstream, such as, for example, a labelling machine or a filling machine. In the case of rotary machines, this arrangement has the advantage that a very large angle of rotation can be used as process time. With such an arrangement, process angles of between 300° and 355° can be achieved. In addition, this is a very space-saving solution, since only one transport star is required as an intake and discharge star, whereas two transport stars are integrated in conventional solutions. 
       FIG. 9  shows an isometric view of another embodiment of a microwave heating device, which is realized in a rotary design. The device has a plurality of receiving units  25 , a plurality of receiving units  25  (at least two) being mounted on a receiving carrier  38  in each case. The receiving carriers each have a movement unit  27 , which is responsible for the lifting movement of the preforms  1  in the direction of the resonators  11 . In addition, the preform holding elements  26  have a drive for inducing a rotary movement in the preforms  1 . The microwave heating device comprises a plurality of microwave compact heads  20 , each consisting of a microwave tuner  23 , a water load  28  and a microwave generator  21 . For reasons of space, in each case one microwave compact head  20  of a receiving unit  25  is located on the upper side of a disc-shaped microwave conductor  22 , and one microwave compact head  20  is located on the lower side. The microwave conductor  22  is realized as a carrier  4  and has a flat disc shape, which is hollow on the inside, to enable it to route the microwaves from the microwave generator  21  in the direction of the resonator  11 . The microwave conductor  22  in this case can be so realized that two discs are mounted upon one another, which discs then form an inner cavity and thereby become the hollow conductor, although the conductor can also be so constructed that only segments are mounted upon one another in each case, such that no continuous, circular disc hollow conductor is produced, but such that each resonator  11  forms its delimited hollow conductor  22 . 
       FIG. 10  shows a section through the microwave conductor  22  according to  FIG. 9 . It can be seen that two hollow conductor segments  39  are mounted upon one another in the resonator plane TE, and thereby form a microwave conductor  22 . The two hollow conductor segments  39  each have an annular structure. Further outwards on the hollow conductor segments  39  in the circumferential direction are the resonators  11  with the hollow cylinders  11   a  arranged above and below same, respectively. The individual components can be seen clearly in the section through the microwave hollow conductor  22 . Located on the lower side of the hollow conductor segments  39  is the microwave generator  21 , as well as the circulator  29  and the water load  28 . By means of the circulator, the microwaves are routed from the generator  21  into the microwave conductor  22 , and there past the tuner  23 , in the direction of the resonator  11 . The tuner is so realized that three tuning pins  23   a ,  23   b  and  23   c  extend into the microwave conductor  22 , thereby affecting the conduction cross-section. In this way, the heating of the preforms  1  in the resonator  1  can be adapted. This representation shows a preform  1  currently in the resonator  11 . 
       FIG. 11  shows a further embodiment of a microwave conductor  22 . Unlike the embodiment according to  FIG. 10 , here the microwave conductor  22  is not constructed from two annular or disc-shaped hollow conductor segments  39 , but as a hollow conductor profile, from which a plurality of longitudinal pieces are joined together to form an entire hollow conductor. Here, likewise, the microwave tuner  23  can be seen to have three pins  23   a ,  23   b  and  23   c . Additionally provided, in the direction from the microwave generator  21 , not visible here, to the resonator  11 , for the purpose of setting an optimum microwave distribution in the resonator  11 , is an orifice plate  41 , which results in a diameter discontinuity of the microwave conductor  22 . A fastening element  42 , to which the orifice plate  41  is fastened, is provided for mounting purposes. The fastening element  42  in this case has passage dimensions similar to those of the orifice plate  41 .