WO2006105769A1

WO2006105769A1 - Method and device for controlling the temperature of blanks

Info

Publication number: WO2006105769A1
Application number: PCT/DE2006/000590
Authority: WO
Inventors: Horst Muegge; Klaus-Martin BAUMGÄRTNER; Rudolf Emmerich; Matthias Graf; Karl-Heinz Balkau; Klaus Hartwig
Original assignee: Sig Technology Ltd.
Priority date: 2005-04-07
Filing date: 2006-03-30
Publication date: 2006-10-12
Also published as: DE112006001499A5; EP1868785A1

Abstract

The invention relates to a method and a device for controlling the temperature of blanks consisting of a thermoplastic material. At least sections of the blanks are at least temporarily exposed to microwave radiation. The microwave radiation generates a thermal profile in the wall (55) of the blank. The blank is heated in a heating zone (44) that covers only part of the blank and is delimited by the wall. During at least part of the temperature-control process, the blank and the heating zone are displaced in relation to one another.

Description

Method and device for tempering preforms

The invention relates to a method for controlling the temperature of preforms made of a thermoplastic material, in which the preforms are exposed at least partially and at least temporarily to microwave radiation.

The invention furthermore relates to a device for tempering preforms made of a thermoplastic material which has at least one microwave generator and at least one heating zone bounded by a wall.

Such methods and devices are used, for example, for the tempering of preforms, which are converted into a container after thermal conditioning by a blowing process. For example, bottles are made of plastic in such a container molding. In a container forming by. Blasdruckeinwirkung be preforms made of a thermoplastic material, for example, preforms made of PET (polyethylene terephthalate), supplied to different processing stations within a blow molding machine. Typically, a blow molding machine of the type having a heating device and a blowing device, in the region of which the previously tempered preform is expanded by biaxial orientation to a container. The expansion takes place by means of compressed air, which is introduced into the preform to be expanded. The procedural sequence in such an expansion of the preform is explained in DE-OS 43 40 291. The introductory mentioned introduction of the pressurized gas also includes the introduction of compressed gas into the developing container bubble and the introduction of compressed gas into the preform at the beginning of the blowing process.

The basic structure of a blowing station for container molding is described in DE-OS 42 12 583. Possibilities for tempering the preforms are explained in DE-OS 23 52 926.

Within the blow molding apparatus, the preforms as well as the blown containers may be transported by means of different handling devices. In particular, the use of transport thorns, onto which the preforms are attached, has proven to be useful. The preforms can also be handled with other support devices. The use of tongs for handling preforms and the use of spreaders for mounting in a mouth region of the preform are also part of the available constructions.

A handling of containers using transfer wheels is described, for example, in DE-OS 199 06 438 in an arrangement of the transfer wheel between a blowing wheel and a discharge path.

The already described handling of the preforms takes place firstly in the so-called two-stage process, in which the preforms are first produced in an injection molding process, then temporarily stored and later conditioned in terms of their temperature and inflated to a container. On the other hand, there is an application in the so-called one-step process, in which the preforms are suitably tempered immediately after their injection-molding production and sufficient solidification and then inflated.

With regard to the blowing stations used, different embodiments are known. In blow stations, which are arranged on rotating transport wheels, a book-like Aufklappbarkeit the mold carrier is often-meet. But it is also possible to use relative to each other ver-displaceable or differently guided mold carriers. In fixed blowing stations, which are particularly suitable for receiving a plurality of cavities for container molding, typically plates arranged parallel to one another are used as mold carriers.

Before the heating is carried out, the preformed rings are typically attached to transport mandrels which either pass the preform through the entire Transport blowing machine or circulating only in the heating device. In a stationary heating of the pre-moldings such that the mouths of the preforms are oriented in a vertical direction downwards, the preforms are usually plugged onto a sleeve-shaped holding element of the transport mandrel. In a hanging heating of the preforms, in which they are oriented with their mouths in a vertical direction upwards, expanding mandrels are introduced into the mouths of the preforms, which clamp the preforms in the rule.

An essential problem with the use of conventional infrared radiators for heating the preforms is that the predominant radiation component is already converted into heat in the immediate vicinity of the surface of the preform and that a temperature control of the inner wall portions of the preform only by Heat propagation occurs within the thermoplastic material. Since the thermoplastic material has pronounced thermally insulating properties, sufficient time for the preforms to heat up for about 20 seconds results in adequate heat propagation. To avoid overheating of the surface areas of the preform takes place at the same time for heating and blowing with cooling air. This results in a relatively high energy consumption for the implementation of the heating.

To support as even as possible active heating of the preforms by the wall thickness of the preform, it is also known to carry out heating with HF radiation or microwave radiation as an alternative or in addition to heating with infrared radiators. From US Pat. No. 3,830,893 it is already known to heat preforms made of nitride polymers with microwaves. The preforms are in this case transported through waveguide rectangular or round cross section through and thereby heated. The entire area of the preform body is provided here with a homogeneous temperature distribution. In the entire waveguide region, a comparatively low electric field strength of the microwave field is provided. Such low field strengths are sufficient for tempering nitrile polymers, but such methods and devices are not applicable to a variety of thermoplastic materials having low microwave absorbance.

Object of the present invention is to improve a method of the type mentioned in the introduction so that with low mechanical engineering effort a high-quality heating is supported at the same time high throughput rates.

This object is achieved in that the microwave radiation, a heat profile in the wall of the preform is produced, that the preform ling is heated in at least one bounded by a wall heating zone, which receives only a portion of the preform and that during at least one part the temperature is generated relative movement between the preform and the heating zone.

A further object of the present invention is to design a device of the type mentioned in the introduction in such a way that high throughput rates are supported with a simple structural design. This object is achieved in that the heating zone is dimensioned such that it receives only a portion of the preform and that a positioning device for generating a relative movement between the preform and the heating zone adjacent to the heating zone is arranged.

By means of the method and the device according to the invention, it is also possible to generate a finely tunable temperature profile in the area of preforms made of low-absorbing plastics. The microwaves have a high penetration depth in the material of the preform, so that a volumetric heating of the preforms is supported. It can thereby be generated a favorable radial temperature profile in the wall of the preform. Due to a high conversion rate of the energy used into heat energy within the preform wall, the efficiency of the heating is substantially increased.

The effectiveness of the heating can be further increased by the fact that the microwave radiation is concentrated in a portion of the heating zone.

In particular, a shortening of the heating time can be achieved despite the use of a small-sized microwave generator in that the preform is arranged in the region of the concentrated microwave radiation.

A typical field of application is defined by tempering the preform with microwave radiation in the frequency range from 0.8 to 12 GHz. For this purpose, optionally a microwave generator with predetermined ter fixed frequency or variable frequency is used.

With regard to the construction of the mechanical engineering it proves to be advantageous that the preform is moved relative to a stationary heating zone.

According to a further variant of the invention, it is also contemplated that the heating zone is moved relative to the preform.

The generation of a desired temperature profile is assisted, in particular, by carrying out the relative movement between the preform and the heating zone in the direction of a preform longitudinal axis.

To shorten the heating time is further proposed that the preform is heated by several heating zones simultaneously.

The implementation of a continuous heating process is facilitated by several heating zones in a transport direction of the preform are sequentially passed through the preform.

The generation of an advantageous field distribution within the heating zone can be assisted by superposing two microwaves in the region of the heating zone. In principle, however, only a microwave can be used.

According to one embodiment, it is contemplated that at least two microwaves of the same frequency are superimposed. In addition, it is also possible that at least two microwaves of different frequency are superimposed. These different frequency microwaves can be generated by a plurality of microwave generators.

Back-reflection damage to the microwave generator can be avoided by dissipating microwave radiation reflected back from the heating zone in the direction of the microwave generator.

With regard to typical applications, it proves to be advantageous that the microwave radiation generates a temperature profile in the material of the preform in the direction of the preform longitudinal axis.

Alternatively or additionally, it is also possible that the microwave radiation generates a temperature profile in the material of the preform in the circumferential direction.

A particularly preferred field of application is that the tempered preform is transformed into a container following the heating with the microwaves by a blow molding process.

An optimization of the heating process can be carried out by performing an impedance matching during the execution of the microwave heating.

The change in material parameters as a result of the heating can be taken into account by performing the impedance matching as a function of a measured preform dielectric constant. In the drawings, embodiments of the invention are shown schematically. Show it:

1 is a perspective view of a blow molding station for the production of containers from preforms,

2 shows a longitudinal section through a blow mold, in which a preform is stretched and expanded,

3 shows a sketch to illustrate a basic structure of a device for blow-molding containers,

4 shows a modified heating section with increased heating capacity,

5 shows a schematic representation of the coupling of a microwave generator with a heating zone using typical coupling elements,

6 shows a comparison with FIG. 5 modified arrangement,

7 shows a schematic longitudinal section through a preform arranged in the region of the heating zone,

8 shows a longitudinal section through a preform, which is simultaneously introduced into a plurality of heating zones, 9 shows an illustration of an arrangement having a plurality of heating zones arranged one behind the other and at a distance,

10 is a diagram illustrating a plurality of blowable temperature profiles and

11 shows a field-like arrangement of a plurality of heating zones for simultaneous heating of a plurality of preforms.

The basic structure of a device for forming preforms (1) in container (2) is shown in FIG. 1 and in FIG. 2.

The device for forming the container (2) consists essentially of a blowing station (3) which is provided with a blow-mold (4) into which a preform (1) can be inserted. The preform (1) may be an injection-molded part of polyethylene terephthalate. To allow the preform (1) to be inserted into the blow mold (4) and to allow the finished container (2) to be removed, the blow mold (4) consists of mold halves (5, 6) and a bottom part (7), which is a lifting device (8) is positio-nierbar. The preform (1) can be held in the region of the blowing station (3) by a transporting mandrel (9) which, together with the preform (1), passes through a plurality of treatment stations within the device. But it is also possible to use the preform (1), for example via pliers or other handling means directly into the blow mold (4).

To enable a compressed air supply line, a connecting piston (10) is arranged below the transport mandrel (9). assigns the compressed air to the preform (1) and at the same time performs a seal relative to the transport mandrel (9). In a modified construction, it is basically also conceivable to use solid compressed air supply lines.

A stretching of the preform (1) takes place in this exemplary embodiment by means of a stretch rod (11) which is positioned by a cylinder (12). According to another embodiment, a mechanical positioning of the stretch rod (11) is carried out over curve segments, which are acted upon by Abgriff rollers. The use of curve segments is particularly useful when a plurality of blowing stations (3) are arranged on a rotating blowing wheel

In the embodiment shown in Fig. 1, the stretching system is designed such that a tandem arrangement of two cylinders (12) is provided. From a primary cylinder (13), the stretch rod (11) is first moved to the area of a bottom (14) of the preform (1) before the beginning of the actual stretching operation. During the actual stretching process, the primary cylinder (13) with extended stretching rod together with a carriage (15) carrying the primary cylinder (13) is posi-tioned by a secondary cylinder (16) or via a cam control. In particular, it is envisaged to use the secondary cylinder (16) in such a cam-controlled manner that a current stretching position is predetermined by a guide roller (17) which slides along a curved path during the execution of the stretching operation. The guide roller (17) is pressed by the secondary cylinder (16) against the guideway. The carriage (15) slides along two guide elements (18). After closing the mold halves (5, 6) arranged in the region of carriers (19, 20), the carriers (19, 20) are locked relative to one another by means of a locking device (20).

To adapt to different shapes of a mouth portion (21) of the preform (1), the use of separate threaded inserts (22) in the region of the blow mold (4) is provided according to FIG.

Fig. 2 shows in addition to the blown container (2) and dashed lines drawn the preform (1) and schematically a developing container bladder (23).

3 shows the basic structure of a blasting machine, which is provided with a heating section (24) and a rotating blowing wheel (25). Starting from a preform input (26), the preforms become

(1) of transfer wheels (27, 28, 29) transported in the region of the heating section (24). Along the heating route

(24) heating elements (30) and blower (31) are arranged to temper the pre-moldings (1). After a sufficient temperature control of the preforms (1), they are transferred to the blowing wheel (25), in the region of which the blowing stations (3) are arranged. The finished blown containers (2) are fed by other ϋberga- berädern a discharge line (32).

In order to be able to transform a preform (1) into a container (2) in such a way that the container (2) has material properties which ensure a long usefulness of foodstuffs filled inside the container (2), in particular drinks, special ones must be used Procedural steps in the heating and Ori- Respect the Vorfoπnlinge (1) are respected. In addition, advantageous effects can be achieved by adhering to special dimensioning regulations.

As a thermoplastic material different plastics can be used. For example, PET, PEN or PP can be used.

The expansion of the preform (1) during the orientation process takes place by compressed air supply. The compressed air supply is in a pre-blow phase in which gas, for example compressed air, is supplied at a low pressure level and subdivided into a subsequent main blow phase in which gas at a higher pressure level is supplied, during the pre-blow phase typically Compressed air is used with a pressure in the interval of 10 bar to 25 bar and during the main blowing phase compressed air is supplied with a pressure in the interval from 25 bar to 40 bar.

From Fig. 3 it can also be seen that in the illustrated embodiment, the heating section (24) is formed of a plurality of revolving transport elements (33) which are strung together like a chain and guided by Um-steering wheels (34). In particular, it is thought that the chain-like arrangement would open up a substantially rectangular basic contour. In the illustrated embodiment, in the area of the transfer wheel (29) and an input wheel (35) facing the expansion of the heating section (24) a single relatively large-sized guide wheel (34) and in the area of adjacent deflections two comparatively smaller dimensioned deflection wheels (36) used. In principle, however, any other guides are conceivable.

To enable a possible dense arrangement of the transfer wheel (29) and the input wheel (35) relative to each other, the arrangement shown to be particularly useful, since three deflection wheels (34, 36) are positioned in the region of the corresponding extent of the heating section (24) , in each case the smaller deflection wheels (36) in the region of the transition to the linear courses of the heating track (24) and the larger deflection wheel (34) in the immediate transfer area to the transfer wheel (29) and to the input wheel (35) , As an alternative to the use of chain-like transport elements (33), it is also possible, for example, to use a rotating heating wheel.

After a finished blowing of the containers (2), they are led out of the region of the blowing stations (3) by a removal wheel (37) and transported via the transfer wheel (28) and a delivery wheel (38) to the delivery route (32).

In the modified Heizstrek- ke (24) shown in Fig. 4 can be tempered by the larger number of heating elements (30) a larger amount of preforms (1) per unit time. The fans (31) introduce cooling air into the region of cooling air ducts (39), which in each case oppose the associated heating elements (30) and deliver the cooling air via outflow openings. Due to the arrangement of the outflow directions, a flow direction for the cooling air is realized essentially transversely to a transport direction of the preforming rings (1). Fig. 5 shows the basic structure of a heating element (30). The heating element (30) has a microwave generator (41), which is typically designed as a magnetron. The microwave generator (41) can be coupled to a heating zone (44) via the series connection of a circulator (42) and a tuner (43).

The circulator (42) serves to prevent re-radiation of microwaves into the microwave generator (41). Optionally, microwave radiation reflected from the heating zone (44) is discharged via the circulator (42) into a water load (45) and absorbed there. The circulator (42) and the tuner (43) may be formed as a waveguide. Likewise, training as a coaxial conductor is possible. The circulator (42) has a design similar to a T-piece, wherein the middle leg of the T-piece opens into the water load (42).

The absorption of microwave energy by the preform (1) is essentially dependent on the dielectric properties of the material of the preform (1). These dielectric properties are temperature-dependent and change during the performance of the heating. Using the tuner (43), it is possible to perform an impedance matching and thereby to optimize the heating process during the heating process, even with changing dielectric properties of the preform (1) and thereby maximize both the energy efficiency and the energy efficiency required to minimize heating time. The impedance matching using the tuner (43) can be either controlled or regulated, including impedance detection. A measurement of a current impedance can be performed using diode elements. Temperature measurement can be done using typical infrared sensors. A high-quality temperature measurement is possible in particular because essentially only the preform and not adjacent to the preform components are heated and thereby stray radiation is largely avoided.

The heating zone (44) is designed as a coupling-in system for the microwave radiation. In the area of the heating zone (44), a microwave field of high power density is generated. The high power density can be achieved, for example, by reducing the height of the waveguide, by dielectric or metallic auxiliary bodies and / or by suitable shaping of walls of the heating zone (44). According to one embodiment, a resonator chamber with an E010 resonance is used. When using a single heating zone (44), a height of the waveguide is increased by 30%, and when using a stack of heating zones (44), a height of the waveguide is reduced by 60%. These constructive measures increase the power density by a factor of 2 or by a factor of 9.

Fig. 6 illustrates an embodiment of the arrangement in Fig. 5. The circulator (42) is realized here with a docked water load (45). Between the circulator (42) and the tuner (43) an analysis unit (46) is arranged. Through the heating zone (44) extends through an antenna (47). Behind the door ner (43) is a transition element (48) arranged to allow a transition from the waveguide to a coaxial conductor. The transition element (48) is terminated by a short gate valve (49). Another connection of the heating zone (44) opens via a further transition element (50) in a waveguide (51) which is connected to a short-circuiting slide (52) and a water load (53). Also in this arrangement, the analysis unit (46) detects the impedance of the microwave system and the tuner (43) and the short shifters (49, 52) adjust the impedance of the microwave generator (41) to the present microwave path.

Fig. 7 shows a longitudinal section through a preform (1), which is arranged in the region of the heating zone (44). In the illustrated embodiment, the preform (1) is moved both around a preform longitudinal axis (54) and also positioned in the direction of the preform axis (54). The illustrated embodiment shows a stationary heating zone (44), which is partially enclosed by a wall (55). The heating zone (44) is formed like a chamber and has two openings (56, 57), which are adapted to an external dimensioning of the preform (1). Typically, the openings (56, 57) have a circular configuration and are provided with a diameter that is slightly larger than an outer diameter of the preform (1).

After retraction of the preform (1) into the heating zone (44), a field distribution within the heating zone (44) forms as a temperature profile on the preform (1). By a movement of the preform (1) relative to the heating zone (44), preferably with variably adjustable speed, one can adjustable temperature profile along the parison longitudinal axis (54) are generated.

8 shows an embodiment in which a plurality of heating zones (44) are arranged in a stack in the direction of the preform longitudinal axis (54) one behind the other. Even with such an arrangement, either the preform (1) can be moved relative to the heating chambers (44) or the heating chambers (44) relative to the preform (1).

The power input into the preform (1) can be regulated by various methods. In principle, it proves to be advantageous to use a microwave generator (41) with controllable power. Another control parameter is the specification of the relative speed between the preform (1) and the heating chamber (44). With a variable speed specification, it is possible, for example, to generate locally higher temperatures by means of lower speeds of movement. It is also possible to use the tuner (43) to vary the power flow in the feeder by an impedance change. When stacking a plurality of heating chambers (44), the impedance change is expediently carried out in all feeds used. When using a plurality of heating zones (44) next to or above each other, there is a minimization of the high-frequency technical coupling of the individual heating zones (44).

9 shows an arrangement in which a plurality of heating zones (44) are arranged one behind the other in a transport direction (58) of the preforms (1). The individual heating zones (44) each have a spacing relative to one another. The preform (1) will be included of this embodiment with its preform longitudinal axes (54) in the transport direction (58) moves. An arrangement of the preforms (1) can take place on transport elements (59). In this embodiment too, a relative movement between the preforms (1) and the heating zones (44) is generated by the transport speed of the preforms (1) and / or a movement of the heating zones (44). In particular, it is intended to move the heating zones (44) independently of one another.

10 shows a compilation of blowable temperature profiles. The course (60) shows the temperature distribution over the product length for the blown container (2), the course (61) the corresponding temperature distribution after carrying out the microwave heating for the preform (1) and the course (62) the corresponding temperature distribution in use a conventional infrared heater instead of microwave heating according to the invention. It can be seen in particular that the unwanted temperature control of the mouth region of the preform in the length range from 0 to 20 mm can be significantly reduced by microwave heating.

Fig. 11 shows a field-like arrangement of a plurality of heating zones (44) to allow parallel heating of a plurality of preforms (1). As a result, the temperature of a very large number of preforms per unit time is supported. The heating elements (30) according to the invention make it possible, in particular, to cause a local concentration of the microwaves within the heating zone (44). It is thereby possible to carry out the corresponding concentration of the microwaves in that region of an inner space of the heating zone (44) in which the preform ling (1) is arranged. This concentration makes it possible to minimize the resulting heating time without using extremely powerful microwave generators (41).

According to a further embodiment, both IR emitters and microwaves can be used to control the temperature of the preforms (1). Particularly preferred is a heating of the mouth of the preform (1) opposite ground area with IR radiation in order to install sufficient heat energy here.

Claims

Patent claims

1. A process for the tempering of preforms made of a thermoplastic material, in which the preforms at least partially and at least temporarily exposed to microwave radiation, characterized in that the microwave radiation, a heat profile in the wall of the preform (1) is generated, that the Preform (1) is heated in at least one of a wall bounded by a heating zone (44) which receives only a portion of the preform (1) and that during at least a portion of the temperature control relative movement between the preform (1) and the heating zone ( 44) is generated.

2. The method according to claim 1, characterized in that the microwave radiation is concentrated in a partial region of the heating zone (44).

3. The method according to claim 1 or 2, characterized in that the preform (1) is arranged in the region of the concentrated microwave radiation.

4. The method according to any one of claims 1 to 3, characterized in that the preform (1) is tempered with microwave radiation in the frequency range of 0, 8 to 12 GHz.

5. The method according to any one of claims 1 to 4, characterized in that the preform (1) is moved relative to a stationary heating zone (44).

6. The method according to any one of claims 1 to 5, characterized in that the heating zone (44) is moved relative to the preform (1).

7. The method according to any one of claims 1 to 6, characterized in that the relative movement between the preform (1) and the heating zone (44) in the direction of a preform longitudinal axis (54) is performed.

8. The method according to any one of claims 1 to 7, characterized in that the preform (1) of a plurality of heating zones (44) is heated simultaneously.

9. The method according to any one of claims 1 to 8, characterized in that a plurality of heating zones (44) in a transport direction (58) of the preform (1) are consecutively traversed in time by the preform (1).

10. The method according to any one of claims 1 to 9, characterized in that in the region of the heating zone (44) two microwaves are superimposed.

11. The method according to claim 10, characterized in that at least two microwaves of the same frequency are superimposed.

12. The method according to claim 10, characterized in that at least two microwaves of different frequency are superimposed.

13. The method according to any one of claims 1 to 12, characterized in that from the heating zone (44) in the direction of the microwave generator (41) reflected back microwave radiation is derived.

14. The method according to any one of claims 1 to 13, characterized in that by the microwave radiation, a temperature profile in the material of the preform (1) in the direction of the preform longitudinal axis (54) is generated.

15. The method according to any one of claims 1 to 13, characterized in that a temperature profile in the material of the preform (1) is generated in the circumferential direction by the microwave radiation.

16. The method according to any one of claims 1 to 15, characterized in that the tempered Vorform- ling after heating with the microwaves by a blow molding process in a container (2) is formed.

17. The method according to any one of claims 1 to 16, characterized in that during the implementation of the microwave heating, an impedance adjustment is performed.

18. The method according to claim 17, characterized in that the impedance matching in dependence on a measured temperature of the preform (1) is performed.

19. A device for tempering preforms made of a thermoplastic material having at least one microwave generator and at least one heating zone limited by a wall, characterized in that the heating zone (44) is dimensioned such that it receives only a portion of the preform (1) and that a positioning device for generating a relative movement between the preform (1) and the heating zone (44) adjacent to the heating zone (44) is arranged.

20. Device according to claim 19, characterized in that the heating element (30) has a concentrator for the spatial concentration of the microwaves within the heating zone (44).

21. The apparatus of claim 19 or 20, characterized in that the heating element (30) has a positioning device for the arrangement of the preform (1) in the concentration range of the heating zone (44).

22. Device according to one of claims 19 to 21, characterized in that the microwave generator generates microwaves in a frequency range of 0.8 to 12 GHz.

23. Device according to one of claims 19 to 22, characterized in that the heating element (30) has a positioning device for moving the preform (1).

24. Device according to one of claims 19 to 22, characterized in that the heating element (30) has a positioning device for movement of the heating zone (44).

25. Device according to one of claims 19 to 24, characterized in that the positioning device for carrying out a relative movement in the direction of a preform longitudinal axis (54) is formed.

26. Device according to one of claims 19 to 25, characterized in that the heating element (30) has at least two heating zones (44).

27. Device according to one of claims 19 to 26, characterized in that at least two heating zones (44) in a transport direction (58) of the preforms (1) are arranged one behind the other.

28. Device according to one of claims 19 to 27, characterized in that the heating zone (44) is designed for the superposition of at least two microwaves.

29. The device according to claim 28, characterized in that the heating zone (44) is formed for the superposition of at least two microwaves with the same frequency.

30. The device according to claim 28, characterized in that the heating zone (44) is designed for the superposition of at least two microwaves with different frequencies.

31. The device according to any one of claims 19 to 30, characterized in that in a connection region of the microwave generator (41) with the heating zone (44), a circulator (42) for discharging reflected microwave energy is arranged.

32. Device according to one of claims 19 to 31, characterized in that the heating zone (44) for generating a temperature profile in the direction of a Vorformlingslangsach.se (54) is formed.

33. Device according to one of claims 19 to 31, characterized in that the heating zone (44) for generating a temperature profile in the circumferential direction of the preform (1) is formed.

34. Device according to one of claims 19 to 33, characterized in that the heating element (30) is formed as part of a blow molding machine.

35. Device according to one of claims 19 to 34, characterized in that the heating element (30) has a control for performing an impedance change.

36. Device according to one of claims 19 to 35, characterized in that the heating element (30) has a measuring device for detecting a temperature of the preform (1).