DEVICE FOR HEATING PLASTIC PREFORMS WITH SHIELDING PLATE

Description

[0001] The present invention relates to a device and method for heating plastic preforms. It is known in the prior art that plastic preforms are formed into plastic containers, for example, by stretch blow molding. For this purpose, the plastic preforms are first heated and then, in a heated state, moved into the actual forming device. Microwave ovens are sometimes used to heat the plastic preforms in the prior art, but infrared ovens are also commonly used.

[0002] The plastic preforms are moved through these ovens in an upright position so that the base bodies of the plastic preforms are heated for the subsequent stretching process. However, the plastic preforms also have threaded areas that should not be heated. Without appropriate facilities, excessive heating of the threaded areas could occur, which in turn is detrimental to further processing.

[0003] Therefore, shielding plates are known from the prior art, which are usually mounted on the holders and are intended to prevent excessive heating of the threaded areas and in particular of the support rings. In the prior art, the plastic preforms are usually gripped and moved through openings in these shielding plates or within them. However, in the prior art, this means that these shielding plates must always have a larger cross-section than the largest cross-section of the plastic preforms, which is usually located in an area of ​​the support ring of the plastic preforms. This, in turn, creates comparatively large gaps between the plastic preforms and the respective shielding plates, through which heat can nevertheless escape.

[0004] In other words, the existing light gap between the shielding plate and the support ring of the plastic preforms in the prior art still leads to a certain degree of overheating of the support ring. This, in turn, can lead to problems with a blow molding nozzle imprint, which can form in the support ring, during subsequent processing, for example, by a blow molding machine.

[0005] Such shielding devices are known, for example, from WO 2017 / 140 660 A1, FR 2 950 284 A1, AT 000 520 U1, US 7,121,821 B1, WO 2015 / 181 738 A1 or US 5,066,222.

[0006] WO 2017 / 140 660 A1, for example, discloses a heat shielding device with two heat shielding elements that are movable relative to one another. AT 000 520 U1, for example, discloses a device for producing hollow bodies from tubular, thermoplastic preforms, wherein the preforms are transported through a heating device and subsequently fed to a blow molding device. Furthermore, FR 2 950 284 A1 discloses a protective device for thermally protecting a neck region of a preform when the preforms are transported along a predetermined transport path through a furnace.

[0007] The present invention is therefore based on the object of enabling improved heat shielding, which also protects the support ring and / or the thread of the plastic preforms more effectively than is achieved in the prior art. This object is achieved according to the invention by the subject matter of the independent patent claims. Advantageous embodiments and further developments are the subject matter of the dependent claims.

[0008] A device according to the invention for heating plastic preforms comprises a transport device that transports the plastic preforms at least partially along a predetermined transport path. This transport device comprises a plurality of holding devices for holding the plastic preforms. Furthermore, the device comprises a heating device for heating the plastic preforms at least temporarily during their transport by the transport device. The device comprises at least one heat shielding device that is suitable and intended to counteract heating of the mouth regions of the plastic preforms.

[0009] According to the invention, the heat shielding devices comprise at least a first heat shielding element and a second heat shielding element, which are movable relative to one another and which, in at least one relative position to one another, at least partially and preferably substantially surround a portion of the plastic preform to be heated in its circumferential direction. Preferably, the heat shielding devices also each comprise a first heat shielding element and a second heat shielding element.

[0010] In contrast to the one-piece heat shielding plates of the prior art, the invention proposes that these be constructed in two parts and be movable relative to each other. This allows these shielding elements to be positioned more precisely or closer to the plastic preforms, thus reducing the occurrence of light gaps between the plastic preform and the cross-section of the shielding plate. This also reduces unwanted heating of the mouth areas of the plastic preforms.

[0011] In the context of the present invention, a heat shielding device is understood to mean, in particular, a device which is suitable and intended to at least partially interrupt a heat flow running in the direction of the mouths of the plastic preforms.

[0012] Substantially surrounding or largely surrounding the plastic preforms is understood to mean surrounding by at least 160°, preferably by at least 180°, preferably by at least 200°, preferably by at least 220°, preferably by at least 240°, preferably by at least 260°, preferably by at least 280°, preferably by at least 300°, preferably by at least 320° and particularly preferably by at least 340° and particularly preferably by at least 350°. It is preferably possible for the two shielding elements together to completely surround the plastic preform or except for a small gap at most. Advantageously, the two shielding elements form a hole or in particular an at least partially circular hole which is adapted in particular to the outer contours of the plastic preform so that the gap between the plastic preform and the two shielding elements is small.

[0013] In a further advantageous embodiment, a plurality of heating elements are arranged laterally next to the transport path of the plastic preforms. These heating elements are preferably arranged in a region in which the plastic preforms are conveyed in a straight line.

[0014] In a further advantageous embodiment, at least one shielding element, and preferably both shielding elements, can be advanced toward the plastic preform by a movement that occurs perpendicular to a longitudinal axis of the plastic preform. Advantageously, the shielding elements can be advanced toward the plastic preform below a support ring. This means that the support ring of the plastic preform itself is no longer heated. The shielding elements are preferably moved in a horizontal direction.

[0015] In a further advantageous embodiment, a movement device is provided which can advance the shielding elements laterally or with a movement component perpendicular to the longitudinal direction of the plastic preform. However, said advance movement is particularly preferably carried out by a conveyor chain or another circulating conveyor means (such as a belt) on which the shielding elements are arranged. It is conceivable that, if this conveyor means runs in a straight line, the individual shielding elements are advanced towards one another, and if the conveyor follows a curved path (such as around a deflection wheel), the individual shielding elements move apart. However, it would also be conceivable for the shielding elements to be advanced by means of a separate drive.It would also be conceivable that this or generally a movement device is selected from a group of movement devices which includes guide curves, electrical movement devices, pneumatic movement devices and hydraulic movement devices.

[0016] In an advantageous embodiment, these holding devices comprise mandrels that can be inserted into the mouths of the plastic preforms. The plastic preforms are held by these mandrels. These holding devices are advantageously active holding devices that can activate and deactivate a grip on the plastic preforms.

[0017] In a further advantageous embodiment, these mandrels are designed to be rotatable, so that during heating of the plastic preforms, rotation of the same with respect to their longitudinal direction is also possible.

[0018] In a further advantageous embodiment, at least one and preferably exactly one or exactly two heat shielding elements are preferably assigned to each holding device. It is also advantageously possible for receiving segments of two adjacent heat shielding elements to be assigned to each holding device. Receiving segments of two adjacent heat shielding elements are also advantageously assigned to each plastic preform. In a further advantageous embodiment, the transport device has a circulating transport means and in particular a circulating transport chain. The holding devices for holding the plastic preforms can also be arranged on this transport chain, and furthermore, the shielding elements are preferably also arranged on this circulating transport means. Each plastic preform (or its threaded region) is preferably shielded by exactly two successive shielding elements.

[0019] In a further advantageous embodiment, it is possible for the shielding devices to be arranged on the same holding device as the aforementioned gripping elements or the aforementioned mandrels. In this case, it would be conceivable for at least one or both heat shielding elements, i.e. either the first or the second or even both heat shielding elements, to be arranged on the same holding device as the mandrels. It would thus be possible for both the holding device for holding the plastic preforms and the shielding device to be arranged on the transport chain by means of the same holding device. Preferably, at least one shielding device is arranged on the same holding device as the gripping elements, preferably a plurality of shielding devices are arranged on the same holding devices as the gripping elements, and particularly preferably all shielding devices are arranged on the same holding devices as the gripping elements.

[0020] In a further advantageous embodiment, a transport path along which the plastic preforms are moved is curved at least in sections and preferably also rectilinear in sections. The shielding devices are preferably designed such that they are closed in a rectilinear section of the transport path, and the plastic preforms are located in a heated state in the shielding elements.

[0021] Preferably, adjacent shielding elements are separated from each other in the curved regions of the transport path.

[0022] Preferably, the plastic preforms are fed in a curved section of the transport path. Preferably, the plastic preforms are also removed in a curved section. Preferably, both the feeding and removal of the plastic preforms take place in the same curved section of the transport path. In a further advantageous embodiment, the shielding elements are also arranged on the transport device.

[0023] In a further advantageous embodiment, the heat-shielding elements are plate-like. It is possible for these plates to be positioned underneath the support rings of the plastic preforms.

[0024] In a further advantageous embodiment, at least one heat shielding element has a first recess for receiving at least one portion of the plastic preform.

[0025] In a further advantageous embodiment, at least one heat shielding element and the heat shielding elements are made of a reflective material (in particular of a (heat) radiation reflecting and / or heat reflecting) material.

[0026] In a further advantageous embodiment, at least one heat-shielding element, and preferably the heat-shielding elements, are made of a material suitable and intended for absorbing IR radiation. Particularly preferably, at least one heat-shielding element, and preferably the heat-shielding elements, are made of an aluminum alloy, such as stainless steel. Alternatively or additionally, it would also be possible to use a polished material as the material for the heat-shielding element(s).

[0027] This recess is particularly preferably a trough into which a section of the plastic preform, and in particular a section of its base body, can enter. This trough advantageously has a circular segment-shaped profile. In this way, the plastic preform can reach very close to the respective trough, or the heat shielding element can be moved very close to the plastic preform. A radius of curvature of this recess is particularly preferably adapted to an outer circumference of the plastic preform to be heated. It is possible for this radius of curvature to be slightly larger than a corresponding radius of curvature of the plastic preform, so that the shielding element can nevertheless be moved very close to the plastic preform.

[0028] In a further advantageous embodiment, at least one shielding element has a second recess for receiving a section of the or a further plastic preform. In this embodiment, at least one shielding element has two such recesses. These two recesses are advantageously arranged on opposite sides in the transport direction or along the transport path of the plastic preforms. It is possible for the first receiving device to receive regions of a first plastic preform and the second receiving recess to receive regions of a second plastic preform, which are transported in particular adjacent to one another or one behind the other. Therefore, sections of two successive plastic preforms are preferably shielded by a specific shielding element.

[0029] In a further advantageous embodiment, the first recess of the first shielding element and the second recess of the second shielding element are designed such that they interact at least temporarily to substantially surround a portion of the plastic preform in its circumferential direction. In this embodiment, it is particularly proposed that two adjacent shielding elements be positioned toward one another such that they surround the plastic preform during its heating.

[0030] In an advantageous embodiment, at least one shielding element has a first recess for receiving a portion of a first plastic preform, as well as a second recess for receiving a portion of a second plastic preform. Advantageously, these are plastic preforms transported adjacent to one another.

[0031] In a further advantageous embodiment, at least two heat shielding elements are of identical design. In particular, the two heat shielding elements have the same geometric configuration. Preferably, in particular, two shielding elements that interact to surround the same plastic preform are also of identical design. Advantageously, two shielding elements arranged consecutively in a transport direction or along the transport path are also of identical design. In a further advantageous embodiment, all shielding elements are of identical design.

[0032] In an advantageous embodiment, at least one of the recesses mentioned is suitable for surrounding the plastic preform in its circumferential direction at an angle that is greater than 70°, preferably greater than 90°, preferably greater than 120°, preferably greater than 140°, preferably greater than 150°, preferably greater than 160° and particularly preferably greater than 170°. Alternatively or additionally, at least one recess is suitable and intended for surrounding the plastic preform in its circumferential direction at an angle that is less than 270°, preferably less than 250°, preferably less than 230°, preferably less than 210°, preferably less than 200° and particularly preferably less than 190°.

[0033] In a further advantageous embodiment, in a state in which the recesses surround a plastic preform, an area surrounded by these recesses is smaller than or equal to the cross-section of a support ring of this plastic preform. These surface dimensions, or also this specific design of the respective shielding elements, allow a gap between the plastic preform and the shielding element to be efficiently reduced.

[0034] The present invention is further directed to a shielding element for shielding heat from uprightly transported plastic preforms. This shielding element has a holding device for at least indirectly fastening the shielding element to a transport device for transporting plastic preforms, as well as a first recess which is suitable and intended for receiving at least a portion of a first plastic preform, wherein this recess extends over a circumferential angle of less than 180°. According to the invention, the shielding element has a second recess which is suitable and intended for receiving at least a portion of a second plastic preform, wherein this second recess extends over a circumferential angle of less than 180°.A shielding element is therefore proposed which is suitable and intended for receiving two plastic preforms and in particular two consecutive plastic preforms.

[0035] Advantageously, the shielding element has a plate-shaped section which can be inserted below the support ring of the plastic preforms.

[0036] In a further preferred embodiment, at least one recess has a circular segment-shaped profile. Preferably, both recesses, viewed together, sweep an angle of more than 160°, preferably more than 180°, preferably more than 200°, preferably more than 220°, particularly preferably more than 240°, particularly preferably more than 260°, and particularly preferably more than 300°. In a further preferred embodiment, the two recesses point in opposite directions. These two recesses are preferably spaced apart from one another. In a further advantageous embodiment, the holding device has at least one (and preferably two) rod-like bodies which are arranged, for example screwed, on the shielding plate (which has the recesses). This rod-like body is preferably arranged between the two recesses.

[0037] The present invention further relates to a method for heating plastic preforms, wherein the plastic preforms are transported along a predetermined transport path by means of a transport device and are heated at least temporarily during this transport. Furthermore, heating of the mouth regions of the plastic preforms is counteracted by means of heat shielding devices.

[0038] According to the invention, the heat shielding devices comprise at least a first heat shielding element and a second heat shielding element, which are movable relative to one another and which, in at least one relative position to one another, at least partially and preferably substantially surround a section of the plastic preform in its circumferential direction.

[0039] Advantageously, the plastic preforms are conveyed at least partially in a straight line and / or at least partially along a curved, in particular circular, path. Advantageously, the heat-shielding elements are also moved relative to one another. These heat-shielding elements are particularly preferably arranged on the transport device and are also moved toward or away from one another by the transport device itself. It is possible for adjacent heat-shielding elements to have a minimum distance from one another in the straight sections and a maximum distance, at least temporarily, in the curved sections.

[0040] Advantageously, adjacent shielding elements move toward or away from each other in a pincer-like motion. In a further advantageous embodiment, the plastic preforms are inserted between two adjacent heat-shielding elements by a movement in a direction perpendicular to the longitudinal direction of the plastic preforms. Preferably, the removal of the plastic preforms also occurs by a corresponding movement perpendicular to the longitudinal direction of the plastic preforms.

[0041] Further advantages and embodiments can be seen from the attached drawings: Fig. 1: A schematic representation of a system for producing containers; Fig. 2: A detailed representation of a transport device for transporting plastic preforms; Fig. 3a and 3b: Two representations of shielding elements; Fig. 4a and 4b: Two representations of shielding elements mounted on holding devices; Fig. 5: A further view of a device according to the invention; Fig. 6: A representation illustrating the shielding elements.

[0042] Figure 1shows a schematic representation of a system 50 for producing plastic containers. From a storage device 52, plastic preforms are first fed to a heating device 1 by means of a transport device 54. This heating device 1 has a transport device, designated 2, for transporting the plastic preforms, as well as at least one heating device 4, which heats these plastic preforms during their transport by the transport device 2. A forming device 58, such as a blow molding machine, is provided downstream of the heating device 1. This forms the heated plastic preforms into plastic containers, in particular plastic bottles.

[0043] Figure 2shows a detailed view of the transport device 1. A plurality of shielding elements 62, 64 are provided. It can be seen that these shielding elements 62, 64 each serve to shield a mouth region 10a of the individual plastic preforms 10. The shielding elements 62, 64 are arranged on chain links and are designed such that they abut one another, at least during rectilinear transport, in a section I of the transport device.

[0044] In this way, it is possible, on the one hand, to introduce the plastic preforms into a space between two adjacent holding elements in a deflection area of ​​the transport device, and, on the other hand, these can be conveniently closed along a straight path. It can also be seen that the shielding element 62 has a first recess 62a and a second recess 62b. Likewise, the shielding element 64 also has a first recess 64a and a second recess 64b.

[0045] Reference numeral 6 designates the shielding device as a whole, meaning here the entirety of all individual shielding elements. In a closed state, i.e., particularly during linear transport, the recess 62a of the shielding element 62 and the recess 64b of the subsequent shielding element 64 serve to surround the plastic preform in its circumferential direction. This makes it possible for the shielding elements to have a smaller cross-section than, for example, the support ring of the plastic preform.

[0046] The Figures 3a and 3bshow two representations of shielding elements 62. Here, the recess 62b and the recess 62a opposite it can be seen in particular. The reference numeral 66 denotes support devices with which the shielding element can be fastened to a holding device. The reference numeral 68 denotes a recess for locking the shielding elements to a holding device. In this way, a quick change of the holding elements is also possible. Due to the designs or the two-part design of the shielding elements, it is possible for the holding elements to be inserted below the support ring during the transport process. In this way, it is possible for a very large circumferential angle of the plastic preform to be directly shaded by the shielding elements. Experiments have shown that a blow nozzle imprint for the downstream blow molding machine is considerably reduced.The shielding elements described here also allow for a more favorable neck extraction after the heating process and also more process freedom due to reduced heating of the mouths of the plastic preforms.

[0047] The Figures 4a and 4b show a composition of a holding arrangement 5 on which a shielding element 62 is arranged. It can be seen that the shielding elements 62 are plug-in elements that can be inserted into areas of the holding device 5. The reference numeral 52 denotes fastening means, such as screws or the like, which serve to hold the shielding elements 62. This holding arrangement also has (not shown) the individual holding devices for holding the plastic preforms. This holding arrangement can be arranged on a conveyor chain that serves to transport the plastic preforms.

[0048] Figure 5shows a further illustration of a device according to the invention. Here, it can be seen that the successive shielding elements 62 and 64 do not yet completely surround the plastic preforms, but rather a certain angular area remains empty or unshielded. In this embodiment, a further shielding element 65 is provided, which here is in the form of an elongated plate and which covers the remaining area in the circumferential direction around the plastic preforms. In this case, the shielding device 6 for each plastic preform is composed of the two shielding elements 62 and 64 and the aforementioned shielding plate 65. This shielding plate 65 is preferably arranged stationary, i.e., the plastic preforms move relative to this shielding plate.In contrast, the shielding elements 62 and 64 move with the plastic preforms and are therefore particularly preferably also arranged on the transport device 2, i.e., the transport chain. Preferably, the shielding elements are arranged stationary relative to the plastic preforms, at least during the transport of the plastic preforms in a straight section.

[0049] Figure 6shows a further illustration of the device according to the invention. In this illustration, in addition to the recesses 62b and the shielding elements 62 and 64, the gripping element 74 can also be seen, which engages the plastic preforms to hold them. This gripping element is designed here as a mandrel that can be inserted into the mouths of the plastic preforms. Also visible is the carrier 66, by means of which the shielding elements are attached to their holding device. These gripping elements are therefore the aforementioned holding devices for holding the plastic preforms.

[0050] The applicant reserves the right to claim all features disclosed in the application documents as essential to the invention, provided they are novel, individually or in combination, over the prior art. It is further noted that the individual figures also describe features that may be advantageous in and of themselves. The skilled person will immediately recognize that a specific feature described in a figure may be advantageous even without adopting additional features from that figure. Furthermore, the skilled person will recognize that advantages may also arise from a combination of several features shown in individual or different figures. List of reference symbols

[0051] 1 Heating device 2 Transport device for transporting the plastic preforms 4 Heating device 5 Holding device 6 Shielding device 10 Plastic preforms 10a Mouth area of ​​the plastic preform 10 50 System for producing plastic containers 52 Storage device 54 Transport device 56 Fastening means 58 Forming device 65 Shielding plate 66 Support devices 68 Recess for locking the shielding elements 74 Gripping element 62, 64 Shielding elements 62a First recess 62b Second recess 64a First recess 64b Second recess 66 Support 74 Gripping element, holding device I Section of the transport device

Claims

1. Apparatus (1) for heating plastic preforms (10), having a transport device (2) which transports the plastic preforms (10) at least in sections along a predetermined transport path, wherein the transport device (2) having a plurality of holding devices (74) for holding the plastic preforms (10), with a heating device (4) in order to heat the plastic preforms (10) at least temporarily during their transport with the transport device (2), wherein the apparatus (1) having at least one heat shielding device (6) which is suitable and intended for counteracting heating of mouth regions (10a) of the plastic preforms (10), characterized in that the heat shielding devices (6) have at least a first heat shielding element (62) and a second heat shielding element (64), which are movable relative to one another and which, in at least one relative position to one another, at least partially surround a portion of a plastic preform (10) in the circumferential direction thereof and wherein the holding devices (74) have mandrels, which can be inserted into openings (10a) of the plastic preforms (10) and these mandrels are designed to be rotatable, so that during the heating of the plastic preforms it is also possible to rotate them with respect to their longitudinal direction.

2. Apparatus (1) according to at least one of the preceding claims, characterized in that the holding devices (74) are active holding devices which activate and deactivate a grip of the plastic preforms (10).

3. Apparatus (1) according to at least one of the preceding claims, characterized in that at least one heat shielding element (62, 64) is assigned to each holding device (74).

4. Apparatus (1) according to at least one of the preceding claims, characterized in that exactly two heat shielding elements (62, 64) are assigned to each holding device (74).

5. Apparatus (1) according to at least one of the preceding claims, characterized in that both heat shielding elements (62, 64) can be fed to the plastic preform (10) by a movement which is perpendicular to a longitudinal direction of the plastic preform (10).

6. Apparatus (1) according to claim 1, characterized in that the heat shielding elements (62, 64) are plate-like.

7. Apparatus (1) according to at least one of the preceding claims, characterized in that at least one heat shielding element (62, 64) has a first recess (62a, 64a) for receiving at least one section of the plastic preform (10).

8. Apparatus (1) according to at least one of the preceding claims, characterized in that the heat shielding elements (62, 64) are made of a reflective material.

9. Apparatus (1) according to at least one of the preceding claims, characterized in that the heat shielding elements (62, 64) can be fixed to the holding devices via a quick-change connection.

10. Apparatus (1) according to claim 7, characterized in that at least one heat shielding element (62, 64) has a second recess (62b, 64b) for receiving a section of the plastic preform (10).

11. Apparatus (1) according to claim 10, characterized in that the first recess (62a, 64a) of the first shielding element (62, 64) and the second recess (62b, 64b) of the second shielding element (64) are formed such that they cooperate at least temporarily to substantially surround a portion of a plastic preform in the circumferential direction thereof.

12. Apparatus (1) according to at least one of the preceding claims, characterized in that at least one heat shielding element (62, 64) has a first recess (62a, 64a) for receiving a section of a first plastic preform and a second recess (62b, 64b) for receiving a section of a second plastic preform (10).

13. Apparatus (1) according to at least one of the preceding claims, characterized in that at least two heat shielding elements (62, 64) are of the same design.

14. Apparatus (1) according to at least one of the preceding claims 7 - 11, characterized in that at least one recess (62a, 62b, 64a, 64b) is suitable for surrounding the plastic preform (10) in its direction at an angle which is greater than 70°, preferably greater than 90°, preferably greater than 120°, preferably greater than 140°, preferably greater than 150°, preferably greater than 160° and particularly preferably greater than 170° and / or that at least one recess (62a, 62b, 64a, 64b) is suitable for surrounding the plastic preform (10) in its direction at an angle which is less than 270°, preferably less than 250°, preferably less than 230°, preferably less than 210°.

15. Apparatus (1) according to at least one of the preceding claims 9 - 12, characterized in that in a state in which the recesses surround a plastic preform (10), an area surrounded by these recesses (62a, 64b) is smaller than or equal to the cross-section of a supporting ring of this plastic preform.

16. Apparatus (1) according to at least one of the preceding claims, characterized in that the heat shielding elements (62, 64) are arranged on the same holding device as the holding devices (74).

17. Shielding element (62, 64) for shielding heat from plastic preforms (10) transported in an upright position, with a holding device for fastening the shielding element (62, 64) at least indirectly to a transport device (2) for transporting plastic preforms, which has a plurality of holding devices (74) for holding the plastic preforms (10), having a first recess (62a, 64a) which is suitable and intended for receiving at least a portion of a first plastic preform (10), wherein this recess (64a) extending over a circumferential angle of less than 180°, characterized in that the shielding element (62, 64) has a second recess (62b, 64b) which is suitable and intended for receiving at least one portion of a second plastic preform, wherein this second recess (62b, 64b) extends over a circumferential angle of less than 180° and wherein the holding devices (74) have mandrels which can be inserted into mouths (10a) of the plastic preforms (10) and these mandrels are designed to be rotatable, so that during the heating of the plastic preforms it is also possible to rotate them with respect to their longitudinal direction.

18. Method for heating plastic preforms (10), wherein the plastic preforms (10) are transported along a predetermined transport path by a transport device (2), which has a plurality of holding devices (74) for holding the plastic preforms, and are heated at least temporarily during this transport, wherein heating of mouth regions (10a) of the plastic preforms (10) is counteracted by a heat shielding device (6), characterized in that the heat shielding devices (6) each have a first heat shielding element (62) and a second heat shielding element (64), which are movable relative to one another and which, in at least one relative position to one another, at least partially surround a portion of the plastic preform (10) in the circumferential direction thereof, and wherein the holding devices (74) have mandrels which can be inserted into mouths (10a) of the plastic preforms (10) and these mandrels are designed to be rotatable, so that during the heating of the plastic preforms it is also possible to rotate them with respect to their longitudinal direction.