DE20020150U1 - Warming section for stretch blow molding - Google Patents

Description

Law firm GbR

PO Box 860624
81633 Munich

Advanced Photonics November 28, 2000

Technologies AG M/IND-063-DE/G

Bruckmühler Str. 27 MB/BO/HZ/hk

83 052 Bruckmühl-Heufeld
Federal Republic of Germany

Heating section for stretch blow molding

Description

The invention relates to a heating section of a stretch blow molding system for stretch blow molding PET containers (in particular beverage bottles).

Drinks bottles made of stretch-blown PET (polyethylene terephthalate) form a large and expanding segment of the drinks packaging market. They have achieved this market position thanks to their very good performance properties - in particular the extremely low empty weight in relation to the filling volume - and the ease and cost-effectiveness of their manufacture. There are a number of large suppliers who are in tough competition with each other and are therefore subject to strong cost pressure. This forces the production process to be increasingly rationalized. The focus here is on shorter machine cycle times and energy savings.

PET bottles are manufactured in a two-stage process. First, preforms are produced from a PET mass by injection molding. In a second step, these preforms are heated to a stretching temperature of around 110 ⁰ C in a relatively short period of time, and finally, in the heated state, they are fed into a stretch blow mold, in which

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of compressed air into the interior of the preform the container (the bottle) is formed.

It is known and common practice today to use heating sections equipped with infrared radiators through which the preforms pass to heat the preforms. The use of elongated halogen lamps with radiation characteristics that have a significant active component in the near infrared range is also known. These methods and arrangements are superior to the use of medium and long-wave infrared radiation in that the radiation spectrum, which is mainly in the near infrared, is advantageously adapted to the transmission characteristics of plastics in general and PET in particular.

This fact is illustrated by the graphic representation in Fig. 1. Here, the radiation wavelength is plotted on the X-axis and the transmittance (for the transmission curves) or the normalized spectral radiation distribution (for the various radiation spectra) is plotted on the Y-axis.

The thick solid line schematically indicates the typical transmission behavior of plastics and the dotted line the transmission spectrum of PET ₇ , while the thin solid lines marked with the numbers 1, 2 and 3 indicate the radiation spectrum of a medium-wave infrared heater (1), a short-wave infrared heater (2) and a radiator emitting in the near infrared (3).

However, according to the inventors' findings, the known heating processes and lines that use halogen lamps as radiation sources do not yet work optimally, both in terms of energy and with regard to the further processing properties of the preforms.

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The invention is therefore based on the object of specifying an improved heating section for stretch blow molding, which in particular enables an overall more efficient plant construction and operation.
5

This object is achieved by a heating section having the features of claims 1, 2 or 3.

According to a first aspect, the invention includes the essential idea of using near infrared radiation to heat the walls of the preforms as evenly as possible, i.e. to achieve the smallest possible temperature gradient between the outer and inner surfaces. According to the findings of the invention, a preform heated in this way can be further processed particularly easily, in particular with a significantly reduced blowing pressure. This in turn enables considerable energy savings in the actual molding step as well as smaller dimensions of the compressed air units and stretch blow molds and thus considerable cost savings in the production of the system.

In known stretch blow molding systems, several heating sections and compensation sections arranged between them are provided, so that a multi-stage heating/compensation curve is used overall, as is sketched in Fig. 3 in the form of two dotted lines that show the temperatures on the outer and inner walls of a preform as a function of time. It can be seen that in the course of these heating and compensation steps, the initially very large temperature differences between the outer and inner walls equalize with one another. However, in addition to the aforementioned provision of corresponding heating and compensation areas, which entails a correspondingly large-volume construction of the heating section, this also requires a long heating phase. This procedure is therefore not only due to the complex construction of the heating

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distance and because of the long cycle times. The solid lines in the left-hand area of the graphic show that the method proposed here offers significant advantages in these aspects. 5

To further illustrate the method according to the invention, reference is made to Fig. 2, in which the temperature distribution on the outer and inner walls of an injection-molded preform for a 2 1 PET bottle is plotted over the height of the preform. It can be seen that with the implementation of the invention, a temperature gradient of less than 4 K was achieved practically over the entire height of the preform. (The larger deviation near the height zero is not to be taken into account here, because the preforms should remain "cold" near the base point.)

In a preferred process, an overall energy efficiency of the heating step of over 15%, in a particularly advantageous embodiment of over 18%, is achieved. This is due on the one hand to the high efficiency of the energy input of the NIR radiation into the preform walls with a suitable design of the irradiation device and on the other hand to the shortening of the heating section and the elimination of the aforementioned compensation sections with active air supply, in which of course a considerable proportion of the process heat is uselessly discharged in a conventional design.

According to a first aspect of the invention, the design of the heating section according to the invention provides an actively cooled counter-reflector, which is preferably arranged at a short distance from the preforms. In contrast to the conventionally used ceramic reflectors without active cooling, the cooled reflector does not cause a shift in the radiation spectrum towards the long-wave range and therefore fully exploits the advantages of NIR radiation for preform heating. The short distance from the preforms also increases

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lent the efficiency of the energy input from the side facing away from the halogen lamps.

The counter reflector is preferably made of metal - like the main reflector - and cooling is achieved in particular by a cooling air fan.

According to a further important aspect of the invention, the heating section forms a substantially closed radiation space, so that radiation losses are minimized. This is achieved on the one hand by providing an additional "head reflector" which essentially closes off the space between the halogen lamp main reflector group and the counter reflector at the top, with the exception of a ventilation slot. A largely radiation-technical closure of this space at the bottom is achieved by a highly reflective design of the preform holders in the foot area of the preforms or by attaching special reflectors to them.

According to a further essential aspect of the invention, the active surface of the main reflector has a special geometry that ensures a particularly uniform radiation input into the preforms over their entire height - while at the same time "sharply" closing off the radiation field to the base area, which is to be kept as cold as possible. For this purpose, an essentially W-shaped cross-sectional geometry of the main reflector is selected, with respect to the individual halogen lamp, which is known per se from the applicant's DE 199 09 542 A1.

In a preferred embodiment of a heating section according to the aforementioned aspects, a quartz glass disk is arranged between the infrared radiator main reflector group and the preforms, which separates the assembly from the preform conveying area in terms of flow.

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Convection flow from the halogen lamps towards the preforms is prevented, so that they are not surrounded by hot air in an uncontrolled manner, but are heated essentially solely (and thus in a controlled manner) by the absorption of the incident NIR radiation.

Active cooling of both the main and counter reflectors is also preferred, with the main reflector in particular being water-cooled. The counter reflector can be air-cooled to save cooling water, but can also be water-cooled for high-performance applications.

For certain applications, it may be useful to provide an additional line radiator in the heating section, which is directed at the foot areas of the preforms and additionally heats them in a targeted and locally limited manner. Such a line radiator comprises in particular a single elongated halogen lamp in an additional reflector that is essentially partially elliptical in cross-section, the halogen lamp being arranged in a focus of the elliptical reflector cross-section and the assembly being positioned in such a way that the foot areas of the preforms (which rotate during their transport through the heating section) rotate through the second focal point of the elliptical cross-section.

Furthermore, it is preferred that the main and/or counter reflector be made of solid aluminum, which contributes to a particularly homogeneous radiation and temperature field due to its good reflection properties and high heat capacity and also enables the flexible implementation of different heating section configurations in a cost-effective manner. The profiles are manufactured in particular as extruded or injection molded profiles, if necessary also by milling. To easily implement active cooling, the profiles are

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In particular, cooling fluid flow channels are formed. An analogous design is also useful for the additional reflector of the line emitter mentioned above.

Advantages and benefits of the invention are apparent from the dependent claims and embodiments which are described below with reference to the figures. The figures show:

Fig. 1 a graphical representation of the spectral transmittance of plastics compared to the spectral emission of various infrared radiators,

Fig. 2 a graphical representation of the temperature profile in PET preforms,

Fig. 3 is a graphical representation of the time dependence of the temperature in PET preforms in a conventional and an inventive heating process, 20

Fig. 4 is a schematic cross-sectional view of a heating section of a stretch blow molding system according to a first embodiment of the invention (as a schematic cross-sectional view) and 25

Fig. 5 is a schematic cross-sectional view of a heating section of a stretch blow molding system according to a second embodiment of the invention (as a schematic partial cross-sectional view). 30

With regard to Figures 1 to 3, reference is made to the explanations above.

Fig. 4 shows a simplified, schematic cross-sectional view of a heating section 1 for heating

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Injection molding preforms (for which typical outlines are shown in the figure with dot-dash lines, designated Pl and P2) as part of a stretch blow molding system for producing PET bottles. The preforms are fed on a holder 3 into a substantially closed radiation chamber 5. This is delimited on one side by a quartz glass pane 7 and on the other side by a solid aluminum counter-reflector 9, which reflects the radiation passing through the quartz glass pane 7 from a radiation source (described further below) from the side opposite the radiation source onto the preforms.

Above the preforms, a metallic head reflector 11 is provided with a reflective surface inclined to the quartz glass pane 7, which closes off the top of the radiation chamber 5 except for a ventilation gap 13. The preform holders 3, which carry the preforms through the radiation chamber 5, have a highly reflective surface area 3a around the outer contour of the preforms. The reflective surface areas 3a, which are lined up in a row in a plurality of continuous holders, form a lower closure of the radiation chamber 5.

A radiation source main reflector unit 15 is arranged adjacent to its second lateral end (the quartz glass pane 7). In the embodiment shown here, this comprises three main reflector modules 17, each with three approximately W-shaped reflection surface areas 19 and two cooling water channels 21, as well as nine elongated halogen filament lamps 23. The main reflector modules 17 are each designed as extruded aluminum profiles with a pressed-in cooling water channel 21.

When dimensioning the radiation space, it is important that the distance between the radiation source main reflector unit 15 and the preforms is as homogeneous as possible.

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gene of the radiation field, while the distance between the counter reflector 9 and the preforms is set as small as possible, depending on the diameter of the preform, in particular in the range between 5 and 20 mm. The power density of the radiation source main reflector unit 15 is in the range between 150 and 300 kW/m ² and the residence time of the preforms in a heating field of the heating section is between 5 and 10 s. A heating section can comprise several heating fields of the type shown in Fig. 1 with a length in the range between 100 and 120 cm, but according to the inventors' findings one heating field is sufficient for standard applications.

For special applications, a modified heating section 1' is intended, as shown in Fig. 5. This differs from the heating section according to Fig. 4 by the provision of a so-called "line radiator" 25 arranged in the lower region of the (modified) counter-reflector 9 ^Λ .

This comprises a solid additional reflector 27 with a rectangular outer cross-section and a partially elliptical inner cross-section and an elongated halogen filament lamp 23A arranged in one of its focal lines with essentially the same structure as the halogen lamps 23 of the radiation source main reflector unit 15. The line radiator is arranged at an angle behind the plane of the counter reflector such that its other focal line lies on the surface area 3a of the preform holders 3, specifically in the closest section of the walls of the preforms. It is used for additional targeted heating of the base area of the preforms if this cannot be brought to a sufficient temperature by the radiation source main reflector unit 15.

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The implementation of the invention is not limited to the aspects and examples described above, but is also possible in a variety of modifications within the scope of the claims, which are within the scope of professional practice. 5

List of reference symbols

1; 1 ^�Lgr; Heating section

3 Preform holder

3a highly reflective surface area

5 Radiation room

7 Quartz glass disc

9 Al counter reflector

11 Head reflector

13 Ventilation gap

15 Radiation source main reflector unit

17 Main reflector module

19 Reflection surface area

21 Cooling water channel

23; 23A halogen filament lamp

25 line arrays

27 Additional reflector

Pl, P2 outline of a preform

Claims (9)

1. Heating section ( 1 ; 1 ') of a stretch blow molding system for stretch blow molding PET containers from injection molded preforms (P1, P2), with a plurality of elongated halogen lamps ( 23 ) arranged essentially in one plane as infrared radiation sources, which emit infrared radiation with a significant effective portion in the near infrared range, in particular in the wavelength range between 0.8 µm and 1.5 µm,
a main reflector ( 17 ) adjacent to the infrared radiation sources and
a counter-reflector ( 9 ) arranged opposite the infrared radiation sources with respect to the passing preforms,
characterized in that
the counter reflector has cooling surfaces and/or is actively cooled. 2. Heating section ( 1 ; 1 ') of a stretch blow molding system for stretch blow molding PET containers from injection molded preforms (P1, P2), with a plurality of elongated halogen lamps ( 23 ) arranged essentially in one plane as infrared radiation sources, which emit infrared radiation with a significant effective portion in the near infrared range, in particular in the wavelength range between 0.8 µm and 1.5 µm,
a main reflector ( 17 ) adjacent to the infrared radiation sources and
a counter-reflector ( 9 ) arranged opposite the infrared radiation sources with respect to the passing preforms,
in particular for carrying out the method according to one of the preceding claims,
characterized in that
the main and counter reflectors together with a head reflector ( 11 ) and foot reflectors associated with the preform holders ( 3 ) form a substantially closed radiation space ( 5 ). 3. Heating section ( 1 ; 1 ') of a stretch blow molding system for stretch blow molding PET containers from injection molded preforms, with a plurality of elongated halogen lamps ( 23 ) arranged essentially in one plane as infrared radiation sources, which emit infrared radiation with a significant effective portion in the near infrared range, in particular in the wavelength range between 0.8 µm and 1.5 µm,
a main reflector ( 17 ) adjacent to the infrared radiation sources and
a counter-reflector ( 9 ) arranged opposite the infrared radiation sources with respect to the passing preforms,
in particular for carrying out the method according to one of the preceding claims,
characterized in that
the main reflector ( 17 ) has a number of reflector sections which are approximately W-shaped in cross-section and which corresponds to the number of halogen lamps ( 23 ), the center of a halogen lamp being located in the center plane of each "W". 4. Heating section according to one of claims 1 to 3, characterized in that the counter-reflector consists of metal at least on the surface, in particular entirely, and its surface is arranged at a small distance of less than 25 mm from the nearest point of the preforms. 5. Heating section according to one of claims 1 to 4, characterized in that a quartz glass disk ( 7 ) is arranged between the infrared radiation sources ( 23 ) and the preforms (P1, P2), which separates an infrared radiator reflector region and a preform conveying region from one another in terms of flow. 6. Heating section according to one of claims 1 to 5, characterized in that the main reflector ( 17 ) and the counter reflector ( 9 ) are actively cooled, the main reflector in particular having a water cooling system ( 21 ). 7. Heating section according to one of claims 1 to 6, characterized by a line radiator ( 25 ) arranged outside the plane of the infrared radiation sources, in particular a single elongated halogen lamp ( 23 A) with an additional reflector ( 27 ) which is essentially partially elliptical in cross section, for additional targeted heating of the foot regions of the preforms (P1, P2). 8. Heating section according to one of claims 1 to 7, characterized in that the main reflector ( 17 ) and/or the counter reflector ( 9 ) and/or the additional reflector ( 27 ) of the line radiator ( 25 ) has at least one solid aluminum profile, in particular with at least one molded-in cooling fluid flow channel ( 21 ), preferably a parallel arrangement of several such profiles. 9. Heating section according to one of claims 1 to 8, characterized in that a cooling air blower for active cooling is assigned to the counter-reflector.