Patent Application
20110294086
This claims the benefit of German Patent Application DE 10 2010 021 445, May 25, 2010 and hereby incorporated by reference herein.
The present invention relates to a method for the temperature control and/or regulation of a preform heating device. The invention furthermore relates to a heating device for controlling the temperature of preforms.
Beverage containers made from thermoplastic materials, especially from the most widely used PET, are commonly produced in a stretch blow molding process. In this mostly two-stage stretch blow molding process the containers are typically produced from injection-molded, rotationally symmetric preforms. The said preforms consist of an elongated, cylindrical, lateral body section with a rounded, closed bottom and a neck section with an upper opening, which can also be referred to as mouthpiece section. Positioned close to this opening there is usually a thread section, which can be delimited toward the bottom by a collar or the like. Already during the injection-molding process of the preform is the said thread section produced to final dimensions as will be required for later use. During the stretch blow molding process it continues to keep its original shape and later forms the thread for the screw cap of the finished beverage container. The remaining sections of the preform are, in contrast, deformed and stretched. During the manufacturing process the said preforms are heated to a predefined magnitude of process temperature in order to enable forming by stretch blow molding in the desired manner. The heating is mostly performed by means of infrared radiation, because in this manner it is possible to ensure defined and uniform temperature control of the preforms.
The plastic material intended for further processing (in general PET) is of such a nature that it will strain harden as it is stretched. Of decisive importance in this process is the forming temperature. The strain hardening effect is normally put to use in the production of PET containers for the purpose of controlling and optimizing wall thickness distribution. Depending on the production process, it is possible to apply the infrared radiation in such a way that the preforms are heated according to a temperature profile. The aim of this is to have the warmer sections deformed with priority to the other parts as long as is required for the stretching resistance resulting from strain hardening to become greater than the resistance of the adjacent cooler sections. Commonly, the temperature profile is uniformly distributed around the circumference of the preforms and can vary process-dependently along the longitudinal axis of said preforms.
In order to apply the desired temperature profile to the preforms it is possible to use a number of zones, for instance up to nine or more zones. It is possible to control this plurality of different zones individually, whereby the selected setting is maintained constant over a longer period of operating the heating apparatus. In order to respond to changes in the ambient conditions it is possible to use a regulating system for recording the preforms' temperature at least one measuring point. This regulating system is intended for keeping the preforms' temperature at the selected measuring point constant. The controlled variable represents the manipulated input variable for all heating apparatuses so that in the instance of measuring a temperature that is below a pre-set nominal temperature, the manipulated variable and thus the heat output to all heating apparatuses will be increased.
In general, this regulating system is indispensable, as the preforms' temperatures may vary over time, for instance after a certain period of operation and with the heating apparatus gradually warming up. The production hall may also warm up during an operating day, for instance, possibly causing heat build-up to the heating apparatuses and the preforms. Although it may still be possible to influence and modify the different heating zones each proportionally to the same extent, modifying the manipulated variable too much, however, may result in the total heat increase to deviate more than desired because the entire heating profile may change. Temperatures deviating too much from a nominal temperature may have negative influences on container quality.
In order to avoid these problems the various heating zone controls are, in practice, manually adjusted. In addition, it is also possible to have different programmes to make allowances for temperature differences between summer and winter operation.
It is an object of the present invention to provide an improved method for the temperature control of preforms in connection with a stretch blow molding process, whereby the said preforms are heated according to a desired temperature profile with said temperature profile complying as exactly as possible to the nominal values, even under changing external conditions. A further aim of the invention is to provide an improved preform heating device which allows setting the temperatures and temperature profiles required for affecting the preforms as accurately as possible.
For achieving the first-mentioned object, the invention provides a method for controlling a heating device for preforms made from a thermoplastic material, especially from PET, whereby the method for controlling makes it possible to adjust the heating device in the desired manner for the purpose of bringing the preforms to the optimal temperature prior to a subsequent blow molding or stretch blow process. The temperature control process comprises two separate, consecutive heating stages, each with different tasks to fulfill. The first heating stage is intended for achieving a nearly uniform base temperature of the entire preform, with said base temperature corresponding approximately to the maximum heating temperature for maintaining the dimensional stability of the thread section at the preform's open-topped neck section. The second heating stage is intended for achieving a profiled temperature according to a predefineable thermal profile, with said profiled temperature being the forming temperature required for blow molding or stretch blow molding at least the preform's body section located below the thread section and/or a collar area located therebelow. The present invention provides an improved concept for thermal layering for the preforms. The heating is performed in two stages, allowing the temperature to be regulated more immediately and better adjusted in the individual stages.
The heating may be commonly achieved in at least two consecutive sections or heating stages of the heating device. The first section provides the preforms with basic heating in order to heat them to a base temperature that is as uniform as possible and that is below the softening temperature of the plastic material, in order to avoid, as far as possible, that the thread section is unduly heated. Thermal layering is not yet applied during this basic heating phase, as the intention is to achieve a uniform temperature distribution over the entire body of the preform. For this purpose it is possible, if required, to use a suitable algorithm to predefine a zonal layering depending on the preform's geometry (wall thickness, distance to radiator, length). The regulation system in the method according to the invention is intended to achieve a defined base temperature, as far as possible throughout the process, whereby said base temperature can range between at least 50° C. and up to 90° C. In this manner the method allows to compensate for different input and storage conditions of the preforms. As the said preforms are likely to have been stored in different locations at different temperatures before being supplied to the stretch blow molding process, it is necessary to provide uniform input conditions for the preforms in order to achieve the best forming results possible. Accordingly, a basic heating phase constitutes the first temperature control stage and a subsequent temperature profiling phase constitutes the second temperature control stage.
During the temperature profiling phase that constitutes the second section, the preforms may be heated with the temperatures being variably layered, i.e. in direction of the longitudinal axis of the said preforms. According to a preferred embodiment of the method according to the invention it is possible to provide at least one temperature reading at the exit of the first heating stage in order to record the preforms' temperature after the basic heating and to appropriately adjust the first heating stage. It is optionally possible to allow for a further temperature reading after the second heating stage in order to record the preforms' final temperature after the first heating stage or after the temperature profiling phase and to accordingly take the recorded temperatures into account when adjusting the first heating stage. In the course of this, at least the temperature measured after the first heating stage is processed for further regulating the heat output of the said first heating stage. Provisions may also be made for processing both temperatures for regulating the heat output of the first stage, i.e. the temperature measured after the first stage or the basic heating phase as well as the final temperature measured after the second stage or the temperature profiling phase. Also, provisions may be made for processing both temperatures for regulating the heat output of the first stage and/or of the second stage, i.e. the temperature measured after the first stage or the basic heating phase as well as the final temperature measured after the second stage or the temperature profiling phase.
Due to the regulated temperature in the basic heating phase, it can be assumed throughout that the preforms are in the same initial condition when they enter the temperature profiling phase. Ideally, temperature layering would not change afterwards so that the temperature or the heat output of the heating stages would require no readjustments, thus making a control loop unnecessary. The temperature should nevertheless be measured at the exit of the oven as well in order to be able to monitor the actual temperature of the preforms upon entry into the blow molding station and in order to compensate for side effects, such as aging to the heating devices, for instance to the infrared radiators. The measurement value acquired in this process can be used for accordingly adjusting the oven control's setting value for the basic heating process.
In an alternative embodiment it is further possible to use the value measured at the oven's exit for regulating and controlling the second heating stage, provided that this is necessary for reasons of the heating devices' aging or other side effects. Preferably, the measurement value taken after the first heating stage is additionally taken into account for regulating and controlling the second heating stage.
It is moreover advantageous for the preforms to be heated, in the section of the first heating stage or the basic heating phase, to a largely uniform base temperature ranging between approximately 50° C. and approximately 90° C. This temperature depends primarily on the maximum allowable temperature for the neck section of the preforms made from PET or another suited thermoplastic material, because this section with its thread that is to be used later is not to be changed and deformed during heating and the subsequent stretch blow molding process, but rather to remain unaltered and maintain its size and shape throughout all processing stages.
An advantageous variant of the method according to the invention allows for the preforms to be heated to the base temperature in the section of the first heating stage by means of inserting heating elements into the open-topped preforms. These heating elements function as so-called boosters in that they require only a very short time for bringing the respective preform from storage temperature to the desired base temperature, which is brought to a yet higher temperature level in the subsequent heating stage by means of applying a temperature profile. This booster or heating element may be of a typical length that corresponds to an individual radiant heater, thus making a largely homogeneous heating of the preforms possible. Moreover, it is also possible for further radiators to function as components of this booster, with said components applying heat radiation to the outside of the preforms for achieving the desired basic heating. Another advantageous variant of the invention is to utilize part of an oven's exhaust heat, which would otherwise be conducted outside, for producing the energy for the basic heating. The oven's exhaust heat can be taken advantage of by, for instance, deflecting the warm exhaust air and/or conducting this exhaust air through suitable heat exchangers for cooling it, thus representing a potential for energy saving.
As already mentioned, the second heating stage essentially serves to apply a temperature profile to the preforms in this temperature profiling phase, with said temperature profile being adjusted and/or varying along the length of said preforms. In order for the preform's thread section to maintain dimensional stability throughout the subsequent process steps, special attention should be paid not to apply too much heat to the thread section when heating the neck section and the remaining preform. As the thread section and the so-called neck ring are required for handling and transport purposes, it is important not to modify these sections of the preform. In the section of the second heating stage, the preforms can be heated in particular by means of radiant heating devices. In order to avoid overheating, said radiant heating devices may be provided with a regulated surface cooling system, if required.
The method regulated in compliance with the configuration according to the invention allows the controlled variables, i.e. the heat output of the first heating stage, to be adjusted very quickly, because the input variable to be considered for temperature regulation is the measured value from the temperature reading immediately after the first heating stage. In contrast to the already known measurement methods, the regulation in this process is already performed after about half of the heating line, thus preventing the controlled variables from deviating too much. This results in more accuracy for the temperature regulation, thus improving, in an effective manner, procedure quality and reducing the scrap rate resulting from improperly formed preforms. In the second heating stage temperature layering is preferably applied under constant conditions, which also prevents the process parameters from drifting and contributes to maintaining a constant quality. The described heating system of the basic heating stage with the optionally employable boosters or heating elements can be operated in a particularly energy-efficient manner, as it allows the preforms to be brought to the required process temperature very quickly.
A further advantage lies in fact that the processes and process parameters can be implemented in the machinery without restrictions, whereby there are no limitations whatsoever with regard to transferability to machinery of the same or a similar kind, even if the said machinery may have respectively different configurations. Furthermore, all conceivable influences connected with the installation or location of the machinery are eliminated as far as possible, thus accelerating time to machinery startup. Such a location factor may also be, for instance, an ambient parameter such as a typical hall temperature that can perceptibly influence the heating process of the preforms.
In order to achieve the above mentioned object of the invention, a heating device may also additionally be provided for controlling the temperature of preforms made from a thermoplastic material for a subsequent blow molding or stretch blow molding process. The heating device according to the invention comprises at least two distinct and respectively consecutive heating stages, whereby at least the first heating stage is provided with a heating device for the largely uniform basic heating of the preforms. A further embodiment variant of the heating device according to the invention can intend for at least the first heating stage to be formed by at least one heating element that is inserted into the preforms so that the preforms are heated and brought to the base temperature from inside.
Furthermore, it is possible to provide at least one temperature sensor, which is disposed downstream of the first and upstream of the second heating stage and which is coupled via signal transmission to a control unit for regulating the heat output of the first heating stage with the result that the first heating stage can be regulated very quickly. It is moreover possible for a further temperature sensor to be disposed at the exit of the second heating stage and coupled to a control unit. This helps to further improve the control quality. Other aspects, embodiment variants, and advantages in the configuration and operation of the heating device according to the invention are to be seen in the context of the method variants already mentioned above, as all the method variants are to be regarded as options for operating the heating device.
Furthermore, it must be pointed out here that the present invention is generally suited for use in microwave ovens, rotary ovens, linear ovens, stationary ovens, etc. It is furthermore possible to use individual heating jackets, whereby each preform is selectively temperature-controlled in a separate heating jacket. For purposes of completeness, it should be noted that in addition to the two mentioned, separate heating stages, it is possible to provide further heating stages, as the case may be, without requiring a detailed explanation here.
In the following passages, the attached figures further illustrate exemplary embodiments of the invention and their advantages. The size ratios of the individual elements in the figures do not necessarily reflect the real size ratios. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention.
The diagram in
Another diagram in
The same or equivalent elements of the invention are designated by identical reference characters. Furthermore and for the sake of clarity, only the reference characters relevant for describing the respective figure are provided. It should be understood that the detailed description and specific examples of the device and method according to the invention, while indicating preferred embodiments, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The qualitative diagram given in
Another diagram in
The schematic block diagram in
In order to enable the heating device 10 to be regulated, a first temperature sensor 16 is provided for recording an actual temperature 18 at the exit of the first heating stage 12 so that the preforms' temperature after their first basic heating can be recorded and the first heating stage 12 accordingly adjusted as accurately as possible. The output signal of the first temperature sensor 16 provides a value for the actual temperature 18. It is moreover possible to include a second temperature sensor 20 downstream of the second heating stage 14 for a further temperature reading, which serves to record the final temperature of the preforms after the temperature profiling phase in the second heating stage 14. The value measured by means of the second temperature sensor 20 for a nominal temperature 22 is processed, together with the actual temperature 18 provided by the first temperature sensor 16, in a summing circuit 24 and an amplifying stage 26 arranged downstream thereof, in order to control the first, basic heating stage 12. Due to the advantageous positioning of the two temperature sensors 16 and 20, it is possible, in the described manner, to implement a regulating system for controlling the oven that reacts very quickly and reliably prevents the heating temperatures of both heating stages 12 and 14 from deviating too much.
In the section of the first heating stage 12, or the basic heating phase, the preforms can be heated to a largely uniform base temperature ranging between approximately 50° C. and approximately 90° C. This temperature must at least be below the flow or softening temperature of the thermoplastic material used for the preforms, because in particular the thread section is required to retain its dimensional stability during the temperature control phase in the first heating stage 12, which provides no thermal shielding or cooling for the neck and thread section, in contrast to the second heating stage 14 that commonly does provide such shielding or cooling. It is possible to heat the preforms to the base temperature in the section of the first heating stage 12 by means of, for instance, inserting heating elements into the open-topped preforms. Subsequently, a temperature profile is applied to the preforms in the section of the second heating stage 14, with said temperature profile being adjusted and/or varying along the length of said preforms, this being achieved, for instance, by radiator rails with varying radiation from varying heights, thus producing an appropriately adapted infrared radiation.
The schematic illustration of
The schematic illustration in
The subsequent adjacent second heating stage 14 also comprises a radiator section 54 with infrared radiators, which are, however, variably regulated in order to create the desired temperature profile, with the result that, on the one hand, the necessary forming temperature T3 of approximately 100° C. is achieved, but, on the other hand, the thread section of the preforms 46 is kept at the temperature level of T2.
The illustration in
Just behind the entry area 32 the heating line 34 comprises the booster 52 or the first heating stage 12 for the basic heating of the preforms 46. Downstream of the booster 52 are the radiator areas 54 of the second heating stage 14, which is indicated in the presented exemplary embodiment by altogether six consecutively arranged heating boxes. Upstream of the rotating entry area 32 with the entry star wheel is a linear feed path 58 for feeding the preforms 46 to the container forming device 30.
According to
Both variants according to
The detailed view in
The invention has been described with reference to a preferred embodiment. Those skilled in the art will appreciate that numerous changes and modifications can be made to the preferred embodiments of the invention and that such changes and modifications can be made without departing from the spirit of the invention. It is, therefore, intended that the appended claims cover all such equivalent variations as fall within the true spirit and scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| DE 102010021445.0 | May 2010 | DE | national |
