Method and device for blow-molding containers

p0001The invention relates to a method for the blow molding of containers, in which a preform of a thermoplastic material is converted into the container by blast pressure action after thermal conditioning along a transport path in the region of a heating section within a blow mold.

p0002The invention also relates to a device for the blow molding of containers from a thermoplastic material which has at least one heating section arranged along a transport path of a preform and a blowing station provided with a blow mold.

p0003In the case of container forming by blast pressure, preforms made of a thermoplastic material, for example preforms of PET (polyethylene terephthalate), are fed to different processing stations within a blow molding machine. Typically, such a blow molding machine has a heating device as well as a blowing device, in the region of which the previously tempered preform is expanded by biaxial orientation to a container. The expansion is carried out with the aid of compressed air, which is introduced into the preform to be expanded. The process procedure during such expansion of the preform is described in the<patcit id="pcit0001" dnum="DEOS4340291A"><text>DE-OS 43 40 291</text></patcit> Is explained. The introduction of the pressurized gas mentioned in the introduction also includes the pressure gas introduction into the developing container bubble as well as the pressure gas introduction into the preform at the beginning of the blowing process.

p0004The basic design of a blow station for container forming is described in the <patcit id="pcit0002" dnum="DEOS4212583A"><text>DE-OS No. 42 12 583</text></patcit> Described. Possibilities for tempering the preforms are described in the<patcit id="pcit0003" dnum="DEOS2352926A"><text>DE-OS 23 52 926</text></patcit> Is explained.

p0005Within the blow-molding device, the preforms as well as the blown containers can be transported by means of different handling devices. The use of transport mandrels, onto which the preforms are pinned, has proven particularly effective. However, the preforms can also be handled with other support means. The use of gripping tongs for handling preforms and the use of spreader mandrels that can be inserted into a mouth region of the preform are also among the available constructions.

p0006A handling of containers using transfer wheels is described, for example, in the <patcit id="pcit0004" dnum="DEOS19906438A"><text>DE-OS 199 06 438</text></patcit> In an arrangement of the transfer wheel between a blow wheel and an output section.

p0007The previously described handling of the preforms takes place on the one hand in the so-called two-stage processes in which the preforms are first produced by means of an injection molding process, are then intermediately stored and only subsequently conditioned with respect to their temperature and inflated to form a container. On the other hand, the so-called one-step process is used, in which the preforms are suitably heated immediately after their injection-molding process and sufficient solidification, and are subsequently inflated.

p0008Various embodiments are known with regard to the blowing stations used. In the case of blow stations, which are arranged on rotating transport wheels, a book-like folding-open capability of the mold carriers is frequently encountered. However, it is also possible to use mold carriers which are displaceable relative to each other or guided differently. In the case of stationary blowing stations, which are in particular suitable for accommodating a plurality of cavities for the container formation, plates arranged parallel to one another are typically used as mold carriers.

p0009Before carrying out the heating, the preforms are typically placed on transport mandrels, which transport the preform either through the entire blowing machine or which circulate only in the region of the heating device. In the case of a stationary heating of the preforms such that the mouths of the preforms are oriented downwards in the vertical direction, the preforms are usually placed on a sleeve-shaped holding element of the transport mandrel. In the case of suspended heating of the preforms in which they are oriented with their mouths upwards in the vertical direction, spreading mandrels are usually introduced into the mouths of the preforms, which clamp the preforms.

p0010When carrying out a blowing-type container formation, an essential task is to achieve a predetermined material distribution in the container wall. An essential parameter for the determination of the resulting material distribution is the distribution of the heat distribution realized before blow molding in the preforms.

p0011The heat distribution is typically realized in such a way that a temperature level is produced in a circumferential direction of the preforms and a temperature profile is produced in a longitudinal direction of the preforms. In addition, the pre-selection of a suitable temperature profile also takes place through the wall of the preform from outside to inside. In principle, it is to be assumed that regions of the preform with a lower temperature lead to thicker wall regions of the blown container, and that the warmer regions of the preform are more strongly stretched during the blow molding operation, resulting in thinner wall regions of the blown container.

p0012The temperature in the region of the preforms can be measured with so-called pyrometers. A measurement-technical detection of a concrete wall thickness in the region of the blown containers can take place with so-called wall thickness sensors which operate, for example, optically or using sound waves.

p0013However, it is found that only a suitable specification of the heat distribution within the preforms is not sufficient to produce optimum material properties in the blown container. On the contrary, there are comparatively complex interactions between the execution of the stretching process, the pressure build-up within the developing vessel bubble of the material distribution in the preform and the temperature distribution in the preform.

p0014From the <patcit id="pcit0005" dnum="US2005194705A1"><text>US 2005/194705 A1</text></patcit> It is known to measure and regulate process parameters of the blowing process, for example the supply pressure of the blowing gas or the temperature of the blow mold.

p0015In the <patcit id="pcit0006" dnum="WO03078136A"><text>WO 03/078136 A</text></patcit> The measurement-technical detection and control of such process parameters of the blowing process is also already explained.

p0016The <patcit id="pcit0007" dnum="WO2006108380A"><text>WO 2006/108380</text></patcit> Describes the regulation of an electrical linear drive for performing a stretching operation. However, it is not the concrete stretching of the preform, but the position of the stretching bar.

p0017The object of the present invention is to improve a method of the type mentioned in the introduction in such a way that high-quality blow molding is assisted at a simultaneously high throughput rate with a low machine-related outlay.

p0018This object is achieved with the features of claims 1 and 21.

p0019A 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 and good product quality.

p0020This object is achieved according to the invention in that at least one sensor for detecting at least one parameter characterizing the transformation of a developing container bubble during the shaping of the preform into the container is connected to a control device which has an evaluation unit for this parameter and which has at least one influencing process Which is arranged in a closed control circuit for adapting the measured parameter to an associated setpoint value.

p0021According to the invention, it has been recognized that an extremely advantageous procedure can be achieved by not measuring the finished blown container, but that the development of the container bladder which is formed during the transformation of the preform into the container is already monitored within the blow mold. By evaluating the development of the container bubble and by directly influencing the parameters influencing this development, the material properties of the blown container can be controlled more precisely and more effectively and adapted to desired properties.

p0022The concrete shaping of the developing container bubble can be detected, in particular, by the fact that a contact of a developing container bubble to an inner side of the blow mold is measured at least temporarily and at least in sections.

p0023The detection of a further parameter characterizing the development of the bubble is possible in that a position of the stretching rod is measured.

p0024It is also contemplated that a speed of the stretching rod is measured.

p0025A further variant consists in measuring a stretching force.

p0026Moreover, it is also possible that a blow pressure is measured.

p0027Furthermore, it is also possible for a blow gas volume to be measured.

p0028A defined observance of a predefined development of the container bladder can be achieved, in particular, by the fact that the parameter is regulated according to a time-variable setpoint profile.

p0029In general, it is expedient to regulate the installation of the container bubble at the blow mold.

p0030According to an exemplary embodiment, the position of the stretching rod is regulated.

p0031It is also contemplated that the speed of the stretching rod is controlled.

p0032In addition, it is also possible that the stretching force is controlled.

p0033A further embodiment variant is that the blowing pressure is regulated.

p0034Finally, it is also possible that the volume of the blow gas is regulated.

p0035Influencing the development of the bubbles can be effected in that the stretching speed is used as the manipulated variable.

p0036Another embodiment consists in the fact that the stretching rod position is used as the manipulated variable.

p0037In addition, it is contemplated that the stretching force is used as the manipulated variable.

p0038Likewise, it is possible that the blow pressure is used as the manipulated variable.

p0039Finally, it is also possible that the blow-gas volume is used as the manipulated variable.

p0040A consideration of the complex relationships and mutual influencing of the individual factors can take place by the measured value being fed to a control-technical model which generates the manipulated variable.

p0041Embodiments of the invention are schematically illustrated in the drawings. Show it:<dl id="dl0001"><dt>FIG</dt><dd>A perspective view of a blow station for producing containers from preforms,</dd><dt>FIG</dt><dd>A longitudinal section through a blow mold in which a preform is stretched and expanded,</dd><dt>FIG</dt><dd>A sketch for illustrating a basic construction of a device for the blow molding of containers,</dd><dt>FIG</dt><dd>A modified heating path with increased heating capacity, </dd><dt>FIG</dt><dd>A schematic representation for illustrating the temporal course of the installation of the container bubble on the inner wall of the blow mold,</dd><dt>FIG</dt><dd>Diagrams illustrating the relationships between the temporal pressure profile within the preform, the container bladder and the blown container, the stretching force development as well as the stretching rod positioning,</dd><dt>FIG</dt><dd>6 is an enlarged partial illustration of the course according to FIG <figref idrefs="f0005">FIG</figref> With additional reference lines and</dd><dt>FIG</dt><dd>3 is a schematic representation of a control concept for controlling the development of the container bubble.</dd></dl>

p0042The basic structure of a device for converting preforms (1) into containers (2) is described in <figref idrefs="f0001">FIG</figref> and in <figref idrefs="f0002">FIG</figref> Respectively.

p0043The device for shaping 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. The blow mold (4) consists of mold halves (5, 6) and a bottom part (7), which is provided by a lifting device, to enable the preform (1) to be inserted into the blow mold (4) and to allow the finished container (2) (8) is positionable. The preform (1) can be held in the region of the blowing station (3) by a transport mandrel (9), which together with the preform (1) passes through a plurality of treatment stations within the device. However, it is also possible to insert the preform (1) directly into the blow mold (4) via pliers or other handling means.

p0044A connecting piston (10), which feeds compressed air to the preform (1) and at the same time performs an anchoring relative to the transport mandrel (9), is provided for the purpose of enabling a compressed air supply line, under the transport mandrel (9). In a modified construction, however, it is also basically conceivable to use fixed compressed air supply lines.

p0045In this exemplary embodiment, the preform (1) is stretched using a stretching rod (11) which is positioned by a cylinder (12). According to another embodiment, a mechanical positioning of the stretching rod (11) is performed by means of curved segments which are acted on by tapping rollers. The use of curve segments is particularly useful when a plurality of blowing stations (3) are arranged on a rotating blowing wheel

p0046In the case of the <figref idrefs="f0001">FIG</figref> , The drawing system is designed in such a way that a tandem arrangement of two cylinders (12) is provided. The stretching rod (11) is first driven into the region of a bottom (14) of the preform (1) by a primary cylinder (13) before the actual stretching process begins. During the actual stretching operation, the primary cylinder (13) is positioned with a stretched stretching rod together with a carriage (15) carrying the primary cylinder (13) from a secondary cylinder (16) or via a curve control. In particular, it is intended to use the secondary cylinder (16) in a curve-controlled manner in such a way that a current stretching position is predetermined by a guide roller (17), which slides along a curved path during the stretching operation. The guide roller (17) is pressed against the guide track by the secondary cylinder (16). The carriage (15) slides along two guide elements (18).

p0047After locking of the mold halves (5, 6) arranged in the region of carriers (19, 20), the carriers (19, 20) are locked relative to one another with the aid of a locking device (20).

p0048For adaptation to different shapes of an opening section (21) of the preform (1), according to FIG <figref idrefs="f0002">FIG</figref> The use of separate threaded inserts (22) is provided in the region of the blow mold (4).

p0049<figref idrefs="f0002">FIG</figref> Shows the preform (1) in addition to the blown container (2), also schematically, a developing container bubble (23).

p0050<figref idrefs="f0003">FIG</figref> Shows the basic structure of a blowing machine which is provided with a heating section (24) and a rotating blowing wheel (25). The preforms (1) of transferring wheels (27, 28, 29) are transported out of a preform inlet (26) into the region of the heating section (24). Heating elements (30) and blowers (31) are arranged along the heating section (24) in order to heat the preforms (1). After a sufficient temperature control of the preforms (1), these 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 additional transfer wheels to an output section (32).

p0051In 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 shelf - life of foodstuffs, especially beverages, filled inside the container (2) Of the heating and orientation of the preforms (1). In addition, advantageous effects can be achieved by adhering to specific dimensioning rules.

p0052Various plastics can be used as the thermoplastic material. For example, PET, PEN or PP can be used.

p0053The expansion of the preform (1) during the orientation process takes place by compressed air supply. The compressed air feed is fed into a pre-blowing phase in which gas, for example compressed air, is fed with a low pressure level and divided into a subsequent main blowing phase, in which gas is fed with a higher pressure level. During the pre-blowing phase, compressed air is typically used with a pressure in the interval from 10 bar to 25 bar, and during the main blowing phase compressed air is fed at a pressure in the interval of 25 bar to 40 bar.

p0054Out <figref idrefs="f0003">FIG</figref> It can also be seen that, in the illustrated embodiment, the heating section (24) is formed from a multiplicity of circulating transport elements (33) which are arranged in a chain-like manner and are guided along deflecting wheels (34). In particular, it is intended to clamp a substantially rectangular basic contour by means of the chain-like arrangement. In the illustrated embodiment, a single relatively large-sized deflection wheel (34) is used in the region of the expansion of the heating section (24) facing the transfer wheel (29) and an inlet wheel (35), and two comparatively smaller dimensioned deflection wheels (36) are used in the region of adjacent deflections . In principle, however, any other guides are also conceivable.

p0055The illustrated arrangement proves to be particularly suitable for providing the most densely arranged arrangement of the transfer lead (29) and the inlet lead (35) relative to one another since three deflecting wheels (34, 36) are positioned in the area of ​​the corresponding expansion of the heating section (24) (34) in the immediate transfer area to the transfer wheel (29) and to the feed wheel (35). (DE). WIPO Home services World Intellectual Property Organization Alternatively to the use of chain-type transport elements (33), it is also possible, for example, to use a rotating heating wheel.

p0056After the bubbles (2) are completely bubbled, they are led out of the region of the blowing stations (3) by a take-off wheel (37) and transported to the discharge section (32) via the transfer wheel (28) and an output wheel (38).

p0057In the <figref idrefs="f0004">FIG</figref> , A larger quantity of preforms (1) per unit of time can be tempered by the larger number of heating elements (30). The blowers (31) in this case feed cooling air into the region of cooling air ducts (39), which each lie opposite the associated heating elements (30) and emit the cooling air via outflow openings. By the arrangement of the outflow directions, a flow direction for the cooling air is realized substantially transversely to a direction of transport of the preforms (1). The cooling air ducts (39) can provide reflectors for the heating radiation in the region of the surfaces opposite the heating elements (30), it is also possible to provide cooling of the heating elements (30) via the delivered cooling air.

p0058<figref idrefs="f0005">FIG</figref> Shows the allocation of a mold half (5) and a container blow (23) in a schematic representation. The container bubble (23) develops during the blowing process in the direction of a container longitudinal axis (41) and transversely to the container longitudinal axis (41). As a result of the expansion of the container bubble (23) transversely to the container longitudinal axis (41), the container bubble (23) rests against the inner wall of the blow mold (4). The system typically begins in a region of the blow mold (4) facing the mouth section (21) of the container (2) and then advances towards the bottom part (7).<figref idrefs="f0005">FIG</figref> Illustrating the progress of the installation of the container bubble (23) on the blow mold (4) at various points in time.

p0059<figref idrefs="f0005">FIG</figref> (42), a stretching force development (43) as well as a stretching rod positioning (44). The time axis is scaled in milliseconds and the amplitude values ​​are scaled in bar, newton or millimeter.

p0060<figref idrefs="f0005">FIG</figref> Shows that, with regard to the positioning of the stretching rod (11), the latter is first fed into the preform (1) essentially without developing a stretching force. After an abutment of the stretching bar (11) on the bottom of the preform (1), the latter is stretched and a comparatively great unfolding of the stretching force takes place. Upon completion of the longitudinal stretching, the remaining stretching force results from the stabilization of the preform in the stretched positioning taking into consideration elastic restoring forces within the material of the preform. The blowing pressure profile (42) shows the division into a pre-blowing phase during the stretching operation and into a main blowing phase after a substantial completion of the stretching process.

p0061The arrangement of the container bubble (23) on the blow mold (4) can be detected, for example, using contact-sensitive sensors. Likewise, non-contact detection is possible, for example, by means of an electric field measurement or by optical sensors. Additional measurement information is provided by the detection of the pressure profile, the gas volume supplied to the container bladder (23), the stretching speed, the stretching force or the temperature in the material of the preform to be reformed. Optionally, one or more of the above parameters can be evaluated. The measurement-technical detection of the abovementioned parameters also includes an indirect determination of the above parameters by the measurement-technical detection of an auxiliary parameter which can be assigned to the above parameters and a corresponding conversion.

p0062Using the provided measurement information, it is in particular possible to determine the deformation work or deformation energy supplied to the material during blow molding.

p0063According to a preferred embodiment, the measured or measured parameters, which characterize the development of the container bladder (23), are fed to a control-technical model and are evaluated here in a suitable manner for generating manipulated variables. Preferably, a stretching system is used in which the stretching speed and / or the stretching rod positioning and / or the stretching force can be controlled or regulated. For example, recking systems with ball screw spindles, controlled pneumatic recking systems or recking system based on electric linear motors or servomotors can be used.

p0064According to an exemplary embodiment, it is possible to detect the velocity of the bubble development and, for example, to increase the speed of the stretching process in the event of too rapid a bubble development so that the stretching rod positioning can follow the evolution of the bubbles and thereby guide and center the developing container bubble through the stretching rod (11) is ensured.

p0065According to another embodiment, it is possible to reduce the stretching speed if the development speed of the container bladder (23) is too low, thus realizing a predetermined temporal assignment of transverse and longitudinal stretching.

p0066According to a further embodiment, the position of the stretching bar (11) is measured and the blowing pressure supply is controlled. This can be done, for example, using a proportional valve. According to another exemplary embodiment, the volume of blow air supplied is not the blowing pressure but the volume of blowing air supplied to the developing container bubble (23).

p0067According to another exemplary embodiment, the stretching force is measured. If the stretching force decreases at a predetermined stretching speed, it can be concluded from this that the development of the container bladder (23) is too rapid and the longitudinal stretching is at least partially carried out by the stretching rod (11) and at least partly by the internal pressure in the container bladder (23) ). When a state of such a state is detected, the volume of blowing gas supplied per unit of time can be reduced. Conversely, the inflow of blowing gas can be increased with an increase in the stretching force which is attributable to a too low development speed of the container bubble (23).

p0068According to a further preferred embodiment, the control engineering model mentioned above includes a multidimensional parameter space of setpoints. As long as the resulting actual values ​​lie within a corresponding multi-dimensional target space, an optimal process control can be concluded. If at least one of the relevant parameters leaves the setpoint range, a correction of at least one of the available actuating variables is required.

p0069<figref idrefs="f0006">FIG</figref> For the pre-blowing phase, which lies between the instants t1 and t2, shows possible variations in the pressure rise speed by means of suitable manipulated variables. The interval between t1 and t2 typically has a duration of 50 to 100 milliseconds. The influencing of the bubble development is effected by the two illustrated variants of the gradient of the blowing pressure profile (42) during the execution of the first phase of the stretching process or after a first pressure build-up.

p0070<figref idrefs="f0006">FIG</figref> Shows schematically and in a highly simplified embodiment a possible control circuit for influencing the development of the container bubble (23). In the illustrated exemplary embodiment, a two-loop cascade control is provided. In an internal control circuit, a volume (45) of the container bubble (23), which is influenced by a blowing gas supply (46), is taken into account. The shaping system (47) of the container bubble (23), which is additionally influenced by the drawing system (48), is taken into account in the outer control loop. The actual control system is much more complex due to the complex and partly non-linear relationships of the individual control and manipulated variables.

p0071The detection of the above-mentioned measured variables can be effected, for example, using a flow sensor (49) whose measured value is converted into the resulting volume, and the molding system can be detected using a position sensor (50).