Abstract:
A method and apparatus are used to transfer workpieces, in particular preforms or bottles. The workpiece is subjected to pressure by a gripper-like holding device, which has at least one gripper arm arranged pivotably relative to a gripper carrier. The gripper arm is pivoted in order to perform gripping and releasing movements. The pivoting motion of the gripper arm is controlled by at least one magnet, particularly a permanent magnet.

Description:
The present application is a 371 of International application PCT/DE2009/001266, filed Sep. 7, 2009, which claims priority of DE 10 2008 062 315.6, filed Dec. 11, 2008, the priority of these applications is herby claimed and these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention pertains to a method for transferring workpieces in which the workpiece is handled by a gripper-like holding device comprising at least one gripper arm mounted with freedom to pivot relative to a gripper carrier and in which the gripper arm is pivoted to perform gripping and release movements. 
     The invention also pertains to an apparatus for transferring workpieces comprising at least one gripper-like holding device for handling the workpiece, wherein the holding device is provided with at least one gripper arm mounted with freedom to pivot relative to a gripper carrier, the gripper arm comprising a first pivoted position for holding the workpiece and a second pivoted position for releasing the workpiece. 
     These types of methods and apparatuses can be used in conjunction with, for example, the production of containers by the blow-molding process. In these types of applications, the apparatus is installed in a blow-molding machine. The workpiece in question, which is handled by the holding device, can be a preform, a blow-molded container, or a support element which itself holds the preform or the blow-molded container. 
     In the molding of containers by the action of blown air, preforms of a thermoplastic material such as PET (polyethylene terephthalate) are sent to various processing stations within a blow-molding machine. A blow-molding machine of this type usually comprises a heating device and a blowing device, in which the previously tempered preform is expanded to form the container by a process of biaxial orientation. The expansion is produced by compressed air, which is introduced into the preform to be expanded. The course of the technical processes which take place during this type of preform expansion is explained in DE-OS 43 40 291. The previously mentioned introduction of the compressed gas also comprises the introduction of compressed gas into the developing container bubble as well as the introduction of compressed gas into the preform at the beginning of the blow-molding process. 
     The basic design of a blow-molding station for molding containers is described in DE-OS 42 12 583. Possible ways of tempering the preforms are explained in DE-OS 23 52 926. 
     Within the blow-molding machine, the preforms and the blown containers can be transported by means of various handling devices. The use of transport mandrels, onto which the preforms are set, is known. The preforms can also be handled by other types of carrying devices, however. The use of grippers to manipulate the preforms and the use of clamping mandrels, which can be inserted into the mouth of the preform to hold it, also number among the available designs. 
     A way of handling containers and preforms by the use of transfer wheels is described in, for example, DE-OS 199 06 438, according to which a transfer wheel is arranged between a blow-molding wheel and a discharge section, and another transfer wheel is arranged between the heating section and the blow-molding wheel. 
     The previously explained handling of the preforms takes place in a so-called two-stage process, in which the preforms are first produced by an injection-molding step and then stored temporarily. In the second stage, after a certain period of time, they are conditioned to the proper temperature and blow-molded into containers. A so-called single-stage process is also used, in which the preforms are produced by injection-molding, allowed to solidify sufficiently, suitably tempered, and then blow-molded immediately. 
     Various designs of the blow-molding stations are known. In the case of blow-molding stations which are arranged on rotating transport wheels, the mold carriers are often designed to open up like books. It is also possible, however, to use mold carriers which are guided in such a way that they can be shifted relative to each other or guided in some other way. In the case of stationary blow-molding stations, which are especially suitable for accepting multiple cavities for container molding, plates arranged parallel to each other are typically used as mold carriers. 
     The preforms can be handled either exclusively by grippers or by grippers in combination with other carrying or handling elements. For example, the preforms can be held by transport mandrels over a portion of their transport distance, after which they can be handled by grippers over at least one an additional portion of the transport distance. The grippers in use up to now, however, do not fulfill all of the requirements to be imposed on grippers to ensure the non-damaging and simultaneously reliable handling of the preforms. 
     The essential requirement is that the preforms must be handled reliably and without breakdowns at high transport rates per unit time. Advisably, the grippers should be able to handle both the preforms and the blow-molded bottles. In addition, mechanical damage to the preforms as a result of the handling by the grippers must be avoided or minimized. 
     Many different types of gripper-like holding devices are known. There are, for example, actively controlled grippers, in which the opening and closing movements are controlled by a mechanical cam control unit or by pneumatic or electrical actuating elements. There are also grippers in which the gripper arms are not actively controlled but rather in which the gripper arms are provided with slanted infeed surfaces, which, when the workpiece is pushed against them, cause the gripper arms to open. After the area of the workpiece to be gripped has been fully introduced, the grippers then snap back into place under the action of springs and thus hold the workpiece. In conjunction with the use of slanted guide surfaces, the workpiece can be removed again by the application of previously determined tensile forces. 
     Both the use of actively controlled grippers and the use of passively actuated grippers lead to advantages and disadvantages. Actively controlled grippers require suitable actuating elements, which take up space, lead to corresponding costs, and increase the amount of required maintenance work and servicing. Passively controlled grippers are usually very simple in design, but because of their direct contact with the workpieces, they lead to scratches or abrasion of the workpiece. 
     SUMMARY OF THE INVENTION 
     The goal of the present invention is to improve a method of the type described above in such a way that reliable, non-damaging transport of the workpieces at high transport speeds is facilitated. 
     This goal is achieved according to the invention in that the pivoting movement of the gripper arm is controlled by at least one magnet. 
     An additional goal of the present invention is to design an apparatus of the type described above in such a way that a simple mechanical design is achieved while at the same time non-damaging transport of the workpieces is ensured. 
     This goal is achieved according to the invention in that a positioning device for the gripper arm comprises at least one magnet, the distance of which from a counterelement can be varied. 
     Through the use of magnets, advantage can be taken of magnetic forces of attraction or repulsion, which act without contact. If, for example, the gripper arm is connected to an actuating element on which a permanent magnet is arranged so that its magnetic north pole points in a contact direction, this permanent magnet will be subjected to a compressive force when approached by another permanent magnet, the magnetic north pole of which points toward the first magnet. In the presence of a suitable mechanical arrangement, this compressive force leads either to an opening movement or to a closing movement of the gripper. Conversely, upon the approach of a second magnet the south pole of which points toward the first magnet, a tensile force will act on the first magnet, advantage of which can also be taken to execute an opening or a closing movement of the gripper. 
     In general, it is necessary to use two magnets to generate appropriate compressive forces, these magnets being arranged with the same magnetic poles facing each other. If it is desired to generate tensile forces, however, it is sufficient to use only one magnet. This magnet can cooperate with a ferromagnetic material such as iron. The use of two magnets to generate tensile forces, however, leads to a stronger effective force, and thus an apparatus of more compact design can be obtained without a loss of actuating force. 
     The handling of preforms by the use of gripper arms as described above and in the following pertains equally to the handling of blow-molded containers, although this will not be pointed out separately each time. Spring-loading the gripper arms relative to a central element or relative to each other facilitates the automatic holding of the preforms or bottles without the need for separate interlocking elements or actively controlled actuating elements. It is also conceivable that, rather than directly, the gripper arms could act indirectly on the preforms or bottles by way of carrier elements, which themselves hold the preforms or bottles. The availability of a large amount of force can be facilitated by the cooperation of at least two magnets. 
     The gripper can be controlled as a function of the stage of the transfer process by moving the magnet and a counterelement relative to each other to vary the distance between them. 
     The transfer of a large number of workpieces per unit time can be facilitated by using a transfer arm of a rotating transport wheel to position the gripper carrier. 
     To generate compressive forces, it is provided that the magnet and a counterelement designed as a magnet are positioned with the same magnetic poles pointing toward each other during the performance of a gripper actuation. 
     The generation of tensile forces is made possible in that the magnet and a counterelement designed as a magnet are positioned with the opposite magnetic poles facing each other during the performance of a gripper actuation. 
     Wear-free operation is achieved in that the magnet and the counterelement are conducted past each other without contact. 
     To increase the operational reliability, it is proposed that the gripper arm be connected to an additional mechanical actuating device. 
     It is also conceivable that the gripper carrier could be arranged in the area of a container-filling machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  shows a perspective view of a blow-molding station for the production of containers from preforms; 
         FIG. 2  shows a longitudinal cross section through a blow-mold, in which a preform is stretched and expanded; 
         FIG. 3  shows a sketch which illustrates a basic design of an apparatus for the blow-molding of containers; 
         FIG. 4  shows a modified heating section with increased heating capacity; 
         FIG. 5  shows a perspective diagram of a transport element with two gripper arms and a central element in the closed state of the grippers; 
         FIG. 6  shows the arrangement according to  FIG. 5  after the grippers have been opened; 
         FIG. 7  shows a schematic diagram of an opened gripper, which comprises a permanent magnet in the area of an actuating element, and of an associated positionable countermagnet; 
         FIG. 8  shows the arrangement according to  FIG. 7  after the movable magnet has approached the magnet located in the area of the gripper; and 
         FIG. 9  shows the arrangement according to  FIGS. 7 and 8  after completion of the closing movement of the gripper. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive handling of workpieces is explained below on the basis of the example of a blow-molding machine. 
     The basic design of an apparatus for molding preforms  1  into containers  2  is illustrated in  FIGS. 1 and 2 . The arrangement can be the same as that shown, or it can be carried out after rotation by 180° in a vertical plane. 
     The apparatus for molding the container  2  consists essentially of a blow-molding station  3 , which is equipped with a blow-mold  4 , into which a preform  1  can be introduced. The preform  1  can be an injection-molded part of polyethylene terephthalate. So that the preform  1  can be introduced into the blow-mold  4  and so that the finished container  2  can be removed from it, the blow-mold  4  consists of mold halves  5 ,  6  and a bottom part  7 , which can be positioned by a lifting device  8 . The preform  1  can be held in place in the area of the blow-molding station  3  by a holding element  9 . It is possible, for example, to introduce the preform  1  directly into the blow-mold  4  by means of grippers or other means of manipulation. 
     So that compressed air can be introduced, a connecting piston  10  is arranged underneath the blow-mold  4 ; this piston supplies compressed air to the preform  1  and simultaneously provides a good seal. In a modified design, it is also possible, however, to use fixed compressed air feed lines. 
     In this exemplary embodiment, a stretching rod  11 , which is positioned by a cylinder  12 , is used to stretch the preform  1 . According to another embodiment, the mechanical positioning of the stretching rod  11  is achieved by the use of cam segments, which are actuated by pickup rollers. The use of cam segments is especially advisable when a plurality of blow-molding stations  3  is mounted on a rotating blow-molding wheel  25 . 
     In the case of the embodiment shown in  FIG. 1 , the stretching system is designed with a tandem arrangement  12  of two cylinders. Before the start of the actual stretching operation, a primary cylinder  13  moves the stretching rod  11  into the area of the bottom  14  of the preform  1 . For the actual stretching operation, a secondary cylinder  16  or a cam control unit positions the primary cylinder  13 , which carries the extended stretching rod, jointly with a slide  15 , which carries the primary cylinder  13 . In particular, a cam can be used to control the secondary cylinder  16  in such a way that a guide roller  17 , which slides along the control surface of the cam during the performance of the stretching operation, defines the stretching position at the time in question. The secondary cylinder  16  presses the guide roller  17  against the control surface of the cam. The slide  15  slides along the two guide elements  18 . 
     After the mold halves  5 ,  6 , which are arranged in the area of carriers  19 ,  20 , have been closed, the carriers  19 ,  20  are locked together by a locking device  20 . 
     So that the apparatus can be adapted to differently shaped mouth sections  21  of the preform  1 , separate threaded inserts  22  can be used in the area of the blow-mold  4  as shown in  FIG. 2 . 
       FIG. 2  shows not only the blow-molded container  2  but also the preform  1 , shown in broken line, and in schematic fashion the developing container bubble  23 . 
       FIG. 3  shows the basic design of a blow-molding machine equipped with a heating section  24  and a rotating blow-molding wheel  25 . Starting from a preform feed unit  26 , the preforms  1  are transported into the area of the heating section  24  by transfer wheels  27 ,  28 ,  29 . Heat radiators  30  and blowers  31  are installed along the length of the heating section  24  to temper the preforms  1 . After the preforms  1  have been tempered sufficiently, they are transferred to the blow-molding wheel  25 , in the area of which the blow-molding stations  3  are mounted. The finished blow-molded containers  2  are sent by transfer wheels  37 ,  28 ,  38  to a discharge section  32 . 
     So that a preform  1  can be molded into a container in such a way that the container  2  comprises material properties which guarantee a long shelf life for the food product, especially a beverage, packaged inside the container  2 , special processing steps must be carried out during the heating and orientation of the preforms  1 . In addition, advantageous effects can also be achieved by following special dimensioning guidelines. 
     Various plastics can be used as the thermoplastic material. PET, PEN, and PP, for example, are suitable. 
     The preform  1  is expanded during the orientation step by the infeed of compressed air. The compressed air feed process is divided into a pre-blowing phase, in which gas, such as compressed air, is supplied at low pressure, and a following main blowing phase, in which gas is supplied at a higher pressure. During the pre-blowing phase, compressed air is used typically at a pressure in the range of 10-25 bars, and during the main blowing phase, compressed air is supplied at a pressure in the range of 25-40 bars. 
     It can also be seen from  FIG. 3  that, in the case of the embodiment shown here, the heating section  24  is formed by a plurality of endlessly traveling transport elements  33 , which are arranged in a chain-like row and guided around deflecting pulleys  34 . In particular, the chain-like arrangement can be set up to define an essentially rectangular outline. In the present embodiment, a single, relatively large deflecting pulley  34  is used in the area of the part of the heating section  24  facing the transfer wheel  27 , whereas, in the area where the two adjacent deflections occur, two comparatively smaller deflecting pulleys  36  are used. In principle, however, any other desired type of guide configuration can be used. 
     So that the transfer wheel  27  and the blow-molding wheel  25  can be arranged as closely together as possible, the illustrated arrangement proves to be especially effective, because here the three pulleys  34 ,  36  are positioned in the relevant area of the heating section  24 ; that is, the two smaller pulleys  36  are located in the area of the transition to the linear portion of the heating section  24  and the larger pulley  34  directly in the area of the transfer to the transfer wheel  27  and to the blow-molding wheel  25 . As an alternative to the use of chain-like transport elements  33 , it is also possible to use, for example, a rotating heating wheel. 
     After the blow-molding of the containers  2  is finished, the containers are removed by the transfer wheel  38  from the area of the blow-molding stations  3  and transported to the discharge section  32 . 
     In the modified heating section  24  shown in  FIG. 4 , the larger number of heat radiators  30  makes it possible to temper a larger number of preforms  1  per unit time. The blowers  31  conduct cooling air into the area of cooling air channels  39 , which are arranged opposite their assigned heat radiators  30  and release the cooling air through outlets. Because of the way in which the outflow directions are oriented, the cooling air flows in a direction which is essentially transverse to the transport direction of the preforms  1 . The surfaces of the cooling air channels  39  opposite the heat radiators  30  can be provided with reflectors for the heat radiation; it is also possible for the discharged cooling air to cool the heat radiators  30 . 
     The preforms  1  and the containers  2  can be transported through the blow-molding machine in various ways. According to one design variant, the preforms are carried along over at least a large portion of their transport distance by transport mandrels. It is also possible, however, for the preforms to be transported by grippers, which grip the external surface of the preform, or by internal mandrels, which are introduced into the mouths of the preforms. There are also various ways in which the preforms can be oriented in space. 
     According to one variant, the preform is supplies in the area of the preform feed unit  26  with its mouth pointing vertically upward; it is then rotated and conveyed along the heating section  24  and around the blow-molding wheel  25  with its mouth pointing vertically downward; and finally it is rotated again before it reaches the discharge section  32 . According to another variant, the preform  2  is heated in the area of the heating section  24  with its mouth pointing vertically downward but rotated again by 180° before it reaches the blow-molding wheel  25 . 
     According to a third design variant, the preform travels through the entire blow-molding machine without ever being rotated, i.e., with its mouth pointing vertically upward at all times. 
       FIG. 5  shows a perspective view of a preform  1  being held by a gripper-like transport element  41 . The transport element  41  can also position a blown container  2  in a similar manner. The transport element  41  comprises a gripper carrier  42 , which is connected to the gripper arms  45 ,  46  by pivot joints  43 ,  44 . The gripper arms  45 ,  46  are connected to a central element  49  by levers  47 ,  48 . The levers  47 ,  48  are connected to the gripper arms  45 ,  46  by pivot joints  50 ,  51  and to the central element  49  by pivot joints  52 ,  53 . The central element  49  comprises an opening  54 , into which a retaining element  55  connected to the gripper carrier  42  is inserted. According to an alternative embodiment, the retaining element  55  can also be formed as an integral part of the central element  49 . 
     At least one of the pivot joints  43 ,  44 ,  50 ,  51 ,  52 ,  53  can be realized as a pin-in-hole combination, as a hinge, as a film joint, or as a solid joint. When film joints or solid joints are used, it is possible for all of the components of the transport element  41  except for the springs  56 ,  57  to be designed as an integral one-piece unit or for some of the components to be combined into one or more modules. 
     The gripper arms  45 ,  46  are also connected to the retaining element  55  by springs  56 ,  67 . The springs  56 ,  57  are preferably designed as tension springs and are held by pins  58 ,  59  at the ends facing away from the retaining element  55 , the pins being formed as extensions of the pivot joints  50 ,  51 . 
       FIG. 5  shows the transport element  41  in the closed state, in which the preform  1  is held by the gripper arms  45 ,  46 . The springs  55 ,  57  and the central element  49  stabilize the gripper arms  45 ,  46  in the closed position shown. It can be seen in particular that both the longitudinal axes  60 ,  61  of the levers  47 ,  48  and the longitudinal axes  62 ,  63  of the springs  56 ,  57  are arranged at an angle to the central longitudinal axis  64  of the central element  49 . With respect to the central longitudinal axis  64 , the longitudinal lever axes  60 ,  61  and the longitudinal spring axes  62 ,  63  extend forward toward the preform  1  at an angle. 
     In the embodiment shown in  FIG. 5 , the central longitudinal axis  64  represents the axis of symmetry of the transport element  41 . The central element  49  is located between the gripper arms  45 ,  46 ; and, in the closed position shown, the gripper arms  45 ,  46  rest laterally against the central element  49 . 
       FIG. 6  shows the same arrangement as that of  FIG. 5  except that now the gripper arms  45 ,  46  have been opened. According to the exemplary embodiment shown here, the retaining element  55  is rigidly connected to the central element  49  and is shifted jointly with the central element  49  relative to the gripper carrier  45 . As a result of the positioning movement of the central element  49 , the levers  47 ,  48  and thus also the gripper arms  45 ,  46  are pivoted and release the preform  1 . 
       FIG. 7  shows a detailed design embodiment of a transport element  41 . The gripper carrier  42  is for its own part connected to an element base  67  so that it can be deflected. This arrangement makes it possible for the carrier to get out of the way in the event of a collision. In the area of the central element  49 , a magnet designed as a permanent magnet  68  is arranged. The permanent magnet  68  in the exemplary embodiment shown here is arranged with its magnetic north pole pointing away from the element base  67 . A countermagnet  69 , which can also be designed as a permanent magnet, is arranged a certain distance away from the permanent magnet  68 . In the example shown, the countermagnet  69  is arranged with its magnetic north pole pointing toward the permanent magnet  68 . 
     When the countermagnet  69  approaches the permanent magnet  68 , a compressive force is exerted on the permanent magnet  68 , which transmits it to the central element  69 . This actuating force causes the gripper arms  45 ,  46  to close. 
       FIG. 8  shows an operating state in which the countermagnet  69  has been moved up to within a very short distance of the permanent magnet  68  and is thus much closer to it than it was in the arrangement of  FIG. 7 . The mouth section  21  of the container  2  has already been introduced almost completely into the receiving area of the gripper arms  45 ,  46 , and the gripper arms  45 ,  46  are ready to begin their closing movement. 
       FIG. 9  shows the transport element  41  after the completed closing of the gripper arms  45 ,  46 . The mouth section  21  of the container  2  is held firmly in place by the gripper arms  45 ,  46  in a position above, for example, a support ring. 
     According to a typical embodiment, the gripper carriers  42 , i.e., their element bases  67 , are mounted on a rotating transfer wheel. The containers  2  or the preforms  1  or, in general, the workpieces are also conveyed to or away from the area of the transport elements  41  by rotating transport elements or elements which move in some other way. The countermagnets  69  are therefore advisably arranged in the area of one of these transport devices for the workpieces. 
     The embodiment explained above ensures that the permanent magnet  68  occupies a defined, constant position relative to the gripper carrier  42  and also that the countermagnet  69  occupies a defined, constant position relative to the workpiece to be transferred. The threading-in or threading-out of the part of the workpiece to be gripped between the gripper arms  45 ,  46  can thus be coordinated precisely with the movement by which the countermagnet  69  approaches the permanent magnet  68  or moves away from it, so that the movements in question can be executed precisely in relation to each other. 
     According to another embodiment, it is proposed in particular that the permanent magnets  68  and the countermagnets  69  do not touch each other at any time during the course of the process. This makes it possible to achieve an especially high resistance to wear. 
     According to another embodiment, it is provided that contact between the permanent magnets  68  and the countermagnets  69  is allowed, but that, because of the forces of magnetic repulsion which are in play, the magnets exert only a small amount of force on each other. As a supplement to the idea just explained above, the permanent magnets  68  and/or the countermagnets  69  can be provided with thin coatings such as coatings of plastic to prevent damage in the even that they come into contact with each other. 
     According to another embodiment, it is proposed that, adjacent to the permanent magnet  68  and/or adjacent to the countermagnet  69 , mechanical guide elements be provided, which, in cases where the magnets do not exert enough force, forcibly open or close the gripper arms  45 ,  46 , so that, in the event of a possible functional breakdown, the transfer processes can nevertheless be completed reliably. 
     As an alternative or supplement to the above, it is also proposed that, in cases where the magnets do not exert sufficient force, the opening and/or closing movements of the gripper arms  45 ,  46  be brought about directly by the workpiece to be transferred, so that the transfer processes can be carried out reliably even when problems occur. 
     In the case of the stationary magnets, it is possible in all of the embodiments to replace the permanent magnets or to supplement them with electromagnets or electrically actuated magnet systems.