BACKGROUND OF THE INVENTION 
   The present invention relates to a sorting device for flat items of mail, having at least one input station, in each case having a mail separating device and a following transport device for transporting the separated items of mail past process appliances, such as readers, bar code printers, to a loading station of a mail buffer store in each case, the mail buffer store comprising a plurality of storage pockets which circulate in a conveying loop and are moved past the loading stations, and which are loaded with the mail in the loading stations. 
   In order to sort flat items of mail, such as letters, postcards, small packets and the like in accordance with the, distribution information specified on the surface, it is known to accommodate these items of mail, to transport them and to discharge them in a controlled manner in specific pocket-like containers. 
   For example, EP 0 608 161 A1 discloses a salting device in which flat objects are transported laterally into storage pockets from outside by means of a channel-like, locally fixed feed device. These storage pockets, circulating in a closed loop, are moved horizontally past the feed device and the items of mail then fall into sorting containers located underneath by means of specific opening of flaps accordance with the sorting stipulation. In order to put the items of mail securely into the moving storage pockets, either the items of mail in the feed device must have a very high transport speed, which can lead to damage when braking the items of mail in the container, or the speed of the storage pockets is very low or the containers are at a standstill during the input, which means a reduction in the throughput of the machine. 
   For the purpose of ordering in a specific sequence, and solution has been disclosed (EP 820 818 A1) which uses an intermediate store, which comprises storage pockets circulating on a conveyor device in a conveyor loop having at least two semicircular conveyor sections, it being possible for said storage pockets each to pick up an item of mail and to deposit is in the actual tray again when instructed. In this case, initially all the items of mail to be ordered are accommodated in any desired sequence in the storage pockets of the intermediate store. The items of mail are then removed from the storage pockets of the intermediate store and transferred into the trays in such a way that they are located in the latter in the order to be produced. The trays, which are open at the top, are located along the straight sections of the conveyor device, underneath the storage pockets. 
   In order to improve the input of the items of mail into the storage pockets, according to WO 97/10904 the end section of the transport means of the feed device is designed such that it can be pivoted so that, doing the input of the respective item of mail, this end section is pivoted together with the moved storage pockets in substantially the same direction and at substantially the same speed and, following the input, is pivoted back again. 
   The throughput of these sorting devices is limited by the throughput of the circulating storage pockets. An increase in the throughput can theoretically be achieved only by increasing the speed and/or reducing the pitch of the transport system of the storage pockets. However, as outlined (input into the storage pockets), this is possible only to a very limited extent. 
   A sorting device (EP 0 949 015 A2) has also been disclosed which has a plurality of input units, at least one mail buffer store with continuously circulating storage pockets, mail containers as final sorting points, which are filled from the different mail, buffer stores, and with an input and output device for mail containers. In this case, it is possible to manage with fewer final sorting points than there are sorting destinations. 
   SUMMARY OF THE INVENTION 
   The invention is placed on the object of providing a sorting device having circulating storage pockets which discharge items of mail in a controlled manner at final sorting points in accordance with the read destination addresses, which has a higher throughput and with which the expenditure for automatically changing the mail containers and for measuring their filling levels can be reduced. 
   According to the invention, the object is achieved by the features of claim  1 . 
   Here, the sorting device can have a plurality of mail buffer stores with storage pockets in each case circulating continuously in a conveyor loop, arranged one after another and moved past one or two loading stations. Underneath the straight transfer sections of the conveyor loops of the mail buffer store or stores there are mail containers moving on a transport path, circulating continuously and open at the top, as final sorting points assigned to the destination addresses or destination address groups. The items of mail are unloaded into the mail contains in accordance with the read destination addresses by means of controllable opening of the respective storage pocket at the time at which this pocket is located in the appropriate position above the associated mail container. The container transport path has at least one input device for empty mail containers and at least one output device for filled mail containers. The transport speeds of the mail buffer stores and of the mail containers and also the lengths of the transfer sections of the circulation paths of the mail buffer stores which are arranged above the transport path for the mail containers for loading the mail containers are defined in such a way that, during the time interval in which a storage pocket passes through a transfer section of a mail buffer, each mail container has moved through underneath this storage pocket. By means of the new principle— the mail containers are transported to the items of mail— it is possible to increase the throughput without increasing the transport speeds, as a result of parallelization with a flexible arrangement. It is therefore possible to reduce the speed of the storage pockets substantially, which means that the time available for reading the destination addresses as far as reaching the first transfer section is increased. 
   Advantageous refinements of the invention are presented in the dependent claims. 
   For example, in order to increase the stacking quality, it is advantageous to incline the storage pockets counter to the transport direction of the mail containers to accommodate the items of mail in an upright position. 
   Likewise, it is advantageous to incline the circulating mail containers in the same direction, so that an inclined base with one or two preferential stacking edges is produced. 
   If, in the case of specific items of mail, the destination address cannot be read before the first transfer section is reached, these storage pockets preferably pass through a defined number of transfer sections and if, during this time, the destination addresses could still not yet be read, these items of mail are output to a container for unread items of mail. 
   It is advantageous if the mail containers transported along directly underneath the transfer sections of the mail buffer stores in each case have the same transport direction as the storage pockets. As a result, the stacking can be carried out cleanly and in a defined manner. 
   In order to achieve particularly high throughputs, in an advantageous development a plurality of mail buffer stores whose conveyor loops in each case have two straight transverse sections and two semicircular sections with the loading stations are arranged beside each other. The transport path for the circulating mail containers is guided in the shape of a meander underneath the mail buffer stores so that the mail containers transported along directly underneath the transfer sections of the mail buffer stores in each case have the same transport direction as the storage pockets. 
   In a beneficial embodiment, the storage pockets are open at the side and are led with the open side past the end sections of the transport devices. As a result, the transport direction out of the input station and the orientation of the mail can be maintained as far as the storage pocket. 
   Furthermore, it is advantageous if the transport path for the mail containers has at least two narrow finger belts running beside each other at a fixed to distance and, in order to put the mail containers into the transport path or to remove them from the latter, transport rollers which can be raised and lowered under control are arranged at the side of the transport path and between the finger belts. The largest directional component during the input or output in this case runs at right angles to the transport direction of the transport path. As a result, the actions of putting the mail containers in and out can be implemented with little effort. 
   In a further advantageous refinement, stationary actuators that can be driven in order to open the storage pockets are arranged along the transfer sections of the mail buffer stores. In each case a closing element is provided at the end of the transfer sections. 
   In order to determine the height of the stack of mail in the mail containers advantageously, a sensor arrangement for determining the stack height is arranged above the transport path of the containers, after the transfer sections and before the device for putting the mail containers in and out of the transport path. 
   As opposed to this, hitherto an appropriate sensor had to be arranged at each end point or a calculation of the stack height had to be carried out by means of the measured thicknesses of the individual items of mail. 


   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     In the following, the invention will be explained in more detail in an exemplary embodiment by using the drawing, in which: 
       FIG. 1  shows a schematic plan view of a sorting device having 3 mail buffer stores; 
       FIG. 2  shows a schematic side illustration of the storage pockets above the mail containers; 
       FIG. 3  shows a schematic illustration of an input and output device for mail containers (view from below). 
   


   DETAILED DESCRIPTION OF THE INVENTION 
   As can be seen from  FIG. 1 , 3 mail buffer stores  1  are arranged beside one another. Each mail buffer store  1  comprises storage pockets which circulate continuously in a conveyor loop, are arranged one after another and can be unloaded downward under control by means of an opening mechanism, it order to accommodate the items of mail in an upright position. Each conveyor loop has two straight transfer sections and, between the latter, two semicircular sections. The storage pockets are open naturally to the outside and on each semicircular section of a conveyor loop there is a loading station  7 , in which items of mail are transported individually through the lateral openings into the empty storage pockets. The loading station  7  receives the items of mail in each case individually one after another from an input unit  2 , in which the items of mail from stacks of mail, separated by means of a mail separating device  3 , are aligned in an alignment section  4  and are then transported with the aid of a transport device  8  past process appliances, such as an address reader  5  or a bar code printer  6  for applying an identification code, to the loading station  7 . In each semicircular section of the conveyor loops of the mail buffer stores  1 , items of mail are put into the storage pockets, that is to say the system has 6 input units  2  and loading stations  7 . Underneath the circulating storage pockets, mail containers open at the top are likewise moved circulating continuously on a transport path  9  as final sorting points. This is done in such a way that the mail containers are transported directly under all the straight transfer sections of the mail buffer stores  1  in the same conveying direction as the storage pockets, that is to say the transport path rdns in the shape of a meander here. The transport speed of the mail containers is fixed such that each mail container is moved through underneath each storage pocket during its passage through a transfer section. The speed of the storage pockets is therefore relatively low (e.g. 0.1 m/s) as compared to the mail containers (e.g. 1 m/s), which has a beneficial effect on the reliable operation of the input units and the loading stations. Because of the parallelization of the input processes of a plurality of mail buffer stores  1  as illustrated, which is not possible to the same extent in the prior sit with stationary end points, the throughput of the sorting device can be increased. A further advantage is that no relatively long mail passage sections after the address reader are need it in order to provide the necessary time for determining the address. If it is assumed that the diameter of the semicircular sections of the conveyor loops of the mail buffer stores  1  is 2 m, then about 15 s are available after the storage pocket has been loaded until it enters the straight transfer section. The basic construction of the mail buffer stores  1  with the circulating storage pockets  10  and the unloading of the items of mail  11  lying horizontally into the mail containers  12  are illustrated in  FIG. 2 . The direction of movement of the storage pockets  10  and of the mail containers  12  is identified by the arrows. In order to measure the filling level of the mail containers  12  and, on this basis, to determine when the filled mail container  12  is to be the removed and replaced by an empty mail container  12 , there is a sensor arrangement  14  for measuring the stack height for example as a laser sensor, above the transport path after the mail buffer stores  1  and before an input and output device  16 . If the maximum stack height has not yet been reached, then the relevant mail container  12  passes through a further circuit. If it has been reached or exceeded, the output is carried out. The basic structure of the input and output device  16  is illustrated in  FIG. 3 . 
   The storage pockets  10  of the mail buffer store  1 , inclined rearward in the direction of movement for the purpose of better unloading, circulate at a relatively low speed as compared with the mail containers  12 . The number of storage pockets  10  needed is determined from the throughput of the mail separating device  3  and the time for a complete container circuit plus any possibly necessary process times for the determination of the sorting destination. This ensures that each item of mail  11  is kept in the mail buffer store  1  until the mail container  12  assigned in accordance with the destination address and sorting plan reaches the storage pocket  10 . 
   The storage pockets  10  are constructed in such a way that they can be loaded with the separated items of mail  11  from the side, and that they can discharge the respective item of mail  11  downward, driven by the force of gravity, into the mail container  12  running past on three spaced-apart finger belts  17 , following the actuation of a flap construction  13  as a pocket base. The flaps in this flap construction  13  are equipped with a lever of a certain longitudinal extent, such that time-controlled actuation, for example in accordance with a previously determined mail characteristic (e.g. thickness) can be carried out and the triggering time can be varied slightly with the effect of an optimal stacking quality. The location and number of stationary actuators  15  for opening the storage pockets  10  (e.g. lifting magnets) must be defined in such a way that timely triggering of each storage pocket  10  in combination with each mail container  12  running past is ensured. 
   The following further design variants are possible or advantageous:
         the mail containers  12  can also be transported inclined, so that an inclined base with one or two preferential stacking edge/s is produced. Accordingly, the mail buffer stores  1  can also be inclined.       

   In the input and output devices  16  for mail containers  12  there are, between the individual finger belts  17  of the transport path  9  and at the side of the transport path  9 , driven rollers  18  which are set at an angle, which can be raised and lowered above the fingers of the finger belts  17  under timed control by means of suitable actuators (for example pneumatically or electrically driven levers), so that they lift the mail container  12  located about the rollers  18  at that instant above the finger belts  17 , while maintaining the transport speed component of the finger belt  17 , move it transversely with respect to the finger belt  17  and therefore put it into the stream (empty container) or remove it from the stream (full container). In order to increase the process reliability, additional mechanical guide devices can also be used. 
   In order to have sufficient time available to remove adjacent mail containers  12  as well, and also to shorten the discharge time, two input and output devices  16  are provided. The filled mail containers  12  that are removed finally pass via a storage section to labelling modules  19 , where they are provided with appropriately printed labels for the purpose of identification and coding (address range).
The invention relates to a sorting device which includes a mail item buffer storage having storage pouches, circulating in a conveying loop and moving past charge station. Below straight handover sections of the conveying loops, circulating mail from containers, displaced in a conveying path and open to the top, are provided as the sorting terminal points. The mail items are charged into the mail item containers according to their read-out target addresses by controlled opening of the respective storage pouch at that time at which the respective pouch arrives in the respective position above the associated mail item container.