Abstract:
A printer apparatus has a contact pressure device for flat articles on a continuously moving transport belt, in particular letters in a franking and/or addressing machine. In order to increase the usable region and/or of the throughput of the printer apparatus, flat articles of different thicknesses are processed with predetermined speed, without slippage, in succession with arbitrarily small gaps between successive articles. An elastic, bellows-shaped, resiliently supported air bag has a low-friction, wear-resistant cover surface that is in non-positive contact with the transport belt. With its associated retention and air supply devices the bellows serves as a mobile contact pressure module.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a printing apparatus according of the type that is equipped to print to flat goods to be printed during the passage of flat goods by a printer unit. The printing apparatus can be used in a mail processing system, in particular in a franking and/or addressing machine. The goods are letters and other mail items or flat articles. 
     2. Description of the Prior Art 
     In known franking machines, contact pressure elements are used that press the mail good from below against a transport direction that is situated above, and direct the mail items past the print head for printing. For example, elastically borne counter-pressure rollers; counter-pressure elements made of elastic elements in the form of leaf springs; elastically borne flat ribbon belts are used as counter-pressure elements. 
     A device for printing to flat goods at a transport module is already known from U.S. Pat. No. 7,810,810 B2, which has a number of contact pressure elements that act on the flat good with a spring force through an opening in a feed table of an apparatus. The contact pressure elements are designed as lowerable elastic elements and anchored in a lower housing shell of the apparatus. For example, an actively driven flat ribbon belt for the transport of the flat item is provided as a transport direction in the upper housing shell of the apparatus. Due to the suspension of every single contact pressure element, the possibility advantageously exists of mixed mail processing of different mail goods of 0.1 to 10 mm and more that can follow closely in series (i.e. with a short interval between the mail goods) and enable a high throughput of the franking machine. 
     In this contact pressure device, it is disadvantageous that the mail item is pressed from below against the flat transport belt only in a line, or in points. Even with a number of contact pressure elements, a flat contact pressure against the flat transport belt situated above is not achieved; only an approximation of this ideal state is possible. Transport disruptions (such as shocks and delays) that negatively affect the print quality occur upon running into the individual contact pressure elements. 
     A device to lower, position and raise contact pressure elements of a printing apparatus is known from the German Utility Model DE 20 2010 015 351 U1. This device is arranged in a box-shaped module that can be slid like a drawer into the printing apparatus or can be slid out from the printing apparatus. The contact pressure device is elastically mounted on a base plate in the housing of the box-shaped module. The box-shaped module has two guide holders for guide rods or rails for sliding in the box-shaped module, a rocker to lower the contact pressure elements, and shaped parts that interact with the contour of the guide rods for lowering, positioning and raising the contact pressure elements of the contact pressure device in order to initially move the contact pressure device backwards into the printing apparatus while sliding the box-shaped module into the printing apparatus, and to move the contact pressure device forwards towards a transport direction upon reaching a predetermined position. The contact pressure device has brush-like contact pressure elements. 
     A contact pressure device with sprung elastic contact pressure elements is likewise known from the German Utility Model DE 20 2011 109 208 U1. This contact pressure device presses the mail item over its entire surface against the overlying flat transport belt with a double-spring brush element. A transport disruption no longer occurs in the printing process due to the many, densely arranged brush elements, and a high print quality is achieved even given letter thicknesses up to 10 mm. 
     Brush elements of a brush body are mechanically coupled with a spring system that in turn has a number of spring elements. The brush elements themselves are typically flexurally elastic to a limited extent. The brush elements compensate for the thickness difference up to a thickness of the mail goods of 3 mm. The entirety of the brush body is additionally elastically lowered at thicknesses as of 3 mm. The brush body is attached to a base plate that is in turn borne in a spring-biased manner on a floor plate that is elastically connected with a chassis. The spring elements are arranged between the base plate and the floor plate as well as between the base plate and the chassis. When a letter arrives between brush body and transport belt, the brush body as a whole is distanced from the transport belt by the letter thickness minus the brush element curvature. Given letters of approximately the same thickness, the detection of the subsequent letter is certain, even if the preceding letter has not yet left the contact pressure region. However, if a thin letter (1 mm thick) follows a thick letter (10 mm thick), the risk exists that the thin letter will not be detected as long as the thick letter is still located in the contact pressure region. For such cases (mixed mail), it must be ensured that a letter may only arrive in the contact pressure region when the preceding letter has exited said contact pressure region. 
     Otherwise, given the processing of a thick mail items and thin mail items in immediate succession, a flat contact pressure of the thin mail item is not ensured. This leads to poorer printing results. In order to process the mail items of different thickness with a uniformly good print quality, an interval (for instance in brush body lengths) between the successive mail items is necessary. The design of the brush element therefore requires a minimum gap, i.e. a minimum letter interval of approximately one brush body length between the flat mail items in order to ensure the uniform contact pressure. This means either reducing the letter throughput and increased control cost, or exclusion of mixed mail. Given significantly non-uniform letter contents, a complete compensation of such non-uniform thicknesses does not occur, such that the print quality can suffer. 
     Given subdivision of the brush body analogously to the roller bearing, the risk in turn exists of transition shocks and slippage, with subsequent consequences for the print quality. 
     A printing apparatus of modular design is described in addition to the contact pressure described above in DE 20 2010 015 354 U1. In an upper part of the printing apparatus, a transport module is arranged which has an actively driven, revolving transport belt. In the lower part of the printing apparatus, the box-shaped module is provided with a contact pressure device to press against flat articles (letters). During the printing, the flat article is transported clamped between the transport belt and the contact pressure device. The module is slid into the operating mode and can be removed from the printing apparatus in service mode. This arrangement is therefore subsequently designated as a mobile contact pressure module. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to increase the usage range and the letter throughput of a printing apparatus. 
     The invention is based on the object of developing a contact pressure device suitable for mixed mail processing—in particular for letter thicknesses from 0.1 to 10 mm—that enables a high print quality of the printing apparatus. 
     It is a further object to achieve a contact pressure device of the aforementioned type in a modular design with which flat articles of different thicknesses can be processed more quickly than the one-piece-at-a-time manner described above. 
     A high throughput of flat goods should be achieved by the printing in the printing apparatus. The printing apparatus should be characterized by an affordability and functional security, inexpensive servicing and user-friendliness, and a low noise emission. 
     The printing apparatus according to the invention has a mobile contact pressure module with a contact pressure device that is provided to press flat articles against a roller driven, continuously moving transport belt, wherein the transport belt is arranged in an upper part of the printing apparatus and the mobile contact pressure module is arranged in its lower part. A flat article is transported between the transport belt and the mobile contact pressure module. The contact pressure device has a gas-tight shell that can be elastically inflated such as an elastic, bellows-shaped, resiliently supported air bag that has a low-friction, wear-resistant cover surface that is not firmly connected with the transport belt. The air bag and the associated retention and air supply devices are a component of the mobile contact pressure module, which can be exchangeable. A floor frame, at least two tension springs and a function arm with a microswitch mounted thereupon, as well as a compression swing, are mounted as a retention device inside a housing of the mobile contact pressure module. The function arm is attached at one of its ends to the floor frame so as to be rotatable around a bearing axle, and is pressed at its other end against a stop by the spring force F 1  of the compression spring in the operationally ready state of the mobile contact pressure module. In the removal-ready state of the mobile contact pressure module, upon a movement counter to the action of the spring force F 1 , the function arm can be brought out of its stop. The elastic, inflatable, gas-tight shell has an upper part and a lower part, as well as a middle part, with the middle part being attached to an upper part of the housing of the contact pressure module, and the upper part of the shell penetrating through a window opening into the upper part. A floor plate is mounted at the floor of the lower part of the contact pressure device, and the at least two tension springs are tensioned between the floor plate and the floor frame or the upper part, so the tension springs together exert an spring force F 2  on the surface of the floor plate in order to urge the floor plate onto stops to limit travel, and therefore onto the microswitches. Each microswitch is activated until the spring force F 2  is greater than or equal to the sum of pre-tension F 2   pre  and the resulting force effect F=ΔP·A (wherein ΔP is the gas pressure difference between internal gas pressure and external pressure, and a is the effective surface of the shell). The minimum contact pressure force of the contact pressure device is established by the pre-tension F 2   pre  of the tension springs. The elastic constant c of the tension springs is selected so as to be equal to the quotient of the difference ΔF=F 2   max −F 2   pre  and the maximum deflection a max  of the floor plate. 
     A circuit board, equipped with a time delay circuit, is arranged inside the housing. The circuit board has a power connection to supply the circuit board with an operating voltage when the mobile contact pressure module is in an inserted state, and a connector electrically connected to the output of the time delay circuit so as to provide that output to the contacts of a motor of a pump. The circuit board also has a connector that electrically connects an input of the time delay circuit with the contacts of the microswitch. The time delay circuit of the circuit board detects activation of the microswitch and omits, as an output a time-delayed signal to the motor of the pump; but, in the state in which the mobile contact pressure module is removed, the pump remains unpowered, so the pump is started with a time delay when the function arm is brought to a stop, and gas is then pumped into the shell of the contact pressure device as long as the output signal is emitted. The operationally ready state of the mobile contact pressure module thus is set with a time delay. 
     The stops for travel limitation are provided at a predetermined distance D from the running surface of the flat articles on the upper part of the housing. The operation of the microswitch is interrupted when the floor plate moves away from the stops for travel limitation and a minimum distance a min  from the stops is thereby exceeded. 
     The printing apparatus allows printing of mixed mail with letter thicknesses in a region from 0.1 to 10 mm, even when a mail piece with a minimum letter thickness follows a mail piece with a maximum letter thickness in immediate succession, with a minimum gap between the mail pieces. The contact pressure module has a shell filled with a gas as a component of the contact pressure device, advantageously an air-filled, rubber elastic bellows system. The side walls of the shell are designed to be dimensionally stable while the cover surfaces (contact pressure surface and floor surface) execute a travel movement when the internal gas pressure rises. The width of the contact pressure surface of the contact pressure device transverse to the transport direction is smaller than or equal to the length of the straight segment of the transport belt. The inventive contact pressure device advantageously has the same dimensions as the contact pressure device with brush elements that are known from the German Utility Patent DE 20 2010 015 351 U1. In that known pressure control device, however, the contact pressure device is supported on the lower part of the contact pressure module housing. In contrast to this, in accordance with the invention a middle part of the bellows is mounted in the upper part of the contact pressure module housing, and the bellows has two assembled halves that rest gas tight on the middle part or on one another and internally allow a pressure compensation. Given an increase of the internal gas pressure, such a bellows system allows an expansion of the shell essentially in a preferred direction, so a current flat article to be transported that rests with its underside on a contact pressure surface of the bellows and is pressed against a transport device. The transport direction proceeds across the contact pressure module and the article is actively driven. 
     Upon intake of flat articles—for example mail items—the pressure relationships in the bellows system change. The contact pressure surface of the bellows is deformed depending on the mail item thickness. A higher air pressure within the bellows system thereby arises that is compensated by the elastically mounted, lowerable floor plate that forms the floor of the bellows. The tension springs that are tensioned between the floor plate and a floor frame are thereby forced to extend (stretch) beyond their nominal extent. 
     If mail pieces with lower thickness are transported again, the contact pressure surface can rapidly adapt its shape solely by the stretched tension springs acting between the floor plate and the floor frame. Via the air pressure in the bellows system, the contact pressure surface of the bellows can promptly rest on flat transport belt after the mail item. Smaller intervals (gaps) between the mail items are thereby possible and a higher throughput is achieved. Particularly in the processing of mixed mail, this is an advantage compared to a contact pressure device with brush elements. 
     A slide coating with low friction is applied to the contact pressure surface, while the transport belt is equipped with a surface that has a higher friction. A mail item is therefore safely transported. The noise emission remains below the average in such printing apparatuses. 
     Upon inflation of the bellows by means of a pump, for example a small electric air piston pump or membrane pump, the bellows presses its contact pressure surface against the actively driven flat transport belt. The bellows and the transport belt are in non-positive connection, meaning that despite the contact between the bellows and the transport belt, the transport belt still moves substantially unimpeded. At the same time, a travel movement of the floor plate is executed downward. A predetermined force F 2  that counteracts the travel movement is exerted by the tension springs. 
     Given transport of a thick mail item and an immediately following thin mail item, the contact pressure surface of the bellows is deformed more significantly as the gap between the mail items is reduced. For example, the flat mail items can be letters that immediately follow one another at a distance of approximately 50 mm. The length of the contact pressure device in the transport direction is, for example, four times the gap, and thus approximately corresponds to the mean letter length (235 mm) of a standard or compact letter. The throughput is also consequently increased. Given a minimum letter gap and short letter lengths of approximately 160 mm, a doubling of the throughput of letters with different thicknesses but the same format can thus be achieved. 
     If the internal air pressure P intern  has reduced after some time, due to a leak of the bellows system or due to an increase of external air pressure P extern , a required overpressure in the bellows system can then be established again with the aid of a delayed two-point regulation. As used herein, “regulation” is a process in which the control variable—the gas pressure P intern  within the shell—is changed in a desired manner due to external influences or disruptions given a deviation from a desired value F desired =F 2 +A·P extern  with A=active surface. The energy of the control variable itself is sufficient to pneumatically produce an adjustment of the control device via the lower part of the shell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a franking machine with inserted contact pressure module, from the front left. 
         FIG. 2  is a perspective view of the franking machine according to  FIG. 1  without the upper apparatus housing and with the contact pressure module removed. 
         FIG. 2   a  is a sectional view of the basic design of the inventive contact pressure device from the left (feed side). 
         FIG. 2   b  is a view of the basic design of the operating means of the contact pressure device from the left. 
         FIG. 2   c  is a sectional view of the basic design of the printing apparatus with a contact pressure device, from the front. 
         FIG. 3  is a perspective view of the contact pressure module from the rear above left, partially in an exploded representation. 
         FIG. 4  is a perspective view of the predominant upper part of the contact pressure module from the front above left, in an exploded representation. 
         FIG. 5  is a perspective view of the predominant upper part of the contact pressure module from the front bottom left, in an exploded representation. 
         FIG. 6  is a perspective view of the lower part of the contact pressure module from the front lower left, in exploded representation. 
         FIG. 7  is a perspective view of the upper part of the contact pressure module from the front lower left. 
         FIG. 8  is a perspective view of the upper part of the contact pressure module from the rear lower left. 
         FIG. 9  shows an incremental cross-section presentation of the contact pressure module in the operationally ready state taken along section A-A shown in  FIG. 7 . 
         FIG. 10  shows an incremental cross-section presentation of the contact pressure module in a removal-ready state taken along section A-A shown in  FIG. 7 . 
         FIG. 11  shows an incremental cross-section presentation of the contact pressure module in the state before operational readiness taken along section A-A shown in  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The representation is executed schematically at least in part for simplification and for an easier comprehension. For the purpose of brevity, “letter” is used instead of the term “flat article” in the following. 
     A franking machine  0  with an apparatus housing  01 , a contact pressure module  2  and a letter  4  in the output region (see transport direction arrow) is shown in  FIG. 1 . The apparatus housing  01  is composed of an upper part  011  and a lower part  012  and encloses an apparatus carrier  03  with accessories; see also  FIG. 2  in this regard. 
     The contact pressure module  2  is a mobile component of the lower part of the franking machine  0 . The contact pressure module housing  20  comprises an upper part  201  and a lower part  202 . 
     The input region  02  of the franking machine  0  forms what is known as the letter thickness sluice. The letter thickness sluice  02  is bounded at the top by a shaped part  031  and at the bottom by a shaped part  2010 ; see also  FIG. 2 . 
     The air supply to the bellows-shaped air bag  210  is interrupted when the grip  2062  of the function arm  206  is pressed (see in this regard  FIG. 7 ), and the air bag  210  deflates. The air bag  210  sinks downward, such that a clearance from the transport belt  10  is created. The contact pressure module  2  can be extracted toward the front at the grip  2062 . 
     A franking machine  0  with removed upper apparatus housing  011  (see  FIG. 1 ) and removed contact pressure module  2  is shown in  FIG. 2 . In the lower apparatus housing  012 , an apparatus carrier  03  is attached in which the transport module  1  with the transport belt  10  and the printing module  3  with the print heads  31  are arranged. The contact pressure module  2  can be connected with the lower region of the franking machine mechanically via guide rods  032  and electrically via a connector bushing  033 . 
     The contact pressure module  2  has contact pressure device  21  in a two-part housing  20 . The horizontal surface of the upper part  201  of the contact pressure module housing  20  simultaneously forms the letter travel surface in addition to the cover surface  21011  of the air bag  210 . The grip  2062  of the aforementioned function arm  206  is located in the lower part  202  of the contact pressure module housing  202 . 
     The cover surface  21011  of the upper part  201  of the air bag  210  is lowered like a wedge in the forward region, counter to the letter travel direction (see arrow), and is provided with a low-friction, wear-resistant coating (Teflon®, for example); see also  FIG. 4 . 
     A principle design of the contact pressure device is shown from the left (feed side) in section presentation (as viewed from the feed side) in  FIG. 2   a . A floor frame  205 , at least two tension springs  2052  and a function arm  206 , with a microswitch  2052  and a compression spring  2064  mounted thereupon, are provided inside a housing  20  of the mobile contact pressure module  2 . At one of its ends, the function arm  206  is attached to the floor frame  205  so as to be rotatable around a bearing axle  208 , and at its other end it is pressed against a stop by the spring force F 1  of the compression spring  2064  in the operationally ready state of the mobile contact pressure module  2 . The stop is formed by a rubber pad at the floor frame  205  and by a portion of the handle  2062 . In the removal-ready state of the mobile contact pressure module  2 , given a movement counter to the action of the spring force F 1  the function arm  206  can be brought out of its stop. The contact pressure device  21  has an elastic, inflatable, gas-tight shell  210  with an upper part  2101  and a lower part  2102 , as well as a middle part  2103 , wherein the middle part  2103  is attached to an upper part  201  of the housing  20  of the contact pressure module  2 . The upper part  2101  protrudes through a window opening into the upper part  201 . A floor plate  204  is mounted at the floor of the lower part  2101  of the contact pressure device  21 . The at least two tension springs  2052  are tensioned between the floor plate  204  and the floor frame  205  or the upper part  201 . The tension springs together exert an spring force F 2  on the surface of the floor plate, wherein the spring force F 2  is sufficiently large to draw said floor plate  204  onto stops  20531  (and therefore onto the microswitch  2061 ), at least in the inserted state of the mobile contact pressure module  2 . The microswitch  2061  is activated until the spring force F 2  is greater than or equal to the sum of pre-tension F 2   pre  and the resulting force effect F=ΔP·A, with the gas pressure difference ΔP between internal gas pressure and external pressure, as well as with the effective surface A of the shell, wherein the minimum contact pressure force of the contact pressure device  21  is established by the pre-tension F 2   pre  of the tension springs, and wherein an elastic constant c of the tension springs is selected which is equal to the quotient of the difference ΔF=F 2   max −F 2   pre  and the maximum deflection a max  of the floor plate  204 . At the maximum spring force F 2   max , a maximum contact pressure force is achieved that leads to maximum deformation of the shell. The pre-tension F 2   pre  of the tension springs can be variably selected. The maximum elastic tension F 2   max  that is reached upon maximum deflection a max  of the tension springs can be selected with regard to a maximum weight and/or dimension of the flat article. For example, the maximum spring force F 2   max =14 N, the pre-tension F 2   pre =4 N and the maximum deflection a max =10 mm. An elastic constant c=1 N/mm results from this for the tension springs. 
     Arranged inside a housing  20  is a circuit board  2017  with a time delay circuit. A power connector  20171  of the circuit board to supply the circuit board with an operating voltage in the inserted state of the mobile contact pressure module  2 ; a connector  20172  of the circuit board for electrical connection of an output of the time delay circuit with the contact of a motor of a pump  209 ; and a connector  20173  of the circuit board for electrical connection of an input of the time delay circuit with the contacts of the microswitch  2061  are provided, wherein the time delay circuit establishes an activation of the microswitch  2061  and outputs a time-delayed signal to the motor of the pump, but remains unpowered when the mobile contact pressure module  2  is in the removed state. It is provided that the time delay circuit is designed for a separate adjustment of the time delay of the activation delay and the deactivation delay. Upon the function arm  206  being brought into a stop, the pump  209  is started with a time delay and gas is pumped into the shell of the contact pressure device  21  as long as the signal is emitted, wherein the operationally ready state of the mobile contact pressure module  2  is set with a time delay. Stops  20531  are provided at a predetermined distance D from the travel surface  200  of the flat articles on the upper part  201  of the housing  20 . The activation of the microswitch  2061  is interrupted when the floor plate  204  moves away from the stops  20531  (backwards travel in the direction of the floor of the housing) and a minimum clearance a min  from the stops is thereby exceeded (see  FIG. 2 ). At the same time, the upper part  2101  of the elastic, inflatable, gas-tight shell  210  extends upward in the direction of a transport belt (see  FIG. 3 ). The forwards travel of the upper part  2101  is upwardly limited by the transport belt in that the surface of the upper part  2101  arrives at a stop with the transport belt. A wear-resistant, coated cover surface  21011  on the upper part  2101  of the elastic, inflatable, gas-tight shell  210  serves as a stop surface. The coating increases the sliding capability between the stop surface and the surface of the actively driven transport belt or, respectively, of the flat article. 
     A hose connector  2071  can be inserted into a gas intake and gas outlet opening in the floor of the lower part  2102  of the elastic, inflatable, gas-tight shell  210  and be connected via at least one hose  207  with the valve to relieve a gas overpressure. Alternatively, a hose connector  2071  can be omitted if a T-part  2073  is connected at one side (via hose  207 ) with the hose connector  2071  and at the other side directly with the hose connector  2072 . A gas outlet opening of the pump  209  is likewise connected with the T-part via a hose  207 . Before achieving the operating mode, a lower edge of the microswitch  2061  lies at a minimum distance a min  from the floor plate  204 , in contrast to  FIG. 2   b.    
     A view of a principle presentation of the operating means  206 ,  2062  and  20621  of the contact pressure device from the left is shown in  FIG. 2   b . Due to the gas overpressure achieved in the operating mode, the floor plate  204  experiences a deflection a; a=3 mm is advantageously the clearance from the stops  20531 . The microswitch  2061  is no longer activated given a deflection a&gt;a min . 
     In the operating mode of the printing apparatus, the contact pressure module  2  cannot be removed from the printing apparatus. In this case—in the shown manner—a rocker that can be moved in rotation around a bearing axle is engaged in notches of two guide rods  032 . An engagement in notches of two guide rods already arises in principle from the German Utility Model DE 20 2010 015 351 U1. 
     An opening of a ventilation valve takes place manually via the operation of the function arm  206  by means of the grip part  2062 , wherein the function arm  206  is borne such that it can rotate around the bearing axle  208 . The ventilation valve comprises a sealing surface  20621  and a hose bushing  2051 , wherein the hose bushing  2051  is arranged on one leg of the handle  2062 . 
     Upon operating the handle  2062  in the arrow direction (white arrow), the following three functions are executed: 
     1. opening of the valve to release the overpressure in the shell, 
     2. movement of the microswitch away, out of its operating position, whereby the pump is deactivated and the shell remains unpressurized, 
     3. disengaging of retention means of the contact pressure module from the notches in the two guide rods before removal of the contact pressure module. 
     A principle design of the printing apparatus is shown in  FIG. 2   c  with the contact pressure device in section presentation, as viewed from the front side. The contact pressure device  21  has an elastic, inflatable, gas-tight shell—advantageously a bellows system filled with air. The shell has an upper part  2101  and a lower part  2102  that are connected with one another via a middle part  2103  so as to be gas-tight. A transport belt  10 —shown in simplified form—is realized as a flat transport belt, for example, and is mounted in the printing apparatus at a defined height interval H from the running surface  200  of the flat articles on the upper part  201  of the housing given a deflated bellows system, wherein the height interval is defined by the maximum possible letter thickness. 
     From the feed side, an upper part  201  of a housing of the contact pressure module  2  has a shaped part  2010  at the mail input side, which shaped part  2010  forms a slope in the letter travel surface. A cover surface  21011  of the upper part  2101  of the bellows forms the contact pressure surface of the contact pressure device. The cover surface likewise has at the mail input side a slope on which an edge of a flat mail good runs, whereby a force is exerted on the contact pressure device  21 . In the bellows, the air pressure consequently increases (see also  FIG. 2   a ). If the contact pressure module  2  is slid into the printing apparatus (the manner is not shown), the non-sloped letter travel surface of the upper part  201  lies at a fixed height interval H from the approximately parallel segment of the transport belt that is over this. In the upper part  201 , the two bellows parts  2101 ,  2102  are arranged relative to another and attached with their middle part  2103  to a floor frame  205  so that a gas-tight void (represented with a dot pattern) arises between the upper bellows part and the lower bellows part. An opening (which cannot be shown in this Figure) is introduced into the letter travel surface. The upper part  2102  of the bellows protrudes through this opening in the direction of the transport belt  10 . 
     The lower part  2102  of the bellows has a hose connection  210211  with hose connector  2071  to the air inlet and outlet, and is connected with a hose (not shown). Arranged below the lower part  2102  of the bellows is the sprung floor plate  204  with spring suspension. Guide clips  2053  are curved up from the floor frame  205 . These interact with slots (which cannot be shown in this Figure) in the floor plate  204  that serve to guide said floor plate  204  when the bellows system is moved due to gas pressure. The guide slips have shoulders that form stops  20531  in order to limit the travel upon movement of the lower part  2102 , with the movement directed toward the travel surface. In the operating mode of the printing apparatus, the contact pressure module  2  cannot be removed from the printing apparatus, as has already arisen from the German Utility Model DE 20 2010 015 354 U1. In this case, the U-shaped plate rocker  206  that is movable in rotation around the bearing axle (see  FIG. 2   a ) is engaged, as arises from  FIG. 2   b.    
     A perspective view of the contact pressure module from the upper rear left—partially in an exploded view—is shown in  FIG. 3 . Both parts  201 ,  202  are attached to one another by means of connection bolts  203 . Openings  2021  for the guide rods  032  (see also  FIG. 2 ) are provided on the back side of the lower part  202 . A plug  2011  as a counterpart to the connection bushing  033  is present on the back side of the upper part  201 . Moreover, a box-shaped recess  2022  for the grip  2062  is introduced into the lower part  202 . 
     The upper part of the contact pressure module  2  (shown in  FIG. 1 ) in addition to the floor frame  205  is visible in an exploded presentation in  FIG. 4 , from the front upper left. The bellows-like air bag  210  comprises an upper part  2101  and a lower part  2102 . The sealing surface  21013  of the upper part  2101  and the sealing surface  21023  of the lower part  2102  are adapted to one another. The side parts  21012  of the upper part  2101  and the side parts  21022  of the lower part  2102  are executed in a folded manner. A hose connector  210211  protrudes from the floor surface  21021  of the lower part. 
     The upper part  201  of the contact pressure module housing  20  (see also  FIG. 2 ) accommodates the bellows-like air bag  210  and the floor frame  205 . A shaped part  2010 —letter thickness sluice, below—is provided at the intake region for the letters  4 . Furthermore, a recess  2012  is molded for the upper part  2010  of the air bag  210 . Bores  2054  to accommodate a bearing axle  208  for the function arm  206  (see also  FIG. 6 ) are present on both sides in the floor frame  205 . 
     The parts according to  FIG. 4  are presented in exploded form from the front lower left in  FIG. 5 . All parts are assembled with accurate fit by means of the connecting bolts  203  and the associated guide elements (not designated in detail) and are attached to the upper part  201  of the contact pressure module housing  20  (shown in  FIG. 2 ). The connection between upper part  2102  and lower part  2012  of the air bag  210  is air-tight. 
     The air bag  210  could also be a single (unitary) part. 
     Moreover, a support pocket  2014  for a pump  209  (see also  FIG. 7 ) is molded into the upper part  201 . 
     The lower part of the contact pressure module  2  (shown in  FIG. 2 ) is presented in exploded form from the rear lower right in  FIG. 6 . It comprises the floor plate  204 , the aforementioned floor frame  205  in addition to the function arm  206 , and associated bearing axle  208  and hose  207 . Dog-shaped mounts  2041  for tension springs  2052  are provided at the side angles of the floor plate  204 . An exposure  2043  for the hose connector  2071  to the hose connection  210211  at the air bag  210  (see also  FIG. 5 ) is introduced in the middle of the floor plate  204 . The other end of the hose  207  is connected via a T-shaped hose connector  2072  with a pump  209  (see  FIG. 7 ) whose middle part descends into a rubber elastic hose bushing  2051 . The hose bushing  2051  is grasped in a bend of the floor frame  205 . The outgoing end of the hose bushing  2051  can be sealed air-tight at the grip  2062  of the function arm  206  by means of an elbowed sealing surface  20621 . 
     The grip  2062  is attached to a U-shaped part of the function arm  206  by means of bolts  203 . The free ends of the U-shaped part have bearing holes  2063  for a bearing axle  208  that is in turn borne in lateral bends of the floor frame  205  (see also  FIG. 4 ). 
     A microswitch  2061  for the activation of the pump  209  is attached to the arm of the U-shaped part at the input side of the apparatus. Floor plate  204  and floor frame  205  are elastically connected with one another via the tension springs  2052 . Angled guide clips  2053  in the floor frame  205  serve for defined positioning relative to one another, which guide clips  2053  dip on the one hand into slots  2042  of the floor plate  204  and on the other hand serve as a stop for said floor plate  204 . For this purpose, the guide clips  2053  have shoulders  20531 . The floor plate  204  slides on the free ends of the guide clips  2053 , whose length with the shoulders  20531  establishes the amount of travel (stroke). The combination of slots  2042  and guide clips  2053  requires that the air bag  210  can be displaced only in the vertical direction. 
     The arrangement and attachment of the upper part of the contact pressure module  2  (shown in  FIG. 2 ) together with accessories (such as circuit board  2017 , plugs  20171 ,  20172 , microswitch  2061  and pump  209 ) are visible in  FIG. 7  in the upper part of the contact pressure module housing  201 . 
     In  FIG. 8  it is clear how the pump  209  including bearing bracket (not designated in detail) is attached to the upper part  201  so as to damp structure-borne sound. For this purpose, a vibration damper  2015  that prevents a sound transmission to the upper part  201  is provided in the support pocket  2014 . Also for such damping, a retention angle  2016  that is firmly bolted to the upper part  201  is provided, on its end facing away, with a vibration damper  20161  that is positively and non-positively connected with the other side of the pump  209 . Both pump noise and oscillation transmission from the contact pressure module housing  20  to the transport belt  10 —which can have the consequence of disadvantageous effects on the letter transport, and therefore on the print quality—are therefore prevented. 
     The circuit board  2017  is provided with its own power connection  20171  and with an electrical connection  20172  for the pump  209 , and a connection  20173  for the microswitch  2061 . The circuit board  2017  is furthermore provided with an electronically adjustable deactivation delay for the pump  209 . 
     In  FIG. 9  the contact pressure module  2  (shown in  FIG. 1 ) is shown in the operationally ready state. The air bag  210 —see also FIG.  2 —protrudes with its upper part  2101  upward until the cover surface  21011  non-positively rests on the transport belt  10 . 
     A compression spring  2064  is borne in a support pocket  213  in the upper part  201  and rests non-positively on a bearing point  20622  on the grip  2062  of the function arm  206 , so this is always pushed back into the initial position. 
     The grip  2062  of the function arm  206  is pivoted by the compression spring  2064  around its bearing axle  208  downward until this rests non-positively with its sealing surface  20621  on the rubber elastic hose bushing  2051  that is inserted into the floor frame  205 , and thus seals this air-tight (see Detail C). As a result of this, the fitted hose connector  2072  is also contained in the hose bushing  2051 . 
     The tension springs  2052  are drawn far apart from one another by the inflated air bag  210 —see Detail B—until the microswitch  2061  is triggered and a clearance from the floor plate  204  exists. The deactivation signal is directed from the microswitch  2061  via the circuit board  2017  with deactivation delay to the pump  209  and deactivates said pump  209  with a time delay (see also  FIG. 7 ). 
     The lower part  2102  of the air bag  210  (shown in  FIG. 5 ) rests on the floor plate  204  that, in turn, has a clearance from the shoulders  20531  at the guide clips  2053  that form the stop  20531  for travel limitation. 
     An approximately constant contact pressure with the transport belt  10  can be achieved—even for mixed mail—with greater tolerance (thickness and weight) with the combination of tension springs  2052  and elastic air bag  210 . The heavier the letters that are permitted, the higher the spring constant that is selected. 
     The contact pressure module  2  (shown in  FIG. 1 ) in the removal-ready state is shown in  FIG. 10 . Upon raising (arrow) the grip  2062 , the compression springs  2064  are compressed and the sealing surface  20621  assumes a clearance from the hose bushing  2051  in the floor frame  205  that is therefore open (see Detail C). 
     The air bag  210  (see also  FIG. 2 ) is deflated and—with its upper part  2101 —dips so far into the upper part  201  of the contact pressure module housing  20  that the cover surface  21011  has a clearance from the transport belt  10 . The lower part  2102  of the air bag  210  is lowered until it rests on the floor plate  204 , which in turn is drawn by the tension springs  2052  along the guide clips  2053  until it rests on their shoulders  20531 . As a result of this, the trigger button (not designated in detail) of the microswitch  2061  is contacted to the greatest possible extent without triggering the latter. This means that the pump  209  remains deactivated. 
     In  FIG. 11 , the contact pressure module  2  is shown before assuming operational readiness. The contact pressure module  2  is slid into the apparatus housing  01  (see also  FIG. 1 ), and the grip  2062  of the function arm  206  is released (and therefore free). The spring force of the tension springs  2052  is measured so that the microswitch  2061  is triggered by the floor plate  204 , and the pump  209  is activated with delay via the circuit board  2017  after the hose bushing  2051  has been sealed by the sealing surface  20621  of the grip  2062  of the function arm  206 . The air bag  210  is inflated until operational readiness is established. 
     If a letter  4  arrives in the intake region (letter thickness sluice  02  shown in  FIG. 1 ) of the franking machine  0 , this initially strikes the wedge-shaped region of the cover surface  21011  and is slid by the running transport belt  10  into the contact pressure region of the air bag  210 , and after passing is ejected by the same. 
     Due to the elastic properties—elastic air bag  210  and its suspension in the form of the floor plate  204  and the tension springs  2052 —approximately the same contact pressure forces are achieved independent of the letter thickness, wherein the contact pressure force is even adjustable to the desired degree via the selection of the spring force. 
     If letters  4  of different thickness are simultaneously located in the contact pressure region of the air bag  210 , due to its elastic properties said air bag  210  immediately adapts to these. The letters  4  of different thicknesses—thick after thin or vice versa—can follow one another at short intervals. 
     The contact pressure device  21  has proven itself precisely when an approximately 10 mm thick mail good follows a thin mail good of approximately 0.1 mm thickness, wherein the letter gap can be minimal. The minimal letter gap amounts to approximately 50 mm from the following mail good. The width of the contact pressure device  21  corresponds to the width of the transport belt, and the length of the contact pressure device  21  is smaller than or equal to the length of the straight segment of the transport belt. 
     The bellows has the advantage that an upward and downward expansion is enabled given a relative dimensional stability of its side walls. In the preceding description, a bellows has been addressed in simplified terms. However, a different suitable embodiment of a gas-tight casting that can be filled with air or with another suitable gas, which shell has a flexible contact pressure surface which can rapidly adapt its shape, should not therefore be precluded. 
     Instead of a bellows (bellows-shaped air bag), a flexible, inflatable, air-filled shaped part or, respectively, a shell can be used. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventors to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of their contribution to the art.