Patent Publication Number: US-7913641-B2

Title: Apparatus for coating a cylinder in particular a wiping cylinder of an intaglio printing press

Description:
This application is the U.S. national phase of International Application No. PCT/IB2006/053201 filed 11 Sep. 2006 which designated the U.S. and claims priority to European Patent Application No. 05108567.8 filed 16 Sep. 2005, the entire contents of each of which are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention generally relates to an apparatus for coating a cylinder, (particularly but not exclusively a wiping cylinder of an intaglio printing press) with a plastic composition. 
     BACKGROUND OF THE INVENTION 
     In intaglio printing presses, it is commonly known to use a wiping cylinder contacting the plate cylinder carrying the intaglio printing plate or plates as a wiping device for wiping and cleaning the surface of the intaglio printing plate or plates. The purpose of such a wiping cylinder is to simultaneously press the ink deposited onto the printing plates into the engravings and clean the excess ink from the plenum of the printing plates, i.e. the unengraved area of the printing plates outside the engravings. 
     In order to achieve good printing quality, the wiping cylinder is commonly designed in such a way that its outer surface contacting the printing plates is both physically and chemically resistant, i.e. is adapted to sustain the high contact pressure and friction with the printing plates and can withstand the physical and chemical contact with the ink components and pigments, as well as with the cleaning solutions which are used to clean the surface of the wiping cylinder. 
     It has already been proposed to provide such a wiping cylinder with an outer layer of resilient synthetic composition, namely a heat-hardenable plastic composition such as PVC. U.S. Pat. Nos. 3,785,286, 3,900,595 and 4,054,685 for instance disclose methods for making such wiping cylinders as well as apparatuses for implementing the said methods. These publications are incorporated by reference in the present application, especially in respect to the material used for forming such cylinders and to the machines and methods used for building such wiping cylinders. Referring for instance to the coating apparatus described in U.S. Pat. No. 4,054,685, means are provided for mounting a cylinder to be coated for horizontal rotation about its axis of rotation. Coating is performed by rotating the cylinder past a coating unit consisting of a straight-edged scraper blade mechanism disposed at one side of the cylinder and which extends parallel to the cylinder axis, this blade mechanism being adapted to be moved towards and away from the cylinder. The blade mechanism consists of two blades mechanically coupled to each other, namely a lower blade and an upper blade which are jointly designed to ensure a proper supply of heat-hardenable plastic material to the surface of the cylinder to be coated and allow adjustment of the thickness of the material to be deposited. The blade mechanism is adapted to be moved towards and away from the cylinder while maintaining the straight edge of the lower blade (i.e. the edge which extends along the length of the cylinder) parallel to the axis of rotation of the cylinder. The plastic material is supplied to the blade mechanism on top of the upper blade which is disposed, during coating of the cylinder, in an inclined relationship with respect to the cylinder so as to form a reservoir between the upper side of the upper blade and the periphery of the cylinder to be coated. Means are provided for restraining flow of the plastic material sideways from the reservoir. The blade mechanism can be translated towards and away from the cylinder in order to maintain a desired uniform spacing (a couple of millimeters or less) between the straight edge of the lower blade and the periphery of the cylinder along the full length of the cylinder. The cylinder is rotated in a direction to cause its periphery to move downwardly past the blade mechanism to thereby apply to the periphery of the cylinder a thin uniform layer of plastic composition having a thickness determined by the spacing between the straight edge of the lower blade and the periphery of the cylinder. This layer of plastic material is heat-cured by applying radiant heat to the cylinder throughout its length as the cylinder is rotated so as to cause hardening of the deposited layer of plastic material and produce a hardened layer of the desired hardness. Several layers with different hardnesses and thicknesses are preferably formed in this way onto the cylinder surface. 
     According to the solutions described in U.S. Pat. No. 4,054,685, the heating means for applying radiant heat to the cylinder are disposed in a movable hood part that can be displaced vertically on top of the cylinder. The hood part is designed in such a way that the cylinder is completely hidden below the hood part when the later is in place. Further not only is the cylinder completely hidden by the hood part, but also the coating unit. As a consequence, the coating process must be performed with the hood part moved up vertically so as to allow the user to supply the plastic composition on the coating unit and to visually monitor the coating process. It is only after the coating process has been performed that the hood part can be lowered onto the freshly coated cylinder to harden the deposited layer of plastic material. This prior art solution has a number of disadvantages including in particular a poor ability to aspirate the fumes generated during the coating process. Further, as heat has to be applied to the cylinder during the coating process, the operator is subjected to the heat generated by the heating means which has to be kept at a somewhat high level to compensate for the quick cooling effect caused by cool air flowing from all sides of the machine. 
     U.S. Pat. No. 5,180,612 discloses another type of apparatus for coating a wiping cylinder with a layer of plastic material which, in contrast to the previous apparatuses, makes use of a twin-roller coating unit for the application of the plastic material onto the surface of the cylinder. Rather than a vertically-moving hood part, there is provided a hood part that can be pivoted onto or away from the cylinder mounting location. A disadvantage of this solution however also resides in the fact that the operator cannot monitor the cylinder during the coating process, because the hood part completely hides the cylinder as well as the part of the coating unit which cooperates with the cylinder where the plastic composition is applied onto the cylinder. Further, the visibility of the cylinder is much more restricted with this solution due to the substantially greater size of the coating unit with its two application rollers. As a consequence, the operator must again open the hood part by an amount sufficient for him to be able to visually inspect the surface quality of the deposition, to the detriment of the efficiency of the aspiration of the fumes and of the heating. 
     Another disadvantage of the solution described in U.S. Pat. No. 5,180,612 resides in the structure of the heating means and aspiration system. Firstly, the heating means are disposed on a common reflector plate which constitutes an obstacle to the flow of air within the interior space of the hood part. Secondly, the part of the aspiration system disposed on the machine is entirely located in the hood part, an exhaust pipe being coupled directly to the hood part. This construction can cause problems because the exhaust pipe (as well as the other pipe elements connected thereto) will be subjected to the same rotational movement as that of the hood part. 
     SUMMARY OF THE INVENTION 
     An aim of the invention is to improve the known devices and methods. 
     It is an aim of the present invention to provide an apparatus for coating a cylinder with a plastic composition of the type comprising a movable hood part which is of more adequate construction that the known apparatuses. 
     Another aim of the present invention is to provide a coating apparatus which allows simplification of the required operations to manipulate the apparatus. More precisely, one wishes to propose a solution which does require displacement of the hood part during the coating process. 
     Still another aim of the present invention is to provide a coating apparatus allowing the manufacture of cylinders exhibiting an increased coating quality. 
     Yet another aim of the present invention is to improve the operating conditions of the apparatus for the operator, especially with respect to the aspiration of the fumes generated during the coating and heating processes. 
     These aims are achieved thanks to the apparatus defined in the claims. 
     According to the invention, the hood part is constructed so as to include a hood body and a window panel mounted on a front side of the hood body to allow a human operator to monitor deposition of the plastic composition onto the surface of the cylinder during the coating process. The hood body and window panel are constructed in such a manner that, when the hood part is moved on top of the supporting means holding the cylinder, the window panel lies above the position where the coating unit cooperates with the cylinder during coating. 
     During coating of the cylinder, the hood part can thus be left in a closed state, the operator still having a good visibility of the cylinder thanks to the window panel. Keeping the hood part in place ensures an efficient aspiration of the fumes, improving as a consequence the operating conditions for the operator. In addition, as the hood part is kept in place during the whole coating process (which process typically includes a pre-heating phase of the cylinder to be coated, a coating phase per se, and a heat-curing phase) heat losses are limited, the heated air being confined within the interior space of the hood part. This permits a greater control of the heating profile and, as a consequence, leads to a better quality of the coating. 
     According to a preferred embodiment, the window panel is further adapted to be moved between a closed position, closed onto the hood body, and at least one open position (advantageously a plurality of open positions). This addition enables the operator to adjust his viewing angle of the cylinder depending on his location with respect to the machine. This provides even greater flexibility for the operator without substantially impairing the efficiency of the aspiration of the fumes. 
     According to still another preferred embodiment, the window panel itself incorporates a plurality of aspiration inlets distributed along an edge of the frame of the window panel to aspirate the fumes through an aspiration channel embedded within the frame of the window panel and which is coupled to the aspiration system. Preferably, these aspiration inlets are provided at least on a bottom edge of the frame of the window panel. 
     Other advantageous embodiments of the invention are the subject-matter of the dependent claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will appear more clearly from reading the following detailed description of embodiments of the invention which are presented solely by way of non-restrictive examples and illustrated by the attached drawings in which: 
         FIG. 1  is a perspective view of an embodiment of the coating apparatus showing the hood part in an open position; 
         FIG. 2  is a perspective view of the coating apparatus of  FIG. 1  showing the hood part in a closed position; 
         FIG. 3   a  is a side view of the coating apparatus of  FIG. 2  taken perpendicularly to the axis of rotation of the cylinder to be coated; 
         FIG. 3   b  is a front view of the coating apparatus of  FIG. 3   a  taken perpendicularly to the window panel (when closed onto the hood part); 
         FIG. 4  is a side view of the coating apparatus taken perpendicularly to the axis of rotation of the cylinder to be coated showing the hood part in an open position, window panel closed onto the hood part; 
         FIG. 5  is a side view of the coating apparatus taken perpendicularly to the axis of rotation of the cylinder to be coated showing in greater detail the part of the aspiration system preferably used to aspirate the fumes from the interior space of the hood part; 
         FIG. 6  is a side view of the coating apparatus taken perpendicularly to the axis of rotation of the cylinder to be coated showing in greater detail the part of the aspiration system preferably used to aspirate the fumes from within the window panel; and 
         FIG. 7  is a side view of the coating apparatus showing a preferred way of mounting the heating elements in the hood part so as to facilitate maintenance operations. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
       FIG. 1  shows a perspective view of an embodiment of a coating apparatus according to the invention, designated globally by reference numeral  1 . The coating apparatus  1  comprises a main machine body  2  which supports means  3  for horizontally mounting a cylinder to be coated (cylinder not shown in this Figure) for rotation about its axis of rotation, a coating unit  4  consisting, in this illustrative example, of a blade mechanism with a single blade  40  disposed on one side of the cylinder for the application of the heat-hardenable plastic composition (the blade mechanism is shown in  FIG. 1  in a rest position which is pulled back away from the cylinder mounting location), driving means  5  (e.g. an electric motor or the like) for rotating the cylinder in a direction to cause its periphery to move past the coating unit  4 , and heating means  6  for applying radiant heat to the cylinder throughout its length as the cylinder is rotated to cause hardening of the deposited layer of plastic composition. 
     Not shown is the centralized computer interface, known per se in the art, that is coupled to the functional parts of the machine and enables the operator to operate and interact with the machine. This computer interface preferably included a touch screen mounted on a pivotable supporting arm coupled at the frontal side of the machine body  2  (preferably on the right-hand corner of the frontal side of the machine  2 ) so that the operator can adjust and monitor the various parameters of the machine while facing the cylinder from the frontal part of the machine. 
     In this embodiment, the heating means  6  are located in a movable hood part  7  which can be pivoted onto or away from the cylinder location by an actuation mechanism  70  (such as a pneumatically-actuated arm coupled at one extremity to the main machine body  2  and at the other extremity to the hood part  7 ). The hood part  7  is advantageously provided with a hood body  71  and a window panel  72  comprising a window frame carrying a transparent heat-resistant glass window  73 . In this example, the window panel  72  is preferably mounted rotatably at its upper part onto the hood body  71  by a pair of hinge members  72   a ,  72   b , the window panel  72  being shown in an open position in  FIG. 1 . This window panel  72  enables the operator to have a clear view of the cylinder surface during both coating and heating of the cylinder when the hood part  7  is in its closed position (even when the panel  72  is closed onto the hood part  7 ). In the preferred embodiment as shown, the window panel  72  is further coupled to the hood body  71  by a pair of piston-like supporting members  74   a ,  74   b  enabling the window panel  72  to remain in any of a plurality of open positions. 
     The heating means  6  include a plurality of individual heating elements  60  (preferably ceramic heating elements shaped like tiles) mounted on a curved supporting frame  62  located inside the hood part  7 . In this illustrative example, the heating elements  60  are arranged so as to form an array of eight columns of six heating elements each that are mounted on the curved supporting frame  62  so as to follow the curvature of the cylinder to be coated and extend along the full length of the cylinder. 
     Aspiration means, not shown in detail in this Figure, are further provided in the hood part  7  so as to suitably aspirate the fumes that are generated during the coating and heating processes. These fumes are preferably evacuated to an external condensation and/or filter unit (not shown) before disposal. 
     The means  3  for mounting the cylinder to be coated for horizontal rotation about its axis of rotation include a pair of bearings  3   a ,  3   b  that resemble the head-stock and tail-stock, respectively, of a lathe. The head-stock  3   a  holds a revolving spindle driven by the driving means  5  for coupling with one extremity of the cylinder to be coated and for driving the cylinder into rotation. The tail-stock  3   b  can be moved axially along the axis of rotation of the cylinder to be coated to be secured to the other extremity of the cylinder and to accommodate different lengths of cylinder. If necessary, shaft extensions can be secured to one or both of the head-stock  3   a  and tail-stock  3   b  in order to mount short cylinders. 
     As mentioned hereinabove, the coating unit  4  is shown in  FIG. 1  in a rest position (or cleaning position). The blade  40  is mounted on the coating unit  4  so as to be able to rotate about a rotation axis which is substantially parallel to the axis of rotation of the cylinder to be coated. More precisely, in the rest position, the blade  40  is rotated in such a manner that waste material from the coating process can be cleaned away from the blade into a collecting receptacle  45  disposed underneath the blade  40  (in this example the blade  40  is rotated in such a way that its upper side is oriented towards an operator which would face the frontal part of the machine). This collecting receptacle  45  is advantageously secured to the coating unit  4  so as to follow its movement toward and away from the cylinder to be coated. The collecting receptacle could alternatively be fixedly secured to the machine body  2 . 
     The coating unit  4  is adapted to be moved towards and away from the cylinder to be coated. To this end, the coating unit  4  is coupled to translation means comprising a pair of guide members  8   a ,  8   b  located on each side of the coating unit  4 . Translation of the coating unit  4  onto the guide members  8   a ,  8   b  is induced by suitable driving means, preferably electrical motors. The translation means ensure appropriate displacement of the coating unit  4  between the cleaning position, shown in  FIG. 1 , and the operating position (or coating position), shown in  FIG. 2 , as well as micrometric retraction of the coating unit  4  away from the surface of the cylinder during the coating operation. 
       FIG. 2  is a perspective view of the embodiment of  FIG. 1  showing the hood part  7  in its closed position (the window panel  72  being still shown in an open state) and the coating unit  4  in its coating position.  FIG. 2  also shows the tail-stock  3   b  moved axially towards the head-stock  3   a  as this would be the case after having mounted a cylinder to be coated between the head-stock  3   a  and tail-stock  3   b  (no cylinder being again shown in  FIG. 2  for the purpose of simplification). 
       FIG. 2  further shows that the blade  40  of the coating unit  4  is rotated towards the cylinder to be coated, the straight edge  40   a  of the blade  40  (see  FIG. 1 ) being directed towards the periphery of the cylinder. More precisely, the blade  40  is disposed, during coating of the cylinder, in an inclined relationship with respect to the cylinder so as to form a reservoir between the upper side of the blade  40  and the periphery of the cylinder for receiving a supply of heat-hardenable plastic composition. 
     Rotation of the blade  40  between the cleaning position shown in  FIG. 1  and the coating position shown in  FIG. 2  is advantageously performed by means of an actuator  42  (such as a pneumatic piston) actuating a rotating arm  43  coupled to the underside of the blade  40  via a shaft member  44  (the shaft member  44  being mounted between two bearings  44   a ,  44   b  supported at each side of the coating unit  4  on the guide members  8   a ,  8   b ). The means  42 ,  43 ,  44  for causing rotation of the blade  40  form means for discontinuing the application of the plastic composition at the end of the coating process. 
       FIG. 3   a  is a side view of the coating apparatus taken perpendicularly to the axis of rotation of the cylinder to be coated (which cylinder is indicated in dash-dotted lines and designated by reference C). This Figure shows in greater detail the inner space of the hood part  7  (with the window panel  72  in an open state) and the disposition of the heating means  6  within the hood part  7 . The side view is taken from the right-hand side of the apparatus and shows in particular the head-stock  3   a  of the supporting means  3  with the driving means  5 , the curved supporting frame  62  supporting the heating elements  60  and the actuation mechanism  70  for opening or closing the hood part  7 . 
     The coating unit  4  is not shown in  FIG. 3   a  (nor in  FIGS. 3   b  to  7 ) but it will be understood that, during coating of the cylinder C, the coating unit  4  would be displaced forward as shown in  FIG. 2  to be brought close to the peripheral surface of the cylinder C. In the closed state of the hood part  7 , as shown in  FIG. 3   a , the window panel  72  lies above the position where the coating unit  4  cooperates with the cylinder C during coating when the window panel is completely closed or slightly open. As this will be appreciated hereinafter, the window panel  72  is preferably provided with integrated aspiration means for aspirating air and fumes from a bottom part of the window panel  72 , thereby efficiently aspirating any fumes or vapours coming out of the coated cylinder or of the plastic composition supplied to the coating unit  4 . 
     In  FIG. 3   a , one may already notice that the supporting frame  62  carrying the heating elements  60  is advantageously coupled to the hood part by means of an articulated mechanism  65 . As this will be seen hereinafter with reference to  FIG. 7 , this articulated mechanism  65  is used to facilitate maintenance operations, especially replacement of defective heating elements. As already mentioned, the heating elements  60  are arranged in the form of a matrix (six rows of eight elements each in this illustrative example). The heating elements  60  are advantageously supported onto the supporting frame  62  so as to facilitate the flow of air through the heating means  6 , in-between the heating elements  60 . In this preferred example, the supporting frame  62  comprises a pair of curved members  622  disposed on the left-hand side and right-hand side of the matrix of heating elements  60 . These curved members  622  are each coupled approximately at a mid-position to one extremity of a twin-arm articulation  650  fixedly secured by its other extremity to the hood part  7 . As this will be seen hereinafter, each curved member  622  is fixed at its two ends  622   a ,  622   b  to the hood part  7 , the upper front fixation  622   a , near the window panel  72 , being releasable, while the bottom rear fixation  622   b  is designed in such a way as to allow the curved members  622  to rotate relatively to the hood part  7  upon release of the front fixation  622   a.    
     The pair of curved members  622  support in turn eight supporting rails  625  by their ends, each supporting rail  625  carrying a corresponding one of the rows of heating elements  60 . Preferably, each row of heating elements  60  shares a common reflector  600  which is supported by the corresponding supporting rail  625 . The function of these reflectors  600  is to orient the radiant heat produced by each row of heating elements  60  towards the cylinder C and help to prevent excessive heating of the top-rear end of the hood part  7 . As this is schematically illustrated in  FIG. 5 , the preferred configuration of the heating means  6  allows air to flow in-between each row of heating elements, favouring a better flow of air within the hood part  7  and, as a consequence, an improved aspiration efficiency. 
     Also shown in  FIG. 3   a  is an aspiration chamber  90  forming part of the aspiration system of the machine which is fixedly secured to the machine frame  2 . Fumes which are aspirated out of the interior space of the hood part  7  are evacuated through this aspiration chamber  90 , at least one outlet  90   a  at the rear end of the chamber  90  being provided for coupling to an external aspiration unit (not shown). At the front side of the chamber  90  there is provided at least one coupling section  90   b  for coupling with at least one corresponding aspiration conduit (see conduits  93  and  94  in  FIGS. 5 and 6 ) provided within the hood part  7 . Thanks to this configuration, external pipes are connected to a part of the machine that does not move, the aspiration system consisting of two parts that are operatively coupled to each other upon closure of the hood part  7 . 
     In the preferred embodiment, the hood part  7  includes a pair of aspiration conduits  93  disposed at the left-hand side and right-hand side of the hood body  71  (see  FIG. 6 ) which are coupled at one end to the window panel  72  (through the hinge members  72   a ,  72   b ) and a main aspiration conduit  94  which opens into the hood part  7  (see  FIG. 5 ). The aspiration conduits  93 ,  94  are preferably distinct from each other and do not communicate directly. More precisely, each aspiration conduit  93 ,  94  leads to a corresponding coupling section  93   b ,  94   b  (not shown in  FIG. 3   a ) at the bottom-rear end part of the hood body  71  for coupling to the coupling section  90   b  of the aspiration chamber  90  (see  FIGS. 4 ,  5  and  6 ). Preferably, the chamber  90  is subdivided into three parts, two lateral parts for coupling with the pair of conduits  93  and a central part for coupling with the main aspiration conduit  94 . 
     A removable receptacle  95  is provided at the bottom side of the chamber  90 . The purpose of this receptacle is to collect waste fluid resulting from the condensation of the aspirated fumes which occurs within the aspiration conduits of the hood part  7 . In use, this receptacle  95  is coupled in a sealed manner to the chamber  90  (under the action of an eccentric actuation mechanism  96  which cooperates with the bottom side of the receptacle  95 ). Upon release of the actuation mechanism  96 , the receptacle  95  can be removed from the rear side of the apparatus for cleaning, as schematically illustrated in  FIG. 3   a . Absorbing material (such as a sponge like member) can advantageously be placed in the receptacle  95  in order to absorb the waste fluid and facilitate disposal thereof. 
       FIG. 3   b  is a schematic front view of the apparatus of  FIG. 3   a  taken approximately perpendicularly to the window panel  72  (in the closed position), while  FIG. 4  is a side view of the coating apparatus  1  showing the hood part  7  in an open state, pivoted backwards by the actuation mechanism  70 . The elements already mentioned hereinabove in connection with  FIGS. 1 ,  2  and  3   a  are again designated by their corresponding reference numerals. 
     In  FIG. 4 , one can see the aspiration chamber  90  being decoupled from the aspiration conduits  93 ,  94  of the hood part  7 . In this open state, the coupling sections  93   b ,  94   b  of the aspiration conduits are decoupled from the coupling section  90   b  of the aspiration chamber  90 , the waste fluid resulting from condensation being able to drop under the effect of gravity at the bottom rear end part of the machine. A pair of receptacles  97  located on the left-hand and right-hand sides are therefore provided below the coupling sections  93   b ,  94   b  in order to receive the waste fluid flowing out of the aspiration conduits  93 ,  94 . Preferably, the extremity of each aspiration conduit  93 ,  94  at the coupling section  93   b ,  94   b  is shaped so as to facilitate the flow of the waste fluid, both in the state where the hood part  7  is opened and in the state where the hood part  7  is closed. To this end, the coupling section  93   b ,  94   b  of each conduit  93 ,  94  is shaped so as to exhibit an inclined guiding surface  930  as illustrated. 
     With reference to  FIGS. 5 and 6 , one will now describe in greater detail the configuration of the aspiration conduits  93 ,  94  located within the hood part  7  according to the preferred embodiment.  FIG. 5  shows in particular the configuration of the main aspiration conduit  94  for aspirating the fumes out of the interior space of the hood part  7  while  FIG. 6  shows the configuration of the aspiration conduit  93  located on the left-hand side of the hood part  7  (the aspiration conduit  93  on the right-hand side being the mirrored image of the one illustrated in  FIG. 6 ). 
     As shown in  FIG. 5 , the main aspiration conduit  94  opens at an upper part of the hood part  7 . The aspiration inlet  94   a  of the main conduit  94  preferably extends along the length of the hood part  7  (parallel to the axis of rotation of the cylinder C). In this example, the lower part of the aspiration conduit  94  is divided into two portions (this subdivision being not illustrated) which communicate with the same aspiration inlet  94   a . These portions extend to the bottom-rear of the hood part  7  at the left-hand side and right-hand side and open as two separate coupling sections  94   b  for coupling with the aspiration chamber  90 . 
     As shown in  FIG. 6 , the window panel  72  is provided with a number of aspiration inlets  720  distributed along an edge (preferably the bottom edge) of the window frame in order to aspirate the fumes. These aspiration inlets  720  also appear on the illustration of  FIG. 2 . The aspiration inlets  720  communicate with an aspiration channel  92  provided inside the frame of the window panel  72 . This channel  92  is coupled at each side of the window panel  72  to the corresponding one of the two aspiration conduits  93 . Coupling is realized by providing each hinge member  72   a ,  72   b  with a hollow portion communicating on the one hand with the aspiration conduit  93  and on the other hand with the aspiration channel  92  as illustrated in  FIG. 3   b . In a closed state, air and fumes can be aspirated through the aspiration inlets  720 , via the aspiration channel  92 , the hinge members  72   a ,  72   b  and the aspiration conduits  93  as schematically illustrated by the arrows in  FIGS. 3   b  and  6  in order to be then evacuated through the aspiration chamber  90 . Advantageously, the hinge members  72   a ,  72   b  are configured in such a way as to ensure a communication between the aspiration channel  92  and the aspiration conduits  93  over a certain angular displacement of the window panel  72  and to close the communication between the aspiration channel  92  and the aspiration conduits  93  when the window panel  72  is opened to a greater extent. This can be achieved by suitably designing the hollow portion of the hinge members  72   a ,  72   b  so that it exhibits an opening communicating with the aspiration channel  92  over a limited rotation angle. 
     Turning now to  FIG. 7 , one will briefly describe the articulated mechanism  65  used in the preferred embodiment to couple the supporting frame  62  to the interior of the hood part  7 . As mentioned hereinabove, the supporting frame  62 , or more precisely the front fixation  622   a  (which fixation can be any sort of releasable mechanical fixation means such as a screw member) of each supporting member  622 , can be disconnected from the hood part  7 . Even after disconnection of the front fixation  622   a , the supporting frame  62  remains coupled to the hood part  7  through the articulated mechanism  65  and through the bottom rear fixation  622   b  of each supporting member  622 . Upon disconnection of the front fixation  622   a , the supporting frame  62  is however capable of rotating with respect to the hood part  7  about an axis of rotation defined by the bottom rear fixation  622   b  as illustrated in  FIG. 7 . 
     Disconnection of the support frame  62  for maintenance purposes would occur as follows. Starting with the hood part  7  in a closed stated (as shown in  FIG. 3   a ), the front fixation  622   a  of each supporting member  622  is disconnected so as to release the front part of the support frame  62  from the hood part  7 . Under the effect of its own weight the supporting frame  62  is free to rotate (in a counter clockwise direction in  FIG. 7 ). While the supporting frame  62  is held by an operator, the hood part  7  is driven to its open state as illustrated in  FIG. 7 , the actuation mechanism  70  pushing the hood part  7  so that it rotates backwards. In the process, the pair or twin-arm articulations  650  unfold and straighten. In the open state, both twin-arm articulations  650  are completely unfolded and hold back the supporting frame  62  suspended in the air. The operator can now access the rear end of the supporting frame  62 , where the electrical connections of the heating elements  60  are located. One will understand that this specific supporting arrangement greatly facilitates maintenance operations, especially replacement of any defective heating element. 
     The Figures shows that each row of heating elements  60  is disposed at equal distance with respect to the cylinder surface, i.e. the heating means  6  lies concentrically with the axis of rotation of the cylinder C. It may alternatively be advantageous to dispose some rows of heating elements closer to the cylinder surface that other rows. More particularly, it may be advantageous to dispose the first rows of heating elements which are proximate to the window panel  72  so that their distance with respect to the cylinder is smaller than the rows of heating elements  60  lying further back in the hood part  7 . This would have the advantage of increasing the heating efficiency in the vicinity of the front area of the hood part  7  where the window panel is located  72  so as to compensate for temperature differences within the interior space of the hood part  7  as well as temperature losses occurring when opening the window panel  72 . This solution could also improve the flow of air and increase the efficiency of the aspiration of the fumes. 
     It will be understood that various modifications and/or improvements obvious to the person skilled in the art can be made to the embodiments described hereinabove without departing from the scope of the invention defined by the annexed claims. For instance, the hood part  7  could perfectly be mounted so as to be moved by other means than by pivoting, such as by translating the hood part. A pivotable mounting however remains the preferred solution due to its relative simplicity. Similarly, other means than a pneumatically-actuated piston could be used in order to perform opening and closing of the hood part. 
     It will also be appreciated that various modifications and/or improvements could be made to the aspiration system without departing from the scope of the invention. For instance, while the preferred embodiment provides for a separation between the aspiration conduits for the window panel and for the hood part, it could be envisaged to provide a common aspiration conduit. The proposed solution is however preferred as it enables to completely decouple the two parts of the aspiration system and adjust the power or flow of the aspiration separately for each part, a greater aspiration force being comparatively necessary for the hood part than for the window panel.