Apparatus for the wet processing of photographic sheet material

Abstract

An apparatus for the wet processing of photographic sheet material comprises at least one treatment vessel (12, 12', 12") having upper and lower openings (17, 18), one of the openings constituting a sheet material inlet and the other of the openings constituting a sheet material outlet, the inlet and outlet defining there-between a substantially vertical sheet material path (20) through the vessel. The vessel comprises a pair of rotatable rollers (28, 30) biased towards each other to define a nip (36) there-between through which the sheet material path extends. The apparatus is characterised in that the end faces of one roller lie in substantially the same plane as the end faces of the other roller and each roller is in sealing contact with a stationary sealing means (38) having a continuous contact line (52) which extends along the length of each roller (28, 30) and over the end faces of each roller, at least on the fluid side of said nip (36).

Description

This application claims the benefit of U.S. Provision Application No. 60/008589, filed Dec. 13, 1995.

FIELD OF THE INVENTION

The present invention relates to an apparatus for the wet processing of photographic sheet material such as X-ray film, pre-sensitised plates, graphic art film and paper, and offset plates. More particularly the invention relates to improvements in apparatus in which photographic material is transported through one or more treatment units along a vertical path.

BACKGROUND OF THE INVENTION

As a rule, a processing apparatus for photographic sheet material comprises several vessels each of which contains a treatment liquid, such as a developer, a fixer and a rinse liquid. As used herein, the term sheet material includes not only photographic material in the form of cut sheets, but also in the form of a web unwound from a roll. The sheet material to be processed is transported through these vessels in turn, by transport means such as one or more pairs of drive rollers, and thereafter optionally to a drying unit. The time spent by the sheet material in each vessel is determined by the transport speed and the dimensions of the vessel in the sheet feed path direction.

In a conventional processing apparatus the sheet material is transported along a generally horizontal feed path, the sheet material passing from one vessel to another usually via a circuitous feed path passing under the surface of each treatment liquid and over dividing walls between the vessels. However, processing machines having a substantially vertical orientation have also been proposed, in which a plurality of vessels are mounted one above the other, each vessel having an opening at the top acting as a sheet material inlet and an opening at the bottom acting as a sheet material outlet or vice versa. In the present context, the term "substantially vertical" is intended to mean that the sheet material moves along a path from the inlet to the outlet which is either exactly vertical, or which has a vertical component greater than any horizontal component. The use of a vertical orientation for the apparatus leads to a number of advantages. In particular the apparatus occupies only a fraction of the floor space which is occupied by a conventional horizontal arrangement. Furthermore, the sheet transport path in a vertically oriented apparatus may be substantially straight, in contrast to the circuitous feed path which is usual in a horizontally oriented apparatus. As a consequence of the straight path, the material sensitivity for scratches becomes independent of the stiffness and thickness of the material.

In a vertically oriented apparatus, it is important to avoid, or at least minimise leakage of treatment liquid from one vessel to another and carry-over as the sheet material passes through the apparatus. U.S. Pat. No. 4,166,689 (Schausberger et al. assigned to Agfa-Gevaert AG) describes such an apparatus in which liquid escapes form the lower opening and is intercepted by the tank of a sealing device with two squeegees located in the tank above a horizontal passage in line with the lower opening. One or more pairs of drive rollers in the vessel close the lower opening and also serve to transport the sheet material along a vertical path which extends between the openings of the vessel.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a vertically oriented processing apparatus in which the sealing of one vessel from the next and of the rollers to the housing of the associated vessel is achieved in a simple and reliable manner.

We have now discovered that this objective, and other advantageous characteristics of the apparatus, can be achieved when the rollers which close the lower opening of a treatment vessel are so positioned relative to each other such that end faces of one roller lie in the same planes as end faces of the other roller and each roller is in sealing contact with a stationary sealing means.

According to the invention there is provided an apparatus for the wet processing of photographic sheet material comprising at least one treatment vessel having upper and lower openings, one of the openings constituting a sheet material inlet and the other of the openings constituting a sheet material outlet, the inlet and outlet defining there-between a substantially vertical sheet material path through the vessel, the vessel comprising a pair of rotatable rollers biased towards each other to define a nip there-between through which the sheet material path extends, characterised in that:

(i) the rollers are positioned relative to each other such that end faces of one roller lie in substantially the same planes as end faces of the other roller; and (ii) stationary sealing means are provided in contact with each roller, having a continuous contact line which extends along the length of each roller and over the end faces of each roller, at least on the fluid side of the nip.

Preferably at least one of the rollers comprises a core carrying a covering of elastomeric material. By the term "core" we mean an axially inner member, which is usually cylindrical and which is relatively rigid compared to the elastomeric material covering. The core may be solid or hollow. Usually, drive to the roller will be applied to the core. The elastomeric material may extend beyond the ends of the core, the sealing means being in contact with the end faces of the covering. The extension of the covering beyond the end of the core defines a space into which the elastomeric material of the covering may be deformed as a result of a sealing force between the covering and the sealing means.

It is a requirement of the present invention that the end faces of one roller lie in substantially the same planes as the end faces of the other roller. By the term "end face" we mean the face at the end of the roller, adjacent The outer surface thereof. Thus, where the roller comprises a core provided with an elastomeric material, the term "end face" as used herein means the end face of the elastomeric material covering. An end face of one roller lies in exactly the same plane as an end face of the other roller, or in such a closely adjacent plane that an effective seal can be made between the end faces and a planar portion of the stationary sealing means, taking into account the resilience in the material of which the roller and The sealing means may be formed. As a consequence of this requirement, the elastomeric coverings of the rollers are substantially equal in length.

The stationary sealing means my contact each roller along a straight line parallel to the associated roller axis and preferably contacts the surface of the associated roller at a location which is between 45.degree. and 270.degree. , most preferably between 80.degree. and 100.degree. from the centre of the nip, on the fluid side.

The stationary sealing means may be in a unitary or multi-part form. In particular, a unitary sealing member may comprise a central portion in the form of a substantially horizontally disposed flat plate, the under faces of which contact the surface of each roller, the sealing member further comprising substantially vertically disposed end plates which bear against the end faces of said rollers.

The sealing member preferably exerts a spring force of between 2 and 500 g/cm of roller, perpendicular to the roller surface. The spring loading may be derived from the geometry of a sealing member, from a separate spring incorporated in a sealing member or simply from compression of the elastomeric material covering of the associated roller.

The end plates are preferably biased against the end faces of the rollers with a force of from 2 to 500 g/cm of contact between the end plate and the end face of the roller, measured on the surface of the roller. Thus, the end plates may be urged against the end faces of the rollers by springs so shaped to ensure the desired location of the contact line between the end plates and the end faces of the rollers. Alternatively the elastomeric material covering of the rollers is somewhat oversized, the necessary spring force then being derived from the elasticity of the elastomeric material itself.

The sealing member is formed of, or is provided with a roller-contacting surface formed of a material which preferably has a coefficient of friction (as measured against stainless steel) of from 0.05 to 0.3, preferably from 0.09 to 0.2. The sealing member material in contact with the associated roller surface may comprise a polymer material such as PTFE (poly tetra fluoro ethylene), POM (polyoxymethylene), HDPE (high density polyethylene), UHMPE (ultra high molecular weight polyethylene), polyurethane, PA (polyamide), PBT (polybutyl terephthalate) and mixtures and composites thereof. Alternatively or additionally, those surfaces of the roller which contact the sealing member may be coated with such a low-friction material.

Each vessel may be of modular construction and be provided with means to enable the vessel to be mounted directly above or below an identical or similar other vessel. Alternatively, the apparatus may take an integral form or semi-integral form. By the term "semi-integral form" we intend to include an apparatus which is divided by a substantially vertical plane passing through all the vessels in the apparatus, particularly the plane of the sheet material path, enabling the apparatus to be opened-up for servicing purposes, in particular to enable easy access to the rollers.

The apparatus according to the invention may include a substantially closed connection between adjacent vessels. Thus, for example, the rollers may serve to close the lower opening of the treatment vessel.

Each vessel of the apparatus may comprise a housing having an upper housing part and a lower housing part, the upper housing part being so shaped in relation to the lower housing part of the next higher vessel as to provide the substantially closed connection between adjacent vessels. For example, the upper and lower housing wall parts may be provided with flanges, means being provided to secure the flange of the upper housing wall part with the flange of the lower housing wall part of the next higher vessel thereby to provide the substantially closed connection. Optionally, a gasket may be positioned between the vessels to improve the reliability of this connection.

The top-most liquid-containing vessel of the apparatus is preferably provided with similar closure means for reducing the evaporation, oxidation and carbonization of treatment liquid therefrom (and any other undesirable exchange between the treatment liquid and the environment).

The upper part of the housing of each vessel (optionally other than the top-most) is preferably so shaped as to define a leakage tray so positioned that any treatment liquid which passes, for example, through the roller nip of the next higher vessel drips from the rollers of that vessel and falls into the leakage tray, for collection and recirculation as desired.

By the use of a vertical configuration, the cross-section of the vessel can be low, such as less than 3 times the roller diameter. The volume of the vessel can therefore be low. Indeed, for a given sheet material path length, the volume of one vessel of a vertical processing apparatus can be many times smaller than the volume of an equivalent treatment bath in a horizontal processing apparatus. This has advantages in terms of the volume of treatment liquids used and the efficiency of their interaction with the sheet material.

Nevertheless, one or more of the vessels of the apparatus may include additional features if desired. Cleaning means may be provided for acting upon the rollers to remove debris therefrom, as described in European patent application EP 93202862 (Agfa-Gevaert NV), filed 11 Oct. 1993. Additional rollers, such as a roller pair or staggered rollers may be provided for transporting the sheet material through the apparatus, and these rollers will normally be driven rollers. Additional roller pairs may be provided for breaking the laminar fluid at the surface of the sheet material as it passes through the apparatus, and these rollers may be driven rollers or freely rotating rollers. Even when additional roller pairs are present, the rollers to which the (.phi./L) criterium applies and their associated stationary sealing means will usually constitute the lower roller pair, serving to close the lower opening of the vessel. Spray means may be provided for applying treatment liquid to the sheet material. Guide means may be included for guiding the passage of the sheet material through the apparatus. Heating means may be provided in one or more vessels so that the vessel becomes a sheet material drying unit, rather than a wet treatment unit. While liquid pumping, heating, cooling and filtering facilities will normally be provided outside the vessels, it is possible for some elements of these features to be included in the vessels themselves. Any combination of these additional features is also possible.

In one embodiment of the invention, one or more of the vessels includes at least one passage through the housing thereof to constitute an inlet and/or outlet for treatment liquid into and/or from the associated vessel. One or more vessels may not contain processing liquid, these vessels providing a dead space where diffusion reactions can occur on the sheet material as it passes there-through.

Typical rollers have a core provided with a covering of elastomeric material, although it is possible for the roller to be elastomeric throughout its cross-section. As the sheet material leaves a given liquid treatment vessel it is necessary to remove any liquid carried on the sheet material as efficiently as possible, to reduce edge effects which arise from non-homogeneous chemistry on the sheet material after squeegeeing. To do this job properly, the rollers must exert a sufficient and homogeneous pressure over the whole width of the sheet material. Also, to reduce edge effects, it is desirable that the opposite roller surfaces are in contact with each other beyond the edges of the sheet material. To put this problem in context, rollers used in conventional processing apparatus for example having a length of 400 mm or more and a diameter of from 24 to 30 mm. The sheet material typically has a width of from a few millimetres up to 2 m and a thickness of 0.05 mm to 0.5 mm. In view of the nature of elastomeric material, it is in fact impossible to totally eliminate any gap between the roller surfaces at the edges of the sheet material as it passes through the nip. It is desirable that the roller surfaces be in contact with each other within as short a distance as possible from the edges of the sheet material i.e. that the size of the leak zone should be minimised. It is important however that the force between the rollers is sufficient to prevent leakage when no sheet material is passing through. However, the force must not be so high as to risk physical damage to the sheet material as it passes through the nip.

The objective of a minimum leak zone referred to above can be achieved if the ratio of the diameter of the roller to its length is above a critical limit.

To enable this objective to be achieved, the ratio of the diameter of the roller to its length should be above a critical limit In particular, at least one of the rollers, and preferably each roller, comprises a rigid core carrying a covering of elastomeric material, the ratio (.phi./L) of the maximum diameter (.phi.) of the elastomeric material covering to the length (L) thereof being at least 0.012, most preferably between 0.03 and 0.06. This arrangement also serves to reduce the torque required to rotate the rollers, the ratio of the roller diameter .phi. to the roller length is preferably greater than 0.012. Preferably both rollers conform to this requirement, although it is possible that the diameters (.phi.), and therefore the ratios (.phi./L), of the two rollers need not be identical.

The elastomeric material covering preferably has a thickness of between 1 mm and 30 mm. The elastomeric material may be selected from ethylene/propylene/diene terpolymers (EPDM), silicone rubber, polyurethane, thermoplastic rubber such as Santoprene (Trade Mark for polypropylene/EPDM rubber), styrene-butyl rubber and nitrilebutyl rubber. The hardness of the elastomeric material may be between 15 Shore (A) and 90 Shore (A), as measured on the roller surface. In one embodiment of the invention, the diameter (.phi.) of the elastomeric material covering is constant along the length of the roller. Alternatively the roller may have a radial dimension profile which varies along the length thereof. In the latter case, the diameter (.phi.) in the expression .phi./L is the maximum diameter. In a preferred embodiment, such a roller comprises a non-deformable core, the thickness of the elastomeric material covering varying along the length thereof. Alternatively or additionally, the diameter of the core varies along the length thereof.

Ideally, the radial dimension profile of such a roller is such in relation to the force applied by the roller to sheet material passing through the nip as to be substantially even over the width thereof.

The radial dimension of the roller ideally decreases towards the ends thereof i.e. a convex profile, especially a parabolic profile.

Preferably, the core has a flexural E-modulus of between 50 GPa and 300 GPa. Suitable materials for the rigid core include metals, such as stainless steel, non-ferrous alloys, titanium, aluminium or a composite thereof.

In one embodiment of the invention, the core is hollow. Alternatively the core may be solid.

One or both of the rollers may constitute drive rollers for driving the sheet material along the sheet material path. Alternatively, the rollers may be freely rotating, alternative drive means being provided to drive the photographic sheet material through the apparatus.

The rollers may be biased together by a variety of methods, for example by making use of the intrinsic elasticity of the elastomeric material, by the use of fixed roller bearings. Alternatively, use may be made of resilient means such as springs which act on the ends of the roller shafts. The springs may be replaced by alternative equivalent compression means, such as e.g. a pneumatic or a hydraulic cylinder.

PREFERRED EMBODIMENTS OF THE INVENTION

The invention will now be further described, purely by way of example, by reference to the accompanying drawings in which:

FIG. 1 is, in solid lines, a cross-sectional view of one vessel of a vertical processing apparatus according to the invention, with adjacent vessels being partly shown in broken lines:

FIG. 2 is a partly schematic exploded drawing showing the rollers and the sealing member of the vessel shown in FIG. 1 and the contact line therebetween: and

FIG. 3 is a longitudinal cross-sectional view showing the detail of the construction of one roller used in the vessel shown in FIG. 1.

Although only one specific embodiment of a treatment vessel according to the invention is shown in the Figures, the invention is not restricted thereto. The apparatus for the wet processing of photographic sheet material such as X-ray film as shown in the Figures comprises a plurality of treatment vessels mounted one above another. These vessels may be arranged to provide a sequence of steps in the processing of sheet photographic material, such as developing, fixing and rinsing. The vessels may be of a modular structure as shown or may be part of an integral apparatus.

As shown in FIG. 1, each vessel 12 comprises a housing 14 which is of generally rectangular cross-section and is so shaped as to provide an upper part 15 having an upper opening 17 and a lower part 16 having a lower opening 18. The upper opening 17 constitutes a sheet material inlet and the lower opening 18 constitutes a sheet material outlet. The inlet and outlet define there-between a substantially vertical sheet material path 20 through the vessel 12, the sheet material 22 moving in a downwards direction as indicated by the arrow A. The sheet material preferably has a width which is at least 10 mm smaller than the length of the nip, so as to enable a spacing of at least 5 mm between the edges of the sheet and the adjacent limit of the nip, thereby to minimise leakage. Each vessel 12 may contain treatment liquid 24, a passage 26 in the housing 14 being provided as an inlet for the treatment liquid 24. The lower opening 18 is closed by a pair of rotatable rollers 28, 30 carried in the apparatus.

Each roller 28, 30 is of the squeegee type comprising a stainless steel hollow core 32 carrying an elastomeric covering 34. The core 32 is in cylindrical form having constant internal and external diameters along the length thereof. The rollers 28, 30 are of identical length biased towards each other with a force sufficient to effect a liquid tight seal but without causing damage to the photographic sheet material 22 as it passes there-between. The line of contact between the rollers 28, 30 defines a nip 36. The nip 36 has a length which extends beyond the limits of the lower opening 18. The rollers 28, 30 are substantially equal in length. The rollers 28, 30 are coupled to drive means (not shown) so as to constitute drive rollers for driving the sheet material 22 along the sheet material path 20.

Each roller 28, 30 is in sealing contact along its length with a stationary sealing member 38 which is secured to the housing 14 of the vessel 12, the treatment liquid 24 being retained in the vessel 12 by the rollers 28, 30 and the sealing member 38. The sealing member 38 is of unitary form, having a central portion in the form of a substantially horizontally disposed flat plate 70 having an aperture 39 therethrough, the under faces of the plate contacting the surface 71 of each roller 28, 30 along contact lines which are located at about 90.degree. from the centre of the nip 36 on the fluid side, that is from the plane joining the axes of rotation of the rollers 28, 30. The sealing member 38 also includes, at each end thereof, vertically disposed end plates 62 which bear against the end faces of the rollers 28, 30, as shown in more detail in FIG. 3. The sealing member 38 is formed of steel plate having a thickness of 300 to 1500 .mu.m, the roller contacting surfaces being coated with PTFE.

The upper and lower housing parts 15, 16 are provided with flanges 19, 21 respectively to enable the vessel 12 to be mounted directly above or below an identical or similar other vessel 12', 12", as partly indicated in broken lines in FIG. 1. The upper housing part 15 is so shaped in relation to the lower housing part 16 as to provide a substantially closed connection between adjacent vessels. Thus, treatment liquid from vessel 12 is prevented from falling into the lower vessel 12" by the rollers 28, 30 and sealing member 38, while vapours from the lower vessel 12" are prevented from entering the vessel 12 or escaping into the environment. This construction has the advantage that the treatment liquid in one vessel 12 is not contaminated by contents of the adjacent vessels and that by virtue of the treatment liquids being in a closed system evaporation, oxidation and carbonization thereof is significantly reduced.

The upper part 15 of the housing 14 is so shaped as to define a leakage tray 42. Any treatment liquid which may pass through the roller nip of the next higher vessel 12', in particular as the sheet material 22 passes therethrough, drips from the rollers of that vessel and falls into the leakage tray 42 from where it may be recovered and recirculated as desired. The distance H between the surface 25 of the liquid 24 and the nip of the rollers of the next upper vessel 12' is as low as possible.

Referring to the schematic exploded drawing of FIG. 2, the two rollers 28, 30 can be shown in contact with each other to form a nip 36 therebetween. The contact line between the stationary sealing member 38 and the rollers is indicated by the broken line 52. This contact line is continuous, having straight longitudinal portions 53 which extend along the length of each roller 28, 30 and arcuate portions 54 which extend over end faces 68, 69 of the rollers. The lowest point 55 of the contact line 52 lies below the level of the nip 36. The contact line therefore extends over the end faces of the rollers 28, 30 on the fluid side i.e. the upper side, of the nip 36. In practice, as will be seen from FIG. 1, the contact between the sealing member 38 and each of the rollers 28, 30 amounts to a surface-to-surface contact rather than a line-to-surface contact as such, especially since the contact surface of the sealing member 38 may be flat as shown, or profiled to the circumference of the roller. Nevertheless, a continuous contact line can be envisaged which extends along the length of each roller and over the end faces of each roller, at least on the fluid side of said nip.

The construction of roller 28 is shown in more detail in FIG. 3. The construction of roller 30 is similar. The roller 28 comprises a hollow core 32 of stainless steel, having a constant outside diameter of 25 mm and an internal diameter of 19 mm. The stainless steel core 32 has a flexural E-modulus of 210 GPa. The core 32 is provided with a covering 34 of EPDM rubber, an elastomer having a hardness of 30 Shore (A). The elastomeric covering 34 has a thickness varying from 7 mm at the roller ends to 7.5 mm at the roller centre. The roller 28 has a length of 750 mm and a maximum diameter of 40 mm. The maximum .phi./L ratio is therefore approximately 0.053. The core 32 is welded to the boss 46 of a roller shaft 50 which extends axially out of the roller, the far end of the roller shaft 50 being retained in a bearing (not shown) or coupled to a drive wheel (not shown) to provide drive to the roller.

The horizontally disposed flat plate portion 70 of the sealing member 38 extends along the surface 71 of the roller 28 and in contact therewith. The end face portion 62 of the sealing member 38 lies against the end face 68 of the elastomeric covering 34.

The covering 34 extends beyond the end of the core 32 to define a space 44 into which the elastomeric material of the covering 34 may be deformed as a result of a sealing force between the covering 34 and the sealing member 38.

As can be seen from FIG. 2, the rollers 28, 30 are positioned relative to each other such that end face 68 of the first roller 28 lies in the same plane as end face 69 of the other roller 30. Each roller is in sealing contact, not only along its length with the longitudinal portion of the sealing member 38 but also by its end faces with the end plates 62. Each of the end plates 62 is so shaped as to have a lower edge 66 which follows around the shaft 50 of the first roller 28 and a around the shaft 51 of the second roller 30 to enable the end plate to be in face-to-face contact with the end face 68 of the first roller 28. At its lowest point however, the edge 66 is below the level of the nip 36. The circumferential distance over which the end plate 62 is in contact with the end face 68 of the first roller 28 and the end face 69 of the second roller 30 is as low as possible, consistent with the contact line between the end plate 62 and the end faces 68, 69 of the rollers 28, 30 extending below the level of the nip 36.

The end plates 62 are urged against the end faces 68, 69 of the rollers 28, 30 by plate springs 64 so shaped to ensure the desired location of the contact line 52. A suitable spring force is from 2 to 500 g/cm of contact between the end plate 62 and the end face 68 of the roller 28 measured at the surface of the roller.

The end plates 62 each include an aperture 74, the lower edge of which is positioned below the level of the top of the rollers 28, 30, enabling the bulk of the treatment liquid 24 to flow out of the vessel at each end thereof and to be recirculated as desired.

Claims

1. An apparatus for the wet processing of photographic sheet material comprising at least one treatment vessel (12, 12', 12") having upper and lower openings (17, 18), one of said openings constituting a sheet material inlet and the other of said openings constituting a sheet material outlet, said inlet and outlet defining there-between a substantially vertical sheet material path (20) through the vessel, the vessel comprising a pair of rotatable rollers (28, 30) biased towards each other to define a nip (36) there-between through which the sheet material path extends, characterised in that:

(i) said rollers are positioned relative to each other such that end faces (68) of one roller (28) lie in substantially the same plane as end faces (69) of the other roller (30); and

(ii) stationary sealing means (38) are provided in contact with each roller, having a continuous contact line (52) which extends along the length of each roller (28, 30) and over the end faces of each roller, at least on the fluid side of said nip (36).

2. An apparatus according to claim 1, wherein said sealing means (38) exerts a spring force of between 2 and 500 g/cm of roller, perpendicular to the roller surface.

3. An apparatus according to claim 1, wherein said sealing means is in the form of an unitary sealing member (38).

4. An apparatus according to claim 3, wherein said sealing member (38) comprises a central portion in the form of a substantially horizontally disposed flat plate (70), the under faces of which contact the surface (71) of each said roller 28, 30, said sealing member further comprising substantially vertically disposed end plates (62) which bear against the end faces (68, 69) of said rollers (28, 30).

5. An apparatus according to claim 4, wherein said sealing member (38) contacts the surface (71) of the associated roller (28) at a location which is between 45.degree. and 270.degree. from the center of said nip (36).

6. An apparatus according to claim 4, wherein said end plates (62) are biased against said end faces (68, 69) of each of said rollers (28, 30).

7. An apparatus according to claim 6, wherein said end plates (62) are biased against said end faces (68, 69) with a force of from 2 to 500 g/cm of contact between the end plate (62) and the end face (68, 69) of the roller (28, 30) measured on the surface of the roller.

8. An apparatus according to claim 1, wherein at least one of said rollers comprises a core (32) carrying a covering (34) of elastomeric material extending beyond the ends of the core, said sealing means being in contact with the end faces of said covering.

9. An apparatus according to claim 1, wherein the material of the sealing means which is in contact with the roller surface has a coefficient of friction (as measured against stainless steel) of from 0.05 to 0.3.

10. An apparatus according to claim 9, wherein the material of the sealing means in contact with the roller surface comprises a polymer material selected from poly tetra fluoro ethylene, polyoxymethylene, high density polyethylene, ultra high molecular weight polyethylene, polyurethane, polyamide and mixtures and composites thereof.

11. An apparatus according to claim 1, wherein said rollers (28, 30) constitute drive rollers for driving said sheet material along said sheet material path (20).

12. An apparatus according to claim 1 wherein at least one of said rollers (28, 30) comprises a core (32) carrying a covering (34) of elastomeric material, the ratio (.phi./L) of the maximum diameter (.phi.) of the elastomeric material covering to the length (L) thereof being at least 0.012.

13. An apparatus according to claim 1, wherein said lower opening (18) is closed by said pair of rotatable rollers (28, 30).