Slit coating head

Abstract

 A jet coating head has at least one body (11, 12) defining an inner chamber (5), which communicates with a coating liquid inlet and is elongated along a longitudinal axis (31) to distribute the coating liquid axially; the jet coating head has two lips (13, 14) defining a longitudinal slit (17), which connects the inner chamber (5) to the outside to dispense the coating liquid in use; the at least one body (11, 12) of the jet coating head comprises a body shell (18, 19) comprising a first composite material with a polymeric matrix and a body core (20, 21) encased completely in the body shell (18,19).

Description

TECHNICAL FIELD

[0001] The present invention relates to a jet coating head for coating sheet paper, film, metal or similar.

[0002] More specifically, the present invention relates to a jet coating head for surface coating a paper strip.

BACKGROUND ART

[0003] Commonly used papermaking machines normally feature a coating device for liquid coating a surface of a paper strip to improve the appearance and print quality of the paper.

[0004] A typical coating device, as described, for example, in EP0403845, comprises a jet coating head for applying the coating liquid to one surface of the paper strip, as the strip is fed with the opposite surface resting on a rotating supporting roller.

[0005] The jet coating head extends along a longitudinal axis parallel to the axis of the supporting roller, and defines an inner chamber, which receives the pressurized coating liquid from one or more lateral inlets, and distributes the coating liquid over an axial length at least equal to the width of the strip. The coating liquid issues from the chamber through a longitudinal slit defined by two facing lips, and is fed onto the surface of the strip to form a coating over the whole width of the strip.

[0006] In the EP0403845 solution, the outlet of the longitudinal slit faces upwards, so the jet coating head is commonly known as a coating "fountain".

[0007] Downstream from the jet coating head in the travelling direction of the paper strip, the coating device also comprises a scraper blade for removing surplus liquid applied to the strip and so achieve an evenly thick coating.

[0008] The jet coating head is normally made of stainless steel, which has several drawbacks.

[0009] On the one hand, the jet coating head is particularly heavy and therefore difficult to handle for assembly and/or maintenance. The handling problem is further compounded by the size of the jet coating head, which, having to be at least as long as the width of the strip, may be several metres in length.

[0010] On the other hand, in use, the longitudinal slit the coating liquid comes out of is not exactly straight, and the distance between the lips is not stable or constant over the whole width of the paper strip, due to thermal expansion of the material the jet coating head is made of, and flexural deformation caused by the weight of the head itself.

[0011] The geometrical instability of the slit has a negative effect on the quality of the finished product, and unevenness in the coating deposited on the paper strip often occurs despite the scraper blade. Known technology tends to solve these drawbacks by strengthening the structure of the jet coating head to increase its flexural rigidity, and by providing a heat regulating system to reduce the temperature, and hence thermal expansion, of the head. Both solutions, however, on the one hand, further increase the weight of the jet coating head, and, on the other, complicate and increase the cost of the coating device, especially the addition of the heat regulating system.

OBJECT OF THE INVENTION

[0012] It is an object of the present invention to provide a coating liquid jet coating head designed to solve the above problems in a straightforward, low-cost manner.

[0013] According to the present invention, there is provided a coating liquid jet coating head, as claimed in Claim 1.

[0014] The present invention will be described with reference to the attached Figure 1, which shows a schematic section of a preferred embodiment of the coating liquid jet coating head according to the present invention.

DESCRIPTION

[0015] Number 10 in Figure 1 indicates as a whole a jet coating head for applying a coating liquid 4 to one surface of a paper strip 3, as paper strip 3 is fed with the opposite surface resting on a supporting roller 2 rotating about its own axis. The axis of supporting roller 2 is not shown, and is perpendicular to the Figure 1 plane.

[0016] Jet coating head 10 is fixed to supporting flanges not shown, extends along a longitudinal axis 31 parallel to the axis of supporting roller 2, and has a cross section of constant shape and size along substantially its whole length.

[0017] Jet coating head 10 comprises one or more bodies, e.g. two bodies 11, 12, having respective opposite surfaces 23, 24 defining a chamber 5. Chamber 5 receives the pressurized coating liquid 4 from one or more lateral inlets (not shown) at the axial ends of jet coating head 10, and defines a manifold elongated along longitudinal axis 31 to a length at least equal to the width of paper strip 3.

[0018] Bodies 11, 12 comprise respective portions 32, 33 having surfaces 34, 35 contacting each other to fluidtight seal the whole length of chamber 5, and are fixed to each other by appropriate fasteners, e.g. bolts or screws 38 holding together portions 32, 33.

[0019] Diametrically opposite portions 32 and 33, bodies 11, 12 have respective lips 13, 14. Lips 13, 14 have respective faces 15, 16 spaced apart to define a longitudinal slit 17, and are fixed to bodies 11, 12 in fixed positions, but which can be adjusted to adjust the distance between faces 15, 16.

[0020] Slit 17 extends to a length at least equal to the width of paper strip 3, extends in cross section in a substantially straight direction radial with respect to longitudinal axis 31, and connects chamber 5 to the outside to feed coating liquid 4 onto paper strip 3 and form a coating 6 over the full width of the strip.

[0021] A scraper blade 7 is fitted to a support 8 downstream from jet coating head 10 in the travelling direction of paper strip 3, and serves to remove surplus liquid applied to the strip and so obtain an evenly thick coating 6.

[0022] According to the invention, bodies 11, 12 are made at least partly of composite polymer-matrix material.

[0023] More specifically, bodies 11, 12 have respective "sandwich" structures. That is, bodies 11, 12 comprise respective shells or skins 18, 19 made of composite polymer-matrix material; and respective cores 20, 21 made of foam material and encased completely in shells 18, 19 along the periphery shown in cross section.

[0024] "Polymer-matrix composite material" is intended here to mean a material comprising a polymer phase, in which a number of fibres are embedded.

[0025] Epoxy or vinylester or polyester resins may be used as the polymer phase. The polymer phase preferably comprises an epoxy resin with a vitreous transition temperature T_g of over 70°C, which is higher than the operating temperature of jet coating head 10. Epoxy resin has the major advantage of enabling the above vitreous transition temperature to be achieved without recourse to painstaking post-curing treatments, and is also highly compatible with carbon fibres.

[0026] By way of example, the resin known by the trade name ELANTAS EC 57 - K31 may be used.

[0027] The fibres embedded in the polymer phase may be carbon, Kevlar®, glass or basalt fibres, or hybrid strengtheners of combinations of these. The fibres are preferably continuous carbon fibres of a length ranging between a few centimetres and the whole length of the coating head or, if this comprises a number of connected modules, the whole length of each component module. The quantity of fibres used conveniently equals over 45% by volume of the total volume of composite polymer-matrix material.

[0028] Shells 18, 19 preferably comprise a number of superimposed layers of continuous fibres within the polymer matrix. The layers may be of two types, used singly or combined : layers of one-way fibres, e.g. parallel to axis 31; and woven layers.

[0029] The fibres in the superimposed layers are conveniently oriented in various directions with respect to longitudinal axis 31, to achieve the necessary mechanical strength in all possible stress directions.

[0030] For example, using an epoxy-matrix composite material and carbon fibres, shells 18, 19 can be obtained with a Young's modulus of over 30 GPa, over 30 MPa tensile strength, and less than 3% extensibility. As regards flexural properties, shells 18, 19 can be achieved with a modulus of over 5 GPa and flexural strength of over 250 MPa.

[0031] Cores 20, 21 may be made of polyurethane, PET, polystyrene or PVC foam, or other commonly used sandwich structure cores may be used. Preferably, cores 20, 21 are made of closed-cell polyurethane foam, such as the foam known by the trade name Divine Cell h80.

[0032] Lips 13, 14 are made from the same composite polymer-matrix materials described above for shells 18, 19. In the case of lips 13, 14, too, the reinforcing fibres are one-way, are arranged in layers, and are oriented in different directions with respect to longitudinal axis 31 to achieve rigidity and dimensional stability of the distance between faces 15, 16.

[0033] Alternatively, lips 13, 14 may be made of steel with a solid structure, as in known technology : the smaller thickness of lips 13, 14 with respect to bodies 11, 12 and, possibly, the presence of threaded inserts for assembling and adjusting the distance between lips 13, 14, make a solid structure preferable.

[0034] In another alternative, lips 13, 14 may also have a sandwich structure identical to that of bodies 11, 12, i.e. comprise respective shells and cores made of the materials described above with reference to bodies 11, 12.

[0035] In another alternative, lips 13, 14 form one piece with bodies 11, 12, i.e. the composite polymer-matrix materials of bodies 11, 12 and lips 13, 14 are seamless.

[0036] Coating liquid 4 deposited on paper strip 3 is erosive both by nature and due to the outflow speed from chamber 5 and, especially, through slit 17. To avoid erosion problems, surfaces 23 and 24 and/or faces 15 and 16 are protected with a corrosion-resistant surface coating (in particular, more corrosion-resistant than the composite polymer-matrix material).

[0037] The coating of chamber 5 and/or slit 17 comprises a gel phase with a filler of ceramic, e.g. alumina, particles. For example, the coating is applied directly to the shell surfaces and lip faces during the composite polymer-matrix material production process.

[0038] Alternatively, the coating comprises plasma-deposited amorphous (diamond-like) carbon or alumina

[0039] Alternatively, the coating comprises epoxy resins with a filler of fullerene-type nanoparticles of over Shore 80 surface hardness.

[0040] For reasons of simplicity, the coating of faces 15, 16 and surfaces 23, 24 is preferably a gel with a ceramic particle filler, e.g. an epoxy gel with a filler of preferably over 10% by weight of alumina particles ranging in average size between a few nanometres and hundreds of microns.

[0041] The surface coating is particularly advantageous where the flow speed of coating liquid 4 is fastest, i.e. at faces 15, 16 of lips 13, 14.

[0042] The composite polymer-matrix material shells are made using so-called "liquid molding" techniques. Once formed, the sandwich structure undergoes a temperature-activated curing cycle. And, finally, post-curing treatment is performed to raise the vitreous transition temperature T_g of the composite polymer-matrix material resin to over 70°C.

[0043] In the example described, lips 13, 14 are fixed to bodies 11, 12, and jet coating head 10 is fixed to the supporting flanges using threaded fasteners, by embedding threaded inserts, e.g. threaded bushings, in the composite polymer-matrix material. The inserts may be embedded directly at the laminating stage of the composite polymer-matrix material production process, or later by inserting the inserts inside holes and fixing them with adhesives or the composite polymer-matrix material resin itself. The holes are formed directly at the laminating stage or after forming the sandwich structures, by numerical-control machining.

[0044] In a variation not shown, to simplify manufacture, jet coating head 10 comprises a number of modules made separately, placed together along longitudinal axis 31 to ensure fluidtight sealing along chamber 5 and slit 17, and fixed to one another using adhesive and/or mechanical fastening devices.

[0045] As will be clear from the above description, jet coating head 10 as a whole has superior mechanical properties, particularly in terms of flexural strength, is lightweight, and has a low coefficient of thermal expansion, by virtue of the properties of the composite polymer-matrix material and, in particular, of the sandwich structure of bodies 11, 12.

[0046] The rigidity of the composite polymer-matrix material, in fact, is combined with a density of less than 2 kg/dm³, i.e. less than a quarter of that of steel normally used in this application. Moreover, the composite material of epoxy resin and carbon fibres has a high degree of corrosion resistance and dimensional stability, by virtue of the low thermal expansion of the carbon fibres.

[0047] Using foam, with a density of less than 0.25 kg/dm³, for the core also aids in reducing the overall specific weight of the sandwich structure of bodies 11, 12.

[0048] The mechanical characteristics of jet coating head 10 as a whole substantially depend on the thickness of shells 18, 19, which is preferably over 2 mm.

[0049] In short, jet coating head 10 is highly resistant to bending stress, without recourse to complicated temperature regulating systems.

[0050] In coating applications, these properties are vital to achieve a straightforward coating device, and to ensure the position, size, and shape of slit 17 are as required, are constant over the whole width of paper strip 3, and remain stable over time, to ensure even distribution of the coating over paper strip 3.

[0051] By way of example, a jet coating head in accordance with the invention has been designed with the following characteristics:

• shell material : composite epoxy-matrix material reinforced with carbon fibres;
• core material : closed-cell polyurethane foam;
• shell thickness : 4 mm;
• length of jet coating head 10 along longitudinal axis 31 : 5600 mm;
• coating liquid feed pressure : 1.5 bar.

[0052] Maximum deformation of the above jet coating head is less than 0.2 mm, determined by finite-element analysis.

[0053] Clearly, changes may be made to jet coating head 10 as described herein without, however, departing from the scope of the present invention as defined in the accompanying Claims.

[0054] In particular, jet coating head 10 may have a differently shaped and/or proportioned cross section from that shown by way of example; and/or lips 13, 14 may comprise metering appendixes or devices at the outlet of slit 17.

Claims

1. A jet coating head extending along a longitudinal axis (31) and comprising:

- at least one body (11, 12) defining a distribution chamber (5), which communicates with at least one inlet for a coating liquid and is elongated along said longitudinal axis, said body (11, 12) comprising a first composite material with polymeric matrix;

- at least two lips (13, 14) fixed with respect to said body and reciprocally defining a longitudinal slit (17) communicating with said distribution chamber and defining an outlet (22) for dispending, in use, said coating liquid;

characterised in that said body (11, 12) comprises
a body shell (18, 19) comprising said first composite material with a polymeric matrix; and
a body core (20, 21), said body core being encased completely in said body shell (11, 12).

2. A jet coating head according to claim 1, characterised in that said core (20, 21) comprises a foam material chosen among polyurethane foams, PET foams, polystyrene foams, PVC foams.

3. A jet coating head according to claim 2, characterised in that said foam material is a closed-cell polyurethane foam.

4. A jet coating head according to anyone of the preceding claims, characterised in that said lips (13, 14) are made of steel.

5. A jet coating head according to anyone of the claims from 1 to 3, characterised in that said lips (13, 14) are made of a second composite material with polymeric matrix.

6. A jet coating head according to claim 5, characterised in that said lips (13, 14) are integrally formed with the first composite material with polymeric matrix of said body (11, 12).

7. A jet coating head according to anyone of the preceding claims, characterised in that said first composite material with polymeric matrix comprises a polymeric phase including a material chosen among epoxy resins, vinyl-ester resins and polyester resins.

8. A jet coating head according to claim 7, characterised in that said polymeric phase comprises an epoxy resin having a vitreous transition temperature (Tg) higher than 70°C.

9. A jet coating head according to anyone of the preceding claims, characterised in that said first composite material with polymeric matrix comprises fibres chosen among carbon fibres, Kevlar®, glass, basalt and combinations thereof.

10. A jet coating head according to anyone of the preceding claims, characterised in that said lips (13,14) have corresponding faces (15, 16) provided with a corrosion-resistant coating layer defining said longitudinal slit.

11. A jet coating head according to anyone of the preceding claims, characterised in that said bodies (11, 12) have corresponding surfaces (23, 24) provided with a corrosion-resistant coating layer defining said distribution chamber (5).

12. A jet coating head according to claim 10 or 11, characterised in that said coating layer comprises a gel phase filled with ceramic particles.

13. A jet coating head according to claim 10 or 11, characterised in that said coating layer comprises amorphous carbon or alumina.

14. A jet coating head according to claim 10 or 11, characterised in that said coating layer comprises an epoxy resin filled with fullerene-type particles.

Drawing