EP0071149A1 - Apparatus for the weight control of coatings on a travelling web - Google Patents

Abstract

A device for the regulation of the wet application weight of coated webs, the web being supported on its uncoated side by a roll. The device has a stationary, inherently rigid wiping device with a concavely curved long entrance flank so that between web surface and wiping device a sickle-shaped entrance gap is formed. The radius of curvature of the entrance flank of the wiping device is equal to or only slightly greater or smaller than that of the support roll. In order to avoid wear and tear at the edge of the wiping device additional wear elements are provided at the wiping device.

Description

The invention relates to a device for adjusting the wet application weight of coatings on material webs, in particular of conventional pigment dispersions on paper or cardboard webs.

In order to improve the printability of paper surfaces and to adapt them to the special printing processes, such paper surfaces are coated with pigment / binder dispersions. Depending on the intended use, the dry application weights are between 5 and 35 g / sqm. The coatings are usually made from an aqueous dispersion with different dry matter contents, so that the wet application weights are considerably higher. It has proven to be expedient first to apply an excess of coating material to the running material web and then to remove it again from the surface by means of metering devices, the aim being at the same time to smooth the coating surface. Known metering devices include roller applicators in connection with metering rollers, so-called reverse roll coaters, roller scraper metering devices, also known as squeegees, and smoothing scraper devices, better known under the abbreviation Blade.

Osiervorgang When _D is the coated material web by _ a support means, carried, for example, a roll and exerted by the metering unit, as blade or doctor blade, a pressure to the web, which counteracts the coating composition.

A coating device is described in US Pat. No. 2,051,403, in which the essential part consists in the formation of the scraper. This curved scraper designed in a special shape has two different radii in the course of its curvature. The end of the inlet flank is not a sharp edge, but rounded, because sharp edges should lead to a faulty surface and unsatisfactory results. The subsequent inlet flank is convexly curved and has a larger radius than the radius of curvature of the rounded edge.

In U.S. Patent No. 2,534,320 a stripper with a sheet thickness of 3 12 is described mm, whose inlet edge ending in a sharp edge, the radius of curvature of _E inlaufflanke about the radius of curvature of 150 mm which the web supporting roll having a resilient surface matches. The curvature is convex in shape. The scraper is arranged in a rotatable holder and can be pressed against the web surface with different pressure, so that the surface of the support roller is deformed.

US Pat. No. 1 925 092 describes a wiper device which is directed against a freely guided web, the web being guided a little around the curved inlet flank of the wiper up to its sharp-edged end. The scraper inevitably serves as a web guide and deflection element at the same time. The gap that forms is limited by the convex curved surface of the wiper and the concavely curved coated surface of the web. Since the running web is discharged tangentially from the sharp end edge of the scraper, it is not possible to set a defined inlet gap. The geometry of the inlet gap that forms does not only depend on the web speed, but also on the amount and the rheological properties of the coating composition and on the elongation behavior of the paper web. The gap geometry is therefore not defined by the device, but is more or less undefined depending on the respective process conditions and cannot be set.

In principle, this also applies to the known metering devices with a blade blade with a material web supported by a roller.

_J e of pressure altitude and position of the metering device to the surface of the coated web of material, there remains a more or less thick layer on the web. _The loading blade and doctor blades are not, by themselves, sufficiently stable tools for setting a gap geometry and also require brackets and guides which give them the required straightness, bending strength and resistance to the hydrodynamic back pressure of the coating material. Particularly high demands are made on the designs of such mounts. With today's common web widths of several meters and web speeds up to 1,200 m / min. the requirements can only be met up to a certain straight line by appropriate design. The consequence of this is that the known metering devices are unsatisfactory in particular if, in order to achieve high application weights, the known blade blades are applied to the web with a small angle of attack and low contact pressure. Under these conditions, a wiping surface or zone forms, in which wipers abut the web surface. A condition very soon arises in which blade forces sometimes cannot withstand the higher color printing and a different application weight arises across the web. Even slight deviations in straightness, due to manufacturing tolerances or other causes lead to considerable fluctuations in the order quantities across the web after dosing. Attempts have been made to improve the straightness by supporting, for example, the blade with the help of pressure hoses. These results are unsatisfactory, especially with larger web widths.

The currently Possible and usual coating systems can have working widths of up to 6,000 mm. The Working speeds and application weights strongly depend on the types and the desired properties. For LWC paper (L _W C is derived from lightweight coated), for example, they are up to 1,300 m / min. with application weights up to 10 g / qm and side; for cardboard, for example, up to 250 m / min. with application weights of approx. 30 g / qm per side and for so-called art paper up to 600 m / min, with application weights of approx. 25 g / qm and side.

As part of systematic investigations, it was found that the exact gap geometry is of considerable importance and it is therefore desirable to develop this gap geometry in a defined manner and to use it to control the hydrodynamic pressure in the entire inlet gap. The object of the invention is therefore to provide a stripping device which makes it possible to set the wet application weight on coated webs with great accuracy, by controlling the hydrodynamic pressure of the coating material between the running web and the fixed stripper.

This object is achieved by a device for regulating the application weight of coatings on material guided in the running web, in particular paper or cardboard webs with a roller supporting the running web on the uncoated side, which has a radius of 150-500 mm and an elastic surface has a hardness of 82-34 units Shore A and a scraper which is at rest during the action on the coating, which has a curved infeed flank on the infeed side, the end of which adjoins the web is sharp-edged and the scraper over the contact point to the Material web is designed as a pivot point. The characteristic of the invention contemporary solution is that the rigid scraper has a concave curved inlet flank with a radius of curvature equal to or slightly smaller or larger than the radius of curvature of the support roller and the length of the inlet flank is 10-250 mm.

Preferred embodiments of the device are described in the subclaims.

The advantage of the inventive design of the metering device is that the geometry of the inlet gap between the support roller and the scraper is fixed for every angle of the scraper and at the end of the inlet gap there is a narrow contact line and no contact zone of the scraper is formed when this is applied with pressure the web is pressed on, which is sufficient to compensate for the hydrodynamic pressure of the coating color. Apart from the angle of attack, the gap geometry is therefore independent of the particular process conditions during dosing.

The invention is explained in more detail below with the aid of schematic sectional drawings and further illustrations.

• Figure 1 shows in section schematically an arrangement and design of a metering device according to the invention with a web coating system.
• Figure 2 shows schematically in section the combination of the stripper block with inserted steel sheet.
• Figure 3 shows schematically the arrangement of punch tongues in the insertable steel sheet.
• Figure 4 shows in section the design of the scraper with attached end strip.
• FIG. 1: A material web, for example a paper or cardboard web 1, is attached to the web via a deflection roller 3 Support roller 2 brought up, wraps around this and is removed again via a further deflection roller 3 and reaches subsequent parts of the coating system, for example a dryer, not shown. With the help of a customary and known application unit 4 with a trough 9 for the coating material, an excess of coating material is applied to the web and removed from the web 1 with the stripper 5. The stripped coating mass passes into a drip pan, not shown. On the web 1, the set coating thickness 8 remains behind the stripper 5. The stripper 5 lies at point 6 on the web surface. The support roller 2, the scraper 5 and the application unit 4, which is only shown schematically and by way of example, are accommodated in a stable frame, which contain the necessary drive devices for the application unit 4 and the support roller 2. The scraper 5 is positioned on the web surface at point 6 via bearings of the scraper 5 on both sides in the frame. The bearings are designed so that the scraper can be pivoted around the application point (6) as a fulcrum in order to be able to precisely adjust the inlet gap between the web surface and the concave curved inlet flank (7) of the scraper. The facilities required for the exact and reproducible setting of the scraper (5) are housed in or on the chair. The desired contact pressure on the web on the support roller is generated mechanically, hydraulically or pneumatically via the ends of the stripper (5) on both sides. Such devices are known. In practice, pneumatic systems have proven themselves better than mechanical or hydraulic systems and are therefore preferred when using the device according to the invention. By placing the scraper (5) with the end (6) of the inlet flank (7) on the web and setting a flat contact angle, a longer distance progressive approach of the inlet flank (7) to the coated web (1) carried by the support roller (2) and a crescent-shaped inlet nip is formed, the geometry of which is determined by the radii of curvature of the support roller (2) and the inlet flank (7) of the scraper and the angle of attack becomes. Due to the concave curvature, a slow narrowing of the wedge gap is achieved, on the one hand, on the other hand, this curvature prevents the formation of a larger contact surface at the end of the gap, which is inevitably formed with a blade doctor knife and a flat angle of attack. The gap geometry is determined by the device, can be set in a reproducible manner and is independent of the process conditions such as working speed, reological properties and amount of the coating slip on the paper web, etc. Due to the rigid design of the stripper (5) by means of a corresponding cross section, it is uniform The scraper's contact across the entire web width is guaranteed, even with larger working widths. The exact cross-section of the scraper (5) is not critical, it is crucial that it has a concavely curved inlet flank (7) of corresponding length with a sharp-edged end (6). A wedge-shaped or rectangular cross section is preferred. The scraper can be made solid or as a hollow profile. The wall thickness of the hollow profile and size are dimensioned in the same way as for full material so that for a given working width the stripper (5) is rigid in itself and deflections in the middle of the web are avoided. The length of the inlet flank can be 10-250 mm, preferably 25-230 mm, very preferably 50-200 mm or 80-150 mm. With larger diameters of the support roller, larger lengths of the inlet flanks are favorable. As the working width increases, larger flank lengths are also advisable in order to achieve a rigid design of the scraper (5). Where applicable additional stiffening elements are provided to make the stripper rigid in itself. The scraper (5) consists of a stable material of sufficient mechanical strength, for example steel or coated metal alloys or correspondingly stable plastic.

The radius of the support roller (2) is 150-500 mm, depending on the working width, whereby larger working widths generally require larger radii in order to avoid deflections. The support roller (2) has an elastic covering on a hard core. Synthetic rubber types or synthetic polymers with the necessary elasticity are suitable for this. The surface hardness of the backup roller can be determined using various measuring methods. Pusey & Johnes plastometers (P + J. units, measured with the 1/8 "ball) or Shore hardness measuring devices (Shore A, measured with truncated cone) are commonly used. The surface hardness of the support roller should be between 82 and 34 units Shore A. (= 40-197 P + J. units), preferably 56-41 units Shore A (= 108-16 ₀ P + J. units). Surface hardness of 40-44 units Shore A (= 127 -149 P + J).

The formation of a narrow crescent-shaped wedge gap of corresponding length builds up a very high hydrodynamic color pressure in the inlet gap, which requires correspondingly high counterpressures, ie contact pressure of the scraper (5) against the web surface in the contact point (6). In order to keep the required contact pressures of the scraper (5) within reasonable limits, the angle of attack of the scraper is increased at desired low application weights, so that the initial width of the inlet gap increases and the pressure build-up in the sickle gap takes place in a shorter range. In order to achieve a high pressure build-up in the gap with high application weights and thus accordingly To enable high contact pressures, a flat angle of attack, ie a narrow, long sickle gap, is set. Between the web surface and the pressed scraper (5), a hydrodynamic pressure forms in the coating mass, which produces a sliding bearing effect. That is, the scraper (5) practically floats on the coating slip and carries out a kind of pre-metering. The actual wiping surface at the end of the inlet flank is extremely narrow, almost linear. This contact zone gives the final dosage of the application weight and leads to a final smoothing of the coating surface. The large hydrodynamic pressures that develop when the scraper is set up also require high application weights, i.e. dry application weights of between 15 and 30 g / m2, the wet application weights are correspondingly higher depending on the dry matter content of the coating material, and the contact pressure of the scraper is still considerable, making it an exact setting possible. Due to the rigid design of the scraper, a partial evasion of the metering unit is not possible.

FIG. 2 shows an embodiment of the scraper (5), in which a steel sheet (11) is inserted into the inlet flank (7), which can be replaced if necessary. _' In this way, errors caused by signs of wear on the actual wiper edge (6) at the end of the inlet flank (7) can be avoided. Microscopic examinations of this edge have shown that a sharpening analogous to the known blade blades is not necessary, but the normal sharp edge provides the best results. However, it is also possible to use correspondingly ground blades. The steel sheet is inserted into the convexly curved inlet flank so that its edge is flush with the scraper edge (6). But it is also possible to insert the steel sheet so that it is low protrudes slightly beyond the edge of the scraper, for example 1-10 mm, to extend the inlet flank. In this case, the scraper is placed with the end of the inserted steel sheet on the web surface and this support line serves as the axis of rotation for the scraper. The blade (11), preferably made of spring steel, inserted into the inlet flank (7) should rest on the scraper (5) over the entire surface and be held in this position. According to one embodiment, this can be done in that the inlet flank (7) has an undercut groove (10), engage in the punching tongues (12) of the steel sheet and hold the blade under tension. However, other fastening and holding devices can also be used, provided that the sheet is held in flat contact with the curved inlet flank (7) of the scraper (5).

Figure 3 shows the arrangement of the punch tongues (12) in the steel sheet (11) in the middle at a distance from each other.

Figure 4 shows a further embodiment of the scraper (5) in which a sealing strip (13) is arranged and fastened to the scraper (5) as a wear element and forms the final edge (6) of the scraper (5). Since the actually scraping edge is very narrow, the end strip (13) can be chosen so thin that no grinding corresponding to the radius of curvature of the inlet flank (7) is required. It is sufficient and possible to use square strips made of a correspondingly abrasion-resistant material. Such materials are, for example, steel alloys, particularly hard steel alloys or other abrasion-resistant materials.

• 100 Teile Pigment (Clay, Kreide, Satinweiß)
• 10 Teile Bindemittel, z.B. carboxylierter Styrol-Butadienlatex mit 50% Trockensubstanz.
• 4 Teile lösliches Bindemittel, z.B. Kasein
• 1 Teil Alkalilauge, z.B. 25%ige NaOH
• 74 Teile Wasser

The operation of the device according to the invention is described in more detail using the following example: A so-called coating base paper with a weight per unit area of 75 g / m 2 should be coated with A) 10 g / m 2 or B) 25 g / m 2 of a coating composition of the following composition will:

• 100 parts pigment (clay, chalk, satin white)
• 10 parts of binder, eg carboxylated styrene-butadiene latex with 50% dry matter.
• 4 parts of soluble binder, e.g. casein
• 1 part of alkali solution, for example 25% NaOH
• 74 parts of water

The coating color also contains the necessary proportions of dispersing aid defoamer and other customary additives.

The coating system has the arrangement shown in Figure 1. The web width is 315 cm. A design according to FIG. 2 with an inserted sheet of spring steel is used as the scraper. The scraper has a concave curved inlet flank with a radius of curvature of 250 mm and is 200 mm long. The support roller has a diameter of 500 mm, the surface hardness is 46 units Shore A (140 P + J. units).

At a web speed of 500 m / min. an excess of coating material is first applied by means of a conventional dipping roller and removed again by the wiper. For this purpose, the sharp-edged end of the stripper is pressed against the coated web surface at a flat angle with such pressure that the hydrodynamic pressure of the coating material building up in the crescent-shaped inlet gap is compensated so that in the operating state between the stripper edge and the support roller surface a narrow gap is created through which the coated web of a defined application thickness and smooth surface passes.

The low application weight A of 10 g / sqm is an amount for which the known blade has proven to be particularly suitable for dosing. This type of scraper develops its advantages with a steep angle of attack of the blade and high contact pressure. A comparable effect is achieved with the device according to the invention if the angle of attack is larger, so that a short inlet zone with pressure build-up in the coating color is formed. With a contact pressure of about 3 kN / m generated pneumatically at the side end of the scraper in the frame against the support roller (line pressure in the present case, a wet application is achieved which, when dry (6% moisture), amounts to 10 g / m² was determined as a control at the same web height on the outer edges and in the middle of the web The mean values from several measurements were: outer web edges 10.0 g / qm and 10.1 g / qm, web center 10.0 g / qm Individual measurements with each other were 0.5 g / sqm This evaluation shows the good weight consistency across the web.

• Bahnaußenkanten 25 g/qm und 24,5 g/qm, Bahnmitte-24 g/qm.

The higher application weight B of 25 g / sqm is an amount for which a blade is less suitable due to the required low angle of attack and the low contact pressure. Other application devices are usually required for this, for example a roller application unit. According to the invention, a higher application weight is now possible with the same metering device with which a low application weight was achieved, by flattening the scraper and forming a long crescent-shaped inlet gap in which a hydrodynamic pressure builds up in the coating composition, without the contact pressure being significantly reduced must become. The good adjustability of the scraper means that only acceptable weight fluctuations occur across the web. The high quality requirements have the tolerable range of weight deviations very: strong. constricted. For reasons of rationalization, the working widths of the systems have been continuously increased, whereby weight deviations across the web are inevitably accepted with the conventional metering devices had to be. This disadvantage is now overcome by the invention. With a contact pressure of about 3 kN / m (line pressure) generated at the lateral ends of the stripper in the frame against the support roller, a wet application is achieved in the present case, which in the dried state (6% moisture) is 25 g / m2. The control weight was determined for checking at the same web height on the outer edges and in the middle of the web. The mean values from several measurements gave:

• Outer web edges 25 g / sqm and 24.5 g / sqm, web center-24 g / sqm.

The maximum deviations between the individual measurements were 2 g / m2. The evaluation shows the good weight consistency across the web. An important advantage of the device according to the invention is that, regardless of the desired application weight, such a contact pressure can always be selected that enables the gap to be formed exactly across the web, even with larger working widths. The prerequisite for this is the correct elasticity of the support roller surface, which must enable the formation of a crescent-shaped, reproducibly adjustable inlet gap and a short actual wiping area on the contact edge of the wiper, and furthermore the rigid wiper with a concave curved inlet flank. The contact pressure should be as low as possible with regard to the design effort of the scraper support roller and its bearings in the frame, but it must be so large that at least the current web speeds of up to 1,200 m / min. build-up hydrodynamic pressure of the coating is compensated and the setting is possible with sufficient reproducibility. Another advantage of the device according to the invention is that with only one device very different order weights with less fluctuations than previously common, especially possible with large working widths. In this way, the surface and transparency of the coating are achieved in a way that was previously only possible with different metering devices specially adapted to high or low application weights. The advantage of the rigid scraper according to the invention with a concavely curved, long inlet flank is that it is not only easy to produce with the required accuracy without great difficulty, but can also be adjusted with great accuracy and reproducibility across the web. The crescent-shaped inlet gap, which is filled to a greater or lesser extent with coating compound during operation between the support roller and stripper, is of equal thickness at all points, in particular at the end edge of the stripper, and enables smaller deviations from the target weight than was possible with the previously known metering devices.

Reference symbol list

• 1 material web, for example cardboard or paper
• 2 support roller with elastic surface
• 3 deflection rollers, guide rollers
• 4 commissioned work
• 5 wipers
• 6 sharp-edged end of the inlet flank, support line, axis of rotation
• 7 curved inlet flank of the scraper
• 8 set application thickness on the coated web
• 9 open trough for the coating material
• 10 Groove in the scraper
• 11 inlaid steel blade, blade blade
• 12 punch tongues in the steel sheet
• 13 strip, end strip

Claims (5)

1. Device for regulating the application weight of coatings on material running in the web, a) with a roller supporting the running web on the uncoated side, aa) which have a radius of 150-500 mm and ab) has an elastic surface with a hardness of 82-34 units Shore A, and b) a scraper which is at rest during the action on the coating, ba) which has a curved inlet flank on the inlet side, bb) whose end abutting the web is sharp-edged and c) the scraper is designed to be pivotable as a fulcrum over the contact point on the web, characterized in that d) the rigid scraper (5) has a concave curved inlet flank (7), da) with a radius of curvature which is the same or slightly smaller or larger than the radius of curvature of the support roller (2), e) the length of the inlet flank (7) is 10-250 mm. 2. Device according to claim 1, characterized in
that the length of the inlet flank (7) is 50-200 mm. 3. Device according to claims 1 or 2, characterized in
that in the concavely curved inlet flank (7) of the scraper (5) a steel sheet (11) lying against it is inserted. 4. The device according to claim 3, characterized in
that the steel sheet (11) has spaced-apart punching tongues (12) which engage in a recessed groove (10) in the inlet flank (7) of the scraper (5) and hold the steel sheet (11) in position. 5. Device according to claims 1 or 2, characterized in
that the end (6) of the inlet flank (7) forming the contact point is formed by an end strip (13) attached to the wiper (5).

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