Method and assembly for controlling the coat profile in coaters based on short dwell time application

Abstract: The present invention relates to a method and apparatus for controlling coat profile in short dwell time coaters and similar apparatuses. In these coaters the coating mix is applied on the web (2) in an application chamber (11) disposed immediately in the rear of the doctor element (5), and the coating mix contacts the web (2) only for a short time. An excess amount of the coating mix is fed into the application chamber (11) and the mix is arranged to flow over the front edge (12) of the chamber (11) counter to the running direction of the web (2). The invention is based on controlling the cross-machine coat profile by altering the distance between the web (2) and the front edge (12) of the application chamber (11) in the cross-machine direction of the web (2).

Description: The present invention relates to a method according to the preamble of claim 1 for controlling the coat profile in short dwell time coaters and similar equipment. The invention also concerns an apparatus according to the preamble of claim 3 suited for implementing the method. Paper web coating is today implemented using a variety of different methods. Short dwell time coaters are employed particularly as on-machine coaters owing to their good running properties. Good running characteristics are based on a low application pressure and a short distance between application and doctoring. In short dwell time application, the coating mix cannot penetrate deep into base web, whereby the required doctoring force remains low and, owing to the short time interval between application and doctoring, wetting of the base web and resulting loss of base web strength remain smaller than in other coaters. In the art, the short dwell time coater is generally denoted by the abbreviation SDTA (Short Dwell Time Applicator). In this type of coater, the coating mix is passed into an application chamber located immediately in the rear of the doctor blade, whereby one wall of the chamber is formed by the web moving supported by a backing roll, one wall by the doctor blade and one wall on the incoming side of the web by the front wall of the chamber. The coating mix is fed into the application chamber via a feed slit disposed at the bottom of the chamber, and a major portion of the infed mix is routed past the front wall edge back to coating mix recirculation. The purpose of the outflow of the coating mix past the front edge, counter to the web travel direction, is to reduce the access of the air layer travelling along with the running web into the application zone, to assure even distribution and replenishment of the coating mix flow and to keep the mix outflow slit open. Such a coating apparatus is described in US-A-4, 250, 211, for instance. To achieve an even surface profile of the web, some kind of coat profile control may be required. Such need for control is typically dictated by the unevenness of the base paper web profile or various factors related to equipment employed and prevailing process conditions. Conventionally, the profile control is implemented by altering the doctor blade settings and a number of different control methods and equipment are known in the art. US-A-4,405,661 describes a coater of the above-mentioned type having the distribution of the coating mix in the application chamber as well as the evenness of application improved by a specific arrangement of the mix feed slit. In this apparatus the doctor blade is loaded by a pressurized loading hose, and the linear loading of the blade is controlled by adjusting the hose pressure and adjustment screws attached to the hose. The cross-machine profile of the doctor blade can be adjusted in certain limits by altering the control screw settings. The distance of the front edge on the incoming side of the web from the web is adjusted by means of a servo-controlled screw. However, this apparatus offers rather limited control facilities of the coat profile, because the pressurized hose absorbs a portion of the profile adjustment exerted by means of the control screws. US-A-5,109,792 discloses an apparatus in which an improvement in coat quality is attempted by way of tighter sealing of the application chamber. This goal is achieved by means of an elastic seal element attached to the front edge of the chamber. The purpose of the seal element is to prevent the backflow of the coating mix and thus increase'the internal pressure of the application chamber. This apparatus offers reduced backflow combined with lower risk of damage to the backing roll in the case of a web break. The doctor blade loading and profile control are implemented in the same fashion as in US-A-4,405,661. FI-C-91,025 (counterpart of GB-A-2 252 926) describes a method of measuring the coat profile and adjusting the doctor blade profile in a coater. According to this method, the cross-machine profile of the coating mix quantity applied on the web in the coating station is measured continuously, and on the basis of the measurement result, the blade profile is adjusted by means of elements controlling either the blade directly or via the blade support beam. This method is not limited to any particular doctor blade construction, but rather, is suited for controlling a variety of equipment having a doctor blade profile adjustment facility. Coat profile control based on doctor blade profile control and doctor blade loading adjustment has several shortcomings. For instance, as the doctor blade loading changes with the adjustment of the blade profile, uneven doctor blade wear results. Such uneven wear causes a need of more frequent blade replacement particularly when blade profile control must be used correcting a coat profile error caused by a persistent defect of the base paper web. In such a case, the coater must be run with the same control settings for long times, whereby the blade wear will eventually reflect the controlled blade profile. A similar situation occurs if the profile control must be used to compensate for constructional errors of the applicator apparatus such as deflections of the blade support beam. Due to the resulting blade wear, the controlled profile will not stay stable over a longer period, but rather, relatively rapidly tends to assume the condition prevailing prior to the profile control operation thus necessitating frequent readjustment of the blade profile. Hence, methods based on blade profile control are not particularly desirable due to the short-term effect of the applied control measures. Profile errors traceable to the base paper defects may also be corrected by calendering the web prior to coating. Naturally, while calendering improves paper quality, it also requires higher investments in equipment. It is an object of the present invention to achieve a method and an apparatus offering coat profile control with improved efficiency and long-term stability over the prior art in short dwell time coaters. The goal of the invention is accomplished by controlling the cross-machine profile of applied coat through varying the distance between the web and the front edge of the application chamber in the cross-machine direction. More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1. Furthermore, the apparatus or assembly according to the invention is characterized by what is stated in the characterizing part of claim 3. The invention offers significant benefits. The distance between the application chamber front edge and the web has been found to exert a strong effect on applied coat weight when other application parameters are maintained constant. The gap width of the backflow slit controls the internal pressure of the application chamber and thus the amount of coating mix applied onto the web from under the doctor blade. Such a control method exerts a long-term effect and does not contribute to blade wear, whereby the blade replacement intervals will not be affected. By virtue of its long-term effect, the control method according to the invention is significantly more advantageous than a control scheme based on blade profile adjustment. The profile control of the application chamber front edge may be combined with the blade profile control, whereby extremely wide-range and flexible control possibilities are obtained. Further, the control of the distance between the application chamber front edge and the web makes it possible to perform permanent correction of profile errors caused by the straightness deviations of the coater blade support beam, blade holders or application chamber front edge and to compensate for persistent profile errors of the base paper web. In the following the invention is described in greater detail with reference to the appended diagrams in which Figure 1 illustrates diagrammatically the cross section of a short dwell time coater according to the invention; and Figure 2 illustrates the controllable application chamber front edge of a short dwell time coater according to the invention. Referring to Fig. 1, the SDTA or short dwell time applicator illustrated therein is representative to the construction of short dwell time coaters employed today. This kind of prior-art apparatus is shown complemented with a controllable front edge 12 of the application chamber 11 according to the invention. Attached to its body 3, the coater of Fig. 1 has a blade holder 4 serving to clamp the lower edge of the doctor blade 5 to the body 3. To the blade holder 4 is connected loading means 6, 7, 8 of the blade 5. The loading means comprises connected to the blade holder 4 a movable support carriage 7 to which a hose 6 filled with a pressurized medium is attached. The hose 6 rests against the doctor blade 5 so as to fill the gap between the upper edge of the blade and attachment of the hose, whereby the hose serves to load the blade 5 against a web 2 running over a backing roll 1. The machine direction of the web 2 and the rotational direction of the backing roll 1 are indicated by the large curved arrow in the diagram. The support carriage 7 of the loading hose 6 is advantageously divided into segments in the longitudinal direction of the blade 5. Each segment is provided with a control means 8 suited for the adjustment of the segments of the support carriage 7. Most typically, the control means 8 is a manually adjustable screw or electrically driven screw jack, while a variety of other types of control means are possible. using such a control means 6, 7, 8, the blade loading is conventionally carried out by first setting a suitable base value of loading through adjusting the control means 8 so as to inflate the hose 6 resting against the doctor blade 5. Run-time blade loading control is effected by dynamically adjusting the inflation pressure of the hose 6. With the help of the segmental support carriage 7 and the control means 8, also the profile of the doctor blade 5 and its linear loading can be controlled in the longitudinal direction of the blade 5. By adjusting the blade profile, the amount of applied coating mix in the cross-machine direction of the web 2 to be coated can be varied thus permitting, e.g., the correction of base paper profile defects. The coating mix is applied to the web 2 via the application chamber 11. The walls of the application chamber 11 are formed between the doctor blade 5, the body 3 and the front edge 12 attached to an auxiliary body 14, while the chamber is sealed by the web to be coated running over the backing roll 1. The coating mix is delivered into the application chamber via a feed channel 9 and a flow-evening and flow-distributing manifold of flow-constricting channels 10. The coating mix is fed to the chamber 11 in excess amount relative to the amount applied to the web 2 from under the doctor blade 5, and the excess flows counter to the running direction of the web 2 over the front edge 12 back to the collection pond, wherefrom it is circulated back to the application chamber 11. The flows of coating mix circulation are indicated by arrows in the drawing. The purpose of overflow past the front edge 12 is to prevent the access of the air layer travelling along with the fast-running web 2 into the application chamber 11 and to assure the adherence of the coating mix on the web 2. In applicator apparatuses of the above-described kind, the application distance is short as is evident from Fig. 1. The coating mix contacts the web 2 only for the length of the distance 1, and as the speed of the web 2 is high, the contact duration between the web 2 and the coating mix is extremely short. Owing to the short contact time and high web speed, the coating result will be strongly dependent on the application pressure and the flow pattern of the coating mix in the application chamber 11 making good control of these parameters extremely important. The internal pressure in the application chamber 11 is determined by, i.a., the volumetric inflow rate of the coating mix and the width of the gap 13 between the front edge 12 and the web 2. When the inflow rate is increased, the pressure increases naturally concomitantly, and correspondingly, if the front edge 12 is moved away from the web 2, the chamber pressure decreases. Usually, the distance between the front edge 12 and the web 2 is made adjustable, and this adjustment effects the amount of coating mix applied to the web and the loading of the doctor blade 5. The present invention is based on the findings that the coating mix profile control can be implemented by varying the distance between the web 2 and the application chamber front edge 12 in the cross-machine direction of the web 2. In the following, this control method is called the profile control of the front edge 12. Now referring to Fig. 2, an embodiment of the front edge 12 is shown suited for profile control. This front edge 12 comprises a stainless sheet steel plate having one of its long sides shaped into a curved overflow edge 16. The front edge 12 is continuous and smooth at the overflow edge 16. The front edge 12 is made sufficiently flexible in the vertical direction by means of material-reducing cuts 17. These cuts may be machined by, e.g., a laser and their volume is filled with polyurethane. Downward from the continuous overflow edge 16 extend comb-like isthmuses 18, which separate the urethane-filled cuts 17 from each other. In this fashion the front edge 12 can be made flexible in the vertical direction and yet sufficiently stiff in the cross-machine direction. The profile control of the front edge 12 is accomplished by means of control screw jacks 15 connected to the isthmuses 18. The profile of the overflow edge 16 can thus be adjusted by pushing/pulling the isthmuses 18 with the help of the screw jacks 15. As even the smallest changes in the gap 13 between the front edge 12 and the web 2 affect the coat weight profile, the required control range need only be a few millimeters, and thus the above-described front edge construction is sufficiently flexible to provide a functional method of coat profile control. The front edge profile control affects the internal pressure of the application chamber 11, and thus the coat weight profile. When the overflow edge 16 is moved closer to the web 2, the local pressure in the chamber 11 increases at this point and more coat is applied to the web 2, because the local increase of the coating mix pressure moves the position of the doctor blade 5 so as to move the blade 5 farther away from the web 2 thus permitting a greater amount of coating mix to pass onto the web. Correspondingly, when the overflow edge 16 is moved farther away from the web 2, the local pressure in the chamber 11 decreases at this point and the doctor blade 5 moves closer to the web 2 thus permitting less coating mix to pass onto the web. As the delivery of coating mix into the application chamber 11 is arranged as homogeneous as possible, the profile control of the front edge 12 affects locally the pressure in the application chamber 11 thus achieving the desired coat profile control. The control method according to the invention is suited for use in all coaters which have the application arranged to occur in an application chamber adapted in the immediate vicinity of the doctoring element just prior to doctoring or smoothing and in which the application pressure can be adjusted by constricting the outflow of the coating mix from the application zone. In practice this definition refers to coating apparatuses having the application chamber bordered by a front edge whose distance from the web can be adjusted to control the overflow in the incoming direction of the web. No other constructional details of coater apparatus are limiting to the adaptation of the invention, and in fact, the invention may be employed in, e.g., apparatuses in which the coat is applied on the web using a coater similar to a short dwell time applicator and is subsequently smoothed to final coat weight by means of a separate doctor blade. Alternatively, a blade as the doctoring means can be replaced by any other element such as a bar, for instance. The final coat profile may be controlled by adjusting both the front edge and the doctor blade profile, or alternatively, the front edge profile alone. Hence, the invention can be readily adapted for coat profile control in coater apparatuses having no profile control of the doctor blade. The construction of the adjustable front edge may be varied from the exemplifying embodiment described above. For instance, the overflow edge may be formed by a bar or rod to which perpendicular rods are attached that can be adjusted so as to control the front edge profile. In principle the overflow edge may even be provided by movable elements forming segments, while such a construction carries the risk of coat striping and stepped profile control. As any pressure steps, however, are smoothed out in the application chamber, and moreover, the doctor blade itself levels away steps in the coat profile, also the stepped control of coat profile may in the end give a good final coat quality. Obviously, the invention can be applied in addition to paper web coating, also to coating paperboard and other similar materials.

Claims: A method of controlling the coat profile of a paper or paperboard web to be coated, in which method coating mix is fed in front of a smoothing doctor element (5) in an application chamber (11) bordered at least by the doctor element (5), the web (2) to be coated and a front edge (12) disposed at a distance from the doctor element (5) and the surface of the web (2), and adjusting the distance between said front edge 12 and the surface of the web, characterized in that the coat profile is controlled by varying said distance between said front edge (12) and the web (2) in the cross-machine direction of the web (2). A method as defined in claim 1, characterized in that the profile control of the front edge (12) is complemented with a profile control of the doctor element (5). An assembly for coating a paper or paperboard web, said assembly comprising a backing roll (1) over which the web (2) to be coated is adapted to pass, a doctor element (5) attached to a body (3) of the assembly so as to permit the adjustment of said element close to the backing roll (1) in order to smooth a coat applied to the web, a front edge (12) disposed in front of the doctor element (5) at a distance from the doctor element (5) and the backing roll (1), an application chamber (11) bordered at least by the backing roll (1), the doctor element (5) and the front edge (12), and means (9, 10) for feeding coating mix into the application chamber (11), and means for adjusting the distance between said front edge (12) and the backing roll (1), characterized by means (15, 16, 17, 18) for varying said distance in the cross-machine direction. An assembly as defined in claim 3, characterized in that the front edge (12) comprises a continuous overflow edge (16), isthmuses (18) extending therefrom and thickness-reducing cuts (17) made thereto between said isthmuses (18) and thereafter filled with a resilient material. An assembly as defined in claim 4, characterized by actuator means (15) connected to said front edge isthmuses (18) for the purpose of altering the shape of the overflow edge (16) by pulling or pushing said isthmuses (18). An assembly as defined in any of claims 3 - 5, characterized in that said assembly is a short dwell time applicator (SDTA coater). An assembly as defined in any of claims 3 - 6, characterized in that said doctor element is a doctor blade (5).

Inventors: LINNONMAA JUKKA, PALOVIITA PETRI, LINNONMAA, JUKKA, PALOVIITA, PETRI
Applicants: METSO PAPER INC, METSO PAPER INC.
Application year: 1995
Main classification: D21H23
IPC codes: D21H23, B05C3, B05C11