US2729192A - Doctor blade for paper coating apparatus - Google Patents

Description

Jan. 3, 1956 E. WARNER 2,729,192

DOCTOR BLADE FOR PAPER COATING APPARATUS Filed Dec. 51, 1952 IIIIIIIIIIIIIIIIIIIIII 'IIIIIIIIIII/III entor 311v EDGAR WARNER Gttorneg United States Patent DOCTOR BLADE FOR PAPER COATING APPARATUS 8 Claims. (Cl. 118-417) This invention relates to the manufacture of coated paper for high grade printing and other uses and consti- .tutes an improvement on the method set forth in U. S.

Patent No. 2,229,621 and it further relates to apparatus for practicing the improved method. The invention has a field of especial utility in the application of coating material comprising pigment and adhesive to one or both surfaces of a moving web of paper during the course of its manufacture on the paper making machine, but it is also useful for applying the same or other types of coating to moving webs either on or ofi? of the paper making machine.

The process of the aforesaid patent comprises the application of an excess of fluid coating composition to a paper web, the formation of a layer of filter cake on the surface, and the wiping off of the liquid overlying the filter cake. 'The patented process has been extensively and increasingly used ever since its introduction and has been found to be highly satisfactory. Some inconveniences have, however, been encountered in connection with its usein cases where the paper machine is functioning imperfectly due to causes completely aside from the coating operation. For example, when there happens to beat loose edge or loose edges on the web or when someother portion of the width of the web runs slack for a greater or lesser time and is not corrected by alength equalizing operation as set forth in the aforesaid patent, too great a weight of coating remains on the slack portions and either fouls the drying cylinders or results in excessive coating weights on localized portions of the web, or both. Also, when a break in the paper web occurs before the web reaches the coater, it results in a loose end being drawn over the coating roll but not under tension as required to wipe off the excess, which therefore remains on the loose end and fouls the drying cylinders, which are often difiicult to clean. This can be extremely aggravating at times when trouble with breaking of the web is being encountered on the machine.

I have now found that I can eliminate the aforesaid diiliculties, even though the paper machine operation itself may be quite imperfect, and that I can render the coating operation more uniform and satisfactory by removing the surplus coating. composition and smoothing the coating remaining on the web by means of a doctor or wiping blade which is flexible and provided with a spring support so that it is adapted to yield where the pressure of the web thereon is too strong and to spring out and maintain properpressure on those parts of the web which would otherwise be slack. The Working edge of this doctor is desirably formed of a small diameter cylindrical rod adapted for rotation in its holder. In many cases I find it is of further advantage to provide a yielding resilient carrier for the web, which supports the web where the spring doctor is pressed thereagainst. The support may be in the form of a rubber covered roll or endless belt and may in many cases advantageously be moistened in order to cause the paper web to adhere 2 V thereto so, that it is placed .under additional tension Where it passes under the spring-pressed doctor, as described in U. S. Patent No. 2,534,321.

The invention will now be more fully described with reference to the accompanying drawing in which:

Fig. 1 is a diagrammatic elevational view partly in section of a coating device employing a flexible spring doctor in accordance with the present invention.

Fig. 2 is a similar view, also partly in section showing the modification in which the paper web is backed up ,by a yielding resilient surfaced roll where the coated web is operated on by the spring docto Fig. 3 is a similar view of a modified form of the present invention.

Fig. 4 is a fragmentary sectional detail of the rotating doctor and its holder, drawn to a much larger scale.

Fig. 5 is a fragmentary view illustrating means for' rotating the doctor rod by power applied at each end thereof.

Referring first to Fig. 1, the paper web 11 to be coated, passes over a roll 12 which is rotating in either direction at a suitable speed, dipping into liquid coating material 13 contained in a pan or trough 14. An excess of coating material is thereby carried up and deposited on the web. A small diameter cylindrical doctor rod 20 is held in a correspondingly formed holder 22 by spring pressure of a clip 23, as will be more clearly seen in Fig. 4. Both holder and clip are continuous throughout the working length of rod 20 and are arranged to expose an arcuate portion 25 of the cylindrical surface of rod 20, which serves as the working edge of the doctor. Holder 22 and clip 23 together constitute what is, in the appended claims, called a socket which embraces the doctor rod around substantially more than half of its circumference. The rod 26 and holders 22 and 23 are carried by spring mount 24. This may be made of a single sheet of spring metal or a plurality of'strips mounted side by side, as desired. They should not, however, be widely separated since i have found it to be desirable that the springing action be substantially evenly distributed across the full width of the paper web rather than being applied at the ends or at considerable intervals along the length of rod 29. The spring 24 is of such stiffness and is so formed and mounted that changes in fiexure of spring 24, which result from changes in web tension such as occur during operation, cause relatively small percentage changes in the pressure of rod 20 against the paper web. It is desir-able that the formation and mounting of spring 24 be such that the directions ofmovements of rod 20, resultingfrom changes in fiexure of spring 24, form angles of not more than about 45 deg, or at most 60 deg, with the direction of the resultant of the tensile forces on the web as it bends over rod'20.

If it is also desired to prevent the heavy coatings on broken ends of; the web, the arrangement illustrated in Fig. 2, may be used. This arrangement differs from that in Fig. l in that the p"per web is held against the doctor rod 20 by a roll 3b which is provided with a yielding resilient covering 31, of rubber or the like. In this case the path of the paper may remain as shown in Fig. l,

or it may advantageously be changed, as shown in Fig. 2,

' so that the paper wraps the roll 30 to a material extent before it passes on to the drying operation. In either event, the paper will always be held against doctor rod 20 regardless of whether or not there is any externally applied tension on theweb, and the still liquid excess of coating will be wiped o of the web to the very end, even though the end is not held back, and fouling of the dryers is thereby prevented. The supporting spring 24 for doctor rod 20 may advantageously, in this case, be mounted at a different angle, but as before, the direction of movement of rod due to fiexure of spring 24 should make a large angle with the path of the web. The spring metal portion 24 of the mounting, whether in the form of a single sheet of spring metal or a plurality of strips mounted side by side, as hereinbefore set forth in describing Fig. l of the drawing, and whether bent as shown in Fig. l or straight as described in connection with the alternative embodiments of the invention illustrated in Figs. 2 and 3, is hereinafter, in the appended claims, called a flexible plate-like structure of resilient material.

If the path of the paper web is made such that it wraps roll as shown in Fig. 2, I find it advantageous to moisten the surface of the yielding resilient cover 31 of roll 30 so that the paper web will adhere slightly to the surface as it passes under doctor rod 20. This, as described in Patent No. 2,534,321 serves to provide a local increase in tension on the web where it passes under doctor rod 20, due to the indentation of the surface of the rubber cover 31 on roll 30, and the consequent elongation of the surface in the immediate vicinity of the doctor. This moistening may be accomplished in any desired manner as, for example, by a shower 35 which serves to maintain a pool 36 of water between roll 30 and another roll 37 held thereagainst by pressure which may be adjusted to give the desired degree of moistening.

Another alternative embodiment of the present invention is illustrated in Fig. 3. The doctor rod 20, and its spring mount 24 retain essentially the relationship to the yielding resilient covering 31 on roll 30, and the paper web supported thereby as described in connection with Fig. 2. It differs in its position relative to the vertical so that in this modification the paper is moving downwardly where it passes under the doctor. This makes it possible to eliminate the applying roll 12 and apply the coating material directly to the paper by means of a shower pipe 40. The coating so supplied forms a pool in advance of the doctor, through which pool one surface of the paper passes before reaching the doctor. In this figure I have also shown screws 42 which may, if desired, be provided at intervals along the length of the doctor mount so that the pressure of rod 20 on roll 30 may be locally adjusted if desired. The roll 37 may in this case be conveniently located beneath the roll 30 and the water shower 45 directed into the nip between these two rolls, as illustrated. Surplus water runs down into pan 46 and is carried away by drain 47. Also shown in this embodiment is a doctor 48 which removes surplus water, coating material which runs out beyond the edges of the paper web, and broken paper which might otherwise tend to wrap around the roll. With the device as illustrated, the angle between the doctor blade and the surface of roll 30 is advantageously of the order of magnitude of 45 deg. since if it is much less than this it does not leave room for free circulation and flowback of the liquid coating material doctored ofl? of the web, and if it is much larger than this, the pressure exerted by the roll on the blade is directed too nearly directly towards the support so that the flexure of the spring to compensate a given difference in level of the paper support surface becomes excessive.

The doctor rod 20 has been stated to be adapted to rotate in its holder. While it may be turned manually or mechanically through a part of a revolution at definite or indefinite intervals in order to distribute the wear and prolong its life, it is preferably rotated continuously at slow speed to distribute the wear more uniformly and to carry away lumps of coating or foreign matter which might lodge on a stationary doctor and cause scratches in the surface of the coated web. The speed of rotation is desirably slow and need be only enough to keep the rod in continuous motion. A surface speed of about ft. per minute has been found satisfactory. This rotation is effective for the desired purpose regardless of direction, but it is generally preferred to rotate the rod counter to the direction of movement of the paper thereover.

The diameter of the rod 20 should be determined with reference to a number of factors. In the first place, the smaller the diameter of the rod 20, the greater its flexibility and the greater its ability to follow variations in web tension and conform to irregularities in the backing roll surface, in response to the springing action provided in accordance with the present invention. It should also be recognized that doctors with cylindrical working surfaces tend to draw the coating material into parallel ridges where it parts from the doctor surface. These ridges are finer and less noticeable the smaller the diameter of the cylindrical working surface of the doctor. These ridges are unimportant in the case of coatings which will flow out after being applied. In the case of coatings comprising mineral pigment and adhesive, however, such as are used for high grade printing papers, when spread into the thickness used for that purpose, ridges and other irregularities will not flow out to any appreciable extent. It is consequently necessary, when using such coatings, to avoid the formation of ridges which are Wide and high enough to be objectionable. I have found that doctor rods larger than about inch in diameter ordinarily leave ridges too prominent to be acceptable in high grade printing paper, though larger rods can be used when the smoothness re quirements are less critical. For maximum smoothness of this type of coatings on the surface of the paper, therefore, it is desirable to have the diameter of rod 20, as small as possible. On the other hand, rods much less than about /3 inch in diameter are difficult to enclose and yet leave sufiicient working surface exposed. Further, small rods, especially when the working length is considerable, do not have adequate stiffness or strength to resist the torsional stresses involved in use since, if the rod holders 22, 23 are sufiiciently close-fitting to prevent coating material from being carried around the rod 20 as it rotates, they impose a certain frictional resistance to rotation of the rod. When the diameter of the rod is too small relative to its length, irregularities in the coated surface of the paper are found to be formed due, it is thought, to excessive torsional vibrations in the rod. These, I find, can be reduced and smaller rods used for any given length (i. e. width of paper web) by applying the driving force to both ends of the rod simultaneously. This may be accomplished by any suitable known method as, for example, that illustrated in Fig. 5. In this case driving force is applied to each end of rod 20 by a flexible connection such as a flexible shaft 51 driven as by chain drives 52 from a relatively large diameter cross shaft 53, which synchronizes the drives on opposite ends of rod 20, and is driven as by a suitable motor reducer 54. The chain drives 52 and the fixed ends of flexible shafts 51 are shown as supported in suitable bearings 55. If desired, the cross shaft can be eliminated and the two ends may be separately driven by separate motor reducers. In practice I have found a rod 20 of A inch diameter to operate highly satisfactorily in lengths up to about 15 feet, when driven from both ends. In devices for coating webs of lesser width, rods of diameters down to about /8 inch, or even less, can be used satisfactorily and give excellent results.

These doctor rods should not corrode readily in contact with the coatings used and they should be hard enough to wear for a satisfactory length of time before requiring replacement. For use with the aqueous, mineral pigment coatings such as those used in the manufacture of printing paper, I find tool steel rods plated with about 0.002 inch of chromium to operate without requiring replacement for wear or corrosion, for periods of continuous operation of 300 to 400 hours.

The spring mount 24 should be stiff enough to provide pressures of up to about 5 to 10 pounds per linear inch of rod 20 with reasonable deflection, but should be flexible enough to adjust itself to compensate for the maximum expected variation in web length, or surface level of the backing roll, without significant change in the pressure which it exerts on the web.

For the rubber cover 31 on roll 30, a Pusey and Jones density of from 80 to 120 has been found satisfactory with doctor rods of about inch diameter.

As one example of the practice of the present invention, the cover 31 on backing roll 30 was about inch thick and had a Pusey and Jones density of about 85. Doctor rod 20 was of polished tool steel inch in diameter plated with hard chromium to a thickness of about 0.002 inch on the radius. The spring mount 24 was of 0.035 inch thick spring steel and projected approximately 2 /2 inches from its support. It was pressed against the surface of the backing roll with a pressure of 6 to 7 pounds per linear inch and was rotated continuously in a direction counter to the direction of movement of the paper web, at a speed of about 10 revolutions per minute, by means of a geared motor reducer on each end. The roll 30 was about 174 inches in length while the length of the doctor rod holders 22 and 23 and spring mount 24 were slightly longer, and the working length of rod 20 was slightly less. The rod, however, projected beyond the ends of the holders 22, 23, and was connected by flexible connectors to the driving means. In this case the parts were arranged and the paper threaded through the device as illustrated in Fig. 3. The coating was an aqueous composition composed primarily of calcium carbonate, coating clay, and casein adhesive dispersed in water and containing about 47 per cent total solids. The paper web was about 156 inches in width and was composed primarily of sized bleached sulfate pulp, and passed directly from the first dryer section of the paper making machine, at a speed of about eight hundred feet per minute, over roll 30 of the coating device which applied the coating to the wire side of the web. From the coater the web passed over another section of dryers and through another coating device of the same kind which applied a second coat to the same side of the web. The web then passed over further dryers until the drying was completed. The coated paper was then calendered. The resulting product had a coated weight of about 60 pounds per ream 500 sheets 25 x 38 inches) while the coating weight was about pounds per ream. It was a high quality of coated-one-side paper with a highly uniform coated surface, free from visible ridges and blemishes, and suitable for receiving high grade printing.

The flexible blade with the working edge formed by a rotating rod has been found to compensate for the normally encountered variations in length of a paper web when operating on an unbacked web as illustrated in Fig. 1. When operating on a web backed by a rubber covered roll as illustrated in Figs. 2 and 3, it has been found to compensate for dents in the roll surface such as are sometimes caused when crumpled scraps of a broken paper web pass between rolls under pressure, or flats formed when a rubber covered roll is carelessly allowed to stand resting on its surface rather than fully supported on its journal-bearings. When the paper passing over such a blade is backed up by a roll as illustrated in Figs. 2 and 3, the present improvement not only compensates for irregularities in length of a paper web, but also completely prevents carrying through of excessive amounts of wet coating on loose ends of a paper web, and completely prevents the former troublesome fouling of the dryers.

If it is desired to apply the exact amount of coating to the paper web by a printing operation, instead of applying an excess to the web and doctoring off the surplus as described, the apparatus illustrated in Fig. 3 can be used by eliminating the water shower 40 and passing the paper web from left to right between rolls 30 and 37, instead of passing it around roll 30 and under doctor rod as shown. The doctor then spreads a uniform film over the surface of the rubber cover 31 on roll 30 and this film of coating is then transferred to the surface of the paper web by the pressure in the nip between rolls 30 and37. Because of the impervious surface of the rubber cover 31, no filter cake is formed on its surface and the film of coating left on it by the doctor is consequently thinner than that left by the same doctor on the surface of the paper web. I havefound its possible, however, by using a wire wound or threaded doctor rod, to leave as heavy a coating as desired on the roll 30 for transfer to the paper web. The number of threads required .is determined by the thickness of coating layer desired. Coatings of a thickness commonly used on printing paper can be applied by using rods having 30 to threads, of approximately Whitworth form, per inch. I have found it possible by this means to successfully coat paper by the printing method with a simpler type of apparatus than that which has heretofore been considered necessary for this purpose.

I claim:

1. A device for making coated printing paper wherein doctoring means is provided for smoothing and limiting the thickness of the coating, which doctoring means comprises: a flexible cylindrical doctor rod; a flexible socket in which said rod is rotatably mounted, which socket embraces said rod around more than half of its circumference and is continuous throughout the working length of the rod; a spring mounting of uniform stiffness throughout the entire working length of said doctor rod, said mounting comprising a flexible plate-like structure of resilient material, to one edge of which plate-like structure said flexible socket is fixedly attached throughout its length; and a rigid support on which the other edge of said plate-like structure is fixedly mounted throughout its length, whereby the position of the doctor rod and the corresponding degree of flexure of said plate-like structure, at any time, depend upon the degree of pressure against said doctor rod at that time.

2. A device as defined in claim 1 wherein the path of the paper web is changed materially in direction at the doctor rod, by contact therewith, whereby the position of the doctor rod is determined by the equilibrium of the forces of tension in the paper web and the forces of resilience in the rod and its flexible socket and platelike mounting structure.

3. A device as defined in claim 1 wherein a roll with a yielding resilient covering is mounted adjacent the doctor rod in a position to support the paper web against pressure exerted thereon by said doctor rod.

4. A device as definied in claim 1 wherein the platelike structure is secured to its support in such proximity and angular relationship to the surface on which coating is being doctored that it is adapted to constrain movements of the doctor rod, from its normal operating position, to take place at an angle of not more than about 45 degrees with the normal to the surface on which coating is being doctored at the midpoint of the contact of said surface with the doctor rod.

5. A device as defined in claim 1 wherein the platelike structure is fixed to its support in such proximity and angular relationship to the surface on which the coating is being doctored, and possesses such degree of flexibility that, when the doctor is in operating position against the surface on which the coating is being doctored, the platelike structure is flexed to a degree which provides doctoring pressure between said rod and said surface and, during normal operation, maintains the doctor rod in operative position against said surface with only small percentage changes in the degree of fiexure and in the resulting pressure between said rod and said surface.

6. A device as defined in claim 3 wherein the doctor rod is between one-eighth and three-eighths inch in diameter and is pressed against the roll with a pressure of between five and ten pounds per linear inch.

7. A device as defined in claim 1 wherein means is provided for continuously rotating the doctor rod in a direction opposite to the direction of movement, beneath References Cited in the file of this patent UNITED STATES PATENTS Potdevin Aug. 7, 1934 Grant Nov. 15, 1938 Kauppi et a1. Nov. 9, 1943 Trist Jan. 30, 1945 Taylor Dec. 19, 1950 Warner June 3, 1952

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