US1951710A - Process and apparatus for drying fibrous sheets - Google Patents

Description

March 20, 1934. A. w. SCHORGER 1,951,710

PROCESS AND APPARATUS FOR DRYING FIBROUS SHEETS Filed NOV. 12, 1931 2 Sheets-Sheet l ATTORNEYS March 20, 1934. w sc o 1,951,710

PROCESS AND APPARATUS FOR DRYING FIBROUS SHEETS ATTORNEYS Patented Mar. 20, 1934 UNITED STATES PATENT OFFICE PROCESS AND APPARATUS FOR DRYING FIBROUS SHEETS Application November '12, 1931, Serial No. 574,547

11 Claims.

This invention relates to the manufacture of paper in the form of continuous webs or discontinuous sheets or boards, usually of considerable thickness. This invention has to do with an improved process for the manufacture of such papers as stencil-paper, beaming paper, bristolboard, and more particularly stereotype dry mats of the type which are produced from a mixture of cellulose fibers and a filler such as clay, as described in Schorger patents, Nos. 1,524,155 granted January 27, 1925 and 1,720,243 granted July 9, 1929.

Briefly, in the manufacture of ordinary paper in continuous web form, a slurry of wood or cellulose pulp fibers is first formed and a web deposited therefrom upon a moving screen. The web is usually transferred to a felt and, while supported upon the felt, is passed between press rolls to remove a portion of the moisture. The web is then transferred to a suitable dryer.

In the manufacture of special products, such as stereotype mats it may be advantageous not to make a continuous sheet. A web is formed in the usual way upon a screen from a slurry comprising a mixture of pulp fibers and filler. The web is next transferred to a felt, and, while supported upon the felt, it is passed through a pair of cooperating press rolls, the web accumulating on one of the press rolls until it is of the desired thickness. This sheet is then stripped from the press roll in limited lengths and transferred to a suitable dryer.

This invention is concerned with the drying operation and has for its primary object the provision of apparatus and method for drying continuous paper webs or discontinuous paper sheets of varying thickness and simultaneously imparting ultra-smooth surfaces to such webs and sheets and without having any stray fibers projecting from or loosely held at the surface.

This invention has for another object the provision of apparatus for and method of drying relatively thick pulp sheets into boards or mats having increased surface smoothness and more uniform thickness throughout their area.

It is also an object of my invention to provide a method and apparatus whereby such increased smoothness and uniform thickness of the mats, boards or continuous webs are obtained without introducing additional operations into the drying process.

It is a still further object of my invention to provide an improved method and apparatus for obtaining the desired density and uniform texture in fibrous sheet products.

Other and further objects will become apparent as the following description progresses, when read in conjunction with the accompanying drawings, in which:

Fig. 1 illustrates in diagrammatic form, my in- 80 vention as used in combination with a single drum dryer;

Fig. 2 similarly illustrates the application of my invention to a multiple drum dryer such as is commonly used in the paper-making art; and

Fig. 3 is a broken side view of the single drum dryer showing the details of mechanism for automatically lowering the press roll as the dryer drum revolves.

The drying drum 10 of Fig. 1 is caused to revolve in the direction indicated by the arrow by driving gear 11. Drum 10 may be heated by steam or by other convenient means and possesses a smooth and preferably polished surface. Felt 12 is in contact with drum 10 throughout a considerable portion of its circumference and is maintained in position by guide rolls 13. Motion may be imparted to felt 12 by its frictional engagement with drum 10, but in order to prevent possible slippage when a sufficient thickness of so paper stock is passed, between the drum and felt, it is preferably separately driven by roll 15 which is driven by gear 14.

In one form of my invention press roll 16 is rotatably supported upon bearings 17 at one end of Bi lever 18, which is pivotally supported by the dryer framework at 19. Press roll 16 may be driven by the surface engagement therewith of the paper stock which is being carried by the heated drum, but it is preferable to provide a separate positive drive, as by means of gear 20, so as to prevent any tendency toward slippage and injury to the uniformity of the paper texture and surface. The end of lever 18 remote from press roll 16 is held firmly in fixed position as is explained hereinafter. Rod 21 penetrates the end portion of lever 18. Rod 21 is threaded at its top end portion to receive nuts 22 and also near its lower end to receive nuts 23. Lever 18 is maintained in position, firmly pressed against nuts 22 by means of spring 24 which is supported by nuts 23. Rod 21 is linked at 25 to hand lever 26 which is fulcrumed at 2'1. By moving lever 26 in the direction indicated by the arrow, press roll 16 is moved away from heated drum 10. Lever 26 may be held in position by a disengageable latch as indicated at 28.

Scraper or doctor 29 extends over the entire width of drum 10 and is supported upon rod or roller 30 which is rotatably supported at 31 upon 11o bracket 32 upon the dryer frame. Crank 33 is connected to rod by means of belt or chain 34. Movement of the crank in the direction indicated by the arrow causes the edge of doctor 29 to move away from the surface of drum 10.

In operation, a length of the wet pulp sheet, as it is received from the press roll of the wet machine, is placed upon felt 12 at point indicated by the arrow. The sheet passes between the felt and heated drum 10 and continues to be so carried during a number of revolutions of the drum until the sheet is practically dry. During this time the doctor 29 is raised from the drum 10 sufficiently to prevent contact with the sheet. If the drying' operation is performed without the use of a press roll, there is a tendency for the sheet, due to uneven drying and the formation of steam pockets, to become warped and wrinkled. Furthermore, any roughness and imprints which the sheet has received upon the wet machine are retained and the surface of the finished sheet is rough and irregular and the surface which is in contact with the felt bears an impression of the weave of the felt.

As the pulp sheet comes from the press roll of the wet machine its moisture content amounts to.aproximately to percent of the total weight thereof and the sheet is said to be tender.

In my improved method of drying the pulp sheets, the roll 16 is lifted at the beginning of the drying operation in the manner heretofore described, so that it is out of contact with the tender wet sheet until its moisture content has decreased enough so that the sheetis strengthened sufficiently to withstand the mechanical action of the press roll. Considerable latitude is allowable in this respect but I aim to bring the press roll into operation while the moisture content is between 45 percent and 65 percent. The lowering of the press roll should not be delayed until after the moisture content has decreased to less than about 35 percent or the benefits will be lost. The sheet is very plastic as it is received from the wet machine and roll pressing during this plastic stage produces the most benefit. As the sheet becomes more dry it gradually loses this plasticity and when the moisture content decreases to about 35 percent it changes from the plastic condition to a more or less set condition.

' This moisture content varies with pulps of different character. After the press roll has been brought into contact with the moist sheet it is.

progressively and gradually lowered as the sheet continues to become more dry, thereby subjecting the sheet to constantly increasing pressure. To do this lever 26 is operated to move the surfaces of roll 16 and drum 10 together slowly, bringing them closer together between each passage of the sheet until lever 26 is secured by latch 28 in its final position. Lever 26 may be moved manually to impart the gradual movement to roll 16 or provision may be made to accomplish the operation automatically. Adjustment for purposes of controlling the thickness of the mats or boards should be made by screwing nuts 22 up or down upon rod 21. Spring 24 is sufficiently stiff so that, under normal conditions, it determines positively and accurately the thickness of the mat or board but at the same time provides the necessary resilience to permit the passage 'of hard foreign objects or accumulated pulp between roll 16 and drum 10 with little injury to their surfaces or without causing failure of any part of the machine. In the methods above described pressure is the agency which operates upon the sheet and as the press roll is progressively lowered it is the progressive increase; of pressure which gradually compresses the sheet to its final dimensions. This method of gradually increasing pressure while moisture content is being decreased is one of the features-of my invention.

The method of operation of my dryer may be changed somewhat from the method which is described heretofore. stiff as to positively and accurately determine the thickness of the mat or board and make the latter equal to the final spacing between the surfaces of roll 16 and drum 10 as fixed by the roll spacing mechanism. In the modified method the press roll may be brought into contact with the moist sheet and the pressure may be increased progressively as the sheet becomes progressively more dry by elevating rod '21 in progressive increments in the same manner as is described heretof ire. After roll 16 has been lowered enough to make contact with the mat as it passes thereunder the roll compresses the mat. At first the mat is very plastic and little or no resistance is offered to the press roll. The sheet becomes decreasingly plastic until its moisture content reaches about 35 percent when it passes from a plastic condition to a more or less set condition after which excessive pressure tends to crush the structure. As rod 21 is elevated periodically, or with each revolution of drum 10, the sheet continues to be increasingly compressed. Gradually the resistance to compression of the sheet increases until it is suflicient to partially overcome the resistance to compression of spring 24. As rod 21 continues to be elevated, spring 24 is increasingly compressed with the result that increasing pressure is exerted upon the sheet. When the desired maximum pressure is reached rod 21 is'arranged to remain stationary and the sheet is thereafter dried under constant pressure until it is dry. In this latter mode of operation rod 21 may be elevated at such a rate that before the desired maximum pressure is reached, the press roll would normally, that is, in the absence of the sheet, press against the surface of drum 10.

In some respects'this latter method of operation is preferred over the first described method since it is more advantageous to operate under less rigid spring conditions and roll spacing can be used to positively determine thickness only for limited thickness regulation. The greater flexibility results in reduced wear and damage to surfaces and mechanism caused by the presence of foreign hard objects and the wadding up of pulp which occasionally occurs.

In a further variation of the method, immediately upon the moist sheet having acquired strength to Withstand mechanical injury by the press roll, the latter is moved downwardly to a position such that it is in contact with the dryer drum. In such method the initial spring stiffness or compression should be considerably less than in the previously described methods and the increments by which the spring stiffness or compression is increased may be greater to result in a final pressure substantially equal to that produced in the previously described methods.

It has been stated heretofore that the progressive lowering of the press roll as the dryer drum revolves may be accomplished automatically. Mechanism for such automatic operation is illustrated in Fig. 3. Press roll 41 is mounted upon one end of lever 42 which is supported upon the dryer framework at 43. The other end of lever Spring 24 need not be so 42 is threaded by rod 44 and is held in position between spring 45 and nut 46. The lower end of rod 44 is positioned laterally by suitable socket- 4'1 and is supported upon cam 48. The lever is of sufficient weight to more than counterbalance theweight of press roll 41. Cam 48 and hand lever 51 are mounted in fixed relation upon shaft 49 which is rotatably mounted upon the framework 56 of the dryer. may be toothed throughout its entire periphery or preferably upon only a portion thereof, as shown in Fig. 3, may also be mounted in fixed relation upon shaft 49. Lever 52 is mounted in freely movable relation upon shaft 49. Preferably, the angular relation of cam 48 or ratchet wheel with shaft 49 is made adjustable for reasons which will be pointed out hereinafter. Pawl 53 is mounted upon lever 52 and is adapted to engage the teeth 54 of ratchet wheel 50 so that as pawl 53 moves toward the left in Fig. 3 it transmits clockwise motion to ratchet wheel 50 while when it moves to the right it merely slides over teeth 54. A further ratchet wheel 62 is mounted fixedly upon shaft 49 rearwardly of ratchet wheel 50. Ratchet wheel 62 may be toothed throughout its entire periphery.

Pawl 55 is mounted in a stationary manner upon frame 56 of the dryer and is adapted to engage the teeth of ratchet wheel 62 in the same manner that pawl 58 engages the teeth of ratchet wheel 50. Link 57 is attached at one end to the end of lever 52 and at the other end to crank 58 which is mounted eccentrically upon the shaft 59 of dryer drum 60.

The operation of the mechanism is as follows: Rotary motion of drum 60 and shaft 59 transmits a reciprocating motion to link 5'? and also to lever 52 and pawl 53. With each movement to the left pawl 53 moves ratchet wheel 50 a distance equal to the distance between one or more teeth 54 and during each movement of pawl 53 to the right ratchet wheel 50 remains stationary. Ratchet wheel 50 transmits its motion to shaft 49 and thence to cam 48 which is adapted to vary the elevation of rod 44. As stated heretofore ratchet wheel 50 may be toothed upon its entire periphery but preferably it is toothed upon only a portion of its periphery and cam 48 is so arranged that as pawl 53 travels from the left end of the toothed portion to the right end, as shown in Fig. 3, the lower end of rod 44 is elevated gradually by means of cam 43. Such elevation of rod 44 may be arranged to correspond with the lowering of press roll 41 from a point where it is normally out of contact with a wet pulp sheet to a point where it exerts pressure upon the surface of drum 60. These points do not represent the lowest and highest points upon cam 48 as will be pointed out hereinafter.

When a fresh wet sheet is to be placedupon drum 60 hand lever 51 may be turned clockwise until a sufficiently low point upon cam 48 is reached that there is no danger of press roll 41 contacting with the wet sheet. Usually several sheets are placed end to end upon the drum and since they are placed while the machine is in operation, overlapping occasionally occurs at the ends, in which case the machine must be stopped and the sheets rearranged. When the sheets are in proper place, lever 51 is again moved in clockwise direction until the desired point upon ratchet wheel 50 is in engagement with pawl 53, moving the press roll closer to the surface of the sheets; This point is chosen from experience to result in press roll 41 coming into contact with Ratchet wheel 50, which the moist sheet at the time it becomes dry enough and strong enough to withstand mechanical injury by the press roll. Then pawl 53 engages teeth 54 and the gradual lowering of press roll 41 continues simultaneously with the further drying of the sheet. During the time that pawl'53 is moving backward to the right over teeth 54 pawl 55 engages the teeth on ratchet wheel 62 and prevents the backward movement of the ratchet wheels and cam 48 which would otherwise take place. The movement of ratchet wheel 50 is so arranged that pawl 53 has reached tooth 61 by the time the maximum pressure has been reached to which it is desired to subject the sheet. Thereafter, it is desired that no further lowering of press roll 41 take place or no increased pressure be exerted upon it. Such a condition is, in fact, realized since pawl 53 merely slides back and forth upon the smooth surface of ratchet wheel 50 adjacent to tooth 61 while pawl 55 holds cam 48 in place. It can be seen that if ratchet wheel 50 were toothed upon its entire periphery cam 48 would have to be so shaped that no further elevation of rod 44 took place after this point was reached. This is not the preferred arrangement since, with sheets of different character it may be desirable to use a higher maximum pressure. The cam is arranged therefore to vary in eccentricity throughout its entire periphery to the limits it is considered it will ever be desired to use and the angular relation of the cam and ratchet wheel 50 so correlated as to give the proper elevation of rod 44 during movement of ratchet wheel 50 from one end of teeth 54 to the other. he maintained until the sheet is substantially dry when lever 51 is again moved to a position such that press roll 41 is removed out of contact with the sheet and back to its initial position, as explained heretofore.

I am aware that smooth surfaced sheets have I been made by subjecting dried or nearly dried sheets to heavy pressure but such method does not produce the structure desired by me. If the heavy pressure is applied at the beginning of moment at which the sheet will Withstand roll j contact until the sheet has passed the plastic stage, that is, until the moisture content has decreased to about 35 percent. Under such conditions, the result is an ultra-smooth mat or board, extremely uniform in texture, density and thickness. While I continue to maintain the press roll in operation until the drying operation is complete, this is not necessary. The pressing operation may be discontinued after the greater portion of the moisture has been driven out of it, say after the moisture content has been reduced to 10 percent or before this time, since the board, after it has passed the plastic stage, is more or less firm and set and only minor benefits are obtained from further roll-pressing. I attribute much of the benefit derived to the fact i The pressure may If the heavy pressure is applied too that both the heating drum and press roll have smooth, substantially non-flexible surfaces. The heating drum is preferably of polished metal and the press roll may be of the same material or. may be of hard rubber or any other similar smooth, non-flexible material. If but only one side of the sheet needs to be smooth, then only one of the surfaces need be polished. Any entrained steam, either within the sheet, or between the sheet and drying drum 10, is pressed out by roll 16 and the fibers and filler-are returned to their normal uniform, compact texture and the smoothness is retained which is imparted by the polished surface while the sheet is plastic.

After the board is dry which is considered to be the case after the moisture content has been reduced to '7 percent or less, the press roll is moved out of contact with the board and doctor 29' is moved into contact with drum 10. When the on-coming edge of the board reaches the doctor it is scraped free of the drum. It is deflected upon felt 12 at about point 36 as indicated by the arrow from which it may be removed when it reaches roll 15.

Thus far, I have described my invention in connection with the production of mats or boards from discontinuous wet pulp sheets. Although it is particularly advantageous in that connection, it isby no means limited thereto but may be employed in the production of a continuous pulp board or of a paper comprising a continuous single web of paper stock. As pointed out heretofore, the compressing feature may be adapted readily to multiple cylinder dryers and it is, therefore, apparent that it may be applied to pulp products in continuous web form.

Single drum dryers, such as illustrated in Fig. 1 and described heretofore, are also adapted for the drying of paper in continuous sheet or web form. The drum is sufficiently large so that if it revolves slowly enough the web may be dried in one revolution. By making a few minor c' anges the dryer illustrated in Fig. 1 could be adapted to the drying of paper products in continuous sheet or web form. In a dryer of this type, it is preferable to have several press rolls cooperating with the drying cylinder at progressive points along its circumference.

However, the more common practice is to employ a multiple drum dryer as illustrated in Fig. 2. The continuous web or sheet 37, indicated by dotted lines, enters the dryer at the right side of Fig. 2 and travels around dryer drums 38, progressing to the left until it leaves the last drum in a suitably dry condition. Felt 39 maintains the sheet in the desired close contact with the lower dryer drums. Press rolls 40 may be arranged to cooperate with the upper dryer drums in much the same manner as is illustrated in Fig. 1. They may be spaced decreasing distances from the dryer drums in the direction of travel of the sheet, or the successive rolls may be otherwise arranged to exert progressively in creasing pressure upon the sheet as it becomes more and more dry. Certain of the press rolls, as, for instance, those at the ends, may be omitted for the purpose of confining the pressing operation to the period while the moisture content of the paper is that necessary to secure the best results. 7

My method results in a paper product having ultra-smooth surfaces and a uniform density and texture. It is superior to the ordinary calendering because the interlacing of the fibers, which takes place while they are felted, is not destroyed 'strength of the paper.

but is enhanced since the compression takes place while the fibers are hot and moist. In dry calendering the compression tears the fibers apart and loosens them, thus lessening the cohesive High cohesive strength is highly desirable in stereotype mats. By my method those surface fibers which normally tend to protrude are pressed back into and firmly held by the body of the paper, while in drycalendering they are merely flattened down upon but not firmly imbedded in the surface. In stereotype mats when the hot type metal is applied to the dry calendered mat, these flattened surface fibers rise and are caught by the solidifying metal. As a result, when the metal is released, the fibers which are anchored in both the metal and mat tear loose a portion of the surface of the mat, thereby injuring the surface and preventing further casts.

The mechanical details of the apparatus illustrated are not of consequence and I do not wish to be limited in this respect as such limitations are specifically incorporated in the appended claims.

I claim:

1. The method of producing a relatively thick smooth-surfaced paper sheet of uniform predetermined thickness from a wet pulp sheet which comprises initially drying said sheet sufficiently to prevent'mechanical injury to said sheet, further drying said sheet in non-adhering contact with a heated smooth surface and simultaneously intermittently roll-pressing said hot moist sheet. said roll pressure being applied at constantly decreasing roll spacing to the predetermined thickness, the predetermined thickness being reached by the time the moisture has decreased to substantially 35 percent.

2. The method of producing an ultra-smoothsurfaced relatively thick stereotype mat from a wet pulp sheet which comprises initially drying said sheet sufficiently to prevent mechanical injury to said sheet, further drying said sheet in non-adhering contact with a smooth heated drum and simultaneously roll-pressing said hot moist sheet, said roll pressure being applied at constantly decreasing roll spacing to the predetermined thickness as the moisture content decreases to between 35 percent and 45 percent, and further drying said sheet to the desired moisture content and simultaneously, intermittently rollpressing said hot sheet with the roll spacing at the predetermined thickness.

3. The method of producing a relatively thick smooth surfaced paper sheet of uniform predetermined thickness from a wet pulp sheet which comprises drying the sheet on a heated smooth surfaced drum to a moisture content between 45 and 65 percent, further drying said sheet in nonadhering contact with said drum and intermittently roll pressing said hot moist sheet, said roll pressure being applied at constantly decreasing roll spacing to the predetermined thickness as the moisture content decreases to between 35 and 45 percent, and further drying said sheet to the desired moisture content while simultaneously intermittently roll pressing said hot sheet with a roll spacing at the predetermined thickness.

4. The method of producing a relatively thick smooth-surfaced paper sheet of uniform'thickness from a wet pulp sheet of known thickness and consistency which comprises initially drying said pulp sheet sufficiently to prevent mechanical injury thereto, further drying said sheet in nonadhering contact with a heated smooth surface and simultaneously intermittently roll pressing said hot moist sheet as the moisture content thereof decreases, said roll pressure being constantly increased by regular increments while said sheet is in the plastic stage.

5. The method of producing a relatively thick smooth-surfaced stereotype mat from a wet pulp sheet which comprises initially drying said sheet sufficiently to prevent mechanical injury thereto, further drying said sheet in non-adhering contact with a heated smooth surface and simultaneously roll-pressing said hot moist sheet, said roll pressing being applied at constantly increasing roll pressure as the moisture content of said sheet decreases to substantially the point below which said sheet ceases to be plastic, and further drying said sheet to the desired moisture content and simultaneously roll-pressing said hot sheet.

6. The method of producing a relatively thick smooth-surfaced paper sheet of uniform density from a wet pulp sheet which comprises drying the sheet on a heated drum to a moisture content of between 45 and 65 percent, further drying said sheet in non-adhering contact with a heated smooth-surfaced drum and intermittently rollpressing said hot moist sheet, said roll pressing being applied at constantly increasing roll pressure as the moisture content of said sheet decreases below the point at which said sheet ceases to be plastic, and further drying said sheet to the desired moisture content while simultaneously roll-pressing said hot sheet with constant roll pressure.

7. In means for making a mat or board from a thick wet pulp sheet, the combination of means comprising a revolving heated drum for drying said sheet, means comprising a press roll cooperating with said heated drum for compressing said sheet therebetween, and means operatively connected with said revolving drum for progressively decreasing the spacing between said heated drum and said press roll as said heated drum revolves, said means including a compression member for progressively increasing the pressure upon said press roll when said press roll comes into contact with said sheet or when said decrease of roll spacing is otherwise reduced or prevented.

8. In means for making a mat or board from a thick wet pulp sheet, the combination of means comprising a revolving heated drum for drying said sheet, means comprising a press roll cooperating with said heated drum for compressing said sheet therebetween, and means operatively connected with said revolving drum for moving said press roll into contact with said sheet after the moisture content of said sheet has been reduced a predetermined amount, for progressively increasing the roll pressure upon said sheet as the moisture content thereof is further reduced to a predetermined lower proportion, and for maintaining said pressure constant as said sheet is further dried to a predetermined minimum or final moisture content.

9. The method of producing a smooth-surfaced stereotype mat from a wet pulp sheet which comprises applying said sheet in non-adhering contact with a smooth-surfaced heated drum, revolving said drum through a number of revolutions while said sheet is becoming dry, and simultaneously roll-pressing said sheet at each revolution of said drum, and progressively increasing said roll pressure as said sheet progressively becomes more dry.

10. The method of producing a relatively thick paper sheet of uniform predetermined thickness and having a highly smooth surface from a wet pulp sheet which comprises initially drying said sheet sufiiciently to prevent mechanical injury to said sheet, further drying said sheet with said surface in non-adhering contact with the heated smooth surface and simultaneously intermittently roll-pressing said hot moist sheet against said heated surface while said sheet contains sufiicient moisture to permit the plastic working of the fibers by said heated surface to greatly increase the smoothness of said surface of said sheet, said roll pressure being applied at constantly decreasing roll spacing to the predetermined thickness.

11. The method of producing a relatively thick smooth-surfaced stereotype mat from a wet pulp sheet which comprises initially drying said sheet sufiiciently to prevent mechanical injury thereto, and further drying said sheet in non-adhering contact with a heated smooth surface and simultaneously roll-pressing said hot moist sheet, said roll pressing being applied at constantlyincreasing roll pressure as the moisture content of said sheet decreases to substantially the point below which said sheet ceases to be plastic.

ARLIE W. SCHORGER.

III

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