US 3011913 A
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Dec. 5, 1961 F. PIERCE ETAL METHOD OF COATING PAPERBOARD CARTON BLANKS 2 Sheets-Sheet 1 Filed Sept. 3. 1959 f/GL/ INVENTORS I LAWSON E PIERCE DAV/D G. EDMMRDS jy Q W ATTORNEYS United States are] 3,011,913 METHOD OF COATING PAPERBOARD CARTON BLAWKS Lawson F. Pierce and David G. Edwards, Concord, (Ialifi, assignors to Fibreboard Paper Products Corporation,
San Francisco, Calif., a corporation of Delaware Filed Sept. 3, 1959, Ser. No. 837,)32 3 Claims. (Cl. 117111) This invention relates to the application of wax coatings to paperboard carton blanks, and most particularly to the coating of paperboard blanks with wax by means of an applicator roll whose surface travels in a linear direction opposite to the direction of travel of the blanks. This application is a continuation-in-part of application Serial No. 577,784, filed April 12, 1956, now abandoned.
Paperboard carton blanks are commonly coated with wax for decorative and protective purposes. One of the most widely used methods of applying such coatings is the well known kiss roll coating method. In this method the wax applicator rolls are conventionally rotated so that they travel in the same direction at the nip between the rolls in order to drive the sheet material through the However, the conventional kiss roll coating method has several disadvantages. When the amount of wax ap plied to the applicator rolls is sufiicient to provide a desirably thick coating on the paperboard blank, ridges of the wax form on the rolls, and ridges and valleys also are formed in the coating that is applied to the paperboard. These ridges limit the effectiveness and appearance of the Wax coating, and also result in a waste of the wax since the protection provided by the coating is only as effective as the thin valleys between the ridges. Furthermore, with both applicator rolls traveling in the direction of the paperboard blank at the nip between the rolls, the molten wax tends to break away from the ridges on the rolls and to splatter onto the paperboard blank which has just passed between the applicator rolls. The resultant spots on the wax coating are undesirable from the standpoint of appearance and occasionally for functional reasons. Consequently, it is necessary to maintain the loadings of wax low enough to avoid the splattering problem.
Summarizing this invention, a smooth wax coating is applied to individual flexible, paperboard carton blanks by conveying the flexible blanks individually in succession to the nip between a pair of rotatable rolls, the first of which is a drive roll that has a higher coefficient of friction than the second roll, causing the drive roll to rotate in a direction to propel the blanks through the nip by frictional contact with the blanks, causing the second roll to rotate so that its surface at the nip travels in a linear direction opposite to the travel of the blanks propelled by the drive roll, the second roll being also called a reverse roll, forming a meter film of molten wax on the reverse roll, and transferring the film of molten wax from the reverse roll onto the blanks during propulsion of the blanks through the nip by the drive roll, the blanks being conveyed up to the nip between the rolls at a linear speed at least about five percent faster than the linear speed of the surface of the drive roll. The reverse roll may be placed either above, to the side of, or below the drive roll without afiecting the quality of the coating.
The second or in other words reverse roll applies a much smoother and superior coating when it is driven so that its surface has a linear speed in the ratio of 4:3 to 5:1 relative to the linear speed of the drive roll. This becomes particularly important in the application of continuous coatings at low wax loads below 2 pounds per thousand square feet of coated blanks. Thin coatings ice of wax below about 2 pounds per thousand square feet are generally not truly smooth and continuous when applied by the conventional kiss roll method, whereas a greatly improved coating is obtained by the method hereof.
Also, substantially thicker and smoother wax coatings may be applied by the reverse roll hereof without encountering a splattering problem than by the conventional kiss roll method, and this method is particularly important for the application of such thick coatings. The resultant smooth, thick coating provides increased protection and a pleasing smooth appearance. Heretofore, it has not been appreciated that flexible paperboard carton blanks could be driven evenly through wax applicator nip rolls when one roll was'turning contrary to the motion of the sheet. It has therefore been considered impossible to achieve a uniform, continuous wax coating on flexible paperboard blanks by using the reverse roll coating method hereof. It is clear that the problems encountered in controlling variables, such as the speed of travel of the paperboard blanks, present an entirely different problem in the reverse roll method hereof in which the individual, flexible sheets are driven by the applicator rolls, than in the case of elongated sheet material or web that is pulled through independently of the speed or direction of travel of the applicator rolls. This invention provides a method for producing a smooth coating on individual blanks that cannot be pulled through the applicator rolls by means external to the applicator rolls. Also, the problems in coating flexible, paperboard carton blanks with wax are quite unlike the coating of large rigid materials with other coatings.
In the drawings:
FIG. 1 is a more or less schematic vertical longitudinal section of apparatus for applying a smooth coating of wax to paperboard carton blanks by the method of this invention showing the apparatus as it would appear after removal of a side plate of the framework for the apparatus.
FIG. 2 is a side elevation of the apparatus illustrating driving means for the apparatus.
FIG. 3 is a vertical section taken in a plane indicated by line 33 in FIG. 2 showing the end of the applicator rolls from which the waxed paperboard emerges.
FIG. 4 is a vertical section taken in a plane indicated by line 4-4 in FIG. 2 illustrating the end of the wax applicator rolls which the unwaxed paperboard enters.
FIG. 5 is a plan view of the wax application portion of the apparatus.
FIG. 6 is a schematic fragmentary sectional elevation of a modification of this invention by which sheet material previously coated on one side may have a heavy coating applied to the other side without injury to the coating already applied. 7
in greater detail the coating apparatus is mounted on a framework of legs 2 which support sides 3. Uncoated paperboard carton blanks 4 are stacked edgewise and inclined relative to support table 6 which is located between sides 3. The usual dam or choke member 7 is provided adjacentthe discharge end of table 6 to insure that only the single outermost sheet or blank is fed at a time. A continuously rotating rubber covered feed roll 8 driven in a direction indicated by the arrow in FIG. 1 by driving means, not shown, eflects feeding of the carton blanks 4 onto a continuously moving endless conveyor 9 mounted on conveyor rolls 11/ Retainer belt 12 mounted on rolls 13 maintains the blanks 4 on the conveyor surface. Both endless conveyor 9 and retainer belt 12 are maintained in position by idler rollers 14. Blanks 4 are conveyed by conveyor 9 to rotating rollers 16, and maintained thereon by freely journalled retaining rollers 17. The various rolls and rollers are journalled in bearings 18 on sides 3 of the apparatus.
Rollers 16 and 17 then convey the blanks to wax applicator rolls 19 and 21. Drive roll 19 is the uppermost roll, and it is advantageously covered with rubber or any material that under operating conditions during application of the wax has a coelficient of friction with respect to the paperboard blanks 4 that is greater than the coefiicient of friction of the lower reverse roll 21 so that the paperboard will be driven between the rolls. Reverse roll 21 is covered with material such as polished stainless steel or chromium plate that has a lower coefficient of friction than the drive roll. I he arrows in FIG. 1 indicate the direction in which the ap plicator rolls are driven. Drive roll 19 travels in the same direction as the carton blank 4 at the nip22 between the rolls. On the other hand, reverse roll is driven in a direction contrary to the direction of travel of the paperboard between the rolls.
Wax is applied to the applicator rolls 19 and 21 by means of manifolds 23 which contain a plurality of apertures for distributing the wax as best illustrated in FIG. 4. Conventional doctor rolls 24 meter the amount of wax that is carried by the applicator rolls to the paperboard, and pan 26 is provided below reverse roll 21 to collect the excess wax which falls off the rolls. As the paperboard blanks 4 are driven through the wax applicator rolls 19 and 21, the rolls deposit a film of molten Wax on the paperboard, and the wax rapidly hardens to form a solid coating. The coated blanks are then deposited on rotating endless belt 28 which conveys the blanks away from the apparatus. A heavy smooth coating is formed by reverse roll 21, whereas drive roll 19 forms a light coating on top of the paperboard similar to that obtained by the usual kiss roll hot wax coating method.
As best illustrated in FIG. 2, the drive for conveyor rolls 11 and 13 is supplied by motor 29 through right angle gear box 31, chain drive 32, sprocket wheel 33, gear 34, and gears 36 attached to shafts 37 on the conveyor rolls. Rotation is imparted by lower gear 36 to sprocket 38 mounted on shaft 37, and sprocket 38 drives chain drive 39 and sprocket wheels 41 which in turn rotate rollers 16. Reverse applicator roll 21 is driven in a direction contra to the travel of the paperboard at the nip by motor 29 through gear box 31, chain drive 42, and sprocket wheel 43 secured to shaft 44 for roll 21. In turn small sprocket wheel 46 on shaft 44 drives chain 47, which rotates drive roll 19 at a slower speed than smooth reverse roll 21 by means of sprocket wheel 48 mounted on shaft 49 for feed roll 19. The relative speeds of rotation of applicator roll 19 and reverse roll 21 are easily varied by changing the relative sizes of sprocket wheels 46 and 48. The overall speed of the apparatus may be controlled by employing a variable speed motor 29. In accordance with conventional practice, the doctor roll 24 for feed roll 19 is driven by gear 51 attached to shaft 49 through motion transmitted to gear train 52. Sprocket wheel 43 drives gear train 54 which in turn rotates the doctor roll 24 for reverse roll 21.
Wax applicator drive roll 19 and smooth reverse roll 21 may be maintained vat an elevated temperature by means of steam directed through conduit 56, which divides and directs the steam through shafts 49 and 44 in the wax applicator rolls. Valves 57 provide a control of the amount of steam passing through the rolls which enables the surface temperatures of the rolls to be regulated.
The effectiveness of the method hereof is due to the application of wax to the paperboard by the reverse roll 21 as the board passes through the wax applicator rolls. The wax metered by lower doctor roll 24 onto reverse roll 21 is carried around the roll and directed towards the oncoming paperboard 4 at the nip 22. From about A2 to 25 pounds of wax per thousand square feet of paperboard may be applied to one side of paperboard by the reverse roll 21, with from 2 to 5 pounds of wax per thousand square feet of paperboard employed for most purposes. In the conventional kiss roll method a maximum of from 2 to 2% pounds of wax per thousand square feet of paperboard can be applied to one side of the paperboard by an applicator roll, and coatings substantially below 2 pounds per thousand square feet often tend to become discontinuous and rough.
On the drive roll 19 as in the conventional kiss roll method, the wax is applied by a roll traveling in the same irection as the paperboard. However, the drive roll 19 hereof is preferably employed to apply only a relatively light coating so that the coating material will not interfere with the driving of the sheet material'between the rolls. Wax loadings on the drive roll 19 of less than about /2 pound of wax per 1000 square feet of paperboard have been most successfully employed.
If it is desirable to coat both sides of the sheet material with a smooth heavy coating, the sheet material may be turned over and coated by the modified form of apparatus disclosed in FIG. 6. The heavy smooth coating is applied by reverse roll 21' traveling in a direction contrary to the travel of the sheet material 4. Wax is applied to the reverse roll through manifold 23' and it is metered by doctor roll 24. Excess wax is collected in pan 26', and the coated board 4 is deposited on endless conveyor belt 28.
In order to prevent damage to the smooth heavy coating previously applied on the other side of the sheet material, drive roll' 60 is maintained at a temperature below the melting point of the coating. Air cooling will provide the necessary temperature reduction, or else a cooling liquid may be circulated through shaft 49'. Drive roll 60 is preferably covered with a material such as rubber to provide the requisite frictional difference to drive the paperboard through the rolls, and no additional coating material is applied by this drive roll. Since drive roll 60 tends to pick up some of the previously applied coating by abrasive action on the coated sheet material, it is desirable to remove this coating material from the drive roll so that the material does not mar the coatings on subsequent pieces of sheet material. 'Scavenging roll 61 maintained above the melting point of the coating effectively removes the coating material from drive roll 60, and doctor blade 62 strips the liquified material from scavenging roll 61. The drive for the modification shown in FIG. 6 is the same as the drive illustrated in FIG. 2 except sprockets 63 and drive chain 64, illustrated in FIG. 6 in phantom lines, drive the scavenging roll 61 so that it is traveling contra to the direction of feed roll 60 where the rolls meet. Although scavenging roll 61 is most effective in cleaning feed roll 60 when the rolls travel in different directions at their point of contact, the scavenging roll also removes coating from the feed roll 60 when it travels in the same direction as the feed roll where they meet.
Several factors affect the coating of wax applied in accordance with this invention, such as the speed at which the paperboard blanks mated to the applicator rolls, the speed of the wax applicator rolls 19 and 21, the ratio of the roll surface speeds, the temperature of the rolls, and the amount and type of wax applied. The overall speed of the wax applicator rolls, as distinguished from the ratio of the roll speeds, is not particularly critical. The upper limit of the wax applicator roll speeds is maintained below the speed at which spattering occurs, and this speed can readily be determined by visual observation. The highest practical speed depends upon the amount. and viscosity of the wax, and the diameter of the applicator rolls. For example with an 8 inch diameter stainless steel reverse roll at 160 F. applying 4.8 pounds of paraffin wax having a melting point range of 143 F. to F. to each 1000 square feet of board surface, spattering of the wax occurred at surface speeds above about 600 feet per minute. However, with more viscous coating compositions, or with larger applicator rolls or differently designed apparatus, even higher speeds may be employed before spattering occurs. If desired, shields may be employed to minimize the effect of spattering at high running speeds.
The lowest practical overall roll speeds are limited only by the economic advantages of rapid operation, and by the desirability of avoiding excessive penetration of porous sheet material such as paperboard, by the coating material. For most applications, the best coatings have been obtained with drive roll surface speeds of about 250 to 300 feet per minute. Usually drive roll speeds are not commercially feasible below about 150 feet per minute, and excessive penetration of the coating into sheet material is obtained below this speed if the sheet material is porous, such as paperboard.
It has been found that best results in obtaining smooth Wax coatings on the individual blanks without jamming of the coating apparatus is obtained when the blanks are fed to the drive rolls at a speed of between five percent and up to one hundred percent faster than the linear speed of the surface of the drive roll. The reverse roll tends to kick the relatively thin, flexible paperboard blanks away from the nip, particularly when the blanks are curled as they generally are after being stacked, and feeding-at a faster speed than the linear speed of the drive roll overcomesthis tendency. In the usual kiss roll method with the rolls travelling in the same direction at the nip, the blanks are generally fed at a lower speed than the linear speed at the surface of the drive rolls since there is no tendency for the blanks to kick out of the nip.
The ratio of the speed of the reverse roll to the speed of the drive wax applicator roll has a marked efiect on the coating. It has been found that the reverse roll 21 should have a surface speed greater than the drive roll 19 for best results in producing a smooth continuous coating. This is particularly important at low wax loadings below about 2. pounds per thousand square feet at which loadings the conventional methods tend to provide discontinuous coatings. Furthermore, at all wax loadings, including those above about 2 pounds per thousand square feet, smoother coatings are provided when the linear speed of the reverse roll is greater than the speed of the drive roll. However, at such higher loadings above about 2. pounds per thousand square feet, satisfactory coatings can be produced when linear speeds of the drive roll and reverse roll surfaces are equal.
When the reverse roll travels in its reverse direction slower than the drive roll in its direction, uneven stringy coatings are produced. Best results are obtained with speed ratios of the reverse roll to the drive roll of 4 to 3 or higher, and a ratio of about 5 to 3 has generally been the most advantageous. In any event, the ratio is maintained sufficiently high to produce a smooth, uniform coating of wax. However, if the ratio of the smooth reverse roll speed to the drive roll speed is too high, the wax tends to be thin at the leading edge of the board and to build up on the trailing edge. This difliculty can readily be overcome by reducing the relative speed of the smooth reverse roll. It has been found that the most uniform coatings are obtained at speed ratios of the reverse roll to the drive roll less than about 5 to 1, although the upper limit of the ratio is not particularly critical.
The speed of the reverse wax applicator roll relative to the speed of travel of the board also governs the amount of wax applied to the board. When the speed of a roll is increased or decreased, the doctor roll is correspondingly closed down or opened in order to maintain the same wax loading. Both modified and unmodified waxes are referred to herein as wax, and they in- 6 elude straight waxes as well as waxes modified with ad ditives such as polyethylene and polybutene.
The surface temperatures of the applicator rolls are.
not afiected by the deposit of additives to the extent of the drive roll, but it is also generally desirable to maintain the surface temperature of this roll above the melting point of the components of the wax mixture. The drive roll should be maintained at a higher temperature than the reverse roll.
A nip clearance between Wax applicator rolls 19 and 21 that is less than the thickness of the paperboard is employed in order to drive the board through the nip and produce a satisfactory coating. The rolls and the paperboard comprises sufficiently to enable the paperboard to pass through the nip. Generally, a nip clearance between about 0.011 inch to 0.004 inch less than the thickness of the paperboard is employed for best results. If the nip opening is too small, wax from the reverse roll tends to transfer to the drive roll when there is no sheet material passing through the rolls. When a wide nip opening is employed that approaches the width of the paperboard, a poor coating is produced.
A specific example of coating paperboard by the method and apparatus of this invention is given in tabulated form in Table I. An excellent thick smooth coat of wax was obtained under the conditions specified. Apparatus of the type disclosed in the drawings was employed. A 6 inch diameter chrome plated steel reverse roll 21, and a 6 inch diameter rubber covered drive roll 19 were employed in the apparatus. The paperboard was 0.015 inch thick, and the nip clearance was 0.007 of an inch. The wax temperature and the temperature of the surface of the reverse roll were the same. The temperature of the drive roll was 220 F. Ninety-eight percent by weight of a paraffin wax having a melting point range of from 143 F. to F. was employed mixed with two percent by weight polyethylene which had a molecular weight range of from 2,000 to 4,000.
Table I Drive Smooth Feeding Smooth Roll, Reverse Speed, Wax Tem- Side Wax Feet Per Roll, Feet Ratio Feet Per perature Loading Min. Per Min. Min. F.) (Lbs/M We claim: 1. The method of coating individual, flexible paperboard carton blanks with a wax coating, which comprises conveying the flexible blanks individually in succession to the nip between a pair of rotatable rolls, the first roll of which has a higher coefiicient of friction with respect to the blanks than does the second roll, causing said first roll to rotate in the same linear direction at the nip as the direction in which the flexible blanks are conveyed to the nip to propel the blanks through said nip by the frictional contact of the surface of said first roll with one of the surfaces of the blanks, causing the second roll of said pair to rotate in a direction so that its surface at said nip travels in a linear direction opposite to the direction of travel of the blanks propelled through the nip by the first roll and at a linear speed at least as great as the linear speed of said first roll, forming a metered film of molten wax on said second roll, and
transferring said film from said second roll onto the adjacent surface of the blanks during the propulsion of the blanks through the nip by said first roll, said blanks being conveyed to said nip prior to application of said Waxcoating at a linear speed of at least five percent greater than the linear speed of the surface of said first roll at said nip.
2. The method of coating individual, flexible paperboard carton blanks with a wax coating, which comprises conveying the flexible blanks individually in succession to the nip between a pair of rotatable rolls, the first roll of which has a higher coefiicient of friction with respect to the blanks than does the second roll, causing said first roll to rotate in the same linear direction at the nip as the direction in which the flexible blanks are conveyed to the nip to propel the blanks through said nip by the frictional contact of the surface of said first roll with one of the surfaces of the blanks, causing the second roll of said pair to rotate (1) in a direction so that its surface at said nip travels in a linear direction opposite to the direction of travel of the blanks propelled through the nip by the first roll and (2) at a linear speed having a ratio in the range of 4:3 to 5:1 relative to the rate of linear speed of said first roll, forming a metered film of molten wax on said second roll, and transferring said film from said second roll onto the adjacent surface of the blanks during the propulsion of the blanks through the nip by said first roll, said blanks being conveyed to said nip prior to application of said wax coating at a linear speed of between about five and one hundred percent greater than the linear speed of the surface of said first roll at said nip.
3. The method of coating individual, flexible paperboard carton blanks with a wax coating, which comprises conveying the flexible blanks individually in succession to the nip between a pair of rotatable rolls, the first roll of which has a higher coeflicient of friction with respect to the blanks than does the second roll, causing said first roll to rotate in the same linear direction at the nip as the direction in which the flexible blanks are conveyed to the nip to propel the blanks through said nip by the frictional contact of the surface of said first roll with one of the surfaces of the blanks, causing the second roll of said pair to rotate in a direction so that its surface at said nip travels in a linear direction opposite to the direction of travel of the blanks propelled through the nip by the first roll and at a linear speed at least as great as the linear speed of said first roll, forming a metered film of molten wax on said second roll, and transferring said film from said second roll onto the adjacent surface of the blanks during the propulsion of the blanks through the nip by said first roll, said film being metered in an amount suificient to provide a wax coating of from about 2 to 9' pounds of wax per 1000 square feet of said adjacent surface of the blanks, said blanks being conveyed to said nip prior to application of said Wax coating at a linear speed of between about five and one hundred percent greater than the linear speed of the surface of said first roll at said nip.
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