The present invention relates to the production of corrugated board and more particularly to a novel and improved method for controlling the application of an adhesive to the flute tips of singleface web in the corrugated board manufacturing process.
In the production of corrugated board a multiplicity of grades of linerboard and medium are used as well as a variety of flute formations. This requires frequent adjustment of the glue machine to maintain quality production of corrugated board. The basic concept of the present invention allows the glue machine to adapt to this changeable environment of corrugated production automatically without operator intervention required after startup of the corrugator.
In the first step of corrugated board production, a machine called a singlefacer is used to flute a given grade of medium (paper) between a pair of corrugating rolls machined to a specified profile. This fluted medium is then bonded to a liner of various grades of paper using a starch based adhesive. After combining the medium and liner in this fashion, the resulting singleface web progresses to a bridge storage area where latent heat that has been applied to the medium and liner continue to cure the starch based adhesive securing the bond.
The next step in the corrugating process takes the singleface web produced at the singlefacer and combines it with another (bottom) liner. This bottom liner becomes the exterior surface of a corrugated container and is usually a finer grade of paper. This surface (the box exterior) will normally have flexographic printing applied in the process of creating a finished box. Alternatively, a preprinted liner can be used or a label can be affixed to the outer surface of the corrugated box blank to create the finished box. Bonding the second liner, known as the “doubleface” liner, requires an even application of adhesive onto the medium flute tips across the full width of the singleface web.
The application of adhesive to the singleface web flute tips occurs in a machine referred to as a glue machine. A primary feature of this machine is a glue applicator roll, which may have an engraved surface. A film of adhesive is picked up by the glue applicator roll as it rotates through a glue pan filled with starch based adhesive. The adhesive is metered onto the glue roll, typically using a contra rotating metering roll, so that a consistent glue film thickness is applied across the working width of the glue applicator roll surface. Other methods of metering are applicable, such as those defined in U.S. Pat. No. 6,008,701 dated May 30, 2000.
The glue applicator roll usually runs at a speed some small percentage less than the speed of the singleface web passing in contact with the roll, commonly 95%-98% of singleface web speed. The roll underspeed is crucial to achieve starch application centered on the flute tip allowing for proper bonding to the doubleface liner. Maintaining proper glue roll rotational speed is achieved through the use of a drive with a regenerative feature. This regenerative feature is critical to maintaining the proper speed ratio between the singleface web and glue applicator roll surface.
A glue machine can be equipped with a rider roll designed to bring the flute tips of the singleface web into intimate contact with the adhesive film on the glue applicator roll. The rider roll must be positioned to create an adjustable gap between it and the glue applicator roll through which the singleface web passes. This gap ensures the singleface web flute tips pick up the desired amount of adhesive. Improper setting of the rider roll to glue applicator roll gap can create two undesirable conditions. If the gap setting is too loose, areas along the flute tips may pick up too little starch or no starch at all. This will result in the formation of a blistered and undesirable exterior surface of the corrugated box. If the rider roll to glue roll gap is set too tight, the singleface web passing through this nip will be deformed and damaged. This compression of the board past its elastic range can result in a significant loss in the mechanical strength of the corrugated box deeming it unacceptable to its application. Significant singleface web compression also results in excessive starch application with several negative effects beyond the cost of the starch consumed. For example, excess starch application will cause wash boarding that is difficult to print on and that shows up as undesirable striated lines through a preprinted or labeled surface. Excessive starch application also results in increased energy consumption required to gel the starch and drive the moisture from the glue line.
Rider roll/glue applicator roll gap setting has been normally left to the operator on prior technology glue machines. This can lead to improper gap setting, particularly on corrugators that involve a lot of paper grade changes. It is desirable, therefore, to implement a means of automatic adjustment of the rider roll to glue applicator roll gap.
Automatic rider roll gap setting means have been described in the prior art. Several contact and non-contact means have been disclosed in the literature for direct measurement of singleface web caliper upstream of the glue machine for purposes of command positioning of the rider roll gap. U.S. Pat. No. 4,360,538 discloses, for example, a contact singleface web caliper sensing device that derives a signal that is used to adjust the rider roll gap setting to achieve a desirable compression of the singleface web between the rider roll and glue applicator roll. US Patent Publication 2008/0317940 A1 discloses several non-contact singleface web flute height sensing techniques that use a curtain of visible, infrared or ultraviolet light or laser beams. Any of these upstream flute height measurement techniques, when used in conjunction with a rider roll to glue applicator roll gap measurement, can be used for automatic setting of the desired gap. Also disclosed is a contact automatic singleface web caliper sensor as shown in FIG. 10 of the same publication. This means of sensing singleface web caliper will be discussed in more detail in ensuing paragraphs. All of these contact and non-contact methods for singleface web flute height measurement and subsequent rider roll gap setting add complexity and require absolute calibration of the singleface web flute height sensing means as well as the rider roll gap adjustment hardware that can drift out of tune with time creating a maintenance issue.
Concepts have been described in the literature for use of pressure loading of the rider roll to force the flutes of the singleface web into contact with the glue applicator roll. Means of actuation and sensing of pressure force, for example, are described in U.S. Pat. No. 6,602,455 B2. The pressure loading of the rider roll causes deflection of the singleface web flutes as clearly shown in FIG. 4 of US Patent Application Publication 2008/0317940 A1. There are several problems with the pressure loading concepts. First, the required pressure loading must be empirically determined based upon the strength of the flute tip. The flute tip strength varies considerably with type of flute formation as well as within a flute type as a function of the medium basis weight and even the manufacture of the medium. As a consequence it is difficult to select the desired pressure setting without getting too much or too little deflection of the flute tips. Too little deflection can cause poor starch adhesive transfer, and too much deflection can cause permanent crush to the flute tips causing degradation in the quality of the corrugated board manufactured. In addition, the means of sensing and controlling the pressure are complex and suffer from performance issues related to the bad environment of the glue machine. Starch adhesive is caustic and, as is well know in the art, splashes about the glue machine contaminating operating mechanisms and requiring frequent clean up. The contamination can affect the precision of the pressure loading mechanisms making them difficult to use in practice.
As a consequence, there is still a need in the art for an improved means of automatically controlling the rider roll to glue applicator roll gap to a precise setting to achieve sufficient and necessary adhesive transfer to the singleface web flute tips without compressing the flutes such that permanent damage occurs. Furthermore, it is desirable to achieve these objectives without unduly complex addition of mechanical mechanisms that require maintenance and frequent cleaning to keep them operating. In particular it is desirable to avoid requirement for periodic absolute calibration of measuring and controlling sensors to keep them functional and operable.
The essence of the present invention is a method for precise adjustment of the rider roll to glue applicator roll gap in a glue machine that involves recognizing that regenerative glue applicator roll drive current is reactive to compression of the singleface web within the gap. The glue applicator roll drive, under normal operating conditions, must provide a positive output current to achieve adequate torque out of the glue roll motor to overcome the inertial and frictional bearing drag loads on the glue applicator roll to keep it turning at some desired set point speed. Nominally, speed is set at a range of 95%-98% of operating corrugator speed. This underspeed is required to get proper transfer of starch adhesive onto the center of the flute tips of the singleface web. The rider roll compresses the singleface web against the glue applicator roll to insure transfer of starch adhesive onto the flute tips that are traveling at operating corrugator speed. The singleface web creates a frictional drag on the glue applicator roll proportional to the normal force of the flute tips as they are compressed by the rider roll and the coefficient of friction between the singleface web medium and the adhesive coated glue applicator roll. This frictional drag of the singleface web on the glue applicator roll adds torque on the roll so that less glue roll motor torque is required to maintain the speed of the roll. As more compressive force is added by reducing the rider roll gap, the drive on the glue applicator produces negative current causing the regenerative glue roll drive and motor to act as a brake to maintain the glue applicator roll at the 95%-98% underspeed.
In the preferred embodiment of the present invention, a target glue applicator drive current setting is entered by the operator on the glue machine interface touch screen and an appropriate feedback control loop adjusts the rider roll gap to achieve a variable singleface web compression that will cause the gap to be set where there is just a slight compression when the target current is achieved. The target current is based upon empirical understanding of the impact of singleface web drag on drive roll current output. Empirical data show that with no compression, the glue roll drive current will be some level as simply required to overcome inertia and frictional bearing drag of the roll. As singleface web compression is added, glue roll drive current will decrease. As compression is increased further, the glue roll drive current will go negative indicating the drive and motor are braking the glue applicator roll. Understanding of this singleface web compression/glue roll drive current relationship allows selection of a set point drive current that results in a very slight singleface web compression that allows desired starch adhesive transfer but no permanent flute tip deformation or damage. Experience has shown that target currents can be chosen that will result in singleface web compression of less than one percent. Studies indicate that there is no permanent deformation or damage to the flute tips on the singleface web until compression reaches the range of four to five percent.
A primary advantage of the present invention is that rider roll control is achieved with no additional mechanical hardware or mechanisms as are prevalent in prior art rider roll gap setting concepts. This ultimate simplicity means that there is no penalty related to clean up or maintenance to keep the automatic rider roll control concept of the present invention operational.
Yet another advantage of the present invention is that it can operate perfectly with no sensing of absolute value of gap or singleface web caliper as is prevalent and required by prior art technology. This means that periodic calibration of sensors is not required eliminating an operational reliability issue associated with prior art rider roll gap solutions.
It should be well understood by those skilled in the art that other embodiments of the preferred solution are possible and within the scope of this current invention. For example, the operator could enter a desired target compression of the singleface web and a suitable feedback control loop could adjust the rider roll gap to achieve variable drive current that will cause adjustment of the gap to achieve the drive current that will just yield the required compression. Additionally, the method of the present invention could include the use of any means for direct measurement of glue applicator roll motor output torque or any variable proportional to the torque for purposes of sensing and adapting to singleface web frictional drag force for purposes of controlling compression of the web between the rider roll and the glue applicator roll.
During start-up and other phases of corrugator operation involving acceleration, the glue applicator roll drive current reacts to the requirement to change the speed of the roll rather than to compression of the singleface web. According to the present invention, a filter on the drive current feedback senses the acceleration causing the gap to be latched at its current setting or an initial setting. At start-up, for example, the operator could enter the flute being run and a nominal gap setting would be selected based upon this flute type. This setting could be manually adjusted by the operator based upon his knowledge of the paper combination being run. After reaching a cruise speed, the automatic gap adjustment mode of the present invention would take over based upon settings in the drive current filter and adapt automatically to the caliper of the product being run as well as to ensuing paper changes.
Alternately, according to another aspect of the present invention, an optimal initial start-up gap setting solution would be based upon the feed forward of a singleface web caliper measured using a simple contact means. This singleface web caliper measurement would be calculated using the respective angular velocities of two idler rollers, one with the singleface web flutes down on the idler roll and the other with the singleface web wrapped such that the flutes faced outward away from the idler roll.
In yet another aspect of the present invention, this same means of measuring singleface web caliper could be used to reset the rider roll gap when a splice is made changing paper during a corrugator acceleration or deceleration phase when the rider roll gap setting would otherwise be latched. As soon as the splice enters the glue machine, as evidenced by a splice signal, the rider roll gap would adjust based upon the measured singleface web. Then, when the corrugator reached a quiescent (non-acceleration) period, the drive current feedback would be unlatched and any error associated with the gap setting based upon the sensed singleface web caliper would be corrected.
It should be noted that the automatic gap control using drive current feedback reacts very quickly to a change of paper caliper at a splice, normally adjusting the gap to the correct setting within less than a second. The use of upstream singleface web caliper is not a requirement for automatic gap control but a refinement that reduces operator workload.
U.S. Pat. No. 5,785,812 discloses the use of sensing of glue applicator roll speed change to set the gap between the glue applicator roll and the corrugating roll of the singlefacer. This glue application concept involves sensing of a glue applicator roll speed change wherein the flute tips of the medium are wrapped around the corrugating roll flute tips and the glue applicator roll is powered into direct contact with the corrugating roll. The glue applicator roll speed adjustment is affected thereby with hard contact between the corrugating roll and the glue applicator roll with the fluted medium caught in between simply acting like a slight cushion between the two hard bodies. A key difference between this disclosed method of glue applicator roll gap setting is that the method of the present invention uses drive current as a means of sensing torque on the roll as opposed to speed change of the roll. Further, it is a precept of the present invention that no speed change on the glue applicator roll is allowed to occur due to feedback control of drive current to avoid the speed change. Also, in the present invention, singleface flute compression affects the torque on the roll as opposed to the interference between two hard rollers.
The foregoing and other aspects of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the drawings below:
Primary and essential elements of a corrugated glue machine are shown in
The singleface web 5 comprised of a top liner 4 and a fluted medium 3 (See
Rider roll 40 is attached to side bars 70 that pivot about axis of rotation 60. The side bars are loaded pneumatically or by other suitable means forcing stop blocks 75 against screw jack positioners 80. There are screw jacks located on each side of the machine that are timed by timing rod 90 that is powered by screw jack actuators, not shown. The screw jack actuation system can be controlled to achieve a desired rider roll to glue applicator roll gap.
The control system schematic describing the automatic gap control system of the present invention is shown in
Upon reaching a cruise speed as evidenced by exceeding a creep speed and achieving reasonable steady state operation, software Latch L1 (110) closes initiating the auto glue gap control. At this point rider roll jack screw 80 responds to the initial gap command gc (120), the outer loop of the glue applicator roll drive current feedback if (185) and the drive current command ic (100) to set the gap g (45) (
The drive current io (105) is used in the outer loop of the automatic glue gap control as input to the filter, gain and latch 180. The filter is designed to eliminate noise in the feedback loop as well as any short term fluctuation in the current signal as the singleface web caliper is constant except at paper change. The gain is chosen to provide a stable rider roll gap solution. The latch is logic based software that opens latch L1 (110) during rapid acceleration periods of corrugator operation and opens latch L2 (115) if the corrugator is momentarily stopped for purposes of clearing a dry end jam-up, for example. During stops of this type, wet end papers do not change so there is no reason to act upon gap commands or current feedback signals.
During normal corrugator operation, the latches are closed and the filtered drive current feedback if (185) is compared to the command set point drive current ic (100). A plot of typical output drive current io (105) versus singleface web compression c is shown in
Using empirical data of this type, it is possible to select a target commanded drive current ic (100) that will result in a desired small compression c of the singleface web. This commanded current setting ic (100) combined with the outer loop glue applicator roll feedback current if, (185) as shown in the
A plot showing paper basis weight versus singleface web caliper is shown in
The automatic rider roll gap control of the present invention works well to adapt the gap setting to a corrected level in less than one second as a splice enters the glue machine with a change in singleface web caliper. Although uncommon, it is possible that a splice could be made during a period where the corrugator is accelerating. During corrugator accelerations, according to the present invention, the filter, gain and latch 180 of
A simple means of measuring singleface web caliper is schematically depicted in
It should be noted that the means of estimating singleface web caliper of the present invention as shown in