MANUFACTURE OF CORRUGATED FIBREBOARD

Abstract: To manufacture warp-free corrugated fibreboard, apparatus includes capacitive probes (26) adjacent each travelling web (6a, 7, 8) to monitor moisture content before and after combining. Each probe (26) has multiple electrodes (34) in co-planar configuration parallel to the web and a ceramic coating (38) which has its outer face (39) in continuous contact with the travelling web (6a, 7, 8) giving a consistent spacing between the electrodes and the web. The gap between the electrodes (34) is such that the capacitance of the probe is sensitive to variations in the moisture content of only the nearer web. Control means (50) coupled to the probes (26) and moisture adjustment means (13, 15, 24) compares the monitored moisture contents of the webs with pre-determined set points and/or other moisture contents and actuates the moisture adjustment means (13, 15, 24) of either web as required.

Description: "MANUFACTURE OF CORRUGATED FIBREBOARD" Field of the Invention This invention relates to the manufacture of fibreboard, especially corrugated fibreboard, and in particular to the control of moisture content during manufacture of such board. The term "fibreboard" is intended to embrace, inter alia, cardboard and- paperboard but in some circles fibreboard is known as "•cardboard" or the two terms are used interchangeably. Corrugated fibreboard is manufactured by first applying a flat paper web to a corrugated paper substrate to form what is referred to as a single face web and then adhering a second paper web, known as the double face liner, to the other side of the substrate. The result is a continuous board comprising a core of multiple corrugations between and adhered at their tips to respective liners comprising webs of fibrous material in the form of paper. O H Background to the Invention A long recognized problem in this process is that of imbalance in relative moisture content between the two paper liners in the finished board. If there is a significant imbalance, the board will warp shortly after manufacture, but conversion machines used to fold the board into boxes and other forms will generally only accept flat board. The operation of such machines includes the high frequency extraction of the bottom member of a descending stack of cut lengths of board. Warped board is thus discarded as waste. Although wastage rates are often excessively high, efforts to date to minimize wastage have usually relied upon the ability of skilled operators to recognize moisture imbalance and to take appropriate corrective action utilizing water sprayers and heated drums incorporated into the production line. A significant element of the difficulty in achieving proper moisture control resides in the monitoring of the moisture content of the travelling board or webs. Infra-red moisture detectors have been proposed for this purpose but it has been found that such detectors are excessively sensitive to airborne vapour, which is prevalent as steam in corrugated fibreboard plants, and to variations in the colour of the paper. More generally, it has also been proposed to monitor the moisture content of various substrates, including travelling paper webs, by detecting the OMPI ^Sm^ capacitance of a probe in the vicinity of the substrate. Probes which are sensitive to changing dielectric constant or dielectric loss or both have been disclosed. In some cases, such probes have employed pairs of electrodes between which the sheet material is passed, while others have relied on the concept of fringe capacitance to position the probe at one side only of the material. Such configurations are prone to errors caused by dust and other material build-up on the electrodes, by moisture condensation and by ambient moisture content. Mechanical alignment of electrodes is an additional problem, especially with those relying upon the fringe capacitance concept. Attempts to solve these difficulties by using a sliding contact are prima facie not appropriate for fibreboard in view of the high speed, of the order of 600ft per minute, at which webs move in modern plants and of the highly corrosive nature of paper. A further problem for the fibreboard application is that it is a primary aim to compare the moisture contents of the two paper liners of the assembled board but conventional probes proposed for paper webs can generally only detect the moisture content of the whole board. Summary of the Invention According to a first aspect of the invention, it has been realized that the difficulties may be resolved by utilizing capacitive moisture probes having multiple electrodes in co-planar configuration parallel to the respective web being monitored, wherein, firstly, the electrodes are overlaid by a ceramic coating which has its outer face in substantially continuous contact with the face of the travelling web and which determines a consistent spacing between the electrodes and the web. Secondly, it is possible to set the gap between the electrodes so that the capacitance of the probe is sensitive to variations in the moisture content of the web it is contacting but relatively insensitive to variations in the moisture content of the other liner web, even where the webs are the liners of an assembled board. The invention accordingly provides, in its first aspect, apparatus for manufacturing corrugated fibreboard comprising: means to guide respective travelling webs of fibrous material and to bring them together to form travelling board including a core of multiple corrugations between and adhered to the respective webs, and moisture adjustment means for separately varying the moisture content of the respective webs; characterized by: a plurality of capacitive probes positioned so that at least one is adjacent each travelling web for continuously monitoring the moisture content of the respective web, wherein each probe has multiple electrodes in co-planar configuration substantially parallel to the respective web and includes a ceramic coating over the electrodes which in use of the apparatus has its outer face in substantially continuous contact with a face of the respective travelling web and which determines a consistent spacing between the electrodes and the web, and wherein the gap between the electrodes in each probe is such that the capacitance of the probe is sensitive to variations in the moisture content of the nearer web but relatively insensitive to variations in the moisture content in the more remote web; and by control means coupled to the probes and to the moisture adjustment means for comparing the monitored moisture contents of the webs at the probes with pre-determined set points and/or with other moisture contents, and, if necessary, actuating the moisture adjustment means in order to vary the moisture content of one or both webs. Each capacitive probe advantageously comprises a ceramic tile encasing the multiple electrodes, which preferably comprise plural parallel fingers embedded in and electrically isolated from each other by the ceramic. These fingers are alternately electrically coupled to form two mutually isolated groups of electrodes. The invention further provides in its first aspect, a method of manufacturing corrugated fibreboard comprising guiding respective travelling webs of fibrous material and bringing them together to form a travelling board including a core of multiple corrugations between and adhered to the respective webs; characterized by: continuously monitoring the moisture content of the respective said webs by contacting each travelling OMPΪ web with at least one capacitive probe having multiple electrodes in co-planar configuration parallel to the web, wherein each probe includes a ceramic coating over the electrodes which has its outer face in substantially continuous contact with a face of the respective travelling web and which determines a consistent spacing between the electrodes and the web, and wherein the gap between the electrodes in each probe is such that the capacitance of the probe is sensitive to variations in the moisture content of the nearer web but relatively insensitive to variations in the moisture content in the more remote web; comparing the monitored moisture contents of the webs at the probes with pre-determined set points and/or with other moisture contents; and, if necessary, varying the moisture content of one or both webs. A second aspect of the invention concerns the actual positioning of the probes with respect to the webs prior to formation of the board and with respect to the board after formation. More particularly, in this aspect the invention provides a method of manufacturing corrugated fibreboard comprising guiding respective travelling webs of fibrous material and bringing them together to form a travelling board including a core of multiple corrugations between and adhered to the respective webs; characterized by: continuously monitoring the moisture content of the respective said webs before and after they are brought together; comparing the moisture contents of the webs before they are brought together with respective set points and, if necessary, adjusting the moisture content of the respective web; and adjusting said set points in dependence upon the moisture contents monitored after the webs are brought together. In its second aspect, the invention also provides apparatus for manufacturing corrugated fibreboard comprising: means to guide respective travelling webs of fibrous material and to bring them together to form travelling board including a core of multiple corrugations between and adhered to the respective webs, and moisture adjustment means for separately varying the moisture content of the respec ive.webs; characterized by: a plurality of moisture sensitive probes positioned so that at least one is adjacent each web before they are brought together and at least one is to either side of the board, wherein each probe is rendered sensitive to variations in moisture content of the nearer web but relatively insensitive to variations in moisture content of the more remote web; and by control means coupled to the probes and to the moisture adjustment means for actuating the moisture adjustment means in order to vary the moisture content of one or both webs, the set points for moisture content of the webs at the probes before the webs are brought together being determined in dependence upon moisture contents detected at the probes at the board. Brief Description of the Drawings The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a highly schematic perspective view of a segment of a corrugated fibreboard manufacturing plant, modified to form apparatus in accordance with both aspects of the invention; Figure 2 is a plan view of one of the capacitive moisture probes forming part of the apparatus of Figure 1; Figure 3 is a cross-section on the line 3-3 in Figure 2; Figure 4 is a view similar to Figure 2 of an alternative electrode configuration for the probes; and Figure 5 is an end elevation of a representive articulated assembly to which each moisture probe is mounted Modes of Carrying out the Invention The segment 10 of a corrugated fibreboard production line depicted in Figure 1 includes respective upper and lower guide roller sets 12, 14 in advance of a glue applicator 16 and a glue fixing device comprising steam chest 18 with an overlying press belt 20. Roller sets 12, 14, which also serve OMPI sΛ-s. W1PO as heat application rollers for purposes to be further described, and belt 20 jointly serve as means for guiding an upper single face web 6 and a lower double face liner web 7. Web 6 consists of a flat paper web 6a and a corrugated or fluted paper web 6b which are glued together at an earlier station in the line. Liner web 7 is unwrapped from a feed roll 11. Webs 6, 7 are drawn, by means not shown, through glue applicator 16, wherein the tips of the corrugations or flutes of web 66 are coated with a suitable glue, and brought together at belt 20 to form a board 8 which has a core of multiple corrugations 6b between and adhered to the respective paper webs 6a, 7. Steam box 18 acts to dry and set the glue while the webs are held firmly together fay belt 20 on flat steam heated plates (not visible in the drawings) , aided by weighted rollers 22 on the belt. For the purpose of selectively varying the moisture content of the webs 6a, 7, moisture adjustment means includes roller sets 12, 14 and a water spray 24 adjacent roller set 12. Roller sets 12,14 include a primary roller or drum 13, 15 which may be heated to a substantial temperature, for example by means of internal steam passages, and respective pairs of small adjustable rollers 13a, 15a about which the webs are guided to contact a substantial part of the circumference of the respective drums but which are controllably movable so as to determine the extent of wrap about the drums and therefore the degree of heating and drying of the webs. Water spray 24 comprises a multi-nozzle boom extending above the upper web 6. It may also be desirable to provide such a spray for the lower web 7, but such has practical difficulties in the present embodiment. In conventional practice, supervisory personnel would operate the moisture adjustment devices in accordance with their assessment of the condition of the board exiting from the line, with a view to producing moisture balance as between the paper liners or webs 6a, 7 and so avoiding subsequent warping of the board. In accordance with a preferred practice of the invention, however, multiple capacitive moisture probes are positioned in contact with the paper webs both before and after formation of the board 8. Specifically, there are four sets 30, 31, 32, 33 of three probes 26 each: in each case, the three probes are spaced evenly across the face of the web. Probe set 30 is disposed immediately in advance of roller set 12, and set 31 immediately in advance of roller set 14, while probe sets 32, 33, are disposed on opposite sides of the board just after it exits from belt 20. With reference to Figure 2 and 3, probes 26 are essentially identical tiles, each comprising multiple thin conducting electrodes 34 in a co-planar configuration on a ceramic substrate 36 and overlaid with a ceramic coating 38. This structure would typically be formed by precisely printing the electrodes onto the substrate. After oven-firing, ceramic coating 38 is applied to the surface of the composite and the total assembly fired again. It will be observed that the electrodes 34 comprise plural parallel fingers embedded in and electrically isolated from each other by ceramic. As is best seen in Figure 2, the finger electrodes are alternately electrically coupled to respective terminals 40 at the exterior of the ceramic, thereby forming two mutually isolated groups of electrodes. In operation, the outer flat face 39 of ceramic coating 38 is held in substantially continuous contact with an exposed face of the respective travelling web 6a, 7 by ' light biasing of an articulated assembly 42 (not shown in Figure 1 but detailed in Figure 5) to which the probe is mounted and which is hereinafter described in greater detail. Coating 38 thereby determines a consistent spacing between the electrodes 34 and the respective web and prevents inaccuracies which have previously arisen from variations in the spacing, material build up, moisture condensation and misalignment. Sustained contact is only made practicable by employing hard-wearing temperature-resistent ceramic faced probes - paper is highly corrosive and would rapidly wear out most materials, and high local temperatures occur in fibreboard lines. By virtue of the multiple co-planar electrode configuration, the capacitance of the probe is rendered highly sensitive to variations in the dielectric properties of media adjacent the plane of the electrodes, that is the ratio of the fringe or edge capacitance to inter-electrode capacitance is maximized. Variations in moisture content of the OMPI paper cause marked changes in dielectric properties. The lateral range of sensitivity from the electrode plane can be adjusted by varying the range of the electric field, which is achieved by adjusting the inter-electrode gap. This is important in the present application since it is necessary that the capacitance of the probe is sensitive to variations in moisture content of the web it is contacting but relatively insensitive to variations in moisture content of the more remote web: at probe sets 32, 33 the inter-electrode gap must be adjusted to take account of the close proximity of the other web. Nevertheless, it has proven quite practicable to set the probe parameters such that the measured capacitance changes by 100% for a change in web moisture content from 5% moisture by weight to 15%. A suitable ceramic is aluminium oxide and an appropriate tile is of the order of 50mm square and 0.7mm thick. Electrodes 34 are typically about lOJim thick and between 0.5 and 2.0mm wide, the gap between them being between 0.25 and 2.0mm. Coating 38 is preferably between 50 and 150|im thick. Probes 26 are monitored by a control device 50 which is also coupled to actuate the moisture adjustment devices 12, 14, 24. Control device 50 is typically a micro-processor controller programmed with necessary empirical and calibration data to permit it to determine web moisture contents at the respective probes from the signals received, to compare these contents with programmed set points and/or with each other and, if necessary, to actuate the moisture OM ^R A adjustment devices in order to vary the moisture content of the webs. For example, it may be known by experiment and experience that moisture contents at probe sets 32, 33 should differ by a given percentage, which may e.g. be zero where the paper grades are substantially identical, in order to achieve proper balance at the end of the line and so avoid warping, and that related moisture contents are desirable in the separate webs on approach to the glue applicator. Control device 50 would typically need to be programmed with a regime relating actuation parameters for the heat drums 13, 15 and sprayer to observed deviations in the moisture content. It is found that a particularly effective control configuration is to: (i) treat the respective probes at 30,31 and respective aligned and separately controllable segments of moisture adjustment devices (30, 12, 24; 31, 14) as local control loops for each of which there is a programmed moisture content set point, and then to (ii) control or vary each of those set points according to a pre-determined algorithm in dependence upon the magnitude of and/or difference between, the moisture contents monitored by sets 32, 33 the respective probes of which are at a transverse position, with respect to the board, corresponding to that of the probe at 30,31 and of the associated segment of the moisutre adjustment device. The moisture content is then controlled in each of three longitudinally extending zones or strips, a 14 significant feature since substantial local moisutre variations, e.g. very wet or very dry strips can occur. The number of zones is not necessarily three as illustrated and indeed it is thought a greater number of zones, e.g. twelve, might give optimum performance. As with other data provided to the micro-processor, it is not possible to be specific as to the form of the algorithm(s) or of the dependence of the set point or the probe readings at 32,33 since these are largely matters of empirical determination for a particular line. However, it is found that the aforedescribed control procedure is effective in achieving a satisfactory balance in the moisture content of the produced board, as reflected in the reduced wastage arising from warping. In determining moisture contents from the- capacitive probes, a number of known signal processing techniques can be employed. One arrangement relies upon the principle that if a high frequency alternating voltage is applied across a pair of adjacent electrodes, the amplitude and phase of the current drawn will be determined by the dielectric properties of materials near the electrodes. Using a phased locked loop circuit approach, the system can be configured so that the measurements are sensitive to material dielectric constant rather than dielectric loss. An advantage of this approach is that dielectric constant is less temperature dependent than dielectric loss. An alternative approach is to employ the probe as a feed-back element in an oscillator 15 circuit, whereupon the frequency of oscillation of the probe voltage is proportional to the dielectric constant of material adjacent the probe. The articulated assembly 42 to which each probe 26 is mounted and by which the probe coating is held in light contact with the respective web sufrace, is shown in Figure 5. The probe 26 is secured as a facing for a rectangular head 44 which is hinged at 46, on an axis parallel to the web, to the arms 47 of a bifurcated bracket 48. Bracket 48 is in turn hinged at 50, on a second axis orthogonal to the first, to a block 52 at the free end of an arm 54 rotatably suspended at 55 from one end of an overhead mounting channel 56. The contact pressure at the web is controlled with a double acting pneumatic or hydraulic cylinder 58 pinned between respective clevises 59a,59b at the other end of channel 56 and the top end of arm 54. * A suitable electrical interface 60 for the probe projects from bracket 48. Figure 4 depicts an alternative electrode configuration designed to sense the glue application rate in the manufacture of corrugated board, relying on the fact that the starch glue typically used contains a high proportion of water. It will be noted that the electrodes 34 ' are again co-planar but that, firstly, they are connected in alternative pairs and, secondly, the adjacent electrodes of opposite pairs are much closer together than the electrodes of the same pair. The sets of close-spaced electrodes are themselves spaced at the pitch of the board flutes and the measured capacitance is then at maximum when the " S- m E m OMPI glue strips on the flute tips are adjacent to these sets, and falls to a minimum when the glue strips are midway between the electrode sets. The difference between the maximum and minimum values represents the contribution of the applied glue, the effect of the moisture content of the liner being substracted out: it will be noted again that the lateral range of sensitivity must be adjusted to a suitable value. The particular advantage of the configuration illustrated in Figure 4 is that the alternate pair arrangement with like electrodes opposed results in a sequence of very localized electric fields so that the response at the sets of close-spaced opposite-polarity electrodes is emphasized. OMPI ^^?NATl

Claims: CLAIMS :- 1. Apparatus for manufacting corrugated fibreboard comprising means to guide respective travelling webs of fibrous material and to bring them together to form travelling board including a core of multiple corrugations between and adhered to the respective webs, and moisture adjustment means for separately varying the moisture content of the respective webs; characterized by: a plurality of capacitive probes positioned so that at least one is adjacent each travelling web for continuously monitoring the moisture content of the respective web, wherein each probe has multiple electrodes in co-planar configuration substantially parallel to the respective web and includes a ceramic coating over the electrodes which in use of the apparatus has its*outer face in substantially continuous contact with a face of the respective travelling web and which determines a consistent spacing between the electrodes and the web, and wherein the gap between the electrodes in each probe is such that the capacitance of the probe is sensitive to variations in the moisture content of the nearer web but relatively insensitive to variations in the moisture content in the more remote web; and by control means coupled to the probes and to the moisture adjustment means for comparing the monitored moisture contents of the webs at the probes with pre-determined set points and/or with other moisture contents, and, if necessary, actuating the moisture ' ϋRlA OMPI Λ. ΛPO ^fiNAT . adjustment means in order to vary the moisture content of one or both webs. 2. Apparatus according to claim 1 further characterized in that two of the probes are positioned so as to be at opposite sides of the travelling board. 3. Apparatus according to claim 2 further characterized in that there are further said probes positioned so as to be in contact with the respective travelling webs prior to their being brought together, and in that the control means is arranged to vary set points for moisture content of the webs at the further probes in dependence upon moisture contents detected at the probes adjacent the board. 4. Apparatus according to claim 3 further characterized in that, relative to the direction of travel of the webs and board, said further probes and the moisture adjustment devices are located in advance of a glue applicator while the probes adjacent the board are located behind a glue fixing device. 5. Apparatus according to any one of claims 1 to 4 further characterized in that each of said capacitive probes comprises a ceramic tile encasing the multiple electrodes. 6. Apparatus according to claim 5 further characterized in that said multiple electrodes comprise plural parallel fingers embedded in and electrically isolated from each other by the ceramic, the fingers being alternately electrically coupled to form two mutually isolated groups of electrodes. 7. Apparatus according to claim 6 further characterized in that the electrodes are of width between 0.5 and 2.0mm, the inter-electrode gap is between 0.25 and 2.0mm, and the thickness of the ceramic coating is between 50 and 150μm. 8. Apparatus according to claim 5 further characterized in that said multiple electrodes comprise plural parallel fingers embedded in and electrically isolated from each other by the ceramic, the fingers being electrically coupled in alternate pairs to form two mutually isolated groups of electrodes, and in that the gaps between the adjacent fingers of each coupled pair are substantially greater than the gaps between adjacent fingers of uncoupled pairs. 9. Apparatus according to any one of claims 1 to 8 further characterized in that each capacitive probe is one of a plurality spaced across the path of travel of the respective web for monitoring moisture content at spaced locations across the web, the control means operating separately in each of the associated longitudinally extending zones of the webs. 10. Apparatus according to any one of claims 1 to 9 further characterized in that said moisture adjustment OM?l__ 20 means comprises one or more spray devices disposed to direct liquid onto an associated said web, and one or more drying devices for extracting or driving off moisture from an associated said web. 11. Apparatus according to any one of claims 1 to 10 further characterized by means to lightly bias each probe against the face of the respective web whereby to maintain said contact without deforming the web. 12. A method of manufacturing corrugated fibreboard comprising guiding respective travelling webs of fibrous material and bringing them together to form a travelling board including a core of multiple corrugations between and adhered to the respective webs; characterized by: continuously monitoring the moisture content of the respective said webs by contacting each travelling web with at least one capacitive probe having multiple electrodes in co-planar configuration parallel to the web, wherein each probe includes a ceramic coating over the electrodes which has its outer face in substantially continuous contact with a face of the respective travelling web and which determines a consistent spacing between the electrodes and the web, and wherein the gap between the electrodes in each probe is such that the capacitance of the probe is sensitive to variations in the moisture content of the nearer web but relatively insensitive to variations in the moisture content in the more remote web; OMPI comparing the monitored moisture contents of the webs at the probes with pre-determined set points and/or with other moisture contents; and, if necessary, varying the moisture content of one or both webs. 13. A method of manufacturing corrugated fibreboard comprising guiding respective travelling webs of fibrous material and bringing them together to form a travelling board including a core of multiple corrugations between and adhered to the respective webs; characterized by: continuously monitoring the moisture content of the respective said webs before and after they are brought together; comparing themoisture contents of the webs before they are brought together with respective set points and, if necessary, adjusting the moisture content of the respective webs; and adjusting said set points in dependence upon the moisture contents monitored after the webs are brought together. 14. Apparatus for manufacturing corrugated fibreboard comprising: means to guide respective travelling webs of fibrous material and to bring them together to form travelling board including a core of multiple corrugations between and adhered to the respective webs, and moisture adjustment means for separately varying the moisture content of the respective webs; characterized by: a plurality of moisture sensitive probes positioned so that at least one is adjacent each web before they are brought together and at least one is to either side of the board, wherein each probe is rendered sensitive to variations in moisture content of the nearer web but relatively insensitive to variations in moisture content of the more remote web; and by control means coupled to the probes and to the moisture adjustment means for actuating the moisture adjustment means in order to vary the moisture content of one or both webs, the set points for moisture content of the webs at the probes before the webs are brought together being determined in dependence upon moisture contents detected at the probes at the board. 15. Apparatus according to claim 14 further characterized in that each probe is one of a plurality spaced across the path of travel of the respective web for monitoring moisture content at spaced locations across the web, the control means operating separately in each of the associated longitudinally extending zones of the webs. 16. Apparatus according to claim 14 or 15 further characterized in that said moisture adjustment means comprises one or more spray devices disposed to direct liquid onto an associated said web, and one or more drying devices for extracting or driving off moisture from an associated said web. - ■ KEA O PI ^& 23 17. A capacitive probe having multiple electrodes consisting of parallel co-planar fingers electrically coupled in alternate pairs to form two mutually isolated groups of electrodes, the gaps between adjacent fingers of each coupled pair being substantially greater than the gaps between adjacent fingers of uncoupled pairs. ^ EA O M£L . -vπ?o ΪERNA-

Inventors: FOX JAMES CHARLES, FOX, JAMES, CHARLES
Applicants: FIBRE CONTAINERS, FIBRE CONTAINERS LIMITED
Application year: 1983
Main classification: B31F1
IPC codes: B31F1