"June 1992.""Submitted to TAPPI 1992 Engineering Conference, September 14-17, 1992, Boston, Massachusetts.

Burns, James Robert

Conners, Terrance E.

Lindsay, Jeffrey D.

Scholarly Materials And Research @ Georgia Tech

Institute of Paper Science and TechnologyAtlanta, GeorgiaIPST TECHNICAL PAPER SERIESNUMBER 446DYNAMIC MEASUREMENTS OF STRATIFIEDCONSOLIDATION IN A PRESS NIPJ.R. BURNS, J.D. LINDSAY, AND T.E. CONNERSJUNE 1992JUL -8 1992P. . i. c-At'i;, - " ltForce TransducerFalling Press Headc.~..~ .~w~.~ ~~ ~ Porous CeramicPlateI I-- $ i Flow-Exiting5th I I ZoneProximity I d. MiddleDetector I +1 I ZoneLoe Ps -I iTop, I . \,Zone/..... . Head....^ , ;,: Ale :" v' ,SolidTo determine the true beginning and end of the nip, a fifth proximity detector was added to track, .,', ,,'"', " · ':' - " ~' l:, a,>,>, Displacement >lers" ,",,'-,, Transducersp ssesse se ssure and opening difficult.The typical nip residence time for this press simulator was on the order of 3 to 4 s, similar to thatLEectrower ress 'Simulor''a Head '''',Figure 3. Schematic of new e press h ads with a sheet in which the solid platen zone (SPZ), the mid-dle zp one (MZ), and the flow-exiting zone (FEZ) are defined.To determine the true beginning and end of the nip, a fifth proximity detector was added to trackthe location of the falling platen. The nip is closed when the upper platen position corresponds tothe position of the upper target on the surface of the paper. In practice, mechanical flexing of thetransducer support assembly or of the entire platen assembly sometimes made detection of nip clo-sure and opening difficult.The typical nip residence time for this press simulator was on the order of 3 to 4 ms, similar to thatof a high-speed industrial wet press. The peak pressures generated in this nip ranged were typically2 to 3 MPa in this study, well below those found in commercial press nips (7to 10 MPa).Electro-Hydraulic Press SimulatorIn addition to tests with a falling-weight press simulator, a series of tests was also conducted usingan electrohydraulic press apparatus based on an MTS (Materials Testing Systems) ram. This presssimulator allowed longer nips to be studied (>10 ms) than was possible in the falling-weight presssimulator and offered a variety of instrumentation advantages. For these experiments, the upperpress head was modified by the addition of an extension shaft. The upper press head and extensionshaft were screwed directly onto the hydraulic piston of the MTS. This upper press head consistedof three circular aluminum plates and a porous ceramic plate as shown in Figure 4. The largestplate served as the target for the fifth proximity detector. The lower press head was mounted on thefixed base of the MTS. Carbon paper-paper nip impressions were made to ensure that the pressheads were parallel.Figure 4. Upper press head with extension shaft mounted on the MTS.Eddy Current Proximity DetectorsThe five proximity detectors used in this study were of the eddy current type manufactured byKaman Instrumentation Corp. (Colorado Springs, Colorado). These transducers have a measuringrange of 4.0 mm (0.160 inches) with a resolution of 0.01% of full scale calibration. A full descrip-tion of these proximity detectors, their specific requirements for proper operation, and the methodswe used for calibration are described in Burs (18).Data Acquisition SystemA new data acquisition system was chosen to replace the two eight-bit resolution data acquisitionsystems used by Burton (16). An MDAS 7000 data acquisition system (TransEra Corp., Provo,Utah) was chosen. The 12-bit data acquisition system resolved the signals into 4096 different lev-els. The system was set up for each run to acquire 1000 data points, of which 450 were recordedbefore the system was triggered. The sampling frequency was typically 10 kHz or greater.Dynamic Density Profile DeterminationTo press the handsheets in the press simulators, the 127-mm (5-in diameter) handsheets were cutwith a die cutter to a diameter of 96 mm (3.75 in) to match the size of the falling press head, placedon the lower press head, and aligned with the eddy-current transducers. A small pressure of 17.9kPa was applied to the handsheet prior to pressing to improve the contact of the bottom target withthe solid ceramic platen. The feedback control system of the MTS was adjusted to yield the desiredpeak applied pressure and nip residence time, and the handsheets were pressed. (The pressure pro-file generated by the MTS unit was always a slightly asymmetric haversine pulse. This asymmetryis typical of the pressure profiles measured in industrial wet press nips.)During the pressing of the sheet, the targets moved with the surrounding fiber network in responseto dewatering and densifying forces. The instantaneous density for each zone was calculated fromthe corresponding zonal thicknesses and the zonal basis weights using Equation (2) above.To obtain a consistent moisture content in the porous ceramic flow receiver for each handsheet, itwas necessary to adjust the moisture content of the ceramic flow receiver. After the first sheet waspressed in each handsheet series, the moisture content in the porous ceramic flow receiver wasreduced by pressing blotters in the press nip until it was not possible to detect any further waterpickup by the blotters.RESULTS AND DISCUSSIONA large set of data was acquired for 150 g/m2 handsheets. Much of the data was acquired in theprocess of improving the methods and equipment for this study. The data set acquired after thetechniques and equipment reached their final state of development comprises measurements in 22samples. These samples include both high- and low-freeness softwood handsheets and low-freeness hardwood handsheets which were pressed with either the falling-weight press simulatoror the MTS servo-hydraulic press. A sampling of the results will be presented here; completedetails are given by Burns (18). Results for each set of conditions were typically replicated threetimes.Below, we refer to the zone adjacent to the porous ceramic plate (flow receiver) as the flow-exitingzone (FEZ). The next zone is the middle zone (MZ), followed by the solid platen zone (SPZ).IiIFalling-Weight Press SimulatorThe displacements of the targets embedded in a softwood handsheet are shown in Figure 5. Thehandsheet was formed from a low-freeness (353 CSF) kraft softwood furnish and had an initialmoisture ratio of 4.31. The displacement profiles of the falling press head (target E, labeled"Head") and the target on the top of the sheet (A) are similar in shape. The head appears to havemade good contact with this handsheet and remained in contact with the sheet for a long period oftime, but this was not always observed. In many cases, rocking or tilting of the falling head mayhave occurred. Even in this figure, the head and target A seem to have moved at different rates (thelines have different slopes), which is an indication that the falling press head hit the handsheet firston one side and then leveled off before rebounding. There is also some motion seen in target D,though it is much less than that seen in Burton's results and has little effect on the results. It ispossibly due to deformation of the lower head or imperfect initial contact between the target and thelower platen.6.0125- \ T21990E SWDA \ 353 CSF MRin 431Imt~~~~~~~~~~~~~~~ 1 < 5.01,004.0: l $\ /-5 o20'°0.25 o.0'0.00 I .. _.......' ' I ;'; ' ' I ;'' 'I 0.00.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030Time (sec)Figure 5. Target displacement histories for a low-freeness kraft softwood handsheet. Basis weight =150 g/m2 ; moisture ratio = 4.31; freeness = 353 CSF.The rates and extents of target displacements in this handsheet were significantly less than thoseseen in a high-freeness softwood handsheet for the same nip residence time and peak appliedpressure. There was little compression of the handsheet after the initial rapid compression, and thesheet reached a minimum compression height of 0.62 mm just after the point of maximumpressure. Subsequently, the sheet expanded slowly to a height of 0.68 mm as the pressure in thenip reached zero. After the pressure was reduced to zero, the target displacement profiles becameerratic. This behavior is typical of unrestrained expansion of the sheet and lift off of the sheet fromthe solid ceramic plate.The corresponding zonal densities (mass of dry fiber per unit volume) of the low-freenesshandsheet discussed above (see Figure 5) are shown in Figure 6. There was little difference in theiIJ17. Burton, S.W.; Sprague, C.H. The Instantaneous Measurement of Density Profile Devel-opment During Web Consolidation. J. Pulp and Paper Sci., 13(5): J145-J148 (1987).18. Burns, J.R. An Investigation of Z-Direction Density Profile Development During WetPressing. Ph.D. Thesis, Institute of Paper Science and Technology, Atlanta, Geogia (1992).19. Bums, J.R.; Conners, T.E.; Lindsay, J.D. Dynamic Measurement of Density GradientDevelopment During Wet Pressing. Tappi J., 73(4):107-113 (1990).20. Cowan, W.F. An Investigation of the Hot Surface Drying of Glass Fiber Beds. Ph.D. Thesis,The Institute of Paper Chemistry, Appleton, Wisconsin (1961).21. Jaavidaan, Y. Experimental and Theoretical Study of Two-Phase Flow Through CompressiblePorous Media With Application to Sheet Rewetting in Wet Pressing of Paper. Ph.D. Thesis,University of Maine at Orono (1987).22. Zotterman, C.; Wahren, D. A Technique for Simulating Production Quality of High-SpeedPress Runs. Paper Trade J., 162(16): 37 (1978).I

Dynamic measurements of stratified consolidation in a press nip

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Dynamic measurements of stratified consolidation in a press nip

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