The Effect of Press Felt Non-uniformity on Sheet Smoothness and Dewatering --- PaperCon 2011 The Effect of Press Felt Non-uniformity on Sheet Smoothness and Dewatering John Xu, AstenJohnson Inc 48 Richardson Side Rd., Kanata, ON, K2K 1X2 Rick Phillip, Daniel Hedou AstenJohnson Inc 400 Asten Road, Clinton SC, 29325 ABSTRACT Non-uniformity with three different scales exists in a press felt. Micro-scale non-uniformity has a size of batt fiber (10 to 80 μm) and is created by the coarseness of batt fiber. Lab two-roll press testing indicated that with the increase of micro-scale uniformity (decrease of the batt fiber size), the sheet solids content went up, and sheets became smoother. Medium-scale non-uniformity has a size of 0.5 mm to 3mm. This type of non-uniformity is mainly associated with topography of the felt (e.g. needling track) and felt base fabric. After felt break-in, felt surface becomes smoother and the voids (or pockets) on the felt surface become smaller and shallower. The reduction in void size will lead to the reduction of the rewetting and provides a sheet with higher solids. With the increased cycles of break-in, felt will become smoother and thus lead to a smoother sheet. If the base is coarse and there is no sufficient batt to cover the base yarns, base fabric will create non-uniformity of pressure application during the wet pressing and lead to a lower solids content. Large-scale non-uniformity has a size of 3 mm to 15 mm. The non-uniformity could be the result of non-uniform distribution of batt, the interference between the top and bottom layer of the base fabrics, or the non-uniform shedding of the batt fibers. Lab testing indicated that there was a significant difference in dewatering between the “flocs” and “voids” area of the felt. The local pressure in the ‘flocs” area can be as twice high as that of the “voids” area. INTRODUCTION For printing and writing grade, sheet smoothness is one of the most important specs that a paper mill must achieve. A smoother sheet after the press section should require less calendaring to reach the target smoothness and this could promote higher level of sheet bulk and give the opportunity to the papermakers to reduce their fiber consumption and hence generate significant savings. Since press felts play an important role in determining the sheet dryness and smoothness after wet pressing, it is highly desired that a paper machine clothing company can provide a press felt which can enhance sheet dewatering and in the mean time improve sheet smoothness. The effect of press felt properties on dewatering has been widely studied in the past. Bliesner [1] reported that the felt’s compression properties, its uniformity in pressing the sheet, its flow resistance (in some cases), and its resistance to filling and compaction all contribute to water removal from the sheet. Through laboratory two-roll press study, Jackson [2] found that finer batt on the paper side of a felt improves water removal in a pressure controlled dewatering. It was also found that hardening of the felt due to the decreased basis weight and/or compaction could also improve water removal. In a similar lab dewatering study, Helle [3] found that at constant felt basis weight, felts with finer cap batt produced a higher solids sheet, while at the same batt surface texture, thicker felts gave higher solids content. Felt surface texture or uniformity also plays an important role in water removal during pressing and have been studied theoretically and experimentally [4, 5, 6]. Two mechanisms have been proposed which take into account the influence of the felt surface: rewetting, and loading uniformity. Because the surface property of the felt is so important, different techniques have been developed to characterize the felt roughness and pressure uniformity. Common examples are carbon paper, prescale impression film, resin casting, and imprint on a thin foil [5, 6, 7]. It is generally believed that non-uniformity in the felt will cause non-uniformity of pressure application, and thus lead to lower dewatering. Because the sheet is in direct contact with the felt during the manufacturing process, the fabric non-uniformity will not only affect the water removal, but also the sheet structure and smoothness. Smart [8] and Fekete [9] provided photomicrographs that revealed large uncomparessed regions in the paper felt interface. Oliver and Wiseman [10] showed that only 25% to 33% of the felt surface is load bearing against a hard plate when considered at the batt and paper fiber dimension. It is obvious that those uncompressed regions are different from those pressed areas, which may increase sheet roughness. Busker [11] found that the smoothness of the sheet pressed against the wet press felt was affected by the uniformity of pressure provided by the felt. McDonald and Pikulik studied the effect of felt construction on print quality [12,13, 14]. They showed that the coarseness of felts affected the overall quality of gravure printing. It should be noted that when studying the effect of surface property or pressure uniformity on dewatering, majority of the authors focused on the micro scale roughness or non-uniformity, which is mainly caused by the size of the batt on the paper side of the felt. Bernard [7] proposed a lab technique to quantify medium and large scale of non-uniformity. However, the effect of medium and larger scale non-uniformity on water removal and sheet smoothness are rarely reported [7]. In addition, although the press felts plays an important role in determining sheet smoothness and dewatering, no one studies the effects at the same time. In this paper, we will investigate how press felts non-uniformity with different scales affects sheet smoothness and dewatering simultaneously. PaperCon 2011 Page 2143
Transcript
The Effect of Press Felt Non-uniformity on Sheet Smoothness and Dewatering --- PaperCon 2011
The Effect of Press Felt Non-uniformity on SheetSmoothness and Dewatering
John Xu,AstenJohnson Inc
48 Richardson Side Rd., Kanata, ON, K2K 1X2
Rick Phillip, Daniel HedouAstenJohnson Inc
400 Asten Road, Clinton SC, 29325
ABSTRACT
Non-uniformity with three different scales exists in a press felt.Micro-scale non-uniformity has a size of batt fiber (10 to 80µm) and is created by the coarseness of batt fiber. Lab two-rollpress testing indicated that with the increase of micro-scaleuniformity (decrease of the batt fiber size), the sheet solidscontent went up, and sheets became smoother.
Medium-scale non-uniformity has a size of 0.5 mm to 3mm.This type of non-uniformity is mainly associated withtopography of the felt (e.g. needling track) and felt base fabric.After felt break-in, felt surface becomes smoother and the voids(or pockets) on the felt surface become smaller and shallower.The reduction in void size will lead to the reduction of therewetting and provides a sheet with higher solids. With theincreased cycles of break-in, felt will become smoother and thuslead to a smoother sheet. If the base is coarse and there is nosufficient batt to cover the base yarns, base fabric will createnon-uniformity of pressure application during the wet pressingand lead to a lower solids content.
Large-scale non-uniformity has a size of 3 mm to 15 mm. Thenon-uniformity could be the result of non-uniform distribution ofbatt, the interference between the top and bottom layer of thebase fabrics, or the non-uniform shedding of the batt fibers. Labtesting indicated that there was a significant difference indewatering between the “flocs” and “voids” area of the felt. Thelocal pressure in the ‘flocs” area can be as twice high as that of
the “voids” area.
INTRODUCTION
For printing and writing grade, sheet smoothness is one of themost important specs that a paper mill must achieve. A smoothersheet after the press section should require less calendaring toreach the target smoothness and this could promote higher levelof sheet bulk and give the opportunity to the papermakers toreduce their fiber consumption and hence generate significantsavings. Since press felts play an important role in determiningthe sheet dryness and smoothness after wet pressing, it is highlydesired that a paper machine clothing company can provide apress felt which can enhance sheet dewatering and in the meantime improve sheet smoothness.
The effect of press felt properties on dewatering has been widelystudied in the past. Bliesner [1] reported that the felt’s
compression properties, its uniformity in pressing the sheet, itsflow resistance (in some cases), and its resistance to filling andcompaction all contribute to water removal from the sheet.Through laboratory two-roll press study, Jackson [2] found thatfiner batt on the paper side of a felt improves water removal in apressure controlled dewatering. It was also found that hardeningof the felt due to the decreased basis weight and/or compactioncould also improve water removal. In a similar lab dewateringstudy, Helle [3] found that at constant felt basis weight, feltswith finer cap batt produced a higher solids sheet, while at thesame batt surface texture, thicker felts gave higher solidscontent.
Felt surface texture or uniformity also plays an important role inwater removal during pressing and have been studiedtheoretically and experimentally [4, 5, 6]. Two mechanisms havebeen proposed which take into account the influence of the feltsurface: rewetting, and loading uniformity. Because the surfaceproperty of the felt is so important, different techniques havebeen developed to characterize the felt roughness and pressureuniformity. Common examples are carbon paper, prescaleimpression film, resin casting, and imprint on a thin foil [5, 6, 7].It is generally believed that non-uniformity in the felt will causenon-uniformity of pressure application, and thus lead to lowerdewatering.
Because the sheet is in direct contact with the felt during themanufacturing process, the fabric non-uniformity will not onlyaffect the water removal, but also the sheet structure andsmoothness. Smart [8] and Fekete [9] providedphotomicrographs that revealed large uncomparessed regions inthe paper felt interface. Oliver and Wiseman [10] showed thatonly 25% to 33% of the felt surface is load bearing against ahard plate when considered at the batt and paper fiberdimension. It is obvious that those uncompressed regions aredifferent from those pressed areas, which may increase sheetroughness. Busker [11] found that the smoothness of the sheetpressed against the wet press felt was affected by the uniformityof pressure provided by the felt. McDonald and Pikulik studiedthe effect of felt construction on print quality [12,13, 14]. Theyshowed that the coarseness of felts affected the overall quality ofgravure printing.
It should be noted that when studying the effect of surfaceproperty or pressure uniformity on dewatering, majority of theauthors focused on the micro scale roughness or non-uniformity,which is mainly caused by the size of the batt on the paper sideof the felt. Bernard [7] proposed a lab technique to quantifymedium and large scale of non-uniformity. However, the effectof medium and larger scale non-uniformity on water removaland sheet smoothness are rarely reported [7]. In addition,although the press felts plays an important role in determiningsheet smoothness and dewatering, no one studies the effects atthe same time. In this paper, we will investigate how press feltsnon-uniformity with different scales affects sheet smoothnessand dewatering simultaneously.
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EXPERIMENTAL
The AstenJohnson Gravity Sheet Former (GSF) was used tomake a handsheet. The sheet was pressed by a laboratory presswhich is shown schematically in Figure 1. The top roll is a plain,stainless steel, 168-mm-diameter roll and the bottom roll is agrooved stainless roll with the same diameter. Both rolls aredriven at a speed of 1.3m/s. During the pressing, three differentpress levels were selected: 44kN/m, 66kN/m and 88kN/m. Basedon the dynamic compressive stress-strain curves for a typical1500gsm press felt, the nip generates a peak pressure ofapproximately of 850psi, 1200psi and 1500psi at three differentloads.
A typical dewatering test proceeded as follows:
(1) A 75mm by 80mm handsheet with a basis weight about60gsm was made by Gravity Sheet Former.(2) The excess water was removed by pulling the sheet and itssupporting (a forming fabric) together over a vacuum slot. Aftervacuum dewatering, the sheet solids was about 20%.(3) The felt to be tested was fully wetted, then repeatedlypassed through the nip of a two-roll laboratory press at load of88KN/m for about 10000 cycles.(4) The wet paper was laid over the wet felt and passed throughthe nip at speed of 1.3m/s. Once the sheet passed through thenip, it was taken off from the top press roll.(5) Put the sheet in a box to prevent the evaporation ofmoisture. Weigh the sheet together with the box.(6) Dry the sheet for 30 seconds by sandwiching the sheetbetween a forming fabric and a curved hot plate. A small tensionwas applied to the forming fabric during drying to prevent theshrinkage of the sheet.
After the sheet was dried, it was weighed and its Sheffieldsmoothness was measured. To reduce the variation, eachexperiment was repeated three times. It was found that therepeatability of the dewatering experiment was better than 0.4%and the standard deviation of Sheffield smoothness values werenormally less than 5. In order to remove the break-in effects ofthe new felts, all press testing was done after a 10000 cycles ofconditioning run.
Figure 1. The laboratory two-roll press.
RESULTS AND DISCUSSIONS
The press felt is a composite structure comprised of a wovenbase and batt fibers which are needled onto the woven base.Non-uniformity exists in the composite structures as a result ofcoarseness of batt fiber, non-uniformity of mass distributions(batt or base), and non-uniformity of the felt density associatedwith the manufacturing process. When talking about felt non-uniformity, one must understand the spatial scales of the non-uniformity, which are associated with different felt properties ormanufacturing processes. Three different types of non-uniformity exist in a press felt: micro-scale, medium-scale andlarge scale [7].
Effect of micro-scale non-uniformity on dewatering and sheetsmoothness
The first type of non-uniformity is mainly created by thecoarseness of batt fiber and in the size of batt fibers (10 to 80µm). To study the effect of the batt fiber size on dewatering andsheet smoothness, three felt samples (H, C, E) with the samebase and same batt weight (700gsm) were studied. These threefelts have different size of batt fiber in the cap layer. The size ofcap batt fiber for sample E and C is 58 µm (30 dTex) and 43µm(17dTex), respectively. For sample H, the cap batt fiber is amixture of 19 µm (3 dTex) and 29µm (8dTex) fibers.
Three different pressing levels (44kN/m, 66kN/m and 88kN/m)were used during the pressing. With the increase of the load, thesheet dryness went up and the smoothness went down. In orderto show how fiber size affects the dewatering and sheetsmoothness simultaneously, sheet solids and sheet Sheffieldsmoothness were plotted in the same graph (Figure 2). It is clearfrom Figure 2 that with the decrease of the batt size (from rightto leht), the sheet solids goes up and in the meantime, sheetsbecome smoother (Sheffield smoothness values go down).
Figure 2. Effect of batt fiber size on sheet solids andsmoothness. H: 19/29 µm; C: 43 µm; E:58 µm.
The effect of fiber size on dewatering has been reported widelyin the literature [2, 3]. It was generally believed that batt with
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smaller fiber will provide a more uniform pressure and has lessrewetting, thus increases dewatering. Gullbrand [15] suggestedthat there is an interaction layer between sheet web and feltsurface. This layer consists of compressed areas anduncompressed areas. The absence of mechanical stress in theuncompressed areas implies that these areas will have a lowerdryness as water is only partially pressed out of them.
Although no literature was found regarding the effect of fiberbatt size on sheet smoothness, it is not surprise to see that sheetsbecome smoother with the decrease of the batt fiber size. Figure3 shows the topography of a sheet pressed at 88kN/m by twodifferent felt H and E. It is obvious from the photos that fibers inthe batt will create dents on the paper surface and reduce thesmoothness of the sheet. The larger dents caused by the coarserbatt fibers lead to a rougher sheet surface.
(a) (b)
Figure 3. Topography of the sheet pressed by felt H with finebatt fiber (a) and felt E with coarse batt fiber (b).
Effect of medium-scale non-uniformity on dewatering and sheetsmoothness
The second type of non-uniformity is in the size of 0.5mm to3mm (medium- scale). This type of non-uniformity is associatedwith felt surface roughness or batt density difference in the felt(e.g. needling track). Another cause of the medium-scale non-uniformity is the base fabric. The areas with base fabric yarnswill have different compression properties from the areaswithout base yarns and thus create stress variations. In thissession, we will study the effect of felt break-in and batt weightto demonstrate how medium-scale non-uniformity affects sheetsmoothness and dewatering.
To study the effect of felt break-in on sheet dewatering andsmoothness, the dewatering testing was run on a felt after itpassed through the nip for different cycles. Figure 4 shows thesheet solids and smoothness after the felt passed through the nipfrom 60 cycles to 43000 cycles.
As shown in Figure 4, both sheet smoothness and sheet solidsincrease as the pressing cycle increases. One of the reasons thatthe sheet solids increases with the break-in cycle is that the feltbecomes harder and denser with the increase of pressing cycle[2], which leads to a higher nip peak pressure. However, thechange of felt hardness alone can not explain the smoothnesschange of the sheet. With the increase of nip peak pressure, onemay expect the sheet becomes rougher. However, what we found
from Figure 4 is that the sheets become smoother when thepressing cycle increases.
Jackson[2] estimated the sheet solids change with the felthardness and found that the estimated sheet solids values are lessthan the results from the experiment. He suspected that theuniformity of the pressure application improved and therewetting decreased with the increase of pressing cycle. Toexplain the effect of break-in on sheet dryness and sheetsmoothness, we examined the change of felt surface uniformitywith the break-in. Figure 5 compares the contact areas betweenthe new felt and the same felt after 43000 cycles of nip pressing.As shown in Figure 5, in the micro scale, there is no significantchange on batt contact area and support index, although the battfibers become a little larger because of the fiber flatten (Figure 5(b)).
Figure 4. Effect of break-in on sheet solids and smoothness.
(a) (b)
Figure 5. Effect of break-in on contact area and micro-scale non-uniformity. (a) new felt; (b) felt after 43000 cycles.
Figure 6 shows the surface images of the felt before and afterbreak-in (43000 cycles). The images were taken under low anglereflection light. From Figure 6, it is easy to find that fibers tendto bundle together and form peaks and valleys in the scale ofmillimetre (0.5mm ~ 3mm). Although it is hard to quantify thedifference in dewatering between the peak and valley area, it isreasonable to assume that the peak area would have higherpressure and thus higher solids, while the valley area would havelower pressure and higher moisture content. In the extreme case,if the valley areas are not in contact with the paper surfaceduring the wet pressing, the voids created between the paper and
1 mm
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the felt could be filled with water and thus increase therewetting.
(a) (b)
Figure 6. Effect of break-in on felt surface uniformity. (a)new felt; (b) felt after 43000 cycles.
Compare image (a) and (b) in Figure 6, it is easy to find that,after break-in, felt surface becomes smoother and the voids (orpockets) on the felt surface become smaller and shallower.Generally speaking, a smoother surface tends to provide a moreuniform pressure application, and the reduction in void size willlead to the reduction of the rewetting after mid-nip. It is notsurprise to see that the sheet solids increases with the break-incycle.
Because of the voids/pockets on the felt surface, wet sheetpressed against the felt will experience the non-uniform stressthus deforms differently. Figure 7 shows the images of a papersurface when it was pressed against a new felt (a) and the samefelt after break-in (b). Simliar to the felt surface, there are peaksand valleys on the paper suface with a similar scale as that of thefelt. Apparently, the sheet pressed aginst a new felt appearsrougher than that of the sheet pressed aginst a conditioned felt. Itis clear from Figure 7, with the increased cycles of break-in, feltwill become smoother and thus lead to a smoother sheet.
(a) (b)
Figure 7. Surface images of sheets pressed by (a) new felt; (b)conditioned felt (after 43000 cycles)
Another common cause of the non-uniformity at medium scale isthe base effect. If the base is coarse and there is no sufficient battto cover the base yarns, the compression properties between thebase yarn areas and the areas without base yarns will bedifferent. Areas with base yarns will have a higher compressionstress than those areas without the base yarns. The variations in
compression stress will lead to the so called base fabric mark. InFigure 8, the paper side batt weight of felt A is 600gsm and thepaper side batt weight of felt B is 1000gsm. Although both feltshave the same base fabrics and the same cap batt fibers, thedewatering of felt B is better than felt A. Figure 9 compared theimprint of felt A and felt B when pressed at 1200 psi. In felt A,base fabric mark is shown, which should lead to the non-uniformity of pressure application during the wet pressing. Weactually observed the base fabric mark on the sheet when felt Awas used.
Figure 8. Effect of batt weights on sheet dryness andsmoothness. A:600gsm; B:1000gsm.
(a) (b)
Figure 9. Compare the pressure uniformity of felt A (a) andfelt B (b). Sample size is 2” by 2”.
Effect of large scale non-uniformity on dewatering and sheetsmoothness
The larger scale non-uniformity (5 -15mm) is not as common asthat of micro and medium-scale non-uniformity. It should benoted that non-uniformity at such scale does exist especially forthose felts with low batt weight. The non-uniformity could be aresult of non-uniform distribution of batt, the interferencebetween the top and bottom layer of base fabrics, or non-uniformshedding of batt fibers. How the large-scale non-uniformityaffects dewatering and sheet smoothness has not been reportedin the literature.
5mm 5mm
5mm
10mm 10mm
10mm 10mm
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Figure 10 shows the image of a lab-made felt under transmittedlight. The total batt weight is 600gsm. Similar to the formationof a paper, there are “flocs” and “voids” in the felt. The size ofthe flocs and voids is in the range of a few millimetres to 15mm.
To find out the difference in dewatering properties between theflocs and voids areas, a small newsprint sheet sample was used.The newsprint sample was cut into small pieces with the size of15mm by 15mm. It was weighed and then soaked in water for 24hours. After removing the surface water from the sample, weplace the sample over the flocs or voids area and then press itusing the laboratory two-roll press at the load of 44kN/m. Afterpressing, samples were weighed and their solids content werecalculated.
Figure 10 shows the average sheet solids at flocs areas (location1 & 3) and voids area (location 2 & 4) after six repeatingmeasurements. These numbers are also shown in Figure 11 forvisual comparison. It is easy to find that solids in the floc areasis 4.6 to 8% higher than that in the void area.
Figure 10. Images of two felts under transmitting light. Thesize of red box is about 15 to 20mm.
Figure 11. Sheet solids at different positions of the felts.
In order to find out the pressure difference between flocs andvoids area, Tekscan pressure profiling sensor was used to studythe non-uniformity application of the pressure. The Tekscanpressure profiling sensor measures the pressure by means of anultra-thin sensor of a flexible printed circuit. The circuit usesinterlacing columns and rows of pressure sensitive ink whichchanges in electrical resistance as the pressure changes. Eachintersections between the rows and columns serves as a sensingpoint. Figure 12 shows the pressure distribution of the felt inFigure 10. The sensing area is about 5.6cm by 5.6cm with aspatial resolution of 1.3mm. As shown in Figure 12, the averagepressure in two floc areas (location 1 and location 3) is 369 psiand 302 psi, respectively, while in the voids area (location 2 andlocation 4), the average pressure is only 207 psi and 177 psi.The significant difference in the applied pressure leads to thedifference in solids content shown in Figure 10 and Figure 11.
Figure 12. Pressure distributions in the floc and voidareas in a felt.
The results from this study indicated that the felt large scale non-uniformity has a significant effect on the pressure distributionand thus it will affect the sheet dewatering. Unfortunately, wewere not able to compare the sheet smoothness with such a smallsample. However, one may expect that the sheet will becomerougher after drying if there is a big difference in local moisturecontent.
CONCLUSIONS
Three types of Non-uniformity exist in a press felt as a result ofcoarseness of the fiber, non-uniformity of the felt densityassociated with the manufacturing process, and non-uniformityof mass distributions(batt or base). These three types of non-uniformity have different scales and affect the sheet smoothnessand dewatering differently.
It was found from lab two-roll press study that, with theincrease of micro-scale uniformity (decrease of the batt fibersize), the sheet solids content went up, and sheets becamesmoother. After break-in, the pockets (or voids) on the feltsurface became smaller and felt became smoother. The improveddewatering and pressure uniformity will provide higher sheet
1(47.6%)
4(39.7%)2(39.6%)
3(44.3%)
3(43.8)10mm
10mm
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solids and smoothness. On the other hand, if the batt is not heavyenough to cover the base, dewatering will decrease because ofthe non-uniform distribution of the pressure application causedby the base. No-uniform distribution of the batt can create large-scale non-uniformity. There is a significant difference indewatering between the “flocs” and “voids” area of the felt. Thelocal pressure in the ‘flocs” area can be as twice high as that inthe “voids” area.
REFERENCES
1. Bliesner, W., Sheet Water Removal in a Press: the Role ofWet Felt Properties, Pulp&Paper, p76-79, October, 1978.
2. JACKSON, G.W., “Press Felt Characterization and SheetDewatering”, TAPPI Journal, Vol. 72, No. 9, September 1989
3. Oliver, J.F., and Wiseman, N., Water removal in WetPressing: the Effect of Felt Roughness, Transactions: TR104-109, December 1978.
4. Helle, T., and Forseth T., Influence of felt structure on waterremoval in a press nip, Tappi Journal 77(6): 171-178 (1994).
6. Olsson, J.L. and Hanarp, L.R., Surface Structure andUniformity of Paper Machine Clothing, Pulp and Paper Canada93(5): T127-T131, (1992).
7. Bernard, G., UPA --- A New Method to Assess PressureUniformity of Press Felts, PAPTAC 94th Annual Meeting, Feb.,2008
8. Smart, F.R., “Water removal on a Grooved Second Press”,part I, 1975 International Water Removal Symposium, London,18th -20th March 1975. Proceedings, p.89-115.
9. Fekete, E.Z., “Water removal on a Grooved Second Press”,part II, 1975 International Water Removal Symposium, London,18th -20th March 1975. Proceedings, p.117-141
10. Oliver, J.F., Wiseman, N., “Water removal in wet Pressing:the effect of felt Roughness”, Pulp Pap. Can., vol 77(9):T-149,(1976).
11. BUSKER, L., H., “The Effect of Wet Pressing on PaperQuality”, TAPPI Proceedings, 1985 Engineering Conference,p117 to 129
12. McDONALD, J., D., PIKULIK, I., I., “The effect ofpressing on the print quality of newsprint”, Pulp & PaperCanada, 89:3, 1988, pT76 to T83
13. McDONALD, J., D., PIKULIK, I., I., “Felt construction andprint quality”, Paper Technology, August 1990, p20 to 23
14. McDONALD, J., D., PIKULIK, I., I., “The effect of feltconstruction on surface properties of newsprint”, TAPPI Journal,November 1989, p71 to 76
15. MacGregor, M. A., “Wet pressing research in 1989 – Anhistorical Perspective, Analysis and commentary”, Transactions
of the 9th Fundamental Research Symposium, Vol. 2,Mechanical Engineering publications, London, 1989, P. 511-586.
16. Gullbrand, J., Vomhoff, H., “The influence of press feltmicro-scale stress variation on dewatering”, Nordic Pulp andPaper Research Journal, 20(3): 289, 2005.
17. McDONALD, J., D., PIKULIK, I., I., TAPPI 1992Engineering Conference Proceedings, TAPPI Press, Atlanta,P.869
