CD shrinkage profiles of paper - experiments on a commercial paper machine

S. M. Hoole, S. J. I'Anson, M. Ora, T. N. Ashworth, D. Briggs, B. Phillips, R. W. Hoyland

    Research output: Contribution to journalArticlepeer-review

    Abstract

    This paper has been refereed according to the standard Paper Technology procedure. The extensive summary is italicised to differentiate it from the main content. Paper and board suffer dimensional changes as they are transformed, on the paper machine, from a dilute suspension of fibres and minerals into the end product. The degree of dimensional change can be very different at the edges compared to the middle, with the degree of width reduction (CD shrinkage) being particularly strongly affected. No current machine is immune to this effect and the most modem, highest speed paper machines tend to demonstrate the biggest middle-to-edge differences, although total shrinkage is low. It is also well-known that the strength properties of paper and board can vary by large percentages between middle and edge. It has been suggested that the extreme values of paper properties which can occur at the edges of the machine can lead to problems of dimensional stability and runnability on high-speed, high quality, multi-colour offset printing presses, but this is certainly not always the case. It is true, however, that maximising the reeled-up sheet width (even where gains are measured in millimetres) can lead to additional customer reels being available and substantial economic benefits. It is clear that there is much to be gained from a better understanding of the details of sheet shrinkage profiles for both of the above reasons. This paper describes a series of experiments performed on a high-speed newsprint manufacturing machine (UPM-Kymmene, Shotton Paper Company, PM2) to investigate the origin of the CD shrinkage profile. The method used was to measure CD relative shrinkage profile by measuring changes in dimension of forming fabric marks in the sheet using image analysis. It has been suggested that the CD shrinkage profile could occur due to wet-straining in the press section, with a "necking" effect being responsible for the difference between middle and edge. This hypothesis has been tested by splitting the sheet just after the press section so that subsequent drying takes place in two separate webs. The result of this exper-iment was that the sheet dried as if it was made in two parts and this demonstrated that the profile was not created before the first dryer cylinder. A second hypothesis, which has previously been suggested, is that the CD shrinkage profile appears very suddenly at the point in the dryer section when the moisture content reaches the value at which the fibres collapse into flat ribbons, rather than cylindrical tubes (the fibre collapse point or FCP). The Shotton PM2 furnish is composed of 40 % re-cycled fibre (RCF) and 60 % thermo-mechanical pulp (TMP) and this means that it is unlikely that the FCP will be at one precisely defined dryness. The second experiment was designed to determine whether the profile had been formed in the area of the machine where the FCP might be expected to occur. As before, the sheet was split but, this time at points just before and just after the breaker stack, so that all subsequent drying look place as two webs. The shrinkage profile was unaffected by these splits, showing that the shape of the shrinkage profile had already been determined. It appears that the shape of profile has already been formed and that, although some shrinkage certainly occurs after the FCP, this must be distributed across the width of the machine. A third experiment demonstrates the effect of restraint during drying on the CD shrinkage profile. Samples were collected with and without the blow-box fans operating, with the second of these conditions providing much reduced sheet restraint. The overall reduction in width of the sheet was measured as increasing by about 0.5 % with the fans off. By comparing the profiles, it was shown that shrinkage had increased right across the width of the machine except at the edges, which were unchanged. This has been taken as an indication that restraint was already low at the edges, which has allowed correspondingly higher edge shrinkage values, and that further reduction in external restraint has little effect on the shrinkage profile. This expert-ment also suggests that it is indeed the degree of dryer section restraint which determines the shape of the CD shrinkage profile and, in this case, the higher the restraint, the more severe the profile.
    Original languageEnglish
    Pages (from-to)63-70
    Number of pages7
    JournalPaper Technology
    Volume40
    Issue number10
    Publication statusPublished - 1999

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