Adapting DPS type forming fabrics to run on high speed newsprint machines
Andrew White, Huyck (UK) Ltd
Duplex fabrics are however more prone to wire mark and do not offer the best levels of fibre support to give improved sheet quality and sheet structure. Complex fabric structures such as triple weft and triple layer forming fabrics give improved paper quality, but often at the expense of reduced solids at the pick-up roll. Indeed, many machines have trialled triple layer forming fabrics. However, results with triplex have been mixed. Over duplex, triplex fabrics have higher levels of fibre support. This meant retention, formation and sheet density often improved. However, these quality benefits were usually accompanied by a reduction in sheet solids at the pick-up roll. This of course is not acceptable in high-speed newsprint production. Triple weft fabrics have also been trialled, particularly where fabric stiffness and stability are required. Once again results have been mixed with some failures due to the high caliper and void volume of triple weft fabrics.
The development of DPS (double pick stitch) forming fabrics
Figure 1. TXT fabric utilising DPS yarns
The TXT design proved particularly successful in North America where many machines reported improved printability and reduced mark. Many machines also reported improved retention, better formation and improved drainage. The launch of TXT in Europe at the end of 1996 was less successful however. Wire mark was invariably worse, occasionally resulting in press room complaints. Many machines showed problems at the trim squirts resulting in poor machine runnability. Image analysis and FFT enhancement of the wire mark in the sheet showed that the diagonal wire mark was caused by the twill weave of the DPS yarns (on the paperside the DPS yarn passes over 2 machine direction yarns). Returned fabric samples also showed that fibre stapling was also occurring around the twill weave DPS yarns. This was reason for the deterioration in trim squirt performance. However, as seen in North America, most European TXT runs gave improved retention along with good drainage.
The introduction of DPS forming fabrics to SC papers
Experience with TXT showed the DPS concept could be used to solve both the stability and mark problems inherent with conventional triplex. Figure 2 shows the structure of this evolved DPS fabric known within Huyck as huytexx.
Figure 2. Huytexx
Since the stitch yarn in DPS provides fibre support and actually enhances drainage more yarns can be used. This greatly increases the fabric stability. Both the conventional paperside CD yarn and the stitch yarn are plain weave at the paperside of the fabric. This minimizes wire mark.
This design was first run in 1997 and has proven to be very successful on hybrid formers making SC papers. Wire mark was much improved over all other designs. Other benefits seen included improved paper profiles, higher retention, reduced retention aid consumption, improved Z direction ash and fines distribution, reduced twosidedness and improved printability. Some machines recorded increases in wire life of between 33% and 50%. Drainage was also good resulting in high sheet dryness values before the pick-up roll.
Applying huytexx to newsprint, hybrid formers
Table 1. Fabric parameter comparison
Results exceeded all expectations. Once again the high fibre support levels gave quality and retention improvements. Drainage, especially final drainage was excellent resulting in improved solids at the pick-up roll. This, combined with improved former cleanliness resulted in less breaks throughout the life of the forming fabric. Table 2 summarises the improvements seen.
Table 2. Benefits seen during huytexx run
Whilst the improvements seen may seem too good to be true, it should be pointed out that the machine conditions reverted back when the huytexx fabric was removed and improvements returned once again when the next huytexx fabric was installed. It should also be noted that identical benefits have been seen on many other similar machines making newsprint and improved newsprint grades.
Applying huytexx to newsprint, gapformers
Table 3. Performance of 1st generation huytexx on newsprint
The development of 2nd generation huytexx
Figure 3. 1st (upper) and 2nd (lower) generation huytexx. Machineside CD yarns (bottom and in cross section) increased and of smaller diameter to reduce fabric void volume and caliper
Table 4. Property comparison for 1st and 2nd generation huytexx
SCA Ortviken Pm5
Table 5. Conventional duplex fabrics on Ortviken Pm5
Trials were made with a pair of DPS forming fabrics to improve wire mark and formation. Whilst wire mark and formation did improve, paper machine runnability was poor resulting in the fabrics being removed after less than 24 hours. Specifically, problems were encountered with both initial drainage, rewet and water carry. It was seen however that wire mark had improved. Despite a short run time on the machine it was felt that formation had improved.
Acknowledging that many other machines were having successes with DPS type forming fabrics Ortviken invited Huyck to discuss their thinking on DPS fabrics with a view to running another trial. A new trial was agreed using 2nd generation huytexx. Objectives were to improve wire mark and formation with no compromise in machine runnability. 2nd generation huytexx was chosen to meet these objectives. Mill experience and Huyck experience of over 30 runs on newsprint gapformers shaped the choice of forming fabric design. As well as low void volume it was identified that there was a requirement for a higher fabric permeability with DPS. Experience has shown that, at the high fibre support levels of huytexx, thinking has to be modified and higher permeabilities can be required, especially on newsprint. Table 6 shows the parameters for the chosen fabrics.
Table 6, 2nd generation huytexx on Ortviken Pm5
Start-up was on the 29th March 2001 and was trouble free. Immediate improvements in retention were seen due to the high fibre support levels. The fabrics ran for 83 days before planned removal. Machine personnel reported significant improvements in former cleanliness. Drainage analyses indicated identical dryness to duplex fabrics with grab samples confirming the absence of rewet. Retention increased initially but was then controlled to the same retention as before the trial resulting in a 25% reduction in retention aid consumption for the same couch dryness. The fabrics were much cleaner running with less fibre carry on both machineside and paperside of the fabrics. This contributed to a reduction in break rate of around 50%. Less breaks and higher average machine speeds contributed to an increased daily tonnage of around 3.3%.
2nd generation huytexx was giving the same benefits on newsprint gapformers as 1st generation huytexx was giving on more forgiving hybrid formers.
Why does 2nd generation huytexx work on gapformers
Figure 4. Drainage holes through various fabric structures
1st generation huytexx
2nd generation huytexx
Fabric structures can contain many different shaped holes within which water is available for rewet. However, to simplify analysis, the largest and most common holes are considered. With duplex and 2nd generation huytexx, the shape of the hole is similar. For both designs there is a point within the fabric structure where there is a constriction in open area. It would be more difficult for any water beyond this point to return back into the sheet as rewet. This constriction can be considered as a barrier to rewet. With 1st generation huytexx the hole is more regular and open. This means it is easier for water held anywhere within the fabric structure to return into the sheet causing rewet.
Both absolute void volume and z-directional void volume distribution are therefore important when considering rewetting tendency. Table 7 shows caliper, void volume and rewetting index for the main newsprint machine fabrics. Rewetting index is a measure of the water volume available for rewet and is the volume above any constriction within the fabric structure. The number of holes over the surface area of the fabric is also factored in to give a rewetting index. Also, for duplex, the large and small surface holes are considered. The values are normalized with duplex having a rewetting index of 1. It can be seen that duplex and 2nd generation huytexx have a similar rewetting index. Both do not give rewet on sensitive machines. 1st generation huytexx has a higher rewetting index and absolute void volume and can give rewet on some machines.
Table 7. Void volume and rewet index