Adapting DPS type forming fabrics to run on high speed newsprint machines

Andrew White, Huyck (UK) Ltd

Introduction
Most high-speed newsprint gapformers find that the un-mitigating need for very high operational efficiency is best met by conventional duplex type forming fabrics. These fabrics give the best drainage behaviour and highest sheet solids at the pick-up roll, often the sole technical target on newsprint gapformer wet ends. High sheet solids has become even more important as machine speeds have increased. Many machines now run at well over 1500m/min with 1900m/min the new benchmark.

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
Huyck began working on DPS fabrics in the early nineties in North America. The objective was to reduce wire mark and improve printability, specifically on newsprint grades. DPS fabrics can be considered as combining the best elements of x-weave and triple layer technology to form a new type of fabric structure. As with x-weaves, small paperside CD yarns were used to increase levels of fibre support whilst maintaining open area. These small CD yarns also act as stitching yarns. However, unlike with conventional triplex fabrics, the stitch yarns actually contribute to fibre support and drainage. This is achieved by "pairing up" stitch yarns so that, whilst one DPS yarn is providing fibre support, its partner DPS yarn is providing the stitching function. Huyck called this fabric design TXT. The yarn path of the DPS yarns is identified in Figure 1 along with the paperside structure of a TXT fabric.

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
Triplex forming fabrics were becoming increasingly more common on SC paper machines in Europe. These machines are very demanding on the forming fabric. High filler levels mean the sheet is very sensitive to wire mark hence the need for the plain weave surface of triplexes. However the same high filler levels also mean very high abrasion levels causing the stitch yarns of conventional triplexes to wear. This causes poor fabric stability giving poor basis weight profiles. In the most extreme cases the fabric can delaminate. The stitch yarns of conventional triplex also cause drainage restrictions, which give a wire mark. The mark is however better than with duplex fabrics.

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
It was seen very quickly that the huytexx fundamentals of very high fibre support and high permeability could be applied to newsprint. Also, the plain weave surface of huytexx could be utilized on the machines where TXT had given problems with wire mark and cleanliness. The first run was on a Symformer making standard newsprint. This machine normally runs double layer forming fabrics. Fabric parameters are compared in Table 1.

Table 1. Fabric parameter comparison

Parameter	Duplex	huytexx
Fibre support index	91	166
Permeability	410 cfm	430 cfm
Caliper	0.7 mm	0.82 mm

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

Variable	Change
productivity	5.2% increase
lost time due to breaks	2.2% less lost time
couch dryness	1-2% increase
former drive load	14% reduction
retention	3% increase
retention aid usage	25% reduction
smoothness	improved by 15ml/min or 15%

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
The application of huytexx to newsprint gapformers has proven to be a little more challenging. After the almost universal successes on hybrid formers, identical designs were applied to newsprint gapformers. Huytexx was often running against similar eight shaft duplex designs on both hybrid and gapformers so this was considered the logical first step. These initial huytexx runs on newsprint gapformers were not however universally successful. The paper quality benefits on gapformers matched those seen on hybrid formers. This is because most of the quality benefits due to huytexx are due to the paperside weave and levels of fibre support. However, on gapformers couch solids was often up to 1% lower which proved unacceptable. Table 3 summarizes the benefits and problems with initial huytexx runs on newsprint machines.

Table 3. Performance of 1st generation huytexx on newsprint

	Newsprint hybrid formers	Newsprint gapformers
formation	+	+
wire mark	+	+
smoothness	+	+
retention	+	+
retention aid consumption	+	+
cleanliness	+	+
couch solids	+	-/0
speed/runnabilty	+	-/0

The development of 2^nd generation huytexx
At elevated machine speeds conventional press sections cannot tolerate this increased dewatering load. In one instance the lower solids content resulted in premature removal of a pick-up felt. Analysis of gamma gauge surveys, on-line flow meter data from individual dewatering elements and sheet grab samples indicated the reason for the lower solids content, rewet. Rewet is caused when water from within the forming fabric is drawn back into the sheet. The main parameter affecting fabric rewetting potential is void volume. A 2^nd generation huytexx was therefore developed to minimize void volume. The design was targeted in the main for top positions where excessive void volume causes water spray giving cleanliness and runnability problems. Second generation has an identical paperside structure to 1^st generation huytexx to maintain the paper quality benefits. The machineside structure is modified to significantly reduce the void volume of the fabric. Figure 3 shows the differences in structure between 1^st and 2^nd generation huytexx. Table 4 shows the respective fabric properties of 1^st and 2^nd 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

Parameter	1st generation huytexx	2nd generation huytexx
fibre support index	152-185	152-173
permeability	320-500 cfm	340-480 cfm
caliper	0.8-0.9 mm	0.7-0.75 mm
void volume	51 mm³/cm²	43 mm³/cm²

SCA Ortviken Pm5
Ortviken Pm5 is a Bel Baie IV making standard newsprint from 100% TMP. Machine speed is around 1400m/min. They normally run 8 shaft x-weave fabrics in the top position and 8 shaft double layer fabrics in the bottom position. Table 5 shows the respective fabric properties for the top and bottom fabrics normally used.

Table 5. Conventional duplex fabrics on Ortviken Pm5

	Top fabric	Bottom fabric
design	8 harness duplex, x-weave	8 harness duplex
fibre support index	109	82
permeability	540 cfm	430 cfm
caliper	0.77 mm	0.74 mm

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 2^nd generation huytexx. Objectives were to improve wire mark and formation with no compromise in machine runnability. 2^nd 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

	Top fabric	Bottom fabric
design	2nd generation huytexx	2nd generation huytexx
fibre support index	150	157
permeability	470 cfm	430 cfm
caliper	0.71 mm	0.70 mm

Start-up was on the 29^th 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%.

2^nd generation huytexx was giving the same benefits on newsprint gapformers as 1^st generation huytexx was giving on more forgiving hybrid formers. 

Why does 2^nd generation huytexx work on gapformers
As a general rule, when considering high speed newsprint gapformers, low caliper duplex designs do not give rewet, 1^st generation huytexx can give rewet and 2^nd generation huytexx does not. Analysis of fabric internal structure with Cyberfab offers an explanation. Cyberfab is a computer software package developed by Huyck, Ecole Francaise de Papeterie et des Industries Graphique and Centre Technique du  Papier at Grenoble in France. It enables complex fabric structures to be accurately modelled in three dimensions. Figure 4 shows individual holes in duplex,1^st generation huytexx and 2^nd generation huytexx. The top of each column is the paperside of the forming fabric.

Figure 4. Drainage holes through various fabric structures

Duplex

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 2^nd 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 1^st 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 2^nd generation huytexx have a similar rewetting index. Both do not give rewet on sensitive machines. 1^st 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

	duplex	1st generation huytexx	2nd generation huytexx
caliper	0.65 mm	0.82 mm	0.70 mm
void volume (absolute)	38 mm³/cm²	51 mm³/cm²	42 mm³/cm²
rewetting index	1.0	2.3	1.3

Further developments
Experience at Ortviken Pm5 and other high speed newsprint machines has enabled Huyck to develop a greater understanding in the use of DPS type forming fabrics. Permeability, fibre support index, drainage index, void volume and weave structure are all important in giving the required paper quality and machine performance. Our thinking has been refined and 3^rd generation huytexx designs are now running on gapformers at speeds over 1750m/min. These 3rd generation designs are a continuation of the direction taken on Ortviken Pm5 and extend the principles of high fibre support, the correct permeability level, correct void volume and good z-directional void volume distribution. Called huytexx synergie, these designs give the same paper quality benefits as 1^st and 2^nd generation huytexx but have been seen to further reduce water carry and rewet giving higher couch solids figures when compared to duplex.

Acknowledgements
Acknowledgements to Pär Erikkson, Rickard Johansson and the machine crews on SCA Ortviken Pm5 for their support during the trial. Also to Görgen Nordlander from SCA Research for information and results from SCA Research fabric tests.