A comparison of SSB forming fabrics concepts

Not since the introduction of the x-weave or extra weft design in the mid 80's has a new forming fabric concept caused such a revolution in the paper industry.  The introduction of SSB designs in the late 90's and the subsequent development to date have established the concept as a market leader, across most paper grades and machine types.

The term SSB is the industry classification for "Sheet Support Binder" fabrics and is described as "a multi-layer forming fabric, in which the binding yarns become a part of the structure linking top to bottom and helping to support the sheet".  The industry now sees these designs as the premier design across all Fine and Publication grades and has even begun to enter the so-called Brown or Packaging sector.

The following paper aims to highlight the difference between the current SSB's on the market and demonstrate the benefits and limitations of each.

The major difference between the numerous designs available to papermakers is a direct result of the way they are woven. Most SSB designs are woven on standard 20 harness loom technology and are developments from the standard triple layer designs which were usually woven on 20 harnesses - this weaving concept is a traditional textile technology.  INTEGRA from Heimbach is totally unique in that it is woven on 24 harnesses - the weaving concept here is "state of the art".  The term harness refers to the repeat in the CMD pattern, and in all woven cloth is controlled by the loom.  For 20 harness looms the repeat in the woven structure must be a factor divisible by 20 - hence the 1x1 plain weave over 5 harness of these types of SSB's.  No other realistic configuration is available for the 20 harness designs.  If we consider INTEGRA from Heimbach, the design is woven on 24 harnesses, so the variation available is much more as 1,2,3,4,6,8,12 are all multiples of 24.  In reality the variation available is limited to 1x1 plain weave or 1x2 twill on the paper side (although a 6 harness inverted may be applicable for tissue!!) and 1x4 (4 harness) or 1x6 (6 harness) on the machine side.  It is instantly recognisable that the 24 harness technology offers greater flexibility to the PMC manufacturer to better tailor these bespoke products to the machine type and paper grade.

No. of Harnesses

 

1x1 2 shaft

1x2 3 shaft

1x3 4 shaft

1x4 5 shaft

1x5 6 shaft

20

Paperside

x

x

 

Machineside

x

x

24

Paperside

x

 

Machineside

x

Table 1

Table 1 demonstrates the variation available in both 20 and 24 harness designs, clearly showing the greater variation with the 24 harness weaving technology.

If we now consider these implications for the papermaker and how these variations will benefit him/her, it will be apparent that for the same MD mesh and MD diameter, a 6 shaft on the machine side will have a higher wear volume than a 5 shaft.  If you consider Figure 2 it is instantly visible that this fact is indisputable. 

Figures 1 and 2

The photomicrographs in Figure 1 show two CMD machine side yarns - one from a 20-harness design and one from INTEGRA. The 6-shaft float is approximately 10-15% longer, meaning longer life on the paper machine.  This has been proven numerous times with comparative runs against 20 harness products. In one example, on an Valmet HSSC former making SC with ~30% ash, the INTEGRA doubled the normal 3 weeks life to run 6 weeks - a life time unachievable with the 20 harness SSB previously run on the same position!

In conjunction with the longer float length the burial of the MD machine side yarn is much higher. Now this may not be clear on first read but if you consider the fact that the structural integrity or failure point of any multi-layer fabric is compromised when the MD yarn is damaged or worn. Therefore if the MD yarn is further away from the abrasive rolls and drainage elements it follows that the fabric life will be higher.

The photomicrograph in Figure 2 again shows same two CMD machine side yarns. The position where the MD yarn would be is much higher (~20%) in the 24 harness structure and enables more CMD yarn material to be worn away before the MD is touched by the abrasive elements.

In short, this again means a longer life potential on the paper machine.

It can be argued, although by who is unsure, that improved life potential is no reason to change your purchasing policy.  One of the main market drivers, in conjunction with reducing /tonne of paper produced, inevitably comes back to improvements in sheet quality. This need for better formation, improved profiles, less wire mark etc etc have been the main design drivers in the development of SSB designs.  If we consider the impact that extra weft designs had in the mid 80's, which was in part due to the increase in Fibre Support Index (F.S.I.) compared to previous single and double layer designs.  Increasing the F.S.I will tend to increase the mechanical retention.  This allows greater flexibility to the papermaker in achieving an improvement in either retention, formation and wire mark or in some cases all three.

It seems feasible to suggest that if you increase the mechanical retention you can either reduce the retention aid addition rate or increase the headbox dilution - machine parameters permitting.  Either way this will tend to promote an improvement in formation. By how much is dependent on the starting point, the machine type and furnish used.

If we consider how this mechanism is related to the forming fabric design - and in particular 20 and 24 harness SSB's. It may be safe to assume that if the PMC supplier increases the FSI of the fabric the mechanical retention of the PM will increase and the papermaker will be happy!! Unfortunately, as the FSI increases the air permeability tends to decrease - yarn for yarn, mesh for mesh. This reduces the drainage index and increases the potential for a reduction in sheet dryness.

In a 20 harness SSB the repeat on the machineside is every five MD yarns; so in a 29/cm MD 20 harness SSB the number of machineside MD/CMD knuckles is 29/5 = 5.8/cm.  Whereas in a 24 harness SSB the number of machineside MD/CMD knuckles is 29/6 = 4.8/cm.  These knuckles simply reduce the drainage paths through the fabric structure - in effect reducing the air permeability and drainage index. The benefit may not be instantly recognisable to the poor papermaker. However, if you consider Figure 3 below it is instantly visible that for the same diameters and mesh the air permeability of a product with a 6 shaft machineside will be higher than that with a 5 shaft - due to the reduction in the number of these MD/CMD knuckles. A good comparison is the industry change in DL products from 7 shaft to 8 shaft.  These 8 shaft DL designs were standard on numerous machines - and in some applications still are.  The same benefits associated with the difference in 7 shaft to 8 shaft in DL designs can be expected in the difference between 5 shaft and 6 shaft SSB designs,

Figure 3

Figure 3

It would therefore be correct to say that the PMC supplier can either increase the number of yarns on the paperside for the same air permeability - thus increasing the FSI, or increase the machineside yarn diameter - thus increasing the already ample life potential and increasing the CMD stiffness to promote an improvement in sheet profiles. Either way, significant benefits to the papermaker.

All SSB's, in either 20 or 24 harness versions, are available in various weft (CMD yarn) ratios with the three predominant designs being 1:1, 2:1 and 3:2.  The term weft ratio describes the ratio of paperside and machineside weft yarns. Figure 4 below demonstrates the variations available.

Figure 4

Figure 4

Each variation is designed for different machine types and objectives. As a general rule the 2:1 ratio is a 20-harness product and was one of the first SSB concepts, but further developments have seen this product superseded in some applications.  The 3:2 ratio offers a significant increase in yarn or wear volume and CMD stiffness, without losing fibre support on the paperside, and tends to predominate in the 24 harness products like Heimbach's INTEGRA range. The 1:1 ratio is available in both 20 and 24 harness products and is generally aimed at high-speed gapformer applications due to the reduction in caliper and hence void volume.

In conclusion it can be stated that a 24 harness SSB forming fabric, like Heimbach's INTEGRA, offers greater flexibility to the PMC supplier to better tailor these bespoke products to suit the papermaker requirements.