The New STFI-Former

The New STFI-Former
Bo Norman, Lennart Hermansson and Daniel Söderberg
STFI-Packforsk, Stockholm, Sweden
ABSTRACT
In high-speed twin-wire roll-blade formers, a suction
pulse is generated at wire exit from the forming roll
(table roll suction effect), which may locally deflect the
inner wire. This can generate large- and small-scale formation
defects. Such defects have also been observed in
high-speed hybrid roll-blade formers.
By insertion of a machine wide blade with a thin,
water meniscus breaking front edge, between inner wire
and roll surface at roll exit, the suction pulse, and thus
also the formation defects, can be avoided. This blade is
named the STFI–Blade.
The flow stability in the suspension between the wires
at exit from the forming roll is very important, to avoid
formation defects. This sometimes requires an improved
headbox jet quality in comparison with industrial
standards. The jet quality can be improved by a headbox
nozzle with higher than traditional contraction ratio.
The original twin-wire roll-blade STFI–Former has
been modified to the New STFI–Former by the introduction
of an STFI–Blade and the application of a headbox
with high nozzle contraction. In this way, formation damages
can be avoided even at high speeds.
INTRODUCTION
New high speed paper machines aimed at producing
different kinds of printing paper qualities based on
mechanical pulp or entirely on chemical pulp, are all
relying on the twin-wire roll-blade forming principle. In
such industrial machines, the current designs don’t seem
to be principally stable, in the sense that different
running parameters can be changed, without creating any
formation problems. Typically, jet landing in the twinwire
nip has to be on the outer wire, or formation
damages will inevitably occur.
One paper mill comment is: “To run our forming unit
is like balancing on a knife edge”. Any change in pulp
quality, grammage level, speed, slice opening etc may
result in formation defects.
At the 2007 PAPTAC Annual Meeting in Montreal,
hybrid roll-blade forming was discussed [1], that is
fourdrinier dewatering followed by a top wire unit
including an initial dewatering roll, see Fig. 1.
The maximum roll dewatering pressure p [kN/m 2 ] is
nominally
p = T/R (1)
where T [kN/m] is wire tension and the wire curvature
R [m] is approximated by the roll radius.
In reference [1], it is stated:
“With high running speed various types of web defects
start to show up, typical defects include large-scale
formation defects, crushing of the web, and light spots in
the web.”
Fig. 1. Hybrid roll-blade former principle [1].
It was suggested that the roll should be replaced by a
curved suction shoe, to avoid such defects [1], see Fig. 2.
Fig. 2. Valformer top suction shoe [1].
The main advantage of a suction shoe in comparison with
a dewatering roll is claimed to be the larger freedom in
choice of radius of curvature R and dewatering area
length.
One common process feature for a twin-wire rollblade
former and a hybrid roll-blade former is the
separation phase between wires and roll surface (note the
suction pulse at the pressure pulse end in figure 1). This
is an analogy with the basic effect of table rolls in the
traditional fourdrinier machine see Fig. 3.
Fig. 3. Traditional fourdrinier section with table rolls.

Table rolls were applied already on the first paper
machine in the 1820s, but the task was then only to keep
the wire horizontal along the wire section in a frictionless
way, while gravity generated dewatering.
A suction pressure zone is generated in the downstream
expansion zone between wire and roll. This
causes a local downward deflection of the wire, which is
followed by a corresponding upward movement to the
next table roll. The vertical motions of the wire generate
instabilities in the fibre suspension on the wire, “activity”,
which may improve fibre deflocculation in the mix
on the wire, and thus improve final web formation.
During the 1950s, the table roll suction effect became
stronger at increased machine speeds, and generated formation
disturbances by excessive activity,. Wrist [2]
studied the mechanisms, and showed that the suction
pulse amplitude increases with the square of the machine
speed.
The solution to this problem was the introduction of
the foil blade. This was a stationary, mainly flat element
mounted below the wire, with a small angle against the
wire. It was then possible, at every stage along the wire
section, to choose a suitable angle to optimise activity in
the suspension on the wire.
The difference between table roll and foil blade pulse
amplitudes is demonstrated in Fig. 4.
Fig. 4. Suction pulses generated by a table roll (top)
and a foil blade (bottom), [2].
We now suggest that the change from twin-wire roll to
suction shoe dewatering according to the Valformer
concept is in analogy with the change from table roll to
foil blade on the fourdrinier wire. The main advantage
would then be to avoid the suction pulse after the
forming roll.
Based on the present study, it is suggested that suction
pulse problems in high-speed roll-blade dewatering can
be avoided with an alternative, less expensive design.
This new design will be discussed in some detail below.
ROLL / LOADABLE-BLADES FORMING
Based on traditional roll forming technology [3], com–
bined with Dörries loadable blades concept, [4], the first
twin-wire roll/loadable-blades former, the STFIFormer
[5] was designed, and started up in June 1992, see Fig. 5.
Fig. 5. The twin-wire roll-blade STFI-Former [5].
The main idea behind this design was to make 3-ply
forming possible. The outer layers should initially be
dewatered over the forming roll. After this, the blade
pulses should break down fibre flocs remaining at the
centre, but mixing between the layers should be avoided,
since the outer layers were already formed. An im–
portant part of the design was that dewatering at each
stage should be symmetrical. It was soon realised that
the design would also be well suited for producing all
types of single-layered printing papers.
This basic roll/loadable-blades principle was later
adopted by Voith as the Duoformer CFD, the TQv and
the TQb designs, see Fig. 6.
Fig. 6. The Voith twin-wire roll-blade former TQb
(right), specifically designed for rebuilds of
twin-wire roll formers (left).

The corresponding Valmet/Metso machines are Speedformer
MB and Optiformer, see Fig. 7.
Fig. 7. Valmet/Metso twin-wire roll-blade Optiformer.
SUCTION PULSE EFFECTS
The formation produced with the STFI-Former never
reached the high levels expected. Different kinds of
defects appeared in the products. One defect was a faint
pattern similar to that of the forming roll shell surface.
This was of the original KMW design, where wavy
plates formed the space for dewatering to the roll side,
see Fig. 8. It was suspected that fluid filled the areas
below the “wave maxima”, and therefore locally accentuated
the table roll suction effect, compared to the
surrounding, not completely water filled areas.
Fig. 8. Wavy plates at the KMW forming roll surface.
Circles indicate holes connecting to suction chamber, [5].
An agreement was made with Valmet, to manufacture a
forming roll of the new design, based on conical holes,
replacing the wavy plates, see Fig. 9.
With the new forming roll, the web pattern from the
wavy plates disappeared. However, some formation defects
would still appear. It was therefore suggested that
table roll suction effects still caused some problems.
The dewatering pressure event along the FEX forming
roll was measured, during such running conditions that
left some free suspension between the wires at the end of
roll dewatering, see Fig. 10.
Fig. 9. Conical hole pattern in the new FEX forming roll
from Valmet, [5].
Fig. 10. Pressure event along the FEX forming roll. Roll
diameter 1635 mm, wire wrapping angle 30 degrees,
wire tension 7 kN/m, machine speed 800 m/min.
FEX measurements in co-operation with Valmet, [6].
A pressure sensor at the end of a thin flexible cord was
released into the jet (A), then into the twin-wire roll zone
(B), and finally into the zone where the wires leave the
roll (C). It is evident that some pressure oscillation
occurs around the end of roll dewatering. Such oscillations
have been found by many investigators. It
should be pointed out that in pure roll dewatering (where
such oscillations do not occur), the outer wire rests on a
solid web when leaving the forming roll, while in rollblade
dewatering the wire then still floats on some free
suspension.
At the separation of the wires from the forming roll,
local vacuum is generated by the table roll suction effect.
We assumed that this suction pulse was a source for the
upstream pressure fluctuations. Holm et.al. [7] therefore
studied the pressure event and wire geometry in a model
roll former with a solid roll for one-sided drainage. A
spring-loaded, wire touching displacement sensor was
mounted at the roll surface, by which it was possible to
follow the wire position along the forming zone, see Fig.
11.

Fig. 11. Left: Wire position transducer (bottom) and
pressure probe (top). Right: Wire-to-roll distance along
roll surface. Wire leaves the roll surface the minimum
point to the right. In this example an impermeable wire
was used, wire tension 5.5 kN/m, [7].
It was demonstrated that the vacuum pulse generated
when the wire left the forming roll also triggered the
oscillations in wire curvature and pressure upstream, a
phenomenon which could also explain the pressure
pulsations noted in full scale operation (Fig. 10). Wire
tension is here an important parameter.
A suction pulse at roll exit means that the inner wire
may be locally sucked away from the outer wire, when
leaving the forming roll, see Fig. 12.
Fig. 12. Local separation between inner and outer wires
when leaving a forming roll.
The local wire separation demonstrated in Fig. 12 may
generate streaks and/or formation disturbances in the wet
web on the outer wire. Like in the case of table rolls, the
amplitude of the suction pulse increases with the square
of the machine speed. This phenomenon is therefore of
increasing importance in modern machines with
successively higher running speeds.
The first support blade against the inner wire is
normally placed a minimum of 200 mm from the separation
line from the roll. Due to the traditional size of
support blade and mounting frame, it is difficult to
reduce this distance significantly. This is the background
for the insertion of a new blade. The main task for this
new blade is to break the water meniscus between roll
surface and inner wire at a very early stage, thus preventing
the generation of a local vacuum pulse. There
will then be no local inner wire local deformation. To
come close enough to the separation line between inner
wire and roll surface, which is necessary to avoid a
vacuum pulse, the blade front edge must be very thin.
The new blade has one flat surface, to face the inner
wire, and one curved surface (with forming roll radius) to
face the forming roll surface The blade edge may be
tapered. This new blade is named the STFI-Blade [8].
Eq. 2. describes the relationship between the free wire
length L [mm] from the separation line on a forming roll
with diameter D [mm] to the front edge of an STFI-Blade
with front edge thickness d [mm], when the blade is
mounted to just touch both inner wire and roll surface.
L = √d x D (2)
To test the concept, an STFI-Blade of glass fibre reinforced
polymer was manufactured, with a front edge
thickness of ca 0.5 mm, see Fig. 13.
Fig. 13. The STFI-Blade , mounted at the wire exit from
the forming roll in the STFI-Former [8].
It should be pointed out that, contrary to the traditional
deflector blades in a roll-blade former, the STFI-Blade
does not generate any wire deflection. Therefore, no
normal forces act on the blade surfaces, which then
means a low level of frictional forces along the blade
surfaces. The insertion of the STFI-Blade gave a free
wire length of ca 30 mm. The original STFI-Blade was
mounted in the STFI-Former in 2004 and still (in 2007)
does not show significant wear.
Streaks and large scale formation defects, at times
present in the paper produced with the STFI-Former,
were avoided after the introduction of the STFI-Blade.
However, a large number of small scale light spots
unexpectedly appeared in the web, see Fig. 14.

Fig. 14. Light spots appearing in the paper web at the
introduction of the STFI-Blade.
Fine paper, 60 g/m 2 , 800 m/min.
FLOW STABILITY AT ROLL EXIT
It was – at length – concluded that the formation defects
generated by the STFI-Blade could possibly be caused by
flow instabilities in the free suspension between the webs
leaving the roll dewatering section.
It was already in 1982 demonstrated on the FEX
machine that a twin-wire blade former is much more
sensitive than a twin-wire roll former regarding jet velocity
fluctuations. The original headbox on the FEX
machine was a KMW headbox of high turbulence level
design, initially developed for the application in twinwire
roll forming only.
Fig. 15. KMW-HTB (High Turbulence headBox) from
the 1970s.
At the start-up of the FEX machine, this headbox was
applied in the twin-wire roll former as well as in the
twin-wire blade former. The types of formation obtained
are demonstrated in Fig. 16.
Replacement of the KMW-HTB headbox by a Beloit
Converflo headbox (with a very high nozzle contraction
ratio) in the FEX blade former, produced an excellent
paper formation. This clearly demonstrated that in twinwire
blade forming, the level of flow instabilities in the
headbox jet – which is lower, the higher the nozzle contraction
ratio – has to be very low.
Fig. 16. Twin-wire forming on the FEX machine with
unbleached kraft pulp, using the KMW-HTB headbox.
Left: Sample from roll forming, Right: Sample from
blade forming.
When initial roll dewatering, followed by blade dewatering
originally was chosen as the concept for the STFI-
Former, [5], it was based on this knowledge, that roll
forming is much less sensitive than blade forming to
headbox jet instabilities. Since the major part of dewatering
in a roll-blade former takes place already over the
forming roll, it had earlier never been suspected that
instabilities in the suspension leaving the roll could be a
problem.
Only when the web damages according to Fig. 14
appeared, suspicions about the quality of the headbox jet
came up. In the original tests of the STFI-Blade in the
STFI-Former, a traditional hydraulic headbox with a
nozzle contraction ratio of ca 10 (designed in cooperation
between STFI and Valmet and manufactured
by Valmet) was used, see Fig. 17.
Fig. 17. STFI/Valmet headbox with traditional (low)
nozzle contraction ratio.
The STFI/Valmet headbox was replaced by a headbox
with a nozzle contraction ratio of ca 25 (designed by
STFI and manufactured by Uddevalla Mekaniska Verkstad).
The first test run with this headbox and with the
STFI-Blade in the STFI-Former was made on September
6, 2004.

Fig. 18. STFI/UMV headbox with high nozzle contraction
ratio.
It was then found, that the light spot defects which had
been generated at the original tests of the STFI-Blade,
disappeared when a high-contraction headbox nozzle was
used.
In comparison with the traditional STFI-Former, the
New STFI-Former therefore specifies:
• An STFI-Blade mounted at wire exit from the
forming roll, [8].
• A headbox with significantly higher nozzle
contraction ratio than traditional, [9].
CONCLUSIONS
In twin-wire roll-blade formers, a local suction pulse is
created when the wires leave the forming roll. This
suction pulse may locally deform the path of the inner
wire, and then create various kinds of formation defects.
The effect of the suction pulse will increase with the
square of the machine speed, which means that the
problems will accelerate with the increasing speeds of
modern machines.
The local suction pulse can be avoided, if the water
meniscus between inner wire and roll surface is broken
by a machine wide, thin blade edge, close to the exit line
from the forming roll. The related web formation defects
can then be avoided.
The insertion of such a blade is also an alternative to
replacing the forming roll by a vacuum shoe in hybrid
roll-blade forming [1].
Flow instabilities in the suspension between the webs
when leaving the forming roll may also generate
formation defects. Such instabilities can be reduced, if
the headbox nozzle contraction ratio is sufficiently high –
which may require a higher than traditional ratio. In
hybrid roll-blade forming, the level of activity in the
suspension on the fourdrinier wire in front of the twinwire
roll nip should be limited.
REFERENCES
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former technology to improve sheet quality and printability.
In: PAPTAC 93rd Annual Meeting 2007. Proceedings A,
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Transactions of the 2nd Fundamental Research Symposium
held at Oxford, 1961. Vol 2, 839-888.
3. Webster, D., Continuous web forming machine, Patent U.S.
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