O o d h e a d p u b L i s h I n g L i m I t e d

partially strips it; the transfer is said to be mostly via air-flows and a more
random orientation is produced. MD/CMD is claimed to come down to 3:1
and with condenser rollers and randomiser down to 1.5:1.
The desire to increase productivity while maintaining quality has led to
nonwoven cards with three doffers being developed. The three doffers further
increase the rate at which fibre can be cleared from the main cylinder, allowing
higher throughput because a limit is usually reached when the card over-
loads and excessive nep generation occurs. The three doffers also provide
Workers
Feed rollers
Breast
Main
cylinder
Doffers
Condensers
Output conveyors
6.2
 Schematic diagram of a typical nonwoven card with condenser
rollers.
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Advances in wool technology
134
three webs and should lead to further averaging of the uncorrelated random
web variations. Correlated web variations, however, would be additive if
they arrive at the point of combination in phase. The reduced recycling due
to the three doffers means less blending and opening within the card and
could lead to poorer web clarity, possibly cancelling the benefit of averaging
the three webs.
6.2.2
Cross-lapping
To produce heavier fabrics from a single card, a cross-lapper as illustrated in
Fig. 6.4
 is often used. This device lays up the lightweight web leaving the
card so that the multi-layered web leaving the cross-lapper is perpendicular
to the card direction. The ratio of the card web speed to the cross-lapper
output speed determines the number of layers and the weight of the cross-
lapped web. In this case the predominant fibre direction is across the fabric;
the MD/CMD ratio is then less than 1.
Care has to be taken to align the edges of the overlapping layers otherwise
step changes in fabric density will occur at the mismatch. This means that
adjustment of the cross-lapper and the card web density together is required
to achieve the desired fabric weight.
6.2.3
Web drafting
The heavy cross-lapped web can be drafted, or stretched, to pull the fibre
orientation towards the machine direction. Web-drafters use multiple closely
Randomiser
6.3
 A random card. There is an extra counter-rotating roller between
the main cylinder and the doffers.
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Advances in the manufacture of nonwoven wool
135
spaced roller-sets moving at successively increasing speeds to draft the web
in several stages (Fig. 6.5). In this way MD/CMD ratios close to 1 can be
achieved but at the expense of fabric uniformity as the thinner areas tend to
stretch more easily than the thicker regions. It is impossible to fully control
the web because the wide rollers need to have a diameter sufficient to withstand
bending under their own weight. Their diameter determines the closest distance
between the roller nips and hence the degree of draft control possible. Web
drafting usually occurs after some pre-needling to provide some draft control.
NSC and Spinnbau have recently developed alternatives to cross-lapping,
known as the Turbo™ and Airweb™ doffing systems. These use a conventional
card followed by a new system that throws the fibres into an air-stream to be
collected by suction onto a drum or belt to give a randomised pattern of
orientation. These devices are claimed to produce MD/CMD ratios down to
From card
To bonding
6.4 
Cross-lapper.
6.5 
Web drafter.
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Advances in wool technology
136
1.2 directly. The output speed can be reduced to produce quite heavy webs
without cross-lapping, and high bulk is also possible with some fibre orientation
in the vertical direction.
6.2.4
Air-lay systems
Air-lay processes are an alternative to carding and cross-lapping. These
machines use high-speed rollers and air-flows to generate and air-lay a stream
of staple fibres into a three-dimensional web with high loft. The blending
action in the air-lay machine is less than a full nonwoven card as it has a
small main cylinder, fewer or no workers, and little or no fibre recycling
around the cylinder. However, the web properties are very different from
conventional card webs, having a random orientation distribution and, in the
high-loft versions, many fibres arranged in the vertical plane compared with
cross-lapped webs. This can give high vertical resilience and loft with high
throughput speeds. The lofty webs are usually spray bonded or thermally
bonded but may be needle-punched or spunlaced. Production rates can be
much higher than for the card cross-lapper or carded air-lay systems (such as
Turbo or Airweb). As little blending and opening occurs in the air-lay process
itself, greater attention must usually be paid to blending and opening prior to
the air-lay machine.
A number of companies manufacture long fibre air-lay machines of various
designs 
(Fig. 6.6).
 A process for adding short-fibre cellulose fluff-pulp
(~5 mm) to nonwovens is also called air-lay but this is not relevant to wool
nonwovens.
6.2.5
Wet-laid systems
Wet-laid systems are similar to air laid except for the method of distribution
and transport of the fibres into a batt or web 
(Fig. 6.7).
 The mode of fibre
High-
speed
roller
Feed
Blower
Output
Suction
6.6 
A schematic diagram of a generic air-lay machine.
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Advances in the manufacture of nonwoven wool
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suspension used is water rather than an air-stream. The wet-laid system is
also evident in paper making. The fibres are generally very short when
compared with conventional textiles and generally fall into the range of 2 to
5 mm. These fibres are suspended in water at a concentration generally below
0.005% which is much lower than for paper making systems. The web or
batt is formed on a mesh or perforated belt, also known as forming fabric,
where the water is removed and the short fibres are distributed randomly.
The forming stage is followed by pressing and drying for bonding of cellulosic
fibres. Bonding for wet laid can also be achieved using the same methods as
used for other nonwovens. These are chemical bonding with the addition of
a polymer binder or blending a low melt bicomponent fibre into the blend.
The fabric is then passed through a bonding oven and to further finishing as
required. Wet laid products have very low loft and in fact often resemble
paper in their physical properties. Products that are suitable for the wet laid
process include bedding, linings, medical textiles, wipes and cover stock.
Common wet-laid products are:
• medical textiles;
• interlining fabrics;
• cover stock;
• filter media;
• special papers;
• webs for printed circuit boards;
• serviettes, etc.