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CORE-SPUN THREADS
The solution lay in the development of core-spun threads
that could withstand the extra heat and friction created by
the needle moving at the higher stitching speeds involved.
Early examples used a combination of polyester/cotton to
create a more efficient thread construction. This was
followed at the end of the 1970s by a significant advance
with the appearance of polyester/polyester core-spun
threads, which are still widely used today.
Core-spun threads differ from conventional spun threads.
Spun threads have single yarns consisting of short, cut
staples joined by twisting. Core-spun threads, on the other
hand, have yarns consisting of a multifilament core
surrounded by fine fibres which combine the strength of
multifilament threads with the good sewability and surface
appearance of those made from spun yarn. The core usually
comprises two-thirds of the thread and the cover one third,
with the core sheathed but not fully covered. As with those
made from fibre yarns, core-spun threads have a smooth
grip and textile surface but, due to the combination of
multifilament and spun fibres, both strength and abrasion
resistance are far higher.
These threads also have outstanding sewability even in
difficult applications such as multi-directional sewing
operations. Amann Group from Germany has been
producing sewing threads for more than 150 years and is a
major supplier of core-spun threads. Its sabaC 100%
polyester core-spun thread runs very well in all sewing
applications and on all types of machine from standard high
speed through to high-tech automated systems.
Most footwear uppers, however, are now stitched using
threads made from continuous polyamide or polyester
filaments. Amann’s Serafil is an all-purpose 100% polyester
thread noted for its outstanding breaking strength, high
abrasion and thermal resistance, well-balanced elongation
and good light and weather fastness properties. Having
said that, core-spun sabaC with its textile structure
nevertheless remains a viable alternative to smooth
continuous filament threads.
BONDED NYLON THREADS
There are, however, many applications where greater
tensile strength is needed and this is where nylon, or to give
it its correct name, polyamide fibres score as they can be
anything up to 25% stronger. Core-spun nylon threads also
have better abrasion resistance and this made them ideal
for use in sports shoes. The past 25 years has seen
phenomenal growth in the sports and outdoor footwear
sectors and the quest for ever better performance in turn led
to the use of new high performance upper materials that
sometimes proved difficult to stitch on high speed
automatic machines. Furthermore, this type of footwear is
often exposed to adverse or wet conditions, so the thread
must also be extremely strong, able to withstand abrasion
and be water resistant.
The thread manufacturers responded and invented
‘bonded’ nylon threads with a special coating that allowed
for even higher speed sewing and a wider variety of
footwear materials to be used. Netbond from Italian thread
manufacturer CT Point is one of these. A conventional twist
construction thread, made up from continuous polyamide
filaments, it has then been impregnated with a urethane
resin to bond them together. The filaments are in effect
‘welded’ to each other so that there can be no untwisting or
opening during stitching and no frayed ends when cut. The
thread can be used at high speed on even the most difficult
materials used in snowboarding boots and other advanced
types of sports orientated footwear due to a double coating
of lubricant which stops the needle overheating and
prevents the thread melting.
VERY SPECIAL THREADS
US thread producer American & Efird, among the largest
producers of industrial sewing threads in the world, has
long been regarded as a technological innovator. Its huge
range now includes not only all the regular types of core-
spun and continuous filament threads, which it supplies in
substantial volume to footwear producers, but also highly
specialist products such as Anefil Nylon DRY, a proprietary
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JULY/AUGUST 2012 | www.footwearbiz.com
sabaC polyester core-spun thread
CREDIT: AMANN GROUP
Serafil polyester continuous filament thread produces a neat seam
CREDIT: AMANN GROUP