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Edition 7 – Threads In Focus | HT Polyester Backing Thread / Blind Hemming

HT Polyester Main 1HT Polyester is truly representative of Empress Mills long textile history. It has a purely industrial background and yet is ‘peppered’ all over the place in our sewing thread ranges because of its innate properties.

We first began manufacturing it as a weaving yarn. Modern looms produce fabric so quickly that at the point of weaving i.e. the moment the weft is sent between the warp threads, there needs to be a very strong yarn woven in to the selvedge at each side to really hold the fabric in place.

In order to get the strength needed the selvedge yarns were getting thicker and thicker. No good at all for fine cloth. We needed a very thin yarn with what seemed an impossible strength for something so fine. Along came this polyester. Very fine, extremely strong and non-stretch to stop any movement of the warp and weft – HT Polyester Thread was perfect.

Design & Make Up

First things first. Its full title is ‘High Tenacity Low Shrink Polyester’. We shortened it to just HT Poly in order to differentiate it from other polyester threads in our range.
High Tenacity = Strength. Low Shrink = Stability.

Essentially it is thanks to modern technology that such properties are attainable. The polyester used is a type 712 – a very strong single filament/strand of fibre. It takes 2 filaments twisted together to make the finished thread. This looks like a conventional thread because of the way it is put together i.e. how the strands are plyed. This plying/twisting is essential to its sewability. (See also Edition One of Threads In Focus for an introduction to twisting thread.)

Autosave-File vom d-lab2/3 der AgfaPhoto GmbHThe Twist

In order for a thread to run on a sewing machine the last process must have a Z – often known as a Reverse Twist – and this is why this industrial yarn is suitable for the sewing machine. Each of the 2 filaments of polyester which make up the thread are individually twisted tightly in an S twist. The two are then twisted tightly together the opposite way I.e. Z twist, Reverse. (Remember it is only the number of complete revolutions of twist which will give any strength to a thread – a loosely twisted thread is a weaker thread.) At this stage of manufacture the thread will be a ‘corkscrew’ of different twists and is totally unusable.

Heat Setting

It is only extreme heat that untangles the twist and produces a smooth, even, sewable product. This is called Heat Setting – temperatures of 130 degrees Celsius of dry heat must be evenly achieved throughout the thread. Any more and the thread will melt, any less it will twist back in to unsewable snarls. This process also shrinks the thread as much as it is possible to be shrunk. The natural colour of polyester is just off-white which loses itself in to all light coloured fabrics so Heat Setting is fine but of course we still need a thread for darker fabrics. The dyeing process applies a wet heat (the dyestuff), again to high temperatures and the thread is then dried. Whatever it is used for eventually, there will be no more shrinkage/movement. Complete stability.

The Finished Thread

A stable, fine, strong thread. Unshrinkable. Versatile.


Where do I start. At the beginning of this article I said HT Poly Thread was peppered all over our product range. Well – yes – it is so useful yet so much over looked. It isn’t stunningly beautiful or obviously amazing. It blends in to the background and perhaps this is its very best quality.

Blind Hemming

The stitch needs to be invisible from the outside yet strong enough for purpose. It need be only available in 2 shades – one for light fabric, one for dark. HT Poly Thread exactly suits!

Machine Embroidery Bobbin Fill/Backing

Unobtrusive to the outer design yet the strength to hold all the embroidered stitches. Only 2 shades needed. HT Poly Thread again!!


Economical and easy to use – giving good seam coverage. The even bigger plus is that Polyester is soft. Nylon alternatives will eventually damage even steel machine parts. This thread has almost a ‘kindness’ to sewing machines and more importantly you – the user. HT Poly Thread yet again!!!

Shop For HT Polyester Sewing Thread

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