"n. felt. A stuff of wool, or wool and hair, which is matted into a fabric by beating, rolling, suction and pressure."
The dictionary version tells us little about this fascinating material, which dramatically changed the face of Industrial Manufacturing over 250 years ago and still finds its place today as an engineering material that cannot be paralleled in its usefulness.
Below we have outlined the basic processes in manufacturing Felt and its more common uses today.
However please note the following: - Felt is not the product that is put onto roofs below tiles.
Felt is not the product that covers snooker and pool tables.We are still surprised by the uses to which felt has been put and our education into this material is an ongoing process.
2. Wool Basic Information.
The most common type of wool used in the manufacture of Felt is sheep wool.
Unlike many other animals that have long hairy coats, sheep wool is a unique fibre.
Being hollow in construction, the outside surface of the wool fibre is made up of tiny overlapping scales.
When wet these scales swell and open up slightly. When two wool fibres touch each other the swollen scales interlock and become entangled. The more they are forced to touch the more they interlock and so the resultant mix of fibres becomes "tighter".
This is one of the unique properties of wool that allows it to be formed into a non-woven matrix of fibres, which is called "Felt".
3. Making Felt
a) Cleaning - Specialist cleaning and blending companies process the raw wool. The wool at this stage contains a high percentage of contaminants such as burrs, twigs and seeds that the animal may have picked up in its fleece. Also present are small skin pieces. These companies sort the wool by fineness and length of fibre. The wool once sorted is then scoured to remove a high percentage of these contaminants. It can then be further cleaned by a process called "Carbonising". This process basically burns away residual waste contaminants and leaves the fibre extremely clean. It can however shorten the length of the wool fibre and as such render the material only suitable for certain specialist felting operations.
b) Blending - Newly cleaned wool is then taken and blown through to a large holding vat. This blowing action thoroughly mixes the wool fibres together. At this stage other fibres can be introduced into the mix to obtain certain properties, handle or look of the finished felt. Manufacturers may introduce fibres such as rayon or viscose. This has the effect of whitening the product as well as reducing the final cost of the blend. Introduction of non-wool fibres does however change dramatically the performance of the felt and in no way will a blended felt react or perform exactly like pure wool felt.
c) Carding - The blended wool is then placed into large carding machines which draw the wool fibre out on large rotating combs. This process layers the fibres up to produce a "Bat" or more simply put " a continuous sheet of pre-felted fabric". "Bats" are layered up to form large pre-felted rolls of fibre. Specialist carding machines can also cross layer the bats prior to the next stage of manufacture. Two different types of felt can be manufactured from this processing stage. One is a layered felt (fibres in the felt all run in one direction) and the other being a cross carded felt (fibres run at 180 degrees to each other almost like a weft and a warp on knitted fabrics).
d) Hardening - Bats of pre-felted fibre are layered together under large canvas lined plates (sometimes the impression of the canvas can be seen in the surface of the final product). The two plates come together, compressing the bats. Steam is then forced between the plates to make the scales on the wool fibre swell. The two plates then systematically rotate in opposite directions on their horizontal axis. This causes the wool fibres to begin to interlock and the "felting " process begins. The longer the fabric remains beneath the hardening plates the thinner and harder (denser) the felt becomes as more of the wool is compressed and interlocked into a smaller space.
This process determines the final width, roll length and thickness of the felt to be produced and is governed by specific formulations for each grade of felt. Basically the more bats placed together under the hardening plates the thicker the end product becomes. The longer the felt is hardened the harder and ultimately thinner the felt becomes.
As the fibres interlock with each other they become shorter in length. This causes the width and length of the piece of felt to shrink.
This is a crucial stage in the production of the felt. If the felt is left too long under the plates then it will reduce in thickness and width below the requirement.
e) Milling - For many general engineering felts, the hardening stage produces a near finished product and all that is now required is drying. For more technical products or products of high densities a further more extreme process is required.
Hardening can only produce felts of medium to medium/firm densities. For densities of 40gsm and above the wool fibres need to be shrunk further. This process involves the use of acids. The felt is bathed in an acid solution and then further rolled and pressed. This systematically removes water from the wool and shrinks the fibre lengths therefore increasing their entanglement.
Once the required density has been met the felt is then bathed in an alkali solution to neutralise any residual acid.
f) Drying - All felts must be thoroughly dried prior to shipment. The drying process stabilises the fabric and sets both the thickness and width.
There are 2 types of drying methods incorporated world-wide depending on the country of manufacture.
Countries who are fortunate enough to be bathed in warm sunshine tend to dry felts naturally in the open by literally suspending them over frames in the outdoor sun.
This natural drying technique although time consuming is thought to give the felt greater long term stability as the water is removed slowly allowing the felt to relax and obtain a natural final finish.
The other method of drying is forced drying in warm/hot long gas fired ovens called "Tenters". The felt is placed on a moving conveyor belt. Hooks on either side of the conveyor penetrate the felt at its edge and effectively keep the material taut as it is dried.
(These holes can still be seen on the finished product). The felt is slowly pulled through the oven and all the resilient moisture is forced from the fabric.
Although certainly quicker than the natural method this process involves a higher degree of cost on the final product. Some feel this places a residual tension in the material that can cause problems when the material is further processed into components. Materials that have been Tenter dried have been known to lift or spring when cut as the tension in the roll is released.
g) Dyeing - Some felts can be dyed to a wide variety of colours such as red, yellow, green, blue, orange and black. The majority of dyed felts fall into the handicraft and display products. These are normally very thin felts (approximately 1mm) and provide very little technical functions other than a visual effect on exhibition stands and notice boards or a tactile feel for small soft toys. The most common dyed colour for technical felt is black.
It is very rare that thicker technical felt is offered in any other shade due to the inconsistency of the wool mix to take the dye equally through its structure.
