OPTICAL FIBRES

The principle of all the coating processes described here is the same. Similar to the production of enamelled wire, the fibres are drawn through ink applicator heads with upcoating nozzles and then dried. Today only UV curable resins are used which are dried by curing them with intensive ultraviolet light. The majority of the raw materials used for the production methods described here are UV-based acrylates.
It is possible to achieve particularly high throughput speeds of up to 3,000m/min. when UV inks are used with modern equipment. The various different manufacturing processes described below can be used with modern, state-of-the-art production equipment.

Optical fibre colour coding

As already mentioned, the coating of each individual fibre is applied in the form of a UV curable ink. The thickness of the coating varies between about 4 to 6µm, and is hardened by means of an ultraviolet irradiator with a nitrogen atmosphere inside. Nitrogen is a cheap and environmentally neutral industrial gas used to optimise the curing of the coating surface, in other words, to achieve better cross-linkage of the coat of UV ink applied.
Colour coding serves the following purposes:

Fibre ribbon production

– to simplify further processing, and in case of multiple lead optical fibre cable, for differentiation and measurement;
– to simplify finishing fabrication;
– to offer increased protection against hygroscopy/hydrolysis;
– to achieve better slip effects in the cable (longitudinal compensation with temperature);
– for better strippability of optical fibre ribbons, micro bundles and CFUs.

Tight buffering

Other important criteria include coating thickness, type of resin, curing rate, attenuation increase due to colour coating, winding quality after coating. The coating thickness is 4 to 6µm (resulting increase in diameter approx. 8 to 12µm).
A total of twelve different colours are normally used. Twenty-four colours can also be used, but this is unusual due to the difficulty of differentiating between them. Additional ring marking is sometimes used as an alternative.
Equipment with modular individual components has proved extremely effective here. This makes it possible to adapt the coating system to a wide range of applications quickly and easily. If necessary, the system can even be expanded at moderate cost, for instance with: ring marking, higher UV output, tight buffering, screen proof tester, fibre ribbon production, etc.
Specially developed soft curing UV irradiators offer the following advantages:

Compact fibre unit (CFU)

– perfect curing of the fibre coating with minimal impact on the fibres and coating resin;
– permanent monitoring and automatic regulation of UV irradiator performance depending on the speed of the equipment – this ensures optimal use of the irradiator, and prevents thermal overload of slow-running or stationary fibres; and
– optimized energy consumption compared with competitive products (30% less).

Ring marking of optical fibres

This is a product that is needed principally in the USA and Asia. Europeans generally prefer circular designs. The reasons for manufacturing ribbons are their compact construction and simple method of finishing fabrication; several optical fibres can be spliced in a single operation, and it is possible to achieve a high packing density in cables.
The production process can be briefly outlined as follows. Several optical fibres of different colours are arranged beside one another and coated to form a flat ribbon. Coating or gluing involves the use of a UV cured, usually transparent acrylate, which once again is cured by means of an UV irradiator unit.
A single coating process is generally used for fibre ribbons. The sheathing material is usually transparent, whilst semitransparent acrylates can also be used for additional means of identification.
Commonly encountered products are 4-fibre ribbons, 6-fibre ribbons, 8-fibre ribbons, 12-fibre ribbons and 24-fibre ribbons, which are fabricated from 2 x 12-fibre ribbons.
The production speeds that can be achieved with modern equipment are as follows (sample data for planarity < 50µ):
for 4-fibre ribbons 1000m/min
for 6-fibre ribbons 850m/min
for 8-fibre ribbons 750m/min
for 12-fibre ribbons 650m/min
whereby the design speed is 2100m/min.

Proof test

Since extrusion technology for voluminous single-fibre coatings frequently involves problems such as attenuation characteristics, product switching and material flexibility, it also makes sense to manufacture coated optical fibres with larger diameters using UV acrylates.
These coatings then serve principally to enhance the mechanical properties of the individual fibres.
A clever choice of materials and a possbile “wet-on-wet” multi layer upcoating makes it possible to manufacture a wide range of final products, – from “easy strip” to securely attached materials.
As already mentioned above, a UV acrylate is used for the coating of the optical fibres (coloured or uncoloured), and final diameters are typically 600µm and 900µm. The coating can consist of a single layer (one material) or a multi layer upcoating, e.g. with three materials.
The purpose of manufacturing tight buffered fibres is to achieve mechanical protection, finishing fabrication dimensions, i.e., for easy connection to connector systems, and small dimensions with high mechanical stability.
Another advantage of UV coating over the extrusion process is the fast problem solving and quick continuation of the coating process in case of possible fibre breaks.
By and large we differentiate between the following variants:
– Single-layer solution:
The advantages are simple manufacture, low machine costs and easy material logistics. The disadvantages that can be listed generally include poor strippability and limited control over mechanical properties such as hardness.
– 2-layer solution (wet-on-wet):
In this case two different materials are introduced through an applicator head so that they then form a uniform multilayered product (wet-on-wet upcoating method). The individual layers of this coating can be made to different thicknesses. The materials are not mixed together thanks to the ideal geometry and correct pressure conditions used. This method therefore makes it possible to use materials for the inner layer that do not necessarily have to be mechanically stable and spoolable. The jelly that can be used for this purpose is just one example.
The advantages are again simple manufacture, the possibility of achieving a wide range of mechanical variations in the final product, optimal strippability (e.g. by using lubricant materials such as jellies), no mixing of the materials if an excellent bond is formed immediately.
The disadvantages are the use of several materials and the resulting more complex materials logistics.
– 3-layer solution (wet-on-wet):
Once again, jellies can be used for the inner layer. The second layer can then be a medium-hard zone, while the outer sheathing tends to be a glass-hard layer (mechanically resistant skin layer). After being applied to the optical fibres, the coatings (buffered layer) are transported past a UV irradiator.
Commonly encountered products are:

Other applications of the coating system

– 600µm non-strip (1-layer buffer)
– 600µm medium strip (up to 10cm, 1-layer buffer)
– 600µm easy strip (up to 100cm, 2-layer buffers)
– 600µm easy strip with high surface hardness (up to 100cm, 3-layer buffers)
– 900µm non-strip (1-layer buffer)
– 900µm medium strip (up to 10cm, 1-layer buffer)
– 900µm easy strip (up to 100cm, 2-layer buffers)
– 900µm easy strip with high surface hardness (up to 100cm, 3-layer buffers)
In view of the fact that raw materials are getting increasingly expensive, and cable weight becoming more and more important, today micro cables are fabricated as compact fibre units (CFUs). This makes it possible to achieve geometries with 72 or 96 fibres for cables with outside diameters of 5.4 mm. Thanks to their low weight and high mechanical strength, such cables are suitable as blown cables for duct systems (micro cables for micro ducts).
Once again, it is UV acrylate technology that is used to produce these compact fibre units, since extrusion technology is no longer manageable (shrinkage, etc.).
This makes it possible to manufacture a typical compact fibre bundles with a diameter of only 1.3mm with 12 optical fibres.
During the manufacturing process, several optical fibres are coated with UV acrylate and sometimes also a jelly material. The typical final dimension of a 12-fibre bundle is 1,300µm.
The sheath may consist of one UV coating (rather seldom), 2 UV coatings, or a jelly material and 1 UV coating.
For additional identification, a bar code is generally applied to the outer sheathing by means of a jet printer (1 bar for CFU bundle 1, 2 bars for CFU bundle 2, etc.). Another method is to apply a stripe coding in the form of a UV acrylate. The colour of the stripe serves to identify the cable - the more complex overall materials logistics are a minor disadvantage of this.
The purpose of manufacturing CFUs is to produce an output product for micro cables.
The advantages are low material consumption, small dimensions with high mechanical strength coupled with low overall weight and high optical fibre packing density.
By and large we differentiate between the following variants:
– Single-layer solution:
The entire sheathing is applied at once to several precisely positioned optical fibres by means of an applicator head before being transported past a UV irradiator. Care is taken in the process to ensure that the cable is free of air pockets and uniformly round. The optical fibres are also held exactly in position.
Advantages: simple manufacture, low machine costs, simple materials logistics.
Disadvantages: This design can be used only for a maximum of 8 optical fibres, as the coating may otherwise burst at the side on bending. Limited control over mechanical properties such as hardness.
– 2-layer solution (wet-on-wet):
Two different materials are introduced through an applicator head so that they also form a uniform multilayered product as in the case of tight buffering. The individual layers are arranged in such a way that the coated optical fibres are in a soft bed with a harder skin layer over the top of them.
This method therefore makes it possible to use materials for the inner layer that do not necessarily have to be mechanically stable. The jelly that can be used for this purpose is just one example. The enveloping skin layer is applied at the same time before being transported past a UV irradiator.
Advantages: Simple manufacture, low machine costs, very good attenuation values (even after temperature cycles of -40°C and + 70°C). It is no problem to produce 12 precisely positioned optical fibres with an outside diameter of 1.3mm. The mechanical properties of the sheathing can be controlled.
Commonly encountered products are:
– 900µm 4 optical fibres
– 1,000µm 6 optical fibres
– 1,100µm 8 optical fibres
– 1,300µm 12 optical fibres
These dimensions apply to 1- or 2-layer construction, bearing in mind that technically it only makes sense to the 1-layer version to contain a maximum of 8 optical fibres.
Ring marking serves for additional identification of a fabricated fibre, i.e. to differentiate between several optical fibres (e.g., more than the 12 available colours in a bundle).
Generally speaking the rings are applied using a special black ink and a strongly modified inkjet printer. Other colours could theoretically also be used, but this generally entails a loss of performance due to difficult handling of pigments, reduced contrast, machine modifications, etc. The coating thickness of these rings is generally less than 2µm. The ring structure itself can be selected, though certain standards have become established.
Basically the ring structure can be applied by means of two different technologies:
1. The ring marking is applied directly to the optical fibres before colouring.
The optical fibres are usually coloured for additional identification during the same process (see “optical fibre colouring” above). Due to the fact that it is so thinly applied, the colour coating is sufficiently transparent compared to the ring structure, so that it is clearly visible.
Advantages: the rings are easily identifiable and cannot be removed, as they are protected by the UV colour coating over them, smooth and consistent surface, very good attenuation values.
Disadvantages: slightly higher machine costs, as a second colour applicator with a UV unit is required for an inline process.
2. The ring structure is applied directly to the already coloured optical fibres.
Advantages: The rings are easily recognizable. Machine costs are every slightly lower, as a second applicator station is not required for an inline process. The rings can also be applied at a later stage, in a separate process.
Disadvantages: the rings can generally be removed using aggressive solvents (MEK = methyl ethyl ketone).
The size of the rings themselves is usually 1.5 to 2mm, and they are formed by 5 or 7 closely positioned droplets.
Conventional ring marking structures:
The proof test process is used mainly to pinpoint faults in the glass core that may lead to problems in further processing or in the cable itself. Such faults cannot necessarily be found using OTDR measuring equipment, and hence this process is used. During the “proof testing process”, the optical fibres are subjected to increased tensioning. This can cause any micro cracks that may be in the glass core to be so severely stressed that fibre breakage may occur. The fibre snaps.
This process is carried out using a rewinding system. At the same time, the optical fibres are fabricated on transportable, standardized spools in standardized lengths (e.g. 25,200m or 50,400m).
All the optical fibres from the drawing tower are usually tested by means of this mechanical test.
The tensioning forces may vary depending on the type of optical fibre used. There is a standard for normal optical fibres.
This process can be carried out at test speeds of up to 3000m/min by ensuring the right ratio between testing speed and applied force.
On modern equipment the proof test process can also be carried out in combination with optical fibre colouring – provided the machinery is modular in design.
The process also serves to adjust the length of the optical fibres by winding them on transportable spools suitable for further processing.
The diameter of the optical fibres is usually recorded during the proof test process, and any faults noted or immediately rejected. This process is carrying out by highly sensitive contactless measuring heads.
There are international standards for the application of the proof test force. The test is generally a ratio between time and force. By correctly setting up the machine, it is possible to operate at any “proof test” speed without compromising the quality of the tests. Modern state-of-the-art machines adjust the proof test speed fully automatically.
Thanks to the modularity and flexibility of its systems, Medek & Schörner as one market leader in coating machines for optical fibres was able to implement other applications, including some outside the field of optical fibres.
An example
New manufacturing concept for precision micro flexible flat cables:
For a long time now, flexible flat cables (FFCs) have found widespread use in the automotive and IT industries. Lamination and extrusion are the processes most commonly used for the production of FFCs today. The disadvantage of lamination is its extremely low production speed. Extrusion is unsuitable for the production of micro FFCs due to the high temperatures and pressures encountered in the extruder head: these make it impossible to maintain accurate geometrical dimensions and precise positioning.
Medek & Schörner has now developed a new pressureless cold process for the production of FFCs using UV cured resins, thus ensuring the perfect geometrical accuracy of the cable at high production speeds. The same procedure can either be entirely separate or employed inline with an extruder to position the individual flat cables accurately as they enter the extruder head.

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