Why PTFE

For special-purpose cables

Hightech cable insualtion material

PTFE (Polytetrafluorethylen) is a partially crystalline homopolymer with an exceptionally high molecular weight of 107- 108 kg/kmol. This high molecular weight makes melt-processing of PTFE impossible. The product leaves the polymerization process in a highly crystalline form with a crystallization degree of > 90% providing it with a melting point of 340 - 345 °C. After the first melt, the crystallization degree decreases to approx. 60% and the melting point to 327°C.

It is very suitable for high temperature conductors due to the high temperature resistance, the high mechanical stability and the high durability for usage in harsh environments. To summarize:

excellent all-round chemical resistance
widest service temperature range of -200 °C to +260 °C
superior dielectric properties
no embrittlement or ageing
very good non-stick properties
dimensional stability and stress cracking resistance

PTFE cable production

Generally the processing of PTFE is more complicated than of "conventional" raw materials. Especially the reduction ratio must be taken into account, which the producers of the PTFE raw materials purport. The reduction ratio is the ratio of cross-section of the preform to the cross section of the insulation on the wire. The reduction ratios mostly are within the range of 100 - 4000, depending on the type of PTFE.
The maximum achievable running length for cable, results from the volume of the preform, which in turn results from lenth and diameter of the extruder cylinder. The volume of the preform can be "distrubuted" on the wire. Whereat you always have to consider, that the cross section of the preform must match the cross section of the insulation on the wire (see above).

As the lenth of the cylinder is predetermined due to the extruder design, the only adjustable parameter in the diameter of teh cylinder. You can use smaller as well as larger cylinders in one and the same of our machines, e.g. you can use a diameter 76 mm cylinder instead of the common 64 mm. This enables you to produce a wide range of product diameters with one machine.

For an example let us take a strand of diamerter 0,5 mm and an outer diameter above insulation of 1,2 mm. This would result in the following reduction ratios:

Extruder cylinder 64 mm: 1:2468
Extruder cylinder 76mm: 1:4180

Now if the recommended reduction ratio for the PTFE of our axample would be 3000, in our given example the bigger 76 mm cylinder would be above that, and thus too big.

The total length of our cylinders ranges up to 3000 mm. So you could load 3 preforms of 1000 mm each into these extruders.

However it is not possible to build any size of cylinder, just to reach higher running lengths, das always the ratio from cross-section to insulation cross-section must be considered! Typical running length are 2000 - 6000 m. The line speeds at which the cable is being produced in turn, comply with the thickness of the insulation. As the rsolvent, which is present in the PTFE raw material, must be completely evaporated from the finished product, in order to give the cable it's many positive characteristics. This evaporating process cannot be accelerated and so simply take more time as the nsulation becomes thicker. Common line speed at whcih PTFE-cable extrusion lines oprate are about 60 m/min. At very thin insulations however it would be possible to reach up to 100/min in line speed. However here are more physics and chemistry the limiting factors than the machine technics.

One of the most important characteristics of a PTFE-cable extrusion line is the centricity reached. This means how far the conductor / wire has moved out of the total center of the complete cable. The thinner the insulation wall strength becomes, the more difficult it gets to reach high centricitys. This is because, the thinner the wall thickness of the insulation, the more significant an ex-centricity becomes in proportion. At very thinn wall thicknesses it is almost impossible to reach a centricity of 92%, however even here our extrusion lines reach impressive values.

Last but not least, one has to mention that the production of PTFE-cables, compared to the processing of "conventional" raw materials is technically much more challenging. It is required that the machine operators act with great care at every single step of the production process.

These are the most important steps

Step 1: Make the preforms

The PTFE powder is stored in pouring containers, from where it is withdrawn by small coned pots. With the help of these coned pots the operator fills the preform press. This press is pricipally a long cylinder which is in the center equipped with a mandrel bar. Correspondent to the mandrel bar a round cavity develops in the middle of the preform. After the PTFE material has carefully and without pressure marks been filled into the cylinder, a piston compresses the material with a pre-defined sequence so that a preform is created. This can be unloaded with the help of a plexiglas tube.

Step 2: Load the extruder

A PTFE-Extruder (or paste or ram extruder) is loaded similar to a muzzle loader from the front. For it the extrusion head of the machine swivels out to the front and then to the side. The preforms are now with the help of a loading support being fed into the extruder. As the preforms usually are not in one piece, but in divided into halves or even thirds, at the connections great care is required when the perpendicular and roughened end is created. Additionally the PTFE is moisturized with more of the used resolvent.

Step 3: The extrusion process

Through the very high pressure inside of the extruder cylinder, the PTFE virtually plastifies, thus enabling it to be put around the conductor wire with the help of the used tools, which define the wall thickness (and possible shape). Due to the very high pressure (1300 bar) whichis present inside of the extrusion head during operation, it is technically very difficult to maintain the conductor completely centered inside of the insulation. Our PEX extruders are marked by especially high centiricty values!

Step 4: extracting resolvent and curing

After the extrusion process the PTFE-insulation contains a high portion of resolvent (e.g. Isopar), which must be removed for the biggest part within the vaporizer. To do so, the material is carefully being heated, the resulting vapors are evacuated by any exhaust system. Following the product passes a sintering preocess, where the PTFE receives it's final stability by again heating up to even higher tempartures with subsequent air cooling. To avoid explosions of the resolvent vapors it is mandatory required to to this heating process in these 2 steps.

Why PTFE

Why PTFE

For special-purpose cables

Hightech cable insualtion material

PTFE cable production

These are the most important steps

Step 1: Make the preforms

Step 2: Load the extruder

Step 3: The extrusion process

Step 4: extracting resolvent and curing

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