EoW March 2013

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Technical article
March 2013 90
DuPont Chemicals
and Fluoroproducts
1007 North Market Street
Wilmington, Delaware
USA
Tel: +1 302 774 1000
Website:
Email :
Increasing the gas pressure higher than
1,000 psig will cause too high of a gas
flow resulting in over foaming. This over
foaming condition is often misunderstood
as to be a material or processing problem.
Conversely, if the injector orifice is too
small there may not be enough available
gas pressure to obtain the gas flow
needed. This will result in an inability to
achieve the desired expansion rate and
product capacitance. For this reason it
is typical to have several injectors with
different flow rates available over a wide
range of pressures.
The number of different sizes needed
varies with the product mix and available
gas pressure. Utilising a high-pressure
nitrogen pump increases the gas
pressure range over that produced from a
high-pressure cylinder.
The use of a pump can then help reduce
the number of injector sizes needed for an
operation leading to lower overall costs.
The injector design can also impact
performance. Figure 5 shows the result
of trials comparing four commercially
available injector styles as quantified by
the gas flow variation and the resultant
capacitance variation.
A 50-ohm core with a 23-gauge conductor
foamed to approximately 50 per cent was
used for these trials. The gas flow rate six
sigma variation (± 3 standard deviations)
ranged from 4cc/min to 27cc/min with a
resultant capacitance variation of 0.3 to
3.8pf/ft. These results demonstrate that
performance issues often attributed to
the fluoropolymer material are typically a
processing issue related to the equipment.
Use of the wrong-sized injector or an
unstable design can mask the true
performance benefit of certain materials.
Product cooling
The cooling medium for the extruded core
is usually a combination of ambient air
and water. The distance required for each
of these is dependent on product size and
line speed. Having the correct distances
is critical for cooling prior to wire take-up
to avoid flattening of the insulation on
the reel and impacting the electrical
performance.
By keeping the water quench point
distance as far from the crosshead as
possible will yield the best product. This is
because a long air-cooling distance gives
time for the resin to shrink down onto the
conductor providing a consistent, tight
interface with the conductor without the
use of excessive preheat.
This consistent conductor interface
provides a uniform insulation strip force
even after the initial bond is broken.
The advantage is improved structural
return loss and resistance to stresses
of subsequent processing operations.
Sometimes a long air-cooling distance is
not an option because of total available
cooling distance.
If this is the case cold water should be
avoided in the first cooling section as
excessive ovality of the insulation and low
conductor adhesion can result.
▲ ▲ Figure 6: Cross section of foam core with solid skin outer layer
Tempered cooling is recommended
as it reduces the initial shock on the
insulation improving insulation ovality and
conductor adhesion.
Skinning
Extruding an outer layer of solid material
or skinning provides additional benefits,
such as:
• An easy and efficient way of colouring
the insulation
• Improved dielectric strength, which is
useful on thinner wall cable designs
• Higher foam expansion rates
• Greater resistance to insulation damage
during subsequent processing such as
twinning or braiding
Applying a solid skin coat requires an
initial equipment investment (an auxiliary
extruder and special crosshead) but
provides payback in reduced scrap and
product cost. Both the foam and solid layer
are achieved at the same time through
a single cross- head using standard
processing methods. Figure 6 illustrates a
foam core with a coloured solid outer layer.
Conclusions
There are various foamable fluoropolymer
resin options available, each having
unique capabilities and limitations.
Selecting the correct resin for the
application is important for cost, ease
of processing and desired electrical
performance.
Designing and processing cables within
the materials’ capabilities will produce
quality products with high yields.
Processing equipment selection and
process conditions are critical to ensure
a stable process, maintain minimum
product variation and achieve the lowest
cost operation.
Special techniques, such as the addition
of solid skin layer(s) to foam constructions,
can provide additional improvements to
processing and performance. n
This paper was presented at the IWCS
symposium November 2011.