The shrinkback principle

Shrinkback occurs when the insulation "pulls back" excessively, exposing the conductor either after cutting or after a period of time (normally related to heat exposure). Several tests can determine shrinkback. These tests involve heating a sample in an oven at a specific temperature and for a specific time. Other shrinkback problems become apparent immediately after cutting the insulated wire.

There are many factors that can create shrinkback, so start troubleshooting by asking the following questions:
equipment.

1. Is the problem associated with an increase in line speed above the process standard for the equipment?

2. Have any changes occurred in theextrusion line, i.e., conductor preheat,temperature profile, water trough
temperature or take-up tensions?

3. Remember that efforts to correct shrinkback may adversely affect other properties, e.g., adhesion.
Possible causes of shrinkback and recommended solutions:

4. At what stage of production was the shrinkback noted? This answer is important because excessive tension at
any stage of production can cause the problem.

5. Is the problem evident on product from only one machine or from several? If wire from several machines exhibits shrinkback, the problem may be resinrelated. Has there been a resin change recently?

6. Is the problem related to shrink voids? Slice a length of insulation from the conductor and examine (under
magnification) the conductor/insulation interface for presence of long voids.

7. Is the shrinkback associated with only one color or sequence of colors? If color related, look at the color concentrate or processing parameters of the

Possible causes of shrinkback and
recommended solutions:

A. Excessive Color Concentrate.
! Too much additive can also reduce
physical properties.

B. Conductor Preheat Temperature Too
Low
! Start with 200°F and adjust
accordingly. Too much preheat can
affect the cooling rate of foamed
insulation as well as the blowing rate. A
balance with other parameters must be
achieved for proper capacitance.

C. Foreign Material on Conductor
! Clean or replace wipes, or verify that
the wire does not have a coating on it.
Super-absorbent, paper-based
materials are not recommended as
wipes because they contain
components that may actually cause
D. Excessive Drawdown Ratio
! Try reducing the drawdown ratio.
E. Melt Temperature Too Low
! The foam extruder temperature
profile is crucial for proper blowing
and cell structure. Proper balance
with conductor preheat and cooling
is necessary.
F. Water Bath Too Close to Head or
First Trough Too Cold
! The cooling rate is critical to avoid
shrinkback. Look for shrink voids
along the conductor. If shrink voids
are present, try gradient cooling.
The initial air space length and
subsequent water trough
temperature must be balanced with
preheats and extruder profiles for
proper cell structure and minimum
shrinkback.
Note: If the problem is in cooling too fast,
the physical properties (yield strength) of the
insulated conductor probably test low.
G. Inadequate or Poorly Set Up Cooling
Trough
! If shrink voids are present, check to
see if line speeds have increased.
To eliminate shrink voids, try the
following:

G. Inadequate or Poorly Set Up Cooling
Trough
! If shrink voids are present, check to
see if line speeds have increased.
To eliminate shrink voids, try the
following:
a. Lower the line speed; if voids
disappear, cooling is most likely the
problem.
b. If hot water to the trough is
available, put hot water in the first
trough and try to establish a
gradient cooling system.
c. If slowing the line speed is not an
option, and hot water is not
available, try turning off as much
water to the trough as possible and
still cool the product before it hits
the bobbin. This procedure only
verifies if voids disappear; if the
product is still too hot when it gets
on the bobbin, problems occur later.
If the voids disappear, inadequate
cooling is the problem.
H. Excessive Tension Causing Copper
Drawdown (Improper String-Up?)
! If tension on the insulated conductor
at any stage is high enough to break
the adhesion between the conductor
and insulation, shrinkback can
occur, especially if the insulation is
cooled too fast. This tension can
also reduce the conductor diameter.

A dditional Note:
With HDPE in particular, there have been
instances where shrinkback after insulating
passes, but fails after pairing. But strangely
enough, if the product is jacketed, the
product may pass shrinkback after the
pairing step.
If the problem is not due to copper
drawdown resulting from excessive tension,
the shrinkback may be caused by improper
cooling after extrusion. In this case, the
jacketing step added more heat history and
thereby relieved stresses. Two possible
solutions are:
1. Lengthen the water trough and put hot
water in the first two baths. (see G
above)
2. Use a mixing screw, lower the
temperature profile and use a 40/80/120
screen pack combination. By making
these three adjustments, you are
improving mixing in the extruder, so
make other adjustments accordingly.
A test for improper cooling involves taking
two adjacent samples of insulated wire
before pairing and testing one for tensile and
yield strength and elongation. Place the
second sample in an oven at 100°C for two
hours and then test this sample for tensile
and yield strength. A significant (greater
than 20%) increase in the yield strength of
the oven-aged sample indicates that the
product may have been cooled too quickly
and the heat aging relieved the stresses,
resulting in increased strength and
elongation.

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Generally speaking, insulation shrinkback is a result of inbred stress in the plastic created when the plastic cools in the water bath after extrusion.

Without getting too technical, the two ways shrinkback is reduced are:
(a) By gradient or hot water cooling and/ or,
(b) By cleaning the conductor thereby allowing better adhesion of the plastic to the conductor.

You could also create insulation shrinkback if you accidentally stretched the remaining plastic insulation during the cutting and stripping process.

Assuming that you are sure your process has not created the problem, we recommend that you contact your wire manufacturer and work with them to quickly and permanently resolve this problem.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
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IEC 60811-502:2012
Electric and optical fibre cables - Test methods for non-metallic materials - Part 502: Mechanical tests - Shrinkage test for insulations

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