Surface World May 2020 Surface World May 2020 | Page 74

PRODUCTS & PROCESSES
result, first-class blown film has to fulfil
expectations like exactly specified puncture
and tear resistance, tensile strength, stiffness
and thickness, as well as precisely adjustable
shrink behaviour, high-quality printability and
good sealability.
The highest priority is, of course, maximum
protection of the packaged goods during
transport and storage, and meeting demands
for longest possible storage life and
multifunctional barrier properties. With their
reliable resistance to oxygen, humidity or UV
impact, high barrier films ensure that
products maintain their colour and taste and
that no fluids get in or out. They also ensure
that precious aroma is not lost, that annoying
odours are prevented, and that shelf life is
maximised. Each of these property
requirements may call for additional layers in
the blown film.
Depending on the plastics combinations in
the individual film layers, additional tie layers
may also be needed to promote adhesion.
Furthermore, so that packaging customers
can preserve their competitive edge in spite
of declining batch sizes, intelligent process
optimisation measures are required to
constantly cut down on production costs and
material consumption.
Up to 17 layers
72 MAY 2020
simultaneously
Kuhne responds to all these increasing
demands on the product quality and
productivity of its extrusion lines through
continuous further development of its
technologies and designs. For example,
Kuhne Anlagenbau is the only manufacturer
that offers solutions for all three processes of
blown film production. Whether the multilayer
blown film is to be air-cooled (Smart
Bubble®), water-cooled (Cool Bubble®) or
biaxially oriented without lamination or
coating processes in a single work process
(Triple Bubble®) – Kuhne uses a
sophisticated modular system of standard
components to manufacture optimally
configured customer- and applicationspecific
lines that set worldwide standards in
terms of film quality and output capacity.
The Kuhne core competency here is the
development of multi-layer systems for up to
17 layers that can process practically all
thermoplastics.
In addition to this exceptionally high number
of layer combinations, Kuhne systems also
allow the production of extremely thin films
with thicknesses down to 15 μm – and with
correspondingly low raw material
consumption.
The central component of every blown film
line is the die head, consisting of spirals
which, housed in a vertical cylinder, form the
melt channel. At temperatures of up to 300°
C, the polymer melt is fed into these
rotationally symmetrical components –
mandrels or pancakes – and expelled
through ring-shaped apertures under
pressures of up to 500 bar. A tube – the
socalled “bubble” – up to 20 meters long
forms and is guided, without pressure,
through a calibration cage to the collapsing
frame, where it is laid flat, cut and wound to
customer specifications. For multi-layer films,
as many extruders are used as the number of
layers required. These are arranged in a star
formation on the extruder platform. In a
continuous process, all film layers are
coextruded simultaneously and superimposed
on each other. Nevertheless, for analysis
purposes, each individual layer remains
visible via a microtome section. In the case
of Triple Bubble® technology, three bubbles
are formed in a single work process, with the
first bubble being biaxially oriented inside the
second bubble, and the second bubble being
heat set inside the third.
Down to the last micron
One of the responsibilities of Björn Greis,
Design Manager Extrusion in the Anlagebau
division and Technical Manager of the K-Tool
division, is the complete manufacturing
process of the blown film die heads. On the
basis of the individual customer requirements
regarding line type, film structure and size,
and specifications for output and yield, he
determines whether the job can be fulfilled
with existing modules or whether special
designs will be needed. Greis then discusses
new designs of extrusion tools with the KTool
team on the basis of technical drawings and
CAD models before appropriate flow charts
are drawn up and implementation begins.
Continuous checks with the micrometer clock
ensure that, even in the machining stage, all
the dimensional specifications are being
precisely met.
During the polishing stage, the skill and
experience of the tooling specialists is crucial.
To avoid any resistance to the flow of the
polymer melt, the surfaces are hand polished
and, before coating, measured once again in
a coordinate measuring machine. Even with
all these measurement protocols, Björn Greis
still insists on a final visual inspection: "Every
die head component that comes in or out of
here passes through my hands."
Contour-accurate coating
No wonder then, that his obsession with
precision played a critical role when it came
to selecting a new surface engineering
provider for the chemical nickel plating of his
die heads. Several bad experiences with
imperfect surfaces had made him inherently
sceptical about the value of this technology
for extrusion tools. But still, due to the
geometric complexity of the individual
components and the extreme strain on them
from wear and corrosion, chemical nickel
plating remains a far superior option to all
other galvanic processes. The coating
produced by this chemical, autocatalytic
process has excellent corrosion resistance
and a hardness on a par with hard chrome.
In the watery electrolyte, the coating
precipitates on every component surface that
is in contact with the solution, forming an
absolutely even nickel-phosphorous alloy.
This electrolessly deposited coating enhances
even the most complex geometries with lots
of fits, angles and threads – with absolute
contour fidelity and micron precision.
Through final tempering, the coating attains
a hardness of up to 950 HV. In contrast to
hard chrome plating, chemical nickel plating
does not require any post-processing.
However, the decisive factor for the quality of
the plating is perfect mastery of this decidedly
delicate process. About one and a half years
ago Björn Greis finally found a proven and
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