Surface World May 2020 Surface World May 2020 | Page 78

PRODUCTS & PROCESSES
suitable surface engineering expert in Pallas,
one that also used the sophisticated High-
Phos method that Kuhne was so insistent on.
This particular bath chemistry creates a nickel
coating with a phosphorous content of over
ten percent.
Component surfaces enhanced in this way
are extremely chemically stable and have a
high resistance to corrosion.
Handled with kid gloves
This coating is most often used for the
extrusion dies of Triple Bubble® lines. Their
high-gloss polished die heads – depending
on whether a mandrel or pancake spiral
design has been used – are completely
nickelplated.
In the case of an 11-layer die head, the job
involves plating eleven mandrels, along with
a base and end plate for each of them with
myriads of fine bores, angles and threads.
“This kind of bore has a diameter of only
eight mm and a length of 65 mm,” Björn
Greis explains. “Depending on the
application, we need a poreless plating of 10
to 50 μm. This calls for ultimate precision
and cleanliness!” The challenges that these
specifications imply are enormous, alone in
view of the sheer dimensions of the
components. With an external diameter of
800 mm and a weight of 160 kg per
mandrel – the base plate weighing in at an
impressive 300 kg – these high-gloss
polished components demand a
correspondingly heavyduty infrastructure and,
at the same time, kid-glove handling. The
plating process begins with several stages of
cleaning, which has a decisive influence on
the subsequent bond strength and the optics
of the plating.
Every last trace of grease and oxide deposits
as well as other contaminants like dust has to
be removed. To achieve this, the components
are subjected to a sequence of alkaline and
electrolytic degreasing as well as acid bating
processes. Between each of these stages,
they go through elaborate rinsing cycles. The
next stage is to mask – by hand and with
extreme care – all the threads and bores that
are not to be nickel-plated. “This amounts to
at least 150 masking tasks for just one
plate,” says Pallas Managing
Director Alexander Kalawrytinos. This
intricate task is extremely timeconsuming
and, in addition to long years of
accumulated experience, also requires
numerous aids and appliances like special
baths and speciallymade equipment. Then,
once more, the components go through the
3-stage degreasing process. In the chemical
nickel bath itself, a consistent concentration
– thanks to continuous recycling and
monitoring – of nickel ions washes around
and through the die heads. Each hour, about
10 μm of the alloy are deposited, which
means that Pallas can control the exact
thickness of the coating via the length of time
the components reside in the bath. To ensure
the required consistency of the plating
structure, the bath is continuously filtered and
its nickel and hypophosphite content levels
are analysed. To compensate for the nickel
ions that have been expended to the
workpieces, additional dose of nickel, as well
as hypophosphite, are added, so that the
bath concentration remains constant. For
each die head, Pallas prepares the bath
chemistry from scratch to ensure that the
coating is completely poreless and that there
is no chance of foreign bodies being
deposited on the component surfaces. The
degreasing baths are also completely
renewed for each new die head. Normal
practice in the sector is to only renew the
expensive contents of such baths after six to
eight weeks of service life. After the plating
stage, the die head is dried with compressed
air and the masking is carefully removed.
The components acquire their hard-chromelike
hardness of 950 HV through a
subsequent heat treatment, so-called
hard-tempering. Finally, the parts are
packaged and transported back to Kuhne,
where all they need is a quick polish before
installation.
A rare expert and
reliable partner
For Alexander Kalawrytinos and Dr.
Klaus Möller, who controls and
monitors the workflow at Pallas, this
particular order is currently one of the
most challenging jobs in the
company's chemical nickel plating
department.
In addition to the required precision
and high-end quality of the plating,
the handling of the mirror-polished
parts makes extensive precautionary
measures necessary. Even the tiniest
damage to the sensitive surfaces has
to be avoided at all cost. The
processing time of about fourteen
days for the plating of a single die
head is a long time for Björn Greis,
because the demand for blown film
die heads from Kuhne is huge:
"Currently, we are building one
mandrel a day here." In view of the
stringent requirements he has set for
the plating jobs, he is full of praise for
his surface engineering company of
choice. "Pallas does a super clean
and precise job. There are very few coating
specialists that can produce such reliable
results." He also particularly appreciates the
openness of the interaction between them,
something which, for him, is essential for
success in such complex jobs.
“What I need is not a supplier but a partner
who, even when unforeseen difficulties or
bottlenecks arise, has his eye firmly on
finding a solution. Pallas is exactly that: a
true, highly reliable partner.”
Visit: www.pallaskg.de
76 MAY 2020
read online: www.surfaceworld.com