Continual improvement in productivity in gravure cylinder by ckd11816

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									Continual improvement in productivity in gravure cylinder engraving
using HelioKlischographs

Dr. Siegfried Beißwenger, HELL Gravure Systems GmbH, Kiel, Germany

Over the past 30 years, we have witnessed a continuous increase in
productivity in all sectors of manufacturing. This growth, which has accelerated
even further in the last few years, is based on three factors:
∗ Firstly, the speeding up of individual production processes,
∗ Secondly, the reduction in the complexity of production processes and
∗ Thirdly, the automation of production flows.

Numerous examples of this trend are to be found in all sectors of industry. In
particular, the metal machining sector has achieved particularly dramatic
increases in cutting speeds in recent times. One long-known way to lower
production complexity by reducing the number of process steps is the
introduction of multi-axled, numerically controlled metal working machines
which perform several different steps in a single operation that involves
clamping the work piece only once. In the past, these steps would have had to
be performed one after the other on traditional drilling, milling and grinding
machines and lathes. The microelectronics industry has led the way in
reducing complexity. Today, a single chip integrates the power of millions of
transistors, resistors and capacitors. And these days, electronic I&C
technology allows the majority of production operations to be automated. The
result is a further increase in productivity, reduction of rejects and
improvement of quality. That last point may sound surprising, but it is due to
the reduced error rate found in automated processes compared with manual
operation.

Immense productivity advances in electromechanical engraving
The development of electromechanical engraving technology for gravure
printing cylinders is a prime example of what we have just discussed. We have
chosen to look at the development of productivity in the various processing
stages of magazine and catalog printing and, for purposes of comparison,
have decided to focus on the parameter number of color separations engraved
per man and per shift. In a large German gravure printshop, this figure has
risen from 1.4 in 1965, when all work was still done by etching, to 110 today.
The milestones in this development were the complete changeover to
electromechanical engraving with simultaneous bromide scanning in 1972, the
introduction of OT conversion in 1982, the changeover to digital engraving in
1990 and the introduction of automation technology in conjunction with double-
speed engraving heads in 2000/2001. The number of engraved color
separations is an average taken over a year, the figure for 2001 being a
mixture of statistics from older, non-automatic machines with 4 kHz engraving
heads and new, automatic, high-speed machines.

Since the average number of ribbons per cylinder has risen from 3 to 9 during
the period in question, the changeover from etching to electromechanical
engraving with the various development stages along the way actually
represents an increase in productivity of a factor of 30.


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The simplification of the actual impression cylinder imaging process is
particularly important here. Today, this process basically consists of three
steps: Firstly, digital imposition work on the basis of a „job ticket workflow“,
secondly, the double-speed, automatic engraving process, and thirdly,
chromium-plating. Back in the time of etching, these three steps used to
require 10 process stages. And even in the early days of engraving in
conjunction with bromide scanning (regardless of the time-consuming
production and mounting of the bromide), it still took 6 process steps, since
cylinder correction was usually unavoidable. The high levels of precision
achieved by the latest HelioKlischographs (K406) mean that print proofing and
cylinder correction are often unnecessary. Thus, added to the productivity
increases enjoyed by engraving departments (which now boast 110 engraved
color separations per man and per shift) are other productivity enhancements
not quantified here, namely a much-reduced outlay in terms of proofing and
cylinder correction work.




     This graphic shows the number of engraved color
     separations per man and per shift in a large publication
     printshop over the last 35 years.
     1965 etching, 1972 bromide scanning and engraving, 1982
     OT conversion and engraving, 1990 digital engraving, 2001
     some automatic engraving with double-speed engraving
     heads.


We will take a closer look at the improvements in quality and the reduction in
cylinder remakes achieved by progress in engraving technology below. At this
point, it is important to remember that the importance of shortening the entire
production chain in terms of quality and costs cannot be stressed enough.

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After all, a production step which does not exist cannot go wrong, does not
take any time or require any operators, does not consume any resources and
does not tie up any capital.


The HelioSprint engraving head




                 The double-speed HelioSprint engraving
                 head, introduced by HELL in 1998. Over
                 700 units have since been sold.


One of the major milestones since the introduction of digital engraving in the
early 1990s has been the development of the HelioSprint engraving head,
which was introduced in 1998. While engraving performance had been stable
at between 3,000 and 4,000 cells per second for over 30 years following the
invention of the HelioKlischograph in 1967, HELL managed to increase output
to 7,500 cells per second with the Sprint head. The increased cell volume and
exactly symmetrical shape of the cells demonstrate the improved stability of
the head and allow increased printing density. With its features (high
engraving frequency and stability, with minimal bounce, drag and
responsiveness to temperature change), the Sprint head is unique, and bears
witness to HELL’s core expertise. The head is constantly undergoing
development work by a large team of people, whose objectives are to achieve
even higher engraving frequency and 100% engraving consistency.


Automatic calibration of engraving heads
Alongside the actual engraving, the task of calibrating the engraving heads
used to be the most complicated, time-consuming and error-prone step in the
process. With 14 engraving channels, calibration could easily take an operator

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over half an hour, and required him to perform microscopic measurements of
several test cut cells in each ribbon. With 14 ribbons, this meant measuring
almost 50 cells, moving, positioning and focussing the microscope for each
separate measurement. As well as the time consumed, another problem was
measuring errors. There was also a risk that different operators (and shifts)
would proceed differently in large printshops.

The CellGuard system takes less than 10 minutes to automatically perform
these measurements and set the engraving electronics accordingly. It uses a
single rapid-adjustment camera to measure all the ribbons. This principle
avoids the systematic errors inherent in measuring systems using more than
one camera. The slide used to move the CellGuard camera microscope is also
fitted with a precise linear scale. When measuring the test cut cells, the
system can also use this scale to determine the axial position of the engraving
styli on all the ribbons.




    The CellGuard system automatically measures cells using a
    precision-positioned camera operated by a linear motor. This means
    that all the engraving heads are calibrated with speed and repeat
    precision to ensure ribbon uniformity.


This means that the engraving heads need only rough axial prepositioning,
because absolute register accuracy can be assured electronically by
controlling the engraving data in such a way that it is engraved at the right time
and in exactly the right place. The AutoSpacer function thereby cuts
makeready times and avoids register errors. Intelligent software and HELL’s
own image processing system ensure that measurements are correct to 1 µm
and eliminate false measurements. In this context, it is particularly important to
note that only stylus-engraved cells – and not laser-engraved cells – support
volume measurement using optical distance measurement. This is because of
the fixed relationship between the diameter and the depth of the pyramid-
shaped cell. The relevance of this fact becomes clear if we remember that only
processes which can be measured can be kept constant in the short and long
term.



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The hardware and software in the K406 machine are carefully tuned
The HelioKlischographs from HELL are extremely precise engraving
machines. Their mechanical precision makes it possible to produce regular
engraving screens which are free from problems caused by vibration and the
like, and to achieve exact register accuracy to composite prints from different
cylinders. It is therefore possible to engrave the four cylinders in a color set on
different machines. In fact, in order to delay the start of the engraving process
for as long as possible, this is common practice in many printshops. HELL has
also made sure that even HelioKlischographs of different generations are
compatible with one another.




       Digital engraving machines: The HelioKlischograph K 406


The software modules of the server for engraving data, the cylinder layout
station (HelioForm), the proof station (HelioProof) and the conversion station
(Gipsy) run on standard platforms with Windows NT operating systems.
Imposition work and the many possible engraving-related functions and
parameters (which include customized definition parameters and test cut
settings) are handled using HelioForm, or are adapted automatically via
interfaces from upstream systems and passed to the engraving machine as an
approved form in the shape of a job ticket. Engraving is then started on the
engraving machine by simply selecting the job to be engraved. The test cut
and engraving are then performed automatically. The Gipsy system retrieves
the engraving data from its various storage locations and feeds the data flow
for image and line, plus associated marks, to the individual engraving
channels. Gipsy also integrates print-related adjustments and corrections such
as web shrinkage compensation, shingling and the like, and converts the data
format of the pages (TIFF/IT or PDF) into the relevant engraving screen on-
the-fly.

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Further shortening of engraving cycle times possible
Let’s assume that a printshop has an engraving department with four
HelioKlischograph K406 machines. Thanks to the advanced automation
technology, these machines can be operated by 2 people. With a TR10
cylinder with 12 ribbons, a total of 48 pages and a screen ruling of 70 l/cm, the
cycle time would break down as follows: Loading the cylinder: 3 min; automatic
test cut: 10 min; engraving: 32 min; unloading the cylinder: 3 min; average
waiting time for new cylinder 20 min, average total cycle time: 68 min.

The waiting time between engraving two cylinders is often caused by the fact
that the crane has to take away the finished cylinder before a fresh cylinder
can be brought and placed in the machine.

In order to eliminate this waiting period, HELL has developed the Autoloader.
This device allows the next cylinder to be prepared while the current cylinder is
being engraved. Once engraving is completed, the in-house crane is used to
unload the engraving machine. At the touch of a button, the next cylinder is
then automatically placed in the K406, clamped into position, and engraving
can begin. In this manner, the Autoloader cuts the theoretical cycle time to 50
minutes.




           The K406 engraving machine can be combined
           with an Autoloader. This eliminates waiting
           periods caused by the crane by allowing the
           next cylinder to be prepared in the Autoloader
           whilst the current cylinder is being engraved.


Assuming a waiting period of 20 minutes between two engraving operations,
an engraving department with four K406 Klischographs and a two-man team
has a theoretical top capacity of 28 cylinders or 672 engraved color

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separations per man and per shift. If the waiting time is minimized using the
Autoloader, this top capacity increases to 36 cylinders or 864 engraved color
separations per man and per shift.

The state of the art in digital packaging engraving
Like publication gravure, packaging gravure has also enjoyed immense growth
in productivity. Here too, the engraving machine has developed into a job
ticket-controlled output device, the „copper recorder“. This means that
functions like impositioning and assignment of production parameters (screen,
angle, gradation) have been transferred to front-end devices (e.g. HelioCom).
Carefully tailored proofing solutions for cylinder layout (e.g. HelioFormproof)
now ensure that the required level of production reliability is achieved before
engraving.

The introduction of the HelioSprint engraving head was a great boost to
packaging engraving. Since packaging engraving usually uses only one
engraving head, the engraving time accounts for a relatively large proportion of
the total production time for the impression cylinder. Thanks to HelioSprint and
other functions (automatic fast crossfeed, helical line engraving, sequential
engraving), engraving time has been cut by more than half.




           The HelioKlischograph K500 – fully-automated
           packaging engraving from the data


Automated calibration of the engraving head using a suitable camera system
such as CellGuard is a precondition for automated cylinder production.
Operating an engraving machine now simply involves fitting the cylinder and
selecting a job ticket. The installation of a cylinder loading system, for example
an automatic ceiling-mounted crane, also allows fully-automated cylinder
production. Obviously, this requires specialist software to handle process
control and quality assurance.


Conclusion
With its Klischographs, HELL aims to provide the printing industry with the
simplest, most reliable and most cost-effective engraving systems possible.
The modular technology in these systems allows every user to optimally

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configure his engraving machines to suit his own production conditions. With
this in mind, many customers are particularly interested in being able to
upgrade and retrofit their older equipment. On the basis of its great successes
in recent years, HELL is stepping up its development efforts even further. The
primary objective is to enhance cost-effectiveness to an even greater extent by
further simplifying operation and increasing the degree of automation whilst
improving engraving speed and consistency even more.




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