Complex 2-Component Process Hits the Roof

Complex 2-Component Process Hits the Roof

PRODUCTION
Injection Compression
Molding. Plastic glazing
and large decorative parts
with high-gloss optics are
one of the most attractive
growth markets for plastics in the automotive
industry. The success of
these applications hinges
critically on the availabilThe heart of the production installation for polycarbonate elements is a 2-component swivel-platen
machine with a clamp force of 23,000 kN (photo: Webasto)
ity of manufacturing con-
cepts appropriately tailored to the needs of the product. Only then can the potential of this technology – lightweight construction, safety benefits and design freedom – be fully exploited.
Complex 2-Component
Process Hits the Roof
MARKUS BOSS
BERND KLOTZ
ood optical, mechanical and thermal properties make polycarbonate (PC) a preferred material for
the manufacture of optical parts in the
automotive industry. In 2003, Webasto
AG, Stockdorf, Germany, decided to start
processing PC in its plant in Schierling,
Germany. The aim was to develop PC
roof panels because, with a density of
1.2 g/cm3, the plastic is ideal for use in
lightweight constructions. Compared to
a glass counterpart, a PC roof can be up
to 50 % lighter. The resultant lower
weight of the vehicle lowers its center of
gravity and leads to improvements in
roadholding and handling characteristics – partly because the see-through
G
Translated from Kunststoffe 3/2010, pp. 96–99
Article as PDF-File at www.kunststoffeinternational.com; Document Number: PE110350
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areas in roofs have grown increasingly
larger in recent years.
Many Advantages,
One Shortcoming
The material has other advantages. It has
high impact strength and so is almost unbreakable. It is insensitive to minor damage and stone chipping. In an accident, it
even reduces the risk of injury to vehicle
occupants. It offers designers greater freedom and constructors more scope for
functional integration. One major shortcoming is that PC components are not inherently scratch-proof, so they have to be
spray-coated after molding. This entails
coating the front and rear side in two operations with a primer and a special hard
coat.
What was initially a development project on an experimental mold at Webasto’s Center of Plastics Excellence in
Schierling soon became a commercial
success when the company was awarded
a contract in 2005 to make the panoramic roof for the Smart Fortwo; mass production started two years later. Measuring 1.2 m2, this is the largest automotive
PC roof panel ever made. More than
250,000 units have since been produced.
By virtue of a recently commissioned second production line built around a MXW
2300-17200/8100 SpinForm 2-compo-
i
Contact
Webasto AG
D-82131 Stockdorf / Germany
TEL +49 89 85794-0
> www.webasto.com
KraussMaffei Technologies GmbH
D-80997 München / Germany
TEL + 49 89 8899-0
> www.kraussmaffei.com
© Carl Hanser Verlag, Munich
Kunststoffe international 3/2010
PRODUCTION
Low Level of Residual Stress in
the Molded Part
The installation
boasts flexible injection compression
molding programs for
different process
variants
(figure: KraussMaffei)
nent swivel-platen machine (manufacturer: KraussMaffei Technologies GmbH,
Munich, Germany), with 23,000 kN
clamp force, Webasto has boosted production of PC roof elements to the current level of 320,000 parts per year. The
two injection units are positioned on either side of the clamp unit, one being conventionally arranged, while the second is
located behind the moving platen and
moves along with it.
molded-in stresses in this process configuration would warp the molded part and
make it unusable. Therefore, the large
panes have to be manufactured with very
low molded-in stresses.
This important quality criterion, which
applies to all high-quality optical components, calls for a so-called low-pressure
method – namely, injection compression
molding. In this technique, the plastic
melt is injected into the mold, which ei-
A typical PC roof element demonstrates
these requirements. Given a wall thickness of 4.5 mm, a flow path of 1,200 mm
and one gate, a melt temperature of
300°C and a mold temperature of 95°C,
the maximum cavity pressure is 250 bar,
while internal pressures across the molded part range from 150 to 200 bar. The
holding pressure, too, which in injection
compression molding is applied exclusively via the clamp, is identical across the
entire component, both close to and away
from the sprue. This is therefore known
as the clamp holding pressure. The machine is designed to deliver clamping
force on this basis.
The material composition can be varied to adjust the tinge of the finished
pane and adapt it to all kinds of products and designs. Improved infrared absorbers filter out virtually all UV radiation in spite of the high transparency and
provide a comfortable indoor climate in
the vehicle.
To take full advantage of the functional integration, glazing is increasingly being designed as 2-component parts. The
design provides for a first, transparent
Compression Molding in
Different Process Variants
When installing the new facility, Webasto drew on experience gained with an existing system to further optimize the
process. Up until then, mold construction had been the technology driver – all
workflows had involved the 120-ton
mold and so were correspondingly difficult to modify. Collaboration with the
system supplier, though, has yielded a stable process and shifted the focus more toward system functionality. The new machine features a 40-ton mold for a 0.8 m2
part. The machine setter has four different variants to choose from in the guise
of expansion compression, clamping
compression, opening compression and
force compression.
The whole injection process is ultimately tied to the requirements of the
subsequent coating step. The reason is
that the coating is prepared with solvents
that can cause cracking in the plastic under adverse stress conditions, even during coating itself. However, the solvents
are essential for ensuring that the coating
adheres strongly. The priming and painting stages each require the parts to be
heated for about one hour at 130°C. Any
Kunststoffe international 3/2010
Injection compression mold, injection molding machine and peripheral equipment add up to a finely
balanced production system for PC glazing (photo: Webasto)
ther is not yet fully closed or is opening.
Mold closing is delayed until freezing sets
in. Since the clamping pressure builds up
uniformly, the variants of injection compression molding yield parts which have
an excellent surface and little mechanical
anisotropy, irrespective of the type of
plastic.
component which is back-molded with a
second component (in the case of glass,
this is the black marking for hiding unsightly areas). The process must be highly reproducible, because a substantial part
of the cavity of the second component is
pre-shaped by the first component. Unlike the case for a conventional mold, the >
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PRODUCTION
On the machine control’s display screen, production staff can use four displacement transducers to call up details of the platen parallelism control and
the timing of the compression gap in the parting line – a prerequisite for high-fidelity manufactured optical components (figures: Webasto)
Preventing the Mold from
Tilting Open
The quality requirements imposed on the
part are no lower than those imposed on
the tool and the functionality of the injection molding machine. What is needed is high-fidelity molding, both from
shot to shot and across the entire part surface. The part is molded from the side because it is an optical part. When the melt
enters the mold, higher pressures closer
to the sprue than away from it would
cause the mold to tilt open. The installation must compensate for these forces in
the mold, so as to prevent mold opening.
The difficulty is that fluctuations in
wall thickness over the entire part must
not exceed 0.02 mm. This requires the use
of high-precision, smooth-running, mold
platens with a parallelism in the range of
0.01 mm and so the machine must be
equipped with a parallelism control unit.
KraussMaffei favors recording the com-
64
pression stroke, as measured at the mold
gap – because only there, right at the
molded part, is it possible to effect control directly via measurement, without
the need to consider other possible interferences.
Reflective and Transmission
Optics as Quality Criteria
Only distortion-free surfaces meet the
quality demands which car manufacturers and buyers impose on PC roofs. Two
criteria are crucial. The first is called reflective optics, and refers to the correct
flatness of the surface of the part. For, only an absolutely perfect mirror image conveys a good overall impression of high
quality.An important factor is the 2-component process described here in which
the second component is injected onto
the first and not into a steel mold. The response to the melt, such as one-sided
heating and cooling, can influence the
surface as well as the described changes
in wall thickness. The result is a so-called
window effect, i.e. a physical instability
that is caused by the back-molded material. The human eye perceives a surface
change with a discontinuity of 2 µm as
annoying.
The second concept is called transmission optics and describes a safety feature,
namely, a distortion-free field of view
without interfering specks. This is affected by the stress state of the part (birefringence) and the purity of the material (homogeneity). Therefore, plastication and
compression technology here are crucial.
In the process realized by KraussMaffei
250
Far from sprue
Close to sprue
bar
200
Cavity pressure
cavity for the second component is
shaped by the process outcome of the first
component. In the second process step,
the cavity creates a seal between steel and
transparent plastic – if this part is not
made to an accuracy of hundredths of a
millimeter, this can lead to fluctuations
and even to overpacking of the second
component.
Because the total stress in the molding
is determined by the second component
as well, it is sometimes an advantage to
choose an injection-compression method
for the second component. That, combined with a small number of gates, creates a low level of molded-in stress.
150
100
50
0
0
10
20
30
40
50
Time
60
70
s
80
© Kunststoffe
Typical cavity pressure during an injection-compression molding process, with the same pressure at
the end of compression phase. The flow path between the pressure sensors is roughly 1,200 mm long
(source: Webasto)
© Carl Hanser Verlag, Munich
Kunststoffe international 3/2010
PRODUCTION
Polarization optics
(here: with monochromatic light)
reveals the key advantage of injectioncompression molding: Low molded-in
stresses across the
entire part surface
create ideal conditions for the subsequent scratch-resistant coating
(photo: Webasto)
and Webasto, the mold floats on the plastic melt until freezing is complete. This
leads to distortion-free optical moldings.
Special HPS-AT plastication units are
used for processing the PC on both injection units. They ensure high melt homogeneity and reproducibility.
The compression process makes it possible to maintain a curvature tolerance of
±1.5 mm, as a measure of non-warping
across the entire surface of the panoramic roof (72 measuring points). Side and
rear windows and other trim panels resembling glass can be produced by this
method too. The quality of the PC panes
is so high that at present only the current
legal situation and the lack of resistance
to scratching by wipers prevent its use in
windshields.
Outlook
Injection compression molding is a growing alternative in the production of exte-
rior parts. Compression processes are
now being used for body-panel areas,
such as hoods and fenders.Webasto alone
has acquired eight contracts to start mass
production of roof elements in the next
two years. Other applications which will
go into production in the next few years
are roof shells for convertibles, a fully
opening panoramic roof made of PC and
a rear module with spoiler.
Current machine technology allows for
different compression variants. In all of
them, mold cooling and control thereof
are a crucial factor. Heightened quality demands are moving another issue centerstage that will significantly influence the
process in the future. It is called dynamic
mold heating (DMH). In this, the injection molding machine communicates with
the temperature control devices in real
time via a Varan bus – allowing full flexibility over the temperature profile. THE AUTHORS
DIPL.-ING. MARKUS BOSS, born in 1972, works
in the Polycarbonate Product Group of Webasto AG
in Schierling, Germany.
DIPL.-ING. (FH) BERND KLOTZ, born in 1956,
works in process development at KraussMaffei
Technologies GmbH, Munich, Germany.
High-Performance and Biodegradable
Biodegradable Plastic. BASF SE, Ludwigshafen, Germany, is now offering a
new biodegradable plastic. It goes by the
name Ecovio FS and it has joined the existing Ecovio product line. The company
has optimized this new plastic for two
specific applications: for coating paper
and for manufacturing so-called shrink
films, which serve to easily wrap packaged
goods. For this reason, the first two new
plastic types are called Ecovio FS Paper
and Ecovio FS Shrink Film.
As has been demonstrated in recent
composting experiments, the new Ecovio
FS biodegrades even more rapidly than
its predecessors, and it has a higher content of renewable raw materials. Ecovio
FS consists of the likewise new, now biobased Ecoflex FS (a biodegradable polyester made by BASF) and of PLA (polylactic acid), which is extracted from corn
starch. The use of the new Ecoflex FS raises the proportion of bio-based material
in Ecovio FS Shrink Film to 66 % and that
of Ecovio FS Paper to a full 75 %.
Translated from Kunststoffe 2/2010, p. 49
Kunststoffe international 3/2010
Paper cups and shrink films are the first two applications for a new biodegradable plastic based on
PLA (polylactic acid) and biodegradable polyester (photo: BASF)
The experts who developed the new
Ecovio FS focused on the properties that
are required of these special applications:
In order to obtain effective paper coatings, a film made of the new Ecovio FS
Paper has to be easy to process and exhibit good adhesion to the paper, even when
applied in thin layers. Such coatings are
used, for example, on paper cups or cardboard boxes. Ecovio FS Shrink Film, in
contrast, has a selected ratio of shrinkage
to strength, so that its mechanical load capacity at a film thickness of merely 25 µm
is greater than that of a conventional polyethylene film that is 50 µm thick.
> www.ecovio.de
65