6
SPECIAL:
THE RENAISSANCE OF FOAM PROCESSES
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Foam Injection Molding 2.0
Wittmann Battenfeld and Schaumform Are Developing New Applications
for Foamed I­ njection Molded Parts
A stable system technology that can be extended with further process components as needed is the basis of a
range of innovative applications in foam injection molding. These are solutions for improving the surface quality
as far as high gloss, for the partial combination of non-foamed and foamed molding areas and for foaming thermoplastic elastomers.
Structural foam parts
with high-gloss surfaces
are no longer a black art
(figure: R. Bauer)
L
ightweight construction is a trend
that is increasingly embracing all areas of goods production. Plastics, with
their good ratio of performance data
to low specific weight, play a key role
in this. The lightweight construction
potential of plastic parts can even be
further increased by various foam processes, for example foam injection
molding. One of the pioneers in this
field is the Austrian injection molding
machine manufacturer Wittmann Battenfeld. Its Cellmould process combines efficient performance parameters with a relatively noncomplex, and
therefore robust, system technology
that comes almost entirely from its
own production.
Foam injection molding is not a new
process per se. Applications in which
chemical substances such as azodicarbonamide or phenyltetrazole are added
to the plastic granulate for releasing
blowing gases after plasticizing and injection of the molding compound into
the mold cavity, have been known and
used in production for about 50 years.
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Injection Molding RENAISSANCE OF FOAM PROCESSES
Parameter input and process
control are performed directly
via the machine control unit
Barrier geometry
Pressure
measurement
injector 1
Non-return valve
N2-feed
injector 1
Needle-valve nozzle
Fig. 1. The core components of the Cellmould plastication unit are a 25D melt with a 20D 3-zone
plastication screw and subsequent 5D gassing and mixing zone. The two functional zones of the
screw are separated by a cylindrical barrier (figure: Wittmann Battenfeld)
Since the expansion pressure of the
chemically released gases is only about 15
to 40 bar (= low pressure process), the
process is restricted to relatively thickwalled parts with short flow paths.
To expand the application limits of
foam injection molding, foaming with
the aid of inert gas, chiefly nitrogen, was
developed about 40 years ago. Since nitrogen allows higher expansion pressures of 100 to 200 bar (= high pressure
process), the lightweight construction
potential of foam injection molding can
also be used for thin-walled parts and
components with long flow paths. The
advantages, apart from the weight saving of the parts, are the reduction of the
specific injection pressure needed for
cavity filling, and therefore of the clamping force and compensation of shrinkage
and warpage effects.
Both processes – chemical and physical foaming – are used for processing
thermoplastics, from PP to engineering
plastics such as PC, PA or PBT. The aim of
current development work is to extend
the application potential to thermoplastic elastomers.
Stable System Concept
The main purpose of a foam injection
molding system is, to generate a maximum homogeneously dispersed-phase
polymer-gas solution during plastication.
The technology used, is very similar for all
suppliers. Nevertheless, there are differences in the details of implementation.
Dipl.-Ing. (FH) Wolfgang Roth, head of applications technology at Wittmann Battenfeld comments: “With over 40 years’
practical experience with the technology
7
developed by our predecessor company,
Battenfeld, in Meinerzhagen, Germany,
we had a solid basis for our goal of reducing the system complexity and thereby
making the system more stable, despite
the extension of the applications spectrum. Therefore we have specified our
Cellmould foam injection molding unit as
closely as possible on the standard injection molding unit. Our machines therefore work with a three-zone standard
screw, at the front end with a section for
the gas injection and mixing with the
molding compound.
The specific feature of the Cellmould
technology is that it separates the plastifying zone from the gassing zone with a
fixed cylindrical barrier on the screw. The
alternative would be an additional sleeve
non-return valve. Wolfgang Roth adds:
“The effort for matching two non-return
valves to the particular operating conditions, and making them stable, was our
motivation to look for a simpler solution.
The barrier between the plastication and
gassing zone of the screw has since proven in all sizes. Thus, the wear problems
could be eliminated without needing to
compromise on gas-tightness in the direction of the plastication zone.
In the mixing section of the plastication unit, liquid nitrogen (at a pressure of
up to 300 bar) is metered into the plastic
melt via an injector. In the mixing section of the screw, the splitting of the
melt stream into a large number of individual streams intensifies the nitrogen
distribution (Fig. 1). Since a needle-valve
nozzle keeps the melt cylinder closed
during the plastication and gassing »
Fig. 2. The plastication cylinder (in this case a 1,100 kN machine) is equipped with a gas injector connected to a compact control module (left). The
Cellmould package comprises, besides a gas injector and gas control module, also an injection accumulator on the machine (right, center) and a
central nitrogen generator in combination with a compressor unit (figures: R. Bauer)
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8
SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding
The inert gas is either supplied from a
pressure bottle accumulator or generated from the ambient air by means of a nitrogen generator. In both cases, it is subsequently fed to the gas injector via a
pressure generator. It is part of the Battenfeld plant concept that several machines
can be supplied simultaneously via one
gas supply unit (Fig. 3). Between the pressure supply unit and the gas injector on
the plastication cylinder there is a gas
control module. The integrated valve system and the related gas flow rate is controlled via the Cellmould process software. The corresponding equipment is
available for the entire Wittmann Battenfeld machine spectrum.
Fig. 3. The Cellmould plant concept provides the option that one or more plastication units are
supplied by a central nitrogen generator. One gas control module, actuated via the Cellmould
software, and one gas injector per machine meter the liquid nitrogen into the plastication cylinder (figure: Wittmann Battenfeld)
Fig. 4. Injection mold with dynamic variotherm cooling system for manufacturing a housing
panel of PC/ABS blend with glossy surface (figure: R. Bauer)
process, the melt/gas mixture in the
plastication cylinder is constantly under
pressure. There is thus a single-phase
polymer-gas solution at the end of the
mixing process. Due to the pressure loss
during injection into the cavity, the solubility of the gas in the plastic melt is reduced. The finely dispersed gas nucleates in the melt, thereby creating the
condition for forming a similarly finecelled foam structure.
The effective foam-structure quality
depends on the process conditions of
the injection molding process. This includes the melt viscosity of the plastic,
the injection speed (the higher it is, the
finer is the foam) and not least the defined degree of foaming (material reduction). The latter is set either by corresponding underfeeding into a fixed cavity or by completely filling a cavity, which
is then opened by a preset precision
stroke. To achieve the ­favorable high injection speed for a uniform foam distribution, an injection accumulator is part
of the Cellmould package (Fig. 2).
What Possibilities Are Offered by
High-Pressure Foam Injection Molding?
At injecting in the mold cavity the foam
formation in the outer skin of the melt is
largely suppressed through the immediate viscosity increase of the plastic melt at
getting in contact with the cooled cavity
surface, vice versa the hot core region favors the formation of the cell structure. As
a result, parts with sandwich structures
are produced comprising outer layers of
high density and core regions whose specific weight is 5 to 20 % lower.
The possible density reduction in the
part is in direct proportion to the flow
path/wall thickness ratio for all conventional plastic grades. For example, for
processing PP with a ratio of 100 : 1, the
density is reduced by 15 %, while at 150 : 1,
a density reduction of 10 % can be expected.
Apart from the weight saving, foam
injection molding offers additional potential for improving the part quality; in
particular the uniformly acting expansion
pressure of the foam core compensates
any shrinkage and warpage tendencies.
The effect goes so far that sink marks and
shrinkage warpage can be almost completely avoided. The dimensional stability
is thus generally increased. For processors, this additionally offers tangible process engineering advantages, such as the
reduction of the clamping force demand
(by up to 50 %), and thereby the injection
pressure, caused by the reduction of the
melt viscosity, as well as economic advantages due to shortening of the cycle time,
in particular the cooling time, caused by
the lower part weight.
»
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Not for use in internet or intranet sites. Not for electronic distribution.
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10
SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding
with and without mold temperature control. (Fig. 5).
Elastomers, too, Are Suitable for
­Foaming
Fig. 5.Decorative panel made of a PC/ABS blend, produced with (left) and without (right) activa-
tion of the dynamic mold temperature control (figure: R. Bauer)
High-Gloss Surfaces thanks to Dynamic
Mold Temperature Control
The Author
Dipl.-Ing. Reinhard Bauer is a freelance
writer on plastics technology. His agency
Technokomm is based in Gmünd, Austria;
office@technokomm.at
Contact
Dipl.-Ing. (FH) Wolfgang Roth, M. Sc., is
head of applications technology at Wittmann Battenfeld GmbH, Kottingbrunn,
Austria;
wolfgang.roth@wittmann-group.com
Dr.-Ing. Norbert Müller is CEO at
Schaumform GmbH, Hutthurm, Germany;
Norbert.Mueller@schaumform.com
Service
Digital Version
BB A PDF file of the article can be found at
www.kunststoffe-international.com/948082
German Version
BB Read the German version of the
article in our magazine Kunststoffe or at
www.kunststoffe.de
Even if all parameter variations of the injection processes have been exhausted,
foamed lightweight parts generally show
a characteristic streaking or gray haze on
the surface. The surface effect can be attributed to the fact that gas bubbles push
through the melt flow front during injection. This structure becomes frozen-in on
contact and persists.
Glossy surfaces such as those required for visible parts on housing parts
cannot be achieved by the standard technique. On this point, Dr.-Ing. Norbert
Müller, CEO of Schaumform GmbH in
Hutthurm near Passau, Germany, says: “A
considerable improvement of the surface
quality can be achieved by the combination of foam injection molding with cyclically dynamic mold temperature control,
as offered, for example, by Wittmann Battenfeld in the form of the BFmold and
Variomould techniques.
These are variants of a conformal
cooling integrated in the injection mold
on the visible face of the part with cyclical
hot/cold temperature control units. Only
limited mold areas close to the cavity are
temperature controlled. Due to the heating, for example with pressurized water
heated up to 180 °C, directly before injection of the gassed melt, the material does
not immediately come into contact with
a cold mold wall, so that a continuous
surface without frozen gas bubbles is
formed. (Fig. 4).
“In this way, outstanding surface qualities can be obtained equal to those of
parts made of non-foamed plastics,” says
Müller. The intensity of the effects to influence the surface quality is demonstrated by a comparison of parts produced
The developers’ current agenda includes
extending the application of foam injection molding to thermoplastic elastomers. Norbert Müller says: “While, for example, with polypropylene and polyamide good foam structures can be obtained by both chemical and physical
foaming, our test series shows that most
TPE grades can only be foamed by the
physical method. And of these, only with
TPE, based on thermoplastic polyester
fine cell structure and homogeneity are
achievable so far.”
Müller summarizes the results of his
tests as follows: “The softer a TPE is formulated, the more surface problems become apparent during foaming, in particular if foam injection molding is combined with the precision opening of the
injection mold. Particularly when the
mold cavity has been brush polished, or
even high-gloss polished, the surface often shows numerous dents.”
There are a number of approaches
to explaining that. One is that during
filling, air is trapped between the molding and the cavity surface, which cannot escape. Another assumes that
de-adhesion occurs during precision
opening and the expanding foam part
encloses pointwise air, when it is re-­
positioned in the cavity, resulting in
dents.
Series of experiments show that the
surface problems for TPE processing, unlike those for stiff and solid engineering
thermoplastics, can be reduced by medium to low injection speeds. Texturing of
the cavity surfaces has a similarly positive
effect. A surface roughened by electrical
discharge machining, glass-sphere blasting or by graining permits potential gas
or air accumulations to escape via microchannels in the contact surface between
the injection molded part and the cavity
surface.
Regarding the streaking on the surface, the same considerations apply as
for foam injection molding with engineering plastics. The solution here, too,
is: dynamic conformal temperature
control on the face side. If precision
opening is used at the same time,
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Injection Molding RENAISSANCE OF FOAM PROCESSES
120
Tensile strength
N/mm2
Foamed and Non-Foamed Sections in
One Part
Mechanical Characteristics Can Be
­Accurately Predicted
High-pressure foam injection moldings
have a characteristic sandwich structure
with non-foamed outer layers and a
foamed core layer. The transition between the layers is virtually abrupt. With
low part thickness, the core layer has an
almost constant density throughout its
thickness, while with a large overall thickness a distinct density profile is formed.
The density of the unfoamed outer layer
can be no more influenced by the process control but by the type of gassing
process.
The most important design parameters are thus the chosen density reduction in the central region and the wall
thickness. They can be readily defined by
means of measurements, and serve as
60
8.47
5.70
40
74.27
71.86
70.80
6.46
5.82
5.74
4.43
4.19
4.52
14
12
10
8
6
4
20
2
0
0
5
10
Degree of foaming
%
0
15
© Kunststoffe
Fig. 6. Change of the tensile strength, impact strength and elongation at break of PP-SGF 40
depending on the degree of foaming (0, 5, 10 and 15 %) (figure: Wittmann Battenfeld)
(N mm )/(g/cm )
350,000
100
100
3
380,889
367,719
95
%
95
398,954
381,235
300,000
250,000
90
90
200,000
85
85
150,000
100,000
Resulting weight
450,000
2
Weight-related
stiffness
The fact that sophisticated mold technology is the key for the increasing success of foam injection molding has already been mentioned in conjunction
with the improved surfaces. Another
field of mold and machine technology
that is specially tailored to foam injection molding is the system for partial
opening of the mold via the injection
molding machine. This is permitted by
the combination of non-foamed and
foamed sections in one molded part –
as is essential when functional elements
of nearly non-foamed material, such as
hooks, springs or pins, have to be combined with panel sections made of
foamed material.
To achieve this, a movable cavity section is part of the mold construction. In
the first step, the entire cavity is filled like
a non-foamed injection molded part.
Subsequently only the section to be
foamed is opened with a precision stroke.
Housing parts with complex mechanical
interfaces to the co-parts can thereby be
created in lightweight construction techniques.
80
Tensile strength
Charpy impact strength
Elongation at break
101.07
Charpy impact strength [kJ/m2]
Elongation at break [%]
high-quality soft-foam cushioning can
be achieved inexpensively, e. g. for armrests in automotive engineering or as
shock absorbers for hand-held terminals, that must not be damaged at all if
dropped.
11
80
50,000
0
0
5
10
Degree of foaming
%
75
15
© Kunststoffe
Fig. 7. Change of the flexural strength of the mechanical property (most relevant for housing
parts): the weight-related stiffness of the sample bodies decreases only slightly at 5 % foaming.
At 10 % foaming, it is just as high as for non-foamed parts, and at 15 % density reduction is even
considerably higher (figure: Schaumform)
key data of a theoretical model for predicting mechanical part properties, developed by Norbert Müller as part of his
dissertation.
For the modelling, a symmetrical
sandwich structure is assumed, the outer layers of which are, in simplified
terms, based on the material characteristics of the non-foamed material. For
the foamed core, realistic data for
Young’s modulus and elongation at
break are used (elongation in the case
of ductile materials). The behavior of
the foam core is derived from the behavior of the entire sandwich part,
which works well if the outer layer
thicknesses are known. Studies in which
the foamed core is taken from one part
and mechanically tested are indeed
possible, but lead to highly scattered
data that have only limited value.
Agreement between Theory
and ­Practice
For testing the stiffness and strength,
standard test bars, ideally produced from
injection molded structural foam sheets,
are used. Alternatively, if this option is not
available, standard test bars with a
cross-section of 4 x 10 mm (e. g. Campus
tensile bars) are used. However, for evaluating the measurements, it must be noted that not only the 10 mm-wide standard bar outer layers, but also the 4 mmhigh side surfaces, are non-foamed. Thus,
a foamed standard test bar is similar to a
small four-square tube with about 0.4 to
1.0 mm wall thickness with a fully foamed
core region.
The evaluation of the tensile load
shows, as expected, that the tensile modulus of elasticity and tensile strength »
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SPECIAL RENAISSANCE OF FOAM PROCESSES Injection Molding
450,000
Measured
(N mm2)/(g/cm3)
350,000
Weight-related
flexural strength
12
380,889 380,889
367,719
386,239
Calculated
381,235
398,954 402,581
393,357
300,000
Summary
250,000
200,000
150,000
100,000
50,000
0
foamed part, or in other words: stiff parts
can be realized with lower weight (Fig. 7).
In general, theory and practice agree
very well (Fig. 8).
0
5
10
Degree of foaming
%
15
© Kunststoffe
Fig. 8. The weight-related flexural stiffness determined by simulation (acc. to Schaumform)
conforms with the measured data from trials with injection molded flexural bars (10 x 4 mm
cross-section) (figure: Schaumform)
decrease with increasing foam proportion. That is because material that is no
longer present in the part also does not
contribute to the load transfer. The foam
injection molded product thus stretches
further under the same load and fractures at a lower maximum load. In addition, there are notch effects emerging
from the foam cells close to the outer layer. Overall, the data show that the decrease of tensile strength is always at least
as high as the reduction of the part
weight (Fig. 6).
Under flexural loading, too, the absolute values of the flexural stiffness and
flexural strength are reduced. Since,
however, this loading situation is considerably more favorable for sandwich
structures, the decrease in strength is
much lower here than for tensile loading. The flexural stiffness decreases less
in percentage terms than the part
weight. The results demonstrate that, for
example with a 15 % degree of foaming,
the weight-related stiffness has increased by 4.8 % compared to the non-
Foam injection molding has received a
new boost from the increasing trend
towards lightweight construction applications. The latest innovations include methods for improving the surface quality up to high-gloss, as well as
for combining non-foamed and
foamed sections in one part. The most
important contributions to this are
provided by the development of process and mold engineering, from dynamic mold temperature control
through to one or multi-stage precision opening for entire molds or sections of cavities.
Additional potential is offered by
the available simulation models, providing useful assistance for the parts
designers. Overall, foam injection
molding has achieved a degree of maturity similar to conventional injection
molding. It provides precise and accurate reproducible density reduction
and sandwich structures for a continually extended range of plastics, including thermoplastic elastomers. W
Sensors with a Hard Sensor Front Extend Service Life
Hard Shell – Sensitive Core
Mold cavity pressure and mold wall temperature sensors are key components for
continuous process monitoring and
control in injection molding. This is because they measure in direct contact
with the molding whether quality is being maintained within the required limits or not. In many cases, however, this is
also the weak point of the system because, particularly with abrasive, highly
filled or chemically corrosive polymer
melts, the sensor front becomes damaged with time. The service life of the
sensors is consequently limited in such
applications.
Priamus System Technologies AG,
Schaffhausen, Switzerland, has therefore
developed a new process (patent pending) in which the sensor front is produced
from an extremely hard and chemically
resistant material, which considerably extends service life. Alternative solutions
such as thin titanium nitride coatings or
chromium plated surfaces have not
proved successful in the past because
­either they could not withstand the stress
and were eroded away or they could not
exactly replicate the contours of the sensor front. As a result, they made a visible
mark on the molded part.
Mold wall temperature sensors
with a hard sensor front have been
used successfully in practice for some
while now. These applications have
shown that, even under extreme conditions, a significantly longer service
life can be achieved. Recently, mold
cavity pressure sensors with a hard
sensor front have also become available and can be used reliably, for example, in processing highly filled or
ceramic materials.
Translated from Kunststoffe 12/2014, p. 12.
To the manufacturer’s product presentation:
www.kunststoffe-international.com/960349
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