FINISHING
Microwaves Support
Annealing
Quality Assurance. In conventional
annealing methods, heating proceeds
from the surface to the core of the
article by means of heat conduction.
Due to the poor thermal conductivity
Thermographic false-color image of a hotspot
(photo: Fraunhofer ICT)
of polymers, this process can take
several days where product diameters are large. Microwave heating is independent
of the heat conduction properties of the product, and so the core region can be
heated in a much shorter time.
RUDOLF EMMERICH
SASCHA BAUMANN
nnealing is a process by which a material is heated at a specified rate to
below the crystalline melting range
and is then cooled at a specified rate. Used
for finishing materials, it is widely employed in the plastics industry, e.g. in the
extrusion of semifinished products, injection molding and as an intermediate
step after coarse mechanical machining
of semifinished products.
It is frequently used to relax internal
stresses, which are incorporated into the
material by, e.g., the production process
or downstream machining of the material. Relaxing the internal stresses improves
the dimensional stability of the final
product. Manufacturers say that internal
stresses in very large semifinished goods,
e.g. solid bars 500 mm in diameter, can
even cause a semifinished product to
burst apart. Semicrystalline thermoplastics can be annealed to improve surface
hardness, wear resistance and, through
post-crystallization, also rigidity.
The state of the art consists in heating
semifinished products in hot air to the required temperature in an oven or by
means of paraffin oils and silicone fluids
and then to cool them down to room tem-
A
Translated from Kunststoffe 2/2013, pp. 26–29
Article as PDF-File at www.kunststoffeinternational.com; Document Number: PE111123
Kunststoffe international 2/2013
perature at a specified rate. This process
is very time-consuming because polymer
materials have poor thermal conductivity. In some cases, it can take up to several days if the diameters of the samples to
be annealed are large.
Fig. 1. The polyamide and polyoxymethylene
semifinished materials used in the studies
In conventional annealing, the heat is
transmitted from the surface to the core
by means of thermal conduction. By contrast, it is possible to heat a product volumetrically with the aid of microwaves.
The poor thermal conductivity of the
polymers is thus circumvented.
Microwaves have a frequency in the
range 300 MHz to 300 GHz, and are
probably most familiar to us from their
use in microwave ovens, radar technology and mobile communications technologies. For economic reasons, the frequencies 2.45 GHz and 915 MHz are
mainly used in industrial applications.
Microwaves interact with polar molecules such as water, and engineering plastics such as polyamide (PA) and polyoxymethylene (POM), causing the molecules to constantly rotate as the field oscillates. As the molecules rotate, the
energy which they extract from the field
is converted into heat. This is the principle upon which household microwave
ovens work. Apolar molecules such as
polypropylene (PP) and polyethylene
(PE) do not absorb energy from the microwave field. Additives would need to be
incorporated into them to make them absorbent.
From personal experience with household microwave ovens, microwave
processes often tend to create hot spots in
the material. Hot spots absorb more and
more energy from the microwave field,
and as a result they heat up much more >
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FINISHING
rapidly than the surrounding material.
Just as turntables solved this problem in
domestic microwave ovens, ways need to
be found of achieving uniform heating
on an industrial scale in order that a stable process may be achieved.
12
140
Temperature
A collaborative study with BASF SE investigated microwave-assisted annealing
of engineering plastics. There were two
parts to the study. Part 1 examined microwave-assisted annealing at a frequency of 2.45 GHz and Part 2 at a frequency
of 915 MHz.
Part 1 made a direct comparison between conventional hot-air annealing and
microwave-assisted annealing at a frequency of 2.45 GHz. It was performed on
extruded polyamide and polyoxymethylene semifinished products 60 to 160 mm
in diameter (Fig. 1). The two annealing
variants were evaluated on the basis of
stress relaxation and requisite annealing
time. To this end, the temperature profiles in the samples were recorded during
the heating and cooling processes, and the
internal stresses in the material after annealing were determined. This procedure
was performed for both conventional
hot-air annealing, and microwave-assisted annealing at 2.45 GHz.
The hot-air annealing was carried out
in a conditioning cabinet. The microwave-assisted annealing necessitated
the development and construction of a
demonstrator unit. Finite element analyses assisted with the design of the
2.5 GHz microwave-assisted annealing
oven. These simulated, inter alia, the influence of various types of antennas and
field-homogenization measures on the
resultant heating in the product. Thanks
to the simulation data, it proved possible
to avoid hotspot formation in the material and thus to ensure a stable process.
The temperature profiles in the samples were recorded by means of fiber optic sensors, which facilitate temperature
measurements in both conventional and
microwave-assisted annealing. The sensors were placed on the outer skin of the
sample (Temp1), at a depth of 1/3rd of
the radius (Temp2), at 2/3rd of the radius
(Temp3) and in the bar center (Temp4).
Figure 2 is a plot of temperature against
testing time for conventional annealing
of a PA sample 130 mm in diameter. It can
be seen that the set temperature of 150°C
was approached asymptotically. This
means that the last 10°C of the heating
Outer skin
Depth of 1/3rd of the radius
Depth of 2/3rd of the radius
Bar center
°C
120
100
80
60
40
20
0
0:00:00 0:04:48 0:09:36 0:14:24 0:19:12 1:00:00 1:04:48 1:09:36 1:14:24 1:19:12
Time [d:hh:mm]
© Kunststoffe
Fig. 2. Temperature profiles for conventional annealing of PA (D = 130 mm)
phase take up about 1/3rd of the heating
time. The set temperature in the middle
of the bar was reached after about
17 hours.
Figure 3 is a plot of temperature against
testing time for microwave-assisted annealing of the same material, diameter
and set temperature. Unlike conventional annealing, the temperature inside the
bar clearly precedes the other temperatures by a considerable amount. The set
temperature inside the bar is reached after about an hour. Note also that the set
temperature is not approached asymptotically. The heating rate actually increases as the set temperature is approached, unlike the case for conventional annealing.
The microwave-assisted annealing was
started in a non-preheated oven, i. e. at
room temperature. Temperatures towards the edge tend to be far below the
set temperature, because heat is lost to the
environment and uniform heating cannot occur. Microwave-assisted annealing
without the aid of a preheated annealing
furnace would therefore not save any
time, because now the temperature of the
edge regions of the samples approaches
the set temperature asymptotically.
Frequencies Compared
The experiments show that microwaveassisted annealing at 2.45 GHz without
preheated furnace offers no advantage
over conventional annealing. However, a
combination of conventional heating and
microwave heating shortens the heating
time substantially.
The internal stresses in the material
were determined by a method similar to
that of Davidenkov [1], with rings of defined geometry being fashioned from the
semifinished products. The rings are slit
open and then artificially aged so that
180
Outer skin
Depth of 1/3rd of the radius
Depth of 2/3rd of the radius
Bar center
°C
140
Temperature
Microwaves and Hot Air
in Comparison
180
120
100
80
60
40
20
0
00:00
02:24
04:48
07:12
09:36 12:00 14:24
Time [hh:mm]
16:48
19:12
21:36
00:00
© Kunststoffe
Fig. 3. Temperature profiles for microwave-assisted annealing of PA (D = 130 mm) at 2.45 GHz
© Carl Hanser Verlag, Munich
Kunststoffe international 2/2013
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FINISHING
Fig. 4. Thermographic
false-color image of
homogeneity
they open. The gaps are a measure of the
internal stresses in the material.
In those areas where microwave-assisted annealing produced temperatures
comparable to those of conventional annealing, stress relaxation was comparable.
Part 2 of the study investigated the
thermal behavior of the semifinished
products at a frequency of 915 MHz. The
experiments were carried out directly in
the microwave cavity and no attempts
were made to homogenize the field. The
temperatures were measured in the same
manner as in Part 1. In addition, after annealing was complete and the samples
had been removed from the microwave
cavity, thermographic images were
recorded.
The wavelength of microwave radiation at 915 MHz is three times the length
of that at a frequency of 2.45 GHz.
Figure 4 is a thermographic image reproduced in false colors. The homogeneous temperature distribution is clearly
visible. The sample consisted of POM and
had a diameter of 70 mm. In this experiment, the sample was heated uniformly
to a set temperature of 150 °C in about
8 minutes.
The Title picture shows a PA sample diameter in 180 mm. Clearly, in this experiment, two hotspots formed in the sample.
Kunststoffe international 2/2013
The experiments carried out at
915 MHz show that, for samples with a diameter smaller than half the wavelength,
uniform microwave-assisted annealing is
feasible without the need for additional
heating and conventional field homogenization. Samples whose diameter exceeds
half the wavelength exhibit hotspot formation. It is therefore likely that samples
with large diameters would also require a
combined process of conventional heating and microwave heating as well.
Conclusion
A stable process for microwave-assisted
annealing was developed in the course of
this project. The investigations of the internal stresses revealed comparable stress
relaxation in the conventional and microwave-assisted methods.
i
Contact
Fraunhofer-Institut für Chemische
Technologie ICT
Produktbereich Polymer Engineering
D-76327 Pfinztal
Germany
> www.ict.fraunhofer.de
The investigations of the temperature
distribution in the semifinished products
showed that the problem of asymptotic
approach to the set temperature in microwave-assisted annealing at 2.45 GHz
without a preheated furnace is merely the
opposite of that which occurs in conventional annealing. Instead of the sample
heating up from the edge to the core, it
heats up from the core to the edge. Thus,
for microwave-assisted annealing at
2.45 GHz, a combined process is recommended if the annealing time needs to be
shortened.
Microwave-assisted annealing at
915 MHz of samples of diameter less than
half the wavelength confirmed that there
is no need for a combined method. For
samples larger than half the wavelength,
the combined method is also recommended.
Microwave heating can significantly
shorten the time required for heating the
samples. Where only microwave-assisted
annealing at a frequency of 915 MHz was
performed, the heating time of a sample
of POM 70 mm in diameter was shortened
from about 7 hours to about 8 minutes. ACKNOWLEDGMENTS
Our thanks to BASF SE and Muegge Electronic GmbH
for their support.
REFERENCES
1 Dr. sc. Techn. Horst-Dieter Tietz, Grundlagen der
Eigenspannungen, Springer-Verlag Wien-New
York, 1982
THE AUTHORS
DR. PHYS. RUDOLF EMMERICH, born in 1964, has
been head of the Microwaves and Plasma Technical
Group at Fraunhofer ICT, Pfinztal, Germany, since 1994.
DIPL.-ING. (FH) SASCHA BAUMANN, born in
1977, has been a member of the Microwaves and
Plasma Technical Group at Fraunhofer ICT, Pfinztal,
since 2001.
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