j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3166–3171
journal homepage: www.elsevier.com/locate/jmatprotec
Microwave dewaxing applied to the investment
casting process
Fábio J.B. Brum a,∗ , Sandro C. Amico a , Ivo Vedana b , Jaime A. Spim Jr. c
a
b
c
LAPOL/PPGEM/UFRGS, P.O. Box 15010, Porto Alegre, RS 91.501-970, Brazil
FAQ/PUCRS, Av. Ipiranga, 6681 Predio 12-A, Porto Alegre, RS 90.619-900, Brazil
LAFUN/PPGEM/UFRGS, P.O. Box 15021, Porto Alegre, RS 91.501-970, Brazil
a r t i c l e
i n f o
a b s t r a c t
Article history:
Autoclave processing is commonly used nowadays for dewaxing in the investment casting
Received 2 February 2008
process. However, since the use of microwave is steadily growing in industrial processes
Received in revised form 5 July 2008
and the wax interacts with the electromagnetic energy of the microwaves, the present
Accepted 12 July 2008
work studies the possibility of carrying out dewaxing via microwave. The wax (mineral
wax, vegetable resin, low molecular-weight polymer and anti-oxidant) used in this work
was prepared by melting in an oven equipped with a mineral oil bath. The chemical and
Keywords:
structural stability of the wax were monitored throughout 12 simulated dewaxing cycles,
Dewaxing
via various analyses, namely, volumetric expansion, hardness, Fourier transform infrared
Investment casting
spectroscopy, ultraviolet–visible spectrophotometry, differential scanning calorimetry and
Microwave
viscosity. The results showed that microwave dewaxing is viable, significantly decreasing
Wax pattern
the incorporation of dirt and water, which is inevitable in the autoclave dewaxing process.
© 2008 Elsevier B.V. All rights reserved.
1.
Introduction
The modeling wax pattern used in the investment casting process is a relatively complex mixture of vegetable, mineral and
man-made waxes, natural and synthetic resins, organic fillers,
additives, and, sometimes, water (Piwonka et al., 2000). The
performance of the wax is monitored by the industry so that
it may withstand the demands of the various stages of the
investment casting (Wolff, 2002), namely, manufacturing cycle
of the model, ability to reproduce in details the injection die,
physical properties at green suitable to resist handling during assembly of the clusters, good adherence to the ceramic
and high-dimensional stability throughout the drying process
(Tascioglu and Akar, 2003).
The performance of the wax is usually assessed by evaluating properties like melting point, volumetric expansion,
ash content, filler content, resistance to oxidation, chemical
∗
Corresponding author. Tel.: +55 51 3308 9416; fax: +55 51 3308 9414.
E-mail address: [email protected] (F.J.B. Brum).
0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmatprotec.2008.07.024
inertia, tendency to phase separation, mechanical resistance,
tendency to cavitation, plasticity, elasticity, hardness, viscosity, creep resistance, impact strength, welding and welding
line characteristics (Sabau and Viswanathan, 2003).
In investment casting, dewaxing has been commonly
carried out in industrial autoclaves, in which the molds containing the wax models are placed, being subsequently melted
under wet/warm conditions (high temperature and pressure)
(Olefines, 2004). The wax originated from this process presents
dirt and water, demanding an extra processes, i.e., purification, for its reuse (Bleier and Kukla, 2002).
The wax used in the investment casting process includes
materials with dielectric properties, which are able to be
heated by microwave absorption. In fact, the use of microwave
heating in industrial processes has been growing steadily
due to associated advantages regarding minimization of environmental impact, selective and volumetric heating of the
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3166–3171
materials, and better product quality (Al-Harahsheh and
Kingman, 2004). Indeed, microwave is found in food processing, polymer (Clark and Sutton, 1996; Ku et al., 2001) and
ceramic (Bykov et al., 2001) processing, materials synthesis
and laboratory procedures (Ku et al., 2002; Benitez et al., 2007;
Banik et al., 2003). There are a few published reports in the
literature on the influencing factors, advantages and uses of
microwave energy for the investment casting process, as in
(EPRI, 1993; Anon., 1976; Liu et al., 2007) but this subject still
lacks more investigative work.
In this context, this work evaluates the viability of replacing
an autoclave by a microwave oven for dewaxing. Even though
the latter is a more costly technology, it has the potential of
allowing time and energy savings and higher product quality. The study is based on the comparative evaluation of the
chemical and structural characteristics and recyclability of a
modeling wax after repetitive dewaxing cycles via autoclave
and microwave processing.
2.
Materials and methods
The wax used in this work was prepared by melting in an
oven equipped with mineral oil bath, mechanical stirring
and temperature control, following: (i) melting of the macro
(18%, w/w) and micro (24%, w/w) paraffin, from Petrobras,
along with the polyethylene (LDPE) wax (15%, w/w), from
Megh Ceras e Aditivos, at 120 ◦ C; (ii) addition of ethylene–vinyl
acetate copolymer (EVA) (2.5%, w/w), from Polietilenos Uni
S/A, and pitch (37.5%, w/w), from MBN Prod. Quim. Ltda, and
melting; (iii) cooling to 90 ◦ C under constant stirring; (iv) addition of anti-oxidant 2,6-di-t-butyl-4-methyl phenol (BHT) (3%,
w/w), from Farmaquimica Industrial Ltda; (v) constant stirring
for half an hour, and (vi) natural cooling to room temperature.
The ash content (0.04%, w/w) of the produced wax was
determined according to ASTM D 482-95, and as this value
is below 0.05–0.08% (Olefines, 2004), the wax is considered to
be of good quality for investment casting.
Some of the produced wax (500 g) was submitted to a
dewaxing cycle in an autoclave under 7.4–7.6 kg cm−2 of pressure at 170–175 ◦ C for approximately 10 min. The resulting wax
underwent a purification treatment via hot filtration with 500
mesh sieves, followed by water removal, i.e., heating of the
wax at 120–130 ◦ C under stirring until no bubbles could be
noticed on the wax surface. This dewaxing procedure via autoclave was repeated for up to 12-dewaxing cycles and, at the
end of each cycle, a sample of approximately 30 g was taken.
Another portion of the wax (500 g) was placed on a domestic
type of microwave oven (Sanyo) operating under a frequency
of 2.45 GHz, a power of 1100 W and for a 20-min period at room
pressure (1 atm), reaching a maximum temperature of 120 ◦ C.
In this case, no purification treatment was carried out on the
resulting wax. Dewaxing via microwave was also repeated 12
times and, at the end of each cycle, a 30 g sample was taken.
These samples were tested for:
• Refractive index, in an ABBÉ refractometer (at 80 ◦ C).
• Volumetric expansion (dilation), following the IBER-P-092
standard.
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• Shore D hardness (ASTM D 2240-97), in an Instrument
MFGCo indenter.
• Viscosity, in a Brookfield viscometer (model DV-II+, Spindle
21).
Besides, Fourier transform infrared spectroscopy (FT-IR),
ultraviolet–visible spectrophotometry (UV–vis) and differential scanning calorimetry (DSC) (ASTM D 3418-03) characterizations were carried out using the following equipments:
Spectrum 1000 PerkinElmer, Shimadzu spectrophotometer
(model 1601PC) and TA Instrument (model 2010), respectively.
The methods used here for the characterization of the
waxes may be rigorously applied only to the quantification of
pure substances. This is not the case of the multi-component
wax used in this work. To overcome this drawback, a standard
(virgin) sample was also analyzed, so that reference values and
typical UV–vis, FT-IR and DSC profiles were obtained and compared to those from the different waxes after each dewaxing
cycle.
3.
Results and discussion
Fig. 1 shows the variation of the refractive index with the
number of simulated cycles for both dewaxing techniques,
namely, autoclave and microwave, along with the respective
linear fitting curves. In this figure and the next two ones,
the slope of the linear fitting curves is used as a quantitative
method to evaluate stability, in a way that the higher the slope,
the higher the tendency of the dewaxing method to cause
physical–chemical alterations to the wax during continuous
cycling of the wax.
Fig. 1 shows that microwave dewaxing is a milder treatment to the wax than the autoclave. A similar finding may be
inferred from Fig. 2, i.e., the hardness of the wax is less altered
during microwave processing.
Fig. 3 shows the variation of the percentage increase of the
volumetric coefficient of thermal expansion of the wax with
the number of simulated cycles for both dewaxing techniques.
For this comparison, the 60 ◦ C temperature was chosen, since
in this temperature the wax is already thoroughly molten. In
this analysis, both techniques showed similar results, within
the experimental error and therefore the volumetric expansion of the wax was found to be comparably affected by them.
Fig. 1 – Variation of the refractive index of the wax as a
function of the number of cycles for both dewaxing
techniques.
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Fig. 2 – Variation of the Shore D hardness of the wax as a
function of the number of cycles for both dewaxing
techniques.
From these results, it may be inferred that although
non-specific, these tests are sufficiently sensitive to monitor
structural (physical) changes in the wax following successive dewaxing cycles. Furthermore, the wax, when removed
following a microwave technique, shows less significant structural changes than that found when an autoclave is used.
This may have happened because during autoclave dewaxing, the wax undergoes melting in the presence of water
steam under high temperature and pressure (170–190 ◦ C and
7.4–7.6 kg cm−2 , respectively) inside the autoclave, being then
stirred and heated at 120–130 ◦ C for water removal. This procedure may cause the selective removal of components, e.g.,
the more volatile or hot water soluble ones, or may even cause
physical–chemical changes in some of the components of the
mixture.
In the following analyses, only the results for the virgin
(non-recycled), 6- and 12-cycled waxes will be shown for a
better visualization of the variations. Fig. 4 shows the percentage change in volumetric expansion of the virgin, 6- and
12-cycled microwave dewaxed material (Fig. 4a) and the same
for the autoclave dewaxed material (Fig. 4b). The microwaved
wax, differently from the autoclaved one, appears to show a
decrease in dilation with recycling. This may be attributed
to non-thermal phenomena caused by the microwaves (Clark
and Sutton, 1996), i.e., the successive orientation of the
molecules could result in a more packed molecular structure.
Thus, expansion could be slightly higher for the autoclaved
Fig. 3 – Variation of the percentage increase of the
volumetric coefficient of thermal expansion (at 60 ◦ C) of the
wax as a function of the number of cycles for both
dewaxing techniques.
Fig. 4 – Volumetric expansion (dilation) results for the
virgin, 6- and 12-cycled microwave dewaxed materials (a)
and the autoclaved ones (b).
wax, for which this induced molecular orientation is absent
and also because, in this case, thermal vibration could favor a
somewhat more chaotic (i.e., more expansive) molecular configuration.
Fig. 5 shows the DSC of the virgin, 6- and 12-cycled
microwave dewaxed material (Fig. 5a) and the same for
the autoclave dewaxed material (Fig. 5b). Only one major
phenomenon can be noticed, which is an endothermic transformation related to melting, i.e., the material appears to
behave as a true blend. Besides, the resulting material after
microwave dewaxing is more stable than that of the autoclave
dewaxing, which again may be a consequence of the more
severe process in the latter.
Furthermore, not much difference is noticed between the
different cycles for the microwaved wax compared with the
autoclave one. In fact, in the microwave-cycled wax, all peaks
are close (around 53 ◦ C) whereas the peaks found for the
autoclave-cycled wax moved towards slightly higher temperatures (around 57 ◦ C), which may again be an indication that,
in the latter, the wax is slightly altered during reprocessing.
Fig. 6 shows the viscosity curves of the waxes after
both dewaxing procedures. It can be seen that the wax
shows a thixotropic behavior (Qian et al., 1996) and that the
microwaves increase wax viscosity. This corroborates the volumetric expansion results, i.e., there are structural changes
which increase packing and lead to a greater flowing resistance (Al-Zahrani and Al-Fariss, 1998). The opposite is found
for the autoclave dewaxing process, which decreases wax viscosity due to harsh processing conditions (stirring and high
temperatures), possibly indicating some degree of thermal
cracking.
Fig. 7 shows the ultraviolet spectra of virgin and dewaxed
samples from both methods along with the pitch spectrum
j o u r n a l o f m a t e r i a l s p r o c e s s i n g t e c h n o l o g y 2 0 9 ( 2 0 0 9 ) 3166–3171
Fig. 5 – DSC results for the virgin, 6- and 12-cycled
microwave dewaxed materials (a) and the autoclaved
ones (b).
Fig. 6 – Viscosity results for the virgin, 6- and 12-cycled
microwave dewaxed materials (a) and the autoclaved ones
(b).
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Fig. 7 – Virgin, 6- and 12-cycled microwaved waxes and
pitch spectra in the ultraviolet range (a), and the same for
the autoclaved ones (b).
for comparison, whereas Fig. 8 shows the infra-red spectra of
the wax samples. FT-IR spectra of 6 and 12 cycles were found
identical, hence, to avoid crowding the spectra only the 12cycle spectrum has been provided.
Analysis of Fig. 7, and specially Fig. 8, shows that, in general,
the curves for both dewaxing methods are similar, showing only small variations. The ultraviolet technique is able
to detect conjugated bonds of the abietic acids of the pitch
and their possible oxidations and cycle-additions (Otmmer,
1980). However, these reactions were not found to occur in the
time length, temperature and pressure conditions to which
the waxes have been exposed during either autoclave or
microwave dewaxing. In fact, the UV–vis spectrum was only
able to detect the pitch, with the maximum absorption occurring around 241 nm. This peak was maintained for the 6- and
12-cycled waxes, thus indicating that the double bonds are
still present, i.e., no significant oxidative process has occurred
during wax reprocessing.
Regarding the infrared results, the characteristics peaks
found in the wax were 2922, 1459 and 724 cm−1 , related to
the paraffin fraction, and 2922, 1736 and 1282 cm−1 , related
to the acid wax fraction (pitch) (Socrates, 1994; Pouchet, 1981),
which remains present even after wax reprocessing, ratifying
the UV–vis results. Besides, no contaminants could be found
from the FT-IR spectra.
The absence of wax transformations was expected since
the microwave energy that interacts with the wax molecules
is not sufficiently high to activate significant degradation
reactions under the conditions used in this work (Clark and
Sutton, 1996). In addition, the thermo-mechanical process
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Fig. 8 – Infrared spectra of the virgin wax, 12-cycled microwaved wax and 12-cycled autoclaved wax.
during microwave dewaxing is mild compared to the autoclave
dewaxing process.
4.
Conclusions
The hardness, refractive index, and volumetric expansion
(dilation) analyses of the wax as a function of the number of
reprocessing cycles have shown that:
(i) Although these tests were considered not specific for this
aim, they were sensitive enough to monitor wax degradation occurring due to repetitive cycling of the material
during autoclave processing.
(ii) The wax which was dewaxed on the microwave shows, in
general, less significant physical–chemical changes. Wax
melting due to steam under relatively high temperature
(170–190 ◦ C) and pressure (7–9 kg cm−2 ) conditions and the
subsequent water removal by heating (120–130 ◦ C) and
mechanical stirring for the purification of the autoclaved
wax may be responsible for the noticed wax alterations,
evidencing the advantages of the microwave dewaxing
process, which does not require these post-treatment
steps.
The wax showed a thixotropic behavior and the viscosity
results ratified that microwave dewaxing caused less significant structural changes in the wax. The DSC, FT-IR and
UV–vis curves showed only minor differences of varied magnitude and nature and therefore of difficult identification,
especially because a multi-component wax was used. Nevertheless, there is again an indication that the microwave
dewaxing process is milder and therefore the recycled wax is
more chemically and structurally similar to the original one.
In all, although economical aspects and the presence of
ceramics were not evaluated here, the findings of this work
support the technical viability of substituting the autoclave
for the microwave oven for dewaxing, highlighting the advantages of the latter regarding purity of the resulting wax and its
structural and chemical integrity, even after repetitive cycling.
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