Thermophysical aspects of reclaimed moulding sand addition to the epoxy-SO2 coremaking system studied by Fourier thermal analysis
Thermophysical aspects of reclaimed moulding sand addition to the epoxy-SO2 coremaking system studied by Fourier thermal analysis
Jo´zsef Tama´s Svidro´ 0 1
Attila Dio´szegi 0 1
Judit Svidro´ 0 1
Tibor Ferenczi 0 1
0 Department of Metallurgy, School of Engineering, University of Miskolc , Miskolc 3524 , Hungary
1 & Jo ́zsef Tama ́s Svidro ́
The most important advantage of foundry purpose moulding sand is that it can be reclaimed and reused through the casting manufacturing process. Supplying the foundry with a new source of material, sand reclamation brings along both environmental and economic advantages. Utilization of used sand can be considered as a common technological routine in the production of most types of chemically bound moulding materials. The epoxy-SO2 process is prevalent in the processing of cast iron engine components worldwide. Based on its excellent properties, it is mainly suitable for producing internal sand cores with complex geometry. Even though reclaimed sand addition is an active and well-functioning feature in ferrous foundries, the scientific and thermophysical background of its effects on the casting process is yet to be explored. In this work, the thermal aspects of different reclaimed sand levels in the epoxy-SO2 moulding system were examined. Thermogravimetry and differential thermal analysis of the epoxySO2 and reclaimed sand in focus were carried out to obtain basic understandings about their high-temperature behaviour. A state-of-the-art Fourier thermal analysis method presented in a recent paper was used at temperatures corresponding to actual cast iron production (1300 ± 10 C), contrary to the previous tests at the typical temperature range of aluminium melt processing (660 ± 10 C). By the right of the method, the effects of reclaimed sand addition
Cast iron; TG-DTA; Fourier thermal analysis; Epoxy resin; Heat absorption; Reclaimed foundry sand
Department of Materials and Manufacturing - Foundry
Technology, Jo¨nko¨ping University School of Engineering,
P.O. Box 1026, 55111 Jo¨nko¨ping, Sweden
on the heat absorption (cooling) capacity of the epoxy-SO2
moulding mixtures were investigated.
The epoxy-SO2 process
The epoxy-SO2 process is an organic gas-cured method to
produce sand cores, which means the hardening of the
sand–organic resin mixture is accelerated by SO2 gas
catalyst. The process utilizes a two-part liquid resin. Part I is a
modified epoxy resin containing acrylic and epoxy
functional components, and part II is cumene hydroperoxide as
oxidizer. The mechanism that effectively cures the
epoxyacrylic resin is a combination of acid-induced and free
radical-initiated polymerization reactions [
foundry application, epoxy resins are also used as coatings,
adhesives, laminates, semiconductor encapsulation, and
matrices for advanced composites, based on their
outstanding mechanical stiffness, toughness, chemical
resistance, and superior adhesion [
Sand performance and casting properties are influenced
by the ratio of acrylic and epoxy functional components.
The resin is normally added in the range of 0.6–1.4 mass%,
based on the mass of the sand and determined by the
physical strength requirements of the core or the mould.
The oxidizer is normally used between 30 and 50 mass%
and calculated by the mass of the resin and the preferred
curing rate. In high production, the compaction of the
epoxy-SO2 system is achieved by blowing it into a pattern
(core box) by compressed air in order to form the desired
Specific applications may require preheated core boxes
to improve cycle times. In typical core blowing operations,
relatively low blowing pressures of 275–415 kPa are
possible due to the excellent flow properties of the system. As
components do not react with each other until the SO2 gas
is introduced to the sand–resin mixture, the prepared
material has an extremely long bench life compared to
other cold-box and furan no-bake methods. This feature
minimizes waste sand and decreases the machine
downtime because sand containers and mixers do not have to be
Once the sand is compacted, approximately one second
of curing by SO2 catalyst is done with inert gas carriers
such as nitrogen. It reacts with the cumene hydroperoxide
and oxidizes into SO3 which forms sulphuric acid with the
water in the system. This reaction provides the necessary
acid media to obtain a cured polymer. A simplified version
of the curing mechanism is:
Epoxy resin Cumene hydroperoxide ðoxidizerÞ
! SO2 ðgas curingÞ ! Cured polymer
Hot purging with air at 95 C for around 10 s is
recommended to achieve optimum cure and to remove
residual gas from the core. Collection and neutralization of
residual SO2 is necessary after the process from both safety
and environmental reasons. In other cases, when the SO2
system is used with furan resin, the mixture can also
release considerable aromatic hazardous air pollutants as
they are thermally decomposed during the casting [
Scrubbing of the gas is usually done by a wet scrubbing
unit that utilizes a shower of water and sodium hydroxide.
The 5–10% solution of sodium hydroxide at a pH of 8–14
provides efficient neutralization of the SO2 and prevents
the by-product (sodium sulphite) from precipitating out of
the solution. Higher sodium hydroxide concentration will
cause precipitation of the neutralized product [
Reclamation is defined as the physical, chemical, or
thermal treatment of a refractory aggregate to allow its reuse
without significantly lowering its original advantageous
properties as required for the application. To achieve this
objective, one must evaluate the type of sand entering the
reclamation system, the binder system used, and the area of
reuse. The primary requirement is to remove the resin
coating around the sand grains [
Before the sand is processed by a reclamation system, it
must go through a preliminary preparation process. Sand
lumps must be broken and ground to near individual grain
size to expose the resin layer on individual grains to the
process. Metallic and refuse such as wood and paper and
other trash must be also removed. In some cases, cooling of
the sand is necessary due to the relatively high temperature
immediately after shakeout. Drums and fluid bed coolers
are traditionally used to cool down the sand to adequate
There are three basic types of reclamation systems: wet,
thermal, and dry. The selection depends greatly on the
nature of resin/binder to be removed from the sand grains.
Wet hydraulic reclamation systems are used for clay
(green sand) and silicate bonded mixtures. As these
inorganic materials tend to melt rather than burn in a furnace,
these sands are very difficult to reclaim by dry processes
and are impossible to reclaim by thermal systems.
When the castings are all made in chemically bound
sand moulds and cores, the sand can be reclaimed by
thermal treatment. The gas- or oil-fired calcining ovens are
operated at temperatures of 400–900 C to promote the air
oxidation of the residual binder amount. Following the
calcining, the sand must be cooled down for reuse.
Temperatures must be controlled carefully during thermal
reclamation to avoid sintering reactions that cause the sand
to agglomerate and stick to itself causing flowability
Dry reclamation processes can be divided into
pneumatic and mechanical scrubbing. The pneumatic system
operates by impingement of a high velocity stream of air
and sand grains. The process pulverizes the binder layers,
and the dust debris is removed to a dust collector.
Dustcontaining residual must be collected as it is classified as
In mechanical reclamation, the sand grains are hurdled at
high velocity against a metallic barrier by an impeller
causing sand-to-sand attrition. However, the residual
bonding agents are not completely removed as some chemicals
can be particularly tenacious in sticking to the sand grains.
When this happens, it may be necessary to repeat the cycle
several times. Mechanical reclamation units may be oriented
either horizontally and vertically [
The ‘‘base’’ moulding mixture (without reclaimed sand
addition) studied in this work consisted of washed and
screened silica sand as basic refractory, fresh epoxy resin
suitable for metal casting purposes and cumene
hydroperoxide as oxidizing agent. The silica sand was light brown
coloured and sub-rounded shaped with a medium grain size
of 0.23 mm. Grain size distribution of the investigated sand
is shown in Fig. 1. The measured specific surface area was
148 cm2 g-1, which is a significant parameter of a foundry
aggregate when it comes to the resin demand necessary for
adequate strength properties.
The epoxy resin (Part I) consisted of three main
components: bisphenol A-epichlorohydrin resin, bisphenol
F-epichlorohydrin resin, and trimethylolpropane
triacrylate. It was mixed with the silica sand together with
cumene hydroperoxide as oxidizing agent (Part II) by a
conventional foundry sand mixer. Samples were then
compacted by blowing and cured by SO2 gas. Composition
and production parameters of the ‘‘base’’ moulding mixture
are given in Table 1.
The properties of the ‘‘base’’ mixture and the storage
conditions were maintained carefully. Discrepancy of the
density, the free moisture content, and the loss on ignition
(LOI) can significantly influence the thermophysical and
decomposition behaviour. Samples taken from the ‘‘base’’
epoxy-SO2 system were dried at 105 C for 1 h to measure
the free moisture content. LOI values were then determined
in the dried samples at 900 C for 90 min (Table 2).
The ‘‘base’’ mixture was examined by TG–DTA to have
initial information about the thermal profile of the
moulding material in focus and to reveal its important
decomposition features. TG–DTA was performed on a
MOM Budapest derivatograph C/PC under static air
atmosphere. The heating rate was initially set to
10 C min-1, and the reference material was a-Al2O3.
Samples of 300 mg were placed in ceramic crucibles.
Figure 2 shows the results of the TG–DTA. The epoxy
resin started to decompose around 150 C, when the free
moisture was already vaporized. Minor endotherm peaks
(A and B) on the DTA curve in the temperature interval
between 200 and 550 C show the complex endothermic
process of the resin decomposition, which is overlapping
with the combustion of the degradation products formed.
Therefore, the real endothermic peaks can hardly be
separated from the strongly drifting baseline of the DTA
curve. Based on the transition of the TG curve at
approximately 550 C, the resin has burned out completely until
this temperature. Total mass loss value of *1.1%
corresponded well to the free moisture content and LOI results.
Allotropic transformation of silica sand from a-quartz to
bquartz also appeared on the DTA curve as an endotherm
peak (C) at 573 C.
The reclaimed sand used in this study was the product of
several cycles of mechanical reclamation of the moulding
material described above. It contained silica sand with a
fair amount of (thermally) spent epoxy resin on the surface
of the grains. Nevertheless, certain sections of a used
mould/core positioned far from the liquid metal remain
‘‘thermally untouched’’ by hardly reaching even 100 C.
Purging time (hot
However, these parts likewise enter the reclamation
process. Therefore, the reclaimed sand contained also fresh
resin in addition to spent resin. As of appearance,
reclaimed sand was also sub-rounded but relatively darker
in colour compared to pure silica sand, due to the thermal
effects after several casting cycles and to the presence of
multiple spent resin layers. The sand grains tend to
agglomerate, increasing the amount of coarser fractions
and medium grain size to 0.28 mm, without the formation
of fine particles as the dust debris is removed during the
reclamation process (Fig. 3).
The most important parameter of the reclaimed sand
besides adequate grain size distribution is LOI, which
represents the amount of fresh and spent resin on the
surfaces. The spent resin is from various stages of thermal
decomposition and can significantly affect the thermal
properties and the heat absorption behaviour of moulds and
cores in case of their addition to the fresh moulding
material. Typical LOI value of the reclaimed sand used in
this work was 2.5 ± 0.1 mass%. Free moisture content was
0.02 mass%, much lower compared to the ‘‘base’’ mixture
because the material was heated up several times to at least
above room temperature. The reclaimed sand was also
studied by TG–DTA in order to draw differences between
the thermal behaviour of fresh and spent resin in the
DTA curve in Fig. 4 shows the diverse degradation
mechanism of the reclaimed sand. The fresh resin started to
burn out first, and endotherm peaks A and B representing
this process appeared again, similar to the ones in Fig. 2.
Meanwhile, the secondary/continued degradation of the
spent resin also started. The TG curve shows that the mass
loss taking place between 250 and 600 C is much higher,
compared to the mass loss in the ‘‘base’’ mixture (Fig. 2)
during the same temperature interval. This means that the
major decomposition processes of the spent resin took
0 .036 .009 .2510 .018 .025
place between 250 and 600 C, which is expected to
establish additional heat absorbing processes in the
moulding material. However, the static air atmosphere
allowed the combustion of the spent resin and the
degradation products, contrary to real foundry conditions where
pyrolysis is dominant. Therefore, a major exothermic peak
(C) appeared on the DTA curve between 450 and 500 C.
Endotherm peak ‘‘D’’ indicating the allotropic
transformation of silica sand was less apparent this time, because
of the shadowing of the exothermic peak. The transition of
the TG curve at approximately 600 C showed that the
combustion of chemicals is finished until this temperature.
Total mass loss value of *2.5% corresponds well to the
LOI result of reclaimed sand.
TG–DTA showed valuable initial results; however, the
mixtures were further studied by Fourier thermal analysis
to obtain understandings modelling real foundry
conditions. These conditions were provided by the application of
actual core wall thicknesses and heating rates prevalent in
During sample preparation, clean silica sand was first
mixed with reclaimed sand in different ratios shown in
Table 3. Five different sand mixtures containing clean and
reclaimed sand were then bonded by 1 mass% fresh resin
and cured by SO2 gas. Production properties are given in
The preparation of spherical sand samples made by
mixtures A–E with different diameters of 40, 50, and
60 mm was slightly modified in order to apply the Fourier
thermal analysis method at high temperatures analogous to
cast iron production. N-type mineral insulated
thermocouples with stainless steel sheath were used for temperature
measurements; one temperature measuring point was in the
geometrical centre of the cores, and another lateral
measuring point was near the sample wall (Fig. 5). Exact
locations of temperature reading points concerning all three
sample diameters were akin to the dimensions used in a
previous work (Table 4) [
]. Neither preliminary drying
nor coating of the spheres was applied.
Samples were immersed into liquid cast iron
(1300 ± 10 C) during the measurement. Quartz glass
pipes with outer diameter of 5 mm and wall thickness of
1 mm were used in the initial tests to avoid the direct
contact of thermocouples with the melt during the
immersion of the specimens. However, the thermocouple
readings were disturbed due to the significant gas pressure
built up inside the cores, which was a result of the more
intense heat shock in the cast iron melt compared to the
immersion into liquid aluminium in the earlier work.
Therefore, the outer diameters of the protective pipes (ø p1
and ø p2) with a wall thickness of 1 mm (t1 and t2) were
reconsidered as shown Table 5, to obtain an adequate
evacuation of the gases from the samples.
Results of temperature measurements
Figure 6 shows the temperature distribution versus time in
the 50-mm-diameter samples in case of all five mixtures.
There are significant differences between the heating
characteristics recorded in the centre of a specimen and in
the lateral measuring point. Central temperatures (Fig. 6a)
increase much slower compared to the lateral positions
(Fig. 6b). The difference (gradient) is also evident
comparing 40- and 60-mm sample diameters (Figs. 9, 10 in
Compared to the results from earlier tests [
application of cast iron melt provided higher heating rates than
using aluminium melt. For instance, reaching 500 C in the
centre of a 50-mm sample took approximately 140 s in cast
iron melt, while it took 350–370 s in aluminium melt.
However, only a lower heating rate in aluminium melt
ensured the preliminary observation of several heat
absorbing features on the primary heat distribution versus
time curves. As shown in Fig. 6, neither heat absorbing
processes, nor differences between various mixture
systems could be marked squarely on the results of
temperature measurements due to the higher heating rates obtained
by using cast iron melt. This phenomenon enhances the
role of the thermal analysis in the exploration of heat
absorbing processes taking place in the epoxy-SO2
moulding mixtures with various reclaimed sand additions.
50 mm samples
Results of Fourier thermal analysis
The Fourier thermal analysis (FTA) is mainly used in
nonferrous and ferrous foundries for process monitoring.
Originally, the method is applied to determine the release
of latent heat during solidification in metallic alloys. Its
fundamentals are based on using at least two measuring
points in a 1-dimensional thermal field and the tabulation
of the volumetric heat capacity of the phases taking part in
the solidification [
In this paper, the Fourier thermal analysis used in an
inverse way to study heating curves recorded in core
samples instead of the traditional way used for cooling
curves was developed during earlier works and has been
presented in recent papers [
]. The calculation of total
absorbed heat, fraction of absorbed heat, and rate of heat
absorption by the degradation of the moulding material
gave valuable information about the cooling capacity of the
epoxy-SO2 cores together with the effect of various levels
of additional reclaimed sand.
The most important governing condition for the
transformation from liquid to solid is the temperature gradient,
which depends mainly on the heat transfer between the
melt and the moulding material and takes place at the
liquid metal–mould interface. The heat transfer is strongly
affected by the cooling capacity of moulding materials.
Mixtures with high heat absorption capacity can increase
cooling rates, and mixtures with low heat absorption
capacity will eventuate in low cooling rates. Application of
moulding materials with various cooling capacity can
change the formation of the initial casting skin, which is a
key moment in the relevant stages of the solidification
phenomenon and in the formation of penetration, blow
hole, or even shrinkage-related casting defects.
The calculated total absorbed heat values of all five
mixtures are given in Table 6. Nearly equal result
regardless of the sample diameter is an evidence of the good
reproducibility of the method. Total absorbed heat means
the heat necessary for the overlapping decomposition
processes and phase transitions to take place, which were
studied through the TG–DTA (Figs. 2, 4). These are the
vaporization of free moisture content in the ‘‘base’’ mixture
and in the reclaimed sand at 100 C, the degradation of the
fresh resin between 150 and 550 C, the secondary or
additional decomposition of the spent resin up until
600 C, and the transformation of silica sand from a-quartz
to b-quartz at 573 C.
Results showed that 25 mass% of additional reclaimed
sand content (e.g. the surplus of both fresh resin and the
still combustible spent resin in the system) increased the
total absorbed heat by approximately 10–12%. This means
that the reclaimed sand eventually increased the cooling
capacity of the cores, which is expected to shorten the total
solidification time in the casting, significantly affecting its
final microstructural morphology and may result in a large
variations in mechanical properties.
Earlier authors also investigated the effect of various
mould materials on the cooling rate of cast iron castings
]. They only distinguish metallic, sand, ceramic, and
insulated moulds. According to their findings, the
application of these materials may eventuate in more than 60 C
difference in the temperatures at the end of solidification.
At the same time, other works primarily focused on
different chemically bonded sand moulds and concluded the
significance of the type of sand on the final microstructure
and mechanical properties of aluminium alloys [
works dealing with the thermophysical properties of green
sand concluded the importance of different ingredients on
the thermal conductivity of moulds [
Thus, the method suitable to study the effect of a
specific mixture parameter (such as reclaimed sand
content) on the heat absorption behaviour of a particular
mixture system (epoxy-SO2) is of high importance from a
thermal science point of view. By the right of the inverse
thermal analysis, the effect of spent resin level on the
cooling capacity can be further evaluated versus the
temperature in the specimens. For this purpose, fraction of total
absorbed heat and rate of heat absorption were also
Figure 7 shows the fraction of total absorbed heat versus
the temperature recorded in the centres of the
50-mm-diameter cores. The effects of additional reclaimed sand
appeared clearly, as the fractions of the total absorbed heat
in the mixtures with different additional reclaimed sand can
50 mm samples
200 300 400
50 mm samples
200 300 400
be assigned to a certain temperature in the centre of the
sample. For instance, approximately 70% of the total
absorbed heat was consumed by mixture A (without
reclaimed sand) at 200 C, which corresponded to
*100 kJ. The same amount of heat was absorbed by
mixture E (100% reclaimed sand) up to 200 C, but in this
case, it corresponded to a much smaller part of the total
heat absorbed, around 45%. This shows that the additional
reclaimed sand level in the mixture increased the heat
absorption starting from temperatures even lower than
200 C, as the surplus of fresh resin in the reclaimed sand
has already started to decompose by then. As the
temperature increased, the effect of the secondary/continued
degradation of spent resin became more and more
dominant. As shown in Figs. 7 and 11 in Appendix 2,
decomposition processes were completed until 550–600 C
regardless of sample diameter, confirming the results of
Figure 8 shows the rate of heat absorption versus the
temperature recorded in the centres of the 50-mm-diameter
cores. These curves represent the degradation
characteristics of each mixture variables. Rate of heat absorption
reached a maximum shortly after 100 C in all five
mixtures. The amount of additional reclaimed sand did not
affect this maximum peak of free moisture vaporization,
because reclaimed sand did not add significant surplus of
free moisture to the mixture. On the other hand, maximum
rate of heat absorption was strongly affected by the heating
rate, e.g. the sample diameter. This dependence is
presented in Table 7, according to the curves in Figs. 8 and 12
in Appendix 2.
Thus, the level of additional reclaimed sand did not have
a significant impact on the initial degradation processes of
the epoxy-SO2 mixtures and therefore will not influence the
very early stages of casting solidification. However, the
extra amount of still degradable fresh and spent resin in the
system played a significant role at higher core
temperatures. Figure 8 also shows that mixtures B–E containing
reclaimed sand had several secondary maximum peaks in
the core temperature interval of 250–600 C due to the
secondary/continued degradation of spent resin. Moreover,
curves of mixtures B–E generally had higher heat
absorption rates, compared to mixture A with no additional
reclaimed sand. This confirms the dominance of spent resin
degradation at higher core temperatures, which is expected
to prolong the cooling capacity of the moulds or the cores.
This phenomenon will affect the solidification and the
microstructure formation of the castings, as the moulds and
cores containing reclaimed sand will still have a strong
cooling capacity much longer after the pouring.
In this work, the effect of reclaimed sand addition on the
cooling capacity of epoxy-SO2 mixtures was studied. TG–
DTA was carried out to gain basic information about the
thermal decomposition of the epoxy-SO2 system and the
reclaimed foundry sand, respectively. TG–DTA presented
valuable initial results; however, the materials were further
examined in real foundry conditions by the novel
application of Fourier thermal analysis described in a previous
paper. The method of sample preparation was modified to
run temperature measurements and Fourier thermal
analysis in spherical epoxy-SO2 sand specimens at temperatures
according to their every day application in cast iron
production (1300 ± 10 C).
The results of primary measurements enhanced the role
of thermal analysis at temperatures of cast iron production,
because no clear conclusions about the heat absorbing
processes could be made based only on the heating curves
because of higher heating rates.
The calculated total absorbed heat values showed that
25 mass% of additional reclaimed sand content increased
the total absorbed heat by mixture decomposition by
approximately 10–12%. This means that the combustible
materials on the surface of the reclaimed sand will improve
the cooling capacity of the cores. This phenomenon reflects
on the possibility of controlling the solidification time in a
casting simply by reusing mechanically reclaimed
By the right of calculation of the fraction of absorbed
heat and rate of heat absorption, the effect of reclaimed
sand level on the cooling capacity can also be evaluated
versus the temperature in the core specimens.
The fraction of absorbed heat curves indicated that the
additional fresh and spent resin in the system improved the
cooling capacity of the moulding material at a wide
temperature range (150–600 C). This will affect the
solidification and the microstructure formation of the castings, as
the moulds and cores containing reclaimed sand will have
improved cooling capacity much longer after the pouring.
The rate of heat absorption results showed that
additional reclaimed sand did not influence the heat absorption
by the vaporization of moisture content and the early stages
of mixture degradation. On the other hand, this was
strongly affected by the heating rate, e.g. the sample
diameter or the temperature of the melt. The curves
underlined that the presence of spent resin in the mixture
will predominantly improve the cooling capacity of the
moulds or the cores at higher temperatures.
The outcome of the paper reflects on the future
possibility of controlled solidification achieved by return sand
addition to the epoxy-SO2 mixture and contributes to the
topics of thermal sciences, simulation of casting processes,
and also foundry technology in general.
Acknowledgements The present work was financed by the Swedish
Knowledge Foundation. Cooperating parties in the project were
Jo¨nko¨ping University, Scania CV AB and Volvo Powertrain
Production Gjuteriet AB. External contribution was provided by the
University of Miskolc. Participating persons from these
institutions/companies are acknowledged.
Open Access This article is distributed under the terms of the
Creative Commons Attribution 4.0 International License (http://crea
tivecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give
appropriate credit to the original author(s) and the source, provide a
link to the Creative Commons license, and indicate if changes were
See Figs. 9 and 10.
40 mm samples
60 mm samples
40 mm samples
60 mm samples
40 mm samples
40 mm samples
60 mm samples
60 mm samples
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