Thermal Conductivity of Moulding Sand with Chemical Binders, Attempts of its Increasing

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A R C H I V E S O F M E T A L L U R G Y A N D M A T E R I A L S

Volume 60 2015 Issue 1

DOI: 10.1515/amm-2015-0058

J. ZYCH ∗, J. MOCEK ∗, T. SNOPKIEWICZ ∗, Ł. JAMROZOWICZ ∗

THERMAL CONDUCTIVITY OF MOULDING SAND WITH CHEMICAL BINDERS, ATTEMPTS OF ITS INCREASING

PRZEWODNOŚĆ CIEPLNA MAS FORMIERSKICH ZE SPOIWAMI CHEMICZNYMI, PRÓBY JEJ ZWIĘKSZENIA

The investigation results of the thermal conductivity of the selected group of moulding sands with chemical binders,
mainly organic, are presented in the hereby paper. Studies encompassed also moulding sands into which additions improving
the thermal conductivity were introduced. Two testing methods were applied, i.e. investigations at a steady and unsteady
temperature zone. For investigations at a steady temperature zone the new original experimental stand was designed and built,
adapted also for testing moulding sands with binders undergoing destruction at relatively low temperatures.

_Keywords_ : thermal conductivity, moulding sands, investigation methods, binders

W pracy przedstawiono wyniki badań zdolności do przewodzenia ciepła wybranej grupy mas ze spoiwami, głównie
organicznymi. Badaniami objęto również masy, do których wprowadzono dodatki dla zwiększenia przewodności cieplnej.
Zastosowano dwie metody badań przewodności cieplnej, tj. badania w ustalonym polu temperatury i badania w nieustalonym
polu. Dla badań w stacjonarnym polu zbudowano nowe, oryginalne stanowisko pomiarowe, dostosowane do badań mas ze
spoiwami ulegającymi destrukcji w stosunkowo niskiej temperaturze (spoiwa organiczne). Wyniki badań przedstawiono graficznie w układzie: λ = f(T) - współczynnik przewodzenia w funkcji temperatury. Wykazano istnienie minimum funkcji w
temperaturze niższej niż w przypadku mas klasycznych. Wprowadzanie do masy dodatku grafitu zwiększa jej przewodność.
Wprowadzenie grafitu nie wpływa istotnie na proces wiązania mas furanowych. Masy ze szkłem wodnym charakteryzują się
lepsza przewodnością, szczególnie w wyższych (T >400 ◦ C) temperaturach.

1. Introduction

The intensity of heat exchanging between metal and a
sand foundry mould is one of the factors deciding on the metallographic structure of castings. A majority of foundry alloys
is sensitive to the cooling rate, which means that their structure
depends on the kinetics of the heat transfer to the mould. A
heat flow from a casting to sand mould is of unsteady character, time variable. As it is generally known, in all transient
processes enthalpy changes occur both in the agent giving up
the heat - in this case: metal and in the agent taking the heat

- in this case: moulding sand. A heat flux flowing from metal to the mould depends on the temperature gradient on the
contact surface of metal and the mould and on the processes
of propagation and accumulation of heat in the mould, which
in turn depends on thermophysical properties of the mould
material. These properties are determined by values related to
the material and mould state, which are mutually interrelated.
To the group of these properties belong

_b_ 2 =

~~�~~ λ2
(λ2 · _c_ 2 · ρ2)α2 =
_c_ 2 · ρ2

(1)

α2 - coefficient of temperature conduction, [m 2 /s]
b2 - coefficient of heat accumulation, [(W·s 1/2 )/(m 2 - K)]
ρ2 - specific gravity, [kg/m 3 ]
λ2 - thermal conductivity, [W/(m K)]
c2 - specific heat, [J/(kg K)]
It is known, from the general theory of heat exchanging
in transient processes, that between the mentioned physical
values, connections of a determined physical sense exist and
that they are in a form of equations (1) and (2):

Since the moulding sand is not a homogeneous body,
all mentioned above coefficients determining thermophysical
properties are of a substitutive character. They concern the
moulding sand - of the determined composition prepared under the given conditions - out of which the mould of the
given compaction was made. In addition, they concern the
mould which was poured by the selected alloy (of the determined temperature) either in its wet state or after drying

[2]. The anisotropy of the moulding sand compaction, which
depends on the compacting direction, can also influence these
coefficients values. Due to the above, the coefficients values

- determined by any method - can only serve for relative
comparisons of various moulding sands. Thus, the application
of the obtained results for thermal calculations of the given
sand moulds must be done with a lot of caution, since large

_b_ 2 =

~~�~~
(λ2 · _c_ 2 · ρ2) (2)

where:

∗
AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY, FACULTY OF FOUNDRY ENGINEERING, 23 REYMONTA STR., 30-059 KRAKÓW, POLSKA

352

divergences can occur [1, 2]. The majority of investigations
carried out up-to-date concerned assessments of thermophysical properties of moulding sands with bentonite. However,
in the foundry practice more and more often moulding sands
with chemical binders, organic and inorganic, are used. Organic binders are substances based on carbon compounds, which
undergo the pyrolisis at being heated after the mould pouring
with liquid metal. The heat of the pyrolysis changes the kinetics of the mould heating and casting cooling. At the current
state of knowledge, this problem is poorly investigated and
described. It is even considered, that the thermal conductivity
determination of moulding sands with chemical binders is not
possible, due to the pyrolysis of binders [2].

Descriptions of increasing the heat transfer ability of
moulding sands can be found in references as well as tests
of doing this in the foundry practice occur. It can be achieved
by changing the moulding sand matrix (e.g. from quartz into chromite) or by introducing certain additions (materials)
increasing the thermal conductivity (e.g. graphite, cast steel
shot, etc.).

The series of investigations on the assessment of thermophysical properties of moulding sands with chemical binders
as well as sands with additions improving their thermal conductivity were performed within the hereby study. These own
investigations were preceded by the development of the new
testing method of the moulding sands conductivity and by
building of the new experimental stand.

2. Own investigations

2.1. Object of investigations and their range

The object of investigations constituted moulding sands
with organic binder (furan resin) and the quartz matrix
and with additions improving the sand thermal conductivity (graphite and cast steel shot). An increase of the mould
ability for a fast heat transfer can be realised by the given
below operations.

- Changing of the moulding sand compaction degree.

- Replacing the quartz matrix by another matrix of a higher
ability of taking and transporting heat from the surface of
the casting being cooled into the mould depth and outside
it.

- Introducing additions significantly improving the thermal
conductivity.

- Introducing into the moulding sand additions increasing
its heat accumulation ability and thus, improving general
conditions of a heat transfer from the casting being cooled.

- Application of traditional external chills.

- Making the mould from another material (e.g. applying a
metal mould or core).
The performed investigations concerned the determination
of thermophysical coefficients of moulding sands with furan
binder, in a wide range of temperatures.

- Determination of thermophysical properties of the reclaim
based moulding sand, for two compaction levels.

- Determination of thermophysical properties of the moulding sand with the chromite and magnesite matrices and

comparing them with the moulding sand with the quartz
matrix.

- Determination of properties of moulding sands with the
graphite addition (at two levels), as the component which
can improve the moulding sand ability for the heat transfer.

- Determination of the thermal conductivity of the moulding sand with additions of cast steel shots, increasing the
ability to accumulate heat.
Investigations of thermophysical properties were supplemented by the DTA and DTG tests of moulding sands with furan
resin with the use of the derivatograph. Their aim was to
determine the temperature range, at which the decomposition
process and organic binders pyrolysis occurred and to confirm
thermal effects of these processes.

2.2. Methodology of testing

There are several methods of investigating thermophysical properties of moulding sands but no one is universal.
Generally these methods belong to two categories.

- Methods of testing under conditions of a steady heat flow
(a constant heat flux flowing through the selected moulding sand layer).

- Methods of testing under conditions of an unsteady heat
flow through the selected moulding sand layer.
In the solution belonging to the first category, heating of the
moulding sand layer (sample) by the heating element is used
and then the directional heat flow is forced (unidirectional – for
flat samples, or axially symmetrical - for cylindrical samples
with the heat source situated in the axis). For the determination
of the thermal conductivity (λ2) one of the basic conduction
equations are used. This equation describes the dependence
between the thermal flux intensity ( _q_ ), flowing through the
moulding sand layer of a thickness („ _l_ ′′ ) related to the surface
unit by the temperature gradient (at this thickness) – ( _T_ 2 − _T_ 1)
and by the thermal conductivity coefficient, equation (3):

_q_ = λ2 (3)

_l_ · ( _T_ 2 − _T_ 1)

The thermal conductivity coefficient is determined from the
transformed equation (3). Since moulding sands are porous
materials, on their conductivity decide: the thermal conductivity coefficient of the skeleton of the porous body structure
(matrix) λ _s_ and of the medium filling pores λ _s_ (either liquid or liquid and gas mixture or gas only). In investigations
of moulding sands, which are typical porous materials the
so-called _effective_ _coefficient_ _of_ _thermal_ _conductivity_ λ _sf_ is
experimentally established.

Experimental determination of the thermal conductivity
consists of: measurements of the temperature gradient on the
sample cross-section, determining (either by measurements or
by calculations) the value of the heat flux penetrating the sample, measuring the distance between layer edges (in which the
temperature gradient occurs) and measuring the heat exchange
surface.

Conductivity investigations under conditions of an unsteady heat flow (the second category) consists of recording
and analysing the heating pathway of the moulding sand sample placed in the special mould, after pouring it with liquid
metal and recording the casting cooling the sample thermal

conductivity was preceded by the determination of the coefficient of temperature conduction (α2), calculations of the
coefficient of heat accumulation (b2), followed by calculation
of the thermal conductivity coefficient (λ2) from equation (4):

λ2 = ( _A_ + _B_          - _C_ ) · _m_ (4)

where:

A, B   - constants characterising the given measuring
stand,

C  - heat storage capacity of the tested material,
m  - cooling rate.
Each of the symbols of equation (4) has the mathematically complex structure. The procedure, described in details in
previous studies [2÷5], was applied in calculating the thermal
conductivity coefficient.

2.3. Results of investigations

2.3.1. Conductivity measurements at the steady heat flow

The new experimental stand for investigations of the
moulding sands thermal conductivity, under conditions of a
steady heat flow, was built in the Faculty of Foundry Engineering AGH (Laboratory of Foundry Moulds Technology). This
stand is adapted for moulding sands with organic binders. The
solution is one of the variants of the concept of testing the thermal conductivity proposed by Poensgen [8]. These moulding
sands specificity lies in the fact that after heating to a temperature above 300 ◦ C the organic binder destruction starts. This
process ends at a temperature slightly higher than 500 ◦ C. The
consequence of this process constitutes the sample (mould)
decomposition, which renders difficult the testing procedure.
This effect was taken into account in the designing works of
the new experimental stand. The schematic presentation of the
heating system structure of the sample with the unidirectional
heat flow and the thermocouples system for measuring the
temperature distribution is presented in Figure 1.

Fig. 1. Schematic presentation of the measuring system together with
thermocouples placed in the sample: 1 - cooling plate, 2 - tested
sample, 3 - controlled heating element, 4 - thermocouples, 5 - thermal insulation mounted before the measurement, 5a - stable thermal
insulation

The heating system of the moulding sand sample (2) consists of the controlled heating element (3), cooling system (1),
thermal insulation system (5 and 5a), thermocouples system

353

(4), used for recording the temperature distribution in the measured sample and for control of the heating system. The heating system structure allows to obtain the needed temperature
and the stabilised, unidirectional heat flow through the tested
sample.

The measuring stand diagram is presented in Figure 2.
The heating temperature is controlled by the industrial controller (Fig. 2 - element C). The electronic relay SSR was
applied as the executing element - switching over the heating
system supply. For measuring the current parameters during
the sample heating the universal meter was used (Fig. 2 – element D). This meter was connected in series with the heating
element - recording via the USB port - Sanwa 510a. Voltage
measurements on the heating element were also performed.
From the changes of the current intensity pathways, which
was periodically switched on by the controller, the average
current value was calculated, and then - on this bases - the
power input of the heating element was also calculated.

The heat flow was oriented according to the designing
assumptions of the experimental stand. The heat from the
heating element was flowing through the lower surface of the
moulding sand sample then through the sample itself and was
‘directed’ towards the upper plate, which was intensively water cooled. The directional heat flow was achieved due to: an
application of the flat-shaped sample, thermal insulation of
side surfaces of the sample with modern insulation materials
and the application of the plate, which was intensively cooling
the opposite side of the moulding sand sample.

Fig. 2. Scheme of the measuring stand for investigating the thermal
conductivity of moulding sands with chemical binders. Measurement
under conditions of the steady heat flow

The heating of the heating plate was controlled by the
temperature controller. To limit its corrosion, the plate was
made of the acid resistant steel. Relatively large plate thickness
enabled a uniform heating of the moulding sand sample on its
whole surface. The thermocouple, type ‘K’, was installed in
the lower part of the plate for controlling the heating system.

Fig. 3. a) Sample placed on the heating plate together with thermocouples and cooling plate - a) state before installation of the side
thermal insulation, b) the way of installing measuring thermocouples

354

Temperatures in individual places of the sample were
recorded by means of the multichannel measuring instrument,
Agilent 34970A (Fig. 2. - element B). The way of installing
measuring thermocouples is shown in Figures 1 and 3b. The
typical temperature distribution in the sample, during the thermal conductivity testing is presented in Figure 4. Two periods
are characteristic in the temperature pathway: the first - unsteady (initial heating) and the second - stabilised (steady).
When the temperature stabilised itself in all measuring points,
the current parameters of the heating plate were measured,
since they were necessary in calculating the heat flux intensity. The temperature pathways are displayed on the computer in
a real time, which simplifies the determination of the starting
moment of recording the heating system current parameters.

These current parameters were used in calculation procedures of the heat flux intensity. These were parameters values
at which the temperature zone in the sample is characteristic
for the stabilised heat flow, it means the constant temperature
gradient is maintained.

Fig. 4. Heating pathway of the moulding sand sample prepared with
the application of furan resin as a binder. Recording in individual
moulding sand layers.

2.3.2. Investigation results by the method of the steady heat
flow

A series of measurements of the thermal conductivity of
moulding sands with furan resin was performed. The matrix of
the tested moulding sands was high-silica sand either fresh or
reclaimed. Furanol FR75A resin in amounts of 0.8÷1.0% was
used as a binder, while PTS acid as a hardener in amounts of
0.45÷0.55%. Investigations of moulding sands with graphite
additions in amounts up to 10% in relation to a matrix as well
as with steel shots additions in the same amount were also
performed. Additionally, for comparison assessments of the
matrix influence on the thermal conductivity, investigations of
moulding sands with chromite and magnesite matrices were
performed. Also for comparisons the moulding sand with inorganic binder - water glass, which was added in amount of
3.5%, was investigated. The moulding sand was prepared in
the ester technology - with Flodur, as a hardener added in
amount of 10%, in relation to the water glass amount.

All samples were prepared for measurements in the same
way: before testing their thermal conductivity, they were dried
at a temperature of 120 ◦ C for two hours. The dependence from

the equation (3) was used in the developed measuring methodology of determining the thermal conductivity coefficient.

The instantaneous intensity of the heat flux generated by
the heating plate and flowing through the sample was determined by means of the wattmeter. The heat flux intensity ( _q_ )
was calculated as the ratio of the measured heating power
to the contact surface: moulding sand sample/heating plate.
The obtained results of the thermal conductivity of moulding
sands with chemical binders, including sands with additions
intensifying heat exchanging, are shown in Figure 5. Moulding sands were heated up to a temperature of app. 450 ◦ C.
A higher temperature caused a destruction of moulding sand
with resins, which renders difficult and - in practice - makes
the proper measurement impossible.

Fig. 5. Thermal conductivity of moulding sands of the modified composition based on a furan binder (Furanol FR75A) with a matrix: 1

- reclaim + 10% graphite + steel shot; 2 - reclaim + 10% graphite,
ρ _o_ =1.5 g/cm 3 ; 3 - reclaim + 10% graphite, ρ _o_ =1.6 g/cm 3 ; 4 reclaim + steel shot; 5 - reclaim + steel shot; 6 - moulding sand
with water glass; 7 – reclaim + 5% graphite; 8 – chromite sand, ρ _o_ =
2.9 g/cm 3 ; 9 - reclaim, ρ _o_ =1.6 g/cm 3 ; 10 - reclaim, ρ _o_ =1.5 g/cm 3 .

The analysis of the obtained results leads to the given
below conclusions.

- In a similar fashion as in case of moulding sands with
bentonite (which conductivity investigations are described
in papers [2, 3]) in moulding sands with chemical binders
a non-linear temperature influence on the thermal conductivity is also observed.

- The thermal conductivity minimum occurs at a temperature of app. 300÷350 ◦ C. Both at a lower and at a higher
temperature the moulding sand conductivity is higher.

- A lower thermal conductivity have moulding sands unsatisfactory compacted (ρ _o_ <1.5 g/cm 3 ) prepared from the
reclaim without additions.

- A compaction increase (porosity decreasing) leads to increasing of the thermal conductivity in case of all kinds
of sands.

- The graphite introduction into the sand in amount of 10%
(mass) increases the moulding sand conductivity nearly
twice.

- Even higher conductivity can be obtained when, apart
from graphite, steel shots (which increase the sand ability

for heat accumulation) are introduced into the moulding
sand.

- Moulding sands with water glass are characterised by a
higher ability of the thermal conductivity than moulding
sands with organic binders (resin). The resin pyrolysis at
relatively low temperatures causes decreasing of the temperature gradient and   - in consequence   - decreasing the
heat flux penetrating into the mould.

- The matrix change from quartz into chromite, only slightly
improves the thermal conduction ability of the moulding
sand with furan resin. Similar effects causes an introduction of the magnesite matrix. Thus, this direction of looking for moulding sands of a high ability for the thermal
conductivity is not the proper one.

2.3.3. Investigations under conditions of the unsteady heat
flow

The method of determining thermophysical properties of
moulding sands with chemical binders, proposed in this paper, is based on measuring the temperature zone of the system:
casting – mould, by means of thermocouples placed in various
measuring points. This method belongs to direct methods of
testing the metal cooling and solidification in sand moulds [3].
The diagram of the measuring stand (experimental mould) is
presented in Figure 6.

Fig. 6. Ideological diagram of thermal conductivity measurements of
moulding sands directly in the mould: 1A, 1B – samples of the tested
moulding sand, 2 - mould, 3 - gate, 4 - thermocouple ‘in casting’,
5 - plate casting, 6 - thermocouples in moulding sand samples

Two samples made of the tested moulding sand were
placed in the mould. In each sample 4 thermocouples, allowing to record the heating process after pouring the mould
with liquid metal, were placed. The recording of the casting
cooling pathway is, in this method, necessary for calculations.
Therefore in each measuring series the casting cooling was
recorded by means of two thermocouples properly situated.
The mould view with inserts made of the tested moulding
sand is shown in Figure 7a. The casting with visible ‘traces’
of the tested inserts (of moulding sands with furan resin as a
binder) are shown in Figure 7b.

Halves of the mould, made of the classic moulding sand,
were dried at 220 ◦ C for 24 hours, in order to eliminate the
influence of water vapours - from the wet moulding sand on the obtained results.

355

Fig. 7. a/ Mould for the thermal conductivity testing with two inserts
made of the tested moulding sands with furan resin, b/ Test casting
with visible ‘traces’ of the tested inserts

The casting of dimensions: 33×200×300 mm was made
of grey cast iron and moulds were poured with metal of a
temperature ∼1400 ◦ C. The total recording of temperatures,
both in the casting and in the mould, was performed in each
test. The example is shown in Figure 8.

Fig. 8. Examples of the pathways of the iron casting cooling and
moulding sands heating in samples for the determination of moulding sands conductivity under unsteady conditions. Dotted lines moulding sand with the reclaim matrix, continuous lines – moulding
sand with a reclaim + graphite (10%)

A temperature of the cooled casting (plate) was measured
in a direct vicinity of the insert, which allowed to assess, approximately, the influence of the heat transfer of the moulding
sand – used for making the insert – on the cooling time of the
plate being in the vicinity of this insert.

The analysis of pathways shown in Figure 8 allows to
notice that the casting solidification time of the furan moulding sand (matrix - reclaim) is significantly longer than the
solidification time of the same moulding sand but containing
additions of app. 10% of a graphite dust. These qualitatively
described conditions were confirmed by the thermal conductivity results of the mentioned moulding sands. The lines in
Figure 8 indicate distances between the measuring points and
the contact surfaces: casting/mould.

356

The thermal conductivity results of four tested moulding
sands, which represent sands which could be used for controlling the casting cooling rate, are listed in Figure 9. Moulding
sands with organic binders (furan resin) are characterised by
very similar pathways, in respect of the temperature influence
on the thermal conductivity coefficient value.

Fig. 9. Thermal conductivity of the selected group of moulding sands
with additions increasing the sand ability for heat conduction. Investigations under conditions of an unsteady heat flow

The lowest conductivity reveals the moulding sand based
on the reclaim matrix without any modifying additions. In
a similar fashion as in the previously described tests results
(conductivity under steady conditions), in these studies the
conductivity minimum was also found at temperatures of approximately 300-400 ◦ C.

An introduction of additions increases the thermal conductivity of moulding sands. Already an introduction of 10%
of graphite increases the thermal conductivity in a visible way.
However, a significant increase of the conductivity is obtained
after an additional introduction of steel shots. The thermal
conductivity ability of such moulding sand is nearly twice as
high as of the sand without any additions.

The completely different thermal characteristic have
moulding sands with water glass. A lack of the ‘internal heat
source’, which constitutes the resin pyrolysis, causes that the
moulding sand with water glass has decisively better thermal
conductivity, especially at the increased and high temperatures. At a temperature of approximately 900 ◦ C this conductivity is nearly twice as high. At slightly lower temperature in
moulding sands with water glass melting of water glass occurs. To that effect a better joining of matrix grains by elastic
bridges occurs, which probably facilitates the heat flow.

2.3.4. Derivation studies of furan moulding sands

Heating of moulding sands with organic binders leads
to binders decompositions and heat emissivity. This is confirmed by derivative investigations of moulding sands with
furan resin, the results of which are shown in Figures 10 and
11. Tests were carried out in the air, without the protective
atmosphere. The samples mass losses start at a relatively low
temperature, and after exceeding 300 ◦ C the pyrolysis process
visibly accelerates (Fig. 10). This process is of an exothermic
character, which confirms the temperature pathway in the DTA
curve (Fig. 11).

Fig. 10. Pathway of the sample mass change (moulding sand with
furan resin as a binder) - DTG curve

Fig. 11. Pathway of the sample temperature (moulding sand with
furan resin as a binder) - DTA curve

The heat occurring during heating of furan moulding
sands can be one of the reasons of the thermal conductivity
decreasing at a temperature of approximately 300-400 ◦ C. Such
decrease was found in the thermal conductivity investigations
at steady and unsteady conditions of heat flow (Fig. 5) and
(Fig. 9), respectively.

3. Conclusions

The thermal conductivity of moulding sands with binders,
which are broadly applied at making medium and large castings, is very poorly known. Binders decomposition at a relatively low temperature renders measuring difficult and several
investigation methods are not suitable for investigating moulding sands being in such state. The developed, new methodology of investigating the moulding sands conductivity - by
means of the steady heat flow - allows to study this group of
sands. The performed tests allowed determining the thermal
conductivity in the range up to approximately 450 ◦ C. The
minimum of the conductivity value at a relatively low temperature, (approximately 300-350 ◦ C) is probably related to the
binder pyrolysis and heat emissivity, which disturbs and slows
down the heat flow from the casting into the mould.

It was pointed out in investigations, that the conductivity increase can be achieved by introducing such additions as
graphite or steel shot into moulding sands. A low conductivity
of moulding sands with organic binders can prevent obtaining
a fine structure castings. The conductivity assessment under
conditions of an unsteady heat flow leads to similar conclusions. Moulding sands with organic binders reveal the lowest
conductivity at a temperature slightly above 300 ◦ C. A much
better conductivity characterises moulding sands with inor

ganic binder - water glass. The difference is specially visible
at higher temperatures, above 500÷600 ◦ C.

A conductivity increase at the compaction degree increasing (which measure is the apparent density) is observed in all
kinds of moulding sands.

Acknowledgements

Research was performed within the Project:
POIG.01.04.00-12-116/12-00.

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_Received:_ _20_ _April_ _2014._

357

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[4] W. Longa, Krzepniecie odlewów w formach piaskowych.

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