VŠB - Technical University of Ostrava Faculty of ...katedry.fmmi.vsb.cz/Opory_FMMI_ENG/MMT/Molding...

VŠB - Technical University of Ostrava

Faculty of Metallurgy and Materials Engineering

MOULDING MIXTURES (E-LEARNING)

Jaroslav Beňo

Petr Jelínek

Nikol Špirutová

Ostrava, 2015

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Name: Moulding mixtures

Author: Ing. Jaroslav Beňo, Ph. D.

Prof. Ing. Petr Jelínek, CSc., Dr. h. c.

Ing. Nikol Špirutová

Issue: first, 2015

Number of pages: 168

Educational materials for the branch Modern Metallurgical Technologies (study

programme Metallurgical engineering) of the Master’s study at the Faculty of

Metallurgy and Materials Engineering

Proof-reading: not performed

© Jaroslav Beňo

© VŠB - Technical University of Ostrava

TABLE OF CONTENTS

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TABLE OF CONTENTS

1 BASE SANDS ................................................................................................ 6

1.1 General classification of base sands .......................................................................... 6

1.2 Silica sands ................................................................................................................ 7

1.3 Disadvantages of silica sands .................................................................................... 9

1.3.1 Increased reactivity of silica sands ................................................................... 10

1.3.2 Discontinuous thermal dilatation ...................................................................... 10

1.3.3 Cristobalite expansion ...................................................................................... 12

1.4 Non-silica sands ....................................................................................................... 16

1.4.1 Chamotte schist ................................................................................................. 17

1.4.2 Corundum ......................................................................................................... 18

1.4.3 Chromite ........................................................................................................... 19

1.4.4 Zirconium-silicate ............................................................................................. 20

1.4.5 Olivine .............................................................................................................. 21

1.5 Granulometric composition of sands ....................................................................... 23

1.5.1 Cumulative curve of granularity ....................................................................... 23

1.5.2 Log W criterion ................................................................................................ 26

1.6 Angularity coefficient of sand ................................................................................. 27

1.7 Thermal conductivity (cooling effect) of a foundry mould ..................................... 30

1.7.1 Cooling effect of non-silica base sands ............................................................ 33

2 REGENERATION OF MOULDING SAND MIXTURES ..................... 34

3 STRENGTH OF MOULDING SAND MIXTURES ............................... 35

3.1 Strength of moulding sand mixtures as a result of adhesion – cohesion forces of a

binder ................................................................................................................................. 37

4 BINDER SYSTEMS OF THE IST GENERATION MOULDING

SAND MIXTURES ................................................................................... 38

4.1 Clay binders ............................................................................................................. 38

4.2 Water in clay minerals ............................................................................................. 39

4.3 Bentonite and bentonite mixtures ............................................................................ 40

4.3.1 Natrification of bentonite and its consequences ............................................... 42

4.4 Additives for bentonite mixtures ............................................................................. 44

4.4.1 Saccharides ....................................................................................................... 45

4.4.2 Oxidants ............................................................................................................ 45

4.4.3 Graphite ............................................................................................................ 45

TABLE OF CONTENTS

4

4.4.4 Carriers of lustrous carbon ............................................................................... 46

5 BINDER SYSTEMS OF THE IIND GENERATION SAND

MIXTURES – INORGANIC BINDERS ................................................ 49

5.1 Moulding sand mixtures with water glass ............................................................... 49

5.1.1 Hardening procedures for mixtures with water glass ....................................... 49

5.1.2 Water glass module .......................................................................................... 49

5.1.3 Coagulation threshold of water glass ................................................................ 50

5.2 CO2 process ............................................................................................................. 51

5.2.1 By-products of the hardening reaction ............................................................. 55

5.3 Self-hardening mixtures with water glass ................................................................ 56

5.3.1 Physical – chemical processes of hardening of water glass by liquid hardening

agents 56

5.3.2 Reactivity and optimal concentration of an ester-type hardening agent........... 58

5.4 Problems of collapsibility of mixtures with water glass .......................................... 60

5.4.1 Critical analysis of residual strength of a mixture with water glass and silica

sand (CO2-process) ....................................................................................................... 60

5.4.2 An increase in residual strength and the Ist maximum area ............................. 60

5.4.3 Decrease in residual strength behind the Ist maximum and area of the Ist

minimum (approximately 600°C) ................................................................................ 61

5.4.4 Sharp growth of residual strength behind the Ist minimum and the area of IInd

maximum (800900°C) ................................................................................................ 61

5.4.5 Decrease in residual strength behind the IInd maximum (the IInd minimum). 62

5.4.6 Possibilities of control of residual strength with water glass (CO2 – process) . 62

5.5 Hardening of alkaline silicates by dehydration processes ....................................... 63

5.5.1 Collapsibility of cores ....................................................................................... 63

6 BINDER SYSTEMS OF THE IIND GENERATION SAND

MIXTURES – ORGANIC BINDERS ..................................................... 63

6.1 Organic-based foundry binders ................................................................................ 63

6.1.1 Chemistry of phenolic binders .......................................................................... 64

6.1.2 Shell moulding, the Croning method, “C” method .......................................... 64

6.1.3 Hot-Box method ............................................................................................... 67

6.1.4 Cold-Box method ............................................................................................. 68

6.2 Chemistry of furan binders ...................................................................................... 71

6.2.1 Self-hardening furan mixtures .......................................................................... 71

6.2.2 Cold processes – extrinsic hardening ............................................................... 72

TABLE OF CONTENTS

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6.2.3 SO2 – process (furan resin) ................................................................................ 72

6.2.4 Resol-CO2 ......................................................................................................... 74

6.3 Chemistry of polyurethane binders .......................................................................... 75

6.3.1 Phenolic polyurethane system .......................................................................... 75

Sands – General clasification

6

1 Sands

1.1 General classification of sands

Sand grains, a heat resistant material with a particle size above 0.02 mm, has the main

volume weight apportionment in a mixture. It forms a material skeleton of moulds and cores,

therefore angularity and granulometry of particles belong among its most important properties

aside from the activity of grain surfaces. Angularity and granulometry of particles are decisive

for volume weight, porosity and ensuing permeability and penetrability of a mixture, thermal

dilatation and generation of tension from braked dilatation, thermal conductivity of a mixture

and affect even strength of moulds and cores to a large extent.

A sand particle size of 0.02 mm is considered critical. Smaller particles belong to a

flushable fraction – clays, silica dust, non-plastic particles, spar, other minerals – clay binder

(determination of flush ability by a test, ČSN 721078).

To calculate a sedimentation setting velocity in water (v) and determine flushable

fractions in sands (silica sand can be obtained by washing sands), the Stokes law is used

(applies for particles between 0.001 – 0.1 mm):

122

9

2v

rg (1)

where: g – gravitation

r – a particle radius

2 – base sand specific weight

1 – water specific weight

– water dynamic viscosity

Elutriation of the sand – water suspension is performed in the presence of 5% solution

of NaOH (pursuant to the Czech standard). A dispersing agent prevents aggregation of fine

(colloid) particles. Aggregates have different sedimentation setting velocities and distort

results of the sedimentation analysis.

Washed sands, nowadays used for preparation of synthetic mixtures with inorganic

binders, should not contain more than 1 % of flushable particles. For synthetic resins even

lower concentration is required.

Sands are divided according to the chemical character:

- Acidic (silica sands)

- Neutral (chamotte, corundum)

Sands – General clasification

7

- Alkaline (magnesite, olivine)

Acidic sands react with alkaline oxides of alloyed steels, leading to formation of

compounds with lower heat resistance. As a result of these chemical reactions, burning-on

and burning-in (metal penetration) occur on castings. Therefore Mn steels cannot be cast into

silica sands. For thin-walled castings, the both chemically different oxides can be isolated

from a contact with each other by application of a base or neutral coating (magnesite,

corundum).

Sands are divided according to the origin:

- Natural (silica sands, olivine, disthen-sillimanite, zirconium-silicate)

- Artificial (ground chamotte, corundum, metal beads, Cerabeads)

The selection of a sand type for preparation of a mixture must meet the following criteria:

- A chemical character of the cast alloy (a type of the alloyed material)

- A type of the cast alloy (steel - cast iron); casting temperature, permissible

content of feldspars in sand

- A shape complexity and wall thickness in a casting (susceptibility to

occurrence of casting defects - burning-in, scabs)

- A type of a binder system (fins)

- Economic availability and a price of a mixture also regarding achieving

maximum strength with a minimum binder content

Therefore silica sands are most widely used. Silica is the most common mineral

occurring in nature in an appropriately grainy state and its properties, even at high

temperatures, meet the typical demands. Regarding to higher reactivity of silica sands with

FeO, MnO and other oxides under high temperatures, when manufacturing high-demanding

massive castings it is replaced by sands with a higher melting point (chamotte schist,

chromium-magnesite, corundum, zirconium-silicate, chromite).

1.2 Silica sands

Silica sands belong among the most cost-effective and thus the most widely used

sands for preparation of synthetic mixtures, but they are contained in natural sands. The main

mineral is quartz (SiO2), which crystallizes in the trigonal trapezohedral system (β-quartz). It

has hardness 7, specific weight ranges between 2620 – 2660 kg/m3, pH 6 – 7.2.

Sands - Silica Sands

8

General requirements on silica sands for preparation of synthetic mixtures are as

follows:

- Highly mineralogically pure (SiO2> 98 %) and highly regular

- Less angular (rounded sands are suitable for organic binders – small surface,

minimal binder consumption, however, they do not resist temperature changes

too much and they are susceptible to thermal stress generated defects).

- As minimal concentration of fine fractions as possible, including 0.1mm on a

sieve. For unified bentonite mixtures, 10 – 12 % proportion of the fine fraction

(0.06 – 0.15 mm) is required.

- They should not contain coarser grains above 1.2 mm (does not apply for

massive castings).

- Sands for steel must contain only minimum of feldspars (up to 1 %). Dispersivity

of feldspars is even of higher importance. Sometimes higher concentrations do

not matter, if they are fine-grained. Feldspars have a low melting point and they

strongly decrease the sand sintering temperature.

- Regarding to the demanded smoothness of the surface of castings, the worldwide

tendency is to work with fine-grained sands, about medium grain d50= 0.22 mm

(for massive castings d50> 0.3 mm).

- The grain surface highly active, clean, without coatings and stuck-on non-

flushable particles.

Fig. 1 documents an influence of the above described granulometric processing on

strength of a self-hardening furan mixture for three sorts of silica sands.

Sands - Silica Sands

9

Obr.1. Influence of granularity modification of silica sands on tensile strength of a self-

hardening furan mixture

By removing flushable fractions (below 0.02 mm) and further even fine sand particles

including 0.1 mm, strength of mixtures with an organic binder can be increased significantly,

regardless the silica sand type. An influence of coarse-grained fractions above 0.4 – 0.5 mm

also affects the resulting strength adversely.

Fine fractions have a high surface and increase the binder consumption, thus decreasing the

thickness of the grain envelope of the binder as well as the mixture strength. Then the finest

fractions in the envelope of binder have an adverse effect as internal notches.

1.3 Disadvantages of silica sands

The main disadvantages of sands cover:

- Increased reactivity with Fe oxides and oxides of other metals (alloying elements

of steels) at high temperatures

- Discontinuous thermal dilatation – related to low dimensional accuracy of cast

castings and many thermal stress generated casting defects (fins, scabs etc.)

- Cristobalite expansion in the presence of mineralizers and at high temperatures

(over 900°C)

Sands – Disadvantages of silica sands

10

- Silicosis – a disease caused by silica dust

Considering these main disadvantages, there has been ever increasing effort to replace

silica sands by other minerals (non-silica sands) or by artificial sands.

1.3.1 Increased reactivity of silica sands

Quartz is a markedly acidic compound and reacts with alkaline substances, leading to

formation of compounds of reduced heat resistance. The resulting products of these reactions

are complex oxides (fayalite, pyroxene), typically with a melting point lower than iron.

2 FeO + SiO2 → 2 FeO·SiO2 (fayalite – 1200 °C) (2)

FeO + SiO2 → FeO·SiO2 (pyroxene) (3)

Obr.2. Binary diagram FeO – SiO2

Formation of fayalite relates to penetration of metal into a foundry mould and

occurrence of burning-in (fayalite melting point is specified in literature from 1185 to 1205

°C). The melting point further decreases, when MnO (manganese oxide) enters the reaction,

e.g. in Mn-steels (formation of ternary eutecticum FeO – SiO2 – MnO).

1.3.2 Discontinuous thermal dilatation

Quartz features a discontinuous thermal dilatation, which is highly disadvantageous. In

contrast to other sands (olivine, zirconium-silicate) – Fig. 3, its thermal dilatation is

significantly higher.


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Obr.3. Thermal dilatation of some types of foundry sands

Discontinuousness of a dilatation curve can be caused by modification changes of

SiO2. These changes can be best demonstrated on the dilatation curve of a mixture with water

glass and silica sand (Fig. 4) (a mixture hardened by CO2 – process).

Obr.4. Thermal dilatation of a water glass - silica sand mixture (bauxite

addition)

This is a free thermal dilatation without acting of external forces. The entire course

(curve 1) can be divided into 3 stages:


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Stage I: Discontinuous reversible change of the mixture dilatation to 700 °C is caused by

a reversible change of a modification β ↔ α SiO2 (β → α SiO2 573.3 °C; α → β

SiO2 573.1 – 573.2 °C). The specific weight changes only slightly (ρβ-SiO2 =

2650 kg/m3, ρα-SiO2 = 2630 kg/m

3).

The transformation belongs to “rapid

changes” occurring during several seconds. Changes in the crystal lattice are

small. According to the silica sand sort, a linear increase of the mixture reaches

0.86 up to 1.3 %, i.e. c. 2.58 up to 3.9 % of the volume enlargement.

Stage II: Above 700 °C a mild shrinkage of the mixture occurs, which can be explained

both by a slightly negative thermal dilatation coefficient of α – SiO2 and also by

formation of silicic melts, glasses of the binder system (a decrease in strength).

Stage III: At temperatures above 900 - 1000 °C the so-called “slow transformations” occur

– a permanent expansive growth of the mixture, which reaches up to 5% of a

linear dilatation at 1400°C, (c. 15% of the volume dilatation). (Fig. 4)

The discontinuous dilatation process in stage I and its high value result in an

immediate increase in tension from braked free dilatation in a mould. The highest tension

values can be measured in monofraction sands with rounded grains. A high number of casting

defects on casting surfaces result from an increased thermal stress on a foundry mould face.

They cover scabs, fins, sand inclusions and buckles. For instance, the main factors for scabs

are thermal stresses (tensions) generated by the braked mould dilatation and decreased

mechanical properties in a water condensation zone below the mould surface. The most

effective measure to prevent tension generated defects is to replace silica sand by a sand with

a continuous curve with a lower thermal dilatation value (lower tension), without

modification changes. Besides these effects, the reversible change of the dilatation in stage I

is a source of considerable dimensional inaccuracies of castings, too.

1.3.3 Cristobalite expansion

As has been found before, the primary cause of the continuous expansion growth of

mixtures with water glass above 900 – 1000 °C is formation of cristobalite. As substantial

changes in the crystal lattice (hexagonal → cubic) occur during the transition of α – SiO2

into α – cristobalite, this belongs to the so-called “gradual transformation”. Under normal

conditions they are irreversible and their speed depends on the structure, quartz quality and

the presence of catalysts (mineralizers).

However, α – cristobalite cannot be obtained only by heating pure quartz above 900 –

1000 °C, even for as long time as possible, without mineralizers.


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Under non-equilibrium conditions in the foundry mould, the transformation of α –

SiO2 into β – cristobalite at 900 – 1000 °C is mineralized (catalyzed) by the presence of

cations (Na – from water glass, K – from resols etc.).

Single-valent cations activate cristobalitization much more intensively, because four-

valent ions Si4+

tend to avoid multivalent cations to penetrate into the lattice.

Some oxides, e.g. Al2O3 (aluminum oxide), in contrast to mineralizers, brake the

quartz transformations and also brake the effect of mineralizers present at the same time.

Then, the process runs very slowly without mineralizers and cristobalite with a highly

defective structure forms.

Fig. 5 shows that e.g. a bauxite addition in collapsed mixtures with water glass

hardened by CO2 (curve 2) prevents the expansion growth.

The cristobalitic expansion was measured experimentally on cuts of testing castings

made through cylindrical cores along their longitudinal axes. A linear expansion of cores

(ymax) in the thermal center of the casting was calculated from the relation:

%100maxmax

ja

yy (4)

where: aj stands for an original diameter of the core

Obr.5. Scheme of measuring of the cristobalitic expansion in cylindrical cores

A real expansion of cores is influenced not only by conditions of the mixture chemical

composition (a content of Na+-ion catalyst, a presence of the silicate melt liquid phase, a sort

and fineness of sand), but particularly by temperature conditions in a core and mainly on the

core-metal interface.

After casting the molten metal into a mould cavity, it solidifies on the surface of the

core, which heats up intensely. When the temperature exceeds 900°C, the cristobalitic


14

expansion in the core begins, however, this can only develop in dependence on further

process of solidification of the casting outwards from the core. By measuring the temperature

field of cores, it has been found out that the core temperature at a specific ratio of the casting

thermal content to the core thermal content can exceed the solidification temperature of the

primary solidified crust of the casting, which can re-melt and enable further progression of the

expansion until the crust re-solidifies again (periodical solidification) and has such strength

that the core cannot expand anymore, although the cristobalitic expansion in the core volume

goes on. Then, the expansion only results in the tension growth in the core. The core

temperature remains the highest of the whole casting – core system for a long time (a lower

thermal conductivity of the mixture than of the metal) and the core becomes the thermal

centre of the entire casting.

These thermal relations were expressed by a new dimensionless criterion ψ, which

determines a casting/core interaction, and thus also a degree of the core thermal stressing:

jR

R1 (5)

where 1

11S

VR – relative thickness of a casting

V1 – a casting volume representing its thermal content

S1 – a casting surface representing its heat content removal during cooling

Rj – relative thickness of a core

From the thermal point of view, the criterion ψ expresses how much the casting is

larger than the core and what the conditions for the casting solidification and the core heating-

up are.

It has been validated experimentally that along with an increasing criterion ψ (thermal

stressing of a core), the cristobalitic expansion value increases, evaluated from the core

diameter change (ymax) and also from the growth of the area of the cross-cut through the core

∆P (Fig. 6).


15

Obr.6. Dependence of thermal stressing of cores from mixtures with water

glass (ψ) on the cristobalitic expansion (ymax, ∆P)

Obr.7. Cristobalitic expansion of a

cylindrical core with fins on a longitudinal

cut through the casting

Obr.8. Square hole after pre-casting and

a change to a rounded shape as a result

of the cristobalitic expansion

The result is e.g. a substantial enlargement of a diameter of a pre-cast hole and

occurrence of fins (defects that are known solely in mixtures with organic binders) (Fig. 7), a

square hole pre-cast by a prismatic core becomes round-shaped (Fig. 8) or in the case of

cylindrical cores placed parallel with the level of the molten metal filling the mould, heating-

up the core unevenly, we obtain a pre-cast elliptical-shaped hole with a longer axis of the

ellipse perpendicular to the metal level.

These are practical examples of real castings, not experimental specialties.


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1.3.3.1 Consequences of the cristobalitic expansion of silica sands

- Dimensional as well as shape accuracy of castings cannot be ensured at higher thermal

stressing of moulds and mainly cores made of pure silica sands in the presence of

mineralizers.

- Burning-in, mainly in steel castings, contain a cristobalite fraction. The cristobalitic

expansion plays an important part in all the mechanism of burning-in formation. The

expanding core tears a layer of a surface protection (coating – surface grease). (Fig. 9)

- Where the speed of metal solidification onto the core surface is high and re-melting of the

crust does not occur (cyclic solidification), i.e. on cores with lower thermal stressing, the

cristobalitic expansion shows itself as a residual stress growth, which influences

collapsibility and cleanability of cores adversely. Therefore the residual strength, measured

on standard-rollers after annealing in a furnace to various temperatures, cannot be above

900 °C a collapsibility rate for mixtures with water glass. The cristobalitic expansion

(residual strength after the maximum II), however, the collapsibility gets further worse

due to the residual stress growth in cores. Cristobalite is a biologically very active

modification of SiO2, therefore it becomes more and more related with an initiation of

silicosis today.

Obr.9. Deformation of cores of silica sand with water glass (CO2– process)

1.4 Non-silica sands

Although non-silica sands are generally more expensive than silica sands and their

price is even integrated with higher powder density (thus a lower volume compared to SiO2),

non-silica sands have been used for the following reasons:

- They have higher heat resistance and thermal stability. This contributes to resistance of

moulds and particularly cores against occurrence of casting defects, above all burning-in.

Mixtures have also better collapsibility after casting as well as recoverability of sand.

- They have a lower and linear thermal dilatation. Castings have a higher dimensional

accuracy and tension generated defects are eliminated (fins).

Sands – Non-silica sands

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- Moulds and cores have a higher cooling effect. This prevents formation of deep burning-in

and contributes to higher surface and also inner casting quality.

- Their application leads to lower consumption of binders and improvement of hygienic

conditions in foundry shops (silicosis, vasoneurosis).

- They serve for preparation of modern “hybride sands”, improving some drawbacks of

silica sands (dilatation, defects due to tension).

1.4.1 Chamotte

By burning of highly heat resistant schistose clays (schists) in rotary kilns at

temperatures above 1100 °C, burnt shale is obtained – called chamotte schist. In general, this

is a transformation of the kaolinitic clay to a highly heat resistant form of aluminosilicate –

mullite (3 Al2O3 2 SiO2) (melting point 1850 °C).

By milling and screening (with a maximum grain size 3 – 5 mm), sharp-edged

artificial sand can be obtained, which has a continuous dilatation curve (highly resistant

against scabs). It can be applied for binding by kaolinitic clays (chamotte mixtures), water

glass (hardened with CO2) and partly by organic binders (phenolic and alcydic resins). For

foundry purposes more fusible clays are used, where demands for heat resistance are not so

high, but rather for relative density (absorbability below 3 %).

Obr.10. Phase diagram of Al2O3 - SiO2 system (Aramaki and Roy, 1962):

L – liquid, ss – solid solution

Along with an increasing Al2O3 content, or more precisely with decreasing 32

2

OAl

SiO

ratio, chamotte heat resistance increases.


18

They have a neutral to slightly acidic character. In some foundry plants, ground

fractions of chamotte goods, only suitable for preparation of chamotte mixtures (with clay),

were used formerly.

Obr.11. Chamotte

1.4.2 Corundum

In foundry industry, artificial sand is used – electrocorundum (Al2O3). According to

the modification type the specific weight ranges between 3300 – 4000 kg/m3. The melting

point is 2050 °C, hardness 9.0.

- Al2O3 is a polymorphic oxide. It has 4 modifications in total (α, β, γ, ξ).

- α - Al2O3 (alfa-corundum) – belongs to high-temperature modifications of Al2O3,

which originates from γ – modification above 1000 °C.

- γ - Al2O3 (gama-corundum) – crystallizes in a cubic system and forms through

heating of hydroxide to 1000°C. It transforms into α modification at higher

temperatures.

Obr.12. Electrocorundum

Some authors mention two more modifications (β and ξ), which are not fully

confirmed and opinions on their existence differ.


19

Electrocorundum (α - Al2O3), which is highly indifferent to Fe oxides and stable, is the

most widely used sand.

The electrocorundum is used as sand for preparation of mixtures with clays, water

glass and also with organic binders for special purposes. However, its high price prevents its

more frequent use. Its application as a filler in coatings and surface greases for steel castings

is much more widespread.

1.4.3 Chromite

Chromite represents a solid solution (Fig. 14):

chromite FeO.Cr2O3

picrochromite MgO.Cr2O3 (magnesiochromite)

spinelle MgO.Al2O3

Also present:

hercynite FeO.Al2O3

magnesioferrite MgO.Fe2O3

Chromites are divided according to chromium content:

- Chromite with a high chromium content, the so-called chemical chromites

- Chromite with a low chromium content, but with a higher aluminum content, the

so-called ceramic chromites

Obr.13. Chromite

Ceramic chromites (metallurgical) are used for foundry purposes (base sands). The

chromite sand should contain 45 – 48 % of Cr2O3 at minimum and max. 12 % Fe (75 – 85 %

of the total iron content should be in Fe2+

form and the rest Fe3+

). A special attention is given

to SiO2, namely in a form of free quartz or silicates. For massive steel castings the total SiO2

content should not exceed 2 ÷ 2.5 %. The main mineralogical components are paramagnetic

ferro-chromite (FeCr2O4) and picrochromite (MgCr2O4).


20

A pH value of chromites ranges between 7 ÷ 8 (10). Along with an increasing

alkalinity, a consumption of acidic catalysts increases (a consumption of an acid for furan

mixtures).

- Density: 4450 – 4650 kg/m3

- Hardness: 5.5

- Volume weight (in relationship to granulometry): 2700 – 2900 kg/m3

Chromite primary grains have a polyhedral shape; they are smooth, black to graphite

coloured. Grains start to melt at 1650°C, the sintering point ranges in interval between 1350 ÷

1450 °C.

Obr.14. Mineralogical components in chromites

Chromite sand is widely used especially in steel casting foundry shops (to prevent

penetration from carbon and alloyed Mn steels) in a combination with furan and phenolic

binders. Due to heat exposure (above 1000 °C) and an oxidation or reducing atmosphere of a

mould (core), crucial mineralogical changes in the chromite sand occur, which can even lead

to an increased tendency of regenerated sands to penetration.

1.4.4 Zirconium-silicate

Zirconium sands represent a mixture of ZrO2.SiO2 and ZrO2. The sand specific weight

is a result of an apportionment of both of the components and ranges from 4560 to 4720

kg/m3. The Mohs hardness is 7.5. Chemical composition of the silicate: 32.8 % SiO2 and

67.2 % ZrO2. The melting point of the silicate is 2100 °C, ZrO2 - 2687 °C, but of sands

approximately 1900 °C.


21

Obr.15. Zirconium-silicate

Compared to silica sands, there are some strong points of zirconium sands:

- Linear thermal dilatation, much lower than in other typical sands

- Twofold thermal conductivity and along with high specific weight a high value of thermal

accumulation coefficient (bf); substantially higher cooling effectivity

- High chemical indifference against Fe oxides at high temperatures resulting in a

considerable resistance to penetration and burning in Preferably used for massive castings

or for a production of highly thermally stressed cores (the zirconium sand is replaced by

chromite due to high price)

SiO2 in zirconium sands is firmly bonded; therefore there is no threat for a silicosis

initiation. Sands can be bound by any binder system. In our conditions, zirconium is only used

for special sand mixtures (high price of the sand), but it is used more often as fillers for

various types of foundry coatings and surface greases. The largest deposits of zirconium sands

are in Australia, Brazil, Senegal, Ceylon and Ukraine.

1.4.5 Olivine

Olivine sand is a material of volcanic origin with a high Mg content. In principle, this

is a solid solution of two main silicates.

forsterite Mg2SiO4 (93 %) (melting point = 1900°C)

fayalite Fe2SiO4 (6 %) (melting point = 1205°C)


22

Obr.16. Olivine

Further, it contains some accompanying minerals, such as serpentine, chromite, spinel,

magnetite etc. It is of a slightly alkaline character.

Chemical composition of olivine from the Aaheim deposit (Norway)

MgO 49 % mass

pH = 8.8 ÷ 9.1

SiO2 41.0 %

Fe2O3 7.0 %

Cr2O3 0.40 %

Al2O3 0.50 %

NiO 0.35 %

MnO 0.10 %

CaO 0.05 %

Olivine is of green colour, the specific weight 3200 – 3600 kg/m3, hardness 6.5 – 7.0

and the melting point about 1870 °C (depending on forsterite – fayalite ratio). In the presence

of silica sand the melting point decreases significantly. A beginning of sintering of olivine

sand starts around 1410 °C.

An advantage of olivine sand is almost linear thermal dilatation, lower than in SiO2.

The firm fixation of SiO2 in the structure of fayalite and forsterite causes an alkaline reaction

under high temperatures and then not reacting with MnO and other alkaline oxides. Therefore

it is highly suitable sand for preparation of mixtures for special castings from 12% austenitic

manganese steels, where it can replace magnesite and chromium-magnesite. Replacement of

silica sand by olivine means to avoid a risk of silicosis.

It can be used for mixtures bonded by clays, water glass (CO2 – process, self-

solidifying mixtures, loose and also foamy) and by oils and cement. Deposits can be found in

Western Europe, Scandinavia (Norway), USA and Japan.

Sands – Sieve analysis

23

1.5 Granulometric composition of sands

1.5.1 Cumulative curve of granularity

Properties of a sand mixture depend very much on the granulometric composition of

sand. Results of a sieving analysis are plotted into diagrams, which are a unique

representation of granularity. The most widely used are cumulative curves of granularity (Fig.

17), from which we can read the characteristic criteria of the granulometric composition:

d50 – average granularity (mean size grain), i.e. such a sieve size, through which 50 % of

sand falls down

d25 and d75 – diameters of sieve meshes, to which 25 and 75 % of the total sand weight

corresponds (after flushing away fractions smaller than 0.02 mm)

25

75

d

dS the so-called homogeneity degree (regularity of granularity).

(6)

The sieve analysis is performed using a set of sieves (ČSN ISO 565, a series of

principal sieve sizes R 20/3) with mesh size:

1.0; 0.71; 0.5; 0.355; 0.25; 0.18; 0.063 mm, while the system is vibrating.

Obr.17. Cumulative (integral) curve of the silica sand granularity (0.4 %

flushability)


24

d25= 0.4 mm; d50 = 0.31 mm; d75= 0.25 mm; %5,6210040,0

25,0;

25

75 Sd

dS

The granularity criterion S is satisfying only for natural sands (such as silica sands),

where the cumulative curve in the segment between d25 to d75 has a linear course. The closer

the regularity degree gets to value 1, the more homogeneous the sand is, and vice versa. The

most extreme shapes of the cumulative curve are shown in Fig. 18:

Obr.18. The extreme cumulative curves

In literature we can often meet a classification of sand granularity pursuant to the

American standard AFS (granularity number AFS).A relation of the AFS granularity number

to the average granularity (d50) is shown in Fig. 19.


25

Obr.19. Relationship of the AFS granularity number to the sand average

granularity (d50)

A relation of the number of a sieve (USA) and a mesh size in micrometers is given in

the table below:

Sieve number (USA) Mesh size in micrometers

The rest (20 ÷ 53)

270 Mesh 53

200 75

140 105

100 150

70 210

50 300

40 420

30 600

20 850

12 1700

6 3400

However, this criterion does not assess fractions below d25 and above d75, thus 50 % of

the total mass of the sand. And this assessment is not sufficient at all for artificial sands

(chamotte, schist, alkaline sands, corundum etc.), where the cumulative (integral) curve is

discontinuous, often broken.

Therefore another criterion for assessment of the sand granulometry is implemented –

Log W.


26

1.5.2 Log W criterion

The criterion log W is a thermodynamic function of an arrangement of a system.

According to Planck, the thermodynamic probability (W) is a function of a state and

determines a number of microstates through which the given macrostate is realized.

Boltzmann defines a relation of entropy (S) to W:

S = k·lnW (7)

where k – the Boltzmann constant

AN

Rk ; R – the gas constant; NA – the Avogadro constant

The aforementioned relation implies that the system’s entropy increases along with a

probability of a state and is a criterion of probability. After the mathematical modifications,

equation (7) has a form:

ii NNW log200log (8)

As the logarithm is not defined in zero and 0log0=0, also 1log1=0 and even if the

percentage content is lower than 1, the product NilogNi is negative, then the following rules

are applied for a calculation of log W:

1,10,15,0

5,00

toecirculalizN

calculatednotN

i

i

For the ideal sand, where all grains are of the same size (monofraction), log W is zero,

i.e. log W = 200-100log100=0.

For real sand the value log W ranges between the limits:

RW log0 (9)

Where R is a positive integer number, which is defined by a number of sieves on

which the granulometric analysis is performed. For the sieving analysis according to ČSN

721531, 7 sieves are used and one “sub-sieve“ (a dish for the rest of the fraction 0.020-0.060

mm); the threshold log W value is defined in the following way:

309,908log1008

100log

8

1008200log

%8

100

W

N i

(10)

The closer log W gets to 0, the more grains are concentrated in one fraction; the closer

to 90.31, the more evenly in 8 fractions.


27

1.5.2.1 Application of Log W criterion

The sand arrangement probability criterion log W can be applied for more sensitive

distinguishing of the granulometric composition of natural sands or for assessment of

granularity of artificial sands. The assessments according to the average granularity (d50) and

the degree of regularity (S) are not sufficient enough, because the cumulative curve is

discontinuous and often quite broken.

Tab. 1. Granularity assessment of the sand from the Šajdíkovy Humence locality

The log W criterion can be also used for preparation of special sands by mixing

several fractions for determination of their optimal ratio or a ratio of pre-determined porosity

(volume weight).

1.6 Angularity coefficient of sand

The angularity coefficient of sand WK can be determined on the basis of the ratio of

the outer specific surface SW (from the CARMAN relation by permeability measurement) to

the outer theoretical surface SWT calculated from the sieving analysis:

WT

WK

S

SW (11)

The value is higher than 1, because for the calculation the ideal spherical shape is

assumed, which occupies a minimal surface area. The more spherical shaped the sand grain is,

the less the difference of the outer surface from the spherical theoretical surface and WK is

near to 1; for the ideal sphere WK = 1.

Sands – Angularity coefficient

28

Drift sands are nearest to the ideal sphere shape. The shape differs from the ideal

spherical grain only by 8 – 16 %. Their deposits (localities) are of allochthonous origin, i.e.

they did not originate in the extraction location, but they were transported by wind or water

from far away distances, transported grains knocked against each other and therefore their

shapes became rounded by abrasion. On the contrary, sands from autochthonous deposits are

more angular, e.g. those obtained from kaolin washing differ from the ideal spherical shape

by 12- 52 %. Sands prepared artificially by grinding of refractory materials, such as

chromium-magnesite, electrocorundum, chamotte etc. have a substantially higher angularity

than the above mentioned sands (WK>2.0).

Angularity is of considerable technological importance. Higher angularity of sand

leads to lower compactibility of a mixture (lower volume weight) with all the consequences

for strength, permeability, penetrability and thermal tensions during heating. Angular sands

cannot be combined with organic binders (oils, resins) effectively, because grain envelopes

become ruptured on edges and in corners (loss of compactness), further leading to

inadequately high binder consumption with all the consequences, such as high price of the

mixture etc.

Chromite sand Olivine sand Zirconium sand (zirconium-

silicate)

Obr.20. Different angularity of natural sands (25x magnification)

The different angularity of natural sands is evident in Fig. 20. Grains can be divided

according to the shape (AFS):

- angular

- semiangular

- rounded

- combined

Some authors divide angular grains to:


29

- angular with rounded edges

- sharp-edged

- splintery

Along with grain fineness, their angularity increases (natural sands), Fig. 21:

Obr.21. Relation of grain angularity to their size

The more rounded grains, the higher compactibility of clay mixtures (Fig. 22).

Obr.22. Grain shape influence on compactibility of a mixture

Permeability of green-sand mixtures also depends on the base sand angularity (Fig.

23):


30

Obr.23. Relation of the sand grain shape to permeability of green-sand mixtures

Angularity of sand grains influences binding capacity of green-sand mixtures in

compression and tension (Fig. 24):

A - green compression strength

B – tensile strength

Obr.24. Influence of grain angularity on binding capacity of green-sand

mixtures in tension and compression (1 – rounded grains; 2 – semiangular

grains; 3 - sharp-edged grains)

1.7 Thermal conductivity (cooling effect) of a foundry mould

Increasing demands for surface as well as inner quality of castings require

enhancement of thermally differentiated properties of moulds. Disposable moulds fabricated

of grainy or ceramic materials (base sands) offer many advantages.

An optimal composition of natural (silica sands) or artificial (chamotte schist,

chromium-magnesite, metal beads) sands for moulding mixtures may, to a certain extent,

influence a cooling effect of disposable moulds.

Intensity of heat transfer between a casting and a mould is determined above all by

physical properties of the mould.

Sand – cooling effect

31

Coefficient of thermal conductivity 11 KmW

Specific heat 11 KkgJc

Volume weight 3mkgV

Heat transfer in a foundry mould is affected by:

a. Heat conduction (thermal conductivity)

b. Heat emission (heat radiation)

c. Heat convection (heat flow)

The thermal conductivity in the porous disperse material is realized above all through

places of contacts of individual sand grains. Theoretically, in the ideal base sand, this is a

point contact of spheres, however, in real mixtures, considering the grain angularity, the

presence of binder bridges, water and additives, there are areas of contact increasing the total

conductivity.

Pure quartz (ρ = 2650 kg/m3) has 112,7 KmW , while e.g. a bentonite mixture

(ρv = 1500 kg/m3) has 1133,0 KmW , which is a value 22-fold lower. By addition of 10

% of H2O (ρ v= 1650 kg/m3), λ increases to 1113,1 KmW , i.e. 3.46-times. A clay

addition increases 3-times at minimum.

Therefore castings have a shorter solidification time e.g. in green-sand moulds. Air

and gas in intergrain spaces also shares the heat transfer, however, they have λ approximately

an order of magnitude lower than sand, therefore they need not to be taken into account for

the conduction heat transfer. However, heat transfer does not occur only by conduction, but

simultaneously by convection, and at higher temperatures also by radiation, therefore with

much higher intensity.

From this point of view there are many possibilities to regulate λ in real moulds and

cores, especially through granulometry control, sand type selection and densification degree

(volume weight).

Along with an increased densification degree of a mould the volume weight of the

mould (v) increases, porosity (λ) decreases and thermal conductivity increases (Fig. 25):

%100

2

2

SiO

VSiOm

(11)

where: SiO2 – sand specific weight (SiO2)


32

Obr.25. Relation of thermal conductivity to porosity of mixtures according to

experimental measurements by various authors

An influence of the densification degree on λ = f(t) is demonstrated for a mixture with

water glass (CO2 – process) of a composition:100 weight parts. Silica sand, Provodín 036

d50 = 0.35 mm, S = 0.52, log W = 55.5, 1209,7 kgmSWT

water glass 5 weight parts, 50°Bé (m = 2.4)

A known fact has been validated that along with the increasing densification degree (ρ

v= 1500, 1650 and 1800 kg/m-3

) the thermal conductivity in the entire interval of the observed

temperatures (to 800 °C) increases (Fig. 26).

Obr.26. Relation of thermal conductivity to temperature in mixtures with water

glass (CO2) at different volume weight


33

1.7.1 Cooling effect of non-silica base sands

Cooling effect of moulds with non-silica sands can be evaluated above all through the

heat absorbing capacity coefficient (bf).

ffff cb [Ws0,5

m-2

K-1

] (12)

where: f - thermal conductivity coefficient

cf – specific heat

f – volume weight

bf can be defined as heat amount accumulated (absorbed) by a mould (s

JW ;

watt = amount of heat, J – joule per 1 second) on a mould – metal unit area (m2),

whereas the mould is heated by 1 °C (K).

Besides the base sand type, the sort and amount of a binder plays also an important

part (the content of free water, green-sand mixture), however, the decisive factor remains the

base sand type. The higher bf, the faster metal on the mould (core) solidifies and the lower

possibility for the molten metal to penetrate into intergrain spaces (higher resistance of the

mould against penetration - burning-in).

Tab. 2. Cooling effect of selected sands (according to Halbart)

Heat absorbing capacity coefficient

(according to G.Halbart)

Composition of a mould bf[W. s0,5

.m-2

.K-1

] [%]

1 – magnesite 3510 100

6 - chromite 2765 78.8

4 - chromite 2391 68.1

8 - chromite 2319 66.0

5 - chromite

olivine

1930 55.0

3 - olivine 1821 51.9

2 - dunite 1734 49.4

7 - kerphalite 1665 47.4

Magnesite and chromite give the highest cooling effect to mixtures (mould). Dunite

and kerphalite sands reach approximately half the value.For the comparison, there is a mixture

with silica sand (water-glass 6.4 %): bf = 1550 Ws0,5

m-2

K-1

.

Reclamation

34

2 Reclamation of moulding sand mixtures

Generally the term ‘regeneration (reclamation) of moulding sand mixtures’ means an

obtaining of a substantial part of sand from the used sand mixture back for preparation of new

pattern and core mixtures. Reclamation of sand comprises a series of procedures necessary to

bring the sand into a condition applicable for the re-use.

What are the principal reasons for reclamation?

A Economic Prices of high quality sands increase, especially after special

treatment (drying, sorting of monofractions and composing the

demanded granulometry, activation).

Total cost for 1 t of a reclaim should be more economically

profitable than for 1 t of new base sand (price for depositing

waste sands).

B Transport and

handling

Costs for transportation of sands are a considerable portion of

the total price. High cost relates to transportation of used sands

to dumps. Next to these influences, there is also a favourable

influence of higher self-sustainability of foundries in raw

materials, especially in winter season there is no need to build

such large waste spaces for sand pre-supply.

C Limitation of

localities with new

sands

It is not possible to extract wherever even rich deposits of high

quality sands are located.

D Utilization of sands

for other industrial

branches

Glass-making and building industry.

E Environment

protection

This relates to C article. Exportation of used mixtures (hazardous

waste) has been strictly limited and closely watched recently.

Mixtures contain also many pollutants from binders and

additives (phenol, Na2CO3, ammonia salts and other chemicals),

which are washed off and pollute subsoil waters, river courses

etc.

F Technological A reclaim has a lower absorbability of an organic binder while

reaching the same mechanical properties of moulds and cores. A

decrease in binder consumption closely relates to a reduction of

gas-making ability of mixtures. Mixtures with a regenerate are

even less susceptible to occurrence of defects generated by

tension in a foundry mould due to braked thermal dilatation –

scabs, fins. Grain surface gets rounded and processed. This

means that through an effective reclamation, properties of new

less quality sands can be even improved to a certain extent.

Reclamation

35

A sand grain from a used mixture is enveloped by a layer of a binder, which,

according to a degree of thermal decomposition (distance from a casting), is either in its

original condition (polycondensed resin, silicic acid gel with other products of a hardening

reaction, dried clay paste) or in a condition of destruction / complete destruction (coke

residues from organic binders, silica glass, oolitization layer of clay). According to the used

binder type in a mixture, and adhesive (adherent) forces of a layer to the sand surface, we

must use various effective devices and procedures to obtain a grain surface as clean as

possible. Some hardening reaction products, harmful for next use of the reclaim, are even

soluble in water. The reclamation through water can be used preferably.

Obr.27. Flowchart of regeneration procedures

All types of mixtures cannot be treated by conventional reclamation procedures, so

that 100 % are usable for the new demanding manufacture of moulds and cores. This fact

leads to a need to use only a specific part of a reclaim for preparation of a new mixture or to

proceed to further combined regeneration procedures and to extend the reclamation process

extremely.

Anyway, the sand reclamation requires a construction of a complex system for mixture

grinding, removing metal particles, cooling, drying, screening, exhausting fine fractions and

other special processing in accordance with an actual procedure and equipment.

Combinations of basic procedures are frequently used (Fig. 27).

3 Strength of moulding mixtures

Next to base sand, a binder is the main component of a sand mixture. The sand –

binder interaction generally shows itself in strength of moulding mixtures.

Strength of molding mixtures

36

Binding capacity – strength in a mixture in green condition after densification, which

is decisive for fluidity, compactibility, formability of the mixture and other technological

properties at forming

Strength – after drying (skin drying, burning or hardening), which is the initial

strength in a molten metal contact with a mould (core)

Strength at elevated and high temperatures – decisive for plasticity of a mould

(value of exogenous stresses in castings) and a number of casting defects (cracks, swells).

Strength in the water condensation zone at 100°C temperature belongs here, too.

Residual strength – affects collapsibility of moulds and cores, and thus also

cleanability of castings

The long-time and perspective development of binder systems, and also of sand

mixtures, can be divided into four generations:

Ist generation mixtures – binding is a result of capillary binding forces and van der

Waals forces (sand mixtures with clay binders)

IInd generation mixtures – using chemicals for the process of sand mixture binding

(mixtures with inorganic binders, e.g. water glass) and with organic binders (mixtures with

artificial resins, saccharides and oils)

IIIrd generation mixtures – using physical effects of e.g. force fields for binding

(magnetic mould), vacuum (V-method) and also frozen water – ice (EFF- SET process). This

covers also binderless systems with a gasified pattern (a full-mould process, REPLICAST,

LITGAZ, LOST FOAM).

IVth generation mixtures – where biological procedures are used for binding

(biotechnology)

The oldest Ist generation mixtures (clay binder mixtures) are still used for the

manufacture of moulds (unified bentonite mixtures) and cores for massive steel castings

(chamotte mixtures) and we can see ever increasing interest in application of physical

processes for base sand binding, above all force fields (electric, magnetic), vacuum, crystal

binding with ice etc., which is motivated by an effort for transition to wasteless foundry

technologies.

An objective evaluation of binding properties of binders should be performed for

standard sand (silica sand FRECHEN-BRD, the world standard) according to the reached

specific tensile strength (kPa/1%) and collapsibility after casting, the effectiveness in

Strength of molding mixtures

37

accordance with economic criteria, above all the cost needed to obtain tensile strength of 1

kPa (CZK/1kPa).

3.1 Strength of moulding sand mixtures as a result of adhesion –

cohesion forces of a binder

Strength of a mixture is a result of the binder - base sand interaction. From the point of

view of adhesion – cohesion forces of foundry binders, mixture destruction may be of

following three characters:

Cohesion – cohesion forces of a binder (forces of interaction between homogeneous

atoms – molecules) (designation K) are smaller than adhesion forces between the binder and

the sand (interphase forces between atoms – molecules in surface layers) (designation A); A

>> K.

Adhesion – cohesion – a combination of A÷K

Adhesion A << K

Cohesion work is defined as a total work needed to overcome cohesion forces of the

given material. It is proportional to the energy needed to put a molecule from the liquid or

solid state off to very distant position. It approximately equals evaporation heat.

Adhesion is a comprehensive expression for all factors causing adhesion of two

matters one to another. It originates by effect of chemical, physical and mechanical forces

occurring in molecular distances.

The important adhesion affecting factor is good wetting of the solid phase by a binder.

The work needed for breaking the interphase boundary is called adhesion work (Aadh) and it

can be expressed by the Dupré equation:

adhA l g s g s l (13)

.(l-g) [Pa] The equilibrium surface stress between the liquid and gaseous

matter,

.(s-g) [Pa] The equilibrium surface stress between the solid matter and their

vapours

.(s-l) [Pa] The surface stress between the solid and liquid matter

The work of adhesion can be also expressed using the contact angle (wetting angle)

(cos):

adhA l g 1 cos (14)

Binders of the 1st generation

4 Binders the Ist generation

4.1 Clay binders

Clay binders still belong to the most widespread foundry binders. About 70 % of all the

world’s production of castings are manufactured into moulds of bentonite mixtures*. They are

contained in natural sands and used in a pure condition for binding of washed sands, sometimes

of artificial base sands. They have excellent technological and hygienic properties, they are cost-

effective, have a satisfactory binding capacity, strength after drying, re-usability, they determine

a non–demanding preparation of mixtures and their depositing, e.g. without carbon additions,

and usually do not cause ecological problems.

Three groups of layered clay materials are used as foundry binders:

Kaolinite, illite and montmorillonite – their chemical composition can be given in

oxide or elemental formulas. Illites containing iron are called glaukonites. In natural foundry

sands, illitic binders prevail.

* Bentonite mixtures have been used in foundries since 1920.

SIMPLE LAYERED CLAYS

TWO LAYERS THREE LAYERS

kaolinit

2 3 2 2Al O 2SiO 2H O

nebo

2 2 5 4Al Si O OH

halloysit

2 3 2 2 2Al O 2SiO 2H O 4H O

illit

4 7 20 4KAl Si Al O OH

slídy

muskovit

2 3 10 2KAl AlSi O OH

biotit

3 103 2K Mg,Fe Al,Fe Si O OH

flogopit

3 3 10 2KMg AlSi O F,OH

montmorillonit

3 8 20 4NaAl MgSi O OH

mastek (talek)

2 23MgO 4SiO H O

pyrofylit

2 3 2 2Al O 4SiO H O


When a content of montmorillonite is higher than 7580 %, these clays are called

bentonites. They are the most widely used clay binders allowing “green-sand moulding” into

bentonite mixtures.

In kaolinitic clays, the main component is kaolinite. These refractory clays also serve as

binders for preparation of chamotte mixtures.

4.2 Water in clay minerals

A water molecule has a polar character (r = 0.138 nm) with properties of a dipole (with a

dipole moment = 1.84 of Debye units). The dipole character allows a mutual association.

Through hydrogen bonding, chains with a planar to spatial orientation of polymers are formed

(Fig. 28).

1,01 A

105°

0

1,32 A

0,90 A

0

.H

.

H

H H

PLOŠNÁ ASOCIACE MOLEKUL VODY

.

Obr.28. Dipoles of water molecules and their planar association

A number of hydrogen bonds per one molecule depend on state of matter of water

(temperature and pressure).

neboH

HH OH

OHHOH

HOH

There are four hydrogen bonds per one molecule in ice, and 2 to 3 in liquid at 0°C. It can

be assumed that vapour contains only monomers and at a gradual increase of pressure more and

more polymer molecules form, so normal water does not contain monomer molecules. A

structure of ice is well known, however, a structure of liquid water has not been fully cleared up.

In principle, water in clay materials is of two types:

In 1847 a name „montmorillonite“ was created for this mineral in France. In 1898 an American geologist

KNIGHT called the extracted clay with montmorillonite content „bentonite“ according to the Fort Benton in American Montana.


Molecular water (represents approximately 10% of the mineral mass). It occurs in

interlayer spaces (intracrystalline swelling) or adsorbed on the surface of packets

(intercrystalline swelling) and in pores between packets of particles.

Lattice water - hydroxyl (OH-), as a part of the crystal lattice

Dehydration cannot be performed quantitatively to temperature of 110°C. Complete

evaporation of molecular water occurs no sooner that at 250300°C temperature. The process is

perfectly reversible. Therefore attention should be paid to selection of optimal temperature for

drying of bentonite mixtures for determination of moisture.

In accordance with a content of molecular (free) water in clay mixtures, clay mixtures

can be divided to several types intended for various mould making technologies:

max. content of 0.2 %: Anhydrous mixtures (clay is swelled by other organic matters –

organobentonites)

1.62.5 %: Semi-dry mixtures (for compressing by higher specific compacting pressures

and other technologies requiring high compactibility of a mixture)

max. content of 5 %: Green-sand

67 %: For skin-drying (skin-drying, short-time drying) for instance a mould face by hot

air and casting after a short time (water return transport)

above 67 %: For drying and burning. Chamotte mixtures with high-temperature drying

for removing also chemically bound water (temperature dehydroxylation)

4.3 Bentonite and bentonite mixtures

In literature, various forms of chemical, oxidic and elemental formulas of

montmorillonites are presented, such as:

2 4 10 22E Al Si O OH nH O (15)

2 3 2 2 2Al O 4SiO H O nH O (16)

2 2E Na ,K ,Ca ,Mg

Montmorillonites have a typical three-layer structure (in contrast to e.g. 2-layer

kaolinites). A gibbsite octahedral (AlO6)-is between two silica tetrahedrals (SiO4)

4- (Fig. 29 and

30). In octahedrals and tetrahedrals, atoms are bonded by relatively fixed covalent bonds.

Individual layers are bound by weak van der Waals forces. Structural construction is based on

layers 2:1.


1 (O) 2 (O) 3 (Al, Fe, Mg) 4 (Al, Si)

VÝMĚNNÉ KATIONTY nH2O

O OH Al Si

Exchangable cations H2O

Obr.29. Montmorillonite structure scheme

(R. E. GRIM)

Obr.30. Montmorillonite structure

model (Edelman – Favejee)

Defects of a crystal lattice are another specialty. The individual parallel layers (d001)

(planes) may lie one on top of another on a completely regular basis, or some of them may be

shifted (in the direction of the axis a as well as b) and create an irregular turbostratic lattice

(Fig. 31).

This contributes to penetration of water and

other polar fluids (glycol, glycerol, alcohols,

aldehydes etc.) between silica layers, which move

away from each other and intracrystalline swelling

occurs. Interplanar spacing in Na+ and Ca

2+

montmorillonite are shown in Fig. 32.

Montmorillonites feature frequent

isomorphous substitutions of lattice cations. As a

result of allotropic substitution of central atoms in the

coordinate tetrahedral (Si4+

Al3+

, does not exceed

0.5 at. of Al on 4 tetrahedral positions) or octahedral

(Al3+

Mg2+

, substitution of 2 Al atoms for 3 Mg

atoms can be complete) in the lattice by lower-valent

cations, free valences originate on the surface of

particles (an electrically charged surface with a

negative sign), which become partially saturated in a

space of a substitution base by cations, the most

frequently Ca2+

, Mg2+

and Na+.

A - PRAVIDELNÁ B - NEPRAVIDELNÁ

TURBOSTATICKÁ

Obr.31. Turbostratic lattice of

montmorillonite, totally random

shifting of structural units in the

direction parallel with layers


Therefore the montmorillonite surface has a negative charge (O2-

). This structural effect

is a cause of an extraordinary sorption capacity of montmorillonite. When compensating the

negative charge, cations are bound between particles by different forces, enabling to set a

system of ion exchange capacity (16):

2 2 2 2Li Na H K Mg Ba ,Ca ,Sr Rb (16)

We can perform a substitution of Ca2+

- Mg

2+ ions by Na

+, which is technologically

considerably advantageous in bentonite mixtures. This process is called natrification.

In the presence of Ca2+

ions in the montmorillonite packet, individual particles remain

together even after swelling, while after the ion exchange by Na+, mutual binding forces

decrease so much, that disintegration of packets of cards to individual particles occurs (Fig. 33).

A highly dispersed structure forms, a result of which is an increase in viscosity of the water –

clay system and a support of the swelling ability.

O

Ca2+

O

T

[AlO6]-

[SiO4]4-

15,5

.10-1

0

12,5

.10-1

0

Na+

[SiO4]4-

[AlO6]-

O

T

O

m m

T T

Ca2+ Na+

Ca2+ - montmorillonit - montmorillonitNa+

Obr.32. Scheme of the montmorillonite

layered structure (Hanykýř)

Obr.33. Dispersity of Ca2+

, Na+

packets of

montmorillonite

4.3.1 Natrification of bentonite and its consequences

Ion exchange of cations (Ca2+

- Mg2+

montmorillonite), compensating a charge of the

silica layers for Na+ ions, is called natrification. The process can be expressed by a schematic

equation:

2 2Ca montmorillonit 2Na 2Na montmorillonit Ca (17)

Natrification can be performed by any sodium salt, however, the quantitative course of

the reaction from the left to the right is determined by solubility of calcium salts. The efficiency

is determined by the anion part of the sodium salt. Therefore natrification by NaCl or water

glass cannot be successful. Disodium carbonate is most frequently used with regard to the

minimal solubility of CaCO3 in water.

332

2 2 CaCOonitmontmorillNaCONaonitmontmorillCa (18)

The second important condition is the ion radius of a cation; the smaller, the

higher the efficiency (Fig. 34).


An optimal amount of Na2CO3 addition can be determined from swollen volumes of the

water-bentonite suspension. A maximum swellability in water corresponds to the optimal

amount of the given additive. Deficiency, as well as excess of Na2CO3, reduces the overall

swellability. For Czech bentonites, the optimal amount of Na2CO3 is approximately 4% of the

clay weight. There is a simple test for perfect control even of accuracy of natrification

performed by a manufacturer.

0,001 0,01 0,1 30 1001,0

ROZPUSTNOST VÁPENATÝCH SOLÍ

[g/100g H2O]

MA

X. P

ŘÍR

ŮS

TE

K V

AZ

NO

ST

I [x1

0-1 k

Pa]

0

4

8

12

16

20

24

28

32

101,65 (Cs+)

1,49 (Rb+)1,43 (NH+

4)1,33 (K+)

0,98 (Na+)

0,78 (Li+) 0

4

8

12

16

20

24

28

32

MA

X. P

ŘÍR

ŮS

TE

K V

AZ

NO

ST

I [x1

0-1 k

Pa]

IONTO

VÝ PO

LOM

ĚR

[10

-10 m

]

Cs+

Rb+

NH+4

K+

Na+

Li+

CO2-3 PO3-

4

OH- SO2-4

S2O2-3Cl-

Obr.34. Effectivity of various salts on ion exchange in montmorillonite

Advantages and disadvantages of natrification:

+ Tendency of mixtures to over-moistening decreases, this leads to an increase in strength in a

zone of water condensation (an increase in resistance against scabs). While Ca2+

-bentonite

increases, time to formation of a scab decreases.

+ The clay paste at dehydration begins to shrink at lower temperatures, which favourably

compensates the dilatation growth of silica sand (a decrease in tension and an increase in

resistance against defects generated by tension). On the contrary, loss of water at lower

temperatures leads to the mould surface brittleness and a loss in strength, which results in higher

occurrence of sand inclusions.

+ Dehydroxylation of clay is increased by 150200 °C, leading to an increase in temperature of

degradation of plastic properties and better utilization in particular circulations of unified

bentonite mixture (Fig. 35).

+ Bentonite swells more, which is in a direct relation to strength in the water condensation zone

(Fig. 36). Higher swellability enables to decrease the bentonite content to minimum in foundry

coatings, where it plays a part of antisedimentation additive. Then coatings do not crack then.

- On the contrary, natrification (excess of Na – salts) supports oolitization of sand silica grains

adversely.


PE

VN

OS

T V

KO

ND

EN

ZA

ČN

Í ZÓ

NĚ

[kP

a]0

0,5

1,0

1,5

2,0

Na+

Ca2+

0 200 400 600 800

TEPLOTA ŽÍHÁNÍ BENTONITU [°C]

Obr.35. Influence of bentonite annealing temperature on strength in the water

condensation zone (Na+ - Ca

2+ bentonite)

0 2 4 6 8 10 12 14 16 18 20

OBJEM PO NABOBTNÁNÍ [ml]

PE

VN

OS

T V

KO

ND

EN

ZA

ČN

Í ZÓ

NĚ

[kP

a]

0,4

0,8

1,2

1,6

2,0

2,4

Obr.36. Relationship of strength in the water condensation zone to the

bentonite swellability

4.4 Additives for bentonite mixtures

Carbohydrates (saccharides, pentosans, cellulose)

Wood powder, lignin

Oxidants

Graphite

Lustrous carbon carriers (theory of lustrous carbon)

The above mentioned additives for unified bentonite mixtures (UBM) have different

functions. On the one hand, they have to decrease compactibility and increase strength in the

water condensation zone, thus increasing resistance to tension generated defects (scabs), to

increase permeability, to decrease friability and the mould face drying-off, to increase

toughness of a mixture (improvement of liftability of patterns from moulds), to improve

collapsibility (to prevent formation of lumps) and on the other hand, to enhance surface quality


of castings of graphitizing iron alloys (penetration, roughness of a casting) or to reduce

dangerous exhalations during casting.

The water-bentonite binder system can be only supported by additives developing their

binding abilities in water environment during mixing, not hardening, thus supporting reversible

properties during each circulation of the UBM - unified bentonite mixture. The lower water

content in bentonite, the higher strength of bentonite, but also the higher brittleness. Along with

increasing strength of a mould (hardness), a risk of explosive penetration increases.

4.4.1 Saccharides

In foundry industry, the most frequently used saccharides are monosaccharides

(pentoses, hexoses), oligosaccharides (saccharose) and polysaccharides (starches, dextrins,

cellulose). Starches feature dissolubility under cold condition, however, they have an ability to

take in water at higher temperatures, thus reducing inclination to formation of scabs.

Therefore cold-soluble forms of starches have been developed (cold starches). These

cause a noticeable enhancement of properties in bentonite mixtures, improving lifting-out

patterns from moulds (toughness), decreasing erosion and wear. Cold solubility can be improved

by shortening saccharide chains, which are more water-soluble (starch milling). Thus their effect

in UBM is markedly improved (this is the effect of dextrins and pyrodextrins – starches with

shortened chains).

4.4.2 Oxidants

Using a new technology, through oxidation additives the properties of bentonite mixtures

improve and exhalation volume is reduced at the same time. A mixture of water with ozone (10

ppm in water) and hydrogen peroxide is subjected to ultrasound treatment. An additive prepared

this way (treated water) contains relatively stable radicals, which are able to react with bentonite

and at the same time activate carbon (lustrous carbon carriers), which reduces emissions of

aromas, above all benzene (drop by as much as 74%). The treated water increases binding

capacity of mixtures, which allows reduction of bentonite content in unified bentonite mixtures

as well.

4.4.3 Graphite

Hexagonal layered structure of graphite with glide properties, high anisotropy, water

repellence and also heat resistance, contributes to good fluidity of a mixture even at low

moisture. Through incorporating graphite into bentonite (layered montmorillonite), its faster

intracrystalline swelling occurs. Up to 5 mass % of graphite addition for bentonite is used. Its

presence reduces occurrence of burning-on. An optimal moisture in a mixture decreases (by

0.20.3%) even at an increasing content of flushable substances in UBM. A decrease in water


demand leads to lower oxidation of carriers of lustrous carbon. Uniform densification of a

mould and reduction of the free water content leads to limiting the occurrence of “explosive

penetration”. Compacting (pressing) pressures on forming equipment can be decreased. The

presence of the so-called “process carbon” shows itself above all by an increase in strength of a

mixture in the water condensation zone.

4.4.4 Carriers of lustrous carbon

Graphitizing iron alloys (grey iron – LLG and ductile iron – LKG) are typical alloys with

high fluidity. They closely outline contours of the foundry mould and easily penetrate into

intergrain spaces. If the metal penetration depth is less than an average sand size (d50), grains

can be easily removed from the casting surface and only roughening of a casting shell occurs.

The surface smoothness of castings cast into bentonite mixture moulds is a result of the metal-

mould interaction and the following cleaning method (blasting). Therefore a worldwide

attention has been paid to carbonaceous additives (C-additives) to green-sand mixtures, both

from the point of view of their efficiency for formation of a surface, and also ecological impacts

on working and living environment (depositing waste sands on storage grounds).

Theory of a complex effect of a carbonaceous additive C-additive in mixture exhibits

versatile effects:

Formation of lustrous carbon - LC (surface smoothness)

At the same time, formation of coke (semi-coke), compensating thermal micro-dilatations

of quartz grains (tension generated casting defects), and clogging of intergrain spaces

(penetration prevention)

Formation of gaseous components (volatile fractions) disintegrating bentonite mixture

lumps, thus improving collapsibility of moulds

According to I. Bindernagel retort test, C-additive with complex effects should have the

following composition:

LC (1119)%

Coke residue (4358)%

Volatile substances (2738)%

Determination of lustrous carbon (I.Bindernagel’s retort test)

The International Committee of Foundry Technical Association (C.I.A.T.F.), led by I.

Bindernagel, has worked out a retort test for determination of LC in C-additives. This test has

become a subject in national standards of many states (Fig. 37).


KELÍMEKKŘEMENNÁ VATA

160

40 5

25/30

16°

55

max

10

74°

Obr.37. Conditions for determination of lustrous carbon (I. Bindernagel) C. I. A.

T. F.

Filling in a retort 6 g of quartz wool (for 30x

determination)

Temperature in a muffle

furnace 900°C20°C

Heating of a quartz retort c. 10 min/900°C

Retort with a crucible (C-

matter) 5 min

Cooling in a desiccator (hydroscopic lustrous carbon)

Optimal weighed amount of

C-matter

(0.30.5) g (010)% lustrous carbon

(0.10.3) g (1040)%

(0.060.1) g (4080)%

A retort of quartz glass is filled with quartz wool and heated in a furnace to 117320K.

Then it is cooled in a desiccator and accurately weighted to 10-4

g. The retort is then re-heated in

the furnace to the specified temperature and the tested C-additive is weighed to a crucible

(0.060.5) g. Then the crucible must be put onto the retort as soon as possible and the whole set

is placed into the furnace for 300 s. After this time, the retort and the crucible are cooled in the

desiccator. LC is evolved on the quartz wool fibers, the rest is in the crucible (coke residue) and

to 100% is calculated the gas content, which went through the retort opening without

decomposition.

As you will learn later, the commonly used term lustrous carbon (LC) is in principle a

product of the two-stage pyrolysis, the so-called pyrolytic carbon (PC), which has two forms at

minimum: amorphous carbon (AC) and lustrous carbon (LC), which differs essentially by its

physical as well as chemical properties.

PC LC AC (19)

Optimal amount of LC carrier in the bentonite mixture

Through long-term observation was found out that an optimal amount of LC carrier in

the bentonite mixture ranges between 0.30.5 mass % of the released pyrolytic carbon (PC).


Regulation of LC carrier is performed by mixed bentonite (bentonite and C-additive) or C-

matter (black coal with additives).

The lower concentration does not ensure a smooth surface and, on the contrary, at a

concentration higher than 0.6 mass % many casting defects form (graphite scum, carbon

membranes in the casting structure, cold shuts and misruns in castings).

Influences reducing the theoretical concentration of LC, released from the bentonite mixture

(carriers of LC):

Moisture of a mould Water burns products of pyrolysis of C-additives The higher moisture

of the mixture, the lower active content of pyrolysis products and the worse surface of a

casting

Thickness of a casting Along with a wall thickness, that means along with an increasing

time of solidification, the thermal attack on the mould face increases. The larger the

thickness of the casting, the greater importance has the content of the burnt-out coke (semi-

coke), which “blocks” intergrain spacing in the mould.

Time of casting. This partially relates to the wall thickness of the casting. A certain time is

needed for the two-stage pyrolysis and formation of LC. Therefore in thin-walled castings

with a very short casting time, a type of an additive in coal (pyrolysis velocity) is also

decisive for the surface quality, as well as the content of just released pyrolytic products in

the return mixture (compare the surface quality of castings of grey iron cast into a green-

sand mould, rammed from a synthetic mixture from new raw materials and C-additive).

Binders of the IInd

generation – inorganic binders

5 Binders of the IInd


5.1 Moulding sand mixtures with water glass

5.1.1 Hardening procedures for mixtures with water glass

A. Revesible process dehydration (drying by cold and hot air, by Hot-Box,

microwave hardening)

B. Irreversible process chemical hardening (CO2, esters, cement, slags with C2S).

Dehydration gives the highest strength to mixtures, however, the process reversibility

without the use of special additives does not allow long-term storage, e.g. of cores. A

combination of both the procedures is also frequently used.

A.

2

2

H O

2 2 2 2 2H O

Na O mSiO nH O energie Na O mSiO

(20)

Na2O.mSiO2.nH2O .......................... water-glass Na2O.mSiO2 .................................... disodium silicate

B. 2 322 2 2 4

2 3 3

Na COCONa O mSiO nH O Si OH

H CO NAHCO (21)

n 22n m4nSi OH Si O 2 n m H O

(22)

SinO(2n-m).2(n-m)H2O ....................... hydrated silica gel

2 3 2

OH

2 3

2 3 2

CH COOCH CH OH

I H O 2CH COO I

CH COOCH CH OH

(23)

ethylene glycol dimethyl ester ethylene glycol

3 2 2 2 3 42CH COO Na O mSiO nH O 2CH COONa Si OH (24)

CH3COONa ..................................... sodium acetate

n 22n m4nSi OH Si O 2 n m H O

(25)

2 2 2 2 n 2 2

2 2 4

2CaO SiO Na O mSiO H O 2 CaO SiO kNa O

Na O SiO Si OH

(26)

2CaO.SiO2 ...................................... dicalcium silicate 2(CaOnSiO2kNa2O) ......................... tobermorite hydrosilicates

A different strength of the same mixture hardened by different procedures confirms

that the limiting factor is the cohesion strength of the grain envelope, which depends

particularly on its morphology (gel, particle size, tension and the reaction side-products).

5.1.2 Water glass module

Binders of the IInd


The most frequently used water glasses in foundry industry are sodium glasses:

“thin” - density 36÷38°Bé, m = 3.0÷3.2

“thick” density - 48÷50°Bé, m = 2.2÷2.6

“mixed” density - 40÷44°Bé, m = 2.6÷2.8

Next to density (°Bé – density in Baumé degrees)

144,3

platí pro 1144,3 Bé

(27)

2,18 0,011 W

[kg.m-3

] .................... density W [%] ......................... total water content

Water glass is most frequently characterized by a module

2

2

%SiOm 1,03

%Na O (28)

Water glasses with a higher module are more reactive, e.g. with CO2. A colloid

solution is thermodynamically more labile.

5.1.3 Coagulation threshold of water glass

As evident in Fig. 38, water glasses of the same chemical composition, and also a

module, can have a different coagulation threshold.

MODUL2,0

KO

AG

ULA

ČN

Í PR

ÁH

[% N

a 2O

]

2,5 3,0 3,5 4,01

2

3

4

5

6

7

Obr.38. Relationship between a module and coagulation threshold of water glass (A.

Burian)

A water glass solution is stabilized by sodium ions. There are three kinds of sodium

ions:

Free in an intermicelar solution (only for m < 2.0)

Loosely bonded

Binders of the IInd


Tightly bonded

Not all Na+ participate in stabilization of the colloid solution. Free Na

+ ions in a

diffusion layer can be bonded by a specific amount of an acid (HCl) and thus an observable

coagulation can be initiated, leading to formation of gel. This amount of the acid is

recomputated to a corresponding amount of sodium ions and the coagulation threshold is then

expressed in % of Na2O:

A 2KP 0,31V %Na O (29)

VA [ml] HCl consumption = 1 mol.l-1

Through the coagulation threshold, processability of self-hardening mixtures and

many other properties of mixtures with water glass can be determined. Therefore assessment

of water glass should be based on density, a module and the coagulation threshold.

KOAGULAČNÍ PRÁH [% Na2O]

0 1 2 3 4 5 6 7 8 9

HO

DN

OT

A p

H

11

12

KOAGULAČNÍ PRÁH [% Na2O]

2 3 4 5 6 7

ZP

RA

C. S

MĚ

SI [

min

]

0

10

20

30

Obr.39. Dependence of the

coagulation threshold on pH of water

glass (A. Burian)

Obr.40. Dependence of

processability of a self-hardening

mixture with water glass on the

coagulation threshold (A. Burian)

5.2 CO2 process

Particularly during CO2 flowing through a disperse system of a mixture, intensive

chemisorption (higher pressure)

2 2 2 3CO H O H CO (30)

and drying of generated products (excess of CO2) occurs.

Water glass with a module 2.23.3 is considered a colloid solution with a different

degree of polycondensation of silicate anions. Dispersed phase systems belong among them.

Particles represent an individual base, separated by a dispersed environment. A phase

interface with a large surface is characterized by high surface energy and thus also

considerable thermodynamic instability. The solution pH is higher than 11. The basis of the

colloid solution comprises SiO2 particles with ion adsorption, while the so-called micelle

Binders of the IInd


forms. The scheme of the construction of the colloid micelle of the dihydrogen disilicic acid

in the presence of Na-ions is shown in Fig. 41.

Na+

Na+

Na+

Na+

Na+Na+

Na+

Na+

H+

H+

H+

H+

H+

H+H+

H+

H+

H+

H+H+

H+H+

H+

H+

SiO2-3

Si2O2-5

Si2O2-5

INTERMICELÁRNÍ

ROZTOK

JÁDRO(SiO2)n6

5

4

32

1

SiO2-3

SiO2-3

Obr.41. SiO2 colloid particle in alkaline silicate (1 – diffusion layer, 2 – compensation

layer, 3 –electric double-layer, 4 – micelle, 5 – particle, 6 – granule)

2

2 2 5 2 3H Si O SiO 2H SiO (31)

The main conditions for stability of these solutions include:

Very small dimension of particles (sedimentation of particles)

Electric charge of the same sign

Solvatation of particles (ions)

Both of the last conditions protect the colloid solution against self-coagulation. More

distant adsorbed H+, Na

+ ions form a diffusion layer of the micelle. Due to the motion of the

micelle in the solution, a part of adsorbed H+, (Na

+) ions remain tightly bonded and they

perform with a core the Brown motion, loosely bonded ions form a diffusion layer of a

variable thickness. Between the particles and the solution, a potential difference occurs –

electrokinetic potential (zeta) (Fig. 42).If the colloid particles form an adsorbed envelope of

ions, the relative stability of the system is conditioned by this. In the water-glass colloid

solution micelles in the alkaline environment the negative charge prevails, by which the

particular particles repel each other and the solution is relatively stable. The sole water glass

solution is formed by electrolyte. Ions enter the solvatation layer of particles and influence the

sole properties. Na+

ions dramatically reduce water movability, decreasing strongly the

solvatation sphere (growth of stability of Na-silicates, while the module decreases). The entire

solvate layer is saturated, if the concentration of Na+-ions reaches the value m = 2.0. At

higher concentration, Na+ transfer into the intermicelar solution.

Binders of the IInd


A

B C

PO

TE

NC

IÁL

E

E = +

Obr.42. Total electrokinetic potential of a micelle (A – particle, B – micelle, C – solution)

Adsorption (electric double-layer) is saturated, if the concentration in this layer

reached m = 4.0. These values cannot be exceeded. When increasing ion concentration,

degradation of hydration envelopes occurs by dehydration of ions. Dehydration process does

not occur on Na+, but on Na(OH2)

+ particles. Na

+ ions bind a coordinately bonded water

molecule, which defies dehydration process. When increasing the content of Na+-ions in the

adsorption layer, hydration may decrease max. to 1 mol H2O/mol Na+. Observations have

shown that at low concentrations of Na+ ions in the adsorption layer, the hydration is 1.8 mol

H2O/mol Na+. The growth of Na

+ ions in the diffusion layer leads to a decrease in their

hydration and H2O molecules transfer to the intermicelar solution. At m = 3.3, approximately

40 mol H2O/mol Na+ were measured; at the Na

+ ion growth and a decrease of the module (m

= 2.0), solvatation decreased to 9 mol H2O/mol Na+.

At CO2 hardening, the dihydrogen carbonic acid generates, causing a decrease in pH

value,

2 2 5 2 3 2 3

2

2 5

Na Si O H CO H O 2Na HCO

Si O 2H OH H

(32)

The diffusion layer compresses and in the strongly acidic atmosphere vanishes

completely. Therefore also a decrease in the electrokinetic potential – zeta occurs and a

system of a high value of a change of free enthalpy (G) forms. Its value may only decrease

at spontaneous growth of coarser particles at the expense of the smallest ones (a decrease in

the specific surface). However, very low solubility of colloid substances in the dispersive

phase prevents these processes. Then the only way is an increase of particles by coagulation –

aggregation. Coagulation of water glass is caused by coagulation ions (H+) with an opposite

sign than a colloid particle has. A binding component forms – silicic acid gel.

The coagulation runs most rapidly in a pH interval 5÷6, because particles migrate

through the so-called isoelectric point, they are not charged, therefore they aggregate quickly.

Binders of the IInd


In this pH region, the system has the lowest stability (Fig. 43). The higher mixture alkalinity,

the higher system stability. Water glasses with a low module and highly alkaline mixtures are

relatively more stable and much higher CO2 consumption is needed for hardening.

pH SOUSTAVY0 2 4 6 8 10 12

LOG

. DO

BY

VZ

NIK

U G

ELU

KATALYZÁTOR OH-

MIN

. ST

ÁLO

ST

MA

X. S

TÁ

LOS

T

Obr.43. Influence of pH on the time of coagulation of silicic acid until the gel forms

The main binding component – the gel of the acid – is the decisive strength

determining component after the CO2-reaction, on condition of the cohesion process of the

mixture destruction. The following is determining for strength of the silicic acid gel:

Sizes of particles and their distribution

Coordination number of particles

Stress in the gel (degree of dehydration)

Amount and a form of disengaging of by-products of reactions ( 2 3 2

3

Na CO xH O

NaHCO

,

CH3COONa.3H2O, glycerol).

A particle size growth through polymerization (monomer→dimer→cyclic polymer→

particle) and its aggregation is depicted in Fig. 44.

Binders of the IInd


MONOMER

DIMER

CYKLICKÝ POLYMER

ČÁSTICE

1 HM5 HM

10 HM

30 HM

100 HM

SOL

PROSTOROVÁ SÍŤ

GÉLU

A

B

V PŘÍTOMNOSTI SOLI

pH 7-10

V NEPŘÍTOMNOSTI SOLI

pH 7-10nebo

pH < 7

Obr.44. Stages of monomer polymerization, formation and aggregation of a particle (R.

K. Iler)

5.2.1 By-products of the hardening reaction

As equation (63) implies, the other product of the hardening reaction is the acidic

sodium salt of dihydrogen carbonic acid. This is generated as a result of the neutralization

reaction between NaOH (hydrolysis product) and dihydrogen carbonic acid. The basic

neutralization equation can be expressed as follows:

2 3

2 2 3 2

H CO

NaOH H O CO NaHCO H O (33)

Equation (63) applies for the excess of the acidic component, however, at the

beginning of hardening a reaction occurs, leading to generation of disodium carbonate.

Therefore the main conditions for formation of sodium hydrogen carbonate can be considered

the excess of CO2 and a higher content of free water:

2 3NaOH CO NaHCO (34)

2 3 2 3Na CO H O NaOH NaHCO (35)

By heating to low temperatures (100110°C), sodium hydrogen carbonate decomposes

again, generating disodium carbonate, CO2 and H2O:

3 2 3 2 22NaHCO Na CO CO H O (36)

By CO2 blowing through, the Na2CO3 content decreases at the expense of NaHCO3.

Carbonates disengage from the intermicelar solution and are “blocked” along with free water

in pores of the skelet of the gel. Free water partly acts also as crystalline. Na2CO3 crystallizes

with 1, 7 and 10 molecules:

2 3 2 2 3 2 2 3 2Na CO H O; Na CO 7H O; Na CO 10H O (37)

Binders of the IInd


5.3 Self-hardening mixtures with water glass

Self-hardening mixtures (mixtures without controlled hardening) are hardened by a

reaction occurring between water-glass and a hardening agent. At present, processes of self-

hardening of water glass by liquid hardening agents prevail. They are hardening agents of

ester-type, such as mono-, di-, triacetin (mono-, di- and triacetate glycerol), propylene

carbonate etc.

One of the main advantages, next to

their permanent quality and regulable

reactivity, hygienic unexceptionability,

possibility of application for bonding self-

hardening mixtures with clays prepared in

continuous mixers, is a high degree of

mobilization of binding properties of water

glass at reaching high strength, which allows

a significant decrease in binder content in

the mixture (c. by 50 %). Mixtures contain

23 % of water glass (m = 2.02.5), by

which also a problem of improvement of

collapsibility of moulds and cores as well

as recoverability after casting can be

solved.

For some types of hardening agents –

acetic acid esters, certain problems are

brought by the presence of

Basic types of ester hardening agents:

DIETYLENGLYKOL

DIACETÁT

ETYLENGLYKOL

DIACETÁT

ETYLENGLYKOL

MONOACETÁT

COCH2

CH2 O

CH2 O

CH3CH2COOCH3

CH2

COOCH3CH2

CH2 COOCH3

COOCH3

COOCH3CH2

CH2

CH

COOCH3COOCH3

CH

CH2

OH

CH2 COOCH3COOCH3CH2

OH

OHCH2

CH

CH2COOCH3

CH2

O

CH2

CH2COOCH3

COOCH3

COOCH3

CH2

CH2CH2

CH2 OH

COOCH3

GLYCEROL

TRIACETÁT

GLYCEROL

DIACETÁT

GLYCEROL

MONOACETÁT

PROPYLENKARBONÁTETYLACETÁTPROPYLENGLYKOL

DIACETÁT

CH3COONa.3H2O in the hardened mixture in processes of dry regeneration as well as

temporary plastic state of the mixture, which precedes the final hardening, but it also lasts at

sufficient manipulation strength of the mixture, therefore in larger cores we can see

deformations of their own mass and, under certain conditions, also destructions.

5.3.1 Physical – chemical processes of hardening of water glass by liquid hardening

agents

Coagulation of water glass can be performed by many substances, such as alcohols,

acids etc. From the point of view of colloid – chemical, the hardening process is dehydrating,

conditioning formation of a structural system (gel). However, the highest dehydration effect

Binders of the IInd


has a hydrogen atom (bonds high amount of water). This implies that H+-ion is the most

effective factor of formation of a structure.

The value of the electrokinetic potential decreases by H+-ions entering the

intermicelar solution, while weakening electrostatic repulsive forces between aggregates at

the same time, and after reaching a critical potential (the coagulation threshold value), before

achieving the isoelectric point, coagulation and transition of lyosol in gel occurs.

Gelation of water glass occurs at the increasing concentration of H+-ions in the

intermicelar solution with its dehydration effect at gradual bonding of Na+-ions in a form of

salts. Silicic acid particles do not aggregate only by van der Waals forces, but also by effect of

primary valences with a structure of three-dimensional polymer:

(HO)3 Si OH + HO Si (HO)3- H2O (HO)3 Si OH + HO Si O Si(OH)2 ONa

OH

OH (38)

Only OH- (ONa) groups at the end of the chain impede condensation, another type of

OH- groups forms transverse bonds. As the mixture has to have a certain processibility

(service life), it is necessary for H+-ions to enter the intermicelar solution slowly, gradually,

and be regulated.

Esters are organic matters formed from esterification reaction, by acids acting on alcohols:

wateresteracidalcoholH

OH

(39)

So that the general equation can proceed in the direction of hydrolysis (saponifying),

the alkaline environment and water must be ensured. As a result of hydrolysis of disodium

silicate,

2 2 2 2Na O nSiO mH O 2NaOH nSi m 1 H O (40)

sodium hydroxide forms, which is strongly dissociated, which results in the alkaline

character of the water glass solution. Through alkaline hydrolysis the equilibrium state moves

to the benefit of hydrolysis, as the forming acid is removed from the equilibrium state by

formation of salt.

In foundry operation esters of acetic acid and of two- to five-member polyhydric

alcohols have been most widely used. And from alcohol - glycerol (glycerin), there are

acetates.

Gradually generating acetic acid reacts with NaOH (from water glass).

Binders of the IInd


3 3 2CH COOH NaOH CH COONa H O (41)

while sodium acetate is forming, crystallizing as CH3COO.Na.3H2O, disturbing

compactness of the gel matter. Glycerol monoacetate undergoes hydrolysis most rapidly,

glycerol triacetate most slowly. The water glass hardening speed can be in summer (warm

sands – reaction retardation) or in winter climatic conditions (accelerating) regulated by

mutual ratio of particular acetines.

Obr.45. Structure of a binder envelope

with disengaged glycerol (syneresis,

SEM)

Obr.46. Grain envelope of a self-

hardening mixture hardened by liquid

hardening agents with disengaged

crystals CH3COONa.3H2O (SEM)

5.3.2 Reactivity and optimal concentration of an ester-type hardening agent

The course of the reaction of an ester and alkaline silicate can be divided to three

stages (Fig. 47). Stage I – called also the “incubation period”. In this stage hydrolytic

cleavage of esters (in the alkaline environment) to alcohol (glycerol) and an acid (acetic acid)

occurs. The length of the stage I depends on a free water content (in water glass) and a type of

ester. The entire system reactivity can be controlled by a combination e.g. of monoacetin (the

fastest hydrolysis) and triacetin (the slowest hydrolysis). If there was no free water in the

binder (if coacerate is used), gelation does not occur even after several days.

Binders of the IInd


DOBA REAKCE [min]0

PE

VN

OS

T [M

Pa] ŽIVOTNOST SMĚSI

I. II. III.

ŽÁ

DA

NÁ

PE

VN

OS

T

Obr.47. Course of the reaction of an ester and alkaline silicate

Stage II is characterized by initiation of gelation and beginning of the plastic state of

the self-hardening mixture. Although the mixture has the handling strength already, plastic

deformation of moulds and cores occurs under bending loading. In contrast to CO2-process,

when water in gel is quickly removed by an excess of CO2 (stage II is missing), in a reaction

with esters a considerable part remains blocked by gel and is the cause of the existence of the

plastic state (Fig. 48). Pursuant to some authors, the presence of glycerol in the grain

envelope also contributes to the plastic state. On the contrary, this condition is very

favourable from the point of view of lifting-out patterns and disassembling core boxes. In this

time of stage II the service life of the mixture is over, this means the time from the mixture

preparation, when after compacting the satisfactory final (demanded) strength (minimal wear)

is reached.

Stage III – the plastic state ends here and moulds (cores) obtain the required strength.

Obr.48. Testing of the plastic state of self-hardening mixtures with

water glass Gradual bending of cores to fracture

The commonly used binders (m = 2.22.5) need 812 mass % of the ester hardening

agent (diacetin + triacetin) for water-glass. Then, aside from the free water amount, binder

amount and its coagulation threshold, ester content is decisive for the reaction speed as well.

The higher ester concentration, the more reactive the binder system, the mixture service life

and also the time of the plastic state get shortened, the mixture becomes hardened rapidly,

however, final strength is lower and the mixture is more brittle. Insufficient amount of a

Binders of the IInd


hardening agent leads to an insufficiently hardened binder, to the plasticity growth, and after

its dehydration to deterioration of core collapsibility.

5.4 Problems of collapsibility of mixtures with water glass

5.4.1 Critical analysis of residual strength of a mixture with water glass and silica sand

(CO2-process)

Theoretical pattern of residual strength

Residual strength, in spite of many drawbacks, is still the most widely used

characteristic of collapsibility of mixtures with water glass. In spite of all authors confirming

the existence of two maximums and two minimums on the residual strength curve (mixtures

hardened by CO2), their opinion on the mechanism of occurrence of different residual

strengths depending on heating temperature is not in agreement.

TEPLOTA [°C]0

0

PE

VN

OS

T

V T

LAK

U

t [M

Pa]

200 600 1000 1400

1

2

3

4

0 200 600 1000 1400TEPLOTA [°C]

0,5

OB

JEM

OV

Á H

MO

TN

OS

T

[g.c

m-3

]

1,7

1,5

1,3

1,1

0,9

0,7v

[%]

-4

0

4

8

12

16

ZM

ĚN

A O

BJE

MU

V

[%]

0

10

20

30

40

50

V

v

Obr.49. Residual strength of a mixture with silica sand and water glass (CO2 process)

The entire course of the curve can be divided to several sections (Fig. 49):

An increase in residual strength and the Ist maximum area (around 200°C)

A decrease in residual strength behind the Ist maximum and the Ist minimum area

(around 600°C)

A sharp growth of residual strength behind the Ist minimum and the IInd maximum area

(in the temperature interval 800900°C)

A decrease in residual strength behind the IInd maximum (the IInd minimum)

5.4.2 An increase in residual strength and the Ist maximum area

The strength growth and occurrence of the Ist maximum as a result of additional

hardening of a mixture through dehydration of non-hardened water glass. The residual

strength curve for only dehydrated (Hot-Box, microwave hardening) or self-hardening

mixtures (esters) has not the characteristic Ist maximum, but the initial high value keeps

decreasing with the increasing temperature up to the Ist minimum area (Fig. 50).

Binders of the IInd


A content of Na+ ions in the diffusion layer of micelles determines the coagulation

threshold (% Na2O). For the water glass binding properties to be fully utilized, in self-

hardening mixtures a specific amount of ester (related to coagulation threshold) is needed for

hardening process and, at the same time, also a certain amount of free water (hydrolysis). A

small amount of ester results in a high content of undecomposed “residual” water glass, which

dehydrates by heat and influences particularly the Ist maximum of residual stress

(collapsibility).

600 700 800 900

4

8

12

16

20

24

28

8%

12%

0

TEPLOTA ŽÍHÁNÍ [°C]

0

ZB

YT

KO

VÁ

PE

VN

OS

T

V T

LAK

U [M

Pa]

100 200 300 400 500

Obr.50. Process of residual strength in compression (water-glass 3 %, M = 2.5) in a

mixture with 8 and 12 % of esters (diacetin – triacetin) for the water-glass content

The Ist maximum is completely missing for optimally hardened self-hardening mixtures.

5.4.3 Decrease in residual strength behind the Ist maximum and area of the Ist

minimum (approximately 600°C)

Decreasing of residual strength behind the Ist maximum was most frequently related to

the discontinuous reversible change in the quartz modification →SiO2, occurring at 573°C,

which is in accordance with the curve minimum temperature. Later was shown that other

influences also affect the crack formation in envelopes of a binder. These are above all

dehydration processes in disodium silicate (non-hardened water glass) and silicic acid gel

with regard to their high adhesion strength to the silica sand surface.

5.4.4 Sharp growth of residual strength behind the Ist minimum and the area of IInd

maximum (800900°C)

Behind the Ist minimum a rapid growth of residual strength with a maximum at

800900°C occurs. The IInd maximum value is substantially higher than that of the Ist

maximum. Most of authors explain the strength growth by the liquid phase formation,

occurring according to the binary system SiO2 - Na2O above 793°C, which at first joins-up

Binders of the IInd


formerly formed cracks, the binder envelope transforms into molten glass attacking even the

silica sand surface and under high temperatures the envelope – grain surface interface dies

out, while forming a compact monolith of high residual strength. Strength in the IInd

maximum area is determined not only by water glass content, but also by its module. Some

authors come into a general conclusion that the IInd maximum is a function of the total

content of Na2O in the mixture.

5.4.5 Decrease in residual strength behind the IInd maximum (the IInd minimum)

The original opinion assumes that along with the temperature growth above 800°C

(the IInd maximum), as a result of SiO2 diffusion growth in the direction from the sand grain

to the melt the mixture melting accelerates, the Na2O concentration in the melt slowly

decreases and during cooling to the normal temperature the individual SiO2 crystals begin to

crystallize from the melt. The crystals play a part of “foreign inclusions” in the solidifying

molten glass and act as inner notches generating local stress concentrations. The higher

heating temperatures, the higher concentration of foreign inclusions and the cooled-down

mixture with high internal stress has low residual strength.

The strength of molten grain envelopes is not influenced only by the presence of

“foreign inclusions” and processes on the grain – envelope boundary, but also by permanent

changes in sand volume. A mixture, especially from mineralogically pure washed sands,

begins to expand strongly above 800°C. A cause is a transition of -SiO2 into cristobalite,

which is catalyzed by the presence of Na+ ions; the quartz specific weight decreases from the

initial 2.65 to 2.33 g.cm-3

, while the mixture increases its volume by as much as 40 %. While

the residual strength of the mixture with water glass above 1000°C is relatively low, cleaning

of thermally stressed cores is highly difficult. Expansion of the mixture is very significant,

having two different effects here; it disintegrates testing elements, thus reducing the residual

strength behind the IInd maximum, however, substantial permanent changes in a volume

initiate tensions, which the mixture is not able to relax in spite of all its plasticity, they cause

the residual stress growth, thus leading to impaired collapsibility.

5.4.6 Possibilities of control of residual strength with water glass (CO2 – process)

Additives containing Al2O3 belong to very effective substances influencing the IInd

maximum area. As the cause of the IInd maximum is formation of melt, the move to high

temperatures is explained by transition of the binary system Na2O - SiO2 to the ternary Al2O3 -

SiO2 - Na2O with a high melting point (viscosity change).

Binders of the IInd


Another favourable effect of Al2O3 on collapsibility is stabilization - SiO2 against

transition to cristobalite related to the continuous expansion growth of the mixture with silica

sand.

5.5 Hardening of alkaline silicates by dehydration processes

Strength approximately by an order of magnitude higher than when using chemical

processes (CO2-process, ST mixtures with esters) can be reached by physical dehydration of

mixtures bonded by Na-silicates using Hot-Box or microwave hardening. While the silicate

module decreases, the strength of dehydrated mixtures increases.

This enables to reduce the binder content (Na2O content) dramatically, with favourable

effects on collapsibility and recoverability of silica sand. Dehydration is a reversible process.

However, for conventional procedures a longer storability of cores is required. This depends

on storage conditions (relative air humidity, temperature) and it is necessary to stabilize the

binder system by a modification, such as reduction products of monosaccharides (glucitols) or

organosilanes.

5.5.1 Collapsibility of cores

The binder system dehydration process can be used to de-core castings, e.g. in water,

possibly even with a subsequent wet self-regeneration of base sand. Rehydration causes

sudden drop in strength while increasing volume weight of cores.

The collapse of cores occurs to 60 minutes of immersion in water at the latest. If hot

cores were immersed in water (for instance together with a casting), their self-destruction

occurs to 2 minutes. This may be called the “wet regeneration” of base sand, however, there is

an ensuing problem of waste water salt content (re-crystallization, neutralization, etc.).

6 Binder of the IInd

generation – organic binders

6.1 Organic-based foundry binders

Organic binders have found their use in preparation of core mixtures and have enabled

manufacture of cores by new progressive procedures. The mixtures have high strength after

hardening (above all in bending), low thermal destruction temperature, thus relating rapid

decrease in strength and in resistance of a core (collapse) to metal shrinkage and mainly

excellent collapsibility. Further, cores have high stability (this means adequate storability) and

mixtures are easy to regenerate both mechanically and pneumatically, at dry condition.

Binders of the IInd


Thermal regeneration using expensive base sands can be applied, too. The mixtures feature

excellent flowability, therefore they can be blown (dry coated mixtures) or injected.

Cores have high strength, which is necessary for the manufacture of complicated thin-

walled shapes. The binder efficiency can be expressed using specific strength, this means

strength corresponding to 1% of a binder. The minimum binder content is observed in term of

economical, hygienic, but even technological point of view (minimal development of gases –

exogenous blow holes). Older types of organic binders, such as saccharides and animal and

vegetable oils, have been replaced by artificial resins nowadays. Up to now, phenolic, furan,

urea, alkyd, polyurethane, epoxy, acrylate and other resins have been applied, however, the

development is hobbled by more and more strict standards for living and working

environment, as well as unknowingness of effects of many emissions and immisions on a

living organism at minimum concentration but long-term exposure.

6.1.1 Chemistry of phenolic binders

A survey of types of phenolic binders and their applications in various technologies is

shown in Fig. 51.

FENOL : FORMALDEHYD

POLYKONDENZACE

V PROSTŘEDÍZÁSADITÉMKYSELÉM

FENOL. PRYSKYŘICE

TYP RESOL

FENOL. PRYSKYŘICE

TYP NOVOLAK

TEPLO (+KYSELINA) KYSELINA

POLYKONDENZÁT

VE STAVU REZITU

POLYKONDENZÁT

VE STAVU REZITU

HEXAMETYLTETRAAMIN

+ TEPLO

SMĚSNÝ POLYKONDENZÁT

VE STAVU REZITU

SAMOVOLNĚ TUHNOUCÍ

COLD - BOX (GISAG), NO - BAKEHOT - BOXSKOŘEPINOVÉ FORMOVÁNÍ

C - METODA

(1:1 ÷ 1:3)(1:0,4 ÷ 1:0,9)

VÝROBA

POJIVA

VYTVRZOVÁNÍ

Obr.51. Phenol-formaldehyde binders and various technologies using phenolic resins.

6.1.2 Shell moulding, the Croning method, “C” method

This is a manufacturing method for thin-walled or mainly hollow cores from dry,

perfectly loose mixture, which commonly comprises silica sand, heat meltable and hardenable

resins and possibly other additives. A shell is formed by a gradual melting of a mixture layer

by heat of a heated pattern or core box (240280°C). Further heating has a hardening effect,

and then a finished shell mould (masque) with highly precise dimensions is removed from the

(metal) pattern. A shell wall thickness is reached by removing a loose non-molten mixture

after the time needed to achieve the required wall thickness, or for shell moulds with a

Binders of the IInd


controlled thickness the demanded thickness is reached by blowing a loose mixture into a hot

core box creating a face and back of the mould (Dietert procedure).

A dry and loose mixture is perfectly blowable, so it is possible to form the most

complicated shapes of moulds and cores without using high pressures for blowing and

demanding deaeration of core boxes. Many times perfect moulding can be performed by

pouring the mixture onto a pattern or filling into a core box by freefall. Shell cores can be set

into sand forms and moulds.

6.1.2.1 Mixture for shell moulding

moulding sand mixture with a powder resin (former procedure)

coated mixture

Silica sand is blended with a finely ground powder novolac resin containing

hexamethylenetetramine (Hexa). The proportion of the binder is very low (57 weight

parts). Hexa acts as a catalyst. Formaldehyde reacts with ammonia according to the equation:

H2 H

2CH2 = O2NH3

2 CH2

NH

NH

KONDENZACE

-2H2O

N

NH

C

H2

CH2

NC

H (42)

The reaction is not stopped in this state; further circular condensation occurs and

hexamethylenetetramine C6H12N4 forms:

6HCHO + 4NH3

KONDENZACE

-6H2ON

CH2

CH2

CH2N

N

N

CH2

CH2

H2C

(43)

Decomposition of “hexa” to formaldehyde and ammonia occurs above 117°C. An

optimal addition content is 1214 % for the resin mass.

The coated mixture represents a dry loose blend of silica grains coated by a thin

binder film. Coating can be performed:

under cold condition or under warm condition

under hot condition

A resin alcohol solution is used for coating under cold condition (under warm

condition). Alcohol (solvent) must be removed from the mixture, which is performed through

100°C hot air. “Hexa” is usually contained in the binder and only calcium stearate is then

Binders of the IInd


dosed into mixtures. Stearate acts as a separating agent – “grease”. The melting point is

148°C. Disadvantages of this former procedure are as follows:

Low output of preparation

Alcohol, representing 30 % of the resin mass, needs to be removed from the mixture,

which is difficult for pollution of air by alcohol vapours, for a danger of fire, explosion

(occupational safety). Complete removal of alcohol is mostly unperformable, its residues

cause many defects, such as “sintering of sand” in reservoirs, “shell bits peeling-off”, local

thinning of a shell.

Advantages are a possibility to make the equipment by own, lower price as well as

energy demands.

Coating under hot condition – formation of a resin film is reached by melting a

novolac resin just in a mixer by effect of heat of hot sand at temperature about 130150°C

(sand temperature must be by 5060°C higher than the resin melting point). Then no solvent

is needed, the resin can be even roughly ground (cheaper). There are no problems with

residual alcohol. When mixing, the mixture temperature is decreasing; “hexa” in an aqueous

solution and hard wax is added at 120100°C. The binder content ranges between 2.74.4

weight parts.

Disadvantages of this procedure are as follows:

Expensive equipment (e.g. Fordath 2000)

Higher energy demands, the mixtures have a higher tendency to crack

formation

Advantages – a higher quality mixture is obtained, the preparation is highly cost-

effective, less problems with air cleanliness, high output of the equipment, the mixture is

storable actually without any limits.

6.1.2.2 C-method present-day development

Binders with high collapsibility even for light metals casting have been developed.

Formerly whole cores could be only removed by heat treatment (annealing). Step by step, free

phenol proportion in resins below 1% occurs and next to just described preparation

procedures of coated mixtures, the fusion coating (under hot condition) has been applied,

which reduces the resin proportion markedly, thus leading to emissions reduction as well. C –

method has found its use also in the manufacture of the most demanding castings:

Cores of hydraulic elements

Binders of the IInd


Thin-walled cores of cooling water jackets

Shell cores and forms for ribbed cylinders and crankshafts

6.1.3 Hot-Box method

In years 19591960 a new method of the manufacture of castings started to be

widespread for large-series production through automatization of the whole process. The Hot-

Box method makes precise cores just in metal Hot-Boxes, where injected bonding mixture is

hardened by heat. In contrast to coated mixtures the HB method uses bonding mixtures, often

based on water soluble binders injected into a core box.

For the HB method the following binders are used:

Urea-formaldehyde

Melamine-formaldehyde

Furan

Phenol-formaldehyde (resoles)

Modified Na-silicates and salt solutions (see inorganic binders)

Hardening of cores begins from the hot core box surface (180300°C, according to the

binder type) and with regard to high thermoreactivity of binders the rapid surface hardening

occurs (1445 sec) and a core gains an adequate handling strength. After taking the core out,

final hardening throughout the whole cross section by accumulated heat occurs. The Hot-Box

method must be used for the manufacture of full volume cores, which is the main

difference in comparison with the C – method. On the one hand, a very expensive coated

blend, on the other hand, a cheaper material for Hot-Boxes, from which full cores can be

made. However, the core is harder and is cast in a casting device without dusting. All

common types of mixers can be used for preparation of the mixtures. However, the mixed

mixture needs to be stored for as short time as possible. It is important to realize, that mixture

hardening begins just from the moment of adding a binder into the mixture of a catalyst and

sand. Acidic catalysts ensure especially short production cycle (latent hardening agents, salts

of inorganic or organic acids, e.g. ammonium nitrate). Resoles belong to the most widely used

binders.

Binders of the IInd


6.1.3.1 Other variants of the Hot-Box technology

Hot-Box Plus – a special procedure for an extreme long service life of a mixture. A

disadvantage is a relatively long hardening time and also higher thermal stability of cores

during casting.

Warm-Box – the hardening temperature is decreased to 140200°C in this

technology. The time of core hardening is comparable to the Hot-Box procedure. Modified

furan resins and a special hardening agent based on sulfonic acid salts are used as binders.

The binder content is reduced to 0.91.2% (hardening agent content 2030 % for the binder

mass). The advantage of this procedure is a lower energy demandingness and reduction of gas

content during casting. Due to advantageous tensile strength under hot condition, the cores are

used for lightweight alloys castings (cylinder heads).

Thermal shock – hardening is performed for a short period (by a shock) at

temperatures of 280300°C and the hardening reaction is finished by effect of the

accumulated core heat. Highly condensed phenolic resins and ureas serve as a binder, acidic

salts serve as a hardening agent. This technology is used e.g. for radiator cores manufacturing.

6.1.4 Cold-Box method

Cold-Box methods were developed for small batch production with the use of special

wood core boxes - PUR Cold-Box technology in 1967 and GISAG Cold-Box in 1968. The

development trend in the present days lies in expanding of cold procedures – NO BAKE in a

mould

self-hardening mixtures (without controlled hardening); a continuous preparation process

of immixing a hardening agent or a catalyst (Alfa-set, furan mixtures)

with controlled hardening, extrinsic hardening (CO2, SO2, TEA, air, formiate etc.)

From the point of view of chemical process of hardening, the self-hardening mixtures

NO BAKE can be divided to acidic or alkaline. While furan and phenolic resins are hardened

by strong acids, alkaline Na-silicates and alkalized phenolic resins are ester-hardened. The

phenolic resol can be self-hardened in a strong acid environment under cold condition. This is

the Cold-Box Gisag principle.

6.1.4.1 Cold – Box GISAG

A resol resin is mixed with silica sand. This mixture is put into a special core-making

machine. This incorporates a quick-mixer, where during several seconds (210) the mixture is

mixed with a catalyst (GH - GISANOL H), which is actually a strong acid. Then the entire

Binders of the IInd


mixture volume is injected into a core box, where hardening exothermic reaction runs during

1530 seconds (a core colour changes from white to deep purple). The core after 515 min

has strength adequate to be set into a mould. Along with the catalyst content the hardening

rate increases, while the final strength, after 4 hours, decreases. The higher catalyst content

also shortens the core storability.

A certain disadvantage of the above mentioned core making procedure is higher

consumption of the mixture, which must be prepared in excess to the volume of the made

core. This mixture cannot be used anymore.

6.1.4.2 Cold-Box AlpHaset technology (BORDEN CHEMICAL, England)

Cold-Box AlpHaset technology belongs to a group of mixtures with alkaline binders.

It is based on a two-component binder system, where a binder is an alkaline phenolic resin

containing sodium or potassium, replacing hydrogen in the hydroxyl group in phenol, and a

hardening agent is ester (20 % for the binder amount).

The hardening reaction can be described in a scheme:

RCOOR HOH RCOOH ROH (44)

Esters dissociate in water to form acid and alcohol. Alkaline catalyzed hydrolysis is

irreversible (88) and the acid is consumed,

2RCOOH OH RCOO H O (45)

while the reaction equilibrium is being moved constantly. The acid releases sodium

(potassium) ions, which is another driving motor for polymerization of the resin. If

neutralization of the existing ions did not occur, hardening does not occur.

The reaction can be controlled by disassembling the core box (lifting-out the pattern)

lasting within a period of several seconds to several hours. The mixtures are characterized by

the existence of an incubation period (for acid hardened binders hardening occurs

immediately). A positive of this is that a pattern can be taken our very easily. The technology

can be applied for silica sands from various localities (1.21.4 weight parts in a binder), but

also for non-silica sands (chromite, zirconium, olivine).

Due to water being a solvent of the binder, the technology is not noticeably dependent

on sand and air humidity. An influence of environment temperature on hardening (to +5°C) is

not noticeable either. The technology is applicable for casting of both ferrous and non-ferrous

metals. Thermoplasticity before the very thermal hardening of the resin is an important

Binders of the IInd


characteristic. So the binder compensates the braked thermal dilatation generated tension of

base sands, thus preventing formation of tension generated defects (fins, cracks). As to

hardening, this is even two-stage hardening of moulds, cores. The first stage is characterized

by low cross-linking of the binder and when heated, the plastic state occurs. Only at higher

temperatures spatial cross-linking occurs, thus achieving high strength, dimensional stability

and erosion resistance of the mould. The absence of sulfur, phosphorus and the minimum

nitrogen content gives priority to this technology for steel castings and spheroidal graphite

iron (SGI).

A big problem for the development of this technology was recoverability of base sand.

An assessment of a regenerate quality is performed according to loss on ignition (the amount

of residues of a resin and a catalyst) and granulometry changes (particulate matter proportions

below 0.125 mm). The loss on ignition indicates also an increased level of alkaline salts,

resulting in shortening of the service life of a new mixture with a regenerate. A type of the

present salts depends on ester types. Salts formed at higher temperatures are not soluble. At

low temperatures, they are soluble in resins applied when re-using the regenerate. A ratio of

soluble to dissoluble salts depends on thermal exposure of moulds. The present alkali salts

easily bind to newly added resins, the number of the present ions increases, thus accelerating

neutralization processes and shortening the service life of the mixture (the incubation period).

This negative effect of salts can be reduced by addition of water immediately before addition

of a resin. An addition of c. 0.2 % of water decelerates the neutralization process and returns

the original equilibrium to the binder system again.

The mixture can be regenerated successfully by a dry mechanical method.

Approximately 8590 % of a regenerate can be used. The mixture does not contain organic

solvents and hygienic conditions during casting are more favourable than for acid hardened

systems. Advantages of this technology:

Reduced tendency to occurrence of fins

Reduced tendency to formation of pinholes and blow holes

Reduced occurrence of cracks

Low sensitivity to temperature and humidity

Easier disassembling of core boxes

Quality surface of castings

Binders of the IInd


Absence of sulfur, nitrogen and phosphorus does not cause surface and subsurface

defects in castings of SGI (pinholes, degradation of the graphite shape, blow holes)

6.2 Chemistry of furan binders

Three types of furan resins are used as foundry binders:

Furan amine aldehyde resins based on fural, furfuryl alcohol, formaldehyde and urea.

They are intended for the manufacture of moulds and cores for castings of grey cast iron

(they contain nitrogen).

Furanresins based on fural, acetone and formaldehyde. These are nitrogen-free binders,

significantly more expensive, intended for the manufacture of moulds and cores for steel

castings.

Fural resins

Generally, furan resins are sorted according to the content of nitrogen (011 %) and

water (030 %). The higher the concentration of furfuryl alcohol in the binder, the better the

technological properties of the self-hardening mixture, however, the higher the price is. For

steel castings, resins containing to 1.5% of nitrogen are advisable.

The furan binder characteristics:

Content of furfuryl alcohol

Free and disengaging formaldehyde (in the resin 0.2 %)

Content of nitrogen and water

6.2.1 Self-hardening furan mixtures

Polycondensation is running, while acidic catalysts are acting (2060 % for the resin

mass):

Trihydrogen phosphoric acid H3PO4 (7580 %),

Arylsulphonic (benzenesulphonic) acid,

SO3H

Para-toluenesulphonic acid (PTS).

CH3 SO3H

The hardening reaction is exothermic, water is generated. This must evaporate,

therefore the hardening process runs in the direction from the surface inwards the core. For

Binders of the IInd


the manufacture of large moulds and cores, drying off by hot air is recommended. The

hardening process is accompanied by a mixture colour change – the original white colour

changes to dark-green to black. This causes troubles when checking indefectibility of

covering of the cores with black graphite coatings. An over-catalyzed mixture is usually

brittle, friable and of “pepper and salt” colour.

Rapid hardening can be reached by benzoic acid, xylene-sulfonic acid, benzene-

sulfonic acid and a mixture of inorganic acids. To increase reactivity, an addition of H2SO4 in

a catalyst (0.515 % of free H2SO4) is used, too.

Catalysts are a source of sulfur (SO2), which is also a part of a regenerate. The sulfur

content growth leads to:

Casting defects, degradation of nodular graphite (reduction of Mg modificate in cast

iron, MgS forms as a result of reactions); this can be solved also by special coatings,

allowing bonding of SO2

Deterioration of the work environment (SO2)

Exogenous blow holes

The hardening reaction speed increases along with increased temperature, therefore we

must work with cold sands. Regarding to acidic catalysts, alkaline base sands cannot be used

(chromium magnesite, magnesite). An advantage of the self-hardening furan mixture is that

coke forms at temperatures of 1100 to 1200°C without air access. The coke number ranges

from 43 to 53 (4353 % of coke from the resin mass). A large proportion of pyrolytic carbon

after thermodestruction of the resin improves the surface quality of castings cast even into

non-coated moulds. The mixture has also excellent collapsibility and easily regenerates by a

dry mechanical way.

6.2.2 Cold processes – extrinsic hardening

Advantages of CO2-process performed in the 50s are more and more often utilized

today in new cold processes in controlled hardening from outside the core box by gaseous or

aerosol media. At present these procedures belong to the most widely used processes of the

manufacture of moulds and mainly cores.

6.2.3 SO2 – process (furan resin)

When using furan resins, the principle lies in their polycondensation in the acidic

atmosphere of sulfuric acid. Supplied sulfur dioxide is oxidized to sulfur trioxide, which

forms sulfuric acid by chemisorption in water:

Binders of the IInd


2 2 3

katalyzátor1

SO O teplota SO2

tlak

(46)

3 2 2 4SO H O H SO (47)

For SO2-process, special furan binders are prepared, differing from common foundry

resins particularly by a higher proportion of furfuryl alcohol and they are more or less

anhydrous with minimum nitrogen content. Their viscosity ranges between 0.180.55 Pa.s.

Methyl ethyl ketone peroxide (MEKP) and hydrogen peroxide (H2O2) are used as

catalysts. SO2 along with air is fed into a core box from pressure tanks of maximum pressure

of 6 bar. A hardening equipment scheme is shown in Fig. 52.

ODVLHČOVAČ

ČISTÍCÍ

MEDIUM

ČISTICÍ ROZTOK

NaOHSO2 SO2 VZDUCH JADERNÍK

VYTVRZOVACÍ

DESKA

PRAČKA

Obr.52. Scheme of equipment for core making using SO2-process

An excess of SO2, determined by a batcher, is absorbed in a special sorption tower

(washing machine). Particularly it is sorbed in NaOH solution (max. 15 %):

2 2 3 2SO 2NaOH Na SO H O (48)

After further oxidation (H2O2 or ozone)

2 3SO oxidační činidlo SO (49)

3 2 4 2SO 2NaOH Na SO H O (50)

sulfur trioxide is neutralized in sodium hydroxide, while sodium sulfate

generates. For 1 kg of SO2, 3.8 kg of NaOH are needed, whereas 8.3 kg of the used solution

are not needed.

The mixture is recoverable by a dry method. For instance, a new mixture is prepared

from 80 % of a regenerate and 20 % of new sand.

Binders of the IInd


Assessment in term of hygiene and work safety - SO2 is poisonous. It can be detected

at as low concentration as 2 ppm. It is strongly aromatic (the worst smelling technology). In

spite of the problems mentioned above, the method is widely used in USA (more than 70

foundries), but also in Europe, China, Japan and even in South Africa.

Advantages of this technology:

Long service life of the prepared mixture

Unlimited storability of prepared cores

High specific strength (3 MPa in bending per 1% of the resin)

Low generation of vapours and gases during casting

Excellent collapsibility of cores after casting

The mixture sticks on patterns only at a minimum extent, gives smooth casting surfaces

with a decreased occurrence of surface defects

One of the shortest times of hardening (35 sec)

6.2.4 Resol-CO2

The binding process principle is in the alkaline condensation of phenol-formaldehyde

resin hardened by CO2-process. The resin is modified by a complex additive (pH = 14). A

high pH value is reached by the presence of KOH and K2CO3 in a binder. During hardening of

CO2, a decrease in pH towards the acidic side occurs. By the 1st stage CO2 hardening, an

immediate strength occurs, then final hardening follows after lifting the core out from the core

box.

In the first stage, adsorption of CO2 in water occurs, while dihydrogen carbonic acid

generates, reacting with alkaline resin in the presence of boritane ions, followed by gelation

and cross-linking of the binder. The reaction is exothermic. The reaction rate is influenced by

the mixture temperature and the pressure in the core box. The amount of the binder ranges

between 1.23.0 weight parts in modified phenol resin (for base sand).

The hardening rate for CO2 is approximately 56 l.min-1

at 0.030.05 MPa pressure.

Strength of cores is lower than for CO2 - water-glass, therefore this technology is applicable

for simple shaped cores. After casting of cast iron and steel castings, collapsibility is

noticeably better. A combination of the cores to bentonite moulds is not recommended.

“Excessive natrification” of bentonite occurs, therefore also strength decreases in a water

condensation zone. The use of olivine sands is not recommended, either.

Binders of the IInd


6.3 Chemistry of polyurethane binders

6.3.1 Phenolic polyurethane system

It is used as:

PEP-SET, three-component self-hardening system

COLD-BOX Ashland, with controlled hardening using tertiary amines

6.3.1.1 PEP-SET system (polybenzylic ether phenolic)

The three-component self-hardening system consists of:

Special phenol-formaldehyde resol dissolved in a mixture of organic solvents

Polymer isocyanate (4,4-difenylmethanediisocyanate)

Special amine catalyst regulating the reaction rate between the basic components

During hardening, urethane polymer binder is formed and no other by-products.

Therefore hardening runs inside the entire core volume. This also implies the ideal course of

the hardening curve Fig. 53.

DOBA OD NAMÍCHÁNÍ

A UPĚCHOVÁNÍ SMĚSI

0

PE

VN

OS

T [M

Pa]

I. II. III.

inkubační

periodavytvrzování

konečné

vytvrzení

Obr.53. PEP-SET system hardening process (I - incubation period, II – hardening, III –

final hardening)

A relatively long incubation period (mixture service life) I, an abrupt increase in

strength II and high final strength III. At various ratios and concentrations of a catalyst, the

time for the core box disassembling can be controlled from several tens of seconds up to 3

hours.

The Pep-Set binder system is suitable for all kinds of sands, strongly acidic or alkaline

sands accelerate hardening of the system. The maximum permissible sand moisture is 0.25 %.

Mixing is recommended to be performed in a continuous mixer with zero rest of a non-

processed mixture. The Pep-Set system contains 3.03.8 % of N2, which represents 0.04% for

sand.

Binders of the IInd


6.3.1.2 Ashland - Cold-Box (amine hardened phenolic urethane system)

(PUR Cold-Box)

The binder consists of two liquid components and after injecting the mixture into the

core box, the core is hardened by a "gaseous” catalyst (tertiary amine).

The first binder component is a hardenable resin, the second one is a hardening agent.

As a hardenable component, the patent protects epoxy, polyester, alcydic, phenol-

formaldehyde resins or oil polymers. In operation, a modified phenolic resin has found the

widest utilization. Difenylmethanediisocyanate is a hardening agent.

The resin and the hardening agent are not favourable matters from the hygienic and

safety point of view. Both of the matters are slightly flammable.

The phenolic resin and polyisocyanate react together only very slowly. To accelerate

the reaction, triethylamine is used as a catalyst. It acts solely as a catalyst, because it comes

out of the reaction without a change. The reaction is then immediate. A strong urethane resin

forms.

1. Polyisocyanate complex

CH2OCN N C O

N

CH3

CH3

CH3

(51)

2. Intermediate complex

CH2OCN N C O

N

CH

CH3 CH3

(52)

3. Polyol components

Binders of the IInd


OHCH2

OH

CH2 O CH2

OH

n

C O H

H

H

CH2OCN N C O

N

CH

CH3 CH3

(53)

4. Polyurethane binder

CH2OCN N

H

C O

O

CH2

OH CH2 O CH2 OH

m

CH2OH

n

N

CH3

CH3 CH3

(54)

A general formula of these resins assumes a sum m + n = min. 2 and m : n ratio = 1 : 1

at minimum.

OH + O

R

R'

C N R"(C2H5)3N

O

R

R'

C N

O H

R"

pevná uretanová

pryskyřice (55)

TEA (C2H5)3N, triethylamine, is an organic liquid of an alkaline reaction, pH = 10 (h

= 730 kg.m-3

). It volatilizes easily, smells as ammonia, is flammable and explosive.

Flammability ranges between that of petrol and ethyl alcohol. The lower limit of

explosiveness is 1.2 volume parts, corresponding to 50 g.m-3

. In foundry plants, up to 0.2

volume parts are permitted. TEA is 3.5-times heavier than air – accumulates near the ground.

The strictest hygienic and safety regulations apply here. The reaction rate depends on velocity

of the amine complex formation. The amine molecule size plays a great part here. Amines

condense in cold parts of cores. Other tertiary amines can be used for hardening, too.

Binders of the IInd


N

NC2H5 C2H5

C2H5

TEA

N

DMEA

CH3 CH3

N

TMA

CH3 CH3

CH3C2H5

CH3 CH3

CH

CH3 CH3

DMIA

trietylamin dimetyletylamin trimetylamin dimetylisopropylamin (56)

In comparison with DMEA, hardening using TMA is shortened by 78%. Both

economic and ecological benefits are evident. The intensive bad smell is a disadvantage.

These are liquid matters (TEA), a catalyst aerosol needs to be formed by air at first; this is led

to a core (or with a carrier gas N2, CO2). There is one part of amine for 7 parts of the carrier

gas.

Hardening runs either as a closed process or an open one. In the closed procedure, at

first the unconsumed catalyst is neutralized in phosphoric acid (1 part of H3PO4 + 3 parts of

H2O) or it is burnt in a natural gas burner in a stack with refractory lining. There are also

washers with H2SO4, where amine sulfate salt generates, from which amine for a new use can

be obtained by chemical processing. Recently there has been an effort to work with an open

system, using amine feeders or activated carbon adsorption, or using biological washers with

microorganisms. In operation, the consumption of 1 t of the mixture corresponds to 0.51 kg

of amine.

Various dry silica sands can be used for preparation of the mixture, because water

decomposes isocyanate and the mixture service life is shortened (maximum permissible

moisture limit is 0.1 %). Alkalis also shorten the mixture service life, temperature of sands

above 35°C is inadmissible.

Continuous mixers are the most suitable, the order of components is not important,

maximum mixing time 4 minutes. The mixture service life is 12 hours. The mixture is highly

liquid, suitable for injecting.

An increase in temperature from 13 to 25°C accelerates hardening by one third. To

accelerate hardening and also the manufacturing sequence, core boxes are preheated to 100°C

(Cold-Box - plus). Cores may be set immediately after hardening for several seconds (90 %

of final strength). Cores are very precise, well storable and well collapsible after casting.

Castings have good surfaces. Formation of fins can be prevented by an addition of

0.751.5 weight parts of Fe2O3 or by special coatings. Cores somewhat stick to core boxes, to

Binders of the IInd


use a polyurethane insert is the easiest way to withdraw them from the core box. The

production cycle for cores of a weight from several grams up to 130 kg is about 30 sec.

The core hardening is two-fold faster than when using the Croning technology. The

technology has the following disadvantages:

Sorption needs to be used for the excess of amine.

Completely dry silica sand or regenerate needs to be used. Just the presence of 0.1 % of

water in sand means a strong decrease in strength (isocyanate decomposition).

Only alcoholic (more expensive) coatings may be applied (hygiene).

When using water coatings, additional drying is needed (microwave drying).

A limited possibility of a regenerate (mechanical regenerate max. 70 %). Therefore a high

proportion goes away for depositing. Regenerates from other binder systems are not

recommended to use.

Recommended literature for further study

[1.] BROWN, J. Foseco Ferrous Foundryman's Handbook, 11. Vydání, Butterworth-

Heinemann, 2000, 384, ISBN 9780080506791

[2.] BEŇO, J. et.al. Archives of Metallurgy and Materials, 59 (2014), 2, 745-748

[3.] TILCH, W, et al. Giesserei. 2006, 8, s. 12-24. ISSN 0175-1034.

[4.] SPADA, A. Modern Casting, 1998. č. 4, s.45-48.

[5.] GIELISSEN, H. Foundry Trade Journal, February 2002, s.7-11.

[6.] HWANG, J.Y. AFS Transactions, vol. 99, s. 807-815.

[7.] ROSCHIER, M.: Fonderie, tom 33, Nr. 382, 1978, s. 286-288

[8.] TROY, E.C. a kol.: AFS - Transactions, vol. 79, 1971, s. 213-224

[9.] BOENISCH, D.: International Conference 1988, BCIRA, England

[10.] HOFMANN, F.: Technologie der Giessereiformstoffe. Georg Fischer Actiongesellschaft,

Schaffhausen Schweiz, 1965

[11.] LEVELINK, H. G. a kol.: Giesserei 62, 1975, Nr. 5, s. 93-99

[12.] HEINE, R. W.,SCHUMACHER, J. S. ,GREEN, R. A.: AFS Transaction 84, 1976, s. 97-

100

[13.] ANDREWS, J. a kol.: Modern Casting, 90, September 2000, s. 40, 43

[14.] HEADINGTON, F. a kol.. AFS – Transactions 98-01, s. 271-291

[15.] BRÜMMER, G.: Gisserei 87, 2000, Nr. 3, s. 62-68

[16.] KLEIMANN, W.:. Giesserei 85, 1998, Nr. 1, s. 87-90

[17.] BEŇO, J.; et. al.. Archives of foundry engineering. Vol. 11., No. 1., 2011. s. 5-8. ISSN

1897-3310.

[18.] KOLORZ, A., LÖHBERG, K.: Giesserei techn. Wiss. Beihefte, 15, 1963, H. 4, s. 191

[19.] BINDERNAGEL, I., KOLORZ, A.: Giesserei, 51, 1964, s. 729-730

[20.] WÖRMANN, H. ad.: Giessereiforschung, 34, 1982, H. 4, s. 153-159

[21.] BINDERNAGEL, I. a kol.: Giesserei, 61, 1974, č. 8, s. 190-197

[22.] BAIER, J.: Giesserei, No. 12, 1979, s. 480-482

[23.] JELÍNEK, P., BEŇO, J. Archives of Foundry Engineering, April - June 2008. Vol. 8, No.

2, pp. 67 – 70, ISSN 1897-3310

[24.] SACHARUK, L. a kol.: Giessereiforschung 53, 2001, Nr. 3, s. 104/109

[25.] BALINSKI, A.:. Institut Odlewnictwa Kraków, 2000, ISBN 83-911283-5-0

[26.] JELÍNEK, P. – ŠKUTA, R. Acta Metallurgica Slovaca, 4/2002, roč. 8, s. 431/439, ISSN –

1335 – 1532

[27.] HÄNSEL, H.Giesserei, 89, 2002, Nr. 2, s. 74/76

[28.] ILER, R. K.: The Chemistry of Silica, New York, 1968

[29.] FLEMMING, E. a kol.. Giesserei Praxis, 1996, Nr. 9/10, s. 177

[30.] SCHNEIDER, H. a kol.Giessereiforschung, 1991, Nr. 1-2, s. 10

[31.] SVENSON, I. L. a kol. J. Chem. Soc. 1986, t 82, s. 3636

[32.] WARREN, D.The British Foundryman, 1971, Nr. 12, s. 449

[33.] JELÍNEK, P., POLZIN, H. Giesserei Praxis, č. 2, 2003, s. 51/60

[34.] LE SERVE, F. L. - LEMON, P. H.: Modern Casting, 1969, Vol. 56, November 2, s. 146-

150.

[35.] SCHAARSCHMIDT, E. Giessereitechnik, 1971, 17, No.2, s. 43-50

[36.] GARDZIELLA, A. - KWASNIOK, A.. Sonderdruck aus Giesserei, 1988, H. 13

[37.] WOLF, K.: Giesserei – Erfahrungsaustausch, 41, 1997, Nr. 9, s. 391/394

[38.] STÖLZEL, R. a d.: Giesserei 86, 1999, Nr. 6, s. 154/157

[39.] STÖLZEL, R. – GENZLER, Ch.Giesserei – Praxis, 1999, Nr. 12, s. 568

[40.] LEDEGOURDIE, G. a kol.: Foundry Trade Journal 1997, october, s. 434

[41.] ADAMS, P.Litějnoje proizvodstvo, 1999, No. 2, s. 20

[42.] SCHREY, A. a d.Giesserei 88, 2001, Nr. 6, s. 51/56

[43.] JELÍNEK, P.; DUDA, J.; MIKŠOVSKÝ, F.; BEŇO, J. Archives of Foundry Engineering.

Vol. 12 (2012), 1/1012. p. 47-52

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