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G a b r i e l e G a l a n t e

O v i d i o M i c h i l l i

R u g g e r o M a s p e r o

No-BakeAS WE SEE IT

1


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INDEX

PREFACE page 11

1. THE NO BAKE PROCESS page 13

1.1 PROCESSCOMPATIBILITY page 13

2. THE RESINS AND THEIRPOLYMERISATION page 15

2.1. THERESINFAMILIES page 16

2.1.1. GENERALCHARACTERISTICS page 16

2.2. THECLASSIFICATIONOFFOUNDRY

RESINS page 19

2.2.1. FIRSTGROUP page. 19

FURANRESINS page 19

PHENOLRESINSANDFURAN-PHENOLRESINS page 26

UREA-PHENOLANDUREA-FURANRESINS page 27

2.2.2. SECONDGROUP

ISOCYANATES-URETHANESYSTEM page 28

THETHREECOMPONENTSTYPE page 28

THETWOSOLUTIONSTYPE page 29

2.2.3. THIRDGROUP

ALKALINEPHENOLRESINS page 30

2.2.4 RESINAGEING page 32


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2.3 ADDITIVES page 33

SIILANES page 33

WATER page 33

IRONOXIDE page 33

2.4 PHYSICALANDCHEMICALCHECKSONRESISNS page 35 VISCOSITY- DENSITY page 36

REFRACTIVEINDEX page 36

3. CATALYSTS AND HARDENERS page 39

3.1 CATALYSTS page 39

3.2 HARDENERS page 42

3.2.1 ESTERS page 43

THEUSEOFESTERSINTHEALKALINE

NO-BAKESYSTEM page 43

THEPOURINGPROCESS page 43

THEREGENERATIONPROCESS page 44

4. SODIUM SILICATE page 45

4.1 THEBASICPRINCIPLESOFTHEPROCESS page 45 THESILICATE-ESTERREACTION page 45

4.1.1 SETTINGTIMES page 46

4.2 THETYPEOFSODIUMSILICATE page 47

4.3 THETYPEOFESTER page 47

4.4 ADDITIVES page 49

4.5 CARRYINGOUTTHEWORK page 49


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4.5.1 MIXPREPARATION page 49

4.5.2. MOULDING page 50

4.6 SILICATECHECKS page 50

CHEMICALCHECKS page 51 PHYSICALCHECKS page 51

MECHANICALCHECKSONTEST

PIECESOFBONDEDSAND page 51

5. THE SANDS page 55 SILICASAND page 57

OLIVINESAND page. 57

CHROMITESAND page 57

ZIRCONSAND page 58

6. THE PHYSICAL AND CHEMICALCHARACTERISTICS OFTHE SAND MIXTURES page 59

6.1 SANDCHARACTERISTICS page 59

GRANULOMETRYANDFINENESSINDEX page 59

SPECIFICSURFACEAREAOFTHEGRAINS page 60

MOISTURE page 61 FINES FRACTIONS page 61

LOSSONIGNITION page 62

THEACIDDEMANDVALUE(ADV) page 62

THEBASEDEMANDVALUE page 62

CLAY page 63

OOLITECONTAMINATION page 63

TEMPERATURE page 63

6.2 THEHARDENINGPHASES page 64

6.2.1. THEWORKTIME page 64


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9.1.1 CASTIRONSOLIDIFICATION page 88

THEVOLUMETRICCHANGEINCASTIRONS page 89

NO-BAKEANDDUCTILEIRONCASTING page 91

9.2 THECONTRIBUTIONOFTHERMALANALYSIS

TOTHEEVALUATIONOFTHETENDENCYOF CASTIRONSTOSHRINK page 93

9.3 MOULDINGBYPRESSURESHOOTING page 97

9.3.1 THEPROCESSCHEMISTRY page 98

9.3.2. DESCRIPTIONOFTHEMOULDSHOOTING

PLANT page101

THEMIXER page101

THEMOULDINGPLANT page101

GASSING- PURGING page106 THEBUFFERPOSITION page106

MOULDSTRIPPING page107

PASSINGTHEMOULDSTOTHE

POURINGLINES page107

HORIZONTALLYPOUREDMOULDS page107

VERTICALLYPOUREDMOULDS page107

9.3.3. THEADVANTAGESOFMOULDINGWITHA

MOULDSHOOTER page108

9.3.4 THEADVANTAGESOFVERTICALPOURING page109

9.3.5. FIELDSOFAPPLICATION page109

10. SAND RECLAMATION (REGENERATION) page111

10.1 THEDEGREEOFREGENERATION page114


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7

CAV. GALANTE GABRIELE

Gabriele Galante belongs to a family which, in the best traditions of the Luino

industrial class, continues to carry out a very significant role.His grandfather founded a construction company and his father started his own

foundry where young Gabriele, learnt the basic techniques founding stet, once

his studies has been completed.

Since 1972, when IMF was founded, he has demonstrated his innate design

and entrepreneurial capabilities, through the development of the technology,

which is the hall mark of IMF in todays world markets.

As President of IMF he can offer machines and equipment for the application

of proven processes, marked by precision, flexibility, modularity and adaptabi-

lity: suitable for a wide variety of operating conditions.

His successful commercial strategies have led to expansion abroad, and he isalso the President of EPF, the French subsidiary; and President of IMF North

America, the USA subsidiary.

AMAFOND, the Italian Association of Foundry Machinery Makers, elected him

as Association President from 1983 to 1987, a period of integration with analo-

gous associations, within a wide International context.

He followed this success by becoming President of the European Committee

of Foundry Materials Producers (CEMAFON) from 1988 to 1991. In this posi-

tion he took a broad view and forged connections with similar organisations

outside Europe, thus creating wider horizons for exports.

Today, as a member of the Executive Commission of AMAFOND, he is respon-

sible for the development of its representative role within CEMAFON, at a cru-

cial moment in the process of industrial globalisation.


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8

No-Bake as we see it- PARTONE

DR. OVIDIO MICHILLI

He was born in S. Valentino (Pescara), on the 9th July 1925.

In 1943 he joined the chemical laboratory of the Fonderia Ansaldo in Genoa.

In 1944 he transferred to the melting departments of the section concerned with

cast-iron, light alloys and copper alloys. He contributed on the perfection of the

process for the spheroidisation of graphite, through the introduction of magne-

sium metal. This process resolved the serious problem of spheroidisation.

His innovative approach was a great success both in Italy and abroad.

In 1952 he joined the Fonderie Getti Speciali Colombo Giuseppe di Carlo at

S. Giorgio Legnano.

Under the competent management of the owner, and with the professional

capacity of Dr. Michilli, this foundry became highly proficient in the production

of special cast-iron castings. The metallurgic techniques employed and theiroriginality, became standards for the industry, both in Italy and abroad.

In 1956 he was awarded a degree in Industrial Chemistry at the University of

Pisa.

In 1980, he started his activity as a consultant, both in Italy and abroad.

Characteristics: during his working career, entirely spent in the foundry sector,

he has worked with enthusiasm and competence, to progressively free foundry

techniques from empiricism.


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9

DR. RUGGERO MASPERO

He was born at Carimate (Como), on the 4th May 1932.

He was awarded a degree in Industrial Chemistry at the University of Bologna.

Following his military service, he worked in Duesseldorf, Germany, with the

Huetteness-Albertus GmbH (at that time Gebrueder Huetteness), until 1972. He

was initially a research worker, and later was the manager of the Research and

Control Laboratory.

After returning to Italy, he directed the Technical Laboratory of the Research

and Development sector of the Satef Huettenes Albertus SpA, from 1972 to

1994.

In this period he edites several technical publications and was a speaker at

many Congresses and Fairs.

In 1992 he was awarded the A Dacco prize, for Italian foundry work, followingwhich he addressed the International Congress held at the Hague (1993), giving

the official Italian paper.


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10



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11

PREFACE

PREFACE

In the face of a continuous and increasing pressure to produce

quality castings; the foundry technician has a daily need to reduce

production costs. He is also faced with a lack of skilled labour. The

empirical approach, which nowadays is increasingly being repla-ced by technology, is the uncertainty element still typical of found-

ry work.

The No-Bakemoulding process has made a great contribution to

resolving these uncertainties.

IMF has been working in the plant sector of No-Bake technology,

as a partner of casting foundry technicians, for more than twenty

years. This partnership extends from the foundry floor to the

technical offices, and has resulted in technical solutions and plants

which are widely recognised for their quality.

Through this practical guide IMF aims to widen its contribution, by

classifying its experiences and by uniting them with the latest spe-cific publications in the field. Through this work IMF also expects

to make a significant contribution to the training of foundry techni-

cians.

The manual has three parts, each contained in a separate volume.

They are all easily consulted and are complementary to one ano-

ther.

The first volumeis instructional. It contains the essential basic theo-

retical concepts and the more important technical subjects. This

volume will be of most interest to foundry engineers, to methods

office technicians and to those students who intend to specialise

in the foundry sector.(1)The second volume is a practical manual, which contains the most

important technical information connected directly or indirectly

with the No-Bake process.

It is divided into two parts; the first of these describes the charac-

teristics of typical materials and their uses, the second consists of

easy reference technical schedules.

The third volumedeals with equipment.

We have left out some technical information on the basis that it is

common knowledge, whilst other information has been deliberate-

ly repeated, partly in order that it should be completely understood

and partly to make the various parts of the manual independent of

(1)In the appendix which follows the Part I text, there is a glossary of chemical terms and com-pounds, and of physycal phenomena, connected with the No-Bake process.


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12


one another. The on-going developments in the field of foundry

binder chemistry, mean that this treatise cannot be considered to

be final; nevertheless we believe that its contents will enable you to

follow the development of No-Bake technology correctly.


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13

THE NO BAKE PROCESS

1. THE NO BAKE PROCESS

Moulding using the No-Bakeprocess started in the sixties, using

sand cold bonded with either urea-furan resins, or inorganic bin-

ders of the silicate type.

Its introduction made important changes in the production ofcores; and later in the production of flasks and flaskless moulds.

The first obvious effect, much appreciated by the foundry operator,

was theIncrease in Productionby 40 to 60%, especially in the moul-

ding process for large castings.

The second advantage was in the Quality Factorand the increased

certainty that the casting would be a good one, despite the fact

that the typical moulding defects were replaced by a series of new

and for the moment, unrecognised faults.

The third advantage was the possibility, in general, of using less

skilled labour.

1.1 PROCESSCOMPATIBILITY

Moulds can be made using sand and binders with different

characteristics and also, mixed with sand reclaimed from cores. It

is therefore necessary to check the compatibility of the processes,

especially when regenerated sand is used, or when new processes

are introduced.

The characteristics of the sands from the different processes must

be assessed, especially for the amount of fines and the degree ofacidity or basicity in the sand.

The morphology of new sand needs to be evaluated for grain

fragility characteristics, to avoid the need to use greater amounts

of binder and hardener as the number of reclamation cycles

increases. In fact, if the sand has sharp angles and is fragile, there

is a saving in the use of binder at first, due to the small interstitial

volume.

As the number of regeneration cycles increases, the amount of

binder used increases due to grain break down and the increased

fines fraction. In a well regenerated sand this problem is much

reduced.


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14


You are referred to the technical schedules R9/a/b/c, S2, S3, in

Part II, for a check of process compatibility.


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15

THE RESINS AND THEIR POLYMERISATION

2. THE RESINS AND THEIR POLYMERISATION

Foundry resins are organic compounds, normally liquid, and have

molecules which mainly consist of carbon, oxygen, hydrogen and

nitrogen atoms. These molecules, called monomers, are simple

molecules which can be likened to rings. In the resin productionphase active centers form and the molecules join together in long,

mainly two dimensional chains, under the combined action of heat

and a catalyst. In the application phase this reaction continues due

to the addition of a second catalyst; and a rapid and three dimen-

sional chain formation is achieved; resulting in a rigid and dense

network.

The macromolecules thus formed have a very high molecular

weight and are known as polymers(if they are formed from identical

molecules) or copolymers (if they are formed from more than one

type of molecule). Their configurations give rise to the term reticula-

tion.When this reaction takes place in a sand, the network formed holds

the sand grains together in such a way that a rigid skeleton is

formed.

The reaction of chain formation by monomers as described above,

is called polymerisation.

The resins mainly used in the No-Bake process are formed by a

polymerisation of monomers (poly-condensation) and the

co-polymers formed are phenoplasts (from phenol and

formaldehyde), aminoplasts(from urea and formaldehyde) and furfu-

ryl copolymers(from furfuryl alcohol, phenol, urea and formaldeyde).

According to the type of polycondensate, a further polymerisationis necessary at the point of use in the foundry; and this may be one

of two types :

Addition polymerisation: also known as poly-addition, is the pro-

cess in which the reaction product repeats the monomer

unit and the molecular weight of the product is equal to the

sum of the number of monomer units which form the poly-

mer.

Condensation polymerisation is the process in which organic

molecules with individual low molecular weights (mono-

mers), form heavy macromolecules (polymers). The mono-

mer units which are repeated in the polymer chain contain


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16


fewer atoms than the original monomer molecules. This is due to

the elimination of subsidiary compounds at chain formation, usual-

ly water.

Polymerisation starts slowly when the resin is mixed with the sand

and speeds up continuously until the reaction is completed.

The polymerisation process is disturbed by the movement of thesand particles due to the mix being handled, as these movements

break parts of the polymerised mesh during its formation. This

wastes resin, reduces the flow characteristics of the sand mixture

and reduces the mechanical strength of the cured mould.

It is therefore advisable to control the polymerisation (or hardening)

process during the mixture preparation phase, in order to prevent

premature resin chain formation. This means that the catalyst

addition needs to take account of the mixture preparation time, so

that the polymerisation takes place in harmony with the several

phases of moulding.

2.1. THERESINFAMILIES

The rapid growth of the number of types of binders, coupled with

the plant developments in the traditionally complex and divergent

foundry sector, makes it impossible to give precise indications or

off-the-shelf solutions, for the choice of the most suitable binders.

This statement is also true in the context of this publication, given

its instructional and informative nature.

Nevertheless, the data and information given below make a usefulcontribution for the best choice of products and plant, having taken

account of specific factors such as : moulding materials, the type

of casting, the production rate required, the equipment, the skill of

the labour force and environmental impact.

2.1.1. GENERALCHARACTERISTICS

The choice of binders is largely determined by the required produc-

tion rate and the dimensions of the mould and/or the core.


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High production rates require rapid and constant hardening times,

whilst diametrically opposite conditions are required by large and

heavy moulds and cores. These require long initial hardening times

and therefore a long working life (bench life) to enable the mould

to be filled in an acceptable working time, that is before the

polymerisation has appreciably progressed towards completion.The flow characteristics of the sand/resin mixture must allow the

pattern to be copied faithfully and allow a satisfactory level of com-

paction. In the case of complicated pattern models, with so-called

shadow zones (parts where the sand compaction is not easy), it is

advisable to use vibrating compaction tables. Good compaction

enables the percentage of binder to be reduced without reducing

the mechanical strength of the mould.

Resin viscosity plays an important role as it governs the capacity

of the binder to cover the grains of sand.

The inability to Rapthe pattern, except by vibration, means that

special removal equipment is needed. This enables the maximumuse to be made of one of the No-Bake system advantages, the

minimum deformation of the mould cavity impression.

The decomposition rate of the resin during pouring determines the

amount of gas produced in the mould cavity.

The gas quantity cannot be easily controlled due to the organic

nature of the binder. It is therefore necessary to minimise the binder

quantity and the gas contact with the liquid metal (see fig. 2). This

is particularly important when hydrogen and nitrogen are present

as they are in the very reactive nascentstate. In these conditions

they are easily absorbed by the liquid metal, and may cause small

blow holes in steel and cast iron castings.Sulphurous gases arising from the decomposition of the catalysts,

may cause morphological changes of the graphite on the surface

of ductile iron castings.

However, not all cast alloys are affected to the same degree by

gases.

Environmental considerations require that the bonded sand has to

be reclaimed (regenerated) after use and recycled. This process

consists of removing the hardened resin film which covers the sand

grains.If this is to be carried out by a attrition process, it is essential

that the film should be easily removed.

This requirement must be borne in mind when choosing the type of

17



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resin to be used.

The possible need to use a reinforcing framework, is a further fac-

tor to be added, when selecting the most suitable binder.

A good resin does not normally require the mould to go through a

drying stage.

The safety and environmental protection problems arising from the

several stages; from moulding, to manipulation, to pouring and

knocking out, must be minimised. In any event, safety precautions

and environmental pollution levels must conform to the legal limits

and regulations.

In the appendix of Part II, the technical schedules and their usage

instructions are given, both for the resins described and the sand/

resin mixes; together with advice for their best use.

The same schedules also give the safety precautions to be taken

during the handling of the binders, together with other information

about safety and environmental problems. Finally, there is also

information and data about release agents and paints, to assist in

selecting types which are compatible with every type of binder

described.

18


Fig. 2- Gas evolution at 1,010C by different resin types

1. 1,5% alkyd resin/20% isocyanate2. 1,5% polyurethane resin3. 1,5% furan resin/30% toluene sulphonic acid4. 1,5% phenol resin/30% toluene sulphonic acid5. 1,5% alkaline phenol resin

ccOFGASPERGRAMMEOFSAMPLE

TIME, in seconds

15 SECONDS CONTACT

1. alkyd resin

2.polyurethane resin

3. furan resin

4.phenol resin with ind. acid

alkaline phenol resin


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19


2.2. THECLASSIFICATIONOFFOUNDRYRESINS

The most popular resins used for the No-Bake process can be

classified into three groups:

The first of these is composed of resins catalysed by acids,

such as furan resins (see fig. 2.1), phenol resins and phenol-furan resins (see fig. 2.2), and urea-phenol resins (see fig.

2.3). These can be used singly, or as combinations if speci-

fic characteristics are required to meet production needs.

The second group is composed of isocyanates, which poly-

merise by addition with poly-alcohols to form polyurethanes

(see fig. 2.4).

The third group of resins, has only been in use for a short

time and consists of alkaline (basic) phenol resins (see fig.

2.5). This group completes the range of the resins most

commonly used in the No-Bake process; and any type of

sand can be used with them including olivine sand (giventhe basic nature of this sand it cannot be used with resins

catalysed by acids).

2.2.1. THEFIRSTGROUP

Furan, phenol and phenol-furan resins, are those most commonly used

in the No-Bake process.

These definitions are only generic and indicate the type of basic

resin components. These are respectively furfuryl alcohol, phenoland mixtures of these. Normally other compounds are also used to

complete the formulation and modify the resin, to obtain the requi-

red characteristics in the final product.

FURANRESINS

The adjective furan describes the basic component of the resin.

This is furfuryl alcohol and its polymerisation reaction (condensation)

is shown in fig. 2.1. It is soluble in water and has a low viscosity. It

is therefore easily mixed with sand and gives optimum coverage of

the sand grains.


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Fig. 2.1- Furan resin formation by the polymerisation of furfuryl alcohol. The furan nuclei are connected by methy-

lene links to form linear chains. The condensation reaction is exothermic

CH

CH

HC

C

HC

CH

CH2OH

O

O

O

O

O

O

O

C

C CH

HC

CH

HC

CH

H2C

CH2

HC

HC

CH

C

C

C

CH2

CH

CCH2

HC

C

H2

CH

HC

C CH

O

O

CH2

C

CH2

nx

+

CAT

-

H2O


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21


m

HOH2C

H O CH2

OH

CH2OH

+

OH

H

CH2OH

n

HOH2C

+

CAT

-

H2O

OH H H O CH

2

HOH2C

HOH2C

OH

CH2OH

CH2OH

OH

H OH

HOH2C

HOH2C

CH2

O H

CH2OH

CH2OH

OH

H H O CH2

HO

H2C

HO

H2C

OH

CH2OH

CH2OH

OH

CH2OH

+

CH2OH

+C

A

T

O

OH

CH2

C

H2

O

O

H2O

Fig. 2.2- Polymerisation by condensation reactions: A) phenol resin=polymerisation ofphenylmethanol(formed by the condensation of phenol with formaldehyde). B) phenol-furanresin=polymerisation of phenylmethanol with furfuryl alcohol.

A B


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Fig. 2.3- Urea-phenol resin formed by the polymerisation of mono methyl urea and phenylmethanol

CH2OH

OH

+

+

CAT

N CO

CH2OHC

H2

NH

CO

CH2

N

OH

CH2OH

+

H2O

H

N

CH2


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23


Fig. 2.4- Polymerisation reaction by addition between a resol and an isocyanate.

+CAT

[C2H5]3N

R

R1

OH

+

OCN

R2

R

R

1

O

C

H N

O

R2

Poly-isocyanate

Polyurethane

Phenolresin

benzylethertype


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Fig. 2.5- The hardening reaction of an alkaline resin with methyl formate (procedure using agaseous ester) and the pH changes in the several phases.

a)Indicativereaction

Alkalineresol

Methylformate

Potassiumf

ormate

Methanol

In

solublemacro-molecule

b)Schematicphasereactions

M+OPF-

+

ROOCH

pHvalue

Alkalineresin

Ester

12-14

transitionphase

12-14

(HOPF)n

+

M+OOCH-+ROH

7

polimer

alkalinesalt

alcohol

Complex

OK

HO

CH2

OK

OH

OK

CH2

CH2

O

OH

CH2

CH2

O

CH2

OK

O CH2

CH2

CH2

CH2

O

CH2

OK

O

C

H3

+

H

C

O

HO

CH2

OH

OH

OH

CH2

CH2

O

OH

CH2

CH2

O

CH2

OH

CH2

CH2

O

CH2

OH

CH

2

CH2

O

+CH3

OH

+

H

C

OK

O


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We list their main characteristics below :

The sand / furan resin mixture has excellent flow characteri-

stics;

There is a low content of chemical reaction water. This

means that the curing time is less severely affected than

with other resins, both as regards the quantity and type ofcatalyst required and the quality and temperature of the

sand. The tendency for the mould to skin harden, is also

reduced. This skin hardening phenomenon is due to loss of

water by evaporation. This happens more quickly at the sur-

face than in the centre of the piece;

The moulds bonded with furan resin maintain their mecha-

nical characteristics well even when hot. This enables the

ratio of sand to casting to be improved with a consequent

cost saving. This partly offsets the greater cost of this type

of resin.

The polymerised resin film on the sand grains is easily remo-ved during mechanical regeneration.

PHENOLRESINSANDPHENOL-FURANRESINS

The phenol resinsused in the No-Bake process are produced by a

poly-condensation reaction between phenoland formaldehyde under

basic conditions. The formaldehyde is in a small excess; and this

leads to the initial formation of phenylmethanol (phenoplast). The

resolformed polymerises by further condensation, due to the addi-

tion of an acid catalyst in the foundry. This gives a polymer (resin)

with excellent mechanical and heat resistance characteristics.These resins were initially used in the hot process; and their use

was later extended to the No-Bake process in cost competition

with the furan resins. This was the result of a series of modifications

which improved their technical and environmental characteristics.

Specifically, the viscosity was reduced to enable the sand grains to

be coated more easily, the free phenol was reduced to around or

below 5% and the free formaldehyde to below 0.5%. These resins

give off an unpleasant smell.

Compared with the furan resins, their polymerisation and harde-

ning are affected by several factors, principally any temperature

variation of either the sand or the pattern plates, or by exposure of


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the mould to air.

This is because the resin sets very quickly and the setting time is

strongly affected by temperature changes. It is therefore difficult to

control the setting time, or to keep it constant. Again, the surface

of the mould sets more quickly than the internal parts.

Good phenol resins do not contain any nitrogen (which cannot the-refore increase when the sand is regenerated), and they are also

relatively cheap. Phenol resins may contain furfuryl alcohol as a

solvent in many formulations. This improves the mixing characteri-

stics. When furfuryl alcohol is used, it also acts as a binder due to

its monomeric nature, whilst if it is used in appropriate concentra-

tions in the resin formulation, it forms compounds known as phenol-

furan resins. These resins combine the characteristics of each com-

ponent, and are therefore nitrogen-free, and withstand heat well.

Condensation with urea and addition of silane improves cold

strength and therefore the knock-out characteristics as well.

When the absence of nitrogen must be matched with low cost,either phenol resins or phenol-furan resins with a low urea content

are indicated.

Phenol resins are slightly hygroscopic and withstand heat well.

However, the expansion of the sand may not be contained and this

may lead to surface cracks in the mould.

These resins have high mechanical strength when cold and this

reduces the friability of the mould. However, this may create pro-

blems when knocking out. The quantity of gas produced and the

speed at which it is produced at pouring, on colling and at sha-

keout, are modest. The phenol resins have their maximum gas

production slightly later than that of furan resins.The polymerisation reactions of phenol and phenol-furan resins are

both shown in fig. 2.2 (page 21).

UREA-PHENOLRESINSANDUREA-FURANRESINS

Urea resinsare formed by the reaction between ureaand formalde-

hyde. This gives monoand di-methylurea(both anhydrous and hydra-

ted), and these polymerise through reciprocal and complex reac-

tions to give urea resin.

Phenol and furan poly-condensed are normally part of the formu-

lation in which they form urea-phenol(see fig. 2.3 page 22) and urea-


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28


furan co-polymers.

These resins have a relatively low heat resistance and also develop

nitrogen-rich gases. Figures 6.3, 6.4, 6.5 and 6.6 (page 71-72),

show casting defects which can be caused by nitrogen.

2.2.2. THESECONDGROUP: ISOCYANATES-URETHANESYSTEM

The second group of resins is made by reacting poly-isocyanates

with polybenzylphenylether (aresol), using pyridine or an amine as

catalyst. The polymerisation reaction is an addition reaction (poly-

addition); a resol of the benzylether type reacts with the isocyanate

to form polyurethane, without any secondary products being formed

(see fig. 2.4 page 23). The name polyurethane resin is derived from

this compound.

There are two types of formulations on the market : one with threeseparate components, one with two solutions.

THETHREESEPARATECOMPONENTSTYPE

The resin, the isocyanate and the catalyst, are supplied separately

and are added to the mixer through three separate metering

pumps, one for each component (see fig. 2.4/A page 29).

This presentation is the one which is most widely used as it gives

very flexible moulding cycles. It therefore enables the widest range

of requirements to be satisfied. For example :

- different types of patterns;- different types of alloys;

- different production cycles from moulding to pouring;

- climatic variations both seasonal and daily;

- variable moulding programmes (automatic, semi-automatic, and

manual);

The amount of catalyst to be used is always small, and the mete-

ring pump for it must be a high precision type.

THETWOSOLUTIONSTYPE

The resin and the catalyst form a separate solution to that of the


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isocyanate. The procedure with two solutions requires constant

moulding conditions: for the production cycle, the type of casting

and the environment.

The reason for this uniformity, lies in the polymerisation time and

this governs the production cycle. The quantity of catalyst added

to the binder must also be standardised. The work time is there-

fore fixed by these when the binder is used.

The two solutions system is recommended when the plant is not

equipped with a high precision metering pump for catalyst addi-tion, however, but this restricts the system flexibility.

Normally, the two solutions method requires as many resin types

as there are moulding cycles, or temperature variations.

The supply programme must take account of the fact that whilst

the resin appears to stop the action of the catalyst, over a longer

period the polymerisation continues to completion, prior to use.

The reaction of the two parts begins slowly and leads to the forma-

tion of polyurethanes, after they have been added to the sand.

There are therefore a few minutes during which the mixture runs

well. The initial slow reaction rate then accelerates quickly and the

hardening occurs almost simultaneously at the surface and in the

Fig. 2.4/A - Plant layout for the urethane No-Bake system, showing the three separate compo-nents storage and components feeds.

From the tank

Continuous mixer

Dosing pump

200 l. drum of

catalyst

Gear pumpsIsocyanate

Resin

From the tank


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30


centre of the piece. This means that the pattern stripping leaves a

perfect mould; and that the work time / pattern stripping time ratio

is very good.

Total polymerisation, that is the attainment of the maximum mecha-

nical strength, takes less than one hour. During the pour, the polyu-

rethane mould releases lustrous carbon and this may lead to anincrease of carbon in the surface layers of cast steels.

This problem can be overcome by adding 2 to 3% of black iron

oxide to the sand mixture, when the sand is being mixed.

Isocyanate releases nitrogen at pouring and this may cause pinho-

lesin cast-iron and steel castings.

The characteristics of the sand / urethane resin mixture are :

Production flexibility due to the excellent ratio of work time

to pattern stripping time;

The mould is not affected by moisture, nor by water paint,

impurities, variation of temperature or the pH of the sand;

It has a good resistance to heat; It is easily knocked out.

Specific isocyanate solvents are compatible with polystyrene.

Therefore polystyrene patterns can be used with the isocyanate

process.

The flow properties which the binder confers to the sand need to

be improved by using a vibrating table.

We recommend that 2 to 3% of black iron oxide or 1 to 2% of red

iron oxide should be added to the mould mixture, to reduce the risk

of gas defects in the casting when pouring steel. The binder should

be considered to be toxic.

2.2.3. THETHIRDGROUP:ALKALINEPHENOLRESINS

The components used in the so-called alkaline resinprocess are

an esterand an alkaline resoland they are basic.

The resol is a resin which is formed at the first stage of the conden-

sation process and it is a complex mixture of isomers and/or other

compounds.

The chemical reaction of these components is not catalysed. In

contrast to catalysed reactions therefore, the amount of reaction


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31


product and the speed of its formation are directly linked to the

quantity and the type of the reagents used. The reaction products

are a phenol resin, an alkaline salt and an alcohol.

The phenol resin formed polymerises partially at normal tempera-

tures and this action is completed by the heat of the pour.

In fig. 2.5 (page 24), the development of this reaction is shown,together with the pH of the phases.

The special characteristics of basic resins are :

They are less affected by the acid or alkaline nature of the

sand than acid catalysed resins. They can therefore be used

without encountering problems, even with olivine sand.

The setting time varies with the quality and not the quantity

of the hardener, as the chemical reaction which gives harde-

ning is not a catalysed reaction. It follows therefore that the

dosage of the hardener need not be precise, within reaso-

nable limits. Again the hardening reaction can be started

with a wide range of hardeners and these enable the harde-ning process reaction to be controlled. In practice, the

work timeof the sand mixture can be fixed at value betwe-

en a few minutes and an hour.

The mould is not rigid as the total polymerisation only hap-

pens when the mould is strongly heated by the casting pour.

The poured metal therefore finds the mould in a thermopla-

stic condition.

This has the following advantages :

The mixers can be cleaned more easily;

Optimum knocking out as the mould still has a degree of

flexibility; There is compensation for the heat expansion of the sand

and consequently there are fewer of the defects called fins

or veins caused by superficial cracks in the mould;

There is improved resistance to erosion, thanks to the

instant rigidity of the surfaces when they come into contact

with the liquid metal;

The amount of gas formed at pouring and the speed of its

formation are lower than with traditional resins; and the

gases do not contain either nitrogen or sulphur. These cha-

racteristics make this binder ideal for moulds used for

casting steel and spheroidal cast-iron.


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We wish to stress that as the moulds only harden completely when

metal pouring takes place, they are not very strong mechanically.

The mechanical regeneration of the sand used requires a very

effective regeneration plant as the alkaline residues stick firmly to

the sand grains.

2.2.4 RESINAGEING

As we have already said, resins are solutions of macromolecules,

and in the solvent mixture there are molecules which continue to

polymerise slowly at normal temperatures.

When the resins are used, the poly-condensation reactions are

stimulated by the catalyst used and speed up.

It is therefore essential to comply with the suppliers warnings and

storage advice to prevent the resins polymerising before they areused.

The most obvious marker of polymerisation, is that the resin beco-

mes more viscous.

The drums must be sealed hermetically to prevent solvent loss to

air.

The following analyses enable a rapid check to be carried out to

determine the state of the stored resin :

Refractive index;

Viscosity (to register any changes);

Decrease of the bending resistance over 24 hours, of a

sand/resin mixture, when compared with a standard mixtu-re;

A mixing check to verify that the resin mixes well with the

sand.

As ageing changes the concentration of free formaldehyde and

water, it is advisable to determine their concentrations chemically.

The changes registered when a resin ages are irreversible.

Furan resins age more slowly than the other types.

2.3 ADDITIVES


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A number of additives are used in the No-Bake system formula-

tions. They are used to meet technical and cost requirements, by

altering the characteristics of the basic components of the mix. The

most important of these are listed below, with indications of their

action and their use.

SILANES

The silanes used in foundry resins have a common formula of

R.Si (OR)3.

They are added to reduce the hygroscopic characteristics of the

mould and to improve the resin wetting of the sand grains.

The latter improves the mechanical strength of the finished

mould.

WATERWhilst there has to be water present as it is a product of polymeri-

sation, it must be kept as far as possible, within the limits imposed

by the process. Therefore any increases due to the addition of moi-

sture with other essential materials, must be kept to the absolute

minimum.

Apart from the cost aspects, water reduces the hardening time of

the mixture, can create blow hole defects in the castings and dra-

stically reduces the mechanical strength of the moulds (see fig.

2.6).

If follows that moulds tend to harden more rapidly at the surface

than in the centre due to the loss of water through evaporation (seefig. 2.7).

The application of water based paints must also be carried out as

late as possible, in order to guarantee the greatest degree of poly-

merisation of the resin.

IRONOXIDE

Iron oxides are mixed with the sand to reduce the occurrence of

the following defects in castings :

Pinholes due to gas absorption in the surface layers;

Cracks due to the heat expansion of the sand;


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34


Fig. 2.7- The differences of hardening time in surfaces exposed to air and not exposed to air inproducts using different amounts of binder.

HardnessGF

Mixture D Mixture C Mixture BMixture with

self setting

oil

Time in minutes from packing the core

Hardness of exposed surfaces

Hardness of surfaces against the corebox

Fig. 2.6- The effect of water on the resistance to bending stress, of a sand mixture containing1.3% of resin.

Transversestrenght

lbf/in2

H2O%


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Carbon enrichment in the surface layers due to the forma-

tion of lustrous carbon.

The action of iron oxide is the subject of some debate. The widest

held theory is that a compound which is highly resistant to heat is

formed on the surface of the sand grains. It is also believed that

Fayalite is formed due to a reaction with the iron oxide arising fromthe oxidisation of the casting metal. This oxide is always present in

regenerated sands. The compound formed has a low melting point

and fills the spaces between the sand grains, thus preventing

penetration by the poured metal.

The iron oxide used in the production of cores and moulds can be

one of two types :

Magnetite (Fe3O4), which is black. 2 to 3% is added with respect

to the amount of sand.

Haematite (Fe2O3), which is red. 1 to 2% is added with respect to

the amount of sand.

In choosing the most suitable type of iron oxide to use, there is apreference for using the black oxide, especially for the production

of large steel castings.

2.4 PHYSICAL-CHEMICALCHECKSONRESINS

The uniformity of the resins characteristics is clearly important for

the maintenance of production quality and production rates. It is

therefore necessary to determine both the tests to be carried out

and their frequency. It is also necessary to agree the methods andthe acceptable test results variations with the supplier, so that

there is agreement on an acceptable quality level.

This is indispensable given the variety of products, their different

uses and the differences between the analytical methods emplo-

yed. The checks carried out are both physical and chemical and

some of them require special equipment and have complex proce-

dures.

Simple checks are given below, which enable sufficient information

to be gathered to judge whether essential characteristics conform

to requirements.


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36


VISCOSITY-DENSITY

It is advisable to carry out the determination of the viscosity of very

viscous resins (above 1100 m. Pas.) by using a roto-viscometer

instead of Fords cup.

Variations in excess of 10% above the specified viscosity value,

and in excess of 3.5% above the specified density, constituteunacceptable quality.

The density is read using an instrument with an appropriate scale.

REFRACTIVEINDEX

The refractive index is one of the significant characteristics of resin

quality. It is a very useful indicator for evaluating the degree of poly-

merisation, the impurities and the amount of water (see figs. 2.8

and 2.9).

The simplicity of the instrument and of the analysis, means that an

initial selection of products can be easily carried out. Variation inthe refractive index of more than +/- 0.05 units, indicates the need

to carry out further tests.

Fig. 2.8 - The change in the refractive index of resin, as a function of condensation, measured atconstant temperature.

Refractiveindex2

0CD

Time in minutes at 70C 3

resin 1

resin2

resin

3


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The refractive index measurements are carried out with an Abbe

refractometer fitted with a constant temperature prism.

Fig. 2.9- Changes in the refractive index of resin, as a function of the percentage of water.

% water in the resin

Refra

ctiveindex(20CD)


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38



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39

CATALYSTS - HARDENERS

3. CATALYSTS - HARDENERS

Moulding with the No-Bake process is used for a wide variety of

types and quantities of castings. Castings may be very small or

very large, they may be one-off or in long series made on high

rate automated equipment. The binders are therefore required tohave constant hardening times which can be synchronised with the

various phases of the moulding production cycle.

The catalysts and the hardeners produce this conformity, and take

into account the external variables such as temperature, environ-

mental humidity, etc.

3.1 CATALYSTS

The term catalysis is used to describe the influence of substancescalled catalysts on the activation energyvalue and consequently on

the speed of chemical reactions. The action of the catalyst does

not alter the free energy (heat) involved in the reaction in any way,

even though they probably take a direct part in the reaction. At the

end of the reaction the catalyst is unchanged(2).

The term catalyst is often replaced (incorrectly !) by a synonym

such as acid or hardener or accelerator. In practical terms the

catalyst carries out an important function in speeding up the poly-

merisation of the binders, and in its effect on specified technical

moulding times.

The temperature of the sand, of the patterns and the environment(if high), all act synergically with the catalyst on the speed of the

reaction, whilst the quantity of water present acts in opposition to

the effect of the catalyst (see fig. 2.6 page 34).

The action of both factors must therefore be taken into considera-

tion when determining the type and quantity of catalyst to be used,

to achieve a polymerisation rate compatible with the phases of the

productive process; and especially with the moulding process.

Remember that a too rapid polymerisation leads to premature har-

dening and the mould will be friable, especially at the external

angles. In jargon this is called burnt.

The catalyst needs to be added to the sand before the resin and it

(2) - The action of positive catalysts on particles, is comparable to that of lubricating oils: theyreduce their resistance to motion.


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must be mixed very evenly throughout the mass.

Never mix resin and catalyst without sand as the exothermic reac-

tion is so violent, it is like an explosion.

If the catalyst is diluted this makes the distribution easier, however

this slows down the reaction rate.

If it is necessary to dilute the catalyst, always add the catalyst tothe water, never the contrary. The best solution is to buy the

catalyst at the correct dilution.

An excess of acid can accumulate in sands used many times

without correct regeneration.

In addition to its effect on the mould hardening process, it can

cause superficial defects of the type shown in fig. 3.1, due to a

reaction between the metal and the mould. As can be seen, the

upper part of the casting has a normal appearance (it was made

Fig. 3.1- Superficial casting defects (known as the orange peel effect) due to reaction between themetal and the mould, arising from the use of sand which has not been correctly regenerated.

using new sand

using highly

contaminated


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with new moulding sand), whilst the lower part is pitted (it was

made with poor quality recycled sand).

Figs. 3.2 and 3.3 show the variations in mechanical strength of a

sand mixture as a function of time, when different percentages of

41


Fig. 3.2- Changes in mechanical strength with different catalyst levels, as a function of time(PTSA=paratoluensulphonic acid).

Fig. 3.3- The mechanical strength of samples (all made from the same sand resin mixture., (1) with30% catalyst at 25C - (2) with 80% catalyst at 8 C), as a function of time.

Time in hrs

Time in hrs

30% catalyst at 25C

80% catalyst at 8C

Compressivestrenghtlb/in2

Compressivestrenghtlb/in2

(I)

(II)


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42


catalyst are used, both at constant temperature and variable tem-

perature.

Each lot of catalyst must have its concentration checked, either by

direct titration, or indirectly by measuring the density.

The dosage must always be checked at the time of use. This is

essential given the determining effect of the catalyst on the poly-merisation process and on the uniformity of the production process

rate.

Organic sulphonic acidsare the most commonly used catalysts with

furan and phenol resins, either paratoluenesulphonicacid or benzene-

sulphonic acid. These are replacing the use of mineral acids such as

phosphoric acid or sulphuric acid to an ever greater extent.

Phosphoric acid at a concentration of 70 to 80% is still used with

furan resins, however its use is dying out due to the difficulty of

removing it both by heat (starting with the pour) and by the regene-

ration processes. Furthermore, it has longer reaction times than

those which can be achieved using organic acids.Sulphuric acid is used as an activator in synergic combination with

other acids.

All sand types are compatible with acid catalysts, except olivine

sand (due to its basic characteristics).

The urethane resins are catalysed by pyridine derivatives (a basic

organic compound) and these can be added to the resin at the

binder production phase. This determines the speed of reaction

and it cannot be altered during use.

The viscosity of the phenol resin/catalyst mixture is affected by

temperature; and it is recommended that when it is used there

should be temperature control. It is also recommended that itshould be placed in the mixer before adding the isocyanate.

The technical schedules in Part II give the characteristics of the

catalysts normally used with the resins described. The normal pre-

caution in handling acids should be followed for catalysts.

3.2 HARDENERS

Hardeners are chemical compounds which, unlike catalysts, take

part in the chemical reaction as specific components. They are


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43


used in the correct stoichiometric amountsto form a solid compound

with the other components. Esters are the most commonly used

hardeners.

3.2.1 ESTERS

The combination of alkaline phenol resin - ester appeared during

the mid 80s and took a small share of the No-Bake market. Its

market share was limited by the limited mechanical strength of the

moulds made with it. This was due to the low concentration of the

resin in its natural solvent (water).

The typical arrangement of the atoms in an ester is shown by the

following formula :

R-C-0-R1

II

0

R and R1 are aliphatic radicalswhose precise nature can be varied.

Esters which are typically used are : acetates (di-acetates and tri-

acetates of glycols and triacetin) and propylene carbonate.

Generally speaking, esters are formed by the reaction of an alcohol

or glycol with an organic acid, and water is produced as a by-

product.

THEUSEOFESTERSINTHEALKALINENO-BAKE SYSTEM

The quantity of ester used is 15 to 20% of the quantity of resin. The

quantity of resin used is between 1,4 and 1,8% of the quantity of

sand.

The types of hardener which can be used, enable the mould to be

stripped out from 12 to 15, at 20C.


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44


THEPOURINGPROCESS

The heat given out by the metal breaks down the resin and

hardener molecules. The combustion products are those normally

produced by the strong oxidation of organic binders : carbon-di-

oxide, carbon-mon-oxide, water vapour, saturated and unsatura-

ted hydrocarbons (both aliphatic and aromatic).It is not possible to make either qualitative or quantitative forecasts,

it is necessary to make specific analyses. These compounds are

volatile and leave the system, while the alkaline ions from the ester

salts remain behind. These diminish the refractory properties of the

silica sand and make its regeneration difficult.

THEREGENERATIONPROCESS

Heat regeneration is not possible as the alkaline ions do not leave

the system and are therefore not eliminated. The alkali stays atta-

ched to the fissures and roughness of the sand particles and redu-ces their refractory characteristics.

Concentrations of these alkaline oxides must not exceed 0.12%

as this would affect the work time of successive cycles.

Alkaline sand can be regenerated mechanically, and wet regenera-

tion is also possible. It needs to be remembered that the wet pro-

cess produces contaminated water and this must be treated befo-

re discharge.


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46


The reaction can be represented as follows :

2Na+ + SiO3-- + H2O SiO2+2Na

+ + 2 (OH-)

The siliceous acid has a very low dissociation constant, so when

its negative charge is neutralised it tends to precipitate as a colloid.

That is to say it precipitates in an extremely dispersed state withinthe sand mass and finally coagulates.

2) The ester-soda ionic reaction.

For the sodium silicate-ester reaction the ester has been chosen

for its solubility in the silicate. The hydrolysis of the ester proceeds

rapidly as shown below and liberates its salt and alcohol. Both of

these exert a strong gelling action :

R- COO - R1+NaOH R COO Na+R1OH

ester salt alcohol

The dynamics of this reaction are as follows :

Decrease of the pH value due to the removal of sodium ions

by salt formation

Transformation of the silicate ions into poly-siliceous acid

which precipitates, due to its high instability

Chemical drying due to the removal of water by the alcohol.

This reduces the amount of water available for silicate dilution

and leads to a stronger agglomeration of the colloid

Strong silica gel stability due to the dehydration by the alco-

hol

4.1.1 SETTINGTIMES

The silicate neutralisation is not instantaneous therefore the setting

reaction is progressive and gives a long work time to the sand

mixture (that is a long period in which it can be worked).

The mould hardening reaction is affected by external factors such

as the sand temperature, the environment temperature and the

humidity. In the case of the manufacture of widely differing moulds

it may be opportune to control the hardening process by blowing


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47

SODIUM SILICATE

with hot air, possibly enriched with CO2.

4.2 THETYPEOFSODIUMSILICATE

Sodium silicate is an amorphous mineral substance consisting of

silica, soda and water in varying amounts. Its composition is defi-

ned by its modulus or weight ratios of SiO2/ Na2O. For example,a silicate with the following composition :

SiO2=30%; Na2O=10%; H2O=60%

This is a silicate with the following characteristics :

Modulus, or weight ratio : SiO2/ Na2O : 30/10 = 3

Dry extract = 40%

Foundry practice shows that in the silicate-ester process, the sili-

cates with a modulus higher than those used with the silicate-CO2system give the best results.

In choosing the correct modulus for the type of production to be

carried out it must be remembered that as the modulus value incre-

ases, the setting rate increases, the initial mechanical strength ofthe mould increases, and therefore the difficulty of knocking out

increases.

The amount of silicate to be mixed with the sand is determined by

a number of factors including :

The modulus

The fineness of the sand (in relation to the surface area of the

grains to be coated);

The type and quantity of the additives

The temperature and humidity

The eventual presence of clay in the sand (if this is more than

1% it means that more dilute silicates with a lower modulusmust be used).

4.3 THETYPEOFESTER

The silicate hardening can be achieved with two procedures which

use different hardeners. Briefly the processes are :

Variation of the silica-soda ratio by neutralisation of part of the

free soda using either an acid ion or radical. Substances

which can be used for this are : carbon-di-oxide, silicon,


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48


esters, zirconium fluosilicate and similar substances and

glyoxal;

Removal of the formulation water and therefore the dehydra-

tion of the silicate, using cement, blast furnace ash, plaster,

calcined dolomite and also by the action of carbon-di-oxide.

In the No-Bake moulding process the use of esters is preferredas these are the best compounds for the neutralisation of the free

soda, they increase the silica-soda ratio (by the ester free radical),

and remove water from the silicate.

Several formulas are used, with a well established preference for

using mixtures of acetates of poly-alcohols.

For example we show the reaction of glycerol triacetate with

soda:

C3H5(OO CC H3)3+ 3NaOH3 CH3COONa+ C3H5(OH)3Setting takes place in the cold and the setting time depends on the

type of acetate used. Glycerol di-acetate gives rapid hardening

whilst glycerol tri-acetate is relatively slow (see fig. 4.1).Using measured mixtures of the two reagents, a series of interme-

Fig. 4.1- Changes in the mechanical strength of sand bonded with silicate as functions of time andthe type of hardener used.

Time in hrs

Group 1

Group 2 Group 3

Type A

Type A

Rc,indaN/cm2


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49

SODIUM SILICATE

diate setting times can be established.

4.4 ADDITIVES

The method of binding the sand with silicate may have the fol-

lowing drawbacks :

Maintenance of mould characteristics during storage Difficult mould shake-out for certain types of casting

The need to improve the surface quality of castings with

certain types of alloys

The size of the problems inherent in this procedure mainly relate to

the type of metal cast in the mould. At pouring, the sand grains

become coated with a film of glass consisting of anhydrous sodium

silicate and this effectively sinters the sand grains.

This sintering is determined by the intensity of the heating to which

the sand grains are subjected, the length of the heating time and

the size of the sand grains.

In contrast to many organic binders, silicate increases its cohesiveproperties after heating. Therefore, to make knocking out easier,

various organic additives are used. These burn and leave disconti-

nuities in the glassy mass. This device is particularly important to

make the break-down and removal of closed cores easier.

Additives can therefore be divided into knocking-out additives and

additives with binding properties :

Knocking out additives may be : coal, pitch, sugar, molasses,

glucose and sawdust

Additives for increasing binding properties : specially treated

phenolic resins increase the storage strength and improve the

flow properties of the sand mixtures Black iron oxide improves the moulds resistance to molten

metal penetration

4.5 CARRYINGOUTTHEWORK

4.5.1 MIXPREPARATION


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The silicate-ester process requires the use of dry sand which is not

hot, or which is at least at a controlled temperature.

The addition sequence is as follows :

additives

esters

silicate : as the silicate is relatively viscous it is recommen-ded that it should be dosed using a volumetric pump, and

that it should be mixed effectively, whilst avoiding overhea-

ting the sand to prevent water loss. In hot weather a little

water may be added to compensate for the evaporation

which takes place during mixing. Alternatively a less con-

centrated silicate can be used.

4.5.2 MOULDING

During hot weather, it may be advisable to add a little water (0.5 to

1.0%), to compensate for evaporation, or to use a less concentra-

ted silicate solution.

The hardened mould has very little elasticity, therefore the pattern

must have a carefully designed draft angleand smooth surfaces.

The relatively poor flow properties of the sand-silicate mass, means

that there must be an adequate compacting action to ensure that

the mixture fills all the space around the pattern.

Painting moulds with water based paints weakens the strength of

the painted surfaces. Alcohol based paints are more suitable for

this purpose..Moulds exposed to the air deteriorate, as their mechanical charac-

teristics are weakened by water absorption (due to their hygrosco-

pic nature).

4.6 SILICATECHECKS

The maintenance of regular production cycles requires that in addi-

tion to the silicate checks (physical and chemical), the sand quality

must be checked. This is particularly important if regenerated sand


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is being used.

The checks to be carried out are :

The sand temperature

The alkalinity

The quantity of fines in the sand

CHEMICALCHECKS

The determination of SiO2and Na2O to calculate the modulus of

the sodium silicate, can be quickly carried out by a simple titra-

tion.

The method is acceptable for quality conformity checks. The deter-

mination of the amount of dry weight, is carried out by determining

the difference of weight before and after ignition at 600C.

PHYSICALCHECKSSodium silicate is composed of three compounds. However, the

determination of any two of the characteristics listed below will be

enough to establish the conformity of the product :

Viscosity;

Density (degrees Baume);

Solids content;

The silicate modulus, that is the ratio SiO2/ Na2O;

The silica content (SiO2);

The sodium oxide (Na2O) content.

The diagrams shown in figs. 4.2 and 4.3 show the relationships

between density, degrees Baum, the sodium content, the SiO2/Na2O ratio of the sodium silicate and the solids content.

You are referred to Part II for the analytical methods (see schedules

M6 and M7).

MECHANICALCHECKSONTESTPIECESOFBONDEDSAND

The technical tests carried out to determine the mechanical cha-

racteristics of the various mixtures, enable indirect evaluation of

the silicate characteristics; together with direct evaluation of its

mixture with sand.

To carry out a correct evaluation, the mixture must be made without

51

SODIUM SILICATE


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any heating and ensuring that no water is lost by evaporation

during the various stages. This means that the sample of the mix-

ture must be quickly placed in a sealed container, and that the test

pieces must be prepared rapidly.

An exposure to air of about two minutes may cause hardening to

52


Fig. 4.2- The viscosity of sodium silicate as a function of the R modulus, for different concentra-tions of dry material (MS%).

R modulus

Non

viscousliquids

Viscousliquids

pastes

solid

Viscosity

in

poises


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53

SODIUM SILICATE

begin, due to the evaporation of water and absorption of atmos-

pheric CO2.

5. THE SANDS

Fig. 4.3- The relationship between density, the Baum degrees, viscosity, the dry material content,the soda level and the silica/soda ratio in the sodium silicate.

ModulusSiO2or Ratio -------

Na2O

Drymaterialcontentasa%age

Density Degrees Baum

Visco

sit


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54



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mely important for the No-Bake process . The acid catalyst can

react with any alkaline compounds which may be present in seasands, or with metallic particles or oxides. In this event even an

excess of acid will not prevent the unwelcome consequences ari-

sing from their presence.

This is due to the following reasons :

The development of CO2due to the decomposition of car-

bonates, may break the hardened resin film.

The acid-oxide reaction is slower than the setting action of

the binder and the newly formed resin film will be damaged

by it.

The presence of clay reduces setting times and also lowers the

mechanical strength of moulds.

56


Fig. 5.0- Crests due to sand expansion not contained by the binder content.


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THE SANDS

A dry sand with rounded grains, a low percentage of fines and a

clay content of less than 0.2% is acceptable for the No-Bake

moulding process if proper allowance is made.

The optimum sand characteristics, both new and regenerated, for

use in mould and core production; are given in table A (page

117).

OLIVINESAND

This material is 93% magnesium orthosilicate (known as Forsterite).

Its chemical composition is Mg2SiO4.

Olivine sand is the main sand used for casting steel containing

12% manganese, as it does not support a metal-mould reaction.

The castings do not therefore suffer from the surface defects which

are so typical when silica sand moulds are used.

This sand is basic (pH = 9 approximately). It cannot be used with

acid catalysts, as these attack it, even in the diluted state. The acidconsumed by the attack would be removed from its role as a

catalyst and this would affect the hardening rate of the resin.

This sand has optimum refractory characteristics and these make

it ideal for steel casting.

Its grain fragility limits the number of times it can be regenerated

mechanically.

CHROMITESAND

Chromite sand is a sand with very angular grains. It consists of a

mixture of spinels :

FeO Cr2O3, MgO Cr2O3, MgO Al203

This sand has a very high thermal conductivity, a low thermal

expansion and has excellent refractory properties.

It is mainly used to solve metal penetration problems and as a chill,

in those parts of the casting which might be most affected by

microporosity.

It is also used for making cores whenever the use of silica sand

might lead to difficulties of removal.

Whilst it is slightly basic (pH = 7 to 8), it is compatible with all typesof binder as it has a marked chemical inertia.


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Chromite reacts with silica both in sand and in paints, and is itself

changed if polluted with silica sand.

This is due to the following reaction which takes place at tempera-

tures above 600C FeO Cr2O3decomposes into FeO and Cr2O3.

Part of the FeO oxidises to Fe2O3. This in its turn forms a solid

solution with the Cr2O3. This forms a sealant barrier against metalpenetration and eliminates defects such as metal penetration and

fins.

If any SiO2is present it can react with the above iron oxide to form

Fe2SiO4 (fayalite) which is a low melting point compound. Large

amounts of fayalite at high temperatures may cause sand encru-

sted castings.

Therefore, moulds in chromite sand protected with quartz based

paint and chromite sand contaminated with silica sand in excess of

2%, can give rise to castings with sintered surface adhesions. (See

Vol. II, S-5).

Fayalite formation also occurs when silica sand is contaminatedwith chromite sand. These reciprocal contamination problems

must be remembered when designing sand regeneration plants.

ZIRCONSAND

This is a sand composed of zirconium silicate (ZrSiO4). This is che-

mically and thermally inert and does not react with metals.

It has a high thermal conductivity and a high thermal capacity due

to its density. This increases the cooling speed of castings to about

four times the rate when silica sand is used.

The grains are rounded and there are no fines. This means that themoulds have high mechanical strength. However, zircon sand is

extremely uniform in grain size, these being distributed across very

few mesh sizes. This can cause metal penetration defects.


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THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SAND MIXTURES

6. THE PHYSICAL AND CHEMICALCHARACTERISTICS OF THE SANDMIXTURES

This section gives the characteristics common to all the sand mix-

tures, which affect the properties of moulds/cores and the qualityof castings.

You are referred to Part II for the control methods.

6.1 SANDCHARACTERISTICS

The morphology and composition of the sand strongly affect the

quantity of binder required and its polymerisation process. This has

a marked effect on the mechanical strength of the mixtures. The

acidity or basicity of the sand is a very important factor, whetherdue to its original nature or induced by accumulated impurities.

Olivine sand is a good example - it is not compatible with acid

catalysed binders due to its basic nature.

Angular sand grains of the same grain size as rounded grains, have

a greater surface area and therefore require more binder, they have

worse flow characteristics; and due to the breakage of the grain

projections they produce more fines fractions.

GRANULOMETRYANDTHEFINENESSINDEX

The fineness indexof a sand has significance when examined toge-ther with the granulometric spectrum (see fig. 6.1). In fact, if two

sands with the same fineness index are compared, and one has a

grain distribution across only two sieves, and the other has a grain

distribution across four or five sieves, the former will require less

binder. This is because it has a smaller surface area to be covered.

It should be remembered that a variation of five units of fineness,

can result in a fall of a few kg./cm.2in the value of the mechanical

strength of a sand/binder mixture. This variation can easily be attri-

buted to an instrument error, by mistake.


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THESPECIFICSURFACEAREAOFTHEGRAINS

It is important to extrapolate the total surface area of the sand

grains from the granulometric values. This enables the binder

requirement to be assessed (see the technical schedule MO1 in

Part II).

The influence of the specific surface area of the grains, and there-

fore of the fineness index on the consumption of binder is quite

clear. The calculation enables us to follow the deterioration of a

sand through its regeneration cycles; and the following formulaenables us to determine the effect of the variation of the fineness

index of a sand, on the consumption of binder.

Where :

AR = the percentage of binder on the recovered sand

AN = the percentage of binder on the new sand

IFR = the fineness index AFS of the regenerated sand

IFN = the fineness index AFS of the new sand

60


Fig. 6.1- Sands with the same fineness index but with very different granulometric distribution.

FINENESS INDEX: 67,70 FINENESS INDEX: 67,70

IFRAR = AN

IFN


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MOISTURE

Moisture values above 0.1% are very detrimental to the mechanical

properties of the binder mixture (see fig. 2.6-page 34)

FINES FRACTIONSThe sand impurities are usually concentrated in the fines. The fines

may interfere with chemical reactions and certainly increase the

binder requirement due to the increase of surface area, which

needs to be covered with binder. If the quantity of binder is kept

constant, the fines reduce the mechanical strength of the moulding

mixture (see fig. 6.2).

The fines increase in sand with poor thermo-mechanical characte-

ristics (i.e. a fragile sand). This increase is made worse by pneuma-

tic transport and by regeneration processes.

A poorly regenerated sand which has a high fines content, is also

probably contaminated with acid residues, oxides and oolites.These may cause casting defects, due to reactions between the

poured metal and the mould, as shown in fig. 3.1 (page 40).

The flow properties and the permeability, of a sand, both decrease

as the fines fractions increase.

61


Fig. 6.2- Loss of resistance to bending of a mixture with 1.2% of resin, as the percentage of finesvaries.

Fines %

Resistance to bending

Res

istance

to

bend

ingl

b/in2

bar


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LOSSONIGNITION

Sand recovery is necessary both for cost and environmental rea-

sons. It is essential that recovered sand has a low percentage of

fines. As already mentioned, these consume part of the binder and

reduce the mechanical strength of the mould. Moreover, at pouring

the volatile component of the fines develops gases which can beabsorbed by the metal.

The volatile component consists of unburned or cracked binder

and catalyst from the previous cycle(s). One of the parameters for

the evaluation of volatile residues is the loss on ignition (LOI). The type

of metal being cast, the composition of the above residues and

their nitrogen and hydrogen contents, set the limit for acceptable

loss on ignition.

THEACIDDEMANDVALUE

It is important to know the degree of alkalinity of the sand, in orderto assess the extra acid to use, over and above that required by the

resin. In other words it is necessary to know the acid demandvalue

(ADV) to neutralise the sand.

The pH value, or the acid demand value, enable us to evaluate the

degree of sand contamination with basic materials. These neutrali-

se part of the catalyst and slow down the hardening process (as

the catalyst is normally a weak acid).

The acid demand value of a new sand should be below 0.5 cc of

N/10 HCl per 100 g. of sand. The acid demand value of a regene-

rated sand should be less than 5 cc of N/10 HCl per 100 g. to give

a pH of 6 to 8.The acid demand value of a sand must be continually monitored,

in order to be sure that the amount of catalyst used will lead to

hardening in the required time.

THEBASEDEMANDVALUE

The above considerations also apply, to evaluating the degree of

sand contamination with acids. These acids speed up the harde-

ning process.


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CLAY

A clay content above 0.1% is enough to alter the work time of a

sand mixture. If the sand is washed with recycled water with a high

clay content, the sand grains become coated with a clay film which

is very difficult to remove. In this event it is necessary to install

equipment for moving the sand mass so that this film can be bro-ken by attrition.

THEDEGREEOFOOLITECONTAMINATION

Sand which has been regenerated several times, has a degree of

contamination such that oolites may form due to the heat at pou-

ring and at heat regeneration. These oolites may be siliceous, side-

riferous (iron carbonate), phosphatic (tri-calcium phosphate) or

ferruginous (iron silicates and oxides).

All these compounds have low melting points and react with cer-

tain metals. They therefore cause surface defects on castings dueto a metal-mould reaction.

TEMPERATURE

The rate of chemical reactions, including catalysed reactions, is

directly related to the temperature of the reagents. This means that

the uniformity of the various hardening stages in mould forming, is

absolutely dependent on temperature control and stability.

This is true for the sand, the pattern plates, the reagents and the

work environment. The degree of response to variation in reaction

temperature, is a characteristic of each binder and each catalyst.To stress the importance of temperature control : one can make a

general statement that a 10C. increase will halve the hardening

time, and a 10C. fall will double it.

Fig. 3.3 (page 41) shows the relationship of time to mechanical

strength, in mixtures with different percentages of catalyst, added

to mixtures at different temperatures. The diagram clearly shows

that the effect of temperature on mechanical strength is greater

than that of the amount of catalyst used.

6.2 THEHARDENINGPHASES


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The resin hardening reaction must never be disturbed and this is

the reason why the sand mixture must be made and used as quic-

kly as possible, before the polymerisation really takes hold.

To assist processing, continuous mixers are recommended. The

components are added to these in the sequence : sand, additives,hardener and finally resin.

The addition process for the hardener should be automated, com-

puterised if possible.

Vibrating tables or shooting machines ensure that the sand is com-

pacted as quickly as possible.

The critical phases of the hardening process are : the work time

(bench life) and the strip time.

6.2.1. THEWORKTIME

The resin hardening reaction must be slowed up as much as pos-

sible in its early phase, to enable the mould to be completed. The

parameters which govern the reticulation process must therefore

allow a sufficient work time (bench life).

The evaluation of the work time fixes the maximum time for using

the resin mixture and it should not be used beyond this. Use after

this time will result in very poor of flow properties and excessive

reduction of the mechanical strength of the final product. The work

time shortens as the polymerisation of the resin-catalyst mixture

becomes faster at normal environmental temperatures.The work time of a sand resin mixture is evaluated in the labora-

tory and is controlled empirically in the moulding shop.

There are two laboratory methods :

by reactometry (see the technical schedule M4 in Part II).

by measuring the reduction of the resistance to bending.

The test consists of making a series of test pieces from the

mixture, at regular time intervals, under constant temperatu-

re conditions. After 24 hours the test pieces resistance to

bending is checked and the reduction trend is noted. The

time between the preparation of the mixture and the timewhen the test piece is prepared, which corresponds to a

64



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pre-selected value of resistance reduction (normally around

30% reduction compared with the maximum value obtaina-

ble), is the work time of the mixture being examined, under

those conditions.

Refer to technical schedule R for the data on work times of sand-

binder mixtures, with the most commonly used resins.The evaluation of the work time can be carried out empirically by

watching the changes in a lump of mixture exposed to the air under

dry ventilated conditions. The time taken for the surface of the

lump to form a weak crust, which is slightly resistant to the touch,

but is clearly evident, is the work time.

6.2.2. THESTRIPTIME

The second characteristic of a good sand resin mixture, is that it

should harden quickly to enable the mould to be stripped. This

length of time is called the strip time.

Once the mould box has been filled, the mixture should set as

quickly as possible to a consistency which enables the mould to

be stripped without causing any dimensional changes or excessive

deformation of the piece.

Clearly there is a conflict between the requirements of work time

(bench life) and strip time. The ratio between these must be as

high as possible, as the moulding phase normally requires

more time than the preparation and stripping.This ratio depends on the resin type, on the catalyst type and

quantity, on the sand quality and temperature, and is a key factor

for the production rate.

In practice an evaluation of the strip time can be made by asses-

sing the resistance to penetration by a wire probe.

The handling time of the mixture is also affected by its flow charac-

teristics, and by the mechanical equipment (vibrating tables or

shooting machines).

The binder should therefore have properties which permit the sand

mix to form the precise shape of the model and to fill the most

inaccessible corners of the mould quickly. A mould or core which


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has been well compacted has a better chance of lasting unchan-

ged, even during a long storage period.

6. 3 MOULDAGEING

It is advisable to store moulds before use, to ensure that the poly-

merisation process is complete and that the excess solvent has

evaporated. This precaution also reduces the amount of gas pro-

duced during pouring. In automated moulding cycles, it is therefo-

re necessary to ensure that there is a sufficient time lapse between

moulding and pouring.

It is also necessary to take into consideration a possible degenera-

tion of the mould or core characteristics during ageing. When the

air has a high relative humidity and its temperature is low, the resin

reticulation deteriorates. The degeneration phenomenon is moremarked when there is a low percentage of binder in the mix.

The damage caused by moisture absorption is irreversible. For

more details see point 3.2 in the second volume. A number indica-

ting

no+bake+ing.+vol+ii

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