Metal Casting: Appropriate Technology in the Small Foundry

240 Pages·1996·14.58 MB·English

by  Steve Hurst

This technical introduction to small-scale non-ferrous casting provides practical
information and advice for metalworkers, engineers and manufacturing
entrepreneurs who are interested in metal casting on a small scale.
Illustrated instructions are provided on how to make equipment using local materials,
and on the various casting techniques. These include green sand moulding - low-
cost and relatively simple, encompassing different methods of sand casting
appropriate to various types of product - and investment casting, which is an older,
more versatile and precise technique but is more time-consuming. These and other
methods are described and compared along with advice on design and pattern
making, core mixtures, the use of wax, metal preparation and melting, fault finding
and remedies, and kilns and furnaces.
The emphasis is on local construction of equipment, often using recycled materials
- there is a useful section on scrap materials and how to identify and use them -
and on presenting a wide range of methods, traditional and modern, based on the
author's extensive practical experience.
This is an essential addition to the bookshelf of the small rural workshop or
urban engineering business.
ISBN 1 85339 197 2
Steve Hurst has his own small foundry in Oxfordshire, UK. He works as a foundry consultant
to various overseas charities and as a visiting lecturer in the UK, and produces sculpture
which is exhibited widely.
The Intermediate Technology Development Group was
founded by the late Dr E.F.Schumacher. Intermediate
Technology enables poor people in the South to develop
and use skills and technologies which give them more control
over their lives and which contribute to the sustainable
development of their communities.
Intermediate Technology Publications is the publishing arm of the
Intermediate Technology Development Group and is based at
103/105 Southampton Row, London WC1B 4HH, UK.
Pouring gun-metal (bronze LG2) into moulds for Pelton turbine wheels, using the
Roman technique of lost-wax casting
METAL CASTING
Appropriate technology in the small foundry
STEVE HURST
INTERMEDIATE TECHNOLOGY PUBLICATIONS 1996
Intermediate Technology Publications Ltd,
103-105 Southampton Row, London WC1B 4HH, UK
© Intermediate Technology Publications 1996
A CIP record for this book is available from the British Library
ISBN 1 85339 197 2
Typesetting by Diamond People, Bromyard
Printed in UK by SRP, Exeter
Contents
Preface ix
Acknowledgements x
1 Introduction 1
Making a choice of production method 1
Advantages and disadvantages of each process 4
Examples 8
2 Setting up the workshop 10
The small workshop or sand foundry 11
The workshop for a lost-wax foundry 12
Examples of foundry designs 13
3 Pattern making 15
Materials 15
Skills 18
Designing the pattern 18
Storing patterns 23
4 Flexible moulds for the mass-production of wax patterns . . .. 25
Making the case for a flexible mould 26
Making a rubber mould 28
Gelatine moulding 32
Mixing plaster 35
Summary 36
5 Sand casting 38
Essential moulding equipment 39
Making a simple, solid casting 40
Making a simple casting using a split pattern 47
Hollow casting 49
The nature of moulding sand 54
Other sand processes 58
6 Lost-wax casting 60
Waxes 62
Moulding the wax 65
The block investment process 67
Kiln firing (de-waxing) 76
The clay slurry investment method 78
Inca silver 83
4
7 The ceramic shell process 85
Equipment 86
The ceramic shell process in outline 87
The process in detail 87
8 Die casting 102
Examples of the technique 102
Finding and correcting faults in the castings 104
9 Melting and pouring metal 107
Summary 107
Safety precautions 110
10 Metallurgy and the low budget foundry Ill
The structure of metals Ill
Selection of scrap metals for casting 114
Alloying 117
Metal melting 119
Metal flow and solidification 122
11 Kilns and furnaces 126
The furnace drum 127
Kilns 135
Crucibles 141
Conclusion 146
12 Faults, fettling and finishing 147
Common faults in castings 147
Fettling and finishing 151
Appendices
A Newar casting in Patan, Nepal 157
B Bamoum lost-wax casting, western Cameroon 163
C The Dedza crucible 171
D Sand conditioning and testing 175
E Other sand processes 181
F Summary of ceramic shell investment processes 185
Further reading 195
Glossary 199
VI
"The founder is always like a chimney sweep, covered with charcoal and distasteful
sooty smoke, his clothing dusty and half burned by the fire, his hands and face plastered
with soft muddy earth. To this is added the fact that for his work a violent and
continuous straining of all a man's strength is required, which brings much harm to his
body and holds many definite dangers to his life. In addition, this art holds the mind of
the artificer in suspense and fear regarding its outcome and keeps the spirit disturbed
and almost continuously anxious. For this reason they are called fanatics and are
despised as fools. But, with all this, it is a profitable and skilful art and in large part
delightful.'
From Biringuccios De la Pyrotechnia, Venice, 1559
vii
Traditional Newar method of lost-wax casting using a clay-dung slurry investment
viii
Preface
Dr E.F. Schumacher was one of the first economists to understand and consider the
wisdom and skills which were cast aside or destroyed in the rush to develop higher and
higher technology, and to have the wisdom to revitalize and record these skills. In Small
is Beautiful, Dr Schumacher used the symbols of capital value to illustrate levels of
technology: 'If effective help is to be brought to those who need it most, a technology
is required that would range in some intermediate position between the £1 technology
and the £1000 technology. Let us call it- symbolically speaking - a £100 technology'.
He continued: 'Such a technology would be immensely more productive than the
indigenous technology (which is often in a condition of decay), but it would also be
immensely cheaper than the sophisticated highly capital-intensive technology of modem
industry'.
If you look at the small investment casting foundries of India, South-East Asia and
West Africa you find models of the type of technology that Dr Schumacher describes.
Using the minimum capital, abundant skilled or semi-skilled labour, simple machinery
and materials found locally, these traditional co-operatives and small family businesses
manufacture objects of great precision. The objects that they make, usually religious
or tourist sculpture, have sometimes blinded engineers and business advisers to the
possibility of making other objects, such as irrigation, agricultural, micro-hydro
electric or medical items, which are within their wide range of skills.
The technique we now call sand casting, or green sand moulding, was developed
much later than investment (or lost-wax) casting. The former is now common to cities
and small towns all over the developing world. It was introduced in the nineteenth or
early twentieth century, invariably for the repair or replacement of railway or river-boat
engines, or the replacement of weapon or vehicle parts.
Investment casting, by contrast, is indigenous. Though it developed in all the early
civilizations, it is now only found in its original form in a few places. Its grandchild,
the ceramic shell technique, is to be found wherever there is an aerospace or weapons
manufacturing industry.
If I concentrate on Nepal, South-East Asia, North and West Africa, it is because I
observed and researched foundries in these areas. Another observer might have cited
Sri Lanka, China or Indonesia. Although in the pre-Colombian civilizations South
America possessed sophisticated and highly developed metal casting industries, these
were destroyed, together with agricultural and irrigation systems. Today only small
fragments of these technologies remain.
iX