Metal Casting

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a thorough how to with plenty of detail on most methods used in a small foundry

Text of Metal Casting

Pouring gun-metal (bronze LG2) into moulds for Pelton turbine wheels, using the Roman technique of lost-wax casting

METAL CASTINGAppropriate 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

ContentsPreface Acknowledgements 1 Introduction Making a choice of production method Advantages and disadvantages of each process Examples Setting up the workshop The small workshop or sand foundry The workshop for a lost-wax foundry Examples of foundry designs Pattern making Materials Skills Designing the pattern Storing patterns Flexible moulds for the mass-production of wax patterns Making the case for a flexible mould Making a rubber mould Gelatine moulding Mixing plaster Summary Sand casting Essential moulding equipment Making a simple, solid casting Making a simple casting using a split pattern Hollow casting The nature of moulding sand Other sand processes Lost-wax casting Waxes Moulding the wax The block investment process Kiln firing (de-waxing) The clay slurry investment method Inca silver ix x 1 1 4 8 10 11 12 13 15 15 18 18 23 . . . . 25 26 28 32 35 36 38 39 40 47 49 54 58 60 62 65 67 76 78 83

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The ceramic shell process Equipment The ceramic shell process in outline The process in detail Die casting Examples of the technique Finding and correcting faults in the castings Melting and pouring metal Summary Safety precautions metals casting

85 86 87 87 102 102 104 107 107 110 Ill Ill 114 117 119 122 126 127 135 141 146 147 147 151 157 163 171 175 181 185 195 199

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10 Metallurgy and the low budget foundry The structure of Selection of scrap metals Alloying Metal melting Metal flow and solidification 11 Kilns and furnaces The furnace drum Kilns Crucibles Conclusion 12 Faults, fettling and finishing Common faults in castings Fettling and

for

finishing

Appendices A Newar casting in Patan, Nepal B Bamoum lost-wax casting, western Cameroon C The Dedza crucible D Sand conditioning and testing E Other sand processes F Summary of ceramic shell investment Further reading Glossary

processes

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"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

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Traditional Newar method of lost-wax casting using a clay-dung slurry investment

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PrefaceDr 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 i t - 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-hydroelectric 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.

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AcknowledgementsThis book owes its existence to so many people that it is difficult to know where to start. Firstly, and most recently, to the staff and consultants of ITDG with whom I worked overseas during the last five years, particularly to David Poston whose energy and enthusiasm started the process of collecting and writing down the techniques needed to set up and run a small foundry. To go back to my own early experience, my gratitude and thanks to the late, great bronze caster Albert Angeloni who taught me lost-wax investment casting at the foundry of the Royal College of Art (RCA), London; to the staff of the Midland Foundry Industries Training School, West Bromwich, who taught me the theory and basics of sand casting; to Ernie Nelson, Senior Lecturer in charge of the foundry, Belfast Technical College, Northern Ireland, for his experience of heavy industry and large iron castings, and for wise and tolerant advice. For the important and rewarding task of 'taking technology off the back shelf, museums have been vital stopping places, and I should like to thank Janet Peatman, Peter Smithhurst and Nicola Moyle of the Abbeydale Industrial Museum and Kelham Island Museums, Sheffield. My thanks to Sir Eduardo Paolozzi, at the RCA, for encouraging my interest in traditional West African casting over many years. Partner organizations form an essential part of the work of ITDG overseas, so I wish to thank Tony Wilce who worked for both GTZ and ITDG in Malawi; Rajan Tappah and the staff of the Agricultural Development Bank in Nepal; Bob Buckner, Engineer IC workshops. United Missions Nepal at Butwal, Nepal; Thomas Scheutzlich, Engineer of GTZ, Cuzco, Peru; the Director Jose Muniz and the staff of Asiacion Civil PROMIDHEC in Cuzco, Peru. I shall not forget, nor cease to be grateful for, the help given me by colleagues in ITDG: David Poston, Rod Edwards, Greg Wishart and Godfrey Cromwell, all based in Rugby; Bikash Pandy in Kathmandu; Teodoro Sanchez, Jorge Segura and Edgardo Rojas in Peru. Dr John Burton of Reading University gave valuable advice on the first castings of Pelton wheels that we made for ITDG. In the foundry business, by tradition trade secrets are well guarded, and so I am especially grateful to my colleagues in the industry for letting me look around their foundries, both in the UK and overseas. My thanks to Frank Soko of Engineering Services Foundry, Lilongwe, Malawi; to Chris Stevens of Dedza pottery, Malawi, for making ITDG's first crucible; to Shakya Sahib of Nepal Dhalout Udyog Foundry, Patan, Nepal, for showing me the traditional Newar technique of casting; to Gopal Mital Sahib of National Structures Foundry, also in Patan; Vijay Shrestha Sahib of Shrestha Engineering, Bhairawa, Nepal; to Professor Justo Aparichio, Fundicion Ocupacional, Cuzco, Peru; and to Professor Mauricio O'Fallon of the Engineering

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Faculty, University of Bogota. Thanks to the Caballero propellor foundry, Callao; EURQIA propellor foundry, Pucalpa; Carena foundry, Cuzco; Arnie Vince, Buena Fortuna Engineering, Callao; and Alberto Colmanarez Electrical Engineering, Lima, all in Peru. My thanks to the many engineers, foundries and bronze and aluminium casters in Europe, who gave me practical experience, or who helped with my research; to John Payne of F.J. Payne Engineering, Oxford for common-sense advice in both business and engineering; to Rungwe and Claude Kingdon of Pangolin Editions Foundry for a fruitful association which has lasted many years; to Tissa Ranasinghe at the RCA for invaluable advice on the Indian clay slurry process; to Ray and Dave Amatt, neighbours and colleagues. Thanks to David Reed of Red Bronze Foundry, London; Monsanto Chemicals; British China Clay Co.; and to Rolls Royce Aero Engines, Bristol, UK for information about the ceramic shell process. Thanks also to Herman Noak Bildgeiserie, Berlin