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DELMER-Lost wax Investment Casting Process

  2. 2. WORLD GOLD COUNCIL HANDBOOK ON INVESTMENT CASTING THE LOST WAX CASTING PROCESS FOR CARAT GOLD JEWELLERY MANUFACTURE By Valerio Faccenda Consultant to World Gold Council with Chapter 3 written by Dieter Ott Formerly at FEM, Schwäbisch Gmünd, Germany
  3. 3. Copyright © 2003 by World Gold Council, London Publication Date: May 2003 Published by World Gold Council, International Technology, 45 Pall Mall, London SW1Y 5JG, United Kingdom Telephone: +44 20 7930 5171. Fax: +44 20 7839 6561 E-mail: Produced by Dr Valerio Faccenda, Aosta, Italy Editor: Dr Christopher W. Corti Translated by Professor Giovanni Baralis, Turin, Italy Originated and printed by Trait Design Limited Note: Whilst every care has been taken in the preparation of this publication, World Gold Council cannot be responsible for the accuracy of any statement or representation made or the consequences arising from the use of information contained in it. The Handbook is intended solely as a general resource for practising professionals in the field and specialist advice should be obtained wherever necessary. It is always important to use appropriate and approved health and safety procedures. All rights reserved. No part of this publication may be reproduced, translated, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior permission in writing of the copyright holder.
  4. 4. CONTENTS Preface 6 Glossary 7 3 Alloys for Investment Casting 59 3.1 Yellow and red gold alloys 59 3.1.1. Metallurgy and its effects on physical 1 Introduction 13 properties 3.1.2 High carat golds with enhanced 55 1.1 Development of the modern process 13 1.2 The modern process and product quality 14 properties 64 1.3 Choice of equipment and consumables 15 3.2 White gold alloys 64 1.4 Health and safety 16 3.3 Influence of small alloying additions 67 3.3.1 Improving properties 67 2 The process of investment casting 19 3.3.2 Effect of individual additions 68 2.1 Design 20 2.2 Making the master model 21 4 Equipment 75 2.2.1 Alloy of manufacture 21 4.1 Vulcanisers 76 2.2.2 Feed sprue 21 4.2 Wax injectors 77 2.3 Making the rubber mould 23 4.3 Investing machines 78 2.3.1 Types of mould rubber 24 4.4 Dewaxers 79 2.3.2 Making the mould 25 4.5 Burnout ovens 79 2.3.3 Cutting the mould 26 4.6 Melting/casting machines 81 2.3.4 Storing and using the mould 28 4.6.1 Comparison between centrifugal 2.3.5 Common problems 29 and static machines 81 2.4 Production of the wax patterns 29 4.6.2 Centrifugal machines 82 2.4.1 Types of waxes 29 4.6.3 Static machines 83 2.4.2 Wax injection 30 2.4.3 Common problems 2.5 Assembling the tree 33 33 5 Sources of equipment and consumables 89 2.5.1 Bases and sprues 33 2.5.2 Tree design 35 2.6 Investing the mould 36 6 Further reading 97 2.6.1 Flasks 36 2.6.2 Investment powders 2.6.3 Safety and storage of investment 36 7 Acknowledgements 102 37 powders 2.6.4 Checking the condition of the 8 World Gold Council technical investment: the ‘gloss-off’ test 38 publications 103 2.6.5 Mixing the investment 39 2.7 Dewaxing the flask 41 2.8 Burnout 42 2.8.1 The burnout cycle 42 2.8.2 Behaviour of calcium sulphate-bonded investment during burnout 44 2.9 Melting 45 2.10 Casting 46 2.10.1 Test for system temperature 48 2.10.2 Inspection criteria 49 2.10.3 Test for best feed sprue design 50 2.10.4 Casting with stones in place 51 2.11 Cooling and recovery of cast items 52 2.12 Summary of the basic guidelines for each step of the process 53 2.13 Schematic list of possible defects 56
  5. 5. PREFACE Investment (or Lost Wax) Casting is one of the earliest processes developed by man, dating back 6,000 years or more. Today, it is the most widely used process in jewellery manufacture but probably the least understood by practitioners of the art. It comprises a series of process steps, each of which must be performed properly, if good castings are to result. It never ceases to surprise me just how many casters do not realize what quality of casting it is possible to achieve consistently, if each process step is done carefully in a controlled manner. There are comparatively few good manuals on investment casting. Many date back some years and focus on centrifugal casting. Our first WGC technical manual, the Investment Casting Manual, was published in 1995 and has proved popular. Since then, there have been substantial developments in the technology and our understanding of the process. Thus, we considered it timely to update it, particularly as stocks of the original are running out. This Handbook is the result. It has given me great pleasure to work with Valerio Faccenda and Dieter Ott (Chapter 3) in the production of this Handbook. Both Valerio and Dieter are well known to many of you as experts in jewellery technology, especially in investment casting, with each contributing several articles to Gold Technology magazine over the years and presenting at several WGC International Technology Symposia at Vicenza, Italy. Valerio, as a technical consultant to World Gold Council, has also presented at many WGC technical seminars in countries around the world. He is, of course, author of the Finishing Handbook. Dieter has made major contributions to our understanding of the Investment Casting process and to the metallurgy of the carat gold alloys and is author of the Handbook on Casting and Other Defects which complements this Handbook. Both have presented at the prestigious Santa Fe Symposia, Dieter on many occasions. I know that this Handbook will become a classic in the jewellery field and meets a demand for a good comprehensive and authoritative book on the subject. I know you will find it useful and enjoyable. I must also mention Giovanni Baralis who translated this Handbook from Italian into English. Whilst known to relatively few of you, Giovanni has been responsible for translating Gold Technology into Italian over many years. He certainly makes my job easier. This Handbook is the seventh in the range of technical publications published by World Gold Council. These are designed to assist the manufacturing jeweller and goldsmith to use the optimum technology and best practice in the production of jewellery, thereby improving quality of the product, reducing defects and process time which, in turn, results in lower costs of manufacture. We believe that it is important for the practising jeweller to understand the technology underpinning his or her materials and processes if he or she is to achieve consistent good quality. That is one aim of these Manuals and Handbooks – not only to provide good basic guidelines and procedures but to explain, in simple terms, why they are important and how they impact quality. Armed with such knowledge, a jeweller should be better able to solve problems that will inevitably arise from time to time. Christopher W. Corti, London, April 2003 6 Handbook on Investment Casting
  6. 6. GLOSSARY Note: Certain technical terms exist in two spellings (e.g. carat or karat, mould or mold), reflecting English and American common usage. In this Handbook, the English versions have been used, although both are given in this Glossary. Accelerator: Compound which speeds up setting of investment, mainly to increase the productivity. In general it is based on crystalline substances such as sodium chloride, sodium citrate, Rochelle salt. Alloy: A combination of two or more metals, usually prepared by melting them together. They are designed to have certain desired properties, e.g. strength, hardness, ductility, colour, etc. Annealing: Restoration of softness and ductility to metals and alloys after cold working by heating to a temperature that promotes recrystallisation. Assay: The testing of items to determine their precious metal content, e.g. by fire assay or other analytical technique. Base metal: The non precious metals in a jewellery alloy. For instance in a gold-silver-copper-zinc alloy, copper and zinc are the base metals. Binder: A substance used to hold together the investment powder, e.g. for casting jewellery, this can be the Plaster of Paris (q.v.) or acid phosphate. Burnout: The firing of the invested flask at temperature in an oven after dewaxing (q.v.), to condition the mould for casting and to completely eliminate any residual wax or other model materials. CAD/Computer Assisted Design: A sophisticated software system for bi-dimensional or three- dimensional designing. CAM/Computer Assisted Machining: A software system for automated machining of a component, driven by computer software, typically from a CAD system. Carat/Karat: A unit for designating the fineness of gold alloys, based on an arbitrary division into twenty-four carats. Pure gold is 24 carat or 100% pure. A 75% gold alloy is 18 carat and so on. (The carat is also a unit of weight for gemstones, equal to 0.2 grams). Carat/Karat gold: A gold alloy which conforms to national or international standards of fineness and can be legally marked or hallmarked. Castability: The ability of a molten alloy to be poured into a mould, retaining sufficient fluidity to fill the mould completely and to take up an accurate impression of the details of the mould cavity. Casting: This word can have two different meanings: 1) the process of pouring a molten metal in a mould; 2) the metallic object taken out from the mould, after solidification of the cast metal. Casting grain: Metals or, more usually, alloys prepared for melting and subsequent casting by dividing the charge material into small particles (like gravel) by pouring a melt into water to form shot or grains. Casting temperature: Temperature at which power is switched off and the molten alloy is poured into the mould. Centrifugal casting: A method for casting metals in which the molten metal is driven by centrifugal and tangential forces from the crucible into a heated mould whilst both are rapidly rotated. Chilling factor: Cooling capacity of a mould calculated from volume specific heat of the mould material and the mould/melt temperature difference. Value for gypsum binder - low; for silica - medium; for cold copper - very high. Cold work/working: Deformation of a piece of metal or alloy to effect a change in shape at temperatures sufficiently below the annealing temperature to cause work (strain)-hardening, usually with a loss in residual ductility. The amount of cold work imparted is often measured in terms of reduction in cross-sectional area (e.g. wire drawing) or in thickness (e.g. rolling of strip). Cristobalite: The highest temperature phase of silica, stable and with high strength retention from 1470°C (2678°F) to the melting point, 1700°C (3092°F). De-airing: Removal of air bubbles from an investment slurry, to avoid bubble defects on the final casting. Assisted by vibration and/or vacuum. Devesting: Separation of the cast tree from the refractory mould. This can be done by quenching the flask in water or by hammering or with high pressure water jets, depending on the refractory type. Dewaxing: The removal of the largest part of the wax from the invested mould. It can be done dry, in an oven, or wet, with steam. Handbook on Investment Casting 7
  7. 7. Dross: The scum that forms on the surface of molten metals, due largely to oxidation, but sometimes also to the rising of impurities and inclusions to the surface. Feeding: The necessary process of introducing molten metal through suitable channels (the sprues) and into the cavities of the mould to fill them and to compensate for contraction (shrinkage) as castings solidify. Can be gravity assisted, or otherwise pressurised. Feed sprue: A system of wax rods connecting the central sprue to the pattern to be cast. It forms the channel for the melt to fill the mould cavity. It should be kept as short as possible and must not freeze prematurely. Its junction with the pattern is called the “gate”. Fineness: Precious metal content expressed in parts per thousands (‰). 18 carat is 750 fineness. Flask: The outer metal container of an investment casting mould, used from the investment process through to extraction of the cast tree. It is available in standard sizes and reusable. It may be a solid cylinder or a cylinder perforated with holes to allow escape of air from the mould under vacuum. Fluidity: Complex property describing the ability of a molten alloy to run into a mould and take up an accurate impression of the mould cavity. It generally increases with superheat, freedom from oxidation and some alloying additions (such as zinc & silicon). Flux: Inorganic mixture applied to melt surface to protect the melt from oxidation. It should melt at a temperature lower than melting temperature of the alloy. Form filling: The ability of a molten metal to fill the mould cavity completely. Fuel gas to oxygen ratio: The volumetric flow ratio matching the molecular ratio for complete combustion. With a hydrogen/oxygen flame a ratio of 2 gives a neutral flame with a sharp inner cone. A lower ratio gives an oxidising flame; a higher, a reducing flame. Furnace: See Oven Gate: The part of the feed system that controls the flow of metal from the feed sprue to the pattern. When it freezes, it is closed and no more metal can pass that point into the pattern. Gloss time /Gloss-off time: The time between the addition of the investment powder to the water and the moment where the slurry surface loses its “gloss”. It denotes the start of setting of the investment. Gloss-off test: A test for determining the gloss-off time of a batch of investment powder. Useful in defining or eliminating possible causes of casting problems and defects. Grain: See “casting grain”. It can also refer to the tiny crystals - or “grains” - forming the bulk or microstructure of a metal or alloy. Graining: The process of preparing casting grain, normally by pouring the molten alloy into water. Grain control/ grain size control: The metallurgical procedure to keep the grain (crystal) size of an alloy under control, by addition of particular metals or compounds (grain refiners, q.v.). Grain refiner: An addition of suitable metals or compounds to control the grain size of an alloy during solidification or annealing (recrystallisation). Grain size: Dimension of the crystalline grain of metals and alloys. In jewellery alloys, a fine grain size is usually preferred. Gypsum: Calcium sulphate, used as a binder in investment. Gypsum-bonded (investment): The traditional refractory investment based on silica powder bonded with Plaster of Paris (selected hemihydrated calcium sulphate) mainly used by jewellery industry for investment casting of gold alloys. Hallmarking: The stamping of precious metal articles by an independent assay office to show the fineness of that article. Term derives from the U.K. for marks applied by Goldsmiths Hall -’marked by the Hall’. Term is often used loosely to describe a mark applied by a manufacturer to show fineness in non-hallmarking countries. Heat treatment: A treatment given to metals and alloys, involving a combination of temperature, time, heating and cooling, to effect a change in microstructure and other properties. Hot shortness: Brittleness at high temperature, often intergranular, caused by either low melting point segregates or other non-ductile grain boundary constituents. Hygroscopic: A material possessing a marked ability to absorb water vapour from the atmosphere. Some compounds can react with atmospheric water vapour to form new compounds (e.g. calcium sulphate hemihydrate forms calcium sulphate dihydrate). Gypsum-bonded investment is hygroscopic and should not be left exposed to the atmosphere. 8 Handbook on Investment Casting
  8. 8. Inclusion: Non-metallic particle that is found in a metallic body. It can be generated from fragments of extraneous materials (e.g. refractory from furnace crucible or mould) or from the reaction of the metal with foreign materials (e.g. atmospheric oxygen, sulphur compounds generated from investment reaction, etc.). Induction melting: Heating to above the melting point by generating eddy currents within a conducting material surrounded by a water-cooled copper coil carrying an alternating current at low (<150 Hz), medium (>500 Hz) or high (>100 kHz) frequency. Also creates a stirring effect in the melt by induced electromagnetic forces. Investment/Investment powder/Investment mould: The investment is a mixture of fine silica powder and a binder, formulated to withstand the high temperatures of burnout and casting. For the investment casting of gold jewellery, the commonest binder is gypsum, in its hemihydrated form (Plaster of Paris). Besides these main ingredients, commercial investment contains small amounts of other chemicals (modifiers), designed to impart to the investment the required characteristics for optimum performance. When mixed with water to form a slurry, the binder undergoes a hydration reaction (like cement) to set the investment into a solid mould. Liquidus temperature: The temperature above which an alloy is completely liquid, i.e. no more solid metal is present. Below liquidus temperature there is an increasing proportion of solid phase until at the solidus temperature no liquid remains in equilibrium. Lost wax: Original name for investment casting. A wax model (or pattern) forms the cavity in the investment. Then the wax is melted out before firing of the mould. So the wax is "lost", from which the name of the process derives. Master alloy: A premixed metal alloy (q.v.) that is added to fine gold to produce the final carat gold alloy. Generally contains silver and copper with other additions, e.g. zinc, nickel, palladium, deoxidisers and grain refiners. Master model: The master model is the reference model for the design and can be made of wax or plastic or metal. Nickel silver or silver alloys are frequently used. Metal models can be rhodium plated to improve wear and corrosion resistance. CAD/CAM systems can also be used to produce master models. Melting range: The temperature interval between the solidus and liquidus temperatures (see Solidus temperature and Liquidus temperature). Mould/Mold: A hollow object, containing a cavity that is the outer form of the piece(s) to be reproduced by wax injection or by metal casting. In the case of investment casting, the mould can be made of various materials, e.g.: metal, rubber (for wax patterns) or refractory investment (for casting). Mould clamp: A pneumatic device for maintaining a constant clamping pressure to a rubber mould during wax injection. Mould/Mold Frame: A metal frame, usually rectangular (but can be circular), used to contain the rubber layers and master model during production of the rubber mould in the vulcanising press. Negative tolerance: Used in the context of standards of fineness. It implies a small allowance in precious metal content below the specified minimum that is acceptable in some countries. Oven: A furnace where a controlled and relatively uniform temperature can be held for the required length of time. It can be heated by combustion of a suitable fuel (e.g. natural gas, propane, etc.) or by electrical resistance elements. The temperature is controlled through suitable regulators. For burnout, the oven should be of the muffle type with a large volume to contain several flasks. It may be fan-assisted and/or have a rotary hearth to aid temperature uniformity. Overheating: When the temperature of the material becomes too high. Not to be confused with superheat (q.v.). Overheating is an unwanted and potentially detrimental occurrence. The overheated material can begin to decompose or react with other materials into which it comes in contact. Overheated melts can oxidise more readily. Pasty zone: The pasty zone corresponds to the temperature range between the liquidus and the solidus (q.v.). In this temperature range the metal is not fully liquid nor fully solid. It is in a "pasty" state. Compensating shrinkage by feeding liquid alloy under these conditions may be difficult. Pure metals and eutectic alloys do not show a pasty zone. Pattern: A master (usually metal) or consumable (lost wax process) model of a component that is to be reproduced by casting. Pattern dimensions may need to allow for net shrinkage or expansion over the whole casting process. Phosphate-bonded (investment): Investment with acid-phosphate and magnesia, which first gels silica flour and then bonds it by subsequent dehydration. It is used preferably for high melting point alloys, e.g. palladium white gold and platinum. Pickling: The process of dissolving away surface oxides and flux by immersion in a suitable dilute acid bath (‘pickle’). Normally used for cleaning cast trees, soldered or welded parts or scrap (before melting). Handbook on Investment Casting 9
  9. 9. Plaster of Paris: White powder of calcium sulphate hemihydrate (2CaSO4.H2O). It is obtained by suitable heat treatment at relatively low temperature of the mineral gypsum (calcium sulphate dihydrate - CaSO4.2H2O). It reacts with water to form the more stable dihydrate. This reaction is used for investment setting. Porosity: Network of holes in castings, often at the surface, caused by entrapped or dissolved gas or by shrinkage on solidification. Protective atmosphere: An oxygen-free or low oxygen gas atmosphere used to protect a material from oxidation during melting, casting, soldering, welding or heat treatment. Quenching: Fast cooling of a hot material by rapid immersion in a suitable fluid, such as water, oil, or even air or other fluid mixture. The quenchant is usually water for carat gold alloys. Rapid prototyping: Modern technique for producing prototypes with automated machines driven by CAD/CAM systems. Many quite different techniques of rapid prototyping have been developed. A modern method for producing a master model. Reducing flame: A torch flame with excess fuel gas in comparison with available oxygen. Often used for shielding molten metal from oxidation. Refractory: High melting point inorganic (ceramic) material used for furnace linings, crucibles or moulds, usually based on graphite, oxides, nitrides or silicates. Often needs a suitable binder to hold the refractory particles together. Preferably, it should also be resistant to thermal shock and chemical attack. Retarder: Many organic compounds and colloids retard the start of setting of gypsum-bonded investment. This increases the available working time (q.v.). Scrap: Any redundant or reject metal/alloy from a manufacturing operation, that may be suitable for recycling as feedstock to the primary operation. Segregation: The non-uniform distribution or localized concentration of alloying elements, impurities or precipitates within the alloy microstructure, originating from solidification or heat treatment. Setting time: The length of time the investment slurry requires to set, harden or cure. Shot: See Casting grain. Shrinkage: Volume contraction of a molten metal during solidification, typically about 5% for carat golds. Can give rise to porosity in investment castings. Silica: Silicon dioxide selectively processed for producing refractory and abrasive materials. Exists in the vitreous state or as quartz, tridymite or cristobalite phases in equilibrium at increasing temperatures. Silicone rubber: A heat stable, flexible material containing organic radicals and silicon. Can be used in the place of natural rubber for making rubber moulds. Can also be used for heat resistant sealing gaskets. Silicosis: A serious lung disease caused by the inhalation of very fine silica (SiO2) particles. Precautions must be taken when handling investment powders. Soaking: Holding the material in an oven at a constant temperature for the purpose of obtaining a uniform temperature throughout the mass. Solidus temperature: The temperature below which an alloy is completely solid, i.e. finishes freezing on cooling or begins to melt on heating. Above the solidus temperature there is an increasing proportion of liquid phase until at the liquidus temperature no solid remains in equilibrium. Spalling: The breakaway of the surface of the mould due to internal or external stresses, mechanical and/or thermal. Can be a sign of a weak investment. Sprue: Main, central pouring channel in the mould. It forms the stem of the tree and is connected to the castings through the feed sprues (q.v.). It is obtained by melting the wax sprue used to build the wax tree (q.v.). Sprue base: A pad, often of rubber, that makes a bottom for the flask during mould making. The cone (or hemisphere) on a sprue base makes the recess that will be the pouring basin for the molten metal. Superheat: Difference between the melting point / liquidus of an alloy and the casting temperature, required to allow the molten metal to fill the mould without premature freezing. Experience shows that it should be as low as possible to avoid overheating (q.v.). Third hand: Mechanical device, usually fixed to the workbench, to assist in rubber mould cutting. 10 Handbook on Investment Casting
  10. 10. Tree: See Wax Tree Vacuum: A space in which the pressure is much lower than normal atmospheric pressure. Vacuum can be applied to remove air from the mixed investment slurry or to "suck" molten metal in the flask. Vulcanisation/Vulcanization: A chemical reaction of sulphur (or other vulcanising agent) with rubber to cause cross-linking of polymer chains. It increases strength and resiliency of the rubber. Performed as a step in making rubber moulds from the master model. Vulcaniser/Vulcanizer: A piece of equipment used to carry out the vulcanisation, i.e. to produce a rubber mould around a metal master model. Essentially, a press with heated platens. Water blasting: Surface treatment in which high pressure water jets are used to remove the investment from a cast tree. Water quality: The content of ionisable (dissolved) salts and organic matter in the water. Important for mixing of investment slurry, it should be accurately controlled, because it affects the working time and gloss-off time (q.v.). Deionised water is the preferred water quality. Water temperature: The temperature of the water mixed with investment powder. It should be accurately controlled, because it affects the working time and gloss-off time (q.v.). Wax: Any of a group of organic substances resembling beeswax. In general they are formed by esters of fatty acids with higher alcohols. Mixtures of different composition are used to obtain the required properties for making patterns for lost wax casting (melting point, hardness, flexibility, etc.). Usually, the different wax types are differentiated by colour. Wax injector/ Wax pot: Equipment containing molten wax under pressure for injecting into rubber moulds to replicate the desired patterns. Often has a vacuum facility for removing air from the mould prior to injection of wax. Wax pattern: Wax replica of a master model, usually produced by injection of molten wax in a rubber mould. The solidified wax patterns are removed and used in the assembly of a wax tree, which is then invested, to form an investment mould. Wax tree: The assembly of wax patterns on a central sprue, from which the investment mould will be made. Usually shaped like a tree, hence the name. Wettability: The ability of a solid surface to be wetted when in contact with a liquid. Wettability is high when the liquid spreads over the surface. It is related to surface or interfacial tension. Working time (investment): Time available for the preparation of the invested flask. It includes: mixing investment with water, de-airing, pouring the slurry in the flask and de-airing again. In the whole it should be about 1 minute shorter than the gloss time (q.v.). Handbook on Investment Casting 11
  11. 11. The oldest example of a gold investment casting: The Onager or wild ass, cast in electrum (the natural alloy of gold-silver), part of the rein ring from the sledge-chariot of Queen Pu-Abi. From the royal tomb at Ur, Mesopotamia, dating to about 2,600 B.C.
  12. 12. INTRODUCTION 1 1 INTRODUCTION Investment casting is probably the first process used by man for jewellery production, dating back over 6,000 years. This happened long before man used the same process for manufacturing weapons or other objects. Perhaps uniquely, investment casting is the only manufacturing process that has been used first for jewellery production and then subsequently for other production fields, like the mechanical engineering industry. Investment casting is also named lost wax casting: this latter name reminds us that we start from a wax pattern that is invested with a refractory material to form a mould. The wax pattern is then removed by melting (the wax is ‘lost’!) leaving a negative impression in the mould, into which the molten metal is subsequently poured. The word “investment” in the context of investment casting has nothing to do with financial investment. It refers to the fact that the wax patterns are “invested”, i.e. Figure 1.1.1 Greek ring, fourth century B.C. (Schmuckmuseum, Pforzheim) coated, with a refractory material. After setting of the refractory, the wax is melted out and molten metal can be poured in the cavity that accurately reproduces the shape and size of the wax pattern. The cast metal item accurately reproduces also the fine details of the wax model. 1.1 DEVELOPMENT OF THE MODERN PROCESS All past civilisations left us wonderful examples of investment cast jewellery. Such jewellery specimens have been found in the treasures of the Pharaohs of Egypt and in Atzec and Inca tombs of Central and South America. Also, in Europe, the ancient Etruscans, the Greeks, Figure 1.1.1, the Romans and the Byzantines, Figure 1.1.2, left us investment cast jewellery, and later, during the Renaissance, the great Masters created wonderful masterpieces. The starting point for the utilization of investment casting in industry has been the application of the refractory investment in the form of a fluid slurry, invented near the end of 1800. But, until the middle of the past century (I refer to the 20th century!), investment casting has been used almost exclusively for the production of one-off Figure 1.1.2 Byzantine earring, sixth century A.D. pieces for the very few persons who could afford it. (Schmuckmuseum, Pforzheim) Around the middle of the past century, three major breakthroughs made investment casting an industrial process, usable for mass production. The first breakthrough has been connected to automatized chain making. Whilst this process is not related to investment casting, it enabled production of large quantities of jewellery (chain and bracelets) and favoured the access of jewellery to the field of fashion and to an ever wider market. The second breakthrough has been the invention of flexible rubber moulds, for the mass production of wax patterns, by the Canadian, T.G. Jungersen. This invention was rapidly patented in the USA, in 1944, Figures 1.1.3 and 1.1.4, and allowed goldsmiths to reproduce intricate objects, with marked undercuts, without problems or limitations. Finally, the third breakthrough has been the realization that the casting machines developed for use in dentistry, with minor modifications, could be used also for the industrial production of jewellery. These were spring-driven centrifugal casting machines and explain why, even today, centrifugal casting machines are widely used for jewellery production, in spite of the advent of static casting machines, particularly Figure 1.1.3 Patent for elastic rubber moulds, registered in USA in 1944 in the last decade. Handbook on Investment Casting 13
  13. 13. 1 INTRODUCTION After flexible rubber moulds and casting machines were made available, a simple optimisation of the consumable materials has been sufficient to allow a profitable industrial utilisation of investment casting. In particular, we refer to the wax and investment powder. The wax types used for dental applications were too brittle and cracked easily during the extraction of the wax pattern from the rubber mould, especially when marked undercuts were present in the pattern. In this case, a correct balance of properties had to be sought, to develop a product that could be used without particular problems. The investment used in dentistry was too expensive for the goldsmith, who didn’t need the high dimensional precision required for dental applications. Therefore, less costly, but in no way inferior quality, investment types have been developed to meet the requirements of the goldsmith. We refer here to silica-based, calcium sulphate- bonded investment powder. Since investment casting developed into an industrial process, it has become ever more widely used. Today, we can say that at least 50% of jewellery worldwide is Figure 1.1.4 Description of the mould and of the centrifugal wax injector, from the patent of produced by investment casting, as a result of the remarkable technical progress Figure 1.1.3 made, whilst the ancient, time honoured basic concepts remain unchanged, Figures 1.1.5-1.1.7. As a result, investment casting has an aura of fascination, still preserving the artistic and craft aspects of jewellery items. 1.2 THE MODERN PROCESS AND PRODUCT QUALITY Investment casting is very versatile: both simple and intricate shapes can be produced in small or large numbers. It is not costly: often, when we take into account the cost of a good die, pieces that could be cold forged are more economically produced by casting. However, investment casting is not a simple process. There are Figure 1.1.5 Modern investment cast many metallurgical principles we must consider and comply with in the many steps jewellery object: hinged pendant with clasp (Pomellato Spa, Italy) of the process, if we are to obtain a good quality product. These steps are made more complicated by the small size of the castings, which makes process control somewhat difficult. Quite often, the goldsmith focuses his attention on the melting and casting stages; these are only the final steps of a multi-stage process but, very commonly, a defective or unsatisfactory product will be obtained, if all process steps preceding the final ones have not been carried out correctly. Some years ago, World Gold Council, with the Santa Fe Symposium, supported research by the German Institute of Precious Metals into the defects occurring in the Figure 1.1.6 Hinged bracelet: the single links production of jewellery pieces. This study showed that about 80% of defective have been investment cast jewellery pieces had been produced by investment casting and that more than 50% (Pomellato Spa, Italy) of the defects were due to porosity, a defect typical of the investment casting process. The most important results of this research were collected together as case studies in the Handbook on Casting and Other Defects, published by World Gold Council, where the most common defect types are described, along with an exhaustive explanation of their origin and useful recommendations for their prevention. This Handbook is a very useful and essential complement to the present Handbook, which is focused on the process. Investment casting is a very ancient process; nevertheless, in its modern form it is not an easy process to control. We mentioned that the small size of the castings we want to produce is a problem. In Figure 1.2.1 we see the progress of solidification in Figure 1.1.7 Investment cast pendants for young people. Their weight ranges from a ring with a large head. From the first to the last picture, only about 10 seconds have 1 to 3 g (Pomellato Spa, Italy) elapsed. Solidification is completed in less than 1 minute. This experiment to observe 14 Handbook on Investment Casting
  14. 14. INTRODUCTION 1 the progress of solidification was conceptually very simple: molten metal has been poured in the mould, and the liquid metal remaining after a pre-established time has been removed by centrifuging. The shortest time has been about 1 second after pouring. After centrifuging, the mould was opened and the amount of solidified metal was evaluated. These pictures show that the solidification process is very fast and, consequently, its control is nearly impossible. Therefore, it is clear that the last steps of the overall casting process should be carried out under the best possible conditions, in addition to the correct execution of all preceding steps. Figure 1.2.1 Development of solidification in We would be foolish to believe that a completely automatised latest generation a gold alloy ring: a - about 1 second after mould filling melting/casting machine, centrifugal or static, with vacuum and pressure assist, can compensate for carelessness in the preceding steps of the process. The machine will help to achieve a consistent quality of the product, but it will never be able to attain a good quality level, if errors have been made in the preceding steps of the process or simple metallurgical principles have been ignored. 1.3 CHOICE OF EQUIPMENT AND CONSUMABLES The modern goldsmith can choose from a wide range of equipment, from the vulcanisers, wax injectors, investment mixers and burnout furnaces, to melting/casting machines, which represent the largest capital investment. b - after 3 seconds With regard to melting/casting machines, two types of equipment are commercially available that differ in the origin of the force that pushes the melt in the mould: centrifugal machines and static machines. There are no special reasons to prefer one type of machine to the other: both types can produce a high quality product. The main differences between centrifugal machines and static machines will be briefly summarized in Chapter 4, devoted to the equipment, but the final choice should be made by the goldsmith, based on his needs. This choice will depend on the amount of money he is willing to invest, on the type and quantity of product to be produced and, particularly importantly, on the level of technical after-sales service guaranteed by the supplier. A fundamental consideration: a decision taken to purchase new equipment c - after 7 seconds because the current product shows too many defects can be a big mistake! Before considering new equipment, it is absolutely necessary to make a thorough scrutiny of the present production process. When (and only when) we are sure of obtaining the best performance from the existing equipment, can we think to make an investment in new equipment. At this time, when the market offers more and more automatised equipment, there is the danger of committing the full responsibility for product quality to the equipment. The results of such an attitude can be disastrous! Therefore, the most important rule for achieving good results is always to engage your brain and to scrutinize your current process constantly and accurately. Investment casting should never be considered as a routine process. No detail of d - after 10 seconds the process should be neglected, even if, at first sight, it could appear unimportant. In the course of the production process, the goldsmith uses not only equipment, but also various consumable materials: the rubber for making the moulds, the wax for the wax patterns, the investment for filling the flask and, lastly, the alloys. All these materials are the outcome of careful study: they should be selected and used correctly, carefully following the recommendations of the producer on their use. Handbook on Investment Casting 15
  15. 15. 1 INTRODUCTION If the results are unsatisfactory, extemporaneous inventions should be avoided. Please refrain from trying to transform your production workshop in a research laboratory! There is the risk of a further worsening of the problem and of increasing mental confusion! Time can be saved and results improved if we involve the producers of the various materials directly in the problem: generally, the producer is the first to be concerned about the results obtained by use of his product. Usually, he will be able to detect possible errors and recommend suitable corrective action, enabling you to save time and money. 1.4 HEALTH AND SAFETY We have discussed the complex nature of the investment casting process and the need to ensure the correct procedures are followed at each stage. There are health and safety issues that need to be addressed. It is vital that the interests of the workforce are protected if good quality and productivity are to be ensured. Some of the materials may be hazardous or toxic. Of particular note is that related to handling of investment powder and its removal after casting. This material causes silicosis! Engineered control of investment dust or the use of a respirator, approved for silica dust protection, is essential. Respirators must be properly fitted to each worker, who should be trained in its care and use. Other hazards include hot metal handling, electrical and chemical, etc. Suitable precautions must be taken, including provision of protective clothing and implementation of rigorous safety procedures. These issues will be discussed later in more detail. 16 Handbook on Investment Casting
  16. 16. INTRODUCTION 1 Handbook on Investment Casting 17
  17. 17. 1 INTRODUCTION 18 Handbook on Investment Casting
  18. 18. THE PROCESS OF INVESTMENT C ASTING 2 2. THE PROCESS OF INVESTMENT CASTING Investment casting is a typical example of a multistage process. We can list at least 13 separate steps from the initial design to the finishing of the jewellery: 1 – Design 2 – Making the master model 3 – Making the rubber mould 4 – Production of the wax patterns 5 – Assembling the tree 6 – Investing the mould 7 – Dewaxing the flask 8 – Burnout 9 – Melting 10 – Casting 11 – Cooling 12 – Cutting the cast pieces off the tree 13 – Assembly and finishing of the jewellery. With the exception of the last two steps, all other steps directly or indirectly involve metallurgical concepts that should be respected, if a good quality product is to result. The process does not tolerate errors: any careless operation, any apparently innocuous shortcut is a potential source of defects in the finished product. Later, if a defect is observed in the casting, very seldom is the root cause readily found and the proper corrective action identified, because of the complexity of the process. Temperature is an important process parameter in many of the process steps; often the goldsmith tries to improve a situation by changing the temperature, for example of the metal and/or the flask. Usually, a simple temperature change does not solve the problem, but it certainly changes the operating conditions and makes defect diagnosis more difficult. When we have to deal with a defect in our castings, we should first consult the Handbook on Casting and Other Defects to help determine the type and possible causes. The number of defect types is not infinite and many of them, particularly the most common ones, are described in the Handbook. In this way, it is usually possible to identify the defect type and its possible causes correctly. The second step is to scrutinize the process parameters to narrow the possibilities by elimination. Finally, we can try to identify the root cause of the problem. Only at this point can we decide the proper corrective action. Because of process complexity, a defect does not usually originate from a single, simple cause, but from a group of causes that are not necessarily located in a single process step, but over several process steps. Therefore, systematic process data recording is very useful and we never should take anything for granted. The human factor is fundamental for achieving good results. I believe it to be not far from the truth by saying that the contribution of the goldsmith to the achievement of good quality is not less than 80%. The remaining 20% is represented by the equipment, which should be well maintained and reliable. Handbook on Investment Casting 19
  19. 19. 2 THE PROCESS OF INVESTMENT C ASTING In this chapter we will discuss each step of the process, with particular attention to the rules or guidelines to follow and to the most common problems that can arise. Later, in separate chapters, we will describe the characteristics of the most commonly used casting alloys and of the different equipment types. We will also give some basic guidelines for making a correct choice. 2.1 DESIGN Design represents the moment of creation, the birth of the idea for a new jewellery Figure 2.1.1 a Design of a ring in 3 parts by product. Although we can cast very complex shapes, thanks to modern technology, the means of CAD technique. (Courtesy of designer should always have a good knowledge of the casting process, so that he/she can Pomellato Spa.) design pieces that are easily cast. In the design phase, it is also important that the designer be in regular contact with the caster on the shopfloor who will produce the casting. Today, the design operation can be facilitated by the use of Computer Aided Design (CAD) systems, which enable a dimensioned drawing to be obtained, used for making the master model, Figure 2.1.1 (a – e). Such CAD software is not easy to use by inexperienced persons. Specialised knowledge is required. Small workshops can seldom afford such facilities, but it is possible to access CAD service through a reliable CAD service centre. Considerable advantages can be obtained with the use of CAD systems, e.g. the ready availability of a dimensioned drawing is a great help to the work of the model maker. Moreover, if we use a CAD system, we can also use a Computer Aided Manufacturing b (CAM) system and/or one of the many available Rapid Prototyping (RP) methods, Figures 2.1.2 – 2.1.4, for making a first master model, typically in wax or plastic or even metal. With regard to the creative design phase, we should remember that many production problems originate from lack of communication between the designer and the caster. This insular approach is no longer acceptable in a modern jewellery company. A good ‘rule’ says that the relevant production staff should be involved when a new jewellery design is discussed, to scrutinise for potential problems that could arise in the production process. This should be done before the new jewellery design is launched on the market. Good quality starts right from the very beginning! c At the Santa Fe Symposium of 1995, in a discussion on the way to shorten the time between the idea and the realization of the product, J. Orrico, Director of Jewellery Manufacturing at Tiffany & Co., said: “Sure a CAD machine will be great. But realize, even though it is an extremely powerful tool, it can only facilitate the process. A round table can do the same thing. If you can justify a CAD machine, great. If not, everyone has a table. The process needs to cut across organisational boundaries to be truly effective. Get started today!” This very simple, easily implemented recommendation should be always present in our mind if we want to achieve a high quality level: it is fundamental to establish a symbiotic relationship among the d different departments in the company. e 20 Handbook on Investment Casting
  20. 20. THE PROCESS OF INVESTMENT C ASTING 2 2.2 MAKING THE MASTER MODEL The quality of the master model is of fundamental importance for the achievement of good quality product: it should be perfect, with a perfect finish. It should not show the slightest defect, because any surface defect will be replicated in the rubber mould and, in turn, on the wax pattern, on the refractory mould and finally on the castings. In most instances, a defect can be removed in the jewellery finishing stage, to obtain the desired quality level, but it requires time and money. However, such a defect limits the use of mechanized finishing. Such finishing is done by hand, with a resulting waste of time and an increased production cost. Figure 2.1.2 Heads of a rapid prototyping machine: the red head builds the supporting structure, which will be removed later, while 2.2.1 Alloy of manufacture the green head builds the actual model The use of an alloy with suitable high hardness is recommended for manufacturing the master model: finishing of the model will be easier, with a better wear resistance. We should remember that, if the jewellery design is a commercial success, the master model will be used for making many rubber moulds. Therefore, good wear and corrosion resistance are important characteristics for a master model. The use of nickel silver (nickel 50%, copper 30%, zinc 20%) is recommended. Many goldsmiths use sterling silver (silver 92.5%) to make the models, because they are accustomed to cast and work this alloy. The only drawbacks to the use of sterling silver are its low hardness and reactivity with the rubber during vulcanising. No matter what alloy is used, rhodium plating of the finished model is strongly Figure 2.1.3 Operating diagram of the rapid prototyping machine shown in Figure 2.1.2 recommended. For silver models, it is essential. Rhodium plating is bright and hard, Vista laterale = Side view enabling better finishing, increased wear resistance and making it corrosion and Passo della goccia = Spacing of the drops Diametro della goccia = Drop diameter oxidation resistant, particularly in the vulcanisation stage, if conventional rubber is Direzione del movimento dei jets = used, Figure 2.2.1. Advancement direction of the jets Up to now, we have discussed metal models. With the modern techniques of rapid Direzione del deposito dei jets = Deposition direction of the jets prototyping, it is now possible, with the aid of CAD-CAM systems, to manufacture Altezza di un layer = Thickness of a layer models in special plastics that can be used directly for making rubber moulds or for Altezza della parete = Thickness of the whole deposit casting a metal master model, in the place of a wax pattern, Figures 2.1.2, 2.1.3 and 2.1.4. Some jewellers use their wax or plastic model produced by Rapid Prototyping to cast the master model in carat gold. 2.2.2 Feed sprue Usually the feed sprue is considered as an integral part of the model. It links the pattern to be cast with the central sprue into which the molten metal is poured. Function of the feed sprue The feed sprue is a very important component of investment casting. It should Figure 2.1.4 Some models manufactured with guarantee perfect filling of the pattern cavities in the mould. Even more important, the rapid prototyping machine it should act as a liquid metal reservoir to compensate for the unavoidable volume contraction of the gold during solidification of the cast items. If the feed sprue cannot perform this second function, a defect will form - shrinkage porosity, with its typical dendritic appearance, Figures 2.2.2, 2.2.3 and 2.2.4. This defect can be entirely contained inside the casting and, if this is the case, there are no aesthetic problems. However, as is more often the case, if it appears on the surface of the cast piece, it must be repaired or the item scrapped. Repairing is a delicate operation that can be difficult or sometimes impossible, Figure 2.2.5. The criticality of the feed system changes in accordance with the type of casting Figure 2.2.1 Master model of a ring made equipment. Feed sprue design is more critical with the traditional equipment for from nickel silver, rhodium plated Handbook on Investment Casting 21
  21. 21. 2 THE PROCESS OF INVESTMENT C ASTING static casting and a little less critical with vacuum assisted static casting. The difficulty of feed sprue design decreases further with pressure and vacuum assisted casting, the more recent evolution of static casting machine technology, and is minimum with centrifugal casting. When we speak of criticality, we usually refer to form filling in metal casting, because the feed system is never critical for wax injection. Therefore, feed sprues should be carefully designed, Figure 2.2.6, as a function of size and shape of the object to be cast. Given that solidification shrinkage, as a physical characteristic, is unavoidable, the feed sprues, in addition to allowing complete Figure 2.2.2 Shrinkage porosity in a cross form filling, should be able to “drive” shrinkage porosity out of the cast object. section: the dendritic shape is evident Design of the feed sprue Basically, a feed sprue system is a tube or a set of tubes, wherein the metal should flow as smoothly as possible. Turbulence should be reduced as much as possible: so abrupt changes of cross-section, sharp angles, etc. should be avoided. Turbulence in the flowing liquid metal can cause gas entrapment and gas porosity results from entrapped gas in the casting. In all cases, turbulence causes a pressure drop, thus hampering form filling. Therefore, it is always important to think in terms of fluid mechanics and try to imagine the behaviour of liquid metal as it flows towards the cavity to be filled. Figure 2.2.3 Shrinkage porosity in a Patterns with complex geometry or with abrupt changes of cross-sectional area metallographic microsection, observed under often benefit from multiple feed sprues. However, the best results are not always the optical microscope obtained with a multiple feed sprue on the master model because, although multiple sprueing can be beneficial during casting, it sometimes does not enable high quality wax patterns to be obtained, in contrast to those obtained with a simpler feed sprue. In these instances, many workshops use models with a single feed sprue for wax injection. Later, the single feed sprue is cut off and the wax pattern is fitted with a multiple feed sprue. A set of rubber moulds of multiple feed sprues of different size and shape can be used for this purpose. These multiple wax sprues can be fitted to the wax patterns as required, in accordance with the type of casting to produce, Figure 2.2.7. Figure 2.2.4 Dendrites in a shrinkage cavity, The “Y” feed sprue design is the simplest and, from the point of view of fluid observed under the scanning electron mechanics, the best type of multiple feed sprue. When the liquid metal gets to the microscope junction, where it splits into two streams, the metal will not favour one side or the other, unless some other force is involved. Therefore a “Y” is a balanced fluid system. The stem of the “Y” becomes the primary feed sprue and must have enough cross-sectional area to supply ample metal to fill the two secondary feed sprues into which it splits. Figure 2.2.5 Shrinkage defects in a ring with a large head in a vertical section cut through the ring half-way across the band width. Two defective zones are seen: a diffused one in the head and another one in the opposite part of the shank, near the junction with the feed sprue. After pouring, the side parts of the shank solidify first, because they are thinner. Thus, when the thicker head solidifies, feeding of more liquid metal is no longer possible. The defect on the opposite side is known as a “hot spot”, because the sprue junction is heated by the flowing metal, causing a delay in solidification. This zone solidifies after the feed sprue and both sides of the shank are already solid. So it is not possible to feed liquid metal to compensate for the shrinkage. 22 Handbook on Investment Casting
  22. 22. THE PROCESS OF INVESTMENT C ASTING 2 If there is the danger of investment erosion at the point of splitting of the secondary feed sprues, or if the shape of the wax pattern requires a large temperature difference between the liquid metal and the investment, the excessive cooling expected where the metal splits off into the two secondary feed sprues of a “Y” can be relieved by using a “V” design. The wax pattern can be produced with a “Y” sprue, with the stem cut off to form a “V”; this junction is attached directly to the main sprue. With all other parameters constant, the “V” feed sprue will deliver molten metal to the pattern with less temperature drop than the “Y”, because the metal Figure 2.2.6 Examples of split feed sprues path is shorter and less tortuous. (coloured in red) for correct feeding of liquid Size of the feed sprue metal in a ring. They should be connected to Another important point, also based on the principles of fluid mechanics, relates to the thicker part of the ring with a heavier head; also, to the model with an inclined the constant cross-sectional area in primary and secondary feed sprues. If, for angle, to reduce turbulence example, the cross-sectional area of the primary sprue is 8mm2, then the cross- sectional area of each of the two secondary sprues into which it splits should be 4mm2 and not 8mm2. The total cross-sectional area remains constant. In this way we can reduce turbulence. There are no formulae to calculate the optimum size of a feed sprue for a given casting. As a practical rule, we can say that the cross-sectional area of the feed sprue should range from 50% to 70% of the cross-sectional area of the pattern it will feed. 2.3 MAKING THE RUBBER MOULD The correct design of the rubber mould is another important step in achieving a Figure 2.2.7 a Moulds for making complex good quality product. We can say that there are nearly no limits to the shape of feed sprues jewellery pieces that can be produced by investment casting with the presently available materials. The only limit is the imagination and the creative power of the person who should design and make the mould. ‘Mould engineering’ is an indispensable skill that should be cultivated inside the jewellery company. By mould engineering, we refer to designing the mould, choosing the correct material, deciding how many parts will form the mould and if metal inserts will be necessary, deciding how the mould will be cut to facilitate the extraction of the wax pattern, with minimum interference with the surface of the pattern itself. In a Handbook such as this, we cannot teach mould-making technology, we can only illustrate it through some examples. Mould-making should be learnt with Figure 2.2.7 b practice and prolonged, assiduous exercise. We recommend practitioners to attend training courses on this particular subject, for example, those given by the producers of mould rubber. In recent years, there has been a steady improvement in the materials, as has occurred also for wax and investment powder. Therefore, regular updating courses meet the need of understanding the new materials and refining the basic technology. Handbook on Investment Casting 23
  23. 23. 2 THE PROCESS OF INVESTMENT C ASTING 2.3.1 Types of mould rubber Many different rubber types are commercially available, both natural and synthetic and also including the silicone rubbers. Each type of rubber has a different balance of properties and should be chosen for use in specific situations, consistent with the objects to be cast. Usually, natural rubber is stronger and more wear resistant. Silicone rubber is less strong, but enables a better replication of fine detail to be obtained. Two component systems, that are not vulcanisable rubber, have been the most recent to become commercially available. Apparently, they are simpler to use, but they show significantly lower wear resistance compared with other rubber types. The advantages and disadvantages offered by the most common rubber types are listed in Table 1. All types of rubber should be used with care and the recommendations of the supplier should be followed accurately. In particular, vulcanisable rubbers have a finite shelf life. Some of their characteristics can gradually deteriorate when this time has elapsed. The producers recommend storage of the rubber (before vulcanisation) away from heat and light sources, at a temperature not higher than 20°C (68°F). If these simple rules are followed, the rubber will keep its favourable properties unchanged for one year at least. This is what producers guarantee. In practice, if correctly stored, a rubber can last much longer, still giving very good results. All batches of vulcanisable rubber are marked with a code number. In the case of Table 1 Advantages and disadvantages of different rubber types for mould making Type Advantages Disadvantages Natural rubber Excellent tear resistance More difficult to cut (requires vulcanisation) Ideal for intricate models Requires more time for filling the frame Requires only few release cuts It is relatively soft Very limited shrinkage Gives a matt surface Requires the use of spray or talcum Tarnishing of silver models Silicone rubber The frame is filled easily Requires more release cuts (requires vulcanisation) Easy to cut Shrinkage slightly higher than natural rubber Different hardness levels available Good tear resistance but lower than Doesn’t require spray or talcum powder natural rubber Gives a polished finishing Room temperature silicone Very fine surface finishing Suitable only for simple wax or metal models, rubber (two components) Short time for preparation without undercuts Negligible shrinkage Moderate tear resistance Doesn’t require spray or talcum powder Difficult to burn (to enlarge feed sprue) Liquid silicone rubber Very fine surface finishing Difficult to burn (to enlarge feed sprue) (two components) Doesn’t require spray or talcum powder Moderate tear resistance Very easy to prepare High cost Can be used with wax models Negligible shrinkage Transparent, vulcanisable Good surface finishing Shrinkage not negligible silicone rubber Transparent Costly Soft and flexible Easily vulcanised “No shrink” pink Very low shrinkage Vulcanising temperature (143°C +1°C) must – Very good surface finishing be strictly complied with 24 Handbook on Investment Casting
  24. 24. THE PROCESS OF INVESTMENT C ASTING 2 complaints, the producer can trace the production date. Therefore, keeping a record of the code number is important. Above all, we should not store large quantities of rubber and we should use the older batches first (‘first purchased, first used’). 2.3.2. Making the mould Before making the mould, the master model should be carefully cleaned with a degreasing solution in ultrasonic cleaning equipment. In the case of vulcanisable rubber, the mould should be prepared by carefully packing the rubber layers inside a Figure 2.3.1 Steps for making a rubber mould suitable metal frame (preferably forged aluminium). The model is placed in the a – The model is positioned in the centre of the mould centre of the rubber layers and is then covered with an equal number of rubber layers, Figure 2.3.1 (a and b). The vulcanising press should have temperature- controlled platens, preferably with independent thermostatic control. The calibration of the temperature controller should be checked periodically with a reference thermocouple or some other suitable device. Two types of test should be done: with the first one, we verify that both heated platens are at the same temperature. The test can be carried out by putting a small wood block, the same size of the mould and with grooves on the upper and lower surface, between the platens of the vulcaniser. The reference thermocouple is then inserted in the grooves and temperature is measured at different points of the upper b – The mould is completed with other rubber and lower surface. The temperature readings should be the same in all positions. layers The second test aims to verify the correct calibration of the temperature controller. In this case we can use a small aluminium block, of the same thickness as the mould, with a mid height hole for inserting the reference thermocouple. Then we turn the vulcaniser on and we verify that the pilot light of the thermostat turns on and off at the desired temperature of 152-154°C (about 305-309°F). If the light turns on and off at a different temperature, we should adjust the temperature setting knob until the correct temperature is obtained. An incorrect vulcanising temperature is the most common cause of poor quality moulds or of excessive shrinkage. The recommended temperature for vulcanising natural rubber moulds is typically 152-154°C (about 305-309°F). For the silicone rubber moulds, this rises up to 165-177°C (about 329-351°F). Vulcanising time varies with the thickness of the mould: usually a time of 7.5 minutes per rubber layer is recommended (a rubber layer is about 3.2mm/1/8 in. thick). Therefore, a mould 19mm (about 3/4 in.) thick will require vulcanising for about 45 minutes. With particularly complex master models, if good results are not obtained under the conditions cited above, we could lower the vulcanising temperature by about Figure 2.3.2 Protective glove made from 10°C (18°F) and double the time. In this way the rubber will remain in a putty-like stainless steel reinforced fibre for mould cutting state for a longer time and will have more time to conform to the model perfectly. a – The glove fits either hand b – Cutting with a protected hand Handbook on Investment Casting 25
  25. 25. 2 THE PROCESS OF INVESTMENT C ASTING 2.3.3 Cutting the mould To cut the moulds after vulcanising (or curing/setting, for non-vulcanising rubbers), we use blades that should be sharpened or replaced frequently, because the cuts must be sharp and perfect, otherwise we will have moulds that will produce defective wax patterns. To make cutting easier, the blade should be wetted frequently with an aqueous solution of surface-active agents. Two important safety recommendations: the blades are very sharp and so we work with the blade moving away from the hand holding the mould. A second recommendation is to protect the hand holding the mould with a cut-resistant glove, knitted with steel wire, Figure 2.3.2 (a and b). As we proceed with cutting, the cut surfaces should be kept well open, by pulling the rubber strongly apart: this is difficult to do with only one hand. For this purpose, it is very helpful to use a simple, but effective device, called a “Third hand”: it will Figure 2.3.3 Bench fixture to facilitate mould facilitate your work significantly, Figure 2.3.3. The mould should be cut in different cutting (third hand) ways, depending on the type of injector used for making the wax patterns. This is to a – The “third hand” b – The third hand in use avoid the presence of air bubbles in the wax patterns, which will unavoidably lead to the formation of defects. Presently, injectors are frequently used which exhaust the air from the mould before injecting the wax. In this case, the moulds should be vacuum tight. However, traditional injectors are still used in many workshops that do not use the vacuum technique. In this case, the moulds should have suitable vents cut, enabling the air in the mould to escape at the moment of wax injection. In workshops where both vacuum and traditional injectors are used, problems can arise if the moulds are interchanged between the two types, with unfavourable consequences on the quality of the wax patterns. Figure 2.3.4 Convex ring, with a pronounced Teaching how to build a perfect mould is quite difficult in a Handbook, but a few internal undercut examples are given to show what can be obtained from taking the ‘mould- engineering’ approach. The importance of having a good mould maker in the factory is clearly evident from the following example: the model, Figure 2.3.4, is apparently very simple: a ring with a smooth surface, which has a marked undercut on its inner side. At the insistence of the production department, the initial solution has been to produce the wax pattern in two halves, Figure 2.3.5 (a and b). So there was a single mould for each half of the ring. To produce an entire ring, either two wax patterns are joined together or two half rings are cast in carat gold and soldered together. As we can see from the figure, the mould had locating pegs for connecting the two Figure 2.3.5 halves, which were removed after soldering. Both solutions showed considerable a – A single rubber mould is used to produce disadvantages and required a long finishing operation to obtain an acceptable – but half of the ring shown in Figure 2.3.4 never perfect – quality level. A better solution was found later, thanks to a skilled mould maker, and is shown in Figure 2.3.5 b – Two halves must be joined to make the entire ring 26 Handbook on Investment Casting
  26. 26. THE PROCESS OF INVESTMENT C ASTING 2 a b c Figure 2.3.6 Mould designed to produce the wax pattern of the ring in Figure 2.3.4 as a single piece d e the Figure 2.3.6. It is a complex mould, formed in several parts, where the part corresponding to the undercut has been cut in such a way as to be easily removed without damaging the wax pattern. The wax pattern is obtained as a single piece, the quality of the product is perfect and finishing labour has been reduced to a minimum. We emphasize an important detail that should always be kept in our mind when cutting a mould. The cut between the two halves of the mould has been done to coincide with an edge of the ring: in this way there are no traces of separation lines on the main surfaces of the wax ring and finishing operations of the casting are simplified. So a significant improvement of product quality and a reduction in manufacturing cost have been achieved. Another example, similar to the one described above, is shown in Figure 2.3.7. In this case, a metal insert has been used to prevent mould deformation during wax a b c d e f g h Figure 2.3.7 Mould made of two types of room temperature-curing silicone rubber with a metal insert, to produce a ring similar to the ring of Figure 2.3.4 a – The metal master model b to h – The mould. The metal insert prevents mould deformation during wax injection Handbook on Investment Casting 27
  27. 27. 2 THE PROCESS OF INVESTMENT C ASTING injection, because two types of two component silicone rubber have been used for making the mould, instead of natural rubber: one type for the inner part of the ring and another one for the actual mould. If we do not have a skilled mould maker in our factory, we can resort to a solution that should never be considered as optimum, i.e. to use self-parting moulds. In this case, the vulcanised mould will be opened with the simple action of the fingers. Before vulcanising, the mould is assembled in the usual way, by packing the rubber layers in the Figure 2.3.8 a Preparation of a self-parting frame. When nearly half of the layers have been packed, we put small cubes of mould, first half. vulcanised rubber or metal pegs at the outer edge of the mould. These rubber cubes or metal inserts act as locating pegs for the two halves of the mould. Then the free surface is dusted with talcum powder, Figure 2.3.8 (a & b), or is sprayed with a suitable silicone product, or is covered with a thin plastic film. Then a further rubber layer is added, on which the master model is positioned, Figure 2.3.9 (a & b). The previous operation of dusting with talcum or silicone spraying or covering with plastic film is repeated. Then we also repeat all other operations in an inverted order for the second half of the mould, Figure 2.3.9c. The mould is then vulcanised. After vulcanising, the mould will open by the simple pressure of the fingers and will comprise four parts. Figure 2.3.8 b – The red hatched zones should be dusted with talcum or protected Two outer parts - the mould shell - and two thin inner parts, formed by the two inner with other means, because they should not layers, which are the true mould. These two parts will easily separate from the wax bond during vulcanising pattern, without damaging it, Figure 2.3.10. In this mould type, the separation line is in the centre and will always leave a ‘witness mark’, which must be removed later. Moreover, this mould type is not suitable for vacuum injectors. 2.3.4 Storing and using the mould After making, the mould should be numbered, referenced and stored in a closed container - a drawer or a cupboard - away from sunlight and dust. The mould should always be carefully cleaned after use. It is recommended that a register of the moulds Figure 2.3.9 Preparation of a self-parting is maintained, where all parameters for the production of wax patterns are recorded mould. a – The model for each mould (wax type, wax temperature, injector temperature, vacuum, pressure, cooling time). With some latest generation injectors, it is possible to record these parameters on an electronic chip that is inserted in the mould and is “read” by the injector at the moment of wax injection. When a new mould is made, manufacturing parameters should be recorded with care. If necessary, specific tests should be carried out to obtain a perfect mould. With an optimised manufacturing process, mould shrinkage can be minimized. Recently, some vulcanisable rubber types have become available on the market that are claimed to be “no shrink”. The shrinkage of these rubbers can really be zero or nearly Figure 2.3.9 b – Positioning of the model in the mould zero, but to achieve this, the recommended vulcanising temperature should be accurately adhered to. If the vulcaniser is not equipped with a very accurate temperature control system, “no shrink” rubber can show some degree of shrinkage, maybe even more conspicuously than with conventional rubber types. This can occur if the temperature is only a few degrees higher or lower than the optimum temperature. Figure 2.3.9 c – Covering the model 28 Handbook on Investment Casting