By Bill Coffery
The question from the start of this project is, "Why make castings in your workshop?" This must be answered in two ways, one- to obtain a part or parts which are required in a number and are not available commercially and two- for the pleasure of doing it yourself. The first thought at times has led to the assumption that after techniques are mastered, it would be feasible and cheaper to buy one commercial part and use it as a master, produce as many as are required. Nothing could be further from the truth as far as practicability is concerned, and it is certainly not ethically correct. Commercial parts are made in high-volume and are therefore relatively cheap, much cheaper than could be made in the home workshop considering the time and materials involved unless a commercial venture is contemplated. I would heartily discourage this line of thought. If a local group can utilize the same parts this assists in bringing the cost per part into an area that is practicable. But even not considering this, at times where a number of like parts are required, it is much simpler and more accurate to spend time making one master pattern to exact size and casting in a mould So it develops that the first and probably the most important step in the entire casting process is the fabrication of a master pattern suitable for casting. The master pattern is, as the name implies, a master in all respects. Consideration must be given to the end use of the pattern from the start. Is the pattern to be used in a vulcanized rubber mould for the manufacture of wax parts for investment and eventually obtaining brass parts from the lost wax investment process, or in it to be used with silicone rubber moulds with an eventual part cast from a lower melting point metal alloy, or is it to be used as an open faced mould pattern for water putty parts? Once this has been decided, the material for the pattern can then be justified.
Soldered brass masters are required for vulcanized moulds since the vulcanizing temperature is sufficient to damage masters made from less heat resistant materials. This does not mean that soldered brass masters could not be used in rubber moulds, but it would mean extra work since the material is harder to fabricate for the average modeller. Metal (soldered or epoxied), plastic+, plaster or even wood is suitable for patterns moulded in silicon rubber since it cures at room temperature and excessive heat is thereby avoided. Since this mould material is the most applicable for home workshop use, this process will be discussed in detail. The use of styrene for master patterns is highly recommended since it is relatively cheap, easily fabricated and readily available. The major advantage is the addition of details in sequence without the worry of losing detail items which have already been attached as sometimes happens in soldering. A great deal of assistance can be obtained by reviewing the articles written for the 'Model Railroader' magazine on styrene fabrication by Alan B. Armitage. (A digest of these articles is published by KEMTON called "Styrene Fabrication", under their Part Number X-650. This book is sometimes obtainable via VICTORS, 166 Pentonville Road, Islington, London N1 9JL - M.G.S.) There is little use in reviewing the entire fabrication technique in this article, however it must be said that some practice is required before proper dexterity and procedures are obtained. So here again it is a matter of practice. Lay out the part or parts that you want to make and fabricate it as accurately as possible. For Cerrobend parts make the pattern the exact size that you desire the cast parts to be. No shrinkage has to be taken into account as is required in lost wax casting. Remember that the casting will be no better than the pattern so take your time and do it right. If you repeat it several times at the beginning, do not feel bad, it happens to a professional that I know. During the pattern fabrication several things must be taken into consideration. The rubber will produce any mark or surface irregularity so if it is to be smooth, make it smooth. All cracks or joints should be filled as the rubber will penetrate even the smallest crack and may form an internal key which will prevent the separation of the master pattern from the mould, with possible mould tearing. Thought must be given to the parting plane ... where the logical place for mold separation both from the point of practicability of removal and also tar appearance since a separation plane mark and also a certain amount of flash may be evident. Depending on the typo of mould and casting techniques used, a number of patterns may be required prior to mould making. Before you do any moulding go back over each pattern checking all joints, be sure that they are firm and check all critical surfaces with a magnifying glass for unwanted surface defects and then and only then are you ready to prepare the mould.
Casting can be done by several methods, three of which wi11 be discussed here with one in greater detail. Each of these methods requires a different type of mould, therefore the casting technique must be determined prior to mould making. Probably the simplest and least effective of the three is the open faced casting which can, but seldom does, give satisfactory parts. In this method, a single mould is made with the top open for pouring the metal directly into the cavity. You cannot be assured of a complete fill and one side has no detail. It does however utilize the least amount of rubber and is the simplest to rake. Experiments will show that some very simple parts can be cast by this method. Water putty castings are made in moulds of this type and are suite good. The second method utilises a two-half mould and a static head pressure to assure complete filling, To the unfamiliar it consists simply of the two-half rubber mould with either an integral or external reservoir of sufficient height to permit the weight of the metal to force itself into the small detail areas. While this system has and can give very satisfactory parts, some disadvantages must be taken into consideration. A relatively large amount of metal must be melted, poured and allowed to cool to obtain the part each time. This is time consuming and will eventually deteriorate the scrap metal as it is returned to the melt pot. A variation of this method uses a heated vertical feed tube which is capable of being shut off at the mould contact point. The tube is attached to the melt pot and thus a continual supply of melted metal is available. This accomplishes two things- one very little metal other than that actually required for the part is chilled and- two a tube of several feet can give a head sufficient to fill the most complex mould. In practice the two-half mould with its sprue opening is laced in contact with the tube outlet. The valve or stopcock is opened and metal flows into the mould. After a static condition is achieved the stopcock is closed and the mould is set aside to cool. The disadvantages of this method are - one, unless an extremely long tube is used, insufficient head will be developed and loss of detail will occur, and two - the system must be cleaned out after a session to prevent breakage of the glass system. Why use glass? Well Cerrobend is a funny metal and is contaminated by all other metals and therefore a glass melt pot and system is required. This applies to any casting or melting method. Why will it break on cooling? Here again that fanny metal, it expands on cooling, unlike other more normal metals which shrink. This serves us to advantage in that it assists in filling small details. The third method utilizes centrifugal force to induce a relatively small amount of metal into the mould cavity under pressure thus achieving the same type of pressure casting as does the static head method, only better. In this system a two piece round mould is used with the metal ladled in from a glass melt pot. The advantages of this system are, one - a small amount of metal is actually induced and two - head pressures can be varied by varying motor speeds thus filling more complex cavities with a more dense metal. There are two disadvantages. First, the time element involved in attaching and removing the tie down fixtures from the mould which are required to prevent the two halves from spreading under the pressure of metal and second, considerably more rubber is required for the moulds than in either of the other two methods. Since this method is the one that I have settled on, I will discuss this method for the rest of the article.
Now to the raking of a two piece round centrifugal mold. Two pieces of one-half inch plywood are sawed and sanded around a centre hole to a perfect circle of the proper diameter. With the two pieces bolted together through the centre hole, three 3/16" holes are drilled through both pieces approximately 3/4" from the edge and on 120 degree centres. Mark the pieces of plywood, "top" and "bottom" and index mark both the plywood and the spin table so that all parts are relative. These holes are for the mould hold down bolts and also provide spacer support while pouring the moulds. Now take that bottom plywood and run through bolts of the correct size and of sufficient length from the bottom up. Place metal spacers over the bolts equal to one-half of the meld thickness. Invert, wing bolts to provide proper spacer alignment. Using gummed paper tape, wrap the plywood with several layers with the lower edge of the tape evenly against the flat surface upon which the spacers rest. When the tape is dry, restore to the upright position and place several ordinary staples through the tape and into the plywood. You now have a housing of proper height in which the clay can be formed.
(TO BE CONTINUED IN THE NEXT ISSUE OF THE "MERCURY")