This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The following remarks by W. H. Cooper in the School of Mines Quarterly, New York, give a very clear outline of the operations of easting and founding: -
We are indebted to the fusibility of the metals for the power of giving to them, with great facility and perfection, any required form, by pouring them, whilst in a fluid state, into moulds of various kinds, of which, in general, the castings become exact counterparts. Some few objects are cast in open moulds, the upper surface of the metal becoming flat under the influence of gravity, as in the casting of ingots, flat plates, and other similar objects; but in general, the metals are cast in close moulds, so that it becomes necessary to provide one or more apertures or ingates for pouring in the metal, and for allowing the escape of air. Moulds made of metal must be sufficiently hot to avoid chilling or solidifying the fluid metal before it has time to adapt itself throughout to every part of the mould. And when made of earthy materials, although moisture is essential to their construction, little or none should remain at the time they are filled. Earthen moulds must also be so pervious to air that any vapour or gases formed either at the moment of casting or during the solidification of the metal may easily escape.
Otherwise, if the gases are rapidly formed, there is danger that the metal will be blown from the mould with a violent explosion, or, when more slowly formed and unable to escape, the bubbles of gas will displace the fluid metal and render it spongy or porous.
The casting is then said to he "blown." It not infrequently occurs that castings which appear good and sound externally are filled with hidden defects, because, the surface being first cooled, the bubbles of air will attempt to break their way through the central and still soft parts of the metal.
The perfection of castings depends much on the skill of the pattern-maker, who should thoroughly understand the practice of the moulder, or he is liable to make the patterns in such a manner as to render them useless. Straight-grained deal, pine, and mahogany are the best woods for making patterns, as they remain serviceable longest. Screws should be used in preference to nails, as alterations may be more easily made, and for the same reason dovetails, tenons, and dowels are also good. Foundry patterns should always be made a little tapering in the parts which enter most deeply into the sand, whenever it will not materially injure the castings, in order that they may be more easily removed after moulding. This taper amounts to 1/16 or 1/8 in. per ft., and sometimes much more. When foundry patterns are exactly parallel, the friction of the sand against their sides is so great that considerable force is required to remove them, and the sand is torn down unless the patterns are knocked about a good deal in the mould to enlarge the space around them.
This rough usage frequently injures the patterns, and causes the castings to become irregularly larger than intended, and defective in shape, from the mischief sustained by the moulds and patterns.
Sharp internal angles should be avoided as much as possible, as they leave sharp edges or arrises in the sand, which are liable to be broken down on the removal of the pattern, or washed down by the entry of the metal into the mould. Either the angle of the mould should be filled with wood, wax, or putty, or the sharp edges of the sand should be chamfered off with a knife or trowel. Sharp internal angles are also very injudicious in respect to the strength of castings, as they seem to denote where they will be likely to break. Before the patterns reach the founder's hands, all the glue remaining on their surfaces should be carefully scraped off, or it will adhere to and break down the sand. The best way is to paint or varnish wooden patterns, to prevent their absorbing moisture and the warping of the surface and sticking of the sand. Whether painted or not, they deliver better from the mould when they are well brushed with blacklead.
Foundry patterns are also made in metal. These are excellent, as they are permanent, and when very small are less liable to be blown away by the bellows used for removing the loose sand and dust from the moulds. To prevent iron patterns from rusting and to make them deliver more easily, they should be allowed to become slightly rusty, and then warmed and beeswax rubbed over them, the excess removed, and the remainder polished after cooling, with a hard brush. Wax is also used by the founder for stopping up any little holes in the wooden patterns. Whiting is also used for this purpose, but is not as good. Very rough patterns are seared with a hot iron. The good workman, however, leaves no necessity for these corrections, and the perfection of the pattern is well repaid by the superior character of the castings. Metallic patterns frequently have holes tapped in them for receiving handles, which screw in, to facilitate their removal from the sand. Large wooden patterns should also have iron plates let into them, into which handles can be screwed.
Otherwise, the founder is obliged to drive pointed wires into them, and thereby injure the patterns.
The principal materials for making foundry moulds are very fine sand and loam. They are found mixed in various proportions, so that the proportion proper for different uses cannot be well defined; but it is always best to employ the least quantity of loam that will suffice. These materials are seldom used in the raw state for brass casting, although more so for iron, and the moulds made from fresh sand are always dried. The ordinary moulds are made of the old damp sand, and they are generally poured immediately, or while they are green. Sometimes they are more or less dried upon the face. The old working sand is considerably less adhesive than the new, and of a dark-brown colour. This arises from the brick-dust, flour, and charcoal-dust used in the moulding becoming mixed with the general stock. Additions of fresh sand must therefore be occasionally made, so that when slightly moist and pressed firmly in the hand it may form a moderately hard, compact lump.
Red brick-dust is generally used to make the parting of the mould, or to prevent the damp sand in the separate parts of the flask from adhering together. The face of the mould which receives the metal is generally dusted with meal, or waste flour. But in large works, powdered chalk, or wood- or tan-ashes are used, because cheaper. The moulds for the finest brass castings are faced either with charcoal, loamstone, rotten-stone, or mixtures of them. The moulds are frequently inverted and dried over a dull fire of cork shavings, or when dried are smoked over pitch or black rosin in an iron ladle.
The cores or loose internal parts of the moulds, for forming holes and recesses, are made of various proportions of new sand, loam, and horse-dung. They all require to be thoroughly dried, and those containing horse-dung must be well burned at a red heat. This consumes the straw, and makes them porous and of a brick-red colour.
In making the various moulds, it becomes necessary to pursue a medium course between the conditions best suited to the formation of the moulds and those most suitable for the filling of them with the molten metal without danger of accident. Thus, within certain limits, the more loam and moisture the sand contains, and the more closely it is rammed, the better will be the impression of the model; but the moist and impervious condition of the mould incurs greater risk of accident both from the moisture present and the non-escape-of the air. The mould should, therefore, be made of sand which is as dry as practicable, to render the mould as porous as possible. Where much loam it used, the moulds must be thoroughly dried by heat before casting the metal.
As castings contract considerably in cooling, the moulds for large and slight castings must not be too strongly rammed or too thoroughly dried, or their strength may exceed that of the red-hot metal whilst in the act of shrinking, and the casting be broken in consequence. If the mould is the weaker of the two, its sides will simply be broken down without injury to the casting.