One of the most important departments of the malleable works is the pattern shop. Malleable castings are comparatively light when considered in connection with general gray-iron practice and many times are erroneously ordered in quantities from the same pattern.
Every refinement in pattern-making for light castings is found in this branch of the foundry industry. When developing a new article, the pattern-maker must practically live in the foundry. As trial castings are made he must measure them up with the pattern and make changes as found necessary; he must try them out again both before and after annealing, and with iron from different parts of the heat, and thus bring out new points calculated to help the molder and to lessen difficulties from shrinkage. It is the rule never to start an order for a quantity until the pattern has been tried out, the hard castings have been broken for evidence of shrinkage, and everyone has been satisfied it is safe to proceed.
The terms shrinkage and contraction, as applied to malleable castings, should be clearly understood before any further mention is made of this subject. While shrinkage in gray-iron practice is often considered as the shortening of a casting in cooling, in malleable practice shrinkage means the tearing apart of the particles of iron in the interior of a larger section, close to a small one, leaving a spongy mass which is weak and very dangerous to the life of a casting. Contraction being simply reduction in size while the casting is cooling, amounts roughly to 1/4 inch per foot in the hard, i.e., before annealing. During the annealing, about half of this is recovered, so that the net result is about the same as in gray-iron practice. It should be plain, however, that this big contraction causes the tearing as above mentioned when there are heavy sections which remain in a molten state a little longer than do adjoining light sections, unless liquid iron can be fed in. Where this is impossible, the use of chills must be resorted to and fillets made much larger.
In gating patterns it is to be remembered that white iron chills easily and must be poured rapidly. To a certain extent, this limits the number of pieces that may be run successfully, unless the gates be made so large there would be danger of dirty castings. The runners should be large and the sprues heavy, the idea being to get a large amount of metal in front of the gate for the individual casting. The use of the match plate is largely resorted to also, as being well adapted to this class of work. Fig. 152 shows a group of three of the gated patterns in daily use in the Arcade Malleable Iron Foundry, Worcester, Massachusetts. In Fig. 153 there are shown 84 small thumb nuts on one gate; also 18 hinge patterns mounted on a match plate.
But little difference from ordinary gray-iron methods, occurs in molding practice other than. in gating and in pouring. Since white iron melts at a somewhat lower temperature, there is less danger of sand burning into the casting, so less attention is paid to facing sand.
A large per cent of the work is made in the Snap flask on the bench, as illustrated in Fig. 154. From the fact that nearly all work is in quantity, here is where molding machines may be used to great advantage. As these are explained elsewhere, no more need be said except that care should be exercised in the selection of types best suited to local conditions. Fig. 155 is a view from the opposite end of the same foundry floor.
While much of the work is handled in snap flasks, it is not unusual to find metal flasks closely conforming to the shape of the pattern, thereby greatly reducing the amount of sand to be handled, with a corresponding reduction in cost of production.
Practically all castings are cored and the problem is to produce a sufficient number of cores that there may be no delay for the molder. As there is no important difference in the preparation of cores - the same sand and binders being used - the large number required warrants the introduction of modern sand-handling and running machinery to a somewhat greater extent than in gray iron.
The batch mixer alone proves of great value by the reduction of binder required due to its more even distribution; oftentimes this amounts to nearly 100 per cent. The type of batch mixer shown in Fig. 156 is made by the Standard Sand and Machine Company, Cleveland, Ohio. Fig. 157 is a view of the mixing paddles.
Under this head the several methods in use will be described, which are as follows: (1) crucible; (2) cupola; (3) air-furnace; and (4) open-hearth.
The quality of metal produced by this method is without doubt of the best. Being melted out of contact of the fuel, there is no danger of absorbing impurities therefrom, but the small amount of metal available at one time limits the production to only the smaller work. Partly for this reason, the excessive cost of production does not admit of competition with other methods which, while lacking somewhat in quality, yet meet actual requirements.
A more detailed description of melting in the crucible is given in the subsequent section on Brass Melting.
As in gray iron, the cupola offers the most economical method of melting iron, not only in cost of installation and saving of fuel, but in ease of manipulation as well. It has some disadvantages which restrict its use, the greatest being the inferior quality of metal produced, which is caused by the contact of metal with the fuel, and also the danger of burned iron which in turn makes sluggish iron, with the result that castings are likely to show pinholes.
There is also greater difficulty in annealing - usually it requires 200 or 300 degrees Fahrenheit higher temperature. This method may be safely used only when the property of bending, rather than strength, is required. Pipe fittings form a large part of the production of the cupola method. However, it makes a convenient melting medium for the production of anealing boxes.