Many existing methods of producing copper base alloy castings may be broadly subdivided into six separate headings as follows.
1. Sand Casting and CO2 Process
The conventional floor molding and bench molding methods of producing castings using either strickle or loose patterns is still very widely practiced in the copper-base alloy casting industry and they have the very big advantage that for small quantities of castings the pattern costs are the cheapest.
However, these castings cannot be produced to the accuracy obtainable by other methods, and where loose patterns are employed these inaccuracies are caused by two factors: firstly, that the patterns must have a reasonable "draw" in order to facilitate the pattern leaving the sand mould. Secondly, it is invariably necessary to rap the pattern in order to withdraw it from the mould, and consequently variations in casting dimensions do occur.
Machine molding, often known as plate molding, is, of course, very widely used for the smaller and medium sized castings. In fact, it is probably true to say that the biggest tonnage of castings is produced by this method. Modern molding machines enable a very high rate of production to be achieved, and also with accurately made metal patterns considerably greater dimensional accuracy of the castings is obtainable than was possible before the Second World War. Such patterns do, however, require a reasonable "draw" allowance to be made, so that the pattern will leave the mould without difficulty.
The CO2 process has been recently introduced into foundry work and has very large potentialities and can be used in lieu of practically all dry sand molding and a large proportion of green sand molding. Instead of using sand bonded with natural clay, sodium silicate is mixed with silica sand and this is converted during gassing with CO2 to a form of silica which acts as the binder for the sand grains. The CO2 process enables existing pattern equipment to be employed and it has the advantage that greater accuracy can be achieved because the mould may be gassed, and thereby partially hardened before the pattern is withdrawn from the mould, reducing the amount of rap required to withdraw the pattern.
2. Shell Molded Casting
The shell molding process employing synthetic thermosetting resin mixed with high grade silica sand has been developed in the last 50 years, and is a method of producing sand castings to very much closer tolerances. It is, of course, a machine molding process. The pattern plates used for this process must be of metal and capable of withstanding temperatures of 250-300°C. The amount of "draw" required for these patterns can be greatly reduced and rapping the pattern is not used for ejecting the mould from the pattern plate.
The main advantages of this process may be claimed in improved surface finish of the casting, and the improved definition obtainable; castings of complicated section and shape may be easily mass-produced without the use of skilled labor, and very much closer tolerances can be achieved.
In the last connection it is recommended that the contracting foundry should be consulted in regard to what tolerances can be held on shell molded castings, because whilst on certain dimensions it is possible to hold tolerances of the order of 0.003 in. (≈0.1 mm), this is not possible on all dimensions of shell molded castings, particularly where these dimensions are across the joint line of the mould.
The pattern plates for shell molding are, for reasons of accuracy, generally rather more costly to produce than the conventional pattern plates for plate molding. The cost of the resin and high grade sand has also to be absorbed in shell molded casting costs, and as a rule, shell molded castings cost more per pound than conventional sand castings.
However, for many applications, machining operations can be eliminated if shell molded castings are employed, and machining allowances can be greatly reduced in almost all cases. The resultant lower weight of metal of the casting shows substantial advantages in many cases over other methods of production.
Shell molding also has the advantage of a greater degree of consistency, dimensionally, with regard to surface finish, and in regard to soundness, than sand castings, once the correct running and feeding techniques have been achieved.
In regard to mechanical properties, shell molded castings exhibit similar characteristics to sand castings, and all copper base alloys, with the exception of the particular die-cast specifications, are suitable for shell molding.
Gravity die-casting has been employed for many years for the production of copper base alloy castings. In recent years pressure die-casting has in certain cases been found to be a practical proposition.
Die-castings have a number of advantages. The combination of excellent surface finish and close dimensional accuracy can result in the reduction or complete elimination of machining. The higher mechanical properties of die-castings allow a safe reduction of casting section with a consequent saving of weight and material cost.
Not all copper base alloys are suitable for die-casting. The principal alloys which are die-cast are aluminum-bronzes, certain brasses and high tensile brass.
Castings of complex shape can present many difficulties to the foundry man; either because alternate thick and thin sections aggravate feeding problems and give rise to unsoundness, or because the castings are difficult to remove from the die.
4. Chill Casting
Chill casting in its present form has been practiced in the copper base alloy field for over 50 years and is principally applicable to the tin bronzes, namely phosphor bronze, gunmetal, leaded bronze and leaded gunmetal.
This process, as its name implies, speeds up the process of solidification and, with the tin bronzes, this increases the proportion of the hard delta constituent in the casting. In the case of the leaded bronzes it also assists in obtaining a finer distribution of the insoluble lead throughout the matrix. With the possible exception of elongation and reduction of area, all mechanical properties are increased by chill casting. The greatest improvement of all, however, is to be found in hardness figures, which may be on average as much as 50% higher than sand castings.
Chill casting is used both for the production of chill cast stick and rods, and also for individual chill castings of irregular shape. Both methods employ the use of permanent (metal) moulds, usually similar to those used for die-casting but of different metal, and castings can be produced weighing from a few ounces up to hundredweights by this method. The same remarks apply as for die-casting in respect of design of the casting for this process.
5. Centrifugal Casting
Centrifugal casting may employ permanent moulds similar to those used for chill casting, or sand moulds. Its purpose is to obtain by centrifugal force a denser casting and consequently a casting more reliable from the viewpoint of soundness.
The tin bronzes, as for chill casting, are suitable for centrifugal casting and, in addition, the high tensile brasses and aluminum bronzes may be centrifugally cast. This process is, of course, generally used on the larger castings and was originally introduced for the production of gears, worm wheels and cored bars.
Mechanical properties which result from this process are generally similar to those obtainable in chill casting, but in the case of the leaded bronzes there is a danger of increasing the lead content on the periphery, compared to the remainder of the section. Horizontal centrifugal casting techniques are sometimes used for quite small bearings.
6. Continuous Casting
This is a relatively new process as developed for copper base alloys and the principal work has been done in regard to the tin bronzes, phosphor bronze, leaded bronze, gunmetal and leaded gunmetal.
The process has very distinct advantages over chill casting when quantities are sufficient to warrant this production. These advantages may be summarized as greatly increased soundness of the bar. It is, indeed, unusual for continuously cast bar to contain any foundry defects. It is a denser material and, as a result, the overall mechanical properties are the highest obtainable in these alloys.
Continuously cast bar is very clean on the outside and, where tube is produced, also on the inside. It can also be made very accurately to size and cast straight enough for use on automatic lathes with bar feed. Machining allowances are generally greatly reduced on continuously cast bar over other production. Square, rectangular and other regular sections can also be produced by this process.
7. Other Processes
Precision Investment Casting. Precision castings are being used to an ever-increasing extent in industry because they are to close tolerances and of good surface finish, so that costly machining and polishing operations are reduced or eliminated and dimensional consistency of the castings is an advantage where machining operations are necessary. As the accuracy and surface finish of the castings cannot exceed that of the pattern used, high quality pattern equipment is essential.
Plaster Mould Casting. These processes use non-expendable patterns, usually metal but may be of plastic or wood. The castings produced by them can vary in weight from ounces up to several hundredweights. In the Shaw process, a special refractory slurry is used instead of plaster; these moulds are expendable. Semi-permanent ceramic moulds can be made using a special refractory slurry. Each mould can be used to produce 100 or more castings to fairly simple shape with a good surface finish and to a close dimensional tolerance.