Considerable advances have been made in the art of centrifugal casting during the past ten years. However, the immensity of the field of application of centrifugal casting has only been scratched. The fact that many different sizes and configurations of castings can be centrifugally cast has not been generally known.
When the subject of centrifugal casting is mentioned, the average engineer ordinarily thinks of cast iron pipe. The next thought is of iron liners and of various cylindrical castings usually of a high alloy or non-ferrous metal. In general, he thinks of centrifugal castings as being made on horizontal centrifugal casting machines, since this process has been in use for years with marked success.
Until recently, the use of centrifugal casting machines with axis vertical (or at any inclination to the horizontal) has been neglected. This neglect has been unwarranted, as the field of application for vertical centrifugal casting is much broader than that for horizontal casting. The Editor of Metal Progress commented on this in his “Critical Points” last May, in giving the principal details of machinery and casting practice for propeller blade bushing cast of aluminum bronze.
The horizontal machine is best applied to castings of greater length than the diameter and which have a cylindrical bore. The yield approximates 100% (that is, there is not waste in runners or risers), but of the general run of castings very few have the proper shape for this type of machine. On the other hand, practically any size and shape of casting may be poured in a vertical casting machine. This is a rather broad statement, but is true technically; however, certain size castings and those having unhandy configurations are not economically poured in vertical machines at the present time.
The question will immediately arise: “Why pour castings in a machine when they may be successfully cast statically?” The primary reasons are (a) to obtain better quality castings, (b) to produce castings more economically and (c) to cast a part which cannot be satisfactorily cast statically. These reasons do not necessarily go hand in hand as certain castings are centrifugally cast to increase the yield or to decrease cleaning room expense without having any idea of affecting the quality. Usually, however, some improvement in quality results.
Steel castings statically poured with a yield of 40 to 55% may be centrifugally cast with a yield of 65 to 85%. Instead of using the force of gravity through the liquid metal in the heads to obtain sound castings, use is made of centrifugal force. Thus, heads are seldom required (but if necessary in rare instances, much smaller heads are used). Cleaning room costs are therefore considerably reduced. Denser castings result with physical properties comparable with those of forgings. This fact is demonstrable, but is reserved for later consideration in an article on metallurgy; at this place it is primarily desired to call attention to the wide field of casting open to foundrymen who use the vertical machine.
Centrifugal castings are pressure castings, akin to forgings. When a piece of steel is forged, it is heated to a temperature at which the metal is relatively soft and pliable and pressure is then exerted upon the metal while it is in the plastic state. A large proportion of the forging work is done to move the plastic metal about to give a proper external shape. In centrifugal casting, pressure is exerted upon the metal while it is in the molten state, requiring less force to obtain a similar effect—namely a proper shape and sound structure. Moreover, no flow lines exist, as in a forging. The centrifugal casting has equal toughness in all directions, which is not true of a forging.
Another advantage is that the percentage of rejected castings is reduced. Bad castings come from human errors and are no more prevalent in centrifugal casting than in static casting. Moreover, centrifugal force assists in counteracting some human errors which otherwise would result in a bad casting. There is a greater leeway in the foundryman’s efforts to obtain good sound steel castings and at the same time do the job much more economically.
Let us consider the various methods which can be used in vertical centrifugal casting. Three methods will be described, whose products are nominally called “centrifugal castings”. Actually, only one of the products is a true centrifugal casting.
Method 1, True Centrifugal Casting: In this method, the castings are rotated about its own axis during solidification. The casting may have a cylindrical hole through its center or may be solid. In the latter case the metal of the solid center will be inferior in quality to that at the periphery. In any event, no central core is used. The metal solidifies from the outside toward the center, which results in the highest quality of metal at the surface. Yields approaching 100% may be obtained – that is, there are no sprues, gates, risers or sink heads to remove and remelt. Typical castings of this type are small gear blanks, gears, cutters, bushings, and liners. Frequently, of course, the central region is open, and finished by machine shop operations.
Method 2, Semi-Centrifugal Casting: This is very similar to Method 1 but, due to the irregular contour of the central bore, a core is used. (A revealing sketch of such molds used by Ford Motor Co. may be found in Metal Progress, May 1940, Page 526.) As cooling takes place from the periphery toward the center and from the center toward the periphery due to the center core, the quality is similar to a static casting made with extremely tall heads. Gates of various types may be used, some of which serve the function of a head, some merely for a pouring basin. Hydraulic heads of over 100 equivalent feet of molten metal are possible by centrifugal force! Method 2 is adaptable to a wide variety of castings such as jaw clutches, sheaves, gear blanks, casing heads and flanges.
Method 3, Pressure Castings: This refers to the use of centrifugal action generating a high pressure to force the molten metal into the mold (rather than to the use of the resulting casting to resist high pressure). This method is used for unsymmetrical castings which cannot be spun about their own axes. As shown in the third halftone on page 433, the mold is arranged so metal is poured into a gate or cavity place at the center of rotation. Molds for the castings are arranged symmetrically about the center of rotation with gates leading to each from this central down gate. The resulting casting looks like a shaft with spokes extending there-from, having individual castings at the ends of the spokes. Method 3 requires a high degree of technique and experience to design properly the gating ands spacing of the individual castings about the periphery. Usually, only small castings are pressure cast, such as unions, valve bodies, gates and plugs and intricate castings which otherwise would yield large quantities of sprue scrap.
It therefore appears that the term “centrifugal casting” is no longer adequate; more specific terms should be used. There is first the true centrifugal casting which is spun about its own axis, has not central core, and in which the cooling takes place from the periphery toward the central axis. Second is the semi-centrifugal casting which is spun about its own axis but does have a centrally located core’ in this variety solidification proceeds from the central axis toward the periphery as well as from the periphery toward the axis. Third is the method of pressure casting in which the castings are arranged asymmetrically about the center of rotation; normal type of cooling takes place in these as in a static casting.
Any metal which can be cast statically may be centrifugally cast as well. Metals which have a relatively large amount of shrinkage upon solidifying and cooling, such as steel, show the greatest savings economically. Very few advances in the casting of metals have been made in the last hundred years. Foundry work is still too much of an art, and the trend certainly is to put foundry work on a scientific basis. Better metal, denser castings and decreased costs are daily being striven for. Centrifugal casting is a step along the path we are seeking. It may even be the answer.