Designing castings with uniform wall thickness. (Casting Design Notebook).

In many castings, functional requirements dictate that walls be uniform or nearly uniform in thickness. The problem is that if these castings are not properly designed, the molten metal will freeze prematurely during casting before all parts of the mold cavity are filled. In addition, even if the mold cavity is properly filled, centerline shrinkage or porosity may be encountered because portions of the casting cannot be fed before the onset of metal solidification.

Incomplete Maid Fill

To alleviate the problem of incomplete mold fill with uniform section thickness castings, walls may be tapered with extra padding during the design and/or ribs and webs may be added to the casting to provide feed paths for the molten metal. Figure i illustrates all three methods to aid uniform section solidification.

When pouring a flat plate with uniform wall thickness as in Fig. la, the metal enters the mold cavity and attempts to flow in all directions. If the distance from the gate to the extremities of the mold cavity is too great, the metal freezes prematurely, resulting in misruns. When ribs are added to the plate (Fig. 1b), the metal flows readily to the extremities and successful cavities are produced.

The same success with the plate is attained when the casting's gate is enlarged and uniformly tapered to the extremities (Fig. 1c). The metal flows into the mold cavity at the heaviest section, thus preventing an initial chilling of the metal by the mold as occurs with a small volume of metal. Since-the molten metal in heavy sections retains more of its heat as it flows through the mold, it can more readily reach the extremities of the mold cavity.

Porosity

Although measures are taken with uniform thickness castings to ensure proper freezing, there is no assurance that freezing will start at points furthest from the gate and progress in an orderly manner to the gate so that the volume of metal lost because of shrinkage can be replaced by the riser designed for that purpose. The result could have metal nearer to the gate freezing first, isolating pockets of molten metal, which results in voids or porosity at these pockets.

One solution is the use of two risers to feed metal from opposite ends of the casting. However, risers add to the cost of producing a casting.

Another option is designing feed paths (Fig. 1b) to allow molten metal to flow easily to all sections of the mold cavity. If the enlarged sections are gradually tapered from the gate to the extremities, solidification follows an orderly progression through the casting, terminating at the riser. Compensation for normal shrinkage is provided throughout the casting, and sound metal is assured.

The same beneficial freezing pattern is obtained by tapering the plate (Fig. 1c), provided the angle of taper is sufficient and the source of the feed metal is at the heaviest section, Under these conditions, a favorable freezing pattern is assured because the metal that has flowed the farthest (and is therefore the coldest) is deposited in the thin section. In contrast to a heavy section, the thin section of molten metal has less heat to transmit to the mold before freezing can start. Thus, the direction of solidification can be predicted in tapered sections, provided the metal enters at the heavy end.

If the need for feed paths is anticipated in the early stages of casting design, padding can be made a functional part of the casting or it can be located so that its removal adds little to no cost.

Examples

Consider the two investment castings shown in Fig. 2. For successful production of the casting shown in Fig. 2a, ribs were required on each of the legs. These provided feed paths that encouraged complete filling of the cavity and resulted in sound castings.

The casting shown in Fig. 2b was designed with each leg tapered. This permitted easy filling and feeding of the legs and induced a satisfactory freezing pattern.

Although the addition of ribs to the first casting example was satisfactory from a foundry point of view, removal of the ribs was necessary for the proper functioning of the part. In the second casting example, the taper was satisfactory in terms of both production and function because the casting was designed with this in mind.

In the magnesium casting shown in Fig. 3, the problem was to distribute metal efficiently to all flow channels and all parts of the mold before freezing, and to accomplish this objective with a minimum amount of metal. (This was an aircraft application in which no excess weight was permitted.) The solution entailed greatly enlarging the two ends into which the metal was gated, thus allowing them to function as manifolds.

Increasing their size kept the metal hotter and permitted unrestricted flow to each connecting member of the casting. As shown, ribs were added to each thin strut section to provide a larger flow channel. Because of the larger mass, the rib and strut junctions developed hot spots in the mold and helped prevent premature freezing. The heavy manifold ends were later machined off, but the small ribs remained intact.

This article was adapted from Casting Design Handbook, American Society for Metals, Metals Park Ohio.