Junction Configurations

Ongoing and past fundamental research have considered junctions to be of five types, generally represented by the letters L, T, V, X, and Y. All other possible configurations at corners are modifications of the basic types. Design in the shape of these letters and their effects on shrinkage defects have been studied in great detail by producing actual castings, and the basic findings are just as valid and instructive to both designers and foundrymen today as they were originally some years ago when first established. The test castings were evaluated radiographically so that the defective conditions could be reproduced on film and measured accurately. It can be seen that fillets and cores, when used judiciously, benefit casting soundness and that authoritative information is available to ensure the delivery of high quality components from the casting supplier.

Junction Configurations : T-junctions

Comparison of defects produced in various T-shaped junctions cast under test-controlled conditions. Section size of test castings is 3x3 in. unless otherwise noted.

T-junctions, on the other hand, can benefit from coring a hole at the center of the junction of the two members. If this is not practical, the junction can be risered adequately to provide sufficient feed metal. In addition, the designer's careful analysis of service stresses incurred in the field by prototypes may suggest different solutions, particularly since computer-aided design now offers innovative approaches to complex problems.

Junction Configurations : L-junctions

Comparison of defects produced in various designs of L-shaped junctions cast with section sizes of 3x3 in. Test castings were produced under controlled conditions to encourage formation of defects.

As an example, note that the unsound condition in L-junctions can be prevented by eliminating sharp corners and substituting radii and making other minor modifications of great benefit.

Junction Configurations : V-junctions

Comparison of defects produced in various V-shaped junctions cast under test-controlled conditions. Section size of test casting is 3x3 in. with junction members intersecting at 45° angle.

V-junctions formed by uniform sections may not be free from shrinkage cavities, primarily because a "hot spot" is developed readily in the mold at the junction of the two sections. Because it is a very poor conductor of heat, molding sand insulates and prevents adequate cooling of hot metal in certain areas, for instance, at the inside corners of all junctions.

Junction Configurations : X-junctions

Comparison of defects produced in various X-shaped junctions cast under test-controlled conditions. Section size of test castings is 3x3 in. unless otherwise noted. Centerlines of offset junctions are 8 in. apart.

It will be noted that any unsoundness in X-junctions can be reduced by coring, adequate risering, offsetting one or more arms, or other design modifications, such as wall thickness adjustment, that tend to ensure improved directional solidification. Economics and practicality in commercial foundry operations will determine the final solution to design problems. Close cooperation between designer and foundry engineer is most important.

Junction Configurations : Y-junctions

Comparison of defects produced in various Y-shaped junctions cast under test-controlled conditions. Section size of test castings is 3x3 in. with junction members intersecting at 60° angle.

Unsoundness in Y-junctions can be reduced or eliminated by the same judicious application of remedial measures. It should not be forgotten that chills, which are metal inserts imbedded in the surface of a sand mold or sand core to increase the cooling rate at that point, can assist greatly in reducing shrinkage defects. However, chills also add to the cost of casting production and may increase the dimensional variation of the area they contact. The designer is well advised not to depend on them to produce acceptable castings. The metalcasting supplier must be the final judge.

Both foundryman and designer know that satisfactory structural behavior in the field depends on the interaction of component design, service conditions, and material properties, and that a diversity of information is needed to begin the design process. Isolated, heavy masses can frequently be redesigned to avoid occurrence of centerline shrinkage by removing the center of a heavy mass through the use of a core, and this so-called lightening technique changes the area of a heavy mass to a more uniform section, thereby eliminating the foundry engineer's problem of feeding this area. Astute design can at times eliminate the junctions that encourage formation of porous areas (shrinkage), particularly if uneven cooling rates causing high internal stresses can be avoided.