The design freedom and the scope of applications associated with the investment casting process cannot fail to impress design engineers seeking better and more cost effective manufacturing methods. The below text on design and applications of investment castings presents some basic casting design concepts, a review of cast-in-features offered by the process and a series of case histories and applications which should clearly illustrate the attractiveness of this process.
To obtain maximum benefit from investment castings, close cooperation between the designer and Investment Caster is essential. It is a question not merely of the best way of producing a given design as an investment casting, but of collaboration in depth so that the designer may take full advantage of the investment casting process. A component is often designed in terms of the more conventional manufacturing processes and only later is thought given to producing this design as an investment casting. It is quite possible that the design may be radically improved by the design freedom offered by this accurate manufacturing process.
Basics of Casting Design
The investment casting process is above all a casting process, and the fundamentals of directional solidification are as significant as in any other casting process. In simplest terms, molten metal should solidify in a progressive manner from the extremities of the mold towards the ingate or feeder. Such a situation ensures that molten metal is always available to feed the casting as solidification proceeds, any shrinkage contraction being confined to the feeder system.
In order to maintain directional solidification and casting soundness, the running system should be designed so that thick sections feed thin sections and not the other way round. Consider the theoretical casting shown in Figure 4:1a. It is obvious that the thin section will solidify first and solidification will proceed to the boss, which can be fed by a suitable feeder. If the casting were inverted (Figure 4:1b), the thin section would again solidify first, and thus isolate the boss from the feed metal.
Castings are often of complex geometry, and in order to establish satisfactory thermal gradients in the mold, the investment founder must exercise his skill by arranging the castings and feeding configuration in a satisfactory manner. Long before this stage is reached, however, attention to the design of the casting can ease the production process, minimizing development and scrap costs. The following are some of the more basic design rules:
(a) Keep section thicknesses constant.
(b) Where a change in section cannot be avoided, ensure that the change is gradual.
(c) Make use of adequate fillets so as to avoid sharp intersections, and avoid sharp exterior edges.
(d) Do not cause a number of sections to meet at the same point.
It is very seldom that a casting is designed which conforms to all these requirements, but if they are recognized a design will be produced which will offer the best possible conditions for a given component.
Dealing with these points in more detail:
(a) Uniform casting section
Consider the casting design shown in Figure 4:2a and b. The design shown in Figure 4:2a creates a number of variations in section which can lead to poor feeding conditions and the generation of hot spots. A simple change in design will produce a more satisfactory situation (Figure 4:2b).
(b) Section change
The difference in thickness between adjoining sections should be kept to a minimum. Where section variations are less that 2:1, the section should be designed in the manner shown in Figure 4:3a. If the section variation is greater than 2:1, the wedge configuration shown in Figure 4:3a. If the section variation is greater than 2:1, the wedge configuration shown in Figure 4:3b should be adopted, provided the section change does not exceed 4:1.
(c) Use fillets and radii where possible
Sharp corners and intersections lead to unsound conditions and poor structural characteristics. Hot spots and severe shrinkage can be caused by sharp intersections (Figure 4:4a). The size of radius selected to avoid sharp corners and intersections is important. If the radius is too large, hot spots and shrinkage cavities can be produced which may be as bad as a design with no radii. As a general guide, the radius should be in the range of one-third to one-half the section thickness. Examples of good design are shown in Figure 4:4b.