Metal Shaping Processes. Vukota Boljanovic
INTRODUCTION
Increased competition for commercial casting products has highlighted the need to provide engineers and managers with a better understanding of the interrelationships between product design, material, and process selection and of economical methods of casting for specific applications. Successful casting practice requires the proper control of numerous variables. These variables pertain to the particular characteristics of the casting shape, the metals and alloys cast, the type of mold, the mold and die materials, and various process parameters.
3.2 CASTING DESIGN CONSIDERATIONS
A number of factors affect the successful final design of a cast component. Often, however, simple design changes can have a significant impact on the cost, speed of production, and usefulness of the part. For example, casting drill spots may be preferred to casting a cored-through hole. Even though an extra step is required, offhand drilling after casting may be simpler and less expensive than creating a complex mold with numerous cores.
All casting processes have the same characteristics: for example, the process begins before the pattern or die is made, at the point when an engineer decides on the best design for the component; the cost of materials is another important design consideration for all type of casting processes. Accurate comparisons require looking beyond the cost per pound or cost per cubic inch to fully analyze the advantages and disadvantages of each competing process. For instance, the relatively greater strength of metals generally allows thinner walls and sections and consequently requires fewer cubic inches of metals for a given application.
The one rule that covers every stage of good design is communication. While there are rules that govern how molten metal will solidify and take shape as a cast component, each casting process will affect the metal differently and will offer its own benefits. Before issuing a final drawing, it is imperative the design engineer consult a foundry team or patternmaker. The engineer must know how to design a casting that will actually have the requisite strength and functional properties, while a foundry team must be able to make the casting so that it has the strength and functional properties the engineer intended. From a foundry point of view it is more important to receive a component design that is practical and efficient than a “perfect” design that cannot be produced commercially without structural weakness.
Consultation will permit consideration of foundry problems that are likely to be encountered and will promote casting soundness. The time and cost of manufacture also should be considered in the preliminary stages of casting design. Hence, it is recommended that before the design or the order is finalized, the design engineer should communicate with the foundry to discuss pertinent issues such as the following:
•the proposed casting design and accuracy required;
•machining requirements;
•method of casting;
•the number of castings;
•the casting equipment involved;
•Any special requirements (for instance, datum target systems, individual dimensional tolerances, geometrical tolerances, fillet radii tolerance, and individual machining allowances);
•whether any other standard is more appropriate for the casting.
3.2.1 General Design Considerations for Casting
The responsibility of the casting engineer is to determine how to produce the part as a metal casting. He or she studies the original design and determines how best to design the part for manufacturability and casting. Once those considerations are defined, the engineer goes through a specific set of decisions in order to develop a casting design and a casting pattern for making the mold or die.
a) Design of Cast Parts
The following guidelines should be considered:
Corners and angles. Avoid designing parts with sharp corners; sharp corners produce a differential cooling rate, creating “hot spots,” the most common defect in casting design; mechanical weaknesses and stress concentration open the way for potential cracks.
In design of adjoining sections, sharp angles should be replaced with radii, and heat and stress concentration should be minimized. However, adding radii that are too large may also result in shrinkage defects. By incorporating small fillet radii, hot spots are avoided, assuring improved strength. Some examples of improved designs are shown in Fig. 3.1.
Fig. 3.1 Suggested design modifications to improve quality of castings: a) and c) incorrect; b) and d) are correct.
Fillets. Fillets (rounded corners) have three functional purposes:
•to reduce the stress concentration in a casting in progress;
•to eliminate cracks, tears, and draws at reentry angles;
•to make corners more moldable by eliminating hot spots.
The number of fillet radii should be the least possible, preferably only one. To fulfill engineering stress requirements and reduce stress concentration, relatively large fillets may be used with radii equaling or exceeding those of the casting section. Fillets that are too large are undesirable. The radius of the fillet should not exceed half the thickness of the section joined.
At an “L” junction, round any outside corner to match the fillet on the inside wall. In the case of “V” or “Y” sections and other angular forms, always design so that a generous radius eliminates localization of heat. Figure 3.2 shows suggested design modifications toward improving the quality of casting, avoiding filleting, and making corners more moldable.
Fig. 3.2 Redesigned part to avoid hot spots and fillet: a), c), e) and g) are incorrect; b), d), f), and h) are correct.
Section thickness. Maintain uniform cross-sections and thickness where possible. Thicker walls will solidify more slowly, so they will feed thinner walls, resulting in shrinkage voids. The goal is to design uniform sections that solidify evenly. If this is not possible, all heavy sections should be accessible to feeding from risers.
Figure 3.3 shows an example of a part that has been redesigned with uniform walls; the weight of the casting was reduced, lowering the manufacturing cost and remedying the shrinkage problem.
Fig. 3.3 Redesigned part with uniform walls: a) incorrect; b) correct.
The inner sections of castings cool much more slowly than the outer sections and cause variations in strength properties. A good rule is to reduce the inner sections to 0.9 of the thickness of the outer wall.
The inside diameter of cylinders and bushings should exceed the wall thickness of the castings. When the inside diameter of a cylinder is less than the wall thickness, it is better to cast the section solid, as holes can be produced by cheaper (and safer) methods than with extremely thin cores.
Section changes. The design should not contain abrupt section changes. Section changes should blend into each other (Fig. 3.4). The difference in the relative thickness of adjoining sections should not exceed a ratio of 2:1. If a section change of over 2:1 thickness ratio is unavoidable, the alternative is