Author: Mike Gwyn; Pelton Casteel, Inc.
One of the basic principles of the “system approach” in engineering design is to define the system boundaries in such a way that conflicting requirements can be recognized and resolved.
Understanding the differing mind-sets (An overview)
Well-designed castings are known for being functional and cost efficient. Yet those directly involved in designing and producing castings know that the principles behind well-designed castings are difficult to pin down. “Rules of thumb” abound that attempt to define fillets, radii, changes of casting section, minimum section thickness, tolerance capability, etc. Yet, there are regularly casting designs that seem to violate the “rules” successfully. These designs typically have combinations of geometry that should not work, but that do.
On the other hand, design engineers observing this attempt to take latitudes with geometry that seem well founded only find that their design is either not consistently castable or is castable, but at a price that is too high.
This has been a mystery for many, many years that has frustrated design engineers, aggravated foundrymen who attempt to produce troublesome designs, and caused other forms of metal products to be designed when a casting would be the best product — if properly designed.
First, we must look at the viewpoint of each engineer:
Design Engineers typically consider functional mechanical elements, loads, function environment, failure modes, mechanical and physical properties, fabricated shapes, automated secondary operations and cosmetics.
The metalcasters, patternmakers, and die engineers see fluid flow, heat transfer and solidification patterns in the mold, including hot spots as the metal changes from the liquid phase to the solid phase; they see possibilities for infinite variability in casting shape. They also see foundry tooling (patterns, dies, and/or core boxes) that are critical to dimensional accuracy and consistency. They see surfaces to be machined; other surfaces that must be consistent dimensionally for machining fixturing and targeting. They see possibilities for specific alloys and heat treatments that are needed for the casting’s mechanical and physical properties. They see the need for pleasant casting as-cast cosmetics.
Finally, when the design geometry and the alloy’s castability are in conflict with each other, the metalcaster must consider “thermal trickery,” which is the use of chills, insulating, exothermic materials and other heat transfer gimmicks to set up necessary solidification patterns in the casting which are not possible from the casting geometry itself.
Based on these widely differing viewpoints, it would be surprising to find good casting designs to be obvious and trivial. In fact, cost-effective casting design is a technically demanding task for the design engineer.
One of the basic principles of the “system approach” in engineering design is to define the system boundaries in such a way that conflicting requirements can be recognized and resolved. This is the principle that we are applying here. As our conceptual framework is explained, it will become apparent that geometry holds the key to resolving the design conflict identified within properly defined system boundaries.
Elements of Conceptual Framework
Physical and Mechanical Characteristics
Four important physical characteristics affect the castability and performance of any given casting alloy. These are:
- Fluid life
- Solidification Shrinkage
- Slag and/or dross formation tendency
- Pouring temperatures
Each of these characteristics vary widely among alloys and can be significantly different among similar alloys. Differences among these four physical characteristics significantly affect the geometry of well-designed castings.
It is also important to understand two important mechanical characteristics affecting the stiffness of any give casting design:
- Modulus of elasticity
- Section Modulus
The former is a function of the stiffness of the alloy itself and the latter is a function of stiffness from the casting’s geometry. These two mechanical characteristics are also at the heart of well-designed geometry.
Using these physical and mechanical characteristics
Recognize that the above six characteristics affect important variables in designing, producing and using metal castings. These variables include:
- Casting method
- Design of casting sections
- Design of junctions between casting sections
- Internal integrity required
- Dimensional tolerances and extent of near-net shape requirements
- Cosmetic appearance
Casting geometry as a tool.
Casting geometry is the most powerful tool available to improve the following:
- Castability of the alloy
- Mechanical stiffness of the casting
Carefully planned geometry can offset alloy problems in fluid life, solidification shrinkage, pouring temperature and slag/dross formation tendency. Section modulus from geometry has the power to offset problems with lower modulus of elasticity.
What to avoid
In developing a sound conceptual framework for casting design, it is important to avoid reliance on some traditional concepts and tools such as:
- Rules of thumb
- General “Do’s and Don’ts” typical of casting design handbooks
- Simple, orthogonal shape thinking; such as building blocks from mill shapes like plates, bars, tubes, I-beams, other kinds of extrusions of constant cross section, etc.The above shapes limit metalcasting’s power of infinite shape variability. Casting geometry can be so much more free-flowing than orthogonal, extruded, and rotated shapes.
Knowledge and Understanding
Just as important as avoiding the above traditional tendencies in design engineering is to know and understand the nature of molten metal and use it to your advantage.
- Embrace the idea of infinitely variable shape.
- Use free-hand sketches for a conceptual designing.
- Move mass around
- Take mass out where it is not needed; put it where it is necessary.
- Use variability of section modulus over length.
- Use the ability to vary section modulus over section length to design for uniform stress.
Systems approach style to design thinking
Develop a “systems approach” style to design thinking. Such an approach encompasses everything, from the original need for a mechanical or structural element, to molten metal flowing into a shape, to the rough casting right out of the mold or die, through casting finishing requirements, secondary processing in the foundry, secondary processing at a subcontractor and/or the customer’s plant, testing, assembly, and final use and abuse of the product which contains the casting.
The only way to resolve conflicting requirements without a “Rube Goldberg” result is to conceive the system needs at the outset. When applied, the “system approach” style of thinking results in truly cost-effective, simple, elegant metalcasting design.