Close inspection of these rules reveals that they sometimes recommend conflicting geometries. For example, the use of gusseting instead of mass for stiffness might be labeled “recommended” in one set of design rules and “poor” in another.
Further, when a design engineer leaves a familiar casting design realm for an unfamiliar one, unexpected trouble may result. For example, let’s say we are moving from ductile iron to aluminum bronze while staying in a familiar metalcasting process, nobake molding. No alarms are sounded among the “rules of thumb,” but there’s likely trouble in the usual “ductile iron-style” geometry. Good aluminum bronze geometry is different than typical ductile iron geometry, and the molding process may need to supplement the different geometry with heat transfer techniques. Not suspecting this, the design engineer’s new casting design may suffer from “no-quotes,” or higher-than-expected prices and requests for design changes.
How are design engineers supposed to know that successfully casting geometry for aluminium bronze should somehow be different? And if a design engineer did know that, what would be the proper course of design action?
The answer lies in a better understanding of the relationship among geometry, various metalcasting alloys and structure.
Six parameters (based on physics) underlie cost-effective casting design: fluid life, solidification shrinkage, slag/dross formation tendency, pouring temperature, section modulus and modulus of elasticity.
All six, applied as a system, drive the geometry of casting design. Geometry is not only the result of casting design but is also the most powerful weapon in creating successful casting design.
This six-faceted system is capable of optimizing geometry for castability, structure, downstream processing (machining and assembly) and process geometry (risering, gating, venting and heat transfer patterns) in the mold. The process geometry forms the casting geometry.
Quickly sorting through possible casting and process geometries by marking up blueprints or by making engineering sketches is the way to find optimal “system” geometry. An elegant result of good sketched brainstorming can be a solid model of the casting and its process geometry, the basis of rapid prototyping and/or computerized testing.
Applying the System
Optimizing casting geometry using the six-parameter system is not difficult. The six casting and structural characteristics influence important variables in designing, producing and using metal castings. These variables include:
• casting method;
• design of casting sections;
• design of junctions between casting sections;
• surface integrity;
• internal integrity;
• dimensional capability;
• cosmetic appearance.
Both the designer and metalcaster possess a vital ally to streamline any casting design. Casting geometry is the most powerful tool available to improve castability of the alloy and mechanical stiffness of the casting.
Carefully planned geometry can offset alloy problems in fluid life, solidification shrinkage, pouring temperature and slag/dross forming tendency. Section modulus, an attribute of structural geometry, has the capability to increase stiffness and/or reduce stress—a capability that can be very important when applied to alloys with lower strength and stiffness. Modulus of elasticity, an alloy’s inherent stiffness, can be combined with section modulus and section length to limit or allow deflection in a casting design.
CASTING PROPERTIES
1. Fluid Life
Fluid life more accurately defines the alloy’s liquid characteristics than does the traditional term “fluidity.” Molten metal’s fluidity is a dynamic property, changing as the alloy is delivered from a pouring ladle, die casting chamber, etc. into a gating system and finally into the mold or die cavity. Heat transfer reduces the metal’s temperature, and oxide films form on the metal front as this occurs. Fluidity decreases most rapidly with temperature loss, and it can decrease significantly from the surface tension of oxide films.
The absolute value of temperature is not the test of fluidity at a given moment. For example, some aluminum alloys at 1,200-1,400F (650-750C) have excellent fluid life. However, some molten steels at 3,000F (1,650C) have much shorter fluid life. In other words, a molten alloy’s fluid life also depends on chemical, metallurgical and surface tension factors.
Fluid life affects the design characteristics of a casting, such as the minimum section thickness that can be cast reliably, the maximum length of a thin section, the fineness of cosmetic detail (like lettering and logos) and the accuracy with which the alloy fills the mold extremities.
It is essential to understand that moderate or even poor fluid life does not limit the cost-effectiveness of design. Knowing that an alloy has limited fluid life tells the designer that the part should feature:
• softer shapes and larger lettering;
• finer detail in the bottom portion of the mold, where metal arrives first, fastest and generally hottest;
• coarser detail in the upper portions of the mold where the metal is slower to arrive and more affected by oxide films and solidification “skin” formation. Even an alloy with good fluidity, when overexposed to oxygen, may form a high surface tension oxide film that makes the fluidity die, “rounding off” the leading metal front as it flows.
• more taper toward thin sections.
Some alloys, like 356 aluminum, have been specifically designed metallurgically to enhance fluid life. In the case of 356, the high heat capacity of silicon atoms “revives” aluminum atoms as their fluid life begins to wane.
本文转自：China Industry News
本文链接：http://news.made-cn.org/post/Cost-Effective-Casting-Design.html