Machinery's Handbook, 31st Edition
1514 SQUEEZE CASTING The origins of squeeze casting can be traced back to squeeze forming, which is a process structured in three phases: 1) Pouring a known amount of molten metal in a preheated die cavity placed at a lower plate of a press. 2) Closing the die, pressurizing the liquid metal, and maintaining the pressure until complete solidification. 3) Casting ejecting and handling are done in much the same way as in closed die forging. As shown in Fig. 17, squeeze casting consists of liquid metal entering a preheated, lubri- cated die and pressurizing the metal while it solidifies. The load is applied shortly after the metal begins to set and is maintained until the entire casting has solidified. A number of variables can be controlled to influence the soundness of the castings. Design Considerations for Casting When designing cast metal components, several characteristics are inherent in the pour- ing and solidification of the alloy and its interaction with the mold. These characteristics define the parameters within which the designer can work and affect the casting method chosen, design of casting sections and junctions between sections, surface integrity and appearance, internal integrity of the cast alloy, and dimensional accuracy. Carefully planned casting geometry allows the foundry to work with the known pour- ing and solidification characteristics to produce high-quality castings that perform to required specifications, while avoiding costly and time-consuming problems. By under - standing the interaction between casting geometry, the material (in both liquid and solid forms), and the casting process, design engineers can anticipate and avoid many iterations to their product design that could otherwise interfere with and delay progress of a project. The following geometry/material/process interactions dictate good casting design. Fluid Life: This refers to the liquid characteristics of the material that give it the ability to flow freely throughout the mold, along narrow sections and into fine surface detail. Fluid life depends on temperature and the unique chemical and metallurgical properties of each metal. At least in part, it determines minimum wall thickness and how long a thin section can be. So, the designer’s choice of alloy, with its associated fluid life, will dictate certain structural and aesthetic elements of the design. Solidification Shrinkage: As molten metal cools, shrinkage occurs in three distinct stages: liquid shrinkage, liquid-to-solid shrinkage, and solid shrinkage. Solid shrink- age is the continued shrinkage that occurs as the metal casting cools to ambient temper- ature in its solid state. Known as “patternmaker’s shrinkage,” it must be compensated for within the tooling or mold design to ensure that specified final overall dimensions are achieved. Slag/Dross Formation: Some metals are more susceptible to slag and dross formation than others, so are more likely to have small, rounded, non-metallic inclusions within the casting. Design, along with quality and process control, can reduce the quantity and effect of inclusions. Where buoyant slag/dross is likely, a designer may ensure that important surfaces or those to be machined sit low in the mold. With awareness of the likelihood of slag/dross formation for a specific alloy, the foundry engineer can set up the running system (sprues, runners, and gates) to minimize oxidation caused by turbulent flow or entrained air. Pouring Temperature: Pouring temperature can have a significant effect, particularly when the molten metal approaches the mold material’s refractory limit. It is important to note that the concentration of heat in one area can cause problems even with some lower temperature alloys. Better geometry design can help heat dissipate into the mold and al- leviate such issues. Fluid Flow: Designers should take in account the flow rate of molten metal entering the mold and issues this can cause. A balance needs to be struck between the need to get
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