| A | B |
|---|
| Expendable mold processes (definition) | mold is sacrificed to remove part |
| Permanent mold processes (definition) | mold is made of metal and can be used to make many castings |
| Expendable mold processes (advantages and disadvantages) | Advantage: more complex shapes possible Disadvantage: production rates often limited by time to make mold rather than casting itself |
| Permanent mold processes (advantages and disadvantages) | Advantage: higher production rates Disadvantage: geometries limited by need to open mold |
| Overview of Sand Casting | Most widely used casting process. Nearly all alloys can be sand casted, including metals with high melting temperatures, such as steel, nickel, and titanium. Castings range in size from small to very large. Production quantities from one to millions. |
| Steps in Sand Casting (5) | Pour the molten metal into sand mold Allow time for metal to solidify Break up the mold to remove casting Clean and inspect casting Separate gating and riser system Heat treatment of casting is sometimes required to improve metallurgical properties,  |
| The Pattern | A full‑sized model of the part, slightly enlarged to account for shrinkage and machining allowances in the casting |
| Pattern Materials | Wood - common material because it is easy to work, but it warps Metal - more expensive to make, but lasts much longer Plastic - compromise between wood and metal |
| Types of Patterns | Figure 11.3 Types of patterns used in sand casting: (a) solid pattern (b) split pattern (c) match‑plate pattern (d) cope and drag pattern,  |
| Core | Full‑scale model of interior surfaces of part It is inserted into the mold cavity prior to pouring. The molten metal flows and solidifies between the mold cavity and the core to form the casting's external and internal surfaces.,  |
| Chaplets | supports to hold core in position in the mold cavity during pouring |
| Flask | Container that holds the cope and drag |
| Foundry Sands (might not need to know) | Silica (SiO2) or silica mixed with other minerals. Good refractory properties ‑ capacity to endure high temperatures Small grain size yields better surface finish on the cast part Large grain size is more permeable, allowing gases to escape during pouring |
| Green‑sand molds | mixture of sand, clay, and water; “Green" means mold contains moisture at time of pouring |
| Dry-sand mold | organic binders rather than clay. And mold is baked to improve strength |
| Skin-dried mold | drying mold cavity surface of a green‑sand mold to a depth of 10 to 25 mm, using torches or heating lamps |
| Shell Molding | Casting process in which the mold is a thin shell of sand held together by thermosetting resin binder,  |
| Steps in shell‑molding | (1) a match‑plate or cope‑and‑drag metal pattern is heated and placed over a box containing sand mixed with thermosetting resin. (2) box is inverted so that sand and resin fall onto the hot pattern, causing a layer of the mixture to partially cure on the surface to form a hard shell; (3) box is repositioned so that loose uncured particles drop away; (4) sand shell is heated in oven for several minutes to complete curing; (5) shell mold is stripped from the pattern;,  |
| Advantages of shell molding | Smoother cavity surface permits easier flow of molten metal and better surface finish. Good dimensional accuracy - machining often not required. Mold collapsibility minimizes cracks in casting. Can be mechanized for mass production. |
| Disadvantages of shell molding | More expensive metal pattern. Difficult to justify for small quantities |
