Fundamentals of Casting

1. Fundamentals of Casting Casting, one of the oldest manufacturing processes, dates back to 4000 B.C. when copper arrowheads were made. Casting processes basically involve the introduction of a molten metal into a mold cavity, where upon solidification, the metal takes on the shape of the mold cavity. Simple and complicated shapes can be made from any metal that can be melted. Example of cast parts: frames, structural parts, machine components, engine blocks, valves, pipes, statues, ornamental artifacts….. Casting sizes range form few mm (teeth of a zipper) to 10 m (propellers of ocean liners). Mechanical Engineering Dept., SJSU

2. Casting Processes • Preparing a mold cavity of the desired shape with proper allowance for shrinkage. • Melting the metal with acceptable quality and temp. • Pouring the metal into the cavity and providing means for the escape of air or gases. • Solidification process, must be properly designed and controlled to avoid defects. • Mold removal. • Finishing, cleaning and inspection operations. Mechanical Engineering Dept., SJSU

3. Sand Casting Terminology Mechanical Engineering Dept., SJSU

4. Sand Casting Process Mechanical Engineering Dept., SJSU

5. Solidification Time Solidification time = C(volume/surface area)2 Where C is a constant that depends on mold material and thickness, metal characteristics and temperature. Mechanical Engineering Dept., SJSU

6. Solidification Time Sphere, cube and a cylinder with the same volume Mechanical Engineering Dept., SJSU

7. Casting Defects Hot tearing – hot tearing, cracking, occurs if casting is restrained from shrinking, during solidification. Mechanical Engineering Dept., SJSU

8. Casting Defects These defects can be eliminated by proper mold preparation, casting design and pouring process. Mechanical Engineering Dept., SJSU

9. Casting Defects Hot spots – thick sections cool slower than other sections causing abnormal shrinkage. Defects such as voids, cracks and porosity are created. Mechanical Engineering Dept., SJSU

10. Casting Defects and Design Consideration Mechanical Engineering Dept., SJSU

11. Design Consideration Mechanical Engineering Dept., SJSU

12. Die Casting Hot chamber, low-melting alloys; tin, zinc, lead. Cold chamber, high-melting alloys; aluminum, copper. Higher pressure needed High production rates, good strength, good dimensional accuracy and surface finish, and good quality parts. Mechanical Engineering Dept., SJSU

13. Centrifugal Casting True centrifugal casting Centrifuging Mechanical Engineering Dept., SJSU

14. Squeeze Casting Cast parts have good mechanical properties, good dimensional accuracy, fine microstructure, good surface finish. Mechanical Engineering Dept., SJSU

15. Plaster Molding Precision Casting Plaster of Paris (gypsum) is used as the mold material, other components are added to improve the strength and reduce the setting time. The pattern is dried at 400 oF and the two halves are assembled and molten metal is poured into the cavity. The process produces fine detail with good dimensional accuracy and surface finish. Used to cast aluminum, zinc and copper-based alloys (low-melting temp.) Ceramic Molding Similar to plaster molding, except it uses ceramic as a mold which is suitable for high temp. applications. Mixture of zircon (ZrSiO4), aluminum oxide (Al2O3), silica (SiO2) and other bounding agents. Mechanical Engineering Dept., SJSU

16. Fabrication of Plastics Injection Molding Mechanical Engineering Dept., SJSU

17. Vacuum Forming Fabrication of Plastics Hot-Compression Molding Mechanical Engineering Dept., SJSU

18. Design Considerations • Wall thickness should be kept uniform if possible. Non uniform wall thickness could lead to warpage and dimensional variations. Minimum recommended; .025 in or .65 mm, up to .125 for large parts. • Round interior and exterior corners to .01-.015 in radius (min.), prevents an edge from chipping. Mechanical Engineering Dept., SJSU