A Comprehensive Guide to Metal Casting Processes

1. Manufacturing Processes Chap. 11 & 12 - Metal Casting Processes

2. Casting Processes • Categorized as: • single use mold & pattern • single use mold, multiple use pattern • multiple use mold • Or also based on mold types: • Expendable, Permanent and Composite • Metals usually cast include: • iron, steel, aluminum, brass, bronze, magnesium, zinc alloys, nickel based superalloys.

3. Casting Processes • Expendable: • Typically made of sand, plaster, ceramics (good refractory qualities). • Usually mixed with bonding agents. • After solidification, mold is broken to remove the casting.

4. Casting Processes • Permanent: • Made of materials that have high strength at high temperatures. • Are used repeatedly/designed such that the part can be easily removed and is ready for next part. • Resulting parts have different microstructure/grain size due to the higher rate of cooling that the mold is subjected to.

5. Casting Processes • Composite: • Made of 2+ different materials (sand, graphite, metal). • Combination allows for more control of mold strength, cooling rates, better economics.

6. Sand Casting • Most popular casting process. Sand is primary mold material. • Sand is mixed with clay,water to give cohesiveness. • Is then packed around pattern with shape of casting. • Can get good detail, wide range of sizes with this method.

7. Sand Casting • See figure 11.9 on steps to create mold. • Create pattern (usually wood.) • Place pattern of desired shape in sand to make imprint. • Incorporate gating system. • Fill resulting cavity with metal. • Allow for solidification. • Break away the sand mold. • Remove casting.

8. Sands • Silica (SO2), other types used due to their low cost and resistance to high temperature. • Should meet the following requirements: • Refractoriness (basic nature of sand) • Cohesiveness (grains coated with moist clay) • Permeability (function of grain size) • round grains for fine packing • not too round to reduce permeability • Collapsibility • There is always a compromise between conflicting factors.

9. Sand Molds • Green-sand: • Uses sand, clay and water (typical mix contains 88% silica, 9% clay, 3% water). • Green means sand is wet during pouring of metal • Least expensive method for molds. • Skin dried: • Mold is dried with air baked or with flames. • Gives more strength to large castings. • Better dimensional accuracy. • Lower collapsibility (may tear the part). • More processing time due to drying.

10. Sand Molds • Cold-mold box: • Binders are blended with sand to bond sand grains for more strength. • More dimensionally accurate that green molds. • More expensive. • No-bake mold: • Resin is mixed with the sand. • Hardens at room temp. • No heat involved in the bonding: cold-setting process.

11. Ceramic Mold Casting • It is a precision casting method. • Material for mold is refractory (zircon, aluminum oxide and silica). • Suitable for high temperature applications (high temp alloys, stainless and tool steels. • Good dimensional accuracy/surface finish. • Expensive; cannot be reused. • Good for super alloys.

12. Expendable-Pattern Casting(Lost Foam) • Also called Lost-Pattern Casting. • Uses polystyrene pattern that evaporates upon contact with molten metal. • The melted styrene leaves a cavity for the casting.

13. Expendable-Pattern Casting- Lost Foam - • Steps: (See fig. 11.12) • Create Pattern • Place polystyrene beads on preheated die. • (Polystyrene expands/takes shape of cavity). • Add heat to help bond beads together. • Cool die and remove pattern. • Coat pattern with refractory slurry and dry.

14. Expendable-Pattern Casting(Lost Foam) • Steps (cont.): • Place pattern in flask (container) and fill with loose sand. (Sand surrounds/supports the pattern). Compact sand periodically. • Pour molten metal into mold. • Molten metal vaporizes pattern by ablation and fills the cavity. • It actually replaces the space occupied by the pattern by de-polymerizing the styrene. • Degradation by-products are vented off through the sand.

15. Expendable-Pattern Casting(Lost Foam) • Steps (cont.): • Let cool and remove part. • Advantages: • Simple: no parting lines, risers. • Inexpensive • Good detail • Minimum finishing • Can be automated.

16. Expendable-Pattern Casting(Lost Foam) • Sample parts • Cylinder heads • Crankshafts • Manifolds

17. Investment Casting(Lost-Wax Process) • Steps: • Make pattern of wax/plastic by molding/other. • Assemble patterns onto a tree. • Coat the pattern tree with refractory slurry. • Recoat with stucco for strength.

18. Investment Casting(Lost-Wax Process) • Steps (cont.): • Place pattern tree in furnace to melt wax out. • Pour molten metal into hollow pattern. Let cool. • Shake off the crust of refractory material. • Cut off parts from tree, finish.

19. Investment Casting(Lost-Wax Process) • Advantages: • Suitable for high melting point alloys. • Good surface finish / dimensional accuracy. • Few finishing operations. • Drawback: Costly.

20. Permanent Mold Casting • Two metal halves are made, usually gray cast iron, steel or refractory alloys, then hinged for opening/closing. • Mold cavity and gating system are machined into the mold. • If part has internal cavities, metal or sand cores are used. • Surfaces of mold cavity are coated with a refractory slurry or splayed with graphite every few castings to increase mold life. • Coating also serves as parting agents.

21. Permanent Mold Casting • Coating also serves as parting agent. • Ejector pins may be required to facilitate the removal of the part. • Molds are clamped mechanically and heated. Metal is poured, metal flows by gravity. • Casting cools and molds are opened.

22. Permanent Mold Casting • Advantages • Mold is reusable. • Produces good surface finish. • Good dimensional tolerances. (0.010”) • Good mechanical properties. • Stronger due to faster cooling rates. • Drawbacks: • Lower melting pt. alloys only. • Mold life limited by metal erosion/thermal fatigue. • Not economical for small runs. • Not good for intricate shapes given the difficulty in removing mold.

23. Permanent Mold Casting • Sample Parts: • pistons • connecting rods • cylinder heads

24. Die Casting • A type of permanent mold casting. • Typical applications are motors, machine components, hand tools, toys. • Two types of processes: • Hot Chamber • Cold Chamber

25. Die Casting • A piston is used to force a volume of molten metal into a die cavity though a nozzle. See figs. 11.20, 11.21. • Hot Chamber Process • A piston is used to force a volume of molten metal into a die cavity though a gooseneck and nozzle. • Pressures used are up to 5000 psi (2000 psi average). • Metal is held under pressure until it cools. • Dies are usually water cooled to improve life.

26. Die Casting • Cold Chamber Process • Chamber is not heated in the process. • Pressures can be 3000 to 10000 psi. • Used with high melting point alloys.

27. Die Casting • Advantages • Can produce strong high quality parts w/complex shapes. • Good dimensional accuracy & surface details. • Little or no subsequent machining / finishing required. • Good thin walls can be made (0.015") due to high pressures used. • Finish marks will remain (parting lines, ejector pins).

28. Die Casting • Limitations • Dies usually made of tool steel, can be quite expensive. • Tool life heavily dependent on pouring temperature: cracking seen due to thermal cycling. • Die material is impermeable: gases cannot escape, leading to porosities, misruns. • Dies should be vented, resulting vents are later trimmed.

29. Summary

30. Typical defects seen in Casting • Shrinkage • Porosity • Cracks • Inclusions • Poor surface finish • Non-fill / misruns • Dimensional Discrepancies • Parting Lines