Manufacturing Processes Chap. 11 & 12 - Metal Casting Processes

Casting Processes Categorized as: Or also based on mold types: 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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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

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.

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.

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.

Permanent Mold Casting Sample Parts: pistons connecting rods cylinder heads

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

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.

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.

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).

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.

Summary

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