EGR 242 Manufacturing Processes:Lecture 07: Metal Casting Processes:

Today: Casting Processes Sand Casting Investment Casting Lost Wax Casting Ceramic Casting Homework: Problem Set 9-1: Problem 1 Problem Set 9-1: Problem 7 Problem Set 9-1: Problem 9 Read Chap 10 and 11.

Video Presentation: SME and GE Casting videos Lab: Make casting forms of plastic and plaster.

An Introduction to Casting Processes:Casting involves placing a liquid metal material into a mold where it will harden and solidify and take the shape of the mold as its final shape.

Advantages of casting over other manufacturing methods:--casting can produce very complex shapes with internal cavities or hollow sections--casting can produce very large parts (1 oz to 200 tons)--casting can utilize work-piece materials that are difficult or uneconomical to process by other means--casting is competitive economically with other processes.

The steps of the casting process:1) create a mold from a pattern2) pour molten metal into the mold 3) allow part to cool4) remove the metal part from the mold5) heat treat and finish machining the part

Factors that affect the overall properties of the cast part:1) type of metal 2) thermal properties of the metal3) thermal properties of the mold

4) geometric relationship between volume and surface area of the casting5) shape of the mold6) porosity of the mold7) metal pour rate

Casting Supply and End-use Markets

Supply Markets   Metal Casting Processes

  End Use Markets

   Gray iron - 42%   Ductile iron - 31%   Aluminum - 10%   Steel - 10%   Copper - 2%   Zinc - 1.5%   Other - 3.5%

   Automotive and light truck - 35%   Pipe and Fittings - 15%   Construction, Mining, and Oilfield Machinery - 6%   Internal Combustion Engines - 5%    Railroad - 5%   Valves - 5%    Farm Equipment - 3% 

   Municipal Castings - 3%    Other - 23%

Material behavior that affects casting quality:1) most metals shrink when solidifying and when cooling to room temperature2) molten metals have a greater solubility for gases than solid metals. 3) cast parts solidify starting at the mold wall4) depending on composition alloys start solidifying at different temperatures 5) large parts cool more slowly than small parts6) cooling rate affect both grain size, structure, orientation, and ultimately strength

Proper design of the casting system are used to:1) ensure adequate fluid flow to mold2) stop contaminants from entering casting3) avoid premature cooling of casting material4) produce laminar flow of molten metal5) avoid gas entrapment6) provide increased cooling to thicker regions

Defects which can occur in cast parts:1) Metallic projections: (swells, rough surfaces, flash, fins)2) Cavities, internal and exposed (blow holes, pin holes, shrinkage cavities)3) Discontinuities: (cracks, cold tears, hot tears, cold shuts)4) Defective surface: (surface folds, laps, scars, adhering sand, oxide scale)5) Incomplete casting: (misruns, insufficient volume, runout)6) Incorrect dimensions or shape: (improper shrinkage allowance, pattern mounting error, deformed pattern, warped casting)7) Inclusions, trapped insoluble particles (oxides, slag, mold and core spalling)

Films: 1) General Electric, Lynn Manufacturing on investment casting, sand casting, and permanent mold casting. Tape 4 at 28:00 min.2) AIM website: lost foam casting and die casting.3) SME Fund. Manufacturing Processes Sampler: Casting and Die Casting

Metal Casting Processes

Expendable mold methods Sand Shell Expendable Pattern or (Lost Foam) Plaster Ceramic Investment

Permanent mold methods Gravity Feed Permanent Slush Low-Pressure Die Centrifugal Squeeze Semisolid

Expendable molds--molds made of sand, plaster, ceramics, or other materials capable of withstanding high temperatures which are broken up and discarded after a single use.

Permanent Molds--molds usually made of metal which maintain strength at high temperatures and are used for making a large number of repeated castings.

Composite Molds--molds which use elements of both expendable and permanent molds. Sand Casting: -- most commonly used method of casting -- inexpensive tooling -- can be used for most metals -- no limit to size, shape, or weight of cast parts -- coarse finish may required extra machining -- limited control of tolerances. Three common types of sand molds: green molding sand -- mixture of sand, clay, and water. -- least expensive -- used for large parts -- require drying cold box sands -- blends in inorganic and organic binders to chemically bond sand grains -- maintains more accurate tolerances of casting -- more expensive than green sand no-bake molds -- liquid resin is mixed with sand which hardens at room temperature. Parts of a sand casting system:1) Flask--the frame and support2) Cope-- the upper half of sand mold3) Drag -- the lower half of the sand mold4) Pouring basin--orifice where molten metal is poured into the mold5) Sprue--vertical channel through which molten metal flows downward into mold6) Runners--channels that carry molten metal from sprue to mold

7) Riser--reservoirs to supply the molten metal needed to make up shrinkage losses during solidification. 8) Gate-- portion of runner through which the molten metal enters the mold cavity, used to trap contaminants9) Vents--openings used to carry off gases given off by the metal and exhaust air10) Cores--insets used to produce interior cavities11) Chills--used to increase the rate of solidification by controlling thermal heat transfer12) Pattern-- shape used to mold the shape of the casting

Sand casting process: Step-by-Step

Shell-mold Casting: -- uses thermal setting resin binder to form thin shell of fine sand -- developed in 1940's -- gives close dimensional tolerances -- creates good surface finish -- easy to automate -- limited part size -- more expensive tooling and patterns required -- shells have poor porosity -- may be used as part of a composite molding process

Sodium Silicate Process: --uses sodium silicate as a binder mixed with sand --hardens by blowing CO2 through it. --more often packed as a sand mold than a shell

Rammed Graphite Molding: -- uses graphite instead of sand in process similar to sand -- used for reactive metals castings such as titanium and zirconium -- requires packing and baking of molds

Expendable pattern casting (evaporative or lost pattern casting): --uses a polystyrene pattern which evaporates with hot metal contact 1) a metal die is used to create polystyrene patterns 2) pattern is coated with water-based refractory slurry and dried. 3) sand compacted around the coated pattern 4) molten metal is poured into the mold replacing the polystyrene 5) sand mold is broken away from casting -- may be used for complex shapes and fine details -- good for most metals -- may be used for large castings -- requires minimal finishing and cleaning operations -- uses inexpensive polystyrene and sand

-- large expense is cost of die to make the patterns

Plaster mold casting: -- mold made of plaster of paris with strengtheners -- applied as slurry over the pattern -- requires drying -- good for intricate shapes -- good dimensional accuracy and finish -- poor porosity -- limited to nonferrous metals -- limited size and volume -- making the mold takes a

comparatively long time Ceramic Mold Casting: -- uses a slurry of fine grained zircon, aluminum oxide, and silica mixed with bonding agents -- slurry is poured over pattern to form upper and lower mold set, then removed, dried, and baked. -- mold is backed by fireclay for added strength before molten metal is poured into mold -- good for high temperature materials such as high temperature alloys, stainless steel, and tool steels. -- give good dimensional accuracy and surface finish -- intricate shapes -- somewhat limited size -- relatively expensive

Investment Casting: -- uses a molten wax pattern shaped by a metal die -- pattern is dipped in slurry of refractory material and dried repeatedly building up a thick skin -- mold is heated to melt wax -- used for wide variety of metals including high temperature alloys -- gives good external and internal surface finish and close tolerances -- give good detail -- relatively expensive process

Permanent Mold Methods: --Refers to methods which employ a mold that is used repeatedly. --Permanent molds require use of draft angles to be able to remove castings. --Coatings or dressings (graphite, silicon, etc) are used between the mold and the cast part for ease of separation. --Ejectors may be used to separate parts--Molds are often made from cast iron, steel, bronze, graphite, or metal alloys

--Molds are two piece construction and are clamped together.--Castings are usually limited to lower temperature alloys.

Gravity Feed Permanent Mold Casting (gravity die casting): -- gravity feed of molten metal -- used for low temperature metal castings including aluminum, copper, magnesium, and zinc. -- gives close tolerances -- creates uniform and good mechanical properties -- low labor costs, high die costs -- Slush Casting: a type of gravity feed permanent mold casting where the casting is inverted before the part has completely solidified, draining off most of the inner molten material, leaving a shell style casting.

Low-Pressure Casting -- molten metal is forced into mold usually from below by gas pressure or is drawn in by vacuum -- uses metal or graphite molds -- used for high quality castings -- mostly used for aluminum, magnesium or copper alloys.

Die Casting: --developed in early 1900’s --molten metal forced into the die cavity under high pressure. --molten metal is forced into mold with piston by hydraulic pressure --Two common processes: Hot Chamber—uses submerged pump in liquid metal Cold Chamber—uses piston with a single load of molten alloy -- primarily uses for zinc, magnesium, brass, and aluminum alloys, although Cold Chamber process can handle higher temperatures than Hot Chamber process. -- dies are made of hot-work die steels or mold steels -- excellent dimensional accuracy and surface finish -- low porosity -- easily automated -- high production rates -- high die cost and long lead time -- limited part sizeType of Die Casting Machines: Hot Chamber Die Casting Cold Chamber Die Casting

Die Casting Steps:

Centrifugal Casting:

-- uses inertial forces caused by rotation to distribute metal in mold Three common processes: 1)True centrifugal casting -- used to make hollow cylindrical, square, or polygonal parts. -- can be used for long horizontal castings of variable wall thickness2) Semicentrifugal casting -- used to cast parts with rotational symmetry 3) Centrifuging -- uses centrifugal forces to control casting properties.

Semi-solid Processing:

-- combines casting and forging processes -- solidifies metal under high pressure -- reduces porosity of casting 1) Squeeze-casting or melt forging -- gives fine grain structure -- near net shape parts 2) Rheocasting -- mixing of semi-solid slurry breaking up dendritic structures -- mixture injected into mold or die. 3) Thixoforming (semi-solid forging) -- forging of metal billets with 30%-40% still liquid

Table 11.1 Summary of Casting Processes, Their Advantages and LimitationsProcess Advantages Limitations

Sand Almost any metal cast; no limit to size, shape, or weight; low tooling cost

Some finishing required; somewhat coarse finish; wide tolerances

Shell mold Good dimensional accuracy and surface finish; high production rate

Part size limited; expensive patterns and equipment required

Expendable pattern Most metals cast with no limit to size; complex shapes

Patterns have low strength and can be costly for low quantities

Plaster mold Intricate shapes; good dimensional accuracy and finish; low porosity.

Limited to nonferrous metals; limited size and volume of production; mold making time relatively long.

Ceramic mold Intricate shapes; close tolerance parts; good surface finish

Limited size.

Investment casting Intricate shapes; excellent surface finish and accuracy; almost any metal cast

Part size limited; expensive patterns, molds, and labor.

Permanent mold Good surface finish and dimensional accuracy; low porosity; high production rate

High mold cost; limited shape and intricacy; not suitable for high-melting-point metals.

Die casting Excellent dimensional accuracy and surface finish; high production rate.

Die cost is high; part size limited; usually limited to nonferrous metals; long lead time.

Centrifugal Large cylindrical parts with good quality; high production rate

Equipment is expensive; part shape is limited.