Chapter 11 - University of North Florida

Chapter 11 - University of North Florida

Metal Casting Processes: Introduction ¾ History: Made for millenia. Used to pour copper into stone and metal molds 4000 – 3000 B.C. ¾ What parts are m...

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Metal Casting Processes: Introduction ¾ History: Made for millenia. Used to pour copper into stone and metal molds 4000 – 3000 B.C. ¾ What parts are made using the casting process:

Chapter 11 Metal-Casting Processes

ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ ƒ

Alexandra Schonning, Ph.D. Mechanical Engineering University of North Florida Figures and figure text by Manufacturing Engineering and Technology Kalpakijan and Schmid

Cameras Engine blocks Automotive components Agricultural components Railroad components Pipes and plumbing fixtures Power tools Gun barrels Frying pans

(a)

(b )

¾ Trends

ƒ Automation ƒ Demand for high quality

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Mold Classifications

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Sand Casting: The Steps

¾ Classification based on ƒ Mold material ƒ Molding process ƒ Method of filling the mold with molten metal

¾ Expandable Molds ƒ Sand, plaster, ceramic. ƒ Mixed with binders or bonding agents ƒ After solidification of casting: the mold is broken and cannot be reused!

¾ Permanent Molds ƒ Made of metals that retain their strength at high temperatures ƒ Casting can be removed without destroying the mold – Can be reused!

¾ Composite Molds ƒ Made of two or more materials (sand, graphite, and metal) ƒ Combining the advantage of each material

¾Placing the pattern in the sand to make an imprint ¾Adding a gating system ¾Filling the cavity with molten metal ¾Letting the casting to cool and solidify ¾Breaking away the sand mold ¾Removing the casting ¾Cleaning the casting

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Sand Casting: Sands

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Sand Casting: Types of Sand Molds

¾ Silica (SiO2) is commonly used ¾ Why Sand?

¾ Green-sand mold ƒ Most common and least expensive ƒ Sand, clay and water ƒ Sand is moist when metal is poured into mold

ƒ Inexpensive ƒ Resistance to high temperatures

¾ Cold-box mold

¾ Types of sand

ƒ Cold setting process ƒ More expensive than green sand molds ƒ Organic and inorganic binders are mixed with the sand

ƒ Naturally bonded ƒ Synthetic • Generally preferred as it can be controlled better

¾ Sand grain

¾ No-bake molds

ƒ Small: nicer surface, higher mold strength, lower permeability (ability to allow gases to escape) ƒ Want good collapsibility (mold shrinkage while casting cools) Page Page11-5 1-5

ƒ Cold setting process ƒ Synthetic liquid resin is mixed with the sand. Mixture hardens at room temperature. Page Page11-6 1-6

1

Sand Casting: Major components

Sand Casting: Patterns

¾ Mold ƒ ƒ ƒ ƒ

Cope: top Drag: bottom Parting line between them Cheeks: when more than two mold parts are used

¾ Flask: Mold support ¾ Pouring basin: metal is poured in here ¾ Spruce: metal flows down through it ¾ Runner system: carries the metal from the spruce to the cavity. ¾ Gates: inlets to the mold cavity ¾ Risers: additional metal supply. ƒ Blind risers ƒ Open risers

¾ Cores:

ƒ Inserts made from sand to form hollow regions

¾ Vents

¾Used to mold the sand mixture into the shape of the casting. This is what you want to make a “copy” of. ¾Used repeatedly ƒ Strength and durability is important ƒ Coated with a parting agent (for easy removal)

¾Made from ƒ Wood, rapid prototyping, other

¾Considerations ƒ Metal shrinkage ƒ Draft angles (ease of removal)

ƒ Carry off gases Page Page11-7 1-7

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Sand Casting: Patterns

Sand Casting: Cores

TABLE 11.3 a

Characteristic

Wood

Aluminum

Rating Steel

Plastic

Cast iron

Machinability E G F G G Wear resistance P G E F E Strength F G E G G Weightb E G P G P Repairability E P G F G Resistance to: Corrosionc E E P E P Swellingc P E E E E aE, Excellent; G, good; F, fair; P, poor. bAs a factor in operator fatigue. cBy water. Source : D.C. Ekey and W.R. Winter, Introduction to Foundry Technology. New York. McGraw-Hill, 1958.

¾Internal cavities or passages ¾Cores are placed in the mold cavity before casting to form the interior surface of the casting ¾Anchored by ƒ Core prints ƒ Chaplets (may be needed to hinder shifting) • Are left in the casting after solidification

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Sand Casting: Machines

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Sand Casting: Machines (con’t)

¾ Compact the sand by hammering ¾ Mold machines: eliminate demanding labor. Most compact at squeezing head ¾ Jolting ƒ Most compact at the parting line ƒ Rapidly jolted upwards

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¾ Vertical flaskless molding ƒ Eliminates need for flasks Æ good for high production

¾ Sandslingers ƒ Used to fill the flask uniformly with sand under a high pressure stream

¾ Impact Molding ƒ Sand is compressed by controlled explosion

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2

Sand Casting: Steps

Sand Casting: The Operation (pg 1 of 2)

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Sand Casting: The Operation (pg 2 of 2)

Shell-Mold Casting ¾ Developed in 1940s ¾ Close dimensional tolerance, good surface finish

¾ Mounted pattern of a ferrous metal or aluminum is heated to 175- 370oC ¾ Coated with parting agent (silicone) ¾ Clamped to a chamber full of sand mixture ¾ Sand mixture consists of thermosetting resin binder ¾ Mixture is coated over the pattern ¾ Heated in oven for curing ¾ Thin shell hardens (5 – 10 mm) ¾ Shell is removed using ejector pins ¾ Two half shells are bonded or clamped together ¾ Advantage: Fine grains are used ƒ Low resistance to flow of metal Æ sharper corners, thinner sections

¾ Cost:

ƒ Decrease: 1/20 of sand compared to sand casting ƒ Increase: Resin binders ƒ Metal patterns are costly but less so for large production runs

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Expandable-Pattern Casting (Lost Foam) ¾ Also called ƒ ƒ ƒ ƒ

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Expandable-Pattern Casting (Lost Foam), Cont.

¾ Disadvantages

Evaporative pattern Lost pattern casting Full Mold Process Expanded polystyrene process

ƒ Molten metal cools faster than if pattern would have been removed • Less fluidity • Directional solidification of the metal

¾ Pattern made of expandable polystyrene (EPS)

¾ Advantages

ƒ Beads are placed in a die ƒ Die is heated – beads expand to form the pattern

ƒ Simple: no parting lines, cores, riser systems ƒ Inexpensive flasks are OK ƒ Polystyrene is inexpensive and can be used for complex shapes and fine surface detail ƒ Minimum finishing and cleaning is required ƒ Can be automated and is economical for long production runs

¾ Process

ƒ Patter is

• coated with a water based refractory slurry • Dried • Placed in a flask

ƒ Flask is filled with fine sand ƒ Sand is compacted ƒ Metal is poured in w ithout removing the pattern ƒ The pattern is depolymerized (degraded) and vented into the the surrounding sand.

¾ Applications ƒ Brake components for automobiles ƒ Machine bases ƒ Aluminum engine blocks

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3

Plaster-Mold Casting ¾ The Process ¾ Mold is made of plaster: gypsum, calcium sulfate ¾ Powder is mixed with water to create a slurry ¾ Slurry is poured over pattern ¾ Plaster sets ¾ Pattern is removed ¾ Mold is dried at 120-260oC ¾ Mold halves are assembled ¾ Molten metal is poured into the mold

Ceramic-Mold Casting

¾ Low Permeability

ƒ Gases can’t escape ƒ Metals needs to be poured in vacuum or under pressure

¾ Patterns are made of: ƒ ƒ ƒ ƒ ƒ

Aluminum alloys Thermosetting plastics Brass alloys Zinc alloys NOT wood (too moist)

ƒ Uses mold materials suitable for high temperatures (zircon, aluminum oxide, fused silica) • Can be used for ferrous and other high-temperature alloys

¾ Plaster molds can’t withstand temperatures above 1200OC

ƒ Use for aluminum, magnesium, zinc, some copper alloys

¾ Fine details, good surface finish ¾ Precision casting: ƒ ƒ ƒ ƒ

¾Also called cope and drag investment casting ¾Similar to plaster-mold casting. Differences are:

Lock components Gears valves Fittings Tooling

ƒ Pattern may be wood material

¾Precision casting: ƒ Good dimensional accuracy ƒ Good surface finish

¾Used for complicated shapes (impeller and cutters for machining operations).

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Making of Ceramic-Molds

Investment Casting ¾ Lost wax process ¾ 4000-3000 B.C. ¾ Pattern: ƒ Wax or plastic (polystyrene) molding • Injection molding in metal die • Wax patterns can be recovered and reused

ƒ Rapid Prototyping

¾ Pattern is repeatedly dipped in a slurry ¾ Tree-like structurecan be used ¾ Advantage/Disadvantage ƒ ƒ ƒ ƒ ¾ ¾ ¾ ¾ ¾ ¾

Labor and material are high costs High melting point alloys Good surface finish Close dimensional tolerance

¾ Application:

Slurry is poured over pattern Plaster sets Pattern is removed Mold is dried at 120-260oC Mold halves are assembled Molten metal is poured into the mold

ƒ Office equipment ƒ Mechanical components: gears, cams, valves

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Vacuum Casting

Permanent Mold Casting

¾ Mold is held with a robot arm ƒ Partially immersed into molten metal ƒ Vacuum reduces the air pressure to 2/3 of atomspheric pressure ƒ Metal is drawn into the mold where it solidifies

¾ Application ƒ Complex shapes ƒ Thin walled structures ƒ Steels, aluminum, other Page Page11-23 1-23

¾ Also called hard-mold casting ¾ Two halves of molds are made from ƒ Cast iron, steel, bronze, graphite, refractory alloys

¾ Examples of permanent mold castings include ƒ ƒ ƒ ƒ ƒ

Slush casting Pressure casting Die Casting Centrifugal casting Squeeze casting and semisolid metal forming

¾ Increase life:

ƒ coat inside with refractory slurry ƒ sprayed with graphite every few castings ƒ Serve as parting agents

¾ Removal

ƒ Ejector pins

¾ Process

ƒ Molds are clamped together mechanically ƒ Mold is heated

• to aid in metal flow and • Reduce thermal damage to mold

ƒ Metal is poured ƒ Mold is cold

• Fins • Water passageways

¾ Casting materials ƒ ƒ ƒ ƒ ƒ

Low melting points Aluminum Magnesium Copper Gray iron (lower melting point)

¾ Cost

ƒ High die costs ƒ Low labor costs ƒ Good for high production runs Page Page11-24 1-24

4

Slush Casting

Pressure Casting ¾ Also called: Pressure pouring, low pressure casting ¾ Molten metal is forced upward by a gas pressure ¾ Pressure is maintained until metal solidifies ¾ Good for

¾Creates hollow castings with thin walls ¾Molten metal is poured into a metal mold ¾A metal skin solidifies. ¾When desired thickness is obtained

ƒ High quality castings • Steel railroad-car wheels

ƒ Pour the remaining metal out ƒ Mold halves are opened and the casting is removed

¾Good for ƒ Small production runs ƒ Decorative objects (lamp bases and steams), toys ƒ Low melting point metals Page Page11-25 1-25

Page Page11-26 1-26

Die Casting

Hot-Chamber Die Casting

¾ Developed early 1900s ¾ Also called: Pressure die casting ¾ Molten metal is forced into the die cavity at pressures of 0.7MPa – 700 MPa (atmospheric pressure is about 0.1Mpa) ¾ Parts: ƒ Motors ƒ Hand tools ƒ Toys

¾ Two types ƒ Hot-Chamber process ƒ Cold-Chamber process

(a)

¾ Piston traps a certain volume of molten metal ¾ Forces metal into die cavity through a gooseneck and nozzle ¾ Pressure up to 35Mpa (usually 15MPa) ¾ Metal is held under pressure until it solidifies ¾ Die is cooled ƒ Circulating water or oil

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Cold-Chamber Process

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Die-Casting Die Cavities

¾ Molten metal is poured (b) into shot sleeve (injection cylinder) ¾ Chamber is not heated ¾ Pressure: 20MPa – 70 MPa ¾ Good for: ƒ High melting point alloys of aluminum, magnesium, copper

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5

Centrifugal Casting

True Centrifugal Casting

¾Utilizes the inertial forces caused by rotation to distribute the molten metal into the cavities. ƒ True Centrifugal casting • • • •

Hollow cylindrical parts Molten metal is poured into the rotating mold Various outer shapes Inner surface is cylindrical

ƒ Semicentrifugal casting • Cast parts with rotational symmetry

ƒ Centrifuging • Mold cavities of any shape are placed some distance away from the axis of rotation Page Page11-31 1-31

Semi-centrifugal Casting, Centrifuging

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Squeeze Casting and Semisolid Metal Forming ¾ Combinations of casting and forging ¾ Squeeze casting ƒ Developed 1960s ƒ Solidification of the molten metal under high pressure ƒ Machinery: die, punch, ejector pins

¾ Semisolid metal forming ƒ 1970S ƒ The metals used change viscosity when agitated (smoother, more fluid like when agitated)

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