Chapter 11 Manufacuting

7/31/2019 Chapter 11 Manufacuting

1/17

Chapter 11: Metal-Casting Process

Metal Casting Processes: Introduction

- History: Made for millennia. Used to pour copper into stone and metal molds4000-3000 B.C.

- What parts are made using the casting process:o Cameraso Engine Blockso Automotive componentso Agricultural componentso Railroad componentso Pipes and plumbing fixtureso Power toolso Gun barrelso Frying pans

- Trendso Automationo

Demand for high qualityMold Classifications

- Classification based ono Mold materialo Molding processo Method of filling the mold with molten metal

- Expendable Moldso Sand, plaster, ceramico Mixed with binders or bonding agentso After solidification of casting; the mold is broken and cannot be reused.

- Permanent Moldso

Made of metals that retain their strength at high temperatureso Casting can be removed without destroying the mold-can be reused

- Composite Moldso Made of two or more materials (sand, graphite, and metal)o Combining the advantage of each material

Sand Casting: The steps

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

Sand Casting: Sands

- Silica (SiO2) is commonly used- Why sand?

o Inexpensiveo Resistance to high temperatures


2/17

- Types of sando Naturally bondedo Synthetic

Generally preferred as it can be controlled better- Sand grain

o

Small: nicer surface, higher mold strength, lower permeability (ability toallow gases to escape)

o Want good collapsibility (mold shrinkage while casting cools)Sand Casting: Types of Sand Molds

- Green-sand moldo Most common and least expensiveo Sand, clay and watero Sand is moist when metal is poured into the mold

- Cold-box moldo Cold setting processo More expensive than green sand moldso

Organic and inorganic binders are mixed with the sand- No-bake moldso Cold setting processo Synthetic liquid resin is mixed with sand. Mixture hardens at room

temperature.

Sand Casting: Major components

- Moldo Cope: topo Drag: bottomo Parting line between themo Cheeks: when more than two mold parts are used

-

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

o Blind riserso Open risers

- Cores: inserts made from sand to form hollow regions- Vents: carry of gases


3/17

Sand Casting: Patterns

- Replica of the object to be cast- Used to mold the sand mixture into the shape of the casting. Used repeatedly

o Strength and durability is importanto Coated with parting agent (for easy removal)

- Made fromo Wood, rapid prototyping, other

- Considerationso Metal shrinkageo Draft angles (ease of removal)


4/17

Sand Casting: Cores

- Internal cavities or passages- Cores are placed in the mold cavity before casting to form the interior surface of

the casting

- Anchored by:o Core printso Chaplets (may be needed to hinder shifting)

Are left in the casting after solidification

Sand Casting: Machines

- Compact the sand by hammering-

Mold machines: eliminate demanding labor. Most compact at squeezing head- Joltingo Most compact at the horizontal parting line

TABLE 11.3

Ratinga

Characteristic Wood Aluminum Steel Plastic Cast iron

Machinability E G F G G

Wear resistance P G E F EStrength F G E G G

Weight 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.


5/17

- Vertical flaskless moldingo Eliminates need for flasks

Good for high production- Sandslingers

o Used to fill the flask uniformly with sand under a high pressure stream- Impact molding

o Sand is compressed by controlled explosion


6/17

Sand Casting: Steps

Sand Casting: The Operation

- The two halves are closed and weighted down.- Gating system is designed

o Minimize turbulence, air and gases must be able to escape, need a pouringbasin (may be used as a riser)

- After solidification: Casting is shaken outo Extra sand is removed through vibration and sand blasting

- Raisers are cut off- Almost all commercially used metals can be sand cast- Sand casting: Can be economical for small production runs- Sand-mold casting: rough surface- Imperfections can be filled with weld metal

o Minor imperfections can be filled with epoxy


7/17

- (A) Mechanical drawing: part shrinkage and draft angle- (B) Patterns are mounted on plates for alignment

o Note presence core of prints to hold the core in place- (C) Same- (D) Core boxes => Produce the core halves- (E) Core halves are pasted together- (F) Cope half of mold is assembled by securing the cope to the flask. Alignment

joins and inserts are added (for the spruce and riser)- (G) The flask is rammed (packed) with sand. The pattern, plate, and inserts are

removed.

- (H) Drag half is produced in a similar manner as the cope- (I) The pattern is removed from the drag half- (J) Core is set in place in the drag cavity- (K) Mold is closed by placing the cope on top of the drag. Securing with pins.

Flask is subject to pressure to hinder the cope to lift due to buoyant forces.

- (L) Metal is poured in and solidified. The casting is removed from the mold.- (M) Spruce and risers are cut off and are recycled. Casting is cleaned and

inspected.

Shell-Mold Casting

- Developed in 1940s- Close dimensional tolerance, good surface finish- Mounted pattern of a ferrous metal or aluminum is heated to 175-370oC- Coated with parting agent (silicone)- Clamped to a chamber full of sand mixture- Sand mixture consists of thermosetting resin binder- Mixture is coated over the pattern- Heated in the over for cutting- Thin shell hardens (5-10mm)


8/17

- Shell is removed using ejector pins- Two half shells are bonded or clamped together- Advantage: Finer grains are used

o Low resistance to flow metal => sharper corners, thinner sections- Cost:

o

Decrease: 1/20 of sand compared to sand castingo Increase: resin binderso Metal patterns are costly but less so for large production runs

Expendable-Pattern Casting (Lost Foam)

- Also calledo Evaporative patterno Lost pattern castingo Full-mold process (trade name)o Expendable polystyrene process (before)

- Pattern made of expendable polystyrene (EPS)o Beads are placed in a dieo Die is heated - beads expand to form the pattern

- Processo Pattern is

Coated with a water based refractory slurry Dried Placed in a flask

o Flask is filled with fine sando Sand is compactedo Metal is poured in without removing the patterno The pattern is depolymerized (degraded) and vented into the surrounding

sand


9/17

- Disadvantageso Molten metal cools faster than if pattern would have been removed

Less fluidity

Directional solidification of the metal- Advantageso Simple: no parting lines, cores, riser systemso Inexpensive flasks are OKo Polystyrene is inexpensive and can be used for complex shapes and fine

surface detail

o Minimum finishing and cleaning is requiredo Can be automated and is economical for large production runs

- Applicationso Brake components for automobileso Machine baseso

Aluminum engine blocks

Plaster-Mold Casting

The Process

- Mold is made of plastero gypsum, calcium sulfate

- Slurry (powder and water) is poured over pattern- Plaster sets- Pattern is removed- Mold is dried

o 120-260oC-

Mold halves are assembled- Molten metal is poured into the mold- Low Permeability

o Gases cant escapeo Metals need to be poured in vacuum or under pressure

- Patterns are made of:o Aluminum alloys


10/17

o Thermosetting plasticso Brass alloyso Zinc alloyso NOT wood (too moist)

- Plaster molds cant withstand temperatures above 1200OCo

Use for aluminum, magnesium, zinc, some copper alloys

- Fine details, good surface finish- Precision casting:

o Lock componentso Gearso Valveso Fittingso Tooling

Ceramic-Mold Casting

- Also calledo

Cope and drag investment casting- Similar to plaster-mold casting. Differences are:o Uses mold materials suitable for high temperatures (zircon, aluminum

oxide, fused silica)

Can be used for ferrous and other high-temperature alloyso Pattern may be wood material

- Precision castingo Good dimensional accuracyo Good surface finish

- Used for complicated shapes (impeller and cutters for machining operations).Making of Ceramic-Molds

Investment Casting


11/17

- Lost wax process- Pattern:

o Wax or plastic (polystyrene) molding Wax patterns can be recovered and reused (plastic patterns cant).

o Rapid Prototyping-

Pattern is repeatedly dipped in a slurry- Tree-like structure can be used- Advantage/Disadvantage

o Labor and material are high costso High melting point alloyso Good surface finisho Close dimensional tolerance

- Application:o Office equipmento Mechanical components: gears, cams, valves

Vacuum Casting

- Mold is held with a robot armo Partially immersed into molten metalo Vacuum reduces the air pressure to 2/3 of atomspheric pressureo Metal is drawn into the mold


12/17

o It solidifies within a fraction of a second as the metal is only about 55degrees above melting.

o Mold is withdrawn- Application

o Complex shapeso

Thin walled structures

o Steels, aluminum, other

Permanent Mold Casting

- Also called hard-mold casting- Mold can be reused!- Two halves of molds are made from

o Cast iron, steel, bronze, graphite, refractory alloys- Examples of permanent mold castings include

o Slush castingo Pressure castingo Die Castingo Centrifugal casting

- Increase life:o Coat inside with refractory slurryo Sprayed with graphite every few castings (serve as parting agents)

- Removalo Ejector pins

- Processo Molds are clamped together mechanicallyo Mold is heated

To aid in metal flow and To reduce thermal damage to mold

o Metal is pouredo Mold is cooled

Fins Water passageways


13/17

- Casting materialso Low melting pointso Aluminumo Magnesiumo Coppero

Gray iron (lower melting point)

- Costo High die costso Low labor costso Good for high production runs

Slush Casting

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

oPour the remaining metal out

o Mold halves are opened and the casting is removed- Good for

o Small production runso Decorative objects (lamp bases and stems), toyso Low melting point metals

Pressure Casting

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

oHigh quality castings

Steel railroad-car wheels


14/17

Die Casting

- Developed early 1900s- Also called: Pressure die casting- Molten metal is forced into the die cavity at pressures of 0.7 MPa 700 MPa

(atmospheric pressure is about 0.1 MPa)- Parts:

o Motorso Hand toolso Toys

- Two typeso Hot-Chamber processo Cold-Chamber process

Hot-Chamber Die Casting

- Piston traps a certain volume of molten metal-

Forces metal into die cavity through a gooseneck and nozzle- Pressure up to 35MPa (usually 15MPa)- Metal is held under pressure until it solidifies- Die is cooled

o Circulating water or oil- High melting point metals are not suitable for hot-chamber die casting.- Not suitable for Aluminum- Typical metals used are: zinc, tin, and lead based alloys

Cold-Chamber Process

- Used when Hot-chamber die casting is not a good option- Metal is melted in a separate furnace- Molten metal is transferred and poured into unheated shot sleeve (injection

cylinder)

- Chamber is not heated

a


15/17

- Pressure: 20MPa 70 MPa- Slower cycle time than hot-chamber D.C. as metal has to be transferred- Good for:

o High melting point alloys of aluminum, magnesium, copper

Die-Casting Die Cavities

Die Casting: The process

- Typical steps:o Die preparation

Spray mold cavity with lubricant Die is closed

- Fillingo Molten metal is injected under high pressure

- Ejectiono Dies are openedo Shot (casting or several castings) is ejected by ejector pinso Shakeout

Separate scrap (gate, runners, etc) from shot Performed

Using trim-die under pressure Sawing, grinding Tumbling (if gates are thin)

Die casting: advantages and disadvantages

- Great dimensional accuracy and smooth surface finish (for casting)- Thin walls can be cast


16/17


17/17

Documents

Chapter 11 Manufacuting