Modul2 Pendahuluan Proses Produksi - Casting

Lecture No 4 1

Goal of Lecture

• To provide an overview of material forming and shaping process fundamentals

• To understand the important factors in casting metals– Physical– Operational– Economics

Lecture No 4 2

Some Useful Links

• American Foundryman’s Society– http://www.afsinc.org

• American Metalcasting Consortium– http://amc.aticorp.org

• Cast Metals Institute– http://www.castmetals.org

• North American Die Casting Association– http://www.diecasting.org

• Ductile Iron Society– http://www.ductile.org

• Ferroalloys Association– http://www.amc.scra.org/tfa

• National Center for Excellence in Metalworking– http://www.ncemt.ctc.com

• Nonferrous Founders Society– http://www.nffs.org

• Steel Founders of America– http://www.sfsa.org

http://www.afsinc.org/

http://amc.aticorp.org/


http://www.castmetals.org/

http://www.afsinc.org/

http://www.ductile.org/

http://www.amc.scra.org/tfa


http://www.nffs.org/

http://www.sfsa.org/

Lecture No 4 3

Casting Processes

• Some Web pages of casting sources– Some cast houses

• http://www.solidiform.com/

• www.reliance-foundry.com/

• http://www.qesc.com/third.htm– this has some nice images of large castings

– The future of casting• http://www.oit.doe.gov/metalcast/

http://www.solidiform.com/

http://www.ncemt.ctc.com/thrustAreas/index.html



http://www.diecasting.org/

http://www.oit.doe.gov/metalcast/

Lecture No 4 4

Casting

• The conversion of raw materials into useful shapes using phase transformations

• One of the first steps in converting raw materials into useful products

• Applicable to most materials– Metals– Ceramics– Plastics– Glass

• Also includes mixtures– Ceramic slips and slurries

Lecture No 4 5Casting - a Form of Phase Change

Forming • The use of changes of phase to create

intricate shapes– Form the shape in the liquid state in a mold or

container• Molten metals

• Monomer solutions

• Slips

• Slurries

• Change the liquid into a solid• Remove heat

• Remove suspending liquid

• Initiate a reaction– apply heat

– inject reactants

– irradiate with photons

Lecture No 4 6Other Forms of Phase Change

Forming

• Plasma spraying

• Spray forming

• Stereolithography– http://www.solutionsin3d.com/main.htm– http://www.protocam.com/art_to_part/fslide2.htm

• Selective Laser Sintering– http://lff.me.utexas.edu/sls.html

• Fused deposition modeling– http://nasarp.msfc.nasa.gov/fdm.html

• Single crystal growing

• Inorganic glass forming

• Liquid metal jet printing– http://arri.uta.edu/lmj/

http://www.solutionsin3d.com/main.htm

http://www.protocam.com/art_to_part/fslide2.htm

http://lff.me.utexas.edu/sls.html

http://nasarp.msfc.nasa.gov/fdm.html

http://www.reliance-foundry.com/

Lecture No 4 7

Casting Fundamentals

• Casting advantages – High shape complexity with internal cavities– Large shape size and variety– Wide variety of materials– Ease of production– Variety of materials that can be cast– Close tolerances (some processes)– High surface finish (some processes)– Excellent mechanical properties (some

processes)– Economics (for some lot sizes)

Kalpakjian pp 262-263

Lecture No 4 8


• Casting disadvantages – High setup costs– Low tolerances (some processes)– Low surface finish (some processes)– Porosity (some processes)– Inhomogeneities (some processes)– Poor mechanical properties (some processes)

Lecture No 4 9


• Overall Process:– Make mold– Pour in liquid– Cool/solidify– Remove shape from mold

Lecture No 4 10

Types of Casting

• Molten materials which solidify on cooling– Metals, ceramics, glasses

• Liquids which solidify by reactions with light, activators/hardeners or moisture– Plastics

• Slurries which solidify by the extraction of the suspending medium (usually water)– Ceramics

Lecture No 4 11Casting Fundamentals for Molten

Material

• Factors affecting solidification characteristics from the molten state– Fluidity

• Flow of molten material into the cavity

Kalpakjian p 265

Lecture No 4 12

Fluidity of Molten Metal

• Fluidity is dependent on:– Characteristics of the fluid– Casting parameters

Lecture No 4 13Fluidity - Characteristics of the

fluid • Basically, Fluidity is the ability of the liquid to

flow into the mold


• Viscosity and sensitivity

to temperature

• Surface tension

• Inclusions

• Freezing range

Fluidity

Lecture No 4 14

Theory of Fluid Flow

• Theory has three components:– Bernoulli's theorem– Continuity law– Laminar vs turbulent flow


Lecture No 4 15

Bernoulli's theorem

• Where: h is the elevation above a reference plane, p is the pressure at that elevation, v is the velocity of the liquid at that elevation, r is the density of the liquid and g is the gravitational constant

– For zero velocity, pressure is proportional to height and density– For a constant height, Velocity is proportional to the square root of

pressure– etc

h + ---- + ---- = constantp

rg

v2

2g

Lecture No 4 16

Continuity Law

• For an incompressible liquid:

• Av = constant, the flow rate

• Where A is the cross sectional area and v is the velocity

Lecture No 4 17

Laminar vs Turbulent Flow

• Laminar flow is preferred– Reynolds number less than 2000

• Turbulent flow (Re >20,000) can cause air entrapment and dross (oxide) formation – results in defects

Lecture No 4 18

Practical Fluid Flow

• Pouring basin– where the molten metal enters the mold

• Gating system – connects the pouring basin to the rest of the

mold through • Sprue connects the pouring basin to the runners

• Runners carry the molten metal to the mold

• Risers – act as reservoirs to supply molten material as

it solidifies and shrinks

Lecture No 4 19

Kalpakjian p 275

Casting Parameters Affecting

Fluidity

• Mold design

• Mold material

• Mold surface characteristics

• Degree of superheat

• Rate of pouring

• Heat transfer


Material



– Heat transfer– During solidification and cooling

Kalpakjian p 265

Lecture No 4 21

Heat Transfer

• Very complex phenomenon

• Very simple process– A cold mold extracts heat from the melt

causing it to solidify

• Critical to design of mold

• Can compute a relative time for solidification

Kalpakjian p 275

Lecture No 4 22

Heat Transfer - Solidification Time

• Proportional to the square of the volume/ surface area ratio – A sphere will have a much longer solidification

time than a complex shape of the same volume

Lecture No 4 23

Effect of Cooling Rate

– Rate of cooling critical for the structure of the material and hence its properties

• Slow cooling (~100K/s) gives large grain sizes

• Fast cooling (~10 k/s)gives small grain sizes

• Very fast cooling rates (>10 K/s)produce amorphous

materials

– Implications:-• Should design artifact to be thin and not massive

• Require "chills" to control cooling rate


Material



– Heat transfer• During solidification and cooling

– Solidification

Kalpakjian p 265

Lecture No 4 25

Solidification Effects

• This is where the material comes important– Plastics

• Not as critical as for metals

– Semiconductors• Specialty crystal growing

• Single crystal so no microstructures

– Glass• No microstructure


Lecture No 4 26

Solidification Effects - Metals

• Molten metal solidification events depend on the type of material– Pure metals– Alloy

Lecture No 4 27

Solidification of Pure Metals

• Solidification occurs from the mold walls to the center in a plane front

• Grains tend to be equiaxed and grow outward from the mold wall in a columnar structure

• Nucleation agents can cause a more equiaxed structure (more uniform grains and size distribution)

Lecture No 4 28

Crystallization Phenomena

• Kalpakjian p 267

Lecture No 4 29

Solidification of Alloys

• Eutectics behave similarly to pure metals

but

• Cast grain structure depends on phase diagram

Lecture No 4 30

Solidification of Alloys

• Alloys with liquidus and solidus temperatures have a physical “mushy zone”

• “Mushy zone” has solid particles and liquid co-existing

• Solid particles tend to be dendritic (tree form) in nature and grow from the mold wall

• Microstructure highly dependent on cooling rate

• Freezing range is the difference between the liquidus and solidus temperatures

• Ferrous alloys tend to have small freezing ranges

• Aluminum and magnesium alloys tend to have wide freezing ranges

Lecture No 4 31

Kalpakjian p 120

Copper-Nickel Phase Diagram

Lecture No 4 32Why is solidification so important

for metals?

• The solidification events determine the microstructure of the product: – Grain size– Grain distribution– Grain morphology– Grain boundaries– Grain composition– Porosity content and type

Lecture No 4 33

More Crystallization Phenomena

Lecture No 4 34Influence of Grain Size on

Microstructure

• Grain size

• Microporosity

Kalpakjian p 269

Cracking tendency

Strength

Ductility

Lecture No 4 35

Structure-Property Relationships

• Slow cooling - uniform composition

• Normal cooling - microsegregation and Macro segregation

• Microsegregation– Segregation of alloying elements within the

grains or dendrites– Dendrites are the columnar grains that

typically grow from the mold surface– Dendrite surface has higher concentration of

alloying elements than core

• Macrosegregation – Segregation of alloying elements across the

casting itself.

Lecture No 4 36

Structure-Property Relationships

• Macrosegregration– Normal

• Lower melting constituents driven away from the

mold wall– higher concentration at center

– Inverse• melt enters the cavities among the dendrites formed

at the surface

– Gravity• Heavy elements sink to the bottom

• Macrosegregation gives rise to poor microstructures

Lecture No 4 37

Avoidance of Macrosegregation

• Use – Nucleation agents– Create more grains by mechanical means

• Rheocasting - stir the metal while it is in the mushy

zone

• Vibration

• Electromagnetic stirring

Lecture No 4 38

Solidification Effects - Shrinkage

• The metal shrinks as it cools – in the melt – as it solidifies as a solid (largest)

Kalpakjian p 279

Aluminum 6.6%

Carbon Steel 2.5-3%

Copper 4.9%

Gray iron -2.55

Volume Solid Contraction of some metals:

Lecture No 4 39Impact of Shrinkage on Mold

Design

• Dimensions of mold

• Molds must be constructed to be larger than the final product because the metal shrinks as it cools– Patternmakers ruler

• Warpage due to differential shrinkage

• Defects due to induced stresses

• Porosity

Lecture No 4 40

Solidification Effects - Defects

• Defects caused by – materials– part design– processing techniques

• No simple answers


Lecture No 4 41

Defect Classes • Projections

– fins, flash, swells (massive), rough surfaces

• Cavities– internal, exposed, blowholes, pinholes

• Discontinuities– cracks, cold and hot tearing, cold shuts

• Defective surface– folds, laps, scars, adhering sand, oxide scale

• Incomplete casts– insufficient metal, leaky molds

• Incorrect dimensions or shape– improper shrinkage allowance, warping, etc

• Porosity– T

See Kalpakjian pp 279-281

Lecture No 4 42

Porosity

• A special kind of cavity

• Caused by shrinkage or gases

• Detrimental to the ductility of the metal, the surface finish and pressure integrity of the part

Lecture No 4 43

Shrinkage Porosity

• Caused by differential cooling

• Thin sections cool faster than thick sections leading to too little material in the thick sections

• When the thick sections begin to solidify, porosity develops

• Mold designers avoid this by the use of chills and proper flow channels and riser placement

Lecture No 4 44

Gas Porosity

• Liquid metals have greater solubility for gases than solid metals

• Any gas in the melt appears as spherical cavities

• Melt treatment must include various degassification processes

• Can also have gases arising from reactions (melt - mold)

Lecture No 4 45Impact of all these factors on mold

design

• Very complex,

• Empirical

• Best left to experts

• There are some general guidelines, see later


Material



– Heat transfer effects• During solidification and cooling

– Solidification effects– The type of mold material

Kalpakjian p 265

Lecture No 4 47

Influence of the Mold Material

• Mold material impacts:– The heat transfer rate– The surface finish– The number and hence grain size of the

microstructure

• Selection of the mold material is strongly influenced by the process

Lecture No 4 48Casting Processes for Liquid

Metals

• There are two classes of processes:– Ingot casting - simple shapes for subsequent

processing– Net shape casting

Lecture No 4 49The Casting Process for Rolling

and Extrusion • Semicontinuous casting of ingot

– No complete mold– Bottom of mold moveable– Skin forms the mold

MeltWater

in

Water

out

Molten

metal

Solid

material

Lecture No 4 50

Ingot Cross-Section

Extrusion IngotRolling Ingot

• Disadvantage of ingots – Often need scalping to provide good surface

finish– Require energy to homogenize/reheat– Handling 20-40 ton slabs is difficult– It is an extra step which adds costs

Lecture No 4 51

Continuous Casting

– Remove bottom of mold and continually pull slab out of mold - a continuous billet

– Advantages• Lowers cost

• Continuous production

– Disadvantage• Difficult to control

• Grain structure of cast material not entirely

eliminated because the hot deformation is less

• Difficult to change material

• Difficult to start and stop

Lecture No 4 52

Continuous Casting Equipment

Kalpakjian p 160

Lecture No 4 53

Net Shape Casting Processes

• Major Categories– Expendable mold

• made of sand, plaster, ceramics with binders

• mold broken up to remove cast shape

– Permanent mold• used repeatedly

• designed for ease of casting removal

• typically fabricated of high temperature metals

• typically provide higher quality castings because of

the high rate of cooling

– Composite mold• uses the advantages of both expendable and

permanent molds

Lecture No 4 54

Expendable Molds

• Sand Casting

• Lost Foam– Expendable pattern casting

• Plaster Mold

• Ceramic Mold– Cope and drag investment casting

• Vacuum casting

Lecture No 4 55

Sand Casting

• Most ancient process

• Still most prevalent

• ~15 million tons produced each year

• Typical products include:– machine tool bases, engine blocks, cylinder

heads, pump housings

Lecture No 4 56

Sand Casting

• Advantages– Can be applied to all commercially used metals– Can obtain intricate shapes– Can apply to large objects– Is economical for small production runs– Equipment costs are generally low

• Disadvantages– Surface finish depends on the mold material– Dimensional accuracy not as good as other

casting processes

Lecture No 4 57

Sand Casting Process

• Create pattern of desired shape

• Place in box of sand to create an imprint (mold)

• Incorporate a gating system

• Fill imprint with molten metal

• Solidify and cool

• Break away the mold and remove casting

Kalpakjian Ch 11.3

Lecture No 4 58

The Sand Casting Process

Kalpakjian p 298

Lecture No 4 59

Overall Shrinkage

Metal PercentGray Cast iron 0.83-1.3

White Cast Iron 2.1

Malleable cast iron 0.78-1.0

Aluminum alloys 1.3

Magnesium 1.3

Yellow brass 1.3-1.6

High manganese steel 2.6

Lecture No 4 60

The Sand

• Mostly silica sand (silicon dioxide)– Naturally bonded or bank sands– Synthetic or lake sand preferred because its

composition can be controlled more accurately

• The factors– Fine round grains give smooth surfaces– Permeability allows gasses to escape– Good collapsibility to avoid defects in the

casting

Lecture No 4 61

Types of Sand Molds • Green sand molding

– Least expensive– Poor dimensional accuracy– Skin drying improves

• strength,

• dimensional accuracy

• surface finish

• Cold Box molding– Uses sand plus binders– Stronger molds– Dimensionally more accurate

• No-bake molding– Sand plus resins– Best strength, dimensional accuracy and finish

Lecture No 4 62

Sand Mold Components

• Flask (container)

• Mold itself– Cope on top– Drag on bottom

• Pouring basin or cup

• Sprue

• Runner system

• Risers

• Cores

• Vents

Lecture No 4 63

Patterns

• Patterns are used to mold the sand to the required shape

• May be made of wood, plastic or metal

• Material selection depends on– casting size and shape– required dimensional accuracy– quantity to be produced– molding process

Lecture No 4 64

Cores

• Used to form interior cavities

• Really just another specialized word for a pattern

Lecture No 4 65

Sand Molding Machines

• Purpose is to compact the sand around the pattern– Hand hammering– Squeezing– Jolting– Vertical flaskless molding (automated)– Sandslinging– Impact molding– Vacuum molding


Lecture No 4 66

Shell Mold Casting

• A mounted pattern is coated with a parting agent and clamped to a box containing fine sand with a binder

• Box is turned upside down to allow the sand to coat the pattern

• Assembly is placed in an oven to allow the binder to act and the sand hardens around the patterns

• Two half shells made by this method are clamped together to form the mold into which the molten metal is poured

Lecture No 4 67

Shell Mold Casting

• Advantages– Can produce casting with sharper corners,

smaller projections, and thinner walls than green-sand molds

– Is somewhat lower in cost than other sand casting techniques

– Produces casting with excellent surface finish– Process can be automated

• Disadvantages– Unless properly vented, can have severe gas

entrapment problems

Lecture No 4 68

Lost Foam Process

• The pattern is formed from polystyrene and the sand is formed around it.

• On pouring the molten metal into the mold, the polystyrene evaporates and is replaced by the melt

• It is probably one of the most important process for the ferrous and non-ferrous metals industry– Particularly important for the automotive

industry.

Lecture No 4 69

Lost Foam Process

• Advantages– A relatively simple process– No parting lines– Flasks or containers can be inexpensive– Requires minimal finishing operations– Can be automated– Can be used for long production runs– Complex patterns may be made by bonding

polystyrene components together

• Disadvantages– Fluidity is lower than in conventional sand

casting because of large temperature gradients

Lecture No 4 70

Plaster Mold Casting

• In this process the mold is made of plaster of paris instead of sand

• The rest of the process is similar to sand casting in that the two halves of the mold are clamped together and the molten metal poured in

• Often referred to as "precision casting"

Lecture No 4 71

Plaster-Mold Casting

• Advantages– Slower cooling gives a more uniform grain

structure and less warpage– Can produce casting with fine details and good

surface finish– Casting can have wall thickness as low as 1

mm– Casting has high dimensional accuracy

• Disadvantages– Can only be used for Aluminum. Magnesium,

zinc and some copper based alloys because of the max temperature capability of the mold

Lecture No 4 72Investment Casting

(Lost wax)• Very old process• Pattern made by injecting molten plastic or wax into a

metal die. • This pattern is then assembled onto a "tree" with

several patterns• This tree is dipped into slurry of refractory material

and coated with it• This mold is then dried and heated to melt the wax

which then runs out leaving the cavity.• The mold is then heated to ~1000oC to "fire" the

refractory and bond it into a solid• The molten metal is poured in, cooled, and the mold

broken to remove the castings

Lecture No 4 73

Investment Casting Process

Kalpakjian p 306

Lecture No 4 74

Investment Casting

• Advantages– Wide range of sizes possible– Intricate shapes possible– Wax can be reused– Good surface finish– Close tolerances– Applicable to a wide variety of metals and

alloys

• Disadvantage– Molds require careful handling because they

are brittle

Lecture No 4 75

Permanent Mold Casting

• Molds made in two halves from cast iron, steel, bronze, graphite or refractory hard alloys

• Surfaces coated with refractories to increase die life, control heat transfer and help separate casting from mold

Kalpakjian Ch 11.9

Lecture No 4 76

Permanent Mold Casting

• Advantages– Good surface finish– Close tolerances– Uniform mechanical properties– Fine details– Thin walls– High production rates– Automated version have low labor costs– Large size range (few gms to >=100kgs)

• Disadvantages– High equipment costs– Not economical for small production lots– Cannot do intricate shapes

Lecture No 4 77

Types of Permanent Mold Casting

• Gravity feed

• Pressure-casting– Die casting

• Hot chambers

– Cold chambers– Insert molding (cast in place inserts)– Centrifugal Casting– Squeeze casting– Semisolid metal working (forging in the slushy

state)

Lecture No 4 78Die Casting

(Pressure Die Casting)

• Molten metal is forced into the die at high pressure (up to 100ksi)

• Two basic types of machines– Hot chamber– Cold chamber

• Typical parts are carburetors, motors, appliance components and hand tools

• Size ranges from 90 gm to 25 kg

Lecture No 4 79

Die Casting

• Hot Chamber – The chamber where the hot molten metal is

kept is next the die machine and automatically fills the "shot" chamber from which it is injected under pressure into the mold

• Cold Chamber– The molten metal is delivered via a ladle to the

shot chamber from which it is injected into the injection machine.

• These machine are very similar to plastic injection molding machines but have to be able to handle higher temperatures.

Lecture No 4 80

Hot Chamber Die Casting

Kalpakjian p 313

Lecture No 4 81

Die Casting• Advantages

– Multiple dies allow higher production rates– Thin wall, intricate parts– Fine surface detail possible– High production rates with automated

machines– Inserts such as fasteners may be die cast

integrally– Good dimensional accuracy– Fine grained, high strength skin results

• Disadvantages– Die costs are high– Clamping force to keep the two halves of the

die together ranges from 25 to 3000 tons

Lecture No 4 82

Casting Practices

• Casting practices are the techniques, methods and operations used in casting.

• It includes things like safety, fluxes, master alloys, furnaces,. All of which contribute to producing a quality part in a safe manner

• Safety is extremely important in a cast house because molten metal is very dangerous

Lecture No 4 83

Safety in Casting

• Concerns– Splashing of molten metal– Fumes from the molten metal– Dust from the sand– Fuels for the furnace, their control and proper

operation of the equipment supplying them to the furnace

– Water• Water and molten metal is extremely explosive since

the high temperature of the melt rapidly converts it to

steam

– Handling of fluxes which can absorb water– Faulty equipment especially cracks in molten

metal containers such as ladles

Lecture No 4 84

Fluxes and Slags

• Fluxes are inorganic compounds that – refine the molten metal by removing dissolved

gases and impurities– perform other functions

• Prevent oxidation (aluminum casting)

• Cleaning

• Wall cleaning

• Slag forming

• Fluxes are mostly compounds of chorides, fluorides and borates of aluminum, calcium, magnesium potassium and sodium.

• Some fluxes form an insulating cover for the melt to prevent oxidation. They form Slags

Lecture No 4 85

Master Alloys

• Really a misnomer

• They are alloys of those alloying elements required for a particular alloy which do not readily dissolve in the melt

• By alloying these elements in a low melting alloy, they can be more readily dissolved into the melt.

Lecture No 4 86

Melting Furnaces

• Most common types are:– Electric Arc

• have higher melting rate and lower pollution than

most others

– Induction• Coreless induction furnaces provide excellent mixing

• Cored induction furnaces typically used for

superheating and holding furnaces

– Crucible• heated by oil, gas or electricity

• may be stationary, tilting or moveable

– Cupola• Large furnaces for making steel with layers of iron,

coke and flux

• Operate continuously

Lecture No 4 87

Single Crystal Casting

• Creates single crystals for special applications– Jet engine blades - superalloys– Semi-conductor material

• Chrochalsky process

Lecture No 4 88

Design Guidelines for all Casting

• General Design Principles– Avoid sharp corners, angles and fillets– Avoid sharp section changes (blend smoothly)– Avoid large flat areas (use ribs and serrations)– Allow for shrinkage– Design parting line in appropriate location– Design in draft angles (for release of casting

from mold)– Do not ask process to deliver higher

tolerances than it can deliver– Allow extra material for finishing– Consider stress relieving– Enlist the help of a casting company

Kalpakjian Chapter 12

Lecture No 4 89

General Casting Economics

Cost Production

Process Die Equip Labor Rate (Pc/hr)

Sand L L L-M <20

Shell-mold L-M M-H L-M <50

Plaster L-M M M-H <10

Investment M-H L-M H <1000

Permanent mold M M L_M <60

Die H H L-M <200

Centrifugal M H L-M <50

Kalpakjian p 347

Lecture No 4 90

Other forms of Casting• Plastics

– solidification occurs by a reaction• with added hardener/activator

• with moisture in the air

• Ceramics– Like molten metals but at much higher

temperatures• Only simple shapes possible

– Slurry casting• Powder ceramic mixed with water to form a fluid

which is poured into the mold

• Mold is porous and extracts the water

• Cast body must be dried and fired at high

temperatures to obtain reasonable strength

Lecture No 4 91

Conclusions

• Casting is a very flexible process with a lot of advantages

• BUT

• It is complex and has a lot of variables

• Enlist an expert before you commit you design to casting

Lecture No 4 92

Summary

• Phase change forming - Casting– Processes

• Ingot casting

• Shape casting– Expendable mold

– Permanent mold

– Composite nold

Documents

Modul2 Pendahuluan Proses Produksi - Casting