Casting Technique Fix

8/10/2019 Casting Technique Fix

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Casting Techniques

Slip casting (e.g. drain casting, solid casting), tapecasting, gel casting, centrifugal casting, etc

Prepare proper slurry (proper viscosity), fill into moldor direct forming, dehydration (or solvent), de-molding,

drying, sintering, post-fabrication, productCan be used to get complex shape products

Uniformity of slurry: very important

Dispersion of slurry: can be judged by sediment height

(an index)anisometric particles, easy to show preferred direction,e.g. plate particle, tend to be parallel to casting surface

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Schematics of Slip Casting

Drain casting: dilute solution (5%), porous mole,particle adsorb on mold wall, form thin layer, pour extrasolution, de-molding and other processing

Rate of dehydration affect particle packing

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Mechanism of Slip Casting

Flow resistance: mold and cake

Darcy law applied to get relationship betweenpressure drop and velocity; usually mold resistance

greater than cake resistance

PT= applied pressure + suction pressure from mold

Filtration theory: sample thickness and time relation

omcT v

L

P)(

t

PkL

mc

T ])1)((

[2

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Viscosity of Slurry

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viscosity too high orlow: not ideal

Also slurry may haveyield strength

Good stability forstorage life


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Schematics of Solid Casting

Solid casting:concentrated

slurry pour intomold,dehydration,de-molding

and otherprocessing;

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Strength of Product

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Product strengthincrease graduallyDue to increasing

density, strengthcomes frominteraction between

particles, with helpfrom binder


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Filter Pressing

In general: slip casting rely oncapillary action to remove water,filter pressing: from applied

pressure, can reach higher densityCake may be compressible: S =compressibility index

another possibility: vacuumfiltering

S

Tc P

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Sedimentation

Casting/Centrifugal CastingDifferent casting process

Sedimentation: slower rate; add centrifuge to

increase rate of sendimentationWell dispersed suspension, very slow sendimentation,can reach higher density; different particle size,different rate different effect, i.e. may be size

distribution inside the cake (sediment)

to product,poor effect always differential shrinkage

Centrifugal casting: suitable for tube making; r2forg as the acting force

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9/379TA Ring, 1996


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TA Ring,

1996;


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Porous Mold Materials

Gypsum: frequently used; plaster of Paris CaSO4. .5H2O +H2O gypsum CaSO4. 2H2O

Todays technology: good smooth surface, high ultimateporosity, micron-sized pores, short setting time, smalldimensional expansion on setting (for easy release), lowcost

Often use hot water to get uniform mold

Porosity: depending on mixing, electrolyte in water, T etc.parametersLimitations : low compressive strength when partiallysaturated with water erosion; when in contact with acidor alcohol, gypsum life becomes shorter

New generations of polymer mold, with similar pore

structure, high strength, corrosion resistant

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when making mold, too much water, low strength, greateradsorptionTaken from JS Reed, 1995; when mold drying, avoid too

high temperature, will cause dehydration


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Dip Coating

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sin

sin

maxg

Uh

g

y

xy

Pull at an angle andspeed to get coating;

film thickness dependon slurry rheology; forNewtonian fluid


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Particle Orientation

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Taken from TARing, 1996;

Dip coating:shear forcemay causespecialarrangement

of particles tominimizeresistance


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Problems of Slip Casting

Implies optimized viscosity and gelation by proper

additives; slurry rheology, casting rate, cast property, dryingand burn-out rate = balance of these parameters

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Taken from JS Reed, 1995; case A, range of good castis wider and favorable; S = soft; H = hard;


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Gel Casting

Refer to cases where binder (monomer) can polymerize,make system a gel; fill in a mold (of complex shape) stimulate reaction to gelation de-mold thermaltreatment to product

Can be used for dense product or porous product, formercase: concentrated slurry to get high density packing

Linear shrinkage is about 0.51.5%

Volume of binding phase/volume of particles ~ 16%

Mold not necessary porous material, can be made of avariety of materials

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Control of Defects in Cast

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Discussions on Defects

Non-uniformity in wall thickness: due to non-uniformityfrom mold, non-uniformity in water adsorption or castingrate; or erosion non-uniformity in mold etc.distortion: come from stress in green body, or differentialshrinkage during drying, e.g. warping

crack: difference in adhesion between object and mold wall,differential volumetric shrinkage, especially at joint; or verylarge agglomerates

void: could be due to trapped air

Bubbles and pin-holes: comes from during pouring; pin-hole

on the surface may come from removing excess slurrySurface irregularity: come from irregularity on mold surface,water adsorption non-uniformity, during pouring of highviscosity slip etc.

Microscopic defects: incomplete dispersion, contamination,

unadsorbed additive; etc.

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Tape Casting

Or called doctor blade method; often used to fabricatethin, plate-like products, such as substrate

High productivity, if continuous process, green tape first,then cut into appropriate sizes; can be multilayer

laminated productsAppropriate viscosity is of vital importance

In general, particle size around 1 m, or specific surfacearea of 25 m2/g

An-isometric particles may align preferentially duringcasting, to form special structure;

Tapes of Tyflon, Mylar are often used as carrier, may becoated with de-molding compound () for easy

removal

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Common tape casting equipment; from JS Reed, 1995


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Doctor Blade

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dydWPgLdxdp

xy

applied

/)(/

Belt moving velocity + pressure gradienteffect of

motion


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MLCC=multilayer ceramic capacitorLTCC = low temperature co-fired ceramics

Takenfrom TA

Ring,

1996


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Additives

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Quantity Effects

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Too much lubricant,lower tensilestrength; increasestrain-to-failure


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Binder Quantity

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Tape casting: Vb/Vp~ 1525%

high MW binder, can offerhigher strength andtoughness green body, notmoving during drying

(migration with solvent)Yet binder has to be

removed during calcination


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Tape Thickness

Pressure flow under the blade + planar laminar flowby the carrier

Thickness = f(height under blade, speed of carrier,drying shrinkage, viscosity of slurry)

Dr = (density of slurry)/(density of as-dried tape);hocast thickness at the blade; L = length of tape

Drying stage: shrinkage mostly in the thickness; notmuch in the lateral direction thus use Dr

)]6/(1[ 2 vLPhhADH soor

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Uniformity Issue

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High rate and highviscosity, beneficialto product uniformity

Industrial scale: 25m long, severalmeters wide, 1500mm/min speed, tape

thickness: 25-1250m are common;


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Drying

Drying rate ~ temperature and solvent content of thedrying air; initial: solvent evaporation, drying rate aboutconstant

Capillary force to transport liquid inside green body:

mechanism during rate decreasing region

Shrinkage occurs: particle closer to each other

Binder often not moving, lubricant can move

Last item: vapor transport may be rate determiningstep, tape becomes more elastic

Final density ~ 55-60% of TD; 35% organics, 15%

porosity

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Shaping and Laminating

Individual steps: scale become smaller and smaller;0402; 0201, etc.

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Binder Burnout

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Need certain gaspermeability to alloworganics to burn out

and vented;Bond strength

between layers:important issue

during burnout stage;Strength related topressure duringforming,temperature and

time


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Defects

Agglomerate inhibit sintering in that region; differential

shrinkage lead to crack; bubbles & poor surface causepoor contact between layers

Difference between metal line and neighboring ceramics;gas pressure from binder burnout may cause problem

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Effects of Some Parameters

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Effectsbubble skinning crack

More solvent decrease NA may increaseHigher temperature decrease increase NAIncrease solventevaporation rate NA increase increase

Faster pouringrate NA NA NAAir rate up NA NA increase

Inorganic NA lower NA

Ideal slip: high solid content, low viscosity, solvent notcausing skinning and trap air bubble, drying system can

remove gas and rapid drying


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JS Reed, 1995;

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Casting Technique Fix