Casting Technique Fix
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Transcript of 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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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;