1. Che5700 陶瓷粉末處理
Casting Techniques
Slip casting (e.g. drain casting, solid casting), tape casting, gel casting, centrifugal casting, etc
Prepare proper slurry (proper viscosity), fill into mold or direct forming, dehydration (or solvent), de-molding, drying, sintering, post-fabrication, product
Can 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

2. Schematics of Slip Casting
Drain casting: dilute solution (5%), porous mole, particle adsorb on mold wall, form thin layer, pour extra solution, de-molding and other processing
Rate of dehydration affect particle packing

3. Mechanism of Slip Casting
Che5700 陶瓷粉末處理
Mechanism of Slip Casting
Flow resistance: mold and cake
Darcy law applied to get relationship between pressure drop and velocity; usually mold resistance greater than cake resistance
PT = applied pressure + suction pressure from mold
Filtration theory: sample thickness and time relation

4. Viscosity of Slurry viscosity too high or low: not ideal
Che5700 陶瓷粉末處理
Viscosity of Slurry
viscosity too high or low: not ideal
Also slurry may have yield strength
Good stability for storage life

5. Schematics of Solid Casting
Solid casting: concentrated slurry pour into mold, dehydration, de-molding and other processing;

6. Strength of Product Product strength increase gradually
Che5700 陶瓷粉末處理
Strength of Product
Product strength increase gradually
Due to increasing density, strength comes from interaction between particles, with help from binder

7. Che5700 陶瓷粉末處理
Filter Pressing
In general: slip casting rely on capillary action to remove water, filter pressing: from applied pressure, can reach higher density
Cake may be compressible: S = compressibility index
another possibility: vacuum filtering

8. Sedimentation Casting/Centrifugal Casting
Che5700 陶瓷粉末處理
Sedimentation Casting/Centrifugal Casting
Different casting process
Sedimentation: slower rate; add centrifuge to increase rate of sendimentation
Well 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; r2 for g as the acting force

9. 取自TA Ring, 1996

10. 取自TA Ring, 1996;

11. Che5700 陶瓷粉末處理
Porous Mold Materials
Gypsum: frequently used; plaster of Paris CaSO4. .5H2O + H2O  gypsum CaSO4. 2H2O
Today’s technology: good smooth surface, high ultimate porosity, micron-sized pores, short setting time, small dimensional expansion on setting (for easy release), low cost
Often use hot water to get uniform mold
Porosity: depending on mixing, electrolyte in water, T etc. parameters
Limitations : low compressive strength when partially saturated with water  erosion; when in contact with acid or alcohol, gypsum life becomes shorter
New generations of polymer mold, with similar pore structure, high strength, corrosion resistant

12. when making mold, too much water, low strength, greater adsorption
Taken from JS Reed, 1995; when mold drying, avoid too high temperature, will cause dehydration

13. Dip Coating Pull at an angle and speed to get coating;
Che5700 陶瓷粉末處理
Dip Coating
Pull at an angle and speed to get coating;
film thickness depend on slurry rheology; for Newtonian fluid

14. Particle Orientation Taken from TA Ring, 1996;
Che5700 陶瓷粉末處理
Particle Orientation
Taken from TA Ring, 1996;
Dip coating: shear force may cause special arrangement of particles to minimize resistance

15. Problems of Slip Casting
Che5700 陶瓷粉末處理
Problems of Slip Casting
Implies optimized viscosity and gelation  by proper additives; slurry rheology, casting rate, cast property, drying and burn-out rate = balance of these parameters

16. Taken from JS Reed, 1995; case A, range of “good” cast is wider and favorable; S = soft; H = hard;

17. Che5700 陶瓷粉末處理
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  thermal treatment to product
Can be used for dense product or porous product, former case: concentrated slurry to get high density packing
Linear shrinkage is about 0.5 – 1.5%
Volume of binding phase/volume of particles ~ 16%
Mold not necessary porous material, can be made of a variety of materials

20. Control of Defects in Cast
Che5700 陶瓷粉末處理
Control of Defects in Cast

21. Discussions on Defects
Che5700 陶瓷粉末處理
Discussions on Defects
Non-uniformity in wall thickness: due to non-uniformity from mold, non-uniformity in water adsorption or casting rate; or erosion non-uniformity in mold etc.
distortion: come from stress in green body, or differential shrinkage during drying, e.g. warping
crack: difference in adhesion between object and mold wall, differential volumetric shrinkage, especially at joint; or very large 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 slurry
Surface irregularity: come from irregularity on mold surface, water adsorption non-uniformity, during pouring of high viscosity slip etc.
Microscopic defects: incomplete dispersion, contamination, unadsorbed additive; etc.

22. Che5700 陶瓷粉末處理
Tape Casting
Or called doctor blade method; often used to fabricate thin, plate-like products, such as substrate
High productivity, if continuous process, green tape first, then cut into appropriate sizes; can be multilayer “laminated” products
Appropriate viscosity is of vital importance
In general, particle size around 1 μm, or specific surface area of 2 – 5 m2/g
An-isometric particles may align preferentially during casting, to form special structure;
Tapes of Tyflon, Mylar are often used as carrier, may be coated with de-molding compound (脫模劑) for easy removal

23. Common tape casting equipment; from JS Reed, 1995

24. Che5700 陶瓷粉末處理
Doctor Blade
Belt moving velocity + pressure gradient  effect of motion

25. MLCC= multilayer ceramic capacitor LTCC = low temperature co-fired ceramics
Taken from TA Ring, 1996

26. Che5700 陶瓷粉末處理
Slurry Compositions
Change to water system: a common trend for environmental reasons; need to overcome drying-shrinkage problem

27. Che5700 陶瓷粉末處理
Additives

28. Che5700 陶瓷粉末處理
Quantity Effects
Too much lubricant, lower tensile strength; increase strain-to-failure

29. Binder Quantity Tape casting: Vb/Vp ~ 15 – 25%
Che5700 陶瓷粉末處理
Binder Quantity
Tape casting: Vb/Vp ~ 15 – 25%
high MW binder, can offer higher strength and toughness green body, not moving during drying (migration with solvent)
Yet binder has to be removed during calcination

30. Che5700 陶瓷粉末處理
Tape Thickness
Pressure flow under the blade + planar laminar flow by the carrier
Thickness = f(height under blade, speed of carrier, drying shrinkage, viscosity of slurry)
Dr = (density of slurry)/(density of as-dried tape);ho cast thickness at the blade; L = length of tape
Drying stage: shrinkage mostly in the thickness; not much in the lateral direction  thus use Dr

31. Che5700 陶瓷粉末處理
Uniformity Issue
High rate and high viscosity, beneficial to product uniformity
Industrial scale: 25 m long, several meters wide, 1500 mm/min speed, tape thickness: m are common;

32. Che5700 陶瓷粉末處理
Drying
Drying rate ~ temperature and solvent content of the drying air; initial: solvent evaporation, drying rate about constant
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 determining step, tape becomes more elastic
Final density ~ 55-60% of TD; 35% organics, 15% porosity

33. Shaping and Laminating
Che5700 陶瓷粉末處理
Shaping and Laminating
Individual steps: scale become smaller and smaller; 0402; 0201, etc.

34. Che5700 陶瓷粉末處理
Binder Burnout
Need certain gas permeability to allow organics to burn out and vented;
Bond strength between layers: important issue during burnout stage;
Strength related to pressure during forming, temperature and time

35. Che5700 陶瓷粉末處理
Defects
Agglomerate inhibit sintering in that region; differential shrinkage lead to crack; bubbles & poor surface cause poor contact between layers
Difference between metal line and neighboring ceramics; gas pressure from binder burnout may cause problem

36. Effects of Some Parameters
Che5700 陶瓷粉末處理
Effects of Some Parameters
Effects
bubble skinning crack
More solvent decrease NA may increase
Higher temperature decrease increase NA
Increase solvent
evaporation rate NA increase increase
Faster pouring
rate NA NA NA
Air rate up NA NA increase
Inorganic NA lower NA
Ideal slip: high solid content, low viscosity, solvent not causing skinning and trap air bubble, drying system can remove gas and rapid drying

37. 取自JS Reed, 1995;