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Casting Techniques

Che5700 陶瓷粉末處理. Casting Techniques. Slip casting (e.g. drain casting, solid casting), tape casting, gel casting, centrifugal casting, etc

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Casting Techniques

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  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. Che5700 陶瓷粉末處理 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. 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. 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. Che5700 陶瓷粉末處理 Schematics of Solid Casting • Solid casting: concentrated slurry pour into mold, dehydration, de-molding and other processing;

  6. 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. 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 forg 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 • 取自JS Reed, 1995; when mold drying, avoid too high temperature, will cause dehydration

  13. 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. Che5700 陶瓷粉末處理 Particle Orientation 取自TA Ring, 1996; Dip coating: shear force may cause special arrangement of particlesto minimize resistance

  15. 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

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

  19. 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 slipetc. • Microscopic defects: incomplete dispersion, contamination, unadsorbed additive; etc.

  20. 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

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

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

  23. MLCC=multilayer ceramic capacitorLTCC = low temperature co-fired ceramics Taken from TA Ring, 1996

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

  25. Che5700 陶瓷粉末處理 Additives

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

  27. 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

  28. 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

  29. 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: 25-1250 m are common;

  30. 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

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

  32. 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

  33. 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

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

  35. 取自JS Reed, 1995;

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