1. Che5700 陶瓷粉末處理
Dry pressing
For industrial operations: powder feed has to be free-flowing
Die can be: rigid or flexible
Often fully automatic; used to produce parts with thickness over 0.5 mm
Stages include:
Filling
Compaction an shaping
Ejection

2. Dry Press Mode Class of pressing difficulty:
Che5700 陶瓷粉末處理
Dry Press Mode
Class of pressing difficulty:
I: one surface level, < 5mm, one direction pressing
II: one level, > 5 mm, two directions pressing
III: two level, any thickness, two directions pressing
IV: multilevel, any thickness, two directions pressing
Movement synchronized

3. Please note the relative position of each component
Lubricant helps to remove product from mold, also protection of equipment

4. In dry pressing, powder is often vibrated or tapped to produce uniform packing before compaction;
Ramming – another type of dry pressing, often used in making crucibles, pressure applied here is between dry pressing and isostatic pressing

5. In dry pressing, upper punch, & die moves, u

6. Some common processing aids

7. Powder Flow and Die Filling
Che5700 陶瓷粉末處理
Powder Flow and Die Filling
Good flow – essential for reproducible filling; uniform fill density; rapid pressing rate
Preferred state: dense, spherical, smooth, non-sticky surface, coarse than 20 m (after granulation)
The presence of more than 5% in fines (smaller than 20 m may impede (or even stop) the flow
Funnel flow rate faster for powders with small angle of repose

8. Flow rate affected by humidity
Che5700 陶瓷粉末處理
Flow rate affected by humidity
If binder is water soluble  flow behavior may be sensitive to humidity (moisture) (surface becomes sticky)
(storage may be important)

9. Filling and Compaction
Che5700 陶瓷粉末處理
Filling and Compaction
Fill density = bulk density in the mold; the higher the better
Good flow powder always get higher fill density
Fill density usually in the range of 25 – 35%; depending on granule density and packing behavior
Rate of compaction is high initially; decrease rapidly for pressure above 5 – 10 Mpa
Three stages for compaction: (a) granule flow and rearrangement; (b) granule deformation; (c) granule densification dominates
CR = compaction ratio = V fill/ V pressed = D pressed /D fill

10. Che5700 陶瓷粉末處理
Tapped Density
Tap density data provides information on particle packing
A general recommendation: frequency 1 Hz, amplitude 1 cm; ultimate tap density may be reached in 1000 – 2000 sec
Influenced by tap frequency and amplitude
V* = (Vo – V)/(Vo – V) = exp (-k N)…. N= number of taps
V* equivalent to density

11. Applied force greater than yield strength of agglo-merates,  stage II, related to binder and plasticizer

12. More on Compaction Usually little densification above 50 Mpa
Che5700 陶瓷粉末處理
More on Compaction
Usually little densification above 50 Mpa
CR < 2.0 desired  high fill density produce low compaction ratio
For ductile particles (such as metal particles), compaction can go up to nearly 100%  CR >> 2.0
Pores : size reduced, then eliminated
Intergranular , intragranular; & persistent interface
PY apparent yield strength of granules – influenced by the environment (e.g. humidity)
In stage II: D compact = D fill + m log (Pa/PY) … m compaction constant or compactability of powder

15. Moisture as Plasticizer
Che5700 陶瓷粉末處理
Moisture as Plasticizer
Hard granule difficult to change shape, causing residual pore, thus lowering product strength

16. Springback Springback: due to elastic energy inside object;
Che5700 陶瓷粉末處理
Springback: due to elastic energy inside object;
Linear springback of 0.75% or less is desirable;
Organics below Tg, the more , springback more serious; improved by adding plasticizer, can reduce sprigback effect;
Springback

17. For elastic body, deformation almost full recovery; visco-elastic body only partial recovery
Pressing: punch speed is an important parameter  air escape rate, binder deformation rate etc.
In addition to simple pressing - depressing, one can also use cyclic process

18. Che5700 陶瓷粉末處理
Ejection Pressure
Ejection pressure: depending on condition of die surface, taper of die; lubrication of die wall; ejection rate; elastic stress within the compact;
Small ejection pressure: beneficial to life of mold

19. To minimize defect formation, some pressure is kept during ejection process;
Particles with high aspect ratio or highly agglomerated tend to store a lot energy during pressing  large springback effect
Air problem can be minimized by de-airing before compression

20. Tensile Strength of Dried Compact
Che5700 陶瓷粉末處理
Tensile Strength of Dried Compact
High forming pressure, good green body strength; well plasticized powder  develop good strength;

21. Die Wall Effects Applied pressure, part of it transmitted to wall;
Che5700 陶瓷粉末處理
Die Wall Effects
Applied pressure, part of it transmitted to wall;
Mean shear stress at wall w related to Aw = adhesion at wall; f = friction coefficient at wall
Shear stress at wall will reduce pressure into object (depth)

22. Che5700 陶瓷粉末處理
Transmitted Pressure
Mean axial pressure transmitted at depth H: in equation A friction = area of friction; A pressing = area of pressing; for simple cylinder, A friction/A pressing = 4H/D
Lubricated system: pressure transmission ratio higher
attention: pressure also related to H depth & D diameter ratio, i.e. shape of object; problem for very thick sample

23. Pressure Profile Pressure gradient density gradient  defects
Che5700 陶瓷粉末處理
Pressure Profile
Pressure gradient
density gradient
 defects

24. From TA Ring, 1996;
Real sample – 3D, not easy to show, numbers refer to packing density

25. Che5700 陶瓷粉末處理
Control of Defects
* Some common defects: many caused by differential springback during ejection, origins: (a) pressure gradient; (b) non-uniformity during filling; (c) poor lubrication at wall, i.e. friction at wall; (d) differential springback between already ejected part and those still in mold

26. Types of defects and strategy
Che5700 陶瓷粉末處理
Types of defects and strategy
Laminations: often caused by wall friction; increase particle plasticity, lubricant to wall
End capping: for large springback, or low compact strength, or adhesion to punch surface  increase particle plasticity
Ring capping: due to corner high differential springback; punch/die wall difference smaller than granule size
Vertical cracks in the exterior region: often caused by compressed air, often at compact center; too thick sample, gas permeability low;  change powder preparation, high fill density, longer pressing cycle should be helpful

28. Defects and strategy (II)
Che5700 陶瓷粉末處理
Defects and strategy (II)
Shape distortion on sintering: caused by non-uniform shrinkage; high compression rate may be helpful
Extreme large pore in compacts: may be caused by small number of large sized granules; granule with sufficient compressibility would be helpful; or try to remove these very large granule before forming

29. Che5700 陶瓷粉末處理
Isostatic Compaction
等均壓成型: for elongated dimension, complex shape, large volume products; not easy to apply ordinary dry pressing
Wet bag (use liquid pressure chamber); dry bag techniques (pressure through liquid, not direct contact) for small and long objects; often axial direction: ordinary pressure, radial direction: liquid pressure
Flexible tooling for isostatic pressing: synthetic rubber, PU, silicone rubber, etc
Pressure can be up to 200 – 500 MPa; or higher (1400 MPa)
Pressure from everywhere, density gradient minimized
Springback of tooling should be considered; slow decompression (below 2 MPa) is important

30. involves: loading (vibration here can help packing), compaction, and ejection

31. Che5700 陶瓷粉末處理
Combination Pressing
Combination of dry pressing and dry bag isostatic pressing  used for production of dinnerware type products, diameter / thickness ~ 50, thickness < 0.5 cm
Flat area by direct pressing; complex-shape area pressed via liquid
Greater dimension control; elimination of drying and drying shrinkage
Roll pressing: 滾壓成型, for production of thin plates
HIP : hot isostatic pressing (special equipment), for systems difficult to sinter

32. 取自JS Reed, 1995;