Dry pressing

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Dry pressing
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• 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

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Dry Press Mode
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• Class of pressing difficulty
• I one level, lt 5mm, one direction pressing
• II one level, gt 5 mm, two directions pressing
• III two level, any thickness, two directions
  pressing
• IV multilevel, any thickness, two directions
  pressing

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

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In dry pressing, upper punch, lower punch die
motions
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Some common processing aids
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Powder Flow and Die Filling
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• 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

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Flow rate affected by humidity
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• If binder is water soluble ? flow behavior may be
  sensitive to humidity (moisture) (surface becomes
  sticky)
• (storage may be important)

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Filling and Compaction
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• 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

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Tapped Density
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• 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

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Applied force greater than yield strength of
agglo-merates, ? stage II, related to binder and
plasticizer
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More on Compaction
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• Usually little densification above 50 Mpa

• CR lt 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 gtgt 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

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Moisture as Plasticizer
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Hard granule difficult to change shape, causing
residual pore, thus lowering product strength
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Springback
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• 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

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• 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

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Ejection Pressure
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• 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

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• 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

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Tensile Strength of Dried Compact
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• High forming pressure, good green body strength
  well plasticized powder ? develop good strength

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Die Wall Effects
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• 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)

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Transmitted Pressure
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• 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

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Pressure Profile
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• Pressure gradient
• density gradient
• ? defects

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From TA Ring, 1996 Real sample 3D, not easy to
show, numbers refer to packing density
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Control of Defects
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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
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Types of defects and strategy
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• 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

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Defects and strategy (II)
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• 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

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Isostatic Compaction
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• ????? 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

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• involves loading (vibration here can help
  packing), compaction, and ejection

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Combination Pressing
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• Combination of dry pressing and dry bag isostatic
  pressing ? used for production of dinnerware type
  products, diameter / thickness 50, thickness lt
  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 ????, for production of thin plates

• HIP hot isostatic pressing (special equipment),
  for systems difficult to sinter

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??JS Reed, 1995