Che5700 陶瓷粉末處理
Plastic Forming: Extrusion and Injection
Molding
Similar to plastic forming technique, suitable for large
scale, continuous production
Need quite some polymer additives, to make
bahaviour similar to plastic materials
Extruded products: tube, paltes (tile), brick, insulator
(ring shape), catalyst extrudate (pellet) etc.
Injected products: mostly small objects, and small,
complex shape with thin walls
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Che5700 陶瓷粉末處理
Extrusion of Ceramics
Usually mix ceramic powder with polymer binder first,
then add solvent to make slurry, then to remove some
solvent (e.g. by filtration) to get cake for extrusion
High shear mixer is often needed; screw feeder also
has good mixing effect
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Che5700 陶瓷粉末處理
Extruded Shapes
Honeycomb shape is a
common shape in
recent years; say 200
channel/cm2, wall
thickness 0.1 mm;
Often contain high
quantity of binder, e.g.
35%, often become
melt during processing,
to solidify after leaving
mold and cool down
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From JS Reed, 1995; sequence of extrusion: (a) feed, (b)
consolidation and flow in the barrel, (c) pass barrel head to
enter die; (d) pass die-land; (e) ejection; [in some cases,
die-land is not necessary]
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pressure of extrusion system: 4 – 15 M Pa
Che5700 陶瓷粉末處理
Flow in Extruder
extruder: piston type or screw (single or twin) type
flow from 3 parts: (a) drag flow by screw surface; (b)
pressure flow induce by axial gradient; and (c)
leakage flow through clearance between barrel and
outside diameter of screw
ceramic paste rheology is quite complex, difficult to
simulate. Unless to use Newtonian fluid model
In die, simple pressure flow
After die, relaxation of stress may cause small
change in objects (in terms of thickness; depending
on its yield strength of course). Gravity may have
some effect
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Che5700 陶瓷粉末處理
Extrusion Mechanics
Slippage at wall may be due to lubrication effect; often
in the die-land region; at high speed, friction effect from
wall can not be neglected, e.g. die entry region
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Che5700 陶瓷粉末處理
A Few Equations
When feed is compressed into “saturation state”,
one can use Herschel-Bulkley equation for description
n : shear thinning index; b yield strength of the
plastic body
In die-land region, assume slippage at wall, m & f
refers to corresponding properties of slippage film
A land = cross section area of this region, A = other
surface area
 n
  Y  K 
 P   Pdie  entry   Pdie  land
 P  ln( Abarrel / Aland )[  b  k b v ]  ( A friction / Aland )[ 
n
 kfv ]
m
f
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Che5700 陶瓷粉末處理
Empirical Parameters
Slippage film’s yield strength: much lower, higher index
meaning more fluid in film
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Che5700 陶瓷粉末處理
Pressure distribution inside
extrusion machine
Major
part:
die
land
region
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Che5700 陶瓷粉末處理
More Comments
As extrudate pass tapered die, it will receive pressure and
shear force; this value will increase while the die entrance
angle becomes smaller; angle increase  shear stress at
wall decrease, after some critical angle , shear occur within
body  to occur static zone around the periphery of the die;
dead zone will cause defect in product
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Che5700 陶瓷粉末處理
Twin Screw Extruder
Twin screw
better mixing than
single screw; can
be used for
mixing purpose
Gas-aided
process: air can
be pumped into to
help forming (e.g.
small hollow fiber)
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Che5700 陶瓷粉末處理
Control of Defects
Insufficient strength and stiffness: cohesive strength
closely related to binder, strength affect processing
afterwards
Cracks and laminations: caused by differential shrinkage;
fundamental reasons: inclusion effect, e.g. agglomerates;
poor mixing, liquid migration during extrusion, differential
drying rate between inclusion and body, etc
surface craters and blisters: dissolved gas become
bubble after pressure release; vacuum before pressure
may help
Periodic surface laminations: extrusion pressure high,
insufficient surface lubrication, more elastic springback
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Crack
produced by
inclusion
(due to
differential
shrinkage);
Bird-foot
crack: small
inclusion of
low drying
rate;
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Che5700 陶瓷粉末處理
Control of Effects (2)
Curling of the extrudate: more often for un-symmetric
objects; or extruding hollow items through core rod,
symmetry is important;
Laminations from unjoined flow streams: S-shaped flow
by screw extruder
Poor skin: insufficiently developed slippage film, cause
rough surface, need polishing afterwards; or dispersing
more uniformly colloids, fines, more lubricants, liquid etc.;
Gradient in extrudate stiffness: liquid migration (bleeding);
Tearing during cutting: due to frictional drag, change
cutting tool;
Curling on drying: due to differential drying
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Development of S crack during auger extrusion
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conventional injection
molding: mixing of raw
materials (ceramic
powder, binder, lubricant,
solvent)  granulation
or pelletization feed to
the molding machine 
injection molding debinding, de-waxing etc.
 sintering to get
product
Uniformity: very
important factor;
recycled parts may join;
often use extrusion to
make pellets;
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Che5700 陶瓷粉末處理
Plunger Type Injection Molding
Machine
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Che5700 陶瓷粉末處理
Compositions
Many binder system contains more than one compound
for better rheology; viscosity < 105 mPa.s on molding
Burning off these organics is an important work
afterwards
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Che5700 陶瓷粉末處理
Material Variables
Materials into the mold, may through multiple cavities;
temperature control may be important
Uniformly packed green body is important to perfect
product (without defect)
Hardness of tool surface: important
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本圖取自JS Reed, 1995;
填料行為: 物料黏度, 溫度,
固化溫度, 填料速度, 填料口
選擇, 物件的大小形狀, 氣體
排出等因素影響
如圖, 在中間的開口比較好
些
Plunger motion與die motion
必須配合, 形成一個cycle;
物料被射入mold後, 多仍需
要維持在一定壓力, 溫度時間
後才脫模.
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Che5700 陶瓷粉末處理
Mechanics of Flow
對於射出成型的配料, 一般用capillary viscometer零測
其黏度與流變行為 (similar to injection molding)
Basically , shear thinning fluid for the mixture
物料被射入mold後, shear rate & temperature都會下
降, 將造成黏度上升很多; 體積收縮也許需要考慮
固化時, 應該先由mold牆壁開始, 會有收縮, 所以此時
仍須維持 mold壓力以彌補之
系統內溫度不均, 易造成thermal stress, 也會形成產品
缺陷
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Che5700 陶瓷粉末處理
Defects and Control
許多工程上需要改進的地方, 需要許多經驗, 不完全可
以模擬預測
大致來說, 進料必須均勻混合, 選對組成, 粒子沒有大
凝聚體, 其他許多屬於模具設計的工程問題; 藉助於高
分子加工的知識與技術
溫度與壓力的控制
是得到理想填料關
鍵之一
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Che5700 陶瓷粉末處理
Removal of Organics
Due to large quantity, a critical step, related closely
to cost, may become rate-determining step;
也因為量多, 所以出現缺陷機會也大
一般採用方法 (機制):
 Liquid flow
 Solvent extraction
 High pressure extraction
 Vaporization or sublimation
 Thermal or oxidative decomposition
前兩者技術多針對low viscosity liquid (low MW
polymers); capillary flow by gravity or capillary
suction force
Solvent extraction可以產生一些連通孔洞, 有助於後續
燒除高分子量的物質
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移除有機物, 除了加溫外, 也多需要抽真空
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