Plastic Forming: Extrusion and Injection Molding

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Plastic Forming Extrusion and Injection Molding
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• 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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Extrusion of Ceramics
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• 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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Extruded Shapes
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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
• pressure of extrusion system 4 15 M Pa

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Flow in Extruder
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• 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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Extrusion Mechanics
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• 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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A Few Equations
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• 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

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Empirical Parameters
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Slippage films yield strength much lower,
higher index meaning more fluid in film
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Pressure distribution inside extrusion machine
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Major part die land region
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More Comments
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• 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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Twin Screw Extruder
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• 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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Control of Defects
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• 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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Control of Effects (2)
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• 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
  ?de-binding, 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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Plunger Type Injection Molding Machine
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Compositions
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• Many binder system contains more than one
  compound for better rheology viscosity lt 105
  mPa.s on molding

• Burning off these organics is an important work
  afterwards

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Material Variables
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• 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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• ????JS Reed, 1995
• ???? ????, ??, ????, ????, ?????, ???????,
  ?????????
• ??, ??????????
• Plunger motion?die motion????, ????cycle
• ?????mold?, ???????????, ????????.

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Mechanics of Flow
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• ?????????, ???capillary viscometer??????????
  (similar to injection molding)

• Basically , shear thinning fluid for the mixture

• ?????mold?, shear rate temperature????,
  ????????? ??????????

• ???, ????mold????, ????, ???????? mold??????

• ???????, ???thermal stress, ????????

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Defects and Control
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• ????????????, ??????, ?????????

• ????, ????????, ????, ????????, ???????????????
  ??????????????

• ???????????????????

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Removal of Organics
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• Due to large quantity, a critical step, related
  closely to cost, may become rate-determining step

• ?????, ??????????

• ?????? (??)
• Liquid flow
• Solvent extraction
• High pressure extraction
• Vaporization or sublimation
• Thermal or oxidative decomposition

• ????????low viscosity liquid (low MW polymers)
  capillary flow by gravity or capillary suction
  force

• Solvent extraction??????????, ??????????????

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