AE447: Short Overview of
Plastic Processing
Material From Dr. Piyawit Khumphong (MTEC)
Outline
Extrusion Process
Cast Film
Blown Film
Thermoforming
Fiber Spinning
Coextrusion
Injection Molding
Blow Molding
Rotational Molding
(Processing)
(Performance)
(Structure)
(Property)
Extrusion History:
1935 Extruder for thermoplastic was built by
Paul Troester in Germany
1965 Entire extrusion process, from the feed hopper
to the die, could be described quantitatively
To extrude mean to push or force out
(from Latin word: extrudere) ~ push out
Solid State
Extrusion
Plasticating
Extrusion
Melt Fed
Extrusion
Types of Extruder
Single screw extruder
Plasticating, Multi stage, Rubber extruders
Multi screw extruder
Twin screw, Planetary roller extruders
Screwless extruder
Disk, Drum, Ram extruder
Single screw extruder
Dia 20-600 mm.
(25-150 mm)
L/D ratio
Vented extruder
Rubber extruder
Transfermix
Multi screw extruder
Planetary gear
Twin screw extruder
Screwless extruder
Drum extruder
Disk extruder
Ram extruder
Why Screw ?
Screw VS Plunger
Ease of temperature control
Better melt quality
Uniform temperature
No unmelt
No overheated
Energy efficiency
Plunger: Conduction + Compression
Screw: Conduction + Friction
Materials used for extrusion process
Practically all thermoplastics
Relatively high MW
high viscosity and melt strength
i.e. PE, PP, PVC, etc.
Highly viscous polymer: PTFE, UHMWPE
screwless (ram) extrusion
Geometry of Conventional Screw Extruder
Pitch
Flight
Flight depth
Channel depth
Channel width
Helix angle
Diameters: ID, OD
Geometry of Conventional Screw Extruder
Flight depth, Channel depth, Channel width
Feed
Feed:
Transition
Metering
Deep flight, mostly solid state
Transition: Channel depth reduce in linear fashion
/Compression
Metering: Shallow flight, mostly molten state
/Pumping
Extruder Screw
Rapid Compression
Vented extruder
Rubber extruder
Feed: Gravity feed through hopper
Screw channel: barrel, screw and screw flight
Forward transport by frictional force:Solid state
Frictional heat + Barrel heat (conduction)
Plasticating (melting)
Melt film: barrel surface
Metering zone :simply pumped to die
Polymer shape: cross-section of die
Die head pressure: pressure required to force
the material through the die
Flow of Plastics Through Extruder barrel
1. drag flow : molten plastic is pushed forward (along screw edges)
2. pressure flow : reverse flow due to high end pressure
3. leak flow : reverse flow over screw edges
Drag Flow (result of frictional force)
Die
Pressure Flow (high end P)
Total flow = drag flow - pressure flow - (leak flow)
Output
as pressure at the end of screw
factors: screw geometry, screw speed, barrel Temp, flow of plastics
Twin Screw Extruder
Profile extrusion
of thermal sensitivematerials i.e. PVC
Specialty polymer processing
i.e compounding, devolatilization,
chemical reaction, etc.
Advantage over single screw extruder
Better feeding and more positive conveying
characteristic =>
can process hard-to-feed materials
i.e. powder, slippery materials, etc.
Short residence time and narrow RTD
Better mixing, larger heat transfer area =>
good control of temperature.
Twin Screw VS Single Screw
Type of transport
Positive displacement
(closely intermeshing)
Drag induced
- Frictional drag
(solid conveying zone)
- Viscous drag
(melt conveying zone)
Velocity pattern
Complex
difficult to describe
Well defined
fairy easy to describe
Complex Flow Pattern in
Twin screw extruder
Good mixing
Good heat transfer
Good devolatilization capacity
Good control over stock temperature
Large melting capacity
Complex Flow Pattern in
Twin screw extruder
Not well developed theory
Difficult to predict performance of a twin
screw extruder based on extruder geometry,
polymer properties and processing
conditions
Difficult to predict screw geometry
when a certain performance is required in a
particular application
Modular Design Twin Screw Extruder
Removable screw and barrel elements
Changing sequence of screw elements
along the shaft
Co-Rotating and Counter Rotating
Intermeshing
Self wiping/ Kneading
Extrusion Die: Basic Flow Pattern
Which is an appropriate
die design ?
________________
Why ??
Die Land
Criteria in Die Design
1. Maintain laminar flow in the melt
(Because change in the die creates ‘Dead Spots’
---> Uneven Heat and Shear History)
Parallel and Converging Flows
Streamlines
parallel
Streamlines
converge
Shear
Tensile
+ Shear
How does Tensile Stress in converging flow affect the die profile design ?
Melt Fracture
Tensile stress exceeds the tensile strength of the melt
===> Irregular shaped extrudate ‘Melt Fracture’
Criteria in Die Design (cont’d)
2. Die entrance is tapered.
3. Long die land
Eliminate dead spots
Minimize tensile stress
(melt fracture)
Maintain steady melt
Eliminate process memory
(screw turning memory, elastic distortion of flow through the bend)
Melt fracture and Process memory are DIE ENTRY phenomena
Die Exit phenomena
Most common defects
1. Sharkskin: surface rupture due to tensile stress built-up by accelerated
velocity at the die wall as the extrudate leaves the die.
(high modulus, low elasticity materials easily show sharkskin)
Vmax
.
Vmin
Same V
2. Orange peel
3. Bambooing sharkskin condition becomes more intense
(excessive pressure, die T drops)
Remedy: Extra heating the die
thermally relaxing the stress lower viscosity
Die Swell
• Polymer swells as it leaves the die
• This results from elastic recovery of the melt
as leaving the die and before cooling.
ID
OD
Die swell in (a) rod and (b) pipe
Post-extrusion and products
• Rod ----> Pelletization
• Profile, Pipe and Tube
• Sheets and Films
• Filament
• Wire and Cable
• Coextrusion (Laminates)
Pipe Extrusion
To produce exact pipe dimension,
a sizing mandrel is used.
Internal sizing mandrel
External sizing
a) pressure sizing
b) vacuum sizing
Profile Extrusion
Profile: extruded products other than
films, sheet and filament
Process optimization
- Require equipment to support and shape the extrudate
during ooling
- 3 important effects:
Die Swell
Thinning effect of hual-off forces
Shrinkage effect of cooling
- Allowance must be made in the die design.