plastic processing material from dr. piyawit khumphong (mtec) ae447: short overview of
TRANSCRIPT
Plastic Processing
Material From Dr. Piyawit Khumphong (MTEC)
AE447: Short Overview of
OutlineExtrusion Process
Cast FilmBlown Film
ThermoformingFiber Spinning
CoextrusionInjection Molding
Blow MoldingRotational Molding
(Structure)(Structure)
(Property)(Property)
(Performance)(Performance)
(Processing)(Processing)
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 StateExtrusion
PlasticatingExtrusion
Melt FedExtrusion
Types of Extruder
Single screw extruderPlasticating, Multi stage, Rubber extruders
Multi screw extruderTwin screw, Planetary roller extruders
Screwless extruderDisk, 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 qualityUniform temperatureNo unmeltNo overheated
Energy efficiencyPlunger: Conduction + CompressionScrew: Conduction + Friction
Materials used for extrusion process
Practically all thermoplastics
Relatively high MWhigh viscosity and melt strengthi.e. PE, PP, PVC, etc.
Highly viscous polymer: PTFE, UHMWPEscrewless (ram) extrusion
Geometry of Conventional Screw Extruder
Pitch
Flight Flight depth Channel depthChannel width
Diameters: ID, ODHelix angle
Flight depth, Channel depth, Channel width
Geometry of Conventional Screw Extruder
Feed: Deep flight, mostly solid state
Transition: Channel depth reduce in linear fashion/Compression
Metering: Shallow flight, mostly molten state/Pumping
Feed Transition Metering
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
Polymer shape: cross-section of die
Die head pressure: pressure required to force the material through the die
Metering zone :simply pumped to die
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
Flow of Plastics Through Extruder barrel
Total flow = drag flow - pressure flow - (leak flow)
Pressure Flow (high end P)
Die
Drag Flow (result of frictional force)
Output as pressure at the end of screwfactors: screw geometry, screw speed, barrel Temp, flow of plastics
Profile extrusion of thermal sensitivematerials i.e. PVC
Specialty polymer processing i.e compounding, devolatilization, chemical reaction, etc.
Twin Screw Extruder
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
Complexdifficult to describe
Well definedfairy easy to describe
Complex Flow Pattern inTwin screw extruder
Good mixingGood heat transferGood devolatilization capacityGood control over stock temperatureLarge melting capacity
Complex Flow Pattern inTwin screw extruder
Not well developed theoryDifficult 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 elementsChanging sequence of screw elements along the shaft
Co-Rotating and Counter Rotating
Intermeshing
Self wiping/ Kneading
Extrusion Die: Basic Flow PatternExtrusion Die: Basic Flow Pattern
Which is an appropriate die design ? ________________Why ??
Die Land
1. Maintain laminar flow in the melt (Because change in the die creates ‘Dead Spots’
---> Uneven Heat and Shear History)
Parallel and Converging Flows
How does Tensile Stress in converging flow affect the die profile design ?
Criteria in Die Design
Streamlines parallel
Streamlines converge
Shear
Tensile + Shear
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. Eliminate dead spotsMinimize tensile stress
(melt fracture)3. Long die land 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
Most common defects1. 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)
.
2. Orange peel3. Bambooing sharkskin condition becomes more intense
(excessive pressure, die T drops) Remedy: Extra heating the die
thermally relaxing the stress lower viscosity
Die Exit phenomena
Vmax
Vmin Same V
Die Swell
• Polymer swells as it leaves the die• This results from elastic recovery of the melt
as leaving the die and before cooling.
Die swell in (a) rod and (b) pipe
ID OD
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 sizinga) pressure sizingb) vacuum sizing
Profile ExtrusionProfile ExtrusionProfile: extruded products other than films, sheet and filament
Process optimization- Require equipment to support and shape the extrudate during ooling - 3 important effects:Die SwellThinning effect of hual-off forcesShrinkage effect of cooling- Allowance must be made in the die design.