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.