Plastic Processing Material From Dr. Piyawit Khumphong (MTEC) AE447: Short Overview of

Outline Extrusion Process Cast Film Blown Film Thermoforming Fiber Spinning Coextrusion Injection Molding Blow Molding Rotational Molding

(Structure) (Property) (Performance) (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 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 mm. ( mm) L/D ratio Vented extruder Rubber extruder Transfermix

Multi screw extruder Planetary gear Twin screw extruder

Screwless extruder Drum extruderDisk extruderRam 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 Diameters: ID, OD Helix 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 TransitionMetering

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 screw factors: 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 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

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 spots Minimize tensile stress (melt fracture) 3. Long die landMaintain 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 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). 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 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 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.