Processing of PolymersProcessing of Solid Polymers:
P l h t d t th lt t t h d d Polymer heated to the melt state, shaped underhigh pressure and cooled down to roomt t (b l T T ) t ittemperature (below Tg or Tm) to preserve itsshape.
Shaping involves shear, bulk and elongationaldeformations of the polymer melt, which havedifferent viscoelastic characteristics.
Processing of Polymers
Modulus-temperature curvesfor (a) polypropylene, (b)and generic thermoplasticg pand cross-linked materials.
Processing of Polymers
Specific volume versusSpecific volume versustemperature, upon coolingfrom the liquid melt, fortotally amorphous (curve A),semicrystalline (curve B),and crystalline (curve C)polymers.
Processing of PolymersBulk density (b) is the density of the
uncompressed polymer particles anduncompressed polymer particles andinterparticle voids, whereas the bulk factor isthe ratio of the solid and bulk densitiesthe ratio of the solid and bulk densities..
Compressibility, is the percent differenceb t th l l k d b lk d it dbetween the loosely packed bulk density andthe packed bulk density.
Processing of PolymersThere are many processes for plastics. Selection of a process
depends on many factors including:
- Quantity and production rate
- Dimensional accuracy and surface finish
- Form and detail of the product
- Nature of material
- Size of final product
In general, plastics processes have three phases:
1. Heating - To soften or melt the plastic.
2. Shaping / Forming.p g g
3. Cooling - So that it retains its shape.
Processing of PolymersThermoplastics start as regular pellets or granules and can be re-
Thermosetting materials start as liquids/syrups, often called
" i " ti ll d d t (" f ") hi h d"resins", or partially cured products ("preforms") which need
heat for the shaping phase. The shaping is accompanied by a
chemical reaction, which means that the material does not
soften on reheating. The reaction may be exothermic (giving
heat out), in which case cooling is required.
Processing of Polymers Extrusion
Films and sheets Fibers and filaments Pipes tubing and profilesPipes, tubing and profiles Wire coating
Injection Molding Injection Molding Thermoforming
Bl M ldi Blow Molding Compression and Transfer Molding.
Extrusion ProcessesExtrusion of plastics, like injection molding, is a
relatively simple concept, but the design andli ti f t d i l fi ldapplication of extruders is a complex field.
Most common polymer processing (60% of worldproduction)production).
Manufacture an endless product of constant cross-section that iscut, sawed, chopped, rolled or reduced to specific length.
Two-part process including plastication and shaping
Extruder and dieExtruderanddie
Hopper designs: (a) standard hopper with gravimetric feed, (b) spiral hopper, (c) crammer feeder, and (d) standard hopper with metered feed.
Barrel The barrel is a metal cylinder that surrounds the screw. One end fastens to the feed throat and the opposite end
connects directly to the die adapter. They are usually made from standard tool steels E t d b l t i ll h l th t di t (L/D) ti Extruder barrels typically have length-to-diameter (L/D) ratios of 24:1 to 36:1 To reduce barrel wear barrels are nitrided or bimetallic liners To reduce barrel wear, barrels are nitrided or bimetallic liners
are inserted into the barrel. Although barrel and barrel liners typically have smooth
surfaces, a liner or barrel with axial grooves can be installed inthe feed section of the extruder .
Grooved barrel liner
The Breaker Plate It acts as a seal between the extruder barrel
and the die adapter th s pre enting leakage ofand the die adapter, thus preventing leakage of the melt.
h kIt supports the screen pack. It develops head pressure (restricts flow), and
converts the rotational motion of the melt to axial motion.
The screen pack filters melt for contamination and gel particles, generate head pressureg p , g p
The Breaker Plate
Screen PackFive or more screens are used in a typical screen
pack.A h b k lA coarse screen next to the breaker plate supports
the finer screens and prevents the melt pressurefrom forcing them through the breaker platefrom forcing them through the breaker plate.
The selection of screen sizes depends on thematerial and extrusion process, increasing thematerial and extrusion process, increasing thenumber of screens or the mesh size increases thepressure developed during extrusion.
Since screen packs become blocked bycontaminants, they must be changed periodically.
Screen changers: (a) sliding plate, (b) auto-screen, and (c) rotary screen.
Extrusion ProcessesPlastication: Cold polymer granules fed into a hopper and supplied
to the screw by gravity. Granules advance between the flights of the screw and
the hot walls of the barrel As the polymer advances along the extruder, it
becomes liquid (by melting or by passing through theglass transition)Th l b i d i l The polymer becomes pressurized at it moves alongthe extruder, so it can easily exit the extruder througha diea die.
Extrusion ProcessesShaping: Die shape dictates the nature of thepolymer product. Circular die: Melt spinning of fibersp g Slit die: Sheet casting Annular die: Film blowing Annular die: Film blowing Pipes ( > 12 mm) and tubing ( > 12 mm) Wire coating