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Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
1
SHAPING PROCESSES FOR PLASTICS
Outline
- Introduction• Extrusion -^ The extruder -^ Extrusion Die -^ Melt flow in extruder - Wire and Cable Coating• Polymer Sheet and Film• Fiber and Filament
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Plastic Products
•^
Plastics can be shaped into a wide variety of products:–
Molded parts – Extruded sections – Films – Sheets – Insulation coatings on electrical wires – Fibers for textiles
-^
In addition, plastics are often the principal ingredient in othermaterials, such as–
Paints and varnishes – Adhesives – Various
polymer matrix composites
•^
Many plastic shaping processes can be adapted to produce itemsmade of rubbers and
polymer matrix composites
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
3
•^
Applications of plastics have increased at a much faster rate thaneither metals or ceramics during the last 50 years–
Many parts previously made of metals are now being made of plastics – For example: plastic containers have been largely substituted for glassbottles and jars
-^
Total volume of polymers (plastics and rubbers) now ………. thatof metals (
tonnage is still less because density of metals is greater
•^
Almost unlimited variety of part geometries
-^
Plastic molding is a ………… process; further shaping is not needed
-^
Less energy is required than for metals because processingtemperatures are much lower–
Handling of product is simplified during production because of lowertemperatures
-^
Why Plastic Shaping Processes are ImportantPainting or plating is usually not required
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Two Types of Plastics
Thermoplastics–^
Chemical structure remains unchanged during heating and shaping
-^
More important commercially, comprising more than 70% of totalplastics tonnage
Thermosets–^
Undergo a
curing process
during heating and shaping, causing a
permanent change (called cross-linking) in molecular structure
-^
Once cured, they cannot be remelted
To shape a thermoplastic polymer it must be heated so that itsoftens to the consistency of a liquid 9
In this form, it is called a
polymer melt
Important properties:–^
Viscosity
-^
Viscoelasticity
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
5
Viscosity of Polymer Melts
Fluid property that relates shear stress to shear rate during flow •^
Due to its high molecular weight, a polymer melt is a thick fluid withhigh viscosity
-^
Important because most polymer shaping processes involve flowthrough small
channels
or
die openings
•^
Flow rates are often
, leading to high shear rates and shear stresses,
so significant pressures are required to accomplish the processesViscosity of a polymer melt decreaseswith shear rate, thus the fluid becomesthinner at higher shear rates
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Viscosity of Polymer Melts
Also, the viscosity of apolymer melt decreaseswith temperature, thusthe fluid becomesthinner at highertemperatures
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
7
Viscoelasticity
Combination of viscosity and elasticity •^
Possessed by both polymer solids and polymer melts
-^
Example: die swell in extrusion, in which the hot plasticexpands when exiting the die opening
Extruded material"remembers" its formershape when in the largercross-section of the extruderand attempts to return to itafter leaving the die orifice
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Extrusion
-^
Compression process in which material is forced to flow through a dieorifice to provide long continuous product whose cross-sectional shape isdetermined by the shape of the orifice
-^
Widely used for thermoplastics and elastomers to mass produce items suchas tubing,
pipes, hose, structural shapes, sheet and film, continuous
filaments, and coated electrical wires
-^
Carried out as a continuous process;
extrudate
is then cut into desired lengths
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
13
Wire and Cable Coating
- Polymer melt is applied tobare wire as it is pulled at highspeed through a die• A slight vacuum is drawnbetween wire and polymer topromote adhesion of coating• Wire provides rigidity duringcooling - usually aided bypassing coated wire through awater trough• Product is wound onto largespools at speeds up to 50 m/s
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Film: •^
Thickness below 0.5 mm
-^
Used for packaging
(product wrapping
material, grocery bags, and garbagebags)
-^
Thicker film applications includepool covers and liners for irrigationditches
Polymer Sheet and Film
Sheet: •^
Thickness from 0.5 mm to ~12.5 mm
-^
Used for products such as flatwindow glazing and
stock for
thermoforming
Materials for Polymer Sheet and Film: All thermoplastic polymers - Polyethylene, mostly low density PE- Polypropylene- Polyvinylchloride- CellophaneProcesses include:• Slit-Die Extrusion of Sheet and Film• Blown-Film Extrusion Process• Calendering
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
15
Slit-Die Extrusion of Sheet and Film
•^
Slit may be up to 3 m wide and as narrow as around 0.4 mm
-^
A problem in this method is
. of thickness throughout width
of stock, due to drastic shape change of polymer melt during its flowthrough die
-^
Edges of film usually must be trimmed because of thickening at edges
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Blown-Film Extrusion Process
•^
Combines extrusion andblowing to produce a tube ofthin film
-^
Process begins with extrusionof tube that is drawn upwardwhile still molten andsimultaneously expanded byair inflated into it through diemandrel
-^
Air is blown into tube tomaintain uniform filmthickness and tube diameter
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
17
Calendering
•^
Feedstock is passed through aseries of rolls to reduce thicknessto desired gage
-^
Equipment is
., but
production rate is high
-^
Process is noted for
.. surface
finish and high gage accuracy
-^
Typical materials: rubber orrubbery thermoplastics such asplasticized PVC
-^
Products: PVC floor covering,shower curtains, vinyl tablecloths, pool liners, and inflatableboats and toys
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Fiber and Filament Products
-^
Most important application of fibersand filaments is in textiles
-^
Their use as reinforcing materials inplastics (composites) is growing, butstill small compared to textiles
-^
Fibers can be natural or synthetic
-^
Synthetic fibers constitute about
75%
of total fiber market today:–
Polyester is the most important
-^
Others: nylon, acrylics, and rayon
-^
Natural fibers are about
25%
of total:
-^
Cotton is by far the most important
-^
Wool production is less than cotton
-^
Definitions:
-^
Fiber
- a long, thin strand whose
length is at least
100 times
its diameter
-^
Filament
continuous
length
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
19
Melt Spinning
•^
Starting polymer is heated tomolten state and pumped throughspinneret, similar to conventionalextrusion
-^
Typical spinneret is 6 mm thickand contains approximately 50holes of diameter 0.25 mm
-^
Filaments are drawn and air cooledbefore being spooled onto bobbin
-^
Significant extension and thinningof filaments occur while polymeris still molten, so final diameterwound onto bobbin may be only 1/
of extruded size
•^
Used to produce filaments ofpolyesters and nylons
Dr. M. Medraj
Mech. Eng. Dept. - Concordia University
Mech 421/6511 lecture 15/
Dry Spinning
•^
Similar to melt spinning, but starting polymer is in solution andsolvent can be separated by evaporation
-^
First step is extrusion through spinneret
-^
Extrudate is pulled through a heated chamber which removesthe solvent, leaving the polymer
-^
Used to produce filaments of cellulose acetates and acrylics
Wet Spinning
•^
Polymer is also in solution, only solvent is non-volatile
-^
To separate polymer, extrudate is passed through a liquidchemical that coagulates or precipitates the polymer intocoherent strands which are then collected onto bobbins
-^
Used to produce filaments of rayon (regenerated cellulose)
