Lecture 1 Flashcards
why use polymers and polymer composites
low cost per unit volume
extrusion used for
for products with constant cross section
extrusion, thermoplastics or thermoset
thermoplastics
why use screw instead of electric heaters
because of viscosity and low thermal condusctivity of the plastic
three sections of screw extruder
feed
melt
pumping
extrusion process
- raw material fed into hopper into barrel
- screw drives material through sections
- molten plastic forced through die and cooled
die swell
(liquid soap and water)
- die swell maximises entropy after lowered entropy when squeezed through
how is extrudate cooled
cooling rate and uniformity important to avoid shrinkage
air, water, vacuum
examples of products that are extruded
pvc tubes, sheet and film, plastic coated electrical wire
co-extrusion
two polymers extruded through single die
extruded film is stronger in which direction
parallel to material flow
polymer molecules are more favourably aligned
extrusion blow moulding
plastic tubes, filled with air to fill die cavity (e.g. bleach bottles, milk bottles)
