Plastics Processing Flashcards
What are some of the reasons why plastic shaping processes are
important?
(1) many of the processes are net shape processes; (2) in general,
less energy is employed than in metalworking processes; (3) lower temperatures are required
to process plastics than metals or ceramics; (4) there is great flexibility in geometry; and (5)
painting and other finishing processes are generally not required.
what parameters does viscosity depend?
n (1) temperature and (2) shear rate
How does the viscosity of a polymer melt differ from most fluids that are Newtonian
A polymer melt exhibits pseudoplasticity, which means that its value decreases with
increasing shear rate.
What does viscoelasticity mean, when applied to a polymer melt?
Viscoelasticity is a combination of viscous and elastic properties which cause the melt
to exhibit memory - the tendency to return to its previous shape, as exhibited by die swell in
extrusion.
Briefly describe the plastic extrusion process
a polymer melt is compressed to flow through a die orifice and
thus the continuous length of the plastic assumes a cross-sectional shape that is approximately
the same as that of the orifice.
The barrel and screw of an extruder are generally divided into three sections; identify the
sections
(1) the feed section, in which the feed stock is fed from the
hopper and heated; (2) the compression section, in which the polymer changes to a viscous
fluid; and (3) the metering section, in which pressure is developed to pump the plastic through
the die orifice.
Define die swell in extrusion
Die swell is the tendency of the extrudate to expand in cross-sectional dimensions
immediately on exiting the die orifice
What are the functions of the screen pack and breaker plate at the die end of the
extruder barrel?
(1) filter dirt and lumps, (2) build pressure, (3) straighten the flow
and remove memory of the polymer melt.
Briefly describe the injection moulding process
Injection moulding is a process in which a polymer is heated to a highly plastic state
and forced to flow under high pressure into a mould cavity, where it solidifies
An injection-molding machine is divided into two principal components. Name them
(1) the injection unit and (2) the
clamping unit.
What is the function of gates in injection molds?
The function of gates in an injection mold is to constrict the flow of molten plastic into
the cavity, which increases the shear rate and reduces the viscosity of the polymer melt
What are the advantages of a three-plate mold over a two-plate mold in injection
molding?
As the mold opens, the three-plate mold automatically separates the molded part(s)
from the runner system
Discuss some of the defects that can occur in plastic injection molding
(1) short shots, in which the polymer melt solidifies before filling
the cavity; (2) flashing, in which the polymer melt is squeezed into the parting surfaces
between the mold halves and around ejection pins; (3) sink marks, in which the surface is
drawn into the molding by contraction of internal material; and (4) weld lines where the melt
has flowed around a core or other convex detail in the mold cavity and met from opposite
directions, thus resulting in mechanical properties that are inferior to those in the rest of the
part.
What are the significant differences in the equipment and operating procedures between
injection molding of thermoplastics and injection molding of thermosets?
The differences in injection molding of thermosets are (1) shorter barrel length, (2)
lower temperatures in the barrel, these first two reasons to prevent premature curing; and (3)
use of a heated mold to cause cross-linking of the TS polymer.
What is reaction injection molding?
Reaction injection molding involves the mixing of two highly reactive liquid ingredients
and immediately injecting the mixture into a mold cavity where chemical reactions leading to
solidification occur
