Polymer and Composite Processing Flashcards
Name the different type of polymers used?
Long chain molecules
Thermoplastics - can be melted and cooled to
form new shapes repeatedly and are easy to process
Thermosets more difficult to process - once cured
do not soften on further heating
Elastomers – rubbers which can be stretched
extensively under low mechanical stress and
recover original shape
Name the different plastic formation processes
Extrusion
Injection Moulding
Compression Moulding
Transfer Moulding
Thermoforming
Describe the basic Extrusion Process
Most common method of continuously processing a
polymer
Raw materials – pellets, granules or powder
Pressure and temperature related process
Material flows under compression through a die – orifice
shape determines cross-section of final product
Mainly used for thermoplastic materials
What is extrusion used to make?
Can be used to make pipe,
window frames, curtain rails,
film, sheet, fibre, rod.
Output can either be stored
on a roll, or cooled and cut to
length
Outline the main and additional zones in a polymer extruder
Feed Zone: Takes in feed stock at a continuous rate, preheats the plastic and moves it to the compression zone.
Compression Zone: Screw depth is decreased, friction prevents the plastic moving back so compression occurs. Any trapped air bubbles are squeezed out and fed back to the feed zone
Metering Zone: Screw depth constant. Melt is homogenised so as to provide homogeneous material to the die at a constant temperature, pressure and feed rate.
Mixing Zone: Additional zone in the metering section to
ensure uniformity of the melt if several ingredients are
mixed.
Venting Zone: Part of the barrel open to a vacuum.
Commonly used to remove water absorbed by the polymer. Can also be used to remove volatiles. Requires that the polymer is decompressed, and completely molten under the vent. Normally two stage process, compression, metering, venting, then
compression and metering again.
Which process are filters used in and why?
Extrusion
Filter out any inhomogeneous material in the melt. Such particles can reduce the strength of the extrudate.
Normally supported by a breaker plate which can help to straighten out the flow from the screw before it reaches the die
Filter itself needs to be replaced when clogged - can be done automatically.
What is multiple screw extrusion name some of the configurations.
Twin screws used in situations where mixing is difficult
(e.g. polymer blends)
Many configurations possible: counter rotating inter-meshing, co-rotating inter-meshing, and non-inter-meshing
What are the different types of extrusion dies? Why is the melt flow filtered before going into the die?
Film and sheet dies
Pipe and tubing dies (hollow dies)
Solid profile dies
Co-extrusion dies
Filters out un-melted resin
Increases back pressure
What are the causes of post extrusion swell? How is it addressed? What does the amount of swell depend on?
Material exiting the die will still be soft and cooling
After the polymer leaves the die it will swell to some degree, as a result of shear and tensile stresses set up in the die
Die geometries often vary from desired final shape to account for swell
The amount of swell depends on the state of strain in the die
It is a function of the die shape, the fluid viscosity and
the fluid velocity
What defects can arise from extrusion?
Melt Fracture: Due to high stresses in material – can be caused by die shape
‘Sharkskin’ or ‘bambooing’: Roughened surface of extrudate
– Tensile stresses caused by friction at interface
creating a velocity profile across cross-section
• Bamboo like appearance occurs where
velocity gradient is very large
Outline Extrusion blow moulding. Draw a diagram.
Normal processing route for plastic bottles
Process starts with a length of extrudate from an extrusion die – often a tube.
This is clamped in a mould and inflated to the required shape using hot air.
The mould can move relative to the die. When
the parison has been clamped, the parison is cut from
the extrudate and the mould moves away. While one
parison is being inflated, further parisons are being extruded.
The stretching of the polymer in the axial and hoop
directions produces molecular alignment along these
axes strengthening the plastic
The limit on manufacturing speed is the rate at which the
polymer swells during exit from the die - if this rate is too
high then surface cracks can appear.
What is the general outline of Injection moulding? Draw a diagram.
Polymer heated and forced to flow at
high pressure into mould
Two types of injection: Plunger or Reciprocating Screw
Polymer is then allowed to cool (thermoplastic) or cure
(thermoset or elastomer) and then removed, normally
by ejection.
What is the operating cycle for injection moulding?
Clamp tool halves
Inject polymer and dwell
Cool or cure
Separate halves and eject
The first and last steps are as short as possible to reduce manufacturing time
What are injection runners- cold runner moulds and why are they needed? What geometry should they have?
Runners are used to get polymer from the nozzle to the mould
Multi-impression moulds can require complex systems of
runners
To prevent solidification of thermoplastic polymers in the
runner, the runner surface area should be small - minimises heat transfer to the polymer
A large runner cross sectional area helps the polymer flow quickly
For a given flow rate, round runners are more efficient
Waste material will solidify in runners – removed and recycled
Why are runner-less moulds sometimes used?
The use of a sprue and runners results in waste material -
this can normally be ground up and re-used but is not necessary
To get rid of the sprue, “runnerless” or “hot runner” moulds can be used
On multi-impression moulds, a “hot runner plate” may be
used to heat the runners which lead out to the various
impressions
