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
Describe the cool/cure time for injection moulding. What effects it?
In the majority of cases this will be the longest part
of the cycle
For thermosets and elastomers, the minimum
length of time this step takes is defined by the time
required to cure the polymer
The time required for a thermoplastic to cool is set
by the thermal diffusivity of the polymer and the
efficiency of mould cooling
What are the limitations of cooling channels?
The ease with which they can be made
The effect they have on tool strength
Whether or not they can be sealed
Positioning large channels too close to the mould will create differential cooling
What are the considerations that should be taken when designing injection moulds?
Wall thickness should be as uniform and as small as possible
– Uniform: changes in thickness can cause differential
cooling and tears
– Small: quicker cooling, lower cycle times
The polymer will shrink on cooling - mould design must take this into account
Provide appropriate fillet and edge radii and draft angles
Ensure, if possible, that holes will not require complex tooling
– holes along the axis of mould closure are easy to provide, holes at an angle to this axis may need moving cores
What is Compression Injection Moulding? What and when is it used for? What are the pros and cons?
Normally used for processing thermosetting materials or
elastomers
A pre-measured volume of polymer is put into a mould
then subjected to heat and pressure until it takes the
form of the cavity and cures
Low capital cost
Produces little or no waste
Disadvantages are that complex shapes are difficult to make, as are large objects
- limited by the amount of pressure which can be applied and the poor flow characteristics of thermoset and elastomeric materials
What is Transfer Injection Moulding? What and when is it used for? What are the pros and cons?
A development of compression moulding which
increases productivity
The material is loaded into a separate heated pot and forced by a piston through runners into the heated mould where curing takes place
Better productivity than compression moulding
More complex parts can be produced
Generates waste and the dies are more expensive
Outline Thermoforming. Include a diagram.
Refers to a number of processes by which sheet or film is drawn into or over a former under the action of pressure and temperature
Process limitation is how far the sheet can be deformed before it cracks
Commonly used to make packaging, plastic panels and housings
Outline Calendering. Include a diagram.
Analagous to rolling in metals
Warm plastic “dough” is fed through a number of heated rolls and then stripped off in sheet form
Other methods for making sheet are extrusion and extrusion film blowing
Outline Rotational Moulding
Used to form large hollow parts
Two part closed mould
Polymer granules encased in the mould
which is then heated and rotated until the polymer has melted and coated the inside of the mould
Can have inserts in the mould (e.g. valves in plastic footballs)
What are the 5 composite manufacturing processes?
Contact moulding
Filament winding
Pultrusion
Resin transfer moulding
Vacuum bagging/autoclaving
What fibres are used for fibre reinforcement?
The fibres may be glass, carbon, aramid, polyester,
nylon.
May come as:
– whiskers (
Outline Contact moulding. What are the pros and cons?
Oldest method of manufacturing composites
Dry reinforcement is placed on a mould and coated with resin manually - compacted using rollers
Edges will normally need trimmed after the resin has cured
There is a limit to the corner radii which the mat
can be made to adapt to
Quality operator dependent
Labour cost high
Geometric flexibility low
Cheap, flexible process best suited to one-off
production
Tooling costs low
Outline Filament Winding. What are the pros and cons?
Commonly used to make pressure vessels and
pipes
A group of fibres are pulled through a resin bath where they are impregnated with resin
The fibres are then wound onto a rotating mandrel
which gives the product its shape
The product will be partially cured in situ and fully
cured in an oven after the basic shape has been
wound
Relatively cheap and automated process
Geometric limitations arise from the geodesic
requirement and the fact that the mandrel is
enclosed
Outline Pultrusion. What are the pros and cons?
Similar to extrusion
Tows are impregnated in a resin bath before being put through a heated die
Generates uniaxially reinforced components
Automated process, low labour costs
Used to make structural shapes, pipe rod, etc.
Outline Resin Transfer Moulding. What are the pros and cons?
Dry reinforcement is held in a mould and resin is injected under pressure to encapsulate and wet out the reinforcement
Capable of creating complex shapes
Tooling expensive, but productivity high
Allows for the reinforcement to be tailored to the
product - fibre direction can be controlled to be in
line with major stresses
Outline Vacuum bagging/autoclaving. What are the pros and cons?
Similar to contact moulding - wet reinforcement is laid down on a mould
The lay-up is encapsulated in a plastic bag and air evacuated from the bag. This generates pressure on the composite layup, consolidating the layers together
If temperature is required the whole mould/reinforcement assembly is put in an autoclave
Good for making large parts to a reasonable quality level - extensively used in aerospace for this reason
Productivity quite low
Autoclaves expensive - everything else cheap
What considerations should be taken for the design of composite components?
The most common reason for using composite
components is the high strength to weight ratio of
composites
However, composite products are nearly always anisotropic to some degree
Techniques such as “volume fraction analysis” can
allow the mechanical properties to be developed
for simple shapes
For complex shapes this can be very difficult and
so finite element techniques are commonly used
Composite products work best when all the reinforcement can be aligned with the largest
stresses which the component will face in service
Unaligned fibres represent an extra weight and cost with no performance benefit
