Extrusion processing Flashcards
What is extrusion?
Extrusion is a continuous process for producing shapes of constant cross-section.
It requires forcing a material through a shaped die by means of
pressure
e.g. pipes/profiles for gas or drinking water and windows or guttering
What components is an extruder made up of?
Modern extruders consist essentially of a hollow barrel, which is kept under a set temperature, inside which one or more screw(s) rotate(s) at controllable constant speed (thus, the machine comprises also a motor, a speed
reduction gear, heaters, sensors and a control system).
The geometry of these machines can vary widely, from single screw
extruders with a screw having a constant square pitch, to multi-screw
machines of intricate design.
Intermeshing twin screw extruders are frequently used as well.
What are advantages of extruding?
Continuous
High production volumes
Low cost per pound
Efficient melting
Many types of raw materials
Good mixing (compounding)
What are disadvantages of extruding?
Limited complexity of parts
Uniform cross-sectional shapes only
What is the feed to extruder? and what is most favourable?
The feed may be fine powders, regrind material, virgin pellets.
Different types mixed together may give feeding problems
Spherical granules are the more efficient, fine powders the worst
Using a pre-blended masterbatch is usually favourable
What are unideal material properties to change post extrusion?
Undesirable changes are oxidation (usually change in colour); degradation (with possible emission of gas); surface contamination
Why should condensation of feed granules be avoided?
even for hydrophobic
materials: water vapour may create bubbles in the melt, and steam (with higher pressures for higher processing temperature)
What is the function of the extruder?
Function of extruder is to convert solid feedstock into a homogeneous melt and to pump it through a die at uniform rate. The extruder converts a solid polymer feedstock into a homogeneous melt and
pump it through the die at constant rate and uniform temperature and pressure.
Process manufactures continuous product of constant cross-section that is sawn, rolled or chopped into portable lengths
* Extruder consists of a screw inside a barrel and plastic (in the form of powder or granules) is fed onto the screw from a hopper
* Polymer is conveyed along the barrel, where is it heated by conduction from barrel heaters and shear due to its movement along the flights
* At the end of the extruder, the melt passes through a die to produce an extrudate of the desired shape
What does the barrel need to be?
Barrel needs to be strong in
order to withstand high pressures generated by molten polymers (up to 100 MPa)
Barrel temperatures are controlled by electrical heaters monitored by
thermocouples
Shear heating is generated by
mechanical working of the polymer and so some extruders are
equipped with a barrel cooling
system.
Why is the barrel diameter important?
Barrel diameter (D) is important
because the theoretical output of
the extruder is proportional to the value of D squared
Typical values of barrel diameter range from 2.5 to 15 cm.
Length to diameter ratio varies from about 5 to 34. The shorter machines (L/D below 20) are used for
processing elastomers and the
longer machines (L/D above 20) are used for thermoplastics.
What functions does the screw need to fulfil?
- Transport solid feedstock
- Compress and melt the solid* Homogenise, convey and develop
sufficient pressure to pump the melt
against the resistance of the die.
The polymer is heated by conduction
from the barrel heaters and shear due to its movement along the screw flights.
What are the 3 zones of a screw?
- Feed Zone – constant flight depth – preheat material and convey it to
subsequent zones - Compression zone – screw depth
decreases gradually to compact the
polymer, squeezing out trapped air and improving heat transfer - Metering zone – constant flight depth but shallow, melt homogenised to supply material of uniform temperature and
pressure to the die
What does the screw design depend on?
The actual design of the screw depends on the envisaged die
design and output rates.
It also depends upon the softening and melting characteristics of
the polymer and the way that the melt viscosity of the polymer
varies as a function of process temperature and output rate.
What are the 3 main screw designs? and how do the processing windows vary between them?
Short compression screw- has a much shorter compression but a longer feed zone and metering is 2nd largest. Has a smaller viscosity?
PVC type compression screw- increases compression across the length
Standard extruder screw- has a larger compression area then the feed and metering zones are similar in length.
Think viscosity and processing window
What is the function of the feed zone?
The function of the feed zone is to preheat the plastic and convey it to subsequent zones. There is constant screw depth in this section.
Sufficient material must be supplied so as to not starve the metering zone.
A thin film of melt develops in contact with the barrel. As the screw rotates it
scrapes off the molten film and so a melt pool is formed on the front face of the screw flight. More solid granules are swept into the molten pool as the screw
rotates
What is feed zone bridging?
Bridging is the resin sticking to the screw in the feed zone and simply rotating with the screw rather than being moved forward, If premature heating occurs.
How can feed zone bridging be limited?
- prevent heating the material too soon
- screw can be cooled by circulating water through inside channel of the screw in feed zone
-the polymer must be completely melted by the end of the compression zone ie the channel depth decreases because teh rate of mass flow rate passing through any screw is constant so the volume of the screw channel (and hence the depth of the screw flights) must decrease so the polymer can transform from solid to liquid.
-screw depth kept constant so the melt is homogenised to supply uniform temperature and pressure at constant rate to the die.
What are the 3 main zones and can you summarise there function and property?
Feed- function is solid conveying. Properties are Constant pitch / channel depth (H).
H = HMAX
Compression- function is melting: solid - melt transition. Properties are ‘Compression’ since channel depth (H) decreases along this zone.
Metering- function is melt pumping; pressure generation. properties are Laminar flow / mixing. High pressure.
Channel depth, H = HMIN(constant)
How can a plastics extruder be specified?
(1) Type of extruder: single-screw, or….??
(2) Size: output (Q) is related to diameter, (D)^2
(3) Screw design:
Length-diameter (L/D) ratio:
Thermoplastics L/D > 20:1
Rubber (hot-feed) 5:1 - 10:1
Rubber (cold feed) 15:1 - 20:1
Why increase L/D ratio ??
Greater capacity to heat / mix
More flexibility of screw design, to include more screw zones
Ability to increase residence time, maintain output rate or increase pressure
What is the compression ratio a function of?
Bulk density differences (solid granules / melt)
Efficiency of solid / melt transport rates
Shear flow conditions in the metering zone
It is related to machine specification parameters.
Why is the depth ratio inaccurate? and so what can replace this?
The depth ratio is technically inaccurate, as compression is three dimensional—the ratio of the volumes of the channels at each end, not just the depths.
However, if the diameter and pitch are the same at both ends, the CR is close to
the depth ratio (it differs because screw root diameters are not the same).
What does the screw do? and where is it located?
The screw is positioned inside a cylindrical barrel of diameter Db. The
screw flights nearly touch the barrel, separated by a small clearance δ,
which is on order of a fraction of a millimeter. For all practical purposes, the screw diameter, Ds
, measured from flight
tip to flight tip, is the same as the barrel diameter.
What are the 2 main zones, function and properties that classify screw design?
Venting / decompression - has a function to remove gases and volatiles and the properties increase channel depth before a second compression zone.
Mixing Zones have properties - Many different purpose-designs. Some can be ‘retro-fitted’ to the extruder.
Has function - Achieve high pressure and shear stress
What is de volatilisation?
Screw / barrel with decompression zone (‘venting’)
How does a venting screw vary from typical screw design?
Vented extruders have a suitable designed opening to allow gas or vapour to escape
Volatiles: gases, including steam (H2O)
Water molecules often cause hydrolytic degradation, or surface defects on
extruded / moulded products
In correspondence of the venting point, the screw root diameter is reduced to allow decompression of the melt and favour the gas escaping
The melt pressure is reduced to atmospheric pressure in the decompression zone and volatiles escape in the vent zone.
The melt is then conveyed to a second compression zone, which prevents air pockets from being trapped.
How does screw design vary for amorphous polymers?
Amorphous polymers that progressively soften through the glass
transition require a compression zone that covers most of the length of the screw.
This is especially true for heat sensitive polymers like PVC.
How does screw design vary for sharp melting point polymers?
With a sharp melting (eg nylon) melting takes place so quickly that the compression of the melt can be performed in one pitch of the screw.
What are the 4 main single screw extruder features and what do each of them do?
Feed zone: preheat polymer; the crew depth is constant, channel is deeper and the length of the zone is adapted to the material used to ensure neither starving nor overfeeding
Compression (plasticating) zone: the channel dept starts decreasing so that air trapped in the granules is expelled, heat transfer is improved, density change during melting is accommodated; material is compacted and melting happens in contact with heated barrel
Metering zone: constant screw depth to homogenize the melt and supply the die with a material at constant temperature and pressure
Die zone: a breaker plate sieves out extraneous material and removes the ‘turning memory’ from the melt before the die
Why is there a turning memory effect in the die zone?
What is the single screw extruder flow mechnaism? -2
Melting: the polymer is conveyed along the screw, melting in a thin film at the
barrel wall, and continuously scraped by the rotation
Conveying: in the ideal case, as the screw rotates, the materials would slip on the
screw and adhere to the inside of the barrel, producing a purely axial
(forward) movement of the polymer.
If the material sticks to the screw and slips on the barrel instead there
would be zero output, as material and screw would rotate together.
In practice, the flow behaviour lies between these two extremes since the
material adheres to both the screw and the barrel.
What are the 3 main flows caused in the single screw extruder?
The friction between screw and barrel causes the drag flow; opposed to that
is the pressure flow (due to the difference in pressure at the two extreme of the
extruder) and the leak flow, due to the space between screw and barrel where
the material can leak backwards.
What is the extrusion breaker section and why do we need it?
AKA the screen packs (commonly supported by a breaker plate which has lots of counter sunk holes allowing passage of the melt and preventing dead spots where particles of melt can gather)
It assists with the build up of back pressure improving mixing along the screw. The back pressure is required for uniform melting and proper mixing of the polymer.
The breaker plate and screen pack also eliminates rotational memory of the molten plastics and creates longitudinal memory.
What is the name of the screw in the typical extruder / barrel and how are the screw length expressed?
The Archimedean screw fits closely the cylindrical barrel, with just enough clearance to allow rotation.
The average time during which the material resides within the barrel is called Barrel Residence Time; measured using coloured plastic
Screws length are usually expressed as a ratio of length/diameter, L/D ratio. Usually for thermoplastic this value is >20 as it gives high melt flow rates and good homogeneity
The helix angle φ is usually 17.65°, arising from a pitch = screw diameter (square screw)
[ref eq and analysis pg40]
What is the volumetric flow rate made up of?
total output volumetric flow rate = drag flow + pressure flow + leakage flow
What is drag flow directly proportional too?
Note that drag flow is directly proportional to the rotational speed of the screw, i.e. for a given screw design, QD is proportional to screw speed (N)
Ref eq pg 42
What is the main counteraction to drag flow and how does it do this?
PRessure flow.
* Overall output is equal to drag flow in open-discharge situations i.e. where no downstream restriction is imposed on the flowing melt.
* Whenever a die or any additional component (such as a breaker plate) which suppresses flow is attached to the extruder, the pressure created acts against the drag forces to create a pressure flow (QP) on the material within the screw channel.
* Hence pressure flow (QP) can be considered to be a negative flow i.e. flow away from the die towards the feed end of the extruder.
ref eq pg44
What are the conditions for laminar shear flow of melt in the metering zone?
the polymer melt in the screw channel adheres to both barrel and screw surfaces
there is relative motion between these surfaces
the helix on the screw allows for forward motion
What can be added to the pressure flow conditions?
Slit ref pg 46 for equation
Leakage flow can occur in the gap between the barrel surface and the screw flight land. This occurs particularly when the
screws are old or become worn.
Leakage flow is a particular type of pressure flow and so its equation is also of the form:-
when is Q (total output) at a maximum?
When Pressure = 0. Pmax would instead be the pressure that stops completely the flow. Q would therefore = drag flow as there is no back pressure.
The die needs pressure to force the melt through, and in general the flow through the die is directly proportional to the pressure P,
through a factor K that depends on the shape of the die.
What are the effects of die restriction?
- The presence of the extruder die induces the pressure flow (back pressure) through the metering zone and hence reduces output from the extruder.
- However, this pressure which causes inefficiencies in the throughput from the extruder is also responsible for driving the polymer melt through the die.
- When the die is coupled to the extruder their requirements are conflicting.
- The extruder has a high output if the pressure at its outlet is low.
However, the outlet from the extruder is the inlet into the die and the
outlet from the die increases with increasing pressure.
What are the two kind of die in this module ?
either circular or slit die [ ref equation for output of newtonian fluid in die pg 57]
How do you find the operating point in a single screw extruder?
By combining the two equations, we can find the operating point of the
machine.
This point defines the output of the process and the pressure generated in
the extruder. It is between the die output and pressure and the extruder output plot and pressure which are opposing relationships. ref pg 59
What does the operating point of the extruder consider?
The ability of the screw to deliver melt and the role of the die in controlling pressure and output.
Screw Speed
Metering zone length
Die radius
Polymer melt viscosity
As far as possible it is beneficial for the process to run at high output and low pressure conditions.
[ref flow eq pg 62]
What is the strain restrained by?
Qd, QE and QL but as the leakage is very small we’d only use Qd and QE - as is on pg 62/63
How does increase in screw speed affect the die?
die characteristics don’t change but the extruder characteristic will move upwards so the output and pressure both increase.
how does increasing the metering zone lengthaffect the die?
Increasing the length of the metering zone reduces the slope of the extruder characteristic
How does increasing die radius effect the output-pressure plot?
Increasing the die radius, R, increases the slope of the die characteristic and hence
increases output and reduces pressure
How does increasing polymer melt viscosity affect the output-pressure plot?
Increasing polymer melt viscosity decreases the gradients of both the extruder and die output equations.
This results in an increase in die-head pressure for an output rate that remains relatively unchanged.
-can be increased by increasing temperature/molecular weight
How can the polymer melt viscosity be reduced?
Reducing polymer melt viscosity will result in a reduction in melt pressure at constant output – which is beneficial.
Increasing the die and/or extruder temperature profiles
Choosing a lower molecular weight (higher MFI) compound Use of additives, such as internal lubricants and plasticisers
What is the trade off of reducing the polymer melt viscosity?
increased cooling times or reduced mechanical properties.
How accurate is the output-pressure relationship?
- This approach is a useful tool for assessing the effects of extrusion parameters and
polymer viscosity on output rates. - However, it is only an approximation because many assumptions have been made. Such as:
The extruder and die characteristics are curved because polymers are pseudoplastic (shear viscosity decreases at
increasing processing rates).
Now with non-Newtonian behaviour and extrusion how can the shear rate decrease?
the die characteristic is non-linear.
Where viscosity is a function of temperature and molecular weight, with this increase, the gradient of each characteristic decreases.
Through the power law: Increase in screw speed will in increase shear rate and therefore reduce shear viscosity.
What is chain entanglement?
if the chain crosses an arbitrary plane 3 times then it is entangled.
Entanglements exist also in solid polymers; they are located in the amorphous component (for semi-crystalline polymers); they will affect the polymer behaviour differently if we are above or below Tg
What does the minimum length a polymer chain has depend on?
this depends on the backbone rigidity and on lateral substituents
What is reptation theory with entanglements?
Reptation theory describes the effect of polymer chain entanglements on the relationship between molecular mass and chain relaxation time (or similarly, the polymer’s zero-shear viscosity). The theory predicts that, in entangled systems, the relaxation time τ is proportional to the cube of molecular mass.
How does entanglement and viscoelasticity interact?
We can see the dramatic effect of entanglement with this log plot of melt viscosity versus MWt(molecular weight). A steady increase of MWt gives initially a linear increase in viscosity Then suddenly at Mc(the Critical Entanglement Weight) viscosity increases by MWt3.4.
What are some types of extrusion products?
Extrusion Products – Packaging
* Tubular (biaxially oriented) films – PE, PP for carrier bags, or snack foods; * Flat films – PE, PP, co-extruded structures; * Cast sheets for thermoforming - PS, PP, PET, PVC or foamed plastics; * Parisons in extrusion blow moulding (EBM) – HDPE, LDPE, PP, PVC; * Multilayer or co-extrusion processes, for sheet and film applications requiring specific
functional properties, such as improved barrier performance.
Define what die design and die function is with what factors need to be optimised and the possible cross section
Die design- fluid is pumped through an orifice to form a product with fixed cross-section
Cross-section shapes
* Simple (circular; rectangular; annular (pipes)
* Complex (profiles)
Die functions- To shape the flowing melt to the correct form, whilst: * Optimising output (Q) * Minimising pressure (ΔP) - hence energy
Consumption
Minimising costs :- capital outlay; - operating costs
What is the mandrel, breaker plate and screen pack for the extrusion die?
Mandrel- For shaping hollow profiles and supported by ‘legs’, through which air flows
Breaker plate - To control pressure
Screen pack - a filtration device – sometimes continuous
What are the specific die zone area in the extrusion die?
Die zones:
Screen pack
Head (die entry)
Tapered zones
Die land
What flow is required for the die entry?
Laminar flow needs to be maintained in the melt, to avoid uneven heat distribution and
shear history, and the development of high tensile stresses that will
create an extrudate of irregular shape
ref diagram pg 84
What the the dimensional changes in the swell that makes die design complex?
Dimensions of the profile are increased due to extrudate swell phenomenon but are decreased by thermal shrinkage (instability) and drawdown (induced).
AKA thermal constraction
How does the die swell change before, during and after the die?
Before die, entropy is minimized by assuming random ‘spherical’ conformation
During passage through die, flow rates increases, polymer chains elongates, disentanglement of chains can happen (function of relaxation time)
After die, flow rates decreases again, to regain entropy polymer tends to go back topre-die conformation; entanglements will favour the process, so less entanglements= less swell
Define Die swell
Time dependent recovery of elastic deformation in a viscoelastic polymer melt
What is the die swell ratio? and how can the swell be interpreted?
Die swell = Dextrudate/Ddie
high pressure and high (shear) stress
molecular alignment (strain) in the polymer, part of which will
be elastic
at the die exit, pressure and shear stress reduce to zero subsequent relaxation occurs, causing a recovery of the
elastic component of strain
lateral swelling occurs in the extrudate, known as die swell
Define die drawdown and what does it result in ?
Axial tensile (elongational) strain imposed by a tensile stress on the polymer melt, outside the die
Can result in axial stretching and balanced by lateral contraction
Why is drawdown applied at the extrudate exit?
Drawing of the extrudate after exit is applied to ‘pull’ the extrudate away from the die: this counteracts the swell and helps orientate the molecules, thus increasing linear strength
What are two main types of drawdown? and give examples of each.
Deliberate - orientation processes:
Uniaxial – fibres, filaments
Biaxial – films, sheets, oriented pipes
Accidental / unintentional / attempting to correct errors….
Balancing effects of die swell
Compensating for process / material variations
Attempting to optimise linear output (vH)
What is the haul off system and why is it important?
The haul-off system is the key to dimensional control and to stability of the profile geometry, once constant output from the extruder has been attained, and cooling has been established.
Usually, this is a pair of variable speed continuous belts whose gap or separation distance can be adjusted.
Pressure on the profile should be sufficient to assure positive contact and no slippage.
This ensures that tension is maintained on the profile,
which promotes straightness (no curling, warping or
distortion).
The extra speed of the haul-off system, normally called
drawdown, should be kept as low as possible.
High drawdown ratios can lead to too much residual stress
in the polymer and can cause high shrinkage or warpage
of the finished profile.
Consistent extrusion profiles can only be obtained when a
sufficiently stable and powerful haul-off system is used.
What are the main dies exit instabilities and downstream operations?
sharskin, thermal contraction, die swell, drawdown, haul off system
What is sharkskin?
The roughening of the surface of extrudate
* One theory for this is that the melt proceeding along the die
channel has a velocity profile with maximum velocity at the
centre and zero at the wall; when leaving the die, the material at
the wall has to accelerate, and this generates tensile stresses that
can rupture the surface
* Another theory is concerned with wall/melt interaction at molecular level causing absorbed chains to get disentangled from the bulk, perturbating the surface of the melt.
