Extrusion processing Flashcards

Question

How does a venting screw vary from typical screw design?

Answer 1

 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.

Answer 2

 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.

Answer 3

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.

Answer 4

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

Answer 5

 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.

Answer 6

 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.

Answer 7

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.

Answer 8

 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]

Answer 9

total output volumetric flow rate = drag flow + pressure flow + leakage flow

Answer 10

 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

Answer 11

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

Answer 12

 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

Answer 13

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:-

Answer 14

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.

Answer 15

* 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.

Answer 16

either circular or slit die [ ref equation for output of newtonian fluid in die pg 57]

Answer 17

 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

Answer 18

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]

Answer 19

Qd, QE and QL but as the leakage is very small we'd only use Qd and QE - as is on pg 62/63

Answer 20

die characteristics don't change but the extruder characteristic will move upwards so the output and pressure both increase.

Answer 21

Increasing the length of the metering zone reduces the slope of the extruder characteristic

Answer 22

Increasing the die radius, R, increases the slope of the die characteristic and hence increases output and reduces pressure

Answer 23

 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

Answer 24

 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

Answer 25

increased cooling times or reduced mechanical properties.

Answer 26

* 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).

Answer 27

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.

Answer 28

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

Answer 29

this depends on the backbone rigidity and on lateral substituents

Answer 30

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.

Answer 31

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.

Answer 32

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.

Answer 33

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

Answer 34

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

Answer 35

Die zones:  Screen pack  Head (die entry)  Tapered zones  Die land

Answer 36

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

Answer 37

Dimensions of the profile are increased due to extrudate swell phenomenon but are decreased by thermal shrinkage (instability) and drawdown (induced). AKA thermal constraction

Answer 38

 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

Answer 39

Time dependent recovery of elastic deformation in a viscoelastic polymer melt

Answer 40

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

Answer 41

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

Answer 42

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

Answer 43

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)

Answer 44

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.

Answer 45

sharskin, thermal contraction, die swell, drawdown, haul off system

Answer 46

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.