Polymers- Extrusion Flashcards
Describe extrusion
Involves forcing a polymer melt through a die to create an extrudate with constant cross section. It is a continuous process so can create parts of infinite length. Relatively efficient- most scrap from out of specification, edge trimming, start-up/shut-down
What is the extrudate?
The material that is extruded out through the die
How does flow direction influence properties of extrudate?
It determines the preferential orientation of the polymer chains within it. Material often stronger in the direction the flow was in than in other directions
Describe the parts of an extruder
Hopper feeds pellets into feed zone. These are fed to the melting zone where 1 or 2 rotating Archimedes screws inside a closely fitting barrel pump molten polymer along through the metering zone to the die. Channel depth along screws decreases causing friction and melting. Breaker plate just before die reduces spiral motion. Screen pack just before die filters out contaminants. Both control flow into die
Why is a single screw relatively inefficient?
Relies on good friction between polymer and screw. There is back pressure and is inefficient pump.
Advantages of twin screw extrusion
More efficient pumping so less residence time (time in machine). Means less chance of Mr being reduced due to overheating causing scorching or chain breakages. Larger screw surface area so more efficient heating. Less back pressure
Modular design of screws
Feed zone: mixture of solid polymer and air fed into barrel.
Compression zone: channel depth decreases causing shearing friction which raises temperature and melts polymer.
Metering zone: melt is conveyed to end of screw at constant channel depth improving homogenisation
Other types of extruder screws
Standard plus venting section (zone of large channel depth) to allow evolved gases to escape. PVC type for amorphous polymers which progressively soften through a glass transition (gradual channel depth decrease over full length). Nylon type for crystalline polymers with sharp melting point (sudden large decrease of depth)
Dimensions of films, sheets, tubes and pipes
Film: t<0.3mm
Sheet: t>0.3mm
Tube: d<12mm
Pipe: d>12mm
Velocity unification
Material touching die surface moves slower than material closer to centre. Drags removed on exit and the material unifies velocity. This causes swelling
Viscoelastic relaxation
While in the die the melt is under pressure. When it leaves the pressure is removed. This causes swelling
How to solve problem of swelling
Use non-orthogonal aperture (so then swells into correct shape). Use longer die. Use tapered die
Problem of residual stresses
Frozen in stresses can warp the part and can cause failure
How to solve problem of residual stresses?
Adjust design so it has a constant wall thickness which reduces the residual stresses. Also can reduce amount of material used and cost of production
How does extrusion of tubes work?
The polymer is forced around a cylindrical mandrel held in place by spider legs which the material splits around then recombines. Leaves die as cylinder. Air flows through centre of mandrel and leaves die in centre of cylinder to support the top part of the tube
Film blowing
Used to make polymer films. Melt forced through a ring die which is pulled up vertically by nip rolls. Air used to inflate the tube into a bubble and also used at sides to support bubble as it rises into the collapsing rolls. Frost line observed when polymer is transparent when molten and opaque when solid. After nip rolls goes to take up roll. Need polymer with good melt strength like branched PE
Uses of film blowing
Makes cylindrical films ideal for plastic bags and packaging. Can be slit into a continuous film
Film and sheet extrusion
Melt is forced through a slit die onto a chill roll which cools and draws the melt from the die. Then goes to nip rolls and take up roll. Poorer quality but cheaper than film blowing. Can make multilayer films using extra extruders (films need gluing together
Slot die for film and sheet extrusion
Coat hanger die. Melt goes in through narrow inlet to the manifold which spreads out into coat hanger shape. Melt then goes up onto a land or island and exits out of the lip
Film and sheet extrusion coating
The extruded film can be coated onto a non-polymeric substrate like paper or metals. An unwind roll of the substrate feeds it to a pressure roll in contact with the normal chill roll. Example is LDPE onto thin card for milk cartons to prevent leakage and add adhesive layer. Or layer of LDPE onto aluminium foil for the adhesive for yoghurt pot lids
Wire and cable extrusion
Insulating coating applied to wire cables from a tubular die. Wire guided through centre of die and comes out the same exit as the polymer. Commonly use LDPE or PVC. For high temperatures use cross-linked PE or polyamides
Fibre spinning
Many fibres (<0.2mm d) extruded through a spinneret die and then drawn using godet rolls to stretch them. Rolls move at increasing speeds. Imparts a unidirectional orientation of polymer chains along fibre axis. Use polyesters, polyamides and PP to be woven into fabrics for clothing and soft furnishings. Fibres used in tyres and composites.
Monofilaments
Made using fibre spinning but have d>0.1mm. Used in fishing lines, racket strings, nets, carpets
Non-woven fabrics
Made by allowing fibres from spinneret die to fall randomly onto a mat and then embossing (fed between embossing roll and anvil roll). Cheaper as no need to weave. No drawing means not stiff or strong. Very absorbant to used in nappies. Uses PP or PET
Circumferential speed of polymer through extruder
Vc=xπD
x is rotational speed (rev/sec)
D is screw diameter
Speed of polymer flowing along extruder
V=VcCosφ
φ is helix angle between screw grooves and vertical
Helix angle formula
φ=tan^-1(C/πD)
C is horizontal pitch between grooves
Formula for width of channel in screws
W=Ccosφ-e
Where e is width of the screw grooves (flights)
Volumetric flow rate of polymer down channel of screw
Q=VHW/2
H is screw flight depth or height
Things wrong with calculation for volumetric flow rate
Neglects pressure anges along metering zone. There is pressure build up from compression zone which drops when metering zone is reached. Pressure rises significantly when die is reached
