Extrusion Processing Flashcards
What is extrusion?
Continuous process for producing shapes of constant cross-section by forcing material through a shaped die with pressure.
What are the forms of products produced by extrusion?
Pipes - water, gas, sewage, plumbing
Profiles - windows, guttering, trunking
Sheet - roofing, cladding, vacuum forming
What are the pros and cons of extrusion?
Pros:
Continuous
High production volumes
Low cost per pound
Efficient melting
Many types of raw materials
Good mixing
Cons:
Limited complexity
Uniform cross section shapes only
Describe the process of extrusion
Solid polymer feedstock is converted into a homogenous melt as it is screwed through the barrel and pumped through a die at a constant rate and uniform temp and pressure.
What considerations should be taken with what material is fed into the extruded?
Spherical granules are more efficient, powders are the worst
Condensation on granules should be avoided as water vapour may create bubbles in the melt
Need to avoid oxidation, degradation and surface contamination post-extrusion
Why do some barrels have cooling elements as well as heating elements?
Shear working can raise the temp. So may need to be cooled.
What is the function of the extruder screw?
Transport solid feedstock
Compress and melt solid
Homogenise, provide sufficient pressure to pump melt against die resistance
What are the three features of an extruder screw?
Feed zone - constant screw depth, preheats material and conveys it to subsequent zones
Compression zone - screw depth decreases to compact, squeeze out air and improve heat transfer
Metering zone - constant shallow screw depth, melt homogenised to supply uniform temp and pressure to the die
What is bridging in the extrusion feed zone?
If the material is heated too quickly, resin can stick to the screw in the feed zone and rotate with it rather than be pushed forward.
To ensure it doesn’t happen, some screws are cooled with circulating water.
Why does the clearance h need to be as small as possible between the screw and barrel in the metering zone?
To prevent back flow as the pressure is high (should be of order x10-2mm)
What is the compression ratio?
Ratio of screw channel depth or volume from feed to metering zones
CR = Hfeed/Hmetering = Vfeed/Vmetering
What is a venting zone in an extruder screw?
Allows for removal of volatiles and gases.
It is a zone with an increased channel depth before a second compression zone. Volatiles and gases escape through a vent port.
How is a screw designed for amorphous polymers?
As amorphous polymers progressively soften through the Tg, they require a compression zone that covers most of the length of the screw, especially for heat sensitive polymers like PVC.
How is the screw designed for polymers with a sharp melting point?
Melting takes place quickly so compression of the melt can be performed in one pitch of the screw.
What does a filter do before an extrusion die? And what is a breaker plate?
Filters any inhomogeneous material that may clog the die. Supported by a breaker plate which has a large number of counter sunk holes to allow the melt to pass, while preventing dead spots where particles could gather.
Breaker plate also straightens the spirally melt flow emerging from the screw (changes rotational memory to longitudinal memory).
They both also help build up back pressure which improves mixing along the screw.
