Extrusion

Question 1

What parts are made by extrusion?

Answer 1

~ geometries uniform in cross-section

Question 2

What is the composition of extrusion paste?

Answer 2

~ 50vol% particle loading
~ binder/plasticizer (polymer or clay)
~ coagulant (converts a slurry to a paste)
~ 35-50vol% water

Question 3

How much do extruded parts shrink from drying?

Answer 3

~ 1-5%

Question 4

What is done for compositions containing clay?

Answer 4

~ a fluid suspension is mixed with a high speed mixer, then filter-pressed into the form of a paste

Question 5

What is done for systems with a polymeric binder?

Answer 5

~ powder slurry and spray-dried granules are mixed together to form a paste

Question 6

What is a pug mill?

Answer 6

~ an auger (helical tool) with broken flights, mixes the feed and forces it through a shredder into an evacuated chamber

~ shredded mat’l is small is cross-section and hence more uniformly de-aired (trapped bubble removal)

Question 7

What are the types of operations for extruders?

Answer 7

~ batch (piston) or continuous (auger) operation

Question 8

What is differential laminar flow?

Answer 8

~ mat’l movement is slowed near the barrel wall because of friction

~ plug flow is typicaly in the die land

Question 9

What is entrance angle?

Answer 9

~ extent of differential flow though tapered die depends on its entrance angle

~ the taper compresses the mix to accurately adopt the desired shape, but above a critical angle, a region of static material forms which is a source of defects

Question 10

What must be done to cavity walls?

Answer 10

~ need to be polished to facilitate slippage

~ lubricant can be injected near the walls

Question 11

How are hollow tubes extruded?

Answer 11

~ a spider is attached to the die and supports the central control rod

~ flow streams must join downstream from the spider

Question 12

What are the characteristics of pastes?

Answer 12

~ typically shear thinning and show a yield stress