Extrusion Processes - Polymers

Question 1

What is Extrusion?

Answer 1

High-volume manufacturing process in which raw thermoplastic is melted and formed into a continuous profile

Question 2

What does Extrusion produce?

Answer 2

Items with constant cross section e.g. pipe

Question 3

How does the Screw Extruder work?

Answer 3

1. Raw material (polymer granulates) is fed into a hopper
2. Hopper delivers material into barrel
3. The screw drives material along the barrel through various sections , feed section, melt section and plumping/metering section
4. Melt passes through filter screen and break plate to remove contaminants and materials rotational memory
5. Filtered metal is forced through the die to produce shaped extrudate
6. Extrudate is pulled and cooled

Question 4

What does the Feed section on the screw do?

Answer 4

Polymer conveyed to melt section, little friction or compression

Question 5

What does the Melt section on the screw do?

Answer 5

Polymer compressed by crew and forced against the wall of the barrel, melt film forms due to shearing along the wall and melt pool is collected against flight of screw

Question 6

What does the Pumping/metering section on the screw do?

Answer 6

Remaining solids disperse in melt, melting much less efficient. Pressure builds up against die

Question 7

What does the Filter screen do?

Answer 7

Captures unmelted polymer, congealed resins, contamination and builds back pressure

Question 8

What does the Breaker Plate do?

Answer 8

change helical movement into longitudinal translation movement, also improves final mixing

Question 9

What happens in the Die?

Answer 9

Polymer is forced through the die to take up final shape

Question 10

What are the main components in the screw design?

Answer 10

• Lengths of each section and geometry can be changed to accommodate for different types of polymers
• Pins can be added to improve mixing
• Vents along screw allow water vapor to escape

Question 11

What two types of extrusion dies are there?

Answer 11

1. Annular - to enable cable coating, film blowing
2. Shaped - to produce complex cross sections

Question 12

What is the Die swell (Barus effect)?

Answer 12

Extrusion forces the polymer into alignment, the soft extrudate relaxes on exit from the die changing its cross section
The die shape is designed to compensate for the relaxation
Swell Ratio = D/D0