ENG1011 Chap.9 Polymers, 3D Printing, Work Hardening and Annealing
Polymers, 3D Printing, Work Hardening and Annealing
Polymers, 3D Printing, Work Hardening and Annealing covers Week 6, which explains how processing changes a material's properties. Polymers are long chain molecules. Thermoplastics are covalently bonded along each chain but held to neighbouring chains by weaker secondary bonds, so they soften on heating and can be remelted; thermosets are cross-linked into one covalent network and do not melt.
Chain shape matters as well: linear chains pack closely, while branched chains pack loosely. Polymers are far less stiff than metals. Fused deposition modelling builds a part layer by layer and suits custom parts and early prototypes, while injection moulding needs a part-specific tool and suits high-volume production. In metals, permanent deformation happens by the movement of dislocations, defects in the crystal structure.
Cold working multiplies dislocations, raising strength and lowering ductility, and annealing heats the metal so they can be removed, restoring ductility.
Heating and cooling also cause thermal strain equal to the expansion coefficient times the temperature change, which sets how much a moulded part shrinks.
Week 6 content is tested mainly through statements to judge as true or false, which reward precise understanding over memorised phrases.
The repeater practice test sets items on suitable uses of 3D printing, on the behaviour of dislocations, on polymer bonding and on the effects of work hardening, followed by a thermal expansion calculation for a moulded part. Each statement usually hinges on one word, such as always, never, increase or decrease, so read every option fully before deciding.
This week also connects to the bridge project.
Team members are printed in PLA, so layer bonding, shrinkage and warping affect how the printed members match their design dimensions and how the joints fit together during assembly.
What this chapter covers
- 01
Polymer chains, covalent and secondary bonding
- 02
Thermoplastics against thermosets
- 03
Linear and branched chains
- 04
FDM 3D printing and the properties of printed parts
- 05
Injection moulding for high-volume parts
- 06
Dislocations, cold work and work hardening
- 07
Annealing to restore ductility
- 08
Thermal strain and mould shrinkage
Worked example · free
Mould size for a shrinking polymer part
- 1Temperature change: 20 minus 180 = minus 160 degrees.
- 1Thermal strain: 70 times 10 to the minus 6 times minus 160 = minus 0.0112.
- 1Mould length: 80.00 divided by (1 minus 0.0112) = 80.91 mm.
Key terms
- Thermoplastic
- A polymer whose chains are held together by secondary bonds, so it softens on heating and can be remelted.
- Thermoset
- A polymer cross-linked into a covalent network that does not melt on reheating.
- Dislocation
- A line defect in a metal crystal whose movement produces plastic deformation.
- Work hardening
- The rise in strength and fall in ductility of a metal deformed plastically at room temperature.
- Annealing
- A heat treatment that removes the effects of cold work and restores a metal's ductility.
Polymers, 3D Printing, Work Hardening and Annealing FAQ
When is 3D printing the right manufacturing choice?
It suits custom parts made for a particular customer and early-stage prototypes, because it needs no tooling. It is a poor choice for very smooth surface finishes or for producing huge numbers of identical items, where injection moulding is far cheaper per part.
Why does cold working make a metal stronger but more brittle?
Plastic deformation at room temperature creates many more dislocations, which tangle and block one another. Further slip becomes harder, so strength rises, but the metal can deform less before it cracks, so ductility falls.
Can the effects of work hardening be reversed?
Yes. Annealing heats the metal so that its crystal structure can recover and recrystallise, removing most of the extra dislocations. The metal regains its ductility, which is why sheet that has been heavily formed is often annealed before further shaping.
Does injection moulding or 3D printing give the smoother surface?
Injection moulding, because the melt takes the finish of a polished tool, while a printed part shows its deposited layers. That is one reason printing suits prototypes and custom parts rather than products where surface finish matters.
Exam move
Learn this chapter as cause and effect pairs: a process, what it does to the structure, and what that does to the properties. Test yourself with true or false statements about polymers, dislocations and work hardening, and give a one line reason for each verdict. Practise one thermal strain calculation so the sign and reference length become routine.
Write each process as a cause and effect chain on one line, then quiz yourself by covering the effect and recalling it.
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