Engineering Polymers — Engineering Reference

See also (Tier 3 family index): Polymers Taxonomy

1. At a glance

Polymers occupy the lightweight + corrosion-resistant + cheap-to-mold niche of the materials selection chart. Global plastic production reached roughly 400 million tonnes in 2024 (PlasticsEurope, Plastics — the fast Facts), about 5× larger than primary aluminum and 1/5 of crude steel. The vast majority is commodity polymer (PE, PP, PVC, PS, PET) destined for packaging, film, fibre, and consumer goods; engineering plastics — the grades structural designers reach for — account for ~5 % of tonnage but capture most of the value (PEEK alone trades at $100–250/kgvsHDPEat$1.20/kg).

Three structural classes. This is the single most-important taxonomy for selection:

Class	Bonding	Behaviour with heat	Examples
Thermoplastics	Linear / branched chains, van der Waals + entanglement only	Soften and re-melt reversibly	ABS, PC, PA, POM, PET, PEEK, PEI
Thermosets	Covalent 3-D cross-link network formed on cure	No melt — degrade if overheated	Epoxy, unsaturated polyester, vinyl ester, phenolic, PU casting resin
Elastomers	Lightly cross-linked, glassy T below room temperature	Large reversible elastic strain	NR, NBR, EPDM, FKM, silicone, polyurethane rubber

Property envelope. Tensile strength typically 10–200 MPa (CF-PEEK approaches Al at 230 MPa; PTFE bottoms at 21 MPa). Modulus 1–4 GPa for unfilled engineering grades; 8–25 GPa with glass/carbon-fibre loading; 130 GPa for continuous-fibre CF-PEEK laminate. Service-temperature ceiling is the dividing line between commodity and engineering: PE/PP cap at ~80–100 °C, ABS/PC at 100–140 °C, PA/POM at 100–150 °C, PEI at 170 °C, PPS at 220 °C, PEEK at 250 °C continuous.

Where polymers sit in the design stack. First pick for:

• Enclosures, housings, consumer-product chassis (ABS, PC, PC/ABS, PA66-GF)
• Sliding bearings, low-friction gears, snap-fit parts (POM, PA, UHMWPE)
• Seals and gaskets (NBR, FKM, silicone, EPDM)
• Composite matrices (epoxy, vinyl ester, polyester)
• Electrical insulation and connector bodies (PBT, PA66-GF, FR-4 epoxy-glass, PTFE)
• Transparent components (PMMA, PC, COC)
• Bio-/chemical-resistant labware and process parts (PEEK, PTFE, PPSU, PPS)
• Mass-produced injection-moldable parts where geometry complexity is high

Where polymers lose. Stiffness-bound deflection problems (E < 1/30 steel), high-temperature structure (> 250 °C), creep-sensitive sustained-load applications without specific creep-modulus design, fire-exposed primary structure (most polymers burn or drip), UV-exposed un-stabilised exterior, anywhere dimensional stability to ±0.05 mm matters in service across wide humidity or temperature swings.

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2. First principles

2.1 What a polymer actually is

A polymer is a macromolecule built from repeat units (monomers) linked by covalent bonds. Polyethylene, for example, is –(CH₂–CH₂)–ₙ with n typically 10³–10⁶. The molecule’s length controls almost everything that matters mechanically: number-average molecular weight M_n, weight-average M_w, and polydispersity index PDI = M_w/M_n. Engineering grades target PDI ≈ 2 for predictable melt processing; ultra-high-molecular-weight grades (UHMWPE, M_w ≈ 3–6 × 10⁶ g/mol) trade processability for extreme wear resistance.

2.2 The two critical transition temperatures

• Glass transition temperature T_g. Below T_g the amorphous polymer is glassy — chain segments cannot rearrange on engineering timescales, behaviour is hard and brittle, modulus is 1–4 GPa. Above T_g the polymer is rubbery — segmental motion unlocks, modulus drops 100–1000× across a narrow window. T_g is measured by DSC (ISO 11357) or DMA (ASTM D7028).
• Melt temperature T_m. Only semicrystalline polymers have one — the sharp first-order transition where crystalline domains melt and the polymer flows as a viscous liquid. Amorphous polymers (PC, PMMA, PS, PEI, PPSU) have no T_m — they just keep softening above T_g until they flow.

Design implication. Maximum useful service temperature is:

• Amorphous polymers: roughly T_g − 20 °C (no crystalline scaffolding to hold shape above T_g).
• Semicrystalline polymers: can exceed T_g by 50–100 °C (crystallites provide a stiffness floor up to T_m). Long-term continuous service is usually quoted as CUT (Continuous Use Temperature) by UL 746B and may sit well below T_m due to oxidative degradation.

2.3 Amorphous vs semicrystalline

Property	Amorphous (PC, PMMA, PEI, PPSU, PS, PVC, ABS)	Semicrystalline (PA, POM, PEEK, PET, PBT, PP, PE, PPS)
Transparency	Yes (often glass-clear)	No (crystal domains scatter light)
Shrinkage on cooling	Low: 0.4–0.7 %	High: 1.5–2.5 %
Chemical resistance	Lower (solvent stress cracking common)	Higher
Modulus above T_g	Drops sharply	Retained until T_m
Anisotropy after molding	Low	High (flow-aligned crystallites + fibres)
Dimensional stability	Better at constant T	Better across T range up to T_m

2.4 Cross-linking and thermoset cure

A thermoset is, after cure, a single molecule spanning the part — covalent cross-links convert the liquid resin into an infinite network. There is no melt point: heating past the degradation temperature simply pyrolyses the network. Cure is governed by:

• Gel point — onset of network connectivity; mixed resin loses pourable viscosity.
• Vitrification — T_g of curing network rises above cure temperature; further reaction is diffusion-limited and stalls.
• Post-cure — secondary thermal cycle above original cure T to drive conversion to ~95–99 % (epoxy aerospace prepregs cure at 121 °C, post-cure at 177 °C).

Elastomers are thermoset rubbers: very lightly cross-linked (one cross-link every ~100 backbone units) so chains can stretch but cannot flow past one another. Vulcanisation (sulfur-cured NR/NBR/EPDM), peroxide cure (silicone, EPDM), or platinum-catalysed addition cure (LSR silicone) all produce the same network topology by different chemistry.

2.5 Viscoelasticity

All polymers are viscoelastic — stress depends on strain history, not just instantaneous strain. The two engineering consequences are creep (strain rises under constant stress) and stress relaxation (stress falls under constant strain). The pragmatic design tool is the creep modulus E_c(t, T), tabulated by the resin manufacturer at the service temperature for 1 h, 100 h, 1000 h, 10,000 h.

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3. Practical math / design equations

3.1 Deflection Temperature Under Load (DTUL / HDT)

The standard high-temperature ranking metric, per ASTM D648-18 or ISO 75:2020:

• Specimen 127 × 13 × 3.2 mm (or 80 × 10 × 4 mm ISO) loaded in 3-point bend
• Two load levels: 0.45 MPa (66 psi) low-load, 1.82 MPa (264 psi) high-load
• Temperature ramped at 2 °C/min until deflection reaches 0.25 mm
• Report the temperature at which deflection criterion is met

Always cite the load. PEI Ultem 1000 HDT @ 0.45 MPa = 210 °C; HDT @ 1.82 MPa = 200 °C. Same polymer, 10 °C apart — but PEEK 450G is 162 °C @ 1.82 MPa unfilled and 315 °C @ 1.82 MPa with 30 % GF. Quoting “HDT = 162 °C” for PEEK without the load is meaningless.

3.2 Polymer melt viscosity — Arrhenius + shear thinning

Newtonian zero-shear viscosity follows Arrhenius:

η₀(T) = A · exp(E_a / RT)

where E_a is the flow activation energy (30–80 kJ/mol for engineering melts; ~26 kJ/mol for HDPE, ~60 kJ/mol for PC). Useful for predicting how much you must raise barrel temperature to drop viscosity to a target.

Under shear, every engineering melt is non-Newtonian (shear-thinning). Power-law model:

η(γ̇) = K · γ̇^(n−1)

with flow index n ≈ 0.3–0.5 for most engineering thermoplastics (n = 1 would be Newtonian; lower n = stronger shear-thinning). Practical consequence: doubling injection screw speed cuts apparent viscosity by 2^(1−n) ≈ 30–50 %, which is how you fill thin walls.

3.3 Mold shrinkage

After injection molding, the part shrinks linearly from mold cavity dimension to room-temperature dimension as the melt cools and crystallises:

• Amorphous polymers (PC, ABS, PMMA, PEI, PPSU): 0.4–0.7 %
• Low-crystallinity semicrystalline (PE-LD): 1.5–2.5 %
• High-crystallinity semicrystalline (POM, PA, PEEK, PP homopolymer): 1.8–2.5 %
• 30 % glass-filled grades (PA66-GF30, PBT-GF30): 0.3–0.6 % parallel to flow, 0.8–1.3 % transverse — fibres lock the shrinkage anisotropically, the cause of warpage in poorly-designed fibre-reinforced parts.

3.4 Coefficient of linear thermal expansion (CLTE)

Per ASTM D696-16 or ISO 11359-2:

Material	CLTE × 10⁻⁶ /°C
Steel	12
Aluminum	23
ABS, PC, PMMA, PA, POM, PET	60–100
PEEK	47
PTFE	130
PA66-GF30 (parallel to flow)	25
PA66-GF30 (transverse to flow)	80
Liquid-crystal polymer (LCP)	5–20

Bimaterial design rule: a 200 mm aluminum plate bolted to a 200 mm PA66 housing rises 70 °C → Al grows 0.32 mm, PA66 grows 1.12 mm, differential = 0.80 mm. Slot the holes or use compliant bushings.

3.5 Creep modulus and creep design

Polymers creep at room temperature under any sustained load. Never substitute short-term modulus E into a deflection formula for a load lasting more than an hour. Use the apparent creep modulus E_c(t, T):

E_c(t, T) = σ_applied / ε(t, T)

Rule-of-thumb for engineering plastics at modest stress (< 25 % yield):

• E_c (1 h, 20 °C) ≈ 0.85 · E_short
• E_c (1000 h, 20 °C) ≈ 0.30–0.60 · E_short
• E_c (1000 h, 80 °C) ≈ 0.15–0.30 · E_short

Manufacturer-published isochronous stress-strain curves are the design source; CAMPUS and UL Prospector databases tabulate them per ISO 899.

3.6 Worked example — PA66 gear, 100 N·m, 60 °C continuous

Problem. Spur gear, PA66 (Nylon 66) unfilled, 50 mm pitch diameter, 8 mm face width, transmitting 100 N·m torque continuously at 60 °C ambient. Tooth count 32, module 1.5 mm. Predict tooth-root stress; compare unfilled vs PA66-GF30; predict 1000-h tooth deflection.

Step 1. Tangential tooth load.

F_t = 2T / d_pitch = 2 (100 N·m) / 0.050 m = 4000 N

Step 2. Lewis bending stress at tooth root.

σ = F_t / (m · b · Y)

For module m = 1.5 mm, face width b = 8 mm, Lewis form factor Y ≈ 0.35 at 32 teeth (20° pressure angle):

σ = 4000 N / (0.0015 m · 0.008 m · 0.35) = 95.2 MPa

Step 3. Material check — neat PA66.

• Dry-as-molded (DAM): σ_y ≈ 85 MPa at 23 °C
• At 60 °C, 50 % RH (typical equilibrium): σ_y drops to ~45 MPa
• 95 MPa applied > 45 MPa yield → fails immediately by tooth-root yield.

Step 4. Material check — PA66-GF30.

• DAM σ_y = 175 MPa at 23 °C
• At 60 °C, 50 % RH: σ_y ≈ 100 MPa
• 95 MPa applied < 100 MPa → marginal; raises factor of safety only to 1.05. Move to 35 % GF or increase face width.

Step 5. Creep deflection at 1000 h. PA66-GF30 short-term modulus E ≈ 9.0 GPa (DAM, 23 °C). At 60 °C, 50 % RH: E_short ≈ 5.5 GPa. Creep modulus 1000 h at 60 °C ≈ 2.5 GPa per Solvay/Lanxess datasheet.

Tooth deflection at root under cantilever bending (tooth treated as 4 mm cantilever beam):

δ_creep ≈ (F_t · L³) / (3 · E_c · I) = 4000 · (0.004)³ / (3 · 2.5 × 10⁹ · 1.71 × 10⁻¹¹) ≈ 0.20 mm

Versus δ_initial (short-term, 60 °C 50 % RH) ≈ 0.09 mm. Creep multiplier ≈ 2.2× over 1000 h. This is why metal gears stay engaged correctly and plastic gears need backlash designed in.

Conclusion. Unfilled PA66 fails on yield; 30 % glass-filled marginally works with 1000-h tooth deflection ~0.2 mm; steel or POM-on-steel gear pair preferred for continuous high-torque service. PA66-GF is appropriate when shock-absorption and low cost outweigh dimensional precision.

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4. Reference data — common engineering polymers

Mechanical values per ASTM D638-22 (tensile, 5 mm/min, dry-as-molded) or ISO 527-1:2019 equivalent unless noted. Modulus is short-term secant. Temperatures are continuous service range, not peak excursions.

Polymer	Class	Crystallinity	ρ (g/cm³)	σ_y (MPa)	E (GPa)	Elong @ break (%)	T_g (°C)	T_m (°C)	T_use (°C)	HDT @ 1.82 MPa (°C)
HDPE	Commodity TP	semicryst.	0.95	25	1.0	800	−110	130	−50 to +80	50
UHMWPE	Engineering TP	semicryst.	0.93	21	0.7	350	−110	135	−200 to +80	45
PP (homopolymer)	Commodity TP	semicryst.	0.91	35	1.5	100	−10	165	−20 to +100	60
PVC (rigid)	Commodity TP	amorphous	1.40	50	3.0	30	80	—	0 to +60	70
PS (GPPS)	Commodity TP	amorphous	1.05	45	3.3	3	100	—	−20 to +80	85
ABS	Engineering TP	amorphous	1.05	40	2.0	25	105	—	−40 to +80	88
PC (Lexan/Makrolon)	Engineering TP	amorphous	1.20	65	2.3	100	145	—	−135 to +120	130
PMMA (acrylic)	Engineering TP	amorphous	1.18	70	3.0	4	105	—	−40 to +80	95
PA6	Engineering TP	semicryst.	1.13	75	2.6	50	50	220	−40 to +90	60 (DAM)
PA66 (dry)	Engineering TP	semicryst.	1.14	85	3.0	40	65	265	−40 to +100	95 (DAM)
PA66 (50 % RH)	Engineering TP	semicryst.	1.15	50	1.6	200	0	265	−40 to +100	70
PA66-GF30	Engineering TP	semicryst.	1.36	175	9.0	4	65	265	−40 to +120	250
POM (Delrin 150, homopolymer)	Engineering TP	semicryst.	1.42	70	3.1	25	−60	175	−40 to +85	110
POM-C (Hostaform / Celcon, copolymer)	Engineering TP	semicryst.	1.41	62	2.7	35	−60	165	−40 to +90	100
PET (semicryst., Rynite)	Engineering TP	semicryst.	1.39	80	3.0	50	75	255	−40 to +100	80
PBT (Valox / Crastin)	Engineering TP	semicryst.	1.31	55	2.5	250	50	225	−40 to +120	65
PBT-GF30	Engineering TP	semicryst.	1.53	135	9.5	3	50	225	−40 to +140	215
PEEK (Victrex 450G)	High-perf TP	semicryst.	1.32	100	3.6	25	143	343	−60 to +250	152
PEEK 450CA30 (CF30)	High-perf TP	semicryst.	1.40	230	24	1.5	143	343	−60 to +250	315
PEEK 450GL30 (GF30)	High-perf TP	semicryst.	1.53	170	10	2	143	343	−60 to +250	315
PEI (Ultem 1000)	High-perf TP	amorphous	1.27	105	3.0	60	217	—	−50 to +170	200
PPSU (Radel R)	High-perf TP	amorphous	1.29	70	2.3	60	220	—	−50 to +180	207
PSU (Udel)	High-perf TP	amorphous	1.24	70	2.5	60	185	—	−50 to +150	174
PES	High-perf TP	amorphous	1.37	85	2.7	40	225	—	−50 to +180	203
PPS (Ryton, neat)	High-perf TP	semicryst.	1.35	85	3.8	3	90	285	−20 to +220	110
PPS-GF40	High-perf TP	semicryst.	1.66	195	16	1.5	90	285	−20 to +230	270
PTFE (virgin)	High-perf TP	semicryst.	2.16	21	0.5	300	−97 / +127	327	−250 to +260	55
Epoxy (DGEBA + amine, cured)	Thermoset	n/a	1.20	65	3.5	4	60–180	—	up to 150 (std)	T_g dependent
Polyester (unsat., cured)	Thermoset	n/a	1.20	55	3.5	2	70–110	—	up to 80	70
Vinyl ester	Thermoset	n/a	1.15	80	3.5	5	100–140	—	up to 110	110
Phenolic (PF, cast)	Thermoset	n/a	1.30	50	6.5	1	n/a (heavily X-linked)	—	up to 200	175
PU casting (90A elastomer)	Elastomer	n/a	1.15	35 (T_b)	0.01	500	−30	—	−30 to +90	n/a
NR (natural rubber)	Elastomer	n/a	0.93	25 (T_b)	0.003	700	−70	—	−50 to +80	n/a
NBR (Buna-N)	Elastomer	n/a	1.00	20 (T_b)	0.005	500	−40 to −10	—	−30 to +110	n/a
EPDM	Elastomer	n/a	0.87	20 (T_b)	0.004	500	−55	—	−50 to +150	n/a
FKM (Viton A)	Elastomer	n/a	1.85	15 (T_b)	0.007	300	−20	—	−20 to +205	n/a
Silicone (VMQ)	Elastomer	n/a	1.15	10 (T_b)	0.005	400	−123	—	−60 to +230	n/a

T_b = ultimate tensile strength at break for elastomers (per ASTM D412 / ISO 37). PA values for “dry” are DAM (dry-as-molded); polyamides absorb up to 2.5 % moisture at 50 % RH equilibrium with major property impact.

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5m. Composition & microstructure

5m.1 Commodity thermoplastics (mass-market, not normally “engineering”)

• PE (polyethylene) — –(CH₂CH₂)–ₙ. The simplest polymer. Distinguished by branching/density:
  • LDPE (ρ 0.92) — high branching, flexible, low strength; films, squeeze bottles, cable jacketing.
  • LLDPE (ρ 0.92) — linear short-chain branches via comonomer (octene, hexene); tougher film than LDPE.
  • HDPE (ρ 0.95) — minimal branching, ~70 % crystalline; σ_y ≈ 25 MPa, FDA-compliant grades for milk jugs, fuel tanks (rotomolded), HDPE pipe (PE100, PE4710).
  • UHMWPE (M_w 3–6 × 10⁶) — extreme entanglement; σ_y 21 MPa low, but highest abrasion resistance of any thermoplastic (10× POM); orthopaedic joint cups (ASTM F648), ice-rink dasher boards, conveyor wear strips, bulletproof vest fibre (Dyneema, Spectra).
• PP (polypropylene) — –(CH₂CH(CH₃))–ₙ. Tacticity-dependent: isotactic PP is semicrystalline (engineering grade), atactic PP is amorphous and gummy. Homopolymer (rigid, brittle at low T); random copolymer (improved clarity, less stiff); impact-copolymer with EPDM phase (automotive bumper fascia). Living-hinge applications exploit PP’s flex-fatigue resistance.
• PVC (polyvinyl chloride) — –(CH₂CHCl)–ₙ. Two product families that share resin but behave totally differently:
  • Rigid PVC (uPVC) — pipe, window frames, siding. σ_y 50 MPa, T_use to 60 °C.
  • Flexible PVC — 20–50 % phthalate or alternative plasticiser added. Cable insulation, hose, medical tubing, garden hose. Plasticiser migration is a long-term failure mode.
• PS (polystyrene) — –(CH₂CH(C₆H₅))–ₙ. GPPS (general-purpose, clear, brittle), HIPS (high-impact, butadiene-rubber-modified, opaque), EPS (expanded foam, insulation/packaging), XPS (extruded foam, building insulation).

5m.2 Engineering thermoplastics

• ABS (acrylonitrile-butadiene-styrene) — graft terpolymer: SAN matrix with butadiene-rubber particles. σ_y ≈ 40 MPa, E ≈ 2.0 GPa, T_g ≈ 105 °C. Opaque, easy to mold, machine, glue (acetone or MEK solvent welds joints as strong as parent), 3D-print on FDM. Standard Lego bricks, automotive interior trim, consumer-electronics housings. Limits: poor UV resistance (use ASA — acrylic-replacing-butadiene — for outdoor); low solvent resistance; modest heat tolerance.
• PC (polycarbonate, bisphenol-A based) — –(O–C₆H₄–C(CH₃)₂–C₆H₄–O–CO)–ₙ. σ_y ≈ 65 MPa, E ≈ 2.3 GPa, T_g ≈ 145 °C, glass-clear (90 % light transmission). Exceptional impact toughness (notched Izod 800 J/m, ~10× PMMA). Bulletproof glazing, eyeglass lenses (Trivex/PC), motorcycle helmet shells, baby bottles (BPA-free regulatory pressure shifting to copolyester or Tritan). Limits: scratches easily (~3H pencil hardness); crazes and cracks in contact with hydrocarbons, alcohols, alkalis under stress.
• PMMA (polymethyl methacrylate, acrylic, Plexiglas, Perspex, Acrylite) — σ_y ≈ 70 MPa, E ≈ 3.0 GPa, T_g ≈ 105 °C. Glass-clear (92 % light transmission, better than PC), excellent UV stability and weatherability. Used in aircraft canopies (cell-cast), large aquarium walls, signage, LED diffusers, dental prosthetics. Limits: brittle; impact resistance ~1/10 PC.
• PA (polyamide, Nylon) — –(CO–R–NH)–ₙ. Many variants:
  • PA6 — –(NH(CH₂)₅CO)–ₙ from caprolactam ring-opening; T_m 220 °C; cheaper than PA66; cast-Nylon stock shapes (Nylatron with MoS₂ for self-lubrication).
  • PA66 — –(NH(CH₂)₆NH·CO(CH₂)₄CO)–ₙ from hexamethylenediamine + adipic acid; T_m 265 °C; stiffer + higher HDT than PA6.
  • PA12 — –(NH(CH₂)₁₁CO)–ₙ; T_m 178 °C; lowest moisture absorption of common nylons (~1.5 % vs 8.5 % for PA6 saturated); pneumatic tubing, fuel lines, SLS 3D-printing powder (PA2200).
  • PA46 (Stanyl) — T_m 295 °C; high-temperature gears.
  • Aromatic polyamides (PPA, aramid) — Nomex, Kevlar fibres; flame-resistant clothing, ballistic vests, brake pads.
  • Glass-filled PA66 (GF30, GF50) — workhorse engineering grades; PA66-GF30 σ_y 175 MPa, E 9 GPa, replaces die-cast aluminum in automotive intake manifolds, brake pedals, e-mobility cooling housings.
  • Moisture caveat. PA absorbs water plasticising the amorphous regions: σ_y drops 40 %, modulus drops 50 %, elongation rises from 50 to 200 % between DAM and 50 % RH equilibrium. Always design with conditioned properties unless service is dry.
• POM (polyoxymethylene, acetal, Delrin homopolymer / Hostaform/Celcon copolymer) — –(CH₂O)–ₙ. σ_y 70 MPa, E 3.1 GPa (homopolymer), T_m 175 °C. Highest crystallinity of common engineering thermoplastics (75–85 %), which gives dimensional stability + lowest coefficient of friction among rigid polymers (μ ≈ 0.15 against steel). Standard for precision gears (electric-window mechanisms, fishing-reel internals), snap-fits, bushings, conveyor parts.
  • Homopolymer (DuPont Delrin 100/150/500) — higher crystallinity, ~10 % higher modulus and HDT.
  • Copolymer (Celanese Celcon, BASF Ultraform, Polyplastics Duracon) — better hydrolysis + hot-water resistance, better chemical resistance to caustics. Standard for water-contact parts.
  • Limits: depolymerises and releases formaldehyde above ~230 °C; do not over-dry, do not stagnate in barrel.
• PET / PBT (thermoplastic polyesters) — –(O(CH₂)ₙOCOC₆H₄CO)–ₙ. Both are condensation polymers of terephthalic acid:
  • PET (Mylar film, Dacron fibre, beverage bottles) — n = 2. Engineering grade is semicrystalline + 30 % GF (DuPont Rynite); high HDT, stiffness, dimensional stability. Bottle-grade PET is amorphous (rapid-quench, low IV) — totally different material.
  • PBT (Valox, Crastin, Pocan, Ultradur) — n = 4. Faster crystallising, easier injection molding than PET; standard for automotive electrical connectors, electronic enclosures, glass-filled grades displaced die-cast zinc in many applications. Hydrolysis-resistant grades (Crastin HR) for under-hood automotive.
• PEEK (polyetheretherketone, Victrex PEEK 450G / Solvay KetaSpire / Evonik Vestakeep) — –(O–C₆H₄–O–C₆H₄–CO–C₆H₄)–ₙ. σ_y 100 MPa, E 3.6 GPa, T_g 143 °C, T_m 343 °C, continuous use 250 °C. Outstanding chemical resistance (resists everything below 200 °C except concentrated H₂SO₄ and oleum), radiation resistance (10× polyimide), wear/abrasion. CF30 grade (450CA30): σ_y 230 MPa, E 24 GPa, modulus exceeds aluminum 6061. Used in aerospace (jet-engine bypass-duct seals, bushings), oil & gas downhole (PEEK-CF compression seals, BHA components), medical implants (spinal cages — FDA-cleared since 1999; replaces titanium where MRI-clarity, modulus matching of bone, and bone-on-growth matter), semiconductor wafer handling, EV battery insulation. Natural-grade ~$100/kg;medicalimplantgrade$200–250/kg.
• PEI (polyetherimide, SABIC Ultem 1000) — amorphous; σ_y 105 MPa, E 3.0 GPa, T_g 217 °C. Naturally UL 94 V-0 with no flame retardant additive, FAA FAR 25.853 compliant for aircraft cabin interior (Ultem 9085 is the FDM 3D-printing grade certified for cabin parts on commercial aircraft). Transparent amber colour. Steam-sterilisable to ~140 °C in 1000+ cycles. Common applications: MRI-compatible medical components, semi-conductor wafer carriers, aircraft seat-back assemblies, sterilisation trays.
• PPSU / PSU / PES (sulfone family) — share the diaryl sulfone unit –(C₆H₄–SO₂–C₆H₄)–. All amorphous, transparent, autoclavable.
  • PSU (Udel) — T_g 185 °C, used in dialysis filter housings, plumbing fittings.
  • PES (polyethersulfone) — T_g 225 °C, hot-water and steam applications.
  • PPSU (polyphenylsulfone, Radel R) — T_g 220 °C; toughest sulfone; surgical instrument trays, dental orthodontic brackets, aircraft cabin interior.
• PTFE (polytetrafluoroethylene, DuPont Teflon, Daikin Polyflon, AGC Fluon) — –(CF₂CF₂)–ₙ. σ_y 21 MPa, E 0.5 GPa, T_use to 260 °C continuous. Lowest coefficient of friction of any solid (μ_s 0.05–0.10 against steel). Chemically inert — resists everything except molten alkali metals, F₂ gas, and ClF₃. Cannot be melt-processed — melt viscosity is ~10¹⁰ Pa·s; sintered from compressed powder, then machined or skived into sheet/tape. Cold flows under sustained compressive load — a 100-h-loaded PTFE gasket can permanently extrude 10–15 % of original thickness. Used in chemical-process valve seats, expansion-joint bellows, PTFE-coated cookware, plumber’s thread tape, electrical-wire insulation in high-frequency aerospace cabling (low ε_r = 2.1).
  • Melt-processable fluoropolymer cousins:
    • PFA (perfluoroalkoxy) — PTFE-like properties, melt-processable, used for chemical tube linings.
    • FEP (fluorinated ethylene propylene) — slightly lower T_use (200 °C), melt-processable, common heat-shrink tubing.
    • PVDF (Kynar, Solef) — semicrystalline, σ_y 50 MPa, piezoelectric in β-phase, used in chemical pipe (Kynar lining), Li-ion battery binder, architectural metal coatings (PVDF paints), 3D-print filament.
    • ETFE (Tefzel) — used in architectural film (cushion roofs, Eden Project, Allianz Arena).
• PPS (polyphenylene sulfide, Ryton, Fortron, Torelina) — –(C₆H₄–S)–ₙ. σ_y 85 MPa, E 3.8 GPa, T_g 90 °C, T_m 285 °C, T_use 220 °C continuous. Inherently flame retardant (V-0). Excellent chemical resistance (no known solvent below 200 °C). Brittle when unfilled — virtually always sold as 40 % GF (Ryton R-4) or 65 % mineral-glass. Used for automotive coolant pumps, EGR valves, oil pump components, electrical connectors under-hood.

5m.3 Thermosets

• Epoxy — workhorse adhesive, composite matrix, electronics encapsulant.
  • Chemistry: diglycidyl ether of bisphenol-A (DGEBA, the standard liquid epoxy) + hardener.
  • Hardeners:
    • Aliphatic amines (TETA, DETA, IPDA) — RT cure, T_g 60–110 °C, common in two-part consumer epoxy adhesives.
    • Aromatic amines (DDS, MDA) — high-T cure (120–180 °C), T_g 180–220 °C, structural aerospace prepreg.
    • Anhydrides (MTHPA, NMA) — long pot life, used in filament-wound composites, electrical encapsulation.
    • Polyamide / amidoamine — flexibilised, lower T_g, marine and coating use.
  • Aerospace structural grades: Hexcel 8552, Toray 3900-series, Solvay/Cytec CYCOM 5320-1 — cure at 121–177 °C, T_g 180–210 °C, used in carbon-fibre prepreg laminates.
  • Modulus 3.0–4.0 GPa neat resin; ultimate stress 60–90 MPa; CTE 50–80 × 10⁻⁶ /°C; widely used as adhesive (Loctite EA 9396, 3M Scotch-Weld 2216).
• Unsaturated polyester (UP) — workhorse low-cost composite matrix.
  • Chemistry: maleic anhydride + glycol + styrene crosslinker; cured by peroxide free-radical initiation.
  • Mechanical: σ_u 55 MPa, E 3.5 GPa, T_g 70–110 °C.
  • Higher shrinkage than epoxy (5–8 % vs 2–4 %); lower chemical resistance.
  • Use cases: glass-fibre boats, building cladding, bathtubs, septic tanks, sheet molding compound (SMC) automotive panels.
  • Styrene emission is a workplace issue — increasingly displaced by vinyl ester in regulated regions.
• Vinyl ester — epoxy-methacrylate backbone, styrene-crosslinked.
  • Bridges polyester (cost, processability) and epoxy (chemical resistance, mechanical).
  • σ_u 80 MPa, E 3.5 GPa. Standard for chemical tank linings (Ashland Derakane), corrosion-resistant industrial gratings, pipe wraps.
• Phenolic (PF, Bakelite, Resol/Novolak) — phenol + formaldehyde condensation.
  • First commercial synthetic plastic (1907, Baekeland).
  • Highly cross-linked, T_use 150–260 °C, low smoke and toxicity on combustion (preferred over polymers in mass-transit rolling stock, FAR 25.853).
  • Brake pads, electrical insulators, foundry binder (no-bake sand binder), ablative heat shields (Apollo command module exterior).
• Polyurethane (PU) — the chemistry that spans rigid to elastomer.
  • Thermoset cast urethanes (Vibrathane, Adiprene) — Shore 50A to 75D; industrial wheels, mining sieve panels, scraper blades, vibration mounts; abrasion resistance 5–10× rubber.
  • Thermoplastic urethanes (TPU) — segmented block copolymer (hard polyurethane + soft polyether/polyester); Shore 70A to 65D; injection-mouldable; cable jacketing, ski boots, smartphone bumpers, footwear midsoles.
  • Rigid PU foam — building insulation, refrigerator-wall insulation.
• Silicone (PDMS, polysiloxane) — Si–O–Si backbone instead of C–C; the only common engineering polymer with an inorganic backbone.
  • Cure systems: peroxide (HCR), addition (LSR, two-part platinum-catalysed), condensation (RTV, one- or two-part).
  • Both rubber (most uses) and resin (semiconductor encapsulation) forms.
  • T_use −60 to +230 °C continuous, peaks to +300 °C; food (FDA 21 CFR 177.2600) and medical (USP Class VI, ISO 10993) compatible; UV and ozone resistant.

5m.4 Elastomers (vulcanised or addition-cured rubbers)

Designation (ASTM D1418)	Common name	Backbone	Key strength	Key weakness
NR / IR	Natural rubber / poly(isoprene)	C-C, unsaturated	Highest tensile (25 MPa) and tear among elastomers; resilience	Poor ozone, UV, oil resistance
SBR	Styrene-butadiene	C-C, unsaturated	Cheap, tyre tread	Poor oil, ozone
NBR	Acrylonitrile-butadiene (Buna-N)	C-C, polar -CN	Standard oil/fuel seal; hydraulic O-rings	Poor ozone unless blended; T_use < 110 °C
HNBR	Hydrogenated NBR	Saturated, polar	NBR’s oil resistance + EPDM’s heat (+150 °C)	3× cost of NBR
EPDM	Ethylene-propylene-diene M-class	C-C, saturated	Ozone, weather, hot water/steam to 150 °C	Poor oil/fuel resistance
CR	Polychloroprene (Neoprene)	C-C, Cl-substituted	Balanced oil + weather; flame retardant	Modest in everything
IIR / BIIR	Butyl / brominated butyl	Saturated	Best gas-permeation barrier; tyre innerliner, gloveboxes	Slow cure, poor strength
FKM	Fluoroelastomer (Viton, Dai-El)	C-C, F-substituted	T_use 205 °C continuous, 260 °C peak; resists fuels, oils, brake fluid, ozone	$100+/kg; poor low-T (T_g ~ −20 °C)
FFKM	Perfluoroelastomer (Kalrez, Chemraz)	C-F backbone	T_use 320 °C; resists everything FKM resists plus amines, ketones	$1000+/kg
VMQ / FVMQ	Silicone / fluorosilicone	Si-O backbone	Widest T range (-60 to +230 °C); food/medical compatible	Low tensile (10 MPa); poor abrasion
AU / EU	Polyurethane rubber (PU/PUR)	C-N-C urethane	Highest abrasion of any elastomer (10× NBR); high tensile	Hydrolysis (esters); T_use < 90 °C
ACM / AEM	Polyacrylate / ethylene-acrylate	C-C, polar	Hot oil to 175 °C, automotive transmissions	Poor water resistance
CSM	Chlorosulfonated PE (Hypalon)	C-C, polar	Ozone + acid resistance; pond liners	Discontinued in 2010, replaced by EPDM-blend or CR

Hardness scales:

• Shore A (rubber/soft plastics) — 30A (rubber band) to 90A (hard rubber wheel); 95A roughly equals 45D.
• Shore D (rigid plastics) — 40D (HDPE) to 90D (PEEK/POM); above 90D the test is not very discriminating and Rockwell M / Rockwell R (ASTM D785) are used.

Compression set (per ASTM D395, 22 h at 70 °C or 100 °C, 25 % compression) is the key elastomer seal-performance metric: lower is better. FKM and silicone typically 15–25 %; NBR 25–35 %; cheap CR up to 50 %. High compression set = seal will leak after a thermal cycle.

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6m. Mechanical properties

Standards in order of citation precedence: ISO 527-1:2019 / ASTM D638-22 (tensile), ISO 178:2019 / ASTM D790-17 (flexural), ISO 75:2020 / ASTM D648-18 (HDT), ASTM D785-08(2021) (Rockwell), ASTM D2240-15 (Shore durometer), ASTM D256-10(2018) (Izod impact), ASTM D5045-14 (plane-strain fracture toughness K_IC), ASTM D7791-22 (axial fatigue).

Stiffness, strength, and toughness

• Modulus — unfilled engineering thermoplastics 1.5–3.8 GPa, glass-filled (GF30) grades 8–10 GPa, carbon-filled (CF30) 20–25 GPa, continuous-fibre composites 50–130 GPa.
• Yield strength — most engineering grades neat at 40–110 MPa; GF30 175–230 MPa.
• Notched Izod impact toughness:
  • PC: 800 J/m (highest of common rigid plastics)
  • ABS: 200–300 J/m
  • PA66-GF30: 100 J/m
  • PMMA: 20 J/m
  • PS, PVC unmodified: 20–30 J/m (brittle)

Fatigue

Polymer fatigue data is sparse compared to metals — UL Prospector / CAMPUS sheets often omit it. Two key behaviours:

• No true endurance limit. S-N curves continue dropping at 10⁷+ cycles; design to a specified life, not a fatigue limit.
• Self-heating from hysteresis. Polymers have high mechanical-damping tan δ; cyclic loading at frequencies above ~5 Hz heats the part. At high strain amplitude / high frequency, fatigue may transition to thermal failure (creep at elevated T) rather than true crack-growth fatigue. Run-out criterion may be 10⁶ cycles to keep heating manageable.

Anisotropy

Critical in fibre-filled grades. PA66-GF30 σ_y is 175 MPa parallel to flow but only 100 MPa transverse. Mold-flow analysis (Moldex3D, Autodesk Moldflow, SIGMASOFT) predicts fibre orientation tensor and drives FEA orthotropic material cards.

Moisture sensitivity (polyamides)

PA66 dry-as-molded (DAM) → 50 % RH equilibrium (~2.5 % moisture absorbed):

• σ_y: 85 → 50 MPa (−40 %)
• E: 3.0 → 1.6 GPa (−47 %)
• Elongation: 40 → 200 % (+400 %)
• T_g: 65 → 0 °C
• Dimensional change: +0.5 % linear (parts grow)

Conditioned (CON) values, not DAM, are the right design basis for most service environments.

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7m. Thermal / electrical / chemical

Thermal

Property	Typical engineering thermoplastic	Engineering metal (reference)
Density ρ (g/cm³)	1.1–1.4 (PTFE 2.16)	2.7 (Al) / 7.85 (steel)
Thermal conductivity k (W/m·K)	0.20–0.30	16–235
Specific heat c_p (J/kg·K)	1300–2000	480–900
CLTE α (10⁻⁶ /°C)	60–120 (GF30 grades 20–30)	12–23

Polymers are thermal insulators. Their k is 100–1000× lower than metals. This drives:

• Slow injection cooling (dominant cycle-time bottleneck).
• High temperature gradients during welding — heat doesn’t dissipate, causing localised over-melt.
• Surface-bias temperature in friction applications (bearing surface runs hot while bulk stays cold).
• Excellent insulators for cryogenic vessels (multilayer PEEK and PI films in spaceflight cryotanks).

Electrical

Most polymers are excellent electrical insulators — bulk resistivity 10¹²–10¹⁸ Ω·cm.

Property	Range
Volume resistivity (Ω·cm)	10¹² (PA, conductive) to 10¹⁸ (PTFE, PE)
Dielectric strength (kV/mm), 3 mm specimen	15–50
Dielectric constant ε_r at 1 MHz	2.1 (PTFE), 2.3 (PE, PP), 3.0 (PEEK), 3.2 (PEI), 4.6 (FR-4 epoxy-glass)
Loss tangent tan δ at 1 MHz	0.0001 (PTFE) to 0.02 (PA)

Common electrical applications: wire and cable insulation (PE, XLPE, PVC, FEP, PTFE), printed-circuit substrate (FR-4 epoxy-glass laminate, ε_r 4.6 — itself a polymer composite), connector bodies (PBT-GF, PA66-GF), capacitor films (PP, PET, PEN, PVDF), motor slot insulation (Nomex aramid paper), high-voltage standoffs (PEEK, PEI, ceramic-filled epoxy).

Chemical resistance

• Polyolefins (PE, PP) — resist most aqueous solutions, alcohols, weak acids and bases; fail on aromatic and chlorinated solvents.
• PVC — resists acids, bases, and salts; attacked by ketones, esters, aromatic solvents.
• PC, PMMA, PS — solvent stress-cracking is the dominant failure mode; avoid contact with hydrocarbons, alcohols, alkalis under stress.
• Polyamides — resist hydrocarbons and most oils; absorb water (the chief drawback); attacked by phenols, strong acids and bases at elevated T.
• POM — chemical resistance generally good; attacked by strong acids (especially copolymer POM-C is much better than homopolymer POM-H in hot water and caustic).
• PEEK — resists nearly all chemicals to 200 °C except concentrated sulfuric, oleum, halogenated and aromatic solvents at high T.
• PTFE, PFA, FEP, ETFE — universal chemical resistance; attacked only by molten alkali metals (Na, K) and elemental F₂ at high T.
• Sulfone family (PSU, PES, PPSU) — autoclave + steam compatible; attacked by ketones, halogenated solvents.

Weathering / UV

Without stabilisation, PC, ABS, PA, and PP yellow and embrittle under UV (sunlight) over 1–5 years.

Stabiliser systems:

• HALS (hindered-amine light stabilisers, Tinuvin 770) — trap radicals.
• UV absorbers (benzotriazoles, hydroxyphenyl-triazines) — absorb 290–400 nm.
• Carbon black — broadband UV screen; standard 2.5 % loading in geomembranes, wire jacketing.

Inherently weather-resistant polymers: PMMA, ASA, fluoropolymers (PTFE, PVDF, ETFE), FKM, silicone.

Flame — UL 94 ratings

The standard global-industry plastic flame test (UL 94-2023) classifies plastics by their behaviour after ignition with a 50 W flame:

• V-0 — self-extinguish within 10 s; no flaming drips. Highest standard for general plastics.
• V-1 — extinguish within 30 s; no flaming drips.
• V-2 — extinguish within 30 s; flaming drips allowed (drip-ignited cotton fails V-1/V-0).
• HB — horizontal burning ≤ 75 mm/min for 3 mm specimen; lowest qualifying rating, suitable for non-critical enclosures.
• 5VA / 5VB — thicker-specimen high-flux test; required for total-enclosure equipment (UL 746C).

Inherently V-0 polymers (no flame-retardant additive required): PEI, PPSU, PES, PEEK, PPS, fluoropolymers. FR-grade polymers (containing brominated/phosphate FR additives): FR-ABS, FR-PC, FR-PA. RoHS/REACH restrict halogenated FRs in EU; phosphorus and nitrogen-based FR formulations are displacing them.

Outgassing (vacuum and aerospace service)

Per ASTM E595-22, polymers exposed to vacuum at 125 °C for 24 h:

• TML (Total Mass Loss) ≤ 1.00 %
• CVCM (Collected Volatile Condensable Material) ≤ 0.10 %

These are the NASA/ESA low-outgassing thresholds. Acceptable: PEEK, PEI, polyimide (Kapton, Vespel), most fully-cured epoxies (post-cured), PTFE. Marginal: PVC (plasticisers), silicone (low-MW siloxanes — important contaminant in optics; cleanroom-grade silicones screened to E595).

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8m. Processing & joining

8m.1 Injection molding (dominant process for engineering thermoplastics)

Heat → inject into closed cavity → cool → eject. Cycle time 10–90 s depending on wall thickness (thermal-diffusion limited).

Tool cost: $5–20k(prototype,single-cavity,low-volume)to$100k–$1M(high-cavitation,hardened-steelproduction).Per-partcostcandropbelow$0.10 for high-volume commodity parts.

Design-for-injection-molding (DFM) rules:

• Uniform wall thickness (2–4 mm typical) — variable wall causes sink, voids, warpage.
• Draft angle 1–3° on all faces parallel to mold-open direction.
• Generous fillets at inside corners; sharp internal corners are stress raisers and cause “knit-line” weakness.
• Gate at thickest section; melt should flow from thick to thin.
• Avoid undercuts unless using side-actions or collapsible cores (adds tool cost).
• Knit lines form where flow fronts meet — design to place them in low-stress regions.

8m.2 Other thermoplastic processes

• Extrusion — pipe, sheet, film, wire jacketing, profiles. Continuous; tool cost $5–30k for a die.
• Blow molding — bottles, fuel tanks, hollow industrial containers. Extrusion blow (HDPE jugs) or injection-stretch blow (PET beverage bottles).
• Thermoforming — heat sheet, drape over mold, vacuum or pressure form. Packaging blister, signage, large refrigerator liners, aircraft cabin interior trim.
• Rotational molding — powder + closed mold rotating in oven, polymer fuses to inner surface. Large hollow parts (kayaks, playground equipment, agricultural tanks, road-traffic barriers); LLDPE dominant.
• Compression molding — thermosets (SMC/BMC sheet/bulk molding compound), bulky composite parts (truck cab hoods).
• 3D printing:
  • FFF / FDM (fused filament) — PLA, PETG, ABS, PA, PC, CF-PA, ULTEM 1010 (Stratasys industrial) for FAA-cleared cabin parts; build temperatures 200–415 °C.
  • SLA / DLP — UV-cured photopolymers (typically methacrylate-urethane blends); excellent surface but UV-sensitive and brittle vs molded equivalents.
  • SLS / MJF — PA12 dominant powder; PA12 (Vestamid, Vestosint) standard for SLS; HP MJF runs PA12 / PA11 / TPU.
  • MJF / FDM with continuous fibre — Markforged Onyx (CF-PA), Anisoprint (continuous CF).

8m.3 Thermoset processing

• Hand layup / spray layup — boats, low-volume composite.
• Resin Transfer Molding (RTM) / VARTM — vacuum-assisted infusion of dry fibre preform; automotive, wind-turbine blades, marine.
• Filament winding — pressure vessels, pipe, rocket motor cases.
• Pultrusion — continuous-profile fibre + resin pulled through heated die; fibre-glass-reinforced rebar, ladders, gratings.
• Prepreg autoclave — aerospace structural carbon-fibre composites; 6–10 bar pressure, 121–180 °C cure; Boeing 787, Airbus A350 fuselage barrels.
• Casting — open-mold cast polyurethane, epoxy potting compound (electronics encapsulation).

8m.4 Machining

• POM (Delrin) — machines like brass; sharp tools, no coolant needed, excellent dimensional stability. The standard machinable engineering plastic.
• PC — machines cleanly; anneal afterwards to relieve machined-in stress (PC is notorious for crack propagation from machined edges in chemical contact).
• PEEK — machines well with carbide tooling; expensive stock means careful CAM.
• PMMA — chips easily; use vacuum to capture swarf; vapor-polish edges (methylene chloride) for optical clarity.
• UHMWPE — slow speeds (gummy); sharp tools.
• PTFE — deforms under tool pressure; sharp positive-rake tools, slow speeds, no coolant; difficult to hold tolerance.
• ABS — machines OK but edges chip; cool tool to avoid melting / smearing.
• Glass-filled grades — abrasive on tooling; expect 1/5 the tool life of unfilled.

8m.5 Joining

• Solvent bonding — for amorphous thermoplastics that dissolve in the solvent. ABS + acetone or MEK, PC + dichloromethane, PVC + cyclohexanone, PMMA + dichloromethane or PMMA cement. Joint strength approaches parent material when designed right (lap joint, scarf joint).
• Adhesive bonding:
  • Cyanoacrylate — fast, brittle bond; works on most rigid plastics except polyolefins and fluoropolymers.
  • Epoxy — structural; needs sanded/abraded surface and clean degrease (IPA wipe).
  • Anaerobic acrylic (Loctite) — fastener thread-locking, retaining compound.
  • Structural acrylic (3M DP 8005, Plexus MA300) — bonds difficult-to-bond polyolefins (PE, PP) without surface treatment; aluminum-grade strength on plastic.
  • Polyurethane structural (Sika, 3M) — flexible structural bond; automotive glazing, panel bonding.
• Surface preparation for low-energy plastics (PE, PP, PTFE):
  • Flame treatment — propane flame pass increases surface energy from 30 to 50 mN/m.
  • Plasma / corona — generates polar groups at surface; standard for bottle-printing, medical-tubing bonding.
  • Chemical etch — sodium-naphthalene complex on PTFE; converts surface to a brown bonded carbon layer.
• Welding:
  • Ultrasonic welding — 20–40 kHz vibration at joint melts thermoplastic in 0.1–1 s; works for amorphous (ABS, PC, PS) and near-amorphous PA. Requires energy directors (small triangular ridges at joint, ~0.5 mm) in part design.
  • Vibration welding (linear or orbital) — 100–300 Hz mechanical reciprocation; larger parts than ultrasonic; automotive intake manifolds (PA66-GF), instrument panels.
  • Hot-plate welding — heated platen melts both parts, then pressed together; large parts (HDPE tanks, automotive coolant tanks, plumbing pipe).
  • Spin welding — circular parts only; rotating one half against the other.
  • Hot-gas welding — extrudate filler rod + hot N₂ or air; PVC ductwork, PP tank repair.
  • Friction stir welding — emerging for thermoplastics (PE pipe lap joints).
  • Laser welding (through-transmission) — IR laser passes through transparent top part and heats absorbing bottom part at the interface; medical devices, microfluidics.
• Mechanical joining:
  • Snap-fits — cantilever beam designed to flex past undercut, snap into engagement; standard for housing closures; design rule: max strain at root ≤ 70 % of yield strain.
  • Self-tapping screws into bosses — PT, Plastite, Delta-PT thread profiles; design boss OD = 2× screw nominal, depth = 2× screw diameter.
  • Heat-set inserts (Heli-Coil, Spirol) — brass insert pressed in by heated probe; preferred for repeated assembly cycles.
  • Ultrasonic inserts — knurled brass insert installed via ultrasonic horn.
  • Press-fit pins, rivets, snap-rings — usable; pre-design for plastic creep relaxing the contact pressure.

8m.6 Surface finishing

• Painting — adhesion-promoter prep on PP/PE; ABS accepts paint directly.
• Vapor polishing — methylene-chloride vapor on PC restores optical clarity to machined edges (banned in some jurisdictions; alternative: flame polish or ethanol-vapor).
• Electroplating — ABS is the preferred plate-on-plastic substrate (chromate etch dissolves butadiene rubber phase to leave mechanical anchor for electroless Cu/Ni then electrolytic Cu/Ni/Cr). Decorative auto trim, sanitary fixtures.
• Anodising — for metals only; not applicable to polymers.
• Pad printing / silk-screen — labels, graphics on housings.
• Laser marking — fibre/CO₂/UV laser permanently marks most thermoplastics; high-contrast on Nd:YAG-doped grades.

8m.7 Annealing

Particularly important for PC, PMMA, and acrylic-blend machined parts: 1 h per 25 mm of thickness at T_g − 25 °C, slow cool. Relieves molding/machining stresses that drive crazing and solvent-cracking in service.

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9m. Applications & selection trade-offs

Quick-pick guide by service requirement

Need	First pick	Reason
Cheap consumer housing	ABS, PP	Cheap, mouldable, paintable
Optical transparency	PMMA (best clarity, weather), PC (impact), COC (cyclic-olefin, optics)	Refractive index 1.49 / 1.58 / 1.53
Impact-resistant transparent	PC	Bullet-resistant glazing, eye protection
Snap-fit / hinge / cable-management	POM, PA66	Stiff, fatigue-resistant
Sliding bearing	POM, PA66 + PTFE, UHMWPE	Low μ, low wear
Gears (light duty)	POM (precision) or PA66 (shock)	Self-lubricating, quiet
Gears (heavy duty)	PA66-GF30 (replace metal at lower torque) or PEEK	Higher modulus, creep margin
Automotive under-hood (engine bay)	PA66, PPA, PPS	Hot, oily, vibrating
Connectors (electrical)	PBT-GF, PA66-GF, LCP	Dimensional stability, dielectric
FR enclosure / battery housing	FR-PC, FR-ABS, PA66-FR, PEI, PPS	UL 94 V-0
Food contact (rigid)	PP, PET, PC (BPA-free copolymer), POM-C	FDA 21 CFR compliant grades
Medical implant	PEEK (load-bearing, MRI-friendly), UHMWPE (joint wear surface), silicone (soft)	Biocompatible, sterilisable
Aerospace cabin interior	PEI (Ultem 9085 FDM, 1000 IM), PPSU	V-0 + FAR 25.853 + low smoke
Aerospace structure	Carbon-fibre / epoxy prepreg	Stiffness + strength + weight
Continuous service > 200 °C	PEEK (best balance), PPS-GF, PEI, PI (Kapton, Vespel)	Thermal stability
Cryogenic seal	PTFE, PCTFE	Stays ductile at LH2 / LN2
Marine boat hull (cost)	Polyester / glass	Cheap, mouldable, OK strength
Marine boat hull (premium)	Vinyl-ester or epoxy / glass	Hydrolysis-resistant
Wind-turbine blade	Epoxy / glass (older) or epoxy / carbon (modern, > 60 m)	Stiffness, fatigue
Pressure-vessel composite (CNG, H₂)	CF / epoxy filament-wound, Type-IV with HDPE liner	Burst pressure, weight
Tyre tread	NR / SBR blend	Wet grip + wear
Hydraulic O-ring (oil to 100 °C)	NBR (cheap), HNBR (longer life)	Oil resistance
O-ring fuel / brake fluid / > 120 °C	FKM	Chemical + thermal margin
O-ring high-purity, semiconductor	FFKM (Kalrez, Chemraz)	Plasma / aggressive chemistry resistance
Weather-strip, roofing	EPDM	Ozone + UV resistance
Industrial wheel, mining screen	Cast PU	Abrasion resistance
Self-lubricating bushing	PEEK + PTFE/CF additive (Ketron PEEK-HPV, PI Vespel SP-3)	Dry-running PV limit

Trade-off discussions

• POM vs PA66 for gears. POM has lower friction coefficient (0.15 vs 0.25), better dimensional stability, and no moisture sensitivity. PA66 has higher fatigue strength, better impact toughness, and lower cost. Industry convention: nylon-on-acetal mating pair, which combines POM’s smoothness with PA66’s resilience. For high-temperature service, swap to PEEK (both gears) — cost climbs 50×.
• PC vs PMMA for transparent parts. PC: 10× impact, scratches easily (3H pencil), yellows in UV. PMMA: brittle, scratch-resistant (2H–4H), UV-stable. Use PC for safety glazing (bullet-resistant, eye protection). Use PMMA for outdoor signage, aircraft canopies (cell-cast, UV stable, optical clarity).
• ABS vs PC/ABS blend. Pure PC is tough but hard to mold (high viscosity, high temperatures, knit-line sensitive). Pure ABS molds easily but lacks PC’s impact. PC/ABS blend (Cycoloy, Bayblend, Lupoy) gets PC’s impact + ABS’s processability + low cost; the workhorse for laptop housings, automotive interior, mobile-device backs.
• PEEK vs aluminum. PEEK density 1.32 vs Al 2.70 → 50 % weight saving. Modulus 3.6 vs 70 GPa → 1/20 stiffness. CF-PEEK closes the modulus gap (24 GPa vs Al 70 GPa). PEEK costs ~$100/kgnaturalvsAl$3/kg; PEEK wins only when (a) service > 150 °C, (b) corrosion or chemical environment kills Al, (c) MRI/EMI transparency required, (d) weight is paramount and aluminum is already optimised.
• NBR vs FKM O-ring. NBR costs $0.10–$0.50 each, fine for hydraulic oil to 100 °C. FKM costs $1.00–$5.00 each, needed for: fuels, brake fluid, > 120 °C continuous, ozone exposure, ATEX-rated equipment. Rule of thumb: spec NBR; switch to FKM only when service genuinely needs it.
• Filler choice for “filled engineering plastic”:
  • Glass fibre (GF) — increases modulus, HDT, creep resistance; reduces CTE; main option for cost-effective stiffness improvement; abrasive on tools.
  • Carbon fibre (CF) — higher modulus per loading (24 GPa at 30 % CF vs 9 GPa at 30 % GF); conductive (ESD/EMI shielding); excellent in sliding-bearing applications (lower wear); 5–10× cost of GF.
  • Mineral (talc, mica, CaCO₃, wollastonite) — cheaper than GF; less reinforcement; mainly for cost + isotropic stiffness + reduced shrinkage; common in automotive PP TPO bumper fascia.
  • PTFE additive (5–20 %) — incorporates as dispersed phase; reduces friction (PV limit doubles); used for self-lubricating bushings (POM + 20 % PTFE, PEEK + 10 % PTFE).
  • MoS₂ (molybdenum disulfide), graphite — friction/wear reduction; classic Nylatron 901 = PA6 + MoS₂.

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10m. Failure modes

1. Creep (cold flow). The #1 plastic-specific failure mode. Polymers strain under any sustained load at any service temperature. Always design with creep modulus E_c(t, T), not short-term E. Symptoms: bolted joint losing preload, seal extruding through clearance, snap-fit boss splaying, gear-tooth deformation. Mitigation: choose semicrystalline polymer above its T_g (POM, PA, PEEK), fibre-fill (GF30 cuts creep ~3×), reduce sustained stress to < 25 % yield, design metal-to-metal load path (use plastic only as guide/cosmetic).

2. Crazing and stress whitening. Visible precursor to failure in PC, PS, PMMA, ABS. Tiny voids form at stress concentrators; under increasing load, crazes coalesce into cracks. Triggered by chemical contact at low stress (PC + isopropanol cleaning is a classic failure: parts that survived in dry assembly crack visibly after one alcohol wipe). Mitigation: anneal machined PC, avoid IPA cleaning, switch to PEI / PPSU / PEEK.

3. Environmental stress cracking (ESC). Combination of tensile stress + chemical environment + susceptible polymer. HDPE in surfactant-bearing detergents is the textbook case (Bell ESCR test, ASTM D1693). PC fails in alcohols, some hydrocarbons; PMMA fails in alcohols, esters; ABS fails in many solvents. Mitigation: stress-relieve, use higher-MW grade, select inherently resistant polymer (PEEK, PE-RT, crosslinked PE).

4. UV degradation. Chain scission, yellowing, embrittlement. PC, ABS, PA, PP fail in 1–5 years of unstabilised outdoor service. Mitigation: stabilise with HALS + UV absorber + carbon black, or select inherently weatherable (PMMA, ASA, PVDF, FKM, EPDM, silicone). Standard accelerated test: ASTM G155 xenon-arc / ASTM D4329 QUV-A.

5. Hydrolysis. Polyesters (PET, PBT), polyamides, polycarbonate, polyurethane (ester-grade) hydrolyse in long-term water/steam exposure. PBT in glycol-water coolant, PET fibre in pressure-cooked food, PU foam in humid attic. Mitigation: hydrolysis-stabilised grades (Crastin HR, Hytrel HSC), or select hydrolysis-resistant polymer (PEEK, PEI, PPS, fluoropolymers, ether-based PU).

6. Brittle fracture below T_g. Cold-temperature impact failure. PC fails brittle at −135 °C; PA66 turns brittle below 0 °C unless impact-modified (PA66-IM grades, rubber-toughened, retain ductility to −40 °C). Mitigation: select polymer with T_g 30 °C below minimum service temperature; for cryogenic, use PTFE, PI, or PEEK.

7. Fatigue. S-N data sparse vs metals; no endurance limit. Thermal heating from hysteretic damping at high frequencies (> 5 Hz) can lead to thermal runaway. Mitigation: design to specified cycle life (10⁶ or 10⁷ cycles), keep frequency low, choose semicrystalline polymer (better fatigue than amorphous), test parts at expected service profile.

8. Plasticiser migration (flexible PVC). Phthalate or alternative plasticiser leaches into adjacent materials over time. Vinyl record sleeves stuck to vinyl records, vinyl wire insulation crumbling after decades, blood-bag DEHP leaching. Mitigation: use non-migrating plasticisers (polymeric, TOTM), or switch to TPU / TPV (Santoprene) thermoplastic elastomer.

9. Filler-matrix debonding in glass-filled grades. Moisture or chemical ingress along glass-resin interface progressively weakens GF parts in service. Mitigation: use silane-coupled GF (standard in modern grades), specify hydrolysis-stabilised resin for hot-wet service, derate strength after long-term aging tests.

10. Outgassing in vacuum service. Plasticisers, low-MW species, residual solvent volatilise in vacuum, contaminating sensitive surfaces (optics, semiconductor wafers, satellite thermal blankets). Mitigation: select per ASTM E595 (TML < 1 %, CVCM < 0.1 %); bake-out parts before vacuum service; select PEEK, PEI, Kapton, Vespel, fully post-cured epoxies.

11. Stress relaxation in elastomer seals. Primary cause of static-seal leakage over time. FKM and silicone relax less than NBR. Compression set per ASTM D395 quantifies relaxation. Mitigation: design for higher initial compression (25–30 %), select low-set elastomer (FKM, silicone, HNBR), avoid temperature cycling that accelerates relaxation.

12. Chemical attack in service. Even “resistant” polymers degrade given enough exposure: PC in alkaline detergent, PA in glycol-water at high temperature, POM in chlorinated water, FKM in amines and ketones. Always reference resin-manufacturer chemical-compatibility chart with exposure duration + temperature + stress, not a simple “resistant / not resistant” matrix.

13. Mold-line / knit-line weakness. Where flow fronts meet during injection, polymer chains are aligned and entanglement across the boundary is weak — knit-line tensile strength is often 50–70 % of bulk. Mitigation: gate placement to put knit lines in compression-only zones, raise melt temperature, add vacuum vent at knit-line location.

14. Galling / cold welding under sliding contact. Like-polymer pairs (POM on POM, PEEK on PEEK) can adhere, tear up, and seize. Mitigation: dissimilar pairs (POM on PA66, PEEK on PI), self-lubricated grades (POM + 20 % PTFE, PA66 + MoS₂), or use polymer against metal (steel, hard-anodised aluminum).

15. Photo-thermo-oxidation in spacecraft service. Combined UV, atomic oxygen (LEO), and thermal cycling degrade unstabilised polymers in months. Kapton (PI) is the LEO baseline; PEEK and PI survive multi-year missions; PE and PP do not.

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11. Cross-references

• [[Engineering/materials-steel]] — metallic alternative for structural / wear-critical applications
• [[Engineering/materials-aluminum]] — metallic alternative when weight + corrosion drive selection
• [[Engineering/materials-composites]] — polymer matrix + fibre reinforcement (CFRP, GFRP); epoxy and vinyl ester are the matrix resins discussed here
• [[Engineering/materials-ceramics]] — high-temperature alternative when service > 250 °C
• [[Engineering/materials-selection]] — Ashby method with polymers in the materials chart
• [[Engineering/mechanics-of-materials]] — beam bending and torsion calculations using polymer modulus values
• [[Engineering/additive-manufacturing]] — FDM, SLS, SLA, MJF all consume polymers; powder + filament grades
• [[Engineering/joining-welding]] — solvent bonding, adhesives, ultrasonic welding, friction welding for plastics
• [[Engineering/pcb-design]] — FR-4 (epoxy-glass), polyimide flex, PTFE high-frequency substrate are all polymer composites
• [[Engineering/Tier3/surface-treatments]] — annealing of PC, PMMA, machined PEEK for stress relief
• [[Robotics/end-effectors]] — soft grippers (silicone, TPU), bumpers (PU rubber), suction cups (NBR, silicone)
• [[Robotics/comm-buses]] — TPU and PA cable jacketing, polyurethane drag-chain cables
• [[Languages/Tier3/construction-bim]] — polymer building products (PVC pipe, EPDM roofing, polycarbonate skylights)

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12. Citations

1. Callister, W. D. & Rethwisch, D. G. Materials Science and Engineering: An Introduction, 10th ed. (Wiley, 2018). Polymer chapters cover MW distribution, crystallinity, viscoelasticity.
2. Ashby, M. F. Materials Selection in Mechanical Design, 5th ed. (Butterworth-Heinemann, 2016). The Ashby polymer chart and selection methodology.
3. Ashby, M. F. & Jones, D. R. H. Engineering Materials 2: An Introduction to Microstructures and Processing, 5th ed. (Butterworth-Heinemann, 2019). Polymer processing chapters.
4. Crawford, R. J. & Throne, J. L. Plastics Engineering, 4th ed. (Butterworth-Heinemann, 2020). Standard plastics-engineering textbook covering thermoplastics, thermosets, and processing.
5. Harper, C. A. (ed.) Modern Plastics Handbook (McGraw-Hill, 2000). Industry-reference for resin selection and processing.
6. Mark, J. E. (ed.) Encyclopedia of Polymer Science and Technology (Wiley, ongoing online edition). Authoritative chemistry-and-properties reference.
7. Brydson, J. A. Plastics Materials, 7th ed. (Butterworth-Heinemann, 1999). Resin-by-resin chemistry and properties.
8. Osswald, T. A., Baur, E., Brinkmann, S., Oberbach, K., & Schmachtenberg, E. International Plastics Handbook, 4th ed. (Hanser, 2006).
9. Domininghaus, H., Elsner, P., Eyerer, P., & Hirth, T. Kunststoffe — Eigenschaften und Anwendungen, 8th ed. (Springer, 2012). German-industry reference with extensive datasheets.
10. ASTM D638-22 — Standard Test Method for Tensile Properties of Plastics.
11. ASTM D790-17 — Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics.
12. ASTM D648-18 — Standard Test Method for Deflection Temperature of Plastics Under Flexural Load.
13. ASTM D785-08(2021) — Standard Test Method for Rockwell Hardness of Plastics.
14. ASTM D696-16 — Standard Test Method for Coefficient of Linear Thermal Expansion of Plastics.
15. ASTM D395-18 — Standard Test Methods for Rubber Property — Compression Set.
16. ASTM D412-16(2021) — Standard Test Methods for Vulcanized Rubber and Thermoplastic Elastomers — Tension.
17. ASTM E595-22 — Standard Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum Environment.
18. ISO 527-1:2019 — Plastics — Determination of tensile properties — Part 1: General principles.
19. ISO 178:2019 — Plastics — Determination of flexural properties.
20. ISO 75-1:2020 — Plastics — Determination of temperature of deflection under load.
21. ISO 11357-2:2020 — Plastics — Differential scanning calorimetry (DSC) — Glass transition temperature.
22. UL 94:2023 — Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances.
23. EU Directive 2011/65/EU (RoHS 2) and Commission Delegated Directive (EU) 2015/863 (RoHS 3).
24. Regulation (EC) No 1907/2006 (REACH).
25. PlasticsEurope. Plastics — the fast Facts 2024. https://plasticseurope.org
26. Manufacturer datasheets: Victrex (PEEK 450G, 450CA30, 450GL30), Solvay (KetaSpire PEEK, Radel PPSU, Udel PSU, Ryton PPS, Torlon PAI), SABIC (Ultem PEI, Lexan PC, Cycoloy PC/ABS, Noryl PPO), DuPont (Delrin POM, Zytel PA, Hytrel TPC-ET, Vespel PI, Kapton PI film), Lanxess (Durethan PA), BASF (Ultramid PA, Ultraform POM), Celanese (Hostaform POM, Celanex PBT), Quadrant EPP (machinable stock shapes), DSM (Stanyl PA46, Arnitel TPC).
27. Krevelen, D. W. van & Te Nijenhuis, K. Properties of Polymers, 4th ed. (Elsevier, 2009). Group-contribution methods for predicting polymer properties.