Commodity and engineering plastics

Polymer / Plastics / Rubber Technology (B.Tech) · Polymer materials · free study notes for exams, placements and interviews

In short

Commodity plastics are the five high-volume, low-cost thermoplastics — polyethylene, polypropylene, PVC, polystyrene and PET — that make up roughly three-quarters of world polymer consumption and go into packaging, pipes, film and housewares. Engineering plastics (nylon, polycarbonate, POM, PBT, ABS, modified PPO, and the high-performance PEEK/PPS/PEI family) cost several times more but carry load, hold dimensions and work above 100 °C, so they replace metal in gears, housings, headlamps and under-bonnet parts. The dividing line is not chemical but a property-and-price line: strength above about 40–50 MPa, modulus above about 2 GPa and continuous service above 100 °C. This topic is examined in Polymer Materials and Polymer Technology papers and in GATE-equivalent polymer science questions on structure–property relations, and it is the core screening topic for interviews at Reliance Industries, GAIL, IOCL, OPaL, Supreme Industries, Nilkamal, Finolex, Astral, Prince Pipes, SABIC, Covestro, Lanxess and the auto-component tiers supplying Maruti, Tata Motors, Mahindra, Bajaj and TVS.

Explained

How plastics are classified

By thermal behaviour: *thermoplastics* soften reversibly on heating (linear or branched chains, secondary bonds only) and are re-mouldable; *thermosets* cross-link irreversibly on cure (phenolics, UF/MF, epoxies, unsaturated polyester) and only char on reheating. Commodity and engineering plastics are almost all thermoplastics.
By volume and price tier: commodity (PE, PP, PVC, PS, PET) → engineering (PA, PC, POM, PBT, ABS, mPPO, PMMA) → high-performance (PSU, PES, PPS, PEI, PEEK, PTFE, LCP). Indicative Indian resin prices run roughly ₹90–150/kg for commodity grades, ₹200–600/kg for engineering grades and ₹3,000–8,000/kg for PEEK — all move monthly with crude and the rupee, so quote them as indicative only.
By chain order: *amorphous* polymers (PS, PVC, PC, PMMA, ABS, SAN) are transparent, shrink little (0.3–0.8%), and lose stiffness sharply at Tg; *semi-crystalline* polymers (PE, PP, PA, POM, PET, PBT, PEEK) are translucent to opaque, chemically more resistant, usable between Tg and Tm, and shrink more (1–3%).
By chain architecture: linear (HDPE), short-chain branched (LLDPE), long-chain branched (LDPE), cross-linked (PEX pipe), atactic/isotactic/syndiotactic (atactic PP is a soft gum; isotactic PP is the crystalline commercial grade, while syndiotactic PP also crystallises but is little used).
Homopolymer, copolymer, blend/alloy: random copolymer (PP-R pipe), block copolymer (impact PP copolymer, SBS), graft copolymer (ABS = SAN matrix with grafted polybutadiene rubber), alloys (PC/ABS, PPO/PS, PBT/PC, PA/ABS, PVC/ABS).

The five commodity plastics

LDPE (ρ 0.910–0.925, Tm ≈ 110 °C): long-chain branched, low crystallinity, very flexible and tough, excellent heat-sealing — lamination and carry-bag film, squeeze bottles, wire insulation.
LLDPE (ρ 0.915–0.925): linear backbone with short branches from an α-olefin comonomer (butene/hexene/octene); higher tensile and puncture strength than LDPE at the same density, so film is down-gauged — the workhorse of Indian agricultural and packaging film.
HDPE (ρ 0.941–0.965, Tm 130–135 °C, ~70–80% crystalline): stiff, tough, good environmental stress-crack resistance in PE100 pipe grades — water and gas pipes, blow-moulded containers, jerry cans, woven sacks.
PP (ρ 0.90–0.91, Tm 160–165 °C, Tg ≈ −10 to 0 °C): the lightest common plastic; homopolymer for stiffness, random copolymer for clarity, impact copolymer for cold toughness; outstanding living-hinge fatigue life (flip-top caps). Used in furniture, battery cases, car bumpers/interiors, BOPP film, raffia. Brittle below 0 °C unless impact-modified.
PVC (ρ 1.38–1.40 rigid, Tg 80–85 °C): the only large-volume polymer that is about 57% chlorine by mass, so it is inherently flame-retarding and cheap; *always* needs heat stabiliser and lubricant because it loses HCl above ~180 °C. Rigid (uPVC) → pipes, window profiles, electrical conduit; plasticised → flexible hose, cables, flooring, leathercloth. CPVC (chlorinated, Tg ~105 °C) takes hot water.
PS (ρ 1.05, Tg ~100 °C): GPPS is glass-clear, rigid and brittle; HIPS is rubber-modified and opaque; EPS/XPS foams for packaging and insulation. Attacked by hydrocarbons; poor UV resistance.
PET (ρ 1.33–1.40, Tg 75–80 °C, Tm 255–265 °C): polyester made by polycondensation of PTA with MEG; graded by intrinsic viscosity — fibre ~0.62, film ~0.64, bottle ~0.76–0.84 dl/g. Bottles are stretch-blow moulded so the wall is biaxially oriented and strain-crystallised; must be dried to <50 ppm moisture or hydrolysis cuts the IV in the melt. Glass-filled PET is itself an engineering plastic.

The engineering plastics

Polyamides (nylons): PA6 (Tm ~220 °C) from caprolactam ring-opening; PA66 (Tm 255–265 °C) from hexamethylenediamine + adipic acid. Tough, self-lubricating, fatigue- and fuel-resistant — gears, bushes, cable ties, intake manifolds, zips. Strongly hygroscopic (2–3% water at equilibrium): water plasticises the amide, dropping Tg from ~60 °C to near room temperature, so strength and stiffness fall and the part grows dimensionally. PA11/PA12 absorb less; PA6/PA66 with 30% glass fibre doubles strength and raises HDT past 200 °C.
Polycarbonate (PC): amorphous, Tg 145–150 °C, bisphenol-A based; outstanding notched impact strength (600–900 J/m) and optical clarity — headlamp lenses, helmet visors (IS 4151 for motorcyclists' helmets), polycarbonate sheet, electrical housings, medical devices. Notch-sensitive and attacked by alkalis and many solvents; must be dried before moulding to avoid hydrolytic chain scission and silver streaks.
POM / acetal: highly crystalline polyoxymethylene, homopolymer (Tm ~175 °C) and copolymer (Tm ~165 °C); lowest friction and best dimensional stability of the common engineering plastics — gears, cams, fuel-system parts, zip sliders. Degrades above ~230 °C releasing formaldehyde; must never be processed with acidic residues or long melt residence.
ABS and SAN: acrylonitrile (chemical resistance) + butadiene (impact) + styrene (processability, rigidity). ABS gives easy plating and the best surface finish — appliance bodies, two-wheeler panels, luggage, moulded furniture; SAN is transparent. Poor UV and weather resistance unless ASA/ASA-PC is used.
PBT (Tm ~225 °C): crystallises fast, so it moulds thin electrical parts with good dimensional stability — connectors, relays, switchgear; PC/PBT alloys for bumpers.
PMMA (acrylic, Tg ~105 °C): best weatherability and light transmission (~92%) of any common plastic — tail lamps, signage, sanitaryware; brittle and scratch-prone.
Modified PPO/PPE (PS-blended): low density, very low water absorption and good dimensional stability — pump housings, electrical parts, automotive wheel covers.
High-performance: PPS (Tm 285 °C), PSU/PES (Tg 185/225 °C, steam-sterilisable), PEI (Tg 217 °C), PEEK (Tg 143 °C, Tm 343 °C, continuous use ~250 °C), PTFE (Tm 327 °C, lowest friction, not melt-processable — sintered). Used where metal replacement must survive heat, steam or aggressive chemicals.

Structure–property relations that explain every selection

Tg sets the useful limit of an amorphous plastic; Tm sets it for a semi-crystalline one. A semi-crystalline plastic used between Tg and Tm (PP at room temperature) is tough; the same plastic below Tg is brittle.
Crystallinity ↑ → density, stiffness, yield strength, chemical and barrier resistance, shrinkage and warpage all ↑; transparency, impact strength and elongation ↓. Controlled by chain regularity, branching, comonomer content, cooling rate and nucleating agents.
Molecular weight ↑ → strength, toughness, melt viscosity and ESCR ↑; flow and cycle time worsen. Hence film/pipe grades are high-MW (low MFI) and injection grades low-MW (high MFI).
Chain stiffness and polarity raise Tg: flexible –CH₂– backbone (PE, Tg ≈ −120 °C) → bulky/aromatic backbone (PC, Tg 150 °C). Hydrogen bonding (nylons) gives high Tm and strength but water uptake.
Reinforcement and fillers (glass fibre, talc, mineral) raise modulus and HDT and cut shrinkage, at the cost of impact and surface finish — a 30% GF nylon is stiffer than many die-cast alloys per unit weight.
Creep, not yield, limits plastics in service. Design to allowable strain (typically 0.5–1% for unreinforced thermoplastics) using an apparent/creep modulus, never the short-term tensile modulus.
Environmental stress cracking (PE in detergents, PC in solvents) and UV degradation are the two most common field failures; so is hydrolysis in PET/PC/PBT/PA if moulded wet.

Processing routes and what each polymer demands

Injection moulding — PP, PS, ABS, PA, PC, POM, PBT. Steps: drying → plasticising → injection → packing/holding → cooling → ejection. Key settings: melt temperature, injection speed, holding pressure (governs shrinkage and sinks), mould temperature (governs crystallinity, gloss, warpage), back pressure.
Extrusion — pipe, profile, sheet, film, wire covering: HDPE, PVC, PP, PS. Screw zones: feed → compression → metering; L/D usually 20–30:1; die swell and draw-down set final dimensions.
Blown and cast film — LDPE/LLDPE/HDPE blown film with blow-up ratio 2–4; BOPP and BOPET are cast then biaxially oriented, which raises strength, clarity and barrier.
Blow moulding — extrusion blow for HDPE/PP containers; injection stretch-blow (ISBM) for PET bottles, where stretching above Tg gives strain-induced crystallinity and CO₂ retention.
Thermoforming — HIPS, PVC, PET sheet for trays, cups, refrigerator liners.
Rotational moulding — PE powder for water tanks (the ubiquitous Indian overhead tank), large, stress-free, hollow parts.
Drying is not optional for PET (<50 ppm), PA (<0.2%), PC and PBT (<0.02%) — the condensation polymers hydrolyse in the melt and lose molecular weight irreversibly.
Rheology note: thermoplastic melts are pseudoplastic (shear-thinning), so viscosity falls as injection speed rises — this is why thin walls can be filled at all.

Testing, standards and Indian practice

Mechanical and thermal tests: tensile (ISO 527 / ASTM D638), flexural (ISO 178 / ASTM D790), notched Izod impact (ISO 180 / ASTM D256) and Charpy impact (ISO 179 / ASTM D6110), hardness (Rockwell R, Shore D), HDT (ISO 75 / ASTM D648 at 0.45 and 1.8 MPa), Vicat softening (ISO 306), MFI/MVR (ISO 1133 / ASTM D1238 — PE at 190 °C/2.16 kg, PP at 230 °C/2.16 kg), density (ASTM D792), water absorption (D570), flammability rating UL 94 (HB, V-2, V-1, V-0) and limiting oxygen index (ASTM D2863). BIS mirrors most of these in the IS 13360 series, "Plastics — Methods of test".
Product standards commonly quoted in India: IS 4984 (PE pipes for water supply, PE63/PE80/PE100 grades), IS 14333 (HDPE pipes for sewerage), IS 4985 (uPVC pipes for potable water), IS 15778 (CPVC pipes for hot and cold water), IS 12818 (uPVC screen pipes for tubewells), IS 14886/IS 9845-type migration testing for food-contact articles, and the BIS food-contact resin series for PE, PP and PVC. If a number is uncertain, cite the subject rather than a possibly withdrawn number — many old IS numbers have been superseded.
Regulatory: food-contact plastics must comply with the FSSAI Packaging Regulations, 2018, which call up the relevant Indian Standards. The Department of Chemicals and Petrochemicals has brought several polymers (including PVC, PP, PET, PC and ABS resin) under Quality Control Orders requiring BIS licence and the Standard Mark; effective dates have been deferred more than once, so check the current notification before claiming a grade is covered.
Waste and plastic use: the Plastic Waste Management Rules, 2016 as amended (EPR registration on the CPCB portal, the single-use-plastic prohibition from 1 July 2022, and the 120-micron minimum for carry bags) and the Solid Waste Management Rules, 2026 (in force from 1 April 2026) govern end-of-life — detailed in the recycling note.
Commercial/tax: most plastic raw materials and articles sit in the 18% GST slab under the 5%/18%/40% structure effective 22 September 2025; confirm HSN-wise. Factory operation, safety and worker records now fall under the four Labour Codes in force from 21 November 2025, which replaced the former Factories Act.

Key points

Commodity plastics — PE, PP, PVC, PS, PET; high volume, low cost, properties adequate for packaging and piping, not for sustained load at temperature.
Engineering plastic — a thermoplastic good enough to replace metal in a load-bearing or dimensionally critical part; roughly >40–50 MPa tensile, >2 GPa modulus, >100 °C continuous use.
Thermoplastic vs thermoset — secondary bonds, re-mouldable, weldable vs covalent network, not re-mouldable, only ground as filler.
Amorphous — transparent, low shrinkage (0.3–0.8%), isotropic, limited by Tg; semi-crystalline — opaque, shrinkage 1–3%, chemically resistant, limited by Tm.
LDPE/LLDPE/HDPE — differ by branching, hence density and crystallinity; same monomer, very different stiffness.
PP — lightest common plastic (ρ ≈ 0.90), best living hinge, brittle below 0 °C.
PVC — ~57% chlorine, self-extinguishing, cheapest rigid pipe polymer, but dehydrochlorinates without stabiliser.
PET — graded by intrinsic viscosity; bottle grade ~0.8 dl/g; biaxial orientation gives its strength and barrier.
Nylon — hydrogen bonding gives strength and Tm, but 2–3% water uptake shifts Tg and dimensions.
PC — best notched impact and clarity among engineering plastics; notch-sensitive, alkali- and solvent-attacked.
POM — lowest friction, best dimensional stability; releases formaldehyde if overheated.
ABS — SAN + grafted rubber; finish and platability; poor weathering.
MFI — inverse measure of molecular weight and melt viscosity; low MFI for pipe/film, high MFI for thin-wall injection.
HDT and Vicat — short-term thermal limits under load; always state the stress (0.45 or 1.8 MPa).
Glass fibre — raises modulus, strength and HDT; lowers impact, raises anisotropic warpage.
Creep governs design — use allowable strain (0.5–1%) and apparent modulus, never short-term strength.
Dry condensation polymers (PET, PA, PC, PBT) before moulding or lose molecular weight permanently.
SDR = outside diameter / wall thickness; the lower the SDR, the higher the pressure rating of a PE or PVC pipe.

Formulas and examples

Degree of polymerisation: DP = Mn / M0 — M0 = repeat-unit molar mass (g/mol)
Mn = Σ NiMi / Σ Ni ; Mw = Σ NiMi² / Σ NiMi ; PDI = Mw / Mn (≥ 1; 1 = monodisperse)
Carothers (step-growth): DP = 1/(1 − p), p = fractional conversion of functional groups
Mark–Houwink: [η] = K·M^a — [η] in dl/g; K, a are polymer–solvent–temperature constants
Crystallinity: Xw(wt) = ρc(ρ − ρa) / [ρ(ρc − ρa)] ; Xv(vol) = (ρ − ρa)/(ρc − ρa) — ρa, ρc = fully amorphous and fully crystalline densities (g/cm³)
Fox equation (random copolymer/miscible blend): 1/Tg = w1/Tg1 + w2/Tg2 — Tg in kelvin, w = mass fractions
Flory–Fox: Tg = Tg∞ − K/Mn — Tg rises with chain length and plateaus
Mould shrinkage: S = (L_cavity − L_part)/L_cavity ; so L_cavity = L_part/(1 − S)
Part mass m = V × ρ_solid ; shot mass = n·m + runner mass
Clamp force F = p_cavity × A_projected — p in MPa (N/mm²), A in mm², F in N; 1 tonne-force = 9.81 kN
Cooling time (plate): t = (s²/π²α)·ln[(4/π)(T_melt − T_mould)/(T_eject − T_mould)] — s = half wall thickness (m), α = thermal diffusivity (m²/s), α ≈ 0.9 × 10⁻⁷ for PP
Pipe pressure rating: p = 2σ_s/(SDR − 1), SDR = D/e, σ_s = MRS/C (C = design coefficient, 1.25 for water)

Example 1 — Molecular weight averages and DP (PP). A sample contains 2 mol of chains of 10,000 g/mol, 5 mol of 30,000 and 3 mol of 60,000.

Σ Ni = 10 mol; Σ NiMi = 20,000 + 150,000 + 180,000 = 350,000 → Mn = 35,000 g/mol.
Σ NiMi² = 2(1.0×10⁸) + 5(9.0×10⁸) + 3(3.6×10⁹) = 2.0×10⁸ + 4.5×10⁹ + 1.08×10¹⁰ = 1.55×10¹⁰.
Mw = 1.55×10¹⁰ / 3.5×10⁵ = 44,286 g/mol; PDI = 44,286/35,000 = 1.27.
Repeat unit C₃H₆ = 42 g/mol → DP = 35,000/42 ≈ 833.

Example 2 — Crystallinity of an HDPE pipe grade. Measured ρ = 0.956 g/cm³; for PE ρa = 0.855, ρc = 1.000.

Volume fraction: Xv = (0.956 − 0.855)/(1.000 − 0.855) = 0.101/0.145 = 0.697 → 69.7 vol%.
Weight fraction: Xw = 1.000 × 0.101 / (0.956 × 0.145) = 0.101/0.13862 = 0.729 → 72.9 wt%. (Weight % always exceeds volume % because the crystal is denser.)

Example 3 — Tg of a vinyl chloride–vinyl acetate copolymer (Fox). 85% VC (Tg 354 K) + 15% VAc (Tg 305 K).

1/Tg = 0.85/354 + 0.15/305 = 0.0024011 + 0.0004918 = 0.0028929 → Tg = 345.7 K ≈ 72.6 °C, i.e. the comonomer has lowered Tg from 81 °C — internal plasticisation for calendered sheet.

Example 4 — Mould cavity size, shot weight, clamp force and cooling time (PP crate handle, 4 cavities). Part length 150.00 mm, shrinkage 1.8%, part volume 96 cm³, wall 2.5 mm, projected area 95 cm² per part plus 20 cm² runner. Melt 230 °C, mould 30 °C, ejection 90 °C, cavity pressure 30 MPa.

Cavity length = 150.00/(1 − 0.018) = 150.00/0.982 = 152.75 mm.
Part mass = 96 × 0.905 = 86.9 g; shot = 4 × 86.9 + 25 g runner = 372.6 g.
Projected area = 4 × 9,500 + 2,000 = 40,000 mm²; F = 30 × 40,000 = 1.2 × 10⁶ N = 1,200 kN = 1,200/9.81 = 122 tonne-force → specify a 135-tonne machine (≈10% margin).
s = 1.25 mm = 1.25×10⁻³ m; s²/(π²α) = 1.5625×10⁻⁶/(9.8696 × 0.9×10⁻⁷) = 1.5625×10⁻⁶/8.883×10⁻⁷ = 1.759 s. Temperature ratio = (230−30)/(90−30) = 3.333; (4/π)(3.333) = 4.244; ln 4.244 = 1.4455 → t ≈ 1.759 × 1.4455 = 2.54 s cooling, so a cycle of about 12–15 s with filling, holding and ejection.

Example 5 — Pressure rating of a PE100 pipe (IS 4984 practice). PE100 has MRS = 10 MPa; design coefficient C = 1.25 for water → σ_s = 8 MPa. For SDR 11 (e.g. 110 mm OD, 10 mm wall): p = 2 × 8/(11 − 1) = 1.6 MPa = 16 bar (PN 16). The same pipe in PE80 (MRS 8, σ_s = 6.4 MPa) gives p = 12.8 bar — which is why PE100 lets a utility use a thinner wall for the same 16 bar duty.

Property	HDPE	PP	uPVC	PS (GPPS)	ABS	PA6	PC	POM
Density, g/cm³	0.95	0.905	1.40	1.05	1.05	1.13	1.20	1.41
Tg / Tm, °C	−120/132	−5/163	82/—	100/—	105/—	50/220	148/—	−60/172
Tensile strength, MPa	22–31	30–40	45–55	35–50	40–50	70–85 (dry)	60–70	65–70
Notched Izod, J/m	100–200	25–60	50–100	15–25	200–400	50–60 (dry)	600–900	70–120
HDT at 1.8 MPa, °C	~45	~55	~70	~80	~95	~65 (dry)	~130	~100
Water absorption, %	<0.01	<0.02	0.1	0.05	0.3	1.6–3	0.2	0.2
Shrinkage, %	1.5–3	1.2–2	0.3–0.6	0.4–0.6	0.4–0.7	0.8–1.5	0.5–0.7	1.8–2.5
Typical use	Pipe, jerry cans	Crates, bumpers	Pipe, profiles	Disposables	Appliance body	Gears, ties	Lenses, visors	Gears, cams

Where it is used

Petrochemicals and polymerisation plants: Reliance (Jamnagar, Hazira, Dahej), GAIL Pata, IOCL Panipat, HMEL, OPaL Dahej and Haldia make PE, PP, PVC and PET — roles in polymer production, grade development, quality control and technical service.
Pipes, fittings and building products: Finolex, Astral, Prince, Supreme, Ashirvad in uPVC/CPVC/HDPE pipe to IS 4985, IS 15778 and IS 4984; rotomoulded water tanks in PE.
Packaging converters: blown and BOPP/BOPET film, PET preform and bottle plants, thermoformed trays — process engineers set MFI, draw ratio, IV and barrier.
Automotive and appliance tiers: PP interiors and bumpers, PA66-GF intake manifolds, PC headlamp lenses, PBT connectors, ABS panels for Maruti, Tata Motors, Mahindra, Bajaj, TVS and their Tier-1 moulders.
Moulded housewares and furniture: Nilkamal, Supreme, Cello — PP and HIPS injection and blow moulding, where cycle time and shot weight decide cost per piece.
Job roles: polymer process engineer, moulding/tooling engineer, compound and grade development chemist, technical service and application development, QA/testing lab (tensile, MFI, HDT, migration), BIS/FSSAI compliance.

Interview questions

Q: What makes a plastic an "engineering" plastic? A: Properties good enough to carry load and hold dimensions in service — roughly tensile strength above 40–50 MPa, modulus above 2 GPa and continuous use above 100 °C — at a price several times commodity resin.
Q: LDPE, LLDPE and HDPE are all polyethylene — why do they behave differently? A: Branching differs: LDPE is long-chain branched and least crystalline, LLDPE has short branches from an α-olefin comonomer, HDPE is essentially linear and 70–80% crystalline, hence stiffest and densest.
Q: Why is PP preferred for a flip-top cap hinge? A: Its crystalline morphology orients at the hinge and survives millions of flex cycles without fatigue cracking; most other commodity plastics craze there.
Q: Why must PET and nylon be dried before moulding? A: They are condensation polymers; water in the melt hydrolyses the chain, permanently lowering molecular weight and IV, which shows up as splay, brittleness and low strength.
Q: Why does nylon's stiffness fall in service? A: It absorbs 2–3% moisture, which plasticises the hydrogen-bonded amorphous phase, dropping Tg towards room temperature and swelling the part.
Q: Why does PVC need a heat stabiliser when PE does not? A: PVC dehydrochlorinates above about 180 °C, and the liberated HCl autocatalyses further degradation and discoloration; PE has no labile chlorine.
Q: Amorphous or semi-crystalline — which would you pick for a transparent, tight-tolerance housing, and why? A: Amorphous (PC, ABS, PMMA): no crystalline shrinkage, so shrinkage is low and isotropic, and the absence of crystallites keeps it clear.
Q: What does MFI tell you, and why does pipe grade have a low MFI? A: MFI is inversely related to molecular weight and melt viscosity; pipe needs high molecular weight for melt strength, slow crack growth and ESCR, which means low MFI.
Q: PC has very high notched impact strength, yet parts crack in the field — why? A: PC is highly notch- and stress-concentration-sensitive, and it is attacked by alkalis and many solvents; sharp corners, moulded-in stress and solvent contact cause brittle failure.
Q: How would you raise the HDT of a PA6 part without changing polymer? A: Add 30% glass fibre — reinforcement restrains the amorphous phase above Tg and can lift HDT at 1.8 MPa to above 200 °C, close to Tm.
Q: Why design plastic parts on strain rather than yield strength? A: Thermoplastics creep under sustained load, so stress falls and deflection grows with time; limiting strain to about 0.5–1% with an apparent (creep) modulus keeps deformation acceptable over design life.
Q: What does SDR 11 mean on a PE pipe? A: Standard dimension ratio = outside diameter/wall thickness = 11; with PE100 (σ_s = 8 MPa) it gives p = 2σ_s/(SDR − 1) = 16 bar.

Common mistakes

Treating "thermoplastic vs thermoset" and "commodity vs engineering" as the same split — the second is a property-and-price classification, and glass-filled PET or PP can cross the line.
Quoting a Tg for a semi-crystalline polymer as its service limit (or a Tm for an amorphous one, which has none).
Giving nylon's dry-as-moulded strength as its design value, and ignoring moisture-driven dimensional growth.
Mixing up weight and volume crystallinity, or forgetting the ρ in the denominator of the weight-fraction formula.
Using the short-term tensile modulus in a long-term deflection or snap-fit calculation — creep is ignored and the part fails.
Unit slips in clamp force and shrinkage: MPa with mm² gives newtons (divide by 9.81 kN for tonne-force), and the cavity is *larger* than the part, so divide by (1 − S) rather than multiplying by (1 + S).
Quoting HDT or MFI without the test condition (0.45 or 1.8 MPa; 190 °C/2.16 kg vs 230 °C/2.16 kg), which makes the number meaningless.