Plastics 101
Thermoplastics vs thermosets, crystalline vs amorphous, and commodity engineering plastics
Plastics are divided into thermoplastics (re-meltable: ABS, PC, nylon, POM, PEEK) and thermosets (cross-linked: epoxies, polyimides). Within thermoplastics, semi-crystalline grades (nylon, POM, PEEK) resist chemicals and wear but shrink more in molding; amorphous grades (ABS, PC, acrylic) are dimensionally stable and transparent but solvent-sensitive. Selection starts with temperature, chemicals and mechanical load.
1. Thermoplastic vs thermoset
Thermoplastics soften and flow when heated, which is why they can be injection molded, extruded and recycled. Thermosets cure irreversibly — they cannot be re-melted and are typically molded in sheet or casting form; examples are epoxy (electrical laminates, composites) and phenolic.
Within thermoplastics, commodity grades (ABS, PP, PS, PVC) cover most consumer and structural-light applications at low cost; engineering grades (nylon, POM, PC, PET) add strength, temperature and wear performance; high-performance polymers (PEEK, PPS, PEI, PTFE) serve demanding thermal and chemical duty.
2. Crystalline vs amorphous
Semi-crystalline polymers (PA, POM, PEEK, PP, PE) have ordered molecular regions: they resist solvents and fatigue, wear well, but shrink more on molding (1–2.5%) and are opaque. Amorphous polymers (ABS, PC, PMMA, PS) shrink less (0.4–0.7%), are dimensionally stable and can be transparent, but stress-crack in some solvents and have lower wear resistance.
The crystalline/amorphous split drives practical choices: gears and wear parts → POM or PA66; transparent covers → PC or PMMA; high-temperature electrical → PEEK or PPS; chemical tanks → PE/PP; snap-fit consumer housings → ABS or PC/ABS.
3. Key engineering properties
Typical benchmark values: ABS density 1.06 g/cm³, UTS ≈47 MPa, heat deflection ~98 °C at 0.46 MPa; PC density 1.20, UTS ≈70 MPa, impact strength ~2× ABS; PA66 density 1.14, UTS ≈83 MPa (dry), tough but moisture-absorbing; POM density 1.41, UTS ≈69 MPa, excellent wear and fatigue; PEEK density 1.30, UTS ≈97 MPa, continuous service to ~250 °C.
Creep matters at temperature: amorphous grades creep more than semi-crystalline ones, so spring/retention features favor PA and POM. UV degrades most plastics — add UV stabilizers or black pigments for outdoor parts; PE and PP require surface treatment before bonding or painting.
4. Selection and quoting notes
Plastic material cost is a small fraction of total part cost; geometry, quantity and process dominate. For machined prototypes of a future molded part, machine ABS/PC first (they machine cleanly) and verify with molded samples later.
Thermal expansion of plastics is 5–10× steel (70–200 µm/m·K): precision assemblies need generous clearances or metal inserts. Keep in-service temperature below the heat deflection temperature with a margin, and check chemical compatibility with cleaners, fuels and lubricants.
Quick reference
| Material | Type | Density g/cm³ | UTS MPa | HDT °C (0.46MPa) | Notable |
|---|---|---|---|---|---|
| ABS | Amorphous | 1.06 | 47 | 98 | Tough, cheap, snap fits |
| PC | Amorphous | 1.20 | 70 | 132 | Clear, impact 2× ABS |
| PMMA (Acrylic) | Amorphous | 1.19 | 72 | 95 | Optically clear, brittle |
| PA66 (Nylon) | Semi-cryst. | 1.14 | 83 | 95 (1.8MPa) | Wear, tough, hygroscopic |
| POM (Delrin) | Semi-cryst. | 1.41 | 69 | 158 | Gears, bearings, low friction |
| PP | Semi-cryst. | 0.91 | 33 | 100 | Living hinges, chemical tanks |
| PEEK | Semi-cryst. | 1.30 | 97 | 315 (glass) | High temp, medical, aerospace |
Sources & references
- ISO 1183 — Density of plastics
- ASTM D638 — Tensile properties
- ASTM D648 — Deflection temperature
- Xometry Learning Center — Plastics 101 (public engineering reference)