Many buyers mix up plastics and rubbers. The part fails. The line stops. I do not want that to happen to you.
Plastics are rigid or semi-rigid polymers shaped by heat or cure. Rubbers are elastic polymers that stretch and return. I compare their structures, properties, costs, and use-cases so you can choose with confidence.
I write as a factory owner. I keep the words simple. I give clear rules, tables, and quick checks. I also share a short story from a project where this choice saved weeks.
What is the basic difference between plastics and rubbers?
Confusion starts when both look similar in black. A hard “rubber-like” plastic fools the eye. Your part comes out too stiff.
Plastics deform and keep shape; rubbers stretch and spring back. Plastics have higher stiffness and lower elongation. Rubbers have lower stiffness and higher elongation with good rebound.
Structure, feel, and numbers—side by side
I explain it this way during design calls. Plastics are long chains that pack tightly or crosslink into rigid networks. Rubbers are long chains with light crosslinks that allow large stretch. This small structural shift changes how parts behave under load, temperature, and chemicals.
I test both on every project. I measure hardness1 on Shore D for most hard plastics and Shore A for rubbers. I pull tensile bars to see strength and elongation. I bend and compress samples to read creep and compression set2.
Key property snapshots
| Property | Typical Plastics (e.g., PP, PA, POM) | Typical Rubbers (e.g., EPDM, NBR, Silicone) |
|---|---|---|
| Hardness scale | Shore D 60–85 | Shore A 30–80 |
| Tensile strength | 40–90 MPa | 7–25 MPa |
| Elongation at break | 5–50% | 200–700% |
| Elastic recovery | Moderate | High |
| Creep under load | Moderate to high | Low to moderate (depends on set) |
| Temperature range (typical) | −20 to +120 °C | −50 to +200 °C (material dependent) |
✅ Quick rule: If the part must flex daily without taking a set, start with rubber.
🛠️ Process tip: If you need thin walls and tight clips, start with plastic.
How do thermoplastics3, thermosets4, and elastomers5 compare?
Projects derail when we choose the wrong processing family. Lead time, tooling, and failure modes all change.
Thermoplastics melt and remelt; thermosets cure once; elastomers (rubbers) crosslink for elasticity. Each family limits your geometry, cycle time, and field performance.
Families at a glance
I divide materials into three buckets when I do DFM.
Definitions
- Thermoplastics (TP): Soften with heat. Injection molded, extruded, thermoformed. Recyclable re-melt. Examples: PP, PE, PA, PC, POM, ABS, TPU (special case).
- Thermosets (TS): Cure to a permanent network. Do not melt again. Examples: phenolics, epoxies, melamine, some cast polyurethanes.
- Elastomers (Rubbers): Lightly crosslinked networks with large reversible strain. Cure by heat/chemistry. Examples: EPDM, NBR, HNBR, FKM, Silicone, CR, NR.
Manufacturing and cost snapshot
| Item | Thermoplastics (TP) | Thermosets (TS) | Elastomers (Rubbers) |
|---|---|---|---|
| Typical process | Injection, extrusion | Compression, transfer, cast | Compression, transfer, injection |
| Tooling cost | Medium–High | Low–Medium | Low–Medium (geometry dependent) |
| Cycle time | Seconds | Minutes–hours | Minutes (injection fastest) |
| Wall thickness | Thin possible | Thicker preferred | Thicker preferred |
| Tolerance capability | Tight | Moderate | Moderate (ISO 3302-1) |
| Recyclability | Good | Poor | Poor (post-cure network) |
I once saw a housing designed in rubber for a snap-fit cover. It kept creeping open. We swapped to PA66 plastic for the cover and kept EPDM for the seal. The problem disappeared in one week.
Where hybrid designs shine
- Plastic body + rubber seal: Common in valves, HVAC dampers, and caps.
- Plastic clip + rubber pad: Good for grip and noise control.
- Rubber-to-metal bonded mount: Best for vibration isolation6.
✅ Use plastics to carry shape and tolerances.
✅ Use rubbers to seal, absorb shock, and decouple vibration.
Which should I choose for an industrial part?
A wrong early choice locks cost and risk. A simple matrix saves time.
Choose rubber for sealing, damping, grip, and large deflection. Choose plastic for rigid structure, thin walls, and snap features. When in doubt, combine both.
Selection rules I use on RFQs
I start with the function. Does the part need to deform in service and return? Does it touch oil, water, or air? What is the temperature? Do we need low friction, high stiffness, or a seal?
Function-to-material matrix
| Function / Need | Plastic candidate | Rubber candidate | Notes |
|---|---|---|---|
| Rigid housing, snap fits | PA66, PC/ABS, POM | — | Tight tolerances possible |
| Static flange seal | — | EPDM, NBR, FKM, Silicone | Pick by media & temp |
| Vibration isolation | — | NR, EPDM, NBR, HNBR | Tune hardness and geometry |
| Low-friction bearing surface | POM, PA + lubricant | — | Plastics win here |
| Soft grip/overmold | TPE (TPU/TPE-S) over plastic | EPDM/CR over metal | Bonding matters |
| High heat exposure (>150 °C) | PEEK, PPS, PC (limited) | Silicone, FKM, HNBR | Check continuous temp |
| Fuel/oil contact | POM/PA (for structure) | NBR/HNBR/FKM for seal | Combine for function |
Environment checks
- Water/steam: EPDM rubber or PPS/PA for housings.
- Oils/fuels: NBR/HNBR/FKM rubber; PA/POM for housings.
- Sun/ozone: EPDM or UV-stable plastics (PC needs UV protection).
- Food contact: Silicone rubber (platinum-cured) or PP/PE with proper certification.
✅ I lock hardness early (Shore A for rubber7, Shore D for plastic8).
🛠️ I confirm tolerance class early to avoid over-promising.
Are thermoplastic elastomers plastics or rubbers?
Teams argue over TPE. It feels like rubber but runs like plastic. The answer guides tooling and bonding.
Thermoplastic elastomers (TPE/TPU/TPE-S/TPE-E) process like plastics but behave like soft rubbers. They sit in the middle. Use them when you need soft touch, fast cycles, thin walls, or two-shot overmolding.
When I pick TPE over vulcanized rubber
I choose TPE for soft grips9, bellows with thin walls, or living hinges that need flexibility without post-curing. I also use TPE in two-shot tools that overmold onto PC/ABS or PP in one cycle. This cuts assembly time and improves cosmetics.
TPE vs vulcanized rubber
| Feature | TPE (e.g., TPU, TPE-S) | Vulcanized Rubber (e.g., EPDM, NBR) |
|---|---|---|
| Processing | Injection, fast cycle | Compression/transfer/injection, cure required |
| Recyclability | Re-meltable | Not re-meltable |
| Thin-wall capability | Excellent | Moderate |
| Elasticity and rebound | Good | Very good |
| High-temp resistance | Moderate (−40 to +120 °C) | Higher possible (e.g., silicone 200 °C) |
| Chemical resistance10 | Good (grade dependent) | Excellent with right rubber (FKM/HNBR) |
| Compression set | Moderate | Often lower with right cure system |
A client once needed a bellows with 0.8 mm walls and a tactile surface. We tried silicone rubber first, but the wall thickness varied at the folds. We switched to TPU with an optimized gate and achieved stable walls, quicker cycles, and easier color control.
✅ Pick TPE when you need soft feel and fast production.
🛠️ Check bonding charts if you plan to overmold onto PC/ABS or PP.
How do I validate performance with tests and standards?
Good drawings fail without the right tests. I never skip this part.
I test hardness, tensile, elongation, compression set (rubbers), creep (plastics), aging, and media compatibility. I match standards and simulate real loads. This prevents field returns.
My validation checklist
I start with material ID, color, and lot tracking. I confirm hardness: Shore A for rubber1s, Shore D or Rockwell for plastics. I cut dumbbells and pull tensile bars. I run compression set on rubbers at service temperature. I check creep for plastics under long-term load. I do heat aging11 and media soak for both.
Core tests and targets (indicative)
| Test | Plastics Target (example) | Rubbers Target (example) |
|---|---|---|
| Hardness | Shore D 70–80 | Shore A 60–70 |
| Tensile strength12 | 60–80 MPa | 10–20 MPa |
| Elongation at break13 | 10–30% | 300–600% |
| Compression set (24h @ temp) | — | ≤25% at service temp |
| Creep (1000 h @ load) | ≤1–2% strain increase | — |
| Heat aging (70–168 h)14 | ≤10% change | ≤15% change |
| Media soak (oil, water, etc.) | Dimensional change ≤1–2% | Volume change per spec (e.g., NBR ≤10% in oil) |
I also map certifications. If the product touches drinking water, I select EPDM grades that can achieve WRAS/NSF. For food contact15, I use FDA 21 CFR 177.2600 silicone or approved plastics. For fire safety, I review UL 94 for plastics or special flame-rated rubber compounds.
Standards summary (selection)
| Need | Plastics | Rubbers |
|---|---|---|
| Flammability | UL 94 | FMVSS 302 (automotive), custom tests |
| Drinking water | — | EPDM with WRAS/NSF options |
| Food contact | EU/US food-contact plastics | FDA 21 CFR 177.2600 silicone/EPDM |
| Dimensional tolerances | ISO 20457 (molded plastics) | ISO 3302-1 (molded rubber) |
✅ I write a control plan before mass production.
🛠️ I store retain samples for traceability.
Conclusion
Plastics keep shape. Rubbers seal and absorb. I often use both in one design to get strength, tolerance, and reliable sealing.
Need a quick review of your part?
I can compare plastic vs rubber, propose a hybrid, and set tests that fit your risk and budget.
- Brand: Julong Rubber
- Website: www.rubberandseal.com
- Email: info@rubberandseal.com
- Country: China
- Products: rubber seals, custom rubber parts, rubber wheels, rubber tubes, rubber gaskets
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Understanding hardness helps in selecting materials that meet specific performance requirements. ↩ ↩
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Compression set affects the longevity and performance of rubber components. ↩
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Thermoplastics have unique properties that make them suitable for specific applications. ↩
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Thermosets offer durability and heat resistance, making them ideal for certain applications. ↩
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Elastomers provide flexibility and resilience, essential for many industrial applications. ↩
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Vibration isolation is crucial for reducing wear and improving performance in mechanical systems. ↩
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Understanding Shore A hardness is crucial for selecting the right rubber for your application, ensuring optimal performance. ↩
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Understanding Shore D hardness is crucial for selecting the right plastic material, ensuring optimal performance in applications. ↩
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Learn how soft grips enhance user experience and product functionality in various applications. ↩
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Explore this link to gain insights on chemical resistance, crucial for selecting the right materials in various applications. ↩
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Learn about heat aging tests and their role in assessing material performance over time. ↩
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Understanding tensile strength is crucial for selecting materials that meet performance standards in engineering applications. ↩
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Understanding elongation at break is crucial for assessing material flexibility and durability in real-world applications. ↩
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Explore this resource to understand heat aging tests, ensuring your materials meet durability and performance standards. ↩
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Learn about the standards and certifications for materials used in food contact applications. ↩
