Material and Primer Matrix
A starting point, not a conclusion: eliminate what clearly will not work, then converge with data and validation.
A starting point, not a conclusion. The ranges below eliminate options that clearly will not work; the supplier datasheet and your own validation still decide.
1. Elastomer comparison
| Material | Hardness | Continuous temperature | Oil / media resistance | Typical use |
|---|---|---|---|---|
| Silicone HCR (VMQ) | Shore A 30–80 | about -50 to 200 °C | Good weathering and water; swells in mineral oil | Gaskets, keys, overmoulded bodies |
| Liquid silicone (LSR) | Shore A 20–70 | about -50 to 200 °C | As VMQ, higher purity, lower odour | Precision overmoulding, medical and food contact |
| Fluorosilicone (FVMQ) | Shore A 40–80 | about -40 to 200 °C | Mineral oil and grease resistance, clearly better than VMQ | Shaft seals, seals near gearboxes |
| TPU | Shore A 70–95 | about -30 to 90 °C | Good oil and abrasion resistance | Dust boots, sleeves, wear corners |
| TPE / TPS | Shore A 40–90 | about -30 to 100 °C | Moderate; creeps under long compression | Handle overmoulding, grip patterns |
Order-of-magnitude only. Filler system, cure method and post-treatment change the outcome, and batches of the same grade can differ.
2. Substrate, pre-treatment and primer
| Substrate | Pre-treatment | Primer direction | Watch out for |
|---|---|---|---|
| Stainless / carbon steel | Degrease → blast or polish → ultrasonic clean | General metal primer for heat-cure silicone | Surface roughness drives adhesion; platings need a compatible primer |
| Aluminium and anodised aluminium | Degrease → alkaline wash → neutralise → dry | Aluminium-specific primer | Uneven anodising thickness causes local non-bonding |
| Copper / nickel-plated | Remove oxide → clean → dry | Metal primer (mind copper ions) | Copper can inhibit silicone cure; verify cure completeness |
| PA / PC / ABS | Plasma activation | Plastic primer or primer-free compound | PA absorbs moisture and needs drying; PC is prone to stress cracking |
| PBT / PPSU | Plasma activation plus local abrasion | High-temperature plastic primer | High crystallinity makes mechanical interlock decisive |
| PCBA | Conformal cleaning → mask non-bonded areas | Low-corrosion, low-volatility primer | Keep clear of components and solder-joint heat zones |
3. Validation items
- Adhesion: peel or push test per substrate, recording the failure mode (interfacial or cohesive) — a number alone is not enough.
- Damp heat: retest adhesion after 85 °C / 85% RH; the item overseas customers ask for most.
- Water or media immersion: set temperature and duration by the actual medium (water, coffee, cleaning agent, sweat).
- Thermal shock: verifies the interface under mismatched expansion.
- Function after ageing: compression set on seals, key rebound, appearance of overmoulded parts.
4. Four common mismatches
- Standard silicone long-term in mineral oil: it swells and loses strength; move to FVMQ or NBR.
- Overmoulding plastic without plasma: adhesion becomes luck and batch variation grows.
- Using primer to fix a structure problem: an interface step or a poorly located insert cannot be primed into place.
- Selecting on hardness alone: two grades at the same Shore A can differ by a factor of two in compression set and tear strength.
5. Related pages
- Material selection — working back from the duty condition;
- Properties library — hardness, temperature, flame retardance, cleanliness;
- Bonding to other materials — pre-treatment, primer and press-cure process;
- R&D and testing — what we can run and how acceptance is judged.
6. Questions
Can this table be used as an acceptance standard?
No. Acceptance depends on the part, the duty condition and the customer specification; the table narrows the options.
Do you formulate the primer or does the material supplier?
Usually the supplier's matched system, which we match and validate against the substrate and process. Custom primer evaluation is possible for unusual substrates.
Can one primer serve both stainless steel and aluminium?
Not recommended. Surface chemistry differs enough that a general primer gives unstable adhesion; validate each separately.