How Automotive Rubber Sealing Strips Are Made: From Compound to Cut Length
Buyers often judge a sealing strip by its price per metre. In practice the performance difference between two visually identical strips comes from four process steps: compounding, vulcanization, profile control and finishing. Here is what happens on the line at YUYANG.
1. Compounding
Raw EPDM polymer is mixed with carbon black or silica filler, process oil, curatives, anti-ozonants and blowing agent (for sponge grades) in an internal mixer. The compound recipe sets hardness, compression set, colour and weather resistance. Every batch gets a rheometer cure curve and a physical-property test before release.
2. Extrusion
The compound is fed through a vacuum extruder with a die machined to your profile. Vacuum venting removes trapped air, which is what prevents surface porosity on sponge profiles. Complex parts use co-extrusion: two or three compounds joined in the die, for example a dense carrier with a soft sponge bulb, or a metal carrier inserted for clip-on seals.
3. Vulcanization (curing)
Continuous microwave or hot-air curing lines cross-link the rubber. Cure temperature and line speed must be balanced: under-cure gives poor compression set and a sticky surface; over-cure makes the strip brittle. This is the single biggest source of quality variation between suppliers.
4. Finishing
- Flocking: electrostatically applied nylon fibre on the surface that contacts glass, lowering friction and noise.
- Coating: silicone or PU spray for easier window movement.
- Adhesive backing: 3M or equivalent acrylic tape for peel-and-stick installation.
- Cutting, punching and notching: to length, with mitered corners, clip holes or end caps.
Quality checkpoints
| Check | Typical method | Frequency |
|---|---|---|
| Dimensions | Profile projector / caliper | Every setup + hourly |
| Hardness | Shore A durometer | Per batch |
| Compression set | 22 h at 70 °C (ASTM D395) | Per batch |
| Heat ageing | 70 – 100 °C, 70 – 168 h | Per compound |
| Ozone resistance | 50 pphm, 20% strain | Per compound |
| Adhesion (tape / flock) | Peel test | Per roll |
What this means for your RFQ
A quotation that only states "EPDM, black, 30 × 20 mm" leaves the compound, cure and finishing undefined. Ask your supplier to declare hardness, density, compression set and any coating — and request a sample from the actual production die, not a lab sample. We provide all of these on request.
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