## Common ground
All options are sealing caps used to seal containers, closures, or joints.
## Options at a glance
**3926.90 – Plastic material**
- **Defining trait:** Made from synthetic polymer materials like polyethylene or polypropylene.
- **Typical examples:** Bottle caps, plastic jar lids, disposable container seals.
- **Best fit when:** The cap is lightweight, rigid, and used in food packaging, consumer goods, or low-pressure applications.
**4016.99 – Rubber**
- **Defining trait:** Made from natural or synthetic rubber, offering elasticity and high sealing performance under pressure or temperature changes.
- **Typical examples:** Gasket seals, industrial pipe fittings, high-pressure container lids.
- **Best fit when:** The cap must provide a tight, flexible seal in demanding environments (e.g., automotive, chemical, or outdoor use).
## Key differences
| Feature | 3926.90 (Plastic) | 4016.99 (Rubber) |
|------------------|--------------------------|----------------------------|
| Material | Rigid plastic | Flexible rubber |
| Sealing pressure | Low to moderate | High |
| Durability | Moderate (can degrade) | High (resists wear, heat) |
| Common use | Consumer packaging | Industrial, mechanical |
## How to choose
- Is the cap made of a rigid, lightweight plastic? → 3926.90
- Does it need to flex and maintain a tight seal under pressure or temperature changes? → 4016.99
## Overlaps & pitfalls
A cap may use both plastic and rubber (e.g., a rubber gasket in a plastic cap), but the primary material determines classification. If unclear, prioritize the **main sealing component**—if rubber is used for sealing, choose 4016.99; if plastic forms the main body and seal, use 3926.90