## Common ground
All options are types of closures (stoppers, lids, caps) categorized by their primary material.
## Options at a glance
**3923.50 – Plastic**
- **Defining trait:** Made primarily from synthetic polymers like polyethylene or polypropylene.
- **Typical examples:** Bottle caps, jar lids, plastic stoppers for lab glassware.
- **Best fit when:** The closure is lightweight, moldable, and used in packaging or consumer goods.
**4016.99 – Rubber**
- **Defining trait:** Made from natural or synthetic rubber, offering elasticity and sealing ability.
- **Typical examples:** Tire valve caps, rubber stoppers for flasks, gasket-style closures.
- **Best fit when:** A flexible, airtight, or shock-absorbing seal is needed.
**4503.10 – Cork**
- **Defining trait:** Made from the bark of cork oak trees; naturally porous and compressible.
- **Typical examples:** Wine bottle stoppers, laboratory stoppers, insulation caps.
- **Best fit when:** Natural material is preferred for sealing or aesthetic reasons (e.g., wine bottles).
**7010.20 – Glass**
- **Defining trait:** Made from silica-based glass; non-reactive and transparent.
- **Typical examples:** Glass stoppers for chemical bottles, perfume bottle caps, lab glassware closures.
- **Best fit when:** Chemical resistance, clarity, or high purity is required.
**8309.90 – Base metal**
- **Defining trait:** Made from iron, steel, aluminum, or other non-precious metals.
- **Typical examples:** Metal caps for fuel tanks, industrial valves, threaded metal lids.
- **Best fit when:** Durability, strength, or resistance to high pressure/temperature is essential.
## Key differences
| Feature | Plastic | Rubber | Cork | Glass | Base Metal |
|------------------|-------------|-------------|-------------|-------------|-------------|
| Material source | Synthetic | Natural/synthetic | Natural (tree) | Silica | Ore-based |
| Flexibility | Low–Medium | High | Medium | None | Low |
| Sealing quality | Good | Excellent | Good | Excellent | Good |
| Chemical resistance | Moderate | Variable | Good | Very high | Varies |
## How to choose
- Is the closure made of plastic? → 3923.50
- Does it need to be flexible and airtight? → 4016.99
- Is it natural, porous, and used for wine or lab glassware? → 4503.10
- Is it transparent and non-reactive (e.g., lab use)? → 7010.20
- Is it metal, strong, and used in industrial settings? → 8309.90
## Overlaps & pitfalls
A closure may combine materials (e.g., rubber gasket in a metal cap), but the **main material** determines classification. For example, a glass stopper with a rubber seal is still classified under 7010.20 if glass dominates. If unclear, prioritize the **primary structural material**