## Common ground
All options refer to pre-impregnated composite materials (prepregs) used in advanced manufacturing, typically for aerospace, automotive, or industrial applications.
## Options at a glance
**3921.90 – Glass fiber epoxy/phenolic prepregs**
- **Defining trait:** Reinforced with glass fiber and impregnated with epoxy or phenolic resin.
- **Typical examples:** Structural panels for marine or construction use.
- **Best fit when:** The material uses glass fiber and either epoxy or phenolic resin, regardless of coating.
**6815.12 – Epoxy coated carbon fiber prepregs**
- **Defining trait:** Carbon fiber fabric with an epoxy coating (surface treatment) applied after impregnation.
- **Typical examples:** High-performance aerospace components like aircraft wings or drone frames.
- **Best fit when:** The product is carbon fiber-based and explicitly described as "epoxy coated" (indicating surface finish).
**6815.13 – Other epoxy coated carbon fiber prepregs**
- **Defining trait:** Carbon fiber prepregs with epoxy coating, but not covered under 6815.12 (e.g., different formulation or application).
- **Typical examples:** Specialty composites for motorsports or medical devices.
- **Best fit when:** The product is carbon fiber-based and epoxy-coated, but does not meet the specific criteria of 6815.12 (e.g., non-standard coating process or additives).
## Key differences
| Feature | 3921.90 | 6815.12 | 6815.13 |
|-----------------------|---------------------|----------------------|------------------------|
| Fiber type | Glass fiber | Carbon fiber | Carbon fiber |
| Resin type | Epoxy or phenolic | Epoxy | Epoxy |
| Coating status | Not specified | Explicitly coated | Coated (non-specific) |
| Specificity of coating| None | Standardized | General/other |
## How to choose
- Is the fiber glass? → 3921.90
- Is it carbon fiber with epoxy coating, and standard in application? → 6815.12
- Is it carbon fiber with epoxy coating, but not standard or different formulation? → 6815.13
## Overlaps & pitfalls
- **Overlap:** 6815.12 and 6815.13 both describe epoxy-coated carbon fiber prepregs; distinction depends on whether the product fits a defined standard (6815.12) or not (6815.13).
- **Pitfall:** "Epoxy coated" may be confused with "epoxy-impregnated." Coating refers to surface treatment, not bulk impregnation. If unsure, prioritize the product’s main function: structural reinforcement vs. surface finish.
- **No priority rule given** — choose based on the product's MAIN defining feature (fiber type and coating status)