Penetrants are broadly classified into two types, depending on the process used to inspect the component:
- Red dye penetrants that are visible against the white of the developer;
- Fluorescent penetrants that show any imperfections under ultra violet (UV) light.
Fluorescent Penetrants can be further classified as to how the surface excess penetrant is removed.
- Method A – Water Washable – the penetrant can be removed with water alone;
- Method C – Solvent Removable – the penetrant needs to be removed using a solvent based cleaner;
- Method D – Post Emulsifiable, Hydropholic – an emulsifier is used to remove the penetrant chemical from the component.
Chemetall have based their newer penetrants on NPE (Nonylphenolethoxylate) Free technology, which have low-odour, low toxicity and have therefore a minimal impact on the environment.
The choice of penetrant affects the associated developer and cleaner/remover required.