Nylon is a typical hygroscopic material. On many molding floors, splay marks, bubbles and brittle parts all turn out to have the same root cause: the material wasn't fully dried. This article explains why drying is necessary and how to do it.
1. Why nylon must be dried
Nylon molecular chains contain amide groups, which readily absorb moisture from the air. When moist pellets are melted at high temperature, the moisture turns into steam and also causes hydrolysis of the material, lowering its molecular weight. The result is cosmetic defects and reduced strength at the same time.
2. What moisture content to target
The industry generally recommends keeping moisture content below 0.2% before injection molding; for parts with high appearance or performance requirements it can be kept even lower. The specific value should follow the material supplier's processing recommendations.
3. Problems caused by excessive moisture
1. Splay marks, bubbles and a hazy surface.
2. Lower melt viscosity, making drooling and flash more likely.
3. Hydrolysis of the material, making parts brittle and weaker.
4. Large fluctuations in dimensions and properties, with inconsistent batches.
4. How to dry
1. Equipment: preferably use a desiccant dryer for more consistent drying; ordinary hot-air drying is strongly affected by ambient humidity.
2. Temperature and time: a common practice is drying at around 80℃ for 4~6 hours, adjusted according to the material and its moisture condition.
3. Avoid over-drying: if the temperature is too high or the time too long, nylon tends to yellow; take particular care with light-colored and natural materials.
4. Prevent moisture regain: use dried material as soon as possible, keep the hopper covered or use a drying hopper, and don't leave material exposed for long periods.
5. Material in original packaging: unopened, sealed material generally has low moisture content, but it absorbs moisture quickly once opened, so reseal leftover material properly.
5. Conditioning after molding
Freshly molded nylon parts are almost dry-as-molded, with low toughness and prone to brittle fracture. For parts that need toughness, such as snap fits and cable ties, conditioning (e.g. boiling in water or storage at constant temperature and humidity) can be applied so the parts absorb an appropriate amount of moisture, restoring toughness and stabilizing dimensions.
6. A simple troubleshooting approach
When splay marks, bubbles or sudden brittleness appear in production, check the drying conditions and hopper residence time first before adjusting injection parameters; this often saves a lot of detours.
RJF Plastics supplies PA6 and PA66 resins and modified compounds and can provide corresponding processing recommendations. The above is general information; specific processes should follow the material supplier's recommendations.