The effects of running wet nylon can look very similar to the effects from running the material too hot or even running contaminated material. The part molded from wet nylon will be dry therefore you cannot check the moisture content of the part to determine if it was run from wet material. Once the material has been run wet, the moisture has burned away. This is then blamed on the material being too dry. Sometimes people alter their processing parameters to try and fill the cavity and end up degrading the material with shear heat. What happens when you dry the material to extremely low moisture content is that the flow decreases which can then make it difficult to fill the mold cavity. It would be great if you could reduce the moisture to zero. This will typically yield acceptable parts but reducing the moisture even further will improve the properties still more.Ī lot of people speak of “over drying” nylon but you cannot really dry it too much. Most nylon manufacturers will recommend drying nylon to below 0.2% moisture content. The moisture content cannot be controlled well enough to insure anything resembling accuracy. This is why you never see melt flow reported on a nylon data sheet. The higher the moisture content, the higher the flow. It should also be noted that moisture in nylon acts as a lubricant which increases the flow of the material. A loose fitting can draw in moist air from the shop and ruin the dry material. It is also very important that all hose fittings on the dryer be secure. ![]() If the material sits open, it will absorb the moisture right back and in my experience, it seems that when the material is warm, it absorbs the moisture even faster. It is important to note that once the material is dried that it must be run immediately. However, you have to use it quick once you open the bag and if you do not use it all, the remaining material in the bag will have to be dried for the full amount of time. I have even heard of some people skipping drying altogether. If the material is packaged in bags or Gaylords that are lined with foil, you can probably cut that time in half. Nylon 6 and 6/6 typically need to be dried for 40 hours at 175F. That liquid water expanding into steam is very destructive to the polymer causing loss of properties and the “splay” appearance that we are all familiar with. The problem with processing nylon or any other material that is wet is that the water will reach boiling point at the processing temperature of the material. I have seen it once or twice in over 25 years in the materials business. Encapsulated moisture is pretty rare though. No amount of drying time or temperature will get that water to evaporate. The only exception to this would be if the material was compounded in such a way that a large amount of water actually gets encapsulated in a small hollow spot in the pellets. This means that the same amount of drying time will be required to dry material that has sat open for 24 hours or 24 years. Under normal conditions, say 20-80% RH, nylon will reach saturation in about 24 hours. Nylon does not have a shelf life there is not a “point of no return” in which the material cannot be dried. Also like a sponge, when nylon is exposed to dryer air, it will give that moisture up and dry out. Similar to a sponge, once the sponge has been soaked in water to saturation, it will not hold more water. At that point no amount of humidity will make it absorb more. However, it only absorbs moisture until it is saturated. Immersing nylon in water will cause it to absorb moisture extremely fast. The higher the humidity, the faster it will absorb moisture. Nylon is hygroscopic, meaning that it absorbs moisture right out of the air surrounding the material or directly from water if it is submersed. I hope in this article to clear some of this up. Moisture in the raw material causes many processing issues and part failures and the affects that moisture has on molded parts seems to confound people as well. ![]() Nylon is one of the oldest and most commonly used thermoplastics but there still seems to be a lot of confusion about properly drying nylon and in general about how nylon is affected by moisture.
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