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. 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. I hope in this article to clear some of this up.
A Few Basic Concepts to Understand
Nylon is hygroscopic, meaning that it absorbs moisture right out of the air surrounding the material or directly from water if it is submersed. The higher the humidity, the faster it will absorb moisture. Immersing nylon in water will cause it to absorb moisture extremely fast. However, it only absorbs moisture until it is saturated. At that point no amount of humidity will make it absorb more. Similar to a sponge, once the sponge has been soaked in water to saturation, it will not hold more water. Also like a sponge, when nylon is exposed to dryer air, it will give that moisture up and dry out. Nylon does not have a shelf life; there is not a “point of no return” in which the material cannot be dried.
Under normal conditions, say 20-80% RH, nylon will reach saturation in about 24 hours. 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. 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 is called encapsulated moisture. No amount of drying time or temperature will get that water to evaporate. Encapsulated moisture is pretty rare though. I have seen it once or twice in over 25 years in the materials business.
Processing
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. 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.
Nylon 6 and 6/6 typically need to be dried for 40 hours at 175F. If the material is packaged in bags or Gaylords that are lined with foil, you can probably cut that time in half. I have even heard of some people skipping drying altogether. 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. It is important to note that once the material is dried that it must be run immediately. 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 also very important that all hose fittings on the dryer be secure. A loose fitting can draw in moist air from the shop and ruin the dry material.
It should also be noted that moisture in nylon acts as a lubricant which increases the flow of the material. The higher the moisture content, the higher the flow. This is why you never see melt flow reported on a nylon data sheet. The moisture content cannot be controlled well enough to insure anything resembling accuracy. Most nylon manufacturers will recommend drying nylon to below 0.2% moisture content. This will typically yield acceptable parts but reducing the moisture even further will improve the properties still more.
A lot of people speak of “over drying” nylon but you cannot really dry it too much. It would be great if you could reduce the moisture to zero. 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. Sometimes people alter their processing parameters to try and fill the cavity and end up degrading the material with shear heat. This is then blamed on the material being too dry. This is a misunderstanding.
Trouble Shooting Defective Nylon Parts
Once the material has been run wet, the moisture has burned away. It will come out of the barrel as steam. 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. The effects of running wet nylon can look very similar to the effects from running the material too hot or even running contaminated material. A thorough investigation will be required to determine the root cause.
Moisture in Molded Parts
Once a part has been molded or extruded, the part will immediately start absorbing moisture from the atmosphere. This moisture absorption has two effects on the part.
First, it will affect the physical properties of the part. The part will get less brittle and more flexible. These properties will change dramatically. Looking at a typical unfilled Nylon 66, here are some properties for Dry as molded specimens and after the specimens have been conditioned at room temperature and 50% relative humidity:
Property Dry as Molded 50% RH
Tensile Strength at Yield 12,000 psi 8,000 psi
Flexural Modulus 420,000 psi 180,000 psi
Izod Impact 1.0 ft-lb/in 3.0 ft-lb/in
Keep in mind that if the part is exposed to intermittent periods of moist and dry environments, it will continue to absorb and give up moisture and the properties will change accordingly for the life of the part. It will never reach any type of equilibrium.
The second change will be dimensional. The dimensions of nylon parts are dramatically affected by temperature and humidity. This can severely affect the functionality of long parts. It is important to take these issues into account when designing a Nylon part. If you cannot afford to have this much change in dimensions, I would recommend that you look at PBT or PET as alternative materials.
Water Packing
Many processors will pack nylon parts in water to improve the impact strength or flexibility. Nylon parts are dry coming out of the molding machine so adding moisture will change the properties.
There is confusion about how this works. The property changes that are caused by the moisture absorption are not permanent. Once the parts are removed from the water, they will start to give up their moisture and the properties will adjust accordingly. Water packing is a good solution for temporarily adding impact strength and flexibility. This might aid in part assembly.
For instance, if you have a problem with tabs on a part cracking during assembly, water packing might be a good solution. Packing the parts in water will make them more flexible to ease assembly and then the parts will stiffen back up as the dry. If the part that you are molding needs more flexibility for ongoing use, you might want to switch to a more flexible grade as water packing is not going to help.
Just remember that it is not a permanent change in the properties.
Conclusion
Many processors have been plagued by problems with Nylon and moisture. Many people in the material business have told me that this is one of the most common causes of customer complaints and material returns. I think that this may be because of some general misunderstandings about how moisture affects Nylon.
weeklypellet.com 
Hi,
Thanks for the information on nylon and it’s interference with water. I do have one question though, what scientific literature did you use for this post? Not that I do not trust it, but I am working on a similar subject and would like to use this information but I am not allowed to use an online post as literature.
I hope to receive a reply.
Best regards,
Arentius Stokkel
LikeLike
it was a little while ago when I wrote that post. Most of the information is from my personal experience however, I have some literature from DuPont that I could send you. Send me an Email at dean@midwestresins.com and i will reply.
LikeLike
Good things
LikeLike
The moisture issue is public knowledge and for those versed in the art acknowledge the information!! We do not need a scientific journal that can also alter facts or ignore facts to know that dessicant drying is important for all condensation polymers. The big news beyond moisture is life time of Nylon and the past and current limits that can be altered with new technologies like Alterin PAM and PES and UVITA SME 3811 that can significantly extend outdoor life of Nylon 6 and 6,6, unfilled and filled for over 25 years thereby reducing waste and increasing sustainability. If you want facts and reference material I suggest M.Kohan book Nylon Plastic Handbook its the so called Bible of Nylons
LikeLike
Can moisture absorption will be out from wet nylon 6 when end product will be in sunlight during days?
Thanks and regards
Shiromani dhar diwedi
Mo. No.+91 7754882249
LikeLike
Nylon 6 parts will absorb and release moisture during the parts lifespan based on relative humidity in the atmosphere and temperature. A part that is outside on a hot sunny day could certainly release a large amount of its moisture.
LikeLike
It is possible to ‘over dry’ Nylon as the manufactures usually set a range or offer a suggested max moisture content.
LikeLike
They never set a minimum moisture content because you cannot dry nylon too much. That being said, if you dry nylon and too high a temperature and/or for too long a time period, you can burn some of the lubricant out of the material (if a lubricated grade) and even start to degrade the material itself. You have be very careful with unfilled Nylon 66 as you will discolor it if you over dry it. It will turn yellow.
LikeLike
So, I’ve read that nylon immersed will absorb to some saturation point and then stop. I am planning to use commercially available (fence gate) nylon rollers for a movable swim platform in such a way that the nylon wheels will remain submerged in freshwater. As long as I can deal with the dimensional change, is there any reason these won’t continue to serve their function?
LikeLike
You should not have any problems. The rollers will swell up a bit when submersed.
LikeLike
Thanks so much for the prompt guidance!
LikeLike
Hello, I’m making guitar picks out of nylon and just received my first samples. The guitar pick is more stiff than I would like it to be, but I understand it will become more flexible as it absorbs moisture. The parts were molded 3 days ago. From what I understand, the parts will absorb moisture for 2 weeks but you said in your article 24 hours. My parts are 3 days old, do you think they will change much more or have they already equalized do you think? Thank-you for any information!
LikeLike
When the picks came out of the molding machine the moisture content was virtually zero. They will begin absorbing moisture from the atmosphere immediately and typically reach equilibrium in 24 hours. After that, they will absorb and release moisture depending on the relative humidity of the air.
If the pick is stiffer than you would like, you could soak it in water overnight. That should make it more flexible.
LikeLike
Hi …I need to cover a shallow open drain (concrete) …I was looking at using nylon dowel (15cm long) as legs for the cover …the legs would sit on the bottom of the drain and the drain remains wet mostly …would nylon dowel be ok in this situation or would POM be more suitable….thanking you Garry
LikeLike
I think in this case, that nylon would work just fine. The dimensional changes that you might see are not going to be enough to cause you any problems.
LikeLike
Hello. I use a MXD6 nylon to create a barrier in a pet bottle preform (injection molding). The MXD6 is dried for 5 hours at 50 °C and the process temperature is 260°C. Preforms are good in quality and are accepted by customer. But I have a lot of problems in the machine due the obstruction of nylon runners. Every 2/3 weeks I have to stop production and replace mold nozzles.
Is possible that high moisture in MXD6 melt causes the formation of carbonized deposits?
Thanks a lot and sorry for my english.
LikeLike
Nylon and PET undergo various transformation depending on fortification. Transamidation for Nylon and Transesterfication for PET which includes crosslinking of PET!! The solution to your problem based on the information you provided is Alterin SP and Alterin PAM or PES for the PET. This will eliminate the thermal damage to both resins during your fabrication and control cross link reactions and especially monomer formation.
LikeLike
I do not think that moisture is causing your problems. This sounds like the material is degrading in the barrel. You might want to check that the screw and barrel are not excessively worn.
LikeLike
For PA 6/ 66 material suggest to dry at 80°C 2 – 4 hours.
LikeLike
Nylon will give up moisture to the atmosphere in low humidity environments. When the material dries out, it will lose flexibility and tensile elongation. You could package the parts with a bit of moisture in the bag but this will only be a temporary fix. You could use a nylon 12 which does not absorb as much moisture but Nylon 12 is really expensive.
I usually use unfilled polypropylene homopolymer for living hinge applications. You can make nearly unbreakable hinges with this material that will last virtually forever.
LikeLike
You did not mention the grade of Nylon being used? This can make a difference!! Most Nylon Monofilaments for 3D printing are Nylon 6,6,6, 11,12 and 6,12. Most are NOT fortified! Cracking of any polyamide can occur from both environmental conditions from metal salts, and chemicals that attack the surface of the amide and carboxyl groups.
Yellowing then crazing and then cracking is a surface oxidation problem. Light will have a detrimental effect as will thermo-oxidative degradation on surface cracking. The issue regarding drying and moisture after fabrication is NOT a big on cracking as it is on impact properties and physical properties. A living hinge from Nylon should be properly lubricated, properly fortified. The solution is Alterin molecular technology to prevent thermo photo oxidation and chemical resistence especially from trace halogens like chlorides.
LikeLike
Do you have an approximation for the ‘life time of drying’ in the room ? My process dictates a 30-min delay between drying PA6 and potting epoxy resin. I’m worried that the poor adhesion might be due to re-adsorbed moisture.
LikeLike
The rough numbers are that dried nylon will go from roughly .2% moisture to 1% in about 8 hours at 50% RH. On a high humidity day that time might be reduced to 3 hours. In my personal experience, I have found that warm dry nylon right out of the dryer can absorb too much moisture for molding in 15-20 minutes if it sits in a pail exposed to the atmosphere.
LikeLike
If nylon is painted ( or coated with PU) after molding – will it still absorb moisture ? Further let us paint gets removed from a portion of part – that exposed portion shall absorb the moisture but will it percolate to entire part or remain limited to only exposed portion . Any thoughts will be appreciated
Best Regards
Ashish
LikeLike
YES
LikeLike
That is a very interesting question. I am not sure of the answer and I cannot find any information about it. I would think that painting or coating after molding would prevent moisture absorption to some degree but I am just not sure.
LikeLike
We are planning to use either a Nylon 6 or PC/ABS for a relative humidity sensor for ambient air. Will the material affect the sensor readings compared to the actual RH in the ambient air? I assume there is a lag time for the material to absorb and release moisture. Is this accurate? Thanks in advance for your help!
LikeLike
Both materials will absorb moisture. The nylon will absorb moisture a lot faster than the PC/ABS but neither will absorb moisture fast enough to affect the sensor. You should be fine with either.
LikeLike