Methods to check Moisture Content in resins

[Rojan]

Hi guys,

Simple question, but what methods do you use to check moisture content in your resins before processing? We use the weight loss method, but I doubt this method has been validated properly at my plant. I doubt the two parameters - temperature and time have been chosen by trial and error... I am sure the values I get are far from the truth as it is not possible to dry PA6 to 0,00% water content by hot air...

Regards,
Robert

[brentb]

AI and Omni weight-loss. The temp and time are not just random from the makers. Thought and experimentation went into them
There are new chemical-free KF units out now.

KOM

brent

[MTUHusky]

Hey Rojan,

Unfortunately, most injection molding operations use the weight-loss method because it's cheaper and on the surface makes sense. The problem is a lot of engineering plastics have a lot of fillers and additives in them that give off false readings for a weight loss system. The best analyzer you can get is a moisture specific tester which only measures the moisture in the material and ignores things like lubricants and other additives. A specific tester off the top of my head would be the Vapor Pro.

Another issue you need to look out for is the max moisture listed on the data sheet. I have seen data from the material supplier for an unfilled nylon that says max moisture is 0.2% and the data for the 50% glass filled nylon is the same 0.2% but how can that be if glass absorbs zero moisture? The max moisture for the 50% glass filled nylon should be 0.1%. Just something else to keep in mind.

Hope this helps!

[Rojan]

Hey MTUHusky,

Thank you for your answer.

BTW, I also noticed that some material suppliers put the same max moisture content for non GF filled and GF filled resins in their TDS which seems weird.

So if most of us use the weight loss method please tell me your procedure for validation of this method.

Robert

[Joel JS]

Hi Robert,

You can get some decent results with a weight loss moisture analyzer. (Zero percent loss does sound suspicious!) I would recommend to do a controlled study... measure it without drying, measure it with 1 hour drying, and with 2 hours drying .... You can also mold parts with each material sample to see the affect on the parts at different moisture loss values.
Hope that helps.
Joel

[MTUHusky]

Rojan,

Unfortunately, the procedure is pretty much making sure your weight loss machine is calibrated and has the appropriate settings for the material you are testing. I have had issues where my pellets were slightly molten after the test and that is a red flag that you have bad results and I had to have the machine re-calibrated. The pellets should still be pellets when you are done. Other than that, it's a guessing game. In my opinion, the best thing to do is bite the bullet and buy a moisture specific analyzer, as this is the only way I am aware of that gives results we can have confidence in.

One of our industry leaders, John Bozzelli, has written an article on this very topic. http://www.ptonline.com/columns/analyze-moisture-before-processing

[Rojan]

Thank you guys for your answers. All your guidelines are very valuable.

I have been 4 years in the industry, but I have noticed many times that people use dessicant dryers with pretty low dew points together with very big hoppers. I calculated residence times in some cases and I came up with values up to 5 hours. That is interesting because even with dessicant beds the resin gains the moisture back from the air. What are your experiences? Selecting the proper size of hopper is crucial.

Robert

[MTUHusky]

Thank you guys for your answers. All your guidelines are very valuable.

I have been 4 years in the industry, but I have noticed many times that people use desiccant dryers with pretty low dew points together with very big hoppers. I calculated residence times in some cases and I came up with values up to 5 hours. That is interesting because even with dessicant beds the resin gains the moisture back from the air. What are your experiences? Selecting the proper size of hopper is crucial.

Robert

Hey Robert,

My experience with desiccant dryers is that they are often not set up correctly or are not sized for the job.

Not setup correctly:
- The dryer temperature is way too high. This will lead to degraded material, never, ever go over the material manufacturers max dryer temp, you will degrade the material. If your dryer temperature is at or below the recommended settings, then you cannot over dry the material period. There is no such thing as material that is "too dry". There is such a thing as oxidized material which is a form of degradation, but you would have to leave the material in the dryer for a month or more to do it.

- The temperature of the desiccant bed is too low or too high. The recommended temperature for the desiccant bed should be between 120 - 150 F. Not sure what it is? Grab a thermocouple and go measure it!

- The temperature drop from inlet to outlet of the desiccant dryer is no more than 50F. As above, not sure what the temperature drop is? That's right, grab a thermocouple and go measure it!

The dyer is not sized correctly:

- I see this one a lot. They have a dryer already at the machine, so why move it right? Well lets say you have a dyer that is rated for 24 lbs of material per hour and you are running a process that is eating 18 lbs an hour. You then change jobs and the mold coming in eats 30 lbs of material an hour. In a few hours, you will start to see moisture problems and wont know why. For this reason, I started adding the amount of material per hour to each molds operating manual so I wouldn’t have to calculate it on the shop floor. To calculate the material per hour I simply take a full shot with runner and weigh it and multiply that weight by the number of cycles per hour. If that number is higher than the number printed on your dryer, you need a bigger dryer.

If I were the owner of a shop and I had a choice, I would pick a vacuum dryer as they are less finicky and more efficient.

I almost forgot one point, make sure that your hopper has a funnel angle of no sharper than 60 degrees. Anything less than 60 degrees can result in funnel flow where the new material coming into the drying hopper will "funnel" right through the center and bipass all the material on the wall of the hopper. So you can get a situation were you are sucking wet material into the drying hopper and the fresh wet material funnels through the center while the dry (sometimes really dry) material sticks or sits on the side walls. This can be very frustrating, as the only fix is buying a 60 degree funnel hopper or also known as a "mass flow hopper".

Hope this helps!

[Joel JS]

Thank you guys for your answers. All your guidelines are very valuable.

I have been 4 years in the industry, but I have noticed many times that people use dessicant dryers with pretty low dew points together with very big hoppers. I calculated residence times in some cases and I came up with values up to 5 hours. That is interesting because even with dessicant beds the resin gains the moisture back from the air. What are your experiences? Selecting the proper size of hopper is crucial.

Robert

Hey Robert,
If you mean the dryer hopper holds 5 hours of material...... no problem, material won't gather moisture while sitting in a properly working dryer.

Hope that helps - Joel

[JayDub]

I have noticed many times that people use dessicant dryers with pretty low dew points together with very big hoppers. I calculated residence times in some cases and I came up with values up to 5 hours.

Molding small medical parts in relatively low quantities, most hoppers are too big. The real problem is the amount of material I need to dry falls below the air inlet, so it sees no air flow and stays wet pretty much indefinitely. Consequently we typically dry two or three different colors in the same hopper, at the same time, in silk bags or stainless mesh baskets.

[Rojan]

I wrote it wrong (English is not my mother tongue). I meant that we can dry the material very well, but if the residence time of the material in the hopper (after it leaves the dessicants) is e.g. 5 hours then the hopper is way too big which I see very often. JayDub thanks for your comment - that's what I meant.

Robert

[Joel JS]

Hi Robert,

So, some of the time 5 hours residence time is not a problem, but that depends upon the material.... I wouldn't try it with nylon, but you might be ok with ABS. We lowered our fill sensors on the hoppers so we get less than 2 hours residence time for our jobs.

Hope that helps,
Joel