Moisture Content in Plastic Beads

pjv

Hi All;

I'm looking for some help.

When injection molding plastic beads, they need to be very dry. By weight pastic beads can contain up to 10% moisture, and for proper performance·nylon should·contain less than 0.1 %, and some others such as polycarbonate less than 0.01%

Because the moisture is embedded into the beads, I can't measure the water content by just sticking a humidity sensor into a container of beads as it takes hours of heated air to drive the water out of the beads.

Also I can't see an easy way of determining the water content by weighing a quantity of beads as I don't know the specific gravity accurately enough.

Is there a way I could fill a container with a quantity of beads and somehow electronically measure the water content indirectly?

How could I use microwave/RF absorption, or capacitance or ????? to achieve the precisions I'm looking for?

Any insight into this would be much appreciated.

Cheers,

Peter (pjv)

Bean

Peter,

I think they make a meter for measuring moisture in lumber that uses some kind of radio waves (Not the kind that puts probes into the wood).

I don't know if they would work for the beads or not ?
Something like this: http://testproducts.com/safecart/product_info.php/cPath/33/products_id/560?osCsid=44e8cdd11bb1014fd4bbc347768d9a2a

Bean.

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Unsoundcode

Hi pjv, I work on metal detectors made by Loma, the head is a coil that looks for a change in flux. They also act as a density meter. Whenever the product becomes overly wet they will kick off the product. In my case I have to convince the operator they have a bad "mix" and not a faulty detector. Loma are pretty high dollar but it makes me wonder if·some cheap kind of metal detector would detect an increase in moisture. If we pull one out any time soon you can have it, it's unlikely but where Im at they scrap high dollar equipment like its household trash and you never know.

Jeff T.

stamptrol

Peter,

A few years ago, the company I worked for needed a way to determine species of a piece lumber as it passed a point in the mill (at 60 pieces per minute). The idea was to separate spruce from fir due to the different drying times in the kiln.

What the guys at the mill came up with was detecting the moisture content of the piece with a capacitance proximity sensor. It was one of the large diameter ( 30 mm ) type and had either an analog output or adjustable set-point, don't quite remember. Anyway, it did work in that application as sort of a go/no-go setup.

The microwave idea might work. Take a set volume of beads, put it in the microwave for a fixed time then check the temperature rise. You'd probably have to build up a lookup table of temperature rise vs moisture content.

Had some of the same issues on big extruders, but the boss was a great believer in pressure relief valves on the extruder screw!

Keep us informed of your progress.

Regards,

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Tom Sisk

http://www.siskconsult.com
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pjv

Hi All;

Terry.. Thanks for the link, will pursue that one. Not sure if it will go low enough. Any idea what technology they are using??

Jeff... I'm not expecting metal detectors which operate on a tuned loop to be sensitive to moisture..... but perhaps this is something different ??

Tom... I just tried the microwave idea. I accurately weighed out a quantity of 100 grams of "moist" nylon beads in a pyrex cup, and put them in a microwave oven. The idea was to boil off the embedded moisture, then re-weigh the beads to get the weight loss. From that I should have the water content by % weight. Well, a lot of smoke as the beads melted and possibly might have caught fire if they had been left longer. They were very hot, soft to the touch, and quite damp. Perhaps we were overdoing it. So we spread them out on a large plate to decrease the concentration, and also reduced the power by using a lower duty cycle. But the results were not encouraging.

Anyone else with some ideas?

Cheers,

Peter (pjv)

QuattroRS4

Have you tried silica packets with a measured quantity of your beads

1) Get weight of new silica packet
2) place silica packet with measured quantity of beads in a sealed container
3) Wait till moisture is extracted - probably needs numerous trials.
4) weigh silica packet after test
5) calculate the before and after

Just a thought ?

Beau Schwabe

pjv,

"...I can't see an easy way of determining the water content by weighing a quantity of beads as I don't know the specific gravity accurately enough...."

Just a thought... I wonder if you used a high voltage electrostatic spark? You set a specific charge rate, that ultimately determines the number of "pops" per minute. The moisture content of the beads within the
spark path should affect the spark rate.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Kevin Wood

This is from the website of a local company: www.ptli.com/testlopedia/tests/karl_fischer-D789.asp

Tracy Allen

How big and how uniform are these beads for packing? Compressible?

How about a parallel plate capacitor packed with sample beads? Perhaps ratiometric to a second one packed with an equal weight of known dry beads. There would be an effect on complex impedance that might be frequency dependent. The effectiveness depends on the exceptionally high dielectric constant of free water compared to plastic. Water strongly bound would have different characteristics. Maybe a waveguide or coaxial resonator instead of a capacitor.

The inverse of the microwave experiment. Put a bag of beads in a vacuum and observe a drop in temperature due to the heat of vaporization. Or freeze dry them?

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Tracy Allen
www.emesystems.com

Beau Schwabe

Tracy,

That's along the same lines as what I was thinking, only without the spark.....(grin) ...a large parallel plate capacitor setup should work nicely.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

pjv

Hi All;

Thanks for your interest.

Quattro... The moisture is imbedded into the beads, and it typically takes hours of hot dry air to drive the water off. In fact we use a regenerating silica bead system to dry the air for this purpose. What I'm looking for is something that can tell me the water content in a quick measurement. I've designed a new dryer for these beads, and the purpose of this tester is to determine when they have been dried enough.

Beau.... Not sure I understand the sparking issue.

Kevin.... Interesting site. I'll pursue that some further. Looks kinda expensive, but who knows??

Tracy.... These are base materials for injection molding plastic items. A bead is roughly 1/8 inch ball, but could also be smaller. Waste "regrind" material is also included, and that can have quite a variation in size including a lot of "fines". The capacitor approach may be workable, but I'm concerned about the inconsistency of the dielectric constant of varying materials (although we could determine "standard" values for these) or variations from batch to batch.··I guess we'll need to build one and see what happens. As part of my RF·tools, I happen to have an HP network analyzer that will show me complex impedance at frequencies·from 300 KHz through 3 GHz, so that could readily show some effects of the moisture.

Will follow up on the commercial unit (Terry). As well s Kevin's suggestion. And of course, since I'm a real strong believer in doing it yourself, we'll have a go at the capacitance route.

Any more suggestions are welcome.

Thanks again.

Cheers,

Peter (pjv)

Post Edited (pjv) : 1/26/2007 5:10:54 PM GMT

Phil Pilgrim (PhiPi)

I presume the beads are light enough to float. (But I haven't seen David Letterman test any, so I can't be sure. ) Anyway, get a graduated cylinder and a plunger large enough that, when plunged into the cylinder with a liquid/bead mixture, the liquid can squeeze past, but not the beads.

• Sieve a quantity of beads to remove the fines.
• Weigh the resulting sample.
• Put a known quantity of liquid and the bead sample into the cylinder.
• Push the beads down into the liquid until the liquid rises to a particular point on the rod connecting the plunger.
• Tap the cylinder to dislodge any air bubbles that my have gotten trapped under the plunger or among the beads, and readjust the plunge depth if necessary.
• Measure the liquid height on the graduated cylinder.
• Compute the beads' specific gravity.

Now, the plunger and its connecting rod are going to displace some of the liquid, too. The idea behind stopping the plunging when the liquid reaches a certain point on the connecting rod is to keep this portion of the displacement constant. This displacement can then be subtracted from the total displacement before the s.g. calculation.

-Phil

Tracy Allen

I'm visualizing the nylon bead as a solid object on a macroscopic scale, but as a tangle of fibrous molecules with spaces and intertices on the microscopic scale. The water diffuses into those interior spaces, and since nylon is hygrophilic, the water bonds to some extent to the molecular structure. That means it has fewer degrees of freedom than free water and that would affect its dielectric constant and also the energy required to free it from the lattice.

I ran across this instrument: www.completeplastics.com/Moisture_Analyzer-2287.html

Measurement of soil moisture is a similar thorny problem. There is another dimension there, because the material being measured is not homogeneous, and varies from highly hygrophilc clays to sands with very low surface energies. The following URL is a clearinghouse for methods of soil moisture measurement:

www.sowacs.com/

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Tracy Allen
www.emesystems.com

steve_b

Not sure if this would work....

But how about putting the beads in a pressure chamber.
Water boils at lower temperatures when there's lower pressure (did I get that right?)...

Anyhow, With a temperature sensor in the same chamber (something you may be able to read)...you could change the pressure to a certain point where a given amount of water would give a certain temperature.

Or, you could put a high vacuum on it and suck the water out....if you have dessicant in the same chamber, ifyou slowly return pressure maybe it would go in to the dessicant before going in to the beads..

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

Bongo

Yup,



The use of twin vane oil seal vacuum pumps is to dehydrate refrigeration plant prior to putting in or releasing the refrigerant.· Most refrigeration plant reacts very badly with moisure, so the vacuum pumps are used to extract molicules rather than drops (of water)



CPS manufacture a guage that measures vacuum in microns, but you can hear the smallest quantities of anything comming out of a twin vane oil seal vacuum pump - they gurgle.



Suggest that you locate a manufacturer of vacuum chambers and get them to demonstrate with your beads inside.· It is possible that it will be much easier to detect moisture my tapping into a CPS guage (mine has flashing LED's to indicate, so this is a no brainer) and using vacuum to dehydrate rather than hot air.



I can forsee two possible problems:

The evaporation pressure of some materials in the plastic itself is too high to allow the plastic to be subject to an adiquate vacuum for the purpose.

The moisture levels are so high that the pump oil gets contaminated too quickly.· Though there is the "gas ballast valve" that will help in this regard.· Vacuum pump/chamber manufacturer can demonstrate and explain.



bongo

T Chap

Since they are so small this may not help at all, but I was thinking that bouyancy could be some help. If they do in fact float, the time it takes then to rise from a position at the bottom of a container could be affected by the moisture. If they do not float, but do sink to the bottom, the opposite test could be applied. Take the time of transit from bottom to top and compare known high moisture beads to known low moisture beads, see if the time is affected. Experimenting with different mediums could produce better test results.

Post Edited (originator) : 1/27/2007 10:48:02 AM GMT

pjv

Hi All;

Wow, lots of answers; thanks.

Phil .... No, I think they don't float, will need to confirm that.

Tracy .... That first link looks like they have exactly what I need. I get errors trying to delve deeper into their site, so will have to contact them by phone to learn more. I fear that the cost of their FASTI instrument may be high. If so, perhaps we can determine the technology employed and build something ourselves.

Steve ..... That is exactly our plan, but that is for the bulk drying of beads for plastic production. We need a simple quick method of confirming the dried beads are dry enough.

Originally we were drying our bulk maretial with hot air dried with a dessicant. The dessicant was recycled by driving off the absorbed (adsorbed ?) moisture with electrically heated regenerators, a slow and costly (electricity) process. We then build a natural gas fired regenerator with a heat exchanger so the combustion products, water (steam actually) and CO2 would not re-wet the dessicant. Then we expanded that to heat the plastic beads directly. this works, but still is a slow process, very dependent on the prevailing relative humidity in the atmosphere. Noting that several commercial equipment suppliers were selling vaccum based driers, we tested that concept by placing a pound or so of heated wet plastic beads in a pressure cooker, and pulled a vacuum on it. WOW... that really worked. The batch was dry in a few minutes. So now we have built a vacuum vessel big enough to hold lots of plastic and are attaching an electric air heater to pre-heat the contents of the vessel, and then activate the vacuum pumps to urge the water out of the plastic beads. No dessicant and regeneration involved.

But we still would like a method to quickly/easily/inexpensively measure our results on every batch so we can be confident that the material is "good-to-go" for molding; and hence this enquiry.

Bongo .... We use vacuum for other processes, including maintaining our building's A/C system, so we are aware of the vacuum issues.

Originator ..... I believe they do not float, but regardless, I'd be concerned about the myriads of microscopic bubbles attached to the beads.

Thanks All for helping, we'll see what the costs of the commercial answers are, and if they are exhorbitant (which is what I fear), we may need to pursue the capacitor approach.

Cheers,

Peter (pjv)

steve_b

What if you were to put a measured quantity of beads in a chamber/pipe. If the pipe was fed with air that has a known humidity, dry beads would pull some saturation out...then measure the humidity in the outgoing pipe.
You'd probably want to heat the beads to a specific temperature to aide in the saturation.

Kinda like the oxygen/exhaust sensor on your car. Measure be and after the cat (I think)!

Cheers
STeve

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Steve

"Inside each and every one of us is our one, true authentic swing. Something we was born with. Something that's ours and ours alone. Something that can't be learned... something that's got to be remembered."

Phil Pilgrim (PhiPi)

Peter,

If they don't float, it would be easy to measure their specific gravity without the plunger that I mentioned above. You just need to find a liquid medium for the volume measurement that's not absorbed quickly. Water might even be adequate if the rate of absorption is slow and you can work fast.

-Phil