posted November 11, 2004 10:46 AM               

As a medical extruder / molder we process many compounded resins and almost all have voids in the pellets. But we have never seen voids in the finished product.

posted November 11, 2004 01:38 PM               

Given that you are running 100% reclaim, I'd guess that the bubble break problem is being caused by gels. Checking the sharpness of your granulator, hangup points in the pelletizing line and extrusion line might help.

Air entrapment generally causes surface defects, and tends to occur on a short L/D extruder running at high RPM's, so to prove a point, you could try running slower than normal and see if the problem disappears.

Steve

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Steve Hodgson

posted November 11, 2004 08:29 PM               

Pellet voids generally do not contain air, but rather likely, low pressure, soluble process related gases. This is not neccesarily true for regrind.

Regarding air passed to the die, if melting is not complete before a pressure peak occurs in the screw, then the air/gas will be trapped and forced out the die.

The point where melting is completed, and where pressure peaks, varies based on screw design, resin, and process conditions.

A short screw being run at high rate is case in point. Melting finishes late, and there is great potential for a high pressure peak at the end of the transition.

My self serving comment; The balance of melting and pressure along the screw can be predicted using extrusion screw simulation. Inquiries are always welcome.

posted November 19, 2004 04:47 PM               

When the pellet comes out of the repelletizing process, it is in the melted form (at the die face). It then cools and contracts (either in air or water, depending on the process). If you visualize the shrinking of the spherical particle, you will see that some of the material is pulled outwards during that contraction, leaving a void (vacuum) in the center of the pellet.

I have never carried out any experiments to confirm this theory.

Because of the thickness of the "outer shell" of the pellet, the diffussion rate would be very very slow, so it would be a long time before the void in the center of the pellet became filled with air.

If this theory is true, when the pellet is melted, the vacuum in the middle of the pellet will have no consequence on the process.

posted November 19, 2004 07:38 PM               

Low pressure and vacuum can be the same thing.

posted November 29, 2004 01:15 PM               

quote:

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Having returned from Thanksgiving, I now find that 50,000 lbs. of re-grind I sent another plant is supposedly wet...not surface moisture but wet "inside " the pellet.

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Have you tried the plate glass trick? That should reveal anything that's in the pellets.

I only get "popping" sounds when I use a purging product called "Aquapurge", moisture in ABS etc(Hydroscopic resins) hasn't made any noise, just shown up as pits and irregular lines on the sheet being extruded.

I take it that your plant is repelletizing the regrind, what equipment are you using?

posted November 30, 2004 08:45 AM               

If you have a fine gram scale you can run a "loss of weight on drying" test on the pellets. From that you can estimate the moisture content. This test is not too reliable at low moisture levels, but at higher levels of moisture it is a good indicator. Typically the pellets weighed first (10-20 grams) and then placed in a vacuum drying oven for and hour. Additionally you can melt the pellets to see if any water (or other gas) is liberated when the pellets are melted. I would keep the temperature of the melted resin as low as possible.