Screw Binding?

[molding]

Hello all,

I am currently a little confused and out of ideas. I was currently processing a new job in with some Xenoy 5720 material, not the first time; only in this particular molding machine. After about 3-5 shots my screw will begin to bind up and not want to turn, works the pumps excessively and the extruder pressure will red line. I have to back the sled off and play around with it to get it to start turning and again to be able to purge it out.

Things I have done:

1. Verified all heater bands are working properly.
2. Checked the press out for proper hydraulic workings.
3. Moisture content of the material.
4. Ran process temperatures on the high side of the recommended temperatures (525 F).

I run this press all the time and never have a problem, only with this material.

Is it possible my screw could be a little worn causing it to bind with this material? I haven't seen any signs of wear with my other materials I run; like PP, HDPE, TPU, and Noryl. Screw recovery has always been acceptable and seems normal.

The press I'm trying this in is a 1986 1000 Ton Cincinnati with a 165 oz. barrel.

Strange thing is last month I ran this material in a similar machine, only a 500 ton 70 oz., and had absolutely no problem like this. Granted it does have a new barrel and screw. I even processed it on the low side (480 F).

Any input on this would be greatly appreciated.

Thanks
Dave

[Chris]

what kind of back pressure are you running.

[molding]

About 75 psi.

[brentb]

What kind of screw,GP?

KOM

brent

[brentb]

I would think that being worn wouldn't cause the screw to bind?

KOM
brent

[brentb]

Have you tried a little decompress before rotation?

KOM

brent

[molding]

Brent,

It's a gp screw with a longer feed zone according to Cincinnati Milacron, if it's still the same screw when it was built. I believe the previous owner ordered it this way. I think it was for some kind of thermo expansion they used to do with it.

Unfortunately the machine doesn't have pre-decompression before screw rotate.

Dave

[rickbatey]

I think there's two issues. The large screw turns faster due to the large diameter so it feeds down the barrel faster. I think you need to run a reverse heat profile, hotter in rear and cooling toward the front. Last point maybe what's the compression ratio? It may be so high or higher than the other press it causes unmelted pellets to stall the screw. Of course it may have mixing elements on the screw that cause the cold pellets to stall the screw as well. I had an issue not too long ago on an 880t Milacron. Turned out it had a special mixing type screw in it. Had to raise the heats much higher but the screw wasn't too large for molds.
Rick.

[molding]

Rick,

What you are saying makes perfect sense. When it was "stalled" the only way I could get it to turn was to retract the screw way back and vacuum out the material in the feed throat. I will have to check on the compression ratio, I don't recall them telling me that part. They only said it had a 24" feed zone, 4" longer that typical. I'll have to ask them about the compression ratio on Monday and post. It was a relayed conversation from my maintenance man.

What type of heat profile would you recommend? Last try I was at the following:

Nozzle 500
Zone 1 520
Zone 2 520
Zone 3 520
Zone 4 520
Zone 5 520

Material is a Sabic Xenoy 5720. Recommended melt temp is 478-520 per data sheet.

Thanks
Dave

[molding]

Also, do you agree with Brent that a worn screw shouldn't make it stall. This was just something my repair technician told me that he had seen in the past. Seems to me that if it was worn a little that it would turn easier. Maybe I'm looking at it wrong.

Dave

[brentb]

Also, do you agree with Brent that a worn screw shouldn't make it stall. This was just something my repair technician told me that he had seen in the past. Seems to me that if it was worn a little that it would turn easier. Maybe I'm looking at it wrong.

Dave

What Rick says makes sense, as usual. I ran many Cincis with mixing sections, but only for PE fast cycles and liquid color.
Maybe the worn screw could cause unmelted pellets as Rick sauys.

KOM
brent

[rickbatey]

I don't think it's worn since is imagine that would give you more clearance. This press may have a low torque/high speed screw with mixing elements for faster cycles.
As far as the heat profiles goes: try these from back to front and feel free to start lower and raise little by little until it keeps turning. 490, 510, 500, 490, nozzle about 480 or so. Third one might need to be the same as the second and the second at 520-515.
Rick.