Intermittent screeching noise during plasticising stage

Hi all,

I'm currently running a clear Polycarbonate material using a Fanuc Roboshot injection moulding machine. For some strange reason an intermittent screeching noise occurs during the plasticizing (metering) stage. No problem during the injection (screw forward) stage, only when rotating back. There doesn't seem to be a cyclical pattern to when this noise occurs, there can be no noise at all on some shots and then the screeching noise occurs. When it does occur it can also vary in loudness.

Barrel heats are setup as per resin manufacturers guidelines. Actual melt temperature of material has been checked and seems ok. I've tried increasing barrel temperatures to see if the material might be a factor but this does not help, the same intermittent screeching occurs.

I'd be interested in hearing some of your thoughts on this, and any help would be appreciated. The screw and barrel are relatively new (1 year), so wear shouldn't be a problem.

Thanks in advance,
DM1977

i bet it is rust between the screw /barrel , just a drop of oil would fix the problem.

I would start by checking the temperature at the feed throat to make sure it's not getting too much cooling. Next you probably should confirm each heater band is working, it is possible for a zone to have a bad heater band or two at the beginning or the end of a zone and still appear to be working correctly because the heater bands nearest to the thermocouple are working correctly. You didn't mention screw speed, if set too fast it will feed material faster then the heat transfer needed to avoid force feeding cold plastic. Objectively, the good melt temperature could be achieved from frictional heat which will cause rapid wear on the screw and barrel and the screeching sounds. Hope this helps.

Does the press have a high torque screw plasticizing settings? If so I would turn it on. I would see if you can see the actual screw drive pressure/energy while plasticizing. I would bet that the melt has cold pellets which is the source of the noise as the shear heating make the slugs melt.
I have only seen this when running very fast cycles or clear resins like acrylic/polycarbonate. UV and FR additives tend to mitigate raising the barrel heats too high due to degradation. One last possible point: the compression ratio of the screw is low and is causing this as the cold pellets are compressed and melted.
Rick

Thanks guys for your help, it's much appreciated. Just a quick update:

I raised the feed throat temperature by about 20 degrees, it seemed to have a positive effect at the start but the intermittent screeching returned. Cycle time is 13 seconds. I'm not sure about the compression ratio of the screw at the moment, will have to check it out later this week.

Screw torque is running consistently between 48% to 50%. No correlation apparent between the screeching and slightly increased torque. There doesn't appear to be a high torque setting on the machine. Screw speed is set to achieve a recommended screw surface speed value for PC. Again, will try to check the heater bands later this week.

I'll also make the suggestion to our maintenance guys re the rust possibility.

Thanks again,
DM1977

DM1977,
All very valid points above.
Also try raising the zone temp next to the feed throat - in fact in the feed zone of the screw and see if that gets better.
Let us know.
Suhas