posted January 23, 2005 06:55 AM            

The machine was purchased 2nd hand and on purchase the screw and barrel was upsized from 115mm to 130mm. There had to be some compensation made because the barrel remained the same length but the screw designer assures me that the screw is generating enough shear. Just some hand written notes I have from a conversation with the screw designer last night (he is in USA and I am in Ireland)show there is a volume compression of 2.5 and a linear compression of 2.7 and a 4.2 /1 compression ratio. Again I was just noting as he spoke so (and I am not an expert on screw design) so forgive me if these make no sense. But I can firm up this data when the working week begins.

The only improvement I could make was by fitting a mixing nozzle (to generate more shear) and by increasing the barrel heats. But this just reduces the reject level it does not eliminate it. Barrel heats now are around the 300 degree centigrade mark (I started at about 265c). The screw speed is about 80rpm.

One other area I looked at was decompression. At the end of the screw recovery the residual pressure can drive the screw back about 6 to 8 mm. I have reduced this by introducing a slight sprue break (carriage back) at the end of screw recovery but it is not 100% constant and by tapering off the back pressure near the end of recovery.

Occaisonally the first hour of production is fine but then the problem re-appears, (the dryer is holding over 3 hours of material, based on current throughput).

posted January 23, 2005 08:46 AM               

When I hear of streaks and splay moisture problems automatically come to mind. I would first make sure the dryer's performance is up to speed. You could test this by taking the dryer output from a machine that you know works, and feed the current dryer with that material. If the problem goes away and stays away, you know the problem source is the current dryer.

If that does not work then I would look at the potential of overworking Noryl. I have experienced where some grades of Noryl will degrade and produce gas when overstressed. We can discuss that further if the dryer proves not to be the source.

Tom Cunningham
www.ExtrusionTechnicalServices.com

posted January 23, 2005 12:22 PM               

Sometimes the issue is the injector nozzel and the injection speed. A small nozzel and high injection speed can work the material to degradation.

If you work the material before the nozzel harder (with the likelyhood that you will raise the temperature of the melt) you will lower the viscosity of the material entering the nozzel. Lower viscosity at the same shear rate will result in lower stress and potentially less degradation. As you can see there may be some optimum point for the least degradation, but in my opinion you are making the wrong play.

I would compare the following from each machine used; nozzel lengths and diameters, melt temperature, and injection rate (cc/sec). Get the melt temperatures using air shots at a very slow injection rate. This will give a better indication of the melt temperature being generated by the screw as opposed to any nozzel heating. Then you could also test at full injection rate to see any heating effects.

It would be possible to run simulations of the screw and the nozzel performance. Simulations help to see inside the process and detect problems and patterns. Let me know if you are interested.

Tom C
www.ExtrusionTechnicalServices.com