posted January 08, 2002 09:28 PM            

Thanks for the suggestion Steve. I will try it out.

posted January 09, 2002 04:45 AM               

With what frequency does the Melt Pump Inlet
pressure change. In most extruders fitted
with a melt pump, the screw speed (RPM) is varied by the gear pump controller, you set
your desired output pressure, the gear pump
motor turns at the required number of RPM
to generate that pressure and then adjusts the screw to keep the pump supplied with material.
Have you had the speed consistancy of the
melt pump drive checked?

The varying output of a single screw
extruder (change in pressure) is known as surging and there are a number of
posibilities:
Screw Design, material being processed (including MBX, fillers, slip additives)
Process Settings (particularly feed zone and
compression-the next zone), changes in feed hopper level,changes in bulk density of feed ( virgin, 50% regrind,virgin etc).

If you run the line slower, does the problem
improve or go away? You can try varying
the settings in feed and compression zones.

If the frequency matches screw RPM, the gauge
is registering the passage of a screw flight
across it (this is normal).

As the melt pump output pressure is constant, I'm assuming the Machine Direction gauge (dimension) of your product is also constant,
is this correct.

Steve H

posted January 09, 2002 10:35 PM            

The melt pump inlet pressure fluctuates inconsistant from 0~120 bar. At 0 and 120 bar the equipment stops. The melt pump speed is consistant at 40 rpm. We have tried running at slow speed an the problem still exist. We have tried varing temperature setting from low to high but the problem still exist.

The extruder is of a 120mm single screw with an ABS screw design. Settings are as below

Barrel (5 zones) - From 195 ~220 C
S/Changer - 210 C
Melt pump temp - 220 C
Pressure before melt pump - 0~120 bar
Pressure after melt pump - 32 bar (consistant)
Screw Speed - 89 rpm
Melt Pump Speed - 40 rpm

The gauge (thickness) is consistant after the T-Die but we there a moisture flow marks on the sheets. We are unable to used the vacuum vent unit as material is flowing out of the venting outlet. We suspect this is due to the back pressure from the gear pump.

posted January 11, 2002 02:40 AM               

You have been running the same material (and grade?) for the last year- the problem has just started recently.

Are you adding any fillers, MBX or other additives to the ABS?- what about regrind.

What thickness material/width are you running? Are you drying it? or relying
on the barrel vent?

I think the problem is melt pump related,
either the M/P control system is faulty.
Or it is incorrectly set- what suction pressure have you got it set at?
Have you checked the calibration of the
suction and discharge pressure gauges.

A worn screw is another posibility, have you pulled the screw and checked it for wear?

32 bar for the discharge pressure seems low, too. What sort of screenchanger and screen pack are you running?