Extrusion problem

Dear Sharma,
I suspect problem is with the cable from melt pressure sensor to plc or amplifire. Is the cable shielded? Is shield connected properly to earth? Try with new shielded cable by earthing cable shield properly. You may have re-route this cable if it is going with some power cables.

Hello Sharma,

I presume the Inoex weight control system you are using does the following:
(a) By gravimetrics, holds a reasonable constant level of pellets in each of the extruder hoppers.
(b) By ultrasonic transducers, determines the thickness of the combined three layers of plastic and trims the line speed accordingly to ensure a minimum of plastic is used.

Please confirm.

If the above is how your system works, then the Inoex system can be ruled out as the phenomenon is only occurring on the inner shield extruder.

The first unusual thing is that the inner shield extruder is 90 mm in size. This indeed seems quite large to me.

Is the inner shield extruder screw turning very slowly and what is the motor speed at that screw RPM? What kind of screw are you using? Is the extruder powered with a static DC drive of standard design?

Proper engineering answers to these questions will give us insight into what is going on.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

Dear Peter J,
Yes the Inoex is working same as you described. But we are measuring the thickness using X-ray. Line speed is not controlled by the thickness. We run machine at constant speed but minimum weight/m is fixed by us, then inoex controls the screw speed to give optimum result. Yes our screw is 90-20D and we run it at moderate speed. (17 to 23rpm)
We are continuously monitoring all parameters related to the extruder to find any changes in parameter related to melt pressure and until now we have not found any parameter out side range.

We have three more CV lines, but this is happening on only one machine. Yes machine has static DC drive.

Hello Sharma,

I suspect your DC motor on the inner shield extruder is running way too slow. Generally speaking, DC drives are 1% regulated drives for extruders because the load is reasonably constant except for some load vaiance due to the plastic pressure pulse created by the screw. Capstans on the other hand usually have 1/10 of 1% DC drives for quick reaction and tight control.

What this means for an extruder is that there is 1% variation between no load and full load at the base motor speed which is often 1750 RPM. Thus between no load and full load, the drive motor can vary by 1% or 17.5 RPM, independent of the actual motor speed.

You should never run below your DC motor during production lower than say 400 RPM and higher than say above 1400 RPM. Call that the proper operating range.

If your motor gearing to your oversize inner shield extruder was say was 10:1 then your motor would be running between 170 RPM and 230 RPM and that is way too slow considering you still have that 17.5 RPM variance between no load and full load (or proportionally as the load varies with the plastic pressure pulse created by the screw). You don't have to be Einstein to realize that the motor speed error can therefore be quite large when compared to the very low speed of the motor. The relatively slow screw speed itself may also create plastic slugging inside the extruder thereby adding another change in the load.

Likewise, short term trimming of the low motor speed by your Inoex system may even create more havoc.

The starting spot is to ensure your motor speed is at least 400 RPM or faster (say around 600 RPM) for the slowest (17 RPM) screw speed.

Let us know what you find out.

Kindest regards,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com

The gear system on extruder has reduction ratio 30. The normal melt pressure remains around 450bar. But some times it can touch down to 270bar but this remains only for 1 to 4 second. Recently I changed pressure transducer and prob. The problem is still there. I closely monitored the change in screw speed, motor current, and throughput of extruder but nothing strange found. All values look normal and constant. We normally run our lines for 7 to 15 days in a single run. Some time this happen after 4 days of run and rest of the run the melt pressure will remain constant and some time melt pressure drop 3 times in a single day. Mean it is not happening after a fixed interval of time and frequency.
Some time back I tried to relate it with material, but it was happening only on one machine. Other machines have shown no sign with any material. I was thinking that it was happening due to release of deposited material from breaker plate. But it is not true. Then my suspicion gone on deform conductor but again I found I was wrong.

Hello Again Sharma,

Now it seems that you are giving us new information and this is what makes it so hard for us.

Are you now saying that this phenomenon only starts to occur sporadically or randomly after 4 days of normal continuous running?

Are there any other surprises we should be aware of?? Please advise. Thank you.

There are few mechanical things that could give you an almost instaneous drop (and recovery) of extruder pressure.

If, these are just random events that can occur anytime there are a couple of electrical things I would check.

- Perhaps there is a weakness or problem with the static drive system? Presumably you have a proper isolation transformer (not an auto-transformer) on the drive system and that your supply voltage is clean. Presumably the drive system is fan cooled and that the cooling system is working properly.

- Perhaps this is an early indication of a DC motor problem. The next time the there is a change over, have the electricians check and clean the motor and the cooling fan filters. Perhaps the commutator could do with a cleaning (stone) as well.

Sincerely,
Peter J. Stewart-Hay
Principal
Stewart-Hay Associates
www.Stewart-Hay.com