foam LDPE extrusion surge problem

[shawn8791]

We are trying to run a solid foam LDPE cord extrusion using a 4-strand die and sizer. The part is 5/32 diameter and the foam is medium density made from a chemical foaming agent in a single screw extruder. We had no problems running the sample for our customer and now that we are running production we can't get a consistant part. The part runs normally (for 2-3 minutes) between .153-.159 on diameter deviation, but every 3.5 - 4.0 minutes the size will cycle from .130 to .170 (starting with .130 for 20 seconds then .156 for several minutes, then .170 for 20 seconds, then the cycle starts back over). We are running 20 RPM on the extruder screw and 70 fpm line speed. You can also see the foam transition line change as the part's size drastically changes. We have tried many varieties of temperature profiles with no change (better or worse). We have a good tested gravimetric blender installed. All the heaters and thermocouples have been checked along with puller and extruder speeds. Nothing seems to corrospond with the variance. Any help would be greatly appreciated.

 

[ornerynorsk]

You say you've checked the heaters and thermocouples, have you verified consistency of temps with a laser thermometer?

Is your blowing or foaming agent an admix, or are you buying blended feedstock?

Just a couple things that came to mind.

There are a couple of regular users of this forum that are exceptionally knowledgable in the field of plastics, and I suspect that they will have far better suggestions to add to this discussion.

Good luck.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[shawn8791]

I have checked thermocouples and verified with another thermocouple and readout. We have found that checking with an IR sensor with barrel heaters is somewhat difficult because of the surrounding heat and reflective material.

The blowing agent is a masterbatch that is added in at 1.6% to LDPE resin with a new gravimetric blender mounted above the feed throat of the extruder.

 

[MatthewL]

Shawn,

Check the timing of your production error cycle with the refill times on the gravimetric feeders. Normally (at least for the feeders at my plant) during refill the feeder switches to volumetric until the hopper is refilled, then goes back to gravimetric. Maybe your screw rate is excessively variable for a constant feed rate? Check the blender (is it a screw?) for blockages, angel hair buildup, fines, etc. Just a thought.

Regards,

Matt

Quality, quantity, cost. Pick two.

 

[Pud]

Mix material, 20Kg by hand, 10Kg + blowing agent at at a time in a plastic bag.
Switch off the "good tested gravimetric blender" and try it.

Some blowing agents also have a shelf-life.

H



www.tynevalleyplastics.co.uk

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