Re: Foam extrusion

Well...never done foam, but have 20 years in plastic films.....I imagine you
are running a flat die of some sort, an extruder, some sort of haul off
device (puller).

My first suggestion is to quantify the variation you have in ***
thickness....is it predominantly in the machine direction, or in the
transverse (across the ***). I'm sure you have both, but you can separate
them out by sampling at across the *** at unifgorm intervals in length.
cross web variations are usually related to poor mixing or temperature and
pressure variations across the ***. Linear variations can be caused by
surging in the extruder (poor feeding of material), which would manifest
itself as variations in die head pressure, or a cyclical variation that
corresponds with a heater or coooling fan cycling on and off...most of these
can be sorted by changing the temperature profile in the extruder.

The addition of a melt pump is a sure fire method of getting more consistant
product and higher outputs.....

also preheating of the pellets prior to conveying lowers the internal shear
and allows you to increase output.

You might also have some speed variation in your haul-off device (bad
brushes in a motor can cause variation, as can slippage through a nip due to
poor speed match between the downstream equipment)

Another thing to consider is die swell, die swell is related to the die gap
and land length, it increases linearly with a decrease in land length, but
it cubes with an increase in die gap...so if all else fails you can gain
some improvemnet in output by increasing the land length (new die
required...ouch)

Not knowing all the specifics of your process...I can only make
generalizations.

you can respond to me privately if you wish...

David W. Beard
"Paxpacis" <paci.andrea@xxxxxxxxx> wrote in message
news:1141652876.465654.232070@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Working on a new line that can produce foamed *** based on physical
expansion (N2) i'm looking for any suggestion able to get the line
yield increased. Actual major problems:
- thickness distribution on the ***
- leakage of the coex layer
- type of test for QC

please give me your suggestions
Thanks



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