This is a typical process development issue for extrusion processes.
One efficient approach is through nonlinear modelling. Several kinds
of extrusion processes have been modelled with nonlinear modelling
from a small series of experiments, and then it becomes easy to
calculate the limits of line speed and quality variables, in presence
of constraints on other variables, with a Lumet system. Two Swiss
companies have good experience of this. Foam extrusion is more
interesting because there are more degrees of freedom.
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
My first suggestion is to quantify the variation you have in sheet
thickness....is it predominantly in the machine direction, or in the
transverse (across the sheet). I'm sure you have both, but you can separate
them out by sampling at across the sheet at unifgorm intervals in length.
cross web variations are usually related to poor mixing or temperature and
pressure variations across the sheet. 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
Not knowing all the specifics of your process...I can only make
you can respond to me privately if you wish...
David W. Beard
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