Hello to all,
I am working on a blown film operation but the film thickness is varying a lot and the elongation properties are not up to the required limits. I am using LDPE and the process is 3 layer co extrusion blown film.
Can this problem be generated due to MFI of the materials as we are using two different materials at a MFI of 0.3 and 0.8.
Also what would be the effect of changing the frost line height?
Dear Alyaan,
Request you to provide more details like a) what are other materials in your 3 layers blown film? , b) your target thickness, c) which machine are you using? do you have rotating die ? d) annular gap, frost line, pull up speed, BUR, air in melt temperature, air flow in the ring, even air circulation near the bubble can affect the thickness.
There are several factors responsible for it. You please explain.
Check if the output die temperature is held constant, as in the sopadlor air, maybe the air is cooling your output die and causing the material to cool slightly.
Test outlet temperature increase
Melt flow index of your LDPE is needed, what sort of LDPE has been used in your foil production?
Low-density polyethylene (LDPE) melt index of 0.3 and 0.8 dg/min corresponds to a high molecular weight, which together with the long chain branching of LDPE, should give a high melt strength suitable for extrusion and blown film formation. However, depending on flow rate from the extruder, melt orientation can occur giving die swell (and hence increased thickness) as the melt emerges from the extruder. Sometimes die swell or melt fracture, due to non-laminar flow, can occur in pulses giving uneven film formation.
Separately, high molecular weight results in increased entanglements and hence lower elongation at break of the film, assuming this is what is meant by elongation properties. Increased elongation can be obtained by using a lower molecular weight LDPE assuming that crystallisation is the same.
However, using of LDPE with lower molecular weight (i.e. with higher MFI) can solve your problem.
try first increasing the temperature at the exit die and reduce the air blowing speed. If this does not work, try using MFI 2 LDPE.
hallo,
I have to do an research on the numerical and analytical description for the blown film die swell behaviour and also parameters that affecting the die swell behaviour. i had search for a long time, but unable to find some suitable material to study. May i know anyone can provide me some useful sources? Thank you very much
You are extruding common and simple to extrude LDPE resins. 90+% of the times you have an extrusion problem rather than a polymer problem. Is your melt temp where recommended or you are overshooting it? Is your pressure constant or varying?Are you blending any LLDPE? Is your die well centered?
Is your gauge variation radial? Then centering of dies and air rings or dirty air rings can cause this . In addition dirty dies, cold heating bands etc make one side of the film to come out too hot., as do screw wear. If you have a temperature gun measure the profile around the bubble.
If you have MD gauge variations then most likely you have surging( an extruder problem)
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