Converting Magazine

Eldridge M. Mount

Coextruding films without defects
April 14, 2008

When we coextrude films, it is necessary to have careful control of the melt rheology of each layer. If we don’t, then we are prone to get a melt disturbance. Now melt disturbance is a rich word and will mean many things to many people so it will be important to decide what we mean when we say melt disturbance. Personally, I recognize three melt disturbances which I call a melt disturbance of the first kind, the second kind and the third kind. So far, I have not found a melt disturbance of the fourth kind. I thought I was close once, but I am still at only three kinds. My nomenclature comes from the three principal process sources I have found to date. For indeed, there are many things which can distort a film or sheet from pinner’s bubbles to die lines to insects, etc. But here I will focus on the three process-driven ones. 

A melt disturbance of the first kind is generally due to increasing line speed, a good thing, until a large area of poor film or sheet is formed. There is a general distortion in the sheet which comes and goes and perhaps appears randomly across the sheet. It may also be associated with a melt pressure and/or melt temperature variation. Sometimes, increasing barrel temperatures might help but sooner or later it will not go away without lowering screw speed. This is due to poor melt quality when the screw is no longer able to completely melt the polymer and produce a homogeneous feed to the die. It can be corrected only by a change in screw design.

In the melt disturbance of the second kind, there suddenly appears a rough and textured melt which appears like apple sauce spread in lanes and may spread across the whole sheet. This may occur at start up from a cold shutdown, an outage or during normal production. Everything had been fine until this point, and then bang everything is bad. The apple sauce appearance will not go away in spite of any process changes made. Usually this is caused by an overriding or underriding temperature zone. It will be traced back to a burned out heater, a misplaced thermocouple or a loose heater along the melt pipe. It can also be caused by a very long melt pipe temperature zone with multiple heaters and a single thermocouple in the middle. What happens is a large temperature profile is induced in the melt pipe, and when the profile of viscosities enters the die and feed block, there is a big flow rearrangement. You will have to find and fix the heater problem.

In the first two cases something was “broken,” the screw was not functioning or a heating zone was broken. So the first thing to do when you have a “disturbed melt” is to go out to the line and make sure everything is working as it should. Check extruder performance and all the heaters. If you do this and you still have a “melt disturbance,” then you likely have a melt disturbance of the third kind.

A melt disturbance of the third kind is due to a mismatch in layer viscosity such that one of the outer layers is now higher viscosity than an inner layer. It is a rule of nature that the outer flow layers have to be lower viscosity than the inner layers, and if not then the layers will rearrange (I will say more as to why in a later blog) and this can come from an increase in line speed, an increase in reclaim level, a change in resin supplier or a new set of extrusion temperatures, etc. It appears as a series of waves across the sheet, as chevron defects in the sheet. At its earliest stage it may cause a visual defect where the see-through of the film is degraded (Figure 1). 



Figure 1

To fix this, you will have to find the source of the viscosity mismatch and correct it. Generally, I try to keep the core melt as cold as practical while heating the outer layers some. This can be done with barrel temperatures for extruders of 4.5 in. (115 mm) and perhaps for 6 in. (150 mm) but at 8 in. (200 mm) and above, this is much harder to accomplish as melt temperature is controlled by screw design. If you are unable to lower core melt temperature, then you have to increase core viscosity by removing reclaim (by not being allowed to do for long periods) or changing polymer molecular weight. Alternatively, you can increase skin temperatures or lower molecular weight of the skin. Melt temperature increases in the skin will be effective up to a point. Then depending on the skin resin and its purpose, you may have several constraints, first is oxidation and carbon buildup in the skin extruders. If it is a heat-sealing layer, then you may see an increase in minimum (initiation) seal temperature and or a decrease in hot tack.

Also in today’s world of high-efficiency catalyst technology for polyolefins, the melt viscosity may not respond greatly to melt temperature changes (I will also discuss more in a future posting), and you are left with changing the materials or changing the internal dimensions of the die.

Posted by Eldridge M. Mount on April 14, 2008 | Comments (0)