Canadian Plastics

Blown film can’t possibly exist

Let me get this straight: you extrude a thermoplastic such as PE through a circular die, then blow it up like a kid chewing bubble gum. Then you collapse the bubble and drag the solidified tube throug...

April 1, 2002   By Jim Anderton,technical editor



Let me get this straight: you extrude a thermoplastic such as PE through a circular die, then blow it up like a kid chewing bubble gum. Then you collapse the bubble and drag the solidified tube through a set of rollers two or three stories overhead. Riiiiight.

I think that hidden beneath every blown film line there’s a cast extrusion setup feeding the bogus “air ring”. It has to be an elaborate deception; “blown” is too unlikely to be true.

Think your extrusion operation is a pain to control? Consider what goes on with blown film. At the front end, it’s basically extrusion, except that there are issues before the resin even hits the hopper. The film business is outrageously competitive, but trim or scrap material is in the form of light, fluffy film, so even regrinding isn’t a no-brainer.

Then you have to consider a range of additives that profile operators generally don’t use — unique combinations of magic potions like anti-stat, slip agents and anti-block. It’s as if gravimetric load cells were invented just for this process. Then the melt has to take a right angle turn and feed an annular die.

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So far you’re controlling additive and pigment loading, possibly recycled material, and screw operation in a barrel feeding a relatively large tool which must deliver melt at a uniform velocity all the way around the die lips. Add micrometer-level control of the die to the variables list.

Assuming the extrudate emerges in a perfect shell of molten resin, you still have to get it stretched into thin gauge layflat before any second op processing like slitting, printing, and occasionally, making the product into things that people want to buy, like bags.

Add low pressure, high volume air to inflate the bubble, and then worry about fisheye, arrowhead, weld lines, clarity, gauge consistency and anisotropy (the word they taught us to say in University when we mean ‘different’) in physical properties because you’re stretching the semi-solid product in two dimensions using completely different methods. Apparently a clean screen pack helps here too.

Which brings us to haul-off. Cooling the tube fast enough to make it solid before it hits the nip rolls means more blower air, possibly on both sides of the bubble, so add that to the headache. At least the collapsing frame is set-and-forget during the run, but the winder dragging the product off the whole contraption has to be controlled accurately, since variations here cascade through the whole system. Too slow and at best you’re throwing away productivity, while too fast can yank the product out of the cage like Kleenex out of the box, which can result in serious downtime in a truly spectacular way.

So far, this sounds like a nightmare. What impresses me even more is that this process existed before computer control, non-contact gauging and phase-controllable AC drives. Incredible. If you’re a profitable blown film operator, you have both my sympathy and my admiration!


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