Page 30 - The Inside Scoop Archive

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The Inside Scoop on Blown Stretch Film Technology

With the recent introduction of our Infinity PX2 blown hand wrap, I have gotten a lot of questions about the difference
between blown and cast stretch film. Without trying to be overly technical on this subject, I would like to share with you some
insights into the manufacturing process of blown film in general; and in particular as it applies to stretch film.

The blown film process consists of one or more extruders supporting a stream of melted resin into a circular die. The die is in
the form of a ring, and the melt is expanded as air is forced into the extruded tube. This forms a bubble much the same way
that you blew bubbles from a bubble pipe when you were a kid, or a grandparent. This bubble is expanded by the air being blown
inside to a larger diameter, usually about three times the size of the original die ring. The bubble is then collapsed by cold air
from the air ring which is just above the die. This turns the melted plastic back to a solid form. Somewhere around 10 to 50
feet above the die, the bubble is collapsed by an "A" frame, slats, or rollers, to flatten it. Passing through nip rollers that pinch
the film in order to keep the air in the bubble. The edges of the flattened bubble are then trimmed off, and the two sides of
the film seperated. Each side is then sent to a separated winder to be wound into stretch film rolls.

Ok, that is an overview of the blown extrusion process as illustrated in the diagram above. The blown extrusion process lends
itself to making good stretch film in that the bubble process orients the film in both the transverse ( across the web ) and in
the machine direction at the same time. This provides a very good balanced tear resistance, and usually better puncture
resistance. As the resin has to support itself and withstand air pressure, you have to use "harder" LLDPE resins than you can
use in cast stretch films. Blown films are best for applications with low elongation, and where film toughness is desirable. This
process is used widely in the manufacturing of hand wrap stretch film.
Cling in blown stretch film has to be added to the film or the layers would not stick to each on the pallet. Note on the diagram
above that a liquid tackifier is added to the melted resin inside the extruder. Most often, this tackifier is Poly iso-butylene, or
PIB as it is commonly known. PIB is a liquid migrating tackifier, which is blended into the hot melt inside the extruder, and
slowly blooms to the surface of the film. This blooming process can take a number of days, and is usually helped along by placing
the extruded pallets of film in a hot room with an elevated temperature to bring the bloom of PIB to the film surface sooner.

Note on the diagram above, the PIB is shown as a liquid tackifier on the surface of the film. Because this liquid tackifier is on
all film surfaces inside the roll of stretch film, too much pressure on the roll because of winding tension etc, can cause the film
to "telescope". This is where the core is pushed out of the film roll, and the remaining film collapses on itself. I once sold some
blown hand wrap to a distributor, and received a call that the lids of the boxes were being pushed open by the film, and the
cores were coming up out of the box. Telescoping can also occur in cast film when too much tackifier is used, and there is to
much winding pressure on the film roll.

Blown film lines cost a lot less than cast lines, but have much slower output rates. They cannot run very fast because of the

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