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Important considerations in Blown film production:
Frost Line Height (FLH): This the distance between the die and the zone where
bubble solidifies to its final diameter. This depends on rate of cooling from cooling
ring. It indicates the point till which the stretching on bubble in both directions is
possible beyond which the plastic solidifies and no further changes possible.
Blow up Ratio (BUR): It is ratio of the diameter of a blown film bubble (at its
largest point) to the diameter of the extrusion die it comes out of. The higher the
BUR, the larger the bubble diameter is in relation to the die. E.g. a 32” bubble from
a 16” die: 32”/16” = 2.0 expressed as 2.0:1
Blow Up Ratio (BUR) = Bubble Diameter / Die Diameter
A blow-up ratio greater than 1 indicates the bubble has been blown to a diameter
greater than that of the die orifice. The film has been thinned and possesses an
orientation in the transverse direction (TD). Minimum BUR should be 2.
BUR indicates the increase in the bubble diameter over the die diameter. The die
gap divided by the BUR indicates the theoretical thickness of the melt after
reduction by blowing. However a more practical formula is:
BUR = 0.637 x Layflat Width / Die Diameter
Draw Down Ratio (DDR): The final thickness reduction in the melt after blowing is
indicated by a drawdown ratio. In other words is the ratio of the thickness emerging
out of die to its final thickness after being formed.
Drawdown Ratio (DDR) = Width of Die Gap / Film Thickness x BUR
A drawdown ratio greater than 1 indicates that the melt has been pulled away from
the die faster than it issued from the die. The film has been thinned and possesses
an orientation in the machine direction (MD).
A third ratio, called the blow ratio (BR), is the increase of layflat width over die
diameter. BR is used less frequently as BUR is more common.
BUR affects the orientation of the polymer molecules. Running a high BUR, the film
is stretched or pulled in the cross direction to reach the ultimate diameter of the
bubble. At the same time, it is being pulled in the machine direction by the
movement of the downstream equipment (Nip rollers). By pulling the film in two
directions, the molecules become oriented in both directions. This gives a better
balance in properties from MD to TD so these films are typically tougher and have
more balanced shrink properties.
In practice these numbers are only approximate because the melt swells as it leaves
the die gap. The above calculations are made using the die gap dimension because
the degree of swell varies with the plastic material used and processing conditions
CO EXTRUSIONS: Co-extrusion is the process of extruding two or multiple layers
of materials through a single extrusion die with two or more orifices arranged so that
the extrudates merge and weld together into a laminar structure before cooling. This
type of extrusion utilizes two or more extruders to melt and deliver a steady
volumetric throughput of different plastics melts. The layer thicknesses are
controlled by the relative speeds and sizes of the individual extruders delivering the
materials.
The advantage of co-extrusion is that each ply of the laminate imparts a desired

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