Polyethylene Blown Film Basics
The goal of this article is to describe specific defects that can appear in tubular blown film lines and to describe probable causes and solutions.
An operator can become so familiar with a given film production line that problems are solved intuitively, but bringing a new line on stream or training new personnel may raise difficulties. Some inherent and half forgotten quirk of the process may be of no consequence under familiar conditions, but can become the unrecognized cause of defects when conditions are changed to process new requirements or accommodate new products.
Polyethylene Blown Film Process Basics
The process of producing polyethylene film by extruding molten resin into a continuous tube seems extremely simple. The elements of the process include the resin pellets which are fed through a hopper into extruders. There, heat and friction convert the pellets to a melt which is forced through a ring-shaped die to form a tube.
The tube is inflated to increase its diameter and decrease the polyethylene film gauge. The tube is also drawn away from the die to decrease its gauge.
The tube, also called a “bubble,” is then flattened by collapsing frames and drawn through nip rolls and through idler rolls to a winder which produces the finished rolls of film.
The blown film extrusion system is one of the most complex and sensitive of all plastics processing technologies. The tubular blown film process is efficient and economical, and can produce from a light gauge, clear converter film to heavy gauge construction film, which when slit and opened, may measure 20 meters or more in width.
The formula to calculate the BUR and draw down ratios and their meanings are as follows:
BUR shows 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.
Since it is not easy to use calipers on the bubble to measure its thickness unless you knock it down, a more practical formula is:
The final thickness reduction in the melt after blowing is indicated by a DDR.
A third ratio, called the blow ratio (BR), is the increase of lay flat width over die diameter.
BR is used less frequently, but can easily be confused in conversation with the more common BUR.
A BUR greater than one shows that 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).
A DDR greater than one 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).
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 is different with the resin used and processing conditions.