Rotation of Die
Rotating the die with or without air ring can help mask errors built into the melt by process faults which cause variations in the polyethylene film thickness, called gauge bands.
By rotating the die and air ring, the gauge bands can be moved around the surface of the film as the bubble is extruded. The bubble itself does not rotate. The gauge bands are thus distributed across the face of the roll, level wound as fish line on a reel, and the result is a cylindrical roll of film of perfect symmetry.
Without rotation, these faults build up in one place on the roll of film, as fish line on a reel without a level wind. The result is a roll of film with a surface looking like a wood turning for a short thick balustrade.
Rotation can introduce problems of its own in that the gauge bands now gradually move across the face of the collapsing frames. Such action causes the web to move back and forth between the frames and the lay-flat to wander back and forth in the line downstream from the main nip rolls. A web guide is required to finally track the lay-flat in a straight line so the film winds up as a good roll.
Generally, broad gauge bands caused by a draft of air or a heat rise off the front end of the extruder against the melt cannot be rotated because the melt itself is not rotating through this fault. So the roll of film may be tapered concave faces or have convex or as the different thicknesses of film build up upon themselves.
As the bubble or die diameter is increased, so is the transverse speed of gauge bands across the faces of the collapsing frames increased for a given rotational speed. This can cause bubble instability, intermittent wrinkling in the nip rolls and web wander downstream. These problems can be corrected by reducing rotational speed.
One rotation should never take less than the time it takes to build a roll of film. Because the gauge bands will not have had time enough to be uniformly distributed across the entire face of the roll of film.
Many problems occur in blown film extrusion in the hot melt between the frost line and the die and where the tube is collapsed at the main nip rolls. Other sections in this booklet deal more specifically with problems such as uneven rolls, gauge bands, wrinkles, maintaining output, optical and physical and problems and solutions.