Web shifting - base paper, misalignment, free span and process
Mar 22, 2016
Web shifting can be caused by the paper or by something mechanically wrong in the web path. The following are some typical causes of sheet shifting.
Web shift caused by base paper
Asymmetric MD tensile stiffness profile
An asymmetric MD tensile stiffness profile causes an unsymmetrical web tension profile. Higher tensile stiffness leads to higher web tension.
The web tension profile indirectly affects the web shift.
Asymmetric web tension profile
The spreading force of a spreader roll is relative to the web tension. If one edge of the sheet has higher tension, the spreading force on that side is higher.
The web will move toward the high tension side.
Web shift caused by misalignments
Unwind and flying splice
The unwind has three different alignments:
primary unwind position
transfer arms when moving
secondary unwind position (transfer arms when stopped against the stoppers)
The effect of unwind alignment can be detected by following the web position before, during and after the transfer arm movement.
If two preceding parent rolls have different web tension profiles, the change in web tension profile is very fast during splicing. This causes fast web shift during splicing.
Paper leading rolls
Misaligned paper leading rolls have three different mechanisms in causing web shift:
uneven web tension
web guiding by bending
web guiding by folding
Fabric leading rolls
Misaligned fabric leading rolls can cause different loop lengths between the front and back sides of the machine. Longer loop length forces the fabric and web to run faster on the longer side.
The difference in the loop length can be detected by inspecting the behavior of the seam. The seam goes in front on the side where the loop length is shorter.
Web shift caused by the process
Asymmetry in the coating heads
Uneven blade loading in the blade coating head has been shown to have a high impact on web shifting. Higher coating color application on an edge requires higher blade loading on that side. Higher coating color application on an edge also causes higher web expansion due to the moisture on that side.
Higher nip load on an edge of a sizer causes higher web speed on that side.
Asymmetry in drying
Asymmetric mass flow in air flotation dryers causes unsymmetrical average air pressure to the web. If the air pressure is higher on one side, the sinusoidal wave on that side has larger amplitude and the web has a longer path on that side. Uneven air pressure can be due to uneven chamber pressure of the nozzles or uneven nozzle gaps.
The effect of air dryers on web shift can be detected by changing the air velocity of the dryers and measuring the web position during the change.
Unsymmetrical drying cylinder temperatures make the cylinders slightly cone-shaped. The web speed is higher on the large diameter end of the dryer can.
Web shift vs. free span
Web shift is typically relative to the third power of the free span (see figure below).
For more information on improving your runnability by avoiding web shifting, contact your Valmet representative.
