This KB article describes how one can avoid banding in gradients (a.k.a vignettes) when making proofs with Pack Proof.
Banding can occur due to various reasons but one important reason is related to the fact that 8-bit data is generated for proofing. The amount of steps/tints available to proof a gradient going from 0% to 100% is limited to 255 for 8-bit digital files.
That's why small "jumps" can occur in case of large gradients. This is what is causing the vertical/horizontal lines in a gradient (i.e., banding).
To avoid the banding, add noise to the input file by using the Automation Engine Export to PDF ticket:
Select the input file (Adobe® or Normalized PDF).
Open the Export to PDF ticket.
Click the Rasterize tab:
Rasterization resolution: the printing resolution (use the maximum in case of an asymmetric resolution).
Enable the Convert vignettes to image option.
Vignette noise: 5%.
Save these settings as a custom ticket and add this ticket before the Proof for Contract Approval ticket.
When above described workaround doesn't solve the banding, then it might be related to any of the following:
Badly created custom EPL files: please make sure to use an Esko EPL!