CorelDRAW Trapping Widths for Screen Printing

Trapping is an essential prepress technique used to prevent visible gaps between touching colors caused by press movement, screen stretch, or substrate shifting. In CorelDRAW, setting the correct trap width creates a small overlapping boundary between adjacent colors or between a white underbase and top colors. This guide details the standard trap widths required to protect against misregistration in screen printing across various substrates, along with instructions on how to set them in CorelDRAW.

Recommended Trap Widths by Application

The appropriate trap width depends on your press type, screen mesh tension, and the stability of the substrate being printed.

• Flatstock and Paper (High-Tension Screens):
  • Trap Width: 0.5 pt to 1 pt (0.007 in to 0.014 in / 0.18 mm to 0.35 mm)
  • Paper and rigid substrates do not stretch, allowing tighter tolerances. Use this range on graphic presses with high-tension mesh.
• Textiles on Automatic Presses:
  • Trap Width: 1 pt to 2 pt (0.014 in to 0.028 in / 0.35 mm to 0.7 mm)
  • Standard cotton and polyester apparel printed on well-maintained automatic presses require moderate trapping to absorb standard fabric pull and mechanical vibration.
• Textiles on Manual Presses or Stretchy Fabrics:
  • Trap Width: 2 pt to 3 pt (0.028 in to 0.042 in / 0.7 mm to 1.05 mm)
  • Manual presses introduce operator variation, platen heat flex, and registration shifts. Stretchy materials like spandex, fleece, or ribbed garments also require this wider boundary to prevent white gaps.

White Underbase Choking

When printing on dark garments, the white underbase must be slightly smaller than the top colors (choked) rather than expanded.

• Underbase Choke Width: 1 pt to 1.5 pt (0.014 in to 0.021 in / 0.35 mm to 0.53 mm)
• Choking the underbase guarantees that the top colors completely encapsulate the white layer, preventing the bright underbase from peaking out around the artwork edges if slight shifting occurs.

Rules for Trapping Direction

1. Light into Dark (Spread): Always expand the lighter color into the darker adjacent color. The human eye tracks darker edges; spreading a light color into a dark color keeps the perceived shape of the design intact.
2. Choke Underbase: Always reduce the underbase perimeter so it sits comfortably inside the boundaries of all overlying spot colors.
3. Black Outlines: Black lines naturally trap underlying colors. Ensure black outlines are set to "Overprint" rather than knocked out, eliminating the need for manual spreads beneath them.

How to Apply Trapping in CorelDRAW

CorelDRAW allows precise vector trapping using outlines and the Overprint setting:

1. Select the Object: Choose the lighter vector element that needs to expand into a darker neighbor.
2. Add an Outline: Open the Outline Pen dialog (F12).
3. Specify Width: Set the outline width to twice your desired trap width (for example, use a 2 pt outline to achieve a 1 pt spread, as CorelDRAW centers outlines by default).
4. Match Colors: Set the outline color identical to the object's fill color.
5. Enable Overprint: Right-click the object and select Overprint Outline. This tells the RIP (Raster Image Processor) to print the expanded outline directly over the adjacent color rather than knocking a hole in it.
6. Verify Trapping: Go to View > Enhanced with Overprints or open the Separations preview in the Print dialog to verify that the overlapping edges register properly without knocking out underlying layers.