Color Trapping is a major aspect in the apparel decorating (screen printing) industry as well as any printing industry for that matter. Most artists supplying designs to screen printers are expected to supply accurate color separations utilizing chokes and spreads (traps). Color Trapping is the spreading, overprinting or under cutting of objects in order to prevent printing inaccuracies such as registration issues. One of the most widely seen issues in the textile screen printing industry is the poor registration where the white underbase (white printer for dark shirt printing) is visible where it should not be visible.
Understanding Color Trapping and the various methods to utilize them will ensure your prints are correct along with ease of setup and in many cases will speed up the overall production.
Trapping and Overprinting can be set in multiple ways within CorelDRAW; directly in your document with each object, on the Separations Tab in the Document Overprints dropdown list or by using the Automatic Trapping option in the print dialog. See your CorelDRAW User Guide for more information on Color Trapping.
The basic methods for Trapping are Overprint, Choke and Spread. Note: Spread is generally described as Trapping in the apparel decorating industry.
Manually setting Traps on objects when designing can ensure an accurate separation for your screen printer. Sometimes you may forget to add traps when designing and a global choke and spread size may not work due to varying sized elements in your design. This is when manual color separations and trapping are needed. Small type with serifs may not allow a .5 choke while other elements in the design warrant a .75 choke or trap.
Fig 3 shows a side view of a Butt registered element where the colors or objects are exactly the same on top of each other. The Spread or Trap shows where the top color has an outline to make the top color slightly larger than the bottom or underlying color and a Choke where the underlying color has a white outline to make it slightly smaller than the top color or object.
The following steps will show you how to apply Chokes and Traps to a manual separation in CorelDRAW. In this design, we will be adding Chokes and Traps to a simple spot color design that will be printed on dark t-shirts where a white underbase (white printer) is needed yet NO white is supposed to show in the print.
Create your design and size it to final print size.
Add registrations marks and labels (ink colors)
Duplicate your design for each color to be printed or copy and paste to a new page for each color to be printed.
Convert each color plate to Black & White or Grayscale if halftones are used in the design.
In some instances, you may only be able to apply one or the other, Choke or Trap, due to the object properties or how the objects are stacked in the design. Practice, printing equipment and the inks used will also play a role in dictating which method is used or if both Chokes and Traps are warranted. Basically, some R&D and trial and error will turn anyone into a good color separator.
In the diagram below, it shows the outlines in the color of the actual plate. For the manual separations you would use black or white outlines for your Traps or Chokes.
Print your separations to film knowing the final print will look as expected and there should be no issues with registration.