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Screenprinting on tshirts generally has a lot of dot gain as the ink spreads and doesn't need trapping, but if you sometimes have gaps between two adjoining colours you can make life easier by adding a trap.
Traps go by a variety of names - overprint, bleed, choke etc. They all involve expanding one colour so that it slightly overlaps it's neighbouring object.
This method only works with objects, not images in Corel. this is common to most Corel versions.

Here is a simple design - Circle & Square. I've drawn the circle with a yellow outline.
I want to avoid the objects separating exactly leaving butt-fit films which leave no room for error.
I've right clicked on the circle and chosen overprint outline.

The outline of an object (by default) is centred on the edge of the object. The effect of this is that the circle colour and the square colour will butt-fit and the outline will sit over the join giving a little wriggle room when printing.

Now if you have the outline the same colour as the object and overprint the outline, the object will expand over it's neighbour. This won't be obvious on the print.

Because it's centred on the edge a 2pt stroke gives a 1pt trap etc. The size of your trap should depend on how accurate your equipment can print.

Illustrator users have the same tools, it's in Attributes and is 'overprint stroke'.
 

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What about using distress overlays? I'm including links to some pictures of what I recently had to work with. It won't let me attach the pictures so I just uploaded them to my Picasa Web Album.

If I'm printing a light color on dark shirts and I want a white underlay, sometimes we can see a little bit of the white underlay around the distressed image.

https://picasaweb.google.com/lh/photo/shaRo3eGaDqIN_7dCoa_ka3plsc5ALedbcRudQ_8RBM?feat=directlink

When the distressed pattern is a monochrome .TIF where I can choose the fill/outline color to alter the black/white colors. I make the background transparent and the distress pattern the same as the background color (for visual purposes).

https://picasaweb.google.com/lh/photo/GntQmNJaCydaeeahCJG9XK3plsc5ALedbcRudQ_8RBM?feat=directlink

If I go in to the separations tab, turn on "advanced settings" and click on the graphic option for the "overprint" for that color:

https://picasaweb.google.com/lh/photo/4iEhR9C1FPxTV_hgoAJA3a3plsc5ALedbcRudQ_8RBM?feat=directlink

....it doesn't really give me an overprint when I actually output the velum.

https://picasaweb.google.com/lh/photo/9PgCfUOgqadvM46LjBvotK3plsc5ALedbcRudQ_8RBM?feat=directlink

When I turn overprint off and try to print the same thing (to be the white underlay) both of the vellums still line up exactly.


So far my only solution for this has been to auto-trace the distressed pattern to vectorize it, which often freezes my computer for a really long time and it doesn't even come out good enough, or it just crashes Corel. I have a 3.40ghz processor and 8GB of RAM, so I'd think my computer could handle all of that. Vectorizing the distressed pattern gets to have way too much detail. So I'd have to eliminate the detail, making the pattern look "fake", just so I could give it all an outline to manually give it an overprint. If I even get that far. Adding an outline around a vector distressed pattern with tens of thousands of nodes will definitely crash Corel if vectorizing it didn't already.

Any ideas on this? Honestly I've suggested printing without an underlay since the idea is to look faded anyway. Our production area, as well as the customers, always insist on the print colors being brighter so we require a white underlay.

If you had to do an "Athletic Gold" print (like in my pictures) on a darker shirt color (even black), and it needed to look distressed and "worn", would you worry about an underlay? Or would you leave it out to give it more of a faded look? What about if the print color was Red? I've seen Red practically disappear on black without a white underlay.

Thanks!
 
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