how do you hold detail on screens for DC printing?

[jvanick]

I keep hearing that in order to run long DC orders that the screens in some cases have to be exposed for longer.

How are you guys holding detailed halftones in your screens if you're exposing for longer than what you'd normally do for plastisol?

I'm still using a FL unit, so maybe that's most of my problem... but when I 'overexpose' even for a few seconds longer on a photopolymer emulsion, i can't rinse out the screens easily enough and start to blow detail.

What's the secret?

Thanks!

[alan802]

Halftones, flouro bulbs and DC ink is not a good combination.  My secret would be to get another light source because you're fighting a tough battle.  I don't know that you'll be able to get a durable DC screen from a flouro unit while holding fine detail.  The best thing you could do would be try a dual cure emulsion, that will give you a fighting chance.

[kingscreen]

Dual cure emulsion, hardener, and post exposure.

The print below was burned using a fluorescent exposure unit.

[Inkworks]

I don't think longer initial exposure is the right way to go, complete exposure maybe, but you should be trying for that with plastisol too. We block out DC screens with emulsion and expose that after it dries, which gives the washed out stencil a post exposure anyways.

[jvanick]

I think we do pretty good with complete exposure on our plastisol screens...  and always post expose, usually in the sun for 20-30 minutes...

blocking out with a 2nd coat of emulsion sounds interesting.  I assume you're just blocking out the edges, or are you doing a thin coat over everything that's not where your stencil is?

-J

PS.  Still waiting to see how others do with the UV LED units...  would rather not invest in a MH unit if the LED stuff works out well.

[blue moon]

I don't think longer initial exposure is the right way to go, complete exposure maybe, but you should be trying for that with plastisol too. We block out DC screens with emulsion and expose that after it dries, which gives the washed out stencil a post exposure anyways.

this would be my thinking too. You can just leave them out in the sun after they are exposed.

pierre

[Inkworks]

I think we do pretty good with complete exposure on our plastisol screens...  and always post expose, usually in the sun for 20-30 minutes...

blocking out with a 2nd coat of emulsion sounds interesting.  I assume you're just blocking out the edges, or are you doing a thin coat over everything that's not where your stencil is?

-J

PS.  Still waiting to see how others do with the UV LED units...  would rather not invest in a MH unit if the LED stuff works out well.

Edges, pinholes, regi marks if needed. Sometimes a skim coat over the edges of the squeegee path if it's a big run.

[Frog]

This post is more a screen making post in general, but fits here as well

With good films, and a good exposure unit, one should not have a need to adjust exposure times.
A properly exposed screen is a completely exposed screen.

Whether it stems from weak films or laser vellums, or the fear of losing halftones and fine lines, underexposure is and has been the most common error in this step of the process.

As has been pointed out, fluorescent units are not the best for detail, but with a vacuum, will handle most jobs thrown your way.
As has also been pointed out, besides using an appropriate emulsion, post exposing and/or the application of a hardening agent can increase durability as well.

I should note, that it is my understanding that "post development exposing" only has an effect on photopolymer.

[ZooCity]

Complete initial exposure is very helpful to getting a solid DC screen, not necessary longer time under the light. 

With fluoros you may cross this line where complete exposure is not possible without significant undercutting.  If so, split that difference and stop exposure where you still hold the detail.  After resolving, thoroughly post expose from the squeegee side.  The sun will provide more appropriate UV than the fluoros so use the big expo unit in the sky for this where you can.  Harden after post expo.

I'm a big advocate of thoroughly drying stencils at every step of the process.  Keep an eye on this in humid FL. 

And here's a tip I just got- dry out your screens (as in cook 'em in a hot box) one last time after applying hardener, it increases the effect of the hardener by 2-3x. 

[JBLUE]

In order for post exposure to work it must be set to 4 times the original exposure. Even then it only nets you 15% more hardness. Thats per my Kiwo rep and their website. We just let ours sit in the sun for the day after we expose it the first time with hardener.

[Inkworks]

For post exposure we generally do 2 times original time to account for the extra thickness of the emulsion used as block-out, and expose from squeegee side as someone pointed out above.

I think part of the challenge is having a strong enough light source to get a full thickness exposure before you get significant undercutting. We use an 8K MH at 53" or a 3K MH at 20" here

[mimosatexas]

Zoo described the process we use pretty well.  We print lots of discharge and we print runs into the multiple hundreds without breakdown regularly.  For jobs with higher quantities where we are using white we will usually create a second white screen just in case, but rarely need it unless there was an issue with the screen reclaiming prior to the coating/exposure.  The issue never seems to be with exposure.

Our setup/process is:

homemade 26"x38" exposure unit (max exposure area/interior dimensions.
3/8" uncoated low iron glass
12 24" T8 daylight bulbs spaced roughly 3" apart and 8 inches from the bottom side of the glass.

films are waterproof inkjet printed on an epson 1400 or my buddies 60 inch epson, printed with all channels maxed cmyk.  No all black, no rip, etc.

We use Aquasol HV, usually 1/1 with sharp side of the coater for discharge.

It takes 2:15 to expose the screen sufficiently for holding 40lpi halftones on a 160 mesh when soaked and washed out with a 1500 psi pressure washer on the fan setting.  This is our bread and butter screen and we use it for basically everything we do with discharge with white, colors, whatever.  we use higher mesh occasionally for certain things.  We use a much wider variety of screens and coatings for plastisol, and alter other parts of the process as well.

We dry with a fan, then post-expose for 6 minutes with squeegee side down on the same unit, or put it outside for the on a sunny day. 

We then blast it one more time with the pressure washer just to be sure we had no emulsion creep during drying.  Basically a very fine film of emulsion on the squeegee side might "drip" into the stencil here and there while drying initially.  It usually gets pushed through with the first or second test print, but it is easier to just blast out the post exposed screen and towel it down so it dries in 5 minutes under the fan and simply not worry about it.

After dry, we use MS hardener on both sides and let dry.

Our process takes a little longer with the additional pressure wash step, and essentially underexposing initially to compensate for undercutting with the tubes, but it works great and we can do 230 and 280 mesh with 65lpi without much issue.

We don't even have a vacuum top, we use a piece of mdf the size of the interior dimensions of the screen with 1 inch foam covered in black fabric and a a weight or two.  No issues with contact.

[mimosatexas]

and yes it's ghetto.  I want to build my MH unit, but literally havent had the time due to being so busy with actual printing for the last few months.

[mk162]

building a unit is a waste of time, especially for the prices you can find good used units for.  If you can get a 40-1k for about $400-500 you are GOLDEN.  Unless of course you know how to build yours with an integrator.

[ZooCity]

Build the vac frame/glass or find a standalone frame for cheap and buy an Olec AL-53 or similar.  You can find complete lamp/integrator/power setups for cheap, cheap.  I homebrewed a vac frame that ran off a shop vac and later a dedicated vacuum, stuck an olec in the bottom of it and it worked great.