Speaking of Underexposure

[squeezee]

I measured the uv density of all the inks, black was the highest OD, then yellow, not very good but you could get a screen out.  Cyan and magenta were indeed, pretty poor.

[RichardGreaves]

Failure of the positive (positive failure), like a bad raincoat, will always encourage screen makers to under expose, because they haven't protected their image area from exposure.

[yorkie]

Just how good does the raincoat need to be?

[DouglasGrigar]

Failure of the positive (positive failure), like a bad raincoat, will always encourage screen makers to under expose, because they haven't protected their image area from exposure.

And poor quality positives (no RIP, poor ink, laser printer vellum) are common and of course underexposure is what Richard - 98 to 99%....

So can we test this idea - lets see.

An SBQ film product (Cap film) this is basically QTX in a cap film (QT film).

The actual and full exposure was tested and used (the Cap film was to eliminate human error).

So full exposure with three positives - one with a RIP good quality film and special inks, one without a RIP and standard inks (with a good quality film same as the RIP), and one positive from a laser printer.

Mist sprayed to soften, then low pressure fan spray from a hand sprayer (not a pressure washer)
Top is laser printer, mid is no rip, and bottom is RIP and inks.


Three min. hand sprayer


One min. fan spray pressure washer 20 inches.


Full two min. pressure washer


Five min. pressure washer

[yorkie]

Isn't there something missing from your screen?

[DouglasGrigar]

Isn't there something missing from your screen?

No, I use an exposure calculator, I abandon the use of the 21 step scale as I found it difficult for my students to use, and often the numbers used for the 21 are underexposed when compared to the exposure calculator (numbers given not tested).

So for example on Ulano’s site...

QTX Ulano’s very popular emulsion

http://www.ulano.com/emulsn/presensitized.htm#qtx

It lists "Est. 305 mesh BL Speed - 180 seconds” NO WAY IN HELL!!!

Was that white mesh, and a prayer? Was that coated using the glisten method or 1/1 with the sharp edge?

And I know where this may go - yes the pre-sensitised emulsion CAN be used with an exposure calculator (quite accurately and well I may add) if someone cannot see the change the problem is best fixed with a visit to the Optometrist. (Of course I am from the KIWO school of thought on this).

I will likely just buy a scale from Richard at Indy so I can test the results of a scale and exposure calculator as I have abandoned the 21 steps for so long I do not know where my scale actually is...

[DouglasGrigar]

Could be we need a clear and agreed on version of what a complete exposure actually is...

I work with this thought, something I picked up when having Dave Dennings help me out.

Full and complete exposure is where the entire layer of emulsion is exposed into a stencil and the entire thickness from face to well side is linked and will not wash off with water (well side where scumming often comes from).

This does not into account any edge thickness inconsistency from the scoop coater edges.

[ZooCity]

Full and complete exposure is where the entire layer of emulsion is exposed into a stencil and the entire thickness from face to well side is linked and will not wash off with water (well side where scumming often comes from).

Then somebody needs to start making truly clear inkjet film for use on the newer generation epsons.  Or an affordable imagesetter that uses clear film.  Or an affordable cts system. With the milky "waterproof" stuff this is unattainable wherever the positive film is in contact with the stencil.  Well, I guess it's attainable if you're willing to wrangle the massive gain from laying down that much ink on your positive I suppose....

[DouglasGrigar]

Full and complete exposure is where the entire layer of emulsion is exposed into a stencil and the entire thickness from face to well side is linked and will not wash off with water (well side where scumming often comes from).

Then somebody needs to start making truly clear inkjet film for use on the newer generation epsons.  Or an affordable imagesetter that uses clear film.  Or an affordable cts system. With the milky "waterproof" stuff this is unattainable wherever the positive film is in contact with the stencil.  Well, I guess it's attainable if you're willing to wrangle the massive gain from laying down that much ink on your positive I suppose....

Do you have to use the “waterproof” stuff - I switched to the hybrid inks and use the clear.

“Affordable” is dependent on your needs and numbers... when we bought our imagesetter we paid over 45k for it and paid for it in less than three months of use (savings only - increased production was less than a month).

[Evo]

I use one of these:




First I lay a piece of the frosted waterproof inkjet film on the exposure glass.

Then I do three-stepped exposures with the calculator - near the top of the screen, the first exposure at 100% of what I think the correct exposure is, then at the middle of the screen at 150%, then near the bottom at 200%.

Since the calculator has 5 steps of neutral density filter, I get approx 15 exposures (with some over-lap) to work with. I cross reference the best of the three and average out the best one.

Since I've laid down a piece of blank inkjet film under all this, the exposures are usually very close.

I typically add about 5% to whatever is calculated. I use diazo or dual cure so I have the extra exposure latitude to work with - the extra 5% never hurts.


With all the above as my procedure I've been able to dial in the exposures pretty quickly and accurately. Now I'm just searching for the perfect water resistant emulsion.

[DouglasGrigar]

Indy, talked to Richard, have a 21 and will start testing and comparing with the KIWO exposure calculator and will report the results when I have time to go over this (possibly the next class).

Part of our problem could very well be the issue with some (not-so) professional “screen printing experts” who are clearly giving out erroneous information (a testament to doing something for 50 plus years wrong continues to be wrong each and every year and never improves). We have some real drama queens and parasites in this industry.

In a sea of ignorance the island of simple competence is a glowing oasis.

[RichardGreaves]

The key use of the Stouffer T2115 is to help judge the cure or hardness (resistance to dissolving) of a stencil - not judging the fineness of line or copy properties of your image. It's a tiny positive that could last a lifetime and should be on EVERY stencil you expose so you can prove to any boss that you exposed screens equally or properly. It's accountability. It costs US$10. What do you have to lose?

Plastisol ink is like salad dressing and doesn't attack under exposed stencils - but water-based and solvent inks will break them down in minutes. Completely cured stencils reclaim easily, underexposed stencils don't. Once you can make a durable stencil, then it's time to graduate to refining your copying skills with an exposure calculator.

I've been fielding screen making questions for the last 30 years and only one question in 100 is about reproductive problems.

I've written and spoken for those thirty years that "Since every exposure unit will choke fine lines and halftones, the best art department understands how to tune/calibrate fine lines so they survive the screen making and printing process. If you want to print a 0.030" line on a shirt, you may have to use a 0.045" line. Art (in their ivory tower air conditioned room sitting on comft chairs), can make it easy for printers & screen makers to do their job, rather than expect screen makers and printers to FIX art with under exposure or printing tweeks and tricks".

[inkman996]

Doug I have been messing with the TLX emulsion I got. I printed out my own eight step step wedge test for it, I wanted to use a step wedge test from my own printer to get more accurate results with my own printer. The problem I am having even after the step test I cannot get a full exposure no matter what time I advance to, remember I am only using a UV exposure unit. Tho i cannot get a full exposure I still get good actually very good stencils all be it it takes twenty minutes, even tho the stencils are coming out really nice I have to fight the scumming and poor wash out. I am going to retire the emulsion because i am convinced its not viable with a UV light source. Hope fully soon we will have a new MH unit in its place?

[ZooCity]

I too use this model Stouffer grayscale strip, Richard told me to start doing it years ago.  With an integrator and other controls in place you don't actually need it on every screen.  The above post really lays it out as to why you would have one of these around. 

To calibrate exposure I actually have a slightly laborious but very effective practice:

• grid out 9 boxes on a fresh screen with a sharpie or blockout pen
• now, using your best estimate pick a low and high ltu value and number the squares with these values
• place the stouffer strip, with a strip of your positive material behind it, a 0-100% strip about the same size (made on your film outputter) as well some text and stroke lines at various points again, from your printer that you use in production, not someone elses
• move this operation from square to square, blocking out the others with some ruby lith or whatever's around and exposing at the light values for each square
• resolve as you normally do and do it as aggressively as usual

Now you've got nine options that, if you had a good estimate of where the emulsion was going to expose properly, will allow you to find that sweet spot between complete stencil exposure through to the squeegee side of the screen and detail. 

If you use your own output device and imitate your device by placing the clear piece of film behind the stouffer strip you're now confident of what %'s will hold on an (almost) fully exposed screen of various mesh/emulsion combinations and can use this information from art generation on down the line. 

A step wedge is faster but this is the most accurate method I've arrived at and gives you full control over the calibration. 

I've mentioned it before but it's literally not possible to "fully" expose some stencils with the milky "waterproof" films and still hold your detail.  So, until some better tech comes out regarding this you need to find that place in the middle and hang out there.  Post exposure, while it should be needless is a real saving grace when using these films. 

Doug, did you say that yer using clear film on an Epson?  If so I really, really want to know how to set this up!

[Evo]

Indy, talked to Richard, have a 21 and will start testing and comparing with the KIWO exposure calculator and will report the results when I have time to go over this (possibly the next class).

Part of our problem could very well be the issue with some (not-so) professional “screen printing experts” who are clearly giving out erroneous information (a testament to doing something for 50 plus years wrong continues to be wrong each and every year and never improves). We have some real drama queens and parasites in this industry.

In a sea of ignorance the island of simple competence is a glowing oasis.

I hope your statements here coming right after my post are a coincidence and not directed my way...