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"Standards" for Dot gain/trap/etc?

Posted on Fri, 7 May 2004 at 15:47



Greetings.

Having come from the world of offset printing on absorptive substrates (paper) I'm used to having industry-accepted standards for things like dot gain, trap and the like. The SWOP and GRACOL standards for sheetfed and web-fed offset are scripture in my experience, but I'm having a hard time finding a similar organization for the screen print industry.

I've perused the SGIA website, and contemplated becoming a member, but no mention of such an organization exists anywhere in the litterature I've seen. (Even when studying printing in college)

Does such an organization exist? Are the standards (if in existance) accepted by industry?

Surely the entire industry can't be stumbling around in the dark without guidance from a "higher power."

-Brian Dieckman
Statistical Quality Services
Digital Audio Disc Corporation

Location of Opportunity or Item

Comments

Anonymous says: Greetings Brian, To reply to your question about stumbling in the dark is difficult, for the most part the SGIA is the higher power of our industry. The SGIA has the ability to guide and fill in the blanks ...

Greetings Brian,
To reply to your question about stumbling in the dark is difficult, for the most part the SGIA is the higher power of our industry. The SGIA has the ability to guide and fill in the blanks with the SPTF as it's technical out reach. The information your talking about is not commonly discussed, most screen printers do not measure trap of the overprint and dot gain is quite varing between press to press not to mention from emulsion to emulsion.
Speaking personally my unadjusted films print with the 1/4 tone haveing loss, the mids not haveing enough gain and the 3/4 have to much gain. The overprints I measure with my X-rite and find that depending on which sequence the c,m,y is run the trap numbers come between 77-89, which by my understanding of offset trap those numbers are way out of line.
Contemplating about joining the SGIA is a simple decision. I am a member thru my employer and the SGIA allows me to network with some of the most experiance people in the industry who openly talk about these questions, it is a give and take relationship. More and more this industry is matureing with people looking at screen as a science first then as a art.
I look forward to seeing your reply and communicating further in depth.
-Ken Ferguson

posted on: Fri, 05/07/2004 - 10:04pm
Anonymous says: Pur printing process is as "science" as we can get. Our operators (over a hundred of them) are not trained as traditional printers, more like machine oprerators. As a result we have "engineered out" as ...

Pur printing process is as "science" as we can get. Our operators (over a hundred of them) are not trained as traditional printers, more like machine oprerators. As a result we have "engineered out" as much of the art as we can. What printing or color theory knowledge they have has come from experience at other jobs or they've just been around so long that it's second nature to them. We've only just started formal training of operators in the theory of printing.

We have standard setups per image type in both screenmaking and at the printing press. Screen mesh opening and angle are determined by image properties, as are off-contact, squeegee pressures and squeegee durometer on press. In general the sytem works well. It's only when images come to us as 4-color process do our problems arise.

I've not been with the company for very long, so much of what I speak are assumptions, sometimes based on observations, sometimes direct information from the engineers. What I do know is this; during some kind of TQM push dot gain in the process was addressed and since no "standards" were published, (like the SWOP standard) the engineers decided that a "linear" dot gain profile was the best. As a result, our halfotnes are sometimes washed out, and in order to match our proofs (which are SWOP) we have to wack our presses out and go outside that "standard setup" causing repeatability problems. (and customer returns)

My opinion of the whole mess is a little low at this point, and I hope to improve it by bringing some "new" ideas to the table. Knowing that there is no true industry standard helps quite a bit. We can stop striving for that "holy grail" of standars and start doing what makes sense in our process.

As a side note, I'm the only person in the plant with a college degree in printing. Not that the degree qualifies my position or verifies my opinion, but I'm noticing some real lacking in the basics of print that have lead to (in my opinion) poor decisions like this one. For instance; the screen room is currently "under exposing" the stencil as a means of dot gain control. It's best, of course, to find the exposure level that fully cures the emulsion layer without introducing undue gain or loss. It's my opinion that dot gain control should happen on the front end in the RIPs.

If the company is not a member of SGIA, I will request a membership this week. My involvement (and experience) are primarily in offset printing and my focus is presently in this arena. I have a feeling, however, that I will be asked to move into the screen print process very soon.

Thank you, Ken for your reply.

posted on: Mon, 05/10/2004 - 9:49am
Anonymous says: Hi Brian, Your situation sounds like the norm in the industry. The lack of education of the printers brings frustrations to a head when things go wrong. You are well educated in the printing ...

Hi Brian,

Your situation sounds like the norm in the industry. The lack of education of the printers brings frustrations to a head when things go wrong. You are well educated in the printing industry. So you should not have a hard time fixing the problems that you have .The SGIA site is a great place to start to improve your overall knowledge of the screen-printing process. There are so many different types of substrates that we as screen printers print on that a "standard" that can be followed is hard to come by. The standard for a billboard printer in their set-ups will most definitely be different that for a C.D printer. The key is consistency, and an absolute understanding of the fundamental variables involved I the process. Once the fundamentals are mastered, you can apply the knowledge to any type of screen-printing. It sounds like you are making the same mistakes allot of printers are... you are not matching the proof to the print. If you have a match print, make sure that the proof has colour bars on it, just like an offset one, (10,25,50,75,90) and a patch for the solid densities that you can read with a densitometer. Your typical screen print gain with linear film will read some thing like this / 10=7, 25=23, 50 =52, 75=86, 90=96.the linear film that you print with will never match a proof if you are not hitting the same densities, ink hue, and gain as the SWOP standard proofs that you are trying to match to. You have to curve your film to replicate the gain that you would get in the offset world (10=18,25=38,50=70,75=85,90=95). But in order to do that you have to print a piece of linear film, with the tonal ranges from 1 to 100 % with whatever line counts that you use and every substrate that you use, with every mesh count that you use. When you set-up your presses to print these tests, make sure that you are not using excessive pressure to get the image to print. Check the dot with a loupe to make sure that you are not getting to much dot loss/ gain. It also helps to have a band of 50% tone around the entire image to make sure that you are printing with even pressure by checking the tone with your densitometer .try to make the 50% print at about a 68 % tone +- 2% all around the print After you have printed the linear film all you have to do is take a look at where the target gain that you want printed on each of your mesh count /line count/substrate type combinations So… if your 18% gain that you wanted to mach your proof to, printed in the 16% area on you linear film, you would make the 10%=16% on your curved film to hit the target. Your SWOP standard can then be still used as your SWOP standard. And each time you set-up your presses you should be able to hit the numbers that you want to match the proof .you should be able to come close to colour matching every time, with in about a 3%+- range. Your presses should have no problem adjusting for that amount of play, with little down time and fiddling about.
One thing that you need to remember though, is that you have to make your art, screens, ink, set-ups, etc… the same every time Consistency in your process is a must or you will fail.
Do you have any quality control devices or testing equipment in your pre-press areas (art room, screen department) and your press floor? To do top quality 4-colour process work, you might need to make purchases of some of the testing equipment that is out there. With out it consistency is very hard to achieve.

Here is a list of thing that you might need
Transmission densitometer
Reflection densitometer
Moisture meter
, Rz meter
Thickness gauge
Newton tension meter
Off contact gauge
Mesh counter
Exposure test kit
X60 loupe

I hope this helps a bit more. Never under expose your stencils. Kick some butt in the screen department!! Make the adjustment in the artwork to hold detail like you said. Under-exposing will cause more gain in the shadows due to the emulsion not having a good foot hold of the mesh and could cause the dots to fall off the screen. Also it will limit the life of the stencil in regards to its solvent, moisture and abrasion resistance.

Well I could go on … but I think that is enough for one night.

I hope this will help.

Just remember,” you can’t make chicken soup, out of chicken crap” LOL.

Best regards, Peter Duff.

posted on: Mon, 05/10/2004 - 10:57pm
Anonymous says: For Our screen process, the only real "tool" we use is a loupe. Our offset machines, however, use X-Rite 520's to take L*a*b and Density readings for quality control logs. We could start measuring solid ...

For Our screen process, the only real "tool" we use is a loupe. Our offset machines, however, use X-Rite 520's to take L*a*b and Density readings for quality control logs. We could start measuring solid ink density or even halftone density if the digital file were available.

Our position is not so different, I imagine, from the remainder of the industry. In fact I'm certain we've got it easier than most. Our substrate varies in thickness less than .1 mm from job to job, and there is really only one surface profile to speak of. (polycarbonate or polyenamel topcoat) The real challenge comes in the form of customer files. We still have customers who insist on sending film, of course. Not to mention all the re-orders from 20 years ago when this all got started. (You wouldn't believe how often we print "Born in the USA" by the Boss.)

So looking at what we have do deal with (as an industry) I'm not surprised that no widely accepted standard exists. Even within a limited range of products such as our situation, it's unlikely we could satisfy every customer with a single way of doing things.

Treading in the screenmaking waters is a dangerous ordeal here. A particularly strong personality "runs" the area. The mentality in this facility is that since we did it all ourselves, we must be doing it right. (Our facility produced the first CD in the U.S. when only asian and european equipment was available. Strange that a country with as many resources as the U.S. still falls behind in most areas of the printing industry)

Our graphics area does not presently work closely enough with printing. They send us a proof and a digital file to burn to plate (in offset) and tell us to match it. Since (They think) their proof is "SWOP" we should match it. Of course that's not the way to go, but that's the way it's always been done. A similar situation exists in screen printing. In the case of customer supplied film, or customer supplied proofs, we have the flexibility to "run as is" but that doesn't help consistency. In the case of digital file, we soft proof with an iMac and the results are usualy very good. The extent of our "measurement" is an A/B comparison of proof to disc.

As far as your list goes, we have all of the tools you list available. Most of them are out of the hands of the operators, however. It's going to be an uphil battle, but what isnt?!

-Brian

posted on: Tue, 05/11/2004 - 11:12am
Anonymous says: Hi Brian, With the offset SWOP, the gains are quite similar from press to press as well as one manufacture of plates to another. As you are aware most proofs are med.-high gain proofs in offset. So now ...

Hi Brian,
With the offset SWOP, the gains are quite similar from press to press as well as one manufacture of plates to another. As you are aware most proofs are med.-high gain proofs in offset. So now lets take it to the next step. We can make screens quite repeatable with the mesh and the emulsion( almost as accurate as the Kodak Gold plates ) so we have one variable we can reasonablely depend on.
Now it's time to foot print the press and compensate for the excess gain in the 3/4 tone and open it up in the 1/4 tone. Standardising the process is a group effort lots of communication and learning about each others limitations.Unfortunatly strong personalities are always around, if management want to proceed they need to remind the players that there are NO soloiests in the symphony. As you say it is a uphill battle, but remember were all here and most of us have gone thru the same battle and made it, you can do it as well.

- Ken Ferguson

posted on: Thu, 05/13/2004 - 2:03am
Anonymous says: BD, Visit ISO online at www.iso.org and look up the following published standards: ISO 2846-4:2000 ISO 12647-5:2001 ISO 12637-5:2001 posted on: Tue, 06/08/2004 - 2:40pm
Anonymous says: Thank you, Ben. I'll order these straight away. posted on: Fri, 06/25/2004 - 11:08am
Anonymous says: Hi Peter, Your e-mail is fundamental. Just let me ask you several quetions as I am not sure to got the hole correctly. Do you mean that in order to make a match to offset proof you must adopt offset curves ...

Hi Peter,

Your e-mail is fundamental. Just let me ask you several quetions as I am not sure to got the hole correctly. Do you mean that in order to make a match to offset proof you must adopt offset curves and add to them the screen deviations in order to get correct separation. It means if you have to input % 10=18-3;25=38-2;50=70+2;75=85+11!;90=95+6! After screen printing at optimal conditions you must compare prints with proof in order to adjust ink color densities (change color concentration). Next step should be to come back to curves to fix any deviations in gray.

Valentin

posted on: Wed, 08/04/2004 - 9:45am
Anonymous says: HI Valentin, I had a bit of trouble understanding your email. Your translations were not that clear on there meaning. You must be from some where in the U.S.S.R or near to that. But ...

HI Valentin,

I had a bit of trouble understanding your email. Your translations were not that clear on there meaning. You must be from some where in the U.S.S.R or near to that. But I will try to make an effort to answer your questions as best I can. To make it simple, you have to match the offset print in its gain characteristics to match colour. An offset print will match an analog proof with linear film (1%=1% to 99%=99%). They will both have the same gain characteristics 10=18, 25=38, 50=68, 75=85 90=95. In screen-printing we have to mimic the gain of the offset world to match colour of offset prints. We do this by curving the film that we out put .If we were to print with the 10=10,25= 25, 50=50,75=75, and 90=90 our prints would be washed out looking. WE NEED GAIN to make it look rich and full. We don’t want excessive gain on the press though. We want to print as clean as we can and make good crisp prints. All we want to do is curve the film to give us the same gain as offset on press when we print. Print a one hundred step (1% to 100 %solid) test. Find out what tonal ranges give you the numbers you need to match the offset gain.

Here is an example

%Linear film 10 25 50 75 90

Offset gain 18 38 68 85 95

Curved film 16 30 56 74 88

Screen print with curved film,

18 38 68 85 95

Typical screen print densities, C= 1.30 M= 1.35 Y= .90 K= 1.65

Grey balance is achieved by having the densities in the correct ratio to one another within the CMY colours. Over print these three colours and see if they make a black. If they don’t, find out what colour is most dominant and then neutralize it with the other colour to make a grey. Document the densities at this point, and this will be your standard density in proportion to each other that you will want to hit to achieve grey balance. Don’t adjust density on press to match colour. It will only help in the ¾ tones. Adjust gain to hit the colour you want.

Hope this helps.

Best regards, Peter Duff.

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