Printing Photos…Halftone Specs?

Depends on what paper is used. In the “old days” 60-65 screen for newsprint. 120-180 screen for letter papers and never higher than 320 screen for coated papers. Alot depends on light and showdows of the original, ink, type of press etc.

Printing photographs (halftones) by letterpress is not easy. A couple of thoughts…..

It is desirable to have a halftone screen ruling which is fine enough so that the human eye cannot easily see the individual halftone dots. To achieve this, it is my recommendation that you use a 110 line screen or above. Regarding the choice of a finer screen, it is true within reason that the finer the screen, the better the halftone will look, but the problem is that the finer the screen is, the harder it is to print.

As far as presses go, it is easier to achieve good results with a cylinder press than with a platen press, because a cylinder press only has to exert the required printing pressure in the small area under the cylinder at any given time, whereas a platen press has to exert enough pressure to print the entire halftone at once, and this is not always possible.

Regarding paper, the smoother the paper, and the better the coating or sizing so the ink doesn’t absorb into it, the more likely you are to achieve the quality you are seeking. The rougher the paper, the more likely you are to get an image where many individual halftone dots will not print because they fall in areas of tiny depressions in the paper, so no ink will transfer. The halftone will then appear “snowy,” if you want to call it that. If the paper is not well coated or sized, more ink will absorb into it, leaving less ink on the surface, so the halftone will appear lighter, as they look in newspapers.

Other halftone specs which would be good to know are the minimum size dot which can be faithfully reproduced on your plate, and the maximum size dot which you can print without having the plate print solid black. For instance, if your file has 1% highlight dots, but your plate can’t reproduce a dot smaller than 3%, then you should adjust your file to have no highlight dots smaller than 3%.

You should really make a file for a gray scale, and make a plate of it and print it, maybe with a few different screen rulings (screen counts) as well, to see what your whole process is capable of reproducing, and then make your actual plates using the info you find from the test.

I print halftones on a Heidelberg Cylinder constantly.

Your paper is one of the biggest factors. As mentioned the highest grade, smoothest surface will give you far better results. I’ve recently been doing comparisons between our standard stock and some samples from another paper vendor. No change in settings on the press and the better papers produce FAR better results.

Ink is another big factor. You really need very stiff. I run the stiffest black ink I can get, though it’s truly an offset ink, and I get much better results from that ink than say from Reflex which is always very thin. Thin ink will be much more likely to “smear” off the face of the dot and into the spaces in between plugging your halftone. This give you a very mottled look especially in the quarter tones. Also I’ve found the best way to set up a halftone is to run almost no ink in the roller train, then increase it very little at a time until it looks dark enough without plugging. Starting off with too much ink in roller train will make you very frustrated very quickly.

Packing/ Make ready is THE BIGGEST factor in printing halftones. The best method I’ve found for what I do is to set my base packing so that the quarter tones print clean with very little drop out of the dots and without a any mottle, or filling in of the open spaces between dots. Then I build make ready for the halftone, three quarter tone and shadows so they all print with out drop outs in the dot pattern. I use a full .001” thick sheet with .001” thick make ready sheets pasted to it which gets placed underneath my base packing. To figure out how to cut my make ready I print the halftone to a piece of mylar laid on top of the paper I’m printing on and dry it with a heater or hair dryer. I then lay my .001” make ready sheets over this on a light table to mark each layer for cutting. As I build my make ready I can lay the mylar sheet over the make ready sheet to double check what I’ve cut. If you search the internet you should be able to find some articles on how to produce make ready for halftones. I know one of the Heidelberg books for operators has an extensive section on it.

As stated line screen will make a big difference in how difficult a halftone is to print. I originally started with 150 line screen, but found that because the quality of stock I’m using is very low it was just to difficult to keep decent quality on. Since I’m printing halftones up to 13” x 21” for posters I wasn’t very concerned about seeing the dots with the naked eye. I actually like the look for the style of artwork I’m working with so I’ve settled on the lowest line screen my plate provider can make. I believe it’s 85 line.

As far as image prep, I’ve been using a highlight dot of between 5% and 10%. It sounds very dark compared to commercial offset, but works fairly well. for me. For the shadow end I try and go no darker than 90%. Beyond that it tends to fill in the small spaces between dots. I don’t ever print a halftone that has a full 100% except as part of the Unsharp Masking. Unless of course it’s supposed to be a solid block with a knocked out looking halftone in it. Think moon in the night sky. I also tend to lighted the image overall with a curve that removes weight from about the mid-tone to the highlights. This seems to help with the mottle and gain in the quarter tone from ink smearing off the face of the dot.

You didn’t say if your working with Photo Polymer or or metal engravings. I print exclusively from Photo Polymer for halftones. I can say there is a big difference between new plates and old plates. New plates are far more forgiving when it comes to make ready.

You can always contact your plate/ engraving supplier and see if they can send you a half tone test. It will be a gradation typically from 1% to 100% in several different line screens. You’ll print this and then determine what the smallest and largest don’t you can print well at as well as which line screen is optimal for you. I know the plate supplier I use, Concord Engraving, will do this for you.

Lammy talked about ink:

His statement holds true on all type printing, text only or text and artwork together.

“I’ve found the best way to set up a halftone is to run almost no ink in the roller train, then increase it very little at a time until it looks dark enough without plugging. Starting off with too much ink in roller train will make you very frustrated very quickly.”

Lammy talked about ink:

His statement holds true on all type printing, text only or text and artwork together.

“I’ve found the best way to set up a halftone is to run almost no ink in the roller train, then increase it very little at a time until it looks dark enough without plugging. Starting off with too much ink in roller train will make you very frustrated very quickly.”

I would say 60 for halftone, if the halftone printing also includes any solid color areas you would need to do it in two different passes as the halftone requires very little ink.