Problem with halftone screens

BartS

Member
Dear fellow Screen users,
My company produces films for screen printers and other graphic specialists, I do this on a Screen Katana 5055 with an LDM-1060 film developing machine.
Until recently, I had film from Agfa, type HND (hard dot), for this purpose. Agfa has stopped producing HND film and only supplies HNS film. But since the change from HND to HNS there are problems with the halftone screens. These show horizontal and vertical banding. I have already taken the necessary actions to try to solve the problem, a shortlist of these:
  • Laser intensity adjusted from 120 to 185 due to the reduced sensitivity of the HNS film (density > 4.0).
  • Created new correction curves due to the different properties of the film.
  • The temperature of the development bath checked.
  • Replenishment checked.
  • Dot shape changed from round to elliptical.
  • Set the resolution from 2400 to 3000 dpi
  • Cleaned the pump of the developing section and checked it for proper functioning.
  • Optics of the imagesetter cleaned.
  • HNS films used with a different batch number.
  • Another developer (G101c instead of ACD developer).
  • Imagesetter replaced by a spare machine (I bought this one 2 years ago just to be sure).
None of the above solutions solved the problem. Coincidentally there was still a roll of film in the spare machine, brand unknown, but I suspect also of Agfa because it has the same blue color. On this film, the problem does not occur, so the prime suspect is the HNS film.
My question to you is if this problem is recognizable and if there is a possible solution. Otherwise I would like to hear if there are alternatives to the HNS film, preferably again a hard-dot film like the HND.
With kind regards, Bart
 

gordo

Well-known member
Do you see the problem in the print as well as the negative? Or just the negative?
If you return to the round dot does the problem persist?
Could you better describe what you mean by horizontal and vertical banding? Can you export and post a bitmap file that shows the problem?
 

BartS

Member
Hello Gordo,
Thanks for the reply.

Last Saturday I exposed 2 HNS films and 1 brand unknown film with the same chemistry and with the same software (harlequin rip). Both HNS films show banding, left and right also more as in the middle of the film, but just not exactly in the same place. The unknown film has no banding.
The batch numbers of the HNS films are 00654270 (expiration date 2024-04) and 01204190 (expiration date 2023-09). I don't know the expiration date of the unknown film, but I assume it has already expired. I can produce a bitmap file from the harlequin rip, but in the preview, I see no strange things, so I think that's not the problem.
Positive, negative film or dot shape doesn’t make any difference. The first suspect is an uneven photosensitive emulsion of the film.

In the attachment 2 pictures.
Unknown_film.jpg: presumably Agfa film, no problems, the screen is even, no differences when measured with a densitometer. Any differences you see in the picture are from the light table.
Hns_film.jpg: HNS films with banding, both horizontal and vertical, measurement of the film show differences.
I could try to make an offset plate to see if the banding also shows up in print. I will let you know the results. if you have other suggestions about what the problem could be, I would be glad to hear them.
With kind regards, Bart
 

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Repro_Pro

Well-known member
We have moved from HND to HNS ourselves with no noticeable changes in image quality.
There was a period of weeks we had both types at the same time in different film widths cassettes in our Dolev 800, but we noticed no difference in image quality between jobs ran on either, so from my standpoint these two film types are interchangeable.
We rarely output flat screen tints but I am sure we would have noticed such artifacts as you show in the very delicate graphic objects we routinely output.
You didn't mention the screen ruling you were using, but since these tints were made for silk-screen printing, they would likely be 75 lpi or less, so in my view the dots are quite large and not very sensitive to small variations in exposure or processing.

Your increasing laser intensity so much seems odd to me, so I would suggest you check if the emulsion side on the HNS films you used is where you need it to be, EI (Emulsion IN) or EO (Emulsion Out) - I don't know which side is needed for Katana.
 

BartS

Member
We have moved from HND to HNS ourselves with no noticeable changes in image quality.
There was a period of weeks we had both types at the same time in different film widths cassettes in our Dolev 800, but we noticed no difference in image quality between jobs ran on either, so from my standpoint these two film types are interchangeable.
We rarely output flat screen tints but I am sure we would have noticed such artifacts as you show in the very delicate graphic objects we routinely output.
You didn't mention the screen ruling you were using, but since these tints were made for silk-screen printing, they would likely be 75 lpi or less, so in my view the dots are quite large and not very sensitive to small variations in exposure or processing.

Your increasing laser intensity so much seems odd to me, so I would suggest you check if the emulsion side on the HNS films you used is where you need it to be, EI (Emulsion IN) or EO (Emulsion Out) - I don't know which side is needed for Katana.
Most of my work is indeed line art for screen print, sometimes with small flat screens, 45-85 lpi, round dot. Sometimes for offset, 140-160 lpi, elliptical dot, or flexo 150 lpi, round dot. The dimensions of the screens aren't very large so the problem of banding isn't noticeable. Now I have a customer who wants a full-size flat tint, on my image setter that is 55,8 x 116 cm. (22 x 45,667 inches) and there the problem shows up.
The emulsion side for the Katana is In, code CDH, checked this on old empty HND packaging and the new HNS packaging, both have the same code.
The increase of the laser intensity is indeed odd. Agfa claims there is more silver in the HNS film, so it should be more sensitive to light. However, the results prove otherwise. The Katana uses a hardware variable regulated laser intensity setup, which results in a film with seven 100% black rectangles and seven adjacent rectangles of an (ideally) 50% screen tint. I always choose the one with the highest density (but at least more than D4.00 and the screen tint closest to 50%. The HND film used laser intensity 120, the max. D was 4.3, screen tint 55%. The HNS film uses a laser intensity of 185, with max. D 4.1 and a screen tint of 68%. With correction curves on the harlequin rip, these value is down-regulated to 50%. So in the case of large flat screens, the HNS film isn't an improvement.
 

Repro_Pro

Well-known member
Try to compare the results of the dots on the HND and HND films under a high powered magnifier/microscope (x25 at least).
Can you describe the difference?
If possible post an image file, an image taken with a USB microscope would be very helpful.
Additionally I would re-calibrate aiming at a lower DMAX, around D3.5 should be enough for almost all uses.
 

BartS

Member
Try to compare the results of the dots on the HND and HND films under a high powered magnifier/microscope (x25 at least).
Can you describe the difference?
If possible post an image file, an image taken with a USB microscope would be very helpful.
Additionally I would re-calibrate aiming at a lower DMAX, around D3.5 should be enough for almost all uses.
Inspected the dots with a 25x loupe. It's hard to see any difference, but the HNS film has a softer dot, but that was to be expected.
D3.5 could be an option, I'll give it a try, let you know the result as soon as possible.
 

Repro_Pro

Well-known member
You can also try to slow down your processor so as to prolong the development time (you may also need to lower developer temperature if you suspect over-development).
This should remedy the softness in those dots.
BTW, I don't believe HNS emulsion contains more Silver then HND.
Quite the apposite makes more sense to me. For years manufacturers of films have invested heavily in experiments trying to lower Silver content without sacrificing quality, so it makes no sense for Agfa to do the opposite.
Good luck.
 
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BartS

Member
Ok, a test with D2,95 didn't work out. Dot sizes 2-60% seems fine, after these values the banding shows up. My options at this moment:
- outsourcing the work to a colleague
- try a different brand of film (anyone has any experience with lecai-film from china?)
 

Repro_Pro

Well-known member
I will be very surprised if this problem is entirely from the new HNS film.
Try to re-calibrate at your normal laser intensity (120?) and normal resolution (2400?), and run some tests with different spinner speeds.
 

ansoft

Well-known member
It looks like a problem with dirt of the mirrors. You can also check the rubber rollers and the tension of the film.
 

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