Broken lines on photopolymer plates

I am having trouble with fine lines around the borders of my image areas that seem to have “bubbles” rounded shoulders on the plate. The inside images on the plate seem to be OK and clean, but borders keep getting broken on press. Has anyone experienced this? Thanks for your input

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How thick is the line weight? Are you processing them yourself?

Daniel Morris
The Arm Letterpress
Brooklyn, NY

If you’re using a deep relief plate are you performing a back exposure? The deep relief plates need the back exposure especially to hold swashes and thin rules.

Casey McGarr


I’m not sure what you are describing. Please provide more processing details. What type of plate, machine, design details, etc. Any pics?


Inkylipspress, thank you so much… for your comment. I read this because I was having trouble holding fine lines on the kf 152 (deep relief) plates. I develop them myself with homemade equipment. I read this and it gave me some ideas. I have experimented with back exposures and can now hold 6 pt type with a half point line quite comfortabley. The exposure latitude is very thin though. 10 seconds can make the difference between a beautiful plate and a mess. Not sure how long they will stand up on the press but great for short runs so far.
Thanks again,

Don, you’ve piqued my curiosity regarding back exposure. Some more details would be appreciated. Do you first expose the polymer as normal with emulsion to emulsion and then remove the film and expose the backside for a short amount of time? It sounds like you’ve found a solution for a common problem.

Back exposure is only used on film-backed photopolymer. You don’t remove the negative to do it, you expose through the (transparent) back of the plate, something you can’t do with metal-backed plates. This exposure is to strengthen the base. And without a strong base thin elements will degrade either in washout or on press.


Not only can you not back-expose a thick steel-backed plate (for obvious reasons), you don’t have to. They are formulated slightly harder than their polyester-backed brethren and the steel backing is somewhat reflective. Steel also provides more structural support to the subsurface relief and eliminates surface area stress (buckling, stretching), which is inherent with film-adhered polyester-backed plates (especially severe and noticeable with large images or solids).


Hi Gregory Willis: I tried a number of techniques - some on purpose and some by accident. It appears that I am under exposing the plates to some extent according to the stouffer guide. But at 6 minutes on my exposure unit I get excellent results and capture the fine details really well and the plate still seems very durable. The line problem has plagued me though. I saw a comment about back exposure on this post and did some experiments. My first experiment was a 40 second post exposure. That is 6 min through the negative and then flip it over and 40 seconds on the back no negative. This was a total disaster. Lost way too much detail. Kept the lines though. Next was a pre exposure for 15 seconds. I lost no appreciable detail and the lines were there but still wavey. Tried 30 second pre exposure and got no noticable detail loss and lines just about perfect ( half point lines). I tried a 40 second pre exposure and it came out thick as mud. Totally unacceptable. Throughout this process I was inconsistent in how I laid things out for the exposure and got some unexpected results. The process I found that works for me is:
Step 1) Take the photopolymer with the UV coating intact place it face down on the vacuum table on a black piece of paper. ( no negative in volved in this step) Turn on the vacuum and make a 30 second back exposure.
Step 2) Remove the UV coating and make a regular 6 minute emulsion to emulsion exposure through the negative.
Step 3) Develop, dry and post expose as normal.

I have theories on why this works. It is a work around to having the proper high end equipment which undoubledly will produce better results. But I have spent very little on my equipment and I can have a plate now in less than 24 hours that is inexpensive and totally acceptable (for most jobs).
Let me know if there is any other information I can give you and good luck.

Don, thanks for the thorough explanation. I’ll give it a try and let you know how my tests turn out. Cheers!

Wavy lines only happen when the plate is in the bath too long. The photopolymer absorbs moisture at the surface and expands, but the base does not, so the surface can only move sideways.


I’ve read this idea that photopolymer plates absorb moisture quite a bit on this list and others but I have never found this mentioned in the technical literature. I find it hard to believe. If the weaker, thinner surface is wavy that is a result of weak exposure and prolonged time in the bath but more the consequence of brush movement. Normally a plate with a perfectly printable straight line will also reveal that the relief at the floor is wavy. This is the result of brush movement but in a correctly processed plate, is of no consequence.

Technically, nylon (used for Toyobo brand plates) can absorb moisture under certain conditions but I just don’t see it happening in the short span of the platemaking process.


Well, Gerald, if you had spent as much time as I have processing by hand you might have some personal experience of wavy lines at the surface of the plate and not the base. Watching a plate develop before your eyes is not the same as putting it in the processor and removing it when it is done. I have watched fine lines as they begin to go wavy in the bath, pulled the plate and let it dry and watch the waves recede. (I was only using Miraclon in those days, can’t say if this is common to all plates.)
Of course exposure is part of the equation, and it is what makes photopolymer insoluble. I think this problem can also happen when negs are not dense enough. Fine isolated lines, the ones that get wavy, need more exposure than heavier or closely-spaced elements, but if the negative is not dense enough you are also exposing the non-image area, which can lead to longer washout. For a given exposure, you can shorten washout times so that there is still unremoved material at the floor of the plate, as long as the upper areas are cleared.