Flexo Plates?

My recollection is that flexo uses rubber plates. I’ve used rubber stamps for letterpress successfully, so if they’re deep enough they would work that way. However, though they’ll take and transfer ink, you sure won’t be able to get that “letterpress bite” into the paper! If the flexo plates are photopolymer and deep enough they’ll work fine for letterpress. The key will be having a deep enough counter to prevent it inking.

The flexo plates are rubber made. Flexo plates allow for great detail and screnning. Many of the cereals boxes (or other packaging) are made in flexo, even cmyk jobs. Photopolymer is hard compared to flexo plates.
As AdLibPress says, it won’t ‘bite’ the paper.

Ok, thanks for the feedback guys; I really appreciate your time.

The plate material we use is a photopolymer but certainly softer than what I’ve seen from Boxcar, which is my only real point of reference on this.

I guess it is just something I would have to experiment with to see if there is any interesting work that could be done with this stuff. We have to run either full or half sheets of the material through our processor so I was thinking I might be able to sneak some small cuts onto the excess once in a while.

Now if I could only get someone around here interested in firing up the old Windmill I wouldn’t have to go through the trouble of finding my own press!

Hello sgirard,

I’ve been in flexo for a long time. To answer your first question, another processing step would most likely not make Cyrel (registered trademark of DuPont), better for letterpress printing. If you post-exposed a plate for a longer time, it would just get less flexible and crack, or be prone to cracking.

I have a Cyrel Process-of-Use Manual in front of me. (It’s from the year 2000 so it’s not right up-to-date, but that’s all I have). In it, there is a Cyrel Plate/Ink Compatibility Guide. Unfortunately, the guide only lists ink chemicals which are apt to be in water and solvent based flexo inks, not oil based litho or letterpress inks.

I seem to remember (although I could be wrong), that litho and letterpress inks are not compatible with at least some, if not all, types of Cyrel plates. However, you can find this out for yourself by using the long-standing method of determining plate compatibility with various substances. During this process, take the necessary health and safety precautions to protect yourself, which you probably know since you work in a plate processing area. If you don’t know the necessary precautions, find out first so you can follow them. Then take a small piece of a processed, completed plate and immerse it in the ink or chemical in question in a closed container for 24 hours. After that, take it out and see if it has swelled or otherwise been degraded. If it is unchanged or only marginally changed, you may be able to use it.

Other things to consider are 1) there are different types of photopolymer plate material and some may be more tolerant of letterpress inks than others, 2) photopolymer plates come in different durometers and of course the harder ones would be better for you. Plates for printing corrugated board are the softest (around 34 durometer), and others for printing paper, plastic film, etc., are usually in the 42-63 durometer range. 3) Photopolymer plates come in many different thicknesses, 4) If you are using analog plates and exposing them with a negative, you can increase the relief (difference in height between the floor or non-image area and the printing surface), by reducing the back exposure, and you may have to change the main exposure as well.

In the 1960’s, I worked for a printer who had a continuing job printing cards for library card catalogs. They used rubber plates for this and printed them by letterpress. I’m not sure why they did it that way, but it worked OK.

Hope this helps.

Ooh lucky I found this thread.

I’m in the same situation as the OP. I work for a large flexo printing company and we use photopolymer plates too.

They’re 67 thou thickness but I guess they are softer than what is needed for letterpress.

We print on flexible plastic so I dont know if that factors much into it.

Any tips on getting the plates to work on my Heidelberg would be great :-D

And what is this talk of shoulders? Guys at my work said we can get a straight shoulder with a deeper impression. They said I should try going to 47 thou and see how it works.

kittymeow: Unless you have some kind of base for mounting thin letterpress plates, you will have to build up the image area of the plate to type high, which is .918 inch. For a rough test, you could try 1 inch pine from the lumber yard, which is supposed to be 3/4 (.750) of an inch. Sort through the boards until you find a really flat one. This plus your .067 plate would be .817 inch, then add pressboard to the bottom of the pine, or chipboard (the heavy paper bottom off a pad of paper), and other thicknesses of paper until you get up to .918.

To answer your question about the shoulder, this usually refers to the edge of the image, where it goes down to the non-image area. It is usually sloped, but can be straight such as on wood type.

.067 flexo plates for printing plastic film usually have a relief of .020 to .025 inch, which means that the image area is raised .020 to .025 above the non-image area. This is probably too little for letterpress. See if your guys can make a plate with a lower floor (the height of the non-image area). The plastic base of an .067 plate is usually .005”, which means there is .062 thickness of polymer. In theory, if they didn’t back expose the plate at all, they could make a plate with the non-image area washed right down to the base, which would give you .062” relief (the difference in height between the printing surface and the non-printing area (the floor). However, I wouldn’t recommend this. If you could get .035” relief, that would probably be plenty. This would mean that the vertical distance from the bottom of the plate up to the non-printing area (floor) would be .032, and the distance from the non-printing area (floor) up to the printing surface would be .035. Thus, .032 + .035 would be the total thickness of the plate, .067.

One other thought: you could try mounting the plate to the wood using foam plate mounting tape, which you could also get from work since it’s used in flexo. It is usually .015 or .020” thick, and comes in different hardnesses. Use the hardest foam they have. They might also have solid vinyl mounting tape (not foam), which you could try as well.

Wow thanks Geoffrey! That all makes a lot of sense :-D

I’m lucky my dad is an expert woodcraftsman (amongst other things ;-P) and he should have some nice flat wood I can use as a base. I intend to get one of the Boxcar bases eventually though.

The guys at work will definitely be able to get me a nice relief too. And I’ll definitely pinch some of the mounting tape.

You’ve been a great help! :-D

kittymeow - If .035 inch relief isn’t enough, you could always ask your guys for more.

If your rollers ink the floor of the plate or the shoulders of the images, try raising the rollers by putting tape on the rails which the roller trucks ride on. Then hopefully the rollers won’t squeeze themselves as well as the soft flexo plate, down far enough to ink the floor. After some experimentation, you may very well get some good results. Good luck!

Awesome! I will take those tips with me to Tassie :-D

Well, yesterday I got my plates made. The guys made them .035 and I’ll be getting some of the mounting tape today. They have some .015 and some .060 so I should be right in taking the .015 right?

Tomorrow I’ll be down in Tasmania hopefully running my press!

Hi! I assume you got an .067 plate with .035 relief. If you use .015 mounting tape, then the total plate thickness (.067) plus the tape thickness (.015) will be .082. Since you need to get up to type high (.918), you need .918 - .082 = .836 more height. Does your father have a wood planer? If he does, perhaps he can plane a piece of wood to .836 inch. Then you could just put the mounting tape and the plate on it, and it would be type high.

If you are going to use .750 wood like that which you can get in the lumber yard (here in Canada anyway), then you would have the plate and tape which is .082, plus the wood which is .750, and this would total .832. Type high .918 - .832 = .086, which is the distance you still have to build it up to get to type high. If the .060 mounting tape is solid (not foam), you could use that and then you would only have .086 - .060 =.026 more to build it up. Two thicknesses of cardboard cut from a fairly light cardboard carton should be about .026. (In the industry it is called boxboard, but most of us just call it cardboard).

You may have to add or subtract a little thickness to get it to work, but this should be fairly close.

Let us know the results!