Photopolymer hardness problem

Hi guys!

I have an home made kit to burn photopolymer plates, and all works fine!
But today I’ve contact with photopolymer plates made by professionals, plates with 5 years I think… these photopolymers are much more hard compared to my results.
I’m using KF95 plates, the professionals were slightly higher with steel base.
this made me think if I can have some more hardness in my plates.

what do you think about that?
the photopolymers can have these differences?
I can improve something in my process?

thank you !

reagards, João

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You don’t want the plates to be too hard- this would make them brittle. Are you talking about a plate exposed five years ago? If you are I am not surprised it is hard- it is probably ready to go in the trash.

Daniel Morris
The Arm Letterpress
Brooklyn, NY

Steelbacked polymer is harder than the flexible plates,

Somewhere in my memory , there lurks an image of a three tier machine , there was a bit where you expose the plate under a vacuum , abit where it was upside down and the vibrating wet brush washed it out and a bit where you baked it after. are you missing the part of the process where you bake it???/
I hope my memory is not that far off ……. If i am wrong sorry i miss lead ,someone will correct me if im wrong !!!!!!

the process I made is complete.
-wash out
-baked / cure

My question is, the photopolymer plates after wash needs to be completely dry for the next step ( baked - cure ) ?

Your processing lacks the final step, which is post-exposure, done after the plate is dried and cooled. Many remove surfaace moisture the plate using roller sponges etc., but the drying really happens in the drying unit, using moving heated air. Baking in static air is not sufficient.
Post-exposure is where the working hardness is fixed. Twice the main exposure time is typical, but you need to know the specifications for the actual material you are using. Every plate has different specs.


There are differences in the “hardness” of the photopolymer plates, and Typenut is correct in sayign that many of the metal-based plates are of a higher durometer (measure of hardness) than the plastic-based plates. You can probably get a steel-backed or aluminum-backed plate from your raw plate supplier.

The best practice is to fully dry the plate after wash-out and prior to the final curing exposure. Doing this will provide the hardest and longest-lasting plate you could make, given the durometer specs of the plate material you are using.

thanks for help!
I’m very grateful !



Hardness is not a result of the photopolymer platemaking process, the hardness is a factor of a specific plate.

The last time I checked, a Toyobo KF95 (polyester-backed) and a Toyobo KM95 (steel-backed) are quite similar in their hardness rating. They do have differing structural support though.

Of more concern is if the hardness rating is specified for letterpress application. A Toyobo KF152 (polyester-backed) and a Toyobo KM152 (steel-backed) DO NOT have the same rating. The KF152 is actually not specified for letterpress usage and its hardness rating is lower than required (again, last time I checked). A lot of the problems associated with this particular plate can likely be attributed to that fact.

Note that as a plate ages it will get brittle and lose its tack and resilience (its printability), but this has nothing to do with hardness rating.


My House Plate is a BASF Nylonprint WS 94, I can hold 4 pt. Type just fine. I use a A&V Orbital VIII (24 x 30 ) and yes:

Expose plate under vacuum (-25) with Film (4.0), the plate washes out in a specific temperature, which is different per manufacturer and exacting notes are to be kept. Brush or Plush. After washout, manually dry and drip for 10 minutes.
Insert in ventilated hot air oven for xx amount of time (Plate specific). Place in Exposure drawer on top of Kreene and post expose.

Plate making is no mystery, it’s an exacting science with little leeway. I’m sure Gerald Lange can attest to that.

By nature, steelbacked polymer is harder, the flexible plates are softer, hence they squash under impression.

If properly stored, plates can be used years after exposure, but they turn bad like Fish in a heartbeat if stored wrong.


By nature or commonsense? I wouldn’t trust the latter (based on forum assumptions) and the former should be technically verifiable. It isn’t.

I haven’t seen polyester-backed plates “squish” in any way (you don’t use them, correct? but I do use them in institutional instruction). They certainly do show warp/distortion based on imaging (vividly) and can stretch out of registration (both primarily due to their less dense, sturdy backing).

I’m not sure how long a previously exposed plate can last but I doubt “years.” A carbon dioxide bath can revive them to an extent but I don’t see it as a way to resurrect their printability qualities.


“The KF152 is actually not specified for letterpress usage and its hardness rating is lower than required …”

When I was a boy and letterpress was still the mainstay of commercial printing, correct impression was considered to be the minimum amount required to transfer the ink to the paper. Impression visible on the back of the sheet was considered bad form and was usually present when a printer was trying to get more use out of a mix of worn and newer type. Type was made out of as hard an alloy as practical, not to emboss the paper, but so that it would be long-wearing.

I have printed quite successfully with some fairly “soft” poly plate material like KF 152 and it is fine if you do not intend a pronounced amount impression, though it will leave a slight tactile impression in soft-surfaced papers like Stonehenge.