Trimming Polymer Plates

I’ve never had the problem you’re describing. Are you using plastic backed plates, or metal backed plates?

I’ve cut my plates both ways, though usually I leave more space and not less. Sounds like a makeready issue to me with the packing, amount of impression. Unless you’re using metal backed plates. Those might be distorted from cutting.

Pictures will help a lot!

Edit: I suppose you are using plastic backed plates, what with the boxcar base.

As Vrooooom Indicated, you may have too soft a packing on your platen if you are seeing an impression of the plate edge on the stock. Cutting the plate edge closer to the image may help you as well, but you need to resolve the problem as if the image were extremely small, you would probably not want to trim to too small a size or the adhesive may not have enough area to grip well.

Try a harder surface packing on the platen. You will still get the requisite impression on the paper stock if indeed you are seeking a deep impression in soft paper.

If the problem continues, you may have to do a careful makeready, cutting away some packing in areas outside the image area, or more likely, enhancing the image areas to leave less imporession in the non-image areas where your plate edge is showing.

There is a danger in offering diagnosis and treatment without seeing the patient, but sometimes it can work.

The first step in letterpress printing is proper inking. You can mess with packing, diddle with platen screws, pray to the spirits of the black art, and curse. BUT, if the inking is not right, none of the other stuff will be of any help.

Any ink on any part of the type or the intended printing surface of the poly or mag cut means there is unintended
contact between the rollers and shoulder area (the part below the part that is supposed to receive the ink and print)
The rollers are getting to close to the form. The inked rollers must just kiss the portion of the form which is intended to print.

Usually (remember the first statement above) the problem is that the rails are worn on the old press and the rollers and trucks must be brought out a bit further from the type.
Another, but less likely possibilty is overinking.

Solve the inking problem and you are on the way to good printing.

I will claim that it was late at night and I had sniffed too much ink. The first sentance of the third paragraph of my post above was not well stated.

Any ink on any part of the plate/cut that is not intended to receive ink and print on the paper means the rollers are too close to the form.

Thanks all - I’ll play around with my packing first and make sure the rollers are type high, I have just got hold of a roller gauge So much to learn.

Go easy on that ink sniffing, inky.

You remark that you will play with the packing first. You might do that, but please re-read my first post. You can play with the packing and dance and shout, but you have to get the inking right first.
May I suggest: Remove all the packing. Place chase with plate in the press. No ink is required. Tear a strip of plain computer copier paper about 1” wide. This is your feeler gauge. Place paper vertically between plate and rollers. You might have to do it one roller at a time if any roller is suspect. Tug on the paper. There should be just the lightest pressure between roller and plate.
If you believe there is too much pressure and you have to tug hard to move the paper, the rollers have to be brought away from the plate. One strip of tape on each rail. Repeat test. Add a strip of tape to each rail until there is no pressure/contact between roller and plate. Then take off the last strip of tape added. There should be just the slightest pressure and tug resistance.

Now ink the press and ink the plate. Remove the chase and carefully inspect. You should have ink only on the raised portions and none on the shoulders.

Now you can mess with the packing.

I wish you happy printing.