Label Printing Snippet Part 29: Rollers in your printing press
There are many types of rollers in your printing press and they all have unique functions and uses.
In this blog I’ll guide you through the various roller types, what they do, the coating of the roller surface, shore hardness and other functions.
The most common and widely used roller is the path roller. It’s responsible for transporting the substrate through the press.
It’s important to keep these rollers unsoiled with regular cleaning and free of any dents or scratches. If you dent or scratch your path roller it can lead to scratches and dents in your web requiring replacement.
There’s a coating on all path rollers to help sustain their long life required. It’s advised not to use sandpaper, steel wool or even a Scotch-Brite pad or similar abrasive pad as they will remove the coating.
Using box cutter knives (used daily as a printer) is a definite no when removing dried ink or even from a wrap around on a web break as they will scratch and ruin your roller. Even when there’s pressure to get the job running, it’s worth a few extra minutes to save your roller.
A good way to clean metal rollers is by using acetate and water to safely remove ink and paper dust, especially if you’re using a paper-backed self-adhesive substrate. A weekly schedule should be made for cleaning rollers and the press. For the impression roller, regular maintenance will help eliminate any issues with marking or poor printing.
Teflon coated path rollers are usually only used in the turner bar part of the press and sometimes swapped in and out when needed. These rollers have a non-stick coating for when you do de-lam/re-lam work and now have the adhesive side of the substrate touching the path rollers.
If you do not use a Teflon coated roller you will get wrap around and will be unable to run with a regular path roller. I’ve seen printers use the backing of the plate mounting tape; this can also create issues, so it’s best to have sufficient Teflon coated rollers available.
Grooved path roller to reduce creasing
Usually there are two nip rollers with adjustable pressure - some machines have three. They’re located at the unwind after the edge guide, and on the last print station towards the rewinder after the die and matrix stripping unit.
Nip rollers can also be added to the lamination/cold foil unit and turner bar for re-lam/de-lam work, which provides the substrate and backing film or paper to re-join. These provide tension in the press by a driven, infeed roller and rubber roller along with the braking system and load cells on more modern presses.
As they’re made from rubber, it’s important not to cut on these rollers and damage them. For older machines that don’t automatically release the pressure during press shut down, make sure to lift them before you shut down or flat spots can occur causing instability and tension problems.
The impression roller has the substrate that passes over the impression roller/cylinder and the printing plate is set to this to transfer the image onto the substrate. This happens on all print stations.
A driven impression roller can act as a nip roller if the printing plate has too much pressure. An MPS press has a free-running impression roller which eliminates this problem and gives good registration with low web tension. This is especially helpful in flexible packaging where film or poly can stretch and be too long with too much pressure on the unit. Overall web tension can also affect this.
It’s important to keep the impression roller clean and free of ink, paper, dust, or any damage. As a critical part of print quality, utmost care and caution must be taken when cleaning.
A method I’ve found to work well for cleaning your impression roller is Ethyl or NP acetate and water. Acetate cleans the ink and water removes the paper dust. Cleaning is effective, quick and does not affect or damage the surface.
The nip roller is not the only rubber roller in your printing press; others can include pan/duct rollers for ink to anilox transfer and impression rollers on hot foil units. For optimum performance, shore hardness is important, as is the diameter of these rollers.
The pan/duct roller shore hardness is important for ink transfer from the rubber roller to the anilox. If this roller is too large in diameter or the distance between the pan/duct roller is too small or even touches - allowing for no adjustment to get the optimal ink transfer – ink starvation to the anilox can occur. This will be evident once the doctor blade is applied. If this was not noticed when the anilox was applied to the plate and impression set, you will see the starvation results.
If you have a damaged pan/duct roller and need to send it out to be re-rubbered, it’s best to use a reputable company. Ask the machine manufacturing company for required specifications of diameter and shore hardness for best results. This information might also be in the machine manual.
The impression roller on a hot foil unit, along with correct heat, is a crucial factor in the release of foil during the foiling operation. These rollers are usually around a 95-shore hardness for durability, though this can also depend on the operator’s knowledge. The printer sometimes applies pressure when the foil is missing or picking, but this can be a temperature issue resulting in damage. Training and job experience is the key to the longevity of the impression roller.
Re-ordering an impression roller may be a better option than re-rubbering, depending on the quality and company you deal with. This consideration is due to past experiences of getting rubber rollers out of spec and too soft; these are almost instantly damaged.
If you don’t have a spare this can be costly and cause job delays. If you do have a spare and need to change, it’s one of the harder rollers to change depending on which unit you are using. It may require a maintenance person or even a trained supervisor to take care of the roller change.
A continuous roller can be a solid roller for the desired web width used for primers applying adhesive in lamination and doing flood coats of colours. The diameter of the roller does not matter with continuous print. These offer the advantage of no visible plate gap.
Depending on the condition of the roller it may not print as well as a flexographic plate. Proper care of these rollers is essential.
The other type of continuous roller is one that will have an image on it (same as a flexo plate) and made to the size of the print cylinders e.g. Z128 or 128-tooth. These can either be an engraved rubber roller or a special photopolymer sleeve that requires an adapter.
These continuous rollers also offer a no-plate-gap solution, are more expensive and only used for special job requirements. When storing your continuous roller, never lay if flat or it will develop flat spots and be ruined. Cover it to protect the image.
The new MPS E-Sleeve enables printers to generate predictable results at increased speeds, with guaranteed quality tolerances and more reliable production cost calculations
The print cylinder is one of the most important parts of your printing press.
They’re available in different types of plastic, composite, steel, and aluminium. It depends on what your requirements are, what came with your machine, if it’s a second-hand or new machine, and what the manufacturer offers.
There are important factors to take into account such as the total indicated runout (TIR), the roundness of the cylinder. This is how true and concentric your cylinder is and, depending on the quality or the material, will it stay this way.
The E-Sleeve offered by MPS offers perfect concentricity and high dimensional stability, yielding perfect register on multiple images. The result is a more consistent and sharper print with a free-running impression roller.
It also offers the benefit of not needing to re-mount the plate if mounted the wrong way; transferring a locking key in the opposite direction is all that’s necessary. It’s also fantastically strong and lightweight with improved adhesion of mounting tape.
This video explains the E-Sleeve advantages for MPS customers:
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This blog provides a brief overview of the different rollers in your printing press. To save time and money, it’s good practice to have spare rollers on hand in the event a roller is damaged.
My other label printing snippets are available here.
Kane Marsh is Regional Printing Instructor Asia Pacific with MPS Systems Asia, providing training on MPS' narrow web flexo presses and specialized label applications. Kane has direct print experience on multiple presses in a range of printing technologies including flexo, gravure, offset, screen and a variety of applications.