This is a continuation of my recent post, Rollers for the Offset Press Market, and will provide additional details about offset press rollers.
What is a nip? It is the point between two rollers that are pressed against each other. Some call the nip setting parallel and others call the nip setting a “stripe,” but the width measured between the two rollers is the part that is technically known as the nip. The purpose in setting a nip is to achieve efficient ink transfer with the least amount of mechanical pressure possible. If the nip is set too wide, the ink will not transfer to the plate efficiently. If the nip is set too narrow, it causes poor ink transfer and ink slinging.
The nip width also has a pressure load measurement associated with its width. Greater widths require greater pressure loads: the higher the pressure, the higher the friction and, in turn, the higher the heat generated, which then affects the ink transfer within the press. Proper nip settings define ink transfer efficiencies and minimize the heat generated at the nip points. Therefore, if a press has excessive misting or heat, the nip setting should be investigated to determine the cause.
Presses are known for the mechanical pressures they endure. Pressure creates heat which, in turn, causes swelling and misting. High-speed rollers require minimal drag while turning through the nip points. Less pressure means less heat, swell, and misting. Since the rubber compound is responsible for physically transferring ink to the plate, its durometer will impede or enhance the ink transfer at the nip.
In addition, the rubber compound can affect the amount of pressure required to set a given nip width. Higher durometer rollers require more energy to create a nip, but they create heat and impede the ink flow through the press. Rollers with lower durometer, on the other hand, require less energy to create a proper nip setting. Less energy creates less mechanical pressure. Bottom line: the amount of energy depends upon the durometer of the compound.
A roller must also retain the profile of its original size in order to mitigate heat generated from nip deflection. If the compound does not return to its profile it will begin to swell, increasing the nip width and creating more heat and misting. Metal press forms require a compound to relieve the torque associated with the forms position, while also reducing the heat from both the forward and the following nip deflection. The compound must provide a sustained ink transfer to ensure high-quality and high-speed offset without increasing heat and swell. If you are seeing increasing amounts of misting or heat, check to see if your rollers are swelling. The compound may be an issue if the nip is set correctly.
Balanced chemical and physical properties of the rubber compound are critical to obtaining maximum production efficiency and consistent print fidelity. Please reach out to us for an independent review of your production challenges and we will be pleased to help.
Stay tuned for part three where I will provide more insight and information into offset presses and their components. If you would like more information, you can contact our marketing manager, Katie Graham.