Over the past several years, the pad printing market has seen a transformation from the use of open ink well systems to sealed ink cup systems. This is especially true in the U.S. marketplace where sealed cup systems have dominated new equipment purchases for the last several years. However, open ink well systems are still very popular throughout the world. Certainly there remains advantages to both systems. We will explore these two systems and look at the actual differences within sealed ink cup systems themselves.
Open Ink Well Systems
An open ink well system has an open reservoir of ink from which a flood bar draws ink onto a printing plate and a sharp blade (doctor blade) wipes the plate clean. With this type of system, the ink is exposed to the air and therefore enables the pad printing solvents to evaporate, slowly changing the ink viscosity. Sometimes, on an open ink well system, the ink needs to be re-thinned with a solvent before the print job even begins. This can create a real problem, especially for inexperienced operators.
An open ink well system is popular for certain applications where there are long images, a number of color changes, short print runs, and where an experienced operator is running the machine. Applications that include golf balls and other short-run promotional products are commonplace for these types of systems.
The print area of an open system is generally much larger than with a sealed system because there are no major limitations as to how wide a doctor blade or plate can be made. Long images on plastic housings (i.e. televisions, computer monitors, vacuum cleaner parts) are usually pad printed with an open well machine. However, recent developments with ink cup technology have made it possible to enable long image printing with the consistency of a sealed ink cup. With this system, a cup slides continuously back and forth over a much longer than normal cliché and the image is longer than the cup diameter. It works well for mid-quality images, such as catheter printing in the medical field. Ben Adner, a major contributor to this article, actually developed and patented this method which is used in many machines today.
Open well machines can create additional set-up times as well. Older machines have a row of small screws along the front of the doctor blade holder, which can be adjusted to create higher-pressure areas along the blade length. This can be a frustrating task unless a very experienced operator is running the machine. An open well machine also has many more components involved in the doctoring process than a sealed ink well system, including the ink sweeper and sometime complex doctor blade support mechanism.
Color adjustments are made quite easily with an open system, especially when the color needs to be modified on the fly. Closed systems present problems with color adjustment because they only have a small access hole for adding ink or thinner. Looking at the ink to adjust color is simply impossible.
Sealed Ink Cup Systems
Conversely, a sealed ink cup system is essentially an inverted cup that is filled with ink and uses the sharp rim of the cup as a printing plate wiping system. The sealed ink cup system floods and wipes (doctors) a printing plate in the same motion, dramatically limiting the ink exposure to the air, thereby limiting solvent evaporation and ink viscosity changes. This helps keep the process much more predictable and manageable, and less experienced operators can run the equipment more efficiently.
Magnetic and Non-magnetic
Sealed ink cup systems can be broken down into two main categories: magnetic or non-magnetic. Magnetic ink cups are basically self-doctoring units, which are attached to the machine via a pin that simply moves the cup horizontally across the printing plate. Non-magnetic cups are designed to have the pressure applied by the pad printing machine to an outer flange or centering hole in the cup. Both cup styles, when integrated into the pad printing machine, can yield excellent results.
The magnetic ink cup, with its extremely powerful magnets, has all the necessary down-force to clean the plate, requiring the machine to simply pull the magnetic ink cup assembly back and forth. Therefore, the magnetic ink cup generally has found its strength in multi-color applications because of its easy and inexpensive construction. The non-magnetic ink cup has pressure applied to it by a spring-loaded system geared for one ink cup assembly. The spring-loaded system is compact and very effective, but is not as simple to deploy on multi-color machines. Non-magnetic cups have found their niche in compact or high-speed applications.
As sealed ink cup systems have continued to grow in popularity, a debate has somewhat emerged on what is the best type of ring material for doctoring printing plates. When sealed ink cup systems were first introduced, steel doctoring rings where made of carbide steel. However, since the early 1990s, ceramic rings have been introduced to the market. It can probably be estimated that the marketplace is nearly evenly divided between the old stalwart carbide ring and the new generation ceramic doctoring ring.
At this point, both carbide rings and ceramic rings work extremely well for most applications. However, one primary advantage of the ceramic ring is its self-lubricating qualities that allow it to work equally well on thin steel and thick steel, as well as softer polymer printing plates. The carbide rings are generally somewhat sharp and abrasive and will wear a polymer plate material quickly. Another advantage of the ceramic doctoring ring is that it is generally thicker and stronger, resisting damage during handling. Most ceramic rings taper from the inside and outside diameter to a point roughly .008, leaving plenty of material near the tip. Carbide rings, on the other hand, taper from the outside diameter to a point with a cross section of .004, leaving a minimal amount of material at the tip. The ring profile, in combination with the brittle nature of carbide, can result in potentially more damage during operator handling.
Many influences are involved in determining the best choice between open well and sealed ink cup systems. It is fair to say that sealed ink cup systems have gained momentum due to their ease of operation. Highly trained operators are hard to come by in todays marketplace. The pad printing market has become relatively mature and the quality of products that are available are all very high, regardless of the type of machine (open or sealed ink cups), the type of sealed ink cup (magnetic or non-magnetic), or ring material (carbide or ceramic). The specific application and operator experience will help decide the right direction to pursue.
Plastics Decorating would like to provide a special thank you to Ben Adner of Ink Cups Now (800-780-8071) for his work on this article and a thank you to Julian Joffe of Pad Print Machinery, Inc. (800-272-7764) for his assistance.