This system is considered by some to be the panacea to all Pad Printing ills, would that it was so. What is surprising is that this â€œNew developmentâ€ has been around for many years. A Swiss manufacturer of Pad Printing machines for the watch dial industry had a system 35 years ago! Although pretty basic it performed very well indeed.
The concept is to contain the ink in a cup that is turned upside down and the ink sealed in by pressing the cup down onto the plate. The cup not only holds the ink but when traversed over the plate acts as a doctor blade and leaves ink just in the etched areas on the plate.
The obvious advantage of the system is that solvent evaporation is reduced to a minimum. This allows much closer control of ink conditions resulting in the opportunity for reduced down time and partial unmanned operation. But, you say; â€œThe machinery salesman told me it will run 24 hours 7 days a week 52 weeks a year.â€ Did he say what the print quality would be, how often do you have to top up with ink, how much will the plate wear, how long does the cup last, does the fact that certain ink corrode the plate matter, using two component inks is acceptable but donâ€™t leave them for an extended period in the cup, with some systems it is better to keep the plates and cups as matched pairs. A question that must be asked is. â€œHow much does a replacement cup costâ€. The answer can come as a surprise. Â£500 is not at all unusual. Fortunately if the damage to the cup is very slight it is possible to carefully hone the contact surface on a fine carborundum stone. Some cups can be re-machined at a quarter of the replacement costs. This all means the cups must be handled very carefully. The plate has to be twice the size of a conventional plate for a given image.
The issue of ink condition is crucial. With closed cup systems it is easy to have the attitude out of sight out of mind. Ink mixing is just as important as when you use an open ink well system. The mechanism of pad printing remains the same, evaporation of solvents being the governing characteristic. Solvents must be weighed into the ink and the mix is dependent on the image being printed. For example when fine detail is being printed the mix of solvent will contain a higher percentage of retarder; otherwise the ink will dry in the etching on the plate.
The most common problem is brought on by the impression that inks have an indefinite pot life when held in the closed cup. This is simply not so. Time and again users who are unhappy with print quality contact me. The solution often is to mix a new batch of ink and take out the old and replace it with new. If print quality is critical it is recommended that the ink is replaced once a day. When high volumes of ink hungry images are being printed regular topping up of the ink will be necessary. As long as correctly mixed inks are added ink condition will be maintained. Addition of solvents whilst in production is fraught with problems as the ink volumes are very low and one squirt can completely upset the solvent balance. Old ink that you remove from the cup should be disposed of and not mixed back in to the new ink. If print quality is not important then ink condition can have a much greater latitude.
Ambient conditions can still effect the performance of an ink but not to the same extent as machines with open ink wells. Differing ambient conditions will mean you will have to adjust the initial mix of ink to control solvent evaporation on the pad and in the etched plate.
Another problem that can occur is that some ink systems when used form a crust of dried ink around the edge of the cup. This crust will drag lines of ink across the image and ruin the print. Ink manufacturers have developed inks that overcome this problem so if your current supplier cannot solve it consider another supplier.
With closed cup systems plates generally have to be twice the size of open ink well systems. If you have a lot of plates this can be a substantial cost increase. Ideally the surface finish of the plate needs to be better than when it is cleared with a conventional doctor blade. The doctoring characteristics of the cup depend on the contact surface, machined from solid, spring steel or ceramic. This is allied to the bearing and clamping mechanism. There is no such thing as a cheap system. The cheaper it is the more expensive it is to run.
At times like these I would love to be able to give you my recommendations as to which manufacturer had the best combination but I have to remain impartial. By the very nature of these cups plate wear can be a problem. With steel harder flatter plates will be more effective.
Photopolymer plates are best used with cups that have ceramic rings. These rings are very flat and run well on the photopolymer material. Different photopolymer materials will perform better or worse and you will need to experiment. Broadly the harder materials are better not just from a wear point of view but also as regards to print quality. Multicolour machines are common place. The quality of engineering will determine the effectiveness of the system. Some manufacturers can offer either sealed ink cup or open inking mechanisms on the same basic machine. Later in this article I show the use of sealed ink cups in sophisticated CNC controlled equipment for multicolour printing.
As mentioned above ink cups have different constructions. The simplest is machined from solid metal. Normally hardened steel. The second type has a ribbon steel contact surface that can be replaced when worn. The third is a metal cup with a ceramic contact surface attached to the metal. This gives a very good life but is more expensive than the conventional cup. To achieve maximum life out of any system minimum pressure must be applied and the cup should be mounted on a stable bearing. Any tendency for the cup to rock will cause uneven wear and a consequential film of ink on the plate, which is transferred by the pad. Some manufacturers rotate the cup during the machine cycle can overcome this. Some systems use internal magnets to hold the cup onto the plate this appears to work very well. Be careful that the pigments in the ink are not effected by magnetism, if so they collect around the magnets and they are virtually impossible to use.
There is no doubt that the system is being refined and the use of larger cups is increasing the print area. Four colour machines with ink cup diameters of 210 mm are now available along with six colour machines with 150 mm cups.
The use of the sealed ink system is becoming more important as the effects of legislation require a much reduced level of solvents in the working environment and expelled into the atmosphere. Machines will either have cups that move backwards and forwards and the plate remains stationary or stationary cups and moving plates. The first system is normally used for larger images at slower speeds whereas the second is for higher speed printing of smaller images. There are exceptions. Whether a closed cup or open ink trough is used the same systems can apply. The moving plate with a single plane pad movement provides a cost saving for the machine supplier because there is one less actuation on the machine. From the users point of view as the pad only has to move in the vertical plane there is less vibration on the pad and the machine can cycle faster. This system is used with high speed coding machines. Here a small plate is used with a sealed ink cup.
Larger machines with plate areas of 200 mm by 450 mm have been produced using this method but they need very substantial bearings to support the plate when it is in the pick up position as the loading exerted by a large pad is very considerable.
Manufacturers claim as the pad strokes below the base of the machine there is a larger work area. The down side to this is that the machine has to be well guarded because of the back and forwards movement of the plate. Automatic loading can take advantage of the shorter cycle times.