Pad printing has had the reputation of being a tough process to nail. Consistency is even more difficult to achieve. This is still true if you think pad printing is an art form or that its success depends on some secret recipe. Instead, pad printing is a scientific procedure. Many variables exist that can affect the outcome, but there are new tricks that can help stabilize the decorating process and give an expected outcome or proven result. It’s not good enough anymore to have any significant rejection rate – a printer must have repeatability. Even in markets and industries where cycle time and speed are more critical than cosmetic appearance, it is advantageous to have a process that gives identical print image and the ability to repeat that process at set-up. At the end of the day, these elements will produce good part rate increases!
1 – Surface Tension Testing
The first piece of information the printer needs to capture and control is the surface tension of the material or product to be decorated. It is important to know the immediate surface to which the ink is going to adhere. Many times ink is specified on a sample part, only to find out that the part is going to have a painted or coated surface. Forget the base material in that case… ink needs to adhere to the coating.
How many printers have a dyne test (surface tension) kit? Would you be surprised to hear that very few decorators use one? However, if the dyne (or surface tension) is less than 35, it is difficult, if not impossible, to get adhesion. These kits are inexpensive and it’s a smart move for any decorating project to know how compatible the product is to available ink systems. There are also materials that require pre-treatment, such as corona electrical discharge, chemical primer, or open flame. These can significantly change the surface tension of the material. Don’t get caught up with the notion that surface tension testing is only for those assumed materials like polypropylene or polyethylene. It may also work on substrates like metal, ceramic, wood, and others. All of these can have oils, varnishes, or coatings that could benefit from pre-treatment or a changed surface tension. You won’t know without a way to test!
A surface tension test also will tell a printer how to set up pre-treatment. What dyne number should be achieved? Dyne number can be controlled by how long, how close, what type of pre-treat, and the intensity of the system. But the practical way to know quickly is… test it! This easy test will save set-up time and collect factual data that will make future set-ups smooth and consistent. Please note that a plethora of formulations exists within a material category, particularly with so many parts and products coming from emerging industrial countries. For instance, there are literally hundreds, if not thousands, of different compositions of polypropylene. Add different re-grind percentages and it becomes clear that printers should not have a preconceived notion of surface tension. Products from different material manufacturers and different lots of manufactured parts should be independently tested so there is a degree of certainty in the decorating process.
2 – Cliché – Purchased or In-House?
This next tip is for printers using polymer plate making equipment. First, if a printer purchases 10 or more clichés a month, it’s time to start producing them in-house. The direct ROI on this type of equipment is just one year when compared to the price of just the 10 mentioned above. After the initial equipment investment, yearly costs will be insignificant. The old method of exposing a cliché and then using a hand brush to remove the unexposed image area is no longer an accepted procedure. A good photopolymer cliché is only 1 mil deep and should have a dot pattern in the image area to hold the cup or doctor blade level across the surface of the image. Imagine hand sanding a piece of wood without any backing and you’ll see the analogy of hand scrubbing to wash out a cliché. The same characteristics are true in the plate making process. Hand brushing the cliché gives an uneven surface across the face and, at the same time, can damage the dot pattern. This compounds the unevenness and it is a slight miracle to produce a cliché that has consistent depth, image, and color. Now before those of you who have hand washed for years start writing me letters – yes, I believe you are good! But can everyone in your department make a good cliché and then a second one exactly the same?
A washout station will take the human element out of cliché making. Anyone who is capable of setting a timer will produce good quality plates. There are savings in time and money when a good plate is produced the first and every time, particularly on multi-color setups. A washout station will improve the quality and attitude of any print making area.
The washout station concept assumes that there is a good vacuum bed exposure unit involved. On a regular basis, the bulbs should be checked and replaced if UV output has diminished. Any unlit or low intensity bulb will expose inconsistently and could cause print problems and/or soft plates that wear quickly.
3 – Maintenance
This next suggestion is a maintenance item. Any type of sealed ink cup must have a parallel and sharp edge with a minimum flat on the edge free from nicks and chips. Even if the cup edge doesn’t have any visible imperfections, it may not clean neatly and an edge touch-up can improve this condition. A lapping kit is a honing plate and a diamond dressing paste. A few figure eights every four to six weeks will help the cut perform in a repeatable, consistent fashion, as well as increasing the life of the ring and printing plate. A lapping kit may be the difference in getting a rush project out the door.