THE PEOPLE OF PRINTING INTERNATIONAL
What is pad printing?
Pad printing is a relatively new printing process compared to screen printing and offset printing. It has evolved from the moment it was possible to manufacture pads from silicone rubber. This process experienced a tremendous growth in the last forty years. This technology has its origins in the watch and ceramics industry.
Pad printing can print all kinds of products that are impossible with other printing processes. These are mainly products that are not flat or vary in size, such as:
- Computer parts, microchips, panels, GSMs
- Rulers, scales on measuring instruments, radio and TV parts
- Electrical equipment, household appliances, advertising products, packaging
- CDs, golf balls, toys, sports equipment
- Medical devices
- Tablets, candies
Printing International focuses especially on specific product groups, such as:
- The pharmaceutical sector for printing on coated tablets and gel capsules
- The medical sector for printing on components of medical devices such as syringes, inhalers, insulin pens, catheters, etc.
- The candy industry where coated sweets and chewing gums are printed.
- Sporting goods
- The cosmetics industry for decorating perfume bottles, lipstick holder, etc.
- Beverage packaging for printing on bottle caps, corks, etc.
Three Working Principles
There are different types of pad printing machines. But all these machines are based on one or two basic operating systems. These two systems are called open and closed system.
The open principle
Step 1: the entire printing plate is covered with ink.
Step 2: scraping the printing plate in such a way that only ink remains in the engraved images. After the printing plate is wiped by the doctor blade, the solvent on the top of the ink starts to evaporate, improving the ink’s ability to wet the pad.
Step 3: the pad is positioned above the printing plate and is pressed onto it to pick up the ink. Due to the changes in the ink, the ink sticks to the pad.
Step 4: the pad is now positioned above the substrate. The ink undergoes the same rheological changes as in step 2. The ink loses its affinity for the pad. Meanwhile, the printing plate is covered with ink again.
Step 5: the pad is pressed onto the product to transfer the image. The adhesion between the ink and substrate is bigger than the adhesion between the ink and pad. During this “step-by-step” impression, the pad is designed to roll on the product from the center to the outside. If the correct tampon is used, the angle at which the pad touches the substrate should never be at an angle of 0°. If this were the case, the air would get trapped between the tampon and the product. This would then result in an incomplete transfer of the image. ●
Step 6: the ink is transferred to the product and the pad returns to its original shape. If the printing went well, the pad should be completely clean afterwards. But often it is pushed extra on an adhesive tape that will remove the last ink residues and dust particles.
Closed chamber system (GKS)
The closed chamber system consists of a doctor blade mounted on a hollow chamber. The ink arrives at the top of the chamber by means of a tube. The ink is distributed over the entire chamber. There is an opening at the rear over the entire length of the doctor blade, through which the ink ends up on the printing plate. After scraping, the excess ink ends up in an ink well. A drain hose is provided on the ink chute, which drains ink to a bucket. The ink is then pumped back to the chamber. This system can be provided with a viscosity system. The closed chamber system is mainly used when large areas have to be printed. Examples of this are the printing of balls and the linear printing of caps.
The closed principle
The closed ink cup system is different from the open-well system in this way, that the ink is not directly exposed to the air. The ink is held in a closed container which also performs the doctoring function. The inking takes place when the ink cup is positioned above the printing plate by a lateral movement. The sharp edge of the ink cup, called scraping ring, behaves in the same way as the doctor blade in the open ink system. The scraping occurs when the ink cup moves over the printing plate. In other machine versions with this principle, the ink cup does not move, but the printing plate moves under the ink cup. In both cases the end result is that the surface of the printing plate is cleaned up and the engraved image is left behind with the ink. Printing International has four different diameters of ink cups as standard, namely 65mm / 90mm / 130mm / 160mm.
step 1: inking the printing plate
step 2: scrape off excess ink
step 3: ink take-up
step 4: positioning for transferring the ink
step 5: transferring the ink onto the product
step 6: end of the printing cycle
The open versus the closed system
Every system has its advantage. With the open system, a larger surface area of the printing plate can be used. This is because the image area is not limited by the diameter of the ink cup. They are also a bit more versatile in terms of changing the ink well and accessories, allowing different sizes of printing plates to be used and multiple colors. The open system is in general cheaper than the closed systems. The main advantage of the closed cup system is that it helps to maintain the solvent balance in the ink. Initial mixing of inks and solvents is as critical as when using an open ink system. So, it happens that too much or too little solvent is added to the ink. Temperature and humidity differences also cause problems in a well-mixed ink. The closed system has been developed to solve these disadvantages and to be able to print at high speed. This system generally ensures better print control because the inks are not directly exposed to the air. Another advantage is that the closed system consists of a smaller number of different components and this results in a quicker color change and faster machine cleaning. The printing plates used for a closed cup system have to be twice the size of an open ink well plates.