What is pad printing?

A step by step guide - print specialist C. Luca

Definition

Pad printing, also called tampography or tampo printing, is an indirect offset (gravure) printing process where a silicon pad takes a 2-D image from a laser engraved (etched) printing plate (also called cliché) and transfers it to a 3-D object. Thanks to pad printing, it is now possible to print all kind of difficult shaped products such as curved (convex), hollow (concave), cylindrical, spherical, compound angles, textures, etc. which were not available with traditional printing processes.

To be able to pad print you need some essential parts:

HOW TO PAD PRINT?

Step 1: The closed (sealed) ink cup sits over the engraved (etched) artwork area of the pad printing plate, covering the image and filling it with ink. This is called the home position.

Step 2: The closed ink cup moves away from the engraved artwork area, taking all excess ink and exposing the engraved image, which is filled with ink. The top layer of ink becomes tacky as soon as it is exposed to the air; that is how the ink adheres to the transfer pad and later to the product to be printed.

Step 3: The transfer pad presses down onto the printing plate momentarily. As the pad is compressed, it pushes air outward and causes the ink to lift (transfer) from the engraved artwork area onto the pad.

Step 4: As the transfer pad lifts away, the tacky ink film inside the engraved artwork area is picked up on the pad. A small amount of ink remains in the pad printing plate.

Step 5: As the transfer pad moves forward, the ink cup also moves to cover the engraved artwork area on the printing plate. The ink cup again fills the engraved artwork image on the plate with ink in preparation for the next cycle.

Step 6: The transfer pad compresses down onto the product, transferring the ink layer picked up from the printing plate to the product surface. Then, it lifts off the substrate and returns to the home position, thus completing one print cycle.

What can you print with pad printing?

Pad printing is used for printing on difficult products in many industries. 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.

Are you looking for the right pad printing equipment for your application?

Pad printing can also be used to transfer functional materials such as conductive inks, adhesives, flavours, nutritional additives, natural dyes and lubricants.

Conductive inks:

Advances in conductive ink technology – including ethyl-cellulose polymers, carbon/graphite, silver or UV curable dielectric inks – allow for pad printing circuit boards, multi-layer membrane switches, and touch-key applications, among other uses.

Benefits include savings in process and materials cost, such as doing away with etched copper, gold plating or soldered wire. For example, replacing gold plating with a carbon ink has proven greater robustness over that extremely expensive metal, and a lower electrical resistance than copper.

A growing economic sector is using conductive inks for Radio Frequency Identification (RFID), which is rapidly streamlining inventory and shipping in the global marketplace. Other applications for conductive inks include medical sensors, transistors, antennas, and electrodes; shielding against electro-magnetic and radio-frequency interference plus dissipating or eliminating static.

Once again, pad printing’s ability to print on uneven, curved or recessed surfaces gives the technology an advantage over standard flatbed methods of materials deposition. However, as with printing adhesives, conductive inks must be solvent based and neither too aqueous nor too hydrophilic to be effectively transferred with pad printing.

Adhesives:

Applying adhesives is considered by many manufacturers to be a time-consuming and labour-intensive process. Small or translucent parts need precision application, which often drives up the bottom line. When factored in with waste and clean up, alternative methods of adhesive application are much sought after. Adhesive film is one method, but also results in waste materials. Pad printing’s transfer method of precise viscous liquid deposition is the ideal solution. A sealed ink cup keeps the glue from drying out, and the printing plate can be engraved to the desired depth and exact line thickness (or thinness).

Once adjusted, the pad then deposits exactly the same amount of adhesive in exactly the same location every time. As part of an in-line automated process, the finished part – for example a bevelled cell phone lens – can be fed into a custom machined shuttle fixture that holds the part in place while glue is applied to its edge. The lens is then conveyed to a pick-and-place robot arm that sets the cell phone body onto the glued area. Further automation can transfer the joined parts onto another conveyor that transports them through a drying tunnel or further down the assembly line. Ultraviolet or heat-cured adhesives are useable with this process, as are many solvent-based glues that aren’t too aqueous or hydrophilic.

Flavours, nutritional additives and natural dyes:

Pad printing on foodstuffs is another area that is only beginning to see its potential being explored. More than just a high-tech way to decorate cookies with natural dyes, pad printing technology could be used to apply flavours or nutritional additives to receptive food surfaces. The pads used in pad printing are of variable hardness’s, so the end user can choose the “durometer” of their pad depending upon the strength of the substrate. For example, a gumball would use a harder pad to wrap 180° around, while printing a flavour into the hollow of an antacid would need a softer pad, so as not to crush the tablet.

Lubricants:

Another exacting and hands-off use for the pad printing of fluids are lubricants. Again, this is an area where precision can be challenging, and waste is costly and messy. Without re-inventing the manufacturing process by integrating new and different materials such as powdered or dry film lubricants (and their associated unknown problems, costs and technologies), pad printing provides a simple, proven mechanical solution.

Whether it’s an oil, grease or other substance that reduces friction: if it can stick to a silicon pad and can be transferred to another substrate – it can be pad printed. Since pad printing is well suited for use on three-dimensional items, such as hinges, bearings, threaded parts and housings. Any production line that manufactures similar components can integrate this technology into the process. Especially where misapplication can cause catastrophic failure of adjacent mechanisms, precise lubricant application can be achieved with pad printing & whether it be miniscule dots, fine lines and squiggles or exact angular confinement.

What is the future of pad printing?

With the fast pace of medical and technological design advances, finding a tool that bridges the past and future is critical, both on the manufacturing floor and in budget meetings. Pad printing is a tried and true technology that is flexible enough to adapt to a vast variety of substrates and substances. The need to customize materials deposition will only increase as market niches continue to increase and narrow.

Designers, manufacturers, engineers and managers would be well served to explore the ways pad printing can be used for non-decorating, non-identifying purposes. It’s proven that the best tool is one with many uses and ideal for turning a problem into a solution: just add a full measure of creativity and a spark of imagination.

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.

open-ink-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.

half open ink system

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.

closed ink system

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.