Pad Print Process: About the Pad Printing Process

The Process

The process of Pad printing is the most versatile of all printing processes with its unique ability to print on three-dimensional objects and compound angles. The theory behind the pad printing process was derived from the screen, rubber stamp and photogravure printing processes.

This is how the process works:

	Step 1: Flooding   The image to be transferred is etched into a printing plate commonly referred to as a cliche'. Once mounted in the machine, the cliche' is flooded with ink. The surface of the cliche' is then doctored clean, leaving ink only in the image area. As solvents evaporate from the image area the ink's ability to adhere to the silicone transfer pad increases.
	Step 2: Pick Up The pad is positioned directly over the cliche', pressed onto it to pick up the ink, and then lifted away. The physical changes that take place in the ink during flooding (and wiping) account for its ability to leave the recessed engraving in favor of the pad.
	Step 3: Print Stroke After the pad has lifted away from the cliche' to its complete vertical height, there is a delay before the ink is deposited on the substrate. During this stage, the ink has just enough adhesion to stick to the pad (it can easily be wiped off, yet it does not drip). The ink on the pad surface once again undergoes physical changes: solvents evaporate from the outer ink layer that is exposed to the atmosphere, making it tackier and more viscous.
	Step 4: Ink Deposit The pad is pressed down onto the substrate, conforming to its shape and depositing the ink in the desired location. Even though it compresses considerably during this step, the contoured pad is designed to roll away from the substrate surface rather than press against it flatly. A properly designed pad, in fact, will never form a 0-degree contact angle with the substrate; such a situation would trap air between the pad and the part, resulting in an incomplete transfer.
	Step 5: Pad Release The pad lifts away from the substrate and assumes its original shape again, leaving all of the ink on the substrate. The ink undergoes physical changes during the head stroke and loses its affinity for the pad. When the pad is pressed onto the substrate, the adhesion between the ink and substrate is greater than the adhesion between the ink and pad, resulting in a virtually complete deposit of the ink. This leaves the pad clean and ready for the next print cycle.

 

Limitations of Pad Printing

 

Versatile as it is, the process does have a few limitations. For example:

1. Image transfer is much more efficient when solvent-based inks are used. The use of water-based or UV curable inks are not recommended.

2. The object to be printed needs to have a higher surface energy than the pad. This rules out printing on silicone and other non-stick materials, as well as wet, greasy parts. Additionally, some types of plastic require pre-treatment.

3. Pad printing is limited to relatively small images compared to screen-printing. Pad printable images are usually less than 100 square inches. Large opened areas (>4.0 sq. in) can be difficult to cover with special, screened cliché’s.

4. Pad printing produces a finished ink film thickness of approximately .00025" to .0003" with a single pass. By screen printing standards, this is very thin.

 

Open and Closed Systems

There are two basic categories or types of systems under which all pads printing machines fall: open and closed.Open systems feature an ink trough or basin or some type which is at least partially exposed to the air. Closed systems have "ink cups" that keep the ink from coming into contact with the air.

Basically open systems allow you to print larger images, since more of the cliché’s surface area can be used for the image. Closed systems allow better process control, especially for longer runs, since the ink isn't exposed to the air and doesn't require the frequent addition of thinner. Closed systems typically use less ink, allow for faster cliche' and color changes, and are easier to keep clean.

Pad Printing Machines

There are several basic types of pad printing machines. The majority of machines in use around the industry are referred to as vertical machines. These machines can be open or closed. In closed systems doctoring is achieved by either sliding the ink cup back and forth over the cliche' surface, or by sliding the cliche' beneath a stationary ink cup. The illustration below shows a typical vertical machine with an ink cup.

Some vertical machines can print on any angle using a special pressurized ink cup. Instead of using gravity to flood the cliche', pressurized ink cups use an inflated diaphragm to ensure the image is properly flooded. Pressurized ink cups can be difficult to install and maintain. The illustration to the right shows how a pressurized ink cup works.

Closed vertical machines have also been modified so that their ink cups slide along the X-axis, allowing the printing of wider images. This type of machine is commonly referred to as a "sliding ink cup" machine. The illustration below shows this variation.

By delaying the pad at the end of the print stroke some vertical machines can print 360 degrees around a cylindrical object. The image is transferred as a gear driven nest rolls the part along the length of the pad. The illustration to the right shows how this works.

Without any special pad delays or gear driven nesting fixtures vertical machines can print a maximum of 120 degrees on a cylindrical object. To accomplish this, the pad's set-down point must be top, dead center. The pad is then compressed 60 degrees in each direction from the set-down point.

Horizontal pad printers are really just another variation of vertical machines. Horizontal machines flood and doctor like vertical machines, then actuate the pad to print using a horizontal stroke, rather than a vertical one. The illustration on the following page shows a typical horizontal pad printer.

Horizontal machines are typically used only when the part is too large to fit into a vertical machine, or when the image is too large to fit into a pressurized ink cup.

Rotary pad printing machines are very different from their vertical and horizontal counterparts. Rotary machines have cylindrical cliché’s, usually referred to as "drums". The cliché’s are flooded and doctored as they rotate. The pad also rotates to pick up and transfer the image.

Rotary pad printers can be configured to print vertically or horizontally. Vertical machines can be self-standing for one-up printing, or mounted on automations for tandem or multiple color printing. The illustration below shows a basic vertical rotary design.

Horizontal rotary pad printers are used to print on the vertical sides of parts that are moving along a conveyor. The following illustration is an example of how cassette tapes would be printed on two sides at once using a horizontal rotary pad printer.

Carousel machines allow a part to be printed with multiple colors without ever being moved. Ink cups and cliché’s are mounted on a rotary indexing table which turns to locate each individual cliche' under rotating pads for image pickup. The pads then rotate around on a separate carousel, locating over and compressing onto the part to be printed. These machines are typically slower than their multiple color, standard vertical counterparts, and are used only for printing parts too large to fit a standard machine.

Pad printing machines are driven by a number of different means. Most machines are driven by electronically controlled pneumatic systems and are thus referred to as being electro-pneumatic.

Some pneumatic machines are controlled by air-logic instead of electronics. Less expensive than electro-pneumatic machines, these can be more difficult to control, especially if your incoming air pressure isn't well regulated.

Electro-mechanical machines operate under electrical power only. These machines are usually more expensive than electronically controlled pneumatic machines. They can hold tight tolerances, even at high cycle speeds.

Electro-hydraulic machines are the most expensive type. These machines are rare, and are typically used only for large format applications where additional compression is necessary.

Manually operated machines are useful for small format and short run applications. Images are usually limited to less than 16 square inches, and multiple color printing capability is limited. Many manufacturers of manual machines offer trade in allowances when upgrading to semiautomatic equipment.