Pad Printing: Controlling Ink Delivery

Controlling Ink Delivery in Pad Printing

Many of the problems printers experience can be traced back to solvent evaporation in the ink well. One route around the problem is to control this evaporation, either by carefully metering the flow of ink to the machine or by using an enclosed ink cup. You can go a long way toward controlling the pad-printing process by using the right type of solvent.

Gravity feeding can be somewhat intrusive. You can purchase more sophisticated ink-pu mping systems from equipment suppliers or build one using peristaItic pumps. As with gravity-feed systems, selection of the tubing is important, but even more so since these pumps operate by compressing a tube in a circular action and forcing the ink along it through positive displacement. This compressing action puts a mechanical stress on the tube that, combined with the attack from the solvents, can cause failure in a very short period of time uniess you use the correct tubing. Pump suppliers offer a range of tubing materials and can suggest the best one for your needs.

You can operate an ink pump at a very low fffow rate to keep your ink well topped off, or you can set it up in a flow-and-return configuration, where ink is pumped through the ink we11 into a reservoir with a capacity up to l/4 gal. Any solvent evaporation that takes place in the ink well will be minimal compared to the total vdume of ink in the system, so the overall inklsolvent balance will be maintained.

Solvent-feeding systems work on a similar principle. Gravity-feed systems have been used with reasonable success, while powered hypodermic syringes have been tried with fittle Iuck because the solvents destroy them gradually. Pumping systems work best, but must provide a means to mix the solvent into the ink. The position in which the solvent is fed into the ink well is very important. Some suppliers provide systems with modified spatulas that help mix the solvent into the ink.

Several solvent dosing systems are available commercially, or you can build one using a peristaltic pump.

Points to took for in a pumping system include flow control, tubing materials (again, solvent resistance is crucial), and ease of cleaning.

Presses with closed-cup ink containers have been available for several years and have become much more popular for in-line plastic-moulding applications and in areas where VOC emissions are a concern. Ink stability in these presses is much better than in models with open ink wells, although they are affected by ambient conditions when the pad picks up and deposits the ink. These presses also allow faster printing speeds because you can use very fast solvents that would evaporate far too quickly in an open ink well. However, the use of two-component inks, pa rticulariy isocyanate-based ones, is not recommended with these presses. Such inks will gel in the cup if not agitated and can reduce the effective operating period of the press from 8 hr to as little as 2 hr.

Introduction .. Upgrades in inks and pads have helped make the process more controllable. Better machine design has enabled better consistency in setting and (more importantly) easily adjusting each phase of the print cycle..
Adding & Using Pad Printing Solvents In most cases, pad printing requires you to add solvents to your ink regularly.
Controlling Ink Delivery Many of the problems printers experience can be traced back to solvent evaporation in the ink well.
Controlling Temperature & Airflow The more stable the environment, the more effectively you can control the process.
Controlling Humidity and Static Another troubleshooting step that some printers overlook is how fast the press operates.
Controlling the Variables We've looked at how ambient conditions can affect your print quality, but these aren't the only variablesin the pad-printing process.
How to Handle Temperature Swings In a warm atmosphere, solvents in the ink will evaporate very quickly.