Controlling Ambient Conditions in Pad Printing

Controlling Ambient Conditions for Better Quality in Pad Printing - Introduction

After working with pad-printing machines for 20 years, I've seen many improvements in the technology. 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. Yet the basic principles of pad printing have remained the same. So have the basic problems.

I've heard one difficulty expressed time and again: My machine prints well one day and badly the next. The sufferer will confirm that nothing has been changed from the day before - same machine settings, same pad, same ink, same thinner, and even the same press operator. But in most cases, one important variable has changed - its the weather, or at least the ambient conditions within the plant.

Figure 1: Principles of pad printing

To understand why temperature swings and other changes in ambient conditions can have a drastic effect on the quality of your prints, you must go back to the basic principles of pad printing. In order for the silicone pad to pick up the ink from the cliche and deposit it on the substrate, very rapid changes must occur in the surface tension of the ink caused by the evaporation of solvents (Figure 1).

Moreover, pad printing is a thin film process. It starts with an etch depth in the cliche of just 25 microns, and only about half of that ink film is picked up by the pad. Of this wet ink, 60% is a solvent that evaporates, leaving only a 5-micron dry ink deposit. You can easily see why such a thin ink film is so susceptible to changing temperatures, humidity levels, static charges, and even variations in airflow.

Controlling the rate that solvents evaporate from the ink is the key to overcoming these changes in ambient conditions. I f the solvents evaporate too quickly, the ink might not even pick up from the cliche because it has dried in the etched portions of the plate. I f the solvents evaporate too slowfy, the surface of the ink may not be tacky enough to stick to the pad and lift from the etch. Taken to extremes, both conditions have the same result - little or no ink on the pad. Once ink is on the pad, the effects are similar. If the solvents evaporate too quickly, the ink dries and stays on the pad. I f they evaporate too sjowly, only some of the ink will release from the pad onto the substrate.