Controlling Ambient Conditions in Pad Printing

Adding & Using Pad Printing Solvents

Adding Pad Printing Solvents

In most cases, pad printing requires you to add solvents to your ink reguiarly. How much you add and how often will vary depending on the ambient conditions and also the type of equipment you are using. (Closed-cup presses, for example, prevent solvents from evaporating in the ink well, so they generally don't require solvents to be added during the run.) In a warm environment, you may need to add solvents every 20-30 min.

Always add a measured amount - don't guess!

Here's an example that shows how well this phiIosophy can work. The company in question was pad printing floppy-disk shutters. They used fully automatic machines in an air-conditioned environment, operating them over three shifts. Their output averaged 3000 shutters per shift with an alarming reject rate of 30%.

Obviously, something had to be done. This is the procedure they tried:

1. At the beginning of every shift, a new batch of ink was mixed and a specific weight of solvent was added (a 25/75% mix of retarder and thinner).
2. At hourly intervals, each press was stopped (even if the print quality was still good) and a specific amount of the solvent blend was added to the ink well.

It took the shop approximately two days to find the correct thinner/retarder ratios, amounts of solvents to add, and the length of time between additions. But by strictly adhering to the procedure, production more than quadrupled to an average of 13,000 shutters per shift. Rejects were reduced to 2%. Both product quality and worker morale improved. The staff made further improvements to this procedure and can now regularly attain 15,000 shutters per shift with less than 0.5% rejects.

Using Different Pad Printing Solvents

You can go a long way toward controlling the printing process by using the right type of solvent. Different solvents have different boiling points. Ones with low boiling points are called fast and evaporate quickly; those with high boiling points are catled slow and evaporate slowly. Suppliers normally differentiate between them by letters or numbers. For example, a No.1 would be a fast solvent (known as a thinner), while a No.9 would be a slow solvent more commonly called a retarder).

Selecting the right solvent speed for your application is essential. What is suitable for New Mexico in the summer will not be appropriate for New England in the winter, unless printers in both regions are working in air-conditioned plants with the same temperature and humidity. Your ink supplier offers a range of standard solvents and will help you setect the most suitable one for your ciimate.

If you can't find the ideal solvent, it is possible to mix your own, as the floppy-disk printer did. Use the fastest thinner offered by your ink manufacturer as a base, and then add a slower solvent before mixing with the ink.

Never add a very slow retarder directly to the ink, or you will experience solvent shock; where the ink looks as if it has curdled and will have to be thrown away. Always mix the retarder with a faster thinner first.

When mixing different soivents and inks, follow the manufacturer's recommendations. Measure the components by weight and record the make-up of each mixture you make. And above all, never mix solvents from different manufacturers.