Doctor Ring

Doctoring rings for ink cups vary in material and finish. Which ring suits which pad printing application, and how can it be maintained to extend its lifespan?

 

The doctor ring is used in the pad printing process with a closed ink cup. It is the ground ring located at the bottom of the ink cup. The doctor ring rests directly on the cliché and scrapes off excess ink from its surface. Generally, the ring is round, but it can also be oval. Doctor rings are made of either ceramic or hard metal, with ceramic rings typically used for polymer clichés and hard metal rings for thin or thick steel clichés.

Shapes of the Ground Edge

The ground edge of the doctor ring can have different shapes. The first significant shape is the V-shape, where both the inner and outer edges of the ring are beveled. The second commonly used variant is the asymmetric shape, where the outer edge is beveled, and the inner edge remains straight. We offer both variants for our pad printing machines, as they each have specific advantages:

  • V-Shape:
    This shape presses the ink more intensely into the cliché, which is beneficial for applications requiring very high printing precision, such as in the watch industry. However, this shape can create an upward force during ink scraping, which may compromise the seal of the ink cup.

  • Asymmetric Shape:
    This variant applies less pressure to the ink in the cliché and does not generate any disruptive upward force. It is ideal for use with highly viscous inks, large ink cups, or when the ink cup moves quickly across the cliché.

Choosing the right shape and material for the doctor ring is essential to achieving optimal printing results and ensuring a smooth printing process.

Maintenance and Troubleshooting

Although the doctor ring is very hard, it can wear out over time. In this case, it no longer removes all excess ink, and the cliché surface remains unclean after scraping. The issue can either stem from a worn-out ring or a damaged cliché. To identify the cause, the following test can be performed: move the ink cup back and forth manually on the cliché, and observe the defect pattern. Then, rotate the ink cup 90° and move it again across the cliché. Compare the defect patterns. If the defect pattern changes, the issue lies with the ink cup or the doctor ring. If the defect pattern remains unchanged, the cliché is the problem.

If the ring is the cause, it does not necessarily need to be replaced. Instead, it can be reconditioned (lapped) on a lapping plate and reused. We offer such lapping plates. In the video below, you can see the lapping process of a doctor ring in detail.

After lapping the ring, another test is required to verify if the ring is functioning properly or if further lapping is needed. To perform this test, place the ink cup with the ring on a smooth plate, apply silicone remover to the bottom, and move the ink cup back and forth several times. Then, rotate the ink cup 90° and move it again. If uneven streaks or smears form, the ring is still not in good condition and requires further lapping.