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Etch-Depth Consistency on Pad-Printing Plates

(March 2002) posted on Sun Mar 31, 2002

Discover the negative effects of inconsistent etch-depth on plates and learn about considerations for sourcing, storing, and maintaining pad-printing pads.

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By Carol Swift, Peter Kiddell

It's a relatively simple process to make molded silicone rubber pads. Molds can be made of vacuum-formed plastic, polished metal, or epoxy. The silicone rubber used in pad making comes from several sources and is either addition cured or condensation cured. The addition-curing process involves pouring silicone rubber into the mold and placing it in an oven to cure in as little as 2 hr. Condensation curing, on the other hand, is carried out at ambient temperatures and can take at least 24 hr to complete. Both curing methods deliver usable pads, but condensation-cured materials tend to give the best quality prints while addition-cured pads are more robust.

When it's time to choose a pad, you should rely on several criteria. First, what shape is most suitable? Second, what hardness? Third, what material type? And fourth, what base plate material will be used and will it be molded or glued onto a wooden or aluminum base?

Assuming you know what pad shape you need, hardness is the first area to consider. The final hardness of the pad is affected by the characteristics of the base materials it is made from. The characteristics of the base materials will determine how much silicone oil the user will need to add to achieve the target hardness on press.

Ideally, you should measure the hardness when you receive new pads. Fortunately, the instrument to do this, called a durometer gauge, costs only $200-$300. It is effectively a calibrated spring in an indicator that measures the deflection caused when a probe presses down onto the surface of the pad. The device will give you a reading of Shore Hardness.

Pads are offered with hardness levels ranging from 15-70 Shore, but most applications require hardness levels from 40-55 Shore. The softest pads are typically used for prints that require extensive wraparound, while the hardest are specified for printing into recessed or concave areas when very high print definition is required.

Why is pad hardness so important? Consider that with a variation of only 4 Shore from the expected hardness level, you will begin to notice changes in the print produced by the pad. Some pads will be much farther from tolerance than this and significantly alter the printed results. In general, softer pads are less efficient in transferring ink. And pads using materials that have not been formulated correctly will harden over time. When pads are too hard, they do not compress and will not pick up the image properly.

Another matter to address concerns proper pad placement. You should ensure that the pad is attached to the base plate centrally and that the vertical centerline is at a right angle to the base. Frequently, when you replace a pad on the machine, you must also realign the new pad because it is not symmetrical to the base plate.

Proper care of pads also is very important. Always store them vertically on their bases, unless they are supplied in plastic shells that support them without touching the print surface. Keep them clean and do not expose them to strong solvents over an extended period. Remove dried ink with adhesive tape and then clean the pad with a mild solvent. Store pads at 65-75°F (18-24°C) and never lay one on top of another.

If you need to cover pads in storage to protect them from dust, use tissue paper and not plastic film, as plasticizers in the film may leach out and contaminate the pads. Some suppliers coat the pad surface with a gelatinous mixture to protect them in transit.

Keeping these considerations in mind, you can assure that your pads deliver the performance you expect. The benefits will be increased productivity and superior quality in the printed products you deliver to customers.

