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Now there's process-color printing for plastic lids & containers.

A new multi-station dry-offset printing press uses an innovative "wet-on-dry" method that reportedly brings true process-color direct-printing capability to high-volume, rigid plastic packaging.

Developed by Automation Equipment Inc., Norwalk, Ohio, the new process is said to overcome print-quality limitations of the conventional dry-offset, wet-on-wet method. Those limitations include the potential for ink contamination and the inability to obtain more than about 40% screen coverage when overprinting.

Model MSP-5000 printing press applies each ink color separately on fiat surfaces such as container lids, closures, and rectangular sidewalls. Each color is uv dried separately. This process allows solid-color images with 100% coverage to be printed over other solid colors without contamination, the manufacturer reports. The press uses standard photopolymer plates or waterless lithographic plates.

Wet-on-wet limitations

In the conventional dry-offset method, all ink colors are applied together on a large-diameter "blanket" cylinder, raising the potential for ink mixing. Care must be taken when designing the image to avoid an overlap of different colors on the blanket.

"True process-color photorealistic images cannot be realized with wet-on-wet printing," says Daniel J. Lachowyn, v.p. of sales and marketing. "High-quality graphics are not obtainable without resorting to costly labeling."

Some "pseudo-process work" is now being done with new tack-rated inks, but the capabilities are limited, says Lachowyn. When they are applied in the proper sequence, these new inks can overlap without mixing for a limited number of impressions-although no more than 40% screen coverage can be attained when overprinting wet-on-wet.

New wet-on-dry approach

A standard package consists of four stations of two colors (one process/one spot), each with a compact, water-cooled uv-drying unit. The machine has an inline conveying system that holds products onto snug-fitting carrier mandrels. This allows for close registration control between print stations.

Using black and the three process colors - yellow, cyan, and magenta - virtually any color can be produced. Two optional flexographic printing stations allow for application of a solid white background beneath the colored graphic, resulting in even brighter graphics on clear and non-white products. The stations can also apply a clear "overvarnish" for additional gloss and protection of the image.

Tests using Toray Waterless Litho plates have demonstrated photographic reproduction at 200-line screen definition, which is higher quality than lithographic preprinted labels.

The MSP-5000's speed is about the same as conventional dry-offset presses. It can print up to 800 parts/min with dual lanes for standard lids and containers with a maximum 5-in. diam., and up to 200 parts/min with a single lane for 6-in. items or larger.

The MSP-5000 costs $400,000 to $600,000, which is competitive with higher-end conventional wet-on-wet presses.
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Author:Sherman, Lilli
Publication:Plastics Technology
Date:Apr 1, 1998
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