Preprint is the term used to define roll to roll printing of the outside liner of a corrugated box, before corrugating and converting operations. In corrugating, the corrugated medium (fluting) is glued between the preprinted liner and the inner liner, giving the structure rigidity, strength and cushioning. During corrugating the liners pass over heated plates curing the glue, and subjecting the face of the liners to high heat and pressure such that preprinted inks must be protected.
The key to this protection are functional top coatings that are applied over preprinted linerboard inks. These coatings are designed to resist high heat, steam, hot plate pressure and scuffing from belts and rollers during combining, as well as rubbing as the stock moves through die-cutting/folder gluer box forming operations. Both aqueous and UV coatings are capable of offering the necessary ink protection with dedicated formulations passing hot rub, dry rub, wet block, and slide angle requirements.
Growth of Preprint
The major reason for the growth of preprint is the recognition that a visually appealing, communicating packaged product will influence consumer buying decisions at the “point of purchase”. This is reinforced by studies that indicate that nearly 80% of buying decisions are made at the “point of purchase”.
Massive changes in merchandizing with the influx of big box stores, centralized checkout, and internet selling, are changes that have essentially eliminated the knowledgeable salesperson, placing the onus on a packages capability to sell itself and the product within.
Packaging as Communication
As such, the package has to have great communicating graphics, offering visual impact, brightness, realistic color, and product information. Value added high quality color graphics are the KEY to garnering the consumer’s attention with protective, high gloss top coatings, the final touch.
Preprinted liner has put a high graphics face on corrugated packaging using expansive visual communicating graphics to effectively sell products without the need of a salesperson.
- Consumer goods
- Food and drink
4 Methods of Adding Graphics
Manufacturers of corrugated have four methods to add graphics to corrugated packaging; post print, litho labels, preprint and a newcomer, digital ink jet. All of these are capable of being enhanced and protected with the addition of aqueous or UV top coatings.
Long used, flexo post print has graphic quality limits in that the printing is done on the wavy surface of combined corrugated board. While much advancement has been made to improve fidelity, the product falls short of the graphic demands of the fast evolving merchandizing marketplace.
Aqueous and UV top coating is done offering protection and glitz to attract consumer attention.
Litho labels have their place, but the process of laminating preprinted coated paper to a finished carton has proven to be costly. Never the less there is widespread use of sheet fed printed litho labels from small to very large, exhibiting high impact graphics finished with either aqueous or UV top coatings.
Digital inkjet sheetfed and web printing processes have been making inroads into the label, folding carton, and flexible packaging markets, and now the corrugated market is being targeted. Advantages are perceived to be short order print run capability to produce individualized customer changes in SKU numbers, models and products, etc.
Flexo preprinting of linerboard for corrugated packaging has evolved to become the process of choice. Substrates include Kraft, Mottled White, Bleached White, and White and color coated, with grades from very light to heavy, ranging from 20# paper to 24 point board plus poly, foils, plastics, metallic’s and Mylar allowing a panorama of graphics.
Multi-color (up to 10 colors plus aqueous and/or UV top coatings) flexo press systems are the commercialized production backbone of preprint. These print/coat systems have proven to be a cost effective, high volume, high fidelity print quality, and registration accuracy means of producing preprint liner board.
The Central Impression
The central impression, or CI web fed flexo press is the mainstay of the preprint and flexible materials industry. CI press architecture arranges print units around a single, large diameter impression cylinder. These presses use photopolymer printing plates featuring a mirror image of the planned printed image. The plates raised image areas receive ink applied to them by an anilox roller for transfer onto a substrate. The anilox roller cylinder is engineered of metal and/or ceramic and is laser engraved imparting small cells at a specific angle, line screen, and volume to produce a precise fidelity of print.
Chambered Doctor Blade
A chambered doctor blade system is utilized to deliver a measure of ink to the cells of the anilox roller. In the printing process, ink is pumped into the chamber of the two blade doctoring system. There, the barrier blade, and the doctor blade work to seal the chamber confining ink within, while in contact with the anilox roller. The cells in contact with the doctor blade system collect ink as the anilox roller rotates with any excess ink being removed by the doctor blade. As the anilox is rotated further, its surface makes contact with the raised image surface of the printing plate mounted to the plate cylinder, transferring its cell contained ink. Next, the printing plate rotates transferring the inked image to the substrate. Hot air, infrared, or UV driers are mounted between the print units to dry/cure inks.
Continuous Run Belt Press
Newer technology in the form of a continuous run preprint flexo belt press offers the opportunity to print for a high quality short order market segment, especially food and beverage called, “Preprint volume on demand”. The press with a capability to vary repeat length features print units equipped with one print cylinder, one impression cylinder and one chambered doctor blade anilox. Unique press capabilities are post print conversions, with one job running while another is set up, and large format repeat length.
Any Questions about Pre-Print Coatings? Speak with the Coatings Experts at Cork Industries: (800) 394-9550 or Contact Us.