Printing on Hot Corrugated

Jason asks,

Could you explain why it’s not recommended to print on hot corrugate fresh from the corrugator?
I know this isn’t common practice, but there are times when it is necessary when a rush order comes in.

Once corrugated sheets come off the corrugator they should be allowed a bit of time to cure. This will allow the sheet time to stabilize and adjust to the relative humidity of the environment, etc. Slight size changes can take place during this time. If you go straight from the corrugator to the converting process, it is possible that you could end up with a finished product that doesn’t meet the customer’s specification.

Basically ink dries by absorption and evaporation. The chemical characteristics of the ink, mainly pH levels, and temperature, greatly influence drying as well. When you induce significant temperature change to the equation, such as hot board, you change the entire drying process. This will most likely result in ink drying before it has time to absorb into the fibres of the sheet, or it may cause the ink to dry on the plate. Both situations can result in poor coverage/print quality and excessive build up on the plates.

Hot sheets can also cause “baking” of the printing plates which will significantly reduce the plates’ ability to properly transfer ink as well as the overall life of the printing plates.

A hot sheet could also possibly experience more crush as it goes through the converting process than a cured sheet would, and therefore result in a degradation of product quality. A sheet that isn’t properly cured may also be more likely to create rolled scores.

Converting equipment can also be affected by hot sheets. The functional characteristics of urethane components such as feed belts or wheels, transfer belts, pull collars, and in some cases even cutting die rubber can be decreased by hot sheets. The coefficient of friction of feed belts and wheels can be reduced causing registration and skew issues. Die cutting rubber can be overheated causing reduced ejection rates and rubber life.

There are probably other issues that we’ve overlooked here, but I hope this gives you some idea of why running hot corrugated is not a good practice.

— Ralph

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4 Responses to “Printing on Hot Corrugated”

  1. Bill Heintz Says:

    Ralph,
    Hot board [not cured] off the corrugator will retain memory when creased in converting, especially die cutting. Scores will not retain their crisp depth with hot board, as they will with cured board. This will adversely affect the folding and gluing at acceptable speeds. I fully agree with your comments.
    Bill

  2. Bill Heintz Says:

    Have a good day, Ralph!

  3. Brett Kendall Says:

    Wet-strength resins added to the starch adhesive also need time, pressure, heat, and moisture to properly cure. With belted single-facers, we find a minimum stack cure time of 2 hours at 130 degrees F does the trick. Longer is required for pressure roll single-facers. Once you start to shingle those sheets at the converting press the heat dumps and the curing process is effectively arrested.

    I like Bill’s comment too. Scores on uncured board do seem to “bounce back” and that includes boxes treated with cascaded wax. Of all the answers, however, I like Ralph’s comment on increased crush best. Hot board is soft board and the effects on flexural stiffness can be brutal.

  4. bertram karoloes Says:

    WE HAVE A VENDOR THAT IS PRINTING 95 PERCENT OF THEIR PRODUCTION STRAIGHT FROM THE CORRUGATOR AND IT IS GIVING ME A LOT OF GRIEF,
    AS VARIOUS PROBLEMS ARE OCCURRING DURING PRODUCTION RUNS………POOR COVERAGE,INK BUILDUP ON PLATES AND TOO MANY TO MENTION.
    PLEASE ADVISE .
    REGARDS

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