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Question and Answer

Challenges with Mounting/Laminating

by Jeff Peterson

November-December, 2000
Mounting/laminating can be found in many graphic finishing shops. The market for interesting, attention-getting P.O.P. and counter displays continues to grow (see Success Story on page 49 of this issue). InsideFinishing has compiled a few of the challenges mounting/laminating creates for finishers and has called upon a couple of experts in the field to discuss solutions to potential problem areas.

Why and how do litho laminated sheets warp and what can be done to control it?

Warping occurs when there is a difference in moisture content, temperature, and relative humidity within the sheets to be processed. Both the printed sheet and the substrate it is mounted to contain fiber. All fiber will “swell” or “grow” as moisture is absorbed into the material. The direction of growth will generally be opposite of the grain direction. In other words, the fiber will generally swell more in the width direction than in the length direction. The dryness or wetness of the substrate will affect the degree of warp because moisture will absorb faster or slower depending on the condition of the sheet. It is recommended to test the raw materials to identify the humidity or moisture content of the materials before they are run on the mounter. This will help to understand the acceptable range of good materials.

The way to reduce warp is to introduce as little moisture as possible into the sheets. The adhesive should contain as high of a solid content as possible while still providing adequate adhesion and metering satisfactorily. The adhesive should be metered to a very thin film and should be introduced only to the flute tips if laminating or to the printed sheet if mounting or labeling. Newer laminating equipment in the market will allow the sheets to be run with the flutes through the machine which is a better method for applying adhesive - to the flute tips only. When flutes are run across the machine, adhesive is wiped by the flute tips and actually will be plowed between the flutes and more moisture than necessary will be applied.

When labeling or mounting printed sheets (normally 70# or 80# litho stock) to double-faced corrugated, the adhesive used is normally a dextrin adhesive. This adhesive should be metered as thin as possible while still producing a good adhesion to the substrate. Applying the adhesive to the label (printed sheet) is generally the most efficient method for minimizing warp. When the adhesive is applied to the substrate, more adhesive is required to keep the surface wet before the label is applied, thus more moisture is introduced because the substrate is more absorbent than the printed sheet.

The following are additional tips to help reduce the possibility of warped sheets:

  • Provide at least 48 hours for the sheets to adjust to the factory conditions before mounting/laminating
  • Have the sheets come to you shrink wrapped and do not unpack until the time for mounting
  • If partial loads are to be run, repack the unused sheets immediately
  • If possible, ask your supplier to supply materials within a set moisture content range
  • Flip the final mounted product over after it has come out of the machine to help break the warp as the product cures
  • Run the printed sheets grain across the machine and run the grain of the substrate that it mounts to the opposite way

How can bubbles and/or blisters be controlled with laminated/mounted sheets?

Blistering can occur when the mounted sheets have sufficient glue coverage yet there is not immediate adhesion across the entire surface. This can be caused when there is heavy ink coverage in different areas of the printed sheet. The heavier areas of ink will be drier and will absorb more moisture, causing those specific areas to grow or swell. Differing moisture content across the label, heavy coatings, and overall poor surface wetability can also contribute to blistering or bubbling. Another term for blistering is tunneling. Tunneling occurs when a bubble at the edge of the sheet moves toward the center as the product sits. This is usually caused by poor storage and/or wrapping of the printed sheets. Moisture in the atmosphere is absorbed into the pile and will only penetrate the first few inches into the side of the pile, thus absorption will be different throughout the sheets and fibers will grow at different rates.

Curing blisters or bubbles can sometimes be solved by using a more aggressive adhesive that will bind the sheets together faster, so the fiber in the sheet does not have time to swell. On the other hand, if the cause of the bubbles is heavy ink or coating coverage, you may want to consider using a dextrin adhesive, and then stack the materials under weight. The blisters will work out as the glue sets up. A dextrin adhesive dries slower so the set-up time is not too quick. A fast drying glue can sometimes lock in the bubbles. As you can see, these two suggestions depict opposite extremes for addressing the problem. Your specific application will determine the best choice.

To help decrease the possibility of tunneling, as discussed earlier, when the printed sheets are exposed before mounting, it is suggested to break the sheets down into small stacks, which can adjust more readily to the factory environment.

As in most situations, it is always recommended to contact the manufacturer or distributor of your machine when having a specific problem on press. However, warping and blistering are two very common challenges that, in many cases, can be worked out by the operator with the proper tools and information.

InsideFinishing would like to thank Tom Davies of Automatän (715-341-6501) and Tim Nells of Diamond Machinery Company (440-286-4969) for their assistance with this article.