Many things are happening during the sleeking process that changes the size of the paper. Some people will say the sheet grows as it runs between the heated rollers of the digital press or laminator. The heat and pressure rolling it out like calendared plastic. Others will say no the sheet shrinks as the fibers are being dried out, as the heat and pressure wring the moister out of them. It could very well be both are true. This could very well be what environment you print in. Wet or Dry, what is the moisture content of the paper? What is the force of the pressure? What I can say for sure is these forces do change the size of the sheet. On a 4-process sleeking job there are 2 trips through the digital press and 2 trips through the laminator. This in turns plays with registration. Now if this is a 2-sided piece then there are more trips through the presses. So, what are the steps to minimize the registration issues? What we find is the registration is usually good on the first crop mark and gradually gets off as you travel down the sheet. The more printed upon a sheet the worse the issue becomes. Some are using smaller sheets 8 ½ X 11 so there is less sheet to change. First only work with one side of the sheet at a time, front then back. Only print in simplex for the best registration. When laminating or sleeking run the sheet landscape. This will have the least amount of time when the sheet is under heat and pressure. It may depend on the direction, which is more critical. Some have been able to creep the second printing to make up for the change of the sheet. You can also trap some of the areas as well.
This is when we see corners of letters or shapes missing. These are usually small edges or fine points. What we need to explain here is that the ink on the sheet is the only thing that is carrying the sheet through the laminator during the sleeking process. If there is not enough ink on the sheet to carry the weight of the sheet, separation happens while the ink is still hot. This causes the corners to pick. What we have found to be the most helpful for this issue is to print a 3/8 box around the edge of the sheet, outside the crop marks. This extra ink will carry the sheet through the laminator. Not depending on theses little points or small edges to carry the weight we get much better results. Time is another factor that plays heavily on picking. The ink has a cure time of 24 hours to cure. Now we all know that in a demanding time we do not always have this luxury. If you can only give some time 2 hours may be enough. To ensure that picking does not happen the ink and the foil mustn’t separate horizontally. The ink and foil need to stay together until the foil is pulled vertically away from the ink. This means that the pull rollers need to be turned off. The tension of the take-up roller is what should pull the film from the hot roller to the take-up angle or 90-degree roller. If the film is separating before this point, then you need to loosen the tension of the take-up roller. (Some machines do not have a flatbed through the laminator. The film will separate on the de-curl bar. There may not be anything you can do to prevent it but try to loosen the tension as much as you can.)
These are lines that run across the large sleeked areas. This happens when the sleeking foil is wrinkled under the hot roller. This is a tension issue. What we have found is less tension is usually better than more. If you loosen the foil until you see a diamond pattern in the foil as it crosses the hot roller these wrinkles will usually disappear. If there are deep wrinkles in the foil before the hot roller then the issue is in the foil, change roll. If your machine is equipped with a wrap less roller kit the spread roller will eliminate wrinkles.
Dots on Sheets
If the sheets are printed on a color printer there could be a anti counterfeit dot pattern printed on the sheets. These dots can pick up the sleeking foil. Some printers will not print this pattern if black only is used or yellow is turned off. Some sheets are coated with a coating that is incompatible with the sleeking process and the whole sheet will be covered with dots. Sometimes sleeking film will not show the dots and stamping foil will. These sheets can be used if your laminate them with a printable laminate first.
Not all Paper Will Sleek
It is best to qualify the paper before you sell the job. We recommend that you find 4-8 sheets that you know work well with your equipment and offer these. You need a 3 star or greater HP qualified paper. You may find others that will work. I have seen felt paper with many hits of ink work very nicely I have seen some paper not work at all. Please test your paper before you print.
I have not seen ink build to be as big of a problem as we once thought it was. What is more important is that we have enough ink to do the job. If the sheet is a smooth coated sheet, then 4 hits is good. If it is an uncoated sheet, then we may need 8 hits of ink. The rougher that sheet the more ink you need to fill the valleys. I say start with these hits and test. If you are satisfied with less then good, if not use more. Remember you are producing a piece that should demand enough margin to pay for the extra hits.
The recommended temperature of stamping foil is 105c or 221f. you can try 100c to 110c some machines are calibrated differently so you may have to run hotter or cooler.
Stamping or Sleeking
Stamping foil is the recommended foil for over printing. You will still need to run a cleaner sheet every 15 to 25 sheets. Sleeking foil is not printable. I think Stamping foil is the best all-around foil. I have found it to work better in most situations than sleeking foil. There may still be some situations where sleeking foil is better. It was once thought sleeking foil over lamination and stamping foil over paper. From what I have witnessed out in the field I would say Stamping foil works better most of the time. It is very easy to tell what kind of film it is. Dull backing is sleeking foil. Shiny foil is stamping foil. Rose gold sleeking foil works great everywhere.