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Question and Answer

Working with Difficult-to-Stamp Surfaces

by Staff

August-September, 2008
Foil stamping can be tricky business, and certain types of inks, stocks, and stamping surfaces can add to the challenge. Recently, Jim Kingsby, tech service manager for ITW Foils, and the lab technicians at ITW have completed a troubleshooting guide that includes solutions to many common questions. This article contains a sampling of information contained in the guide as it relates specifically to difficult-to-stamp surfaces, as well as additional information on the subject gathered by InsideFinishing.

What common signs indicate that the ink is not dry enough for foil stamping? What are the suggested solutions?
If the ink on the printed sheet does not have time to properly dry and an operator begins to foil stamp, many times the foil product itself will begin to smear when a finger is passed along the print area. If an operator can see ink from the sheet appear on the polyester film carrier after the image has been stamped, that is another sign of a sheet that is not completely dry. In addition, when the ink offsets onto the sheet below on the feed table or on the feed table rollers, it is a sure-fire sign that the ink is not ready for foil stamping application.

The solution when working with wet ink is to simply allow the proper drying time before stamping. If these problems are occurring, the operator should double check with the printer that wax-free inks were used on the job. If the inks were not wax-free, or if rubber-based inks were used, the job will be very difficult or impossible to stamp over the litho.

To assist with helping the sheets dry faster, reduce the pile height to smaller stacks and allow air to flow through the pile. If there is little or no time to allow the sheets to dry on their own, they can be sent through a dryer of some type, such as a UV tunnel. Remember that black and dark blue are two of the most difficult inks to dry. Also, if a great deal of black coverage exists, it is recommended that the black be printed with one or two colors. Try to stay away from process black – this will take much longer to dry.

What problems can occur with foil stamping over UV coatings or varnishes? What can be done to work with these problems on press?
The most common side effect of trying to foil stamp on a UV surface is a shark-tooth appearance at the edges of the foil stamped image. The main reason this occurs is a coated surface with less than the required dyne level to accept the foil. UV also causes problems when silicone is added to the UV. This, too, can cause a shark-tooth appearance on the foil stamped image.

If there is a significant amount of silicone in the coating, then there is very little you can do to get the foil to adhere to this surface. The silicone will create a slick surface and simply not allow the foil to adhere. However, if the cause of the coating problem is the lack of a proper dyne level, there are several possible solutions.

First, the operator should check the dyne level by utilizing a set of dyne pens that measures the surface tension of the sheet. To foil stamp or glue, the dyne level must be at least 38 and even a bit higher for a good quality stamp. If the dyne level is not at 38 or higher, one solution is to corona treat the sheets through an off-line corona treatment system. The corona treatment will change the molecular make up of the coating on the sheet and raise the dyne level to a proper level for stamping.

Another solution is to texture the foil stamping die or the substrate. This can be accomplished by asking the die supplier to sandblast the stamping die before it is shipped. The operator also can place a piece of sandpaper on the makeready board with the grit facing the die, make several impressions without the foil product, move the sandpaper into a new position, followed by several more impressions. This will create a sandblasted effect on the die. The texturing of the die will help relieve trapped air on the sheet surface and should allow the foil to adhere without trapping air. Remember that texturing the die will dull the foil and create a satin look even with a bright metallic foil.

There are other steps that can be taken to help relieve trapped air on the coated surface. Dome the makeready to allow any gas or trapped air that has built up between the coating and the die to escape more easily from the center and flow to the edges of the foil stamped image. Also, make sure that the operator is utilizing a foil product that is formulated for UV surfaces or surfaces with lower dyne levels. This will help ensure a higher quality stamp, especially if the dyne level is hovering around the 38 level or a little below. Last, satin foils seem to perform better on UV or varnished surfaces. Consider going with a satin gold if the customer will allow it.

What precautions should be taken when applying foil over foil?
Often when an application includes foil stamping one foil over another foil, the operator is apt to be lax in his concern on press, taking for granted that all hot stamping foils are easily overstampable. This is simply not the case. If the foil is beginning to pick or uneven coverage is occurring on the second pass of foil, it is a clear sign that there is a problem.

Specific hot stamping foils are manufactured to be overstampable, but some formulations are not. It is very important to check this before beginning a job where one foil will be applied over another. However, this may not be the only reason for a potential application challenge. If the first foil stamping pass was stamped with too much impression strength or a soft makeready board, this will leave a deep groove around the image and the operator may have problems with coverage on the second pass. It’s possible that a visible indented line will show where the second pass of foil stamps over the top of the first. This can be prevented by using a hard makeready board such as epoxy glass board. This will allow the first foil to be applied with less pressure, keeping the first pass from being stamped too far into the stock.

Another problem can occur with overstamping if the first pass was applied at a temperature that is too low. If this occurs, it can leave a residual wax release coat on the surface of the image area. This will make the second foil pass adhesion very difficult. It is recommended that the operator run the first pass of foil at a fairly high temperature, higher than usual, to make sure that the wax release coat is completely removed, creating a more suitable stamping surface for the final stamped image.

Foil stamping on plastic sheeting continues to be a popular application. What are some of the precautions that should be taken when foil stamping on a plastic surface?
As with difficult-to-stamp coatings, the first steps when foil stamping on plastic are making sure that the plastic has been properly treated and has a dyne level above 38. Without pre-treating the plastic sheet first, the problems with ‘shark toothing’ or pin-holes will occur when trying to stamp.

Just as there are a variety of foils for various types of paper stock, there are specific adhesive formulations for plastic. The foil stamper needs to check with its foil supplier and order a foil product specifically formulated for plastics – a graphics foil will not be a good choice for these applications.

Because plastic can cause more problems with gassing or air entrapment than most paper stocks, utilizing a cylinder-style machine to apply the foil is a good option, if available. Applying the foil through a cylinder foil stamping machine will allow the foil to be laid across the plastic sheet. This stamping action differs from the vertical platen style machines that press the hot stamping die directly into the substrate, thereby trapping air. Another advantage with the cylinder-style machines is that the rolling action also can help keep the plastic from overheating and melting during the foil stamping process.

Utilizing a hard makeready, such as epoxy glass board, is a good choice for plastic sheets. This will help to keep the die from penetrating too far into the sheet, which can leave an outline on the back of the sheet. In addition, a domed makeready is recommended to help squeeze out any air or gasses that might be trapped between the die and the sheet. This is especially important with a larger flat stamp image area.

InsideFinishing would like to thank Jim Kingsby, tech service manager for ITW Foils, and the entire Lab and Technical Service Team at ITW Foils for their assistance with this article. Kingsby has been a long-time consultant for the industry, conducting hundreds of seminars and writing articles and manuals on foil stamping and embossing processes. For more information on the ITW Foils Troubleshooting Guide, contact ITW Foils at (800) 942-9995 or e-mail jim.kingsby@itwfoilmark.com.