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Question and Answer

Preventing Air Entrapment and Gassing with Foil

by Paul Miller

May-June, 1999
When foil stamping a medium to large solid on a coated or varnished stock, there are two potential problems that can occur on press that look almost exactly the same - air entrapment and gassing. What occurs are small pin holes or bubbles that show through the middle of the image from trapped air between the stock and foil or gas that is activated from the heat of the die that is released from the stock. Although both potential problems look much the same on press, ways to combat each are much different. We'll take a look at both challenges and discuss how to prevent and/or combat air entrapment and gassing when foil stamping a solid area.

Air Entrapment On Press

When air entrapment is occurring, a flat engraving comes directly down on the stock with the foil web running in between and air is trapped in the middle of the image. If the surface tension of the stock or coating is high (or has a low dyne count), the air will not breathe through the stock, meaning it has no where to go except through the surface of the foil stamped image.

One of the initial reactions to air entrapment is to add more pressure, thinking the press isn't applying enough to stamp the image. If trapped air is the problem, more pressure will probably not help. Utilizing the right type of foil is a good first step. Certain hot stamping foils are made with a "filled" adhesive that actually can help absorb the air as the image is stamped. It is recommended to keep a few rolls of this type of foil on your shelf for these types of potential problems. If you are still getting bubbles in the image, the next step is to create a dome makeready that will help squeeze the air out of the sides of the large foil stamped area. Begin by using a soft makeready board like polyurethane that will help absorb some of the air. On top of the board, build a round dome by gluing a small piece of thin onion skin or tissue paper in the middle of the image. Next, tear a larger round piece of tissue and glue it on top. Keep doing this until the dome is about 1/8" away from the outer edge of the stamped image. You want to make sure you do not build the dome outside of the image area to keep the focus of the dome in the middle only.

As a last resort, if none of the above helps, you can consider block sanding the sheets. To accomplish this, you simply tape 600 to 800 grit sand paper over the image area of the die and run all of the sheets through the press without foil. This will breakup the surface tension of the sheet and help the air breathe through. Of course, the downside to this option is that the sheets then must go through the press a second time to apply the hot stamping foil.

Gassing Problems On Press

As we discussed earlier, gassing looks extremely similar to air entrapment, but is a totally different problem. Gassing happens when the substrate that you are stamping has a reaction to the heat that is used to apply the hot stamping foil. The size of the image area is not the culprit as it is with air entrapment, although larger images may have more gassing than smaller images. Gassing can be caused by coatings on the sheet, the inks on the sheet, the substrate itself, or possibly a combination of any of the three. To find out for sure if it is gassing verses air entrapment, try stamping the same image on a different part of the sheet where there is no ink or coating. If it bubbles in both places, you have an air entrapment problem. If it bubbles only over the coating or ink, most likely gassing is occurring. Gassing most commonly happens when inks are not completely dry or if the inks have been "wet trapped" under a coating. Very dark inks are the most difficult to dry and need anywhere from 2 to 3 days longer than lighter colored inks.

To help prevent gassing, as with air entrapment, a good place to start is with the hot stamping foil. As there are foil adhesives to help prevent air entrapment, there are foil adhesives specifically developed to combat gassing as well. Again, having a supply of this type of foil in-house can be beneficial to your press operator. If the ink or coating is still wet and that is what is causing the gassing, you should take the sheets and divide them in smaller stacks to allow them to dry faster. If you have in-house UV coating, you can also run the sheets through the coater, utilizing just the UV light to help cure the sheets faster.

If the coating has caused the gassing, and the sheets seem to be dry, you can consider sending the sheets through and coating or varnishing them a second time with a formulation that will except foil. You must be very careful to stamp the sheet with as little pressure as possible in this circumstance to assure that you do not stamp into the coating that is causing the gassing. It is recommended to test several sheets before resorting to the fairly drastic measure of re-coating the entire job.

Pre-Planning is Best

Everything that we have discussed so far are steps that you can take if the job has already been printed and/or coated. If you are able to be involved from the beginning of the job, potential problems can be cut off at the pass before they get on press.

First, with a potential air entrapment problem on a large solid area, you may want to consider foil stamping the job on a cylinder hot stamping press. With cylinder foil stamping, the action of the press allows the air to be pushed out as the foil is laid down on the sheet, eliminating the problem of air being trapped inside the image. If this is not an option, there are several things that can be done to the die by your engraver that can help with air entrapment. If you know in advance that you are working with a large solid on a coated sheet, you can ask your engraver to have the middle of the die made a few thousands thicker than the outer image area. By doing this, the center of the die will hit the stock first and push the air out around the image. Secondly, if you have a large image with some reversed out areas, have your engraver drill holes in the die where the image does not stamp. This will allow the air that may be trapped between the die and stock to escape through the die. Lastly, you can consider incorporating a texture within the image area. A textured pattern throughout the image will create a unique look and break-up the large solid that can cause the pebble or bubble effect within the foil stamped area.

If you foresee the potential of a gassing problem on a job, work closely with your printer to test the inks and coatings with the chosen foil before the job is run. This is especially important with a medium to large solid foil stamped image. If you find a certain type of coating or ink that seems to accept foil very well, keep a record of the manufacturer and recommend it to your printing customers. This can save everyone an enormous amount of headaches and time. Also, try to have sheets coated off-line rather than in-line with the printing whenever possible. This will provide adequate time for the inks to dry and avoid wet-trapping the inks between the stock and coating.

Of course, all of these pre-planning guidelines are easier said than done. However, the more jobs that the trade finisher can become involved with from the beginning, the better experience your customer and your customer's customer will have with the use of hot stamping foil. And for those jobs where pre-planning is not an option, hopefully the on press tips for both air entrapment and gassing will help. Between all the different formulations of coatings and inks due to continuous EPA regulation changes, and all the different types of stocks to choose from, air entrapment and gassing continues to be a challenge to foil stampers everywhere. The best advice is to try to keep up with all the new changes as much as possible and conduct tests with various foil formulations to determine the best choices for certain applications.