Overcoming Challenges when Applying Heat Transfers
When hot stamping with foils, the stamping die can hit the foil roll anywhere on the roll to apply the image to the part. Basically, it is a continuous pattern throughout the hot stamping foil roll. With heat transfers, there are a few more sequences involved based on the fact that each transfer on the roll must be registered to the part as it passes through the hot stamping machine.
Although more is involved with applying heat transfers than a one-color image of hot stamping foil, there are many advantages that they provide. Heat transfers allow the decorator to apply multi-colored images in one single pass using standard hot stamping machinery, equipped with an electronic indexer to read the position of each transfer. The decorator can provide customers with multi-colored printed images on plastics parts without having to invest in printing equipment that utilizes inks and solvents. It is a completely dry process with minimal training needed to run the machinery and apply the transfers.
Eye Mark and Registration
Probably the most important aspect of heat transfer decorations is the precise registration of pre-printed images on the carrier film (web) to the plastic being decorated. This is accomplished through the use of registration marks printed adjacent to the image itself but toward the edge of the web, outside of the die area. Marks should be fully opaque and preferably a dark color for maximum contrast with the paper or polyester web. Registration devices on the hot stamping equipment can be conventional photo eyes, laser, or fiber optics. Sensitivity adjustments on these devices permit fine-tuning for optimum reaction to printed marks.
Heat transfer manufacturers will typically ask where marks should be printed, since some (usually older) presses do not provide infinite adjustment of sensor to web. Considerations also should be given to which direction the web feeds through the equipment, i.e. left to right, right to left, or possibly front to back. This also affects how the image is oriented on the web and how it comes off the roll (top to bottom).
Where registration marks are detected, relative to where they are being applied, is also extremely important for precise placement. Since heat transfers are printed with multiple colors in close register using gravure, screen, flexo, or digital equipment, the relationship of one image to the next is typically very accurate. Therefore, detecting registration marks some number of images prior to the one being applied is perfectly acceptable in most instances. Where extremely close print to part tolerances must be held, it may be necessary to use a somewhat wider web so the sensor can be positioned to detect marks adjacent to the actual images being applied.
Transferring Large Coverage Images
The first area to analyze when working with a medium to large-sized heat transfer is to determine the correct tonnage needed. If insufficient press tonnage is being used, a complete smooth transfer will not take place. When applying a heat transfer (or hot stamping foil for that matter) with a silicone rubber die, approximately 500 pounds of pressing force is required for each square inch of die contact area. For example, if the heat transfer image is 1¾ x 2¾ inches, it will require a slightly larger 2 x 3 inch silicone rubber die, totaling six square inches x 500 = 3,000 lbs., or 1½ tons of pressure. Using the 500 pounds per square inch rule for vertical presses, it would theoretically be possible to apply an image totaling 40 square inches with a 10-ton capacity press.
However, having the correct tonnage is only a portion of the story when stamping a large image. Large transfers can present an air entrapment problem that cannot be eliminated by adding more pressure. This occurs between the substrate surface and the printed film/web, or between the top surface of the web and the flat silicone rubber die. The trapped area will cause bubbles in the image that no amount of pressure will displace. To help alleviate this problem, the silicone rubber die can be made with a raised (spherical) center of three to five thousands to squeeze the air from the center of the stamped area outward as pressure is applied. This will keep the air from being trapped between the carrier and plastic part. Dies of this type require careful consideration as to durometer, since higher durometer (harder) dies require more pressure to force the air out, while softer dies may negate the pressure necessary for transfer and also can loose heat faster on automatic or semi-automatic systems.
The alternative to applying the heat transfer with a vertical press when air entrapment is inevitable is to utilize a roll-on hot stamping press with the ability to push air out as the heated roller transfers the image onto the part surface. Although essentially the same rules apply as far as pressure per square inch of die contact area, the die area is a heated roller with line rather than total contact. The line, or footprint of the roller, is determined by the diameter of the roller (usually 6 or 8 inches) and its durometer. A roll-on press is also more tolerant of surface irregularities when compared with flat silicone rubber dies.
Applying Transfers to Cylindrical Parts
When applying a heat transfer to a tube or other cylindrical item, an arbor or mandrel is used for hollow, less rigid products, or a dual (cradle) roll fixture is used for more substantial parts. Products of this type may be decorated up to 360 degrees by rolling them under a flat silicone die with the pre-registered web trapped between the die and part. A heated silicone rubber roller can be used for this type of application as well. If the coverage area on the cylindrical part is less than 85 degrees, a curved silicone rubber die can be made to fit the curve of the plastic part, allowing the image to be applied with a vertical press.
Applying pre-printed heat transfers does not need to be any more difficult than applying conventional hot stamping foil if the equipment and transfer manufacturer is involved early on to critique the part design relative to where the image will be stamped, assuring that the right equipment and tools are utilized to stamp the heat transfer, and the proper chemistry is used for compatibility with the plastic substrate. Having the proper communication lines in place before the project is too far along can help maintain a clean application with few challenges.
Plastics Decorating would like to thank Dennis Cook with Kensol-Franklin (508-528-2021, firstname.lastname@example.org) and Byron Wyche with Si-Cal Heat Transfers (800-451-5588, email@example.com) for their assistance with this article.