Back Coating with Hot Melt Machines: Applications and Process Integration
Back coating—applying an adhesive layer to the side of a substrate opposite a primary coating or to the back side of a web for subsequent transfer—is a critical process in the manufacture of double-sided tapes, transfer adhesives, protective films, and certain label constructions. In double-sided tape, for example, a carrier film (e.g., PET) is coated with pressure-sensitive adhesive on both sides, then wound with a release liner between layers to prevent blocking. Back coating can be performed as a separate operation or inline with front coating using multiple coating stations. When using a hot melt machine for back coating, the primary considerations are preventing adhesive transfer to non-target surfaces, managing release liners with differential release properties, and controlling web tension to avoid stretching or wrinkling of the coated substrate. The process typically involves a slot die or gravure coater applying adhesive onto the back side of the web while the front side is protected by a release liner or chill roll.
For double-sided tape production, the back coating process is often executed in two passes or in a single pass with two coating heads. In the two-pass method, the first side is coated onto a carrier (e.g., PET film) and wound with a release liner on the adhesive side. The roll is then unwound, and the second side (the back) is coated using the same coating head, while the first side is protected by the liner. In the single-pass method, the web passes through two coating stations: the first applies adhesive to the front side, a laminating nip applies a release liner, then the web is turned over (using a turning bar or reversing rolls), and the second coating station applies adhesive to the back side. A second release liner is then laminated, resulting in a double-sided tape with liners on both sides. The challenge is maintaining registration between the two coating layers and ensuring that the first liner does not shift during the back coating process. Tension control is critical; the web must be stable but not stretched, as stretching changes the width and would misalign the second coating with the first. Many lines incorporate a tension isolation roll between the two coating stations to decouple the tension zones. The dual unwind and rewind options available on LP Series hot melt coating lines, including double unwinder manual splice and turret unwinder/rewinder with automatic splice, facilitate such multi-pass operations.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Transfer coating is a specialized form of back coating where the adhesive is first applied onto a release liner (carrier) and then transferred to the final substrate in a separate laminating step. This process is used when the final substrate is heat-sensitive, porous, or difficult to handle. The coating procedure involves using a solvent-free hot-melt adhesive coating system and a short period of exposure to the environment of the web being processed between the time the web is coated with adhesive and spooled for packaging. The adhesive is applied to a differential-release-type substrate—one side with a relatively difficult-to-release coat and the other side with a very easy-to-release coat. When the adhesive-coated substrate is eventually unwound for use, the back coating releases easily, leaving the adhesive on the desired surface. The use of a hot melt adhesive system reduces drying time between coating and respooling, which minimizes moisture-induced “tubing” (web curl) and dimensional changes known as “gapping” that affected prior art products. Hot melt PSAs are substantially free from volatile solvent components, eliminating the shrinkage issues associated with older solvent-based systems. Transfer coating is commonly used for label stock, medical plasters, and automotive foam tapes.
Back coating onto release liners requires careful selection of the release coating. Silicone release liners are most common, available in light, medium, and heavy release levels depending on the adhesive’s tack. The release level must be matched: for high-tack adhesives (e.g., medical PSAs), a heavy release liner is needed to ensure the adhesive releases cleanly during use; for low-tack adhesives, a light release liner suffices. The release coating must be applied uniformly and cured completely; any defect in the release coating will transfer to the adhesive surface, causing adhesion problems. For double-coated tapes, the two liners often have different release levels: the “tight” liner (harder release) is intended to be removed first, while the “easy” liner stays on the other side. When back coating, the liner must be properly aligned and tensioned to prevent wrinkles. Edge guides on both the liner unwind and the rewind ensure that the liner remains centered on the adhesive-coated web. Some lines incorporate a slitting unit immediately after coating to trim the edges, removing any adhesive that may have migrated beyond the liner edge, which would cause edge buildup on subsequent converting equipment. The LP Series lines offer accessories such as edge trimming unit, slitting unit, web cleaner, antistatic solutions, and spreader rollers to support such quality-critical operations.
Process parameters for back coating differ from front coating due to the presence of the release liner. The liner’s surface energy is much lower than that of a typical face stock, which can cause the adhesive to dewet or retract, creating voids. To prevent this, the adhesive must have excellent wetting properties, often achieved by using a specific adhesive formulation or by increasing the application temperature to lower viscosity and improve spreading. The backup roll in back coating is typically a chrome-plated steel roll with a release coating or a rubber roll with a low-friction surface. A doctor blade may be installed to clean the roll continuously, preventing adhesive pickup. The cooling section must be placed after the back coating station (and before the laminating nip if a second liner is applied) to solidify the adhesive before winding. If the adhesive is wound while still molten, it may block (stick to the liner’s back side) or ooze out at the edges. The chill roll temperature is typically set to 10-20°C for most PSAs. For transfer coating, the cooled adhesive-coated liner is rewound as an intermediate product, then later unwound and laminated to the final substrate in a separate heating and pressing operation. This decoupling allows the coating line to run at optimal speed (e.g., 300 m/min) while the laminating step runs slower for delicate substrates. By mastering back coating techniques—differential release liners, transfer coating, edge trimming, and temperature control—manufacturers produce high-quality double-sided tapes, transfer adhesives, and specialty laminates that meet demanding performance requirements.
