Transfer hot melt coating machine
A transfer hot melt coating machine uses an indirect coating method: hot melt adhesive is first applied onto a temporary carrier (usually a release liner such as silicone-coated paper or film) and cooled to form a self-supporting adhesive film. In a subsequent operation (either inline or offline), this adhesive film is transferred onto the final substrate by applying heat and pressure. This two-step process is essential when the final substrate is heat-sensitive (e.g., thin polypropylene film, foam, or tissue paper) or when the adhesive cannot be applied directly due to wetting issues. Transfer coating is also used to produce double-sided adhesive tapes, where the adhesive film is sandwiched between two release liners or transferred onto a carrier film.
The machine configuration typically includes an unwind for the release liner, a hot melt coating station (often a slot die or roll coater), a cooling section (chill rolls), optionally a laminating station for the final substrate, and a rewind. In the first stage, the release liner passes under the coating head where a thin film of hot melt adhesive is applied. The coated liner then travels over a series of chill rolls that solidify the adhesive into a non-tacky film (if using a non-pressure-sensitive adhesive) or tacky film (if using PSA with a release liner). The liner with the adhesive film is rewound as an intermediate product. In the second stage, this intermediate roll is unwound, the adhesive side is brought into contact with the final substrate under a heated laminating nip, and the release liner is peeled away, leaving the adhesive on the final substrate. Alternatively, for double-sided tapes, the adhesive film is transferred to a central carrier film.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Advantages of transfer coating: 1) Protects heat-sensitive substrates from the high temperature of molten adhesive (since the adhesive cools before contacting the final substrate). 2) Allows inspection of the adhesive film for defects before application, reducing waste of expensive final substrate. 3) Enables coating of very low coat weights (2-10 gsm) that would otherwise penetrate or cause strike-through on porous substrates. 4) Facilitates the production of adhesive films that can be stored and sold as transfer tapes (e.g., double-sided adhesive film without a carrier). 5) Permits the use of high-viscosity adhesives that would not wet the final substrate directly.
Typical applications: Medical dressings – the adhesive is transfer-coated onto a release liner and then applied to a thin, nonwoven backing without heat damage. Double-sided tissue tapes – adhesive is transfer-coated onto a release liner and then laminated to a tissue carrier. High-performance acrylic foam tapes. Labelstock – where the adhesive is pre-coated on a liner and later transferred to the label face stock. Aerospace film adhesives – thick films applied via transfer to honeycomb structures. The transfer method is also used in the production of no-label-look labels, where the adhesive is applied directly to the product surface using a liner.
Key parameters: The release liner must have a controlled release value (low to moderate, e.g., 5-20 g/25mm for easy peel). The chill roll temperature should be set 10-20°C below the adhesive's softening point. Transfer efficiency depends on the laminating temperature: typically 80-120°C for reactivation. The nip pressure must be sufficient to ensure conformability but not so high as to squeeze out the adhesive. For inline transfer coating (single pass), a second unwind for the final substrate and a peel roll for the liner are added after the chill section. This requires precise alignment and tension control between the liner and the final web.
Maintenance and troubleshooting: The release liner is critical; any defects or wrinkles in the liner will transfer to the adhesive. Check for liner stretch – if the liner stretches during coating, the adhesive film may crack upon release. If the adhesive does not transfer completely (some remains on the liner), increase the laminating temperature or nip pressure, or use a liner with lower release force. If the adhesive film breaks during peeling, the film may be too brittle (cool too quickly) – reduce chill roll cooling or add a warm-up roll. Cleaning involves removing adhesive from the transfer nip rolls and the peel bar. Transfer hot melt coating machines are more complex than direct coaters but offer unparalleled versatility for heat-sensitive or delicate substrates. They are indispensable in medical, label, and high-performance tape manufacturing. When selecting such a machine, consider the release liner compatibility and the ability to handle very low transfer tensions to avoid stretching.
