Gravure hot melt adhesive coating machine
A gravure hot melt adhesive coating machine applies molten adhesive using an engraved gravure roll (also called a rotogravure cylinder). The roll has tiny cells or dimples engraved into its surface in a specific pattern (e.g., hexagon, pyramid, or quadrangular). The adhesive is transferred from a heated doctor blade system into the cells, and excess adhesive is scraped off. The substrate passes between the gravure roll and a rubber-covered backup roller, and the adhesive is drawn out of the cells onto the substrate. This method is particularly suited for low coat weights (1-15 gsm) and patterned coatings (e.g., dots, lines, or grids). Unlike slot die, gravure can apply very thin adhesive layers without requiring high pressure.
The machine consists of a heated adhesive supply tank, a pump, a heated gravure roll (usually chrome-plated steel with laser-engraved cells), a chambered doctor blade or traditional doctor blade, a heated backup roller (optional), and a cooling station. The gravure roll rotates in a heated trough or is supplied by a pumped system that floods the roll. The doctor blade removes all adhesive except that which is inside the engraved cells. When the substrate contacts the roll, the adhesive transfers by splitting action. The coating weight is controlled by the cell volume (expressed in cubic centimeters per square meter, cm³/m²), the line speed, and the adhesive viscosity. Typical cell volumes range from 5 to 50 cm³/m², yielding dry coat weights of 3-30 gsm depending on adhesive density.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Advantages of gravure hot melt coating: 1) Excellent for low coat weights (<10 gsm) where slot die may struggle with film stability. 2) Ability to create discontinuous patterns (e.g., dots for breathable laminates). 3) High speed capability – gravure rolls can run at 300-600 m/min. 4) Simple mechanical design with less precise gap adjustments needed. 5) Suitable for solvent-free hot melts with moderate viscosity (500-10,000 cP). Common applications: silicone coated release liners (thin release layer), hot melt PSA for postal stamps, pattern coating for medical tapes (micro-dots for breathability), adhesive transfer films, and nonwoven lamination for hygiene with open pattern to maintain softness.
Disadvantages: Gravure coating is less uniform than slot die; typical variation is ±5-8% across the web. The engraved roll is expensive to manufacture (typically $5,000-$20,000 per roll) and pattern changes require a new roll. Also, the open adhesive tray or chamber may expose the adhesive to air, causing oxidation and charring if not properly designed with a heated, sealed chamber. Modern gravure hot melt coaters use closed chambered doctor blade systems with nitrogen blanketing to prevent degradation. The cell shape and volume must be matched to the adhesive rheology – low-viscosity adhesives require deeper, narrower cells to avoid premature draining.
Key parameters: The cell volume is specified by the engraving. To calculate expected coat weight: Coat weight (gsm) = Cell volume (cm³/m²) × Adhesive density (g/cm³) × Transfer efficiency (typically 0.6-0.9). For example, a cell volume of 20 cm³/m², density 0.95 g/cm³, and transfer efficiency 0.8 gives 15.2 gsm. The engraving angle (e.g., 45° or 60°) affects release. For hot melt applications, the gravure roll must be heated to 100-180°C to keep adhesive molten in the cells; internal oil heating or cartridge heaters are used. The doctor blade is also heated. Maintenance includes cleaning cells with brass brushes or ultrasonic cleaning to remove dried adhesive. Worn cells lose volume and reduce coat weight; rolls can be re-engraved 2-3 times. Gravure hot melt coating is a cost-effective solution for high-speed, low-add-on applications where pattern flexibility is not frequently required, and it remains popular in label and tape industries.
