Roll Coating Dynamics: Film Splitting and Transfer Mechanisms in Hot Melt Roll Coaters
The hot melt roll coating machine relies on the principle of film splitting between two counter-rotating rolls or between a roll and a web. The simplest configuration is the two-roll coater: an applicator roll picks up adhesive from a reservoir and transfers it to the substrate passing between the applicator roll and a backup roll. The thickness of the adhesive layer on the applicator roll is determined by the metering gap between the metering roll (or doctor blade) and the applicator roll. For a doctor blade, the gap setting typically equals the wet film thickness multiplied by a factor (1.1-1.3) due to blade deflection. The blade loading force F (N/m) and blade angle (30-60°) influence the pressure distribution. The coating thickness h is given by h = (Q)/(v * L), where Q is flow rate from the pump. However, in roll coating, the flow is established by the roll rotation and the gap. For a Newtonian fluid in a narrow gap, the pressure distribution is governed by the Reynolds equation. The resulting film thickness on the roll is approximately h = (1/2) * gap + (μ * U)/(σ) * something; practically, engineers use empirical equations. A more relevant parameter is the “pick-out ratio”: the fraction of adhesive transferred from the applicator roll to the substrate. For a smooth roll and smooth substrate, the transfer ratio is near 100% if the adhesive’s adhesion to the substrate exceeds its cohesion and adhesion to the roll. For rough substrates, the transfer may be less, leaving adhesive on the roll, which builds up and causes “roll glazing.” To ensure complete transfer, the roll surface is sometimes chrome-plated with a specific roughness (Ra 0.2-0.4 µm) to release the adhesive. Alternatively, a rubber-covered applicator roll provides conformability for rough substrates. The film splitting process (when the web separates from the roll) creates a meniscus that can produce a patterned surface (e.g., “orange peel”). The capillary number Ca = μU/σ determines if the splitting is uniform. For Ca < 1, surface tension dominates and the film splits cleanly; for Ca > 1, viscous forces create filaments that lead to ribbing instability. Ribbing appears as parallel ridges along the coating direction. To avoid ribbing, operate at low speeds or use a lower viscosity adhesive. For high-speed roll coating, a “reverse roll coating” configuration is used, where the applicator roll rotates opposite to the web movement. This creates a shearing action that smoothens the coating and eliminates ribbing. Reverse roll coating can achieve coat weight down to 5 gsm with excellent uniformity, but it is more complex.
The doctor blade is critical for precise metering. There are two types: rigid blade (steel) and flexible blade (spring steel). The blade wear is proportional to the square of the blade pressure and the abrasive content of the adhesive. Tungsten carbide-coated blades last 10 times longer than steel. The blade’s angle of attack relative to the roll affects the pressure profile; a positive angle (blade pointing into the roll) increases pressure and reduces film thickness. The gap is set by measuring the distance between blade tip and roll surface with a feeler gauge or a laser sensor. In production, the blade wear causes the gap to increase; closed-loop systems measure the coating weight and move the blade closer automatically using a stepper motor. Another important parameter is the “roll speed ratio” between the metering roll and the applicator roll. In a three-roll coater (metering, transfer, applicator), the speed ratio can create shear that thins the adhesive. For a shear-thinning adhesive, increasing the relative speed reduces viscosity and thus reduces film thickness. This effect is used to adjust coat weight without changing gaps. The rheology of hot melt adhesives in roll coating is non-Newtonian, often following the power-law model η = K * γ^(n-1), where n<1 for pseudoplastic. The film thickness equation for power-law fluids is more complicated, and numerical simulation is used. The backup roll material (rubber hardness) influences the nip width and pressure distribution. A soft rubber (Shore A 40) provides a wider nip (5-10 mm) at lower pressure, which is gentler on the substrate but may cause “negative transfer” (adhesive being squeezed back onto the roll). For non-porous substrates, a harder rubber (Shore A 80) is preferred to minimize squeeze-out. The nip pressure is adjusted by pneumatic cylinders; typical pressures are 10-50 N/mm of roll width. Overpressure can force adhesive through the substrate (bleed-through). The line speed for roll coating is generally lower than slot die, 50-200 m/min, because of ribbing and transfer limitations. However, for wide webs (3-5 meters), roll coating is economical and simple.
Hot Melt Coating Machine - Hot Melt Adhesive Coating Machine
Defects in hot melt roll coating include “skipping” (uneven transfer due to roll runout), “ghosting” (residual pattern from previous adhesive), and “air entrapment” as bubbles. Skipping is caused by roll eccentricity; the runout should be less than 2 µm. Ghosting is prevented by thorough cleaning with a hot solvent wash between runs. Air entrapment occurs when the substrate has surface irregularities; vacuum rolls or pre-nip compaction rolls help. Another critical issue is the “thermal expansion” of rolls. Steel rolls expand by about 0.01 mm per 10°C per 100 mm diameter, which changes the gap. Therefore, roll coating machines must reach thermal equilibrium before starting. Some rolls are heated internally by oil or electrical cartridges to maintain a uniform temperature across the width. For temperature-sensitive adhesives, the roll temperature is held just above the melt point to prevent degradation. The melting system for roll coating is often a “trough” with heated walls and a stirrer to maintain homogeneity. The adhesive level in the trough must be kept constant to ensure consistent pick-up. A level sensor controls a pump that feeds from a main melter. The trough is covered to prevent oxidation and contamination. For patterned coating, engraved rolls (gravure) are used. The engraving cell volume (cm³/m²) determines the coating weight. For a given cell geometry (e.g., pyramid, quad), the theoretical volume is known; the actual transfer efficiency is 60-90% depending on viscosity and roll speed. Patterned roll coating is used for fusible interlinings and breathable laminates. The maintenance of roll coating machines includes regular checking of roll alignment (parallelism within 0.05 mm/m), roll surface finish (Ra <0.1 µm for precise coating), and bearing play. The rolls should be reground every 2-3 years. In summary, the hot melt roll coating machine, while simpler than other technologies, requires careful attention to fluid mechanics, thermal management, and mechanical precision. Mastering these aspects yields consistent, cost-effective coating for many applications.
