What is Flexographic Lamination Knowledge Sharing（2）

III. Flexographic Lamination Related Knowledge Sharing

To provide some reference for the industry, the author conducted in-depth research on flexography, analyzing it from the perspectives of its working principle, cost analysis, its application status in the industry, and personal insights. Due to time and personal ability constraints, omissions and shortcomings are inevitable. Corrections and discussions from peers are welcomed.

(1) Working Principle of Flexographic Lamination

Compared to traditional anilox roller adhesive application, flexography uses a roller coating transfer method, meaning the method of adhesive coating has changed. It shifts from the dot-matrix-style cell transfer of the anilox roller to continuous surface coating, avoiding the infiltration and transfer-out of adhesive within the cells, thereby improving adhesive transfer efficiency and the state of the adhesive coating.

Theoretically, with roller coating mode, the continuity and uniformity of adhesive coating transfer are better, implying that better coating application状态 can be achieved during lamination. This allows for higher working viscosity of the adhesive and appropriately reduces the amount of adhesive applied. The reduction in application amount and increase in working viscosity inevitably lead to cost savings, which is the main reason flexographic coating is currently highly sought after.

To better explain the working principle of flexography, it is necessary first to interpret the relationship between working viscosity and working concentration in lamination.

Working viscosity refers to the viscosity of the adhesive during operation, measured in "seconds" or "cps," which can be understood as the thinness or thickness of the adhesive. Working viscosity is related to the original adhesive's viscosity and the amount of diluent added. It is also a key factor determining the adhesive's transfer state. For example, in anilox roller application, excessively high viscosity affects the infiltration and transfer-out of adhesive within the cells, impacting the coating, transfer state, and application amount. Therefore, it is difficult to use high working viscosity for coating with anilox rollers. Depending on the machine speed, viscosity is typically controlled around 13-25 seconds. This is why traditional dry lamination adhesives require adding large amounts of diluent to reduce working viscosity and improve coating uniformity.

1.Working Viscosity of the Adhesive

In roller coating, the working viscosity of the adhesive same affects its transfer. Flexographic lamination machines operate at higher speeds. If the viscosity is too low, it can cause adhesive splashing and uneven coating; if too high, it can cause stringing between rollers, affecting transfer. However, compared to anilox roller application, the working viscosity can usually be increased to 25-70 seconds, saving a significant amount of diluent usage.

The working viscosity of the adhesive is related to the original adhesive's viscosity, the amount of diluent added, the working temperature of the adhesive, and the degree of self-polymerization (pot life). The most critical factor for maintaining stable coating status is controlling the working viscosity of the adhesive. This is the fundamental reason many adhesive manufacturers promote high-solid-content, low-viscosity adhesives.

Working concentration is the effective solid content of the adhesive, usually expressed as a percentage. It refers to the percentage of adhesive in the working solution after adding diluent. Working concentration determines the amount of solid adhesive applied and is the most critical factor determining the application amount.

Working concentration and working viscosity are highly correlated process parameters in lamination. Generally, a high working concentration means less diluent is added, resulting in a correspondingly higher working viscosity. Conversely, lower working concentration means more diluent is added, leading to lower working viscosity.

Because the viscosity of original adhesives from different manufacturers varies greatly, it is not appropriate to compare data solely based on working viscosity. For example, for adhesives with the same 75% solid content, the market's original adhesive viscosity range is 1000-8500 cps. Then, at the same 40% working concentration, the working viscosity after dilution can range from 14 to 135 secondsa very wide range. Adhesives with excessively high viscosity can only be further diluted with more diluent until a suitable working concentration is reached for production, which in turn lowers the working concentration. The diluent added to the lamination adhesive must evaporate completely, equating to significant VOC emissions and increased coating costs.

Currently, typical flexographic lamination mostly uses a four-roller coating method for the lamination process (see schematic diagram). Two supply rollers for adhesive transfer are set in the adhesive pan. The adhesive is then transferred via a transfer roller, which contacts the lamination application roller to complete the transfer and coating of the adhesive onto the film.

Among them:

Roller 1 is a metering roller (doctor roller) made of rubber material;

Roller 2 is usually a ceramic anilox roller (Bobst 127 lines, domestic equipment uses 170, 180 LPI, specific to the equipment);

Roller 3 is a sleeve-type transfer rubber roller (application roller);

Roller 4 is usually a chrome-plated application steel roller.

The metering rubber roller 1 and the anilox roller 2 (sometimes replaced by a smooth roller) form a pair of counter-rotating rollers immersed in the adhesive pan. They provide adhesive supply through the gap control between the metering roller and the anilox roller, and the rotational transfer between the rollers.

The transfer roller 3 receives adhesive from the anilox roller 2 and transfers it to coat the film, forming a nip with the application steel roller 4 to complete the adhesive coating process.

2. Controlling Adhesive Amount

There are two main methods to control the adhesive amount:

First, through the pressure between rollers, mainly the gap between the metering roller 1 and the anilox roller 2, the pressure between rollers 1 and 2, and the pressure between the anilox roller 2 and the transfer roller 3, to control the adhesive supply. It should be noted that the anilox roller here primarily serves a transfer role, which is different from the principle of cell transfer in doctor blade-style anilox rollers.

Second, through the speed difference ratio between the rollers, i.e., the rotational speeds of the respective rollers, to adjust the adhesive supply amount.

There is a very interesting aspect here. We first consider the metering roller and anilox roller as a supply roller pair, called Pair 1. The transfer roller and application roller are considered a coating pair, called Pair 2.

As shown in the diagram, Pair 2, besides coating, also needs to partially bear the function of film traction. The rotational speed of this pair must synchronize to some extent with the lamination line speed to ensure tension balance in the drying oven. Therefore, the rotational speed of this pair must not only complete adhesive coating transfer but also assist in synchronizing with the lamination traction roller speed to maintain drying oven tension.

This leads to Conclusion 1: The rotational speed of the transfer roller 3 changes synchronously with changes in the lamination machine speed. Higher production speed means faster rotation of Pair 2 (rollers 3 & 4); slower production speed means slower rotation of Pair 2.

Pair 1, the metering roller and anilox roller, is a supply roller pair immersed in the adhesive pan. They control the amount of adhesive transferred through inter-roller pressure (gap) and rotational speed. Pair 1 can be considered the supplier of coating adhesive for Pair 2.

To maintain consistent adhesive application amount, stable supply is necessary to ensure the adhesive amount per unit area does not change. As the lamination machine speed adjusts and changes, the adhesive supply must change promptly to ensure a stable application amount.

From this, Conclusion 2 is drawn: As the supplier, Pair 1 must provide a corresponding supply amount according to changes in line speed to ensure the stability of the application amount.

Thus, it can be deduced: As machine speed changes, causing the speed of Pair 2 to change, Pair 1 must make corresponding follow-up changes to maintain a stable adhesive supply.

3.Lamination Startup Operating Conditions

In actual production operation, the lamination startup conditions include the following:

(1) The speed during startup is usually not constant; there are acceleration, deceleration, and steady-speed states. Acceleration/deceleration are variable states, meaning Pair 2 must adjust accordingly with machine speed changes to stabilize adhesive amount.

(2) The gap and pressure between Pair 1 must be adjusted before startup andin principle remain fixed during production.

(3) The adhesive in the pan is prepared in advance, meaning the working concentration of the solution is kept constant. (For ease of study, viscosity changes due to ethyl acetate evaporation or adhesive self-polymerization are temporary not discussed).

(4) With fixed working concentration, the working viscosity is also fixed, so the adhesive transfer rate can be considered relatively fixed. (Of course, transfer efficiency might vary slightly with machine speed, but this is also not discussed in detail here).

(5) Assuming the above conditions are relatively fixed, the only variables that can follow lamination machine speed changes are the rotational speeds of the metering roller and anilox roller, to provide a stable adhesive supply to the film. That is, with machine speed changes, Pair 1 must undergo corresponding speed changes to stabilize the application amount.

(6) During production, changes in adhesive supply from Pair 1 can only occur in two ways: either the speed difference between the metering roller 1 and anilox roller 2 changes, or the speed difference between Pair 1 as a whole and the transfer rubber roller changes.

(7) Most machine models set the speed difference between the metering roller and anilox roller as a fixed ratio, e.g., 1:3. Thus, in adhesive supply, changes in line speed inevitable lead to simultaneous changes in the speed of Pair 1.

In summary, the key control points for flexographic coating lie in preparing well before startup: first, the pressure and gap values for Pair 1 (the anilox roller is usually not changed, considered fixed); second, adjusting the speed difference and pressure between the transfer roller and the anilox roller; third, the stability of the adhesive solution.

During production, the ratio between the main line speed and the speed difference of the supply rollers must be matched accordingly to produce high-quality products without compromising quality.

Since the essence of flexographic lamination lies in the change in the adhesive application method, other aspects like drying oven drying and tension control are not specifically discussed here. However, it is important to note that the initial tack of the adhesive significantly affects the control of lamination winding tension.