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Troubleshooting: How to eliminate tiny air bubbles in the laminating adhesive? To improve the appearance of the laminated products

During the operation of the laminate machine, tiny air bubbles in the glue plate and on the bearing roller are often raised as a major quality issue because users are concerned that too many bubbles in the glue plate will adversely affect the appearance quality of laminated products.

 


Table of content

1. What are Bubbles in the plates

2. The causing reason for bubbles

3. Influencing factors

4. Troubleshooting


1. Phenomenon

Under normal conditions, the glue just sprayed from the nozzle of the mixer (solvent-free dry laminate machine) does not contain bubbles.  In solvent-based dry laminate adhesive just prepared, if the stirring speed during mixing is too fast, there will be obvious bubbles visible to the naked eye.  However, after a period of static settling, the bubbles in the glue will gradually rise and disappear. Therefore, the glue added to the glue plate is usually free of bubbles.

In some solvent-based dry laminate processes, bubbles in the adhesive plate may gradually increase after a period of operation; when there are more bubbles, they can even cover the surface of the entire glue in the plate; in extreme cases, foam formed by bubbles may overflow from the glue plate.

For solvent-free dry laminate processes, bubbles gradually spread from near the block to the glue on the entire bearing roller, and the volume of individual bubbles gradually increases.

 

2. The causing reason for bubbles in adhesive plates

(1) Bubbles in the solvent-based adhesive

① In the preparation process of solvent-based dry laminating adhesive, the bubbles in the glue are formed by the air brought into the adhesive solution during the stirring process.

② During the operation of the equipment, when using the open adhesive plate / direct gravure coating mode, the excess glue on the surface of the cleaning roll is scraped off at the Doctor, and “hit” the glue into the glue plate at the same rate as the running rate of the coating roll.  In this process, a large amount of air will be brought into the glue, possibly forming more and more bubbles in the glue plate. Moreover, the higher the operation rate of the equipment, the more the number of bubbles produced by the rubber plate per unit of time.


(2) Bubbles in the solvent-free adhesive 

In the five-roll coating system in the lamination process, the air carried into the glue through the metering roll/ bearing roll can produce bubbles.

 The bubbles in the glue on the roller are related to the residence time of the glue on the roller, the actual speed of the measuring roll, and the viscosity (temperature) of the glue itself.


3. Influencing factors

  • The number and size of bubbles in the adhesive in the plates are significantly related to viscosity changes in the adhesive itself, operating speed of equipment, and viscosity changes in the adhesive.

  • If the viscosity of an adhesive solution is relatively low, the defoaming process will be relatively rapid.

  • The higher concentration and viscosity an adhesive has, the more bubbles will appear in a mixing bucket; the volume of bubbles will also increase; the time required for the complete disappearance of bubbles will also be longer.  

  • If an adhesive only “self-circulates” (no external circulation) in a glue plate, then as the viscosity of an adhesive solution increases (due to solvent volatilization), more and more bubbles will accumulate in a glue plate and may even reach an overflow state.

  • The higher the rotation speed of bearing rolls or viscosity an adhesive has itself, the worse defoaming ability an adhesive has; the number of accumulated bubbles will also increase.

 

4. Troubleshooting

(1) For solvent-based dry laminate processes

① Select adhesives with strong defoaming ability under high-speed operation (consult adhesive suppliers);

② Take measures to stabilize the viscosity of adhesive in a glue plate (equip automatic viscosity controllers or add diluents regularly);

③ Use “jacketed-type glue plates”. Under this coating method, circulation rate of the adhesive is above 95%, and bubbles will not accumulate in a glue plate;

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jacketed-type glue plates

④  Use the “closed adhesive box, direct gravure coating” method.

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direct gravure coating

 (2) For solvent-free dry laminate processes

① Set the temperature for adhesives and bearing rollers/metering rollers at 40℃ or above (generally speaking, the higher temperature an adhesive has lower its tackiness);

②  On-premise that there is no shortage of adhesives on bearing rollers, try to reduce liquid level height for each time after gluing (to shorten residence time for adhesives on bearing rollers);

③  Use as low pressure as possible for transfer rollers;

④ On the premise of ensuring the amount of adhesive per unit area, a larger metering roll gap and a smaller transfer roll speed ratio should be used as far as possible (to reduce the absolute speed of the metering roll).

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