There are various coating processes, including micro-concave coating, slit extrusion coating, comma scraper coating, gravure coating, and reverse contact coating. Among them, slit extrusion coating is a process widely used in the car clothing industry, and its working principle is that the coating liquid is extruded and sprayed along the gap of the coating mold under a certain pressure and flow rate and transferred to the substrate.
Image source: Aerospace Huayang
Compared to other coating methods, slit coating has the following advantages:
If the liquid supply pressure or flow rate of the liquid supply system fluctuates, if it is a rotary pump, you can consider increasing the speed, if it is a piston pump, you can consider adding the pulse buffer device to the liquid supply system, and you can use plastic pipes
Roads instead of metal pipes;
If the back roller or platform supporting the substrate is vibrating or the back roller is defective, it should be replaced or adjusted accordingly, and it can be judged whether it is caused by the roller by checking the length of the defect interval and the perimeter of the roller;
Vacuum chamber pressure changes, adjust the chamber gap or vacuum pump;
2. Vertical grain
Vertical grain is similar to horizontal grain, but appears in the length direction, and has the following characteristics:
Lines in the longitudinal direction are coated with a thickness thinner than those in the next position
There are usually multiple lines
Some vertical stripes have only one stripe, and the reason for this type of stripe is different from that of multiple stripes.
When the upstream meniscus moves towards the lip outlet, vertical lines are created. The possible causes and solutions are as follows:
Due to the high shear force formed by the substrate running speed too fast, it can be considered to reduce the substrate running speed;
If the outlet pressure of the coating lip is too low due to the low viscosity of the coating solution or the thickness of the gasket, or the upstream pressure decreases due to the gap between the substrate and the lip coating is too wide, it can be considered to increase the glue flow rate, reduce the gap between the upstream lip and the substrate, reduce the thickness of the gasket, increase the viscosity of the glue, and use a vacuum cavity.
If a local defect in the applicator head or a vortex is formed in the glue during liquid supply, you can consider repairing the damaged applicator, rematching the glue, cleaning the system, etc.
In the wet coating process, vertical bars are common for a variety of reasons, with the following characteristics:
A single line that starts from a point and extends upstream, like a comet.
An extension line with reduced thickness along the coating direction
The causes and solutions are as follows:
Problems on the substrate, such as dust, dirty spots, where the point is located will hinder the flow of glue, you can consider establishing a dust-free workshop, purging the substrate with high-pressure air flow before coating, etc.;
There is a blockage in the slit cavity or lip that causes a thinning of the extension line, or even a lack of glue. These blockages are either condensates in the glue, or large particles of dust stuck between the lip and the substrate, in addition to the substrate dust removal, the solution is to rematch the glue, the glue needs to be filtered and defoamed, the liquid supply speed is accelerated, and the gap between the slits of the glue mouth is increased.
Damage to the slit applicator head can cause permanent lines after application, because the coating beads are unstable at the damaged location, which requires sanding repair or replacement of the applicator.
4. Bubbles
Air bubbles can appear anywhere on the coating on the film and are characterized by:
The shape is round or oval and varies in size
The thickness of the bubble will be thin where it is, or there may not be any coating inside.
The location distribution is random
The causes and solutions are as follows:
The glue is mixed with air before entering the pump system, which requires defoaming before glue coating;
If the liquid supply system is leaking, check whether the connector and adapter are sealed;
The upstream and downstream meniscus are unstable and trap air, which can be solved by reducing the gap between the lip and the substrate, reducing the running speed of the substrate, and increasing the flow of the liquid.
