Definition
A slit coating machine is an automated equipment used to evenly coat liquid functional materials on flat or flexible substrate surfaces. Its core feature is that the fluid is extruded at a preset width and thickness through a pre-machined slit mold to form a continuous liquid film. This equipment is widely used in photoelectric displays, thin-film solar energy, flexible electronics, printed circuits and functional coatings and other industrial fields, and is a key device in the wet film deposition process.
How it works:
The working principle of slit coating machines is based on the movement of slit extrusion relative to the substrate in fluid mechanics. The liquid coating is delivered to the internal flow channel of the mold at a constant flow rate through a feeding system (such as a plunger pump or progressive cavity pump), creating a belt with uniform pressure distribution at the slit at the mold outlet. The substrate passes under or above the mold at a set speed, and the ribbon is drawn and adheres to the surface of the substrate, which is then dried or cured to form a solid film. The entire process needs to ensure that the flow, clearance and speed are accurately matched to maintain the uniformity of the liquid film thickness. Coating thickness usually follows the formula:h = Q / （v · W), among themhis the thickness of the wet film,Qis the volumetric flow,vis the movement speed of the substrate,WThis is the width of the coating.
Measurement method
The measurement of coating quality is mainly done offline or online. Offline measurement is commonly used to detect the thickness distribution of dry film with a profilometer or stepmeter, and the line width can be evaluated by optical microscope or image measuring instrument. Online measurement integrates spectral thickness gauges or laser trigonometric sensors to provide real-time feedback of film thickness data. Process control is based on a proportional-integral-differential algorithm that adjusts the pump flow rate or substrate velocity to keep the set thickness deviation within the target range. Coating surface defects such as streaks, bubbles, or missing coatings can be identified by a visual inspection system.
Key factors affecting the quality of coating film
The uniformity and stability of the coating film are affected by multivariate coupling. In terms of fluid properties, viscosity, surface tension and rheological properties determine the flow behavior, high viscosity fluids require higher pumping pressure, and low surface tension fluids are prone to edge shrinkage. In the process parameters, the coating gap (the distance between the mold and the substrate) needs to be controlled within the range of tens to hundreds of microns, and too large can easily lead to liquid film instability. When the surface energy of the substrate is low, it needs to be pretreated to improve the wettability. In addition, changes in ambient temperature and humidity can affect the volatilization rate and viscosity of solvents, resulting in fluctuations in film thickness. The accuracy of mold processing, including slit straightness, surface roughness and runner symmetry, directly determines the lateral uniformity.
Main application areas:
In the optoelectronic industry, slit coating machines are used to manufacture orientation layers for liquid crystal displays, color filter coatings, and conductive silver paste lines for touch panels. Functional layers of thin-film solar cells, such as perovskite photovoltaic devices, such as electron transport layers or light-absorbing layers, can be prepared over large areas with the help of this device. The field of flexible electronics, including the electrode and buffer coating of organic light-emitting diodes, electronic paper and RFID tags, relies on high-precision slit coating. At the same time, in the manufacture of printed circuit boards, this process is also used for the coating of solder mask ink and conductive ink. In the field of special functional coatings, such as the glass coating of anti-reflective or anti-fog films, its application can also be seen.
Select core elements
The selection of slit coating machine should be combined with mass production needs, coating accuracy and substrate characteristics. First, confirm the thickness range and uniformity requirements of the coating film, and the general system can achieve a film thickness deviation of plus or minus a few percent. The mold width should match the size of the substrate with adjustment margins. The feeding system should be selected according to the viscosity and shear sensitivity of the coating, peristaltic pumps are suitable for low shear sensitive fluids, and plunger pumps are suitable for high viscosity or particulate slurries. The substrate transfer method (roll-to-roll or sheet-to-piece) depends on the line formation. If non-planar substrates or ultra-thin flexible films are involved, tension control and correction devices are required. Finally, the ease of maintenance and the replaceability of parts are evaluated to reduce long-term operating costs.