Definition
A flat coater is a precision mechanical equipment that prepares uniform liquid coatings on the surface of rigid or flexible flat substrates. Its core function is to transfer and attach the fluid coating solution to the surface of the substrate according to preset thickness and uniformity requirements, and then form a solid functional layer through a drying or curing process. This equipment is distinguished from continuous roll-to-roll coating machines by its batch processing method, which is suitable for the processing of single sheet materials, such as sheet metal, glass substrate, ceramic sheet and hard polymer sheet.
In the fields of non-medical and non-pharmaceutical industry and scientific research, flat plate coaters are widely used in the coating of photosensitive adhesives for printed circuit boards, the preparation of optical films for display panels, the deposition of charge transport layers of perovskite solar cells, the coating of catalytic layers for fuel cell electrodes, and the development and pilot production of various functional films.
Principle
The working process of the flat coater is based on the principle of relative motion and fluid metering control. A precise, adjustable gap is maintained between the coating head and the adsorption substrate platform. The coating fluid is delivered through the feeding system to the internal runner of the coating head, where it is extruded under pressure from the slit or scraper port to form a continuous liquid bridge. The platform or coating head moves in a relatively linear motion at a set speed, during which the liquid bridge is stretched and spread over the surface of the substrate, forming a wet film.
The relationship between the thickness of the coating wet film and the process parameters varies depending on the coating method. For scraper coating, the wet film thickness is mainly governed by the coating gap and the rheological behavior of the coating solution, and the solid content does not affect the wet film thickness, but is used to calculate the dry film thickness. For slit extrusion coating, when the feeding system is in the expected mode, the wet film thickness is determined by the material balance and has no direct relationship with the coating gap, which is expressed as: the wet film thickness is equal to the product of the feed flow divided by the coating width and the coating speed. The formula is: h_wet = Q / (W × v).
Among them, Q is the volume of coating liquid output by the feeding pump per unit time, W is the effective coating width of the coating head, and v is the relative movement speed of the platform or coating head. The formula shows that the wet film thickness is inversely related to the coating speed under the condition that the feed flow rate and coating width are fixed. By controlling these three parameters, the wet film thickness can be preset. Further combined with the solid content of the coating solution, the dry film thickness can be calculated as follows: h_dry = h_wet × S.
It should be noted that the solid content S only affects the final thickness of the wet film after drying, but does not change the thickness value of the wet film when the coating is completed. By controlling the above parameters, the final dry film thickness can be predicted and controlled.
Measurement method
The quality evaluation of the flat plate coating process relies on the measurement of multiple indicators before and after coating and during the coating process. The main measurement methods include the following categories.
Thickness measurement: Wet film thickness can be measured non-contact immediately after application by means of an in-line laser thickness gauge or eddy current sensor. The dry film thickness is measured by contact probe profilometer or optical interferometry to obtain the step height difference between the coating profile and the substrate. In high-precision applications, the accuracy of the platform plane is required to reach within 3 μm, and the straightness of the die lip mouth needs to be controlled at the level of 2 μm.
Uniformity evaluation: Measure the thickness by selecting multiple points on the coating, calculate its average value and standard deviation, and obtain the coating uniformity index. The commonly used quantification parameters are coating difference coefficient and uniformity coefficient, which are used to characterize the thickness consistency of the coating in the horizontal and longitudinal directions. It is usually required that the thickness deviation of different locations within the same batch does not exceed ±3 μm of the set value.
Optical and electrical properties test: for transparent conductive film or photoelectric functional layer, use UV-Vis spectrophotometer to measure transmittance and reflectance; Measure the square resistance of the film using a four-probe tester. These indirect measurements reflect the uniformity and functional integrity of the coating's microstructure.
Calibration method: The equipment is calibrated regularly, covering the position and clearance of the coating head (verticality deviation not exceeding ±0.05mm), platform flatness (using laser interferometer), feeding rate (using weighing method to calibrate the discharge volume per unit time), and temperature sensor (fluctuation from room temperature to 150°C does not exceed ±2°C).
Influencing factors
The stability of flat coating quality is affected by the interaction of four factors: equipment, material, environment and operation. Understanding these factors is the basis for process optimization and troubleshooting.
Equipment accuracy: The flatness of the coating platform, the straightness and speed stability of the motion mechanism, and the processing accuracy of the coating head (especially the lip straightness) are the mechanical basis for determining the uniformity of the coating. Insufficient flatness of the platform will lead to changes in the gap between different areas of the substrate, causing thickness fluctuations. The coating head height adjustment resolution needs to reach the level of 1 μm to meet the control requirements of sub-micron level coatings.
Coating fluid properties: The viscosity, surface tension, solid content and volatilization rate of the coating solution directly determine the leveling and drying behavior. Viscosity ranges typically range from 1 to 1000 centipoise, and either too high or too low can result in streaks, orange peels, or shrinkage defects. The solid content affects the shrinkage rate from wet film to dry film, which needs to be considered in the formulation design.
Process parameter setting: coating speed, feed pressure or flow rate, coating gap, and relative motion acceleration between the substrate and the coating head are all key control variables. Excessive speed can introduce air bubbles, and improper clearance settings can lead to discontinuous coating or scratching of the substrate. The determination of the process parameter window should be completed by orthogonal experiments.
Environmental conditions: environmental cleanliness affects the risk of particle pollution; Temperature and humidity affect the solvent volatilization rate and viscosity stability of the coating solution. Precision coating is typically performed in a clean environment with a temperature of 25±3°C and a controlled humidity of 30% to 90%.
Application:
Flatbed coating machine serves basic research and product trial production in many advanced manufacturing fields. Its application is mainly for rigid substrate processing scenarios that require precision film patterning or full-surface coating.
Optoelectronic display field: used for the preparation of solution functional layers such as hole injection layer and light-emitting layer in organic light-emitting diode (OLED); For the coating of orientation films for liquid crystal display (LCD) panels. The coating size can reach 300mm × 400mm to meet the needs of small-sized panels in the R&D stage.
New energy field: In the research and development of perovskite solar cells, it is used for the layer-by-layer deposition of electron transport layer, perovskite light-absorbing layer and hole transport layer. In proton exchange membrane fuel cells, catalyst slurry is used to coat the gas diffusion layer or proton exchange membrane. The equipment is also suitable for the preparation of solid-state electrolyte films.
Printed electronics and sensing: used for circuit printing of conductive inks (such as silver nanowires, graphene) on glass or PET sheets; For the preparation of sensitive layers for flexible sensors such as pressure and temperature. The coating method can be switched to slits, scrapers or wire rods to accommodate functional materials with different rheological properties.
Traditional industrial upgrading field: In the printing and coating of metal sheet printing, the primer or varnish coating used in the printing iron can process has been regulated by industry standards. In ceramic substrate manufacturing, it is used for slurry coating before cast molding.
Selection
Selecting a flatbed coater for specific R&D or production needs requires a systematic evaluation of the following technical dimensions to ensure that the equipment capabilities match the process requirements.
Substrate and size adaptability: Clarify the type of substrate to be coated (glass, metal, silicon wafer, polymer board), thickness range (0.1mm to 3.2mm), and maximum size (commonly used specifications include 200mm×200mm, 300mm×300mm, etc.). The platform needs to be equipped with a vacuum adsorption system to hold the substrate and prevent displacement during the coating process.
Coating method and precision: Choose the type of coating head based on the viscosity of the material and the target thickness. Slit extrusion coating is suitable for low-viscosity fluids and wet films as thin as 5 μm, with high accuracy and a closed feed system and strong anti-contamination ability. The squeegee or wire rod type has a simple and low cost structure and is suitable for medium-thickness coating of higher viscosity slurries. Confirm whether the accuracy of the coating head clearance adjustment is at the micron or sub-micron level.
Functional integration: Assess whether accessibility modules are needed. The heated platform (room temperature to 160°C) allows for in-situ drying, reducing coating leveling time. Air knife systems are used to accelerate solvent volatilization. In addition, integrated liquid film thickness measurement or spectroscopy probes enable closed-loop control and real-time characterization of the coating process.
Automation and operation interface: The control system should adopt a programmable logic controller and a touch screen human-machine interface to support the storage and recall of parameters such as coating speed, stroke, and feeding rate. For scenarios operating in the glove box, the overall dimensions of the equipment must meet the size restrictions of the transition bin of the box. The cleaning convenience of the equipment, especially the disassembly and cleaning design of the feeding pipeline and die, is also an important factor affecting the long-term use experience.