Definition
A small continuous coater is a laboratory-scale or pilot-stage precision coating equipment used to apply liquid or slurry materials to a flexible or rigid substrate surface uniformly and continuously at a preset thickness. Its core function is to simulate the industrial coating process, and to obtain a controllable film layer on a limited size sample by controlling the coating liquid flow, substrate travel speed, and coating head parameters. The equipment is widely used in sample preparation and process development in the fields of electronic functional films, new energy materials, packaging and printing, textiles, optical films, and advanced composite materials, and does not involve medical or pharmaceutical use. Its design complies with relevant industry international standards (such as ISO, ASTM, and other specifications for coating uniformity and thickness measurement) to ensure the reproducibility and scalability of experimental data.
How it works:
The basic principle of a small continuous coater is based on the synergy of shear and extension in fluid mechanics. The substrate passes under the coating head at a constant speed under the traction of the drive system. The coating fluid is typically dosed through a precision feeding unit such as a syringe pump, peristaltic pump, or gravity tank to the internal volume chamber of the coating head. A precise gap is formed between the coating head (e.g., scraper, extrusion head, or microgravure roller) and the substrate, through which the liquid is forced to expand into a thin layer under shear stress on the substrate surface as the substrate moves. The substrate is then entered with a wet film into a drying or curing unit (e.g., hot air oven, IR or UV curing module) to be shaped by solvent evaporation or crosslinking. The entire system is monitored by a programmable controller (PLC) or industrial computer, and the speed, temperature and feed rate are adjusted in real time to achieve precise control of coating thickness and appearance. The gap between the coating head and the substrate can be expressed by Equation (1):
h = h₀ - Δh
where h is the actual wet film thickness, h₀ is the set value of the coating head gap, and Δh is the thickness correction due to the elastic deformation of the substrate or the viscoelasticity of the liquid, which can be obtained by fluid dynamics calculation or empirical calibration.
Measurement method
The evaluation of coating quality relies on systematic measurement and analysis methods. The main measurement methods include:
(1) Thickness measurement: Offline method uses a contact probe thickness gauge or optical profiler to scan the dried coating at multiple points, and the average value and standard deviation are taken. The online method integrates laser displacement sensors or near-infrared sensors to monitor the thickness of the wet film in real time. Thickness uniformity is expressed as coefficient of variation (CV) and calculated as in Equation (2):
CV = (σ / μ) × 100%
where σ is the standard deviation and μ is the average thickness.
(2) Characteristic analysis of coating solution: apparent viscosity is measured by rotating viscometer, shear thinning index is measured by capillary rheometer, and surface tension is measured by surface tension meter. This data is fed into the process model and the coating parameters are optimized.
(3) Substrate wettability: Use a contact angle measuring instrument to evaluate the free energy of the substrate surface to ensure that the liquid can be effectively spread without shrinkage or streaks.
(4) Drying efficiency: Use thermogravimetric analyzer or infrared temperature imaging system to evaluate the uniformity of residual solvent content and temperature distribution after drying.
These measurement methods are closely integrated with national standards (e.g., GB/T 13452.2 on the measurement of coating thickness) and the relevant regulations of the International Electrotechnical Commission for the testing of thin film materials.
Influencing factors
The coating quality of small continuous coating machine is affected by multi-parameter coupling, and the main factors are summarized as follows:
(1) Rheological characteristics of coating fluid: the higher the viscosity, the thicker the coating formed under the same gap; The liquid with severe shear thinning is easy to guide at the inlet of the coating head