Coating instruments are specialized tools used in laboratories to prepare uniform, controllable thickness coatings on substrate surfaces. Its core function is to extend liquid or semi-solid samples (such as coatings, adhesives, slurries, etc.) at specific thicknesses onto the surface of a flat plate to simulate the actual coating process or to prepare test samples. The quality of the coating directly affects the reliability of subsequent test results, such as the evaluation of coating thickness, uniformity, gloss, adhesion and chemical resistance. Therefore, it is important to choose the appropriate coating equipment based on the application criteria and sample characteristics.
Main types and working principles
Laboratory coating equipment is based on two main principles: gap coating and scraper coating. The gap applicator scrapes excess sample onto the substrate through a fixed-thickness gap groove; Scraper applicators utilize adjustable cutter edges or wires to create a uniform coating through mechanical movement. The coating thickness T is usually determined by the physical clearance of the appliance, and for non-Newtonian fluids, the actual wet film thickness also needs to consider the rheological characteristics, and the relationship can be approximately expressed as: Twet = k × G, where G is the nominal gap height and k is the correction factor associated with the rheological behavior of the sample. After coating, the wet film is cured or dried under specific conditions to form a dry film for testing.
Instrument selection and technical parameters
The selection should comprehensively consider the substrate type, sample viscosity, target film thickness range, and required coating accuracy. Key parameters include coating width, thickness range, gap adjustment accuracy, and material compatibility. The common material is stainless steel, which is corrosion-resistant and easy to clean. The following are typical categories and their applicable scenarios.
| Gap applicators | It is suitable for medium to high viscosity liquids and for the preparation of wet films with a single fixed thickness. |
| Adjustable scraper | By adjusting the knife edge with a precision screw, the thickness of the coating film can be continuously adjusted within a certain range. |
| Wire rod applicator | Uses a metering rod wrapped around steel wire for rapid coating of low-viscosity samples. |
| Automatic film coating machine | Provides constant speed and pressure for high repeatability for standardized testing. |
Operational points and influencing factors
Standardized operation is a prerequisite for achieving consistent coatings. First of all, it is necessary to ensure that the substrate is flat and clean. When coating, the utensils should be perpendicular to the substrate and scraped at a uniform speed at one time to avoid pauses or repeated scraping. Ambient temperature and humidity can affect sample leveling and solvent volatilization rate, which in turn can interfere with the final film thickness and appearance. After coating, it needs to be dried or cured according to the time and conditions specified by the relevant standards. Any operational deviations can lead to uneven film thickness, streaks, or defects, affecting test effectiveness.
Maintenance and calibration
Regular maintenance extends the life of the appliance and ensures the accuracy of the coating. Clean thoroughly with a suitable solvent immediately after each use to avoid sample residue curing. When storing, the knife edge or gap should be prevented from bumping. To ensure the accuracy of the coating thickness, it is recommended to regularly verify the nominal clearance of the appliance by a standard gauge block or thickness gauge. If wear or deformation is found, the relevant parts should be replaced in time. Calibration intervals can be determined based on frequency and accuracy requirements, and are performed according to manufacturer recommendations or laboratory quality control procedures.
Application and standard reference
Coating equipment is widely used in the research and quality inspection of coatings, inks, cosmetics, electronic materials and adhesives and other industries. Its use usually follows widely accepted technical standards at home and abroad, which specify coating methods, equipment specifications and preparation conditions. Correct selection and operation are the basis for obtaining comparable experimental data.
| Paints and varnishes | Test plates are prepared to evaluate color, hardness, corrosion resistance, etc. |
| Printing inks | Simulate the printing process to check color strength and wear resistance. |
| Functional coating | Samples of optical, conductive, or protective coatings are prepared for performance analysis. |
References
ISO 1514: Colored paints and varnishes Standard test plates
ASTM D823: Standard Practice for Preparation of Uniform Coatings for Colored Paints, Varnishes, Spray Paints, and Related Products
GB/T 1727: General preparation method of paint film