Definition
Flat wire rod coating machine is an experimental equipment that evenly coats liquid materials on the surface of the substrate based on the principle of relative motion between the measuring rod (wire rod) and the flat substrate. Its core structure consists of a precision-machined coating rod (a metal wire of a specific diameter wrapped around the surface to create gaps) and a flat plate platform that can be adjusted in height or applied to pressure. This equipment is commonly used in laboratory research and development and quality control in the fields of coatings, inks, adhesives, functional films, and electronic pastes to prepare thin coatings with controllable thickness and high uniformity.
How it works:
The operation of the flat line rod coater is based on the "scrape" mechanical model. To operate, the liquid to be applied is placed on the front end of a stationary or slow-moving flat substrate. The coating rod moves relative to the flat plate at a constant linear speed, and the metal wires on its surface create a precise wedge-shaped gap between the rod and the substrate. The liquid is sheared, squeezed and gradually spread in this gap. The decisive parameters of coating thickness are the diameter and winding density of the metal wire on the wire rod: the thicker wire forms a larger gap, resulting in a thicker wet film; Thinner threads form a thinner film. Wet film thickness can be approximated as:
hwet = c × d
Among them,hwet is the thickness of the wet film,d is the diameter coefficient of the metal wire,c It is a correction factor related to material viscosity, surface tension and coating speed (usually determined empirically). The dry film thickness is determined by the thickness of the wet film and the solid volume fraction of the coating solution.
Measurement and parameter setting
When using the flat line rod coater, the following parameters are mainly measured and controlled:
(1) Coating speed: usually adjustable, usually between 5–200 mm/s, the faster the speed, the greater the shear force, but the effect on film thickness is nonlinear.
(2) Wire rod specifications: expressed by "wire number" or "wire diameter", common specifications range from 0.1 mm to 2 mm, corresponding to the thickness range of about 5–500 μm.
(3) Coating volume: controlled by quantitative pipetting or pre-coating device, usually 0.5–5 mL, to ensure that the liquid can completely cover the scraping starting area.
(4) Dry film thickness: After the coating film is dry, it is measured with a rice milling meter, profilometer or gravimetric method (weighing the residual film) to verify the accuracy of the wet film thickness conversion formula.
Influencing factors
The quality of the coating film is affected by the interaction of multiple factors:
Material factors: Liquid viscosity is the primary factor, and high-viscosity liquids are prone to form "grooves" or "orange peel" defects at high speeds. Low surface tension can lead to poor wetting or edge effect. If the particle size of solid particles exceeds 1/3 of the gap between the wire rods, it is easy to block or form scratches.
Equipment factors: The flatness of the substrate, the surface energy and whether it is pretreated (such as corona or plasma treatment) directly affect the adhesion and uniformity of the coating film. The straightness and surface finish of the coating rod are also critical.
Environmental factors: Temperature changes will change the viscosity of the liquid, and too high humidity may cause abnormal solvent evaporation rate or condensation on the film surface. Airflow disturbances can cause uneven film thickness.
Operating parameters: Sudden changes in coating acceleration (acceleration and deceleration at the beginning or end) often produce "start line" or "tailing" defects. In practice, it is recommended to use constant speed mode and reserve sufficient start and end zones.
Applications:
Flat wire rod coating machines are widely used in R&D and process control in the following non-medical fields:
Coatings & Ink Industry: Standard test plates for the preparation of water-based or solvent-based coatings to evaluate hiding power, gloss, leveling and abrasion resistance. In inks, printability and drying speed are often tested.
Electronic and optical thin films: Apply conductive paste, insulation layer or optical reflection enhancement film on flexible electronic substrates (such as PET, PI film) to provide a uniform sample for subsequent tests (such as square resistance, light transmittance).
Adhesives and sealants: After uniform coating, the bonding strength, peel force and curing shrinkage rate are tested, especially suitable for the formulation screening of pressure-sensitive adhesives and structural adhesives.
New energy materials: Simulate the coating process of lithium electrode paste (such as NMP system) in the laboratory to evaluate the dispersion, rheology and coating thickness consistency of the slurry.
Functional coating: Such as anti-fog coating, hydrophobic coating, anti-corrosion coating, etc., the thickness is controlled by the wire rod to study the relationship between coating performance and thickness.
Selection guide
The following technical dimensions should be considered when selecting:
(1) Coating film thickness range and accuracy: select the wire rod specification according to the target dry film thickness, and note that the nominal wet film thickness of most wire rods is the theoretical value, and the actual test needs to be corrected. If high precision is required, a precision machine with a micron-level gap adjustment mechanism is available.
(2) Substrate size and material: The commonly used substrate size in the laboratory is A4 (210×297 mm) or custom size (such as 100×200 mm). Note that the thickness of the substrate (such as thin aluminum foil or thick plastic plate) should match the clamping force range of the plate clamping mechanism.
(3) Coating speed range and stability: Some experiments require low speed (<10>100 mm/s) to simulate production line conditions. Choose models with closed-loop speed control to ensure repeatability.
(4) Environmental adaptability: For solvent-based or volatile liquids, it is recommended to choose a closed coating chamber or local exhaust interface. Flammable materials need to use electrical components with explosion-proof grades that meet the requirements.
(5) Ease of cleaning and maintenance: The wire rod is a consumable part and needs to be cleaned or replaced regularly. The substrate countertop should be removable and cleaned to avoid cross-contamination.
(6) Additional functions: If it is necessary to heat or cool the substrate (to study the effect of temperature on the forming of the coating film), or to integrate an automatic cleaning system (to reduce human operation errors), it needs to be specified at the time of purchase.
Operational precautions
To ensure experimental reliability and equipment longevity, common considerations include:
Thoroughly clean the wire rod and plate with special cleaning agent and soft cloth before each use to avoid residue affecting the subsequent coating film.
The coating liquid should be thoroughly stirred or defoamed to prevent bubble entrapment and film surface defects.
The coating speed should be groped from a low speed, such as 5 mm/s or 10 mm/s, and gradually adjusted to obtain the process window with the lowest defects.
Record the coating conditions (speed, wire rod specification, temperature, humidity) for traceability.
After the operation, the coating rod and platform should be cleaned immediately, especially for materials that are volatile in solvents or easy to cross-link in composition, to prevent them from being difficult to remove after curing.
Technology development trends
At present, the flat line rod coating machine is developing towards digitalization and systematization. For example, integrated laser ranging or in-line microscopic camera systems can monitor film thickness and defects in real time. The software can be embedded in the coating model to assist in process optimization and reduce trial and error. In the field of flexible electronics and perovskite photovoltaics, the demand for ultra-thin coatings (<1 μm) is driving the improvement of wire rod processing accuracy, such as laser engraving or electroforming processes to manufacture micron-sized wire diameter rods. In addition, the modular design allows the coating machine to be combined with functional parts such as automatic film change and vacuum fixation to increase experimental throughput.