1. How it works
The core function of the squeegee vacuum coater is to coat liquid or slurry materials on the substrate surface to form a uniform thin layer. The equipment flattens and fixes the substrate (such as glass, silicon wafer, metal foil or polymer film) through a vacuum adsorption system, and uses a precision scraper to control the coating gap and cooperate with an automatic walking mechanism to complete the coating process. The "vacuum" in the name of the equipment refers to the substrate fixation method, and the "scraper" refers to the film forming method, which work synergistically to ensure the thickness consistency and surface flatness of the coating.
From the analysis of the physical mechanism, scraper coating belongs to the predetermined coating technology. During operation, the slurry is placed in front of the scraper and the lower edge of the scraper maintains a preset slit spacing (h) from the base surface. When the scraper moves relative to the substrate at velocity V, the slurry fills the slit and spreads over the substrate surface under the action of shear force. According to the fluid continuity equation, the slurry volume flow rate Q per unit width is satisfied in a unit of time:
Q = h · v · w
where w is the coating width, h is the thickness of the wet film (determined by the scraper gap), and v is the scraper travel speed. The thickness of the dry film after drying can be estimated δ the following formula:
δ = h · φ · (ρwet / ρdry)
where φ is the volume fraction of the solid object in the slurry, ρwetand ρdryThe density of wet film and dry film respectively. This relationship reveals the quantitative correlation between scraper clearance, slurry solids content and final film thickness, which is the theoretical basis for formulation optimization and process parameter setting.
2. Structure and function modules
The scraper vacuum coating machine is composed of four modules, and the technical indicators of each module jointly determine the film-making ability of the equipment.
2.1 Vacuum adsorption platform
There are microscopic vacuum holes (usually 0.5mm to 1.0mm in diameter) distributed on the surface of the platform, and the hole spacing is designed according to the size of the base (commonly 15mm ×15mm matrix). The vacuum system is composed of a rotary vane vacuum pump and a buffer tank, and the flatness of the platform is controlled within 0.01mm/m. The module needs to establish negative pressure within 3 seconds after the base is placed, so that the base fits smoothly and without local warping. For breathable substrates (e.g. paper, nonwovens), the vacuum level can be reduced or the use of border seals can be assisted.
2.2 Heating System (Optional Function)
Some of the equipment integrates heating elements under the platform, including resistive heating plates and infrared radiation tubes. The temperature regulation range is generally from room temperature to 150°C, and the temperature uniformity is required to be ±1°C to ±3°C. The heating function is mainly for systems that require rapid drying, such as the volatilization of NMP solvents in lithium battery electrode slurries or the crystallization control of perovskite precursor solutions.
2.3 Scraper Assembly
The scraper material can be divided into three categories: stainless steel, cemented carbide and ceramic. Stainless steel scrapers are suitable for conventional water-based or oil-based slurries; Carbide scrapers have better wear resistance and are suitable for slurries containing hard particles (such as ceramic powder); ceramic scrapers are used in strong acid-alkali corrosive systems. The straightness of the blade edge of the scraper is required to be less than 0.002mm/m, and the cutting edge radius is usually 10μm to 50μm. Clearance adjustment methods include manual micrometer adjustment and electric servo adjustment, the latter of which can achieve closed-loop control.
2.4 Travel drive mechanism
The drive system adopts stepper motor or servo motor with linear guide, and the travel speed range is usually 10mm/s to 200mm/s, and the speed accuracy ± 0.5%. The effective coating stroke ranges from 200 mm to 800 mm, corresponding to different substrate sizes. The equipment needs to have a speed segment setting function to adapt to the acceleration control during the start-stop stage and prevent slurry accumulation or tailing defects at the start and end.
3. Technical parameters
There are differences in the technical specifications of different types of scraper vacuum coaters, and the following table summarizes the typical parameter ranges and corresponding applicable scenarios:
| Technical parameters | Typical scope and description |
|---|---|
| Coating gap range | 0.02 mm – 5 mm with ± accuracy of 0.001 mm |
| Coating width | 100 mm – 600 mm |
| Maximum thickness of the base | 10 mm (glass/sheet metal) to 3 mm (flexible film) |
| Applicable slurry viscosity | 1 cP – 50,000 cP (centipo) |
| Heating temperature range | Room temperature – 180°C (optional) |
| Vacuum level | -0.08 MPa to -0.095 MPa (gauge pressure) |
In terms of material adaptability, the equipment covers the following systems: water-based systems (such as acrylic emulsion, PEDOT:PSS conductive polymer), organic solvent systems (such as polyimide solution, PVDF binder), particle suspension systems (such as lithium battery cathode paste, ceramic ribbon paste), and light curing systems (such as UV curing adhesive, photoresist). For slurries containing large particle size particles (> 20 μm), a scraper model with a large gap adjustment range should be selected to prevent streak defects caused by particle jamming.
4. Applications
Squeegee vacuum coaters are used in several laboratory research areas. In the direction of new energy batteries, the equipment is used for the preparation of lithium-ion battery cathode (LiFePO₄/NMC) and anode (graphite/silicon carbon) electrode pieces, and the coating area density can be controlled within the tolerance of ±2%; In the direction of flexible electronics, it is used for the coating of transparent conductive films and graphene films of silver nanowires, and the substrate can be flexible films such as PET and PI. In the direction of functional ceramics, it is used for cast molding to prepare ceramic ribbons, with a film thickness of 0.1mm to 1mm, and is used for MLCC capacitors and solid-state electrolyte films.
The standard operating procedure is as follows:
Step 1: Base preparation – Clean the substrate surface to remove dust and oil, and smooth the flexible film with a flattening treatment.
Step 2: Slurry configuration – Weigh each component according to the recipe, and after stirring and defoaming, the slurry reaches the target viscosity and dispersion.
Step 3: Device settings – Place the base on the vacuum platform and turn on the vacuum adsorption; adjust the scraper to the target clearance; Set the coating speed and stroke.
Step 4: Coating operation – Place the slurry evenly at the front of the scraper, start the running mechanism, and the scraper moves to complete the coating.
Step 5: Drying and post-treatment – If heating is required, turn on the platform heating function for drying; After removing the substrate, slice, roll or sinter as needed.
Common defects and their adjustment methods include: if the coating thickness is uneven, the horizontality of the platform and the parallelism of the scraper should be checked; If longitudinal stripes appear, check whether there is a gap or particle jamming at the edge of the scraper; If there is a bubble pinhole, check whether the slurry defoaming is sufficient and whether the coating speed is high.
5. Maintenance points
Daily maintenance needs to pay attention to the following matters: clean the scraper and platform in time after each coating to prevent the slurry from curing and blocking the vacuum hole; Check the blade edge of the scraper once a week, use a magnifying glass to observe the wear, and replace the scraper if necessary; Platform leveling is tested once a month, calibrated using a laser level; Change the vacuum pump oil quarterly and check the tightness of the pipeline. For devices that use heating functions, it is recommended to calibrate the temperature sensor every six months to ensure accurate temperature control.