Definition
Vacuum adsorption coating machine is a process equipment that uses negative pressure to firmly adsorb a substrate (such as paper, film, foil or fabric) on the coating platform, and evenly apply liquid slurry or coating to the surface of the substrate through a precision coating unit. Its core is to achieve flat fixation of the substrate during the coating process through vacuum apertures or adsorption plates, eliminating coating defects caused by uneven tension or substrate warping, and is often used in lithium battery electrode preparation, optical film manufacturing, flexible electronic devices, coating sample preparation and functional material research and development.
How it works:
The workflow of a vacuum adsorption coater can be summarized in three successive stages. First, the substrate is placed on the porous or stainless steel platform, which has a built-in vacuum cavity, which extracts the cavity air through an external vacuum pump, so that the substrate is tightly adsorbed on the surface of the platform under the action of atmospheric pressure difference, and the adsorption pressure is generally controlled between 50 and 100 kPa. The coating unit (e.g. scraper, slit die or roller coating system) is then moved or fixed to the platform with a preset clearance height, and the defoamed slurry from the reservoir is coated on the substrate in laminar flow. Finally, the coated wet film is dried or cured with the substrate in a vacuum-maintained state, or transferred directly to the next process.
Measurement methods and parameters
When controlling coating quality, the following parameters need to be quantitatively measured:
Coating thickness is measured by an online or offline laser thickness gage, and film thickness uniformity can be assessed using a scanning optical profiler or digital dial indicator for sampling at intervals in the coating direction. The quality of the coated wet film is commonly characterized by the amount of coating per unit area (g/square meter), which is calculated by weighing the difference in the quality of the substrate before and after coating:
Among them M_t is the total mass after coating, M_b is the quality of the substrate, and A is the coating area.
The vacuum adsorption uniformity uses a multi-point pressure sensor array to measure the negative pressure value of each hole on the surface of the platform, and the standard deviation should be less than 5% of the set value. The rheological properties of the coating liquid, such as viscosity and yield stress, need to be measured by a rotating rheometer in the range of 10 to 100 seconds inward at the shear rate to ensure the stability of the coating window.
Influencing factors
The main factors affecting the coating results of vacuum adsorption coating machines include:
The air permeability, surface roughness and hydrophilicity of the substrate directly affect the vacuum adsorption effect and coating spreading behavior. Highly breathable substrates may cause vacuum pressure leakage, so it is necessary to appropriately increase the vacuum pump capacity or use a sealing auxiliary layer. The viscosity and shear thinning characteristics of the coating solution determine the fluid flow state in the gap of the scraper or die, and the viscosity increases with the increase of solid content, and if it exceeds the upper limit of the equipment, it will lead to tailing or orange peel defects. Too low vacuum pressure will cause the substrate to float locally, causing thickness fluctuations. Too high may cause the substrate to become too dented, affecting the flatness of the coating after drying.
The matching relationship between coating speed and scraper clearance can be expressed as the empirical correlation between coating wet film thickness and gap ratio. Ambient temperature and humidity need to be controlled, and temperature fluctuations greater than plus or minus one degree Celsius may change the viscosity of the solution by 3% to 5%. Platform surface cleanliness is also crucial, as residual solid particles can easily scratch the coated surface.
Applications:
Vacuum adsorption coating machine has wide applicability in non-medical and non-pharmaceutical fields. In the field of new energy materials, the coating density deviation is usually controlled within plus or minus 1.5% for the coating of lithium-ion battery anode and cathode slurry. In the field of electronic materials, it is used for photosensitive resin coating on copper-clad substrates of flexible circuit boards and for uniform coating of electromagnetic shielding films. In the field of precision manufacturing, it is used in the pre-coating of underfill adhesives in microelectronic packaging. In the field of basic materials research, it is used for the reproducible preparation of samples such as functional coatings, superhydrophobic surfaces, and catalyst supports. In addition, in the coatings industry, it is used for the preparation of standard test samples to meet the requirements of international standards for thickness and surface condition.
Key points of selection
When selecting a vacuum adsorption coating machine, the following technical characteristics should be examined:
For the effective working size and vacuum channel density of the coating platform, a platform with a porosity greater than 20% should be selected for wide substrates to maintain adsorption stability. The adjustment accuracy of the coating unit requires that the repeatability of the gap control is better than plus or minus five microns, and the scraper material matches the hardness of the substrate. The vacuum system should be able to adjust the pressure independently, and be equipped with a high-precision vacuum gauge and filtration device. For solvent-containing or volatile coatings, the equipment must have solvent-compatible seals and exhaust interfaces. The control software should support multi-stage programming of coating speed, starting position, and platform heating temperature (up to 200 degrees Celsius). In addition, the anti-accumulation design of the platform edge and the ease of cleaning the coating head also affect the long-term operating efficiency.
At the maintenance level, it should be confirmed whether the vacuum pipeline can be removed and cleaned, and whether the scraper supports tool-free replacement. The final selection recommendation is based on the actual sample trial coating verification, and the decision is made by measuring the uniformity of the coating film thickness and the surface defect rate.