Definition
A spray coating machine is a device that evenly covers the surface of the substrate with liquid coating by spraying. Unlike brush or roller coating, it atomizes the paint into fine particles using compressed air or a high-pressure pump, which is then sprayed onto the target surface to form a continuous film. This device can be used on various substrates such as metal, plastic, and wood, mainly to simulate actual coating processes or to prepare standard samples.
Principle
The core principle is actually simple: the coating is delivered to the nozzle, where it mixes with high-speed airflow and atomizes into fine droplets. These droplets travel under inertia and strike the substrate, spreading out and evaporating solvents that leave a solid coating. The atomization quality directly determines the uniformity of the coating film. The droplet diameter d and the surface tension γ, viscosity η, relative velocity v of the gas, and density ρ of the mist droplet roughly satisfy the surface tension of the coating
d ∝ (γ·η)/(ρ·v²)
Of course, the actual nozzle structure also affects the design. Simply put, the higher the air velocity, the thinner the paint, and the finer the droplets.
Measurement method
To evaluate coating film quality, several key indicators are generally measured. Film thickness is the most commonly used, measured directly using a vortex thickness gauge or micrometer. For uniformity, take multiple points on the template to measure thickness and calculate the coefficient of variation. Adhesion should be determined by the grid or pull-out method, following ISO 2409 or ASTM D3359. There's also a atomization effect: observe the particle distribution and use a laser particle size meter to scan the atomization area. Dry film density and surface roughness also require attention; roughness is measured using a probe profilometer. Make sure the substrate is clean before testing, otherwise the data may show strange deviations.
Case study
Once, I met a user who did wood coating and wanted to adjust the varnish application parameters, but they kept getting orange peel. We tested different viscosities and distances with a spray coating machine. With viscosity set to 95 seconds (coating - 4 cups) and spray distance of 25cm, the dry film thickness is 60 microns, surface roughness Ra is only 0.3 microns, and adhesion is grade 0. After adjustment, the orange peel completely disappeared and the gloss improved. Later, they pushed this spec directly to the mass production line, increasing yield by nearly 15%. Note that this example only illustrates the commissioning approach; specific parameters depend on the coating system.
Application:
This equipment is most commonly used in the automotive, home appliance, building materials, and furniture coatings industries. In the laboratory, it is used to create standard templates and verify coating formulas; On the production line, first-piece confirmation or process parameter surveys are conducted. For example, the rheological characteristics of water-based paint greatly affect the spraying effect, so the coating machine can quickly select the appropriate viscosity. It also performs coating tests after powder coating pretreatment, which can handle it well. In aerospace, functional coatings such as anti-corrosion and insulation layers also rely on prototyping. Anyway, as long as it involves liquid coating preparation, it can always get involved.
Selection
When choosing a machine, consider the type of coating: solvent-based, water-based, and high-viscosity coatings have different requirements for spray guns. For atomization, choose between air spraying or airless spraying; the former is suitable for fine surfaces, while the latter uses thick films. The size of the substrate determines the effective spraying area, and large boards require large-stroke robotic arms. Accuracy indicators such as film thickness repeatability should be controlled within ±5%, which is considered reliable. The flow rate can be adjusted widely, from tens of milliliters to several hundred milliliters per minute. Another niche point: exhaust emission systems, and solvent-based coatings must be equipped with explosion-proof fans. Although the budget doesn't specify the exact number, don't be tempted by cheap nozzles that wear out in just three months.