The Science Behind a Perfect Aluminum Profile Finish

Pretreatment

Spray coating treatment for aluminum alloy profiles includes electrostatic powder coating and electrostatic liquid coating. In addition to fluorocarbon paint coating—commonly used on architectural aluminum profiles—electrostatic liquid coating also includes acrylic paint coating and polyester paint coating. The spray coating process for aluminum alloy profiles generally consists of three stages: pretreatment, spray coating, and baking/curing.

Substrate Loading → Pretreatment → Drying → Spray Coating → Baking & Curing → Inspection

(1) Pretreatment

Surface pretreatment generally follows a process line of degreasing followed by chemical conversion treatment.

Degreasing is typically carried out using a dedicated degreasing agent, with the purpose of removing oils, dirt, and residual debris adhering to the surface of the aluminum profile during extrusion, as well as removing the light natural oxide film on the profile surface. Chemical conversion treatment generally uses chromating or phosphate-chromate treatment, with the purpose of forming a chemical conversion film (such as a chromate film or phosphate-chromate film) on the substrate surface. This enhances the adhesion between the substrate and the coating (formed in the subsequent spray coating process) and provides protection to the substrate. The chemical conversion film should have an appropriate thickness; using the mass-loss method to measure film thickness, a chromate film of 600–1200 mg/m² is generally considered optimal, while a phosphate-chromate film of 600–1500 mg/m² is generally considered optimal.

(2) Drying

The purpose of drying is to remove the moisture introduced during pretreatment.

There are generally two drying methods: natural drying and high-temperature drying. Natural drying involves draining or air-drying indoors using a fan; this method takes a long time and has low efficiency, so few enterprises use it. Most enterprises use high-temperature drying, during which the drying temperature must be carefully controlled. Generally, the drying temperature after chromate treatment should not exceed 65°C, while for phosphate-chromate treatment it should not exceed 85°C. If the temperature is too high, the chemical conversion film will lose excessive moisture and be damaged.

(3) Spray Coating

① Electrostatic Powder Coating

Electrostatic powder coating applies powder coating material to the surface of the aluminum alloy profile using a powder spray gun, forming a protective and decorative organic polymer film. It operates on the principle of a high-voltage electrostatic corona field. In electrostatic powder coating, a high negative voltage is applied to the spray gun while the workpiece being coated is grounded, creating a high-voltage electrostatic field between the gun and the workpiece. As the carrier medium (compressed air) transports the powder coating material from the powder hopper through the delivery pipe to the diffuser ring of the gun, the diffuser ring—connected to the high-voltage negative electrode—generates a corona discharge, producing a dense concentration of charge around it that charges the powder coating material negatively. Under the combined action of the electrostatic force and the output force of the compressed air, the powder coating material is propelled from the gun nozzle toward the workpiece and adheres uniformly to its surface. After the subsequent curing process, this forms a uniform, continuous, smooth, and flat coating. In the electrostatic powder coating process, the key process parameters to control are spraying voltage, spraying distance, and air supply pressure.

Powder coating materials generally have a suitable spraying voltage range. Within this range, as the spraying voltage increases, the amount of charge carried by the powder coating material increases, and the amount of material deposited likewise increases. When the voltage is too high, however, electrostatic repulsion occurs, which instead reduces the amount deposited; excessively high voltage can also cause dielectric breakdown of the powder coating, affecting coating quality. The spraying voltage is generally controlled at 60–80 kV.

Spraying distance mainly affects coating film thickness and powder deposition efficiency, because changes in spraying distance alter the electric field strength. Practical experience shows that powder deposition efficiency is best when the spraying distance is controlled at 200–300 mm. If the spraying distance is too small, spark discharge can easily occur, causing dielectric breakdown of the powder coating and affecting coating quality; if the spraying distance is too large, powder deposition efficiency becomes too low. Because electrostatic powder coating relies primarily on charged powder particles adhering to the workpiece surface under electrostatic force—rather than on air pressure blowing the powder onto the workpiece—the air pressure and powder delivery air volume should be kept at the minimum levels required during spraying. When the air supply pressure increases, the air supply volume increases, and the kinetic energy of the powder coating material increases, which can easily cause the powder to bounce off the workpiece surface and reduce powder deposition efficiency. In production, the air supply pressure includes powder supply air pressure, atomization air pressure, and fluidization air pressure; changes in any of these pressures will affect both spraying efficiency and coating quality.

② Electrostatic Liquid Coating

Electrostatic liquid coating applies liquid coating material to the surface of the aluminum alloy profile using an electrostatic spray gun, forming a protective and decorative organic polymer film. Acrylic paint spraying and polyester paint spraying are generally applied as a single coat (i.e., one spraying pass forms one paint film). Fluorocarbon paint spraying, however, generally requires two, three, or four coating passes: a two-coat process refers to spraying a topcoat over the primer after the primer has been applied; a three-coat process refers to spraying the topcoat immediately after the primer, followed by a clear varnish; a four-coat process refers to spraying a barrier coat after the primer, then the topcoat, and finally a clear varnish over the topcoat. The working principle of electrostatic liquid spraying is the same as for powder coating: a high voltage is applied to the spray gun while the workpiece being coated is grounded, creating a high-voltage electrostatic field between the gun and the workpiece.

When the electric field strength is sufficiently high, electrons at the tip of the spray gun needle acquire enough kinetic energy to bombard the air near the nozzle, ionizing air molecules and generating new ions and electrons. This breaks down the air's insulating properties, and the ionized air produces a corona discharge under the action of the electric field. As liquid coating particles pass through the gun nozzle, they become charged, and as they pass through the corona discharge zone, they combine further with the ionized air and become charged again. The charged coating droplets, repelled by like charges, are fully atomized, and under the action of the high-voltage electrostatic field, they move toward the oppositely charged workpiece and deposit on its surface, forming a uniform coating.

The key piece of equipment in electrostatic liquid spraying is the electrostatic spray gun. Based on their atomization principle, electrostatic spray guns can be divided into three main categories: centrifugal electrostatic atomization, air electrostatic atomization, and hydraulic electrostatic atomization. Centrifugal electrostatic atomization can be further divided into disc-type and rotary-cup-type electrostatic atomization, with rotary-cup-type spray guns currently being the most widely used. The main process control parameters for electrostatic liquid coating are spraying voltage, spraying distance, coating viscosity, and spray volume. Spraying voltage is a critically important parameter in electrostatic coating. Figure 1 (referenced in the original) illustrates the relationship between voltage and electrostatic spraying efficiency: when the voltage is below 40 kV, spraying efficiency is only about 20%; beyond this point, spraying efficiency increases rapidly with rising voltage, reaching over 80% at 60 kV. As voltage increases further, the rate of improvement levels off and spraying efficiency increases only slowly; in addition, excessively high voltage can easily break down the air and cause spark discharge. The spraying voltage is therefore generally controlled at 60–90 kV. For a given voltage, the electric field strength is inversely proportional to the gap distance between electrodes. If the distance between the spray gun and the workpiece is too short, spark discharge can occur; if the distance is too great, the paint mist deposition rate decreases. Generally, a 1 cm air gap can withstand an electric field strength of 10 kV; thus, for an electric field strength of 90 kV, the theoretical minimum gap distance is at least 9 cm. Below this value there is a risk of dielectric breakdown of the air, causing spark discharge and fire hazard. In practice, a safety factor of 3 to 4 times this minimum distance must be applied.

For this reason, the distance between the electrostatic spray gun and the workpiece is generally selected within the range of 150–350 mm.

Before liquid spraying, the liquid coating material must first be formulated (commonly referred to as “paint mixing”) so that its viscosity is suitable for spray application. The higher the coating viscosity, the poorer the atomization performance, which adversely affects spraying efficiency; however, if the viscosity is too low, sagging and bubbling can easily occur. Paint mixing should be carried out according to the process requirements provided by the coating supplier, taking into account the day's temperature, humidity, and the coating's electrical conductivity, by adding an appropriate amount of thinner (such as toluene or xylene) to achieve a suitably viscous coating. The smaller the spray volume, the more favorable it is for atomizing the coating particles. When the spray volume is large, some coating droplets are unable to acquire sufficient charge at the gun nozzle, resulting in incomplete atomization and affecting the uniformity of coating thickness. Generally, while ensuring coating quality and spraying efficiency, the largest feasible spray volume is selected in order to maximize production efficiency.

(4) Baking and Curing

Baking and curing is the key stage of the spray coating process and has a very significant impact on coating quality. If the curing conditions fail to meet process requirements, the coating's weather resistance, adhesion, chemical stability, and impact resistance—among other properties—may all be adversely affected. Curing conditions vary for different coating materials, and production should strictly follow the process requirements provided by the coating supplier. For powder coatings, the typical curing condition is a curing temperature of 200°C with a curing time of 10 minutes. Fluorocarbon paint requires a somewhat higher curing temperature, generally around 230°C, with a curing time of approximately 10 minutes as well.