Zenith Aluminum & Brass Casting Foundry
China sand casting and gravity casting manufacturer

Powder coating process

Powder coating can be used both as a decoration and an efficient means of corrosion resistance for aluminum casting parts. Zenith aluminum casting has many aluminum casting parts such as lamp covers, instrument housing, handgrips requested for powder coating. We have three powder coating partner factories providing excellent services for us.

Powder brands available for zenith aluminum casting:

• Akzo Nobel
• DuPont
• Morton

Types of powder coating

1.Thermoplastic powder coating is one that melts and flows when heat is applied, but continues to have the same chemical composition once it cools and solidifies.

2.Thermosetting powder coatings also melt when exposed to heat. After they flow into a uniform thin layer, however, they chemically cross-link within themselves or with other reactive components. The final coating has a different chemical structure than the basic resin, is heat stable and, unlike thermoplastic powder after curing, will not soften back to the liquid phase when re-heated.

The process of powder coating for aluminum casting parts

Powder coating is a dry finishing process, using finely ground particles of pigment and resin, which are electrostatically charged and sprayed onto electrically grounded aluminum casting parts. The charged powder particles adhere to the aluminum casting parts and are held there until melted and fused into a uniformally flowing coating in a curing oven.

Pretreatment

aluminum casting parts to be powder coated first are exposed to a pretreatment operation to ensure that the surface to be coated is clean and free of grease, dust, oils, rust and other residue. The pretreatment process is normally conducted in dip tanks. Powder coating lines usually incorporate a chrome phosphate application step for aluminum casting parts that adds corrosion protection and improves the adhesion of the coating. After the aluminum casting parts have passed through all of the pretreatment steps, they are normally dried in a low-temperature dry-off oven. After drying, the aluminum casting parts are ready to be powder coated.

Powder Application

The powder coating application process uses four types of equipment: the powder delivery system, the electrostatic spray gun system, the spray booth and the powder recovery system.

The delivery system consists of a powder storage container or feed hopper, and a pumping device that transports a mixture of powder and air into hoses or feed tubes. Some feed hoppers vibrate to help prevent clogging or clumping of powders prior to entry into the transport lines.

Electrostatic powder spray guns direct the flow of powder; control the pattern size, shape and density of the spray as it is released from the gun; charge the powder being sprayed and control the deposition rate and location of powder on the target. Spray guns are either manual (hand-held) or automatic (mounted to a fixed stand or gun mover).

The electrostatic process encourages the powder to "wrap" around the product or deposit on the surfaces of the product that are not directly in the path of the gun.

The most common application method is with corona charging guns which generate a high-voltage, low-amperage electrostatic field between the electrode and the product being coated. Powder particles that pass through the ionized electrostatic field at the tip of the electrode become charged and are deposited on the electrically grounded surface of the part.

The powder particles in a tribocharging electric gun receive an electrostatic charge as a result of friction which occurs when powder particles rub a solid insulator or conductor inside the gun. The resulting charge is accomplished by stripping electrons from the powder, producing positively charged powder. Because there is no actual electrostatic field, the charged particles migrate toward the grounded part and can deposit in an even layer over the entire surface of the part.

The powder bell uses a turbine that rotates an enclosed powder bell head. Powder is delivered to the bell head and ejected by centrifugal force. The powder passes through an electric field between the bell head or an externally mounted electrode and either the grounded object to be coated or a counter electrode positioned behind the bell’s head. The powder is subject to the normal corona charging mechanism, ejecting the powder evenly over a large area.

The tribo disk features a non-rotating disk positioned vertically inside a loop and parts are conveyed through the loop as the disk oscillates up and down, applying overlapping layers of powder coating on the parts′ surface. The tribostatic charging method is employed, and the disk forms a uniform, horizontal spray pattern of about 2.5 ft in diameter.

Powder Application

The use of oscillators, reciprocators and robots to control spray equipment reduces labor costs and provides more consistent coverage. Gun triggering_turning the gun on and off using a device that can sense when the part is properly positioned_will reduce overspray material, which means lower material and maintenance costs.

Other improvements made to spray guns involve variations in the spray patterns to improve the efficiency with which they deposit powder on the substrate, and allow one gun to be used on different parts and configurations. There are nozzles more resistant to clogging and ways to continuously clean charging electrodes.

Alternate application methods include fluidized beds and electrostatic fluidized beds. A fluidized bed is a fixed container in which powder is suspended in a continuous stream of air. Preheated objects may be coated by dipping directly into a fluidized bed. Flamespray, used to apply thermoplastic powder, makes use of a heat gun where the powder is propelled through the flame using compressed air. The heat of the flame melts the powder, eliminating the need for ovens.

The powder spray booth is designed to safely contain the powder so that overspray cannot migrate into other areas. The entrance and exit openings must be properly sized to allow clearance of the largest product part. The airflows through the booth must be sufficient to channel all overspray to the recovery system but not so forceful that it disrupts the powder deposition and retention on the part.

The batch booth is designed for coating individual parts or groups of parts that are handled using batch production methods. Conveyorized booths are designed for the continuous coating of the product on an overhead conveyor line in medium to high production operations, coating parts of various sizes or shapes.

Chain-on-edge booths are specifically designed for use with an inverted conveyor that has spindles or carriers for holding the parts. Flat-line booths, primarily designed for the one-sided coating of two-dimensional parts of minimal thickness, use a horizontal conveyor_a belt, a bar and pin, or driven roller type. The conveyor passes through the powder booth carrying the part to be coated on its surface.

The powder recovery systems make use of either cyclones or cartridge filter modules that can be dedicated to each color and easily removed and replaced when a color change is needed. Equipment manufacturers have made significant design improvements in powder spray booths that both allow color changes to be made with minimal downtime and allow the recovery of a high percentage of the overspray, which can raise powder use to nearly 100 percent.

Curing

There are various methods used in the curing of powder coated parts. Convection ovens can be either gas or electric. Air is heated and circulated inside the oven around the powder coated parts. The parts attain the temperature within the oven.

Infrared (IR) ovens using either gas or electricity as their energy source emit radiation in the IR wavelength band. This radiated energy is absorbed by the powder and substrate immediately below the powder but the entire part need not be heated to cure temperature. This allows a relatively rapid heat rise causing the powder to flow and cure when exposed for a sufficient time.

Combination ovens generally use IR as the first zone to melt the powder quickly. The following convection zone can then use rather high velocity currents since there is no danger of disturbing the powder. These higher velocities permit faster heat transfer and a shorter cure time.

Induction ovens are normally used to pre-heat parts prior to powder coating to help elevate film build. These are used in fusion bonded epoxy coating applications, such as re-bar and pipe coating, using high line speeds with a coating thickness of over 10 mils.

Heat sensitive substrates, such as medium density fiberboard, are cured in two ways. Thermal curing uses infrared, convection or a combination of the two, and specially formulated low-temperature-curing powder, which is sprayed onto the MDF board that has been heated to bring some of its moisture to the surface.

With ultraviolet curing, the melt and flow can be separated from the curing process and minimal heat is required to cure the powder. The parts enter an infrared or convection oven first, and then are briefly exposed to ultraviolet light for final curing and hardening of the finish.