Although still very common in the industrial world, conventional liquid painting is becoming less widespread due to the fact that powder coating is virtually harmless to the environment, while liquid paints will emit VOC’s. However, liquid coatings do offer some advantages over powder coating. First of all, liquid paint does not form as thick or heavy of a coat (known as and measured in mil thickness) as powder coating. This is extremely important when working with very tight and specific tolerances that are involved with the final assembly of a product. Secondly, liquid paint is an excellent alternative to powder coating when working with very specialized paint colors that may be too difficult or time consuming to be color matched as a powder. This is especially true when dealing with metallic or high gloss colors. In addition, liquid paint can be applied to any type of surface and does not require the use of heat to achieve curing. Liquid painting is very common when dealing with smaller, short run quantities that can be done in batches. In addition, all of the military work done at Coatings Applications is primed and painted using liquid per MIL specs. All plastics are done in liquid and air dried as they cannot be exposed to the high temperatures involved with powder coating. Finally, any projects done at Coatings Applications that involve the use of conductive coatings (i.e. Electrodag) are done in liquid in our batch booths.
STAGES
Process: Liquid Painting
DRY OFF OVEN
The goal of pretreatment is to create a clean, porous surface which promotes good paint adhesion while also helping to prevent under film corrosion after the coating process is complete. The cleanliness of the surface is as important in liquid painting as it is with powder coating. All parts to be liquid painted will go through this process unless they are yellow chromated, yellow iridized, clear etched, clear iridized, or zinc plated.
Coatings Applications utilizes a three stage iron phosphate system to properly pretreat and prepare parts to be powder coated. This three-stage system is comprised of a degreaser/phosphate in one stage, followed by a water rinse and a seal rinse in the third and final stage. This phosphating process, also referred to as conversion coating, adds significantly to the overall cosmetic qualities, durability, and corrosion resistance of the finished product. This is a direct result of the uniform substrate surface that is created during the phosphate process. Once treated by the phosphate wash system, metal parts will appear an iridescent blue and gray in color, signifying that an iron oxide base has been formed and that the parts are clean, degreased, and free of oils, residues or any other chemicals remaining from the fabrication process.
The next step for all parts is to pass through our dry-off oven. The main goal of the dry off oven is to expose parts to high temperature and heat to ensure that all moisture remaining from the phosphate wash is thoroughly removed. Furthermore, the elevated temperatures will force oils and residues from fabrication out of the metal. This drying stage adds significantly to the quality of the finished product by helping to prevent runs and water marks. Once they have passed through the dry off oven, the parts are laid flat on trays and placed on carts so they can be painted in batches. The size and shape of the parts will determine how many are laid per tray during painting.
Liquid paint is applied using HVLP (high volume/low pressure guns). Liquid paint application involves the use of reducers that lower the paint’s viscosity to prevent any orange peel effect on the finished product. Unlike the powder coating application, liquid paint is not done utilizing our conveyor line, but instead the parts are laid flat on trays and painted in batches.
Once painted, all parts are placed back onto the carts and into the batch oven to be cured.
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The curing process is slightly different for liquid paints than it is for powder coatings. The catalysts and acclerators used in the liquid process allow the parts to be cured at much lower temperatures (low bake) than with powder coatings (high bake). The entire cart of painted parts is placed into a stand alone batch oven. Depending on the liquid paint applied, the cure cycle for liquid paint is 170–180 degrees for approximately 15–20 minutes, unless the substrate is plastic where only air drying is feasible.
The powder application is done using an electrostatic spray gun. Electrostatic guns work because the powder is electrically charged utilizing compressed air and voltage. The powder is held in the hopper and fluidized. Once the trigger is pulled on the spray gun, the powder is pulled to and out of the gun using compressed air. In addition a voltage source creates an electrostatic field at the tip of the gun which imparts the powder with a positive charge. As a result, the powder adheres to the grounded part until heated in our convection oven, which initiates the curing process. Once coated, the parts are sent into our convection oven.
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