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Process & Properties Guide of Powder Coating Thickness

Posted on: December 21st, 2017 by byersbushblog

The Properties of Powder Coating

Powder coating is a method of covering a surface with a dry polymer-based powder that gels when heated and hardens once cooled.  This powder flows freely upon the surface covering its entirety.  No solvents are needed as it is not a liquid.

In traditional liquid paints the solvent keeps the binding agent and the pigmented filler in solution.  Once the solvent evaporates, the paint will have formed a hard coating.  The difference with powder coating is that instead of using the solvent, it is solidified through heat causing the thermoplastic powder to reform around the surface into a coating stronger and more robust than conventional paints.

A process of electrostatic charge is often utilized to hold the powder in place prior to the heating stage.  Traditionally used on metal objects and surfaces like car and bike parts or aluminum excursions, it can also be applied to other materials.

The properties of powder coating give this method an advantage over traditional liquid coatings. VOC or volatile organic compounds are an environmental concern, but unlike liquid paint coatings, the powder coating process emits near zero VOC.  Paints, as a liquid coating, tend to run and sag if the application is too thick, but powder coatings can be applied liberally without this problem.  The results that would take multiple coats of paint to achieve can be accomplished in one session.

Since powder coatings are essentially little beads of plastic, the overspray can be recycled utilizing nearly all of the products.  Additionally, operating costs are less.  Just as winds can draw patterns in the desert sand, unique textures and tones can be achieved in the powder coating.  Powder coatings are corrosion resistant and impact resistant.

corrosion-resistant

Types of Powder Coatings and Powder Coating Production

We have two primary categories.  They are thermoplastic powders and thermosetting powders.  The thermoplastic type is pure.  When heat is applied to the coating, it gels and flows into shape but maintains the same elemental structure.  It’s like melting chocolate chips in a cookie.  The thermosetting type is a bit different.  The formula includes a cross-linker.

During the baking stage, the added chemicals react with the polymers on a molecular level transforming into a new coating with improved performance.

How do you decide which powder coating formula to use in any given situation?  The choice of which powder suits your needs best is determined by the look your hoping to achieve aesthetically and the performance properties needed to withstand environmental factors.  And of course price is a factor as well.

Powder Coat Application Thickness Ranges

The coating thickness is determined by a few different factors.  First is the application equipment for the coating powder.  Most often the tool of choice is the Corona gun.  This is an electrostatic gun.  It imposes an electrostatic charge onto the powder.  The item you are coating is grounded.  Then compressed air forces the powder out of the gun and the natural electro attraction draws the powder forward and onto the object. This is one reason that powder coating is so effective. Complete coverage and appropriate thickness is achieved. This method of application results in a thickness of 6 to 12 mils. Another factor is the choice of spray nozzle. Depending on the shape of the object being coated.

powder-coat-application

The other method is called fluidized dip coating.  When the desired thickness is 15 mils or more this method can be utilized.  A bed or deep container is filled with powder and the objects are then dipped into the bed.  The items are often preheated and the dipping bed is aerated to agitate the powder resulting in surface coverage.  The powder sticks to the heated object.  Items like dishwasher racks and testing equipment are coated using this method.

Film Thickness

Have you ever been to the playground with your child and seen one of those pits full of plastic balls?  Imagine that those balls represent the particles in a powder coat.  How many can you fit in the pit?  Well that depends on how large each ball is.  Similarly, the thickness of a powder coating is achieved by both the number of particles and the particle-size.

An additional factor is the limit of how many particles of like charge can be stacked atop one another.  This is affected by the temperature of the object being coated.  On the other hand, there is a minimum amount of powder particles that are required for the thinnest acceptable coating.

In this relationship, an increase in particle size also increases the maximum thickness achievable in the powder coating.  The size of each particle in the powder is measured in mesh.  Mesh describes measurement.  The fluidized dip process typically uses a 50 mesh or higher powder and the spray gun method uses around 70 mesh particles.  At times companies will load their Corona guns with 50 mesh powder, typically only used in dipping beds, and achieve 9 to 15 mils thicknesses.

Part Preparation Processes and Equipment

One preparation technique is preheating the object you wish to coat.  This method is not used in traditional liquid paint coating due to the danger of premature curing.  The reason this is done is because preheating can up the achievable thickness level when spraying.  Why? Because the object is heated electrically, charged particles will dissipate their charge faster.   As the powder shifts from a solid form into a liquid this charge is lost.  This makes it possible to stack more particles onto the unit before reaching the electrostatic limit.

At around 190F the powder will begin to melt or gloss over and additional powder can be sprayed.  Very high thicknesses are achievable if the part being coated can be preheated to 250F.  The thickness of 100 mils becomes possible.

Recoating

Here is another technique.  You can reheat an already coated part, bringing it back to the melted stage and add additional powder.  It is important when utilizing this method that the original coating has attained molten state or the second layer will not adhere properly.

Coating Techniques

When employing the electrostatic thermoplastic system, the techniques for spraying are basically the same as thermoset powder coating.  These are what you need to take note of.

One, aim the Corona gun at a perpendicular angle to the object you are coating.

Two, watch the inner corners and areas that may resist the electrostatic application, like a Faraday cage effect.

Third, remove all burrs to achieve a smooth subsurface.

Fourth, move slowly across the area and let the electrostatic force do its job.  Powder can also be applied via electrostatic discs.

heating-curing

Heating and Curing

Once the desired thickness is achieved the next step is heating, often in an oven, for the purpose of curing the coating.  The melted polymer reacts chemically forming a network encasing the object.  This process for curing is called cross linking.  Usually the powder must be kept at 200 Celsius for around ten minutes.  This can be accomplished using a convection oven type design or infrared lights.

As a growing industry there are new market opportunities emerging for powder coating technologies.  Currently the powder coatings are cured in ovens and set by infrared lamps.  But recent developments are showing that the polymers can be cured using UV/LED technology.

In the future powder coatings may be gelled and set utilizing simple LED lights.  This is an opportunity for both industries to grow and develop.  UV curable powders that are clear or black pigmented can be applied and cures much faster.

The Final Step: Cooling

The final step is the cooling process. This is an important step because the characteristics that make powder coating so desirable result during the cooling stage of the process.  While the electrostatic spray system for thermoplastic powder coating and thermoset powder coats are practically the same, the curing and cooling process is different.

The thermosetting powders harden into a coating during the curing reaction.  This is irreversible.  The thermoplastic powders on the other hand become hard after they have cooled down.

At that time they possess their complete mechanical properties.  If contact is made with the object being coated prior to a complete cooling, damage may be done to the powder coating.  Some have utilized a water cooling system.  The curing objects are soaked in cold water to bring the temperature down to an acceptable level.

When it comes to discussing the art of powder coating, an understanding of each of these steps is crucial.  The thickness of your coat depends upon the size of the powder particle and the potential of the electrostatic charge.

Thermoset polymers are used for thicker coatings and usually within a fluidized dip bed.

Thermoplastic powders work well for thinner coatings applied with a Corona spay gun.  Heated and properly cooled.  This technology is on the move and is growing, changing and improving every day.

We want you to enjoy the benefits of this technology.  The next time you need a coating, we want to help make it powder. Contact us

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