The key to successful powder coating is starting with a clean part. To maximize the benefits of powder coating and reduce field failures, the use of a powder coating pre-treatment plant is highly recommended.
Powder coating is increasingly accepted as the preferred finishing process for many applications. Increasingly stringent environmental regulations, rising costs in all areas, and demands by consumers for better quality and more durable products are among the challenges facing today’s finishers. Powder coatings provide a solution to these challenges and others. Powder coating is the technique of applying dry paint to the component.
The powdered paint is normally applied by using a powder feed system and gun to electrostatically charge and spray the powder onto the part. For some applications, the part being coated is dipped into a fluidized bed of powder. The coated part is then heated in an oven, or via infrared panels, to melt and cure the paint. During the curing process, a chemical cross-linking reaction is triggered and it is this chemical reaction that gives the powder coatings many of their desirable properties.
Powder Coating unquestionably offers excellent physical qualities when it comes to adhesion, corrosion resistance, and impact resistance. To maximize those characteristics, however, requires effective pretreatment. In order to get the powder coating to adhere to the substrate, the metal must be prepared so that the full quality of the powder-coated finish can be achieved. Proper pretreatment enhances the end result of the powder coating finish. The pretreatment process is comprised of essentially two units of operation: Cleaning and Phosphatizing (or conversion coating).
Powder Coating Pre-treatment Plant
A powder coating pre-treatment plant is a system that is used to clean and prepare metal surfaces for powder coating. The plant typically consists of a series of tanks or chambers that contain different chemicals and solutions. The metal parts are passed through the tanks or chambers in a sequence that is designed to remove dirt, grease, rust, and other contaminants. The pre-treatment process also helps to create a surface that is receptive to the powder coat.
Here are the steps in a typical powder coating pre-treatment plant:
- Degreasing: This step removes oils, grease, and other contaminants from the metal surface.
- Rinsing: This step removes the degreaser from the metal surface.
- Alkaline cleaning: This step neutralizes the metal surface and removes any remaining contaminants.
- Rinsing: This step removes the alkaline cleaner from the metal surface.
- Phosphate conversion coating: This step creates a thin layer of phosphate on the metal surface. The phosphate coating helps to improve the adhesion of the powder coat.
- Rinsing: This step removes the phosphate conversion coating solution from the metal surface.
- Drying: This step dries the metal surface before it is powder coated.
The specific pre-treatment process that is used will depend on the type of metal surface being coated and the desired finish.
Here are some of the benefits of using a powder coating pre-treatment plant:
- Improved adhesion of the powder coat
- Increased durability of the powder coat
- Better corrosion resistance
- Reduced environmental impact
Powder coating pre-treatment plants are an essential part of the powder coating process. By properly pre-treating the metal surface, you can ensure that the powder coat will adhere properly and provide a long-lasting, durable finish.
The Importance of Cleaning and Powder coating Pre-treatment plant
Cleaning and pre-treatment are essential steps in the powder coating process. They are crucial for ensuring the adhesion, durability, and overall quality of the powder coat.
Cleaning
Proper cleaning removes contaminants such as dirt, grease, oils, and rust from the metal surface. These contaminants can interfere with the adhesion of the powder coat, leading to premature peeling or flaking.
Pre-treatment
Pre-treatment involves applying a conversion coating to the metal surface. This coating creates a microscopic crystalline structure that enhances the adhesion of the powder coat. It also provides additional corrosion resistance.
The specific pre-treatment process used depends on the type of metal being coated and the desired finish. Common pre-treatment methods include:
- Iron phosphating: This is the most common pre-treatment method for powder coating. It is suitable for a wide variety of metals, including steel, aluminum, and zinc.
- Zinc phosphating: This method provides superior corrosion resistance compared to iron phosphating. It is often used for applications where corrosion is a major concern
- Passivation: This method is used for aluminum surfaces. It creates a thin oxide layer that protects the aluminum from corrosion.
Benefits of Proper Cleaning and Pre-treatment
Proper cleaning and pre-treatment offer several benefits for powder coating:
- Enhanced Adhesion: A clean and pre-treated surface provides a strong bond for the powder coat, preventing premature peeling or flaking.
- Increased Durability: Proper pre-treatment enhances the durability of the powder coat, making it more resistant to weathering, abrasion, and chemicals.
- Improved Corrosion Resistance: Pre-treatment provides additional corrosion protection to the metal substrate, extending the lifespan of the coated product.
- Reduced Environmental Impact: Powder coating is an environmentally friendly finishing method due to the absence of solvents. Proper pre-treatment further minimizes environmental impact by reducing the need for re-coating due to premature failure.
Conclusion
Cleaning and pre-treatment are fundamental steps in achieving a high-quality powder coat finish. They are essential for ensuring the adhesion, durability, and corrosion resistance of the powder coat, ultimately extending the lifespan of the coated product. Investing in proper cleaning and pre-treatment is an investment in the long-term performance and value of your powder-coated products.
Cleaning is the most important step in the pretreatment process. The performance of the conversion coating and ultimately the powder-coated finished product, will only be as good as the cleaning in this first unit of operation.
In order for conversion coatings to adhere to the metal surface, all impurities must be removed from the surface of the substrate. Soils such as cutting fluids, drawing compounds, rust inhibitors, grease and oil, shop soil and common everyday dirt must be cleaned from the metal.
In fact, the degree of cleanliness required for powder coating is higher than that required for liquid coatings. With liquid coating, if a small amount of oil, such as a fingerprint, is present on the substrate, liquid coating can pick that oil up, emulsify it and bring it into the paint coating itself without inhibiting adhesion. Powder coating, without a thorough cleaning, will plate right on top of the soil and subsequently lose adhesion.
The Fundamentals of Cleaning with a Powder coating Pre-treatment plant
Degreasing
The first step in cleaning is to remove any oils, grease, or other contaminants from the metal surface. This can be done using a variety of methods, including:
- Alkaline degreasing: This method uses an alkaline solution to remove contaminants
- Emulsion degreasing: This method uses an emulsion of oil and water to remove contaminants.
- Solvent degreasing: This method uses a solvent to remove contaminants.
The specific degreasing method that is used will depend on the type of contaminant that is being removed.
Rinsing
After the metal surface has been degreased, it must be rinsed thoroughly to remove any traces of the degreaser. This can be done using a variety of methods, including:
- Dip rinsing: This method involves dipping the metal surface in a tank of clean water.
- Spray rinsing: This method involves spraying the metal surface with clean water.
- Flow rinsing: This method involves passing the metal surface through a tank of clean water.
The specific rinsing method that is used will depend on the size and shape of the metal part.
Alkaline cleaning
After the metal surface has been rinsed, it must be cleaned with an alkaline cleaner. This will help to neutralize the metal surface and remove any remaining contaminants. The specific alkaline cleaner that is used will depend on the type of metal that is being cleaned.
Rinsing
After the metal surface has been alkaline cleaned, it must be rinsed thoroughly to remove any traces of the alkaline cleaner. This can be done using the same methods that were used for degreasing.
Phosphate conversion coating
After the metal surface has been rinsed, it must be treated with a phosphate conversion coating. This will create a thin layer of phosphate on the metal surface. The phosphate coating will help to improve the adhesion of the powder coat.
Rinsing
After the metal surface has been phosphate conversion coated, it must be rinsed thoroughly to remove any traces of the phosphate conversion coating solution. This can be done using the same methods that were used for degreasing.
Drying
The final step in cleaning is to dry the metal surface thoroughly. This can be done using a variety of methods, including:
- Air drying: This method involves exposing the metal surface to air to allow it to dry naturally.
- Forced air drying: This method involves using a fan to blow air over the metal surface to help it dry.
- Oven drying: This method involves placing the metal surface in an oven to dry it.
The specific drying method that is used will depend on the size and shape of the metal part.
It is important to note that the specific cleaning process that is used will vary depending on the type of metal being coated, the desired finish, and the specific requirements of the application.
Here are some additional tips for cleaning with a powder coating pre-treatment plant:
- Use fresh water for all rinsing steps.
- Change the degreasing solution regularly.
- Monitor the pH of the alkaline cleaner solution regularly.
- Use a clean oven for oven drying.
- Allow the metal surface to cool completely before applying the powder coat.
By following these tips, you can help to ensure that your powder-coated products have a high-quality, long-lasting finish.
The labor constraints of mechanical cleaning: sandblasting, shot blasting, air blasting or even wire brush have rendered the approach virtually non-existent in the modern powder coating operation. Chemical cleaning is without a doubt the most popular approach.
Three Chemicals are used:
- Alkalines
- Acids
- Solvents
Alkalines are the most popular. They are the most versatile and most widely used. Acids in general are limited to soils that can be cleaned with acid, mainly corrosion and scale. Acids tend to perform poorly on heavier oils and greases. Solvents, on the other hand, are very good for oils and greases, but not much more.
They are limited in their efficiency in that they will not touch corrosion and scale. The use of solvents also raises a number of environmental and occupational safety concerns. Most importantly for the purposes of this discussion, the degree of cleanliness involved with a solvent cleaner is not suited for powder coating. Effective cleaning involves careful consideration of these issues:
- Substrate
- Soil
- Cleaning System
- Waste Treatment System
- Occupational Safety
- Environmental Concerns
Common Chemical Cleaning Systems with a Powder Coating Pre-treatment Plant
Here are some common chemical cleaning systems used in powder coating pre-treatment plants:
Alkaline Degreasing This method involves immersing the metal part in a hot alkaline solution to remove oils, grease, and other contaminants. The alkaline solution can be either a sodium hydroxide or potassium hydroxide solution.
Emulsion Degreasing This method involves spraying the metal part with an emulsion of oil and water. The oil droplets in the emulsion attach to the contaminants and carry them away. This method is effective at removing light to medium soils.
Solvent Degreasing This method involves immersing the metal part in a solvent, such as mineral spirits or toluene. The solvent dissolves the contaminants and removes them from the metal surface. This method is effective at removing heavy soils.
Spray Wash This method involves spraying the metal part with a high-pressure stream of water or solvent. The spray wash removes contaminants from the metal surface by physical force.
Pressure Wand This method involves using a pressure wand to direct a stream of water or solvent at the metal surface. The pressure wand is effective at removing contaminants from hard-to-reach areas
Ultrasonic Cleaning This method involves immersing the metal part in an ultrasonic bath. The ultrasonic waves in the bath create cavitation, which breaks up and removes contaminants from the metal surface. This method is effective at removing contaminants from crevices and small holes.
The specific chemical cleaning system that is used will depend on the type of metal being coated, the desired finish, and the specific requirements of the application.
The chemical cleaning system can be performed three ways:
- Immersion or soak tank
- Spray wash
- Pressure wand
Hand washing obviously represents the most basic approach to cleaning. It is extremely labor intensive and labor sensitive. The cleaning will only be as good as the person doing the job.
Hand washing is not recommended prior to powder coatings. Soil is moved around, but never completely off, the surface of the substrate. This technique provides such inadequate cleaning that we do not recommend that it be employed prior to coating of any sort.
The next most fundamental system would be immersion or soak tank cleaning. This represents a step up from hand washing in which parts are immersed in a heated solution which helps to emulsify and remove the soils. An immersion system allows the handling of parts larger than those which can easily be hung on a conveyor system through a spray washer.
There is no impingement (physical scrubbing action) involved in an immersion system. The cleaning action is much like dipping dishes into soapy sink water and then simply rinsing them off.
This type of approach would not provide proper cleaning without some physical scrubbing or spray working to remove dirt particles. Occasionally some systems will feature agitation that provides some impingement, but there is nothing in this approach, except the chemicals, that aids in the cleaning action.
Immersion is ideal, however, for cleaning strangely configured parts where the physical spray cleaner is unable to reach every area of the substrate surface due to limitations posed by the configuration. In an immersion system the part is completely submerged ensuring, absent an air pocket, that the cleaning chemical is covering the entire surface.
Immersion tanks are easily contaminated. Soil is always in suspension or in contact with the part being cleaned. To a certain degree this also happens in a spray washer, but spray washers are generally overflowed such that oil and grease soils are skimmed off at a constant rate and only the particles settle out to the bottom of the tank and are not being recirculated.
Pressure wand or nozzle washing is a hand spray system of pressure washers not unlike those found in the do-it-yourself carwash. Pressure wand washing offers the impingement, heat and chemicals important to good cleaning. It is a less than desirable approach because, like hand washing, it is operator intensive. If the operator performs well and reaches every area of the surface; the part will be clean. To miss just a spot puts the ultimate performance of the coated product in jeopardy.
The most popular, all-around system is spray washing. A power spray washer system offers all the factors of a good cleaning. Spray washers hold the chemical in a tank from which cleaner is sprayed onto the metal. This approach provides the combination of impingement with the cleaning properties of the chemical to expedite the cleaning process.
The important contact “Time” component of the W.A.T.C.H System is controlled in this approach ensuring the spray reaches the surface for exactly the same amount of time on each part. When the temperature is involved, the cleaning action is enhanced all the more.
Spray washers can be of any size, and can be configured in a customized manner to suit a variety of pretreatment requirements. One limitation to spray washing can be water quality which, when substandard, can impede the efficiency of the cleaning.
Conversion Coating of Powder Coating Pre-treatment Plant
The next step in the pretreatment process is conversion coating. A conversion coating is used on a metal substrate to provide adhesion and corrosion resistance.
There are three basic types of conversion coatings:
- Iron Phosphatizing
- Zinc Phosphatizing
- Chromic Conversion Coating
Iron Phosphatizing
Iron phosphatizing is the easiest, most commonly used conversion coating for powder-coated products. Iron phosphate coating can be used on steel, aluminum, zinc, and galvanized. It leaves an amorphous rather than a crystalline coating.
Iron phosphate coating converts the surface of the steel to an iron phosphate type coating which will increase adhesion and decrease corrosion under the coating. Iron phosphatizing is easy to use and to maintain. It is also less expensive, safe, and easier to dispose of.
Zinc Phosphatizing
After iron phosphatizing, the most versatile and popular system with powder coating would be zinc phosphatizing. It achieves the quality performance expected from powder coating. Zinc phosphatizing not only converts the surface of the substrate but also overlays a crystalline structure.
It actually grows a zinc phosphate crystal on the surface of the metal. Under the examination of an electron microscope, iron phosphate coating appears smooth while zinc phosphate coating leaves an intricate matrix to which the powder coating can adhere.
This gives the metal a heavier coating than iron which in turn also offers superior corrosion resistance. The same substrates pretreated with iron phosphate coating can be pretreated with a zinc phosphate coating.
Zinc phosphatizing is harder to maintain and there are more critical controls. Zinc is less tolerant to changes in time, temperature, concentration, and pH. It is more expensive. Zinc phosphatizing requires a higher capital expense in that tanks have to be stainless steel or acid resistant. The zinc content also renders it more difficult to dispose of.
Where absolute maximum corrosion resistance is required, zinc phosphatizing would be our recommended approach. Even though powder coatings exhibit very high corrosion resistance as opposed to conventional liquid coatings, zinc phosphate will further improve upon those corrosion resistance characteristics. Products which require long service life or which must endure the torture of harsh environmental or atmospheric conditions are probably best pretreated with a zinc phosphate coating.
How to Phosphatize
Iron phosphate coating can be applied in all three chemical cleaning systems (1. immersion or soak tank, 2. spray wash, or 3. pressure wand.) The cleaning and conversion coating is typically accomplished in stages. There can be as few as two stages:
- Clean & Conversion Coat
- Rinse
Three Stages
- 1-Clean & Conversion Coat
- 2-Rinse
- 3- Seal
Or a five-stage system:
- 1-Clean
- 2-Rinse
- 3-Conversion Coat
- 4- Rinse
- 5- Seal
- Additional stages can be used in custom-designed systems. Zinc phosphatizing must be done in a five-stage operation.
In pretreating powder-coated products, we recommend the use of a five-stage system in which the cleaning stage is separated from the phosphatizing stage. The overwhelming importance of a thorough cleaning prior to powder coating has led us to conclude that a five-stage system works best.
Rinsing
While cleaning and phosphatizing are the principal units of operation in the pretreatment process, the importance of thorough rinsing cannot be overlooked. It is very important that the rinse stages be kept clean to avoid recontamination of the parts.
Any chemical remaining on the part becomes soil which must be removed prior to proceeding to the next stage of the system. The rinsing stage cannot be overlooked. There are two types of final rinses: chromated and non-chromate. The chromated final rinse has always been known as the standard of the industry, but it has come under increasing scrutiny from environmental and occupational safety regulators.
The trend is away from chrome to safer, nonchromated rinses. Without an absolutely clean part, conversion coating will not perform to the extent that offers the subsequently powder-coated parts to realize optimum adhesion and corrosion resistance. A commitment to thorough cleaning is key to maximizing powder coating performance